Solvent Dependent Excited State Behaviors of Luminescent Gold(I)–Silver(I) Cluster with Hypercoordinated Carbon

Polynuclear Au­(I) complexes continues to attract considerable attention because of their bright emissions in the visible wavelength, which hold promise in applications in luminescence, fluorescence sensing, and bioimaging. Despite various spectroscopic investigations on their steady state properties, detailed understanding of the origin of their emissions and excited state relaxations is still lacking. Here, we report femtosecond time-resolved transient absorption experiments combined with quantum chemical calculations on a brightly emissive [Au₆Ag₂(C)­(dppy)₆]­(BF₄)₄ cluster in different solvents. Global analysis on the transient absorption spectra based on a sequential model gives three spectral components: (1) excited state absorption (ESA) of ¹MLCT_Au state (τ = 1–3 ps); (2) ESA of ³MLCT_Au state (τ = 11–40 ps), and (3) ESA of ³MLCT_Ag state (long-lived). By variation of the solvent’s polarity and hydrogen bonding ability, the relative population of the triplet MLCT states and the emission properties can be modulated. Especially in methanol, an additional site specific O–H···π bond is formed between methanol molecules and aromatic rings of ligands, which enhances the ultrafast nonradiative decay from the hydrogen bond stabilized ³MLCT_Au state and reduces the population of the emissive ³MLCT_Ag state. The results presented here about the excited state dynamics of luminescent gold­(I)–silver­(I) cluster allow a deeper insight into the origin of their emissions by monitoring the population of the emissive ³MLCT_Ag state and dark ³MLCT_Au state in different environments

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

Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver-Doped Ag_<i>x</i>Au_{25–<i>x</i>} Clusters

The luminescent ligand protected metal clusters have attracted considerable attentions while the origin of the emission still remains elusive. As recently reported in our previous work, the rod-shaped Au₂₅ cluster possesses a low photoluminescence quantum yield (QY = 0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au₂₅ cluster can drastically enhance the photoluminescence with high quantum yield (QY = 40.1%). To explore the enhancement mechanism of fluorescence, femtosecond transient absorption spectroscopy is performed to determine the electronic structure and ultrafast relaxation dynamics of the highly luminescent silver-doped Ag_<i>x</i>Au_{25–<i>x</i>} cluster by comparing the excited state dynamics of doped and undoped Au₂₅ rod cluster, it is found that the excited state relaxation in Ag_<i>x</i>Au_{25–<i>x</i>} is proceeded with an ultrafast (∼0.58 ps) internal conversion and a subsequent nuclear relaxation (∼20.7 ps), followed by slow (7.4 μs) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in Ag_<i>x</i>Au_{25–<i>x</i>} (∼20.7 ps) compared to that in undoped Au₂₅ rod (∼52 ps). We conclude that it is the central Ag atom that stabilizes the charges on LUMO orbital and enhances the rigidity of Ag_<i>x</i>Au_{25–<i>x</i>} cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around ∼0.8 THz were observed in both clusters, indicating the symmetry preservation from Au cluster to Ag alloying Au clusters. The present results provide new insights for the structure-related excited state behaviors of luminescent ligand protected Ag alloying Au clusters

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

Two Electron Reduction: From Quantum Dots to Metal Nanoclusters

The quantum dots (QDs) and metal nanoclusters (MNCs) have recently attracted increasing interest due to their intriguing physical–chemical properties. Nevertheless, the inherent correlations between them have rarely been explored. In this study, we successfully achieved the conversion of the silver QDs ([Ag₆₂S₁₃(SBu^t)₃₂]⁴⁺) to silver MNCs ([Ag₆₂S₁₂(SBu^t)₃₂]²⁺) via the electrochemical reduction method. A key intermediate could be obtained by setting the voltage at (−0.6) V, and its atomic structure has been determined to be [Ag₆₂S₁₃(SBu^t)₃₂]²⁺ by single crystal X-ray crystallography. After that, the centroid S atom in the Ag₁₄S cubic core can be extruded out of the clusters through the window via an energy favorable route during the reducing process which will be reported for the first time. The detailed conversion process and the accompanying changes of optical properties were studied. Our work revealed a unique case that QDs could be converted to MNCs

Light-Induced Ring-Closing Dynamics of a Hydrogen-Bonded Adduct of Benzo[1,3]oxazine in Protic Solvents

Benzo­[1,3]­oxazine, an organic optical switching compound, is known to be in an equilibrium between its closed form (OX) and its open zwitterionic form (IN). Here we report a light-induced ring-closing mechanism of a hydrogen-bonded adduct (p-IN, partially protonated open isomer of benzo­[1,3]­oxazine) based on the observations of femtosecond and nanosecond transient absorption spectra in protic solvents. Femtosecond transient measurements upon visible excitation reveal the appearance of two states having different dynamical signatures. One corresponds to a conventional intramolecular charge transfer excited state. The other one is a concerted electron–proton transfer product (d-IN, embedded solvent molecule released from p-IN). Without steric hindrance, the main molecular structure tends to be planar upon excitation, and two intermediates, IN and OX, are involved in the sequential thermal transformation before the return to the ground state of p-IN. Specifically, in alcoholic solvents, d-IN converts to the original p-IN compound within 1 ms via the dominant pathway d-IN → IN → OX → p-IN and the side pathway d-IN → IN → p-IN, which is found to be feasible in energy; in contrast, in aqueous solution with increasing strength of intermolecular hydrogen bonding, the rate of the thermal transformation is enhanced by 1 order of magnitude

Toward an Understanding of How the Optical Property of Water-Soluble Cationic Polythiophene Derivative Is Altered by the Addition of Salts: The Hofmeister Effect

Hofmeister ion-specific effects on optical properties of a water-soluble cationic polymer, poly­(3-alkoxy-4-methylthiophene) (PMNT), are investigated by means of absorption, resonance Raman spectroscopy, and molecular dynamic simulations. It is found that the ionochromism of conjugated polyelectrolytes PMNT obeys Hofmeister series with high optical sensitivity, while the spectral changes result from the different electrostatic interactions and the conformational change of the cationic PMNT in different salt solutions. As a result, UV–vis absorption spectra exhibit almost no shift of absorption of PMNT in the presence of SO₄^2–, F^–, etc., whereas a red-shifted absorption of PMNT with I^–, SCN^– is clearly observed. To gain a deeper understanding of the nature of these anion-dependent chromic phenomena, ab initio calculations and molecular dynamics (MD) simulations are carried out to present the microscopic insights, that the Hofmeister effect occurs at the PMNT/water interface through the direct (hydrophobic, hydrophilic, and electrostatic) interactions between the anions and PMNT backbone. It is found that the salting-in anions like I^– strongly suppress the hydrophobic collapse of PMNT backbone, leading to more extended and ordered PMNT backbone with red-shifted absorption, and the salting-out anions like F^– strongly avoid the hydrophobic PMNT backbone, keeping a random-coiled configuration of PMNT backbone without obvious absorption changes in KF solution. The existence of ordered and disordered backbone configurations is further verified by monitoring the main in-plane skeleton Raman modes (CC and C–C stretch) of PMNT in various salt solutions. The results presented here could provide a fundamental understanding of salt effects on chemical and biological processes occurring at the macromolecular/water interface, and then may potentially stimulate many chemical and biological applications

Two Electron Reduction: From Quantum Dots to Metal Nanoclusters

The quantum dots (QDs) and metal nanoclusters (MNCs) have recently attracted increasing interest due to their intriguing physical–chemical properties. Nevertheless, the inherent correlations between them have rarely been explored. In this study, we successfully achieved the conversion of the silver QDs ([Ag₆₂S₁₃(SBu^t)₃₂]⁴⁺) to silver MNCs ([Ag₆₂S₁₂(SBu^t)₃₂]²⁺) via the electrochemical reduction method. A key intermediate could be obtained by setting the voltage at (−0.6) V, and its atomic structure has been determined to be [Ag₆₂S₁₃(SBu^t)₃₂]²⁺ by single crystal X-ray crystallography. After that, the centroid S atom in the Ag₁₄S cubic core can be extruded out of the clusters through the window via an energy favorable route during the reducing process which will be reported for the first time. The detailed conversion process and the accompanying changes of optical properties were studied. Our work revealed a unique case that QDs could be converted to MNCs