INTRAMOLECULAR CHARGE-TRANSFER PROPERTIES OF A MOLECULE WITH A LARGE DONOR GROUP - THE CASE OF 4'-(1-PYRENYL) ACETOPHENONE

Picosecond time-resolved fluorescence and absorption, nanosecond fluor escence decay curves and stationary fluorescence spectra were compared in different solvents at various temperatures for 4'-(1-pyrenyl) acet ophenone (PYRA) and p-N,N-dimethylaminoacetophenone (DMAA). In polar s olvents, a single emission band with large Stokes shifts is observed f or PYRA, whereas DMAA reveals a dual fluorescence. Quantum chemical an d molecular mechanics calculations were performed for PYRA in order to monitor variations of electronic transition energies, oscillator stre ngths and dipole moments upon changing the molecular geometry by twist ing the pyrenyl group with respect to the acetophenone subunit. The ex periments confirm that the TICT state model can explain perfectly the fluorescence behavior of DMAA. However, it appears that a description in terms of an unperturbed TICT state cannot be applied for PYRA. In t his case, either broad angular distributions around more or less shall ow excited state minimum of perpendicular geometry (emission from a (T )ICT state) or twisting of donor and acceptor groups towards a more pl anar geometry should be considered. In alcohol, a two-step relaxation process was postulated involving stabilization of a hydrogen-bonded co mplex.