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Third- and Fifth-order Nonlinear Time-resolved Spectroscopies for Ultrafast Molecular Dynamics in Carotenoids

Miki, Takeshi

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Multipulse optical technique is an essential tool on the direct observation of electron-nuclear motions responsible for various molecular properties. For example, light-energy harvesting or anti-oxidation, which flora and fauna have achieved along the course of evolution, are initiated by the molecular dynamics in conjugated hydrocarbons such as conjugated polyenes or porphyrin rings. In the case of the conjugated systems in photosynthetic pigments, a part of the dynamics has been revealed as an electronic-state dynamics. However, it is required to disentangle the remaining part of the molecular dynamics mainly consisting of the vibronic interactions induced by nuclear motions. In the scope of this thesis, the vibronic interactions between the electronic states having Bu or Ag symmetries in conjugated polyenes were detected by use of pump-probe and pump-degenerate four-wave mixing (DFWM) experiments. In addition, as an example of multimodal time-resolved spectroscopies, the combination of the two optical experiments was demonstrated to overcome analytical problems related to precision and accuracy of functional analysis for pump-probe spectra. The significant influences of the vibronic interactions between the electronic states with Ag-Ag or Bu-Bu symmetries were observed by pump-DFWM experiments for a series of the conjugated polyenes having four different conjugated double bond length N = 9, 10, 11 and 13. The frequency shifts of C–C (1100 cm–1) and C=C (1500 cm–1 for Bu state, 1800 cm–1 for Ag states) stretching modes indicated the features and some difference of the two couplings. The coupling between Ag– states appeared for all polyenes under the existence of the excited state with Ag symmetry. On the other hand, the coupling between Bu states only appeared for the polyenes with N = 9 and 10, in which strong degeneracy of two Bu states can exist. In addition, solvent polarizability changed the coupling strength which was examined for lutein (N = 9.5) in three different solvents (hexane, THF and benzene). While the coupling appeared in hexane and in THF, it was absent in benzene since the degeneracy of the ionic Bu+ state and covalent Bu– state were very sensitive to solvent polarizability. The observation could be connected to environmental effects on the photosynthetic polyenes surrounded by proteins and lipids in photosynthetic apparatus. In addition, an example of multimodal approach, which combines two different optical experiments, was demonstrated by the simultaneous analysis of a pair of data sets recorded by pump-probe and pump-DFWM experiments. This approach overcame conventional analytical problems of rotation ambiguity and local minimum in global target fitting. While the characterization of the relaxation model for rhodamine 6G was not uniquely done by global target fitting, the multimodal approach uniquely determined the appropriate kinetic model by the evaluation of four error functions. Moreover, the interpretation of the spectral and temporal elements were based on the response functions of pump-probe and pump-DFWM experiments. The direct detection of vibronic coupling and the methodological development to disentangle the ultrafast molecular dynamics contributes to the investigation of nonadiabatic processes which is crucial to understand molecular properties.

Item Type: Dissertation
Supervisor: Motzkus, Prof. Dr. Marcus
Place of Publication: Heidelberg
Date of thesis defense: 23 June 2017
Date Deposited: 26 Jun 2017 09:04
Date: 2017
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Subjects: 530 Physics
540 Chemistry and allied sciences
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