Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile.
Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile.
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DOI：
10.1039/c3cp44095h
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发表时间：
2013-02
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通讯作者：
K. Oum;P. W. Lohse;J. R. Klein;Oliver Flender;M. Scholz;A. Hagfeldt;G. Boschloo;T. Lenzer
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作者：
K. Oum;P. W. Lohse;J. R. Klein;Oliver Flender;M. Scholz;A. Hagfeldt;G. Boschloo;T. Lenzer
用瞬态吸收光谱法表征了染料D35在乙腈和tio2、ZrO薄膜上的弛豫动力学。在乙腈中，通过S(0)→S(1)跃迁光激发染料，我们观察到亚皮秒时间常数的超快溶剂化动力学。S(1)激发态吸收（ESA）波段的后续衰减，时间常数为7.1 ps，暂定为激发态的结构弛豫，而内部转换（IC）返回S(0)的时间常数为203 ps的光谱衰减。在TiO(2)上，我们观察到电子从D35的S(1)态快速（<90 fs）注入到TiO(2)导带，随后在界面处瞬态Stark场变化引起双相动力学，时间常数分别为0.8和12 ps，导致S(0)→S(1)吸收带的特征蓝移。有几个过程可以导致这种光谱位移：(i)光激发诱导D35˙（+）自由基阳离子的立即形成，它们最初形成电子-阳离子配合物；（ii）这些配合物解离产生移动电子，当它们开始在介孔tio2(2)中扩散时，局部静电场可能发生变化；（iii）这可能触发D35分子在变化的电场中重新定向。在纳秒时间尺度上，较慢的光谱衰减被解释为局部斯塔克场的减少，因为移动电子向TiO(2)中移动得更深，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，时间常数分别为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Stark位移证据，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传导带电子。时间常数为4,35和550ps的多指数衰减被分配给阳离子和捕获电子之间的复合，以及S(1)中D35分子的一小部分，它们通过IC衰减到S(0)。D35˙（+）在乙腈和二氯甲烷中的微分稳态吸收光谱提供了完整的D(0)→D(1)波段。D35和D35˙（+）的吸收光谱可以用MPW1K泛函的TDDFT计算得到很好的描述。
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.