Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states dose in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.