Category Archives: ChemPhotoChem

Laser Interfaced Mass Spectrometry of the Sunscreen Molecule Octocrylene Demonstrates that Protonation Does Not Impact Photostability

Posted on 8 September 2023 by

Laser photodissociation spectroscopy has been used to characterize the extent to which protonation affects the ability of octocrylene to act as an effective UV sunscreen molecule. We find that protonation results in a significant red shift of the absorption profile compared to non-protonated octocrylene, but does not impact on the ultrafast excited state decay pathways.

Abstract

Octocrylene (OCR) is a widely used organic sunscreen molecules, and is a dominant component of many sunscreen formulations. Here, we perform the first measurements on the protonated form of OCR, i. e. [OCR+H]⁺, to probe whether protonation affects the molecule's photostability. The novel photochemical technique of UV laser-interfaced mass spectrometry is employed from 400–216 nm, revealing that the electronic absorption spectrum of OCR across the S₁ and S₂ states red shift by 40 nm upon protonation. Our measurements reveal that [OCR+H]⁺ predominantly undergoes photofragmentation into the m/z 250 and 232 ionic products, associated with loss of its bulky alkyl side chain, and subsequent loss of water, respectively. We compare the photochemical fragmentation results with higher-energy collisional dissociation results to investigate the nature of the photodynamics that occur following UV absorption. The excited state decay pathways over the S₁ and S₂ excited states of [OCR+H]⁺ are associated with statistical fragmentation in line with dominant ultrafast decay. This behaviour mirrors that of neutral OCR, demonstrating that protonation does not affect the ultrafast decay pathways of this sunscreen molecule. We discuss our results in the context of the known breakdown of OCR into benzophenone, identifying a potential photoactivated pathway to benzophenone formation in solution.

Tightly Connected Poly(3‐Thiophene Boronic Acid)/g‐C3N4 Heterojunctions for Enhanced Visible‐Light Photocatalytic Hydrogen Production

Posted on 8 September 2023 by

Tightly connected poly(3-thiophene boronic acid)/g-C₃N₄ (PBTA/CN) heterojunctions were fabricated, dependent on hydrogen-bonding interactions for enhanced visible-light photocatalytic hydrogen production. The enhanced photoactivities are attributed to significantly enhanced charge transfer and separation by high-level electron transfer from CN to PTBA.

Abstract

Constructing efficient polymer semiconductor/g-C₃N₄ heterojunctions is highly desirable for enhancing the photogenerated charge separation of g-C₃N₄ and further improving the solar-hydrogen production efficiency. Herein, we synthesized poly(3-thiophene boronic acid)/g-C₃N₄ (PTBA/CN) heterojunctions with tight interface contact by a simple wet-chemical strategy. The resulting ratio-optimized 3PTBA/CN heterojunction exhibits 8.7 times enhancement of the visible-light photocatalytic hydrogen production compared to CN_. Based on the steady-state surface photovoltage spectra (SS-SPS), photoluminescence spectra (PL), ⋅OH amount measurements, time-resolved photoluminescence spectra (TR-PL), and single-wavelength photocurrent action spectra, it is confirmed that the enhanced photocatalytic performance is mainly attributed to the promoted photogenerated charge separation resulting from the transfer of high-level electrons from CN to PTBA via the formed tight interface contact, depending on the hydrogen bonding interactions between the boronic acid groups [−B(OH)₂] of PTBA and the amino groups (−NH₂) of CN. Furthermore, the −B(OH)₂ of PTBA facilitates the uniform dispersion of the co-catalyst Pt. This work provides an effective strategy for constructing efficient tightly connected polymer semiconductor/CN heterojunction photocatalysis.

Physicochemical and Nonlinear Optical Properties of a Stilbazolium Family Single Crystal with Third Order Nonlinear Optical Activity

Posted on 7 September 2023 by nSekar Anand, nMuthurakku Usha Ranin

A third-order nonlinear optical 4-[2-(4-dimethylamino-phenyl)-vinyl]-1-methyl-pyridinium 2-nitroaniline-4-sulfonate (DSNA) single crystal was successfully grown for the first time by incorporating a novel counter anion in the stilbazolium cation. Characterization results suggest that the DSNA crystal can be used for optoelectronic and optical limiting applications.

Abstract

A novel counter anion group was incorporated with the organic stilbazolium cation (C₁₆H₁₉N₂ ⁺) to yield a new third-harmonic-generation-active single crystal of 4-[2-(4-dimethylamino-phenyl)-vinyl]-1-methyl-pyridinium 2-nitroaniline-4-sulfonate (DSNA). The slow evaporation solution growth technique is employed to obtain DSNA crystals, whose nonlinear responses are analysed through a continuous wave laser Z scan experiment. The thermally induced strong reverse saturation absorption and self-defocusing behaviour of the DSNA crystal suggests that the material could be used as an optical limiting device. Optical characterization shows that the lower absorption edge falls in the visible region (530 nm), with a broad transparency range of 0.53 to 1 μm. Exploration of other optical constants, such as extinction coefficient (k=10⁻⁴), emission wavelength (602 nm – red light), optical (σ_opt=10¹⁰ Ωm⁻¹) and electrical conductivity (σ_elc=10¹¹ Ωm⁻¹) also concludes that the DSNA crystal could be potentially employed in optoelectronic devices. The various bonds and their corresponding lengths and angles involved in the formation of DSNA ionic crystal are evaluated by means of single-crystal X-ray diffraction (SCXRD). Linear and nonlinear optical property findings undoubtedly affirm that the titular DSNA crystal is an effective nonlinear optical (NLO) crystal.

The Importance of Precise Reaction Condition Control for the Comparison of Photocatalyst Materials on the Example of Hydrogen Peroxide Formation over Polymeric Carbon Nitrides

Posted on 7 September 2023 by

Our study uses the photocatalytic production of hydrogen peroxide by various polymeric carbon nitride materials to demonstrate the importance of precisely adjusting various reaction conditions in the reactor, such as light intensity, oxygen flow, and wavelength. In addition, reaction parameters were chosen to achieve extremely high hydrogen peroxide concentrations.

Abstract

In our study, we aimed to show how different reaction parameters can affect production rates using photocatalytic hydrogen peroxide formation by different polymeric carbon nitrides (PCN). For this purpose, selected materials were first compared under the same reaction conditions and compared with TiO₂ (P25). We also show that different light intensities can have a different influence on seemingly similar materials. Since hydrogen peroxide production in the presence of an electron donor proceeds mainly by reduction of oxygen, we also show an influence of the oxygen flow on the formation rates. Thus, with high oxygen fluxes and high intensities of irradiated light, we were able to achieve an H₂O₂ concentration of 125 mM after about 25 h. Finally, the two best PCN materials were selected to measure light intensity dependence at different wavelengths up to visible light. It was found that they behaved differently at the different wavelengths and thus it could be shown that an exact specification of the reaction parameters is indispensable for comparisons in the literature.

Merging Organocatalysis and Photocatalysis: A New Momentum in Covalent Radical Catalysis

Posted on 6 September 2023 by nGaétan Archer, nKunlong Song, nMaurice Médebielle, nJérémy Meradn

This Concept article describes the contribution of photocatalysis in the field of covalent radical catalysis to control the generation and reactivity of radical catalysts. The synthetic perspectives offered by such catalytic combinations are discussed from a mechanistic point of view.

Abstract

The use of free radicals as organocatalysts constitutes a powerful strategy to activate and functionalize unsaturated carbon chains. Indeed, the unique affinity of open-shell species for alkenes and alkynes can be rerouted to achieve the catalytic covalent activation of the substrate and control a subsequent radical cascade. However, the field of covalent radical catalysis has remained challenging for decades due to important issues in terms of catalyst handling, reaction design and viability. Recently, these pitfalls have been addressed one by one by the use of photocatalysis to control the generation and the reactivity of radical catalysts. This Concept article aims to highlight recent achievements in the field of photocatalyzed covalent radical catalysis and the perspectives offered by such catalytic combinations. The reaction mechanisms and the interconnection between the catalytic cycles are reviewed with the hope of demonstrating the synthetic potential of this approach and foster a rapid growth of this nascent topic.

Photoinduced One‐Electron Oxidation of Estrone Derivatives: A Combined Steady‐State and Time‐Resolved Spectroscopy Investigation

Posted on 4 September 2023 by

Photoinduced one-electron oxidation of a set of estrone derivatives (R-OX) with ammonium persulphate was carried out by means of steady-state and time-resolved spectroscopies. The photogenerated SO₄⋅⁻ oxidizes efficiently the estrones to the radical cations and a mesolytic fragmentation gave the phenoxyl radical with k _frag of 10⁴–10⁵ s⁻¹ that depend on the X−O bond dissociation energies.

Abstract

A detailed investigation of the photophysics and of the redox properties of a set of selected estrone carboxylate and sulfonate esters was carried out by means of steady-state and time-resolved spectroscopies. The observed dual fluorescence was assigned to an efficient intramolecular energy transfer from the aromatic moiety to the carbonyl group. On the other hand, the photoinduced monoelectronic oxidation of R−OX in the presence of persulphate anion is followed by mesolytic fragmentation of the so generated radical cation into the corresponding phenoxyl radical with rate constant values (k _frag) of 10⁴–10⁵ s⁻¹, that depend on the X−O bond dissociation energy values.

Accessing a Long‐Fluorescence‐Lifetime BODIPY Dye via Efficient Förster Resonance Energy Transfer Induced by Host‐Guest Doping

Posted on 8 August 2023 by

Long-fluorescence lifetime: A novel pure organic RTP system with a long fluorescence lifetime has been achieved through an efficient FRET process via host-guest doping, offering great potential for information encryption, fingerprint identification, and bio-imaging applications.

Abstract

The development of pure organic room temperature phosphorescence (RTP) luminophores with high quantum yield and long emission lifetime has attracted considerable attention due to their extensive optoelectronic and biomedical applications. The commonly used BODIPY dyes have strong UV-Vis absorption, a relatively sharp emission peak, high quantum yield, and a short emission lifetime. By expending the molecular doping approach, we herein successfully achieved a long-fluorescence-lifetime BODIPY dye via efficient Förster resonance energy transfer (FRET) from RTP molecule o-AI-Cz to the short-lived fluorescence emitter Bodipy-2I. This host-guest material possessed an ultralong lifetime of up to 595 ms and a long emission wavelength under ambient conditions, which has great potential in data encryption and anti-counterfeiting applications. Moreover, this easily prepared FRET system exhibited an efficient response to two-photon excitation in living cell fluorescence imaging.

ESIPT‐Active Pyrene‐imidazole Fluorophores: GSIPT, Dual Solid‐ and Solution‐State Emission plus Counter‐Intuitive Crystal Packing

Posted on 7 August 2023 by

A novel pyrene-imidazole-based ESIPT-active molecules have been developed that exhibit unique ground state enol to keto transformation and dual solid- and solution-state emission.

Abstract

We report a new excited-state intramolecular proton transfer (ESIPT)-inspired hydroxyl (OH)-substituted pyrene-imidazole that promote unprecedented intramolecular proton transfer in the ground state (GSIPT). The enol and keto isomers are in equilibrium, with a high keto-isomer component in the solid state. In conjunction with DFT calculations, the photophysical studies and crystal structure analysis shred substantial evidence for the ground state transformation. The ESIPT-active compounds also show the rare feature of high dual solution (φ_f=70 %) and solid state (φ_f=41 %) emission characteristics and counter-intuitive stronger intermolecular interactions in the solid state versus the ESIPT-inactive counterparts.

Electron‐Deficient Phenanthrenequinone Derivative for Photoactivated Hydrogen Atom Transfer Mediated Oxidation of Secondary Alcohols

Posted on 2 August 2023 by

We designed a synthetic route for previously unpublished photocatalyst, 3,6-bis(trifluoromethyl)-9,10-phenanthrenequinone (PQ-CF₃), and studied its photophysical properties in comparison with other known phenanthrenequinones. The photocatalytic efficacy was demonstrated in the oxidation of 29 secondary alcohols. Mechanistic studies revealed that regardless of the electronic properties of the substrate, PQ-CF₃ operates rather via highly efficient hydrogen atom transfer (HAT) than single-electron transfer (SET).

Abstract

In 2000, Fukuzumi and co-workers reported a seminal study on the photochemical oxidation of benzylic alcohols with visible-light-excited 9,10-phenanthrenequinone (PQ) under argon atmosphere (J. Am. Chem. Soc. 2000, 122, 8435). We optimized the reaction conditions they reported and were able to oxidize 1-(4-methoxyphenyl)ethanol quantitatively to 4'-methoxyacetophenone in only 15 min with 10 mol % PQ as a photocatalyst under oxygen. However, we observed a significant decrease in the oxidation rate with more electron-deficient benzylic alcohols as starting materials. To improve the photooxidation performance, we designed a high-yielding synthetic route for a novel, more electron-deficient PQ derivative, 3,6-bis(trifluoromethyl)-9,10-phenanthrenequinone (PQ-CF₃). Its efficiency as a photocatalyst in the fast oxidation of secondary alcohols was demonstrated not only with several electronically diverse benzylic alcohols but also with aliphatic substrates. The comprehensive mechanistic studies based on Hammett plot construction, kinetic isotope experiments, and DFT computations suggest that the mechanistic pathway of the alcohol oxidation is dependent on the electronic properties of both the catalyst and the substrate. As the key mechanistic discovery, we showed that the newly developed PQ-CF₃ operates as a highly efficient hydrogen atom transfer (HAT) catalyst.

Triplet Formation and Triplet‐Triplet Annihilation Upconversion in Iodine Substituted Non‐Orthogonal BODIPY‐Perylene Dyads

Posted on 24 July 2023 by

BODIPY-perylene nonorthogonal dyad forms three kinds of triplet species through SOCT-ISC and SO-ISC mechanism. All three kinds of triplet species participate in energy transfer to annihilator molecules. Two annihilator molecules produce one anti-Stokes shifted photon. The presence of iodine on dyad offers 2.6× faster ISC and 8.4× higher upconversion yield.

Abstract

BODIPY-perylene dyads have emerged as useful metal free sensitizers for triplet-triplet annihilation upconversion (TTAUC), these dyads are capable of efficient triplet generation via spin-orbit charge transfer intersystem crossing (SOCT-ISC). This important route to triplet formation requires dyads in which two moieties are oriented perpendicular to each other. In this contribution, we give a deeper insight on the behavior of recently reported BODIPY-perylene dyads, where BODIPY-perylene dihedral exhibits a non-orthogonal dyad geometry. The intersystem crossing of BODIPY-perylene dyads with and without iodine are investigated using femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) spectroscopy. The concurrent decay of the singlet charge transfer state (¹CT) and rise of triplet states in both the iodinated and non-iodinated dyads confirms the SOCT-ISC as the main intersystem crossing pathway despite the altered geometry of the dyads. The presence of an iodine atom on the BODIPY-moiety enables intersystem crossing 2.6-times faster and provides a higher triplet yield with respect to dyad without iodine. The upconversion quantum yield ( ) is 8.4-times higher in the sample containing iodinated dyad as sensitizer and perylene as annihilator. The triplet-triplet energy transfer rate (k _TTET) is ~8×10⁸ M⁻¹ s⁻¹ for both iodinated and non-iodinated sensitizer containing TTAUC systems in 1,4-dioxane.