Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy

The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in solution is studied experimentally and theoretically. Using the pump-supercontinuum probe (PSCP) technique we observed the complete spectral evolution of hot absorption induced by femtosecond optical pumping. In t-stilbene the hot S₁ state results from S_n®S₁ internal conversion with 50 fs characteristic time. The time constant of intramolecular thermalization or intramolecular vibrational redistribution (IVR) in S₁ is estimated as t_IVR<<100 fs. In PNA and DPNA the hot ground state is prepared by S₁®S₀ relaxation with characteristic time 0.3-1.0 ps. The initial molecular temperature is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency n_m and width square G² on molecular temperature T. Within this framework we derive analytic expressions for the differential absorption signal DOD(T(t),n). After calibration with stationary absorption spectra in a low temperature range, the solute temperature T(t) may be evaluated from a transient absorption experiment. For highly polar PNA and DPNA, T(t) is well described by a biexponential decay which reflects local heating effects, while for nonpolar t-stilbene the local heating is negligible and the cooling proceeds monoexponentially. To rationalize this behavior, an analytic model is developed, which considers energy flow from the hot solute to a first solvent shell and then to the bulk solvent. Fastest cooling is found for PNA in water: a time constant of 0.64 ps (68%) corresponds to solute-solvent energy transfer while 2.0 ps (32%) characterizes the cooling of the first shell. In aprotic solvents cooling is slower than in alcohols and slows down further with decreasing solvent polarity. This contrasts with nonpolar t-stilbene which cools down with 8.5 ps both in acetonitrile and cyclohexane. Comparison of the cooling kinetics for PNA in water with those for DPNA in water-acetonitrile mixtures suggests that the solute-solvent energy transfer proceeds mainly through hydrogen bonds.

S.A. Kovalenko, R. Schanz, H. Hennig, N.P. Ernsting