Electronic structure of thiophene and pyrrole dimers: 2,2′-bithiophene, 2,2′-thienylpyrrole, and 2,2′-bipyrrole

Publisher Website
Google Scholar
Abstract
Vibrationally resolved fluorescence and fluorescence excitation spectra have been measured for solid solutions of 2,2′-bithiophene, 2,2′-thienylpyrrole, and 2,2′-bipyrrole in n-hexane at 4.2 K. The vibronic development of the spectra of all three molecules is very similar to what has been reported previously for longer polythiophene oligomers. Bithiophene and thienylpyrrole have similar S0 to S1 excitation energies: 29 603 and 30 006 cm−1, respectively. The S0 to S1 excitation energy of bipyrrole is significantly higher, 32 720 cm−1, indicating nonplanarity of the pyrrole rings. The vibrational development of the spectra and coincident origin energies in excitation and emission establish that the S0 to S1 transition is symmetry allowed for all three molecules, reasonably described in simple molecular orbital theory as the promotion of one electron from the highest energy occupied molecular orbital (HOMO) to the lowest energy unoccupied molecular orbital (LUMO).