Structure–Kinetics Correlations in Isostructural Crystals of α‑(<i>ortho</i>-Tolyl)-acetophenones: Pinning Down Electronic Effects Using Laser-Flash Photolysis in the Solid State

Aqueous suspensions of nanocrystals in the 200–500 nm size range of isostructural α-(ortho-tolyl)-acetophenone (1a) and α-(ortho-tolyl)-para-methylacetophenone (1b) displayed good absorption characteristics for flash photolysis experiments in a flow system, with transient spectra and decay kinetics with a quality that is similar to that recorded in solution. In contrast to solution measurements, reactions in the solid state were characterized by a rate limiting hydrogen transfer reaction from the triplet excited state and a very short-lived biradical intermediate, which does not accumulate. Notably, the rate for δ-hydrogen atom transfer of 1a (2.7 × 107 s–1) in the crystalline phase is 18-fold larger than that of 1b (1.5 × 106 s–1). With nearly identical molecular and crystal structures, this decrease in the rate of δ-hydrogen abstraction can be assigned unambiguously to an electronic effect by the para-methyl group in 1b, which increases the contribution of the 3π,π* configuration relative to the reactive 3n,π* configuration in the lowest triplet excited state. These results highlight the potential of relating single crystal X-ray structural data with absolute kinetics from laser flash photolysis.尺寸范围为200~500 nm的同构α-(邻甲苯基)-苯乙酮（α-(ortho-tolyl)-acetophenone，1a）与α-(邻甲苯基)-对甲基苯乙酮（α-(ortho-tolyl)-para-methylacetophenone，1b）的纳米晶体水悬浮液，在流动体系的闪光光解实验中展现出优异的吸收特性，其瞬态光谱与衰减动力学的测试质量可与溶液中测得的结果相媲美。与溶液体系的测量结果不同，固态下的反应以限速三重激发态氢转移反应为特征，且存在寿命极短的双自由基中间体，该中间体不会发生累积。值得注意的是，晶相中1a的δ-氢原子转移速率（2.7 × 10^7 s–1）是1b（1.5 × 10^6 s–1）的18倍。尽管二者的分子与晶体结构几乎完全一致，但δ-氢提取速率的这一下降可明确归因于1b中对位甲基产生的电子效应：相较于反应活性的最低三重激发态3n,π*构型，该对位甲基会提升3π,π*构型的占比。上述研究结果凸显了将单晶X射线结构数据与激光闪光光解得到的绝对动力学数据相结合的研究潜力。