Emitting Species of Poly(3-hexylthiophene): From Single, Isolated Chains to Bulk

The photophysical properties of films of alkyl-substituted
polythiophenes are governed by a subtle interplay between intra- and
interchain electronic couplings. The intramolecular properties, however,
are still not entirely clear because polythiophenes possess a strong
tendency to form π-stacked aggregate structures with appreciable
interchain couplings. Here we employ low-temperature single-molecule
photoluminescence spectroscopy on isolated regioregular poly(3-hexyl-
thiophene), P3HT, chains with different, but well-defined molecular
weights to reveal the intrachain properties of their emitting sites. We find
that the inhomogeneous distribution function of the zero-phonon lines
(ZPL) is very narrow (<480 cm⁻¹ , 60 meV), which indicates a low degree
of torsional disorder of the P3HT backbone on length scales of the
emitting sites (despite a large mean dihedral angle). Moreover, the single-
chain ZPLs are exclusively located in the high energy tail of the
corresponding spectrum of a disordered ensemble. Using concentration-dependent measurements in combination with time-
dependent density functional theory, we show that this spectral shift stems from aggregation-induced partial planarization and
concomitant electronic coupling between segments of neighboring P3HT chains.