Continuous-wave electron spin resonance studies of porphyrin and porphyrin-quinone triplet states

Abstract

CW ESR studies of the photoexcited triplet states of a series of porphyrins and covalently linked porphyrinquinones have been performed in isotropic and anisotropic (liquid-crystalline) frozen solutions. In frozen nematic solutions, the appearance of the spectra is strongly dependent on the orientation of the sample relative to the external magnetic field. This fact allows a differentiation between the z and x,y axes and hence a more accurate determination of the zero-field splitting parameters D and E. Spectrum simulation and fitting aided in the interpretation of the experimental results and the extraction of the relevant parameters. In the case of some porphyrin-quinones, steady-state triplet ESR signals cannot be observed, which is ascribed to singlet-electron transfer enhanced by folding of the quinone acceptor over the porphyrin donor. The doublet signal of porphyrin or chlorin cation radicals, which is also qbserved, appears in emission under certain conditions, indicative of the radical triplet pair mechanism.

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

“…5, top) depends only on the ZFS parameters and the relative population rates for the triplet sublevels. The fit to the experimental spectntm yields parameters which are similar to those found for ZnTPP [11,16] (see Table 1). The main difference is that D is now slightly larger than 3E.…”

“…For porphyrins with no side groups this assumption is valid because their orde¡ is determined by short-range interactions which are governed by the size and shape of the macrocycle. However, for substituted porphyrins such as TAPD-ZnP-NQ, the model breaks down because the interactions of the side groups with the liquid crystal play a significant role in determining the ordering [16]. Asa result, the assumption that one of the principal axes of the order matrix is perpendicular to the porphy¡ plane is no longer valid.…”

“…We will show that the information provided by the triplet spectra can be used to independently estimate the orientational distribution function which would otherwise bring additional unknown parameters into the simulations of the spectra of TAPD+'NQ -'. In contrast to previous studies of molecular and radical pair triplet states of porphyrins in liquid crystals [9][10][11][12][13][14][15][16], we will base the orientational distribution on a model of the interactions between the solute and solvent and will take the porphyfin side groups explicitly into account. The geometric parameters for the calculation of the radical pair spectra will be taken from the structure of the complex obtained by combining the X-ray structures of the components.…”

Improved method for estimating the ZFS parameterD for delocalized biradicals

“…1,4,[8][9][10][11][12][13][14][15][16][17][18][19][20][21] An approximation which is frequently used by Mukai and co-workers 1,19,20 is…”

“…The spin densities used are those of each monoradical half. It has been noted that this equation provides only crude approximations for delocalized biradicals, 21 which initiated an investigation in our laboratory into finding a simple approximate method that would provide more reasonable D values for delocalized biradicals owing to our interest in moderate-sized nondisjoint organic molecules with S b 1/2. 22 The problem we encountered in trying to use Eqn (1) for delocalized radical systems was that the delocalized system could not be separated into monoradical halves due to shared spincontaining atoms (hereafter referred to as delocalized biradicals).…”