Formation of a long-lived radical pair in a Sn(iv) porphyrin–di(l-tyrosinato) conjugate driven by proton-coupled electron-transfer

Abstract

The novel conjugate 1, featuring two l-tyrosinato residues axially coordinated to the tin centre of a Sn(iv)-tetraphenylporphyrin, is reported as the first example of a supramolecular dyad for photochemical PCET. It is noteworthy that the excitation of 1 in the presence of a suitable base is followed by photoinduced PCET leading to a radical pair state with a surprisingly long lifetime.

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

“…1 crystallizes in the chiral P 2 1 space group and the molecular model show the expected tryptophan Cα stereocenter configurations ( S ), further confirmed by the refined Flack parameter [22] . The coordination sphere around the Sn IV center presents bond lengths and angles in line with those already reported in similar systems (see also the comparison in Table S2) [20a,21c,23] . Arrays of 1 develop along diagonal unit cells concatenated by hydrogen bonds involving the indole ‐NH and the acetylated carbonyl groups of equivalent tryptophan residues pertaining to adjacent unit cells (d NH ⋅ ⋅ ⋅ OC =2.829(5) Å and d NH ⋅ ⋅ ⋅ OC =2.862(5) Å, Figure S12A).…”

“…Two new Sn IV ( l ‐tryptophanato) 2 ‐porphyrin conjugates, differing for the substituents at the porphyrin periphery, alongside with the corresponding appropriate model compounds, i. e., featuring benzoate axial groups, commonly employed for comparative studies, [20,21] were prepared, fully characterized, and studied ( 1 – 4 , Scheme 1). The single crystal X‐ray analysis of 1 , while confirming the H‐donor character of the aa –NH indole side group, evidences some distinct peculiar features with respect to the parent Sn IV ( l ‐tyrosinato) 2 ‐porphyrin conjugate ( 5 ), earlier reported [20a] . A detailed photophysical investigation of 1 – 4 in CH 2 Cl 2 in the presence of pyrrolidine was performed.…”

“…These intra‐ and inter‐molecular interactions efficiently stabilize a bent conformation of the amino acid sidechain on only one side of the porphyrin macrocycle, resulting in the two tryptophan residues coordinated to the same tin center to adopt significantly different indole‐ring orientations (Figure S13). This difference is peculiar and has not been observed for adduct 5 [20a] and analog systems [23] and may be related to packing effects.…”

Photoinduced Proton-Coupled Electron Transfer in Supramolecular SnIV Di(l-tyrosinato) Porphyrin Conjugates

“…In this respect, kinetic requirements to achieve an efficient light-induced reaction are relatively more demanding than those of conventional, thermal PCET. Within this framework, we have recently reported on a simple system consisting of a Sn IV porphyrin bearing two tyrosinato axial ligands ( 1 , Chart ) as a first example of a supramolecular metallo-porphyrin amino acid conjugate . Importantly, differently from the covalent systems typically investigated, − the supramolecular nature of these type of adducts potentially gives simple, yet direct, access to a considerably extended library of fine-tuned photoactive electron/proton donor/acceptor derivatives, most certainly convenient for gaining new insights into PCET reactivity.…”

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

“…15,17,27 The overall reaction up to this point corresponds to long-range electron transfer from Y Z to Q B , coupled to deprotonation of the former and protonation of the latter, yielding Y Z and Q B H. Somewhat surprisingly, this overall reaction sequence had not been emulated in artificial donoracceptor compounds until very recently, 28 even though PCET in artificial Y Z mimics has received much attention. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] The donor-sensitizer-acceptor compound in Fig. 2b contains all the necessary components to emulate the global reaction sequence seen for photosystem II in Fig.…”