Origin of Vibronic Coherences During Carrier Cooling in Colloidal Quantum Dots

Recent two-dimensional electronic spectroscopy experiments
[Tilluck et al. J. Phys.
Chem. Lett. 2021, 12 (39), 9677−9683] indicate the
creation of coherent vibronic wavepackets in the first femtoseconds
of hot carrier cooling in hexadecylamine-passivated CdSe quantum dots.
Here we present a quantum chemical study of the origin of these coherences
in a CdSe nanocrystal. We find that coherent wavepacket motions along
vibrational coordinates with alkylamine character promote nonradiative
relaxation through conical intersections between the exciton states
of the inorganic core. Electronic excitations in the core are found
to pass energy to the vibrations of the ligands via two distinct mechanisms:
excitation of core phonon modes that are coupled to the ligand vibrations
and direct excitation of ligand vibrations by delocalization of the
exciton onto the ligands, both of which naturally arise within a photochemical
framework based on many-electron potential energy surfaces. If these
findings are demonstrated to be general, vibronic coherences may be
leveraged to control photophysical outcomes in colloidal quantum dots.