Unraveling Solvent-Independent Excited State Proton Transfer Dynamics in Sterically Substituted Photoactive Systems

Excited state proton transfer (ESPT) plays a central role in the function of many photoactive molecules; however, it can be strongly influenced by environment polarity. Here, we investigate the excited state photodynamics of two structurally complex triazine- and benzotriazole-based molecules that bear sterically bulky substituents around the proton transfer unit: bis-ethylhexyloxyphenol methoxyphenyl triazine (BEMT) and methylene bis-benzotriazolyl tetramethylbutylphenol (MBBT). Using femtosecond transient electronic absorption spectroscopy in solvents of contrasting polarity and hydrogen-bonding character, supported by mixed-reference spin-flip time-dependent density functional theory calculations, we show that ultrafast relaxation in both systems is governed by a largely solvent-insensitive intramolecular ESPT. We demonstrate that the solvent-independent behavior primarily arises from intrinsic, electronic properties of BEMT and MBBT rather than steric substitution. While bulky substituents are not required for efficient intramolecular ESPT, they offer enhanced solvent-invariant ESPT dynamics. This highlights potential design elements for next-generation photoactive systems for tuning excited state behavior.