Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany.
Phys Chem Chem Phys. 2013 Mar 21;15(11):3906-16. doi: 10.1039/c3cp44095h.
The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO(2) and ZrO(2) thin films. In acetonitrile, upon photoexcitation of the dye via the S(0) → S(1) transition, we observed ultrafast solvation dynamics with subpicosecond time constants. Subsequent decay of the S(1) excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S(0). On TiO(2), we observed fast (<90 fs) electron injection from the S(1) state of D35 into the TiO(2) conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S(0) → S(1) absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35˙(+) radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO(2), the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO(2) and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 μs, 170 μs and 1.4 ms. For D35 adsorbed on ZrO(2), there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons. Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S(1) which decay by IC to S(0). Differential steady-state absorption spectra of D35˙(+) in acetonitrile and dichloromethane provide access to the complete D(0) → D(1) band. The absorption spectra of D35 and D35˙(+) are well described by TDDFT calculations employing the MPW1K functional.
已通过瞬态吸收光谱法在乙腈中和 TiO(2) 和 ZrO(2) 薄膜上对染料 D35 的弛豫动力学进行了表征。在乙腈中，通过 S(0) → S(1) 跃迁光激发染料后，我们观察到具有亚皮秒时间常数的超快溶剂化动力学。随后，S(1)激发态吸收 (ESA) 带的 7.1 ps 时间常数的衰减被暂时分配给激发态的结构弛豫，而 203 ps 时间常数的光谱衰减则归因于内部转换 (IC) 回到 S(0)。在 TiO(2)上，我们观察到 D35 的 S(1) 态快速(<90 fs)向 TiO(2)导带注入电子，随后是界面瞬态斯塔克场变化引起的双相动力学，时间常数为 0.8 和 12 ps，导致 S(0) → S(1) 吸收带的特征蓝移。几个过程可能导致这种光谱位移：(i)光激发立即形成 D35˙(+)自由基阳离子，最初形成电子-阳离子配合物；(ii)这些配合物的解离产生了可移动的电子，当它们开始在介孔 TiO(2)中扩散时，局部电场可能会发生变化；(iii)这可能会引发 D35 分子在变化的电场中的重新取向。纳秒时间尺度上较慢的光谱衰减被解释为局部斯塔克场的减小，因为可移动的电子向 TiO(2)深处移动并逐渐被屏蔽。多指数电子-阳离子复合发生在更长的时间尺度上，时间常数为 30 μs、170 μs 和 1.4 ms。对于吸附在 ZrO(2)上的 D35，没有明显的瞬态斯塔克位移的证据，这表明最初形成的阳离子-电子(陷阱态)配合物不会解离形成可移动的导带电子。4、35 和 550 ps 的多指数衰减被分配给阳离子和被捕获电子之间的复合，以及 S(1)中通过 IC 衰减到 S(0)的 D35 分子的一部分。D35˙(+)在乙腈和二氯甲烷中的差分稳态吸收光谱提供了完整的 D(0) → D(1) 带的通道。D35 和 D35˙(+)的吸收光谱可以通过采用 MPW1K 泛函的 TDDFT 计算很好地描述。
