Clusters and related (sub)-nanoscale systems are promising alternatives to traditional bulk materials as they often exhibit novel properties, stemming from their quantized sizes, that can be developed for energy conversion applications. I will present our recent ultrafast spectroscopy measurements that demonstrate the exquisite tunability of the physical and chemical properties of clusters, through systematic adjustment of their atomic composition, enables new understandings to control energy flow. By tuning the d-electron density of strongly correlated, neutral metal oxides clusters with atomic precision, our ultrafast measurements act as a window to monitor electron correlation and identify the structural features driving carrier localization/separation and polaron formation.1,2 We developed an innovative technique for quantifying the metallicity of molecular sized materials.3,4 The electronic relaxation properties are strongly dependent on both size and local structure, enabling sub-nanometer clusters to exhibit metallic or semiconducting properties, and even transition between the two in direct relation to the ligand-to-metal charge transfer (LMCT) character of the photoexcitation. I will also present new results on the ultrafast ion-pair formation photodynamics of small atmospheric molecules, which have attracted substantial attention due to their prominent role in acid-base chemistry, ozone depletion and acid rain. We find that excited state proton transfer and ion pair formation is an important protection mechanism that stabilizes small formic acid clusters (FA)n against ultraviolet photodissociation on the fs timescale.5 However, proton transfer becomes exponentially less favorable with decreasing size, explaining why it has never been seen in FA2.
(1) Garcia, J. M.; Heald, L. F.; Shaffer, R. E.; Sayres, S. G. Oscillation in Excited State Lifetimes with Size of Sub-Nanometer Neutral (TiO2)NClusters Observed with Ultrafast Pump-Probe Spectroscopy. J. Phys. Chem. Lett. 2021, 12, 4098–4103.
(2) Garcia, J. M.; Heald, L. F.; Shaffer, R. E.; Sayres, S. G. Effect of Oxidation on Excited State Dynamics of Neutral TinO2n-x (n < 10, x < 4) Clusters. J. Chem. Phys. 2021, 155, 211102.
(3) Garcia, J. M.; Sayres, S. G. Increased Excited State Metallicity in Neutral Cr2On Clusters (N<5). J. Am. Chem. Soc. 2021, 143, 15572–15575.
(4) Garcia, J. M.; Sayres, S. G. Tuning the Photodynamics of Sub-Nanometer Neutral Chromium Oxide Clusters Through Sequential Oxidation. Nanoscale 2022, 14, 7798–7806.
(5) Sutton, S. F.; Rotteger, C. H.; Jarman, C. K.; Tarakeshwar, P.; Sayres, S. G. Ultrafast Proton Transfer and Contact Ion-Pair Formation in Formic Acid Clusters. J. Phys. Chem. Lett. 2023, 14, 8306–8311.