The N‐Arylamino Conjugation Effect in the Photochemistry of Fluorescent Protein Chromophores and Aminostilbenes

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: 2 P, 2 PP, 2 OM, and 2 OMB) and the corresponding trans-4-(N-arylamino)-4'-cyanostilbenes (ACSs: 1 P, 1 PP, 1 OM, and 1 OMB) was performed. We have previously shown that the parameter Phi(f)+2 Phi(tc), in which Phi(f) and Phi(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 Phi(f)+2 Phi(tc)<<1.0, because its formation is decoupled with the C=C bond (tau) torsion pathway and its decay is generally nonradiative. Our results show that the same concept also applies to ABDIs 2 with the parameter Phi(f)+2 Phi(ZE) in which Phi(ZE) is the quantum yield of Z-->E photoisomerization. We conclude that the tau torsion rather than the C-C bond (phi) torsion is responsible for the nonradiative decays of ABDIs 2 in aprotic solvents (hexane, THF, acetonitrile). The phenyl-arylamino C--N bond (omega) torsion that leads to a nonradiative TICT state is important only for 2 OM 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 tau torsion-coupled proton transfer are also discussed.