Intramolecular photoinduced electron transfer in a hydrogen bonded zinc(II) porphyrin-dinitrobenzene complex by time-resolved electron paramagnetic resonance spectroscopy

Abstract

A time-resolved electron paramagnetic resonance (TREPR) study was performed on a hydrogen bonded donor-acceptor complex. in which a guanine-functionalized zinc(I1) porphyrin and a cytosine-functionalized dinitrobenzene are assembled via base-pairing in two types of liquid crystal (LC). In the nematic phase, selective photoexcitation of the zinc(I1) porphyrin moiety yields a narrow derivative-like signal, which is not observed when the Watson-Crick complementary dinitrobenzene unit is absent. The rise of the narrow signal is accompanied by the decay of the broad one, which is ascribed to the lowest excited triplet state of the zinc(I1) porphyrin. These findings are rationalized in terms of intraensemble electron transfer (ET) occurring from the lowest excited triplet state of the zinc(I1) porphyrin donor to the dinitrobenzene acceptor, with the narrow EPR signal being attributed to a long distance chargeseparated species. The phase pattern of the derivative-like signal is reversed by substituting the LC with a positive magnetic anisotropy (Ax > 0) for one with the opposite sign (Ax < 0). The observed narrow signal is assigned to a spin correlated radical pair (SCRP). In the isotropic phase at higher temperatures, a narrow, net absorptive EPR signal is observed regardless of the type of LC employed. This latter signal is assigned to a thermally populated SCRP.

Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Electron Transfer in Pd Porphyrin–Quinone and Zn Porphyrin–Quinone Dyads with a Cyclohexylene Spacer

“…In our experimental conditions with impeded diffusion, i.e., in frozen and viscous matrices, two pathways of charge-separated state formation can account for the observed multiplet radical pair (RP) signal, i.e., triplet radical pair (TRP) and weakly coupled spin-correlated radical pair (SCRP) mechanisms. , SCRPs were found in covalently linked arrays with sufficiently long D–A distances (∼18–24 Å) , and hydrogen-bonded D–A complexes. ,, A characteristic feature in SCRP spectra is two antiphase doublets, centered at the g -factors of the individual radicals, whereas exchange and dipolar interactions ( J and D , respectively) determine the splitting in each doublet.…”

“…16,57 SCRPs were found in covalently linked arrays with sufficiently long D−A distances (∼18−24 Å) 58,59 and hydrogen-bonded D−A complexes. 25,26,37 A characteristic feature in SCRP spectra is two antiphase doublets, centered at the g-factors of the individual radicals, whereas exchange and dipolar interactions (J and D, respectively) determine the splitting in each doublet.…”

The Relation between Hydrogen Atom Transfer and Proton‐coupled Electron Transfer in Model Systems

“…This is a noteworthy result, considering that the driving force for ET in 7 (DG s~-43 kJ mol -1 ) is substantially less than that for 6 (DG s~-96 kJ mol -1 ). The complexities of photoinducing a PCET reaction become apparent when W-band time-resolved EPR experiments are performed on 8 [105,106]. EPR signals are observed that are consistent with intersystem crossing from the singlet to the triplet excited state prior to ET.…”

Triplet porphyrins as donors in intramolecular electron transfer and their intermolecular interaction with free radicals

“…These findings are rationalized in terms of intraensemble ET (Fig. 1) occurring from the lowest excited triplet state of the donor (zinc(II) porphyrin) to the acceptor (dinitrobenzene) and by spin polarization effects [21,22]. The narrow EPR signal is attributed to a long-distance charge-separated spin-correlated radical pair (SCRP).…”

“…An approach to model the primary ET events in photosynthesis involves the synthesis and study of preorganized supramolecular aggregates containing donors and acceptors which are not covalently linked. Along these lines, recent studies involved novel supramolecular structures in which molecular recognition was established via Watson-Crick basepairing interaction [21] (Fig. 2).…”