Time-resolved and polarization-sensitive fluorescence and dichroism in absorbing liquids

A detailed understanding of guest-host molecular interactions is required in many topical research fields ranging from biology to the physics of photosensitive materials. Recently, we proved that time-resolved and polarization-sensitive fluorescence and dichroism are effective experimental tools for elucidating the influence of specific intermolecular interactions on the nonlinear optics of anthraquinone-doped liquids and liquid crystals. We review these results in this chapter. In particular, we report on the application of fluorescence and dichroism to the study of the influence of isotope deuterium substitution and photoinduced electronic excitation on the rotational dynamics of anthraquinone molecules in liquids. Our results point to photosensitive hydrogen bonding as the main microscopic mechanism for the large enhancement of optical nonlinearity observed in these materials. Moreover they prove the feasibility of a 'fluctuating-friction molecular motor', a concept which could have wide-ranging implications for controlling the state of matter at the molecular scale. This proof of concept relies on the accurate characterization of the state-dependent rotational dynamics of anthraquinone molecules. To this aim we developed an experimental methodology that combines measurements of transient bleaching and of time-resolved fluorescence and spectroscopy. This approach appears to be the only viable one whenever light absorption of the dye excited states cannot be made negligible.