School of Chemistry, Cantock's Close , University of Bristol , Bristol BS8 1TS , United Kingdom.
Bristol Centre for Functional Nanomaterials, School of Physics , University of Bristol , Bristol BS8 1TH , United Kingdom.
J Phys Chem B. 2019 Jun 6;123(22):4745-4756. doi: 10.1021/acs.jpcb.9b01475. Epub 2019 May 24.
The ultrafast dynamics of a bimolecular excited-state proton transfer (ESPT) reaction between the photoacid 7-hydroxy-4-(trifluoromethyl)-1-coumarin (CouOH) and 1-methylimidazole (MI) base in aprotic chloroform- d solution were investigated using ultrafast transient infrared (TRIR) and transient absorption (TA) spectroscopies. The excited-state lifetime of the photoacid in solution is relatively short (52 ps), which at the millimolar photoacid and base concentrations used in our study precludes any diffusion-controlled bimolecular ESPT reactions. This allows the prompt ESPT reaction between hydrogen-bonded CouOH and MI molecules to be studied in isolation and the "contact" ESPT dynamics to be unambiguously determined. Our time-resolved studies reveal that ultrafast ESPT from the CouOH moiety to hydrogen-bonded MI molecules occurs within ∼1 ps, tracked by unequivocal spectroscopic signatures of CouO* photoproducts that are formed in tandem with HMI. Some of the ESPT photoproducts subsequently π-stack to form exciplexes on a ∼35 ps time scale, minimizing the attractive Coulombic forces between the oppositely charged aromatic molecules. For the concentrations of CouOH and MI used in our study (up to 8 mM), we saw no evidence for excited-state tautomerization of coumarin anions.
在非质子性氯仿- d 溶液中，通过超快瞬态红外（TRIR）和瞬态吸收（TA）光谱研究了光酸 7-羟基-4-（三氟甲基）-1-香豆素（CouOH）和 1-甲基咪唑（MI）碱基之间双分子激发态质子转移（ESPT）反应的超快动力学。在我们的研究中，溶液中光酸的激发态寿命相对较短（52 ps），这使得在使用毫摩尔光酸和碱基浓度的情况下，任何扩散控制的双分子 ESPT 反应都无法进行。这使得可以孤立地研究氢键结合的 CouOH 和 MI 分子之间的快速 ESPT 反应，并明确确定“接触”ESP 动力学。我们的时间分辨研究表明，从 CouOH 部分到氢键结合的 MI 分子的超快 ESPT 在 ∼1 ps 内发生，与同时形成的 CouO*光产物的明确光谱特征相吻合。一些 ESPT 光产物随后在 ∼35 ps 的时间尺度上π-堆积形成激基复合物，从而最小化了带相反电荷的芳族分子之间的吸引力库仑力。对于我们在研究中使用的 CouOH 和 MI 浓度（高达 8 mM），我们没有看到香豆素阴离子的激发态互变异构的证据。
