Structural Dynamics of a Noncovalent Charge Transfer Complex from Femtosecond Stimulated Raman Spectroscopy

被引:20
作者
Fujisawa, Tomotsumi [1 ]
Creelman, Mark [1 ]
Mathies, Richard A. [1 ]
机构
[1] Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA
关键词
PHOTOINDUCED ELECTRON-TRANSFER; PICOSECOND LASER PHOTOLYSIS; ENERGY-GAP DEPENDENCE; DONOR-ACCEPTOR COMPLEXES; RADICAL-ION-PAIRS; EXCITED-STATE; PYROMELLITIC DIANHYDRIDE; SUPRAMOLECULAR SYSTEMS; RECOMBINATION DYNAMICS; ACETONITRILE SOLUTION;
D O I
10.1021/jp3001306
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Femtosecond stimulated Raman spectroscopy is used to examine the structural dynamics of photoinduced charge transfer within a noncovalent electron acceptor/donor complex of pyromellitic dianhydride (PMDA, electron acceptor) and hexamethylbenzene (HMB, electron donor) in ethylacetate and acetonitrile. The evolution of the vibrational spectrum reveals the ultrafast structural changes that occur during the charge separation (Franck-Condon excited state complex -> contact ion pair) and the subsequent charge recombination (contact ion pair -> ground state complex). The Franck-Condon excited state is shown to have significant charge-separated character because its vibrational spectrum is similar to that of the ion pair. The charge separation rate (2.5 ps in ethylacetate and similar to 0.5 ps in acetonitrile) is comparable to solvation dynamics and is unaffected by the perdeuteration of HMB, supporting the dominant role of solvent rearrangement in charge separation. On the other hand, the charge recombination slows by a factor of similar to 1.4 when using perdeuterated HMB, indicating that methyl hydrogen motions of HMB mediate the charge recombination process. Resonance Raman enhancement of the HMB vibrations in the complex reveals that the ring stretches of HMB, and especially the C-CH3 deformations are the primary acceptor modes promoting charge recombination.
引用
收藏
页码:10453 / 10460
页数:8
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