Synthesis, characterization, spectroscopic and electrochemical studies of donor-acceptor ruthenium(II) polypyridine ligand derivatives with potential NLO applications

In this work we report the preparation of six new donor-metal-acceptor (D-M-A) type complexes of ruthenium(II) with the highly absorbing "chromophoric" ligand 4,4′-bis(2-(4-methoxyphenyl)styryl)-2,2′-bipyridine, (L-OCH₃, donor moiety) and substituted polypyridinic ligands with electron acceptor character (NN-A). The NN-A studied ligands were pyrazino[2,3-f][1,10]phenanthroline (ppl), 11-R-dipyrido[2,3-a:2′,3′-c]phenazine (dppz-R; R is H, NO₂, or CN) and 10,11-[1,4-naphtalenedione]dipyrido[3,2-a:2′,3′-c]phenazine (Aqphen). The complexes were characterized by IR, NMR, UV-Vis spectroscopy and cyclic voltammetry. The potential NLO response of the complexes was evaluated by solvatochromic studies. Although the communication between D and A exists, the effect of the change of the acceptor moiety on the properties of the complexes is small and the behavior of the complexes is governed mainly by the donor ligand. The Metal to Ligand Charge Transfer bands (MLCT) exhibited by all complexes in the visible region have dominant electronic density transfer character from the metal to the chromophoric L-OCH₃ ligand. The hypsochromic shift of this low energy absorption band on going from a less polar (benzene) to a more polar solvent (acetonitrile) indicated that a redistribution of the electronic density among the metal and the donor ligand is observed. This behavior permits to predict a NLO response for these types of complexes. The combination of high molar absorptivity with intraligand charge transfer (ILCT) mixing into the MLCT bands are encouraging for the generation of new materials with interesting NLO properties.