Enlightening the Well-Controlled Photochemical Behavior of 1,1-Dicyanomethylene-3-Indanone-Functionalized π-Conjugated Molecules

Abstract

Dicyanomethylene-3-indanone (INCN) is a popular electron acceptor showcased in hundreds of push−pull oligomers, including some of the best nonfullerene acceptor (NFA) materials used in small molecule-based bulk-heterojunction (BHJ) organic photovoltaics (OPVs). Consequences of the configuration (i.e., Z or E) and conformation (i.e., s-cis or s-trans) of the exocyclic olefin that conjugates INCN to π-conjugated molecules have largely been ignored. Two recent reports have implicated Z/E photoisomerization in the photodegradation of popular NFAs like IT-4F when subjected to broad spectrum irradiation. Here, we elucidate through experiments and complementary ground-and excited-state computations the photochemical behavior of a family of eight INCN-functionalized donor−acceptor molecules varying in aryl and heteroaryl substitution, alkyl group substitution, and halogen functionalization on the INCN unit. Well-controlled Z/E photoisomerization using selective wavelengths of excitation spanning the ultraviolet and visible regions is observed in all cases yielding a range of Z/E photostationary state (PSS) distributions with no evidence of a previously reported photooxidation. Z/E photoisomerization followed by sequential pericyclic reactions, consistent with one recent literature report, is identified for just one target molecule upon irradiation at 454 nm. The alkyl group positioning on the thiophene ring neighboring the INCN is found to bias the conformational preferences of the target molecules and modulate access to this reaction pathway. All eight molecules undergo facile Z/E photoswitching over numerous cycles upon selective excitation. Overall, the work reveals the well-controlled photochemical behavior of INCN-functionalized π-systems and encourages their use in the design of future functional and organic materials and photoswitches.

Dicyanorhodanine-Pyrrole Conjugates for Visible Light-Driven Quantitative Photoswitching in Solution and the Solid State

“…Remarkably, no thermal isomerization was observed in either direction for 1- Z and 1- E in tetrachloroethane up to 80 °C (Figures S68–S69) or in acetonitrile- d 3 up to 60 °C (Figures S70–S71) over 120 and 80 h, respectively, which speaks to the bistable nature of the isomers. These findings deviate from our previous work on INCN-based donor–acceptor molecules, where we observed that higher polarity solvent induced faster thermal isomerization compared to a lower polarity solvent, in case of an INCN-monothiophene analog . In an H-bonding competitive solvent such as DMSO- d 6 at 60 °C, no Z → E thermal isomerization was observed within 80 h (Figure S72), but the 1- E isomer relaxes to 1- Z completely within 3 h (Figure S73).…”

“…These observations were repeatable even with the reaction vessels being covered with aluminum foil throughout the entire synthesis and purification processes to minimize the possibility of unintentional photoisomerization by ambient light. This unique isomer accessibility from synthesis is dissimilar to our previous work on RCN and INCN-functionalized thiophene oligomers, where only the more thermodynamically stable Z isomer could be obtained from synthesis.…”

Understanding photochemical degradation mechanisms in photoactive layer materials for organic solar cells

“…Using the simplified 1,1-dicyanomethylene-3-indanon (INCN) compounds as model systems, Castellano et al conducted a series of designed experiments and complementary ground- and excited-state computations. 111 As shown in Fig. 9, they offer an exhaustive examination of the photochemical characteristics of a series of INCN-based molecules, featuring variations of INCN units in (hetero-)aryl substitution, alkyl group positioning, and halogen substitution.…”

“…The rotation of vinyl groups in A–D–A type NFAs is the initial process that induced the photodegradation of NFAs, which is proved by the isolated photodegradation products in ITIC-NFAs. 37,80,111 Although the existence of out-side-chains in BTP-NFAs can suppress such rotation modes, the flexible vinyl groups are still able to rotate under illumination. As shown in Fig.…”

“…24 This process involves a photochemical reaction inducing molecular structural changes at the sub-nm scale. 30,37,80,81,107,109,111,112,143,144,146,148,214,215 Under illumination, morphological changes at nm-scale also occur, either due to molecular alterations or light-induced molecular movements at the nm-scale. 29,216–220 As depicted in Fig.…”