molecules

Review Reactive & Efficient: Organic as Cross-Linkers in Material Sciences

Marvin Schock 1 and Stefan Bräse 1,2,3,*

1 Institute of Organic , Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany; [email protected] 2 Institute of Biological and Chemical Systems—FMS (IBCS-FMS), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 3 3DMM2O—Cluster of Excellence (EXC-2082/1–390761711), Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany * Correspondence: [email protected]; Tel.: +49-721-608-42902

 Academic Editor: Klaus Banert  Received: 22 December 2019; Accepted: 10 February 2020; Published: 24 February 2020

Abstract: The exceptional reactivity of the group makes organic azides a highly versatile family of compounds in chemistry and the material sciences. One of the most prominent reactions employing organic azides is the regioselective (I)-catalyzed Huisgen 1,3-dipolar cycloaddition with yielding 1,2,3-triazoles. Other named reactions include the Staudinger reduction, the aza-Wittig reaction, and the . The popularity of organic azides in material sciences is mostly based on their propensity to release by thermal activation or photolysis. On the one hand, this scission reaction is accompanied with a considerable output of energy, making them interesting as highly energetic materials. On the other hand, it produces highly reactive nitrenes that show extraordinary efficiency in polymer crosslinking, a process used to alter the physical properties of polymers and to boost efficiencies of polymer-based devices such as membrane fuel cells, organic solar cells (OSCs), light-emitting diodes (LEDs), and organic field-effect transistors (OFETs). Thermosets are also suitable application areas. In most cases, organic azides with multiple azide functions are employed which can either be small molecules or oligo- and polymers. This review focuses on nitrene-based applications of multivalent organic azides in the material and life sciences.

Keywords: azides; photochemistry; polymers; thermosets; nitrenes

1. Introduction The characteristics of being explosive, malodorous, and highly volatile are usually adverse properties of a compound in the lab. Therefore, after the preparation of the first organic azide, phenyl azide, by Peter Grieß in 1864, interest in this new family of compounds was low for many years [1]. It was not until the 1950s and 1960s that safer derivatives were prepared which allowed the discovery of reactivities that would soon lead to an impressive library of powerful named reactions. At the same time, chemists at Kodak Inc. developed the first photoresist based on organic bisazides, laying an important cornerstone both for the semiconductor industry and organic azides [2]. After this commercial success, organic azides enjoyed increasing popularity as polymer crosslinking agents. The crosslinking process can be initiated by thermal activation or photolysis of the crosslinking agent, and nitrogen is released under the formation of nitrenes. These highly reactive species can form covalent bonds with both saturated and unsaturated hydrocarbon chains of polymers. The material’s mechanical properties, like solubility and hardness, are changed [3–7]. In recent years, this process has also proven to be a powerful strategy to boost efficiencies of polymer-based devices such as membrane fuel cells, organic solar cells (OSCs), light-emitting diodes (LEDs), and organic field-effect transistors (OFETs). For example, the morphology of the donor and

Molecules 2020, 25, 1009; doi:10.3390/molecules25041009 www.mdpi.com/journal/molecules Molecules 2020, 25, 1009 2 of 29 acceptor material in bulk hetero junction organic solar cells was successfully locked and the long-time stability of the device increased in this way [8]. Due their superior crosslinking efficiency and mild activation methods devoid of the addition of initiators, organic azides have become the compounds of choice in these applications. During many decades, the explosive side of organic azides has never been used in commercial applications. While heavy-metal azides have served as detonators in explosives technology, organic azides as both potent and safe explosives are as yet unknown. As nitrogen-rich components, organic azides are prone to violent decomposition at elevated temperature and sensitive to impact or light. Nevertheless, stable derivatives exist and can be designed obeying certain rules of structural design. In recent years, azidated polymers as insensitive energetic binders and plasticizers in propellants have come into focus. They are used to boost the performance of explosive blends without sacrificing safety by taking advantage of the energy delivered during the scission reaction of the azido group [9]. Azido binders and plasticizers are considered as green materials, because combustion produces only molecular nitrogen and other non-toxic decomposition products.

1.1. General Comments for Azides It should be noted that azides and in particular polyazides are (sometimes) prone to violent decomposition at elevated temperatures (over 100 ◦C, depending on the structure). However, they are stable at room temperature and in the absence of light. For safety precautions, please see [10–12]:

1.2. Synthesis of Azides, General Considerations For the synthesis of (poly)azides, many suitable protocols have been employed [10,11]: azides (e.g., 1,6-diazidohexane, see below): • Nucleophilic substitution reactions ◦ Addition of hydrazoic acid to double bonds ◦ Aryl azides: • Reaction of diazonium salts with azide ions ◦ Sulfonyl azides (e.g., 4,4-oxydibenzenesulphonylazide (SDO)): • Reaction of sulfonyl halides with inorganic azides ◦ Reaction of sulfonyl hydrazides with nitric acid ◦ Carbonyl azides: • Reaction of reactive carbonyl compounds with azides ◦ 1.3. Reaction of Azides—General Considerations For the reaction of (poly)azides, many suitable reaction have been employed [10,11]: Thermal or photolytic generation and further reaction of nitrenes • Cycloadditions • Reaction with nucleophiles • Staudinger reaction ◦ Rearrangements • It should be noted at this point that the (quite different) photochemistries of aryl, acyl, and alkyl azides have been investigated in depth [13]. In particular the groups of Platz [14–18] and Gritsan [19–24] contributed with many examples, reactions, and kinetics. Molecules 2020, 25, 1009 3 of 29

2. Azido-Based High Energy Materials Solid propellants and explosives consist of a mixture of a solid fuel or explosive, binder, plasticizer, and oxidizer [25]. Polymeric binders are used to bind together the high energetic material in order to reduce its sensitivityMolecules 2020 and, 25, 1009 to enable further mechanical processing. Plasticizers are added to enhance3 physical of 29 properties like the plasticity and viscosity of the binder polymer by decreasing the attraction between polymerthe attraction chains. between Ideally, theypolymer should chains. have Ideally, low glass they transition should have temperatures low glass transition and high temperatures decomposition temperatures.and high decomposition For high performance temperatures. of theFor blend,high performance the whole systemof the blend, must the have whole a favorable system must heat of formationhave a favorable and oxygen heat balance. of formation Therefore, and oxygen energetic balance. plasticizers Therefore, and energetic binders plasticizers are preferred and binders over inert ones.are At preferred the same over time, inert the ones. high At stability the same and time, low the sensitivity high stability of the and material low sensitivity are demanded. of the material Moreover, combustionare demanded. should Moreover, not produce combustion hazardous should gases. not As produce a nitrogen-rich hazardous class gases. of compounds, As a nitrogen organic‐rich class azides usuallyof compounds, have positive organic heats azides of formation, usually highhave densities,positive heats and high of formation, burning rates. high Duringdensities, scission and high of the azideburning bond, rates. elemental During nitrogen scission and of considerable the azide bond, amounts elemental of energy nitrogen in the and range considerable of 90 kcal amounts per azide of unit areenergy released in [ the9]. Azido-basedrange of 90 kcal binders per azide and plasticizers unit are released possess [9]. favorable Azido‐based material binders properties and plasticizers such as low possess favorable material properties such as low glass transition temperatures and good thermal glass transition temperatures and good thermal and hydrolytic stability, as well as low impact sensitivity. and hydrolytic stability, as well as low impact sensitivity. Moreover, they are considered green Moreover, they are considered green compounds as their combustion produce only nitrogen and other compounds as their combustion produce only nitrogen and other non‐toxic decomposition products. non-toxic decomposition products. In the next two chapters, organic azides as binders and plasticizers in In the next two chapters, organic azides as binders and plasticizers in advanced propellants and advancedexplosives propellants are presented. and explosives are presented. 2.1. Azido Binders 2.1. Azido Binders SeveralSeveral azido azido polymers polymers have have been been investigated investigated as energetic as energetic binders, binders, such assuch glycidyl as glycidyl azide polymers azide (GAPpolymers) and energetic (GAP) and thermoplastic energetic thermoplastic elastomers (ETPE elastomerss) such ( asETPE poly(3-azidomethyl-3-s) such as poly(3‐azidomethyl methyloxetane)‐3‐ (poly-AMMOmethyloxetane)) and (poly poly(3,3–bis(azidomethyl)oxetane)‐AMMO) and poly(3,3–bis(azidomethyl)oxetane) (poly-BAMO), as well(poly as‐BAMO their copolymers), as well as [26 ]. A commontheir copolymers synthesis [26]. route A forcommonGAP issynthesis shown inroute Scheme for GAP1. Poly-AMMO is shown in Schemeand poly-BAMO 1. Poly‐AMMOcan similarly and be obtainedpoly‐BAMO from can the correspondingsimilarly be oxetanesobtained viafrom ring-opening the corresponding polymerization oxetanes and subsequentvia ring‐opening azidation. So far,polymerization only GAP has and established subsequent as azidation. a commercially So far, usedonly GAP energetic has established binder and as plasticizer a commercially system. used Major advantagesenergetic arebinder its liquidand plasticizer aggregation system. state atMajor processing advantages temperature, are its liquid relatively aggregation high density, state at and positiveprocessing heat of temperature, formation, whilerelatively high high glass density, transition and temperature positive heat (T gof= formation,43 C) and while poor high mechanical glass − ◦ characteristicstransition temperature at low temperatures (Tg = –43 °C) and are mainpoor mechanical demerits. characteristics The homopolymer at low temperatures of the most are prominent main demerits. The homopolymer of the most prominent azido0 oxetane, BAMO1 , has the highest energy azido oxetane, BAMO, has the highest energy output (∆fH = 2.16 kJ g− ) of the listed energetic azide 0 ‐1 polymers,output ( but∆fH it = is 2.16 solid kJ g because) of the oflisted its symmetric energetic azide structure polymers, and thereforebut it is solid only because copolymers of its symmetric can be used as binders.structureBAMO and thereforecopolymerized only copolymers with non-energetic can be used monomers as binders. like BAMO tetrahydrofuran copolymerized (THF )with shows non‐energetic monomers like tetrahydrofuran (THF) shows good processability and mechanical good processability and mechanical properties at the expense of lower energy level and thermoplastic properties at the expense of lower energy level and thermoplastic character. The copolymer of BAMO character. The copolymer of BAMO and azido oxetane AMMO, p(BAMO-AMMO), has very attractive and azido oxetane AMMO, p(BAMO‐AMMO), has very attractive features for use as0 ETPEs in features for use as ETPEs in extruded propellants and explosives because of its high ∆fH of up to 3.44 extruded propellants and explosives because of its high ∆fH0 of up to 3.44 kJ g‐1 although its low 1 kJ gtemperature− although properties its low temperature are not optimal properties [27]. are not optimal [27].

SchemeScheme 1. 1.Synthesis Synthesis of of glycidyl glycidyl azide azide polymerpolymer (GAP (GAP,, top) top) and and structures structures of of poly(3 poly(3-azidomethyl-3-‐azidomethyl‐3‐ methyloxetane)methyloxetane) and and poly(3,3–bis(azidomethyl)oxetane),poly(3,3–bis(azidomethyl)oxetane), also also known known as poly as‐AMMOpoly-AMMO and andand poly and‐ poly-BAMOBAMO (bottom),(bottom), respectively. respectively.

Star azide copolymers with hyperbranched polyether cores and short linear polymeric BAMO arms have been prepared by Zhang et al. In contrast to BAMO homopolymer, these structures had significantly lower crystallinity and thus greatly improved processability and better safety properties, while maintaining a good energy level of up to 1.37 kJ g‐1 [28]. To enhance the mechanical properties of the propellant after blending, binders are usually subjected to a process called curing,

Molecules 2020, 25, 1009 4 of 29

Star azide copolymers with hyperbranched polyether cores and short linear polymeric BAMO arms have been prepared by Zhang et al. In contrast to BAMO homopolymer, these structures had significantly lower crystallinity and thus greatly improved processability and better safety properties, 1 while maintaining a good energy level of up to 1.37 kJ g− [28]. To enhance the mechanical properties of theMolecules propellant 2020, 25, 1009 after blending, binders are usually subjected to a process called curing, in4 of which 29 the polymer chains are crosslinked. are traditionally used to cure popular non-energetic polymericin which binder the polymer hydroxy-terminated chains are crosslinked. polybutadiene Isocyanates (HTPB are traditionally) under the used formation to cure of popular polyurethane non‐ networks,energetic and polymeric commercial binderGAP hydroxyis hydroxy‐terminated terminated polybutadiene for this reason. (HTPB However,) under due the to formation their sensitivity of to moisture,polyurethane side networks, reactions and with commercial GAP energetic is hydroxy plasticizers, terminated and for compatibility this reason. However, problems due with to their sensitivity to moisture, side reactions with nitrate ester energetic plasticizers, and chlorine free oxidizer ammonium dinitramide (ADN), free curing systems have been compatibility problems with chlorine free oxidizer ammonium dinitramide (ADN), isocyanate free demanded. Triazole crosslinking between azides and alkynes via 1,3-dipolar cycloaddition was curing systems have been demanded. Triazole crosslinking between azides and alkynes via tested successfully [29] (Scheme2). For example, GAP-based triazole networks were prepared using 1,3‐dipolar cycloaddition was tested successfully [29] (Scheme 2). For example, GAP‐based triazole bispropargyl succinate and 1,4-bis(1-hydroxypropargyl). networks were prepared using bispropargyl succinate and 1,4‐bis(1‐hydroxypropargyl)benzene.

SchemeScheme 2. 2.Mechanism Mechanism of of crosslinking crosslinking ofof anan azido-functionalizedazido‐functionalized polymer polymer with with a dipolarophile a dipolarophile via via 1,3-cycloaddition.1,3‐cycloaddition.

WhileWhile these these triazole triazole networks networks did did not not showshow good mechanical mechanical properties, properties, a adual dual curing curing system system employingemploying isocyanate isocyanate and and a a small small amount amount ofof produced produced networks networks with with mechanical mechanical properties properties superiorsuperior to both to both triazole-only triazole‐only and urethane-only and urethane crosslinked‐only crosslinked networks networks [30]. Benefits [30]. Benefits of azido crosslinkingof azido includecrosslinking better stability include and better enhanced stability burn and rateenhanced of the burn crosslinked rate of polymers.the crosslinked For more polymers. examples For more of azido polymersexamples as energeticof azido polymers binders theas energetic reader is binders referred the to reader the literature is referred [26 ,to31 the,32]. literature [26,31,32].

2.2.2.2. Azido Azido Plasticizers Plasticizers A recentA recent trend trend is is to to use use energetic energetic plasticizers plasticizers (EPs)s) in in combination combination with with energetic energetic binders binders to to furtherfurther increase increase the the performance performance of the of highthe high energetic energetic material. material. Azido AzidoEPs haveEPs have good good potential potential because because they show better compatibility with azido binders than commonly used nitrate ester they show better compatibility with azido binders than commonly used nitrate ester plasticizers plasticizers due to their structural similarity. Selected structures of azido EPs with different due to their structural similarity. Selected structures of azido EPs with different functional groups functional groups are depicted in Figure 1. A large family of azido EPs are azido acetate . In are depicted in Figure1. A large family of azido EPs are azido acetate esters. In comparison to comparison to non‐energetic plasticizers, they often exhibit better ballistic, mechanical, and thermal non-energeticproperties [33,34]. plasticizers, They possess they often high exhibit chemical better and ballistic, thermal mechanical,stability coupled and with thermal low properties sensitivity [and33,34 ]. Theyglass possess transition high temperatures, chemical and although thermal their stability level coupledof nitrogen with content, low sensitivity oxygen balance, and glass density, transition and temperatures,heat of formation although is not their optimal. level A of large nitrogen number content, of azido oxygen acetate balance, esters have density, been prepared, and heat ofincluding formation is notdiethylene optimal. glycol A large bis(azidoacetate) number of azido acetate(DEGBAA esters), have been glycol prepared, bis(azidoacetate) including diethylene (EGBAA glycol), bis(azidoacetate)trimethylol nitromethane (DEGBAA ),tris(azidoacetate) ethylene glycol bis(azidoacetate)(TMNTAA), pentaerythritol (EGBAA), trimethylol tetrakis(azidoacetate) nitromethane tris(azidoacetate)(PETKAA), and (1,3TMNTAA‐bis (azido), pentaerythritolacetoxy)‐2,2‐bis(azidomethyl) tetrakis(azidoacetate) propane (BABAMP) (PETKAA (see), and Figure 1,3-bis 1) [35]. (azido acetoxy)-2,2-bis(azidomethyl)The most recently reported propane azido (BABAMP) ester, 3‐(seeazido Figure‐2,2‐bis(azidomethyl)propyl1)[ 35]. The most recently 2‐azidoacetate reported azido 3 ester,(ABAMPA 3-azido-2,2-bis(azidomethyl)propyl) has a nitrogen content of 57%, 2-azidoacetate a density of 1.326 (ABAMPA g/cm , and) has a heat a nitrogen of formation content ∆𝐻 of of 57%, −1 3 1 a density1725 kJ of mol 1.326, revealing g/cm , and better a heat performance of formation than∆ mostH f of of 1725 the already kJ mol− known, revealing azido better esters performance and good thancompatibility most of the with already GAP known [36]. For azido an extensive esters and overview good compatibility on azido EPs, with see GAPreference[36]. [37]. For A an method extensive for predicting the glass transition temperature (TG) of azido‐esters through molecular structures was proposed by Zohari et al. using a training set of 21 compounds [38]. They reported paradoxical behavior regarding the effect of the azide group on TG, but their method gave good predictions for a test set and could be of help in designing novel azido‐ester plasticizers. A severe problem of small molecule EPs is the migration and exudation from the composite matrix, which deteriorates the

Molecules 2020, 25, 1009 5 of 29 overview on azido EPs, see reference [37]. A method for predicting the glass transition temperature (TG) of azido-esters through molecular structures was proposed by Zohari et al. using a training set of 21 compounds [38]. They reported paradoxical behavior regarding the effect of the azide group on TG, but their method gave good predictions for a test set and could be of help in designing novel azido-esterMolecules 2020 plasticizers., 25, 1009 A severe problem of small molecule EPs is the migration and exudation5 of from29 the composite matrix, which deteriorates the mechanical properties and energy level of propellant and therebymechanical reduces properties their safety. and Energetic energy level polymers of propellant as plasticizers and thereby possess reduces excellent their migration safety. Energetic resistance duepolymers to the inverse as plasticizers correlation possess between excellent molecular migration weight resistance and due plasticizer to the inverse mobility. correlation However, between linear polymersmolecular get weight too viscous and plasticizer with increasing mobility. weight. However, The linear highly polymers branched get three-dimensional too viscous with increasing structure of hyperbranchedweight. The highly polymers branched circumvents three‐dimensional this issue. Zhang structure et al. of prepared hyperbranched an azido-terminated polymers circumvents multi-arm this issue. Zhang et al. prepared an azido‐terminated multi‐arm copolymer (POGA) with copolymer (POGA) with hyperbranched polyether core and linear azido-terminated glycidyl azide hyperbranched polyether core and linear azido‐terminated glycidyl azide polymer arms. The polymer arms. The terminal hydroxyl groups of a block copolymer of hyperbranched polyether and terminal hydroxyl groups of a block copolymer of hyperbranched polyether and polyepichlorohydrin (POE) were tosylated, followed by azidation of the chloro and tosyl groups. With polyepichlorohydrin (POE) were tosylated, followed by azidation of the chloro and tosyl groups. approximately 17,000 g mol 1, POGA had a significantly higher molecular weight than common With approximately 17,000 −g mol−1, POGA had a significantly higher molecular weight than common 1 plasticizersplasticizers (200–1000 (200–1000 g molg mol− −),1), resulting resulting inin much slower slower exudation. exudation. Although Although its its plasticizer plasticizer efficiency efficiency waswas inferior inferior to to the the commercially commercially available available smallsmall moleculemolecule plasticizer plasticizer butyl butyl nitrato nitrato ethyl ethyl nitramine nitramine (bu-NENA(bu‐NENA), it), possessedit possessed a superiora superior energy energy level level andand goodgood safety performance performance [39]. [39 ].

FigureFigure 1. 1.Lines Lines 1 1 and and 2: 2: Azido Azido energeticenergetic plasticizers for for gun gun and and rocket rocket propellants propellants with with different different functionalfunctional groups. groups. Lines Lines 3 3and and 4: Selected 4: Selected structures structures of azido of acetate azido esters. acetate From esters. left to From right: left1,7‐ to

right:diazido 1,7-diazido-‐N,N,N’,NN,’N‐tetrafluoro,N0,N0-tetrafluoro-4,4-heptanediamine‐4,4‐heptanediamine (DADFAH (DADFAH), ),1,5 1,5-diazido-3-nitrazapentane‐diazido‐3‐nitrazapentane (DIANP(DIANP), 2,2-bis(azidomethyl)propane-1,3-diyl), 2,2‐bis(azidomethyl)propane‐1,3‐diyl dibutyrate ( (ButBAMPButBAMP),), bis(2,3 bis(2,3-diazidopropylene‐diazidopropylene glycol)glycol) (BDAP) (BDAP), 1,1,1-tris(azidomethyl)ethane, 1,1,1‐tris(azidomethyl)ethane ( TMETA).). Triazido Triazido pentaerythrite pentaerythrite acetate acetate (TAP (TAP‐Ac-),Ac ), 1,3-bis1,3‐bis (azido (azido acetoxy)-2,2-bis(azidomethyl) acetoxy)‐2,2‐bis(azidomethyl) propane (BABAMPpropane), pentaerythritol(BABAMP), tetrakis(azidoacetate)pentaerythritol (PETKAAtetrakis(azidoacetate)), trimethylol nitromethane (PETKAA), trimethylol tris(azidoacetate) nitromethane (TMNTAA tris(azidoacetate)), 3-azido-2,2-bis(azidomethyl)propyl (TMNTAA), 3‐azido‐ 2,2‐bis(azidomethyl)propyl 2‐azidoacetate (ABAMPA). 2-azidoacetate (ABAMPA).

3. Cross-Linking3. Cross‐Linking with with Organic Organic Azides Azides (Part (Part I):I): Organic Semiconductor Semiconductor Devices Devices

3.1.3.1. Bisazides: Bisazides: The The Forefathers Forefathers of of Photosensitizers Photosensitizers in Lithography TheThe beginning beginning of of the the 21st 21st century century hashas seenseen tremendoustremendous technical technical development development of ofelectronic electronic equipmentequipment as as a resulta result of of continued continued downscaling downscaling ofof semiconductor integrated integrated circuits circuits (IC (ICs) Thiss) This was was only possible by the evolution of photoresist materials and lithography methods towards higher resolutions. Where the semiconductor industry had once started with ICs at the μm scale, it will soon be conquering the sub‐7 nm region using extreme ultraviolet (EUV) lithography. At the beginning of this evolution, organic bisazides had the leading share as sensitizers in photoresist systems. In 1958, Eastman Kodak Co. filed a patent for an azide resin discovered by Kodak researchers Martin Hepher

Molecules 2020, 25, 1009 6 of 29 only possible by the evolution of photoresist materials and lithography methods towards higher resolutions. Where the semiconductor industry had once started with ICs at the µm scale, it will soon be conquering the sub-7 nm region using extreme ultraviolet (EUV) lithography. At the beginning of this evolution, organic bisazides had the leading share as sensitizers in photoresist systems. In 1958, Eastman Kodak Co. filed a patent for an azide resin discovered by Kodak researchers Martin Hepher Molecules 2020, 25, 1009 6 of 29 and Hans Wagner, also known as the “Kodak thin film resist” (KTFR) [2]. It was composed of the bisazideand Hans sensitizer Wagner, 2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone also known as the “Kodak thin film resist” (KTFR) dispersed [2]. It was in composed a cyclized of rubber the (poly-bisazidecis-isoprene) sensitizer and 2,6 became‐bis(4‐ theazidobenzylidene) first working horse‐4‐methylcyclohexanone for semiconductor industrydispersed until in a it reachedcyclized its resolutionrubber (poly limit‐ ofcis‐ 2isoprene)µm in 1972. and became the first working horse for semiconductor industry until it reachedCommercial its resolution two-component limit of 2 μm photoresists in 1972. for the fabrication of µm- and sub-µm structures in microelectronicsCommercial technology two‐component are composed photoresists of a polymerfor the fabrication resin and of a μ photosensitizer.m‐ and sub‐μm The structures principle in of photolithographymicroelectronics is technology depicted are in Figure composed2. A of silicon a polymer wafer resin is firstand a coated photosensitizer. with an oxide The principle layer. Then, of eitherphotolithography a positive or negative is depicted photoresist in Figure is2. depositedA silicon wafer on it, is dried, first coated and exposed with an tooxide UV layer. light throughThen, a mask.either Irradiation a positive or activates negative the photoresist crosslinking is deposited reaction on between it, dried, the and photosensitizer exposed to UV light and thethrough polymer a resin.mask. This Irradiation changes activates the solubility the crosslinking of the resin reaction in the exposedbetween region.the photosensitizer In the final and stripping the polymer process, dependingresin. This on changes the type the of photoresist,solubility of the either resin the in exposed the exposed (positive region. resist) In the or final the unexposed stripping process, (negative resist)depending area is soluble on the type and isof removed,photoresist, leaving either behindthe exposed the desired (positive pattern. resist) Popularor the unexposed commercial (negative negative photoresistsresist) area consist is soluble of low and molecular is removed, weight leaving polymers behind derived the desired from phenolspattern. andPopular formaldehyde commercial (so negative photoresists consist of low molecular weight polymers derived from phenols and called novolacs) and aromatic bisazides as photosensitizers [40]. formaldehyde (so called novolacs) and aromatic bisazides as photosensitizers [40].

FigureFigure 2. 2.Principle Principle of of photolithography photolithography with positive positive and and negative negative resists. resists.

SchemeScheme3 shows 3 shows the the chemical chemical mechanisms mechanisms in bisazidein bisazide photoresist photoresist systems systems [ 40 [40].]. Irradiation Irradiation of of the photoresistthe photoresist with UV with light UV generates light generates highly highly reactive reactive singlet singlet nitrenes nitrenes which which can react can with react the with polymer the matrixpolymer creating matrix covalent creating bonds covalent via (1) bonds insertion via (1) into insertion C–H bonds, into (2)C–H addition bonds, to(2) double addition bonds to double forming bonds forming aziridine rings which is the most common way, and (3) abstraction followed aziridine rings which is the most common way, and (3) radical abstraction followed by recombination of by recombination of radical polymer chains. To a smaller extent, diradical triplet nitrenes are radical polymer chains. To a smaller extent, diradical triplet nitrenes are generated by fast intersystem generated by fast intersystem crossing. It can form primary by double hydrogen abstraction crossing. It can form primary amines by double hydrogen abstraction and azo compounds by reaction and azo compounds by reaction with azide precursor or by dimerization at high nitrene withconcentration. azide precursor or by dimerization at high nitrene concentration.

Molecules 2020, 25, 1009 7 of 29 Molecules 2020, 25, 1009 7 of 29

N3 X N3 R NO2 R NO

hu O2 O2 - N2 oxidation

ISC dimerisation 1N X 1N R 3N R N N R minor pathway R

H3C H2 C n R CH2 CH3 cyclized rubber matrix H3C NH n CH2 CH3 insertion addition H-abstraction recombination

H C pseudo-insertion 3 H C + R NH2 n R CH cross-linking 2 CH3 NH R NH + H3C H3C H3C H2 H R N C C n n n n CH CH H3C H CH2 CH3 CH2 CH3 2 CH3 2 C CH n H3C 3 CH2 CH3 CH 2 SchemeScheme 3. Crosslinking3. Crosslinking mechanism mechanism of of bisazide bisazide photoresist systems. systems. Reactions Reactions with with triplet triplet nitrene nitrene are are depicteddepicted in grey.in grey.

BothBoth singlet singlet and and triplet triplet nitrenes can can be be scavenged scavenged by oxygen by oxygen (however (however this reaction this reaction is quite isslow quite slowin comparison in comparison to insertion to insertion into C–H into bonds). C–H bonds). This is a This disadvantage, is a disadvantage, because light because exposure light must exposure be mustcarried be carried out under out a undernitrogen a blanket nitrogen or blanketin a vacuum or in to aprevent vacuum loss to of prevent photosensitivity. loss of photosensitivity. However, the However,rate nitrenes the rate reacting nitrenes with reacting oxygen is with slow oxygen compared is slow to the compared reaction with to the carbon reaction based with radicals. carbon Apart based ϕ0.3 radicals.from a Aparthigh quantum from a high yield quantum for photolysis yield ( for photolysis), a good ( φbisazide0.3), photosensitizer a good bisazide should photosensitizer have an absorption maximum adapted to the optimal working wavelength≥ of the light source and good should have an absorption maximum adapted to the optimal working wavelength of the light solubility in order to achieve high concentrations and thus the required film thickness for source and good solubility in order to achieve high concentrations and thus the required film microstructuration. Various negative photoresist systems with different polymers like poly(methyl thickness for microstructuration. Various negative photoresist systems with different polymers like methacrylate) (PMMA) or poly(methyl isopropenyl ) (PMIPK) and bisazides like 3,3′‐ poly(methyldiazidodiphenyl methacrylate) or ( PMMA2,6‐bis(4)′‐azidolbenzylidene) or poly(methyl isopropenyl‐4‐methyl cyclohexanone ketone) (PMIPK exist )(see and Figure bisazides 3) like[41–43]. 3,30-diazidodiphenyl Most of these bisazides sulfone are or 2,6-bis(4 activated0-azidolbenzylidene)-4-methyl by deep UV light between 250 nm cyclohexanone to 300 nm, limiting exist (see Figuretheir3 )[use41 –as43 ].photosensitizers Most of these bisazidesfor polymer are activatedresins like by poly( deepp UV‐vinylphenol) light between (PVP 250) with nm tostrong 300 nm, limitingabsorption their in use this as region photosensitizers [44,45]. Keana for polymerand co‐workers resins found like poly( thatp perfluorinated-vinylphenol) bisazides (PVP) with can strong be absorptioncrosslinked in thisat the region near [UV44, 45region]. Keana (around and 350 co-workers nm) with found high crosslinking that perfluorinated efficiency bisazides for PVP, canPS, be crosslinkedPMMA atpoly(3 the near‐octylthiophene), UV region (around and 350polyimide. nm) with For high example, crosslinking instead efficiency of 20 for PVPwt%, PSof, PMMA3,3′‐ poly(3-octylthiophene),diazidodiphenyl sulfone, and polyimide. only 3.6 For example,wt% of instead 1,5‐bis(4 of 20′‐ wt%azido of‐2 3,3′,3′,50-diazidodiphenyl′,6′‐tetrafluorophenyl) sulfone,‐ only1,4 3.6‐pentadiene wt% of 1,5-bis(4‐3‐one (see0-azido-2 Figure0,3 3)0,5 were0,60-tetrafluorophenyl)-1,4-pentadiene-3-one required to get 95% film retention after development (see Figure3 )of were a requiredPVP resist to get [46–49]. 95% film This retentionwas attributed after to development the enhancement of a ofPVP the resistC–H and/or [46–49 N–H]. This insertion was attributed reactions to theby enhancement the presence of theof C–Hfluorine and /atomsor N–H [50]. insertion The lithographic reactions by performance the presence of fluorine2,2′‐disubstitued atoms [50 ]. diazidostilbene derivatives was optimized by Voigt on the basis of identified structure‐property‐ The lithographic performance of 2,20-disubstitued diazidostilbene derivatives was optimized by Voigtfunctionality on the basis relationships of identified [51]. structure-property-functionality At concentrations of 6–14 wt%, bisazide relationships 4 (see Figure [51]. At 3) concentrationshad the best of 6–14solubility wt%, in the bisazide series, 4allowing(see Figure for a 3film) had thickness the best of up solubility to 10 μm in combination the series, allowingwith novolac for resin. a film The bisazide had a moderate quantum yield of ϕ0.26 (λexc = 313 nm) and an absorption maximum thickness of up to 10 µm in combination with novolac resin. The bisazide had a moderate quantum at 331 nm in methanol, rendering it suitable for lithography at a wavelength of 365 nm, called “i‐line”, yield of φ = 0.26 (λ = 313 nm) and an absorption maximum at 331 nm in methanol, rendering which is referring toexc the spectral line of the Hg lamps which had originally been used. The maximum it suitable for lithography at a wavelength of 365 nm, called “i-line”, which is referring to the resolution at this wavelength was 0.3 μm. A fully waterborne i‐line lithography process for spectralconducting line of thin the films Hg from lamps poly(3,4 which‐ethylenedioxythiophene) had originally been used. was Thedeveloped maximum by Touwslager resolution et at al. this wavelengthThey used was the 0.3waterµm.‐soluble A fully disodium waterborne salt of i-line bisazide lithography 4,4′‐diazido process‐2,2′‐disulfonic for conducting acid benzalacetone thin films from poly(3,4-ethylenedioxythiophene)as a photoinitiator and were able wasto obtain developed all‐polymer by Touwslager integrated etcircuits al. They with used a minimum the water-soluble feature disodiumsize of 2.5 salt μm of wide bisazide lines separated 4,40-diazido-2,2 by 1‐μm0 -disulfonicspacings [52]. acid Other benzalacetone examples in current as a photoinitiator research include and were able to obtain all-polymer integrated circuits with a minimum feature size of 2.5 µm wide lines separated by 1-µm spacings [52]. Other examples in current research include fabrication of

Molecules 2020, 25, 1009 8 of 29

Molecules 2020, 25, 1009 8 of 29 photo-patterned polymer-based field-effect transistors and light-emitting diodes and are presented in fabrication of photo‐patterned polymer‐based field‐effect transistors and light‐emitting diodes and theare following presented chapters. in the following chapters.

FigureFigure 3. Bisazides3. Bisazides used used in photoresist in photoresist systems. systems. Absorption Absorption maxima maxima are given are ingiven brackets in brackets (if available). (if available). 3.2. Photovoltaics—Higher Performance with Locked Morphology 3.2. Photovoltaics—Higher Performance with Locked Morphology Compared to rigid, more fragile inorganic solar cells, the amenability to solution processed manufacturingCompared methods to rigid, of more organic fragile photovoltaics inorganic (OPV)solar cells, such the as ink-jetamenability and roll-to-roll to solution printing processed opens newmanufacturing possibilities for methods applications of organic where photovoltaics flexibility is (OPV) indispensable. such as ink Current‐jet and key roll‐ challengesto‐roll printing are increasing opens performancenew possibilities and long-term for applications stability. where The mainflexibility sources is indispensable. of device instability Current arekey thermal, challenges chemical, are orincreasing mechanical performance stress, and and light long exposure‐term stability. [53]. TheThe main bulk heterojunctionsources of device (BHJ), instability an inter-percolating are thermal, thinchemical, film structure or mechanical of donor and stress, acceptor and materialslight exposure between [53]. the The electrodes, bulk heterojunction is the most common (BHJ), type an of microstructureinter‐percolating in OPVs. thin Currentfilm structure devices of show donor a drastic and acceptor decrease materials in their efficiency between the as a electrodes, result of deleterious is the most common type of microstructure in OPVs. Current devices show a drastic decrease in their changes in the morphology of this microstructure. Crosslinking has been identified as a convenient method efficiency as a result of deleterious changes in the morphology of this microstructure. Crosslinking to control and stabilize BHJ morphology. The goal is to create a conjugated network of donor polymer has been identified as a convenient method to control and stabilize BHJ morphology. The goal is to with a frozen morphology in which the acceptor is embedded in order to prevent diffusion, aggregation create a conjugated network of donor polymer with a frozen morphology in which the acceptor is andembedded macrophase in order crystallization. to prevent This diffusion, process aggregation should be activated and macrophase without perturbingcrystallization. blend This morphologies. process Someshould cross-linkable be activated groups without require perturbing addition ofblend initiators morphologies. or produce Some reactive cross side‐linkable products groups which require can have negativeaddition impact of initiators on device or performanceproduce reactive and side long-term products stability. which The can azide have functionalitynegative impact can on be device activated thermallyperformance or by and UV long light‐term under stability. formation The ofazide highly functionality reactive nitrenes, can be activated which are thermally capable or of by insertion UV andlight addition under reactions formation (see of Scheme highly 3reactive). Popular nitrenes, BHJ blends which consist are capable of of insertion donating and polymers addition like poly(3-hexylthio-phene-2,5-diyl)reactions (see Scheme 3). Popular (P3HT BHJ)[54 blends] or polybenzodithiophenes consist of electron donating such as polymersPTB7 [55 like] and poly(3 fullerene‐ derivativeshexylthio like‐phene (6,6)-phenyl‐2,5‐diyl) C(P3HT60-butyric) [54] acid or polybenzodithiophenes methyl ester (PC60BM )[such54] asas acceptorPTB7 [55] materials. and fullerene derivatives like (6,6)‐phenyl C60‐butyric acid methyl ester (PC60BM) [54] as acceptor materials. 3.2.1. Azido Polymers 3.2.1. Azido Polymers Sidechains of conducting polymers like P3HT can be azido-functionalized for crosslinking of BHJ blends.Sidechains For example, of conducting after UV crosslinking polymers like of aP3HTP3HT-N5 can be/PCBM azido‐blendfunctionalized with 5% for azide crosslinking functionalized of side-chains,BHJ blends. inhibition For example, of fullerene after aggregationUV crosslinking was observed.of a P3HT The‐N5 device/PCBM retained blend with 65% 5% of its azide power conversion efficiency (PCE) after comparatively mild ageing for 40 h at 110 C compared to less than ◦ Molecules 2020, 25, 1009 9 of 29

30% retention using non-crosslinked commercial P3HT which underwent significant phase separation. However, the initial PCE of 1.5% was 0.7% lower than that of the reference, relativizing the higher stability achieved by crosslinking. This could be explained by limited charge separation arising from a compromised morphology or by degradation of the polymers under the harsh crosslinking conditions (60 min UV exposure in air) employed [56]. A comparative study with donor–acceptor-conjugated polymer PBT bearing photo-crosslinkable groups showed that an appropriate crosslinking network greatly improved the thermal stability of the BHJ cells. Devices fabricated with PBT-Br/PCBM and PBT-N3/PCBM blends that were photo-crosslinked by UV light exposure for 30 min retained 78% and 66% of their initial PCE of 2.1% and 1.8% after annealing for 80 h at 150 ◦C, respectively. Also, in this case, the initial PCEs were lowered by 0.6% and 0.9% for the bromo- and azido-polymer, respectively, after UV exposure, indicating some degradation of the blend materials [57].

3.2.2. Small Molecule Azide Crosslinkers Instead of using cross-linkable polymers, a fundamentally different approach is the use of small molecule crosslinkers as additives in BHJ blends. Several bisazides with both aliphatic and aromatic cores have been used in fullerene-type BHJ devices. As mentioned, crosslinking should proceed with high efficiency under mild conditions. The desired reaction of nitrenes is the insertion into alkyl C–H bonds, while attacks on the π-conjugated core of the polymer can degrade semiconductor properties. Png et al. found that sterically hindered bis(fluorophenyl azides) (sFPAs) show very good photo-crosslinking efficiency because C–H insertion is favored over side reactions. Only about 15% of the formed nitrenes insert into aromatic C–H bonds compared to 30% for unhindered bis(fluorophenyl azide) [58]. They crosslinked P3HT with bisazide sFPA (Figure4) and then doped a PC61BM acceptor into the polymer network by solution spinning from chlorobenzene (CB). This method has potential for manufacture on scale as it can be implemented over large film areas and thicknesses. Because crosslinking renders the polymer film insoluble in the CB, a subsequent top acceptor layer could be solution deposited from the same solvent. The crosslinked device showed 20% improvement in PCE over conventional devices [8,59]. Similar results were obtained using bisazide 1,6-diazidohexane (DAZH) in combination with a polysilaindacenodithiophene-benzothiadiazole and (6,6)-phenyl-C71-butyric acid methyl ester (SiIDT-BT/PC71BM) blend. However, PCE losses upon thermal annealing or UV light exposure were significant in most cases, leaving room for improvement [60]. Another example of the superiority of azide crosslinkers in enhancing long-term stability of OPVs compared to other crosslinker types was presented by Chao et al. When crosslinking the active layer with an azide-functionalized triphenylamine derivative, TPT-N3, their device retained 75% of its initial PCE after extended thermal annealing for 144 h at 150 ◦C, whereas the bromo-derivative retained only 60%. The PCE of the reference device without crosslinker dropped to 0.24% under these conditions [61]. Jeng and co-workers synthesized 1 2 1 the low-band gap bisazide cross-linkable bisazide N3-(CPDT(FBTTh2)2) with a D -A-D -A-D -type architecture. This bisazide has a wide absorption range and can harvest additional solar light in addition to stabilizing the BHJ morphology. In contrast to other existing azide crosslinkers, full retention of the PCE after curing and aggressive thermal ageing at 150 ◦C for 24 h could be achieved at a ratio of 1:0.2:1 (P3HT:linker:PC61BM)[62]. However, residual unreacted hydroxy precursor contained in the target compound might have had an additional effect on this remarkable result. Molecules 2020, 25, 1009 10 of 29

Molecules 2020, 25, 1009 10 of 29

Figure 4. Bisazides used as crosslinkers in OPV: ionic 1,4‐bis(perfluorophenyl azide) (AAA+X+‐), Figure 4. Bisazides used as crosslinkers in OPV: ionic 1,4-bis(perfluorophenyl azide) (AAA X−), 1,6‐diazidohexane (DAZH), ethylene bis(4‐azido‐2,3,5‐trifluoro‐6isopropylbenzoate) (sFPA), 4,4′‐ 1,6-diazidohexane (DAZH), ethylene bis(4-azido-2,3,5-trifluoro-6isopropylbenzoate) (sFPA), 4,40- bis(azidomethyl)‐1,1′‐biphenyl (BABP), 1,2‐bis((4‐(azidomethyl)phenethyl)thio)ethane (TBA‐X), bis(azidomethyl)-1,10-biphenyl (BABP), 1,2-bis((4-(azidomethyl)phenethyl)thio)ethane (TBA-X), ethyleneethylene bis(4-azido-2,3,5,6-tetrafluorobenzoate) bis(4‐azido‐2,3,5,6‐tetrafluorobenzoate) (Bis(Bis((PFBAPFBA),), 3,6 3,6-bis(5-(4,4”-bis(3-azidopropyl)-‐bis(5‐(4,4′’‐bis(3‐azidopropyl)‐ [1,1′:3′,1′’‐terphenyl)‐5′‐yl)thiophen‐2‐yl)‐2,5‐bis(2‐ethylhexyl)‐2,5‐dihydropyrrolo(3,4‐c)pyrrole‐1,4‐ [1,10:30,1”-terphenyl)-50-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)-2,5-dihydropyrrolo(3,4-c)pyrrole-1,4- dione (DPPTPTA), 4‐4′‐bis(1‐azido)undecane)dicyclopenta‐(2,1‐b:3,4‐b′)dithio‐phene‐bis(5‐fluoro‐7‐ dione (DPPTPTA), 4-40-bis(1-azido)undecane)dicyclopenta-(2,1-b:3,4-b0)dithio-phene-bis(5-fluoro-7- (5′‐hexyl‐(2,2′‐bithiophene)‐5‐yl)benzo‐(c)(1,2,5)thiadiazole) N3‐(CPDT(FBTTh2)2), bis(6‐ (50-hexyl-(2,20-bithiophene)-5-yl)benzo-(c)(1,2,5)thiadiazole) N3-(CPDT(FBTTh2)2), bis(6- azidohexanoate)silicon phthalocyanine (HxN3)2‐SiPc), and tris(4‐(5′‐(3‐azidopropyl)‐2,2′‐ azidohexanoate)silicon phthalocyanine (HxN3)2-SiPc), and tris(4-(50-(3-azidopropyl)-2,20- bithiophen5- bithiophen5‐yl)phenyl) (TPT‐N3). yl)phenyl)amine (TPT-N3). In 2019, the same group reported for the first time on a tetravalent azide used as crosslinker. This In 2019, the same group reported for the first time on a tetravalent azide used as crosslinker. multifunctional four‐arm star‐shaped diketopyrrolopyrrole‐based conjugated small molecule, This multifunctional four-arm star-shaped diketopyrrolopyrrole-based conjugated small molecule, DPPTPTA has a broad absorption range, high charge mobility and can efficiently inhibit molecular DPPTPTA has a broad absorption range, high charge mobility and can efficiently inhibit molecular aggregation. Incorporating 3 wt% of DPPTPTA into a PTB7‐Th/PC61BM blend produced a significant aggregation.change in Incorporatingthe morphology 3 wt% of oftheDPPTPTA blend filminto with a PTB7-Th well‐defined/PC61BM D/Ablend‐separation, produced successfully a significant changesuppressing in the morphology crystallization of the of blendthe PCBM film withacceptor. well-defined Ladder‐type D/A-separation, energy levels successfully in the ternary suppressing blend crystallizationmight explain of the the improvedPCBM acceptor. charge separation Ladder-type and energy energy levels transport. in the The ternary devices’ blend PCE might was increased explain the improved charge separation and energy transport. The devices’ PCE was increased from 6.7 to 8.2% Molecules 2020, 25, 1009 11 of 29

Molecules 2020, 25, 1009 11 of 29 with 56% retention after thermal stressing at 150 ◦C for 18 h. Higher additive content was found to inducefrom BHJ 6.7 to phase 8.2% with segregation, 56% retention resulting after inthermal lower stressing photovoltaic at 150 °C performance for 18 h. Higher [63]. additive In the same content year, Landererwas found et al. to prepared induce BHJ a novel phase bisazidesegregation, 1,2-bis((4-(azidomethyl)phenethyl)thio) resulting in lower photovoltaic performance ethane [63]. (TBA-X In the) in ordersame to betteryear, Landerer control the et al. crosslinking prepared a reactionnovel bisazide and thus 1,2‐bis((4 the BHJ‐(azidomethyl)phenethyl)thio) morphology in fullerene-type ethane BHJs with(TBA various‐X) in donor order to polymers better control and fullerene the crosslinking acceptor reactionPC61BM and(Figure thus the5 ).BHJ Major morphology advantages in fullerene of TBA-X‐ aretype its moderate BHJs with reactivity various donor and goodpolymers solubility. and fullerene Crosslinking acceptor could PC61BM be ( initiatedFigure 5). thermally Major advantages already at a moderateof TBA‐X temperature are its moderate of 80 reactivity◦C. Selective and good 1,3-dipolar solubility. cycloaddition Crosslinking with could the be fullerene initiated took thermally place at 150already◦C affording at a moderate triazolino temperature (40,50:1,2) fullerenes. of 80 °C. Selective Nitrenes which1,3‐dipolar could cycloaddition engage in random with the reactions fullerene with bothtook the place polymer at 150 donor °C affording and the triazolino fullerene (4 were′,5′:1,2) not fullerenes. formed at Nitrenes this temperature. which could The engage best-performing in random reactions with both the polymer donor and the fullerene were not formed at this temperature. The device based on a PTB7/PC61BM blend using 2 wt% crosslinker possessed a PCE of 6% with 90% best‐performing device based on a PTB7/PC61BM blend using 2 wt% crosslinker possessed a PCE of retention after harsh thermal annealing of entire solar cells at 120 ◦C for 100 h [64]. An overview on the 6% with 90% retention after harsh thermal annealing of entire solar cells at 120 °C for 100 h [64]. An examples presented herein is given in Table1. overview on the examples presented herein is given in Table 1.

FigureFigure 5. Left:5. Left: Conventional Conventional thermal thermal ageing ageing of an amorphous amorphous bulk bulk heterojunction heterojunction (BHJ) (BHJ) blend blend (center) (center) leadsleads to macrophaseto macrophase separation separation over over time. time. Right: Right: Photo-crosslinkingPhoto‐crosslinking of of the the polymer polymer prior prior to toageing ageing fixates the chains in space and prevents fullerene aggregation. fixates the chains in space and prevents fullerene aggregation.

TableTable 1. Performance1. Performance of of organic organic photovoltaic photovoltaic (OPV)(OPV) devices where where active active layer layer crosslinking crosslinking with with organicorganic azides azides was was employed. employed. PCE: PCE: power power conversion conversion eefficiency.fficiency.

Azide ActiveActive Layer Layer CrosslinkingCrosslinking ReferenceReference PCE PCEPCE (%) (%) Post PCEPCE (%) (%) CrosslinkerCrosslinker (1:x(CL):1)(1:x(CL):1) MethodMethod wPCE/o CL PostAnnealing RetentionRetention after after w/o CL Annealing AgeingAgeing (t, (t,T) T) a P3HTP3HT-N5‐N5 P3HT P3HT-N5‐N5:0::0:PCPC6161BMBM DUVDUV (60 min)min) a 2.22.2 1.5 1.567 67 (40 (40 h, h, 110 110 ◦C) N3-CPDT(FBTTh2)2 [62] P3HT:0.2:PC61BM thermally (130–150 - 3.5 >99°C) (24 h, 150 ◦C) ◦C) via CA BAPB [65] P3HT:0.05:PC BM thermally (150 C) 3.3 3.0 92 (122 h, 150 C) N3‐ P3HT:0.2:PC6161BM thermally (130–◦ ‐ 3.5 >99 (24 h, 150 ◦ via CA 90 (120 d, 85 ◦C) CPDT(FBTTh2)2 150 °C) via CA °C) DAZH [60] SiIDTBT:0.1:C BM DUV (10 min) 6.0 5.7 82 (130 h, 85 C, [62] 70 ◦ N2 atmosphere) BAPB [65] P3HT:0.05:PC61BM thermally (150 3.3 3.0 92 (122 h, 150 TBA-X [64] P3HT:0.07:PC61BM thermally (150 ◦C) 3.7 3.3 87 (200 h, 120 ◦C) °C)via via CA CA °C) b 90 (120 d, 85 sFPA [59] P3HT:0.12:PC61BM DUV (5 min, 90 ◦C) 3.3 3.0 - c °C) Bis(BFPA) [66] P3HT:0.1:PC61BM DUV (5 min, rt) - 3.4 88 (20 d in air, rt) DAZH [60] SiIDTBT:0.1:C70BM DUV (10 min) 6.0 5.7 82 (130 h, 85 °C, DPPTPTA [63] PTB7Th:0.05:PC61BM thermally (10 min, 6.7 7.3 55 (18 h, 150 ◦C) N2 atmosphere) 150 ◦C) d (HxN3)2-SiPc [67] P3HT:0.08:PC61BM thermally (10 min, 3.4 3.3 97 (23 h, 150 ◦C) TBA‐X [64] P3HT:0.07:PC61BM thermally80 ◦C) (150 3.7 3.3 87 (200 h, 120 e TBT-N3 [61] P3HT:0.05:PC61BM thermally°C) via (20 CA min, 3.4 3.4 60°C) (24 h, 150 ◦C) 150 ◦C) a b CAsFPA= 1,3-dipolar [59] b cycloaddition;P3HT:0.12: DUVPC61BM= deep UV;DUV CL (5= cross-linker;min, 90 Ambient3.3 fabrication;3.0 ‐ Structured bilayer; c d ; e Used to crosslink an electron transport layer in an inverted°C) device; annealed at 80 ◦C for 10 min annealed at 150 ◦C for 20 min.

Molecules 2020, 25, 1009 12 of 29

In the realm of inorganic solar cells, so far only two examples of azide crosslinking exist. Watson et al. used the tetrahedral four-site crosslinking agent 1,3,5,7-tetrakis-(p-azidobenzyl)-adamantane (TPBA) to transform the soluble and fragile hole-transporting organic semiconductor poly(triaryl amine) (PTAA) into a solvent-resistant and mechanically tough film. Crosslinking also enabled fabrication of perovskite solar cells with increased photovoltaic efficiencies. The crosslinked polymer film showed better adhesion to the perovskite layer, mitigating interfacial device failure [68]. TPBA was also used by Watson et al. in the fabrication of a so-called compound solar cell, a perovskite solar cell partitioned into an array of microcells for increased chemical and mechanically stability [69].

3.3. Multilayered and Photopatternable OFETs and Organic Light-Emitting Diode (OLEDs) Solution processes enable low-cost fabrication of large area multi-layer structured OLEDs. However, sequential deposition without dissolution of the underlying layer is challenging. One way to approach this issue is to transform solution-processed polymers into insoluble films by crosslinking. Park et al. designed a conducting copolymer from photo-crosslinkable poly(azido-styrene) (PS-N3) and hole-transporting material poly(triphenylamine) (PTPA). In contrast to harsh and lengthy thermal crosslinking, extremely short time (5 min) and low power UV light (2 mW/cm2, 254 nm) was enough to transform the polymers into insoluble films. Multi-layer OLEDs with a ratio of 5:95 of PS-N3 to PTPA and a configuration of ITO/PEDOT:PSS/X-PTPA-5/PVK:PBD:Ir(ppy)3/LiF/Al performed twice as good as the control device without cross-linkable film. Furthermore, micro-patterned, pixelated OLEDs were fabricated through photolithography. The versatility of PS-N3 to produce photo-crosslinkable, conducting films by copolymerization was exemplified using poly(carbazole) as one possible alternative hole-transporting material [70]. A different approach was made by Huang et al. Here, a 6-azidohexyl-functionalized carbazole was copolymerized with an alkylated triarylamine to obtain a cross-linkable hole-injection/transporting material named X-PTCAzide (Figure6). Photo-crosslinking of a 15-nm-thick film was completed after 1 h with low power UV light (1 mW/cm2, 365 nm). A solution-processed electroluminescent device with the structure ITO/X-PTCAzide/Ir(ppy)3:PVK/BCP/Alq3/LiF/Al exhibited good performance with an external quantum efficiency of 7.93% and a luminous efficacy of 29.6 cd/A[71]. Molecules 2020, 25, 1009 13 of 29

FigureFigure 6. Chemical 6. Chemical structure structure of (poly of (poly 4-(9-(6-azidohexyl)-9 4‐(9‐(6‐azidohexyl)H-carbazol-3-yl)-N-(4-butylphenyl)-‐9H‐carbazol‐3‐yl)‐N‐(4‐butylphenyl)N-phenylaniline)‐N‐ (PTC).phenylaniline) (PTC).

ModernModern applications applications of of bisazides bisazides asas photoresistsphotoresists were were demonstrated demonstrated in in the the fabrication fabrication of of photopatternablephotopatternable OFETs OFETs and and LEDs. LEDs. ForFor example, the the bisazide bisazide ethylene ethylene glycol glycol bis(4 bis(4-azido-2,3,5,6-‐azido‐2,3,5,6‐ tetrafluorobenzoate)tetrafluorobenzoate) (FPA (FPA)) allows allows crosslinking crosslinking of ofpolymers polymers with with high efficiency high efficiency and is readily and is photo readily‐ photo-patternedpatterned by deep by UV deep and UV a shadow and mask. a shadow It was mask.used to produce It was a chemically used to produce robust dielectric a chemically film robustbased dielectric on PS film that based provided on PS thata providedfavorable aenvironment favorable environment for growth for growthof a of a triethylsilylethynyl–anthradithiophenetriethylsilylethynyl‐‐anthradithiophene (TES (TES-ADT‐ADT) semiconductor) semiconductor film, film,yielding yielding OFETs with OFETs much with higher field‐effect mobility than those with plain PS layers [72]. The sterically hindered bis(fluorophenyl azide) sFPA (Figure 4), was used to fabricate a photo‐patterned poly(9,9‐ dioctylfluorene‐alt‐benzothiadiazole) (F8BT) film. In addition, a stack of eight alternating 50‐nm‐ thick films of poly(9,9 di‐n‐octylfluorene‐alt‐(1,4‐phenylene‐((4‐sec‐butylphenyl)imino)‐1,4‐ phenylene) (TFB) and F8BT was realized by sequential deposition and photo‐crosslinking of the individual polymer layers. In the fabrication of LEDs with the structure ITO/PEDT:PSSH/TFB/F8BT/Ca/Al, this technique allowed fine control of the TFB/F8BT heterostructure simply by spin‐casting TFB on poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDT:PSSH) instead of the previously needed hard‐bake procedure. With a 10‐nm‐thick TFB interlayer, the efficiency of the F8BT light‐emitting film jumped tenfold to 4.4%. Moreover, both hole and electron OC1C10‐PPV diodes with various crosslink densities showing little hole or electron trapping were fabricated. Because the electronic transition of the sFPA does not interfere with transitions of many polymer OSCs, deep UV exposure into polymer films of up to 300 nm thickness is possible with low risk of polymer photo‐oxidation [58]. A common problem in OSC devices is electromigration of the conducting polymer. For example, devices fabricated by Png et al. using PEDT:PSSH as hole‐injection layer (HIL) showed electromigration of PEDT+ cations towards the OSC interface. By crosslinking of the HIL with water‐soluble ionic 1,4‐bis(perfluorophenyl azide) AAA+X‐ (Figure 4), migration was successfully suppressed in unipolar hole‐only diodes. Although PEDT+ electromigration was found to be a minor contributor to the voltage rise in these diodes, it may be important in bipolar OLEDs. Here, a high PEDT concentration at the HIL/OSC interface could promote exciton quenching and prevent recombination of the in the light‐emitting OSC by facilitating electron escape [73].

3.4. Stretchable Polymer Semiconductors The growing popularity of wearable and implantable electronics has stimulated the evolution of intrinsically stretchable polymer SCs. These can be obtained by crosslinking of elastomers. Recently, the importance of the crosslinker crystallinity on the mechanical properties of the polymer SC was highlighted. Wang et al. were able to control the crystallinity of alkyl bridged perfluorophenyl bisazides 7a–d by introducing different functional groups into their alkyl bridge (Figure 7). Incorporation of urethane groups encouraged self‐assembly via H‐bonding, thus yielding a

Molecules 2020, 25, 1009 13 of 29 much higher field-effect mobility than those with plain PS layers [72]. The sterically hindered bis(fluorophenyl azide) sFPA (Figure4), was used to fabricate a photo-patterned poly(9,9-dioctylfluorene- alt-benzothiadiazole) (F8BT) film. In addition, a stack of eight alternating 50-nm-thick films of poly(9,9 di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4- phenylene) (TFB) and F8BT was realized by sequential deposition and photo-crosslinking of the individual polymer layers. In the fabrication of LEDs with the structure ITO/PEDT:PSSH/TFB/F8BT/Ca/Al, this technique allowed fine control of the TFB/F8BT heterostructure simply by spin-casting TFB on poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDT:PSSH) instead of the previously needed hard-bake procedure. With a 10-nm-thick TFB interlayer, the efficiency of the F8BT light-emitting film jumped tenfold to 4.4%. Moreover, both hole and electron OC1C10-PPV diodes with various crosslink densities showing little hole or electron trapping were fabricated. Because the electronic transition of the sFPA does not interfere with transitions of many polymer OSCs, deep UV exposure into polymer films of up to 300 nm thickness is possible with low risk of polymer photo-oxidation [58]. A common problem in OSC devices is electromigration of the conducting polymer. For example, devices fabricated by Png et al. using PEDT:PSSH as hole-injection layer (HIL) showed electromigration of PEDT+ cations towards the OSC interface. By crosslinking of the HIL with water-soluble ionic 1,4-bis(perfluorophenyl azide) + AAA X− (Figure4), migration was successfully suppressed in unipolar hole-only diodes. Although PEDT+ electromigration was found to be a minor contributor to the voltage rise in these diodes, it may be important in bipolar OLEDs. Here, a high PEDT concentration at the HIL/OSC interface could promote exciton quenching and prevent recombination of the electrons in the light-emitting OSC by facilitating electron escape [73].

3.4. Stretchable Polymer Semiconductors The growing popularity of wearable and implantable electronics has stimulated the evolution of intrinsically stretchable polymer SCs. These can be obtained by crosslinking of elastomers. Recently, the importance of the crosslinker crystallinity on the mechanical properties of the polymer SC was highlighted. Wang et al. were able to control the crystallinity of alkyl bridged perfluorophenyl bisazides 7a–d by introducing different functional groups into their alkyl bridge (Figure7). Incorporation of urethane groups encouraged self-assembly via H-bonding, thus yielding a crystalline crosslinker. Ester groups gave semi-crystalline material due to the absence of H-bonding. On the other hand, creating tertiary carbon centers with ethyl side chains suppressed crystallization by steric hindrance. All bisazides had good thermal stability and activation temperatures for nitrene formation at around 172 ◦C. Only with branched, amorphous crosslinker 7d evenly crosslinked elastic films with good cyclability could be obtained (Figure5). Fully stretchable bottom-gate-top-contact OFET devices with crosslinked DPP-10C5DE layers were fabricated. While the initial field-effect hole mobility µh of 0.25 cm2V 1s 1 of the control device with neat polymer decreased with an increase in strain, ∼ − − the crosslinked films displayed stable µh values in the parallel to strain direction. Charge transport in the direction perpendicular to strain was significantly lowered because the polymer chains aligned in the strain direction. Those devices with polymer films crosslinked with crystalline crosslinker showed a significant drop in µh in the perpendicular direction after being subjected to 5000 cycles. This was attributed to the film’s high surface roughness leading possibly to delamination between the layers. Contrastingly, films crosslinked with the amorphous bisazide exhibited stable hole mobilities in most cases [74]. Molecules 2020, 25, 1009 14 of 29

crystalline crosslinker. Ester groups gave semi‐crystalline material due to the absence of H‐bonding. On the other hand, creating tertiary carbon centers with ethyl side chains suppressed crystallization by steric hindrance. All bisazides had good thermal stability and activation temperatures for nitrene formation at around 172 °C. Only with branched, amorphous crosslinker 7d evenly crosslinked elastic films with good cyclability could be obtained (Figure 5). Fully stretchable bottom‐gate‐top‐contact OFET devices with crosslinked DPP‐10C5DE layers were fabricated. While the initial field‐effect hole mobility μh of ∼0.25 cm2V−1s−1 of the control device with neat polymer decreased with an increase in strain, the crosslinked films displayed stable μh values in the parallel to strain direction. Charge transport in the direction perpendicular to strain was significantly lowered because the polymer chains aligned in the strain direction. Those devices with polymer films crosslinked with crystalline crosslinker showed a significant drop in μh in the perpendicular direction after being subjected to 5000 cycles. This was attributed to the film’s high surface roughness leading possibly to delamination Moleculesbetween2020 ,the25, 1009layers. Contrastingly, films crosslinked with the amorphous bisazide exhibited stable14 of 29 hole mobilities in most cases [74].

decreasing crystallinity after annealing:

F N F 3 O O F H O O N F crosslinked domains in film F N O O and poor cyclability H F O O F N 7a 3 (crystalline) F F N F 3 O O F O O F F O O F O O 7b F N3 (semicrystalline) F F N F 3 O O F O O F F O O F O O 7c F N3 (amorphous) F F N F 3 O O F H O O N F F N O O evenely crosslinked film H and good cyclability F O O 7d F N3 F (amorphous)

FigureFigure 7. 7.Influence Influence of of crosslinker crosslinker morphology on on film film morphology. morphology.

4. Cross-Linking4. Cross‐Linking with with Organic Organic Azides Azides (Part (Part II):II): Exchange Membranes Membranes

4.1.4.1. Azides Azides in in Proton Proton Exchange Exchange Membranes Membranes ApartApart from from solar solar cells, cells, proton proton exchange exchange membrane membrane fuel fuel cells cells (PEMFC (PEMFCs)s) and and direct direct methanol methanol fuel fuel cells (DMFCcells s)(DMFC are verys) are promising very promising future power future sources. power sources. Nafion ®Nafion, a perfluorosulfonated®, a perfluorosulfonated polymer, polymer, is currently is thecurrently most commonly the most used commonly polymer used electrolyte polymer membrane electrolyte in thesemembrane devices, in butthese due devices, to its high but cost due and to its poor performancehigh cost atand high poor temperature, performance new at polymers high temperature, are sought. new High polymers proton conductivity are sought.σ isHigh achieved proton with lowconductivity methanol permeability σ is achieved and with hydrophilic low methanol character permeability of the polymer. and hydrophilic However, character hydrophobic of the polymer. membranes However, hydrophobic membranes have higher physical stability. Finding the right balance has have higher physical stability. Finding the right balance has proven to be difficult. Polymer crosslinking proven to be difficult. Polymer crosslinking with organic azides can effectively control the with organic azides can effectively control the hydrophobic nature of the membrane as shown in the hydrophobic nature of the membrane as shown in the following examples. Kim and co‐workers have following examples. Kim and co-workers have used 2,6-bis(4-azidobenzylidene)-4-methyl-cyclohexanone, used 2,6‐bis(4‐azidobenzylidene)‐4‐methyl‐cyclohexanone, a popular crosslinking agent in a popular crosslinking agent in photolithography (see chapter 3.1), to crosslink sulfonated poly( photolithography (see chapter 3.1), to crosslink sulfonated poly(ether sulfone) (SPES) at allyl termini ® sulfone)by thermal (SPES activation.) at allyl Comparedtermini by to thermal Nafion® activation.‐112 (σ = 0.48 Compared S/cm), the tocrosslinked Nafion -112membrane (σ= 0.48 had S an/cm), theexcellent crosslinked proton membrane conductivity had of an 0.79 excellent S/cm at proton 100 °C conductivityand a much lower of 0.79 methanol S/cm at permeability 100 ◦C and of a much5.2 lower methanol permeability of 5.2 10 7 cm2s 1, corresponding to only 17% of that of Nafion®. × − − The membrane withstood oxidation by Fenton’s reagent for up to 4 h compared to around 3 h for the non-crosslinked version [75,76]. The same bisazide was also tested in combination with a poly(fluorenyl ether sulfone) polymer, resulting in lower proton conductivity of 0.38 S/cm at 100 ◦C, but also even 8 2 lower methanol permeability of 3.9 10− cm /s. In both examples however, the proton conductivity × ® at 20 ◦C was still lower than that of Nafion . Therefore, the group applied their terminal crosslinking approach to a SPES-based ionomeric block copolymer with defined blocks. Sulfofluorenyl moieties were incorporated to increase water uptake and proton conductivity (Figure8). This membrane outperformed ® Nafion at temperatures above 40 ◦C, reaching a value of 0.38 S/cm at 100 ◦C[77]. Molecules 2020, 25, 1009 15 of 29

× 10−7 cm2s‐1, corresponding to only 17% of that of Nafion®. The membrane withstood oxidation by Fenton’s reagent for up to 4 h compared to around 3 h for the non‐crosslinked version [75,76]. The same bisazide was also tested in combination with a poly(fluorenyl ether sulfone) polymer, resulting in lower proton conductivity of 0.38 S/cm at 100 °C, but also even lower methanol permeability of 3.9 × 10‐8 cm2/s. In both examples however, the proton conductivity at 20 °C was still lower than that of Nafion®. Therefore, the group applied their terminal crosslinking approach to a SPES‐based ionomeric block copolymer with defined blocks. Sulfofluorenyl moieties were incorporated to Moleculesincrease2020 water, 25, 1009 uptake and proton conductivity (Figure 8). This membrane outperformed Nafion®15 at of 29 temperatures above 40 °C, reaching a value of 0.38 S/cm at 100 °C [77].

FigureFigure 8. Structure8. Structure of theof the sulfonated sulfonated poly(ether poly(ether sulfone) sulfone) (SPES)-based (SPES)‐based ionomeric ionomeric block block copolymer copolymer with terminalwith terminal allyl groups allyl groups for crosslinking. for crosslinking.

A commonA common feature feature of these of these terminally terminally crosslinked crosslinked membranes membranes is their highis their temperature high temperature dependence compareddependence to the compared non-crosslinked to the versionsnon‐crosslinked or Nafion versions®. The authorsor Nafion believe®. The “that authors the hydrothermal believe “that energythe on heatinghydrothermal caused energy a positive on structuralheating caused change, a positive leading structural to a phase separationchange, leading for the to crosslinked a phase separation membrane, for the crosslinked membrane, and this facilitated the crosslinked membrane to overcome a high 1 and this facilitated the crosslinked membrane to overcome a high activation energy (26.49 kJ mol− ) −1 foractivation proton conduction.” energy (26.49 [78 kJ] mol A crosslinked) for proton membrane conduction.”[78] suitable A forcrosslinked high-temperature membrane PEM suitable fuel for cells high‐temperature PEM fuel cells was presented by Papadimitriou et al. It is based on aromatic was presented by Papadimitriou et al. It is based on aromatic polyethers with cross-linkable propenyl polyethers with cross‐linkable propenyl groups as side chains. For crosslinking, 25–30 wt% 2,6‐bis(4‐ groups as side chains. For crosslinking, 25–30 wt% 2,6-bis(4-azidobenzylidene)-4-methyl-cyclohexanone azidobenzylidene)‐4‐methyl‐cyclohexanone (1b) (Figure 3) was thermally activated to react with the (1b) (Figure3) was thermally activated to react with the propenyl side-chains forming propenyl side‐chains forming aziridines and amines. After a major weigh loss between 330 °C and and500 amines. °C due After to decomposition a major weigh of nitrene loss between intermediates, 330 ◦C the and crosslinked 500 ◦C due membrane to decomposition showed superior of nitrene intermediates,thermal stability the crosslinked up to 760 membrane°C compared showed to the superior non‐crosslinked thermal stabilityversion. upCrosslinking to 760 ◦C compared is known toto the non-crosslinkedlead to more compact version. structures, Crosslinking thus is knowndecreasing to lead the material’s to more compact doping structures,ability and thusits conductivity. decreasing the material’sInterestingly, doping the ability crosslinked and its membrane conductivity. showed Interestingly, slightly higher the crosslinked doping levels membrane (160 wt% showed H3PO4) slightly and higherconductivity doping levels (0.088 (160 S/cm) wt% than H 3thePO neat4) and polymer conductivity (195 wt% (0.088 H3PO S/cm)4, σ = than 0.062 the S/cm), neat polymerwhich could (195 be wt% H3PO 4, σ = 0.062 S/cm), which could be attributed to favorable interactions between the phosphoric acid (PA) and the aziridine and amine functionalities formed during crosslinking. During preliminary fuel cell tests the membrane delivered stable performance at 200 ◦C for 48 h [79]. Instead of crosslinking with bisazides, Su et al. introduced sulfonyl azide groups into polybenzimidazole (PBI). Upon heating the PBI chains were cross-linked via the of generated sulfonyl nitrenes into the Ar–H bonds of the benzimidazole units. These crosslinked membranes (see Scheme4 for the structure) had improved migration stability of PA, chemical oxidative stability and improved tensile strength, whereas 4 the doping ability with PA and the proton conductivity decreased, reaching around 10− S/cm at 190 ◦C with 20% PA content [80]. Molecules 2020, 25, 1009 16 of 29

attributed to favorable interactions between the phosphoric acid (PA) and the aziridine and amine functionalities formed during crosslinking. During preliminary fuel cell tests the membrane delivered stable performance at 200 °C for 48 h [79]. Instead of crosslinking with bisazides, Su et al. introduced sulfonyl azide groups into polybenzimidazole (PBI). Upon heating the PBI chains were cross‐linked via the insertion reaction of generated sulfonyl nitrenes into the Ar–H bonds of the benzimidazole units. These crosslinked membranes (see Scheme 4 for the structure) had improved migration stability of PA, chemical oxidative stability and improved tensile strength, whereas the Moleculesdoping2020 ability, 25, 1009 with PA and the proton conductivity decreased, reaching around 10−4 S/cm at 19016 °C of 29 with 20% PA content [80].

SchemeScheme 4. 4.Crosslinking Crosslinking reactionreaction ofof polybenzimidazolespolybenzimidazoles with with sulfonyl sulfonyl azide azide groups groups for for proton proton conductingconducting membranes. membranes.

4.2.4.2. Anion Anion Exchange Exchange Membranes Membranes UntilUntil now, now, very very few few examples examples of of crosslinkingcrosslinking with azides azides for for fabrication fabrication of of anion anion exchange exchange membranesmembranes (AEMs) (AEMs) exist. exist. Tough Tough and and flexible flexible AEMsAEMs based on on poly(2,6 poly(2,6-dimethyl-phenylene‐dimethyl‐phenylene oxides) oxides) (PPO(PPO) with) with azido-functionalized azido‐functionalized side side chains chains werewere produced by by thermally thermally activated activated crosslinking crosslinking via via nitrenes.nitrenes. Illustrating Illustrating the the versatileversatile reactivity reactivity of of the the azide azide function, function, the theside sidechains chains were werealso used also for used forquaternization quaternization of of the the polymer polymer by byCuAAC CuAAC with with an alkyne an alkyne-functionalized‐functionalized ammonium ammonium precursor. precursor. The Theauthors authors found found that, that, despite despite having having a adecreased decreased water water uptake, uptake, their their AEMs AEMs based based on onnitrene nitrene crosslinkingcrosslinking had had substantially substantially higher higher hydroxide hydroxide conductivities conductivities of upof up to 57.7to 57.7 mS /mS/cmcm compared compared to AEMs to AEMs known in literature using other crosslinking techniques. This could be attributed to the known in literature using other crosslinking techniques. This could be attributed to the architecture architecture of the sidechains, providing a continuous conduction pathway between the 1,2,3‐ of the sidechains, providing a continuous conduction pathway between the 1,2,3-triazoles bearing triazoles bearing a quaternized ammonium function and water molecules/hydroxide ions [81]. He et a quaternized ammonium function and water molecules/hydroxide ions [81]. He et al. synthesized al. synthesized a quaternized polysulfone with azide side groups for thermal crosslinking with a quaternizedreduced graphene polysulfone (rGO) withas inorganic azide side nanofiller. groups At for 160 thermal °C, the crosslinking azide groups with reacted reduced with alkenyl graphene (rGOgroups) as inorganic of the rGO nanofiller. under formation At 160 ◦ofC, aziridines. the azide In groups comparison reacted with with the alkenyl pristine groups membrane, of the therGO undercrosslinked formation membranes of aziridines. showed In comparison better mechanical with the pristine strength membrane, and thermal the crosslinkedstability. Moreover, membranes showedimproved better alkaline mechanical resistance strength was observed. and thermal This stability. might be Moreover, the result of improved the crosslinked alkaline structure resistance wasimpeding observed. access This of might the hydroxide be the result ions to of the the interior crosslinked of the structure membrane. impeding On the downside, access of crosslinking the hydroxide ionsadulterated to the interior water of uptake the membrane. and led to the On formation the downside, of smaller crosslinking ionic clusters adulterated in the membrane, water uptake thereby and ledimpeding to the formation continuous of ion smaller transport. ionic However, clusters inusing the as membrane, low as 0.1 wt% thereby rGO impeding already greatly continuous lowered ion transport.the methanol However, permeability using as to low 1.38 as × 0.1 10− wt%9 cm2rGO/s andalready enhanced greatly the ion lowered selectivity the methanol by the order permeability of two to 1.38magnitudes10 9 cm from2/s 7.56and × enhanced 105 to 1.21 the × 10 ion7 S/cm selectivity3, surpassing by the commercial order of two Nafion magnitudes®‐117 by far from [82].7.56 105 × − × to 1.21 107 S/cm3, surpassing commercial Nafion®-117 by far [82]. × 5. Cross-Linking with Organic Azides (Part III): More Applications

5.1. Long-Chain Branching in Polyolefins Long-chain branching (LCB) in polyolefins can enhance melt strength and improve processing properties. However, metallocene polyethylene (PE)[83], the most widely used polymer nowadays, is linear and has a narrow molar mass distribution. The resulting narrow processing window is disadvantageous for industrial application. LCB of PE was achieved by thermal crosslinking with 1,3-benzenedisulfonyl azide (1,3-BDSA)[6]. At concentrations above 100 ppm of 1,3-BDSA, LCP Molecules 2020, 25, 1009 17 of 29

5. Cross‐Linking with Organic Azides (Part III): More Applications

5.1. Long‐Chain Branching in Polyolefins

MoleculesLong2020, 25‐chain, 1009 branching (LCB) in polyolefins can enhance melt strength and improve processing17 of 29 properties. However, metallocene polyethylene (PE) [83], the most widely used polymer nowadays, is linear and has a narrow molar mass distribution. The resulting narrow processing window is effectsdisadvantageous became evident: for industrial the zero application. shear viscosity LCB increasedof PE was fromachieved 2.8 kPaby thermal s with 128crosslinking ppm 1,3-BDSA with 0 to 451,3 kPa‐benzenedisulfonyl s with 1027 ppm azide1,3-BDSA (1,3‐BDSA. The) [6]. steady At concentrations state compliance above Je 100increased ppm of from 1,3‐BDSA 0 to 512, LCP ppm, buteffects then decreasedbecame evident: at higher the zero concentration, shear viscosity which increased might from be attributed 2.8 kPa s with to a change128 ppm of 1,3 the‐BDSA dominant to shape45 kPa from s with linear 1027 to starppm shape. 1,3‐BDSA The. The bisazide steady crosslinker state compliance had a goodJe0 increased efficiency from between 0 to 512 40–60%ppm, but and lowthen propensity decreased for at higher chain scissionconcentration, reaction. which Therefore, might be attributed this LCB to approach a change is of a the viable dominant alternative shape to copolymerizationfrom linear to star of PEshape.with The 1-olefins. bisazide crosslinker had a good efficiency between 40–60% and low propensity for chain scission reaction. Therefore, this LCB approach is a viable alternative to 5.2.copolymerization Dynamic Vulcanization of PE with 1‐olefins.

5.2.1,3-BDSA Dynamic Vulcanizationhas also been used as crosslinking agent for dynamic vulcanization of ethylene– propylene–diene terpolymer rubber (EPDM) and polypropylene blends to produce thermoplastic 1,3‐BDSA has also been used as crosslinking agent for dynamic vulcanization of ethylene– vulcanizates (TPVs). The TPVs belong to the larger family of thermoplastic elastomers (TPEs). They can propylene–diene terpolymer rubber (EPDM) and polypropylene blends to produce thermoplastic be processed by conventional thermoplastic fabrication processes but exhibit more elastomer-like vulcanizates (TPVs). The TPVs belong to the larger family of thermoplastic elastomers (TPEs). They propertiescan be processed than mechanical by conventional blends, thermoplastic making them fabrication interesting processes materials. but exhibit This ismore a result elastomer of the‐like high degreeproperties of crosslinking than mechanical of the elastomeric blends, making chains. them Vulcanization interesting materials. of the elastomer This is a takes result place of the during high its intimatedegree melt of crosslinking mixing with of thethe elastomeric thermoplastic chains. and Vulcanization leads to a fine of dispersion the elastomer of crosslinked takes place smallduring rubber its particlesintimate in themelt thermoplastic mixing with the matrix. thermoplastic In contrast and to leads peroxides, to a fine curing dispersion with of1,3-BDSA crosslinkeddoes small not rubber give rise to chainparticles scission in the of thermoplastic the matrix. 1,3-BDSA matrix. In wascontrast found to toperoxides, be a better curing vulcanization with 1,3‐BDSA agent thandoes elementarynot give sulfur,rise to as chain evidenced scission by of improved the matrix. rheological 1,3‐BDSA and was mechanical found to be properties. a better vulcanization Because of agent its ability than to undergoelementary C–H sulfur, insertion as evidenced reactions, by it does improved not only rheological efficiently and cross-link mechanical the properties. elastomer Because phase butof its also createsability a higher to undergo number C–H of insertion covalent reactions, links to theit does thermoplastic not only efficiently phase, thereby cross‐link aff ordingthe elastomer more rigid phase and stablebut three-dimensionalalso creates a higher networksnumber of [ covalent3]. This mechanismlinks to the thermoplastic was also proposed phase, thereby in another affording study, more where 1,3-BDSArigid andwas stable compared three‐dimensional with two bisazidoformates networks [3]. This (see mechanism Figure9) for wasPP/ EPDMalso proposed-based TPVs in another from the perspectivestudy, where of processability 1,3‐BDSA was andcompared material with properties. two bisazidoformates In contrast (see to all Figure other 9) curingfor PP/EPDM systems‐based tested, bisazidesTPVs from lead the to aperspective significant of reduction processability of the and rubber material particle properties. size with In contrast increasing to all crosslink other curing density. systems tested, bisazides lead to a significant reduction of the rubber particle size with increasing Low 1,3-BDSA content gave the best processability of the vulcanizate. Moreover, 1,3-BDSA was more crosslink density. Low 1,3‐BDSA content gave the best processability of the vulcanizate. Moreover, suitable for curing than the azidoformate compounds, which were too reactive during iPP/EPDM melt 1,3‐BDSA was more suitable for curing than the azidoformate compounds, which were too reactive mixing due to their lower decomposition temperature. during iPP/EPDM melt mixing due to their lower decomposition temperature.

FigureFigure 9. 9.Bisazide Bisazide crosslinkerscrosslinkers for dynamic dynamic vulcanization. vulcanization.

In anotherIn another example, example, 1,12-diazido-dodecane 1,12‐diazido‐dodecane was employed was inemployed the crosslinking in ofthe poly(dimethylsiloxane) crosslinking of (PDMSpoly(dimethylsiloxane)) elastomers and coatings. (PDMS This) elastomers material and exhibits coatings. high temperatureThis material and exhibits chemical high resistance temperature as well as propertiesand chemical making resistance it interesting as well as in foodproperties and biomedical making it interesting industries. in Most food frequently and biomedical used curing industries. systems are,Most however, frequently based used on metal curing catalysts, systems thusare, however, preventing based their on application metal catalysts, in these thus field preventing due to possibletheir application in these field due to possible metal contamination. Free radical curing with organic azides metal contamination. Free radical curing with organic azides circumvents this issue. At 90 ◦C, the azide reactscircumvents with the vinyl-side this issue. chains At 90 of°C,PDMS the azideto form reacts two with main the coupling vinyl‐side products: chains of 1,2,3-triazolines PDMS to form by two 3 + 2 cycloadditionmain coupling and products: aziridines 1,2,3 by rearrangement‐triazolines by of3 + 1,2,3-triazolines 2 cycloaddition or and by aziridines radical addition by rearrangement of nitrenes issued of 1,2,3‐triazolines or by radical addition of nitrenes issued from thermal decomposition. By this from thermal decomposition. By this method, highly crosslinked films could be obtained, although only method, highly crosslinked films could be obtained, although only 10–20% of bisazide molecules 10–20% of bisazide molecules formed elastically active strands. The crosslinking density was lower at higher temperatures, probably due too side reactions resulting from an excess of reactive nitrene radicals as well as evaporation of the bisazide [84].

5.3. Nanoparticles The versatility of bisazides in crosslinking reactions was highlighted by the preparation of well-defined polyester nanoparticles via intermolecular chain collapse using the CuAAC reaction. By varying the number of alkyne sidechains in the linear polyester precursor and the amount of bisazide Molecules 2020, 25, 1009 18 of 29

formed elastically active strands. The crosslinking density was lower at higher temperatures, probably due too side reactions resulting from an excess of reactive nitrene radicals as well as evaporation of the bisazide [84].

5.3. Nanoparticles The versatility of bisazides in crosslinking reactions was highlighted by the preparation of well‐ Molecules 2020, 25, 1009 18 of 29 defined polyester nanoparticles via intermolecular chain collapse using the CuAAC reaction. By varying the number of alkyne sidechains in the linear polyester precursor and the amount of bisazide crosslinkercrosslinker 1,8-bisazide-3,6-dioxaoctan, 1,8‐bisazide‐3,6‐dioxaoctan, the the size size of the of nanoparticlesthe nanoparticles could could be controlled. be controlled. For example, For usingexample, a polyester using witha polyester two alkyne with two sidechains alkyne sidechains per 17 repeating per 17 repeating units and units four and equivalents four equivalents of azide of per alkyneazide moiety per alkyne yielded moiety a particle yielded size a particle with 178size nmwith diameter. 178 nm diameter. Doubling Doubling the amount the amount of azide of azide roughly doubledroughly the doubled particle the diameter. particle Disadvantages diameter. Disadvantages of this CuAAC of this approach CuAAC were approach the long were reaction the long time of 24 hreaction and possible time of Cu(I) 24 h and contamination possible Cu(I) of the contamination product [85 ].of Tothe avoid product this, [85].Jiang To et avoid al. used this, sulfonyl Jiang et azideal. used sulfonyl azide functionalized linear copolymers that form nitrenes upon heating, which insert functionalized linear copolymers that form nitrenes upon heating, which insert into the C–H bonds of the into the C–H bonds of the backbone and lead to intramolecular crosslinking in dilute solution. The backbone and lead to intramolecular crosslinking in dilute solution. The binding mechanisms of sulfonyl binding mechanisms of sulfonyl azides (Scheme 5) are akin to those of aromatic or aliphatic nitrenes azides (Scheme5) are akin to those of aromatic or aliphatic nitrenes (Scheme3). Advantages over thermal (Scheme 3). Advantages over thermal crosslinking using benzocyclobutene (BCB) are the lower crosslinking using benzocyclobutene (BCB) are the lower reaction temperature of 180–200 C vs. 250 C reaction temperature of 180–200 °C vs. 250 °C and the higher reactivity of the nitrene species.◦ The ◦ andnanoparticles’ the higher reactivity diameter of could the nitrene be controlled species. by The the nanoparticles’ content of sulfonyl diameter azide could groups be controlled and by bythe the contentmolecule of sulfonyl weight azideof the groupspolymer and [86]. by the molecule weight of the polymer [86].

intramolecular crosslinking SO2NH

dilute solution

RSO2N NO2S R + N2 RSO2N3 sulfonyl nitrene insertion ISC RSON h or T 3 1 2 3 RSO2 N RSO2 N - N2

1 RSO2 N

R' H R' H RSO2N NO2S R

dimerization

RSO2N R' RSON H + R' RSO2N R' H 2 H

recombination H-abstraction C-H insertion SchemeScheme 5. The5. The crosslinking crosslinking mechanisms mechanisms ofof sulfonyl azides azides are are akin akin to to those those of of aromatic aromatic or oraliphatic aliphatic nitrenesnitrenes (Scheme (Scheme3). 3).

AA promising promising class class of of nanoparticles nanoparticles (NPs)(NPs) for drug drug delivery delivery and and imaging imaging are are block block-copolymer‐copolymer nanoparticlesnanoparticles because because they they can can be be functionalizedfunctionalized both at at the the core core and and the the surface. surface. To To increase increase the the throughputthroughput of massivelyof massively parallel parallel signature signature sequencing sequencing (MPSS), (MPSS), an imaging an imaging method method for DNA for sequencing, DNA Zhangsequencing, et al. synthesized Zhang et al. core-crosslinkedsynthesized core‐ NPscrosslinked as a replacement NPs as a replacement for larger, for micro-scale larger, micro latex‐scale beads. Theylatex crosslinked beads. They the core crosslinked of polystyrene-block-poly(ethylene the core of polystyrene‐block oxide)‐poly(ethylene NPs via nitrene oxide) insertion NPs via by photolysisnitrene insertion by photolysis of encapsulated aryl bisazides 1,4‐diazidobenzene (DAB) and 4,4′‐ of encapsulated aryl bisazides 1,4-diazidobenzene (DAB) and 4,40-bis(azidomethyl)-1,10-biphenyl (BABP). Whenbis(azidomethyl) using BAPB,‐ NPs1,1′‐biphenyl exhibited (BABP improved). When thermal using stability, BAPB while, NPsDAB exhibiteddid not improved function wellthermal in this stability, while DAB did not function well in this regard. The authors attribute this to the different regard. The authors attribute this to the different intramolecular distance of the azide moieties: the longer intramolecular distance of the azide moieties: the longer BABP might more readily span the distance BABP might more readily span the distance between two PS chains within the core of a NP. Nanoparticles between two PS chains within the core of a NP. Nanoparticles that were core‐crosslinked with BABP that were core-crosslinked with BABP did not shrink in size under mock polymerase chain reaction (PCR) did not shrink in size under mock polymerase chain reaction (PCR) conditions and could therefore conditionsserve as and nanobeads could therefore for PCR serve[87] Instead as nanobeads of using for small PCR molecule [87] Instead crosslinkers, of using small Hu et molecule al. prepared crosslinkers, NPs Hu et al. prepared NPs by self-assembly of an amphiphilic block copolymer containing acyl azide groups in a one-pot reaction in aqueous media. The block copolymer PEG-b-P(NIPAMco-AAPMA) was synthesized by RAFT copolymerization of N-isopropylacrylamide (NIPAM) and 4-(acyl azido) phenylmethacrylate (AAPMA) in the presence of macromolecular chain transfer agent PEG5000-CTA at room temperature. In contrast to nitrene and click chemistry of alkyl or aryl azides, acyl azides show a different crosslinking mechanism. Upon heating, they transform into isocyanates via Curtius rearrangement, which readily react with water to form amino groups [88]. Subsequently, crosslinking between the amino groups and unreacted isocyanate groups can occur. Above a certain concentration of acyl azide groups in the copolymer, self-crosslinking without reassembly into bigger micelles was observed and NPs with a diameter of as low as 17 nm were obtained (Scheme6) [ 89]. While the preparation of sub-20 nm particles often is cumbersome, Molecules 2020, 25, 1009 19 of 29

by self‐assembly of an amphiphilic block copolymer containing acyl azide groups in a one‐pot reaction in aqueous media. The block copolymer PEG‐b‐P(NIPAMco‐AAPMA) was synthesized by RAFT copolymerization of N‐isopropylacrylamide (NIPAM) and 4‐(acyl azido) phenylmethacrylate (AAPMA) in the presence of macromolecular chain transfer agent PEG5000‐CTA at room temperature. In contrast to nitrene and click chemistry of alkyl or aryl azides, acyl azides show a different crosslinking mechanism. Upon heating, they transform into isocyanates via Curtius rearrangement, which readily react with water to form amino groups [88]. Subsequently, crosslinking Molecules 2020, 25, 1009 19 of 29 between the amino groups and unreacted isocyanate groups can occur. Above a certain concentration of acyl azide groups in the copolymer, self‐crosslinking without reassembly into bigger micelles was theobserved strategy presentedand NPs with here a is diameter convenient, of as economical low as 17 and nm environmentallywere obtained (Scheme benign. 6) Moreover, [89]. While it canthe be usefulpreparation for the preparation of sub‐20 nm of particles hollow polymer often is cumbersome, nanospheres, the an emergingstrategy presented class of material here is convenient, with exciting economical and environmentally benign. Moreover, it can be useful for the preparation of hollow properties [90]. polymer nanospheres, an emerging class of material with exciting properties [90].

SchemeScheme 6. (6.a) (a Assembly) Assembly of of block block copolymer copolymer PEG-PEG‐b‐-P(NIPAM-P(NIPAM‐coco‐-AAPMA)AAPMA) chainschains at at critical critical micelle micelle concentrationconcentration (CMC) (CMC) followed followed by by heat heat induced induced self-cross-linkingself‐cross‐linking into into sub sub-20‐20 nm nm micelles micelles takes takes place place above a certain concentration of acyl azide groups in the polymer. (b) Otherwise, reassembly into above a certain concentration of acyl azide groups in the polymer. (b) Otherwise, reassembly into larger larger units occurs. Scheme adapted from [89]. units occurs. Scheme adapted from [89].

5.4. Hydrogels Hydrogels are highly water-absorbent three-dimensional networks of crosslinked polymer chains. They can be designed as responsive materials which upon stimulation by external factors can liberate specific compounds. A pH- and thermosensitive hydrogel based on chitosan crosslinked with terephthaloyl bisazide was synthesized by Odinokov et al. as a drug delivery system. Crosslinking was achieved by nucleophilic addition of polysaccharide amino groups to 1,4-phenylene diisocyanate generated in situ by thermolysis of the bisazide (see Scheme7). Inclusion of the bioactive compound taxifolin was provided by formation of hydrogen bonds with the chitosan network. In a second formulation, taxifolin was covalently linked to the chitosan network via the second isocyanate group Molecules 2020, 25, 1009 20 of 29

5.4. Hydrogels Hydrogels are highly water‐absorbent three‐dimensional networks of crosslinked polymer chains. They can be designed as responsive materials which upon stimulation by external factors can liberate specific compounds. A pH‐ and thermosensitive hydrogel based on chitosan crosslinked with terephthaloyl bisazide was synthesized by Odinokov et al. as a drug delivery system. Crosslinking was achieved by nucleophilic addition of polysaccharide amino groups to 1,4‐phenylene diisocyanate Moleculesgenerated2020, 25 in, 1009situ by thermolysis of the bisazide (see Scheme 7). Inclusion of the bioactive compound20 of 29 taxifolin was provided by formation of hydrogen bonds with the chitosan network. In a second formulation, taxifolin was covalently linked to the chitosan network via the second isocyanate group of the adduct formed between taxifolin and 1,4-phenylene diisocyanate. Increasing the content of of the adduct formed between taxifolin and 1,4‐phenylene diisocyanate. Increasing the content of crosslinker for hydrogel formation led to a decrease of taxifolin release. crosslinker for hydrogel formation led to a decrease of taxifolin release.

OH NH3 O O HO O O HO O NH m OH O

N3 N NH C chitosan N 90-100 °C O O 3 H O/AcOH (1.2%) C 2 N O HN terephthaloyl diazide 1,4-phenylene diisocyanate O OH HN O O HO O O HO k NH3 OH

formulation 1

N O C O O H C HO O N O N OH O HO O O C OH dioxane, 101 °C OH N OH O OH OH O

H2O/AcOH (1.2%) taxifolin chitosan 25 °C H O N O HO O O OH N OH H NH NH3 O OH O HO O O HO O HO O OH O NH n OH O OH NH

HN O OH HN O O HO O O HO HN m OH O formulation 2 SchemeScheme 7. 7.Chitosan-based Chitosan‐based drug drug release release systemssystems using terephthaloyl terephthaloyl bisazide bisazide as as a crosslinker. a crosslinker.

WithWith 0.05 0.05 mmol mmol bisazide bisazide per per 20 20 g g of of a a 3% 3% solutionsolution of chitosan, chitosan, more more than than 80% 80% of ofthe the drug drug was was releasedreleased from from the the hydrogel hydrogel within within 1212 h, while only only 49% 49% was was released released with with glutaric glutaric aldehyde as a as a crosslinker.crosslinker. In In case case of of thethe secondsecond formulation, enzymatic enzymatic hydrolysis hydrolysis released released less less than than 3% 3% of taxifolin of taxifolin withinwithin 28 28 h [ 91h [91].].

5.5.5.5. Porous Porous and and Gel-Like Gel‐Like Materials Materials with with Azides Azides

Due to their high specific surface areas, well-defined pore sizes, and functional sites, porous materials are employed in molecular adsorption, storage and separation, sensing, , energy storage, and conversion and drug delivery [92]. Rigid polyazides with tetrahedral cores have been used for the synthesis of porous materials: For example, the fourfold click reaction on tetraphenylazido methane or tetraphenylazido adamantane with 1,4-diethynylbenzene yielded hyper-cross-linked polymers (HCPs) in good yield. Adsorption measurements of N2 and CO2 suggest extensive microporosity with a pore size of around 2 nm as well as presence of pores as large as 8–10 nm which are “likely due to interparticle cavities generated by the aggregation of nanoparticles.” [93] In addition, fullerenes have been used for similar purposes [94,95]. Moreover, surface-anchored MOFs can be functionalized with azide groups and Molecules 2020, 25, 1009 21 of 29

Due to their high specific surface areas, well‐defined pore sizes, and functional sites, porous materials are employed in molecular adsorption, storage and separation, sensing, catalysis, energy storage, and conversion and drug delivery [92]. Rigid polyazides with tetrahedral cores have been used for the synthesis of porous materials: For example, the fourfold click reaction on tetraphenylazido methane or tetraphenylazido adamantane with 1,4‐diethynylbenzene yielded hyper‐cross‐linked polymers (HCPs) in good yield. Adsorption measurements of N2 and CO2 suggest extensive microporosity with a pore size of around 2 nm as well as presence of pores as large as 8–10 Moleculesnm 2020which, 25 ,are 1009 “likely due to interparticle cavities generated by the aggregation of nanoparticles.”21 [93] of 29 In addition, fullerenes have been used for similar purposes [94,95]. Moreover, surface‐anchored MOFs can be functionalized with azide groups and subjected to cross‐linking with alkynes. subjected to cross-linking with alkynes. Dissolution yields surface-anchored polymeric gels (SURGELs) Dissolution yields surface‐anchored polymeric gels (SURGELs) (Figure 10). These water‐stable and (Figure 10). These water-stable and metal-free nanoporous polymer materials may be functionalized metal‐free nanoporous polymer materials may be functionalized postsynthetically photo‐patterned postsyntheticallyand precisely photo-patternedstructured at different and precisely length scales structured from atsub different‐nm to lengthcm appropriate scales from to sub-nmbioactive to cm appropriatefunctions. High to bioactive biocompatibility functions. and Highstability biocompatibility render them suitable and stability for biotechnological render them applications suitable for biotechnological[96]. applications [96].

Figure 10. Transformation of azide-tagged SURMOF into surface-anchored polymeric gel (SURGEL) Figure 10. Transformation of azide‐tagged SURMOF into surface‐anchored polymeric gel (SURGEL) via azide-alkyne cross-linking and subsequent metal removal after dissolution [96]. via azide‐alkyne cross‐linking and subsequent metal removal after dissolution [96]. 6. Polymerization Reactions with Azides 6. Polymerization Reactions with Azides 6.1. The aza-Wittig Polymerization towards Poly(azomethine)s 6.1. The aza‐Wittig Polymerization towards Poly(azomethine)s Poly(azomethine)s are a special class of π-conjugated polymers because they contain double Poly(azomethine)s are a special class of π‐conjugated polymers because they contain imine bonds with a free electron pair on the nitrogen atom that can interact with many electrophilic substances double bonds with a free electron pair on the nitrogen atom that can interact with many electrophilic and chelate metal ions. The most salient properties of these poly(Schiff-bases) are their excellent thermal substances and chelate metal ions. The most salient properties of these poly(Schiff‐bases) are their stability,excellent electrical thermal conductivity, stability, and electrical second- conductivity, and third-order and nonlinear second‐ opticaland third properties.‐order nonlinear As it is impossible optical to controlproperties. the reactivity As it is impossible of the conventional to control the polycondensation reactivity of the conventional between amines polycondensation and , between Chujo and Miyakeamines reportedand aldehydes, another Chujo polymerization and Miyake method reported towards another poly(azomethine)s polymerization utilizing method the aza-Wittigtowards reaction.poly(azomethine)s In this nitrogen-containing utilizing the aza version‐Wittig reaction. of the Wittig In this reaction, nitrogenN-P‐containing-ylides, version formed of from the Wittig organic azidesreaction, and phosphines, N‐P‐ylides, reactformed with from carbonyl organic compoundsazides and phosphines, to give react under with carbonyl mild conditions compounds[97 to,98 ]. The reactivitygive imines of under the aza-Wittig mild conditions polymerization [97,98]. couldThe reactivity be easily of tuned the aza by‐ changingWittig polymerization the steric and could electronic be featureseasily of tuned phosphines by changing employed. the steric In and the electronic presence features of tri-n -butylphosphine,of phosphines employed. 1,4-diazidobenzene In the presence and 2,5-bis(3,7-dimethyloctyloxy)terephthalaldehydeof tri‐n‐butylphosphine, 1,4‐diazidobenzene readily and underwent2,5‐bis(3,7‐dimethyloctyloxy)terephthalaldehyde an A2B2-type polycondensation reaction to yieldreadily a soluble underwentπ-conjugated an A2B2‐ all-typetrans polycondensationpoly(azomethine) reaction with highto yield molecular a soluble weight π‐conjugated of Mn = 31,000all‐trans Da and apoly(azomethine) polydispersity of withPDI high= 2.03 molecular[99]. The weight authors of have Mn = also 31,000 synthesized Da and a fullypolydispersity regioregular, of PDI head-to-tail = 2.03 coupled[99]. poly(azomethine)s The authors have fromalso synthesized AB-type monomers fully regioregular, containing bothhead‐ theto‐tail aldehyde coupled and poly(azomethine)s azide functionality. Remarkably,from AB self-condensation‐type monomers containing was not an both issue the in the aldehyde aza-Wittig and reaction. azide functionality. X-ray diffraction Remarkably, measurements self‐ condensation was not an issue in the aza‐Wittig reaction. X‐ray diffraction measurements indicated indicated a highly crystalline layered order structure of the regioregular polymers, while the absence of a highly crystalline layered order structure of the regioregular polymers, while the absence of any any diffraction peak indicated the amorphous structure of the regiorandom version. The high crystallinity diffraction peak indicated the amorphous structure of the regiorandom version. The high crystallinity could lead to better π–π-stacking and elongation of conjugated length, which is indicated by a greater could lead to better π–π‐stacking and elongation of conjugated length, which is indicated by a greater absorption at higher wavelength. The solution processability and crystallinity of these polymers makes them interesting for optoelectronic device applications including organic field effect transistors and photovoltaic cells [100]. However, their low solubility in toluene resulted in early termination of the polymerization and low molecular weights. Therefore, poly(azomethine)s were end-functionalized with polyhedral oligomeric silsesquioxanes (POSS) units to increase solubility. This did not only result in an increase of the molecular 1 1 weight from 12,000 g mol− to 19,800 g mol− , but also in enhanced electroluminescence and thermal stability of the devices prepared with this polymer. Moreover, the kinetic advantage of the aza-Wittig polymerization over the conventional polycondensation reaction was demonstrated by almost complete consumption of the end-capping unit within 2 h, while the conventional polycondensation reaction remained at about 40% conversion even after 4 h reaction time [101]. Molecules 2020, 25, x 22 of 29

absorption at higher wavelength. The solution processability and crystallinity of these polymers makes them interesting for optoelectronic device applications including organic field effect transistors and photovoltaic cells [100]. However, their low solubility in toluene resulted in early termination of the polymerization and low molecular weights. Therefore, poly(azomethine)s were end-functionalized with polyhedral oligomeric silsesquioxanes (POSS) units to increase solubility. This did not only result in an increase of the molecular weight from 12,000 g mol−1 to 19,800 g mol−1, but also in enhanced electroluminescence and thermal stability of the devices prepared with this polymer. Moreover, the kinetic advantage of the aza-Wittig polymerization over the conventional polycondensation reaction was demonstrated by almost complete consumption of the end-capping Moleculesunit within2020, 25 ,2 1009 h, while the conventional polycondensation reaction remained at about 40% conversion22 of 29 even after 4 h reaction time [101].

6.2.6.2. The The Staudinger Staudinger Reaction Reaction towards towards Polyphosphazenes Polyphosphazenes PolyphosphazenesPolyphosphazenes are are organic–inorganic organic–inorganic hybrid hybrid polymers polymers with with P ═ Nrepeating repeating units. units. They They exhibit exhibit highhigh thermal thermal and and chemical chemical stability, stability, flame flame resistance resistance and remarkable elasticity elasticity and and are are used used in indrug drug delivery,delivery, biosensors, biosensors, superhydrophobic superhydrophobic surfaces surfaces andand tissuetissue engineering. The The most most popular popular synthesis synthesis routeroute via via ring-opening ring-opening polymerization polymerization requires requires highhigh temperaturestemperatures and and vacuum vacuum which which may may degrade degrade thethe polymer polymer [102 [102].]. InIn contrast, the the Staudinger Staudinger reduction reduction offers off aers mild a route mild to route polyphosphazenes to polyphosphazenes using usingphosphines phosphines as azide as azidereducing reducing agents. agents.In the first In step, the a first reactive step, phosphazide a reactive phosphazideis formed by nucleophilic is formed by nucleophilicattack of the attack phosphine of the phosphine to the azide. to The the azide.phosphazenes The phosphazenes resulting from resulting subsequent from subsequentloss of nitrogen loss of nitrogenare usually are usually prone to prone hydrolysis to hydrolysis yielding amines, yielding which amines, are desired which aretarget desired products target of the products Staudinger of the reduction [98]. However, depending on the starting materials, stable polyphosphazenes can be Staudinger reduction [98]. However, depending on the starting materials, stable polyphosphazenes obtained. Sundhoro et al. found that the reaction between perfluoroaryl bisazides and certain can be obtained. Sundhoro et al. found that the reaction between perfluoroaryl bisazides and certain bisphosphines with aromatic or aliphatic linkers yielded polyphosphazenes with molecular weights bisphosphines with aromatic or aliphatic linkers yielded polyphosphazenes with molecular weights close to 60,000 g mol−1 and dispersities as narrow as 1.1. This was a significant improvement over 1 closepolyphosphazenes to 60,000 g mol −preparedand dispersities with non-fluorinated as narrow bisazides.as 1.1. This Moreover, was a significant the novel polymers improvement showed over polyphosphazeneshigh thermal stability prepared with decomposition with non-fluorinated temperatures bisazides. up to 441 Moreover, °C and char the yields novel up polymers to 53% [103]. showed high thermal stability with decomposition temperatures up to 441 ◦C and char yields up to 53% [103]. 6.3. Multicomponent Reactions with Azides 6.3. Multicomponent Reactions with Azides Multicomponent reactions (MCRs) are one-pot reactions with three or more reactants leading to highlyMulticomponent selective products reactions while (MCRs) retaining are the one-pot majority reactions of the withatoms three of the or starting more reactants material. leading MCRs to highlyshow selective good functional-group products while retainingtolerance, the high majority atom economy, of the atoms and of simple the starting isolation material. procedures. MCRs showSimilarly, good functional-group functional polymers tolerance, can be high generated atom economy, in multicomponent and simple polymerizations isolation procedures. (MCPs) Similarly, with functionalthe appropriate polymers choice can be of generated monomers. in multicomponent Based on the copper-catalyzed polymerizations MCR (MCPs) of with terminal the appropriate alkynes, choicesulfonyl of monomers. azides, and Based carbodiimides on the copper-catalyzed by Xu et al. [104], MCR Han of terminal et al. designed alkynes, an sulfonyl MCP to azides, produce and carbodiimidesfunctional polymers by Xu et featuring al. [104 ], multisubstituted Han et al. designed azetidine an rings MCP in to theproduce polymer functional backbone. polymers The featuringreaction multisubstituted was catalyzed by azetidine copper(I) ringsiodide in and the conducted polymer in backbone. dichloromethane The reaction at room was temperature, catalyzed by copper(I)obtaining iodide polymers and conducted with high molecular in dichloromethane weight of up at to room 29,200 temperature, g mol−1 in good obtaining yields (72–89%) polymers (see with Scheme 8). Polymers containing tetraphenylethenyl1 (TPE) moieties showed aggregation-induced high molecular weight of up to 29,200 g mol− in good yields (72–89%) (see Scheme8). Polymers containingemission tetraphenylethenyl with fluorescence ( quantumTPE) moieties yield showed of up to aggregation-induced 27.1% in solid state. emission Surprisingly, with fluorescence the solid powders of then two polymers without TPE fluorophores exhibited visible light emission under 362 quantum yield of up to 27.1% in solid state. Surprisingly, the solid powders of then two polymers nm UV. The crystal structure of a model compound suggests strong intramolecular through-space π-π without TPE fluorophores exhibited visible light emission under 362 nm UV. The crystal structure of and other electronic interactions which was verified by DFT calculations. New polymers with a model compound suggests strong intramolecular through-space π-π and other electronic interactions distinctive properties were produced by transforming the four-membered azetidine rings into whichand was amidine verified moieties by DFT via calculations. acid-mediated New ring-opening polymers with reactions distinctive in a properties mixture of were aqueous produced HCl by transformingsolution and the THF. four-membered All polymers azetidine possess good rings film-forming into amide and ability amidine and good moieties photo-patternability, via acid-mediated ring-openingmaking them reactions promising in amaterials mixture in of electronic aqueous and HCl optoelectronic solution and devices THF. All[105]. polymers possess good film-forming ability and good photo-patternability, making them promising materials in electronic and Moleculesoptoelectronic 2020, 25, 1009 devices [105]. 23 of 29

SchemeScheme 8. Synthesis 8. Synthesis of ofmultisubstituted multisubstituted poly(azetidine)s poly(azetidine)s via via copper-catalyzed copper‐catalyzed multicomponent multicomponent polymerizationpolymerization reaction reaction and and subsequent subsequent acid acid-mediated‐mediated ring-opening ring‐opening reaction. reaction.

7. Summary The first chapter on high‐energy materials makes it clear that organic polyazides should be handled with care, but they show also ample opportunities. As nitrogen‐rich class of compounds, they are employed in explosive mixtures and are prone to violent decomposition upon stimuli such as light, heat, impact, and friction. However, the examples presented herein show that many compounds can be handled safely. GAP is a popular commercial energetic binder and the family of azido esters has been shown to be a green alternative to nitrate ester plasticizers, with better compatibility with azido binders, from the first application of bisazides in photoresists in semiconductor production to morphology control in organic photovoltaics. In material sciences, organic polyazides have been established as highly efficient cross‐linking agents. Two classes of organic polyazide cross‐linkers can be distinguished: small molecule crosslinkers and azido polymers. A major advantage of organic polyazide cross‐linkers is that they can be both thermally and photo‐activated without additive. Cross‐linking proceeds either via heat‐ or light‐induced decomposition of the azides into highly active nitrenes, which readily react with both saturated and unsaturated hydrocarbon chains or via cycloaddition reactions with alkynes. The orthogonality of these approaches opens up even more possibilities in cross‐linking materials. This has led to the development of interesting new materials in the field of life sciences, such as cross‐linked hydrogels for drug delivery. Finally, exploiting the diverse reactivity of the azido group employing organic polyazides with novel polymers provided a number of new features. Using multicomponent reactions as well as more convenient alternative routes led to polydiverse novel polymers. With cross‐ linking currently being investigated as a powerful strategy to increase the stability of organic semiconductor devices, novel organic azido cross‐linkers might become crucial in the commercial breakthrough of organic semiconductors.

Funding: We acknowledge the support of the Cluster of Excellence 3DMM2O (EXC‐2082/1–390761711) funded by the Deutsche Forschungsgemeinschaft (DFG) and the project KeraSolar, funded by the Carl‐Zeiss foundation.

Acknowledgments: We thank the Bräse group for useful comments.

Conflicts of Interest: The authors declare no conflict of interest.

List of Abbreviations (HxN3)2‐SiPc bis(6‐azidohexanoate)silicon phthalocyanine 1,3‐BDSA 1,3‐benzenedisulfonyl azide AAPMA 4‐(acyl azido) phenylmethacrylate ABAMPA azido ester 3‐azido‐2,2‐bis(azidomethyl)propyl 2‐azidoacetate BABAMP 1,3‐bis (azido acetoxy)‐2,2‐bis(azidomethyl) propane BABP 4,4′‐bis(azidomethyl)‐1,1′‐biphenyl BCB benzocyclobutene BDAP bis(2,3‐diazidopropylene glycol) BHJ bulk heterojunction bu‐NENA butyl nitrato ethyl nitramine

Molecules 2020, 25, 1009 23 of 29

7. Summary The first chapter on high-energy materials makes it clear that organic polyazides should be handled with care, but they show also ample opportunities. As nitrogen-rich class of compounds, they are employed in explosive mixtures and are prone to violent decomposition upon stimuli such as light, heat, impact, and friction. However, the examples presented herein show that many compounds can be handled safely. GAP is a popular commercial energetic binder and the family of azido esters has been shown to be a green alternative to nitrate ester plasticizers, with better compatibility with azido binders, from the first application of bisazides in photoresists in semiconductor production to morphology control in organic photovoltaics. In material sciences, organic polyazides have been established as highly efficient cross-linking agents. Two classes of organic polyazide cross-linkers can be distinguished: small molecule crosslinkers and azido polymers. A major advantage of organic polyazide cross-linkers is that they can be both thermally and photo-activated without additive. Cross-linking proceeds either via heat- or light-induced decomposition of the azides into highly active nitrenes, which readily react with both saturated and unsaturated hydrocarbon chains or via cycloaddition reactions with alkynes. The orthogonality of these approaches opens up even more possibilities in cross-linking materials. This has led to the development of interesting new materials in the field of life sciences, such as cross-linked hydrogels for drug delivery. Finally, exploiting the diverse reactivity of the azido group employing organic polyazides with novel polymers provided a number of new features. Using multicomponent reactions as well as more convenient alternative routes led to polydiverse novel polymers. With cross-linking currently being investigated as a powerful strategy to increase the stability of organic semiconductor devices, novel organic azido cross-linkers might become crucial in the commercial breakthrough of organic semiconductors.

List of Abbreviations

(HxN3)2-SiPc bis(6-azidohexanoate)silicon phthalocyanine 1,3-BDSA 1,3-benzenedisulfonyl azide AAPMA 4-(acyl azido) phenylmethacrylate ABAMPA azido ester 3-azido-2,2-bis(azidomethyl)propyl 2-azidoacetate BABAMP 1,3-bis (azido acetoxy)-2,2-bis(azidomethyl) propane BABP 4,40-bis(azidomethyl)-1,10-biphenyl BCB benzocyclobutene BDAP bis(2,3-diazidopropylene glycol) BHJ bulk heterojunction bu-NENA butyl nitrato ethyl nitramine ButBAMP 2,2-bis(azidomethyl)propane-1,3-diyl dibutyrate CA 1,3-dipolar cycloaddition CuI copper(I) iodide DADFH 1,7-diazido-N,N,N0,N0-tetrafluoro-4,4-heptanediamine DAZH 1,6-diazidohexane DEGBAA diethylene glycol bis(azidoacetate) DFT density functional theory DIANP 1,5-diazido-3-nitrazapentane DPP-10C5DE diketopyrrolopyrrole with diethylpentanediyl linker DPPTPTA 3,6-bis(5-(4,4”-bis(3-azidopropyl)-(1,10:30,1”-terphenyl)-50-yl)thiophen-2-yl)-2,5- bis(2-ethylhexyl)-2,5-dihydropyrrolo(3,4-c)pyrrole-1,4-dione DUV deep UV EGBAA ethylene glycol bis(azidoacetate) ETPE energetic thermoplastic elastomers F8BT poly(9,9-dioctylfluorene-alt-benzothiadiazole) FPA bis(4-azido-2,3,5,6-tetrafluoro-benzoate) GAP glycidyl azide polymer iPP isotactic polypropylene Molecules 2020, 25, 1009 24 of 29

LED light-emitting diode N3-(CPDT(FBTTh2)2) 4-40-bis(1-azido)undecane)dicyclopenta-(2,1-b:3,4-b0)dithio-phene-bis(5-fluoro-7- (50-hexyl-(2,20-bithiophene)-5-yl)benzo-(c)(1,2,5)thiadiazole) NIPAM N-isopropylacrylamide NP nanoparticle OFET organic field-effect transistor OLED organic light-emitting diode OPV organic photovoltaics OSC organic solar cell P3HT poly(3-hexylthio-phene-2,5-diyl) PA phosphoric acid PC61BM phenyl-C61-butyric acid methyl ester PC71BM (6,6)-phenyl-C71-butyric acid methyl ester PCBM phenyl-Cn-butyric acid methyl ester (in general C61) PCE power conversion efficiency PDMS poly(dimethylsiloxane) PEDT poly(3,4-ethylenedioxythiophene) PEMFCs proton exchange membrane fuel cells DMFCs direct methanol fuel cells PETKAA pentaerythritol tetrakis(azidoacetate) PFBA propane-1,3-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate) PMIPK poly(methyl isopropenyl ketone PMMA poly(methyl methacrylate poly-AMMO poly(3-azidomethyl-3-methyloxetane) poly-BAMO poly(3,3–bis(azidomethyl)oxetane) POSS polyhedral oligomeric silsesquioxanes pp polypropylene PSSH poly(styrenesulfonic acid) PTAA poly(triaryl amine) PTPA poly(triphenylamine) PVK:PBD poly(vinylcarbazole)-2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol rGO reduced graphene SDO 4,40-oxybis-benzenesulfonyl azide sFPA sterically hindered bis(fluorophenyl azides) SiIDT-BT/PC71BM silaindacenodithiophene-benzotriazole SPES poly(ether sulfone) TAP-Ac triazido pentaerythrite acetate TBA-X 1,2-bis((4-(azidomethyl)phenethyl)thio)ethane TFB poly(9,9 di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene) THF tetrahydrofuran TMETA 1,1,1-tris(azidomethyl)ethane TMNTAA trimethylol nitromethane tris(azidoacetate) TPBA 1,3,5,7-tetrakis-(p-azidobenzyl)-adamantane TPE tetraphenylethene TPT-N3 tris(4-(50-(3-azidopropyl)-2,20-bithiophen5-yl)phenyl)amine PBI polybenzimidazole X-PTCAzide crosslinked poly triarylamine-carbazoyl azide (poly 4-(9-(6-azidohexyl)-9H-carbazol-3-yl)-N-(4-butylphenyl)-N-phenylaniline)

Funding: We acknowledge the support of the Cluster of Excellence 3DMM2O (EXC-2082/1–390761711) funded by the Deutsche Forschungsgemeinschaft (DFG) and the project KeraSolar, funded by the Carl-Zeiss foundation. Acknowledgments: We thank the Bräse group for useful comments. Conflicts of Interest: The authors declare no conflict of interest. Molecules 2020, 25, 1009 25 of 29

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