The Multichromophore Approach: Prolonged Room-Temperature Luminescence Lifetimes in Ru(II) Complexes Based on Tridentate Polypyridine Ligands

A new family of ruthenium(II) complexes with multichromophoric properties has been 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 1a and 1b, respectively. The mono-coupling product 2c was also synthesized as the starting complex for a dimetallic complex under optimized Suzuki coupling conditions. The palladiu(0)-catalyzed homocoupling reaction on complexes 1a and 2c led to dimetallic RuII 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 absorpion spectra and decays of 2a-e have been investigated. The absorption spectra of all the 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 has been attributed to 3MLCT states at room temperature and to the lowest-lying anthracene triplet (3An) 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 takes place in these species, allowing the luminescence lifetime of the RuII-based chromophores to be prolonged to the microsecond timescale, with the anthryl groups behaving as energy storage elements for the repopulation of the 3MLCT 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.