Aromaticity versus regioisomeric effect of β-substituents in porphyrinoids

The photophysical properties of naturally occurring chlorophylls depend on the regioisomeric nature of the β-pyrrolic substituents. Such systems are the "gold standard" by which such effects are judged. However, simple extrapolations from what has been learned with chlorophylls may not be appropriate for other partially reduced porphyrinoids. Here we report the synthesis of a series of cis/trans-porphodilactones (cis/trans-1) and related derivatives (cis/trans 2-5) designed to probe the effect of regioisomeric substitution in porphyrinoids that incorporate degrees of unsaturation through the β-pyrrolic periphery that exceed those of chlorophyll. These test systems were obtained through β-pyrrolic modifications of the tetrapyrrolic core, which included reduction of β-diazalone to the corresponding dilactol moieties and 1,3-dipolar cycloadditions. In the case of cis- vs. trans-3 bearing two pyrrolidine-fused β-rings we found an unprecedented ΔQL up to ca. 71 nm (2086 cm-1), where ΔQL (QL means the lowest energy transfer band, also the S0 → S1 transition band, which is often assigned as Qy(0,0) band) refers to the transition energy difference between the corresponding cis/trans-isomers. The ΔQL values for these and other systems reported here were found to depend on the differences in the HOMO-LUMO energy gap and to be tied to the degeneracy and energy level splitting of the FMOs, as inferred from a combination of MCD spectral studies and DFT calculations. The aromaticity, estimated from the chemical shifts of the N-H protons and supported by theoretical calculations (e.g., AICD plots and NICS(1) values), was found to correlate with the extent of porphyrin periphery saturation resulting from the specific β-modifications. The aromaticity proved inversely proportional to the degree to which the regioisomerism affected the photophysical properties as noted from plots of ΔQLs in cm-1vs. the average NICS(1) values for 1-5. Such a finding is not something that can be easily interpolated from prior work and thus reveals how aromaticity may be used to fine-tune photophysical effects in reduced porphyrinoids.