| Abstract
| - Nanosecond time-resolved absorption studies in a magnetic field ranging from 0 to 2.0 T have been performedon a series of covalently linked donor(PXZ)−Ru(bipyridine)3−acceptor(diquat) complexes (D−C2+−A2+).In the PXZ moiety, the heteroatom (X = O (oxygen), T (sulfur), and S (selenium)) is systematically variedto study spin−orbit coupling effects. On the nanosecond time scale, the first detectable photoinduced electron-transfer product after exciting the chromophore C2+ is the charge-separated (CS) state, D+−C2+−A+, wherean electron of the PXZ moiety, D, has been transferred to the diquat moiety, A2+. The magnetic-field-dependentkinetic behavior of charge recombination (monoexponential at 0 T progressing to biexponential for all threecomplexes with increasing field) can be quantitatively modeled by the radical pair relaxation mechanismassuming creation of the CS state with pure triplet spin correlation (3CS). Magnetic-field-independentcontributions to the rate constant kr of T± → (T0,S) relaxation are about 4.5 × 105 s-1 for DCA-POZ and-PTZ (due to a vibrational mechanism) and 3.5 × 106 s-1 for DCA-PSZ (due to spin rotational mechanism).Recombination to the singlet ground state is allowed only from the 1CS spin level; spin-forbidden recombinationfrom 3CS seems negligible even for DCA-PSZ. The field dependence of kr (field-dependent recombination)can be decomposed into the contributions of various relaxation mechanisms. For all compounds, the electronspin dipolar coupling relaxation mechanism dominates the field dependence of τslow at fields up to about 100mT. Spin relaxation due to the g-tensor anisotropy relaxation mechanism accounts for the field dependenceof τslow for DCA-PSZ at high fields. For the underlying stochastic process, a very short correlation time of 2ps has to be assumed, which is tentatively assigned to a flapping motion of the central, nonplanar ring inPSZ. Finally, it has been confirmed by paramagnetic quenching (here Heisenberg exchange) experiments ofthe magnetic-field effects with TEMPO that all magnetic-field dependencies observed with the present DCA-PSZ systems are indeed due to the magnetic-field dependence of spin relaxation.
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