How General are Changes in Excited State Surface Chemistry of Colloidal Quantum Dots for Photocatalysis?

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. These boundaries also protect nanocrystal surfaces from photochemical degradation and maintain their colloidal stability. In recent work, we demonstrated using time-resolved infrared (TRIR) spectroscopy that certain types ligand-nanocrystal interactions on PbS QDs exhibit marked reduction in surface bonding strength in the excited states of the nanocrystals, which may provide a pathway to modulate ligand bonding during photocatalytic reactions. In this work, we examined changes in the excited state surface chemistry of CdSe QDs passivated with stearic acid ligands to test the generality of this phenomenon. Transient absorption and photoluminescence spectroscopies were used to characterize the vibrational spectra of the ligands in the excited states of the QDs and to compare their time-dependence to the dynamics of the electronics states. The transient vibrational spectra of the symmetric and antisymmetric stretch modes of the carboxylate anchoring groups of the ligands indicated a net reduction of their higher frequency antisymmetric stretch and an enhancement of the lower frequency symmetric stretch modes. These changes were consistent with the influence that image dipoles created by the polarizable excited excitonic states of the QDs have on the transition dipole moments of the carboxylate anchoring groups. Importantly, a lower frequency transient vibrational feature around 1330 cm^-1 appeared in the TRIR spectra that was not present in the ground state infrared absorption (FTIR) spectrum of the QDs. This feature corresponded to the C–O single bond of more weakly bonded carboxylate groups in the monodentate bonding geometry. The shorter excited state lifetime of CdSe in comparison to PbS QDs allowed us to directly observe the reattachment of ligands to their original bonding geometries on much longer time scales. These findings suggest that changes in excited state surface chemistry may be general to metal chalcogenide QDs and motivates continued work to elucidate the origins and means to control the phenomenon for photocatalytic applications.