Picosecond photon echo techniques are used to measure directly vibronic relaxation times in the first excited singlet state of pentacene in naphthalene and p-terphenyl. In regions of low (< 300 cm–1) and high (> 1000 cm–1) vibrational energy, relaxation is fast (τ <2 ps) due to direct phonon emission (low energy) and fast vibrational energy redistribution (high energy). In the intermediate region relaxation times vary drastically. Effects of isotope substitution of guest and host suggest intramolecular relaxation in this region. Changes in local environment are shown to influence the relaxation rate quite drastically, indicating large host-induced anharmonic coupling between different vibrations
