Spectral and photophysical properties of ethylene-bridged side-to-side porphyrin dimers .1. Ground-state absorption and fluorescence study and calculation of electronic structure of trans-1,2-bis(meso-octaethylporphyrinyl)ethene

We have studied a double-bond bridged porphyrin dimer, trans-1,2-bis(meso-octaethylporphyrin)ethene (tbis=OEP), which in solutions exhibits new spectral properties: (i) pronounced absorption bands in addition to the monomer ones are observed in the 480-500 and 600-900 nm regions; (ii) a broad-band fluorescence with a viscosity-dependent intensity and spectral position is detected in the near-IR region (750-1100 nm). An investigation of fluorescence excitation spectra, combined with semiempirical quantum chemical calculations and geometry optimizations suggest that in solution tbis=OEP exists in two main conformations, which we name conformers P and U. The ratio of their concentrations was estimated to be approximately 5:1 in toluene at room temperature. The P conformer was found to be responsible for the ''usual'' monomer-type absorption bands (Soret and Q) in the tbis=OEP absorption spectrum, whereas the U conformer is responsible for the additional absorptions in the 480-500 and 600-900 nm regions. The unusual near-IR fluorescence originates from the U conformer and its quantum yield (Phi) is 6 x 10(-4) in toluene. Increase of solvent viscosity results in a strong blue-shift of the near-IR emission and increase of its intensity (Phi = 4 x 10(-3) in paraffin oil). Both conformers P and U were found to exhibit very short exited-state lifetimes (<10 ps in toluene) which become significantly longer in more v iscous solvents. The calculations suggest that the peculiar groundstate spectral properties of the U conformer result from its particular geometrical structure favoring a common conjugation between the pi orbitals of the porphyrin rings and the ethylene bond, whereas only excitonic interactions exist in the P conformer. Therefore, the U conformer can be considered as a real supermolecule rather than two interacting separate porphyrin macrocycles. Our results point to the key role of the connecting bridge in the formation of the optical properties of the ethylene-bridged porphyrin dimers.