| Abstract
| - The photophysical characterization of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confusedfree-base porphyrin, as well as the tautomer-locked 2-methyl 5,10,15,20-tetraphenyl N-confused free-baseporphyrin, was carried out using a combination of steady state and time-resolved optical techniques. N-Confusedporphyrins, alternatively called 2-aza-21-carba-porphyrins or inverted porphyrins, are of great interest fortheir potential as building blocks in assemblies designed for artificial photosynthesis, and understanding theirexcited-state properties is paramount to future studies in multicomponent arrays. Femtosecond resolved transientabsorption experiments reveal spectra that are similar to those of tetraphenylporphyrin (H2TPP) with eitherSoret or Q-band excitation, with an extinction coefficient for the major absorbing band of 1e that was abouta factor of 5 larger than that of H2TPP. The lifetime of the S1 state was determined at a variety of absorptionwavelengths for each compound and was found to be consistent with time-resolved fluorescence experiments.These experiments reveal that the externally protonated tautomer (1e) is longer lived (τ = 1.84 ns) than theinternally protonated form (1i, τ = 1.47 ns) by ∼369 ps and that the N-methyl N-confused porphyrin wasshorter lived than the tautomeric forms by ∼317 ps (DMAc) and ∼396 ps (benzene). Steady-state fluorescenceexperiments on tautomers 1e and 1i and the N-methyl analogues corroborate these results, with fluorescencequantum yields (ΦFl) of 0.046 (1e, DMAc) and 0.023 (1i, benzene), and 0.025 (DMAc) and 0.018 (benzene)for the N-methyl N-confused porphyrin. The lifetime and quantum yield data was interpreted in terms ofstructural changes that influence the rate of internal conversion. The absorption and transient absorption spectraof these porphyrins were also examined in the context of DFT calculations at the B3LYP/6-31G(d)//B3LYP/3-21G(d) level of theory and compared to the spectra/electronic structure of H2TPP and tetraphenyl chlorin.
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