Spatially resolved thermalization dynamics of electronically photoexcited azulene probed by a molecular integrated thermometer

The vibrational cooling process of photoexcited azulene (Az) was studied by using a covalently connected molecular thermometer (coumarin 151 (C151)). The transient absorption at the red edge of the absorption band of C151 det ected after the photoexcitation of azulene is attributed to the hot band ab sorption of C151. First, the time profile of the transient absorption signa ls of the integrated molecule Az-CH2-C151 was examined by two thermalizatio n models: one of which includes the intermolecular energy transfer through the sigma-bond and through the solvent, and the other includes only the thr ough-solvent path. From these analyses, it is found that the cooling times of azulene in the solvents used here were found to be less than 7 ps. These results indicate that there is a fast cooling process after the relaxation of the electronically excited state compared with that reported before. Th e transient absorption spectrum and its time profile of a compound with a l onger chain (Az-(CH2)(3)-C151) are very similar to those of Az-CH2-C151 exc ept for the much weaker intensity. These facts indicate that the molecular structure of Az-(CH2)(3)-C151 is flexible and the distance between the ther mometer (C151) and the heater (azulene) is short.