Effects of protonation on the spectroscopic properties of tetrapyridoacridine (TPAC) mono- and dinuclear Ru(II) complexes in their ground and 3MLCT excited states

The spectroscopic behavior of mono- and dinuclear Ru(II)) complexes (P, T, PP and TT, Figure 1) that contain the extended planar ligand tetrapyrido[3,2-a:2',3'-c:3 '',2 ''-h:2"',3"'-j]lacridine (TPAC) and either 1,10-phenanthroline (phen) or 1,4,5,8-tetraazaphenanthrene (tap) as ancillary ligands is examined in water and as a function of the pH. These four complexes luminesce in aqueous solution. The analyses of the data in absorption lead to the pK(a), values in the ground state, and the data in emission show that the excited (MLCT)-M-3 states are much more basic than the ground state. When the complex contains tap ligands (T and TT), a decrease in pH transforms the luminescent excited basic form into another luminescent excited protonated species, which emits more bathochromically. In contrast, with phen ancillary ligands (P and PP), the protonated excited state does not luminesce. The rate constant of first protonation of the (MLCT)-M-3 state is diffusion controlled, except for the dinuclear PP complex, whose protonation takes place on the nitrogen of the acridine motif. For P, in which the protonation process is the fastest, it would take place on the nitrogen atoms of the nonchelated phen moiety of the TPAC ligand. These results allow also us to gain information on the localization of the excited electron in the (MLCT)-M-3 state populated upon absorption as well as in the relaxed 3MLCT emissive state. Moreover as these complexes are interesting for their study with DNA, it can be concluded from these data that a portion of the excited species in interaction with DNA will be protonated.