Fluorescence spectroscopic and time-dependent DFT studies for intramolecular excimer formation of Di-9H-fluoren-9-yldimethylsilane: dynamics and energetics for conformational change.

Intramolecular excimer formation of di-9H-fluoren-9-yldimethylsilane in various solvents was studied by means of steady-state and time-resolved fluorescence spectroscopies. Solvent viscosity effect on the kinetics for the monomer-excimer transition was found to be very slight, indicating that the transformation is accompanied by a slight conformational change presumably involving a transition from a near face-to-face conformer to a true sandwich conformer. The excitation energies and the corresponding oscillator strengths were derived with the time-dependent B3LYP/cc-pVTZ method. The simulated absorption spectrum involving the variation of the oscillator strengths in terms of the wavelengths accords with the experimental absorption pattern. The TD DFT calculations also reveal that excitation energy is significantly decreased with decreasing the interchromophore separation and the dihedral angle in a fluorene dimer, as an indication that the excimer state emitting the significantly red-shifted fluorescence corresponds to a sandwich conformer.