Zero-field-splitting and π-electron spin densities in the lowest excited triplet state of oligothiophenes

Abstract

The electronic properties of thiophene oligomers (nT, n=2–8) have been investigated in the lowest excited triplet state. Theoretical calculations of the zero field splitting parameters and of the π-electron spin density have been performed and compared with previous experimental EPR results. The calculations are based on a simple π-electron (one-electron-per-site) model including electron–electron interaction at the extended Hubbard level. Optimized bond lengths result from making them self-consistent to the corresponding bond orders via Coulson’s relationship. The calculated D values decrease from D=0.0959 cm−1 for n=2 to D=0.0597 cm−1 for n=8, in agreement with EPR data. The measured as well as the calculated E values are rather small. Furthermore, we found that ZFS parameters are affected by the torsion angles between the thiophene rings. The chain length dependence of D can be rationalized comparing π-electron spin density calculations and computed bond length distortions. These clearly indicate that the triplet excitation reaches a finite extension over about four thiophene rings.

Photostability of D–π–A nonlinear optical chromophores containing a benzothiazolium acceptor

“…This increasing tendency of a quantum yield with extension of conjugated bridge copies the ability of phosphorescence emission by molecules 1 – 3 although only qualitative conclusions of the emission probability could be done. This behavior is consistent with the fact that the S 0 – S 1 excitation energy changes more with the conjugate chain length than that of S 0 – T 1 49, 50. Enhancement in rate constants for intersystem crossing for longer chains was observed and attributed to a lowering of the S 1 – T 1 energy gap for longer chains 49, 51.…”

“…This behavior is consistent with the fact that the S 0 – S 1 excitation energy changes more with the conjugate chain length than that of S 0 – T 1 49, 50. Enhancement in rate constants for intersystem crossing for longer chains was observed and attributed to a lowering of the S 1 – T 1 energy gap for longer chains 49, 51. Thus, molecules with longer conjugation lengths seem to have a higher triplet quantum yield.…”

Vibrational optical activity of chiral carbon nanoclusters treated by a generalized π-electron method

“…A properly parametrized, effective model can be highly useful for π -electron systems. 23,62,[67][68][69][70][71][72][73][74][75][76][77] Among π -electron models, the first-neighbor approximation (here called first-neighbor tight-binding (TB) or Hückel-model) represents the simplest approach. When applied in a broader sense, the TB approximation may refer to the application of a distance based cutoff.…”

Resonance Raman Spectroscopy of the T1 Triplet Excited State of Oligothiophenes

“…These results agree with those from prior DFT calculations 34 and from semi-empirical extended Hubbard calculations. 41 We propose that the localized quinoidal region and the distributed unpaired electron density disrupt the regular bonding pattern of the ground state, and give rise to the relatively complex vibrational spectra of triplet tetra-and hexathiophene. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Unlike the similar ground-state spectra, we find significant differences between the triplet spectra of tetrathiophene and hexathiophene.…”