Photophysical properties of a highly delocalized allylic cation: Acid-independent and solvent-dependent intramolecular charge transfer

Abstract

Tetraarylallenes that form highly delocalized allylic cations upon activation represent an emerging class of charge-transfer chromophores. Here, we investigate the solvent- and cation-dependent photophysical properties of a donor–acceptor substituted allene that generates an extended polymethine-like cation upon protonation or Lewis acid coordination. Steady-state spectroscopy reveals pronounced solvatochromism of the activated species, and femtosecond transient absorption measurements further demonstrate that the excited-state lifetimes are strongly dependent on solvent polarity. In contrast, variation of the activating acid does not impact the spectral features and excited-state dynamics, indicating that the photophysical response is governed by the intrinsic electronic structure of the delocalized allylic cation rather than specific counterion effects. These results provide fundamental insight into the environmental modulation of charge-transfer character and establish guiding principles for the design of robust, stimuli-responsive optical materials.