Departament of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
J Phys Chem A. 2011 Aug 4;115(30):8392-8. doi: 10.1021/jp204354x. Epub 2011 Jul 14.
An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ⇝ S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).
对 3-氯-4-甲基伞形酮（3Cl4MU）在基态 S(0)、第一激发单线态 S(1)和最低三重态 T(1)中的光物理特性进行了广泛的表征。在水中、中性乙醇、酸化乙醇和堿化乙醇中进行了量子产率（荧光、磷光、系间窜跃、内转换和单线态氧形成）以及寿命的定量测量，在室温下和低温下在水中、二恶烷/水混合物和醇中获得。分配了不同的瞬态物种，并提出了一个一般的动力学方案，总结了 3Cl4MU 的激发态多平衡。在水中，平衡仅限于中性（N*）和阴离子（A*）物种，无论是在基态（pK(a) = 7.2）还是在第一激发单线态（pK(a)* = 0.5）。在二恶烷/水混合物（pH 约为 6）中，观察到 S(1)态动力学的显著变化，出现了附加的互变异构体 T物种。在低含水量混合物（混合物 9:1v:v）中，仅观察到 3Cl4MU 的中性（N）和互变异构体（T*）形式，而在较高含水量混合物（水摩尔分数大于 0.45）中，所有三种形式 N*、T和 A都在激发态中共存。在三重态中，在非质子和非极性溶剂二恶烷中，观察到的瞬态信号被分配为中性形式的三重态-三重态跃迁，N*(T(1)) → N*(T(n))。在水中，观察到两种瞬态物种，并将其分配为中性 N*(T(1))和阴离子形式 A*(T(1))的三重态（也在堿化乙醇中获得）。3Cl4MU 的磷光光谱和衰减在中性、酸化和堿化溶液中证明，只有这两种物种 N*(T(1))和 A*(T(1))存在于最低三重态 T(1)中。辐射通道被发现是阴离子物种失活的主要通道，而对于中性物种，S(1) ⇝ S(0)内转换与荧光竞争。对于 N和 A，系间窜跃产率代表 S(1)的次要失活通道。
