2‑(1,1-Dicyanomethylene)rhodanine-Functionalized\nOligothiophenes: A Structure–Property Investigation of <i>Z</i>/<i>E</i> Photoisomerization Behavior

A series\nof oligothiophenes singly and doubly functionalized with\ndicyanorhodanine (RCN) units have been investigated to understand\ntheir <i>Z</i>/<i>E</i> photoisomerization behavior\nupon structural modulation. Monotopic RCN target molecules (<b>1-</b><i>Z</i>–<b>9-</b><i>Z</i>) were designed to observe the consequences of π-conjugation,\nsolubilizing group substitution, and formylation of the thiophene\nunits. In all cases, the <i>Z</i> isomer is obtained from\nsynthesis as the thermodynamically stable isomer, whereas the <i>E</i> isomer is achieved through selective irradiation (including\nred light, λ<sub>irr</sub> = 628 nm) as a <i>Z</i>/<i>E</i> mixture in solution. For the quarterthiophene\nentries, photoisomerization is inhibited, with photoirradiation resulting\nonly in degradation. The result comports with concentration-dependent\nstudies, which show that increasing π-conjugation results in\ngreater aggregation and muted <i>Z</i>/<i>E</i> photoisomerization. Ditopic RCN targets (<b>10-</b><i>ZZ</i>–<b>12-</b><i>ZZ</i>), mimicking\nacceptor–donor–acceptor (A–D–A) oligomers\nrelevant to OPV materials, also show evidence of photoisomerization\nin solution, with formation of <i>Z,Z</i>/<i>Z,E</i> mixtures at the photostationary state (PSS). Complementary ground-\nand excited-state DFT calculations show excellent agreement with the\nexperimental findings. This comprehensive structure–property\nanalysis is expected to both guide and caution the functional materials\ncommunity with respect to the usage of photoisomerizable RCN-oligothiophenes\nfor optoelectronic applications.