Structure Dependence of Electron Spin Polarization in Zn−Porphyrin−Quinone Ensembles Oriented in a Liquid Crystal

Abstract

Photoinduced intramolecular electron transfer was studied by time-resolved electron paramagnetic resonance spectroscopy in two photosynthetic model systems oriented in a nematic liquid crystal. The donor-acceptor assemblies consisted of differently substituted zinc porphyrins covalently linked to a quinone with different spacers, i.e., triptycyl and cyclohexylene. The orientation of the guest molecules with respect to the director of the liquid crystal was determined by line shape analysis of their triplet spectra and by molecular modeling. It is demonstrated that the polarized pattern of the radical pair spectrum is sensitive mainly to the spacer structure, but to some extent also to the peripheral substituents.

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

“…Probably it is because long substitution on the porphyrin, which is collinear with the cyclohexylene spacer, enhances the spacer's fluctuations, thus affecting donor−acceptor electronic interactions. 65 It should be noticed here that a similar polarization pattern of RP signal with the same kinetic behavior was found in a triad consisting of Zn porphyrin covalently linked to two quinones one after another. 51 The signal was assigned to superposition of the spectra of two RPs, which were formed by photoinduced IET from the porphyrin to the adjacent and far quinones, respectively.…”

“…As to the influence of ZnP peripheral substituents, comparison of the results presented earlier 65 and in this study shows that long-chain substitution of the porphyrin donor periphery, slightly change the parameters of the RPs formed, whereas the RP's spectra in systems with short-chain substituents are similar. Probably it is because long substitution on the porphyrin, which is collinear with the cyclohexylene spacer, enhances the spacer's fluctuations, thus affecting donor−acceptor electronic interactions.…”

“…It was shown in detail for similar systems that the flexibility of the cyclohexylene bridge and the quinone attached to it may account for the obtained spectral line shape and the polarization pattern of the RP. , Moreover, the strong exchange coupling (in a similar system J was found as of −137.0 mT) significantly reduces S ↔ T mixing probability in the RP. Furthermore, taking into account the polarization pattern of the narrow signal, we can propose a triplet precursor of the ET. ,,,, Therefore, TREPR spectrum of RP is the result of transitions occurring only within the triplet manifold; i.e., the precursor of the photoinduced ET is the triplet state of ZnP moiety.…”

“…Furthermore, taking into account the polarization pattern of the narrow signal, we can propose a triplet precursor of the ET. 16,50,56,63,65 Therefore, TREPR spectrum of RP is the result of transitions occurring only within the triplet manifold; i.e., the precursor of the photoinduced ET is the triplet state of ZnP moiety.…”

Orientational Dependence of Cofacial Porphyrin–Quinone Electronic Interactions within the Strong Coupling Regime

“…While it is recognized that donor–acceptor (D–A) mutual orientation plays a role in determining the magnitude of electronic coupling ( H DA ), − studies that probe directly the orientation dependence of electron transfer (ET) reactions are limited. The paucity of such data derives in part from the fact that even slight variations in D–A orientation can change other parameters that modulate the magnitude of ET rate constants such as (i) D–A separation distance, (ii) the number and nature of bonds in the ET pathway, (iii) the tunneling energy, (iv) the nature of D/A solvation, and (v) the manner in which D and A are coupled to the intervening medium.…”