Electron Transfer Across Modular Oligo-p-phenylene Bridges in Ru(bpy)2(bpy鈥損hn鈥揇Q)4+ (n = 1鈥?) Dyads. Unusual Effects of Bridge Elongation

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• 作者：Maria Teresa Indelli ; Michele Orlandi ; Claudio Chiorboli ; Marcella Ravaglia ; Franco Scandola ; Frederic Lafolet ; Steve Welter ; Luisa De Cola
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：January 12, 2012
• 年：2012
• 卷：116
• 期：1
• 页码：119-131
• 全文大小：1246K
• 年卷期：v.116,no.1(January 12, 2012)
• ISSN：1520-5215

文摘

A series of dyads of general formula Ru(bpy)₂(bpy鈥損h_n鈥揇Q)⁴⁺ (n = 1鈥?), based on a Ru(II) polypyridine unit as photoexcitable donor, a set of oligo-p-phenylene bridges with 1鈥? modular units, and a cyclo-diquaternarized 2,2鈥?bipyridine (DQ²⁺) as electron acceptor unit, have been synthesized. Their spectroscopic and photophysical properties have been investigated in CH₃CN and CH₂Cl₂ by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. The experimental study has also been complemented with a computational investigation carried out on the whole series of dyads. The absorption spectra of the dyads show new spectroscopic transitions, in addition to those characteristic of the donor, bridge, and acceptor fragments. DFT calculations suggest the assignment of such bands as bridge-to-acceptor (蟺 ph_n) 鈫?(蟺* DQ) charge-transfer transitions. This assignment is consistent with the solvatochromic and spectroelectrochemical behavior of the new bands. For all the dyads at room temperature in fluid solution, the typical ³MLCT luminescence of the Ru(II) polypyridine unit is strongly (>90%) quenched, supporting the occurrence of an efficient intramolecular photoinduced electron transfer. The study has revealed, however, that the photophysical mechanism is actually more complex than presumed on the basis of a simple photoinduced electron-transfer scheme. For n = 1, very fast (few picoseconds) photoinduced electron transfer from the MLCT state localized on the substituted bpy ligand to the DQ unit has been observed, followed by slower interligand hopping and charge recombination. For n = 2鈥?, MLCT excited-state quenching takes place without transient detection of charge-separated product, indicating that charge recombination is faster than charge separation. This behavior can be rationalized in terms of the superexchange couplings expected through this type of bridges for the two processes. The kinetics of MLCT quenching in the dyads with n = 1鈥? does not follow the usual exponential falloff with bridge length: after a regular decrease for n = 1鈥?, the rate constants become almost insensitive to bridge length for n = 3鈥?. The rationale of this uncommon behavior, as suggested by DFT calculations, lies in a switch in the MLCT quenching mechanism with increasing bridge length, from oxidative quenching by the DQ acceptor to reductive quenching by the bridge.