Electronic Transitions in Highly Symmetric Au130 Nanoclusters by Spectroelectrochemistry and Ultrafast Spectroscopy

Rich and discrete energy states in gold nanoclusters enable different combinations of electronic transitions and correspondingly electrochemical and optical properties for a variety of applications. The impacts on those electronic transitions by the emergence and magnitude/alignment of a band gap and by the contributions from different atomic/molecular orbitals require further study. Au nanoclusters with 130 core Au atoms are of interest in this report because they are at the transition size regime where a small yet well-defined band gap can be resolved along with continuous quantized frontier core orbitals. Here, electrochemical analysis is combined with UV–vis–near infrared optical measurements to unveil previously unresolved electronic transitions. Finite changes in the steady-state optical absorption spectrum are captured by spectroelectrochemistry when the Au nanoclusters are charged to different states via electrolysis. Multiple previously unresolved peaks and valleys as well as isosbestic “points/regions” are observed in the differential spectrum. The detailed spectral features are explained by the respective electronic transitions to those affected energy states. Key features are also well correlated with ultrafast absorption analysis which provides additional insights, such as the lifetime of the corresponding transitions. The experimentally measured energy states and transitions could serve as references for future theoretical study to learn the respective contributions from different atomic orbitals and, importantly, to explore routes to enhance or suppress certain transition so as to modulate the corresponding electrochemical and optical properties for better applications.