SPECTRAL AND PHOTOPHYSICAL PROPERTIES OF ETHYLENE-BRIDGED SIDE-TO-SIDE PORPHYRIN DIMERS .2. FEMTOSECOND TRANSIENT ABSORPTION AND PICOSECONDFLUORESCENCE STUDY OF TRANS-1,2-BIS(MESO-OCTAETHYLPORPHYRINYL)ETHENE

Femtosecond transient absorption and picosecond fluorescence spectra a nd kinetics have been measured for an ethylene-bridged porphyrin dimer molecule, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbis=OEP). In part 1 of this work it was shown with the help of absorption and fl uorescence spectroscopy that in solution the molecule exists in at lea st two different conformers, the P conformer with porphyrin-type spect ral properties and the U conformer with spectral properties atypical f or porphyrin molecules. As our quantum chemical calculations showed, t he U conformer is characterized by a common pi-conjugation through the double-bond bridge. In the present work we have found that the lifeti mes of the lowest excited singlet states of the P and U conformers are extraordinary short, similar to 6 ps for P and similar to 7-9 ps for U in toluene at room temperature. Singlet-state lifetimes of both conf ormers were found to be strongly dependent on solvent viscosity with t hat of the U-form exhibiting the most pronounced dependence; a fluores cence lifetime of approximately 460 ps was measured for the U conforme r in a frozen toluene solution at 77 K. Photoisomerization-like confor mational relaxation in the S-1 state, leading the system to the ''poin t of funnel'' characterized by decreased Delta E(S-1-S-0) energy gap, is proposed as a mechanism to explain the photophysics of the P and U conformers.