Persistent optical hole-burning spectroscopy of nano-confined dye molecules in liquid at room temperature: spectral narrowing due to a glassy state and extraordinary relaxation in a nano-cagePersistent optical hole-burning spectroscopy of nano-confined dye molecules in liquid at room temperature: spectral narrowing due to a glassy state and extraordinary relaxation in a nano-cage
144505-12 , 2018-05 , the American Institute of Physics
Persistent optical hole-burning spectroscopy has been conducted for a dye molecule within a very small (~1 nm) reverse micelle at room temperature. The spectra show a spectral narrowing due to site-selective excitation. This definitely demonstrates that the surroundings of the dye molecule are in a glassy state regardless of a solution at room temperature. On the other hand, the hole-burning spectra exhibit large shifts from excitation frequencies, and their positions are almost independent of excitation frequencies. On the basis of a configuration coordinate model, the hole-burning spectra have been theoretically calculated by taking account of a vibronic absorption band of the dye molecule, under the assumption that the surroundings of the dye molecule are in a glassy state. The calculated results agree with those from the spectroscopy of the dye molecule in a polymer glass performed for comparison, where it has been found that the ratio of hole-burning efficiencies of vibronic- to electronic-band excitations is quite high. On the other hand, the theoretical results do not explain the large spectral shift from the excitation frequency and small spectral narrowing observed in the hole-burning spectra measured for the reverse micelle. It is considered that the spectral shift and broadening occur within the measurement time owing to the relaxation process of the surroundings that are hot with the thermal energy deposited by the dye molecule optically excited. Further, the relaxation should be temporary because the cooling of the inside of the reverse micelle takes place with dissipation of the excess thermal energy to the outer oil solvent, and so the surroundings of the dye molecule do not attain the thermal equilibrium and remain in a glassy state. These results suggest that a very small reverse micelle provides a unique reaction field in which the diffusional motion can be controlled by light in a glassy state.