Thermochromism, Franck–Condon Analysis and
Interfacial Dynamics of a Donor–Acceptor Copolymer with a Low
Band Gap
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Abstract
The
electronic properties of the donor–acceptor (DA) polymer
poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[<i>c</i>]-1,2,5-thiadiazole}
(PTBT) have been investigated using spectroscopic and computational
techniques. Electronic absorption and emission spectra reveal the
presence of an ordered and a disordered phase in solution. Franck–Condon
modeling of the ordered phase yields Huang–Rhys factors of
0.55 (20 °C) and 0.51 (−180 °C), indicating little
structural distortion between ground and excited state. DFT calculations
with resonance Raman spectroscopy are consistent with a lowest energy
excited state that is electronically delocalized and has little charge-transfer
character, unexpected for a copolymer with a low bandgap (∼1.8
eV). Transient absorption spectroscopy of PTBT:fullerene blends reveals
near-unity internal charge-transfer yields in both ordered and disordered
film morphologies. In the disordered blend, charge transfer is complete
within the laser pulse (100 fs), whereas the ordered blend also features
a slower phase due to exciton diffusion in the phase separated morphology.
In the ordered blend, the spectra and dynamics of charge transfer
reveal that excitons and charges promptly occupy delocalized states
on extended polymer chains. The pervasive use of donor–acceptor
structures in polymer devices makes understanding the interplay of
morphology and electronic structure of these polymers essential and
here a spectroscopic and computational investigation gives an extensive
picture of the electronic properties and their effect on charge dynamics
in a DA polymer