Vibrational chirp in the dynamic Stokes-shift process due to ultrafast geometrical relaxation in a polydiacetylene

Relaxation kinetics of photoexcitation in a blue-phase polydiacetylene has been investigated by difference absorption spectroscopy with extremely high time- and energy-resolution using broadband sub-5-fs optical pulses. One-fs-step tracking of an induced emission peak has enabled us to separate the vibrational dynamics during geometrical relaxation and internal conversion into modulations of transition-peak energy and transition probability. The peak energy shows redshift of about 60 meV and the instantaneous frequency of the “CC” stretching mode is positively “chirped” starting from 1600±100 cm⁻¹ (21±2 fs) and ending in 2100±100 cm⁻¹ (16±1 fs). This positive chirp indicates a huge dynamic structural change from (CRCCCR^′)_n–like to (CRCCCR^′)_n–like structured bonds in the polymer backbone during the internal conversion process within 100 fs. This corresponds to the reduction of the bond length from 1.32±0.04 Å to 1.19±0.04 Å by about 0.13 Å, which was estimated from the bond lengths in literatures determined by the x-ray analysis for several polydiacetylenes.