Quantifying Entatic States in Photophysical Processes: Applications to Copper Photosensitizers

The entatic or rack-induced state is a core concept in
bioinorganic
chemistry. In its simplest form, it is present when a protein scaffold
places a transition metal ion and its first coordination sphere into
an energized geometric and electronic structure that differs significantly
from that of the relaxed form. This energized complex can exhibit
special properties. Under this purview, however, entatic states are
hardly unique to bioinorganic chemistry, and their effects can be
found throughout a variety of important chemistries and materials
science applications. Despite this broad influence, there are only
a few examples where entatic effects have been quantified. Here we
extend the entatic concept more generally to photophysical processes
by developing a combined experimental and computational methodology
to quantify entatic states across an entire class of functional molecules,
e.g., Cu-based photosensitizers. These metal complexes have a broad
range of applications, including solar electricity generation, solar
fuels synthesis, organic light emitting diodes (OLEDs), and photoredox
catalysis. As a direct consequence of quantifying entatic states,
this methodology allows the disentanglement of steric and electronic
contributions to excited state dynamics. Thus, before embarking on
the syntheses of new Cu-based photosensitizers, the correlations described
herein can be used as an estimate of entatic and electronic contributions
and thus guide ligand design and the development of next-generation
transition metal complexes with improved or tailored excited state
dynamics. Lastly, entatic energies in some Cu photosensitizers are
the largest yet quantified and are found here to approach 20 kcal/mol
relative to the conformationally flexible [Cu­(phen)2]+. These energetics are significant relative to typical chemical
driving forces and barriers, highlighting the utility in extending
entatic state descriptors to new classes of molecules and materials
with interesting functional properties involving the coupling between
electron and vibrational dynamics.