Photoinduced processes in dyads made of a porphyrin unit and a ruthenium complex

A Ru(II) complex (Ru) containing as ligands a tridentate 4'-p-tolyl-2,2':6',2 ''-terpyridine (ttpy) and a tridentate 2,6-bis(4'-phenyl-2'-quinolyl)pyridine (bpqpy) has been covalently linked to a porphyrin module (PH2) to obtain a PH2-Ru dyad. The corresponding PZn-Ru dyad has then been obtained by metalation of the Gee base porphyrin with Zn(II) acetate. The photoinduced processes which occur on excitation of the PH2-Ru and PZn-Ru dyads, as well as of the PH2 and PZn porphyrin units and the [Ru(ttpy)(bpqpy)](2+) model compound Ru, have been investigated in butyronitrile rigid matrix at 77 K and fluid solution at 295 K. In both dyads at low temperature, the lowest singlet excited state of the porphyrin moiety (S-1) is quenched by energy transfer to give the triplet metal-to-ligand charge-transfer excited state of the Ru complex ((MLCT)-M-3) which, in its turn, is quenched by energy transfer to yield the triplet excited state of the porphyrin moiety (T-1). At room temperature, a charge-transfer (CT) excited state corresponding to the transfer of an electron from the porphyrin moiety to the Ru-based moiety comes into play. For the PZn-Ru dyad, where the CT state lies below the S-1 excited state of the porphyrin moiety, the deactivation of S-1 (k greater than or equal to 5 x 10(10) s(-1)) occurs mainly by electron transfer to give the CT level that then deactivates to the T-1 excited state of the porphyrin moiety (100% efficiency; k = 9.3 x 10(9) s(-1)). Since the T-1 level is intrinsically long lived (tau similar to 210 mu s), its deactivation occurs essentially via an activated process through the upper lying CT level (k = 5.7 x 10(6) s(-1)). The (MLCT)-M-3 excited state of the Ru-based moiety directly formed by light absorption appears to decay unperturbed with its intrinsic lifetime (k = 1.1 x 10(10) s(-1)). In the case of the PH2-Ru dyad, the CT level lies slightly above S-1. As a consequence, only a fraction (ca. 30%) of the S-1 excited states are quenched by electron transfer, the remaining part being quenched by energy transfer to give the (MLCT)-M-3 excited state of the Ru-based moiety. Deactivation of the CT state leads to the formation of T-1 (k = 8.7 x 10(9) s(-1)), whereas the 3MLCT excited state undergoes unperturbed deactivation (k = 1.2 x 10(10) s(-1)) directly to the ground state. For the latter dyad, the T-1 excited state is very long lived (280 mu s) since deactivation via the upper lying CT level is precluded for energetic reasons.