Publications by authors named "Andong Xia"

Reverse intersystem crossing (RISC) has become possible by minimizing the energy gap between the first excited singlet (S) and triplet state (T), which facilitates the thermally activated delayed fluorescence (TADF). Due to the small singlet-triplet energy gap, the S and T states exhibit comparable redox reactivity, leading organic TADF compounds to be potent photocatalysts. Here, we report such TADF compounds with multiple donor units designed as an efficient photocatalyst for the direct C(sp)-H carbamoylation of saturated aza-heterocycles.

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Article Synopsis
  • The rate of triplet state generation in heavy-atom-free organic molecules can be greatly increased (up to 10^10 times) through a process called spin-orbit charge-transfer intersystem crossing (SOCT-ISC), which relies on charge recombination mechanisms.
  • In this study, researchers investigated the behavior of specific organic compounds (arylsubstituted boron-dipyrromethene dyads) using time-resolved spectroscopy to understand how different factors affect the efficiency of triplet state generation.
  • The findings indicate that the energy levels of electron donors and solvent effects play key roles in determining the Gibbs free energy changes during electron transfer processes, which in turn impact the rates of charge recombination and triplet quantum yields
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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 -butyl hydroperoxide (-BuOOH), which may induce DNA damage, were detected experimentally, and metal-independent decomposition reactions of -BuOOH by DCBQ were proposed. It has not yet been confirmed by theoretical calculations.

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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.

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Strongly coupled excimer formation from interchromophoric charge transfer driven by the ultrafast excited-state structural dynamics of a 5,5'-linked 4-amino-1,8-naphthalimide covalent homodimer was investigated by ultrafast transient spectroscopy and chemical calculations. Theoretical calculations indicate that the structural relaxation associated with the dihedral motion leads to significantly enhanced interchromophoric charge transfer (CT) coupling, which favors the formation of an excimer-like symmetry-broken CT state. The formation and relaxation dynamics of the excimer state in the dimer are identified via ultrafast transient absorption and fluorescence spectroscopy.

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Organic dyads with intramolecular charge-transfer (ICT) character are emerging as viable and more sustainable photocatalysts than metal-based complexes. Herein, a carbazole- and naphthalimide-based organic dyad () was designed as an efficient organic photocatalyst for the direct C(sp)-H carbamoylation of saturated aza-heterocycles. Aiming at understanding the effect of environment, especially the solvent polarity on photocatalysis performance, the excited-state dynamics of in different polar solvents were studied by femtosecond (fs) and nanosecond (ns) time-resolved transient absorption (TA) spectroscopy.

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Disulfide bridges common in proteins show excellent photostability achieved by ultrafast internal conversion and maintain the stability of the tertiary structure. When disulfide bonds exist in aromatic compounds, the rigid chemical structure may affect the cleavage and reforming dynamics of disulfide bonds. In this work, a model compound with a disulfide five-membered-ring structure, 4,5-dithiolo--(2,6-dimethylphenyl)-1,8-naphthalimide (DTDPNI), is selected to elaborate the effect of disulfide modification on the excited-state deactivation mechanism.

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The conventional ultrafast pump-probe spectroscopy has primarily focused on examining the formation and decay of the excited state intermediates, but it is very difficult to detect those intermediates while the formation is slow and dissipation is much fast because of the limited concentration during the intrinsic photocycle. To address this issue, a multipulse ultrafast pump-dump-probe spectroscopy was employed to generate and probe the short-lived ground state intermediates (GSIs) in an electronic push-pull pyrene derivative (). This particular derivative undergoes planarized intramolecular charge transfer (PICT) in the excited state upon initial femtosecond pulse excitation.

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Boron-dipyrromethene (BODIPY) derivatives are prospective organic-based triplet photosensitizers. Since the triplet generation yield of the parent BODIPY is low, heavy atoms are widely used to improve the triplet yield. However, the dimerization of BODIPYs can also significantly improve their ability to produce triplets.

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Background: Microbes colonizing lower airways can regulate the host immune profile and consequently participate in lung disease. Increasing evidence indicate that individual microbes promote lung cancer progression and are involved in metastasis incidence. To date, however, no study has revealed the community structure of lung bacteria in metastatic non-small cell lung cancer (NSCLC) patients.

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In this work, the photoinduced charge separation (CS) via symmetry breaking in an orthogonal -β-linked boron dipyrromethene (BODIPY) dimer was investigated by polarized transient absorption spectroscopy. The time constant about 0.76 ps of the CS reaction determined in dimethyl sulfoxide is much faster than the solvation dynamics.

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Article Synopsis
  • The study investigates the photophysical properties of two compounds, bis-ANI (a dimer) and ANI (a monomer), focusing on their behavior in solvents of varying polarity through spectroscopy and computational methods.
  • Results show that bis-ANI exhibits lower fluorescence yields and emission red shifts compared to ANI when solvents become more polar, indicating differences in their electronic interactions.
  • The presence of the amino linker in bis-ANI creates an asymmetric charge separation (CS) state, which is influenced by solvent polarity and leads to noticeable changes in their excited state dynamics and charge recombination rates.
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Modulating the excited-state intramolecular proton transfer (ESIPT) reaction to achieve anticipant performance is always fascinating for chemists. However, feasible methods and a definite mechanism for tuning the ESIPT reaction remain insufficient. In this work, we reported the feasibility of continuously modulating the ESIPT dynamics in 2-(2'-hydroxyphenyl)oxazole (HPO) derivatives with different substitutions on the positions 5 and 5' of the core HPO through steady-state/transient spectroscopy and theoretical calculations.

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Understanding the excited-state charge transfer/separation (CT/CS) of donor-π-acceptor chromophores can provide guidance for designing and synthesizing advanced dyes to improve the performance of dye-sensitized solar cells (DSSCs) in practical applications. Herein, two newly synthesized electronic push-pull molecules, CS-14 and CS-15, that consist of carbazole donor and benzothiadiazole acceptor segments are chosen to explore the ultrafast dynamics of intramolecular CT/CS processes. The theoretical calculation results depict an excited-state intramolecular CT character for both dyes, while the dihedral angle between donor and acceptor of CS-14 is larger than that of CS-15, suggesting a more significant CT character of CS-14.

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The photoinduced intramolecular charge separation (CS) and charge recombination (CR) phenomena in a series of donor-bridge-acceptor (D-B-A) molecules are intensively investigated as a means of understanding electron transport through the π-B. Pyrene (Pyr) and triarylamine (TAA) moieties connected via phenylene Bs of various lengths are studied because their CS and CR behaviors can be readily monitored in real time by femtosecond transient absorption (fs-TA) spectroscopy. By combining the steady-state and fs-TA spectroscopic measurements in a variety of solvents together with chemical calculations, the parameters that govern the CS behaviors of these dyads were obtained, such as the solvent effects on free energy and the B-length-dependent electronic coupling () between D and A.

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Inspired by green plants, artificial photosynthesis has become one of the most attractive approaches toward carbon dioxide (CO ) valorization. Semiconductor quantum dots (QDs) or dot-in-rod (DIR) nano-heterostructures have gained substantial research interest in multielectron photoredox reactions. However, fast electron-hole recombination or sluggish hole transfer and utilization remains unsatisfactory for their potential applications.

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In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, and , by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations.

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Light-harvesting and then intramolecular energy transfer are the crucial steps in natural photosynthesis. Dendrimers are one of the most promising artificial light-harvesting antennas. Insight into the relationship between molecular structure and energy transfer (or delocalized excitation) in dendrimers would help in understanding and mimicking photosynthesis.

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Symmetric quadrupolar molecules generally exhibit apolar ground states and dipolar excited states in a polar environment, which is explained by the excited state evolution from initial charge delocalization over all molecules to localization on one branch of the molecules after a femtosecond pulse excitation. However, direct observation of excited-state charge redistribution (delocalization/localization) is hardly accessible. Here, the intramolecular charge delocalization/localization character of a newly synthesized acceptor-donor-acceptor molecule () has been intensively investigated by femtosecond stimulated Raman scattering (FSRS) together with femtosecond transient absorption (fs-TA) spectroscopy.

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As one of the most promising nonfullerene acceptors for organic photovoltaics, perylene diimide (PDI)-based multibranched molecules with twisted or three-dimensional (3D) geometric structures have been developed, which effectively increase the power conversion efficiency (PCE) of organic solar cells. Understanding the structure-property relationships in multichromophoric molecular architectures at molecular and ultrafast time levels is a crucial step in establishing new design principles in organic electronic materials. For this, photodriven excited-state symmetry-breaking charge separation (SB-CS) of PDI-based multichromophoric acceptors has been proposed to improve the PCE by reducing the self-aggregation of the planar PDI monomer.

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The excited state symmetry breaking charge transfer (SBCT) dynamics of two diacetylide-triphenylamine (DATPA) derivatives with different electron-donating abilities are investigated by femtosecond transient absorption and fluorescence spectroscopy. By tracking the evolution of the excited states by transient absorption spectra and the kinetics of the instantaneous emission dipole moments obtained from transient fluorescence spectroscopy, it is found that, in nonpolar solvent, the relaxed S state is quadrupolar with little change of emission dipole moments for the two molecules within 30 ps, whereas in polar solvent, the quadrupolar state evolves to a symmetry broken S state, in which, the emission dipole moment exhibits a fast reduction in the first few picoseconds. The larger reduction in emission transition dipole moment for the molecule with stronger electron-donating methoxy groups indicates a larger extent of symmetry breaking compared with the one with weak electron-donating methyl groups.

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The supramolecular polymeric radicals are developed to promote the generation efficiency and stability of naphthalenediimide (NDI) radical anions. To this end, a water-soluble bifunctional monomer bearing two naphthalene-viologen end groups and a NDI center is designed and synthesized. The supramolecular polymeric NDI radical anions are fabricated on the basis of host-guest complexation between the NDI-containing bifunctional monomer and cucurbit[8]uril (CB[8]) and followed by the photoinduced electron transfer process under UV light irradiation.

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Chemical reactions that are driven by plasmon-induced hot carriers are a timely topic of interest to chemists and material scientists as they provide catalytic alternatives that may reduce cost and/or waste. Herein, we monitored the localized surface plasmon resonance-induced keto-enol isomerization process of 2-mercapto-4(3H)-quinazolinone (MQ) by time-dependent surface enhanced Raman scattering (SERS), where the MQ molecules are adsorbed on gold nanoparticles (GNP) surface by Au-S bonds. The mechanism of keto-enol isomerization has been successfully investigated, and it is found that the isomerization is induced by hot hole transfer from GNPs to the adsorbed molecules.

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The selective interplay between dimensional morphology transition and signal transfer is an important feature for both nanomaterials and biosystems. While most of those reported examples considered either dimensional transition or signal transfer, the integrated interplay or selectivity for these two aspects in single self-assembled system has been rarely studied. Here, we report that a positively charged chiral π-building block could self-assemble into multidimensional nanostructures, which showed tunable circularly polarized luminescence (CPL).

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