The Journal of Physical Chemistry A最新文献

Per-and polyfluoroalkyl substances (PFAS) are a class of ubiquitouspollutants that do not appreciatively biodegrade. A ban on the mostcommonly used PFAS, perfluoroalkyl carboxylic acids (PFCA) and perfluoroalkylsulfonic acids, created a gap that was filled by several similarlyheavily fluorinated compounds. While chemical and biological studieson several subclasses of PFAS have progressed significantly, testingon the chlorinated PFAS is still lagging. For most PFAS, the maintreatment consists of filtration followed by pyrolysis of the ladenadsorbents. Herein, the pyrolytic degradation pathways of mono- andpolychlorinated PFCA (ClxPFCA) are analyzed. This study showed thatthe bond dissociation energies (BDEs) are slightly lower in the acidform compared to the carboxylate form. The C–Cl bonds are generallythe weakest (∼70 kcal/mol) while the strongest are the C–Fbonds (105-110 kcal/mol). After studying their BDEs, the secondarydegradation pathways of the radicals formed by the first bond cleavagewere analyzed. Five reaction pathways were studied for the chlorinatedradicals generated by the first bond cleavage. The activations energies(E_a) and the enthalpies (ΔH) in the cases of a barrierless reaction are similar amongthe group of radicals. The lowest Ea are those of the Cl-shifts (∼10–25kcal/mol) while the most energetic pathways are the F-losses (50–65kcal/mol). Finally, a kinetic study was performed to evaluate thereaction rate coefficients of each pathway. It showed a preferencefor the β-scission pathways at the highest temperatures analyzedand the F-shift followed by Cl-loss at the lowest temperatures. Thelifetimes of the radicals are generally in the ms-min for the perfluorinatedalkyl radicals and min-hr for the perfluorocarboxyalkyl radicals.

Organic room temperature phosphorescence (RTP) materialsrely onefficient utilization of triplet excitons, yet the microscopic relationshipamong molecular distortion, crystal packing, and triplet decay dynamicsremains insufficiently understood. Herein, we employ multiscale excited-statecalculations combined with crystal-level simulations to elucidatehow molecular substitution and hydrostatic pressure regulate RTP propertiesin three phenothiazine-based molecular crystals (MM-HPT, MH-MPT, andMM-MPT). We demonstrate that methyl substitution induces progressiveconformational distortion, which reconstructs intermolecular packingand frontier orbital compositions, thereby controlling the competitionbetween radiative and non-radiative triplet decay pathways. AlthoughMH-MPT exhibits the strongest spin–orbit coupling and the largestphosphorescence radiative rate due to enhanced n-π orbital mixingand localized triplet excitons, its flexible excited-state geometryleads to severe structural relaxation and accelerated non-radiativedecay. In contrast, MM-MPT achieves more favorable triplet utilizationby rigidifying the excited-state structure through strong intermolecularconfinement, resulting in suppressed non-radiative losses. Furthermore,hydrostatic compression of MM-HPT continuously reduces excited-statestructural relaxation and enlarges the T₁-S₀ energy gap, producing a pronounced phosphorescence hypsochromicshift from 528 to 471 nm. Pressure-induced orbital reconstructionand weakened excitonic coupling further reveal the microscopic originof the emission evolution. These findings establish a molecular-leveldesign principle that efficient RTP emission requires balancing tripletgeneration, radiative transition, and structural relaxation suppressionrather than simply enhancing spin–orbit coupling, providingnew insights into the rational design of stimulus-responsive organicphosphorescent materials.

This study explores the effects of 1-substitution ofperylene bisimides(PBIs) with aryloxy groups on their photophysical features and aggregationequilibria. The main variables considered for this purpose were thesize of the aryloxy substituent, the solvent, and the analytical concentration.In all cases, aggregate formation was monitored by observing the concentrationdependence of the spectral features. Our results led us to concludethat the resulting aggregates exhibit an H-type signature and thatthe substituent hinders self-assembly. Compared to the reference unsubstitutedhomologue, the insertion of an aryloxy group proved to be an optimalstrategy for modulating the photophysical features of both monomersand aggregates. A detailed discussion is presented herein.

Theradiolytic stability of hydroxylamine (NH₃OH⁺) in concentrated nitric acid (HNO₃) is directlyrelevant to plutonium finishing in used nuclear fuel reprocessing,where NH₃OH⁺ serves as both a plutonium reductantand nitrous acid (HNO₂) scavenger under high-radiationconditions. In this study, aqueous 1 M HNO₃ solutions containingNH₃OH⁺ were gamma irradiated to accumulateddoses of up to 100 kGy. The radiolytic loss of NH₃OH⁺ was found to be independent of its initial concentration,pointing to its reaction with HNO₂ and hydroxyl (^•OH) and nitrate (NO₃^•) radicals as theprimary degradation mechanisms. Nitrous oxide (N₂O) wasthe dominant degradation product, with smaller amounts of molecularnitrogen (N₂) and hydrogen (H₂)─the latterexclusively from water radiolysis. A multiscale computer model wasdeveloped, combining stochastic radiation track-structure simulationswith a deterministic kinetic model, and validated against the completeexperimental dataset. The model quantitatively reproduced the radiolyticloss of solute NH₃OH⁺ and solvent nitrate ions(NO₃^–), and the evolution of N₂ and H₂. The N₂O yields, however, are determinedby reactions of the aminoxyl radical intermediate (NH₂O^•), whose rate coefficients under strongly acidic conditionsremain experimentally uncharacterized, representing the principalsource of model uncertainty. Nevertheless, the presented model providesrobust, quantitatively accurate predictions of NH₃OH⁺ stability under conditions relevant to plutonium finishingoperations. Because maintaining a sufficient NH₃OH⁺ inventory is critical for holding Pu(III) prior to oxalateprecipitation, the validated multiscale model is a valuable predictivetool that could be leveraged for process optimization within reprocessingflowsheets.

Asynchronous optical sampling (ASOPS) allows rapid collectionoftransient absorption/reflection data over wide time scales with ahigh dynamic range in both time and signal magnitude. In this paper,a method for recording transient images using ASOPS is presented basedon time-gated integration of the transient signal. This techniqueallows diffraction-limited pump-probe microscopy images to be rapidlycollected at precise, user-specified time delays, with approximately15 picoseconds time resolution. Results are presented for the excited-statedynamics in multilayered WSe₂ flakes, charge-carrier diffusionin a silicon wafer, and images of Au nanoparticle arrays that demonstratethe spatial/temporal resolution and sensitivity of the technique.Potential applications and the ultimate time resolution that can beachieved are also discussed.

Smallorganic molecules are highly promising candidates for circularlypolarized luminescent (CPL) materials owing to their well-definedstructures and excellent tunability, yet achieving high luminescencedissymmetry factors (g_lum) remains a criticalchallenge. In this work, density functional theory (DFT) and time-dependentdensity functional theory (TDDFT) calculations were employed to systematicallyinvestigate the effect of substituent position on the g_lum of helically chiral four-coordinate N,C-boron-chelatedazaborahelicenes. The results reveal that shifting the strong electron-donating-NMe₂ group along the naphthalene ring significantly modulatesthe local torsional angle, which is closely associated with the microscopicangle θ between the transition electric dipole moment (μ) and transition magnetic dipole moment(m). Remarkably, when the -NMe₂ group is located at the C3 position, the strongest π-conjugationwith the naphthalene framework is achieved, resulting in the mostpronounced global helical asymmetry. This geometric imbalance promotesa more favorable alignment between μ and m, thereby reducing the angleθ and substantially increasing the g_lum value to 0.66 × 10^–3 for ABH-N3, approximately1 order of magnitude higher than that of the parent ABH-N (0.07 ×10^–3). The present study elucidates how substituentposition regulates local π-conjugation and global helical asymmetry,thereby modulating the relative orientation of the transition electricand magnetic dipole moments and ultimately the CPL response. Theseinsights provide valuable theoretical guidance for the rational designof helical organic chiroptical materials.

Organic fluorophores combining excited-state intramolecularprotontransfer (ESIPT) and aggregation-induced emission (AIE) offer enhancedphotoluminescence properties. Herein, we have adopted a significantdistance strategy for asymmetric pyrene Schiff bases (HPBC and 1HPBC)to disclose their concurrent fluorescence mechanisms. In DMSO, bothmolecules showed two competing ESIPT and C═N photoisomerizationprocesses at the E*-state, while K*-rotation is kinetically and thermodynamicallyunfavorable. In HPBC, the rigid dimeric Schiff base units endowedstrong intramolecular H-bonding at the S₁ state, whichblocked the N–N assisted rotation of the adjacent C═Nbond and suppressed the nonradiative E*-relaxation. Thus, ESIPT, asthe dominant relaxation pathway, led to a comparatively emissive solution.The enhanced solid-state K*-emission is attributed to the thermodynamicallymore favorable ESIPT, restricted free rotations, and antiparallelstacking. Conversely, flexible 1HPBC, as a monomeric Schiff base,considerably lowered the energy barrier for nonradiative relaxation,thereby leading to more competitive ESIPT and C═N rotationdecay channels. On the basis of this computationally predicted fluorescencequenching mechanism of 1HPBC, it is proposed that the solid-stateemission enhancement may occur through combined restricted intramolecularrotation and ESIPT processes, which requires experimental validation.Hence, it is found that simple structural modifications can efficientlytune the fluorescence mechanisms, providing a theoretical referencefor the regulation of ESIPT-AIE systems.

Ozone-initiated oxidation of cyclohexene bridges thegap betweenclassical alkene ozonolysis and low-temperature combustion chemistry.In this study, it was investigated in jet-stirred reactors using complementarygas-chromatographic measurements over 350–900 K, temperature-dependentsynchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS)over 370–800 K for only ozone-containing mixtures, and isomer-resolvedsynchrotron vacuum-ultraviolet photoelectron-photoion coincidence(SVUV-PEPICO) spectroscopy at 400 K under only ozone-containing conditions.Ozone induces substantial low-temperature oxidation, including about1,000 ppm 1,6-hexanedial, while synchrotron measurements identifycyclic ethers and ketohydroperoxides (KHPs). The temperature-dependentdetection of cyclohex-2-en-1-one provides direct evidence for competitionbetween unimolecular peroxy-radical isomerization and bimolecularradical-recombination pathways: the oxyl-radical channel accountsfor 72.3% of the modeled branching at 370 K, whereas hydroperoxideformation and isomerization account for 51.5% and 29.2%, respectively,near 575 K. A kinetic model reproduces cyclohexene consumption andthe principal product trends, showing that prompt fragmentation dominatesthe initial ozonolysis step and that subsequent cyclic-peroxy chemistryis strongly temperature dependent.

Aggregation-induced emission (AIE) materials have attractedbroadinterest due to their efficient solid-state luminescence, yet a directand predictable conversion of aggregation-caused quenching (ACQ) moleculesinto AIE systems remains a significant challenge. Recent experimentalstudies suggest that water (H₂O) can modulate molecularpacking to induce ACQ-to-AIE transitions, but the underlying molecularmechanism remains elusive. Here, we employ the density functionaltheory (DFT), time-dependent DFT (TD-DFT), ONIOM (QM/MM), and moleculardynamics simulations to systematically unravel the H₂O-mediatedACQ-to-AIE mechanism in 3,5-diacetyl-1,4-dihydro-2,6-lutidine (DDL).Our results reveal that the nonradiative decay of DDL is associatedwith the couplings between the ππ* and nπ* states. In solution, the weak fluorescence of DDL originatesfrom efficient nonradiative decay associated with the strong couplingsbetween the ππ* and nπ* states,where acetyl-group-related vibrational modes make an important contributionto the nonradiative process. In pure DDL aggregates, the nπ* state remains energetically competitive, thereby facilitatingdark-state-mediated nonradiative relaxation and giving rise to ACQ.Upon incorporation of H₂O into the crystal lattice, π–πstacking is disrupted, the slip angles increase, and molecular vibrationsare restricted. More importantly, the ππ*/nπ* energy gap at the Franck–Condon region is enlarged,which weakens the vibronic interaction between the bright ππ*state and the dark nπ* state. As a result,access to the dark nπ* state is suppressed,and radiative decay from the ππ*-min becomes more competitive.Notably, H₂O molecules act not only as hydrogen-bond acceptorsand donors, but also form a stable intermolecular hydrogen-bondingnetwork that restricts the structural relaxation of the acetyl group.The mechanism is consistent with the experimentally observed enhancementof fluorescence quantum yield from 0.95% to 25.6%. This work elucidatesthe molecular principles underlying the H₂O-mediated ACQ-to-AIEtransitions and offers a conceptually significant framework for designingsolid-state luminescent materials without chemical modification.