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 close 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.