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 sugges t the existence of weak intramolecular pi-pi interactions in these sys tems. Low fluorescence quantum yields and short lifetimes were obtaine d for all the ZnTPP-based D-A pairs in comparison with the correspondi ng values for ZnTPP. Multiexponential fluorescence decays obtained in five different solvents for these systems have been interpreted in ter ms of the coexistence of ''closed'' and ''extended'' forms of D-A stru ctures that do not equilibrate within the singlet state lifetime of th e donor porphyrin. A photoinduced electron transfer (PET) from the ZnT PP donor to the appended dinitroaromatic acceptor subunit of ''closed' ' forms has been suggested. The apparent PET rate constants obtained f rom time-resolved (k(et)TR) and steady state (k(et)SS) fluorescence da ta vary as meta<para<ortho. This observation is inconsistent with the known distance dependence of k(et) and indicates an orientation influe nce on the PET reactions.