Simultaneous measurement of electronic and vibrational dynamics to clarify a geometrical relaxation process in a conjugated polymer

Abstract

The real-time vibrational spectroscopy was utilized to obtain both electronic and vibrational dynamics in a conjugated polymer under the same excitation and probing condition by using the same sample and observing at the same time. Most of the conjugated polymers are in an amorphous phase, in which there is a relatively broad distribution of chain length, conjugation length, and degree of interaction strength between neighboring chains. Because of the varieties, a simultaneous measurement of electronic and vibrational dynamics in a broad spectral range is considered to be most powerful to study such systems as conjugated polymers. In the present paper, we performed this type of experiment of the simultaneous probing of electronic and vibrational relaxations at 128 wavelengths. The sample studied here is an amino-moiety-containing conjugated polymer, poly{[3-hexylthiophene-2,5-diyl]-[$p$-dimethylaminobenzylidenequinoidmethene]} (PHTDMABQ), whose monomer is a derivative of a thiophene oligomer. The light source is a few-cycle pulse laser with an ultimate time resolution of 0.2 fs in the visible and near IR ranges. The data containing both electronic relaxation and vibrational dynamics were analyzed to obtain a more reliable relaxation mechanism of the excitations than a combination of individual studies of electronic and vibrational relaxations. By utilizing the ultrafast pump-probe spectroscopic system, we could identify the exciton state with a $\ensuremath{\pi}$-electron delocalized in benzilidene ring. In the polymer PHTDMABQ, the geometrical relaxation from the free exciton to the exciton polaron takes place in 60--100 fs. This time is close to that observed in several polydiacetylenes previously reported. The C-C single bond stretching frequencies of the free exciton and exciton polaron are about 1300 and $1350\text{ }{\text{cm}}^{\ensuremath{-}1}$. The coherent molecular vibration after the geometrical relaxation is considered to be a kind of reaction induced coherence. This coherent vibration decays with the time constant of about 450 fs.