QUENCHING OF FLUORESCENCE IN A SERIES OF COVALENTLY-LINKED PORPHYRIN DINITROBENZENE COMPOUNDS

Isomeric donor-acceptor (D-A) compounds were synthesized by linking a 3,5-dinitroaromatic acceptor subunit, via an ester bridge, to the para, meta or ortho position of one of the aryl groups of meso-5,10,15,20-(tetraphenyl)porphyrin (H2TPP) and its zinc (II) analogue (ZnTPP). UV-visible and proton nuclear magnetic resonance (H-1 NMR) spectra suggest the existence of weak intramolecular pi-pi interactions in these systems. Low fluorescence quantum yields and short lifetimes were obtained for all the ZnTPP-based D-A pairs in comparison with the corresponding values for ZnTPP. Multiexponential fluorescence decays obtained in five different solvents for these systems have been interpreted in terms of the coexistence of ''closed'' and ''extended'' forms of D-A structures that do not equilibrate within the singlet state lifetime of the donor porphyrin. A photoinduced electron transfer (PET) from the ZnTPP donor to the appended dinitroaromatic acceptor subunit of ''closed'' forms has been suggested. The apparent PET rate constants obtained from time-resolved (k(et)TR) and steady state (k(et)SS) fluorescence data vary as meta<para<ortho. This observation is inconsistent with the known distance dependence of k(et) and indicates an orientation influence on the PET reactions.