Electronic absorption and fluorescence of vapor-deposited oligothiophenes: exciton splitting in self-organized molecular films

Hans-Joachim Egelhaaf; Dieter Oelkrug

doi:10.1117/12.200971

9 February 1995 Electronic absorption and fluorescence of vapor-deposited oligothiophenes: exciton splitting in self-organized molecular films

Hans-Joachim Egelhaaf, Dieter Oelkrug

Author Affiliations +

Proceedings Volume 2362, International Conference on Excitonic Processes in Condensed Matter; (1995) https://doi.org/10.1117/12.200971
Event: Excitonic Processes in Condensed Matter: International Conference, 1994, Darwin, Australia

Abstract

The electronic transitions of vapor deposited (alpha) -oligothiophenes nT with n equals 3 to 8 thiophene units and film thickness of d equals 2 to 200 nm are investigated by angular-resolved polarized absorption and fluorescence spectroscopy. The absorption spectra of monolayers are strongly blue-shifted and show extreme dichroism arising from nT molecules oriented with their long molecular axes perpendicularly to the plane of the substrate. The extent of band shifts due to H-aggregation can roughly be quantified with the model of exciton dipole-dipole interaction, but the experimental oscillator strengths are much too low. In multilayers, the uniaxial molecular order is well preserved in 5T up to 10 layers. However, in 6T the order collapses quickly with growing film thickness, leading to small domains with no preferential orientation to the substrate. In the latter case the exciton splittings are reduced and the selection rules are broken up resulting in broader and more intense absorption bands. The fluorescence yields of the films are in the order of (phi) equals 10^-4 to 10^-2, increasing gradually with the film thickness, but still definitely lower than in the monomers where (phi) equals 0.07 (3T) to 0.4 (6T). At least two weak fluorescence emissions are identified in the films originating from the low energy edge of the exciton band and/or from the two-electron excited 2¹A$g)-state.

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Hans-Joachim Egelhaaf and Dieter Oelkrug "Electronic absorption and fluorescence of vapor-deposited oligothiophenes: exciton splitting in self-organized molecular films", Proc. SPIE 2362, International Conference on Excitonic Processes in Condensed Matter, (9 February 1995); https://doi.org/10.1117/12.200971

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