The <i>N</i>‐Arylamino Conjugation Effect in the Photochemistry of Fluorescent Protein Chromophores and Aminostilbenes

Abstract

Abstract To understand the nonradiative decay mechanism of fluorescent protein chromophores in solutions, a systematic comparison of a series of ( Z )‐4‐( N ‐arylamino)benzylidene‐2,3‐imidazolinones (ABDIs: 2P , 2PP , 2OM , and 2OMB ) and the corresponding trans ‐4‐( N ‐arylamino)‐4′‐cyanostilbenes (ACSs: 1P , 1PP , 1OM , and 1OMB ) was performed. We have previously shown that the parameter Φ f +2 Φ tc , in which Φ f and Φ tc are the quantum yields of fluorescence and trans → cis photoisomerization, respectively, is an effective probe for evaluating the contribution of twisted intramolecular charge transfer (TICT) states in the excited decays of trans ‐aminostilbenes, including the push–pull ACSs. One of the criteria for postulating the presence of a TICT state is Φ f +2 Φ tc ≪1.0, because its formation is decoupled with the CC bond ( τ ) torsion pathway and its decay is generally nonradiative. Our results show that the same concept also applies to ABDIs 2 with the parameter Φ f +2 Φ ZE in which Φ ZE is the quantum yield of Z → E photoisomerization. We conclude that the τ torsion rather than the CC bond ( φ ) torsion is responsible for the nonradiative decays of ABDIs 2 in aprotic solvents (hexane, THF, acetonitrile). The phenyl‐arylamino CN bond ( ω ) torsion that leads to a nonradiative TICT state is important only for 2OM in THF and acetonitrile. If the solvent is protic (methanol and 10–20 % H 2 O in THF), a new nonradiative decay channel is present for ABDIs 2 , but not for ACSs 1 . It is attributed to internal conversion (IC) induced by solvent (donor)–solute (acceptor) hydrogen‐bonding (HB) interactions. The possible HB modes and the concept of τ torsion‐coupled proton transfer are also discussed.