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 CC 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 remains 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 spectroscopies. 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 (Φ_iso) 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 rotation around the ethylenic bond, supported by TD-DFT calculations. These findings offer a new understanding of electron-donor substituted TPEs and their potential as tunable photochromic materials.