Hochkorrelierte Wellenfunktionen zur Berechnung der angeregten Zustände von Farbstoffkationen - High-correlated wave functions for the calculation of the excited states of cationic dyes
The thesis with the title 'High-correlated wave functions for the calculation of the excited states of cationic dyes' treats the computation of the spectral characteristics of different conjugated pi-systems with the use of
quantum-chemical methods. A goal was to achieve a deepened insight to the connection between structure and color. Therefor the ab initio methods CASSCF and CASPT2 have been applied; for comparison additionally the methods HF-SCF/CIS and DFT-LR were used.
The computations were accomplished at a set of streptocyanine dyes. The long-wavelength absorption bands were calculated with high accuracy; in particular the characteristic shift of the long-wavelength absorption band (of about 100nm) on extension of the chromophors by one vinyl unit could be reproduced. Methods that take use of only one determinant reference functions to describe the excited states are not able to
reproduce the results. The electronic structure of polyenyl cationens is very similar to the
structure of the isoelectronic cyanine dyes. The experimental long-wavelength absorption bands for a series of those dyes could also be reproduced.
The characteristics of the cyanines and polyenyl cations are also existent in protonted all-trans Schiff Base chromophors. The results of calculations with simple psb-model systems show both, the applicability of the used methods to this type of chromophor and the possibility to scale to model systems. On this basis later
investigations of models of the retinal chromophores could be evaluated.
Two chapters of the thesis are dedicated to the spectral properties of the photo receptor protein rhodopsin and/or its chromophore, the protonated retinal Schiff base. The characteristics of the first photo intermediates and the effect of charged groups to the absorption spectrum were brought up for discussion there.
To investigate the reason for the bathochromic shift of the first photo intermediates the rhodopsin models represent the chromophor in the dark state and/or the first photo product of rhodopsin. Both retinal models were produced on the basis of experimental data. The relevance of these structures were confirmed by the computed bathochromic shift of the long-wave absorption band, which reproduces the experimental shift
almost quantitatively. With the use of models that represent transition structures between the dark state and the photoexcited model, the most
important contribution to this shift could be led back to anti-planar twisting of the Chromophors, i.e. to twisting of the formal double bonds within the rather polyenic region of the chromophores.
In order to examine the effect of the charged chromophore environment to the absorption spectrum of rhodopsin, the excited states of chromophore
models that include parts of the structure of the protein environment were computed. Different chromophore geometries in interaction with the
counter ion Glu113, water, Glu181 (in different protonation states) and Tyr268 were used. Changes to the chromophore structure due to the environment are small but, however, clear. The presence of Glu113/water causes a strong blue shift of the S_1-state. The effect of an
additional carboxyl group (Glu181) is strongly reduced in presence of the complex counter ion.