Enhancing Proton-Coupled Electron Transfer in Blue Light Using FAD Photoreceptor AppA.

The Blue Light Using FAD (BLUF) photoreceptor utilizes a noncovalently bound FAD to absorb light and trigger the initial ultrafast events in receptor activation. FAD undergoes 1 and 2 electron reduction as an enzyme redox cofactor, and studies on the BLUF photoreceptor PixD revealed the formation of flavin radicals (FAD and FADH) during the photocycle, supporting a general mechanism for BLUF operation that involves PCET from a conserved Tyr to the oxidized FAD. However, no radical intermediates are observed in the closely related BLUF proteins AppA and BlsA, and replacing the conserved Tyr with fluoro-Tyr analogs that increase the acidity of the phenol OH has a minor effect on AppA photoactivation in contrast to PixD where the photocycle is halted at FAD.

Two-Dimensional Infrared Spectroscopy Resolves the Vibrational Landscape in Donor-Bridge-Acceptor Complexes with Site-Specific Isotopic Labeling.

Donor-bridge-acceptor complexes (D-B-A) are important model systems for understanding of light-induced processes. Here, we apply two-color two-dimensional infrared (2D-IR) spectroscopy to D-B-A complexes with a -Pt(II) acetylide bridge (D-C≡C-Pt-C≡C-A) to uncover the mechanism of vibrational energy redistribution (IVR). Site-selective C isotopic labeling of the bridge is used to decouple the acetylide modes positioned on either side of the Pt-center.

Time-resolved vibrational spectroscopic study of molecular nanoaggregate photocatalysts.

The controlled aggregation of organic chromophores into supramolecular structures offers a way to control and tune photocatalytic activity. However, the underlying mechanisms of charge transfer and accumulation are still unclear. Time-resolved vibrational spectroscopy is a powerful structural probe for studying photogenerated intermediates.

Competing Nonadiabatic Relaxation Pathways for Near-UV Excited -Nitrophenol in Aqueous Solution.

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited -nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts.

Contrasting Photosensitized Processes of Ru(II) Polypyridyl Structural Isomers Containing Linear and Hooked Intercalating Ligands Bound to Guanine-Rich DNA.

The DNA binding and cellular uptake of the lambda enantiomer of two bis-tetraazaphenanthrene (TAP) Ru(II) polypyridyl complexes containing either a linear dppn () or a hooked bdppz () benzodipyridophenazine ligand are reported, and the role of different charge-transfer states of the structural isomers in the photo-oxidation of guanine is explored. Both complexes possess characteristic metal-to-ligand charge-transfer (MLCT) bands between 400 and 500 nm and emission at ca. 630 nm in an aerated aqueous solution.

Slow vibrational relaxation drives ultrafast formation of photoexcited polaron pair states in glycolated conjugated polymers.

Glycol sidechains are often used to enhance the performance of organic photoconversion and electrochemical devices. Herein, we study their effects on electronic states and electronic properties. We find that polymer glycolation not only induces more disordered packing, but also results in a higher reorganisation energy due to more localised π-electron density.

Methanol-to-Olefins Studied by UV Raman Spectroscopy as Compared to Visible Wavelength: Capitalization on Resonance Enhancement.

Resonance Raman spectroscopy can provide insights into complex reaction mechanisms by selectively enhancing the signals of specific molecular species. In this work, we demonstrate that, by changing the excitation wavelength, Raman bands of different intermediates in the methanol-to-hydrocarbons reactions can be identified. We show in particular how UV excitation enhances signals from short-chain olefins and cyclopentadienyl cations during the induction period, while visible excitation better detects later-stage aromatics.

Simultaneous Surface-Enhanced Raman Scattering with a Kerr Gate for Fluorescence Suppression.

The combination of surface-enhanced and Kerr-gated Raman spectroscopy for the enhancement of the Raman signal and suppression of fluorescence is reported. Surface-enhanced Raman scattering (SERS)-active gold substrates were demonstrated for the expansion of the surface generality of optical Kerr-gated Raman spectroscopy, broadening its applicability to the study of analytes that show a weak Raman signal in highly fluorescent media under (pre)resonant conditions. This approach is highlighted by the well-defined spectra of rhodamine 6G, Nile red, and Nile blue.

Characterization of the Reversible Intersystem Crossing Dynamics of Organic Photocatalysts Using Transient Absorption Spectroscopy and Time-Resolved Fluorescence Spectroscopy.

Thermally activated delayed fluorescence (TADF) emitters are molecules of interest as homogeneous organic photocatalysts (OPCs) for photoredox chemistry. Here, three classes of OPC candidates are studied in dichloromethane (DCM) or N,N-dimethylformamide (DMF) solutions, using transient absorption spectroscopy and time-resolved fluorescence spectroscopy. These OPCs are benzophenones with either carbazole (2Cz-BP and 2tCz-BP) or phenoxazine/phenothiazine (2PXZ-BP and 2PTZ-BP) appended groups and the dicyanobenzene derivative 4DP-IPN.

A stronger acceptor decreases the rates of charge transfer: ultrafast dynamics and on/off switching of charge separation in organometallic donor-bridge-acceptor systems.

To unravel the role of driving force and structural changes in directing the photoinduced pathways in donor-bridge-acceptor (DBA) systems, we compared the ultrafast dynamics in novel DBAs which share a phenothiazine (PTZ) electron donor and a Pt(ii) -acetylide bridge (-C[triple bond, length as m-dash]C-Pt-C[triple bond, length as m-dash]C-), but bear different acceptors conjugated into the bridge (naphthalene-diimide, NDI; or naphthalene-monoimide, NAP). The excited state dynamics were elucidated by transient absorption, time-resolved infrared (TRIR, directly following electron density changes on the bridge/acceptor), and broadband fluorescence-upconversion (FLUP, directly following sub-picosecond intersystem crossing) spectroscopies, supported by TDDFT calculations. Direct conjugation of a strong acceptor into the bridge leads to switching of the lowest excited state from the intraligand IL state to the desired charge-separated CSS state.

Unraveling Excited State Dynamics of a Single-Stranded DNA-Assembled Conjugated Polyelectrolyte.

Conformational templating of conjugated polyelectrolytes with single-stranded DNAs (ssDNAs) has the prospect of tailoring excited state dynamics for specific optoelectronic applications. We use ultrafast time-resolved infrared spectroscopy to study the photophysics of a cationic polythiophene assembled with different ssDNAs, inducing distinct conformations (flexible disordered structures vs more rigid complexes with increased backbone planarity). Intrachain polarons are always produced upon selective excitation of the polymer, the extent being dependent on backbone torsional order.

Tuning of B photochemistry in the CarH photoreceptor to avoid radical photoproducts.

Time-resolved infrared spectroscopy reveals the flow of electron density through coenzyme B in the light-activated, bacterial transcriptional regulator, CarH. The protein stabilises a series of charge transfer states that result in a photoresponse that avoids reactive, and potentially damaging, radical photoproducts.

The Transition from Unfolded to Folded G-Quadruplex DNA Analyzed and Interpreted by Two-Dimensional Infrared Spectroscopy.

A class of DNA folds/structures known collectively as G-quadruplexes (G4) commonly forms in guanine-rich areas of genomes. G4-DNA is thought to have a functional role in the regulation of gene transcription and telomerase-mediated telomere maintenance and, therefore, is a target for drugs. The details of the molecular interactions that cause stacking of the guanine-tetrads are not well-understood, which limits a rational approach to the drugability of G4 sequences.

Correction: Time-resolved infra-red studies of photo-excited porphyrins in the presence of nucleic acids and in HeLa tumour cells: insights into binding site and electron transfer dynamics.

Correction for 'Time-resolved infra-red studies of photo-excited porphyrins in the presence of nucleic acids and in HeLa tumour cells: insights into binding site and electron transfer dynamics' by Páraic M. Keane , , 2022, , 27524-27531, https://doi.org/10.

Nanophase-photocatalysis: loading, storing, and release of HO using graphitic carbon nitride.

A blue light mediated photochemical process using solid graphitic carbon nitride (g-CN) in ambient air/isopropanol vapour is suggested to be linked to "nanophase" water inclusions and is shown to produce approx. 50 μmol HO per gram of g-CN, which can be stored in the solid g-CN for later release for applications, for example, in disinfection or anti-bacterial surfaces.

Photoinduced Electron Transfer from a 1,4,5,6-Tetrahydro Nicotinamide Adenine Dinucleotide (Phosphate) Analogue to Oxidized Flavin in an Ene-Reductase Flavoenzyme.

Recent reports have described the use of ene-reductase flavoenzymes to catalyze non-natural photochemical reactions. These studies have focused on using reduced flavoenzyme, yet oxidized flavins have superior light harvesting properties. In a binary complex of the oxidized ene-reductase pentaerythritol tetranitrate reductase with the nonreactive nicotinamide coenzyme analogs 1,4,5,6-tetrahydro NAD(P)H, visible photoexcitation of the flavin mononucleotide (FMN) leads to one-electron transfer from the NAD(P)H to FMN, generating a NAD(P)H cation radical and anionic FMN semiquinone.

Time-resolved infra-red studies of photo-excited porphyrins in the presence of nucleic acids and in HeLa tumour cells: insights into binding site and electron transfer dynamics.

Cationic porphyrins based on the 5,10,15,20--(tetrakis-4--methylpyridyl) core (TMPyP4) have been studied extensively over many years due to their strong interactions with a variety of nucleic acid structures, and their potential use as photodynamic therapeutic agents and telomerase inhibitors. In this paper, the interactions of metal-free TMPyP4 and Pt(II)TMPyP4 with guanine-containing nucleic acids are studied for the first time using time-resolved infrared spectroscopy (TRIR). In DO solution (where the metal-free form exists as DTMPyP4) both compounds yielded similar TRIR spectra (between 1450-1750 cm) following pulsed laser excitation in their Soret B-absorption bands.

Nonadiabatic excited-state dynamics of ReCl(CO)(bpy) in two different solvents.

We present a study of excited-states relaxation of the complex ReCl(CO)(bpy) (bpy = 2,2-bipyridine) using a nonadiabatic TD-DFT dynamics on spin-mixed potential energy surfaces in explicit acetonitrile (ACN) and dimethylsulfoxide (DMSO) solutions up to 800 fs. ReCl(CO)(bpy) belongs to a group of important photosensitizers which show ultrafast biexponential subpicosecond fluorescence decay kinetics. The choice of solvents was motivated by the different excited-state relaxation dynamics observed in subpicosecond time-resolved IR (TRIR) experiments.

Lithium Insertion into Graphitic Carbon Observed via Operando Kerr-Gated Raman Spectroscopy Enables High State of Charge Diagnostics.

Monitoring the precise lithium inventory of the graphitic carbon electrode within the Li-ion battery, in order to assess cell aging, has remained challenging. Herein, operando electrochemical Kerr-gated Raman spectroscopy measurements on microcrystalline graphite during complete lithium insertion and extraction are reported and compared to conventional continuous-wave Raman microscopy. Suppression of the fluorescence emission signals via use of the Kerr gate enabled the measurement of the Raman graphitic bands of highly lithiated graphite where 0.

Solving the Puzzle of Unusual Excited-State Proton Transfer in 2,5-Bis(6-methyl-2-benzoxazolyl)phenol.

2,5-Bis(6-methyl-2-benzoxazolyl)phenol () exhibits an ultrafast excited-state intramolecular proton transfer (ESIPT) when isolated in supersonic jets, whereas in condensed phases the phototautomerization is orders of magnitude slower. This unusual situation leads to nontypical photophysical characteristics: dual fluorescence is observed for in solution, whereas only a single emission, originating from the phototautomer, is detected for the ultracold isolated molecules. In order to understand the completely different behavior in the two regimes, detailed photophysical studies have been carried out.

Structural Information about the -to- Isomerization Mechanism of the Photoswitchable Fluorescent Protein rsEGFP2 Revealed by Multiscale Infrared Transient Absorption.

RsEGFP2 is a reversibly photoswitchable fluorescent protein used in super-resolved optical microscopies, which can be toggled between a fluorescent On state and a nonfluorescent Off state. Previous time-resolved ultraviolet-visible spectroscopic studies have shown that the Off-to-On photoactivation extends over the femto- to millisecond time scale and involves two picosecond lifetime excited states and four ground state intermediates, reflecting a to excited state isomerization, a millisecond deprotonation, and protein structural reorganizations. Femto- to millisecond time-resolved multiple-probe infrared spectroscopy (TRMPS-IR) can reveal structural aspects of intermediate species.

Ruthenium Assemblies for CO Reduction and H Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO.

Two novel supramolecular complexes ([Ru(dceb)(bpt)Re(CO)Cl](PF)) and ([Ru(dceb)(bpt)PtI(HO)](PF)) [dceb = diethyl(2,2'-bipyridine)-4,4'-dicarboxylate, bpt = 3,5-di(pyridine-2-yl)-1,2,4-triazolate] were synthesized as new catalysts for photocatalytic CO reduction and H evolution, respectively. The influence of the catalytic metal for successful catalysis in solution and on a NiO semiconductor was examined. IR-active handles in the form of carbonyl groups on the peripheral ligand on the photosensitiser were used to study the excited states populated, as well as the one-electron reduced intermediate species using infrared and UV-Vis spectroelectrochemistry, and time resolved infrared spectroscopy.

Excitation-Wavelength-Dependent Photophysics of dd Di-isocyanide Complexes.

Binuclear Rh(I) and Ir(I) TMB (2,5-dimethyl-2,5-diisocyanohexane) and dimen (1,8-diisocyanomenthane) complexes possess dσ*pσ and dπpσ singlet and triplet excited states that can be selectively excited in the visible and UV spectral regions. Using perturbational spin-orbit TDDFT, we unraveled the detailed character and spin mixing of these electronic transitions and found that delocalization of pσ and dπ orbitals over C≡N- groups makes C≡N stretching vibrations sensitive reporters of electron density and structural changes upon electronic excitation. Picosecond time-resolved infrared spectra measured after visible light, 375 nm, and 316 nm excitation revealed excitation-wavelength-dependent deactivation cascades.

Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst.

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H production, poly(dibenzo[,]thiophene sulfone), P10.