Ultrafast dynamics of Porphyrins in the condensed phase: I. Free base tetraphenylporphyrin
Ultrafast dynamics of Porphyrins in the condensed phase: I. Free base tetraphenylporphyrin
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DOI：
10.1021/jp020398g
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发表时间：
2002-10-24
影响因子：
2.9
通讯作者：
Zewail, AH
中科院分区：
文献类型：
--
作者：
Baskin, JS;Yu, HZ;Zewail, AH
在飞秒分辨率下，利用荧光上转换和瞬态吸收，我们对四苯基卟啉（H_2TPP）在苯溶液中进行了测量，在Soret，Q（y）和Q（x）带中分别泵浦了约1300 cm（-1）的过剩振动能量，并且还泵浦了Q（y）的最低振动带。从这些研究中，在不同的激发和不同的检测波长，我们提供了一个模型，用于描述的Soret，Q（y）和Q（x）电子流形的基本分子内过程，以下顺序的时间尺度和耦合：电子（飞秒），振动（飞秒皮秒），和单三重态（纳秒）。这些动态的电子和振动弛豫途径在一个分子中的小偶极子在非极性溶剂中可以研究溶剂重组的干扰，如小斯托克斯位移的荧光。振动激发的Soret --> {Q（y，）Q（x）}和Q（y）--> Q（x）电子弛豫发生在小于100 fs内，在我们的分辨率内，如Soret（397 nm）和Q（y）（514和550 nm）激发后Q（x）荧光的立即上升所证明的。在Q（x）态中通常有三个可区分的超快弛豫时间尺度，它们被分配给导致Q（x）（最低激发单重态）中热平衡的分子内和分子间振动弛豫过程。测得的时间尺度如下：100-200 fs的分子内振动能量重新分布，1.4 ps的弹性碰撞与溶剂分子引起的振动重新分布，和10-20 ps的热平衡的能量交换与溶剂。平衡的Q（x）人口的衰减发生在纳秒时间尺度上的系统间跨越到三重态。
With femtosecond resolution, using fluorescence up-conversion and transient absorption, we have carried out measurements on free base tetraphenylporphyrin (H2TPP) in benzene solution, pumping with similar to1300 cm(-1) of excess vibrational energy in each of the Soret, Q(y), and Q(x) bands, and also pumping the lowest vibrational band of Q(y). From these studies, made for different excitations and at different detection wavelengths, we provide a model for describing the elementary intramolecular processes in the Soret, Q(y) and Q(x) electronic manifolds, with the following order of time scales and couplings: electronic (femtosecond), vibrational (femtosecond-picosecond), and singlet-triplet (nanosecond). These dynamical electronic and vibrational relaxation pathways in a molecule with small dipole in nonpolar solvents can be studied without interference from solvent reorganization, as indicated by the small Stokes shift of fluorescence. Vibrationally excited Soret --> {Q(y,) Q(x)} and Q(y) --> Q(x) electronic relaxation occurs in less than 100 fs, within our resolution, as evidenced by the immediate rise of Q(x) fluorescence after Soret (397 nm) and Q(y) (514 and 550 nm) excitation. There are generally three distinguishable ultrafast relaxation time scales within the Q(x) state, which are assigned to intra- and intermolecular vibrational relaxation processes leading to thermal equilibrium in Q(x), the lowest excited singlet state. The measured time scales are as follows: 100-200 fs for intramolecular vibrational energy redistribution, 1.4 ps for vibrational redistribution caused by elastic collision with solvent molecules, and 10-20 ps for thermal equilibration by energy exchange with the solvent. Decay of the equilibrated Q(x) population occurs on the nanosecond time scale by intersystem crossing to the triplet state.