Photoinduced Proton-Coupled Electron Transfer in Supramolecular Sn^IV Di(l-tyrosinato) Porphyrin Conjugates

Abstract

Proton-coupled electron transfer (PCET) plays a key role in many biological processes, and a thorough comprehension of its subtle mechanistic complexity requires the synthesis and characterization of suitable artificial systems capable of mimicking this fundamental, elementary step. Herein, we report on a detailed photophysical investigation of conjugate 1, based on a tin­(IV) tetraphenylporphyrin (SnTPP) chromophore bound to two l-tyrosinato amino acids, in CH2Cl2 in combination with organic bases of different strength and the preparation of a novel conjugate 3, based on a tin­(IV) octaethylporphyrin (SnOEP) in place of the tetraphenyl analogue, and its photophysical characterization in CH2Cl2 in the presence of pyrrolidine. In the case of compound 1 with all bases examined, quenching of both the singlet and triplet excited states is observed and attributed to the occurrence of concerted proton–electron transfer (CPET). Rates and quenching yields decrease with the strength of the base used, consistent with the decrease of the driving force for the CPET process. Conjugate 3 with pyrrolidine is quenched only at the triplet level by CPET, albeit with slower rates than its parent compound 1, ascribable to the smaller driving force as a result of SnOEP being more difficult to reduce than SnTPP. For both systems, the quenching mechanism is confirmed by suitable blank experiments, specific kinetic treatments, and the observation of kinetic isotope effects (KIEs). Differently from what has been previously proposed, a detailed reinvestigation of the triplet quenching of 1 with pyrrolidine shows that no long-lived radical pair state is formed, as diradical recombination is always faster than formation. This is true for both 1 and 3 and for all bases examined. The kinetics of the CPET pathways can be well described according to Marcus theory and point toward the involvement of substantial reorganization energy as typically observed for PCET processes of concerted nature.

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

“…A kinetic isotope effect (KIE) of 1.4(±0.1) was found comparing the triplet excited state decays of 1 in CH 2 Cl 2 with pyrrolidine in the presence of 1 % v/v of either CH 3 OH or CD 3 OD (Figure S34), implying that H/D transfer is directly involved in the quenching process [38] . This value is consistent with the one found for the parent conjugate 5 and other related systems [20b,30] . Furthermore, application of the kinetic treatment used to account for CPET reactions ([Eq.…”

“…The decay of the triplet excited state of the SnOEP component, populated by intersystem crossing from the singlet, is also affected by increasing addition of pyrrolidine, as measured near the triplet absorption maximum at 440 nm (Figure 7A) [24] . This behavior is characteristic of array 2 since addition of pyrrolidine to model compound 4 in CH 2 Cl 2 has less pronounced effects on the triplet decay [20b] . Also, as observed for the singlet excited state, the triplet decay in 2 with pyrrolidine can be well described using the kinetic formalism of [Eq.…”

“…Furthermore, as a fundamental requirement to promote efficient CPET, pyrrolidine was shown to be involved in hydrogen‐bonding interactions with the indole NH group with an association constant of K A =5.3(±0.7) M −1 in CH 2 Cl 2 , as determined from a spectrophotometric titration experiment on model compound N‐ acetyl‐ l ‐tryptophan using two different methodologies (Figure S30–S32) [35,36] . This value is lower than that measured for the hydrogen‐bonding between the same base and the hydroxide group of a tyrosine amino acid, [20b] consistent with the lower donor ability of the NH vs. the OH group with respect to H‐bond [37] …”

“…[20] For these systems excitation of the chromophore with visible light, in CH 2 Cl 2 and in the presence of organic bases of suitable strength, triggered CPET with reduction of the porphyrin component, oxidation of the tyrosine residue, and concomitant proton transfer to the base. Importantly, CPET quenching rates and yields were strongly dependent on the porphyrin chromophore and on the base used, with pyrrolidine providing the largest quenching efficiency [20b] . For these systems, diradical recombination was always faster than formation thus implying a photoacid behavior of the porphyrin‐tyrosine conjugates [20b] .…”

“…Importantly, CPET quenching rates and yields were strongly dependent on the porphyrin chromophore and on the base used, with pyrrolidine providing the largest quenching efficiency [20b] . For these systems, diradical recombination was always faster than formation thus implying a photoacid behavior of the porphyrin‐tyrosine conjugates [20b] . From these premises, we focused over possible progresses and improvements by variation of the redox active amino acid from tyrosine to tryptophan.…”