SYNTHESIS AND CHARACTERIZATION OF A DIRECTLY LINKED PORPHYRIN-ANTHRAQUINONE MOLECULE

Article abstract of DOI:10.1016/S0040-4020(01)85155-3

We have synthesized a porphyrin-anthraquinone molecule (PAQ) in which the AQ moiety is attached directly at a meso-position of tritolylporphyrin.The absorption spectrum shows pronounced perturbations in the bands of both the P and AQ groups that are independent of solvent polarity; a charge-transfer band was not observed.In contrast, the spectrum of the porphyrin fluorescence in PAQ is only moderately red-shifted, but the lifetimes and intensities depend markedly on solvent dielectric constant: the fluorescence is quenched negligibly in solvents with ε_s<= 4, moderately in solvents with 4 < ε_s <= 6 and strongly in solvents with ε_s >= 7.In methylene chloride, the major emitting component has a lifetime of ca. 30 ps as compared to ca. 9.0 ns for both tetratolylporphyrin (TTP) and an ester-linked TTP-AQ molecule.An electron transfer mechanism is implicated even though the energetics for net electron transfer do not appear to be favorable (the sum of the redox potentials being essentially isoenergetic with the porphyrin S₁ state in benzonitril).We infer that the short distance between the P and AQ moieties (ca. 1.4 Angstroem edge-to-edge) compensates for the otherwise marginal energetics in accord with Marcus theory.However, calculations of the reorganization energy, based on a two-sphere dielectronic continuum model, and estimates of the solvent-dependent reaction energetics, using the Weller equation, do not yield a meaningful correlation with the fluorescence data measured in 19 solvents and binary solvent mixtures.Electron transfer in this intimately linked donor-acceptor molecule may involve an inner-sphere and/or adiabatic mechanism.