Excited-State Dynamics and Photochromism of Extended Tetraphenylethylene Derivatives and Their Control by Amino Conjugation Effect

Abstract

Tetraphenylethylene (TPE) derivatives are key building blocks for solid-state fluorophores, offering tunable emission wavelengths and high quantum efficiencies. While it is nowadays well established, especially in recent literature, that rotation around the central C=C bond dominates deactivation in solution and that its restriction in solid state results in aggregation-induced emission (AIE), the impact of substitution on TPE dynamics in solution has remained largely unexplored. This is likely due to the challenge of efficiently separating E and Z isomers in most synthesized molecules. Here, we report the solution-phase photophysics of stereopure, extended TPE derivatives using both steady-state and femtosecond transient spectroscopy. Introducing triphenylamine (TPA) substituents generates distinct spectral differences between E and Z isomers, enabling modulation of the photostationary state and selective control of the isomeric equilibrium via irradiation wavelength. Notably, this photochromism is accompanied by a pronounced decrease in the photoisomerization quantum yield (Φiso) relative to non-extended TPEs, consistent with the “amino conjugation effect” previously observed in stilbene derivatives. Time-resolved spectroscopies provide mechanistic insight, revealing the substituent’s influence on emissive and dark state lifetimes, as well as access to a conical intersection through S1-state rotation around the ethylenic bond, supported by TDDFT calculations. These findings offer a new understanding of electron-donor substituted TPEs and their potential as tunable photochromic materials.