Observation of breather excitons and soliton in a substituted polythiophene with a degenerate ground state

We investigated the ultrafast dynamics of a unique polythiophene derivative that has a degenerate ground state due to an alternating benzenoid-quinoid resonance. We probed 128 different wavelengths at the same time by using a sub-5-fs pulse laser and a detection system composed of a polychromator and a multichannel lock-in amplifier. The method allowed us to study the electronic relaxation and vibrational dynamics in completely same conditions at the same time. Because the polythiophene derivative has degenerate ground state, solitons are expected to be generated after photoexcitation. The dynamics of a breather composed of a dynamic bound state of solitons generated immediately after photoexcitation was time-resolved to reveal coupling between the vibrational modes and the solitons. The C-C and C-C stretching modes were found to be modulated by the breather, whose lifetime was determined to be 30-50 fs. The results of quantum-chemical excited-state molecular dynamics simulation are consistent with experimental results. Our modeling results allow to identify related vibrational normal modes strongly coupled to the electronic degrees of freedom. Moreover, analysis of calculated trajectories of excited state shows appearance of short-lived breather excitation decaying due to intramolecular vibrational energy equilibration on a time scale of hundreds of femtoseconds, which also agrees well with the experimental results.