Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in SnIV (l-Tryptophanato)2 Porphyrin Conjugates

Aromatic amino acids such as l-tyrosine and l-tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton-coupled electron-transfer, PCET), both ET-only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a Sn-IV tetraphenylporphyrin and Sn-IV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l-tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X-ray analysis. The photophysical investigation of 1 and 2 in CH2Cl2 in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible-light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal-mediated analogues featuring Sn-IV porphyrin chromophores and l-tyrosine as the redox active amino acid and well illustrate the peculiar role of l-tryptophan with respect to PCET.

Automated summary

Aromatic amino acids such as l-tyrosine and l-tryptophan play a role in mediating electron transfer reactions in natural systems. This study prepared and characterized two novel conjugates based on porphyrin chromophores and l-tryptophan as electron/proton donor. Photophysical investigation showed different quenching mechanisms operating upon visible-light excitation, highlighting the unique role of l-tryptophan in proton-coupled electron transfer.