Tetsuro Tayu

Radiative and non-radiative decay processes of the excited state of the colored form of photochromic furylfulgide

Abstract The time evolution of the luminescence of the colored form of a furylfulgide dispersed at various concentrations in a poly(methyl methacrylate) film was measured as a function of the luminescence photon energy. The observed decay time of the luminescence is about 1–2 ns and one order of magnitude shorter than the radiative lifetime (14 ns) estimated from the absorption intensity. The decay time is independent of temperature below 77 K. These results suggest that the non-radiative tunneling process from the excited state to the ground state is responsible for the decay.

Inter-molecular interaction of photochromic furylfulgide dispersed in a polymer film

Abstract Luminescence spectra of the fulgide in a polymer film were measured as a function of the excitation photon energy at various fulgide concentrations. The peak energy and the half width of the lumisnescence spectra decreases with decreasing the excitation photon energy at the concentrations of 0.01 and 0.02 mol/dm 3 , while they are almost independent of the excitation photon energy at the concentration of 0.1 mol/dm 3 . These results suggest that the energy transfer due to the inter-molecular interaction among fulgide molecules takes place when the average distance between fulgide molecules is as short as 30 A.

Exciton-phonon interaction and potential fluctuation effect in PbI 2(lx) Br 2x mixed crystals

Excitonic properties of PbI 2(1- x ) Br 2 x mixed crystals have been investigated with attention to both the changes of the exciton-phonon coupling strength and of the magnitude of the alloy potential fluctuation. Below x =0.2, the exciton-phonon coupling strength becomes larger as x increases and a new broad luminescence band due to a self-trapped exciton state appears above x ≈0.15. Physical parameters such as the excitation energy of the exciton and the exciton-phonon coupling strength change linearly with x following to the virtual crystal model. Above x =0.5, new reflectivity peak and luminescence band corresponding to the “orthorhombic PbI 2 ” are observed in stead of those of the hexagonal PbI 2 . We also observe the luminescence from PbI 2 -clusters because of the large potential fluctuation effect.

Optical properties and photo-coloration of furylfulgide single crystal

Abstract Optical properties of the E-form single crystals of furylfulgide (2-[1-(2,5-dimethyl-3-furyl)-ethylidene]-3-isopropylidenesuccinic anhydride) have been studied through measurements of polarized reflection, absorption and luminescence spectra at 2 K. In the E⊥b configuration, a weak absorption band accompanied by the phonon sidebands is found at the lower energy side of an intense lowest π → π ∗ transition band. Photo-excitation of the weak absorption band does not induce any photo-reaction, but is followed by a radiation of luminescence. On the other hand, photo-excitation of the intense π → π ∗ band gives rise to the E → C photo-isomerization. Polarized absorption and luminescence spectra after the photoisomerization show that the photo-produced C-form molecules form a highly oriented thin film. The thickness of the film, estimated from the peak intensity of the absorption band, is more than 57 nm, which suggests that the E-form molecules inside the crystal can also change to C-form by irradiation of UV light.

Optical studies of photochromic furylfulgide single crystal

Polarized reflection spectra of single crystals of photochromic furylfulgide were measured in the region from 1.5 to 31 eV, and the optical constants were determined by using the Kramers-Kronig analysis. The Davydov splitting are estimated for the several absorption bands locating in the near ultraviolet region. It was confirmed that the reversible transformation between 2E and 2C molecules can be carried out even in crystal by the optical excitation.

Inter-molecular and molecular-matrix interaction of photochromic compound furylfulgides

Inter-molecular interaction of photochromic compound furylfulgide dispersed in a PMMA film was studied from the measurements of luminescence spectra as a function of the concentration of the fulgide and also a function of the excitation photon energy. The peak energy of luminescence decreased as the concentration increased and as the excitation photon energy decreased for the concentration lower than 0.01 mol/dm3. However, the peak energy was the lowest one among the measured samples and independent of the excitation photon energy for a sample of the concentration of 0.1 mol/dm3. These are well understood in terms of an electric dipole-dipole interaction between the fulgide molecules. The critical distance between the molecules was about 4 nm. The conversion rate for the coloration depended strongly on the temperature as well as the excitation photon energy. It was found that the band shape and peak energy of the absorption spectrum for the light-produced C- or E-form fulgides in a PMMA film depends on the excitation photon energy. This means that one can select special free volumes (cage) from the inhomogeneously distributed ones in a polymer matrix by a narrow band excitation.

Localized and self-trapped exciton states in PbI2-PbBr2 mixed crystals

Abstract Time-integrated luminescence spectra and time behaviour of the luminescence have been measured in PbI2(1-x)Br2x mixed crystals in order to elucidate the relaxation processes of free, localized and self-trapped excitons. The luminescence spectra for values of the composition ratio x below 0.05 exhibit recombination lines due to free and bound exciton states. Above x = 0.07, the luminescence coming from the localized exciton states which we denote as L states, becomes dominant. The observed decay time of this L band luminescence at the lowest energy is 100 ps. This rather short decay time is explained as due to the transition of the localized exciton state to lower energy excited states including a self-trapped exciton.