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| - Ultrafast Dynamics of the Azobenzene−Coumarin Complex: Investigation of CoolingDynamics Measured by an Integrated Molecular Thermometer
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| - 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 distancebetween the probe molecule and Az was fixed by covalently linking them together through a rigid prolinespacer. Picosecond dynamics in THF solutions were studied upon excitation into the S1 state by a 100 fs laserpulse at 480 nm. Transient absorption spectra obtained for Az-Pro-ATC combined the S1 state absorptionand 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 majorcomponents were observed in the transient kinetics of Az-Pro-ATC vibrational cooling. It is proposed that inca. 0.25 ps after the excitation, the S1 state of azobenzene decays to form an initial vibrationally excitednonthermalized 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 redistributingthe energy within the molecule. Subsequently, the latter state cools by transferring its energy to the closestsolvent molecules in ca. 5 ps; then, the energy diffuses to the bulk solvent in 13 ps.
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