Architectures and Applications of BODIPY-Based Conjugated Polymers

polymers

Review Architectures and Applications of BODIPY-Based Conjugated Polymers

Yiqi Fan 1,†, Jinjin Zhang 1,†, Zhouyi Hong 1, Huayu Qiu 1,2, Yang Li 1,* and Shouchun Yin 1,*

1 College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, China; [email protected] (Y.F.); [email protected] (J.Z.); [email protected] (Z.H.); [email protected] (H.Q.) 2 Key Laboratory of Organosilicon Chemistry and Materials Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 311121, China * Correspondence: [email protected] (Y.L.); [email protected] (S.Y.) † These authors contributed equally to this work.

Abstract: Conjugated polymers generally contain conjugated backbone structures with benzene, hete- rocycle, double bond, or triple bond, so that they have properties similar to semiconductors and even conductors. Their energy band gap is very small and can be adjusted via chemical doping, allowing for excellent photoelectric properties. To obtain prominent conjugated materials, numerous well- designed polymer backbones have been reported, such as polyphenylenevinylene, polyphenylene acetylene, polycarbazole, and polyfluorene. 4,40-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)- based conjugated polymers have also been prepared owing to its conjugated structure and intriguing optical properties, including high absorption coefficients, excellent thermal/photochemical stability, and high quantum yield. Most importantly, the properties of BODIPYs can be easily tuned by chemical modification on the dipyrromethene core, which endows the conjugated polymers with multiple functionalities. In this paper, BODIPY-based conjugated polymers are reviewed, focusing on their structures and applications. The forms of BODIPY-based conjugated polymers include linear, coiled, and porous structures, and their structure–property relationship is explored. Also, typical applications in optoelectronic materials, sensors, gas/energy storage, biotherapy, and bioimaging are presented and discussed in detail. Finally, the review provides an insight into the challenges in the Citation: Fan, Y.; Zhang, J.; Hong, Z.; development of BODIPY-based conjugated polymers. Qiu, H.; Li, Y.; Yin, S. Architectures and Applications of BODIPY-Based Keywords: BODIPY; conjugated polymers; architecture; structure–property relationship; application Conjugated Polymers. Polymers 2021, 13, 75. https://dx.doi.org/10.3390/ polym13010075 1. Introduction Received: 15 November 2020 Accepted: 21 December 2020 In the 1970s, Shirakawa et al. accidentally synthesized polyacetylene and then Published: 27 December 2020 developed its conductive applications under close cooperation with MacDiarmid and Heeger [1–4]. Since conductive polymers emerged, conjugated polymers, namely, poly- Publisher’s Note: MDPI stays neu- mers with conjugated bond structures, have attracted increasing interest owing to their tral with regard to jurisdictional claims extremely important roles in optoelectronic devices [5–7] and biological diagnosis and in published maps and institutional treatment [8,9]. In general, the energy band gap of conjugated polymers is very small affiliations. and can be adjusted via chemical doping to obtain excellent optoelectronic properties. Furthermore, conjugated polymers offer a unique combination of properties not available from other materials, such as mechanical flexibility, facile solution processing, scalable architecture, and low cost. To develop conjugated materials with better performance, a Copyright: © 2020 by the authors. Li- large number of functional units have been successfully introduced to polymer systems, censee MDPI, Basel, Switzerland. This including phenylacetylene [10], thiophene [11], carbazole [12], fluorene [13], and 4,40- article is an open access article distributed difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) [14]. Among them, BODIPY (Figure1) under the terms and conditions of the Creative Commons Attribution (CC BY) is famous for its versatility as fluorophores and is widely applied in fluorescent sensors, license (https://creativecommons.org/ organic electronics, biotherapy, and imaging [14–18], which provides a possibility for the licenses/by/4.0/). multifunctional development of conjugated polymers.

Polymers 2021, 13, 75. https://dx.doi.org/10.3390/polym13010075 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 32

and low cost. To develop conjugated materials with better performance, a large number of functional units have been successfully introduced to polymer systems, including phenylacetylene [10], thiophene [11], carbazole [12], fluorene [13], and 4,4′- difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) [14]. Among them, BODIPY (Figure Polymers 2021, 13, 75 1) is famous for its versatility as fluorophores and is widely applied in fluorescent sensors,2 of 30 organic electronics, biotherapy, and imaging [14–18], which provides a possibility for the multifunctional development of conjugated polymers.

Figure 1. Structural representation of the 4,4′-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) core and BODIPY inter- Figure 1. Structural representation of the 4,40-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) core and BODIPY intermedi- mediates suited to polymerization via transition metal–catalyzed polycondensation reactions or electrochemical polymer- ates suited to polymerization via transition metal–catalyzed polycondensation reactions or electrochemical polymerization. ization. In 1968, BODIPY was reported for the first time by Treibs and Kreuzer [19], and In 1968, BODIPY was reported for the first time by Treibs and Kreuzer [19], and later, later, Hee and Richard discovered that BODIPY could be used as a fluorescent probe Hee and Richard discovered that BODIPY could be used as a fluorescent probe with high with high affinity for D1 and D2 dopaminergic receptors [20]. Since then, BODIPYs have affinitygained increasingfor D1 and attentionD2 dopaminergic in most fields.receptors In most [20]. cases,Since BODIPYsthen, BODIPYs have excellenthave gained optical in- creasingproperties, attention including in most large fields. molar In extinction, most cases, narrow BODIPYs absorption, have excellent and high optical quantum properties, yields. includingBODIPYs large are also molar stable extinction, to thermal/photochemical narrow absorption, stimulus and high and quantum possess yields. good BODIPYs solubility areto commonalso stable solutions, to thermal/photochemical which is conducive stim toulus the and processing possess good and usesolubility of optoelectronic to common solutions,devices. Most which importantly, is conducive the to optical the processing properties and and use electronic of optoelectronic structure of devices. BODIPYs Most can importantly,be easily adjusted the optical via systematic properties structural and electr modificationsonic structure [21 of,22 BODIPYs]. For instance, can be modifyingeasily ad- justedthe BODIPY via systematic core with structural donor units modificati at 2,3,5,6-positionsons [21,22]. would For instance, cause a bathochromicmodifying the shift BOD- in IPYthe absorptioncore with anddonor fluorescence units at 2,3,5,6-positi spectra. Allons the advantageswould cause of BODIPYa bathochromic make it ashift promising in the absorptioncandidate for and the fluorescence preparation spectra. of conjugated All the polymers advantages with of innovative BODIPY make molecular it a promising structures candidateand unique for electronic the preparation properties of [conjugated22]. polymers with innovative molecular structures Dueand tounique the diverse electronic strategies properties for the [22]. functionalization of the BODIPY, several synthesis methods have been presented to establish BODIPY-based conjugated polymers through a Due to the diverse strategies for the functionalization of the BODIPY, several synthe- wide variety of BODIPY intermediates (Figure1) with reactive groups (halogen, alkynyl). sis methods have been presented to establish BODIPY-based conjugated polymers Taking halogenated BODIPYs as an example, halogen atoms can be easily introduced through a wide variety of BODIPY intermediates (Figure 1) with reactive groups (halogen, into BODIPY core in α-, β-, or meso-positions via electrophilic substitution reactions or alkynyl). Taking halogenated BODIPYs as an example, halogen atoms can be easily intro- using halogenated dipyrromethene precursors [14]. Subsequently, halogenated BODIPYs duced into BODIPY core in α-, β-, or meso-positions via electrophilic substitution reac- can homopolymerize by nickel-catalyzed Yamamoto cross-coupling reaction or undergo tions or using halogenated dipyrromethene precursors [14]. Subsequently, halogenated alternating copolymerization by palladium-catalyzed cross-coupling reactions, including BODIPYsSuzuki, Heck, can homopolymerize Sonogashira, and by Stille nickel-catalyz cross-couplinged Yamamoto reactions. cross-coupling Without halogen reaction atom, or undergoBODIPY-based alternating conjugated copolymerization polymers could by pall beadium-catalyzed obtained by Oxidation cross-coupling and Friedel-Crafts reactions, includingcross-coupling Suzuki, reactions Heck, Sonogashira, and introducing and alkynyl Stille cross-coupling in α- and β-positions reactions. of BODIPYWithout coreshalo- gento support atom, SonogashiraBODIPY-based cross-coupling conjugated reactionspolymers arecould also be feasible obtained methods. by Oxidation Additionally, and BODIPYs combined with electroactive 3,4-ethylenedioxythiophene (EDOT) or bithiophene can be prepared via electrochemical polymerization, as reported by Algi, Cihaner, and

Skabara [23–25]. Current studies have found that by incorporating BODIPYs into the constructions of conjugated polymers, the corresponding polymers inherit the excellent optical properties of BODIPY due to the increase in conjugated length [14,26–28]. For example, to study the correlation between the number of BODIPY molecules and the polymer properties, Allen and coworkers [26] synthesized a monomer, dimer, trimer, and polymer of BODIPYs with Polymers 2021, 13, 75 3 of 30

mesityl groups in the meso-position, and the photophysical, electrochemical, and electrogenerated chemiluminescence of the obtained materials were investigated. The absorption and fluorescence of the materials showed a gradually red shift from monomer to dimer to trimer to polymer, accompanied by increasing molar absorption coefficients and decreasing quantum yields [26]. In this review, we focus on the ample structures of BODIPY-based conjugated polymers (including linear and porous structures) based on different reaction sites and functionality degrees of monomers. Special attention is devoted to the applications of conjugated polymers containing BODIPYs in optoelectronic materials, biotherapy, bioimaging, sensors, and gas/energy storage, wherein the structure–property relationship exerts tremendous influence.

2. Structures of BODIPY-Based Conjugated Polymers According to the functionality degree, structures of BODIPY-based conjugated polymers can be classiﬁed into linear/coiled, porous, dendritic, and crosslinked. Thus, in the following subsections, the synthesis, property, and structure–property relationship of BODIPY-based conjugated polymers are discussed from a structural perspective. Some structures and synthetic methods of BODIPY-based conjugated polymers are summarized in Table 1.

2.1. Linear/Coiled Conjugated Polymers Generally, the monomers forming a linear/coiled polymer should have two functional groups, such as halogen atoms and alkynyl. In the constructed conjugation of linear/coiled polymers, BODIPYs can be incorporated into the polymers as backbone units, pendant side chains, or end groups.

2.1.1. BODIPY as Backbone Units When BODIPYs act as the backbone units, the structure and properties of conjugated polymers are prone to site-selective synthesis; that is, polymerization through either α- or β- positions. The β-connected copolymers possess linear structures, whereas the α-connected ones possess coiled structures [29]. Zade and coworkers [30] presented three alternate copolymers, 1–3 (Figure2), containing BODIPYs and acetylene with different connectivities (α-α-connected, α-β-connected, β-β-connected) and examined the effect of site-selective copolymerization. The decomposition temperatures of polymers 1, 2, and 3 were 266, 312, and 200 ◦C respectively, because of the difference in the compactness of the polymer melt. In other words, the coiled structures contributed to increasing the decomposition temperature. In addition, the solid-state optical band gaps of 1, 2, and 3 were 1.28, 1.42, and 1.67 eV respectively, which demonstrated that the π-conjugation was more efficient in the case of polymerization through the α-position of BODIPYs than in the polymerization through the β-position. The site-selective synthesis mainly altered the highest occupied molecular orbital (HOMO) energy levels of polymers rather than the lowest unoccupied molecular orbital (LUMO) energy levels. Based on the theoretical calculation results, polymers 1–3 were subjected to charge-carrier mobility measurements in field-effect transistors devices, in which 1 and 3 acted as n-type semiconductors and 2 as a p-type semiconductor. However, their transistor property was poor. Nevertheless, this work provided an efficient approach for tuning the band gaps of BODIPY-based conjugated polymers via site-selective polymerization. Likewise, Zade and coworkers [31] synthesized polymers 4–6 (Figure2 ) with BODIPYs and 5,5-bis(hexyloxymethyl)-5,6-dihydro-4H-cyclopenta[c]-thiophene (CPT) as comonomers. The CPT was connected through the α-position or β-position of the BOD- IPY molecule, and 6 had acetylene as the spacers. Polymer 4 exhibited the highest p-channel mobility in polymer field-effect transistors, attributed to the efficient conjugation of the α-connectivity of BODIPYs as well as their planar structure without methyl at β-positions. Due to the existence of the spacers, 6 with planarity of the backbone showed the second- highest host mobilities. Except for charge mobility, the BODIPYs with connectivity at Polymers 2021, 13, x FOR PEER REVIEW 4 of 32

lowest unoccupied molecular orbital (LUMO) energy levels. Based on the theoretical calculation results, polymers 1–3 were subjected to charge-carrier mobility measurements in field-effect transistors devices, in which 1 and 3 acted as n-type semiconductors and 2 as a p-type semiconductor. However, their transistor property was poor. Nevertheless, this work provided an efficient approach for tuning the band gaps of BODIPY-based conjugated polymers via site-selective polymerization. Likewise, Zade and coworkers [31] synthesized polymers 4–6 (Figure 2) with BODIPYs and 5,5-bis(hexyloxymethyl)-5,6-dihydro-4H-cyclopenta[c]-thiophene (CPT) as comonomers. The CPT was connected through the α-position or β-position of the BODIPY molecule, and 6 had acetylene as the spacers. Polymers 2021, 13, 75 Polymer 4 exhibited the highest p-channel mobility in polymer field-effect transistors,4 of at- 30 tributed to the efficient conjugation of the α-connectivity of BODIPYs as well as their planar structure without methyl at β-positions. Due to the existence of the spacers, 6 with planarity of the backbone showed the second-highest host mobilities. Except for charge α-position may produce better photovoltaic properties [29] and more intriguing nonlinear mobility, the BODIPYs with connectivity at α-position may produce better photovoltaic optical properties [32]. properties [29] and more intriguing nonlinear optical properties [32].

FigureFigure 2.2.Chemical Chemical structuresstructures ofof polymerspolymers1–6 1–6(Reproduced (Reproduced with with permission permission from from Reference Reference [ 30[30].]. Copyright Copyright @ @ 2016,2016, JohnJohn WileyWiley and and Sons. Sons. Reproduced Reproduced with with permission permission from from Reference Reference [ 31[31].]. Copyright Copyright @ @ 2015, 2015, American American Chemical Chemical Society). Society).

ApartApart fromfrom site-selectivesite-selective copolymerization,copolymerization, anotheranother similarsimilar synthesissynthesis methodmethod isis site-site- selectiveselective modificationmodification withwith polymerizablepolymerizable groupsgroups beforebefore polymerization.polymerization. ThreeThree polymers,polymers, 7–97–9 (Figure(Figure3 ),3), have have been been constructed constructed through through the the electropolymerization electropolymerization of of BODIPYs BODIPYs functionalizedfunctionalized by EDOT EDOT at at 3,5-positions 3,5-positions or or 2,6-positions. 2,6-positions. Polymer Polymer 8 modified8 modified through through 3,5- 3,5-positionspositions showed showed a bathochromic a bathochromic shift shift of of the the absorption absorption wavelength wavelength and and a decreasedecrease inin bandband gapsgaps thanthan thosethose ofof polymerspolymers 77 andand9 9modified modified throughthrough2,6-positions, 2,6-positions, whichwhich furtherfurther provedproved thatthat thethe connectedconnected positionspositions resultedresulted inin significantsignificant differencesdifferences ininthe theconjugation conjugation Polymers 2021, 13, x FOR PEER REVIEW 5 of 32 degree.degree. PolymerPolymer 88 displayeddisplayed thethe lowestlowest bandband gapgap ofof 0.80.8 eV,eV, withwith reversiblereversible reductionreductionand and

oxidationoxidation processesprocesses [[23,25,33].23,25,33].

Figure 3. ChemicalChemical structures structures of of polymers polymers 7–97–9 (Reproduced(Reproduced with with permission permission from from Reference Reference [23]. [ 23Copyright]. Copyright @ 2009, @ 2009,Else- Elsevier.vier. Reproduced Reproduced with with permission permission from from Reference Reference [25]. [Copyright25]. Copyright @ 2009, @ 2009, American American Chemical Chemical Society. Society. Reproduced Reproduced with permission from Reference [33]. Copyright @ 2014, Elsevier). with permission from Reference [33]. Copyright @ 2014, Elsevier).

The properties of BODIPY-based conjugated polymers also depend on the elaborate design of BODIPYBODIPY monomersmonomers with with functional functional modiﬁcation. modification. For For instance, instance, the the incorporation incorpora- tionof oligo(ethylene of oligo(ethylene glycol)methyl glycol)methyl ether ether residues residues as polymer as polymer side chains side chains into BODIPYs into BODIPYs at the at3,5- the or 3,5- meso-positions or meso-positions led to led a signiﬁcant to a significant increase increase in water in water solubility solubility [34]. In [34]. another In another work, work,Liu and Liu coworkers and coworkers [35] presented [35] presented a series a of series near-infrared of near-infrared (NIR) or (NIR) deep-red or deep-red emissive emis- poly- sive polymeric dyes 10–14 (Figure 4) containing extended π-conjugated BODIPY cores. Polymers 10 and 11 were synthesized via the Sonogashira cross-coupling reaction of 2,6- diethynel BODIPYs with 2,6-diiodo-functionalized BODIPYs bearing aldehyde derivative-modified mono-styryl or di-styryl groups at the 3,5 positions. For better comparison, 12, 13, and 14 were prepared by the same reaction of 2,6-diiodo-functionalized BODIPYs bearing mono-styryl group, di-styryl group, and dodecyl with 2,5-diethynyl-3-decylthiothene, respectively. The fluorescence quantum yields of 12–14 were significantly lower than those of their analog polymers 10 and 11, due to the heavy atom effect of sulfur. In solutions, the absorption and emission maxima of 11 were centered at 738 and 760 nm respectively, with redshifts of 41 and 45 nm compared with the spectrum for 10. Similarly, by increasing the number of conjugated connections at the 3,5-positions of BODIPY cores, the absorption and emission maxima of 14, 13, and 12 successively redshifted, which allowed the tuning of BODIPY-based polymers to the NIR region. As for thin films, the absorption and emission maxima of 10–12 displayed further redshift, attributed to the intermolecular electronic interaction and/or increasing planarity in the solid state. Remark- ably, the extension of conjugated BODIPYs at 3,5-position resulted in deep-red or NIR emission [36,37], and such NIR-emissive BODIPY-containing polymers lay a foundation for the development of NIR-imaging biomedical applications.

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meric dyes 10–14 (Figure4) containing extended π-conjugated BODIPY cores. Polymers 10 and 11 were synthesized via the Sonogashira cross-coupling reaction of 2,6-diethynel BODIPYs with 2,6-diiodo-functionalized BODIPYs bearing aldehyde derivative-modified mono-styryl or di-styryl groups at the 3,5 positions. For better comparison, 12–14 were prepared by the same reaction of 2,6-diiodo-functionalized BODIPYs bearing mono-styryl group, di-styryl group, and dodecyl with 2,5-diethynyl-3-decylthiothene, respectively. The fluorescence quantum yields of 12–14 were significantly lower than those of their analog polymers 10 and 11, due to the heavy atom effect of sulfur. In solutions, the absorption and emission maxima of 11 were centered at 738 and 760 nm respectively, with redshifts of 41 and 45 nm compared with the spectrum for 10. Similarly, by increasing the number of conjugated connections at the 3,5-positions of BODIPY cores, the absorption and emission maxima of 14, 13, and 12 successively redshifted, which allowed the tuning of BODIPY-based polymers to the NIR region. As for thin films, the absorption and emission maxima of 10–12 displayed further redshift, attributed to the intermolecular electronic interaction and/or increasing planarity in the solid state. Remarkably, the extension of conjugated BODIPYs at 3,5-position resulted in deep-red or NIR emission [36,37], and Polymers 2021, 13, x FOR PEER REVIEWsuch NIR-emissive BODIPY-containing polymers lay a foundation for the development6 of of32 NIR-imaging biomedical applications.

FigureFigure 4.4. ChemicalChemical structuresstructures ofof polymerspolymers 10–1410–14 (Reproduced(Reproduced withwith permissionpermission fromfrom ReferenceReference [[34].34]. CopyrightCopyright @@ 2012,2012, RoyalRoyal SocietySociety ofof Chemistry).Chemistry).

BODIPY-fused aromatic rings rings such such as as phenyl phenyl [38], [38 ],furan furan [39], [39 and], and thiophene thiophene [40] [ 40are] arepromising promising candidates candidates for for performance performance development development of of BODIPY-containing BODIPY-containing polymers. Chujo and coworkerscoworkers [[40]40] reported two conjugatedconjugated homopolymers 15 and 16 (Figure(Figure5 5)) consistingconsisting ofof thiophene-fusedthiophene-fused BODIPYsBODIPYs andand synthesizedsynthesized viavia oxidativeoxidative coupling.coupling. WithWith thethe suppressionsuppression ofof torsions,torsions, low-layinglow-laying HOMO,HOMO, smallsmall bandband gap,gap, andand rigidrigid framework,framework, thethe thiophene-fusedthiophene-fused BODIPYsBODIPYs endowed endowed the the polymers polymers with with low low band band gap gap and and high high stability stability in atmospherein atmosphere and and a broad a broad absorption absorption spectrum. spectrum. Polymer Polymer16 showed 16 showed wider wider absorption absorption and lowerand lower band band gaps gaps than thethan methyl-substituted the methyl-substituted analog analog15 due 15 to due the to smaller the smaller steric hindrance.steric hin- Suchdrance. ring-fused Such ring-fused BODIPYs BODIPYs can open can new open approaches new approaches for the construction for the construction of low-band-gap of low- conjugatedband-gap conjugated polymers withpolymers broad with absorption broad absorption and good stability.and good stability.

Figure 5. Chemical structures of polymers 15 and 16 (Reproduced with permission from Reference [40]. Copyright @ 2014, American Chemical Society).

The most common and patterned approach to developing novel polymer materials containing BODIPYs is selecting the appropriate comonomers to facilitate intra-molecular charge transfer excitation [41–47]. To precisely predict the performance of alternating copolymers containing BODIPYs, Thayumanavan and coworkers [48] chose five aromatic groups with different electron-donating capacity (including fluorene (17), carbazole (18), bithiophene (19), cyclopentadithiophene (20), and dithienopyrrole (21)) to copolymerize with BODIPYs and investigated the influence of comonomers on the properties of the ensuing polymers 17–21 (Figure 6), especially the frontier molecular orbital energy levels.

Polymers 2021, 13, x FOR PEER REVIEW 6 of 32

Figure 4. Chemical structures of polymers 10–14 (Reproduced with permission from Reference [34]. Copyright @ 2012, Royal Society of Chemistry).

BODIPY-fused aromatic rings such as phenyl [38], furan [39], and thiophene [40] are promising candidates for performance development of BODIPY-containing polymers. Chujo and coworkers [40] reported two conjugated homopolymers 15 and 16 (Figure 5) consisting of thiophene-fused BODIPYs and synthesized via oxidative coupling. With the suppression of torsions, low-laying HOMO, small band gap, and rigid framework, the thiophene-fused BODIPYs endowed the polymers with low band gap and high stability in atmosphere and a broad absorption spectrum. Polymer 16 showed wider absorption Polymers 2021, 13, 75 and lower band gaps than the methyl-substituted analog 15 due to the smaller6 of 30 steric hindrance. Such ring-fused BODIPYs can open new approaches for the construction of low- band-gap conjugated polymers with broad absorption and good stability.

FigureFigure 5. 5.Chemical Chemical structures structures of polymersof polymers15 and 15 and16 (Reproduced 16 (Reproduced with with permission permission from Refer-from Reference ence[40]. [40 Copyright]. Copyright @ @2014, 2014, American American Chemical Chemical Society).Society).

TheThe most most common common and and patterned patterned approach approach to developing to developing novel polymer novel polymer materials materials containing BODIPYs is selecting the appropriate comonomers to facilitate intra-molecular containing BODIPYs is selecting the appropriate comonomers to facilitate intra-molecular charge transfer excitation [41–47]. To precisely predict the performance of alternating copolymerscharge transfer containing excitation BODIPYs, [41–47]. Thayumanavan To precisely and predict coworkers the [performance48] chose ﬁve aromaticof alternating copolymers containing BODIPYs, Thayumanavan and coworkers [48] chose five aromatic Polymers 2021, 13, x FOR PEER REVIEWgroups with different electron-donating capacity (including ﬂuorene (17), carbazole7 of (18 32), bithiophenegroups with (19 different), cyclopentadithiophene electron-donat (ing20), capacity and dithienopyrrole (including (fluorene21)) to copolymerize (17), carbazole (18),

withbithiophene BODIPYs and(19), investigated cyclopentadithiophene the inﬂuence of ( comonomers20), and dithienopyrrole on the properties (21 of)) the to ensu-copolymerize ingwith polymers BODIPYs17–21 and(Figure investigated6), especially the theinfluence frontier of molecular comonomers orbital on energy the properties levels. The of the en- TheHOMOsuing HOMO energypolymers energy levels 17–21levels of polymers, of(Figure polymers, assessed 6), especially assessed by density by the density functional frontier functional molecular theory (DFT)theory orbital calculations (DFT) energy cal- levels. culationsor cyclic voltammetry,or cyclic voltammetry, showed the showed same tendencythe same tendency as their gas-phase as their gas-phase ionization ionization potential potential(IP) values. (IP) In values. contrast, In thecontrast, LUMO the energy LUMO levels energy were levels almost were invariant, almost invariant, which indicates which that the inﬂuence of BODIPY on HOMO energy levels was dominant. The present study indicates that the influence of BODIPY on HOMO energy levels was dominant. The pre- builds up the key link of polymer design with theoretical predictions, which facilitates the sent study builds up the key link of polymer design with theoretical predictions, which development of BODIPY-based conjugated polymers. facilitates the development of BODIPY-based conjugated polymers.

Figure 6. ChemicalChemical structures structures of of polymers polymers 17–2117–21 (Reproduced(Reproduced with with permission permission from from Reference Reference [48]. [48 Copyright]. Copyright @ 2012, @ 2012, Royal Society of Chemistry).

SamuelSamuel and and coworkers coworkers [49] [49] reported reported two two low-band-gap low-band-gap donor-acceptor donor-acceptor (D-A) (D-A) conju- conju- gatedgated polymers, polymers, 2222 andand 23, containing23, containing BODIPY BODIPY as electron as electron acceptors acceptors and bis(3,4-ethylene- and bis(3,4- dioxythiophene)ethylenedioxythiophene) (bis-EDOT) (bis-EDOT) and bis(3,4-ethylenedithiathiophen and bis(3,4-ethylenedithiathiophene)e) (bis-EDTT) (bis-EDTT) as electron as donors,electron donors,respectively respectively (Figure (Figure7). In 7the). In ultraviolet-visible the ultraviolet-visible (UV-vis) (UV-vis) absorption absorption spectra spec- measuredtra measured in solution, in solution, 22 22displayeddisplayed broad broad absorption absorption from from about about 300 300 to to 1000 1000 nm nm with maximamaxima at at 818 818 nm, while 23 showed a narrower absorption from from around 400 to 900 nm with maxima at 648 nm. This suggested that bis-EDOT possessed stronger donor ability thanthan bis-EDTT. bis-EDTT. Calculated from the absorption spectra, the optical band gaps of 22 and 23 were 1.18 and and 1.35 1.35 eV, eV, respectively. respectively. Furthermor Furthermore,e, the time-of-flight time-of-ﬂight measurements proved thatthat 22 allowed for ambipolar charge transport. Two Two types of devices with the obtained polymers as as electron electron donors donors and and [6,6]-phenyl-C [6,6]-phenyl-C71-butyric71-butyric acid acid methyl methyl ester ester (PCBM) (PCBM) as the as electron acceptor were separately fabricated. When the donor-to-acceptor ratio was 1:4, the device made of 22 exhibited the best performance: a circuit current density (Jsc) of 7.78 mA cm−2, open-circuit voltage (Voc) of 0.31 V, fill factor (FF) of 39%, and power conversion efficiency (PCE) of 0.95% were observed. Clearly, the BODIPY-based conjugated polymer is a promising material for improving the performance of polymer-fullerene devices by rationally decreasing band gaps.

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The HOMO energy levels of polymers, assessed by density functional theory (DFT) calculations or cyclic voltammetry, showed the same tendency as their gas-phase ionization potential (IP) values. In contrast, the LUMO energy levels were almost invariant, which indicates that the influence of BODIPY on HOMO energy levels was dominant. The present study builds up the key link of polymer design with theoretical predictions, which facilitates the development of BODIPY-based conjugated polymers.

Figure 6. Chemical structures of polymers 17–21 (Reproduced with permission from Reference [48]. Copyright @ 2012, Royal Society of Chemistry).

Samuel and coworkers [49] reported two low-band-gap donor-acceptor (D-A) conjugated polymers, 22 and 23, containing BODIPY as electron acceptors and bis(3,4-ethylenedioxythiophene) (bis-EDOT) and bis(3,4-ethylenedithiathiophene) (bis-EDTT) as electron donors, respectively (Figure 7). In the ultraviolet-visible (UV-vis) absorption spectra measured in solution, 22 displayed broad absorption from about 300 to 1000 nm with maxima at 818 nm, while 23 showed a narrower absorption from around 400 to 900 nm with maxima at 648 nm. This suggested that bis-EDOT possessed stronger donor ability than bis-EDTT. Calculated from the absorption spectra, the optical band gaps of 22 and 23 Polymers 2021, 13, 75 were 1.18 and 1.35 eV, respectively. Furthermore, the time-of-flight measurements7 of 30 proved that 22 allowed for ambipolar charge transport. Two types of devices with the obtained polymers as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) as the theelectron electron acceptor acceptor were separatelyseparately fabricated. fabricated. When When the the donor-to-acceptor donor-to-acceptor ratio wasratio was 1:4, 1:4,the the device device made made of of 2222 exhibitedexhibited the bestbest performance:performance: a a circuit circuit current current density density (Jsc )(Jsc) of 7.78 −2 −2 ofmA 7.78 cm mA, cm open-circuit, open-circuit voltage voltage (Voc) ( Vofoc 0.31) of 0.31V, fill V, factor ﬁll factor (FF) (FF) of 39%, of 39%, and and power power conversion conversionefficiency efﬁciency (PCE) of (PCE)0.95% of were 0.95% observed. were observed. Clearly, Clearly, the BODIPY-based the BODIPY-based conjugated conju- polymer gatedis a polymerpromising is a promisingmaterial for material improving for improving the performance the performance of polymer-fullerene of polymer-fullerene devices by devicesrationally by rationally decreasi decreasingng band gaps. band gaps.

Figure 7. Chemical structures of polymers 22 and 23 (Reproduced with permission from Refer- ence [49]. Copyright @ 2012, Royal Society of Chemistry).

2.1.2. BODIPY as Pendants Side Chains Generally, conjugated polymers with BODIPYs as pendants side chain were synthesized through meso-positions connection of BODIPYs containing reactive groups, such as alkynyl [50] electrochemical polymer groups [51,52] or other more complex units [53]. Chujo and coworkers [54] tethered BODIPY dyes to the cardo structures of polyfluorenes in order to achieve advanced light-harvesting antenna (LHA) systems (Figure8). Polymer 24 was synthesized via the polymerization of mono-BODIPY substituted dibromo cardo fluorene derivates through Yamamoto cross-coupling reaction, while polymers 25 and 26 were prepared via the combination of 9,9-didodecylfluorene diboronic acid with dibromofluo- rene containing single or dual BODIPYs at its cardo structures through Suzuki-Miyaura cross-coupling reaction. By comparing the absorption and photoluminescence spectra of polymers and simple mixtures of free BODIPY units and polyfluorene, it was found that the intrinsic optical properties of BODIPYs and polyfluorene main chains were retained. Moreover, 25 and 26 exhibited significant fluorescence emissions in both solution and film states, due to the efficient suppression of the concentration quenching of BODIPYs. The LHA efficiency of 25 was nine times that of the free BODIPY unit, and the energy transfer efficiencies of 24–26 were up to 99% because of the rigid cardo structures of polyfluorene. Based on their excellent properties and good processability, these effective LHA systems with BODIPY-attached cardo polyfluorenes are promising for practical applications. Polymers 2021, 13, x FOR PEER REVIEW 8 of 32

Figure 7. Chemical structures of polymers 22 and 23 (Reproduced with permission from Reference [49]. Copyright @ 2012, Royal Society of Chemistry).

2.1.2. BODIPY as Pendants Side Chains Generally, conjugated polymers with BODIPYs as pendants side chain were synthesized through meso-positions connection of BODIPYs containing reactive groups, such as alkynyl [50] electrochemical polymer groups [51,52] or other more complex units [53]. Chujo and coworkers [54] tethered BODIPY dyes to the cardo structures of polyfluorenes in order to achieve advanced light-harvesting antenna (LHA) systems (Figure 8). Polymer 24 was synthesized via the polymerization of mono-BODIPY substituted dibromo cardo fluorene derivates through Yamamoto cross-coupling reaction, while polymers 25 and 26 were prepared via the combination of 9,9-didodecylfluorene diboronic acid with dibro- mofluorene containing single or dual BODIPYs at its cardo structures through Suzuki- Miyaura cross-coupling reaction. By comparing the absorption and photoluminescence spectra of polymers and simple mixtures of free BODIPY units and polyfluorene, it was found that the intrinsic optical properties of BODIPYs and polyfluorene main chains were retained. Moreover, 25 and 26 exhibited significant fluorescence emissions in both solution and film states, due to the efficient suppression of the concentration quenching of BODIPYs. The LHA efficiency of 25 was nine times that of the free BODIPY unit, and the energy transfer efficiencies of 24–26 were up to 99% because of the rigid cardo structures Polymers 2021, 13, 75 of polyfluorene. Based on their excellent properties and good processability, these effec-8 of 30 tive LHA systems with BODIPY-attached cardo polyfluorenes are promising for practical applications.

Figure 8. Chemical structures of polymers 24–26 (Reproduced(Reproduced with permission from Reference [[54].54]. Copyright @ 2013, 2013, American Chemical Society). American Chemical Society).

Additionally, BODIPYs can be incorporated into conjugated polymers as pendant sideside chains chains through through meso- meso- or or αα-positions.-positions. Yin Yin and and coworkers coworkers [55] [55] synthesized synthesized three three poly- poly- acetylenes 27–29 bearing BODIPYs as pendant side chains through meso- or α-positions (Figure 99)) andand investigatedinvestigated thethe effecteffect ofof connectiveconnective methodsmethods onon thethe propertiesproperties ofof thethe ob-ob- tainedtained polymers. polymers. Polymers Polymers 2828 andand 29 displayed poorer thermal stability than 27 because thethe coplanarity coplanarity of backbone backbone and BODIPY units in polymers 28 and 29 reducedreduced the thermal resistanceresistance of polymers. Moreover, Moreover, 28 and 29 had a broader absorption range, along with a little redshift, than corresponding monomers, while the absorption spectrum of 27 was similar to to that of the monomer owing to a vertical arrangement of BODIPY units with main backbone. All polymers exhibited better nonlinearnonlinear opticaloptical propertiesproperties thanthan polyacety-polyacetylene.lene. Furthermore,Furthermore, thethe third-order third-order nonlinear nonlinear optical optical coefficients coefficients of of28 28(1.27 (1.27× ×10 10−−10 esu) (esu is abbreviationabbreviation forfor electrostaticelectrostatic unit)unit) andand29 29(1.37 (1.37× × 1010−−10 esu)esu) were were ~15 times larger thanthan those those of of 27 (8.5 × 1010−12− 12esu).esu). Accordingly, Accordingly, the the connective connective methods methods of of BODIPYs BODIPYs had had a significanta significant influence influence on onthe the properties properties of th ofe theensuing ensuing polyacetylenes, polyacetylenes, and the and desired the desired pho- Polymers 2021, 13, x FOR PEER REVIEWtoelectricphotoelectric properties properties could could be beachieved achieved by by the the attachment attachment of of polymerizable polymerizable groups groups9 of 32 to BODIPYs at the optimal positions.

FigureFigure 9. 9.Chemical Chemical structuresstructures of polymers polymers 2727––2929 (Reproduced(Reproduced with with permission permission from from Reference Reference [55]. [ 55]. CopyrightCopyright @ @ 2014,2014, RoyalRoyal Society of Chemistry). Chemistry).

2.1.3.2.1.3. BODIPY BODIPY asas EndEnd Groups ToTo gather gather the the schematicschematic information on on the the relationship relationship of of energy energy transfer transfer and and rigid rigid chain,chain, YinYin and coworkers coworkers designed designed a novel a novel poly( poly(p-phenylenep-phenylene ethynylene) ethynylene) (30–19 () 30[56]–19 and)[56 ] anda group a group of oligo( of oligo(p-phenylenep-phenylene ethynylene)s ethynylene)s (30-1, (30-3,30-1, 30-5, 30-3, 30-7 30-5,) [57] 30-7 with)[57 different] with different con- conjugatedjugated lengths lengths (Figure (Figure 10)—they 10)—they were were capped capped with with BODIPYs BODIPYs at their at theirends. ends. According According to tothe the absorption, absorption, excitation, excitation, and and emission emission spec spectra,tra, the the synthesized synthesized polymer polymer or oroligomers oligomers inheritedinherited the the spectroscopic spectroscopic properties of of bo bothth BODIPYs BODIPYs and and phenylacetylene phenylacetylene chain. chain. Both Both thethe absorption absorption andand emissionemission maxima assigned assigned to to the the phenylacetylene phenylacetylene main main chains chains red- red- shifted as the number of phenylacetylene units was increased, however, those assigned to the BODIPY units had almost no change and the relative intensity of the absorption was decreased. The energy transfer efficiency reduced as the conjugated length increased because of the decrease in the energy transfer rate and mass of conformational subunits. Thus, this work showed that the energy transfer efficiency of dye-capped polymers could be tuned by changing the conjugated chain length.

Figure 10. Chemical structure of polymer 30 (Reproduced with permission from Reference [56]. Copyright @ 2011, Else- vier. Reproduced with permission from Reference [57]. Copyright @ 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Wein- heim.).

2.2. Porous Conjugated Polymers

Polymers 2021, 13, x FOR PEER REVIEW 9 of 32

Figure 9. Chemical structures of polymers 27–29 (Reproduced with permission from Reference [55]. Copyright @ 2014, Royal Society of Chemistry).

2.1.3. BODIPY as End Groups To gather the schematic information on the relationship of energy transfer and rigid chain, Yin and coworkers designed a novel poly(p-phenylene ethynylene) (30–19) [56] and a group of oligo(p-phenylene ethynylene)s (30-1, 30-3, 30-5, 30-7) [57] with different con- Polymers 2021, 13, 75 9 of 30 jugated lengths (Figure 10)—they were capped with BODIPYs at their ends. According to the absorption, excitation, and emission spectra, the synthesized polymer or oligomers inherited the spectroscopic properties of both BODIPYs and phenylacetylene chain. Both shiftedthe absorption as the number and emission of phenylacetylene maxima assigned units to was the increased,phenylacetylene however, main thosechains assigned red- toshifted the BODIPY as the number units had of phenylacetylene almost no change units and was theincreased, relative however, intensity those of the assigned absorption to wasthedecreased. BODIPY units The had energy almost transfer no change efﬁciency and the reduced relative as intensity the conjugated of the absorption length increased was becausedecreased. of the The decrease energy transfer in the energyefficiency transfer reduced rate as andthe conjugated mass of conformational length increased subunits. be- Thus,cause this of workthe decrease showed in that the theenergy energy transfer transfer rate efﬁciencyand mass ofof dye-cappedconformational polymers subunits. could beThus, tuned this by work changing showed the that conjugated the energy chain transfer length. efficiency of dye-capped polymers could be tuned by changing the conjugated chain length.

( FigureFigure 10. 10.Chemical Chemical structure structure of polymerof polymer30 30(Reproduced Reproduced with with permission permission from from Reference Reference [[56].56]. CopyrightCopyright @ 2011, Else- Elsevier. vier. Reproduced with permission from Reference [57]. Copyright @ 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Wein- Reproduced with permission from Reference [57]. Copyright @ 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.). heim.). Polymers 2021, 13, x FOR PEER REVIEW 10 of 32 2.2.2.2. Porous Porous Conjugated Conjugated PolymersPolymers Porous organic polymers (POPs) with conjugated structures have attracted pro- nouncedPorous attention organic owing polymers to the (POPs) high ﬂexibilitywith conjugated in their structures skeleton have design attracted and porosity;pro-

consequently,nounced attention their owing applications to the high have flexibility been extended in their skeleton to gas design absorption and porosity; and separation, con- energysequently, storage, their catalysis, applications and have optoelectronic been extended materials to gas absorption [58–64]. Especially, and separation, with facileenergy modi- ﬁcation,storage, diversity catalysis, of and reaction optoelectronic sites, and materi intriguingals [58–64]. optical Especially, properties, with BODIPYs facile modifica- have been widelytion, diversity used as skeletonof reaction units sites, of and POPs. intrig Algiuing and optical coworkers properties [24,] BODIPYs designed have two been BODIPY cores,widely bithiophene used as skeleton and EDOT, units withof POPs. three Al electroactivegi and coworkers function [24] designed groups, two and BODIPY the ensuing polymerscores, bithiophene31 and 32 and(Figure EDOT, 11 )with obtained three elec viatroactive electrochemical function groups, polymerization and the ensuing possessed cellularpolymers structures 31 and and32 (Figure multi-electrochromic 11) obtained via properties. electrochemical polymerization possessed cellular structures and multi-electrochromic properties.

FigureFigure 11. Chemical 11. Chemical structures structures of polymersof polymers31 31and and32 32(Reproduced (Reproduced with permission permission from from Reference Reference [24]. [24 Copyright]. Copyright @ 2013 @ 2013 ElsevierElsevier Ltd.). Ltd).

Kuang and coworkers presented a series of BODIPY-based POPs 33–37 (Figure 12) synthesized by Friedel-Crafts cross-coupling reactions to explore the influences of monomer structures on the porous properties and singlet-oxygen generation properties [65,66]. In comparison with analogs 33–34, 35, with a long space between two BODIPY units, showed higher surface area but worse photocatalytic activity of singlet-oxygen generation. Although 34 and 36 with or without methyl substituents in the BODIPY cores possessed good pore characteristics, 34 presented the coexistence of micropores and mesopores, and 36 only presented mesoporous architecture. Additionally, the singlet-oxygen generation ability of 34 was better than that of 36. Compared with 36, 37 showed poor pore properties and singlet-oxygen generation capacities, implying that the monomeric isomerization was a crucial parameter for the design of BODIPY-based POPs. This work about the relationship between monomeric structures and characteristics can lay a foundation for designing more intriguing BODIPY-based POPs.

Polymers 2021, 13, 75 10 of 30

Kuang and coworkers presented a series of BODIPY-based POPs 33–37 (Figure 12) synthesized by Friedel-Crafts cross-coupling reactions to explore the inﬂuences of monomer structures on the porous properties and singlet-oxygen generation properties [65,66]. In comparison with analogs 33–35, with a long space between two BODIPY units, showed higher surface area but worse photocatalytic activity of singlet-oxygen generation. Al- though 34 and 36 with or without methyl substituents in the BODIPY cores possessed good pore characteristics, 34 presented the coexistence of micropores and mesopores, and 36 only presented mesoporous architecture. Additionally, the singlet-oxygen generation ability of 34 was better than that of 36. Compared with 36, 37 showed poor pore properties and singlet-oxygen generation capacities, implying that the monomeric isomerization was a crucial parameter for the design of BODIPY-based POPs. This work about the relationship Polymers 2021, 13, x FOR PEER REVIEW 11 of 32 between monomeric structures and characteristics can lay a foundation for designing more intriguing BODIPY-based POPs.

FigureFigure 12.12. ChemicalChemical structuresstructures ofof polymerspolymers 3333––3737 (Reproduced(Reproduced withwith permissionpermission fromfrom ReferenceReference [[65].65]. CopyrightCopyright @@ 20172017 WILEY-VCHWILEY-VCH VerlagVerlag GmbH GmbH & & Co.Co. KGaA,KGaA, Weinheim.Weinheim. ReproducedReproduced withwith permissionpermission fromfrom ReferenceReference [[66].66]. Copyright @ 2017, RoyalRoyal SocietySociety ofof Chemistry).Chemistry).

2.3. Other Structures Apart from the above-mentioned categories, some BODIPY-based conjugated polymers also contain branched polymers and crosslinked polymers, whose syntheses and applications are introduced in Section 3.

3. Functional Applications of BODIPY-Based Conjugated Polymers BODIPY-based conjugated polymers are fascinating in that they present a perfect fu- sion of π-conjugated backbone and unique BODIPY dyes. Furthermore, the precise control of structures can be achieved at the molecular level to realize the desired performance. By virtue of various morphologies and excellent photoelectric properties, conjugated polymers containing BODIPYs have emerged as versatile materials for applications ranging from optoelectronic materials, biotherapy and imaging, sensor, gas adsorption, and energy storage. The applications and synthetic methods of BODIPY-based conjugated polymers are summarized in Table 2.

3.1. Optoelectronic Materials

Polymers 2021, 13, 75 11 of 30

3. Functional Applications of BODIPY-Based Conjugated Polymers BODIPY-based conjugated polymers are fascinating in that they present a perfect fu- sion of π-conjugated backbone and unique BODIPY dyes. Furthermore, the precise control of structures can be achieved at the molecular level to realize the desired performance. By virtue of various morphologies and excellent photoelectric properties, conjugated polymers containing BODIPYs have emerged as versatile materials for applications ranging from optoelectronic materials, biotherapy and imaging, sensor, gas adsorption, and energy storage. The structures, applications, and synthetic methods of BODIPY-based conjugated polymers are summarized in Tables1 and2.

3.1. Optoelectronic Materials Conjugated polymers have always been one of the main optoelectronic materials owing to their fine-tuning optoelectronic properties based on the ample electron-rich and electron-deficient aromatics. BODIPYs, classical acceptors, possess excellent photophysical and electrochemical properties, endowing BODIPY-based conjugated polymers with extensive application in optoelectronics, such as solar cells, organic thin-film transistors, and memory devices. To endow solar cells with excellent performance, the molecular design of BODIPY- based conjugated polymers are the following three strategies: (1) the introduction of electron-rich comonomers during polymerization [67–72], (2) fusing aromatic rings on the BODIPY units [73], and (3) the extension of the π-conjugation of side chain [73,74]. Donor-acceptor polymers tend to be utilized as donor materials, and fullerene derivatives such as phenyl-C61-butyric acid methyl ester (PC71BM) as acceptor materials, allowing for the construction of solution-processed bulk heterojunction (BHJ). In 2015, Chochos and coworkers [75] reported a novel ultra-low-band-gap and NIR conjugated polymer 38 (Figure 13), which was synthesized by dibromo-BODIPYs and (E)-1,2-bis(3-dodecyl- 5-(trimethylstannyl)thiophen-2-yl)ethane via Stille cross-coupling reaction. The obtained polymer displayed a panchromatic absorption in the 300–1100 nm range with an optical band gap of 1.15 eV, promoting the construction of NIR BHJ solar cells based on 38 and PC71BM. When the weight ratio of 38 to PC71BM was 1:3, the solar cell showed the best −2 PCE of 1.1% with Jsc of 3.39 mA cm ,Voc of 0.59 V, and FF of 0.56, while the highest charge carrier mobility was (1.05 ± 0.2) × 10−5 cm2V−1s−1 for the 1:4 ratio. In 2008, Sharma and coworkers [74] also presented BJH solar cells based on PC71BM, wherein low-band-gap conjugated polymers 39 and 40 (Figure 13) served as electron donors. Polymer 39 with BODIPYs, thiophene unites, and ethynyl linkers displayed an absorption spectrum ranging from 500 to 800 nm and an optical band gap of 1.74 eV. Polymer 40 with different extended BODIPY unit was achieved by introducing Zinc(II) porphyrins into BODIPY units of 39 via Knoevenagel condensation. Owing to the extension of conjugation, 40 exhibited broadened absorption, ranging from 400 to 800 nm, and decreased optical band gap (1.59 eV). Bulk heterojunction solar cells with 39/PC71BM and 40/PC71BM ratio of 1:2 showed PCE of 3.03% and 3.86%, respectively. The above two works demonstrated that BODIPYs are promising building blocks for devising novel polymeric donors for solar cells conjugated with PC71BM. Polymers 2021, 13, x FOR PEER REVIEW 12 of 32

Conjugated polymers have always been one of the main optoelectronic materials owing to their fine-tuning optoelectronic properties based on the ample electron-rich and electron-deficient aromatics. BODIPYs, classical acceptors, possess excellent photophysical and electrochemical properties, endowing BODIPY-based conjugated polymers with extensive application in optoelectronics, such as solar cells, organic thin-film transistors, and memory devices.

To endow solar cells with excellent performance, the molecular design of BODIPY- based conjugated polymers are the following three strategies: (1) the introduction of electron-rich comonomers during polymerization [67–72], (2) fusing aromatic rings on the BODIPY units [73], and (3) the extension of the π-conjugation of side chain [73,74]. Donor- acceptor polymers tend to be utilized as donor materials, and fullerene derivatives such as phenyl-C61-butyric acid methyl ester (PC71BM) as acceptor materials, allowing for the construction of solution-processed bulk heterojunction (BHJ). In 2015, Chochos and coworkers [75] reported a novel ultra-low-band-gap and NIR conjugated polymer 38 (Fig- ure 13), which was synthesized by dibromo-BODIPYs and (E)-1,2-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)ethane via Stille cross-coupling reaction. The obtained polymer displayed a panchromatic absorption in the 300–1100 nm range with an optical band gap of 1.15 eV, promoting the construction of NIR BHJ solar cells based on 38 and PC71BM. When the weight ratio of 38 to PC71BM was 1:3, the solar cell showed the best PCE of 1.1% with Jsc of 3.39 mA cm−2, Voc of 0.59 V, and FF of 0.56, while the highest charge carrier mobility was (1.05 ± 0.2) × 10−5 cm2V−1s−1 for the 1:4 ratio. In 2008, Sharma and coworkers [74] also presented BJH solar cells based on PC71BM, wherein low-band-gap conjugated polymers 39 and 40 (Figure 13) served as electron donors. Polymer 39 with BODIPYs, thiophene unites, and ethynyl linkers displayed an absorption spectrum ranging from 500 to 800 nm and an optical band gap of 1.74 eV. Polymer 40 with different extended BODIPY unit was achieved by introducing Zinc(II) porphyrins into BODIPY units of 39 via Knoevenagel condensation. Owing to the extension of conjugation, 40 exhibited broadened absorption, ranging from 400 to 800 nm, and decreased optical band gap (1.59 eV). Bulk heterojunction solar cells with 39/PC71BM and 40/PC71BM ratio of 1:2 showed PCE of Polymers 2021, 13, 75 3.03% and 3.86%, respectively. The above two works demonstrated that BODIPYs12 ofare 30 promising building blocks for devising novel polymeric donors for solar cells conjugated with PC71BM.

FigureFigure 13. 13. ChemicalChemical structures structures of ofpolymers polymers 38–3840– 40(Reproduced(Reproduced with with permission permission from from Reference Reference [74]. [Copyright74]. Copyright @ 2018, @ American2018, American Chemical Chemical Society. Society. Reproduced Reproduced with permission with permission from Reference from Reference [75]. Copyright [75]. Copyright @ 2015, Royal @ 2015, Society Royal of SocietyChem- istry).of Chemistry).

There are also many organic solar cells consisting of BODIPY-based conjugated polymers as donors and non-fullerenes as receptors. Sharma and coworkers [76] developed 41 (Figure 14), which is an analogous BODIPY-based conjugated polymer of 39, with monosubstituted thiophene units via Sonogashira cross-coupling reaction. The photovoltaic properties of BHJ solar cell based on 41 as the donor and non-fullerene small molecule (NFSMA) as the acceptor were compared with that of the BHJ solar cell with PC71BM as the acceptor. Polymer 41 and NFSMA exhibited complementary absorption favorable to light harvesting and well-matched frontier energy level favorable to efﬁcient charge transfer and minimal energy loss. In addition, Li and coworkers [77] fabricated all-polymer solar cells, in which BODIPY-based conjugated polymers without (42) or with (43) methyl substituent on the BODIPY cores served as donors, and polymer N2200 consisting of bithiophene and hexahydropyrene derivative served as acceptors (Figure 14). Notably, the complementary interaction between acceptor and donor led to the absorption spectrum extension from 300 to 900 nm in solar cells. After experimental analysis, it was found that 43 with two methyl substituents displayed superior photovoltaic properties compared to polymer 42 without methyl substituent. On account of neat structures, suitable energy levels, and high hole mobility, the PCE of the 43-based solar cell was 5.8%, higher than that of the 42-based solar cell (0.32%). Considering the superiority of 43, Li and coworkers [78] further studied the performances of solar cells composed of electron donor 43 and halogenated fused-ring electron acceptor ITIC-2Cl or BTP-2Cl (Figure 14). Likewise, a wide absorption spectrum from 300 to 1100 nm was obtained, and the external quantum efﬁciencies were above 0.50. −2 Interestingly, a dramatically high Jsc (21.44 mA cm ) was obtained from the BTP-2Cl-based solar cell, and the PCEs increased to 7.56% and 9.86% for ITIC-2Cl- and BTP-2Cl-based solar cells, respectively. Overall, the rational molecular design of BODIPY-based conjugated polymers offers advantages in terms of PCE enhancement, and it is also a good idea to match the polymer acceptor or small-molecule acceptor, replacing fullerene receptors. Polymers 2021, 13, x FOR PEER REVIEW 13 of 32

There are also many organic solar cells consisting of BODIPY-based conjugated polymers as donors and non-fullerenes as receptors. Sharma and coworkers [76] developed 41 (Figure 14), which is an analogous BODIPY-based conjugated polymer of 39, with monosubstituted thiophene units via Sonogashira cross-coupling reaction. The photovoltaic properties of BHJ solar cell based on 41 as the donor and non-fullerene small molecule (NFSMA) as the acceptor were compared with that of the BHJ solar cell with PC71BM as the acceptor. Polymer 41 and NFSMA exhibited complementary absorption favorable to light harvesting and well-matched frontier energy level favorable to efficient charge transfer and minimal energy loss. In addition, Li and coworkers [77] fabricated all-polymer solar cells, in which BODIPY-based conjugated polymers without (42) or with (43) methyl substituent on the BODIPY cores served as donors, and polymer N2200 consisting of bithiophene and hexahydropyrene derivative served as acceptors (Figure 14). Notably, the complementary interaction between acceptor and donor led to the absorption spectrum extension from 300 to 900 nm in solar cells. After experimental analysis, it was found that 43 with two methyl substituents displayed superior photovoltaic properties compared to polymer 42 without methyl substituent. On account of neat structures, suitable energy levels, and high hole mobility, the PCE of the 43-based solar cell was 5.8%, higher than that of the 42-based solar cell (0.32%). Considering the superiority of 43, Li and coworkers [78] further studied the performances of solar cells composed of electron donor 43 and halogenated fused-ring electron acceptor ITIC-2Cl or BTP-2Cl (Figure 14). Likewise, a wide absorption spectrum from 300 to 1100 nm was obtained, and the external quantum efficiencies were above 0.50. Interestingly, a dramatically high Jsc (21.44 mA cm−2) was obtained from the BTP-2Cl-based solar cell, and the PCEs increased to 7.56% and 9.86% for ITIC-2Cl- and BTP-2Cl-based solar cells, respectively. Overall, the rational molecular de- Polymers 2021, 13, 75 sign of BODIPY-based conjugated polymers offers advantages in terms of PCE enhance-13 of 30 ment, and it is also a good idea to match the polymer acceptor or small-molecule acceptor, replacing fullerene receptors.

Figure 14.14. Chemical structures of polymerspolymers 4141––4343 andand non-fullerenesnon-fullerenes N2200,N2200, ITIC-2Cl,ITIC-2Cl, andand BTP-2ClBTP-2Cl (Reproduced(Reproduced withwith permissionpermission from Reference Reference [76]. [76]. Copyrigh Copyrightt @ 2018, @ 2018, American American Chemical Chemical Society. Society. Reproduced Reproduced with permission with permission from Refer- from Referenceence [77]. [Copyright77]. Copyright @ 2019, @ 2019, American American Chem Chemicalical Society. Society. Reproduced Reproduced with with permis permissionsion from from Reference Reference [78]. [78 Copyright]. Copyright @ 2019, Royal Society of Chemistry). Polymers@ 2019, 2021, Royal13, x FOR Society PEER of REVIEW Chemistry). 14 of 32

Perovskite solar cells (PSCs) are considered typical representatives of of the third generation of solar cells owing to to their their high high effi efﬁciency,ciency, low cost, and easy preparation [79,80]. [79,80]. However, their stability could could be be one one of of th thee obstacles obstacles limiting limiting their their development. development. Conse- Con- sequently,quently, Hong Hong and and coworkers coworkers [81] [ 81developed] developed a series a series of dopant-free of dopant-free hole-transport hole-transport mate-

materialsrials (HTMs) (HTMs) based based on BODIPY-containing on BODIPY-containing conjugated conjugated polymers polymers 44–4944 (Figure–49 (Figure 15) to15 )im- to improveprove the the PSC PSC stability. stability. The The selective selective regula regulationtion of ofHOMO/LUMO HOMO/LUMO value value was was achieved achieved by by introducing groups at the positions of BODIPY cores and benzo[1,2-b:4,5-b’]bithiophene. introducing groups at the positions of BODIPY cores and benzo[1,2-b:4,5-b’]bithiophene. Owing to solubilizing groups and nonplanar structure, hydrophobic HTM polymers could Owing to solubilizing groups and nonplanar structure, hydrophobic HTM polymers entirely cover the perovskite layer, allowing it to isolate air and moisture. Because of could entirely cover the perovskite layer, allowing it to isolate air and moisture. Because different HOMO/LUMO energy levels and electrical properties, 44–49 exhibited different of different HOMO/LUMO energy levels and electrical properties, 44–49 exhibited differ- performances in solar cells, among which 44 displayed the highest PCE of 16.02%, with Jsc ent performances−2 in solar cells, among which 44 displayed the highest− 5PCE− of2 16.02%,−1 −1 of 19.22 mA cm ,Voc of 1.06 V, FF of 0.78, and hole mobility of 2.96 × 10 cm V s . with Jsc of 19.22 mA cm−2, Voc of 1.06 V, FF of 0.78, and hole mobility of 2.96 × 10−5 cm−2 V−1 The device stability test showed that 44-based PSC retained 80% of the original PCE under s-1. The device stability test showed that 44-based PSC retained 80% of the original PCE ambient condition after 10 days. under ambient condition after 10 days.

FigureFigure 15.15. ChemicalChemical structuresstructures ofof polymerspolymers 4444––4949 (Reproduced(Reproduced withwith permissionpermission fromfrom ReferenceReference [[81].81]. CopyrightCopyright @@ 2018, AmericanAmerican ChemicalChemical Society).Society).

Apart from solar cells, organic thin-ﬁlmthin-film transistors (OTFTs) areare anotheranother applicationapplication of BODIPY-based conjugated polymers in the area of optoelectronic materials [82–86]. [82–86]. Facchetti and coworkers [[82]82] designeddesigned a group of D-AD-A copolymerscopolymers 5050––5353 consistingconsisting ofof alternating tetramethyl-substituted or dimethyl-substituted BODIPYs and thiophene or 3,3′-dialkoxy-2,2′ bithiophene bridged by Stille cross-coupling reaction (Figure 16). The polymers as thin-films displayed panchromatic absorption spectra up to the NIR region and 60–90 nm redshift compared with the corresponding solution spectra. According to experimental estimation, HOMO energies were determined as −5.04 to −5.59 eV for 50–53. In addition, polymers based on dimethyl-substituted BODIPYs conjugated with thiophene units possessed better planarity and more efficient π-stacking, leading to more potent intra- or inter-chain charge transport. Hence, it was not surprising that in the OTFTs, 52 exhibited p-channel activities with the highest hole mobility of 0.17 cm−2 V−1 s−1 and on/off ratios of 105–106, and favorable stability. Moreover, recently, Sharapov and coworkers [83] presented a series of BODIPY-based conjugated polymers 54–56 containing thiophene-fused BODIPY units and donor groups such as fluorene, BDT, and diketopyrrolopyrrole (DPP) to develop OTFTs (Figure 16). The absorption range of the polymers widened in turn with the maximum absorption redshifted. Particularly, 56 possessed a broad and strong absorption in the range of 500–1600 nm. The cyclic voltammetry results revealed low LUMO energy levels (<−4.0 eV) of polymers, resulting in favorable electron injection and transport. The electron mobilities of 54, 55, and 56 were 4.0 × 10−5, 7.8 × 10−4, and 5.4 × 10−4 cm−2 V−1 s−1, respectively. However, due to inadequate HOMO energy level, hole mobility was not observed for 54, while 55 and 56 with lower HOMO energy levels possessed hole mobilities of 1.0 × 10−3 and 7.2 × 10−4 cm−2 V−1 s−1, respectively. Furthermore, polymer 56 displayed ambipolar charge mobility of 10−3 cm−2 V−1 s−1 in OTFE devices, which is a good parameter for BODIPY-based conjugated polymers. BODIPY-based conjugated polymers also presented potential to serve as n-type semiconducting polymers by the judicious choice of comonomers; for example, the alternating copolymer 57 has been

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alternating tetramethyl-substituted or dimethyl-substituted BODIPYs and thiophene or 3,30-dialkoxy-2,20 bithiophene bridged by Stille cross-coupling reaction (Figure 16). The polymers as thin-films displayed panchromatic absorption spectra up to the NIR region and 60–90 nm redshift compared with the corresponding solution spectra. According to experimental estimation, HOMO energies were determined as −5.04 to −5.59 eV for 50–53. In addition, polymers based on dimethyl-substituted BODIPYs conjugated with thiophene units possessed better planarity and more efficient π-stacking, leading to more potent intra- or inter-chain charge transport. Hence, it was not surprising that in the OTFTs, 52 exhibited p-channel activities with the highest hole mobility of 0.17 cm−2 V−1 s−1 and on/off ratios of 105–106, and favorable stability. Moreover, recently, Sharapov and coworkers [83] presented a series of BODIPY-based conjugated polymers 54–56 containing thiophene- fused BODIPY units and donor groups such as fluorene, BDT, and diketopyrrolopyrrole (DPP) to develop OTFTs (Figure 16). The absorption range of the polymers widened in turn with the maximum absorption redshifted. Particularly, 56 possessed a broad and strong absorption in the range of 500–1600 nm. The cyclic voltammetry results revealed low LUMO energy levels (<−4.0 eV) of polymers, resulting in favorable electron injection and transport. The electron mobilities of 54, 55, and 56 were 4.0 × 10−5, 7.8 × 10−4, and 5.4 × 10−4 cm−2 V−1 s−1, respectively. However, due to inadequate HOMO energy level, hole mobility was not observed for 54, while 55 and 56 with lower HOMO energy levels possessed hole mobilities of 1.0 × 10−3 and 7.2 × 10−4 cm−2 V−1 s−1, respectively. Furthermore, polymer 56 displayed ambipolar charge mobility of 10−3 cm−2 V−1 s−1 in Polymers 2021, 13, x FOR PEER REVIEW 15 of 32 OTFE devices, which is a good parameter for BODIPY-based conjugated polymers. BODIPY- based conjugated polymers also presented potential to serve as n-type semiconducting polymers by the judicious choice of comonomers; for example, the alternating copolymer 57 has been reported by Thayumanavan and coworkers [84] (Figure 16). Thus, BODIPY-based reported by Thayumanavan and coworkers [84] (Figure 16). Thus, BODIPY-based conju- conjugated polymers could play versatile roles in the design of OTFTs. gated polymers could play versatile roles in the design of OTFTs.

Figure 16. ChemicalChemical structures structures of of polymers polymers 5050––5757 (Reproduced(Reproduced with with permission permission from from Reference Reference [82]. [82 Copyright]. Copyright @2013 @2013 WILEY-VCH VerlagVerlag GmbHGmbH && Co.Co. KGaA,KGaA, Weinheim.Weinheim. ReproducedReproduced with with permission permission from from Reference Reference [ 83[83].]. Copyright Copyright @ @ 2020, 2020, American Chemical Society. Reproduced with permission from Reference [84]. Copyright @ 2011, American Chem- American Chemical Society. Reproduced with permission from Reference [84]. Copyright @ 2011, American Chemical ical Society). Society).

Chen and coworkers [87] [87] combined BODIPY units andand 1,4-diethynylbenzene1,4-diethynylbenzene with reducedreduced graphene graphene oxide oxide (RGO) (RGO) via via one-pot one-pot in in situ situ polymerization polymerization to to produce produce the the two- two- dimensional BODIPY-based conjugatedconjugated polymer polymer58 58(Figure (Figure 17 17).). RGO RGO served served as anas electronan elec- trondonor donor and BODIPY-containedand BODIPY-contained polymeric polymeric chain servedchain served as an electron as an electron acceptor. acceptor. The product The productexhibited exhibited good storage good performancestorage performance of rewritable of rewritable memory memory that can that be electricallycan be electrically erased erasedand reprogrammed, and reprogrammed, without without disturbance disturbance by mechanical by mechanical stress. stress. This This may may be causedbe caused by bythe the efﬁcient efficient conjugated conjugated channel channel and and the th favorablee favorable charge charge transfer transfer from from RGO RGO toto 5858.. This work provides an example of a stable high-performance memory device, which not only extends the applicationsapplications ofof BODIPY-based BODIPY-based conjugated conjugated polymers polymers but but also also presents presents a hint a hint for forimproving improving the the stability stability of memory of memory devices. devices.

Figure 17. Chemical structures of polymer 58 (Reproduced with permission from Reference [87]. Copyright @ 2017 Elsevier Ltd).

3.2. Bioimaging and Biotherapy A significant amount of effort has been channeled toward the development of specific imaging and effective therapies, where BODIPY-based conjugated polymers have ample scope due to their high expansibility of optical properties [64,88–95]. Fluorescence imaging can help to provide visualized results with simple operation and low cost. Com- pared with visible light, NIR has been a research hotspot in the field of fluorescence imaging owing to the high penetration depth, small biological spontaneous fluorescence interference, and small light-damage to organisms. Liu and coworkers [92] reported a NIR- emissive BODIPY-containing polymer 60 bearing cancer-homing cyclic arginine-glycine- aspartic acid (RGD) peptide residues. The polymer was synthesized via the post-polymerization functionalization of 59 through click reaction between sulfydryl and halogen (Fig- ure 18). Driven by the existence of tetra(ethylene glycol), 59 and 60 exhibited good water solubility and good photostability, conducive to applications in organisms. Moreover, 59 and 60 showed similar absorption and emission spectra, with the absorption maximum at

Polymers 2021, 13, x FOR PEER REVIEW 15 of 32

reported by Thayumanavan and coworkers [84] (Figure 16). Thus, BODIPY-based conjugated polymers could play versatile roles in the design of OTFTs.

Figure 16. Chemical structures of polymers 50–57 (Reproduced with permission from Reference [82]. Copyright @2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission from Reference [83]. Copyright @ 2020, American Chemical Society. Reproduced with permission from Reference [84]. Copyright @ 2011, American Chem- ical Society).

Chen and coworkers [87] combined BODIPY units and 1,4-diethynylbenzene with reduced graphene oxide (RGO) via one-pot in situ polymerization to produce the two- dimensional BODIPY-based conjugated polymer 58 (Figure 17). RGO served as an electron donor and BODIPY-contained polymeric chain served as an electron acceptor. The product exhibited good storage performance of rewritable memory that can be electrically erased and reprogrammed, without disturbance by mechanical stress. This may be caused by the efficient conjugated channel and the favorable charge transfer from RGO to 58. This Polymers 2021, 13, 75 work provides an example of a stable high-performance memory device, which 15not of only 30 extends the applications of BODIPY-based conjugated polymers but also presents a hint for improving the stability of memory devices.

FigureFigure 17. 17.Chemical Chemical structures structures of of polymer polymer58 58(Reproduced (Reproduced with with permission permission from from Reference Reference [87 [87].]. CopyrightCopyright @ 2017@ 2017 Elsevier Elsevier Ltd). Ltd).

3.2.3.2. Bioimaging Bioimaging and and Biotherapy Biotherapy A significantA significant amount amount of effortof effort has has been been channeled channeled toward toward the development the development of specific of spe- imagingcific imaging and effective and effective therapies, therapies, where BODIPY-based where BODIPY-based conjugated conjugated polymers polymers have ample have scopeample due scope to their due high to their expansibility high expansibility of optical propertiesof optical [properties64,88–95]. [ Fluorescence64,88–95]. Fluorescence imaging canimaging help to can provide help visualizedto provide results visualized with results simple with operation simple and operation low cost. and Compared low cost. with Com- visiblepared light, with NIR visible has light, been aNIR research has been hotspot a resear in thech hotspot field of fluorescencein the field of imaging fluorescence owing im- toaging the high owing penetration to the high depth, penetration small biologicaldepth, small spontaneous biological spontaneous fluorescence fluorescence interference, in- andterference, small light-damage and small light-damage to organisms. to Liu organism and coworkerss. Liu and [ 92coworkers] reported [92] a NIR-emissivereported a NIR- BODIPY-containingemissive BODIPY-containing polymer 60 polymerbearing cancer-homing60 bearing cancer-homing cyclic arginine-glycine-aspartic cyclic arginine-glycine- acidaspartic (RGD) acid peptide (RGD) residues. peptide Theresidues. polymer The waspolymer synthesized was synthesized via the post-polymerization via the post-polymer- functionalization of 59 through click reaction between sulfydryl and halogen (Figure 18). Polymers 2021, 13, x FOR PEER REVIEWization functionalization of 59 through click reaction between sulfydryl and halogen16 of (Fig-32 Driven by the existence of tetra(ethylene glycol), 59 and 60 exhibited good water solubility ure 18). Driven by the existence of tetra(ethylene glycol), 59 and 60 exhibited good water andsolubility good photostability, and good photostability, conducive toconducive applications to applications in organisms. in organisms. Moreover, Moreover,59 and 60 59 showedand 60 similar showed absorption similar absorption and emission and emission spectra, spectra, with the with absorption the absorption maximum maximum at 687 at (59) and 689 nm (60) and emission maximum at 711 (59) and 712 nm (60). Polymer 60 687 (59) and 689 nm (60) and emission maximum at 711 (59) and 712 nm (60). Polymer 60 displayed enhanced NIR signals at breast cancer cells in the perinuclear region, which displayed enhanced NIR signals at breast cancer cells in the perinuclear region, which indicated specific recognition with breast cancer cells. Thus, this research not only presents indicated specific recognition with breast cancer cells. Thus, this research not only pre- a NIR-emissive BODIPY-based polymeric dye for specific cell imaging but also provides a sents a NIR-emissive BODIPY-based polymeric dye for specific cell imaging but also pro- basis for designing specific imaging agents through binding with targeted groups. vides a basis for designing specific imaging agents through binding with targeted groups.

Figure 18. Chemical structures of polymers 59 and 60 (Reproduced with permission from Reference [92]. Copyright @ 2012 Figure 18. Chemical structures of polymers 59 and 60 (Reproduced with permission from Reference [92]. Copyright @ 2012 Elsevier B.V.). Elsevier B.V.).

Water solubilitysolubility is is another another important important favorable favorable factor factor for thefor applicationthe application of polymeric of poly- dyemeric in dye organisms. in organisms. In addition In addition to the aforementionedto the aforementioned introduction introduction of hydrophilic of hydrophilic chains, ionizationchains, ionization is also a feasibleis also a strategy. feasible Cao strategy. and coworkers Cao and [95coworkers] designed [95] a water-soluble designed a water- conju- gatedsoluble polyelectrolyte conjugated polyelectrolyte61 with BODIPYs 61 andwith quaternary BODIPYs ammonium and quaternary salt-modified ammonium fluorenes salt- asmodified monomers fluorenes (Figure as 19 mono). Polymermers 61(Figurepossessed 19). Polymer distinct “turn-off”61 possessed fluorescence distinct “turn-off” character- fluorescence characteristics to DNA due to the electrostatic interaction of positively charged polyelectrolyte and negatively charged DNA of 61-DNA complexes, which allowed it to be a detector agent for DNA. Furthermore, red emission was observed when Hela cells were incubated with 0.1 mg/mL 61, which suggests the potential of 61 for bioimaging applications.

Figure 19. Chemical structure of polymer 61 (Reproduced with permission from Reference [95]. Copyright @ 2015, Springer Science Business Media New York).

Conjugated polymer dots (Pdots) are capable candidates for biological applications, with various advantages such as high brightness, good photostability, flexible luminous range, and biological compatibility. Chiu and coworkers [91] presented yellow-emission

Polymers 2021, 13, x FOR PEER REVIEW 16 of 32

687 (59) and 689 nm (60) and emission maximum at 711 (59) and 712 nm (60). Polymer 60 displayed enhanced NIR signals at breast cancer cells in the perinuclear region, which indicated specific recognition with breast cancer cells. Thus, this research not only presents a NIR-emissive BODIPY-based polymeric dye for specific cell imaging but also provides a basis for designing specific imaging agents through binding with targeted groups.

Figure 18. Chemical structures of polymers 59 and 60 (Reproduced with permission from Reference [92]. Copyright @ 2012 Elsevier B.V.).

Water solubility is another important favorable factor for the application of polymeric dye in organisms. In addition to the aforementioned introduction of hydrophilic Polymers 2021, 13, 75 chains, ionization is also a feasible strategy. Cao and coworkers [95] designed a16 water- of 30 soluble conjugated polyelectrolyte 61 with BODIPYs and quaternary ammonium salt- modified fluorenes as monomers (Figure 19). Polymer 61 possessed distinct “turn-off” fluorescence characteristics to DNA due to the electrostatic interaction of positively istics to DNA due to the electrostatic interaction of positively charged polyelectrolyte and charged polyelectrolyte and negatively charged DNA of 61-DNA complexes, which al- negatively charged DNA of 61-DNA complexes, which allowed it to be a detector agent lowed it to be a detector agent for DNA. Furthermore, red emission was observed when for DNA. Furthermore, red emission was observed when Hela cells were incubated with 0.1Hela mg/mL cells were61, which incubated suggests with the 0.1 potential mg/mL of6161, whichfor bioimaging suggests the applications. potential of 61 for bioimaging applications.

FigureFigure 19. 19.Chemical Chemical structurestructure ofof polymer polymer61 61(Reproduced (Reproduced with with permission permission from from Reference Reference [ 95[95].]. Copyright @ 2015, Springer Science Business Media New York). Copyright @ 2015, Springer Science Business Media New York).

ConjugatedConjugated polymer polymer dots dots (Pdots) (Pdots) are are capable capable candidates candidates for for biological biological applications, applications, withwith various various advantages advantages such such as as high high brightness, brightness, good good photostability, photostability, ﬂexible flexible luminous luminous range,range, and and biological biological compatibility. compatibility. Chiu Chiu and and coworkers coworkers [91 [91]] presented presented yellow-emission yellow-emission Pdots containing the BODIPY-based polymer 62 as the acceptor, poly[9,9-dioctylﬂuorenyl- 2,7-diyl-co-1,4-benzo-(2,10-3)-thiadiazole] (PFBT) as the donor, and poly(styrene-co-maleic anhydride) (PSMA) as the crosslinking agent (Figure 20). When the ratio of BODIPY

polymers to PFBT was 0.54, the Pdots exhibited bright yellow emission based on the efficient Förster resonant energy transfer (FRET) between acceptors and donors. To realize specific cellular imaging, Pdots were further conjugated with streptavidin to obtain Pdot-SA fluorescence probe, which specifically bound biotin anti-EpCAM receptor onto the surface of MCF-7 breast cancer cells. The intensity of the Pdot-SA probe in MCF cells was about five times higher than that of the commercial Qdot 565-streptavidin bioconjugates under the same condition. Overall, Pdots based on crosslinked BODIPY-containing conjugated polymers are qualified for application as bright fluorescent biological probes. Furthermore, Zhang and coworkers [93] reported two hyperbranched BODIPY- containing conjugated polymers 63 and 64 with 2,7-bis(4,4,5,5-tetramethyl-1,3,2- dioxaborolan-2-yl)-9,9-dioctylfluorene, 4,7-dibromobenzothiadiazole, and triiodo-BODIPY as monomers through Suzuki cross-coupling reaction (Figure 21a). Owing to the different reactive abilities of iodine and bromide with boric acid/ether, different mixing procedures were adopted. One was to add all three reactive monomers at once, and in this case, the “local” concentration of BODIPY units in the resultant hyperbranched polymer 63 was high, which would limit the fluorescence emission of 63 due to the concentration quenching effect of BODIPY units. The other was to add triiodo-BODIPYs to the reaction in batches, which resulted in self-quenching reduction. The mixtures of these polymers and PSMA were nanoprecipitated to form small and stable Pdots (63-Pdot and 64-Pdot). 64-Pdot showed a quantum yield of 22%, higher than that of the corresponding linear polymers, whereas 63-Pdot showed a quantum yield of only 11%. The data suggested that hyperbranching was an alternative approach to restrain fluorescence quenching. Similar to the work of Chiu’s group [91], the two Pdots further modified with streptavidin could specifically label MCF-7 cells and exhibited bright fluorescence and good biocompatibility (Figure 21b). These works highlight the potential of Pdots based on different conjugated structures in bioimaging and provide an efficient method to increase brightness and decrease the quenching degree. Polymers 2021, 13, x FOR PEER REVIEW 17 of 32

Pdots containing the BODIPY-based polymer 62 as the acceptor, poly[9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,1′-3)-thiadiazole] (PFBT) as the donor, and poly(styrene-co- maleic anhydride) (PSMA) as the crosslinking agent (Figure 20). When the ratio of BOD- IPY polymers to PFBT was 0.54, the Pdots exhibited bright yellow emission based on the efficient Förster resonant energy transfer (FRET) between acceptors and donors. To realize specific cellular imaging, Pdots were further conjugated with streptavidin to obtain Pdot- SA fluorescence probe, which specifically bound biotin anti-EpCAM receptor onto the surface of MCF-7 breast cancer cells. The intensity of the Pdot-SA probe in MCF cells was Polymers 2021, 13, 75 about five times higher than that of the commercial Qdot 565-streptavidin bioconjugates17 of 30 under the same condition. Overall, Pdots based on crosslinked BODIPY-containing conjugated polymers are qualified for application as bright fluorescent biological probes.

Polymers 2021, 13, x FOR PEER REVIEW 18 of 32 Figure 20. Schematic illustration of the preparat preparationion of crosslinked semiconducting polymers with poly(styrene-co-maleic anhydride) (PSMA), poly[9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,1poly[9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,10-3)-thiadiazole]′-3)-thiadiazole] (PFBT), (PFBT), and and62 62,, and and62 62-based-based poly- pol- ymermer dots dots (Pdot) (Pdot) bioconjugates bioconjugates for for specific specific cellular cellular targeting targeting (Reproduced (Reproduced with with permission permission from from Reference Reference [91 [91].]. Copyright Copyright @ @ 2014 ACS publication). 2014 ACS publication). structures in bioimaging and provide an efficient method to increase brightness and decrease the quenching degree. Furthermore, Zhang and coworkers [93] reported two hyperbranched BODIPY-containing conjugated polymers 63 and 64 with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene, 4,7-dibromobenzothiadiazole, and triiodo-BODIPY as monomers through Suzuki cross-coupling reaction (Figure 21a). Owing to the different reactive abilities of iodine and bromide with boric acid/ether, different mixing procedures were adopted. One was to add all three reactive monomers at once, and in this case, the “local” concentration of BODIPY units in the resultant hyperbranched polymer 63 was high, which would limit the fluorescence emission of 63 due to the concentration quenching effect of BODIPY units. The other was to add triiodo-BODIPYs to the reaction in batches, which resulted in self-quenching reduction. The mixtures of these polymers and PSMA were nanoprecipitated to form small and stable Pdots (63-Pdot and 64-Pdot). 64- Pdot showed a quantum yield of 22%, higher than that of the corresponding linear polymers, whereas 63-Pdot showed a quantum yield of only 11%. The data suggested that hyperbranching was an alternative approach to restrain fluorescence quenching. Similar to the work of Chiu’s group [91], the two Pdots further modified with streptavidin could specifically label MCF-7 cells and exhibited bright fluorescence and good biocompatibility Figure 21. (a) Chemical structures ofof polymerspolymers 63 and 64.(. (b) Illustration of the hyperbranchedhyperbranched BODIPY-based Pdots and their bioconjugate probes for(Figure cellular 21b). imaging These (Reproduced works highlight with permission the potent fromial Reference of Pdots [93].based Copyright on different @ 2017 conjugated Royal their bioconjugate probes for cellular imaging (Reproduced with permission from Reference [93]. Copyright @ 2017 Royal Society of Chemistry). Society of Chemistry). Recently, multifunction dyes have been highlighted for biological applications, espe- Recently, multifunction dyes have been highlighted for biological applications, especially the dyes that have both bioimaging and biotherapy capabili capabilities.ties. Zhao and coworkers [94] [94] proposed proposed a anew new ionized ionized polyfluorene polyfluorene 65 conjugated65 conjugated with with BODPIYs BODPIYs and NIR and phos- NIR phosphorescentphorescent iridium(III) iridium(III) complexes, complexes, and polymers and polymers 66 and66 67and containing67 containing fluorene fluorene and BOD- and IPYsBODIPYs or iridium(III) or iridium(III) complexes complexes respectively, respectively, as control as control groups groups (Figure (Figure 22). Consequently, 22). Conse- quently,the authors the could authors investigate could investigate the FRET proc the FRETess between process BODIPYs between donor BODIPYs and iridium(III) donor and complexesiridium(III) acceptor. complexes Because acceptor. of the Because existence of the of ionized existence monomers, of ionized the monomers, conjugated the poly- con- mersjugated tended polymers to self-assemble tended to self-assembleto establish NPs to establishin water, NPsand the in water,conjugated and the backbone conjugated pro- tectedbackbone the protectedNPs from thephotobleaching, NPs from photobleaching, which was favorable which was for enduri favorableng biological for enduring appli- bi- cations.ological Meanwhile, applications. the Meanwhile, BODIPYs amplified the BODIPYs the light amplified absorption the light ability absorption of NPs, abilityand irid- of ium(III) complexes were responsible for effectively producing singlet oxygen based on the potent FRET. According to in vitro and in vivo experiments, the outstanding inhibition of tumor growth was induced by the high photodynamic therapy (PDT) efficiency based on the high singlet-oxygen quantum yield (97%) (Figure 22). Interestingly, taking advantage of the linear relationship between the phosphorescence lifetime of NPs or the emission intensity ratio and the O2 content, the authors could realize real-time O2 imaging and provide guidelines for PDT assessment. The rational structural design would inspire the progress of more prominent photosensitizers with complementary groups for cancer diagnosis and theranostic.

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NPs, and iridium(III) complexes were responsible for effectively producing singlet oxygen based on the potent FRET. According to in vitro and in vivo experiments, the outstanding inhibition of tumor growth was induced by the high photodynamic therapy (PDT) efﬁ- ciency based on the high singlet-oxygen quantum yield (97%) (Figure 22). Interestingly, taking advantage of the linear relationship between the phosphorescence lifetime of NPs or the emission intensity ratio and the O2 content, the authors could realize real-time O2 Polymers 2021, 13, x FOR PEER REVIEWimaging and provide guidelines for PDT assessment. The rational structural design19 would of 32 inspire the progress of more prominent photosensitizers with complementary groups for cancer diagnosis and theranostic.

Figure 22. Chemical structures of polymers 65–67 and graphical representation of 65 used as photosensitizers with enhanced Figure 22. Chemical structures of polymers 65–67 and graphical representation of 65 used as photosensitizers with en- 1O generation through Förster resonant energy transfer (FRET) for efﬁcient tumor treatment (Reproduced with permission hanced2 1O2 generation through Förster resonant energy transfer (FRET) for efficient tumor treatment (Reproduced with frompermission Reference from [94 Reference]. Copyright [94]. @ Copyri 2019 Royalght @ Society2019 Royal of Chemistry). Society of Chemistry).

Xie and coworkerscoworkers [[90]90] developeddeveloped a a BODIPY-based BODIPY-based conjugated conjugated polymer polymer68 68containing contain- inghexadecane, hexadecane, whose whose structures structures were were similar similar to those to those of bioactive of bioactive fatty fatty acids. acids. The The NP for-NP 68 formulationsmulations of of and68 and human human serum serum albumin albumin (HSA), (HSA), a critical a critical transport transport protein protein in the in body, the were rationally prepared through ultrasonic emulsiﬁcation based on the supramolecular body, were rationally prepared through ultrasonic emulsification based on the supramo- interaction between 68 and HSA (Figure 23). The NPs displayed enhanced water solubility, lecular interaction between 68 and HSA (Figure 23). The NPs displayed enhanced water improved stability, and similar NIR absorption compared with the conjugated polymer solubility, improved stability, and similar NIR absorption compared with the conjugated 68. A certain degree of aggregation quenching was conducive to more potent photother- polymer 68. A certain degree of aggregation quenching was conducive to more potent mal therapy (PTT), which inspired the investigation of the potential phototherapeutic photothermal therapy (PTT), which inspired the investigation of the potential photother- applications of the polymer. The NPs possessed photothermal effect, as evidenced by apeutic applications of the polymer. The NPs possessed photothermal effect, as evidenced the noticeable rise in the NP solution temperature under 685 nm laser irradiation. The by the noticeable rise in the NP solution temperature under 685 nm laser irradiation. The experiment analysis showed that the photothermal conversion efﬁciency of NPs could experiment analysis showed that the photothermal conversion efficiency of NPs could reach 37.5%, and the photothermal reproducibility was prominent. To investigate the reach 37.5%, and the photothermal reproducibility was prominent. To investigate the fur- further application of NPs, a series of in vitro and in vivo experiments were performed. ther application of NPs, a series of in vitro and in vivo experiments were performed. The in vitro experiment showed that the NPs were distributed in the cytoplasm by efficient cytoplasmic endocytosis and displayed significant cytotoxicity under laser radiation. In addition, the in vivo experiment results showed that PTT via intra-tumoral injection with irradiation provided the maximum tumor ablation effect, and adverse effects were almost negligible. Also, owing to their NIR absorption and high photothermal efficacy, NPs have emerged as multi-mode bioimaging materials with readable fluorescence signals and clear photoacoustic (PA) signals. Similarly, in another work [89], Xie’s group prepared a series of conjugated polymers 69–71 (Figure 23) from BODIPY with various numbers of methyl

Polymers 2021, 13, 75 19 of 30

The in vitro experiment showed that the NPs were distributed in the cytoplasm by efficient cytoplasmic endocytosis and displayed significant cytotoxicity under laser radiation. In addition, the in vivo experiment results showed that PTT via intra-tumoral injection with irradiation provided the maximum tumor ablation effect, and adverse effects were almost Polymers 2021, 13, x FOR PEER REVIEWnegligible. Also, owing to their NIR absorption and high photothermal efficacy, NPs20 haveof 32 emerged as multi-mode bioimaging materials with readable fluorescence signals and clear photoacoustic (PA) signals. Similarly, in another work [89], Xie’s group prepared a series of conjugated polymers 69–71 (Figure 23) from BODIPY with various numbers of methyl substituentssubstituents and and DPP. DPP. Subsequently, conjugat conjugateded polymers and F127 were combined to obtainobtain NPs NPs that that exhibited exhibited PTT PTT capacity, capacity, PA PA imaging, and infrared thermal imaging. These studiesstudies usher usher in in a a new generation of multifunctional biomedical materials and develop image-guidedimage-guided therapy. therapy.

Figure 23. Chemical structures of polymers 68–71 and schematic illustration of fabrication self-assembly procedures and applications of NPs (Reproduced with permission from Reference [90]. Copyright @ 2019 ACS publication). applications of NPs (Reproduced with permission from Reference [90]. Copyright @ 2019 ACS publication). 3.3. Sensor 3.3. Sensor Owing to the wide range of absorption and emission spectra of BODIPY derivatives Owing to the wide range of absorption and emission spectra of BODIPY derivatives and the characteristics of conjugated chain amplification signal, BODIPY-based conju- and the characteristics of conjugated chain amplification signal, BODIPY-based conjugated gated polymers are applied to anion or organic vapor sensors [96–99]. Cao and coworkers polymers are applied to anion or organic vapor sensors [96–99]. Cao and coworkers [96] [96] presented a new conjugated polymer 72, synthesized through Sonogashira cross-cou- presented a new conjugated polymer 72, synthesized through Sonogashira cross-coupling pling reaction between BODIPY diiodide and alkynyl fluorene, for the detection of F− and reaction between BODIPY diiodide and alkynyl fluorene, for the detection of F− and CN− CN− ions (Figure 24). When F− was added to the solution of 72, the solution color changed ions (Figure 24). When F− was added to the solution of 72, the solution color changed fromfrom purple purple to to orange orange by by degrees, degrees, and and the the emission emission color color changed changed from from red red to to yellow yellow by by degrees,degrees, which which was was consistent consistent with with the the new emission band at 579 nm. However, when − CNCN− waswas added added to to the the solution solution ofof 7272, the, the solution solution color color gradually gradually changed changed from from purple purple to faintto faint yellow, yellow, and and the emission the emission color color gradually gradually changed changed from red from to redyellowish to yellowish green, which green, agreeswhich with agrees the with new the emission new emission bands at bands 514 and at 514563 andnm. 563Other nm. ions Other did ionsnot cause did not a signif- cause icanta significant change changein the naked in the nakedcolor or color emission or emission color. color.Thus, Thus,probe probe 72 could72 could allow allow the fast the naked-eyefast naked-eye detection detection of F− ofand F− CNand− ions CN without− ions without interference interference from other from ions, other and ions, the andde- tectionthe detection limits of limits 72 toward of 72 toward F− and CN F− −and ions CN were− ions 5.23 were × 10−7 5.23 and× 2.9610 −×7 10and−7 M, 2.96 respectively.× 10−7 M, Nuclearrespectively. magnetic Nuclear resonance magnetic (NMR) resonance demonstrated (NMR) demonstrated that the excellent that the detection excellent ability detection of 72ability was ofderived72 was from derived the nucleophilic from the nucleophilic attack of F− attack and CN of− F to− anddifluoroboron CN− to difluoroboron bridges. Be- sidesbridges. its use Besides as a sensor its use asmedium, a sensor 72 medium, was also72 suitablewas also as suitablean imaging as an agent imaging with agent low cyto- with toxicity. This work not only replenishes the BODIPY-based conjugated polymer library but also provides a feasible method to sense ions through the color response caused by highly responsive structural change.

Polymers 2021, 13, 75 20 of 30

low cytotoxicity. This work not only replenishes the BODIPY-based conjugated polymer library but also provides a feasible method to sense ions through the color response caused by highly responsive structural change.

Table 1. Summary of the structures and synthetic methods of BODIPY-based conjugated polymers.

Structures of BODIPY-Based Conjugated Synthetic Methods Compounds References Polymers Stille Cross-coupling Reaction 4, 5, 22, 23 [31,49] BODIPY as Backbone Sonogashira Cross-coupling Reaction 1–3, 6, 10–14, 17–21 [30,31,34] Oxidation Coupling 15, 16 [40] Linear/Coiled Units Electrochemical Polymerization 7–9 [23,25,33] Conjugated Polymers BODIPY as Pendants Suzuki Cross-coupling Reaction 24–26 [54] Side Chains Coordination Polymerization 27–29 [55] BODIPY as End Groups Sonogashira Cross-coupling Reaction 30-n [56,57] Electrochemical Polymerization 31, 32 [24] Porous Conjugated Polymers Friedel-Crafts cross-coupling reactions 33–37 [65,66] Crosslinked Polymers Yamamoto Cross-coupling Reaction 62 [91] Suzuki Cross-coupling Reaction 63, 64 [93] Other Structures Branched Polymers Polymers 2021, 13, x FOR PEER REVIEW Sonogashira Cross-coupling Reaction 73–75 [9821] of 32

FigureFigure 24. ((aa)) Chemical Chemical structures structures of of polymer polymer 7272. .((b)b Visual) Visual color color changes changes and and emission emission changes changes (λex (λ =ex 365= 365nm) nm) of 72 of (c72 = 10 μM) in tetrahydrofuran/H2O (98:2, v/v) in the presence of different anions (Reproduced with permission from Reference (c = 10 µM) in tetrahydrofuran/H2O (98:2, v/v) in the presence of different anions (Reproduced with permission from [96]. Copyright @ 2015 Elsevier). Reference [96]. Copyright @ 2015 Elsevier).

Valiyaveettil and coworkerscoworkers [[98]98] synthesizedsynthesized aa seriesseries ofof hyperbranchedhyperbranched conjugatedconjugated polymers (Figure 25)25) based based on on BODIPYs BODIPYs for for detecting detecting organic organic vapors. vapors. To Tostudy study the the influ- in- ﬂuenceence of polymer of polymer conformation conformation on vapor on vapor sensing, sensing, the authors the authors developed developed 73 and73 74and as 74A2Bas3- Atype2B3 -typehyperbranched hyperbranched conjugated conjugated polymers polymers with with benzene benzene and and triphenylamine triphenylamine between BODIPY moietiesmoieties respectively,respectively, with with75 75as as an an A2 AB42B-type4-type hyperbranched hyperbranched conjugated conjugated polymer poly- withmer with pyrene pyrene between between BODIPY BODIPY moieties. moieties. These These hyperbranched hyperbranched conjugated conjugated polymers polymers all displayedall displayed solvent solvent effect effect in differentin different solvents. solvents. Polymer Polymer73 possessed73 possessed a more a more planar planar confor- con- mation,formation, which which resulted resulted in emissionin emission quenching quenching at solidat solid state. state. The The twisted twisted conformation conformation of 74of 74and and the the higher higher branching branching degree degree of 75of provided75 provided the the polymers polymers with with weaker weaker aggregation, aggrega- leadingtion, leading to NIR to emissionNIR emission even even at the at solidthe solid state. state. Compared Compared with with73 and 73 and75, 7475,exhibited 74 exhib- higherited higher selectivity selectivity toward toward aromatic aromatic solvent, solv andent, the and high the reproducibility. high reproducibility. Thus, the Thus, system- the aticsystematic research research developed developed a new a vapor new detectionvapor detection utilizing utilizing hyperbranched hyperbranched BODIPY-based BODIPY- conjugatedbased conjugated polymers polymers with better with performance, better perfor enrichingmance, enriching the structures the structures and applications and appli- of BODIPY-basedcations of BODIPY-based conjugated conjugated polymers. polymers.

Figure 25. Chemical structures of polymers 73–75 [98] (Reproduced with permission from Reference [96]. Copyright @ 2016, Royal Society of Chemistry).

3.4. Gas/Energy Storage Generally, conjugated porous polymers (CPPs), owing to their favorable structures, have been extensively applied to gas or energy storage, and BODIPY units are rich in heteroatoms, amplifying their affinity to gas or other substances [100–103]. Patra and coworkers [103] prepared four BODIPY-based CPPs 76–78 (Figure 26) with various alkyl chain lengths at meso-positions via Sonogashira cross-coupling reaction. Polymer 76 possessed

Polymers 2021, 13, x FOR PEER REVIEW 21 of 32

Figure 24. (a) Chemical structures of polymer 72. (b) Visual color changes and emission changes (λex = 365 nm) of 72 (c = 10 μM) in tetrahydrofuran/H2O (98:2, v/v) in the presence of different anions (Reproduced with permission from Reference [96]. Copyright @ 2015 Elsevier).

Valiyaveettil and coworkers [98] synthesized a series of hyperbranched conjugated polymers (Figure 25) based on BODIPYs for detecting organic vapors. To study the influence of polymer conformation on vapor sensing, the authors developed 73 and 74 as A2B3- type hyperbranched conjugated polymers with benzene and triphenylamine between BODIPY moieties respectively, with 75 as an A2B4-type hyperbranched conjugated polymer with pyrene between BODIPY moieties. These hyperbranched conjugated polymers all displayed solvent effect in different solvents. Polymer 73 possessed a more planar conformation, which resulted in emission quenching at solid state. The twisted conformation of 74 and the higher branching degree of 75 provided the polymers with weaker aggregation, leading to NIR emission even at the solid state. Compared with 73 and 75, 74 exhibited higher selectivity toward aromatic solvent, and the high reproducibility. Thus, the Polymers 2021, 13, 75 systematic research developed a new vapor detection utilizing hyperbranched BODIPY-21 of 30 based conjugated polymers with better performance, enriching the structures and applications of BODIPY-based conjugated polymers.

FigureFigure 25.25.Chemical Chemical structures structures of of polymers polymers73–75 73–75 [98 [98]] (Reproduced (Reproduced with with permission permission from from Reference Reference [96]. Copyright[96]. Copyright @ 2016, @ 2016, Royal Society of Chemistry). Royal Society of Chemistry). 3.4. Gas/Energy Storage 3.4. Gas/EnergyGenerally, Storageconjugated porous polymers (CPPs), owing to their favorable structures, haveGenerally, been extensively conjugated applied porous to gas polymers or energy (CPPs), storage, owing and BODIPY to their favorable units are structures,rich in het- Polymers 2021, 13, x FOR PEER REVIEWhaveeroatoms, been amplifying extensively their applied affinity to gasto gas or or energy other substances storage, and [100–103]. BODIPY Patra units and are cowork-22 rich of in32 heteroatoms,ers [103] prepared amplifying four BODIPY-based their affinity toCPPs gas 76 or– other78 (Figure substances 26) with [100 various–103]. alkyl Patra chain and coworkerslengths at meso-positions [103] prepared via four Sonogashira BODIPY-based cross-coupling CPPs 76–78 reaction.(Figure 26Polymer) with various 76 possessed alkyl chain lengths at meso-positions via Sonogashira cross-coupling reaction. Polymer 76 mesoporouspossessed mesoporous structures with structures Brunauer-Emmett-Teller with Brunauer-Emmett-Teller (BET) surface (BET) area surface of 73 area(76a) ofand 73 2 −1 322(76a m)2 and g−1 ( 32276b), m whileg 77(76b and), while78 were77 respectivelyand 78 were microporou respectivelys with microporous BET surface with area BET of 2 −1 2 −1 573surface m2 g area−1 and of ultra-microporous 573 m g and ultra-microporous with BET surface witharea of BET 1010 surface m2 g−1 area. The of different 1010 m poreg . featuresThe different were porerelated features to alkyl were chain related substitution; to alkyl that chain is, substitution;the long alkyl that chain is, thetook long up more alkyl freechain volume, took up resulting more free in the volume, pore blockage resulting effect. in the On pore the blockage contrary, effect.rigid phenyl On the with contrary, non- planarityrigid phenyl led withto small nonplanarity pore diameter led toand small high pore surface diameter area. andOwing high to surfaceprofitable area. pore Owing fea- to profitable pore features, 78 displayed the highest CO uptake of 16.5 wt%, higher than tures, 78 displayed the highest CO2 uptake of 16.5 wt%, higher2 than those of 77 (10.5 wt%) andthose 76b of (6.977 (10.5 wt%). wt%) In addition, and 76b (6.978 showed wt%). In the addition, highest 78isostericshowed heat the of highest adsorption isosteric (30.6 heat kJ of adsorption (30.6 kJ mol−1), due to the rich nitrogen content of BODIPYs, leading to the mol−1), due to the rich nitrogen content of BODIPYs, leading to the good affinity of CO2. good affinity of CO2. Notably, 77 possessed the best selective adsorption to CO2 in the Notably, 77 possessed the best selective adsorption to CO2 in the binary mixtures of CO2 binary mixtures of CO2 and N2, because the appropriate pore size maintained stronger and N2, because the appropriate pore size maintained stronger electrostatic interaction electrostatic interaction between POPs and CO2. BODIPY-based POPs have the capability of between POPs and CO2. BODIPY-based POPs have the capability of producing 1O2, which producing 1O , which can catalyze some oxidation reactions with a quite high conversion. can catalyze some2 oxidation reactions with a quite high conversion.

Figure 26. Synthesis of polymers 76–78. (Reaction conditions: 76a: [Pd(PPh3)2Cl2], toluene, 80 °C; 76b◦ : [Pd(PPh3)4], dime- Figure 26. Synthesis of polymers 76–78. (Reaction conditions: 76a: [Pd(PPh3)2Cl2], toluene, 80 C; 76b: [Pd(PPh3)4], thylformamide, 130 °C; 77◦: M2 monomer, [Pd(PPh3)4], DMF, 130 °C; 78: ◦M3 monomer, [Pd(PPh3)4], DMF, 130 °C) (Repro-◦ dimethylformamide, 130 C; 77: M2 monomer, [Pd(PPh3)4], DMF, 130 C; 78: M3 monomer, [Pd(PPh3)4], DMF, 130 C) duced with permission from Reference [103]. Copyright @ 2016, American Chemical Society). (Reproduced with permission from Reference [103]. Copyright @ 2016, American Chemical Society). Likewise, to exploit superior anode materials of lithium-ion batteries, Kuang and coworkers [102] prepared a new POP 79 (Figure 27) containing anthracene-substituted BODIPYs and 1,3,5-triethynylbenzene, and further obtained carbonized POP (79-C) via a carbonization process. Polymers 79 and 79-C displayed microporous architectures with pore sizes of 20.5 and 6.9 nm respectively, and their BET surface areas were 505 and 638 m2 g−1, respectively. In addition, the existence of heteroatoms allowed POP 79 to possess higher specific capacity, better superior rate performance, and long-term cyclic stability. As for 79-C, heteroatoms doping also made conductivity and the photoelectric property better. Hence, BODIPY-based POPs pave a way for the fabrication of gas/energy storage materials based on the porous structure and richness in heteroatoms of BODIPY units.

Polymers 2021, 13, x FOR PEER REVIEW 22 of 32

mesoporous structures with Brunauer-Emmett-Teller (BET) surface area of 73 (76a) and 322 m2 g−1 (76b), while 77 and 78 were respectively microporous with BET surface area of 573 m2 g−1 and ultra-microporous with BET surface area of 1010 m2 g−1. The different pore features were related to alkyl chain substitution; that is, the long alkyl chain took up more free volume, resulting in the pore blockage effect. On the contrary, rigid phenyl with nonplanarity led to small pore diameter and high surface area. Owing to profitable pore features, 78 displayed the highest CO2 uptake of 16.5 wt%, higher than those of 77 (10.5 wt%) and 76b (6.9 wt%). In addition, 78 showed the highest isosteric heat of adsorption (30.6 kJ mol−1), due to the rich nitrogen content of BODIPYs, leading to the good affinity of CO2. Notably, 77 possessed the best selective adsorption to CO2 in the binary mixtures of CO2 and N2, because the appropriate pore size maintained stronger electrostatic interaction between POPs and CO2. BODIPY-based POPs have the capability of producing 1O2, which can catalyze some oxidation reactions with a quite high conversion.

Polymers 2021, 13, 75 22 of 30 Figure 26. Synthesis of polymers 76–78. (Reaction conditions: 76a: [Pd(PPh3)2Cl2], toluene, 80 °C; 76b: [Pd(PPh3)4], dimethylformamide, 130 °C; 77: M2 monomer, [Pd(PPh3)4], DMF, 130 °C; 78: M3 monomer, [Pd(PPh3)4], DMF, 130 °C) (Repro- duced with permission from Reference [103]. Copyright @ 2016, American Chemical Society). Likewise, to exploit superior anode materials of lithium-ion batteries, Kuang and coworkersLikewise, [102 ]to prepared exploit superior a new POP anode79 (Figurematerials 27 )of containing lithium-ion anthracene-substituted batteries, Kuang and BODIPYscoworkers and [102] 1,3,5-triethynylbenzene, prepared a new POP and79 (Figure further obtained27) containing carbonized anthracene-substituted POP (79-C) via a carbonizationBODIPYs and process.1,3,5-triethynylbenzene, Polymers 79 and and79-C furtherdisplayed obtained microporous carbonized architectures POP (79-C) withvia a porecarbonization sizes of 20.5 process. and 6.9Polymers nm respectively, 79 and 79-C and displayed their BET microporous surface areas architectures were 505 andwith − 638pore m sizes2 g 1 of, respectively. 20.5 and 6.9 nm In addition, respectively, the existenceand their ofBET heteroatoms surface areas allowed were 505 POP and79 638to possessm2 g−1, respectively. higher speciﬁc In capacity,addition, betterthe existence superior of rateheteroatoms performance, allowed and POP long-term 79 to possess cyclic stability.higher specific As for capacity,79-C, heteroatoms better superior doping rate also performance, made conductivity and long-term and the cyclic photoelectric stability. propertyAs for 79-C better., heteroatoms Hence, BODIPY-based doping also POPsmade paveconductivity a way for and the the fabrication photoelectric of gas/energy property storagebetter. Hence, materials BODIPY-based based on the POPs porous pave structure a way andfor the richness fabrication in heteroatoms of gas/energy of BODIPY storage units.materials based on the porous structure and richness in heteroatoms of BODIPY units.

Figure 27. Chemical structures of polymers 79 and diagram of 79-C (Reproduced with permission from Reference [102]. Copyright @ 2018 ACS publication).

3.5. Other Applications In addition to the above-mentioned applications, there are other fields involving BODIPY-based conjugated polymers. Yang and coworkers [104] presented a new linear conjugated polymer 80 via Suzuki cross-coupling reaction (Figure 28). Polymer 80 with the conjugated chains displayed better chemical and thermal stability compared with the monomers. To improve the dispersibility and catalytic efficiency, 80 was combined with silica powder through the F ... Si interactive force, and 80-silica was obtained (Figure 28), which was hardly soluble in common organic solvent, allowing easy recovery from reactions. A series of control experiments proved that the yield of benzylamine oxidation was up to 82% at the reaction temperature of 90 ◦C in the heterogeneous reaction using 1 80-silica as a catalyst. Such excellent catalytic effect depended on the generation of O2 from 80-silica under radiation. The recycling performance of 80-silica was prominent. After it was used three times, the yield only decreased by 5%. Furthermore, BODIPY-based POPs with self-porosity could also serve as photocatalysts without hybridization. Liras and coworkers [105] prepared a new microporous polymer 81 with high surface area (Figure 28). The polymer could be used as a heterogeneous catalyst to oxidize thioanisole to sulfoxide with a rate four times higher than that of homogeneous catalysts. However, polymer 81 could only be reused twice. These heterogeneous photocatalysts based on BODIPY- containing polymers [104–106] represent a new potential class of high-catalytic-efficiency and recyclable catalysts, resulting in the simplification of handling and purification. Polymers 2021, 13, x FOR PEER REVIEW 23 of 32

Figure 27. Chemical structures of polymers 79 and diagram of 79-C (Reproduced with permission from Reference [102]. Copyright @ 2018 ACS publication).

3.5. Other Applications In addition to the above-mentioned applications, there are other fields involving BODIPY-based conjugated polymers. Yang and coworkers [104] presented a new linear conjugated polymer 80 via Suzuki cross-coupling reaction (Figure 28). Polymer 80 with the conjugated chains displayed better chemical and thermal stability compared with the monomers. To improve the dispersibility and catalytic efficiency, 80 was combined with silica powder through the F…Si interactive force, and 80-silica was obtained (Figure 28), which was hardly soluble in common organic solvent, allowing easy recovery from reactions. A series of control experiments proved that the yield of benzylamine oxidation was up to 82% at the reaction temperature of 90 °C in the heterogeneous reaction using 80- silica as a catalyst. Such excellent catalytic effect depended on the generation of 1O2 from 80-silica under radiation. The recycling performance of 80-silica was prominent. After it was used three times, the yield only decreased by 5%. Furthermore, BODIPY-based POPs with self-porosity could also serve as photocatalysts without hybridization. Liras and coworkers [105] prepared a new microporous polymer 81 with high surface area (Figure 28). The polymer could be used as a heterogeneous catalyst to oxidize thioanisole to sulfoxide with a rate four times higher than that of homogeneous catalysts. However, polymer 81 could only be reused twice. These heterogeneous photocatalysts based on BOD- Polymers 2021, 13, 75 23 of 30 IPY-containing polymers [104–106] represent a new potential class of high-catalytic-efficiency and recyclable catalysts, resulting in the simplification of handling and purification.

FigureFigure 28. ChemicalChemical structures structures of of polymers polymers 80,80 80-silica, 80-silica, and, and 81 (Reproduced81 (Reproduced with with permission permission from fromReference Reference [104]. [Cop-104]. Copyrightyright @ 2015 @ 2015 Elsevier Elsevier B.V. B.V. Reproduced Reproduced with with permission permission from from Reference Reference [105]. [105]. Copyright Copyright @ @ 2016, 2016, American ChemicalChemical Society). Society). Given the fast reaction between the 2,6-positions of BODIPY units and iodine, Kuang Table 2. Summary of the applications and synthetic methods of BODIPY-based conjugated polymers. and coworkers [107] synthesized a new POP 82 with 2,6-position-unsubitituted BODIPY Functional Applications of BODIPY-Basedunits as monomers via Sonogashira cross-coupling reaction and verified its capability of Synthetic Methods Compounds References Conjugated Polymerscapturing iodine compared with 2,6-positions-subitituted BODIPY-based POP 83 and the reference compound NBDPStille Cross-coupling without BODIPYs Reaction (Figure 29). 38The, 42 polymerization–49 [75,77 ,process78,81] Solar Cells was accompanied bySonogashira a side oxidation-reaction, Cross-coupling Reaction which consumed39 –sectional41 active[74 sides,76] but Optoelectronic Organic Thin-ﬁlm Stille Cross-coupling Reaction 50–56 [82,83] Materials Transistors Sonogashira Cross-coupling Reaction 57 [84] Memory Devices Sonogashira Cross-coupling Reaction 58 [87] Suzuki Cross-coupling Reaction 59, 60, 63–66, 69–91 [90,92–94] Bioimaging and Biotherapy Sonogashira Cross-coupling Reaction 61, 68 [90,95] Yamamoto Cross-coupling Reaction 62 [91] Sensor Sonogashira Cross-coupling Reaction 72–75 [96,98] Gas/Energy Storage Sonogashira Cross-coupling Reaction 76–79 [102,103] Suzuki Cross-coupling Reaction 80 [104] Other Applications Sonogashira Cross-coupling Reaction 81–83 [105,107] Yamamoto Cross-coupling Reaction 84, 85 [108]

Given the fast reaction between the 2,6-positions of BODIPY units and iodine, Kuang and coworkers [107] synthesized a new POP 82 with 2,6-position-unsubitituted BODIPY units as monomers via Sonogashira cross-coupling reaction and veriﬁed its capability of capturing iodine compared with 2,6-positions-subitituted BODIPY-based POP 83 and the reference compound NBDP without BODIPYs (Figure 29). The polymerization process was accompanied by a side oxidation-reaction, which consumed sectional active sides but reduced the surface area and porous structure for a higher adsorption capacity. The comparison of 82, 83, and NBDP showed an increasing iodine absorption capacity from NBDP to 83 to 82, indicating that not only porous properties but also the presence of afﬁnity function groups impacted the iodine absorption capacity. Moreover, 82 exhibited higher adsorption capability and fast inclusion rate in iodine-hexane solution, and its release rate was also the fastest. Additionally, all POPs showed good regeneration capacity for recycling. These porous materials not only have potential capacity of iodine absorption but also provide a feasible strategy to facilitate the development of other volatile compounds. Polymers 2021, 13, x FOR PEER REVIEW 24 of 32

reduced the surface area and porous structure for a higher adsorption capacity. The comparison of 82, 83, and NBDP showed an increasing iodine absorption capacity from NBDP to 83 to 82, indicating that not only porous properties but also the presence of affinity function groups impacted the iodine absorption capacity. Moreover, 82 exhibited higher adsorption capability and fast inclusion rate in iodine-hexane solution, and its release rate Polymers 2021, 13, 75 was also the fastest. Additionally, all POPs showed good regeneration capacity for24 recy- of 30 cling. These porous materials not only have potential capacity of iodine absorption but also provide a feasible strategy to facilitate the development of other volatile compounds.

Figure 29. Chemical structuresstructures ofof polymers polymers82 82, 83, 83, and, andNBDP NBDP(Reproduced (Reproduced with with permission permission from from Reference Reference [107 [107].]. Copyright Copy- right@ 2017, @ 2017, Royal Royal Society Society of Chemistry). of Chemistry).

Recently,Recently, WuWu andand coworkers coworkers [108 [108]] designed designed two two BODIPY-based BODIPY-based conjugated conjugated polymers poly- mers84 and 8485 and(Figure 85 (Figure 30), and 30), then and further then further prepared prepared Pdots with Pdots uniform with uniform small-size small-size distribution dis- tributionfor super-resolution for super-resolution optical fluctuation optical fluctu imagingation (SOFI). imaging This (SOFI). imaging This technique imaging is technique based on isthe based random on the fluctuation random of fluctuation the fluorescence of the intensityfluorescence of mutually intensity fluorescent of mutually chromogens fluorescent to chromogensachieve resolution to achieve improvement. resolution Polymersimprovement.84 and Polymers85 displayed 84 and emission 85 displayed maxima emission at 565 maximaand 720 nm,at 565 and and due 720 to nm, the and FRET due effect to the between FRET polymereffect between chains, polymer the fluorescence chains, the spectra fluo- rescenceof these polymerspectra of materials these polymer were greatly materials narrow, were with greatly ~39 narrow, nm of emission with ~39 bandwidths nm of emission (full bandwidthswidth at half (full maximum, width FWHM)at half maximum, for 84 and ~50FWHM) nm of for emission 84 and bandwidths ~50 nm of emission (FWHM) band- for 85. widthsCompared (FWHM) with commercially for 85. Compared available with fluorescent commercially dyes available Alexa 555 fluorescent and Alexa 647,dyes84 Alexa-Pdot 555and and85-Pdot Alexa exhibited 647, 84-Pdot stronger and luminescence, 85-Pdot exhibited higher stronger signal-to-noise luminescence, ratio, excellent higher signal- single- to-noiseparticle anti-photobleachingratio, excellent single-particle properties, andanti-photobleaching more pronounced properties, fluctuations; and thus, more they pro- can nouncedbe applied fluctuations; for SOFI. Indeed, thus, they the single-particle can be applied analysis for SOFI. demonstrated Indeed, the thatsingle-particle after eight-order anal- ysisSOFI demonstrated processing, thethat resolution after eight-order of the Pdots SOFI processing, SOFI imaging the significantlyresolution of amplifiedthe Pdots SOFI to 57 imaging(84) and 88significantly nm (85). Furthermore, amplified to after 57 (84 surface) and 88 bio-conjugated nm (85). Furthermore, with streptavidin, after surface the Pdots bio- conjugatedachieved the with specific streptavidin, labeling the of thePdots mitochondrial achieved the membrane specific labeling structure of the and mitochondrial microtubule membranestructure of structure BS-C-1 withoutand microtubule cross-color structure interference of BS-C-1 and without the eighth-order cross-color SOFI interference imaging. Polymers 2021, 13, x FOR PEER REVIEW 25 of 32 andConsequently, the eighth-order the SOFI SOFI imaging imaging. system Conseq baseduently, on BODIPY-containing the SOFI imaging conjugated system based polymers on

BODIPY-containingcan promote the development conjugated of polymers higher-resolution can promote imaging the development systems in the of future.higher-resolution imaging systems in the future.

FigureFigure 30. ChemicalChemical structures structures of of polymers polymers 8484 and 85 (Reproduced with permission fr fromom Reference [108]. [108]. Copyright @ 2016,2016, Royal Royal Society Society of of Chemistry). Chemistry).

Table 1. Summary of the structures and synthetic methods of BODIPY-based conjugated polymers.

Structures of BODIPY-Based Conjugated Polymers Synthetic Methods Compounds References

Stille Cross-coupling Reaction 4, 5, 22, 23 [31,49]

Sonogashira Cross-coupling Re- 1–3, 6, 10–14, [30,31,34] BODIPY as Backbone Units action 17–21 Oxidation Coupling 15, 16 [40] Linear/Coiled Conjugated Electrochemical Polymerization 7–9 [23,25,33] Polymers

BODIPY as Pendants Side Suzuki Cross-coupling Reaction 24–26 [54] Chains Coordination Polymerization 27–29 [55]

Sonogashira Cross-coupling Re- BODIPY as End Groups 30-n [56,57] action

Electrochemical Polymerization 31, 32 [24]

Porous Conjugated Polymers Friedel-Crafts cross-coupling 33–37 [65,66] reactions

Yamamoto Cross-coupling Re- Crosslinked Polymers 62 [91] action

Other Structures Suzuki Cross-coupling Reaction 63, 64 [93]

Branched Polymers Sonogashira Cross-coupling Re- 73–75 [98] action

Table 2. Summary of the applications and synthetic methods of BODIPY-based conjugated polymers.

Functional Applications of BODIPY- Synthetic Methods Compounds References Based Conjugated Polymers

Stille Cross-coupling Reaction 38, 42–49 [75,77,78,81] Optoelectronic Materials Solar Cells Sonogashira Cross-coupling Reaction 39–41 [74,76]

Polymers 2021, 13, 75 25 of 30

4. Conclusions Significant advances have been achieved in BODIPY-based conjugated polymers due to the joint effect of conjugate structures and the fluorescent dye BODIPY, which encouraged us to comprehensively summarize the establishments and applications of BODIPY-based conjugated polymers. BODIPYs possess the advantages of facile synthesis and chemical structure modification and can be synthesized through the conventional preparation methods of conjugated polymers; among the preparation methods, metal-catalyzed cross- coupling polymerization is most frequently used, especially Suzuki cross-coupling or Sonogashira cross-coupling reaction. Due to the numerous active sites of the BODIPY core, the structures of BODIPY-based conjugated polymers are flexible, comprising linear, coiled, porous, branched, and crosslinked structures; among these, linear and coiled conjugated polymers polymerized through α-α-connected, α-β-connected, or β-β-connected account for the largest proportion. In this respect, we summarized design of new polymers that can be initiated from three aspects: (1) the rational selection of polymerization sites, (2) the modification of BODIPY building units before polymerization, and (3) the appropriate selection of comonomers. Furthermore, some significant conclusions are as follows. First, π-conjugation is more efficient in the case of polymerization through the α-position of BODIPYs than in the polymerization through the β-position, allowing for low band gap. Second, the extension of conjugated BODIPYs at the 3,5-position through classical Knoeve- nagel condensation reaction results in deep-red or NIR emission. Third, D-A structures promote intramolecular charge transfer, leading to low band gap and broad absorption. Fourth, the incorporation of BODIPYs as side chains into polymers can effectively inhibit the concentration fluorescence quenching and even result in FERT. Finally, BODIPY-based conjugated polymers possess high flexibility in the design of skeletons and porosity. Owing to flexible design and diverse structures, conjugated polymers containing BODIPYs exhibit remarkable potential for applications in several fields, including optoelectronic materials, biotherapy, bioimaging, sensors, gas/energy storage, and photocatalysis. They are most widely utilized in photoelectric materials, especially solar cells, owing to the excellent photoelectric properties both of BODIPY dyes and conjugated backbones. Furthermore, significant achievements have been made in the development of BODIPY- based conjugated polymers, from structural design to practical applications. However, there is still much room for further growth. Moreover, water solubility and NIR emission are imperative for biomedical applications. Despite that some water-soluble polymeric BODIPYs modified by the introduction of hydrophilic chains or biological molecules have been obtained, there are limited cases and no certain regulations for designing suitable polymers; thus, this area is an open proposition. For greater tissue penetration and less biological damage, NIR region II absorption and emission are the better choice, which is a challenging direction. In addition, designing new BODIPY-based conjugated POPs with higher BET surface areas, anti-degradation backbones, and specific binding groups may afford the BODIPY the capability to adsorb or store more chemical substances and energy. Furthermore, POPs containing BODIPYs are promising electron donor scaffolds for electron-accepting compounds, and their exceptional light-absorption properties and high-dimensional geometry are conducive to amplified charge generation, splitting, and transport. These advantages indicate that POPs are reasonable candidates for photoelectric materials, but there are few related examples and linear polymers are in the majority. It may be that solubility restricts the POP applications; therefore, certain effort should be channeled toward designing rational POPs and uniform film formation to integrate them into photoelectric devices. Moreover, BODIPY-containing conjugated polymers have been 1 applied in photocatalysis, due to the generation of O2 with the action of BODIPYs. It is a question of whether these polymers with energy class levels are allowed to catalyze reduction reactions, such as hydrogen reduction, when they agree with the oxidation and reduction potentials required for the reaction. It can be anticipated that BODIPY-based conjugated polymers may continually broaden the application range of photoelectric materials, biological materials, and other smart materials. This review can inspire chemists, material Polymers 2021, 13, 75 26 of 30

scientists, and engineers to jointly develop new BODIPY-based conjugated polymers and explore their broader practical applications.

Author Contributions: S.Y. and Y.L. initiated the work upon the invitation, designed the proposal and revised the manuscript. Y.F. and J.Z. wrote the first draft of the manuscript. Z.H. and H.Q. prepared the figures. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant numbers 21971049 and 51903070, and National Training Programs of Innovation and Entrepreneurship for Undergraduates grant number 202010346031. Acknowledgments: The authors thank College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, for equipment support. Conflicts of Interest: The authors declare no conflict of interest.

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