Transient features of optical bleaching as studied by photochemical hole burning and fluorescence line narrowing

This paper presents various laser excitation experiments on matrix-isolated 1,4-dihydroxyanthraquinone (DAQ). The molecular spectra are studied in two different organic glasses, namely a hydrogen-bonding matrix of 3:1 EtOH/MeOH and a nonhydrogen-bonding matrix of 3-Me-pentane. The experimental techniques are photochemical hole burning (PHB) and fluorescence line narrowing (FLN). The main difference between the two organic glass matrices is the host-induced photoreactivity of DAQ in the alcohol matrix and the photostability of DAQ in the pentane matrix. This difference leads to "anomalous" spectral features in the photoreactive alcohol-DAQ system. In this system, the zero-phonon transition appears to be more intense at higher temperatures, contrary to the well-proven theories on electron-phonon coupling. The unusual spectral line shapes are explained as transient features due to the efficient photochemistry of DAQ in hydrogen-bonding matrices.