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.

Simultaneous Observation of an Intraband Transition and Distinct Transient Species in the Infrared Region for Perovskite Solar Cells

“…Dispersive spectral features of this type are common when the band of a photogenerated transient such as a molecule in an excited state is blueshifted with respect to the ground-state band, as observed in our previous work. 32 However, the electrons are directly photoexcited in the present experiment, not the MA cation itself. Another effect that might account for the dispersive shapes is that thermal artifacts arise from transient heating of the sample induced by the laser irradiation.…”

“…We recorded time-resolved IR spectra of a MAPbI 3 thin film deposited on BaF 2 excited at 532 nm with laser fluence ∼25 μJ cm –2 , using the laboratory-built highly sensitive time-resolved IR spectrometer (see Supporting Information for details of the apparatus). ,− Unlike the steady-state FTIR spectrum of MAPbI 3 (Figure a, upper panel), which exhibits several sharp IR bands, the observed transient IR spectra (Figure a, lower panel) are dominated by extremely broad and structureless absorption that increases monotonically toward smaller wavenumber; it decays almost completely within a few microseconds. Such a broad transient absorption cannot be due to a vibrational transition and is attributed to the intraband transition of free electrons in the conduction band (CB) of the perovskite.…”

Infrared identification of the σ-complex of Cl-C6H6 in the reaction of chlorine atom and benzene in solid para-hydrogen

“…Rapid development of advanced experimental techniques and computational methods enables verification and detailed reinterpretation of many chemical reaction mechanisms, including identification and structural characterization of their transient intermediates [1][2][3] and explicit demonstration of realistic and not-so-fully-intuitive reaction pathways. [4][5][6][7] This development provides scientists a novel methodology for detailed investigations of fundamental organic reactions, which often seem to be a victim of oversimplified interpretations in organic chemistry textbooks.…”

Solvent effects on the structure of the triplet excited state of xanthone: a time-resolved resonance Raman study

“…Furthermore, inspecting these bond displacements with increasing polarity suggests that it has major vector projections on the v″ 6 , v″ 9 , v″ 14 and v″ 18 modes, and therefore, these modes undergo blue shifts with increasing polarity. Additionally, these observations cast doubt on the general applicability of resonance‐canonical structures to explain the excited states (in this case, triplet excited state) structure …”