Strong S1−S2 Vibronic Coupling and Enhanced Third Order Hyperpolarizability in the First Excited Singlet State of Diphenylhexatriene Studied by Time-Resolved CARS

Abstract

A strong S1−S2 vibronic coupling effect is observed in the time-resolved coherent anti-Stokes Raman spectroscopy (CARS) spectra originating from the first excited singlet state of diphenylhexatriene. As determined from picosecond CARS measurements, the excited state spectrum appears on a subpicosecond time scale. An extremely high excited state hyperpolarizability |γ|excited state ≅ 3 ×10-31 is derived from a CARS line shape analysis and is attributed to the increased delocalization after excitation in accordance with semiempirical calculations of bond lengths. We observe two strongly frequency-broadened vibrations being upshifted with respect to the CC double bond stretching region of the ground state and assign them to the totally symmetric CC stretching motion of the chain. Both frequencies depend on the solvent polarizability, giving evidence of strong S1−S2 vibronic coupling in the lowest excited singlet states. We discuss a model of S1−S2 vibronic coupling via an asymmetric low frequency mode. According to this model a double-well potential for the respective vibrational coordinate is generated in the first excited singlet state, resulting in two frequencies originating from the same type of vibration.