Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.
J Phys Chem B. 2013 Mar 7;117(9):2695-704. doi: 10.1021/jp3093379. Epub 2013 Feb 25.
To understand the effects of solvent-solute hydrogen bonding (SSHB) on the excited-state dynamics of two GFP-like chromophores, p-ABDI and p-CFABDI, we have determined the quantum yields for fluorescence (Φf) and the isomerization Z → E (ΦZE) and the femtosecond fluorescence and transient infrared absorption in selected solvents. The behavior that ΦZE ≅ 0.50 in aprotic solvents, such as CH3CN, indicates that the E-Z photoisomerization adopts a one-bond-flip mechanism through the torsion of the exocyclic C═C bond (the τ torsion) to form a perpendicular species (τ ∼90°) in the singlet excited state followed by internal conversion (IC) to the ground state and partition to form the E and Z isomers with equal probabilities. The observed ΦZE decreased from 0.50 to 0.15-0.28 when CH3CN was replaced with the protic solvents CH3OH and CF3CH2OH. In conjunction with the solvent-independent rapid (<1 ps) kinetics for the fluorescence decay and the solvent-dependent slow (7-20 ps) kinetics for the ground-state recovery, we conclude that the SSHB modifies the potential energy surface for the τ torsion in a way that the IC occurs also for the twisted intermediates with a τ-torsion angle smaller than 90°, which favors the formation of the Z isomers. The possibility of IC induced by torsion of the exocyclic C-C bond (the φ torsion) is also considered but excluded.
为了理解溶剂-溶质氢键（SSHB）对两种 GFP 类似发色团，即 p-ABDI 和 p-CFABDI 的激发态动力学的影响，我们在选定的溶剂中测定了荧光量子产率（Φf）和异构化 Z→E（ΦZE）以及飞秒荧光和瞬态红外吸收。在非质子溶剂（如 CH3CN）中，ΦZE≈0.50 的行为表明 E-Z 光异构化通过外环 C═C 键（τ 扭转）的扭转采用一键翻转机制，在单重激发态下形成垂直物种（τ≈90°），随后进行内部转换（IC）回到基态，并以相等的概率分配形成 E 和 Z 异构体。当 CH3CN 被质子溶剂 CH3OH 和 CF3CH2OH 取代时，观察到的 ΦZE 从 0.50 降低到 0.15-0.28。结合荧光衰减的溶剂独立快速（<1 ps）动力学和基态恢复的溶剂依赖性缓慢（7-20 ps）动力学，我们得出结论，SSHB 以一种 IC 也发生在扭转角度小于 90°的扭曲中间体的方式修饰了τ扭转的势能面，这有利于 Z 异构体的形成。还考虑了外环 C-C 键（φ 扭转）扭转诱导 IC 的可能性，但被排除。
