COMPLEXATION AND INTERCHROMOPHORIC INTERACTIONS IN SELF-ORGANIZED PORPHYRIN AND CHLORIN TRIADS

Spectral. properties and equilibrium constants of multimolecular compl exes (triads) formed by 2-fold coordination of dipyridyl-substituted f ree bases of porphyrin, chlorin, or tetrahydroporphyrin with Zn-porphy rin and Zn-chlorin dimers bridged by either of two different spacers b etween the monomeric entities have been studied in methylcyclohexane a t room temperature. The ability of the dimers bridged by a -CH2-CH2-sp acer to form complexes with the free bases is found to be much lower t han that for the dimers with a phenyl spacer having complexation const ants up to K-c = 5 x 10(7) M(-1). The complexation equilibrium is also affected strongly by the chemical nature of the free base and the pos ition of the pyridyl substituents. Spectral effects occurring upon com plexation in absorption spectra of the dimers are mainly due to Ligati on effects. Strong quenching of the dimer fluorescence in the complexe s is attributed to effective singlet-singlet energy transfer (ET) to t he ligands.