Advances in Homogeneous Photocatalytic Organic Synthesis with Colloidal Quantum Dots

catalysts

Review Advances in Homogeneous Photocatalytic Organic Synthesis with Colloidal Quantum Dots

Dan-Yan Wang, Yu-Yun Yin, Chuan-Wei Feng, Rukhsana and Yong-Miao Shen *

Department of Chemistry, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou 310018, China; [email protected] (D.-Y.W.); [email protected] (Y.-Y.Y.); [email protected] (C.-W.F.); [email protected] (R.) * Correspondence: [email protected]; Tel.: +86-13735368502

Abstract: Colloidal semiconductor quantum dots (QDs) have been proven to be excellent photocatalysts due to their high photostability, large extinction coefﬁcients, and tunable optoelectrical properties, and have attracted extensive attention by synthetic chemists. These excellent properties demonstrate its promise in the ﬁeld of photocatalysis. In this review, we summarize the recent application of QDs as homogeneous catalysts in various photocatalytic organic reactions. These meaningful works in organic transformations show the unique catalytic activity of quantum dots, which are different from other semiconductors.

Keywords: quantum dots; organic synthesis; homogeneous photoredox

Citation: Wang, D.-Y.; Yin, Y.-Y.; 1. Introduction Feng, C.-W.; R.; Shen, Y.-M. Advances The efficient use of clean, sustainable, and pollution-free solar energy provides an in Homogeneous Photocatalytic effective solution to the energy crisis and environmental pollution. Photocatalytic organic Organic Synthesis with Colloidal synthesis has become one of the most promising technologies to direct conversion of solar Quantum Dots. Catalysts 2021, 11, energy into chemical energy. Approximately 40% of sunlight is visible light, so an ideal 275. https://doi.org/10.3390/ photocatalyst needs to extend the maximum absorption wavelength to 800 nm. Over the catal11020275 last few decades, a large number of visible light catalysts have been reported and applied, such as transition metal ruthenium and iridium coordination compounds [1–4], organic Academic Editors: Frédéric Dumur dyes [5], and all kinds of novel bulk semiconductor materials, such as TiO2, WO3, CdS, and Jacques Lalevee etc. [6–13]. However, solar energy cannot be used very effectively by using heterogeneous semiconductor catalysts. Fortunately, colloidal quantum dots (QDs) have exhibited great Received: 27 January 2021 potential with precisely controlled compositions, structures, and surface properties [14]. Accepted: 11 February 2021 Colloidal semiconductor quantum dots, one of the semiconductor nanocrystals, are Published: 18 February 2021 mostly between 2 and 20 nm and are smaller than the Bohr exciton diameter, which produce their superior properties including wide and strong absorption for light harvesting, Publisher’s Note: MDPI stays neutral size-dependent band gap for tunable and selective driving force of redox reaction, large with regard to jurisdictional claims in surface-to-volume ratio for charge extraction, rich reaction sites, and potentially good published maps and institutional affil- iations. photochemical stability due to size effects and surface effects [15]. Initially, quantum dots were loaded on other semiconductor catalysts and used as heterogeneous catalysts in the reaction systems. Normally, QD-based photocatalysts were mainly used for H2 evolution [16–22] and CO2 reduction [23–25], and then extended to organic synthesis. In 2001, Peng et al. [26] found that photocatalytic dimerization of thiol Copyright: © 2021 by the authors. ligands could generate disulfides when they were studying the photochemical instability Licensee MDPI, Basel, Switzerland. of thiol-coated CdSe nanocrystals in water. In 2016, Jensen [27] proved that CdS QDs were This article is an open access article much better than CdS powder for the rate of photocatalysis nitrobenzene degradation. distributed under the terms and conditions of the Creative Commons A 2018 report from Weiss et al. [28] mentioned that small-size CdS QDs (<20 nm) behaved Attribution (CC BY) license (https:// much better than CdS nano particles (NPs) (>20 nm) for photocatalytic conversion of creativecommons.org/licenses/by/ 2-phenoxy-1-phenylethanol. Especially when the QDs size was 4.4 nm, the conversion 4.0/). of 2-phenoxy-1-phenylethanol and the yield of acetophenone and phenol were over four

Catalysts 2021, 11, 275. https://doi.org/10.3390/catal11020275 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 275 2 of 14

times than that of NPs. Additionally, the energy bond position increased from 2.35 eV (NPs, >20 nm) to 2.74 eV (4.4 nm-QDs). Besides, CdS QDs exhibited higher catalytic efﬁciency than CdS NPs, and could be reused 10 times without any loss of catalytic activity. It is clear that QDs have great potential in visible-light-catalyzed organic reactions. Here, we present the recent advances in QDs as homogeneous photocatalysts in organic reactions. The literature on the degradation of organic pollutants and hydrogen generation, as well as the reduction of carbon dioxide, is not the focus of this review.

2. Cadmium Containing QDs 2.1. CdS QDs CdS QDs have good photosensitivity properties, less dosage, and high efficiency, and absorb visible light to stimulate reactants to produce free radicals, thus catalyzing organic synthesis reactions. In early 2016, Weiss et al. [27] explored the reduction reaction of nitrobenzene to aniline with CdS QDs (diameter = 4.5 nm) as visible-light photocatalysts and 3-mercaptopropionic acid (MPA) as sacrificial agents. Notably, nitrobenzene experienced six sequential photoinduced, proton-coupled electron transfers. MPA here captured the hole on the QDs to form QD•− and then the electron was transferred to nitrobenzene or the intermediates nitrosobenzene and phenylhydroxylamine to the final product aniline (Scheme1). Interest- ingly, the reaction system maintaining an acidic pH not only solved QDs poisoning due to Catalysts 2021, 11, x FOR PEER REVIEW 3 of 14 adsorption of the photoproduct aniline on the QDs surface, but promoted protonation of intermediates. + H H +

S R R H

R S - R + R H + + H S - + H S R

O + N H H - + S

R H R

S R S

R - S

SchemeScheme 1. Mechanism1. Mechanism of of the the entire entire process process of of nitrobenzenenitrobenzene convertingconverting to to aniline aniline on on the the acidic acidic condition. condition. Reprinted Reprinted with with thethe permission permission of of ref. ref. [27 [27],], Copyright Copyright 2016 2016 @American @American ChemistryChemistry Society.

In 2017, Weiss et al. [29] developed a ligand-dependent C–C coupling reaction between 1-phenyl pyrrolidine and phenyl trans-styryl sulfone, with the participation of photocatalyst CdS QDs (diameter = 3.7 nm). No sacrificial agents or co-catalyst were needed (Scheme 2). Holes transferred to 1-phenyl pyrrolidine through the surface of QDs were proven by kinetic analysis to be a rate-limiting step, and that the product increased linearly with the concentration of 1-phenyl pyrrolidine (that is, the initial rate) in the first 15 min. For this reason, the ligand shell of the QDs was key to the rate of entire reaction. They added octylphosphonate ligands to replace some native oleate ligands of CdS QDs by a one-step ligand exchange. The ligand exchange disordered the ligand shell and increased the initial reactions rate by a factor of 2.3, and the energy efficiency by a factor of 1.6 when QDs pre-treated by 250 eq. of octylphosphonate (OPA) ligands were used.

Scheme 2. The reaction for the photocatalytic reaction of 1-phenyl pyrrolidine and phenyl trans-styryl sulfone.

Later, Weiss et al. [30] reported that the photocatalysis oxidation of benzyl alcohol to benzaldehyde with 99% selectivity, or to hydrobenzoin by carbon–carbon coupling,

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+ H H +

S R R H

R S - R + R H + + H S - + H S R

O + N H H - + S

R H R

S R S

R - S

Catalysts 2021, 11, 275 In 2017, Weiss et al. [29] developed a ligand-dependent C–C coupling3 of 14 reaction between 1-phenyl pyrrolidine and phenyl trans-styryl sulfone, with the participation of photocatalyst CdS QDs (diameter = 3.7 nm). No sacrificial agents or co-catalyst were neededIn 2017, (Scheme Weiss et 2). al. [Holes29] developed transferred a ligand-dependent to 1-phenyl C–C pyrrolidine coupling reaction through be- the surface of tween 1-phenyl pyrrolidine and phenyl trans-styryl sulfone, with the participation of photocatalystQDs were proven CdS QDs by (diameter kinetic =analysis 3.7 nm). to No be sacrificial a rate-limiting agents or step, co-catalyst and that were the product in- neededcreased (Scheme linearly2). Holes with transferredthe concentration to 1-phenyl of pyrrolidine 1-phenyl through pyrrolidine the surface (that of is, QDs the initial rate) in werethe first proven 15 by min. kinetic For analysis this reason, to be a rate-limitingthe ligand step,shell and of thatthe theQDs product was key increased to the rate of entire linearlyreaction. with They the concentration added octylphosphonate of 1-phenyl pyrrolidine ligands (that to is, replace the initial some rate) innative the first oleate ligands of 15 min. For this reason, the ligand shell of the QDs was key to the rate of entire reaction. CdS QDs by a one-step ligand exchange. The ligand exchange disordered the ligand They added octylphosphonate ligands to replace some native oleate ligands of CdS QDs by ashell one-step and ligand increased exchange. the The initial ligand reactions exchange disorderedrate by athe factor ligand of shell 2.3, and and increased the energy efficiency theby initiala factor reactions of 1.6 rate when by a factor QDs of pre-treated 2.3, and the energy by 250 efficiency eq. of byoctylphosphonate a factor of 1.6 when (OPA) ligands QDswere pre-treated used. by 250 eq. of octylphosphonate (OPA) ligands were used.

Catalysts 2021, 11, x FOR PEER REVIEW SchemeScheme 2. 2.The The reaction reaction for the for photocatalytic the photocatalytic reaction reac of 1-phenyltion of pyrrolidine1-phenyl pyrrolidine and phenyl trans- and4 ofphenyl 14

styryltrans-styryl sulfone. sulfone. Later, Weiss et al. [30] reported that the photocatalysis oxidation of benzyl alcohol to reactedbenzaldehyde withLater, 91% withWeiss selectivity 99% et selectivity, al. using[30] reported or colloidal to hydrobenzoin thatCdS theQDs by photocatalysis carbon–carbon(diameter = 3.6 coupling, oxidationnm) as reacted the of photo- benzyl alcohol catalyst.withto benzaldehyde 91% The selectivity control of usingwith selectivity colloidal99% selectivity, was CdS mainly QDs (diameter oraffected to hydrobenzoin = by 3.6 the nm) amount as the by photocatalyst. of carbon–carbon Cd0 photo de- coupling, 0 positedThe control on the of surfaces selectivity of was the mainly QDs in affected situ, and by the then amount by the of Cdconcentrationphoto deposited of benzyl on the alco- surfaces of the QDs in situ, and then by the concentration of benzyl alcohol. The deposition hol. The deposition of Cd0 on QDs, that is, the addition of Cd(ClO4)2, enabled the ben- of Cd0 on QDs, that is, the addition of Cd(ClO ) , enabled the benzaldehyde to undergo zaldehyde to undergo the re-reduction to the4 C–C2 coupling products. However, the ad- the re-reduction to the C–C coupling products. However, the addition of anthroquinone-2- ditionsulfonate of anthroquinone-2-sulf (AQ), an electron scavenger,onate (AQ), led to aan second electron hole fromscavenger, a second led photoexcitation to a second hole fromof thea second QD transferring photoexcitation to radical intermediateof the QD transferring to form benzaldehyde to radical with intermediate high probability to form benzaldehyde(Scheme3). When with using high the probability thiolate-free (Schem system,e there3). When is no concernusing the about thiolate-free the co-catalyst system, therebeing is no poisoned concern by about thiols orthe low co-catalyst yield due being to the photo-oxidizingpoisoned by thiols of the or thiolate low yield ligands due to to the photo-oxidizingform disulﬁdes. of In the that thiolate case, the ligands CdS QDs to form can recycle disulfides. for four In times that withoutcase, the the CdS loss QDs of can recyclecatalytic for efﬁciency.four times without the loss of catalytic efficiency.

SchemeScheme 3. Selectivities 3. Selectivities achieved achieved for either for either C–C C–C coupling coupling or oroxidation oxidation of ofbenzyl benzyl alcohol alcohol catalyzed catalyzed by by CdS quantum dots (QDs).dots (QDs).

Cheng et al. [28] found that CdS QDs (diameter = 4.4 nm), as efficient photocatalysts, not only broke the β–O-–4 bonds in various lignin models (e.g., 2-phenoxy-1-phenylethanol (PP-ol)), but also transformed native lignin within biomass to functionalized aromatics under visible light without affecting other components in lignocellulose when they studied the full used of the lignocellulosic biomass (Scheme 4). The photogenerated electrons and holes of QDs about the cleavage of β–O–4 bonds were used fully. Additionally, a reversible aggregation-colloidization strategy could separate QDs from the reaction system and then be reused.

Scheme 4. The reaction for the photocatalytic cleavage of β–O–4 bond in lignin model compounds.

As shown above, this group effectively manipulated the organic ligands on the surface of CdS QDs (diameter of about 4.4 nm) for the photocatalytic reaction from native lignin to functionalized aromatics [31]. The hydrophilicity/oleophilicity of ligands is the key to the interaction between QDs and lignin, which facilitate the electron-transfer process. These researchers changed the type of coordination group and the length of ligands, and finally found that short-chain ligands or ligands with low barrier potential boosted the photocatalytic conversion. The effect of 3-mercaptopropionic acid-capped CdS QDs (CdS-C3 QDs) was better than CdS-C6 QDs and CdS-C11 QDs.

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reacted with 91% selectivity using colloidal CdS QDs (diameter = 3.6 nm) as the photocatalyst. The control of selectivity was mainly affected by the amount of Cd0 photo deposited on the surfaces of the QDs in situ, and then by the concentration of benzyl alcohol. The deposition of Cd0 on QDs, that is, the addition of Cd(ClO4)2, enabled the benzaldehyde to undergo the re-reduction to the C–C coupling products. However, the addition of anthroquinone-2-sulfonate (AQ), an electron scavenger, led to a second hole from a second photoexcitation of the QD transferring to radical intermediate to form benzaldehyde with high probability (Scheme 3). When using the thiolate-free system, there is no concern about the co-catalyst being poisoned by thiols or low yield due to the photo-oxidizing of the thiolate ligands to form disulfides. In that case, the CdS QDs can recycle for four times without the loss of catalytic efficiency.

SchemeCatalysts 2021 3. Selectivities, 11, 275 achieved for either C–C coupling or oxidation of benzyl alcohol catalyzed by CdS quantum4 ofdots 14 (QDs).

Cheng et al. [28] found that CdS QDs (diameter = 4.4 nm), as efficient photocatalysts, not onlyCheng etbroke al. [28 ] foundthe thatβ–O-–4 CdS QDsbonds (diameter in = 4.4various nm), as efﬁcientlignin photocatalysts, models (e.g., β 2-phenoxy-1-phenylethanolnot only broke the –O—4 bonds (PP-ol in various)), but ligninalso transformed models (e.g., 2-phenoxy-1-phenylethanolnative lignin within biomass (PP-ol)), but also transformed native lignin within biomass to functionalized aromatics to functionalized aromatics under visible light without affecting other components in under visible light without affecting other components in lignocellulose when they studied lignocellulosethe full used when of the lignocellulosicthey studied biomassthe full (Schemeused of 4the). The lignocellulosic photogenerated biomass electrons (Scheme and 4). Theholes photogenerated of QDs about theelectrons cleavage and of β holes–O–4 bondsof QDs were about used the fully. cleavage Additionally, of β–O–4 a reversible bonds were usedaggregation-colloidization fully. Additionally, a strategyreversible could aggregation-colloidization separate QDs from the reaction strategy system could and then separate QDsbe from reused. the reaction system and then be reused.

SchemeScheme 4. The 4. The reaction reaction for for the the photocatalytic photocatalytic cleavage ofofβ β–O–4–O–4 bond bond in in lignin lignin model model compounds. compounds.

AsAs shown shown above, thisthis group group effectively effectively manipulated manipulated the organic the ligandsorganic on ligands the surface on the of CdS QDs (diameter of about 4.4 nm) for the photocatalytic reaction from native lignin Catalysts 2021, 11, x FOR PEER REVIEW surface of CdS QDs (diameter of about 4.4 nm) for the photocatalytic reaction5 of 14 from na- to functionalized aromatics [31]. The hydrophilicity/oleophilicity of ligands is the key tiveto lignin the interaction to functionalized between QDs aromatics and lignin, [31]. which The hydrophilicity/oleophilicity facilitate the electron-transfer process.of ligands is theThese key to researchers the interaction changed between the type QDs of coordination and lignin, group which and facilitate the length the of ligands,electron-transfer and process.finally These found thatresearchers short-chain changed ligands the or ligandstype of with coordination low barrier group potential and boosted the length the of Feng et al.ligands, [32]photocatalytic presented and finally conversion. the found first that The exampl effectshort-chain ofe 3-mercaptopropionicof photocatalyzed ligands or ligands acid-capped cyclization with low CdS ofbarrier QDs func- (CdS- potential tionalized difluoromethylboostedC3 QDs) the was photocatalyticchlorides better than and CdS-C6 co inactinversion. QDsvated and The CdS-C11olefins effect QDs.to of afford 3-mercaptopropionic the desired difluo- acid-capped romethylated CdSproducts QDsFeng (CdS-C3 with et al. [CdS32 QDs)] presented QDs was (diameterbetter the first th examplean CdS-C6= 3.5 of photocatalyzed± QDs 0.3 andnm) CdS-C11 as cyclization a photocatalyst QDs. of functional- (Scheme 5) by breakingized difluoromethyl the carbon(sp chlorides3)–chlorine and inactivated bond olefinsof difluoromethyl to afford the desired chlorides. difluoromethy- The lated products with CdS QDs (diameter = 3.5 ± 0.3 nm) as a photocatalyst (Scheme5) by reaction system couldbreaking tolerate the carbon(sp different3)–chlorine difluoromethyl bond of difluoromethyl chlorides as chlorides. fluorine The sources reaction and system be appropriate forcould a series tolerate of differentsubstrat difluoromethyles, which was chlorides efficient as fluorine for preparing sources and CF be2-containing appropriate for a series of substrates, which was efficient for preparing CF -containing azaheterocycles. azaheterocycles. 2

Scheme Scheme5. The reaction 5. The reaction for CdS for CdS QDs QDs photocatalytic photocatalytic cyclizationcyclization of ClCFof ClCF2COOEt2COOEt and oleﬁns. and olefins.

2.2. CdSe QDs 2.2. CdSe QDs Wu et al. [33], in Wu2013, et al. showed [33], in 2013,a highly-eff showed aicient highly-efficient and easily and controlled easily controlled process process for forthe the coupling of thiols using CdSe QDs (diameter = 1.9 nm) as a photocatalyst quantitatively coupling of thiolsand using selectively, CdSe withoutQDs (diameter sacrificial agents= 1.9 nm) or oxidants. as a photocatalyst Usually, the synthesis quantitatively of disulfides and selectively, withoutfrom thiols sacrificial needs stoichiometric agents or oxidants,oxidants. such Usually, as Oxone the or synthesis 2,3-Dichloro-5,6-dicyano-1,4- of disulfides from thiols benzoquinoneneeds stoichiometric (DDQ) [34,35]. CdSe QDsoxidants, are reusable such and suitableas for waterOxone systems or and 2,3-Dichloro-5,6-dicyano-1,4-benzoquinonestable in some reactions under aerobic(DDQ) conditions. [34,35]. CdSe Ingeniously, QDs are CdSe reusable QDs can and absorb disulfides on the surface to keep them from oxidizing. This work also proposed the suitable for water systems and stable in some reactions under aerobic conditions. Ingen- mechanism for photocatalytic transformation of thiols to disulfides and clear H2 (Scheme6). iously, CdSe QDsThe can process absorb can disulfides be facilitated on by promotingthe surface H2 toevolution keep them with the from addition oxidizing. of nickel This (II) ions work also proposedwith the turnovermechanism number for (TONs) photocat of thealytic QDs more transformation than 2000. Isotope of experimentsthiols to disul- showed that the hydrogen source was the solvent of water, not thiols in the H produced above. fides and clear H2 (Scheme 6). The process can be facilitated by promoting2 H2 evolution with the addition of nickel (II) ions with the turnover number (TONs) of the QDs more than 2000. Isotope experiments showed that the hydrogen source was the solvent of water, not thiols in the H2 produced above.

Scheme 6. The reaction for the photocatalytic synthesis of disulfides along with H2.

Then in 2017, Wu et al. [36] found that alcohols could selectively convert to aldehydes/ketones without external oxidants in water using 3-mercaptopropionic acid (MPA)-coordinated CdSe QDs (diameter = 2.3 nm) as a photocatalyst and hydrogen as the only by-product. (Scheme 7). The photocatalytic process is clean and highly efficient, with Ni2+ ions as cocatalyst. In this paper, a reactive radical was formed by a relay process with MPA, and water as relay reagents established a new opportunity to explore QDs as a photocatalyst for organic transformations. Additionally, benzyl alcohols were more easily oxidized compared with aliphatic alcohols, due to the less bond dissociation energy of the benzylic C–H bonds (Scheme 8).

Scheme 7. The conversion of alcohols to aldehydes/ketones with CdSe QDs.

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Catalysts 2021, 11, x FOR PEER REVIEW 5 of 14 Feng et al. [32] presented the first example of photocatalyzed cyclization of functionalized difluoromethyl chlorides and inactivated olefins to afford the desired difluo- romethylated products with CdS QDs (diameter = 3.5 ± 0.3 nm) as a photocatalyst (SchemeFeng 5) et by al. breaking [32] presented the carbon(sp the first3)–chlorine example ofbond photocatalyzed of difluoromethyl cyclization chlorides. of func- The tionalizedreaction system difluoromethyl could tolerate chlorides different and difluoromethyl inactivated olefins chlorides to afford as fluorine the desired sources difluo- and romethylatedbe appropriate products for a series with of substratCdS QDses, which(diameter was efficient= 3.5 ± for0.3 preparingnm) as a CF photocatalyst2-containing (Schemeazaheterocycles. 5) by breaking the carbon(sp3)–chlorine bond of difluoromethyl chlorides. The reaction system could tolerate different difluoromethyl chlorides as fluorine sources and be appropriate for a series of substrates, which was efficient for preparing CF2-containing azaheterocycles.

Scheme 5. The reaction for CdS QDs photocatalytic cyclization of ClCF2COOEt and olefins.

2.2. CdSe QDs SchemeWu 5. etThe al. reaction [33], in for2013, CdS showed QDs photocatalytic a highly-eff cyclizationicient and of easily ClCF 2controlledCOOEt and process olefins. for the coupling of thiols using CdSe QDs (diameter = 1.9 nm) as a photocatalyst quantitatively 2.2.and CdSe selectively, QDs without sacrificial agents or oxidants. Usually, the synthesis of disulfides fromWu etthiols al. [33], inneeds 2013, showedstoichiometric a highly-eff icientoxidants, and easilysuch controlled as processOxone for theor coupling2,3-Dichloro-5,6-dicyano-1,4-benzoquinone of thiols using CdSe QDs (diameter (DDQ) = 1.9 nm)[34,35]. as a CdSephotocatalyst QDs are quantitatively reusable and andsuitable selectively, for water without systems sacrificial and stable agents in so orme oxidants. reactions Usually, under theaerobic synthesis conditions. of disulfides Ingen- fromiously, CdSethiols QDs canneeds absorb stoichiometric disulfides on the surfaceoxidants, to keepsuch them fromas oxidizing.Oxone Thisor 2,3-Dichloro-5,6-dicyano-1,4-benzoquinonework also proposed the mechanism for photocat (DDQ)alytic [34,35]. transformation CdSe QDs areof thiols reusable to disul- and suitablefides and for clear water H 2systems (Scheme and 6). Thestable process in some can reactions be facilitated under by aerobic promoting conditions. H2 evolution Ingen- iously,with the CdSe addition QDs canof nickel absorb (II) disulfides ions with on the the turnover surface tonumber keep them (TONs) from of oxidizing. the QDs moreThis workthan 2000.also proposedIsotope experiments the mechanism showed for photocatthat the hydrogenalytic transformation source was theof thiols solvent to ofdisul- wa- fidester, not and thiols clear in H the2 (Scheme H2 produced 6). The above. process can be facilitated by promoting H2 evolution with the addition of nickel (II) ions with the turnover number (TONs) of the QDs more Catalysts 2021, 11, 275 than 2000. Isotope experiments showed that the hydrogen source5 of was 14 the solvent of water, not thiols in the H2 produced above.

Scheme 6. The reaction for the photocatalytic synthesis of disulfides along with H2.

Then in 2017, Wu et al. [36] found that alcohols could selectively convert to alde- SchemeSchemehydes/ketones 6. The 6. The reaction reaction forwithout the photocatalyticfor theexternal photocatalytic synthesis oxidants of disulﬁdes synthesis in alongwater of with disulfides Husing2. along3-mercaptopropionic with H2. acid (MPA)-coordinatedThen in 2017, Wu et al. CdSe [36] found QDs that (diameter alcohols could = 2.3 selectively nm) as convert a photocatalyst to aldehy- and hydrogen as des/ketonesthe onlyThen by-product. without in 2017, external Wu (Scheme oxidantset al. [36] in7). water Thefound using photocatal that 3-mercaptopropionic alcoholsytic process could acid isselectively (MPA)- clean and convert highly efficient,to alde- coordinatedhydes/ketoneswith Ni2+ CdSeionsQDs aswithout cocatalyst. (diameter external = 2.3 In nm) this as oxidantspaper, a photocatalyst a reactivein andwater hydrogenradical using was as the3-mercaptopropionic formed only by a relay process acid by-product. (Scheme7). The photocatalytic process is clean and highly efﬁcient, with Ni(MPA)-coordinatedwith2+ ions MPA, as cocatalyst. and water In CdSe this as paper, relay QDs a reactivereagents(diameter radical esta = wasblished2.3 formednm) a as bynew a a photocatalyst relay opportunity process toand explore hydrogen QDs asas withthea photocatalyst MPA,only andby-product. water asfor relay organic(Scheme reagents transformations. established 7). The photocatal a new opportunity Additionally,ytic process to explore is QDsbenzyl clean as a andalcohols highly were efficient, more photocatalystwitheasily Ni oxidized2+ ions for organic as comparedcocatalyst. transformations. withIn this Additionally,aliphatic paper, aalco benzylreactivehols, alcohols radicaldue were to wasthe more lessformed easily bond by dissociation a relay process en- oxidized compared with aliphatic alcohols, due to the less bond dissociation energy of the benzylicwithergy MPA,of C–H the bonds andbenzylic (Schemewater C–H 8as). relay bonds reagents (Scheme esta 8).blished a new opportunity to explore QDs as a photocatalyst for organic transformations. Additionally, benzyl alcohols were more easily oxidized compared with aliphatic alcohols, due to the less bond dissociation energy of the benzylic C–H bonds (Scheme 8). Catalysts 2021, 11, x FOR PEER REVIEW 6 of 14

SchemeScheme 7. The 7. conversionThe conversion of alcohols of to alcohols aldehydes/ketones to aldehydes/ketones with CdSe QDs. with CdSe QDs.

Scheme 7. The conversion of alcohols to aldehydes/ketones with CdSe QDs.

SchemeScheme 8. 8.The The selective selective oxidation oxidation of polyhydroxy of polyhydroxy compounds. compounds.

Early in 2017, Weix and Krauss [37] reported that oleate-capped CdSe QDs (diameter Early in 2017, Weix and Krauss [37] reported that oleate-capped CdSe QDs (diame- of about 3.3 nm) behaved well in visible-light-induced C–C bond-forming reactions. They thenter of conducted about 3.3 ﬁve nm) organic behaved transformation well in reactions visible-light-induced using lower loading C–C of CdSe bond-forming QDs to reactions. replaceThey then the traditional conducted catalysts five oforganic Ru or Ir transforma complexes (Schemetion reactions9). In some using cases, thelower TON loading of CdSe andQDs turnover to replace frequency the traditional (TOF) already catalysts surpassed of the Ru best or reported Ir complexes traditional (Scheme dyes. 9). In some cases, the EgapTON et and al. [ 38turnover] reported frequency that CdSe QDs (TOF) (diameter alread =y 3.0 surpassed nm) were successfully the best reported used traditional for light-mediated radical polymerization of (meth)acrylates and styrene to construct block copolymersdyes. with high conversion and narrow polydispersity (Scheme 10). QDs were applied as a photocatalyst for atom transfer radical polymerization (ATRP) to achieve various types of polymerization reactions. The authors also pointed out that QDs are a promising catalyst in the ﬁeld of photopolymerization due to their facile synthetic

Scheme 9. Five kinds of photoredox reactions using CdSe QDs.

Egap et al. [38] reported that CdSe QDs (diameter = 3.0 nm) were successfully used for light-mediated radical polymerization of (meth)acrylates and styrene to construct block copolymers with high conversion and narrow polydispersity (Scheme 10). QDs were applied as a photocatalyst for atom transfer radical polymerization (ATRP) to achieve various types of polymerization reactions. The authors also pointed out that QDs are a promising catalyst in the field of photopolymerization due to their facile synthetic and purification preparation, low catalyst loading, and ability to easily tune redox and electronic properties.

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Scheme 8. The selective oxidation of polyhydroxy compounds.

Catalysts 2021, 11, 275 Early in 2017, Weix and Krauss [37] reported that oleate-capped CdSe QDs (diame-6 of 14 ter of about 3.3 nm) behaved well in visible-light-induced C–C bond-forming reactions. They then conducted five organic transformation reactions using lower loading of CdSe QDs to replace the traditional catalysts of Ru or Ir complexes (Scheme 9). In some cases, andthe puriﬁcationTON and turnover preparation, frequency low (TOF) catalyst alread loading,y surpassed and ability the best to easilyreported tune traditional redox and electronicdyes. properties.

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SchemeScheme 9. 9.Five Five kinds kinds ofof photoredoxphotoredox reactions using using CdSe CdSe QDs. QDs.

SchemeScheme 10.10. ControlledControlled photomediatedphotomediated ATRPATRP for for different different monomer monomer facilitated facilitated by by CdSe CdSe QDs. QDs.

MaitiMaiti etet al.al. [[39]39] reportedreported aa newnew methodmethod forfor borylationborylation ofof diazoniumdiazonium salts,salts, whichwhich waswas catalyzedcatalyzed 3-Mercaptopropionic3-Mercaptopropionic acidacid (MPA)-capped(MPA)-capped byby CdSeCdSe QDsQDs (diameter(diameter == 4.04.0 nm)nm) inin waterwater withoutwithout anan externalexternal additive.additive. TheyThey appliedapplied anan acidicacidic conditioncondition ofof pHpH == 6,6, notnot onlyonly preventingpreventing thethe formationformation ofof by-productby-product disulﬁde,disulfide, but butalso alsoavoiding avoidingthe thedecomposition decompositionof of diazoniumdiazonium saltsalt inin organicorganic solventssolvents byby aa biphasicbiphasic mixturemixture ofof waterwater andand dichloromethane.dichloromethane. TheThe correspondingcorresponding biphenylbiphenyl productsproducts werewere obtainedobtained byby photocatalyticphotocatalytic couplingcoupling ofof ﬁnalfinal boraneborane productsproducts oror boricboric acidacid byby usingusing CdSeCdSe QDsQDs (Scheme(Scheme 11 11).). Additionally,Additionally, thethe photophoto 5 catalyticcatalytic reactionreaction systemsystem hashas aa veryvery highhigh TON TON value value of of larger larger than than 10 105.. In 2019, Cossairt et al. [40] disclosed a selective photocatalysis with the aid of CdSe QDs (diameter = 3.3 nm) for the fracture of the Cα–O bond in lignin model substrates (Scheme 12). Compared to the iridium complexes, less than 333 times the QDs were needed to produce the same reaction rate. Another highlight of this work is that the joining of shorter-chain trans-4-cyanocinnamic acid ligand on the QDs’ surface can greatly improve

the conversion rate due to its better rigidityB(OH)2 and conjugated structure. MPA-CdSe QDs

DCM-water Blue LED isolated, 87% BPin B(OH)2 MPA-CdSe QDs + DCM-water Blue LED [1:1] OMe only product

(a) (b) Scheme 11. (a) The photocatalytic borylation of diazonium salts using MPA-capped CdSe QDs in water; (b) MPA-CdSe QDs catalyze the coupling of borylation.

In 2019, Cossairt et al. [40] disclosed a selective photocatalysis with the aid of CdSe QDs (diameter = 3.3 nm) for the fracture of the Cα–O bond in lignin model substrates (Scheme 12). Compared to the iridium complexes, less than 333 times the QDs were needed to produce the same reaction rate. Another highlight of this work is that the

Catalysts 2021, 11, x FOR PEER REVIEW 7 of 14

Scheme 10. Controlled photomediated ATRP for different monomer facilitated by CdSe QDs.

Maiti et al. [39] reported a new method for borylation of diazonium salts, which was catalyzed 3-Mercaptopropionic acid (MPA)-capped by CdSe QDs (diameter = 4.0 nm) in water without an external additive. They applied an acidic condition of pH = 6, not only preventing the formation of by-product disulfide, but also avoiding the decomposition of diazonium salt in organic solvents by a biphasic mixture of water and dichloromethane. The corresponding biphenyl products were obtained by photocatalytic coupling of final Catalysts 2021, 11, 275 borane products or boric acid by using CdSe QDs (Scheme 11). Additionally, the photo7 of 14 catalytic reaction system has a very high TON value of larger than 105.

B(OH)2 MPA-CdSe QDs

DCM-water Blue LED isolated, 87% BPin B(OH)2 MPA-CdSe QDs + DCM-water Blue LED [1:1] OMe only product

Catalysts 2021, 11, x FOR PEER REVIEW 8 of 14

(a) (b)

Scheme 11. (a) The photocatalyticphotocatalyticjoining of borylation shorter-chain of diazonium trans-4-cyanocinnamic salts using MPA-capped acidCdSe QDs ligand in water; on the ( b) MPA-CdSeQDs’ surface can QDs catalyze the coupling of borylation. QDs catalyze the couplinggreatly of borylation. improve the conversion rate due to its better rigidity and conjugated structure. In 2019, Cossairt et al. [40] disclosed a selective photocatalysis with the aid of CdSe QDs (diameter = 3.3 nm) for the fracture of the Cα–O bond in lignin model substrates (Scheme 12). Compared to the iridium complexes, less than 333 times the QDs were needed to produce the same reaction rate. Another highlight of this work is that the

SchemeScheme 12. 12.TheThe oxidation oxidation and and photochemical photochemical reductionreduction of of a lignina lignin model model substrate. substrate.

PhotoinducedPhotoinduced enantioselective enantioselective intermolecularintermolecular [2+2] [2+2] photocycloadditions photocycloadditions have have been been extensivelyextensively researched. researched. Weiss Weiss et al.al. [41[41]] achieved achieved regioselective regioselective and and diastereoselective diastereoselective intermolecular [2+2] cycloadditions of 4-vinylbenzoic acid derivatives using CdSe QDs intermolecular [2+2] cycloadditions of 4-vinylbenzoic acid derivatives using CdSe QDs (diameter = 2.8 nm) as photocatalysis (Scheme 13). Apart from acting as photosensitizer, (diameterthe QDs = here 2.8 werenm) alsoas photocatalysis triplet exciton donors (Scheme and 13). self-assembly Apart from scaffolds, acting ofas which photosensitizer, triplet theenergy QDs here was were tunable also by triplet controlling exciton QDs’ donors size to selectivelyand self-assembly sensitizeone scaffolds, component of which among triplet energythe reaction was tunable system ofby cycloadditions. controlling InQDs’ this case,size 1.4to nmselectively CdSe QDs sensitize selectively one transferred component amongenergy the to reaction1 without system sensitizing of 3,cycloadditions.while 1.0 nm CdSe In QDsthis andcase, noble 1.4 nm metal CdSe complexes QDs selectively could transferrednot achieve energy that (Scheme to 1 without 13). Notably, sensitizing CdSe 3, QD while photocatalysis 1.0 nm CdSe provided QDs and non-normal noble metal complexessyn products could with not a diastereomericachieve that (Scheme ratio of >40:1, 13). Notably, while the metalCdSe complexesQD photocatalysis tended to pro- videdgenerate non-normal antiproducts syn withproducts diastereomeric with a ratiodiastereomeric of <12:1. They ratio found of that>40:1, it resulted while fromthe metal the carboxylate of the substrate absorbed to the QD surface, creating an intermolecular π–π complexes tended to generate antiproducts with diastereomeric ratio of <12:1. They interaction among the 4-vinylbenzoic acid derivatives occur. Additionally, head-to-head found(HH) that products it resulted or head-to-tail from the (HT) carboxylate products wereof the selectively substrate generated absorbed by to adjusting the QD thesurface, creatingposition an of intermolecular carboxylate on the π– substrates.π interaction It was among also associated the 4-vinylbenzoic with the interaction acid derivatives of the occur.carboxylate Additionally, and Cd 2+head-to-headon the QD surface. (HH) products or head-to-tail (HT) products were se- lectivelyIn generated 2019, Weiss by et al.adjusting [42] presented the position a series of of carboxylate aqueous acrylamides on the substrates. and acrylates It thatwas also associatedpolymerized with thoughthe interaction photoinduced of the electron carboxylate transfer and reversible Cd2+ on addition-fragmentationthe QD surface. chain transfer (PET-RAFT) with high efﬁciency, low dispersity, and ultralow catalyst loading (Scheme 14). The water-soluble MPA-capped CdSe QDs (diameter = 2.8 nm) photocatalyst could be separated from the system by using protein concentrators and

Scheme 13. Cycloadditions of 4-vinylbenzoic acid derivatives by the photocatalysis with CdSe QDs.

Catalysts 2021, 11, x FOR PEER REVIEW 8 of 14

joining of shorter-chain trans-4-cyanocinnamic acid ligand on the QDs’ surface can greatly improve the conversion rate due to its better rigidity and conjugated structure.

Scheme 12. The oxidation and photochemical reduction of a lignin model substrate.

Photoinduced enantioselective intermolecular [2+2] photocycloadditions have been extensively researched. Weiss et al. [41] achieved regioselective and diastereoselective intermolecular [2+2] cycloadditions of 4-vinylbenzoic acid derivatives using CdSe QDs (diameter = 2.8 nm) as photocatalysis (Scheme 13). Apart from acting as photosensitizer, the QDs here were also triplet exciton donors and self-assembly scaffolds, of which triplet energy was tunable by controlling QDs’ size to selectively sensitize one component among the reaction system of cycloadditions. In this case, 1.4 nm CdSe QDs selectively transferred energy to 1 without sensitizing 3, while 1.0 nm CdSe QDs and noble metal complexes could not achieve that (Scheme 13). Notably, CdSe QD photocatalysis provided non-normal syn products with a diastereomeric ratio of >40:1, while the metal

Catalysts 2021, 11, 275 complexes tended to generate antiproducts with diastereomeric ratio of <12:1. 8They of 14 found that it resulted from the carboxylate of the substrate absorbed to the QD surface, creating an intermolecular π–π interaction among the 4-vinylbenzoic acid derivatives occur. Additionally, head-to-head (HH) products or head-to-tail (HT) products were se- lectivelyreused four generated times with by adjusting high activity, the position and the of monomer carboxylate in the on reactionthe substrates. mixture It couldwas also be associatedisolated selectively with the byinteraction different of pore the sizescarboxylate of protein and concentrators Cd2+ on the QD in the surface. same way.

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 14 Scheme 13. Cycloadditions of 4-vinylbenzoic acid derivativesderivatives by the photocatalysis withwith CdSeCdSe QDs.QDs.

In 2019, Weiss et al. [42] presented a series of aqueous acrylamides and acrylates that polymerized though photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) with high efficiency, low dispersity, and ultralow catalyst loading (Scheme 14). The water-soluble MPA-capped CdSe QDs (diameter = 2.8 nm) photocatalyst could be separated from the system by using protein concentrators and reused four times with high activity, and the monomer in the reaction mixture could be SchemeScheme 14. 14.Photoinduced Photoinduced electron electron transfer transfer radical radical reversible reversible addition–fragmentation addition–fragmentation chain chain transfer trans- isolated selectively by different pore sizes of protein concentrators in the same way. Catalysts 2021, 11, x FOR PEER REVIEW(PET-RAFT)fer (PET-RAFT) polymerization polymerization catalyzed catalyzed by CdSe by CdSe QDs. QDs. 9 of 14 Following that, Egap et al. [43] also designed a PET-RAFT polymerization to syn- thesize2.3. ColloidalFollowing core-shell Core/Shell that, polymer-QDs Egap QDs et al.[ 43(diameter] also designed = 3.8 nm) a PET-RAFT nanocomposites polymerization using CdSe to synthe- quan- tum dots in a polar solvent. Herein, CdSe QDs (diameter = 3.2 nm) were not only the size2.3.1. core-shell ZnSe/CdS polymer-QDs QDs (diameter = 3.8 nm) nanocomposites using CdSe quantum dots photocatalyst,in a polar solvent. but Herein,also the CdSe inorganic QDs (diameter building = block 3.2 nm) for were nanoparticle–polymer not only the photocatalyst, hybrid In the abovementioned reactions, plain-core QDs were used as photocatalysts. nanocomposites.but also the inorganic CdSe buildingQDs were block stable for in nanoparticle–polymer polar solvents such as hybrid Dimethyl nanocomposites. Formamide However, aggregation and etching strongly influence the stability of the plain-core QDs, (DMF)CdSe QDs and were dimethylsulfoxide stable in polar solvents (DMSO) such due as to Dimethyl the ligand Formamide exchange (DMF) by chain and dimethyl- transfer so in most works, they are needed to design the ligands carefully in the reaction to im- agentssulfoxide on (DMSO)the surface due of toCdSe the ligandQDs. exchange by chain transfer agents on the surface of Schemeprove the14. Photoinduced stability of electronthe catalyst. transfer The radical preparat reversibleion addition–fragmentation of QDs into core-shell chain structurestrans- is an CdSefer (PET-RAFT) QDs. polymerization catalyzed by CdSe QDs. alternative way to stabilize QDs. In 2017, König and et al. [44] applied ZnSe/CdS 2.3.2.3.core/shell Colloidal QDs Core/Shell Core/Shell (diameter QDs QDs = 4.0 nm) as the photocatalyst to activate the C–X bonds to obtain arylation products. (Hetero)Aryl halides obtained electrons from the active QDs and 2.3.1.2.3.1. ZnSe/CdS ZnSe/CdS QDs QDs then dehalogenations to form the corresponding (hetero)-aryl radicals. They followed by In thethe abovementionedabovementioned reactions, reactions, plain- plain-corecore QDs QDs were were used used as as photocatalysts. photocatalysts. How- obtaining a hydrogen atom from the radical cation of DIPEA to be the reduced product, ever,However, aggregation aggregation and and etching etching strongly strongly inﬂuence influence thethe stability of of the the plain-core plain-core QDs, QDs, so in mostsoor into works,mostform works, the they C–H they are arylated neededare needed to product design to design thewith the ligands the ligands participation carefully carefully in inof the the some reaction reaction pyrroleto to improveim- derivatives the stabilityprove(Scheme the of stability15). the catalyst. of the catalyst. The preparation The preparat of QDsion of into QDs core-shell into core-shell structures structures is an is alternativean wayalternative to stabilize way to QDs. stabilize In 2017, QDs. König In 2017, and König et al. and [44] et applied al. [44] ZnSe/CdS applied ZnSe/CdS core/shell QDs (diametercore/shell QDs = 4.0 (diameter nm) as = the 4.0 photocatalystnm) as the photocatalyst to activate to theactivate C–X the bonds C–X bonds to obtain to ob- arylation products.tain arylation (Hetero)Aryl products. (Hetero)Aryl halides obtained halides obtained electrons electrons from the from active the active QDs andQDsthen and dehalo- then dehalogenations to form the corresponding (hetero)-aryl radicals. They followed by genations to form the corresponding (hetero)-aryl radicals. They followed by obtaining a obtaining a hydrogen atom from the radical cation of DIPEA to be the reduced product, hydrogenor to form atomthe C–H from arylated the radical product cation with of the DIPEA participation to be the of reducedsome pyrrole product, derivatives or to form the C–H(Scheme arylated 15). product with the participation of some pyrrole derivatives (Scheme 15).

Scheme 15. The reaction for dehalogenation and C–H arylation reactions.

2.3.2. CdSe/CdS QDs In 2018, Shen et al. [15] aimed to use a one-pot protocol for photoreduction of imines to corresponding amines in the presence of CdSe/CdS core/shell QDs (diameter = SchemeScheme3.0 nm) 15. 15. and TheThe reactionthiophenol reaction for for dehalogenation as dehalogenation the hydrogen and andC–H donor. C–Harylation arylation The reactions. reaction reactions. system has the advantage of wide substrate applicability, high yield, easy amplification, and a high TON value, 2.3.2.and canCdSe/CdS be used QDs for synthesis of butenafine (Scheme 16). In 2018, Shen et al. [15] aimed to use a one-pot protocol for photoreduction of imines to corresponding amines in the presence of CdSe/CdS core/shell QDs (diameter = 3.0 nm) and thiophenol as the hydrogen donor. The reaction system has the advantage of wide substrate applicability, high yield, easy amplification, and a high TON value, and can be used for synthesis of butenafine (Scheme 16). Scheme 16. The reduction of imines with CdSe/CdS core/shell QDs.

There are many reports on the photoreduction of carbon dioxide using quantum dots in the recent years. In 2019, Wu et al. [45] reported the combination of the photore- Schemeduction 16. of The CO reduction2-to-CO of iminesconversion with CdSe/CdS and oxidative core/shell QDs.organic transformation (Scheme 17). They used stable and high-activity CdSe/CdS core/shell quantum dots (diameter = 2.0 nm) as aThere photocatalyst are many reportsto produce on the photoreduca gas prodtionuct of ofcarbon CO diandoxide dimerization using quantum product of dots in the recent years. In 2019, Wu et al. [45] reported the combination of the photoreduction of CO2-to-CO conversion and oxidative organic transformation (Scheme 17). They used stable and high-activity CdSe/CdS core/shell quantum dots (diameter = 2.0 nm) as a photocatalyst to produce a gas product of CO and dimerization product of

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 14

Scheme 14. Photoinduced electron transfer radical reversible addition–fragmentation chain transfer (PET-RAFT) polymerization catalyzed by CdSe QDs.

2.3. Colloidal Core/Shell QDs 2.3.1. ZnSe/CdS QDs In the abovementioned reactions, plain-core QDs were used as photocatalysts. However, aggregation and etching strongly influence the stability of the plain-core QDs, so in most works, they are needed to design the ligands carefully in the reaction to improve the stability of the catalyst. The preparation of QDs into core-shell structures is an alternative way to stabilize QDs. In 2017, König and et al. [44] applied ZnSe/CdS core/shell QDs (diameter = 4.0 nm) as the photocatalyst to activate the C–X bonds to obtain arylation products. (Hetero)Aryl halides obtained electrons from the active QDs and then dehalogenations to form the corresponding (hetero)-aryl radicals. They followed by obtaining a hydrogen atom from the radical cation of DIPEA to be the reduced product, or to form the C–H arylated product with the participation of some pyrrole derivatives (Scheme 15).

Catalysts 2021, 11, 275 Scheme 15. The reaction for dehalogenation and C–H arylation reactions. 9 of 14

2.3.2. CdSe/CdS QDs 2.3.2. CdSe/CdSIn 2018, QDsShen et al. [15] aimed to use a one-pot protocol for photoreduction of iminesIn 2018, to corresponding Shen et al. [15] aimed amines to use in a the one-pot pres protocolence of for CdSe/CdS photoreduction core/shell of imines QDs (diameter = to corresponding amines in the presence of CdSe/CdS core/shell QDs (diameter = 3.0 nm) and3.0 thiophenolnm) and thiophenol as the hydrogen as donor.the hydrogen The reaction donor. system The has reaction the advantage system of wide has the advantage substrateof wide applicability,substrate applicability, high yield, easy high ampliﬁcation, yield, easy and a amplification, high TON value, and can a high be TON value, usedand forcan synthesis be used of for butenaﬁne synthesis (Scheme of butenafine 16). (Scheme 16).

SchemeScheme 16. 16.The The reduction reduction of imines of imines with CdSe/CdS with CdSe/CdS core/shell core/shell QDs. QDs.

There are many reports on the photoreduction of carbon dioxide using quantum dots Catalysts 2021, 11, x FOR PEER REVIEWin the There recent years. are many In 2019, reports Wu et al. on [45 ]the reported photoreduc the combinationtion of of carbon the photoreduction dioxide using10 of quantum14

Catalysts 2021, 11, x FOR PEER REVIEWofdots CO 2in-to-CO the recent conversion years. and In oxidative 2019, Wu organic et al. transformation [45] reported (Scheme the combination 17). They10 used of 14 of the photore-

stableduction and of high-activity CO2-to-CO CdSe/CdS conversion core/shell and quantumoxidative dots or (diameterganic transformation = 2.0 nm) as a (Scheme 17). photocatalystThey used stable to produce and a high-activity gas product of COCdSe/CdS and dimerization core/shell product quantum of mono-alcohol dots (diameter = 2.0 nm) simultaneously,mono-alcohol whichsimultaneously, made full use which of excited made electrons full use and of excited holes. It electrons must be emphasized and holes. It must asmono-alcohol a photocatalyst simultaneously, to produce which made a gasfull useprod of exciteduct of electrons CO and and holes.dimerization It must product of thatbe emphasized appropriate introduction that appropriate of CdS layersintroduction simultaneously of CdS facilitated layers simultaneously CO2 reduction and facilitated be emphasized that appropriate introduction of CdS layers simultaneously facilitated CCOα–H2 reduction activation. and Cα–H activation. CO2 reduction and Cα–H activation.

Scheme 17. 17. Photoreduction Photoreduction of COof CO2-to-CO2-to-CO conversion conversion and oxidative and oxidative coupling coupling of mono-alcohol. of mono-alcohol. Scheme 17. Photoreduction of CO2-to-CO conversion and oxidative coupling of mono-alcohol.

Later,Later,Later, in in 2021,in 2021, 2021, Shen Shen Shen et al. et [46etal.] al. used[46] [46] used the used same the CdSe/CdSsamethe same CdSe/CdS core/shellCdSe/CdS core/shell QDs core/shell(diameter QDs (diameter QDs = 3.0 nm)(diameter = = to3.0 selectively nm)nm) toto selectively selectively produce either produce produce alcohols either either or alcohols pinacol alcohols productsor pinacol or pinacol from products aryl products aldehydes from aryl from and aldehydes ketones.aryl aldehydes Here,and ketones.ketones. thiophenols Here, Here, are thiophenols not thiophenols only proton are and arenot hydrogen notonly only proton donors, proton and but hydrogenand also hydrogen the holedonors, scavengers donors, but also of but thethe also the excitedhole scavengers QDs. of the excited QDs. hole scavengers of the excited QDs. CdSe/CdSCdSe/CdS core/shell QDs QDs were were used as photocatalystsphotocatalysts inin thisthis selectiveselective reductionreduction reaction.reaction.CdSe/CdS Yet,Yet, it it is is surprisingcore/shell surprising that QDsthat the thewere reaction reacti usedon selectivity selectivity as photocatalysts could could be controlledbe controlled in this by selective controllingby control- reduction thereaction.ling amount the amount Yet, of thiophenol it ofis thiophenolsurprising or use or ofthat use base the of (Scheme basereacti (Schemeon 18 ).selectivity Additionally, 18). Additionally, could the be TON controlledthe of TON reduction of byre- control- tolingduction alcohols the to amount isalcohols up to of 4 is× thiophenolup10 4to, while4 × 10 that4or, while use of pinacol ofthat base of couplingpinacol (Scheme coupling is up18). to Additionally, 4is× up10 to5. Additionally,4 × 10 5the. Addi- TON of re- CdSe/CdSductiontionally, CdSe/CdSto quantum alcohols quantum dotsis up can to dots be4 × reused 10can4, bewhile ten reused times that ten whileof timespinacol maintaining while coupling maintaining the is same up catalytictothe 4 same × 10 5. Addi- effectstionally,catalytic in theeffectsCdSe/CdS pinacol in the coupling quantumpinacol reactions.coupling dots can reactions. be reused ten times while maintaining the same catalytic effects in the pinacol coupling reactions.

SchemeScheme 18.18. TheThe selectiveselective transformationtransformation of of aryl aryl aldehydes aldehydes and and ketones ketones by by the the amount amount of of thiophenol thio- usingphenol CdSe/CdS using CdSe/CdS core/shell core/s QDshell as QDs photocatalysts. as photocatalysts. Scheme 18. The selective transformation of aryl aldehydes and ketones by the amount of thio- phenol3. Cadmium-Free using CdSe/CdS QDs core/shell QDs as photocatalysts. 3.1. Colloidal Core/Shell QDs 3.3.1.1. Cadmium-Free InP/ZnS QDs QDs 3.1. ColloidalAlthough Core/Shell there have QDs some exciting results reported using QDs as the photocatalyst for organic reactions, most of them used toxic metal-ion-based QDs such as cadmium. 3.1.1. InP/ZnS QDs Even though the amount of catalyst is limited, it may cause some environmental problems.Although Some examples there withhave moresome environmen exciting resulttally sfriendly reported QDs using were QDs demonstrated as the photocatalyst in forrecent organic years. reactions, In 2019, Pillaimost etof al.them [47] used presented toxic metal-ion-baseda C–C coupling reactionQDs such between as cadmium. Even1-phenyl though pyrrolidine the amount and phenyl-trans-styryl of catalyst is lim sulfone,ited, it maywhich cause was somecatalyzed environmental by cadmi- prob- lems.um-free Some QDs InP/ZnSexamples (diameter with more = 2.9 environmen ± 0.3 nm) withouttally friendlycocatalysts QDs or sacrificial were demonstrated agents. in recentWhite-light years. illumination In 2019, provedPillai etthat al. InP/ZnS [47] presented QDs can be a usedC–C to coupling solar photocatalysis. reaction between 1-phenylThey also pyrrolidine transformed and ferricyanide phenyl-trans-styryl to ferrocyanide sulfone, with which the washelp catalyzed of anionic by cadmium-free11-mercaptoundecanoic QDs InP/ZnS acid-capped(diameter = InP/ZnS 2.9 ± 0.3 QDs nm) (Scheme without 19). cocatalysts or sacrificial agents. White-light illumination proved that InP/ZnS QDs can be used to solar photocatalysis. They also transformed ferricyanide to ferrocyanide with the help of anionic 11-mercaptoundecanoic acid-capped InP/ZnS QDs (Scheme 19).

Catalysts 2021, 11, 275 10 of 14

3. Cadmium-Free QDs 3.1. Colloidal Core/Shell QDs 3.1.1. InP/ZnS QDs Although there have some exciting results reported using QDs as the photocatalyst for organic reactions, most of them used toxic metal-ion-based QDs such as cadmium. Even though the amount of catalyst is limited, it may cause some environmental problems. Some examples with more environmentally friendly QDs were demonstrated in recent years. Catalysts 2021, 11, x FOR PEER REVIEW 11 of 14 In 2019, Pillai et al. [47] presented a C–C coupling reaction between 1-phenyl pyrrolidine and phenyl-trans-styryl sulfone, which was catalyzed by cadmium-free QDs InP/ZnS (diameter = 2.9 ± 0.3 nm) without cocatalysts or sacriﬁcial agents. White-light illumination Catalysts 2021, 11, x FOR PEER REVIEWproved that InP/ZnS QDs can be used to solar photocatalysis. They also transformed 11 of 14 ferricyanide to ferrocyanide with the help of anionic 11-mercaptoundecanoic acid-capped InP/ZnS QDs (Scheme 19).

Scheme 19. InP/ZnS QDs photocatalyzed the ferricyanide reduction and the C–C coupling reaction.

SchemeScheme3.1.2. 19. CuInS19.InP/ZnS InP/ZnS2/ QDsZnS QDs photocatalyzed QDs photocatalyzed the ferricyanide the ferricyanid reduction ande reduction the C–C coupling and the reaction. C–C coupling reaction. Recently, Weiss et al. [48] reported a photocatalytic deprotection of the aryl sul- 3.1.2. CuInS2/ ZnS QDs fonatesRecently, by Weiss using et al. [CuInS48] reported2/ZnS a photocatalyticcore/shell QDs deprotection (diameter of the aryl= 3.2 sulfonates nm) as photocatalysts 3.1.2. CuInS2/ ZnS QDs by using(Scheme CuInS 20).2/ZnS It was core/shell found QDs that (diameter a substrate = 3.2 nm)containing as photocatalysts a QD-binding(Scheme group20). improved the It wasrateRecently, found of deprotection. that aWeiss substrate et Thus, containingal. [48] the reportedsubstrate a QD-binding awas phot group easilyocatalytic improved absorbed thedeprotection rate on ofthe depro- surface of the of thearyl QDs sul- to tection. Thus, the substrate was easily absorbed on the surface of the QDs to promote the fonatespromote by theusing two CuInSelectrons’2/ZnS transfer. core/shell The QDsresults (diameter showed that= 3.2 the nm)deprotection as photocatalysts is selective. two electrons’ transfer. The results showed that the deprotection is selective. The sulfonyl The sulfonyl groups with electron-withdrawing substituents are easily removed when groups(Scheme with 20). electron-withdrawing It was found that substituents a substrate are easily containing removed a when QD-binding they coexist group with improved the tert-butoxycarbonylratethey of coexistdeprotection. with (Boc-) tert-butoxycarbonyl Thus, and toluenesulfonyl-protecting the substrate (Boc-)was easily and groups, toluenesulfonyl-protecting absorbed even withon the proximate surface of groups,the QDs even to ketones.promotewith proximate the two electrons’ ketones. transfer. The results showed that the deprotection is selective. The sulfonyl groups with electron-withdrawing substituents are easily removed when they coexist with tert-butoxycarbonyl (Boc-) and toluenesulfonyl-protecting groups, even with proximate ketones.

SchemeScheme 20. Reaction20. Reaction for CuInS for 2CuInS/ZnS core/shell2/ZnS core/shell QDs photocatalyzing QDs photocatalyzing deprotection deprotection of the aryl of the aryl sul- sulfonates.fonates. 3.2. Halide Perovskite QDs Scheme3.2.The Halide perovskite20. Reaction Perovskite quantum for CuInSQDs dots 2/ZnS have core/shell received extensive QDs photocatalyzing attention from researchersdeprotection in of the aryl sul- recentfonates. yearsThe due perovskite to their high quantum absorption dots coefﬁcient have received and long extensive carrier lifetime, attention and they from researchers in actrecent as the staryears materials due to intheir photovoltaic high absorption ﬁeld. In 2017,coefficient Tüysüz and et al. long [49] carrier showed lifetime, the and they act photocatalytic polymerization reaction of 2,20,50,2”-ter-3,4-ethylenedioxythiophene was 3.2.as Halide the star Perovskite materials QDs in photovoltaic field. In 2017, Tüysüz et al. [49] showed the photo- triggered by CsPbBr3 QDs (length of long side = 8.9 nm, length of short side = 7.8 nm) undercatalyticThe visible perovskite light,polymerization with 1,4-benzoquinonequantum reaction dots have as of the 2,2received electron′,5′,2″ acceptor-ter-3,4-ethylenedioxythiophene extensive that maintainedattention QDs’from researchers was trig-in recentgered years by CsPbBr due to 3their QDs high (length absorption of long sidecoef ficient= 8.9 nm, and length long carrier of short lifetime, side = and7.8 nm) they under act asvisible the star light, materials with 1,4-benzoquinone in photovoltaic field. as the In electron2017, Tüysüz acceptor et al. that [49] maintained showed the QDs’ photo- cubic catalyticphase (Schemepolymerization 21). The reaction polymerization of 2,2′,5 reacti′,2″-ter-3,4-ethylenedioxythiopheneon was promoted as the concentrations was trig- of geredthe CsPbIby CsPbBr3 QDs3 QDs increased. (length It of is long notable side that= 8.9 thenm, CsPbI length3 QDsof short entered side =into 7.8 nm)the polymerunder visiblenetworks light, that with had 1,4-benzoquinone formed to change as intothe electronthe QD–polymer acceptor composite.that maintained QDs’ cubic phase (Scheme 21). The polymerization reaction was promoted as the concentrations of the CsPbI3 QDs increased. It is notable that the CsPbI3 QDs entered into the polymer networks that had formed to change into the QD–polymer composite.

Scheme 21. Photocatalytic polymerization reaction of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene using CsPbI3 QDs.

SchemeIn 21. 2020, Photocatalytic Chen et polymerizational. [50] reported reaction that ofthey 2,2 ′,5employed′,2″-ter-3,4-ethylenedioxythiophene CsPbBr3 nanocrystals (NCs,us- ingedge CsPbI length3 QDs. < 10 nm) of the longer-lived charge separated state to form a C–C bond between 2-bromoacetophenone and octanal (Scheme 22). In situ enamine formed by octanal andIn dicyclohexylamine 2020, Chen et al. [50] had reported holes to thatgenerate they employedradical cation, CsPbBr to 3create nanocrystals a reaction (NCs, with edge length < 10 nm) of the longer-lived charge separated state to form a C–C bond between 2-bromoacetophenone and octanal (Scheme 22). In situ enamine formed by octanal and dicyclohexylamine had holes to generate radical cation, to create a reaction with

Catalysts 2021, 11, x FOR PEER REVIEW 11 of 14

Scheme 19. InP/ZnS QDs photocatalyzed the ferricyanide reduction and the C–C coupling reaction.

3.1.2. CuInS2/ ZnS QDs Recently, Weiss et al. [48] reported a photocatalytic deprotection of the aryl sulfonates by using CuInS2/ZnS core/shell QDs (diameter = 3.2 nm) as photocatalysts (Scheme 20). It was found that a substrate containing a QD-binding group improved the rate of deprotection. Thus, the substrate was easily absorbed on the surface of the QDs to promote the two electrons’ transfer. The results showed that the deprotection is selective. The sulfonyl groups with electron-withdrawing substituents are easily removed when they coexist with tert-butoxycarbonyl (Boc-) and toluenesulfonyl-protecting groups, even with proximate ketones.

Scheme 20. Reaction for CuInS2/ZnS core/shell QDs photocatalyzing deprotection of the aryl sulfonates.

3.2. Halide Perovskite QDs The perovskite quantum dots have received extensive attention from researchers in recent years due to their high absorption coefficient and long carrier lifetime, and they act as the star materials in photovoltaic field. In 2017, Tüysüz et al. [49] showed the photocatalytic polymerization reaction of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene was trig- Catalysts 2021, 11, 275 11 of 14 gered by CsPbBr3 QDs (length of long side = 8.9 nm, length of short side = 7.8 nm) under visible light, with 1,4-benzoquinone as the electron acceptor that maintained QDs’ cubic cubicphase phase (Scheme (Scheme 21). 21). The polymerizationpolymerization reaction reacti wason promoted was promoted as the concentrations as the concentrations of 3 3 ofthe the CsPbI CsPbI3 QDsQDs increased. increased. It is notableIt is notable that the that CsPbI the3 QDs CsPbI entered QDs into theentered polymer into the polymer networksnetworks that that had had formed formed to change to change into the QD–polymerinto the QD–polymer composite. composite.

0 0 Catalysts 2021, 11, x FOR PEER REVIEWSchemeScheme 21. 21.Photocatalytic Photocatalytic polymerization polymerization reaction of reaction 2,2 ,5 ,2”-ter-3,4-ethylenedioxythiophene of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene using 12 of us- 14

CsPbIing CsPbI3 QDs.3 QDs.

Catalysts 2021, 11, x FOR PEER REVIEW In 2020, Chen et al. [50] reported that they employed CsPbBr nanocrystals (NCs, 12 of 14 3 edge lengthIn 2020, < 10 Chen nm) ofet the al. longer-lived [50] reported charge that separated they employed state to form CsPbBr a C–C3 bondnanocrystals (NCs, acetophenone free radical that formed by 2-bromoacetophenone obtaining electrons from betweenedge length 2-bromoacetophenone < 10 nm) of the and longer-lived octanal (Scheme charge 22). separated In situ enamine state formedto form by a C–C bond be- octanalCsPbBrtweenand 2-bromoacetophenone3 nanocrystals dicyclohexylamine (NCs) had and and holes releasing octanal to generate (Schemebromine radical 22).anions. cation, In tositu create enamine a reaction formed by octanal withandacetophenone acetophenone dicyclohexylamine free free radical radical had that that formedholes formed to by 2-bromoacetophenonebygenerate 2-bromoacetophenone radical cation, obtaining to obtaining electronscreate a electronsreaction fromwith fromCsPbBr CsPbBr3 nanocrystals3 nanocrystals (NCs) (NCs) and and releasing releasing bromine bromine anions. anions.

Scheme 22. The construction of the C–C bond between 2-bromoacetophenone and octanal.

SchemeSchemeVery 22. 22.The recently, The construction construction Chen of the et C–Cof al. the bond [51] C–C between used bond 2-bromoacetophenonezwitterionic between 2-bromoacetophenone ligand-capped and octanal. CsPbBr and octanal.3 perovskite QDs (edge length = 10.6 ± 1.2 nm) as photocatalysts to realize the stereoselective C–C Very recently, Chen et al. [51] used zwitterionic ligand-capped CsPbBr3 perovskite QDsoxidative (edgeVery length dimerizationrecently, = 10.6 Chen± 1.2of etnm)α -arylal. as [51] photocatalysts ketonitriles. used zwitterionic to 3-(N,N-dimethyloctadecylammonio) realize theligand-capped stereoselective CsPbBr C–C 3 perovskite pro- oxidativepanesulfonateQDs (edge dimerization length as the of=α zwitterionic10.6-aryl ketonitriles.± 1.2 nm) ligand 3-(N,N-dimethyloctadecylammonio)as photocatalysts not only overcame to realize the QDsthe propane- stereoselectivestability and reuse C–C sulfonateissues,oxidative but as the dimerizationalso zwitterionic reduced ligand ofthe α reaction-aryl not only ketonitriles. overcametime and the improved3-(N,N-dimethyloctadecylammonio) QDs stability the and yield reuse while issues, maintaining pro- the buthighpanesulfonate also stereoselectivities reduced theas reactionthe zwitterionic (>99%) time and improvedof ligand dl-isome not the yieldr.only In while overcameaddition, maintaining theit QDswas the highstabilityfound thatand reuseelec- stereoselectivities (>99%) of dl-isomer. In addition, it was found that electron-donating groupstron-donatingissues, on but the para-positionalso groups reduced on of the thethe aryl reactionpara-position ring or atime larger and of conjugated the improved arylπ ringsystem the or wasyield a larger conductive while conjugated maintaining π sys- the totemhigh allowing was stereoselectivities conductive dimerization to to occurallowing (>99%) (Scheme dimerization of 23 ).dl-isome tor. occurIn addition, (Scheme it23). was found that electron-donating groups on the para-position of the aryl ring or a larger conjugated π system was conductive to allowing dimerization to occur (Scheme 23).

SchemeScheme 23. 23.The The reaction reaction of the of stereoselective the stereoselective C–C oxidative C–C dimerizationoxidative dimerization of a-aryl ketonitriles. of a-aryl ketonitriles.

4. Conclusions 4.Scheme Conclusions 23. The reaction of the stereoselective C–C oxidative dimerization of a-aryl ketonitriles. In general, the application of quantum dots as colloidal photocatalysts to complex organicIn reactions general, has the shown application some surprising of quantum results. dots Colloidal as colloidal quantum dotsphotocatalysts have the to complex advantagesorganic4. Conclusions reactions of high yield has and shown selectivity some as visible surprising light catalysts, results. and Colloidal the TON value quantum is much dots have the higheradvantagesIn than general, that of of high traditional the yieldapplication photocatalysts. and selectivity of quantum The quantumas visible dots dot as light size colloidal has catalysts, a strong photocatalysts inﬂuenceand the TON to complexvalue is on the reaction. However, the use of quantum dots as a homogeneous photocatalyst formuchorganic organic higher reactions conversion than hasthat is still shown of an traditional on-going some researchsurprising photocatalysts. theme, results. with The both Colloidal quantum opportunities quantum dot and size dots has havea strong the influenceadvantages on ofthe high reaction. yield andHowever, selectivity the us ase visibleof quantum light catalysts,dots as a andhomogeneous the TON value photo- is catalystmuch higher for organic than thatconversion of traditional is still photocatalysts.an on-going research The quantum theme, with dot sizeboth has opportuni- a strong tiesinfluence and challenges. on the reaction. The advantages However, ofthe quan usetum of quantum dots as both dots homogeneous as a homogeneous and hetero- photo- geneouscatalyst forcatalysts organic have conversion not been is well-develstill an on-goingoped. Theresearch recovery theme, and with reuse both of opportuni- quantum dotsties andneed challenges. to be further The studied advantages to improve of quan thetum TON dots of quantumas both homogeneous dots. At the moment, and hetero- the applicationgeneous catalysts of Cd-free have quantum not been dots well-devel will beoped. a future The trend recovery due andto the reuse toxicity of quantum of cadmium.dots need The to structure, be further morphology,studied to improve and surfac the TONe ligands of quantum of quantum dots. At dots the moment,need to thebe well-designedapplication of andCd-free researched quantum to makedots willfull usebe a of future solar energytrend due for organicto the toxicity conversion. of cad- It followsmium. thatThe thestructure, preparation morphology, and application and surfac of quantume ligands dots of quantumalways promote dots need each to oth- be er’swell-designed development. and researched to make full use of solar energy for organic conversion. It follows that the preparation and application of quantum dots always promote each oth- Authorer’s development. Contributions: Investigation, R.; writing—original draft preparation, Y.-y.Y.; writing, review and editing, D.-y.W., C.-w.F., R.; supervision, Y.-m.S. All authors have read and agreed to the publishedAuthor Contributions: version of the Investigation,manuscript. R.; writing—original draft preparation, Y.-y.Y.; writing, re- Funding:view and Thisediting, work D.-y.W., was funded C.-w.F., by R.; thesupervision, National Y.-m.S.Natural All Science authors Foundation have read andof China agreed (NSFC to the 21202101),published versionZhejiang of Provincial the manuscript. Natural Science Foundation (LY16B020006), and Science Founda- tionFunding: of Zhejiang This workSci-Tech was University funded by (18062144-Y). the National Natural Science Foundation of China (NSFC Institutional21202101), Zhejiang Review ProvincialBoard Statement: Natural Not Science applicable. Foundation (LY16B020006), and Science Founda- tion of Zhejiang Sci-Tech University (18062144-Y). Informed Consent Statement: Not applicable. Institutional Review Board Statement: Not applicable. Data Availability Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable.

Catalysts 2021, 11, 275 12 of 14

challenges. The advantages of quantum dots as both homogeneous and heterogeneous catalysts have not been well-developed. The recovery and reuse of quantum dots need to be further studied to improve the TON of quantum dots. At the moment, the application of Cd-free quantum dots will be a future trend due to the toxicity of cadmium. The structure, morphology, and surface ligands of quantum dots need to be well-designed and researched to make full use of solar energy for organic conversion. It follows that the preparation and application of quantum dots always promote each other’s development.

Author Contributions: Investigation, R.; writing—original draft preparation, Y.-Y.Y.; writing, review and editing, D.-Y.W., C.-W.F., R.; supervision, Y.-M.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National Natural Science Foundation of China (NSFC 21202101), Zhejiang Provincial Natural Science Foundation (LY16B020006), and Science Foundation of Zhejiang Sci-Tech University (18062144-Y). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: There are no conﬂicts to declare.

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