Thin-Film Morphology and Optical Properties of Photoisomerizable Donor–Acceptor Oligothiophenes

Abstract

It was recently reported that the most popular electron-accepting units introduced to π-conjugated oligomers studied for organic photovoltaic applications are susceptible to unwanted and even destructive photochemical reactions. The consequences of Z/E photoisomerization of the popular 2-(1,1dicyanomethylene)rhodanine (RCN) unit on the optical and morphological properties of a homologous series of RCNfunctionalized oligothiophenes are studied here. Oligomers consisting of one, two, or three thiophene units were studied as pure Z isomers and with E isomer compositions of 25, 53, and 45%, respectively, for Z/E mixtures. Solutions of Z isomers and Z/E mixtures were characterized by UV−vis and photoluminescence spectroscopy, wherein changes to optical properties were evaluated on the basis of E isomer content. X-ray diffraction of thin-film Z/E mixtures reveals crystalline domains of both Z and E forms after thermal annealing for mono-and bithiophene oligomers, with greater interplanar spacing for E crystalline domains than the Z counterparts along the substrate normal direction. The surface morphology viewed by atomic force microscopy also shows fiberlike structures for the E form with a much larger aspect ratio than for the Z domains in the bithiophene oligomer. Optical characterization reveals drastic changes in the solid state upon introduction of the E form for the mono-and bithiophene derivatives, whereas subtle consequences are noted for the terthiophene analogue. Most notably, a 132 nm redshift in maximum absorption occurs for the bithiophene oligomer films containing 53% E isomer compared to the pure Z counterpart. Finally, although solid-state photoisomerization experiments find no evidence of Z → E isomerization in polycrystalline Z films, E → Z isomerization is observed and becomes more restrictive in films with higher crystallinity (i.e., after thermal annealing). This structure−property study, which elucidates the consequences of the RCN configuration on solid-state packing and optical properties, is expected to guide the development of more efficient and stable organic optoelectronic devices.

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

“…We recently reported the ability of dicyanorhodanine (RCN) functionalized thiophenes to undergo Z / E configurational isomerization upon photoirradiation and observed that the solid-state morphologies and optoelectronic properties of thin films from RCN-containing oligomers can be significantly affected by configurational composition . Our structure–property studies could encourage opportunities for solar cell optimization through the consideration of this stereochemical feature.…”

“…It is well-appreciated that small changes in NFA structure can alter the morphology of the organic solar cell active layer and affect overall device efficiency . Common molecular-level approaches to optimizing OPV thin film morphology and charge transport – include modifying the π-conjugated backbone structure and/or planarity and the structure and/or placement of solubilizing side chains. – Modifications specific to INCN-containing molecules include altering the solubilizing alkyl chain placement along the π-conjugated backbone or on the indanone ring and introduction of halogen atoms or methoxy groups to various positions. The structural changes, while sometimes seemingly minor, can induce a higher film absorption coefficient, larger crystalline coherence, and higher electron mobility, ultimately leading to higher PCE.…”

“…We recently reported the ability of dicyanorhodanine (RCN) functionalized thiophenes to undergo Z/E configurational isomerization upon photoirradiation 29 and observed that the solid-state morphologies and optoelectronic properties of thin films from RCN-containing oligomers can be significantly affected by configurational composition. 24 Our structure− property studies could encourage opportunities for solar cell optimization through the consideration of this stereochemical feature. In the same spirit, it is surprising that the structural and functional consequences of Z/E stereoisomerism with respect to the exocyclic double bond that links INCN to a πconjugated moiety have generally been overlooked (Figure 1c).…”

Editorial for Forum on Applied Supramolecular Materials

“…Apart from mechanical features, intriguing designs for tuning the photophysical properties of the self-assemblies toward achieving intriguing applications are also important. Xue and co-workers synthesized oligomers consisting of one, two, or three thiophene units as pure Z isomers and with E isomer compositions of 25%, 53%, and 45%, respectively, for Z/E mixtures . The photophysical properties of solutions of Z isomers and Z/E mixtures were characterized, wherein changes to optical properties were evaluated on the basis of E isomer content.…”

“…Xue and co-workers synthesized oligomers consisting of one, two, or three thiophene units as pure Z isomers and with E isomer compositions of 25%, 53%, and 45%, respectively, for Z/E mixtures. 12 The photophysical properties of solutions of Z isomers and Z/E mixtures were characterized, wherein changes to optical properties were evaluated on the basis of E isomer content. A novel supramolecular perovskite nanoassembly containing a Mn 2+alloyed (PEA) 2 PbBr 4 nanosheet (referred to as Mn:PbPeV NS) and [Cr(1,10-phenanthroline) 2 (5-nitro-1,10-phenanphroline)] 3+ (CrPS) was reported by Das et al toward oxidize biomolecules such as guanine and guanine-rich DNA by using light.…”