Electronic Excitations in Oligothiophenes

Whereas polymers are generally immiscible unless there is a specific advantageous chemical interaction, it is possible to incorporate small molecular semiconductors and oligomers within soluble polymeric hosts [10-12, 14, 15], such as polystyrene and poly(9-vinylene carbazole) (PVK). A further development of this technique is to chemically graft the oligomers as pendant side-chains of a polymer [16, 17], These methods retain the advantages of solution-processing properties of polymers and are discussed later in more detail in the section on LED applications. 

In oligomers and polymers, neutral excited states, or excitons, can be produced by photo-excitation or charge recombination (capture of electrons and holes in LEDs). These can either decay radiatively, as desired for light-emitting diodes or nonradia-tively, with the possibility of yielding mobile charge carriers, for photoconductive and photovoltaic cells. 

We discuss here some of the physical issues involving organic LEDs and photocells, so that with this background, we can better appreciate the technological strategies for optimising device performance in the later sections on LEDs and photocells. 

In the following discussion, we consider first the intramolecular nonradiative decay channels, which can occur for isolated oligomers, then intermolecular nonradiative decay channels, which may also operate in solid state thin films, where the oligomers or polymers are densely packed. We also consider the effects of interring torsion and coplanarity of the 7r-conjugated chains, which give rise to both intramolecular and intermolecular effects. 

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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. 

Although the electronic properties of oligothiophenes have already been reviewed [29, 30], the picture of the nature and ordering of the electronic states in the solid state is still diffuse and incoherent. Therefore we will cover recent results on the optical properties especially in the soUd state and the outcome may lead to a clearer picture of the excited state ordering. This may even contribute to a better understanding of the electronic states for other rigid-rod-like conjugated materials. 

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). 

Using TD-DFT at the B3LYP/6-31G(d,p) level to calculate the electronic transition yields a band at 1.68 eV (/ = 1.77) for H4T4CN4, which corresponds well with the experimental band at 790 nm (1.57 eV) with a shoulder at 871 nm. No other electronic transition with a significant intensity was calculated below 3.25 eV. This band arises from HOMO-LUMO excitations. The energies are in fact similar to those observed for aromatic oligothiophenes in the dication or bipolaron state [37b, 99], for which quinonoid... 

Thus, the long lived photoexcitations of the oligothiophenes in highly polar solvents are a combination of neutral triplet excitations and charged excitations, where the neutral triplets still dominate. The interaction of oligothiophenes with C o in polar solvents such as ODCB is photoinduced electron transfer from the excited state of the oligomers onto C o (or hole transfer from excited C6o onto the oligomer). 

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