Hillel Coral, Rough Sara, Barrett Christopher J, Pietro William J, Mermut Ozzy
Department of Physics and Astronomy, York University, Toronto, ON, Canada.
Department of Chemistry, McGill University, Montreal, QC, Canada.
Commun Chem. 2024 Nov 1;7(1):250. doi: 10.1038/s42004-024-01321-0.
Many studies of azobenzene photoswitches are carried out in polar aprotic solvents as a first principles characterization of thermal isomerization. Among the most convenient polar aprotic solvents are chlorinated hydrocarbons, such as DCM. However, studies of azobenzene thermal isomerization in such solvents have led to spurious, inconclusive, and irreproducible results, even when scrupulously cleaned and dried, a phenomenon not well understood. We present the results of a comprehensive investigation into the root cause of this problem. We explain how irradiation of an azopyridine photoswitch with UV in DCM acts not just as a trigger for photoisomerization, but protonation of the pyridine moiety through photodecomposition of the solvent. Protonation markedly accelerates the thermal isomerization rate, and DFT calculations demonstrate that the singlet-triplet rotation mechanism assumed for many azo photoswitches is surprisingly abolished. This study implies exploitative advantages of photolytically-generated protons and finally explains the warning against using chlorinated solvent with UV irradiation in isomerization experiments.
许多关于偶氮苯光开关的研究在极性非质子溶剂中进行，作为热异构化的第一性原理表征。最方便的极性非质子溶剂之一是氯代烃，如二氯甲烷（DCM）。然而，在此类溶剂中对偶氮苯热异构化的研究导致了虚假、不确定且不可重复的结果，即使经过严格清洁和干燥也是如此，这一现象尚未得到很好的理解。我们展示了对该问题根源进行全面调查的结果。我们解释了在DCM中用紫外线照射偶氮吡啶光开关不仅是光异构化的触发因素，还通过溶剂的光分解使吡啶部分质子化。质子化显著加速了热异构化速率，密度泛函理论（DFT）计算表明，许多偶氮光开关所假定的单重态 - 三重态旋转机制令人惊讶地被消除。这项研究揭示了光解产生的质子的利用优势，并最终解释了在异构化实验中反对使用氯代溶剂与紫外线照射的警告。
