Direct Observation of the Solvent Effects on the Low-Lying nπ* and ππ* Excited Triplet States of Acetophenone Derivatives in Thermal Equilibrium

Abstract

Low-lying excited triplet states of aromatic carbonyl compounds exhibit diverse photophysical and photochemical properties of fundamental importance. Despite tremendous effort in studying those triplet states, the effects of substituents and solvents on the energetics of the triplet manifold and on photoreactivity remain to be fully understood. We have recently studied the ordering of the low-lying nπ* and ππ* excited triplet states and its substituent dependence in acetophenone derivatives using nanosecond time-resolved near-IR (NIR) spectroscopy. Here we address the other important issue, the solvent effects, by directly observing the electronic bands in the NIR that originate from the lowest nπ* and ππ* states of acetophenone derivatives in four solvents of different polarity (n-heptane, benzene, acetonitrile, and methanol). The two transient NIR bands decay synchronously in all the solvents, indicating that the lowest nπ* and ππ* states are in thermal equilibrium irrespective of the solvent polarity studied here. We found that the ππ* band increases in intensity relative to the nπ* band as solvent polarity increases. These results are compared with the photoreduction rate constant for the acetophenone derivatives in the solvents to which 2-propanol was added as a hydrogen-atom donor. Based on the present findings, we present a comprehensive, solvent- and substituent-dependent energy level diagram of the low-lying nπ* and ππ* excited triplet states.

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

“…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.…”

Visible-Light Photocatalytic E to Z Isomerization of Activated Olefins and Its Application for the Syntheses of Coumarins

“…It is likely that these solvents stabilize the radical cation intermediate involved, while acetonitrile helps to elongate the lifetime of triplet excited states [25][26][27][28][29] or stabilize biradical intermediates in an energy transfer mechanism as suggested for our work. Acetone, a typical solvent which can be used as a sensitizer, was not effective enough by itself to achieve good transformation in this reaction.…”

Structure and Electron Configuration of Imidazole-2-carboxaldehyde and Its Excited Triplet: Resonance Raman and Transient Absorption Spectroscopy and DFT Calculation Investigations

“…This result implies that 3 IC may not have a pure nπ* or ππ* character. The mixed nπ* and ππ* configurations of 3 IC may allow it to undergo H-atom abstraction via reversion from the ππ* to nπ* triplet state . If this is the case, then the rate constant of H-atom abstraction is determined by the energy gap between the ππ* and nπ* triplet states.…”