Is our way of thinking about excited states correct? Time-resolved dispersive IR study on p-nitroaniline

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. The third structure: A combination of time-resolved dispersive IR spectroscopy and DFT calculations reveals that the lowest excited triplet state of p-nitroaniline has a partial quinoid structure, in apparent contradiction to the conventional picture based on characteristic resonance structures, such as neutral and zwitterionic charge-transfer forms (see figure).