High-Energy Hybridized States Enable Long-Lived Hot Electrons in Cobaloxime-Silicon Nanocrystal System

Strong electronic coupling is achieved between the molecular
catalyst
cobaloxime ([Co]) and silicon nanocrystals (Si NCs) bridged by an
ethylenepyridine group derived from vinylpyridine (vpy) covalently
bound to the Si NC surface (Si-vpy-[Co]). The ethylenepyridine tether
in Si-vpy-[Co] is key to dramatic changes to the system’s physical
propertieswhich are not observed in the corresponding formylpyridine
(fpy) system (Si-fpy-[Co])consistent with strong electronic
coupling previously observed only in dark electrochemical systems.
UV–vis absorption spectroscopy reveals new [Co]-centered electronic
states in Si-vpy-[Co], and transient absorption spectroscopy finds
a strong absorption feature appearing within 250 fs and persisting
for at least 5 ns. Astoundingly, spectroelectrochemical measurements
reveal that this absorption feature is consistent with both the singly
reduced [Co]− and doubly reduced [Co]2– complexes, leading to the conclusion that these long-lived charges
are derived from high-energy “hot” electrons residing
in [Co]-centered states. Detailed analysis using cyclic voltammetry,
spectroelectrochemistry, electron paramagnetic resonance spectroscopy,
and density functional theory (DFT) calculations provides insight
into the unique electronic structure created in Si-vpy-[Co]. DFT reveals
that the new electronic states arise from hybridization between deep
Si NC band states and high-energy molecular orbitals of the ethylenepyridine
tether and the [Co] catalyst and are facilitated by σ-bonding
character at the ethylenepyridine linkage. This study demonstrates
that strong electronic coupling achieved through precise molecular
chemistry can change the paradigm of otherwise fixed energy levels
in hybrid photoelectrochemical systems for artificial photosynthesis
and related applications.