Oligothiophenes electronic excitations

In all systems, the lowest energy transition is mainly described by an electronic excitation between the HOMO and LUMO levels (with an additional contribution arising from the H - 1 L - - 1 excitation) [45]. Considering the oligothiophenes in their planar conformation, the cyano substitution leads to a red shift of the lowest optical transition because the derivatization gives rise to an asymmetric stabilization... 

The low fluorescence quantum yield and the fact that no phosphorescence could be observed in oligothiophenes leads to the conclusion that most of the electronic excitation energy decays radiationless. There are different views of the participation of triplet states in this decay process in thin films [39, 236, 247, 249, 257, 259, 267-274]. In solution, however, the relaxation via triplet states is well agreed. 

Alkyl-substitution influences the absorption spectra in thin films mainly by their influence on the co-planarity of the molecules, particularly in the excited state, and only to a minor extent by their electron pushing (inductive) effect (see [181]). The latter should result in a red shift which is not observed in absorption spectra of thin films but can be detected in the fluorescence spectra. In absorption spectra, major blue shifts of the absorption peaks occur if larger differences in the torsion angle between different rings are induced by the substituents. As long as the co-planarity is not distorted, quite similar absorption spectra are observed if compared to unsubstituted oligothiophenes [13,182]. 

Recently Egelhaaf et al produced oligothiophene radical cations on silica gel by two-photon ionization with laser intensities I > 5x 106 W cm and laser flash energies above 15 mJ [38]. The electron is excited into a continuum state and no anion is formed. The absorption spectra are very similar to those observed for radical cations in solution. The ESR signal exhibits a signal with, g = 2.0028 at room temperature. At T= 77 K an anisotropic ESR spectrum is found. 

An oligothiophene-flillerene pentad 2.116 (Chart 1.23) was recently synthesized, in which four pendant fullerenes are coupled to the two termini of long oligothiophene chains [223]. An efficient intramolecular electron transfer process was observed in the excited state for these pentads. In a sandwich device Al/2.116 (n = 6)/Au, a photocurrent generation of 14 % was reported under monochromatic illumination, which is similar to the value for triad 2.115 (n = 3). 

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