A core-shell spectroscopic perspective on solution phase charge transfer in photoexcited push-pull chromophores

Push-pull chromophores typically have a D-π-A molecular architecture with an electron-rich donor group D and an electron-poor acceptor group A, linked by aromatic π units. Here, using the prototypical push-pull chromophore 4-(dimethylamino)benzonitrile (DMABN) in solvents of varying polarity, we demonstrate that time-resolved soft X-ray absorption spectroscopy (XAS), combined with state-of-the-art ab initio quantum chemical simulations, provides quantitative insight into how changes in frontier molecular orbital interactions determine the charge-transfer properties of the molecule. Exploiting the element-specific sensitivity of XAS, we characterize the electronic structures of the locally excited and charge-transfer states within the first singlet excited-state manifold and, in addition, unambiguously identify the early-time population of a triplet state. Our results suggest a common underlying mechanism governing the charge redistribution and population dynamics of the excited electronic states in DMABN, with implications for controlling the photophysical properties of push-pull chromophores in charge-transfer applications.