Janus-Type Photophysics of Rotational Isomers in a Diphenylanthracene Dimer

Organic molecular dimers serve as valuable model systems for manipulating and exploring interchromophore interactions. However, the structural flexibility introduced by linker or chromophore rotation gives rise to conformational isomers with distinct atomic arrangements, complicating the interpretation of photophysical processes. In this study, we investigate 9,9',10,10'-tetraphenyl-2,2'-bianthracene (TPBA) to elucidate the distinct electronic characteristics of its two primary rotational isomers: syn- and anti-TPBA. To address the "Janus-type" photophysical behavior of these isomers, we employed a comprehensive suite of static and time-resolved spectroscopic techniques, including excitation-wavelength-dependent time-resolved photoluminescence, transient absorption, and time-resolved electron paramagnetic resonance, complemented by theoretical calculations. syn- and anti-TPBA exhibit markedly different emissive properties and charge-transfer characteristics, reflecting their unique exciton coupling behaviors. Additionally, they showcase distinct triplet formation rates and exhibit environment-dependent triplet formation mechanisms. This in-depth study of the Janus-like electronic properties of TPBA underscores the critical role of conformational isomerism in organic molecular dimers. Neglecting these structural variations can obscure the true photophysical landscape and lead to misinterpretations of mechanistic processes, highlighting the necessity of considering conformational heterogeneity in molecular design and photophysical studies.