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 = Me(2)N; X = OH, Y = Me(2)N; Me CH3; Et = C2H5; Ph = C6H5). All the complexes 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-donating substituents compared to that of the parent Ru(tpy)(2)(2)+ complex. At room temperature, electron-accepting substituents increase the luminescence quantum yield and the excited state lifetime, whereas electron-donating substituents show an opposite effect. The temperature dependence of the emission lifetime has been investigated for some representative complexes, and the role played by activated and activationless nonradiative transitions is examined. It is shown that the values of rate constants for radiationless decay from the luminescent excited state to the ground state are governed not only by the energy gap but also by the nature of the substituents, which presumably affects the changes in the equilibrium displacement or frequency between the two levels: 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 substituents have a larger stabilization effect on the LUMO pi* ligand-centered 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. Heteroleptic 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-centered pi(t(2g)) orbitals.