X‐and K‐band transient EPR of the light induced radical ion pairs in photosynthetic model systems

Abstract

The light induced electron transfer in the complex 2-tetraalkylphenylenediamine-Zinc porphyrin-2-naphthoquinone (TAPD-ZnP-NQ) is investigated using transient EPR at X-band (9 GHz) and K-band (24 GHz). Spin polarized spectra are presented for the radical pair TAPD+'NQ-' which is generated by light excitation of the complex at 30 K in 2-methyl-tetrahydrofuran and the liquid crystal BDH E7. In the liquid crystal, the complex is partially ordered and the spectra depend on the orientation of the sample with respect to the magnetic field. A qualitative interpretation of the spectra on the basis of the coupled, correlated radical pair (CCRP) model is presented. The spectra of the partially oriented samples clearly show that: (i) the exchange interaction, J, makes a substantial contribution to the coupling between the radicals and is of the same order of magnitude as the dipole-dipole coupling (ii) when this coupling is taken into account, the experimental spectra are consistent with the expected geometry of the complex. This system is one of the few cases in which the two couplings are of similar magnitude and allows their influence on the polarization pattern to be investigated. The value of J z 0 . 1 mT obtained from the spectra is at least two orders of magnitude larger than that found in the analogous radical pair, P+'Q-: in photosynthetic reaction centres, although the average distance between the spins in the two systems differ at most by a factor of 1.5. Possible reasons for this difference are discussed.

Separation of the exchange and dipolar contributions to the spin-spin coupling in the donor-acceptor complex TAPD-ZnP-NQ

“…The decay times of the signals at c and d are also different with a slower decay at position d, probably as a result of an orientation dependence of the spin relaxation. At the outer wings of the spectrum there is a broad, weak E/A pattern (positions a and g) which is also observed at X-band [6,7]. In [6] these features were assumed to be contributions to the speetrum of TAPD+'NQ -" from TAPD +.…”

“…The synthesis and purification of TAPD-ZnP-NQ and the details of the sample preparation for the EPR experiments are reported elsewhere [3,7]. The details of the X-band and K-band spectrometer have also been described previously [17][18][19].…”

“…The details of the X-band and K-band spectrometer have also been described previously [17][18][19]. The transient EPR spectra presented here have been obtained by integrating the signal intensity over a given time window and are referred to as digital boxcar spectra [7,17,19]. The data are presented with positive signals in absorption (A) and negative signals in emission (E).…”

“…These effects contain information about the spin dynamics of the system and can be used to deduce the nature of the precursor states which are too shortlived to be observed directly. Such measurements have been carried out on TAPD-ZnP-NQ [3,[5][6][7] and the observed signals have been assigned to the radical pair TAPD+'NQ -" which is clearly generated from a singlet precursor. The complex can also be partially oriented in a frozen liquid crystal so that the orientation dependence of the spectra can be investigated [7].…”

“…Such measurements have been carried out on TAPD-ZnP-NQ [3,[5][6][7] and the observed signals have been assigned to the radical pair TAPD+'NQ -" which is clearly generated from a singlet precursor. The complex can also be partially oriented in a frozen liquid crystal so that the orientation dependence of the spectra can be investigated [7]. A qualitative analysis [7] of the spectra of TAPD+'NQ -" in a liquid crystal solvent at 9 GHz (X-band) and 24 GHz (K-band) suggests that this radical pair is one of the few cases in which the exchange coupling, J, is of the same order of magnitude as the dipolar coupling, d. By comparison, J is at least two orders of magnitude smaller than d in the corresponding pairs P~'65QA" and P~ 91 in bacterial reaction centres and plant photosystem I, respectively [8].…”