Photophysical characterization of free-base N-confused tetraphenylporphyrins

We report the photophysical characterization of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin, potential building blocks in assemblies designed for artificial photosynthesis, using a combination of steady state and time-resolved techniques. Tautomer le was found to have a significantly higher quantum yield of fluorescence than tautomer 1i (Phi(Fl) = 0.036 vs Phi(Fl) = 0.0016, respectively), despite the fact the fluorescence lifetimes were quite similar (i.e., 1.98 ns vs 1.60 ns, respectively). These differences were attributed to a more rapid rate of internal conversion or intersystem crossing in 1i due to steric conditions in the interior of the macrocycle. The absorption spectra of these porphyrins were also examined using the results from B3LYP/6-31G(d)//B3LYP/3-21G(d) calculations and interpreted using the four electron four orbital model.