[page 100↓]

9  Final Conclusion

The investigations of the donor-acceptor DMABN derivatives have been characterized by absorption and fluorescence at both room- and low-temperature, as well as time-resolved absorption and fluorescence spectroscopic methods. Besides, additional supporting evidence has been gained by performing quantum chemical calculations mainly using the semiempirical method with AM1 method. In most of the compounds investigated in this work, intramolecular charge transfer (ICT) formation could well be explained by twisted intramolecular charge transfer (TICT) state through the decoupling of the donor-acceptor moieties in the excited state.

Although, the tetrafluoro analogue of DMABN, DMABN-F4, is closely related to the parent compound spectroscopically, the short wavelength B-band is not observed in the fluorescence spectrum even at 77 K in this compound. This has been explained by the fact that there is an ultrafast access to the CT conformation in the excited state. This could possibly be linked to the pretwisted ground state geometry and the increased acceptor strength. The analysis of the time-resolved measurements indicates that the emission of DMABN-F4 is strongly forbidden and is consistent with the formation of a TICT state with high dipole moment.

New insight was gained by the investigation of other fluorinated derivatives of aniline and phenyl pyrrole. The low fluorescence quantum yield values and the absence of phosphorescence in all of these fluorinated derivatives suggest that the high rate of non-radiative decay takes place through internal conversion rather than intersystem crossing. A possible internal conversion photochemical reaction path could be the folding (butterfly motion) of the benzene ring either towards a Dewar or a prefulvene deformation. It is tentatively concluded that the F-atoms increase this photoreaction tendency already present in the parent benzene (channel III).

From the studies on the photophysical properties of meta- and para-cyano-N-phenylpyrrole (m- and p-PBN), it has been found that both compounds show highly red shifted and strongly forbidden emission in polar solvents, assigned to a TICT state. Comparison to quantum chemical calculations indicates that the twisted structure is connected with an antiquinoid distortion of the benzonitrile group. It has been concluded that m-PBN differs from p-PBN by a less exergonic formation of the TICT state from the LE/ICT quinoid state, and it therefore shows only single LE/ICT fluorescence in nonpolar alkane solvents, [page 101↓]whereas p-PBN shows dual fluorescence (LE/ICT and TICT).

Furthermore, the investigation on dimethyl derivatives of N-phenylpyrrole such as p-DPBN and m-DPBN gave additional insight into the way the photophysical properties would be affected by changing the position of the acceptor group and by increasing the donor-strength and the ground state twist angle. The results led to the conclusion that they emit from a TICT state similarly to the nonmethylated PBN pair, but that this emission is even more strongly forbidden.

Finally, a new perspective regarding the CT state has been gained from compounds with a different linkage position on the donor such as MP2BN and additionally by changing the orientation of the acceptor part and by increasing its strength such as MP2-B25CN. The mesomeric interaction between donor and the different acceptor units has been investigated, and it was found that the behaviour could switch between ICT states with large mesomeric interaction (MICT -Mesomeric Intramolecular Charge Transfer) such as in MP2BN, and with a minimum mesomeric interaction (TICT – Twisted Intramolecular Charge Transfer) states such as in p-PBN and MP2-B25CN. The important factors, such as the relative energies of LE/ICT (MICT) and TICT state and the strength of the mesomeric interaction in the MICT state have been mainly considered.

As a whole, this dissertation mainly deals with the investigation on the photophysical properties of donor-acceptor molecules. All the observations help to understand the ICT processes taking place in the excited state. Throughout this study, the TICT/MICT model could reasonably well explain the CT processes occurring in these molecules. The low fluorescence quantum yields of the fluorinated derivatives of DMABN should be further explored by other techniques such as transient absorption and photo-acoustic spectroscopy as well as preparative photochemistry to get a clear idea on the non-radiative decay pathways involved. A possible clue for this non-radiative decay pathway can also be obtained by performing higher-level quantum chemical calculations such as ab initio and time-dependent density functional theory (DFT) calculations with bending, folding or twisting as a reaction coordinate. Concerning the N-phenylpyrrole compounds, fluorescence polarisation spectroscopy can further help to clarify the nature of the CT excited states observed.


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