Effect of Intramolecular Twisting on the Fluorescence Spectra of Sterically Hindered Tetraphenylbutadienes

Absorption and fluorescence spectra of trans-1,1,4-triphenylbutadiene, trans-1,1,4,4-tetraphenylbutadiene (TPB), and trans-1,1,4,4-tetraphenyl-2-methylbutadiene (TPMB) were measured in an isopentane-methylcyclohexane mixture at room temperature and in rigid glass at 77°K. Absorption maxima exhibit a polarization red shift while fluorescence maxima exhibit a large blue shift upon freezing the samples at 77°K. Thus the intense blue fluorescence of tetraphenyl-2-methylbutadiene appears as a weak yellow fluorescence as soon as the rigid glass begins to melt. In these sterically hindered molecules the equilibrium configuration in the excited state is different from that in the ground state, and an intramolecular twisting relaxation process occurs before emission in fluid media. In rigid media and in the crystal, the twisting relaxation is slowed down, and emission originates from a Frank–Condon state which lies at higher energies. Quantum yield and energy of fluorescence of sterically hindered molecules in general are expected to be viscosity sensitive.