Vibrational Energy Transfer in Highly Excited Bridged Azulene-Aryl Compounds: Direct Observation of Energy Flow through Aliphatic Chains and into the Solvent

Intra- and intermolecular vibrational energy flow in vibrationally highly excited bridged azulene-(CH 2 ) n -aryl ( n = 0,1,3; aryl = benzene or anthracene) compounds is observed using time-resolved pump−probe laser spectroscopy. Light absorption in the azulene S 1 -band, followed by fast internal conversion, leads to vibrational excitation at the azulene side of the molecules. Subsequent energy flow through the aliphatic chain to the aryl group at the other side of the molecules and vibrational energy transfer into a surrounding liquid solvent bath are measured either by probing the red edge of the azulene S 3 -absorption band at 300 nm and/or the anthracene S 1 -absorption band at 400 nm. The data are analyzed by representing the intramolecular energy flux as a diffusion process and using hot absorption spectra of the two chromophores of the compounds for measuring their energy contents. A fit to all of the experimental signals leads to an energy conductivity of a single C−C bond of κ CC = (10 ± 1) cm -1 K -1 ps -1 (with energies measured in cm -1 ). Depending on the substituent and the length of the chain, this models yield intramolecular energy transfer times of 1.2−4 ps. Energy transfer to the solvent 1,1,2-trichloro-trifluoro-ethane, on the other hand, is characterized by an exponential loss profile with a cooling time constant of (21 ± 2) ps, independent of the substituent and the same as for bare azulene.