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

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: 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.

Fate of photoexcited trans-aminostilbenes

“…This has been demonstrated with the green fluorescence protein (GFP) chromophore, 4-hydroxybenzylidenedimethylimidazolinone (p-HBDI), by replacing the para-OH group with para-N-arylamino or meta-amino groups (Chart 9) [80,81]. Unlike the strong fluorescence of GFP (K f $ 0.8), the chromophore p-HBDI is nearly nonfluorescent (K f < 0.001) in bulk solutions at ambient temperatures as a result of ultrafast exocyclic C¼C (i.e., the Z !…”

“…In the case of N-arylamino systems such as 17 and 18, fluorescence enhancement (K f = 0.001-0.056) was also observed, albeit to a smaller extent. Since the relationship K f + 2K ZE % 1.0 holds for p-HBDI, p-ABDI, m-ABDI, 17, and 18 in acetonitrile [80], the TICTforming pathway either by twisting the exocyclic CÀ ÀC or CÀ ÀN bond should be rather unimportant in their excited-state deactivation. This has provided a new approach in resolving the controversy of the C¼C vs. CÀ ÀC torsion in accounting for the fluorescence quenching of p-HBDI [80].…”

Strong electron donation induced differential nonradiative decay pathways for para and meta GFP chromophore analogues

“…[1][2][3][4][5][6][7][8] This proposition has been extended to other unconstrained GFP chromophore analogues. [9][10][11][12] For example both p-HBDI and p-ABDI (Chart 1) have a Z-E isomerisation quantum yield (F ZE ) B 0.5 and F f B 0.0001.…”

o‐Amino Analogs of Green Fluorescence Protein Chromophore: Photoisomerization, Photodimerization and Aggregation‐induced Emission

“…Such a position‐dependent amino substituent effect resembles that in trans ‐aminostilbenes . The photochemical correlation between p ‐ABDIs and trans ‐4‐aminostilbenes in terms of the N ‐arylamino conjugation effect has also been reported . It should be noted that, unlike the single exponential decay profiles of the p‐ and m‐ ABDIs and all the trans ‐aminostilbenes, fitting of the decay curves for the o‐ ABDIs requires biexponential functions.…”