PHOTOINDUCED PROCESSES IN DYADS AND TRIADS - AN OSMIUM(II)-BIS(TERPYRIDINE) PHOTOSENSITIZER COVALENTLY LINKED TO ELECTRON-DONOR AND ACCEPTOR GROUPS

We have synthesized the luminescent Os(ttP)22+ photosensitizer (P) and the supramolecular species P-A, D-P, and D-P-A where the photosensitizer is covalently linked to the electron acceptor MV2+ (A) and/or the electron donors PTZ or DPAA (D); ttp is 4'-(p-tolyl)-2,2':6',2"-terpyridine, MV2+ is methylviologen, PTZ is phenothiazine, and DPAA is di-p-anisylamine. The absorption spectra, emission spectra and lifetimes in the nano- and picosecond time scales (in the 90-298 K temperature range), nanosecond transient absorption spectra and lifetimes (at 155 and 298 K), and the electrochemical behavior of the P, P-A, D-P, and D-P-A species have been investigated in acetonitrile or butyronitrile solutions. The main results can be summarized as follows: (i) the excited photosensitizer *P displays luminescence and decays with tau = 540 and 220 ns at 155 and 298 K respectively; (ii) the behavior of the D-P dyads, whose D+-P- charge-separated state lies at higher energy than the luminescent D-*P level, is essentially the same as that of P; (iii) in the P-A dyad, whose P+-A- charge-separated level lies at lower energy than the *P-A luminescent level, the luminescence intensity of the photosensitizer reduces to 1.9% and exhibits a lifetime of 4 ns at 155 K (260 ps at 298 K); the decay of the charge-separated species occurs with lifetime 27 ns and < 10 ns at 155 and 298 K respectively; (iv) the D-P-A triad with D = PTZ behaves essentially as the P-A dyad; (v) for the D-P-A triad with D = DPPA, the luminescence intensity reduces to approximately 1% at 298 K (tau = 190 ps). No direct evidence is obtained for the presence and survival of the fully charge-separated state D+-P-A-. In a rigid matrix at 77 and 90 K, no quenching of the luminescence can be observed in all the dyads and triads examined.