Time-Dependent Density Functional Theory Study of Low-Lying Absorption and Fluorescence Band Shapes for Phenylene-Containing Oligoacenes

Low-lying band shapes of absorption
and fluorescence spectra for
a member of a newly synthesized family of phenylene-containing oligoacenes
(POA 6) reported in J. Am.
Chem. Soc. 2012, 134, 15351 are studied theoretically with two different
approaches with TIPS–anthracene as a comparison. Underlying
photophysics and exciton–phonon interactions in both molecules
are investigated in details with the aid of the time-dependent density
functional theory and multimode Brownian oscillator model. The first
two low-lying excited-states of POA 6 were found to exhibit excitation
characteristics spanning entire conjugated backbone despite the presence
of antiaromatic phenylene section. Absorption and fluorescence spectra
calculated from both time-dependent density functional theory and
multimode Brownian oscillator model are shown to reach good agreement
with experimental ones. The coupling between phonon modes and optical
transitions is generally weak as suggested by the multimode Brownian
oscillator model. Broader peaks of POA 6 spectra are found to relate
to stronger coupling between low frequency phonon modes such as backbone
twisting (with frequency <300 cm–1) and optical
transitions. Furthermore, POA 6 exhibits weaker exciton–phonon
coupling for the phonon modes above 1000 cm–1 compared
to TIPS–anthracene owing to extended conjugated backbone. A
significant coupling between an in-plane breathing mode localized
around the antiaromatic phenylene segment with frequency at 1687 cm–1 and optical transitions for the first two excited-states
of POA 6 is also observed.