The Role of Spin–Orbit Coupling in the Linear Absorption Spectrum and Intersystem Crossing Rate Coefficients of Ruthenium Polypyridyl Dyes

The successful use of molecular dyes for solar energy
conversion
requires efficient charge injection, which in turn requires the formation
of states with sufficiently long lifetimes (e.g., triplets). The molecular
structure elements that confer this property can be found empirically,
however computational predictions using ab initio electronic structure
methods are invaluable to identify structure–property relations
for dye sensitizers. The primary challenge for simulations to elucidate
the electronic and nuclear origins of these properties is a spin–orbit
interaction which drives transitions between electronic states. In
this work, we present a computational analysis of the spin–orbit
corrected linear absorption cross sections and intersystem crossing
rate coefficients for a derivative set of phosphonated tris(2,2′-bipyridine)ruthenium(2+)
dye molecules. After sampling the ground state vibrational distributions,
the predicted linear absorption cross sections indicate that the mixture
between singlet and triplet states plays a crucial role in defining
the line shape of the metal-to-ligand charge transfer bands in these
derivatives. Additionally, an analysis of the intersystem crossing
rate coefficients suggests that transitions from the singlet into
the triplet manifolds are ultrafast with rate coefficients on the
order of 1013 s–1 for each dye molecule.