Oligothiophenes transition energies

Fig. 7. Observed electronic transition energies of a-oligothiophenes in various states A, neutral species , radical cation o, dication.

We display in Fig. 1.4 the evolution with chain length in oligothiophenes of the 5o 5i transition energy as well as the 5o T energy difference, as calculated at the INDO/MRD-Cl level. Strikingly, we observe the 5q... 

Table 2. Energy (in eV) of the HOMO and LUMO levels, lowest transition energy (Eu, in eV) and related oscillator strength (OS, in arbitrary units) of the coplanar cyano-substituted oligothiophenes in a planar conformation, as provided by INDO/SCI calculations. We present in parentheses the INDO shifts of the frontier levels (with respect to a reference compound in each group) that are compared to the corresponding experimental values appearing in bold [80].

Figure 4. Optical transition energies [70] of oligothiophenes in dichloromethane solution as a function of reciprocal chain-length (1/n) [71]. In addition to the unsubstituted T (Fig. 2), the data for dodecyl-substituted oligothiophenes are included (DDnT) [71], The dotted line shows a linear extrapolation (1/n —> 0), whereas the solid line corresponds to a model calculation according to [79]. Note the difference for small 1/n values.

Summarizing all the optical spectroscopy on isolated molecules, 6T can well be assumed to be a model compound for longer oligothiophenes and especially for polythiophene from all the experimental data as well as theoretical approaches. The similarity between 6T and polythiophene was not only revealed by the transition energies, but also appeared from many other spectroscopic experiments, e.g. photoinduced absorption (PA) spectra [91], or photoemission spectroscopy on the valence levels [92], and have been discussed elsewhere [29, 30]. 

The oligothiophenes show within this series the strongest TPA, while oligofuranes are comparable with polyenes. Experiments confirmed the large TPA of several thiophene oligomers (29). These compounds have TPA cross sections greater than 1000 GM with an excitation energy of more than 4 eV for the optical TPA transition [407]. 

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... 

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... 

A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers (Chart 8.17 and 8.19) were investigated for comparison with a previously studied quinoid oligothiophene that demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices [103]. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one electron second oxidation between 1.37 and 1.96 V and a reversible two-electron reduction between - 0.05 and - 0.23 V. The UV-Vis-NIR spectrum of each compound in solution is dominated by an intense (s = 100000 cm ) low-energy jt-jt transition that has a kmax ranging... 

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