Departamento de Quı́mica Fundamental, Instituto de Quı́mica, Universidade de São Paulo, São Paulo, SP, Brazil.
J Org Chem. 2013 May 3;78(9):4432-9. doi: 10.1021/jo400426y. Epub 2013 Apr 16.
The induced decomposition of 1,2-dioxetanes results in the efficient formation of singlet-excited carbonyl compounds. This transformation has been assumed to involve two sequential electron-transfer steps, and the viscosity dependence of the chemiexcitation efficiency (solvent cage effect) has been considered as evidence for the occurrence of an intermolecular electron back-transfer, despite the very high chemiexcitation quantum yields observed. However, all other chemiluminescent reactions assumed to occur according to the entirely intermolecular mechanism, referred to as CIEEL, are inefficient, except for the peroxyoxalate system. Therefore, we have investigated the solvent cage effect on the singlet quantum yields in both the induced decomposition of 1,2-dioxetanes and the peroxyoxalate reaction. Analysis of the viscosity effect observed for both systems, using a collisional as well as a free-volume model, indicates a very distinct behavior, which was interpreted as the occurrence of intramolecular chemiexcitation in the induced 1,2-dioxetane decomposition. We propose a general mechanism for efficient chemiluminescence in which the required electron back-transfer and C-C bond cleavage are concerted and compete with conformational changes that compromise the chemiexcitation. This mechanism is in agreement with both experimental and theoretical data available on the induced 1,2-dioxetane decomposition as well as with the high quantum efficiency of this transformation.
1,2-二氧杂环乙烷的诱导分解导致单重激发羰基化合物的高效形成。这种转化被认为涉及两个连续的电子转移步骤，而化学激发效率的粘滞依赖性（溶剂笼效应）被认为是发生分子间电子反向转移的证据，尽管观察到非常高的化学激发量子产率。然而，所有其他假定根据完全分子间机制发生的化学发光反应，称为 CIEEL，都是低效的，过氧草酸盐体系除外。因此，我们研究了溶剂笼效应对 1,2-二氧杂环乙烷的诱导分解和过氧草酸盐反应中 singlet 量子产率的影响。使用碰撞和自由体积模型对这两个体系观察到的粘度效应进行分析表明，其行为非常明显，这被解释为在诱导的 1,2-二氧杂环乙烷分解中发生了分子内化学激发。我们提出了一种通用的机制，用于高效的化学发光，其中所需的电子反向转移和 C-C 键断裂是协同的，并与破坏化学激发的构象变化竞争。这种机制与诱导的 1,2-二氧杂环乙烷分解的实验和理论数据以及这种转化的高量子效率是一致的。
