The multichromophore approach:: Prolonged room-temperature luminescence lifetimes in RuII complexes based on tridentate polypyridine ligands

A new family of ruthenium(11) complexes with multichromophoric properties was prepared based on a "chemistry-on-the-complex" synthetic approach. The new compounds are based on tridentate chelating sites (tpy-type ligands, tpy=2,2':6',2"-terpyridine) and most of them carry appended anthryl chromophores. Complexes 2a and 2b were synthesized through the Pd-catalyzed Suzuki coupling reaction between 9-anthrylboronic acid and the chloro ligands on the presursor species la and 1b, respectively. The monocoupling product 2c was also synthesized as the starting complex for a dimetallic complex under optimized Suzuki coupling conditions. The palladium(0)-catalyzed homocoupling reaction on complexes la and 2c led to dimetallic Ru-II species 2d and 2e, respectively. The solid structures of complexes 2a and 2b were characterized by X-ray diffraction. The absorption spectra, redox behavior, luminescence properties (both at room temperature and at 77 K), and transient absorption spectra and decays of 2a-e were investigated. The absorption spectra of all new species are dominated by ligand-centered (LC) bands in the UV region and metal-to-ligand charge-transfer (MLCT) bands in the visible region. The new compounds undergo reversible metal-centered oxidation processes and several ligand-centered reduction processes, which have been assigned to specific sites. The complexes exhibit luminescence both at room temperature in fluid solution and at 77 K in rigid matrices; the emission was attributed to (MLCT)-M-3 states at room temperature and to the lowest-lying anthracene triplet ((3)An) at low temperature, except for 2c, which does not contain any anthryl chromophore and whose low temperature emission is also of MLCT origin. The luminescence lifetimes of complexes 2a-d showed that multichromophoric behavior occurs in these species, allowing the luminescence lifetime of the Ru-II-based chromophores to be prolonged to the microsecond timescale, with the anthryl groups behaving as energy-storage elements for the repopulation of the (MLCT)-M-3 state. Nanosecond transient-absorption spectroscopy confirmed the equilibration process between the triplet MLCT and An levels at room temperature. Thermodynamic and kinetic factors governing the equilibration time and the lifetime of the equilibrated excited state are discussed.