Direct Observation of Hole Shift and Characterization of Spin States in Radical Ion Pairs Generated from Photoinduced Electron Transfer of (Phenothiazine) sub()nAnthraquinone (n = 1, 3) Dyads

Photoinduced intramolecular electron transfer of dyad PTZ3-PTZ2-PTZ1-B-AQ consisting of phenothiazine trimer (PTZ3-PTZ2-PTZ1), bicyclo[2.2.2]octane (B), and anthraquinone (AQ) was investigated. After excitation ( similar to 20 ps) of the AQ moiety in THF, a metastable radical ion pair (RIP) PTZ3-PTZ2-PTZ1 super(+)-B-AQ super(-) appeared at similar to 620 nm. From 500 ps to 6 ns the spectrum changed to a new absorption ( similar to 950 nm), which was assigned to the hole-shifted stable RIP state PTZ3-PTZ2 super(+)-PTZ1-B-AQ super(-). The time constant of the hole-shift process was determined to be 6.0 ns. The hole-shifted RIP state had a lifetime ( tau ) of 250 ns and was characterized by spin-polarized signals as a spin-correlated radical pair (SCRP) by means of time-resolved ESR. These results were compared with those for the phenothiazine monomer analog PTZ-B-AQ, which also produced the RIP state PTZ super(+)-B-AQ super(-) with tau = 1.9 mu s. Time-resolved ESR showed an all emission signal pattern showing the triplet mechanism of PTZ-B- super(3)AQ* arrow right super(3)[PTZ super(+)-B-AQ super(-)]. The origin of the difference in the lifetimes between the trimer and the monomer RIP states was discussed from various points of view, including free energy difference in the RIP states, reorganization energy difference in the charge recombination process, and the spin-state difference. Of these, the spin-state difference effect provided the most reasonable explanation.