Characterizing Excited States of Single Donor-Acceptor Molecule by High-Resolution Raman Images

The excited states of charge transfer (CT), local donor (LD) and local acceptor (LA) for donor-acceptor molecules play an important role in many applications. However, these excited quantum states are still impossible to be characterized by conventional experimental techniques. Optical spectroscopy with the supplement of density functional theory calculations is widely adopted to investigate excited states, but is often inefficient for the donor-acceptor molecule owing to substantial errors of functionals in describing CT states. Our calculations show that the resonance tip-enhanced Raman scattering images that utilizes the confined plasmonic field for CT, LD and LA states possess their exclusive imaging characterizations, giving unambiguous identification of different excited-state properties. In addition, the resonance Raman images provide a benchmark to reasonably estimate the accuracy of different functionals in describing excited states of the charge-transfer system. These findings extend the emerging Raman images to the characterization and improvement of functional donor-acceptor molecular materials.