  The optoelectronic properties of amorphous conjugated polymers are sensitive
to conformational disorder and spectroscopy provides the means for structural
characterization of the fragments of the chain which interact with light -
"chromophores". A faithful interpretation of spectroscopic conformational
signatures, however, presents a key challenge. We investigate the relationship
between the ground state optical gaps, the properties of the excited states,
and the structural features of chromophores of a single molecule
poly(3-hexyl)-thiophene (P3HT), using quantum-classical atomistic simulations.
Our results demonstrate that chromophoric disorder reflects an interplay
between excited state de-localization and electron-hole polarization, and is
controlled by torsional disorder that is specifically associated with the
presence of side chains. Within this conceptual framework, we predict and
explain a counter-intuitive spectral signature of P3HT: a red-shifted
absorption, despite shortening of chromophores, with increasing temperature.
