Ultrafast Dynamics of the Azobenzene−Coumarin Complex:  Investigation of Cooling Dynamics Measured by an Integrated Molecular Thermometer

Abstract

The energy dissipation mechanism from photoexcited azobenzene (Az) was studied by femtosecond time-resolved UV absorption spectroscopy using 7-amino-4-trifluoromethylcoumarin (ATC) as a probe. The distance between the probe molecule and Az was fixed by covalently linking them together through a rigid proline spacer. Picosecond dynamics in THF solutions were studied upon excitation into the S1 state by a 100 fs laser pulse at 480 nm. Transient absorption spectra obtained for Az-Pro-ATC combined the S1 state absorption and vibrationally excited ground-state absorption of ATC. Correction of the transient spectrum of Az-Pro-ATC for the S1 absorption provided the time-resolved absorption spectrum of the ATC hot band. Three major components were observed in the transient kinetics of Az-Pro-ATC vibrational cooling. It is proposed that in ca. 0.25 ps after the excitation, the S1 state of azobenzene decays to form an initial vibrationally excited nonthermalized ground state of Az-Pro-ATC that involves vibrational modes of both azobenzene and coumarin. This hot ground state decays in ca. 0.32 ps to the next, vibrationally equilibrated, transient state by redistributing the energy within the molecule. Subsequently, the latter state cools by transferring its energy to the closest solvent molecules in ca. 5 ps; then, the energy diffuses to the bulk solvent in 13 ps.

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

“…In most cases of molecular cooling analysis it is assumed that the formation of local temperature is nearly instantaneous, which is based on the idea that the intramolecular energy redistribution mediated by nonlinear interactions inside the molecule is much faster than the energy transfer from the molecule to its surroundings. Consequently, the temperature equivalent of the deposited excitation energy is considered to be the starting value of local temperature 21,29,53,54. Here, we see that due to the details of the actual build-up of the temperature, its maximal values can be expected to be much smaller than the mentioned predictions based on instantaneous heating.…”

Direct Observation of Vibrational Energy Flow in Cytochrome c

“…An attempt was made to observe the energy transfer between a pair of molecules by utilizing molecular heater–thermometer integrated systems, where two different molecules, namely, a heater that absorbs the visible radiation and a thermometer that probes the temperature by changing the absorption in the vicinity of its hot band, are brought into proximity by covalently linking the two discrete molecules. − However, for systems with longer linkers, the flexibility of the linker makes it difficult to keep the distance and relative orientation between the heater and thermometer molecules well-defined.…”

Surface hopping modeling of charge and energy transfer in active environments

“…This process is referred to as intramolecular vibrational redistribution (IVR). 245,246 IVR happens in the excited state after photoexcitation, which lifts the ground-state wave function to an excited state, where it is no longer an eigenvector of the Hamiltonian, yielding a wave packet evolving with time among the vibrational modes. IVR also happens after internal conversion to a lower electronic state, converting electronic energy into vibrational energy and creating a thermally unequilibrated molecular system.…”