Ultrafast Relaxations in Ruthenium Polypyridyl Chromophores Determined by Stochastic Kinetics Simulations

Maximizing the efficiency
of solar energy conversion using dye
assemblies rests on understanding where the energy goes following
absorption. Transient spectroscopies in solution are useful for this
purpose, and the time-resolved data are usually analyzed with a sum
of exponentials. This treatment assumes that dynamic events are well
separated in time, and that the resulting exponential prefactors and
phenomenological lifetimes are related directly to primary physical
values. Such assumptions break down for coincident absorption, emission,
and excited state relaxation that occur in transient absorption and
photoluminescence of tris­(2,2′-bipyridine)­ruthenium­(2+) derivatives,
confounding the physical meaning of the reported lifetimes. In this
work, we use inductive modeling and stochastic chemical kinetics to
develop a detailed description of the primary ultrafast photophysics
in transient spectroscopies of a series of Ru dyes, as an alternative
to sums of exponential analysis. Commonly invoked three-level schemes
involving absorption, intersystem crossing (ISC), and slow nonradiative
relaxation and incoherent emission to the ground state cannot reproduce
the experimentally measured spectra. The kinetics simulations reveal
that ultrafast decay from the singlet excited state manifold to the
ground state competes with ISC to the triplet excited state, whose
efficiency was determined to be less than unity. The populations predicted
by the simulations are used to estimate the magnitudes of transition
dipoles for excited state excitations and evaluate the influence of
specific ligands. The mechanistic framework and methodology presented
here are entirely general, applicable to other dye classes, and can
be extended to include charge injection by molecules bound to semiconductor
surfaces.