Theoretical Study on the Reaction between Carcinogenic 2,5-Dichloro-1,4-benzoquinone and <i>tert</i>-Butyl Hydroperoxide: Self-Catalysis and Water Catalysis

The potentially carcinogenic halobenzoquinones (HBQs) have been recently identified in drinking water as disinfection byproducts. Several radical intermediates in the reaction of 2,5-dichloro-1,4-benzoquinone (DCBQ) and t-butyl hydroperoxide (t-BuOOH), which may induce DNA damage, were detected experimentally, and metal-independent decomposition reactions of t-BuOOH by DCBQ were proposed. It has not yet been confirmed by theoretical calculations. The theoretical study in this work provides insights into the details of the reaction. An unprecedented self-catalysis mechanism of organic hydroperoxides, that is, the reactant t-BuOOH also has a catalytic effect, was uncovered at the molecular level. Moreover, as the solvent, water molecules also clearly have an efficient catalytic effect. Due to the catalysis of t-BuOOH and water, the metal-independent reaction of t-BuOOH and DCBQ can occur under moderate conditions. Our findings about the novel catalytic effect of organic hydroperoxides t-BuOOH could offer a unique perspective into the design of new catalysts and an understanding of the catalytic biological, environmental, and air pollution reactions. Furthermore, organic hydroperoxide t-BuOOH could serve as a proton shuttle, where the proton transfer process is accompanied by simultaneous charge transfer. Therefore, organic hydroperoxides may disrupt the vital proton transfer process in biological systems and may give rise to unexpected toxicity

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump–Dump–Probe Spectroscopy

4′-<i>N</i>,<i>N</i>-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump–dump–probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump–probe and pump–dump–probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization

Solvent Polarity Dependent Excited State Dynamics of 2′-Hydroxychalcone Derivatives

The excited-state properties of 4-(dimethylamino)­methoxychalcones (<b>DEAMC</b>) and its derivative 4-(dimethylamino)­hydroxychalcones (<b>DEAHC</b>) were investigated in various solvents with different polarities by using steady-state and femtosecond transient absorption spectroscopy combined with quantum chemical calculations. It is found that their photophysical parameters such as fluorescence quantum yields, lifetimes, and excited-state relaxation paths strongly depend on the solvent polarity. Quantum-chemical calculations elucidate that the geometry of <b>DEAMC</b> in the ground state is slightly torsional whereas <b>DEAHC</b> adopts a near planar conformation stabilized by O–H···O chelated hydrogen bonds. Steady state spectra show that <b>DEAHC</b> is weak fluorescent in all solvents due to nonradiative relaxation in the excited enol and keto states, whereas the fluorescence quantum yield of <b>DEAMC</b> increases with the increasing of solvent polarities, and the emission yield is as large as 0.16 in acetonitrile. Femtosecond and nanosecond transient absorption spectra further prove that in nonpolar solvent the deactivation of S₁ in <b>DEAMC</b> is strongly governed by efficient formation of triplet states, whereas in polar solvent, stronger solvation induced energetically stabilization of ICT state, limiting the intersystem crossing to triplet state. The stabilization of ICT state not only leads to a higher fluorescence quantum yield for <b>DEAMC</b> but also restricts intramolecular twisting process in the enol form of <b>DEAHC</b>, facilitating efficient excited-state intramolecular proton-transfer reaction. These results clearly illustrate the dominant role of excited state solvation in modulating the emission behavior and deactivation mechanisms of fluorophores

Odd–Even Effect of Thiophene Chain Lengths on Excited State Properties in Oligo(thienyl ethynylene)-Cored Chromophores

In a self-assembly material system, odd–even effects are manifested from long-range periodic packing motifs. However, in an amorphous material system, due to long-range disorder, such phenomena are less prone to appear. Here, we report the discovery of a remarkable odd–even effect on the excited state properties of a series of conjugated thienyl ethynylene (TE) oligomers with truxene as end-capping units, Tr­(TE)_<i>n</i>Tr (<i>n</i> = 2–6), in solution. Using steady-state and time-resolved spectral measurements, we found the fluorescence quantum yield and excited state dynamics, both showing odd–even alternation with increasing thiophene–ethynylene chain lengths in apolar cyclohexane (CHX). It is found that the symmetry properties with different torsional modes dominate the excited state processes. In polar tetrahydrofuran (THF), solvation lowers the twisting barriers, leading to symmetry breaking without special odd–even alternation over structures. The results presented here will be helpful for understanding odd–even effects of conjugated polymers and designing novel photoelectric materials

Conformational Relaxation and Thermally Activated Delayed Fluorescence in Anthraquinone-Based Intramolecular Charge-Transfer Compound

A novel donor-π-acceptor-π-donor-type (D-π-A-π-D-type) chromophore, 2,6-bis­[4-(diphenylamino)­phenyl]-9,10-anthraquinone (AQ­(PhDPA)₂), has been reported as an efficient red thermally activated delayed fluorescence (TADF) emitter. Molecular structure and conformation, which directly determine the nature of excited states of a TADF emitter, are critical for obtaining efficient reverse intersystem crossing (rISC) and TADF. In this work, a series of excited-state deactivation processes of AQ­(PhDPA)₂, from the optical excitation to fluorescence and TADF emitting, have been investigated by theoretical calculations and ultrafast transient absorption (TA) spectroscopy. Theoretical calculations and steady-state spectra suggest that the TADF emitter appears to have conformational twisting in the excited state. Both the relaxed S₀ and S₁ conformations have a small energy difference between the lowest singlet and triplet excited states (Δ<i>E</i>_ST) in favor of rISC, whereas Δ<i>E</i>_ST increases at the relaxed T₁ conformation. Ultrafast TA spectra reveal that the intramolecular charge transfer (ICT) state of AQ­(PhDPA)₂ emits efficient fluorescence after a solvation-stabilization process in nonpolar toluene, while the fluorescence from the solvation-induced conformational relaxed ICT state is quenched in polar tetrahydrofuran. Additionally, we further reveal that the suppression of the conformational relaxation in long-lived triplet states contributes to maintaining a small Δ<i>E</i>_ST, which is critical for efficient rISC and TADF. These results provide a guidance for understanding the relationship between TADF and conformational relaxation dynamics, as well as for designing and synthesizing advanced TADF emitters

Accelerating Intersystem Crossing in Multiresonance Thermally Activated Delayed Fluorescence Emitters via Long-Range Charge Transfer

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters are excellent candidates for high-performance organic light-emitting diodes (OLEDs) due to their narrowband emission properties. However, the inherent mechanism of regulating the rate of intersystem crossing (ISC) is ambiguous in certain MR-TADF skeletons. Herein, we propose a mechanism of accelerating ISC in B/S-based MR-TADF emitters by peripheral modifications of electron-donating groups (EDGs) without affecting the narrowband emission property. The long-range charge transfer (LRCT) stems from the introduced EDG leading to high-lying singlet and triplet excited states. The ISC process is accelerated by the enhanced spin–orbital coupling (SOC) between the singlet short-range charge transfer (SRCT) and triplet LRCT manifolds. Meanwhile, the narrowband emission derived from the MR-type SRCT state is well retained as expected in the peripherally modified MR-TADF emitters. This work reveals the regulation mechanism of photophysical properties by high-lying LRCT excited states and provides a significant theoretical basis for modulating the rate of ISC in the further design of MR-TADF materials