Nanosecond time-resolved resonance CARS spectroscopy of triplet excited polycyclic aromatic compounds

We use time-resolved resonance CARS spectroscopy as a diagnostic approach to study the structure and the dynamics of photoinduced transient molecular species. Transient CARS spectra of several highly fluorescent organic molecules in the lowest excited triplet state are reported and discussed. Several fundamentals of anthracene and some monosubstituted derivatives are observed and assigned unambiguously by comparison of the S-0 and T-1 spectral features. Moreover, it is verified that the most intense CC stretching band of anthracene in the lowest excited triplet state exhibits an upward wavenumber shift upon deuteration. Nanosecond time-resolved transient CARS spectroscopy opens up the possibility to study the triplet relaxation process of the sample molecules. Furthermore, it is shown by means of transient CARS spectroscopy that the lowest excited triplet state of 9-fluorenone exhibits pipi* character. The wavenumbers of the transient CO and CC stretching modes differ considerably from the appropriate Raman wavenumbers of the npi* triplet state of benzophenone. Moreover, time-resolved CARS spectroscopy is used to demonstrate that the intersystem crossing of 1,2-benzanthracene occurs on a nanosecond time scale in deaerated solutions.