Ultrafast dynamics of excitons and solitons in disubstituted polyacetylene

We studied the ultrafast photoexcitation dynamics in disubstituted polyacetylene (DPA). We found two distinctively different relaxation channels for the photogenerated excitons; ionic and covalent pathways. In DPA films the photogenerated odd-parity ${(B}_u)$ excitons that are responsible for the high photoluminescence quantum efficiency follow the ionic relaxation pathway. During the hot exciton thermalization process, however, a fraction of the $B_u$ excitons undergo a phonon-assisted transition to the covalent ${2A}_g$ state, which consequently decomposes into two neutral soliton-antisoliton pairs that are subject to ultrafast recombination or dissociate into stable neutral solitons. In DPA solutions the ionic channel remains unchanged, however, the covalent channel becomes ineffective due to the different hot exciton thermalization pathways.