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Results and dicussion

Numerous vibronic bands were observed in the LIF excitation spectra. The vibronic bands are heavily congested and cover the whole wavelength region. The longest wavelength band is observed at 589.81 ± 0.02 nm and is assigned to the electronic origin of the S1 state.

LIF excitation spectra measured for DPBBTD in a supersonic free jet. The expanded spectrum in the region of 585-595 nm was measured in order to determine the position of the electronic origin.
LIF spectrum from 510-550 nm LIF spectrum from 545-595 nm

Vibrational progressions were observed with constant frequency intervals (60-70 cm−1) from the origin to the higher energy region. The constant intervals indicate harmonic, torsional vibrations of the two phenyl groups. Therefore, the potential energy surface in the S1 state is expected to have a single minimum where the two phenyl rings and the BBTD plane in DPBBTD are coplanar. In this case, the potential energy surface in the S0 state should have an energy barrier at the coplanar conformation. The phenyl groups and the BBTD skeleton probably have a torsion geometry in the S0 state due to their intramolecular interference. In the S1 state, however, the DPBBTD molecule has a planar conformation just as does the biphenyl molecule because the π-interaction is enhanced and the intramolecular interference is ineffective.

LIF excitation spectrum of DPBBTD in a supersonic free jet shown using an expanded scale in the range 16945 to 17550 cm−1. Band progressions associated with torsional vibrations of the phenyl groups and indicated. The progressions are labelled as numbers (1-6) for the present.
LIF spectrum from 16945-17550 cm-1
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