Photophysics of AgCl doped with [Cl5Ir(N-methylpyrazinium)](-): I. EPR, ENDOR and structural calculations

By applying a combination of multi-frequency electron paramagnetic resonanc e spectroscopy, electron nuclear double resonance spectroscopy and advanced computational techniques, an understanding of the effects of [Cl5Ir(NMP)]( -) complexes on the photophysics of AgCl dispersions is emerging (NMP = N-m ethylpyrazinium). There is indirect spectroscopic evidence that this dopant is incorporated intact during AgCl precipitation. Calculations predict its substitution for an (Ag2Cl7)(5-) sub-unit of the host lattice, with the NM P ring rotated 45 degrees with respect to the equatorial chloride ligands. Calculations also shaw that, in grains with edge lengths greater than or eq ual to 0.05 mu m, the majority of dopant centres will be fully charge compe nsated by association with four silver ion vacancies. A (101)((1) over bar 01)(<01(1)over bar >)(<0(11)over bar>) geometry is favoured, where the Cl-I r-(NMP) axis defines z. Photo-EPR experiments suggest a small population of undercompensated {[Cl5Ir(NMP)](-) 3V} centres also exists in most of the d ispersions studied. During exposure to actinic light, these dopant centres trap electrons. Since the dopant's LUMO is primarily a pi* NMP orbital, the initial photoproducts are ligand-centred, one-electron donors. Experimenta l H-1 hyperfine data obtained by powder ENDOR spectroscopy for the favoured vacancy geometry of {[Cl5Ir(NMP)](2-) 4V} are consistent with the unpaired electron distribution calculated by Hartree-Fock methods. Vacancy binding energies are so large that the over-compensated donor ionizes before the ex tra vacancy diffuses away.