title: Is our way of thinking about excited states correct? Time-resolved dispersive IR study on p-nitroaniline.
abstract: Low-lying excited electronic states of an important class of molecules known as push-pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high-sensitivity nanosecond time-resolved dispersive IR spectroscopy and DFT calculations on p-nitroaniline (PNA), a prototypical push-pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure. In this structure, the quinoid configuration is restricted to a part of the phenyl ring adjacent to the NO(2) group. The partial quinoid structure of PNA cannot be explained by a commonly used hybrid of a neutral form and a zwitterionic charge-transfer form. Our findings not only cast doubt on the general applicability of the classical way of looking at excited states, based exclusively on characteristic resonance structures, but also provide deeper insights into excited-state structure of highly polarizable molecular systems.
authors.affiliation: Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan.
authors.forename: Sudhakar
authors.initials: S
authors.lastname: Narra
authors.forename: Shu-Wei
authors.initials: SW
authors.lastname: Chang
authors.forename: Henryk A
authors.initials: HA
authors.lastname: Witek
authors.forename: Shinsuke
authors.initials: S
authors.lastname: Shigeto