Greenough Simon E, Horbury Michael D, Thompson James O F, Roberts Gareth M, Karsili Tolga N V, Marchetti Barbara, Townsend Dave, Stavros Vasilios G
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.
Phys Chem Chem Phys. 2014 Aug 14;16(30):16187-95. doi: 10.1039/c4cp02424a.
Using a combination of ultrafast solution- and gas-phase spectroscopies, together with high-level theory calculations, we demonstrate that we are able to track conformer-specific photodissociation dynamics in solution through solvent choice. We reveal this phenomenon in guaiacol (2-methoxyphenol), a key subunit of the natural biopolymer lignin. In cyclohexane, the first electronically excited (1)ππ* (S1) state in guaiacol relaxes with a time-constant of τ = 4.5 ± 0.2 ns, mediated through intersystem crossing to lower lying triplet (Tn) states and internal conversion and fluorescence back to the ground state (S0). In contrast, in methanol, a further relaxation channel is also present; the S1 state relaxes with a time-constant of τ = 2.9 ± 0.1 ns, which is now additionally mediated through coupling onto a dissociative (1)πσ* (S2) state and subsequent O-H bond fission, evidenced through the appearance of a spectral signature for the guaiacoxyl radical after ∼250 ps. With the aid of complementary calculations, we attribute this to the now absent intramolecular H-bond between OH and OMe moieties, which now favours intermolecular H-bonding to methanol, lowering the barrier to O-H dissociation and facilitating H-atom loss via tunnelling.
通过结合超快溶液和气相光谱技术以及高水平理论计算，我们证明能够通过溶剂选择追踪溶液中特定构象体的光解离动力学。我们在愈创木酚（2-甲氧基苯酚）中发现了这一现象，愈创木酚是天然生物聚合物木质素的关键亚基。在环己烷中，愈创木酚的第一电子激发态（1）ππ*（S1）以τ = 4.5 ± 0.2 ns的时间常数弛豫，通过系间窜越至较低的三重态（Tn）以及内转换和荧光回到基态（S0）来介导。相比之下，在甲醇中，还存在另一个弛豫通道；S1态以τ = 2.9 ± 0.1 ns的时间常数弛豫，现在还通过耦合到解离的（1）πσ*（S2）态以及随后的O - H键断裂来介导，约250 ps后愈创木酚自由基的光谱特征出现证明了这一点。借助互补计算，我们将此归因于现在OH和OMe基团之间不存在分子内氢键，这现在有利于与甲醇形成分子间氢键，降低了O - H解离的势垒并促进了通过隧穿的氢原子损失。
