Singh Ajay K, Mondal Jahur A, Ramakrishna G, Ghosh Hirendra N, Bandyopadhyay Tusar, Palit Dipak K
Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
J Phys Chem B. 2005 Mar 10;109(9):4014-23. doi: 10.1021/jp045481d.
The dynamics of ultrafast photoinduced intermolecular electron transfer (ET) from the excited singlet (S1) state of perylene (Pe) to an electron-accepting cationic surfactant molecule, N-cetylpyridinium chloride (CPC), in aqueous micellar solutions has been investigated using the femtosecond transient absorption spectroscopic technique with temporal resolution of 120 fs. The Pe molecule is localized at or near the micellar surface, where it coexists with the pyridinium moieties (headgroups of the micelle) of the surfactant molecule. Following photoexcitation of Pe, an electron is transferred to the neat and geometrically restricted headgroup of the micelle. Dynamics of the forward ET process as well as the geminate recombination or back ET (BET) process have been followed by monitoring the temporal evolution of the S1 state of Pe and the cation radical of Pe (Pe*+), respectively. The multiexponential forward ET process indicates that the ET dynamics is highly correlated with the spatial distributions of the micellar headgroups around a donor Pe molecule and thus dependent on the donor-acceptor distance. The distance-dependent ET and BET rates have been calculated following the method of Weidemaier and Fayer (J. Chem. Phys. 1995, 102, 3820) to get the best fit parameters for the multiexponetial temporal profiles for the S1 state of Pe as well as Pe*+. Because the acceptor is a constituent of the neat micellar medium, their confinement on the surface of the microheterogeneous medium provides a very large concentration such that, even though the forward transfer rate is 0.06 ps(-1) at the distance of closest approach, the ET process is complete within a 200-ps time domain. If the concepts of distribution of ET distances are utilized, the possible role of material diffusion on the kinetics of forward ET is ruled out. This is an experimental study to show, for the first time, the ultrafast distance-dependent light-induced ET dynamics following both the excited state of the donor and the cation radical formed in an ET process using the transient absorption spectroscopic technique in a self-reactive restrictive environment.
利用时间分辨率为120飞秒的飞秒瞬态吸收光谱技术，研究了在水性胶束溶液中，从苝（Pe）的激发单重态（S1）到电子接受性阳离子表面活性剂分子N-十六烷基吡啶氯化物（CPC）的超快光诱导分子间电子转移（ET）动力学。Pe分子定位于胶束表面或其附近，在那里它与表面活性剂分子的吡啶鎓部分（胶束的头基）共存。在Pe光激发后，一个电子转移到胶束纯净且几何受限的头基上。分别通过监测Pe的S1态和Pe的阳离子自由基（Pe*+）的时间演化，跟踪了正向ET过程以及双电子复合或反向ET（BET）过程的动力学。多指数正向ET过程表明，ET动力学与供体Pe分子周围胶束头基的空间分布高度相关，因此取决于供体-受体距离。按照Weidemaier和Fayer的方法（《化学物理杂志》1995年，102卷，3820页）计算了距离相关的ET和BET速率，以获得Pe的S1态以及Pe*+的多指数时间轮廓的最佳拟合参数。由于受体是纯净胶束介质的组成部分，它们在微非均相介质表面的限制提供了非常高的浓度，以至于即使在最接近距离时正向转移速率为0.06 ps-1，ET过程在200 ps的时域内也能完成。如果利用ET距离分布的概念，就排除了物质扩散对正向ET动力学的可能作用。这是一项实验研究，首次展示了在自反应性受限环境中，使用瞬态吸收光谱技术，在供体的激发态以及ET过程中形成的阳离子自由基之后，超快的距离相关光诱导ET动力学。
