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The hydrogen bonding network that surrounds the flavin in Blue Light Utilizing FAD (BLUF) photoreceptors plays a crucial role in sensing and communicating the changes in the electronic structure of the flavin to the protein matrix upon light absorption. The network contains a highly conserved tyrosine that is essential for photoactivation. Using time-resolved infrared spectroscopy (TRIR) and unnatural amino acid (UAA) incorporation, we investigated the photoactivation mechanism and the role of the conserved tyrosine (Y6) in the forward reaction of the photoactivated adenylyl cyclase (AC) from Oscillatoria Acuminata (OaPAC). Our work elucidates the direct connection between the photoactivation process in the BLUF domain and the structural and functional implications on the partner protein for the first time. The TRIR results demonstrate formation of FADH● as an intermediate species on the photoactivation pathway which decays to form the signaling state. Using fluorotyrosine analogs to modulate the physical properties of Y6, the TRIR data reveal that a change in the pKa and/or reduction potential of Y6 has a profound effect on the forward reaction, consistent with a mechanism involving proton transfer or proton-coupled electron transfer from Y6 to the electronically excited FAD. Decreasing the pKa from 9.9 to

Ultrafast two-dimensional infrared (2D-IR) spectroscopy of Escherichia coli Hyd-1 (EcHyd-1) reveals the structural and dynamic influence of the protein scaffold on the Fe(CO)(CN)2 unit of the active site. Measurements on as-isolated EcHyd-1 probed a mixture of active site states including two, which we assign to Nir-SI/II, that have not been previously observed in the E. coli enzyme. Explicit assignment of carbonyl (CO) and cyanide (CN) stretching bands to each state is enabled by 2D-IR. Energies of vibrational levels up to and including two-quantum vibrationally excited states of the CO and CN modes have been determined along with the associated vibrational relaxation dynamics. The carbonyl stretching mode potential is well described by a Morse function and couples weakly to the cyanide stretching vibrations. In contrast, the two CN stretching modes exhibit extremely strong coupling, leading to the observation of formally forbidden vibrational transitions in the 2D-IR spectra. We show that the vibrational relaxation times and structural dynamics of the CO and CN ligand stretching modes of the enzyme active site differ markedly from those of a model compound K[CpFe(CO)(CN)2] in aqueous solution and conclude that the protein scaffold creates a unique biomolecular environment for the NiFe site that cannot be represented by analogy to simple models of solvation.

Time-resolved IR spectra of porphyrins bound to biomolecules are reported. DFT calculations aid porphyrin vibration assignment while distinctive IR bands reveal interactions with therapeutic targets such as quadruplex DNA and proteins in HeLa cells.

Summary Host-guest complexation is an important supramolecular route to materials. Clear design rules have been developed for complexation in solution. This has proved more challenging for solid-state host-guest co-crystals because they often exhibit polymorphism, leading many researchers to focus instead on bonded frameworks, such as metal-organic frameworks. Here, we report an anthracene-based organic cage (1) that forms isoskeletal host-guest co-crystals with five similarly sized solid organic guests. The co-crystals were designed using inexpensive computational methods to identify appropriate guests that have packing coefficients (PCs) ranging from 44% to 50%, coupled with consideration of the guest shape. By complexing highly emissive BODIPY guests into the host structure, we enhanced its two-photon excited photoluminescent properties by a factor of six. Our crystal design approach was also transferrable to hard-to-design ternary organic crystals that were accessed by inserting specific guests into different sized voids in the host.

Binding of drugs to blood serum proteins can influence both therapeutic efficacy and toxicity. The ability to measure the concentrations of protein-bound drug molecules quickly and with limited sample preparation could therefore have considerable benefits in biomedical and pharmaceutical applications. Vibrational spectroscopies provide data quickly but are hampered by complex, overlapping protein amide I band profiles and water absorption. Here, we show that two-dimensional infrared (2D-IR) spectroscopy can achieve rapid detection and quantification of paracetamol binding to serum albumin in blood serum at physiologically-relevant levels with no additional sample processing. By measuring changes to the amide I band of serum albumin caused by structural and dynamic impacts of paracetamol binding we show that drug concentrations as low as 7 μM can be detected and that the availability of albumin for paracetamol binding is less than 20% in serum samples, allowing identification of paracetamol levels consistent with a patient overdose.

Raman spectroscopy has found its way into a wide range of applications and is successfully applied for qualitative and quantitative studies. Despite significant technical progress over the last few decades, there are still some challenges that limit its more widespread usage. This paper presents a holistic approach to addressing simultaneously the problems of fluorescence interference, sample heterogeneity, and laser-induced sample heating. Long wavelength shifted excitation Raman difference spectroscopy (SERDS) at 830 nm excitation combined with wide-area illumination and sample rotation is presented as a suitable approach for the investigation of selected wood species. Wood as a natural specimen represents a well-suited model system for our study as it is fluorescent, heterogeneous, and susceptible to laser-induced modifications. Two different subacquisition times (50 and 100 ms) and two sample rotation speeds (12 and 60 r/min) were exemplarily assessed. Results demonstrate that SERDS can effectively separate the Raman spectroscopic fingerprints of the wood species balsa, beech, birch, hickory, and pine from intense fluorescence interference. Sample rotation in conjunction with 1 mm-diameter wide-area illumination was suitable to obtain representative SERDS spectra of the wood species within 4.6 s. Using partial least squares discriminant analysis, a classification accuracy of 99.4% for the five investigated wood species was realized. This study highlights the large potential of SERDS combined with wide-area illumination and sample rotation for the effective analysis of fluorescent, heterogeneous, and thermally sensitive specimens in a wide range of application areas.

Manganese-mediated borylation of aryl/heteroaryl diazonium salts emerges as a general and versatile synthetic methodology for the synthesis of the corresponding boronate esters. The reaction proved an ideal testing ground for delineating the Mn species responsible for the photochemical reaction processes, that is, involving either Mn radical or Mn cationic species, which is dependent on the presence of a suitably strong oxidant. Our findings are important for a plethora of processes employing Mn-containing carbonyl species as initiators and/or catalysts, which have considerable potential in synthetic applications.

The ability of carboxylate groups to promote the direct functionalization of C-H bonds in organic compounds is unquestionably one of the most important discoveries in modern chemical synthesis. Extensive computational studies have indicated that this process proceeds through the deprotonation of a metal-coordinated C-H bond by the basic carboxylate, yet experimental validation of these predicted mechanistic pathways is limited and fraught with difficulty, mainly as rapid proton transfer is frequently obscured in ensemble measures in multistep reactions (i.e., a catalytic cycle consisting of several steps). In this paper, we describe a strategy to experimentally observe the microscopic reverse of the key C-H bond activation step underpinning functionalization processes (viz. M-C bond protonation). This has been achieved by utilizing photochemical activation of the thermally robust precursor [Mn(ppy)(CO)4] (ppy = metalated 2-phenylpyridine) in neat acetic acid. Time-resolved infrared spectroscopy on the picosecond-millisecond time scale allows direct observation of the states involved in the proton transfer from the acetic acid to the cyclometalated ligand, providing direct experimental evidence for the computationally predicted reaction pathways. The power of this approach to probe the mechanistic pathways in transition-metal-catalyzed reactions is demonstrated through experiments performed in toluene solution in the presence of PhC2H and HOAc. These allowed for the observation of sequential displacement of the metal-bound solvent by the alkyne, C-C bond formation though insertion in the Mn-C bond, and a slower protonation step by HOAc to generate the product of a Mn(I)-catalyzed C-H bond functionalization reaction.

Changes in the structural dynamics of double stranded (ds)DNA upon ligand binding have been linked to the mechanism of allostery without conformational change, but direct experimental evidence remains elusive. To address this, a combination of steady state infrared (IR) absorption spectroscopy and ultrafast temperature jump IR absorption measurements has been used to quantify the extent of fast (∼100 ns) fluctuations in (ds)DNA·Hoechst 33258 complexes at a range of temperatures. Exploiting the direct link between vibrational band intensities and base stacking shows that the absolute magnitude of the change in absorbance caused by fast structural fluctuations following the temperature jump is only weakly dependent on the starting temperature of the sample. The observed fast dynamics are some two orders of magnitude faster than strand separation and associated with all points along the 10-base pair duplex d(GCATATATCC). Binding the Hoechst 33258 ligand causes a small but consistent reduction in the extent of these fast fluctuations of base pairs located outside of the ligand binding region. These observations point to a ligand-induced reduction in the flexibility of the dsDNA near the binding site, consistent with an estimated allosteric propagation length of 15 Å, about 5 base pairs, which agrees well with both molecular simulation and coarse-grained statistical mechanics models of allostery leading to cooperative ligand binding.

Migratory insertion (MI) is one of the most important processes underpinning the transition metal-catalysed formation of C-C and C-X bonds. In this work, a comprehensive model of MI is presented, based on the direct observation of the states involved in the coupling of alkynes with cyclometallated ligands, augmented with insight from computational chemistry. Time-resolved spectroscopy demonstrates that photolysis of complexes [Mn(C^N)(CO)4] (C^N = cyclometalated ligand) results in ultra-fast dissociation of a CO ligand. Performing the experiment in a toluene solution of an alkyne results in the initial formation of a solvent complex fac-[Mn(C^N)(toluene)(CO)3]. Solvent substitution gives an η2-alkyne complex fac-[Mn(C^N)(η2-R1C2R2)(CO)3] which undergoes MI of the unsaturated ligand into the Mn-C bond. These data allowed for the dependence of second order rate constants for solvent substitution and first order rate constants for C-C bond formation to be determined. A systematic investigation into the influence of the alkyne and C^N ligand on this process is reported. The experimental data enabled the development of a computational model for the MI reaction which demonstrated that a synergic interaction between the metal and the nascent C-C bond controls both the rate and regiochemical outcome of the reaction. The time-resolved spectroscopic method enabled the observation of a multi-step reaction occurring over 8 orders of magnitude in time, including the formation of solvent complexes, ligand substitution and two sequential C-C bond formation steps.

Ultrafast time-resolved infrared (TRIR) is used to report on the binding site of the [Ru(phen)2 (dppz)]2+ "light-switch" complex with both bimolecular (Oxytricha nova telomere) and intramolecular (human telomere) guanine-quadruplex structures in both K+ and Na+ containing solutions. TRIR permits the simultaneous monitoring both of the "dark" and "bright" states of the complex and of the quadruplex nucleobase bases, the latter via a Stark effect induced by the excited state of the complex. These data are used to establish the contribution of guanine base stacking and loop interactions to the binding site of this biologically relevant DNA structure in solution. A particularly striking observation is the strong thymine signal observed for the Na+ form of the human telomere sequence, which is expected to be in the anti-parallel conformation.

Ultrafast two-dimensional infrared (2D-IR) spectra can now be obtained in a matter of seconds, opening up the possibility of high-throughput screening applications of relevance to the biomedical and pharmaceutical sectors. Determining quantitative information from 2D-IR spectra recorded on different samples and different instruments is however made difficult by variations in beam alignment, laser intensity, and sample conditions. Recently, we demonstrated that 2D-IR spectroscopy of the protein amide I band can be performed in aqueous (H2O) rather than deuterated (D2O) solvents, and we now report a method that uses the magnitude of the associated thermal response of H2O as an internal normalization standard for 2D-IR spectra. Using the water response, which is temporally separated from the protein signal, to normalize the spectra allows significant reduction of the impact of measurement-to-measurement fluctuations on the data. We demonstrate that this normalization method enables creation of calibration curves for measurement of absolute protein concentrations and facilitates reproducible difference spectroscopy methodologies. These advances make significant progress toward the robust data handling strategies that will be essential for the realization of automated spectral analysis tools for large scale 2D-IR screening studies of protein-containing solutions and biofluids.

Break down of the Born–Oppenheimer approximation is caused by mixing of electronic and vibrational transitions in the radical cations of some conjugated polymers, resulting in unusually intense vibrational bands known as infrared active vibrations (IRAVs). Here, we investigate the mechanism of this amplification, and show that it provides insights into intramolecular charge migration. Spectroelectrochemical time-resolved infrared (TRIR) and two-dimensional infrared (2D-IR) spectroscopies were used to investigate the radical cations of two butadiyne-linked conjugated porphyrin oligomers, a linear dimer and a cyclic hexamer. The 2D-IR spectra reveal strong coupling between all the IRAVs and the electronic π–π* polaron band. Intramolecular vibrational energy redistribution (IVR) and vibrational relaxation occur within ∼0.1–7 ps. TRIR spectra show that the transient ground state bleach (GSB) and excited state absorption (ESA) signals have anisotropies of 0.31 ± 0.07 and 0.08 ± 0.04 for the linear dimer and cyclic hexamer cations, respectively. The small TRIR anisotropy for the cyclic hexamer radical cation indicates that the vibrationally excited polaron migrates round the nanoring on a time scale faster than the measurement, i.e. within 0.5 ps, at 298 K. Density functional theory (DFT) calculations qualitatively reproduce the emergence of the IRAVs. The first singlet (S1) excited states of the neutral porphyrin oligomers exhibit similar IRAVs to the radical cations, implying that the excitons have similar electronic structures to polarons. Our results show that IRAVs originate from the strong coupling of charge redistribution to nuclear motion, and from the similar energies of electronic and vibrational transitions., Break down of the Born–Oppenheimer approximation is caused by mixing of electronic and vibrational transitions in the radical cations of some conjugated polymers, resulting in unusually intense vibrational bands known as infrared active vibrations (IRAVs).

Ultrafast time resolved infrared (TRIR) is used to report on the binding site of the "light-switch" complex [Ru(phen)2(dppz)]2+1 to i-motif structures in solution. Detailed information is provided due to perturbation of the local base vibrations by a 'Stark-like' effect which is used to establish the contribution of thymine base loop interactions to the binding site of 1 in this increasingly relevant DNA structure.

Proteins achieve efficient energy storage and conversion through electron transfer along a series of redox cofactors. Multiheme cytochromes are notable examples. These proteins transfer electrons over distance scales of several nanometers to >10 μm and in so doing they couple cellular metabolism with extracellular redox partners including electrodes. Here we report pump-probe spectroscopy that provides a direct measure of the intrinsic rates of heme-heme electron transfer in this fascinating class of proteins. Our study took advantage of a spectrally unique His/Met ligated heme introduced at a defined site within the decaheme extracellular MtrC protein of Shewanella oneidensis. We observed rates of heme-to-heme electron transfer on the order of 109 s-1 (3.7-4.3 A edge-to-edge distance), in good agreement with predictions based on density functional and molecular dynamics calculations. These rates are amongst the highest reported for ground state electron transfer in biology. Yet, some fall 2-3 orders of magnitude below the Moser-Dutton ruler because electron transfer at these short distances is through-space and therefore associated with a higher tunneling barrier than the through-protein tunneling scenario that is usual at longer distances. Moreover, we show that the His/Met ligated heme creates an electron sink that stabilizes the charge separated state on the 100 microsecond time scale. This feature could be exploited in future designs of multiheme cytochromes as components of versatile photosynthetic biohybrid assemblies.

The signaling protein calmodulin (CaM) undergoes a well-known change in secondary structure upon binding Ca2+, but the structural plasticity of the Ca2+-free apo state is linked to CaM functionality. Variable temperature studies of apo-CaM indicate two structural transitions at 46 and 58 °C that are assigned to melting of the C- and N-terminal domains, respectively, but the molecular mechanism of domain unfolding is unknown. We report temperature-jump time-resolved infrared (IR) spectroscopy experiments designed to target the first steps in the C-terminal domain melting transition of human apo-CaM. A comparison of the nonequilibrium relaxation of apo-CaM with the more thermodynamically stable holo-CaM, with 4 equiv of Ca2+ bound, shows that domain melting of apo-CaM begins on microsecond time scales with α-helix destabilization. These observations enable the assignment of previously reported dynamics of CaM on hundreds of microsecond time scales to thermally activated melting, producing a complete mechanism for thermal unfolding of CaM.

Herein, we exemplify that time-resolved multiple-probe spectroscopy (TRMPS) with infrared detection can be used to observe and quantify the dynamic processes occurring during the solvation of a cat...

Photolysis of trypto-CORM, fac-[Mn(tryp)(CO)3(NCMe)] (tryp = tryptophanate) at 400 nm results in controlled CO-release which may be utilised to inhibit the growth of Escherichia coli (E. coli). An investigation into the fundamental processes which underpin the CO-release event is described. Time-dependent density functional theory (TD-DFT) indicates that irradiation at 400 nm results in LMCT from the indole group of the amino acid to orbitals based on the metal as well as the carbonyl and NCMe ligands. Ultra-fast time-resolved infra-red spectroscopy (TRIR) demonstrates that in NCMe solution, photolysis (400 nm) results in loss of CO in under 3 ps with the sequential generation of three new states with two carbonyl ligands and a coordinated tryptophanate. The first species is assigned to vibrationally hot 3[Mn(tryp)(CO)2(NCMe)] which undergoes cooling to give the complex in its v = 0 state. This triplet state then undergoes solvation (τ ≈ 20 ps) with a concomitant change in spin to give [Mn(tryp)(CO)2(NCMe)2] which persists for the remainder of the experiment (800 μs). These data indicate that following the initial photochemically induced loss of CO, any thermal CO loss is much slower. Related experiments with trypto-CORM in a mixture of DMSO and D2O gave analogous data, indicating that this process also occurs in the medium used for the evaluation of biological properties.

A new approach to increasing the faradaic efficiency of dye-sensitised photocathodes for H2 evolution from water, using integrated photocatalysts, furnished with ester groups on the peripheral ligands, [Ru(decb)2(bpt)PdCl(H2O)](PF6)2 (1) and [Ru(decb)2(2,5-bpp)PtI(CH3CN)](PF6)2 (2), (decb = 4,4′-diethylcarboxy-2,2′-bipyridine, bpp = 2,2':5′,2′′-terpyridine, bpt = 3,5-bis(2-pyridyl)-1,2,4-triazole) is described. Overall, 1|NiO is superior to previously reported photocathodes, producing photocurrent densities of 30–35 μA cm−2 at an applied bias of −0.2 V vs. Ag/AgCl over 1 hour of continuous white light irradiation, resulting in the generation of 0.41 μmol h−1 cm−2 of H2 with faradaic efficiencies of up to 90%. Furthermore, surface analysis of the photocathodes before and after photoelectrocatalysis revealed that the ruthenium bipyridyl chromophore and Pd catalytic centre (1) were photochemically stable, highlighting the benefits of the approach towards robust, hybrid solar-to-fuel devices.

We report the synthesis, electrochemical, and photophysical properties of a series of rhenium complexes, [Re(CO)3Cl(NN)] (NN = dppp2, dppp3, and dppp2Br; dppp2 = pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline, dppp3 = pyrido[3′,4′:5,6]pyrazino[2,3-f][1,10]phenanthroline, dppp2Br = 8-bromopyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline), which have potential for use as CO2 photoreduction catalysts. The complexes are related to [Re(CO)3Cl(dppz)] (dppz = dipyrido[3,2-a:2′,3′-c]phenazine) but have an additional nitrogen heteroatom in the distal phenazine (phz) aromatic ring of the dppz ligand which is known to promote potential photoinduced CO2 binding and reduction in related complexes. We use Fourier transform infrared (FTIR), UV/Visible, electron paramagnetic resonance (EPR) and time-resolved infrared (TRIR) spectroscopies to investigate how the location of the additional nitrogen atom, and how the nature of the substituents affects the photophysical properties. In addition, the one electron reduced catalysts are generated following bulk electrochemical reduction and characterised in CH2Cl2. TRIR experiments in d6-DMSO, CH3CN, CH2Cl2, THF and toluene reveal that excitation generates a 3MLCT(phz) (phz = phenazine) excited state in all compounds. In CH3CN and d6-DMSO (not for [Re(CO)3Cl(dppp2)]), we observe the concomitant formation of an 3IL ππ* (IL = intra ligand) excited state. For [Re(CO)3Cl(dppp2)] and [Re(CO)3Cl(dppp3)] in toluene, a mixture of 3MLCT(phen) (phen = phenanthroline) and 3MLCT(phz) states appear to form together, and decay at the same rate. Inclusion of an nitrogen atom in the phz moiety of the dppz ligand lowers the ‘phz’ localised lowest unoccupied molecular orbital (LUMO) energy potentially favouring population of the 3MLCT(phz) state. This effect is more pronounced for [Re(CO)3Cl(dppp3)] with a more positive reduction potential (E½ = −1.16 vs Fc+/Fc in CH2Cl2) compared to [Re(CO)3Cl(dppp2)] (E½ = −1.20 V vs Fc+/Fc in CH2Cl2). For [Re(CO)3Cl(dppp2Br)], lowering of the LUMO energy results in an even more positive potential (E½ = −1.10 vs Fc+/Fc in CH2Cl2) and this can be seen to affect the photophysics. We find the excited state lifetime for each complex increases with decreasing dielectric constant, where their lifetime in toluene is greater than 100 times longer than that observed in d6-DMSO.

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragment 3CpMn(CO)2 formed upon photolysis of CpMn(CO)3. Experimental data are provided for reactions in n-heptane and perfluoromethylcyclohexane (PFMCH), as well as in PFMCH doped with C2H6, Xe and CO2. In PFMCH we find that the conversion of 3CpMn(CO)2 to 1CpMn(CO)2(PFMCH) is much slower (τ = 18 (±3) ns) than the corresponding reactions in conventional alkanes (τ = 111 (±10) ps). We measure the effect of the coordination ability by doping PFMCH with alkane, Xe and CO2; these doped ligands form the corresponding singlet adducts with significantly variable formation rates. The reactivity as measured by the addition timescale follows the order 1CpMn(CO)2(C5H10) (τ = 270 (±10) ps) > 1CpMn(CO)2Xe (τ = 3.9 (±0.4) ns) ∼ 1CpMn(CO)2(CO2) (τ = 4.7 (±0.5) ns) > 1CpMn(CO)2(C7F14) (τ = 18 (±3) ns). Electronic structure theory calculations of the singlet and triplet potential energy surfaces and of their intersections, together with non-adiabatic statistical rate theory, reproduce the observed rates semi-quantitatively. It is shown that triplet adducts of the ligand and 3CpMn(CO)2 play a role in the kinetics, and account for the variable timescales observed experimentally. ispartof: FARADAY DISCUSSIONS vol:220 issue:0 pages:86-104 ispartof: location:England status: published

Ultrafast 2D-IR spectroscopy has proved to be a powerful analytical tool for the detection and differentiation of Bacillus spores as dry films on surfaces. Here, we expand on these findings by employing 2D-IR spectroscopy to study spores from B. atrophaeus (BG) in aqueous solution. Specific vibrational modes attributable to the calcium dipicolinate trihydrate biomarker for spore formation were observed alongside distinctive off-diagonal spectral features that can be used to differentiate spores from different Bacillus species, indicating that 2D-IR has potential for use as a sensing platform with both solid and liquid phase samples. The ability of 2D-IR to enhance the protein amide I band relative to the overlapping water bending vibration was exploited to compare the nature of the protein component of spores to that of solution phase protein molecules. The vibrational lifetime for the amide I band of the BG spore in H2O was 1.4 ± 0.1 ps, longer than those reported for the proteins in H2O solution. The nature of a band at 1710 cm-1 was also investigated. Collectively these results show the potential advantages of 2D-IR spectroscopy, with successful detection and classification of spores under different conditions being based on detailed molecular understanding of the spore state.

Ultrafast 2D-IR spectroscopy is a powerful tool for understanding the spectroscopy and dynamics of biological molecules in the solution phase. A number of recent studies have begun to explore the utility of the information-rich 2D-IR spectra for analytical applications. Here, we report the application of ultrafast 2D-IR spectroscopy for the detection and classification of bacterial spores. 2D-IR spectra of Bacillus atrophaeus and Bacillus thuringiensis spores as dry films on CaF2 windows were obtained. The sporulated nature of the bacteria was confirmed using 2D-IR diagonal and off-diagonal peaks arising from the calcium dipicolinate CaDP·3H2O biomarker for sporulation. Distinctive peaks, in the protein amide I region of the spectrum were used to differentiate the two types of spore. The identified marker modes demonstrate the potential for the use of 2D-IR methods as a direct means of spore classification. We discuss these new results in perspective with the current state of analytical 2D-IR measurements, showing that the potential exists to apply 2D-IR spectroscopy to detect the spores on surfaces and in suspensions as well as in dry films. The results demonstrate how applying 2D-IR screening methodologies to spores would enable the creation of a library of spectra for classification purposes.

We report the synthesis and photophysical properties of a neutral BODIPY photosensitizing copolymer (poly-8-(4-hydroxymethylphenyl)-4,4-difluoro-2,6-diethynyl-4-bora-3a,4a-diaza-s-indacene) containing ethynylbenzene links between the BODIPY units. The copolymer absorbs further towards the red in the UV-vis spectrum compared to the BODIPY precursor. Photolysis of the polymer produces a singlet excited state which crosses to the triplet surface in less than 300 ps. This triplet state was used to form singlet oxygen with a quantum yield of 0.34. The steps leading to population of the triplet state were studied using time-resolved spectroscopic techniques spanning the pico- to nanosecond timescales. The ability of the BODIPY polymer to generate a biocidal species for bactericidal activity in both solution- and coating-based studies was assessed. When the BODIPY copolymer was dropcast onto a surface, 4 log and 6 log reductions in colony forming units/ml representative of Gram-positive and Gram-negative bacteria, respectively, under illumination at 525 nm were observed. The potent broad-spectrum antimicrobial activity of a neutral metal-free copolymer when exposed to visible light conditions may have potential clinical applications in infection management.

Glycine (Gly) is used as a model system to evaluate the ability of ultrafast two-dimensional infrared (2D-IR) spectroscopy to detect and quantify the low-molecular-weight proteinaceous components of blood serum. Combining data acquisition schemes to suppress absorption bands of H2O that overlap with the protein amide I band with analysis of peak patterns appearing in the off-diagonal region of the 2D-IR spectrum allows separation of the Gly spectral signature from that of the dominant protein fraction of serum in a transmission-mode 2D-IR measurement without any sample manipulation, e.g., filtration or drying. 2D-IR spectra of blood serum samples supplemented with varying concentrations of Gly were obtained, and a range of data analysis methods compared, leading to a detection limit of ∼3 mg/mL for Gly. The reported methodology provides a platform for a critical assessment of the sensitivity of 2D-IR for measuring the concentrations of amino acids, peptides, and low-molecular-weight proteins present in serum samples. We conclude that, in the case of several clinically relevant diagnostic molecules and their combinations, the potential exists for 2D-IR to complement IR absorption methods as the benefits of the second frequency dimension offered by 2D-IR spectroscopy outweigh the added technical complexity of the measurement.

A novel 4,4-difuoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) copolymer with diethynylbenzene has been synthesised, and its ability to act as a photosensitiser for the photocatalytic generation of hydrogen was investigated by time-resolved spectroscopic techniques spanning the ps- to ns-timescales. Both transient absorption and time-resolved infrared spectroscopy were used to probe the excited state dynamics of this photosensitising unit in a variety of solvents. These studies indicated how environmental factors can influence the photophysics of the BODIPY polymer. A homogeneous photocatalytic hydrogen evolution system has been developed using the BODIPY copolymer and cobaloxime which provides hydrogen evolution rates of 319 μmol h-1 g-1 after 24 h of visible irradiation.

Ruthenium polypyridyl complexes which can sensitise the photo-oxidation of nucleic acids and other biological molecules show potential for photo-therapeutic applications. In this article a combination of transient visible absorption (TrA) and time-resolved infra-red (TRIR) spectroscopy are used to compare the photo-oxidation of guanine by the enantiomers of [Ru(TAP)2(dppz)]2+ in both polymeric {poly(dG-dC), poly(dA-dT) and natural DNA} and small mixed-sequence duplex-forming oligodeoxynucleotides. The products of electron transfer are readily monitored by the appearance of a characteristic TRIR band centred at ca. 1700 cm−1 for the guanine radical cation and a band centered at ca. 515 nm in the TrA for the reduced ruthenium complex. It is found that efficient electron transfer requires that the complex be intercalated at a G-C base-pair containing site. Significantly, changes in the nucleobase vibrations of the TRIR spectra induced by the bound excited state before electron transfer takes place are used to identify preferred intercalation sites in mixed-sequence oligodeoxynucleotides and natural DNA. Interestingly, with natural DNA, while it is found that quenching is inefficient in the picosecond range, a slower electron transfer process occurs, which is not found with the mixed-sequence duplex-forming oligodeoxynucleotides studied., Efficient electron transfer requires the complex to be intercalated at a G-C base-pair. Identification of preferred intercalation sites is achieved by TRIR monitoring of the nucleobase vibrations before electron transfer.

Ultrafast time resolved infrared (TRIR) is used to report on the binding site of the "light-switch" complex [Ru(phen)

Correction for ‘Time-resolved infra-red spectroscopy reveals competitive water and dinitrogen coordination to a manganese(i) carbonyl complex’ by Jonathan B. Eastwood et al., Dalton Trans., 2020, DOI: 10.1039/c9dt04878b.

Ultrafast time-resolved infrared (TRIR) is used to report on the binding site of the [Ru(phen)

Time-resolved infra-red (TRIR) spectroscopy has been used to demonstrate that photolysis of [Mn(C^N)(CO)4] (C^N = bis-(4-methoxyphenyl)methanimine) in heptane solution results in ultra-fast CO dissociation and ultimate formation of a rare Mn-containing dinitrogen complex fac-[Mn(C^N)(CO)3(N2)] with a diagnostic stretching mode for a terminal-bound NN ligand at 2249 cm−1. An isotopic shift to 2174 cm−1 was observed when the reaction was performed under 15N2 and the band was not present when the experiment was undertaken under an atmosphere of argon, reinforcing this assignment. An intermediate solvent complex fac-[Mn(C^N)(CO)3(heptane)] was identified which is formed in less than 2 ps, indicating that CO-release occurs on an ultra-fast timescale. The heptane ligand is labile and is readily displaced by both N2 and water to give fac-[Mn(C^N)(CO)3(N2)] and fac-[Mn(C^N)(CO)3(OH2)] respectively. The fac-[Mn(C^N)(CO)3(heptane)] framework showed a significant affinity for N2, as performing the reaction under air produced significant amounts of fac-[Mn(C^N)(CO)3(N2)]. Kinetic analysis reveals that the substitution of heptane by N2 (k = (1.028 ± 0.004) × 109 mol−1 dm3 s−1), and H2O is competitive on fast (

Shifted excitation Raman difference spectroscopy (SERDS) is a powerful tool for the investigation of fluorescent samples such as biological materials. In case of rapidly changing emission backgrounds the efficiency of SERDS can however be limited as alternating detection of spectra excited at the two shifted laser wavelengths is usually restricted to sampling rates of less than 10 Hz. To overcome this issue, a novel optical lock-in detection approach enabling rapid SERDS operation in the kilohertz range using a custom 830-nm dual-wavelength diode laser and a specialized CCD enabling charge shifting on the CCD chip is presented. As an example of fluorescent and heterogeneous natural specimens, six mineral samples were selected and moved irregularly during spectral acquisition. Compared to conventional CCD read-out (operated at 5.4 Hz) the fast charge-shifting read-out performed at 1,000 Hz demonstrated superior reproducibility between repeat spectra. Using partial least squares-discriminant analysis an improved classification performance of the charge-shifting mode (sensitivity: 99 %, specificity: 94 %) over conventional read-out (sensitivity: 90 %, specificity: 92 %) was achieved. Translating the charge-shifting concept to sub-surface analysis using spatially offset Raman spectroscopy (SORS) enabled also the successful detection of charge-shifting SERDS-SORS spectra from a polytetrafluoroethylene layer concealed behind a 0.25 mm thick opaque heterogeneous layer. Chargeshifting SERDS-SORS results demonstrate two-fold improvement in signal-to-background-noise-ratio and match reference spectra much more closely. The charge-shifting approach shows large potential when rapidly changing background interference due to sample heterogeneity, dynamically evolving systems and ambient light variations presents a major challenges, e.g. in biological and biomedical applications.

Detailed understanding of the mechanistic processes that underpin transition metal-catalysed reactions allows for the rational and de novo development of complexes with enhanced activity, efficacy and wider substrate scope. Directly observing bond-cleaving and -forming events underpinning a catalytic reaction is non-trivial as the species that facilitate these steps are frequently short-lived and present at low concentrations. Here, we describe how the photochemical activation of a manganese precatalyst, [Mn(ppy)(CO)4] (ppy = 2-phenylpyridine), results in selective loss of a carbonyl ligand simulating entry into the catalytic cycle for manganese-promoted C–H bond functionalization. Time-resolved infrared spectroscopy (on the ps–ms timescale) allows direct observation of the species responsible for the essential C–C bond formation step and an evaluation of the factors affecting its rate. This mechanistic information prompted the discovery of a new photochemically initiated manganese-promoted coupling of phenylacetylene with 2-phenylpyridine. This study provides unique insight into the mechanistic pathways underpinning catalysis by an Earth-abundant metal, manganese. Although mechanistic understanding can drive new reactivity development, the key bond-forming and -breaking steps in catalytic cycles are often sufficiently fast to elude observation. Here, the authors photochemically produce a key intermediate in Mn-catalysed C–H functionalization, and follow the subsequent steps—spanning processes occurring over seven orders of magnitude in time—using time-resolved infrared spectroscopy.

Ultrafast time-resolved electronic and infrared absorption measurements have been carried out on a series of Ru(II) polypyridyl complexes in an effort to delineate the dynamics of vibrational relaxation in this class of charge transfer chromophores. Time-dependent density functional theory calculations performed on compounds of the form [Ru(CN-Me-bpy) x(bpy)3-x]2+ ( x = 1-3 for compounds 1-3, respectively, where CN-Me-bpy is 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine and bpy is 2,2'-bipyridine) reveal features in their charge-transfer absorption envelopes that allow for selective excitation of the Ru(II)-(CN-Me-bpy) moiety, the lowest-energy MLCT state(s) in each compound of the series. Changes in band shape and amplitude of the time-resolved differential electronic absorption data are ascribed to vibrational cooling in the CN-Me-bpy-localized 3MLCT state with a time constant of 8 ± 3 ps in all three compounds. This conclusion was corroborated by picosecond time-resolved infrared absorption measurements; sharpening of the CN stretch in the 3MLCT excited state was observed with a time constant of 3.0 ± 1.5 ps in all three members of the series. Electronic absorption data acquired at higher temporal resolution revealed spectral modulation over the first 2 ps occurring with a time constant of τ = 170 ± 50 fs, in compound 1; corresponding effects are significantly attenuated in compound 2 and virtually absent in compound 3. We assign this feature to intramolecular vibrational redistribution (IVR) within the 3MLCT state and represents a rare example of this process being identified from time-resolved electronic absorption data for this important class of chromophores.

A combined experimental and theoretical study of the infrared (IR) spectra of 2-naphthaldehyde and 9-fluorenone in their ground and first excited singlet and triplet electronic states is presented. IR studies have also been carried out using supercritical krypton (scKr) as a solvent to measure spectra in the ground and triplet excited states. This solvent provides a weakly interacting environment that is closer to the gas phase and allows a direct comparison with the calculated spectra for a single molecule. The IR spectra for the three different states are computed with Kohn-Sham density functional theory. For the first excited singlet state it is necessary to use an overlap procedure that allows the excited state to be studied by preventing variational collapse to the ground state. This allows the excited singlet state to be studied in an analogous approach to the ground and excited triplet state, in contrast to using time-dependent density functional theory. The good agreement between the calculated excited state spectra and the experiment provides insight into the nature of the excited states. For the ground and excited triplet state the anharmonic vibrational frequencies are computed using the transition optimized shifted Hermite method, and for these molecules the hybrid B97-1 functional is found to provide the closest agreement with experiment in the ground state.

The effect of oligomer length on the vibrational mode coupling and energy relaxation mechanisms of AT-rich DNA oligomers in double- and single-stranded conformations has been investigated using two-dimensional infrared spectroscopy. Vibrational coupling of modes of the DNA bases to the symmetric stretching vibration of the backbone phosphate group was observed for oligomers long enough to form duplex-DNA structures. The coupling was lost upon melting of the duplex. No significant effect of oligomer length or DNA secondary structure was found on either the timescale for vibrational relaxation of the base modes or the mechanism, which was consistent with a cascade process from base modes to intermediate modes, some of which are located on the deoxyribose group, and subsequently to the phosphate backbone. The study shows that vibrational coupling between base and backbone requires formation of the double-helix structure while vibrational energy management is an inherent property of the nucleotide.

A series of [Donor-Acceptor-Anchor] Platinum(II) bipy (bis)acetylides has been developed in order to systematically compare the effect of conjugated vs. electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of these light-harvesting units. This first example of "decoupled" phosphonate-diimine Pt(II) complexes presented here are compared and contrasted to their carboxylate analogs. Ultrafast TRIR and femtosecond transient absorption spectroscopies revealed that all complexes possess a charge transfer lowest excited state, with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of charge-transfer states have been identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy re-distribution accompanied by structural changes was detected, indicating, unexpectedly, a significant difference between the structures of the electronic ground and charge-transfer excited states, as well as the differences in the structural reorganisation in the complexes bearing directly attached vs. decoupled anchoring groups. This work demonstrates that decoupling of the anchoring group from the diimine via a saturated spacer is an easy approach that permits combining higher reduction potential and 10-fold longer charge-transfer excited state lifetime with the possibility of surface attachment, whilst retaining essential charge transfer properties.

Binuclear complexes of d^8 metals (Pt^(II), Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M–M bonding is strengthened upon dσ* → pσ excitation. We report the bridging ligand dynamics of Ir2(1,8-diisocyanomenthane)_4^(2+)(Ir(dimen)), investigated by fs–ns time-resolved IR spectroscopy (TRIR) in the region of C≡N stretching vibrations, ν(C≡N), 2000–2300 cm^(–1). The ν(C≡N) IR band of the singlet and triplet dσ*pσ excited states is shifted by −22 and −16 cm^(–1) relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ^1dσ*pσ state depend on the initially excited Franck–Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir–Ir contraction and 5 ps Ir–Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ∼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ^3dσ*pσ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν(C≡N) IR bandwidth oscillates with the frequency of the ν(Ir–Ir) wave packet, ca. 80 cm^(–1), indicating that the dephasing time of the high-frequency (16 fs)^(−1) C≡N stretch responds to much slower (∼400 fs)^(−1)Ir–Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal–metal unit and that the coherent Ir–Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation.

Weinheim Key to the development of DNA-targeting phototherapeutic drugs is determining the interplay between the photoactivity of the drug and its binding preference for a target sequence. For the photo-oxidising lambda-[Ru(TAP)2(dppz)]2+ (Λ-1) (dppz=dipyridophenazine) complex bound to either d{T1C2G3G4C5G6C7C8G9A10}2 (G9) or d{TCGGCGCCIA}2 (I9), the X-ray crystal structures show the dppz intercalated at the terminal T1C2;G9A10 step or T1C2;I9A10 step. Thus substitution of the G9 nucleobase by inosine does not affect intercalation in the solid state although with I9 the dppz is more deeply inserted. In solution it is found that the extent of guanine photo-oxidation, and the rate of back electron-transfer, as determined by pico- and nanosecond time-resolved infrared and transient visible absorption spectroscopy, is enhanced in I9, despite it containing the less oxidisable inosine. This is attributed to the nature of the binding in the minor groove due to the absence of an NH2 group. Similar behaviour and the same binding site in the crystal are found for d{TTGGCGCCAA}2 (A9). In solution, we propose that intercalation occurs at the C2G3;C8I9 or T2G3;C8A9 steps, respectively, with G3 the likely target for photo-oxidation. This demonstrates how changes in the minor groove (in this case removal of an NH2 group) can facilitate binding of RuIIdppz complexes and hence influence any sensitised reactions occurring at these sites. No similar enhancement of photooxidation on binding to I9 is found for the delta enantiomer. Irish Research Council Science Foundation Ireland University College Dublin Biotechnology and Biological Sciences Research Council Royal Irish Academy/Royal Society

Ultrafast electron transfer in condensed-phase molecular systems is often strongly coupled to intramolecular vibrations that can promote, suppress and direct electronic processes. Recent experiments exploring this phenomenon proved that light-induced electron transfer can be strongly modulated by vibrational excitation, suggesting a new avenue for active control over molecular function. Here, we achieve the first example of such explicit vibrational control through judicious design of a Pt(II)-acetylide charge-transfer donor–bridge–acceptor–bridge–donor ‘fork’ system: asymmetric 13C isotopic labelling of one of the two –C≡C– bridges makes the two parallel and otherwise identical donor→acceptor electron-transfer pathways structurally distinct, enabling independent vibrational perturbation of either. Applying an ultrafast UVpump(excitation)–IRpump(perturbation)–IRprobe(monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. The findings deliver a new opportunity for precise perturbative control of electronic energy propagation in molecular devices. With recent and improved understanding of how nuclear and electronic degrees of freedom can interact with each other comes the opportunity to directly control electronic processes. Now it has been shown that ultrafast vibrational excitation can direct light-induced intramolecular electron transfer along a specific path.

We report the excited state photophysics and photochemistry of [ReCl(CO)3(dppz-(CH3)2)] (dppz-(CH3)2 = 11,12-dimethyl-dipyrido[3,2-a:2’,3’-c]phenazine) in CH3CN using timeresolved infrared (TRIR) and Fourier transform infrared (FTIR) spectroscopy. Excitation of the 1MLCT (metal-to-ligand charge transfer) band of [ReCl(CO)3(dppz-(CH3)2)] populates a 3MLCT excited state which rapidly interconverts on a timescale < 1 ns to a long lived IL (intra-ligand) π-π* excited state with a lifetime of 190 (± 5) ns. In the presence of an electron donor (NEt3), the IL excited state of [ReCl(CO)3(dppz-(CH3)2)] can be reductively quenched to [ReCl(CO)3(dppz-(CH3)2)]− with the radical in the latter localised on the distal phenazine (phz) portion of the dppz ligand. The phz based electron in [ReCl(CO)3(dppz-(CH3)2)]− has minimal interaction with the rhenium metal centre which increases the stability of the photosensitiser in its reduced form. In non-dried, non-degassed CH3CN (1 M NEt3), [ReCl(CO)3(dppz-(CH3)2)]− shows no significant change in the carbonyl region of the IR spectrum for at least 2 hours during continuous photolysis. In addition, we investigate the use of [ReCl(CO)3(dppz-(CH3)2)]− to reduce the previously studied catalyst [NiFe2], with facile electron transfer from [ReCl(CO)3(dppz-(CH3)2)]− to form [NiFe2]–.

Understanding what influences the formation and lifetime of charge-separated states is key to developing photoelectrochemical devices. This paper describes the use of time-resolved infrared absorption spectroscopy (TRIR) to determine the structure and lifetime of the intermediates formed on photoexcitation of two organic donor–π–acceptor dyes adsorbed to the surface of NiO. The donor and π-linker of both dyes is triphenylamine and thiophene but the acceptors differ, maleonitrile (1) and bodipy (2). Despite their structural similarities, dye 1 outperforms 2 significantly in devices. Strong transient bands in the fingerprint region (1 and 2) and nitrile region (2300–2000 cm−1) for 1 enabled us to monitor the structure of the excited states in solution or adsorbed on NiO (in the absence and presence of electrolyte) and the corresponding kinetics, which are on a ps–ns timescale. The results are consistent with rapid (