Cooperative Conformational Change and Excitation Migration of Biphenyl-PMO Amorphous Film, As Revealed by Femtosecond Time-Resolved Spectroscopy

Excited state dynamics of biphenyl-bridged mesoporous organosilica (Bp-PMO) film was investigated by femtosecond transient absorption and dichroism measurements at various excitation intensities. Under the excitation condition with low intensity (ca. 0.1 mu J/pulse), the relaxation from the excited Franck-Condon state with skewed structure of the two phenyl rings to the preplanar state occurred with a time constant of 550 fs, followed by the excimer formation with two time constants of 9.0 and 140 ps. Under higher excitation condition with 1.0 mu J/pulse, very rapid excimer formation within 500 fs was observed. From the analysis of the transient absorption spectra, it was revealed that the cooperative geometrical relaxation from skewed to planar structures, in addition to the energy migration, led to the rapid excimer formation under the high excitation condition. By integrating these results with the fluorescence dynamics, the photoprimary processes in Bp-PMO, such as energy migration, annihilation, and excimer formation, were discussed.