Intramolecular charge-transfer properties of a molecule with a large donor group:: the case of 4′-(pyren-1-yl) benzonitrile

Picosecond transient absorption, nanosecond fluorescence decays, stationary absorption, and fluorescence spectra have been recorded in different solvents at various temperatures for 4'-(pyren-1-yl)benzonitrile(Py- BN) and compared with corresponding data for two structurally similar compounds: 4'-(pyren-1-yl) - N, N-dimethylaniline (Py-DMA) and 4'-(pyren-1-yl) acetophenone (Py- BK). Quantum chemical and molecular mechanics calculations were performed for Py- BN in order to monitor variations in the electronic transitions, energies, oscillator strengths, and dipole moments upon changing the molecular geometry by twisting the pyrenyl group with respect to the benzonitrile subunit. Both experimental results and quantum chemical calculations indicate that after excitation Py- BN relaxes by mutual twisting of the two subunits towards a more planar geometry.