Shimizu Okiyasu, Watanabe Jun, Naito Shizuo, Shibata Yasushi
Photobiophysics Laboratories, Graduate School of Natural Sciences, Nagoya City University, Nagoya 467-8501, Japan.
J Phys Chem A. 2006 Feb 9;110(5):1735-9. doi: 10.1021/jp056073s.
The quenching rate constants of the excited triplet state of Rose Bengal (RB) by oxygen (k(obs)) were measured in ethylene glycol (EG) at different temperatures using nanosecond laser flash photolysis. Although a plot of the quenching rate constant k(obs) for RB triplet state vs oxygen concentration is linear at 20 degrees C, the oxygen dependence of k(obs) does not exhibit linearity but upward curvature at high temperatures from 130 to 140 degrees C. The upward curvature at high temperatures is not well-described by a kinetic scheme first postulated by Gijzeman et al., which is characterized by exciplex formation and a unimolecular dissociation of the exciplex to products, but instead by a more comprehensive mechanism involving a bimolecular dissociation in addition to a unimolecular one. The measurements of the oxygen dependence of k(obs) for RB triplet state at different temperatures yielded a reaction enthalpy for the exciplex formation of 150 kJ mol(-1). Due to the large exothermic reaction enthalpy, equilibrium was obtained for the exciplex at 20 degrees C even at low oxygen concentration and the bimolecular quenching by oxygen became the major dissociation process. The equilibrium attainment and bimolecular dissociation provide a linear oxygen dependence of k(obs) to all outward appearances. Therefore, linearity does not always mean that exciplex dissociation proceeds solely through a unimolecular mechanism.
利用纳秒激光闪光光解技术，在不同温度下的乙二醇（EG）中测量了孟加拉玫瑰红（RB）激发三重态被氧淬灭的速率常数（k(obs)）。尽管在20℃时，RB三重态的淬灭速率常数k(obs)对氧浓度的曲线是线性的，但在130至140℃的高温下，k(obs)对氧的依赖性并非呈线性，而是向上弯曲。高温下的向上弯曲不能用Gijzeman等人最初提出的动力学机制很好地描述，该机制的特征是激基复合物的形成以及激基复合物向产物的单分子解离，而是由一个更全面的机制来描述，该机制除了单分子解离外还涉及双分子解离。在不同温度下对RB三重态k(obs)的氧依赖性测量得出激基复合物形成的反应焓为150 kJ mol(-1)。由于反应焓的大量放热，即使在低氧浓度下，在20℃时激基复合物也能达到平衡，并且氧的双分子淬灭成为主要的解离过程。平衡的达到和双分子解离从表面上看使k(obs)对氧呈线性依赖。因此，线性并不总是意味着激基复合物的解离仅通过单分子机制进行。
