Unveiling the Photophysical and Photochemical Reaction Process of Naproxen via Ultrafast Femtosecond to Nanosecond Laser Flash Photolysis

Ultrafast Time-Resolved Spectroscopic Study on the Photophysical and Photochemical Reaction Mechanisms of ortho-Methylbenzophenone in Selected Solutions

The photophysical and photochemical reaction pathways of ortho-methylbenzophenone (o-MeBP) in different solutions were investigated by employing femtosecond to nanosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopy methods. In pure acetonitrile, neutral or pH 1 aqueous solutions, o-MeBP exhibit similar excited-state evolutions upon excitation in which o-MeBP will experience excitation to an excited state then undergo intersystem crossing and solvent arrangement followed by 1,5 hydrogen atom transfer processes to form the first singlet excited state, triplet state (n, π*), biradical intermediates, and enol form transients, respectively. However, in a pH 0 acidic solution, the protonation of o-MeBP will form the cation biradical intermediate that facilitates radical coupling to generate a benzocyclobutanol product, which causes a dramatic reduction of the lifetime of the enol form transients. In contrast, in sodium bicarbonate solution, the biradical intermediate may be quenched by the bicarbonate ion to construct a C-C bond and form the carboxylic acid that causes a fast decay of biradical intermediate. These results demonstrate that the photophysical and photochemical reaction pathways of o-MeBP are pH-dependent in aqueous solution which may be very useful for the capture of CO₂ capture by photoexcitation of aromatic ketones.

Boosting Near-Infrared Photothermal Conversion by Intermolecular Interactions in Isomeric Cocrystals

Organic cocrystal exhibits excellent photothermal conversion (PTC), but how the intermolecular interactions of cocrystals regulate the PTC is obscure. Here, two isomeric donor molecules (phenanthrene and anthracene) and two electron-withdrawing molecules (7,7,8,8,8-tetracyanodimethylquinone and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinone dimethane) are self-assembled into the four cocrystals (PTQ, PFQ, ATQ, and AFQ). By changing the molecular configuration of the donor and the electron-withdrawing ability of the acceptor, the intrinsic influencing factors of the intermolecular interaction on the PTC were explored. Under near-infrared laser (808 nm) irradiation, the PTC efficiencies of PTQ, PFQ, AFQ, and ATQ are 35.85, 44.74, 57.00, and 60.53%, respectively. Based on the single-crystal X-ray diffraction, ultrafast time-resolved transient absorption, and excited-state theoretical calculations, we found that the π-π stacking in ATQ and AFQ is conducive to promoting the near-infrared light-harvesting ability and the p-π interaction of cocrystals can regulate the nonradiative rotation of -C(C≡N)₂ groups, resulting in a tunable near-infrared PTC via the isomeric cocrystals. Accordingly, the evaporation rate of the porous polyurethane-AFQ foam can reach 1.33 kg·m^-2·h^-1 in the simulated solar-driven water evaporation system. This work provides a strategy to boost the PTC by the intermolecular interactions of cocrystal materials.

Revealing the Photophysical and Photochemical Reaction Processes of Carprofen in Different Solutions via Ultrafast Femtosecond to Nanosecond Transient Absorption

Carprofen (CP), one kind of a nonsteroidal anti-inflammatory drug, exhibits phototoxic side effects in physiology, while its phototoxic mechanism is ambiguous. To uncover CP's photophysical and photochemical reaction processes, femtosecond to nanosecond transient absorption spectroscopies were employed to directly detect excited states and transient intermediates of CP upon UV irradiation in pure acetonitrile (MeCN), MeCN/water 1:1, and acid/alkaline buffer solutions. The transient absorption data together with DFT calculations were integrated to elucidate mechanisms for photochemical reactions of CP in different solutions. The associated photophysical and photochemical reaction pathways are dependent on various solution environments. In a pure MeCN solvent, CP is excited to a singlet state (S₁) and rapidly interacts with the solvent to proceed solvent rearrangement (SR). It then undergoes vibrational cooling (VC) and proceeds intersystem crossing (ISC) to produce the lowest triplet state (³CP). ³CP finally decays to the ground state. While in a MeCN/water 1:1 solution, deprotonated S₁ of CP experiences SR and VC processes, and then it is promoted to a deprotonated triplet state (³CP^-). ³CP^- undergoes the parallel reactions: dechlorination to a phenyl radical (²CP^-) and decarboxylation to a T₁ anion (³CP^-(de-CO₂)). Finally, both intermediates produce the radical anion species ²CP^-(de-CO₂). In a pH = 7.4 (MeCN/PBS 1:1) solution, ³CP^- can be converted into ²CP^-(de-CO₂) more quickly. Interestingly, we found that the dechlorination step can be promoted in an alkaline solution. Phenyl and chlorine radicals produced in an aqueous solution may be the root cause of the drug's harmful side effects on the human body. This may be useful to guide the design of related CP drugs with minimal phototoxicity in the pharmaceutical process.

Charge Transfer, Intersystem Crossing, and Electron Spin Dynamics in a Compact Perylenemonoimide-Phenoxazine Electron Donor-Acceptor Dyad

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor-acceptor dyad (PMI-PXZ) to study the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S₀ → ¹CT transition, was observed (ε = 2840 M^-1 cm^-1 at 554 nm, FWHM: 2850 cm^-1), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI-PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τ_T = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (Φ_Δ = 76%) is higher than that of the previously reported analogue dyad PMI-PTZ (Φ_Δ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI-PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor-acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor-acceptor dyads.

Unprecedented Improvement of Near-Infrared Photothermal Conversion Efficiency to 87.2% by Ultrafast Non-radiative Decay of Excited States of Self-Assembly Cocrystal

Near-infrared (NIR) photothermal conversion is of great interest in many fields. Here, a self-assembly organic cocrystal (N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and pyromellitic dianhydride (PMDA)) with strong absorption in NIR range is constructed, with widespread absorption (200-1500 nm) and very high NIR photothermal conversion efficiency (87.2%). Essentially, in this cocrystal, a small HOMO-LUMO gap of donor-acceptor pair boosts the absorption ability of this cocrystal in the NIR range. The mixed stacking structure significantly enhances the intermolecular interactions as well as the electron-hole delocalization, suppressing the emission processes, leading to nonradiative decay processes from excited states. Strong intermolecular interactions enable the cocrystal to have dense electronic energy levels, leading to a high proportion (94.4%) vibrational cooling and internal conversion processes with ultrafast excited-state relaxation (0.12 ps), which contributes to high NIR photothermal conversion efficiency. Furthermore, the cocrystal has exhibited capable ability for being an excellent candidate for a NIR photothermal therapy agent.

Disentangling Multiple Effects on Excited-State Intramolecular Charge Transfer among Asymmetrical Tripartite PPI-TPA/PCz Triads

By utilizing the bipolarity of 1,2-diphenylphenanthroimidazole (PPI), two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) were designed with triphenylamine (TPA) and 9-phenylcarbazole (PCz). These triads are deep-blue luminescent materials with a high fluorescence quantum yield of nearly 100 %. To trace the photophysical behaviors of these triads, their excited-state evolution channels and interchromophoric interactions were investigated by ultrafast time-resolved transient absorption and excited-state theoretical calculations. The results suggest that the electronic nature, asymmetrical tripartite structure, and electron-hole distance of these triads, as well as solvent polarity, determine the lifetime of intramolecular charge transfer (ICT). Interestingly, PPI-PCz triads show anti-Kasha ICT, and the charge-transfer direction among the triads is adjustable. For the PPI-TPA triad, the electron is transferred from TPA to PPI, whereas for the PPI-PCz triad the electron is pushed from PPI to PCz. Exploration of the excited-state ICT in these triads may pave the way to design better luminescent materials in the future.

Can UV-C laser pulsed irradiation be used for the removal of organic micropollutants from water? Case study with ibuprofen

A novel approach based on the direct pulsed irradiation of UV-C light onto ibuprofen (IBP) solutions was evaluated in this work, as proof of concept for the direct removal of micropollutants. The experiments confirmed that laser irradiation is able to completely degrade IBP in 15 min in distilled water, with a DOC depletion of ca. 25% and with transformation products (TPs) remaining in solution and estimated to represent ca. 10% of the initial IBP concentration. In wastewater spiked samples, removal efficiency is slightly lower but still significant (ca. 5% IBP remaining after 15 min). Hence, this work suggests that low power solid state pulsed lasers, emitting at 266 nm wavelength, show promise for the removal of these type of micropollutants from water. These results open new opportunities towards the development of chemical-free water treatment methods based on direct, selective irradiation using state of the art, miniaturized laser devices.

UV Light Generation and Challenging Photoreactions Enabled by Upconversion in Water

Sensitized triplet-triplet annihilation (sTTA) is the most promising mechanism for pooling the energy of two visible photons, but its applications in solution were so far limited to organic solvents, with a current maximum of the excited-singlet state energy of 3.6 eV. By combining tailor-made iridium complexes with naphthalenes, we demonstrate blue-light driven upconversion in water with unprecedented singlet-state energies approaching 4 eV. The annihilators have outstanding excited-state reactivities enabling challenging photoreductions driven by sTTA. Specifically, we found that an aryl-bromide bond activation can be achieved with blue photons, and we obtained full conversion for the very energy-demanding decomposition of a persistent ammonium compound as typical water pollutant, not only with a cw laser but also with an LED light source. These results provide the first proof-of-concept for the usage of low-power light sources for challenging reactions employing blue-to-UV upconversion in water and pave the way for the further development of sustainable light-harvesting applications.

Revealing Ultrafast Energy Dissipation Pathway of Nanocrystalline Sunscreens Oxybenzone and Dioxybenzone

Two widely used ultraviolet filters, oxybenzone and dioxybenzone, are applied in a variety of areas, particularly in sunscreen cosmetics. Ultrafast femtosecond transient absorption is utilized to trace the excited states and transient states of the nanocrystalline suspension and solution phase of these two molecules. The analysis reveals the intriguing discovery that the transient species of the oxybenzone nanocrystalline suspension have shorter lifetimes than that in solution. The energy dissipation mechanism of these molecules is simulated by density functional theory calculations, and the potential energy surface calculations and the single-crystal structure can well explain the fast decay dynamics of the nanocrystalline transient states of these two molecules.

Tunable surface plasmon polaritons and ultrafast dynamics in 2D nanohole arrays

High-quality and unique surface plasmon resonance (SPR) with a narrow linewidth and controllable resonance energy plays a key role in wide applications including ultrahigh-resolution spectroscopy, on-chip sensing, optical modulation, and solar cell technology. In this work, the response of surface plasmon polariton (SPP) modes in Au nanohole arrays has been effectively tuned by properly adjusting the sample orientation without changing the geometrical parameters, and a very narrow linewidth down to 8 nm is achieved via the strong interference of two (0, -1) and (-1, 0) SPP modes in the Γ-M direction under transverse magnetic polarization. These results have been validated excellently by finite-element-method numerical simulations. More importantly, we have quantitatively investigated the contribution of conduction-band electron distribution to the SPP intensity of the array within a 20 ps timescale with ultrahigh sensitivity by utilizing home-built femtosecond transient absorption spectroscopy, and observed the minimum SPP intensity at ∼700 fs following excitation with a 0.2 μJ pulse. This study may help enhance the understanding toward the intrinsic micromechanism of SPR, thus offering opportunities for potential applications in strong coupling and new-style optical wave manipulations.