Wachsmann-Hogiu, Sebastian: Vibronic coupling and ultrafast electron transfer studied by picosecond time-resolved resonance Raman and CARS spectroscopy


Chapter 6. Summary

In this thesis vibronic coupling effects occurring at conical intersections have been studied. Strong vibronic coupling between two excited electronic singlet states in a polyene-like molecule, DPH, and the role of vibrational modes in photoinduced ultrafast back-ET in B-30 have been studied with a picosecond time-resolved CARS and Raman set-up, respectively. The experimental apparatus developed in our laboratory is a 50 Hz repetition rate system based on laser amplification in a dye medium. The high pulse energy (200 µJ) which can be obtained allows easily converting the wavelength by SHG or SRS, and thus confer the necessary variability for the experiments. The results obtained show a new effect of excited state vibronic coupling in diphenylpolyenes and allow a view into the mechanism of ET.

6.1 Vibronic coupling in DPH

In a picosecond time-resolved CARS experiment it was possible to observe for the first time two very broad and unusual up-shifted vibrational frequencies in the excited singlet state of DPH, which have frequencies higher than frequency region of the C=C stretching mode [1]. This effect was not observed in the two closest related diphenylpolyenes, DPB and DPO, where the gap between the first and second excited singlet states is larger than in DPH. Time-resolved and polarization sensitive CARS measurements carried out for DPH dissolved in a large variety of solvents differing in their physical properties [1, 2] gave evidence of vibronic coupling between the first and second excited singlet states. Quantum-chemical calculations and time-resolved CARS measurements allowed to assign the two broad vibrational frequencies to the C=C symmetric stretching mode. Two explanations have been discussed: (i) the simultaneous existence of two rotamers, where the two frequencies originate from „different molecules“ and (ii) a model of vibronic coupling by an asymmetric low frequency mode [3]. According to this model, the two up-shifted frequencies originate from the double-well potential formed along the C=C coordinate due to the vibronic mixing between the nearly degenerate low lying excited electronic states [3, 4]. The experimental findings are thus related to a pseudo-Jahn-Teller effect due to nonadiabatic coupling between the first and second excited singlet states in DPH.

6.2 Back-ET in B-30

Excitation of high-frequency vibrational modes after photoinduced b-ET and subsequent IVR has been observed for the first time in B-30 [5]. In the primary event, high frequency Raman active modes are most effective in accepting energy. This is in accordance with predictions derived from Fermi‘s Golden Rule.


However, for complete explanation of the vibrational population kinetics, additional channels of IVR have to be taken into account [6]. Although energy transfer between the Raman active modes has been finished after about 10 to 15 ps, thermalization is not yet complete in the whole molecule. Interplay between vibrational excitation and intramolecular vibrational redistribution in the electronic ground state after back-ET has also been observed. It has been demonstrated that the time-scale for IVR is about 10 ps.

An assignment of the observed Raman vibrations allowed to determine the role of the respective vibrations in the ET transfer process at a molecular level [7]. In particular, the torsional mode between the pyridinium and phenoxide rings and the N-inversion mode are expected to play a significant role in the geometry changes in the excited electronic state and in the back-ET.

In conclusion, this thesis tries to give a microscopic view of two specific photophysical processes occurring in condensed matter. The results should contribute to an understanding of the mechanism of energy transfer between the excited states of polyenes, and to help to elucidate the role of vibrational modes in ET reactions. The experimental results are also a test for quantum-mechanical calculation performed in large molecules.

By developing a system with higher temporal and spectral resolution, it will be possible to obtain more details about these processes.


6.3 References

[1] S. Hogiu, W. Werncke, M. Pfeiffer, A. Lau, T. Steinke; „Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene“, Chem. Phys. Lett. 287 (1998) 8

[2] S. Hogiu, W. Werncke, M. Pfeiffer, A. Lau; „Evidence of strong vibronic coupling in the first excited singlet state of diphenylhexatriene by picosecond CARS spectroscopy“, Chem. Phys. Lett. 303 (1999) 218

[3] M. Pfeiffer, W. Werncke, S. Hogiu, A. Kummrow, A. Lau; „Strong vibronic coupling in the first excited singlet state of diphenylhexatriene by an asymmetric low-frequency mode“, Chem. Phys. Lett. 295 (1998) 56

[4] W. Werncke, S. Hogiu, M. Pfeiffer, A. Lau; „Strong S1-S2 vibronic coupling and enhanced third order hyperpolarizability in the first excited singlet state of diphenylhexatriene studied by time-resolved CARS“, J. Phys. Chem. A, 104 (2000) 4211-4217

[5] S. Hogiu, W. Werncke, M. Pfeiffer, T. Elsaesser; „Mode specific vibrational kinetics after intramolecular electron transfer studied by picosecond anti-Stokes Raman spectroscopy“, Chem. Phys. Lett. 312 (1999) 407

[6] S. Hogiu; W. Werncke, M. Pfeiffer, J. Dreyer, T. Elsaesser; „Mode specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer“, accepted for publication in J. Chem. Phys.

[7] S. Hogiu, J. Dreyer, M. Pfeiffer, K.-W. Brzezinka, W. Werncke; „Vibrational analysis and excited state geometrical changes of betaine-30 derived from Raman and infrared spectra combined with ab initio calculations“, accepted for publication in J. Raman Spectrosc. .

[Front page] [Acknowledgements] [1] [2] [3] [4] [5] [6] [7] [8] [Vita] [Declaration]

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