COMPLEXES OF THE RUTHENIUM(II)-2,2' 6',2''-TERPYRIDINE FAMILY - EFFECT OF ELECTRON-ACCEPTING AND ELECTRON-DONATING SUBSTITUENTS ON THE PHOTOPHYSICAL AND ELECTROCHEMICAL PROPERTIES/

We have investigated the luminescence properties of 14 [Ru(tpy-X)(tpy- Y)](2+) complexes (tpy = 2,2':6',2''-terpyridine; X = Y = MeSO(2), Cl, H, Ph, EtO, OH, or Me(2)N; X = H, Y = MeSO(2); X = OH, Y = MeSO(2); X = Cl, Y = EtO; X = OH, Y = Ph; X = MeSO(2), Y = Me(2)N; X = Cl, Y = M e(2)N; X = OH, Y = Me(2)N; Me CH3; Et = C2H5; Ph = C6H5). All the comp lexes examined display a strong luminescence in rigid matrix at 77 K, with lifetimes in the 1-10 mu s time scale. The energy of the emission maximum is red shifted for both electron-accepting and electron-donat ing substituents compared to that of the parent Ru(tpy)(2)(2)+ complex . At room temperature, electron-accepting substituents increase the lu minescence quantum yield and the excited state lifetime, whereas elect ron-donating substituents show an opposite effect. The temperature dep endence of the emission lifetime has been investigated for some repres entative complexes, and the role played by activated and activationles s nonradiative transitions is examined. It is shown that the values of rate constants for radiationless decay from the luminescent excited s tate to the ground state are governed not only by the energy gap but a lso by the nature of the substituents, which presumably affects the ch anges in the equilibrium displacement or frequency between the two lev els: Correlations of the electrochemical redox potentials, the Hammett sigma parameter, and the energy of the luminescent level are reported and discussed. Such correlations show that electron-accepting substit uents have a larger stabilization effect on the LUMO pi ligand-center ed orbital than on the HOMO pi(t(2g)) metal orbital, whereas electron- donating substituents cause a larger destabilization on the HOMO pi(t( 2g)) metal orbital than on the LUMO pi ligand-centered orbital. Heter oleptic complexes carrying an electron-accepting group and an electron -donating group always show lower emission energies when compared with the parent homoleptic complexes because the pi orbital of the tpy-A Ligand is stabilized, and the tpy-D ligand destabilizes the metal-cent ered pi(t(2g)) orbitals.