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Carbamate is an emerging class of polymer backbone for constructing sequence-defined, abiotic polymers. It is expected that new functional materials can be de novo designed by controlling the primary polycarbamate sequence. While amino acids have been actively studied as building blocks for protein folding and peptide self-assembly, carbamates have not been widely investigated from this perspective. Here we combined infrared (IR), vibrational circular dichroism (VCD) and nuclear magnetic resonance (NMR) spectroscopy with density functional theory (DFT) calculations to understand the conformation of carbamate monomer units in a non-polar, aprotic environment (chloroform). Compared with amino acid building blocks, carbamates are more rigid, presumably due to the extended delocalization of π-electrons on the backbones. Surprisingly, cis configurations can be energetically stable for carbamates while peptides typically assume trans configurations at low energies. This study lays an essential foundation for future developments of carbamate-based sequence-defined polymer material design.

Although potentially hazardous chemical elements (e.g., Cu, Cr, Pb, Sb, Ti, Zn) have been studied in clothing textiles, their presence in cleaning textiles is unknown. In this study, 48 cleaning cloth products (consisting of 81 individual samples) purchased in Europe, and consisting of synthetic (petroleum-based), semi-synthetic or natural fibres or combinations of these different types, have been analysed for 16 chemical elements by X-ray fluorescence (XRF) spectrometry. Titanium was detected in most cases (median and maximum concentrations ~3700 and 12,400 mg kg

While crystallization is a ubiquitous and an important process, the microscopic picture of crystal nucleation is yet to be established. Recent studies suggest that the nucleation process can be more complex than the view offered by the classical nucleation theory. Here, we implement single crystal nucleation spectroscopy (SCNS) by combining Raman microspectroscopy and optical trapping induced crystallization to spectroscopically investigate one crystal nucleation at a time. Raman spectral evolution during a single glycine crystal nucleation from water, measured by SCNS and analyzed by a nonsupervised spectral decomposition technique, uncovered the Raman spectrum of prenucleation aggregates and their critical role as an intermediate species in the dynamics. The agreement between the spectral feature of prenucleation aggregates and our simulation suggests that their structural order emerges through the dynamic formation of linear hydrogen-bonded networks. The present work provides a strong impetus for accelerating the investigation of crystal nucleation by optical spectroscopy.

One of the most characterized bioluminescent reactions involves the firefly luciferase that catalyzes the oxidation of the luciferin producing oxyluciferin in its first excited state. While relaxing to the ground state, oxyluciferin emits visible light with an emission maximum that can vary from green to red. Oxyluciferin exists under six different chemical forms resulting from a keto/enol tautomerization and the deprotonation of the phenol or enol moieties. The optical properties of each chemical form have been recently characterized by the investigations of a variety of oxyluciferin derivatives, indicating unresolved excited-state proton transfer (ESPT) reactions. In this work, femtosecond pump-probe spectroscopy and time-resolved fluorescence spectroscopy are used to investigate the picosecond kinetics of the ESPT reactions and demonstrate the excited state keto to enol conversion of oxyluciferin and its derivatives in aqueous buffer as a function of pH. A comprehensive photophysical scheme is provided describing the complex luminescence pathways of oxyluciferin in protic solution.

Crystallization is an important process in a wide range of disciplines from fundamental science to industrial application. Despite the importance of controlling the crystallization and its morphology (e.g. polymorphism), the lack of microscopic description of crystal nucleation often limits the rational approach to its engineering and control. The biggest challenge to experimentally track the nucleus formation is the stochastic and heterogeneous nature of the nucleation occurring at nanometer scale. To overcome this challenge, we developed a method we call “Single Nucleus Spectroscopy” or SNS and use it to follow the formation of single crystal glycine nucleus by Raman spectroscopy at 46 ms time resolution. The spectral evolution was analyzed by non-supervised spectral decomposition algorithm which unraveled the Raman spectrum of prenucleation aggregates. In order to gain microscopic insights into the structure of these aggregates we have established a direct comparison between the experiments and theoretical works. The outcome of our analysis is a new hypothesis of glycine crystal nucleation mechanism.

The Born-Oppenheimer adiabatic limit applies broadly in chemistry because most reactions occur on the ground electronic state. Photochemical reactions involve two or more electronic states and need not be subject to this adiabatic limit. The spectroscopic signatures of nonadiabatic processes are subtle, and therefore, experimental investigations have been limited to the few systems dominated by single photochemical outcomes. Systems with branched excited-state pathways have been neglected, despite their potential to reveal insights into photochemical reactivity. Here we present experimental evidence from coherent three-dimensional electronic spectroscopy that the E to Z photoisomerization of phytochrome Cph1 is strongly nonadiabatic, and the simulations reproduce the measured features only when the photoisomerization proceeds nonadiabatically near, but not through, a conical intersection. The results broaden the general understanding of photoisomerization mechanisms and motivate future studies of nonadiabatic processes with multiple outcomes arising from branching on excited-state potential energy surfaces.

International audience; In this paper, we report on the Photoinduced Electron Transfer (PET) reaction between a donor (adenine analog) and an acceptor (3-methoxychromone derivative) in the context of designing efficient fluorescent probes as DNA sensors. Firstly, Gibbs energy was investigated in disconnected donor-acceptor systems by Rehm-Weller equation. The oxidation potential of the adenine derivative was responsible for exergonicity of the PET reaction in separated combinations. Then, the PET reaction in donor-π-acceptor conjugates was investigated using steady-state fluorescence spectroscopy, acid-mediated PET inhibition and transient absorption techniques. In conjugated systems, PET is a favorable pathway of fluorescent quenching when an electron-rich adenine analog (d7A) was connected to the fluorophore (3MC). We found that formation of ground-state complexes even at nM concentration range dominated the dye photophysics and generated poorly emissive species likely through intermolecular PET from d7A to 3MC. On the other hand, solution acidification disrupts complexation and turns on dye emission. Bridging an electron-poor adenine analog with high oxidation potential (8d7A) to 3MC presenting low reduction potential is another alternative to prevent complex formation and produce highly emissive monomer conjugates.

Numerous time-resolved studies of the Pr to Pfr photoisomerization in phytochrome Cph1 have revealed multiphasic excited-state decay kinetics. It remains unclear whether these kinetics arise from multiple ground-state conformational subpopulations or from a single ground-state conformation that undergoes an excited-state photoisomerization process—either branching on the excited state or relaxing through multiple sequential intermediates. Many studies have attempted to resolve this debate by fitting the measured dynamics to proposed kinetic models, arriving at different conclusions. Here we probe spectral signatures of ground-state heterogeneity of Pr. Two-dimensional electronic spectra display negligible inhomogeneous line broadening, and vibrational coherence spectra extracted from transient absorption measurements do not contain nodes and phase shifts at the fluorescence maximum. These spectroscopic results support the homogeneous model, in which the primary photochemical transformation of Pr to Lumi-R...

We report experimental results on the ultrafast photo-isomerization of the ASRs-PSBR where point mutations lead to an up to 5-fold increase of the photo-isomerization speed, due to electrostatic modifications of the retinal binding pocket.

We report new experimental results on the ultrafast photo-isomerization of ASR - PSBR where, unlike other retinal proteins, point mutations lead to a 2-fold increase of the photo-isomerization speed for the all-trans isomer. Prominent low-frequency vibrational coherences are reported for both the excited and photo-product ground states.

We introduce a two-dimensional spectroscopy setup based on a pair of near-UV-blue (360 - 430 nm), 8.4-fs, phase-locked, collinear excitation pulses and a nearly collinear UV-Vis supercontinuum probe pulse.

International audience; The electronic excited states populated upon absorption of UV photons by DNA are extensively studied in relation to the UV-induced damage to the genetic code. Here, we report a new unexpected relaxation pathway in adenine–thymine double-stranded structures (AT)n. Fluorescence measurements on (AT)n hairpins (six and ten base pairs) and duplexes (20 and 2000 base pairs) reveal the existence of an emission band peaking at approximately 320 nm and decaying on the nanosecond time scale. Time-dependent (TD)-DFT calculations, performed for two base pairs and exploring various relaxation pathways, allow the assignment of this emission band to excited states resulting from mixing between Frenkel excitons and adenine-to-thymine chargetransfer states. Emission from such high-energy long-lived mixed (HELM) states is in agreement with their fluorescence anisotropy (0.03), which is lower than that expected for p–p* states (0.1). An increase in the size of the system quenches p–p* fluorescence while enhancing HELM fluorescence. The latter process varies linearly with the hypochromism of the absorption spectra, both depending on the coupling between p–p* and charge-transfer states. Subsequently, we identify the common features between the HELM states of (AT)n structures with those reported previously for alternating (GC)n : high emission energy, low fluorescence anisotropy, nanosecond lifetimes, and sensitivity to conformational disorder. These features are also detected for calf thymus DNA in which HELM states could evolve toward reactive p–p* states, giving rise to delayed fluorescence.

Conical intersections are molecular configurations at which adiabatic potential-energy surfaces touch. They are predicted to be ubiquitous, yet condensed-phase experiments have focused on the few systems with clear spectroscopic signatures of negligible fluorescence, high photoactivity, or femtosecond electronic kinetics. Although rare, these signatures have become diagnostic for conical intersections. Here we detect a coherent surface-crossing event nearly two picoseconds after optical excitation in a highly fluorescent molecule that has no photoactivity and nanosecond electronic kinetics. Time-frequency analysis of high-sensitivity measurements acquired using sub-8 fs pulses reveals phase shifts of the signal due to branching of the wavepacket through a conical intersection. The time-frequency analysis methodology demonstrated here on a model compound will enable studies of conical intersections in molecules that do not exhibit their diagnostic signatures. Improving the ability to detect conical intersections will enrich the understanding of their mechanistic role in molecular photochemistry.

Accurate characterization of the high-lying excited state manifolds of organic molecules is of fundamental importance for the interpretation of the rich response detected in time-resolved nonlinear electronic spectroscopies. Here, we have characterized the singlet excited state manifold of benzophenone (BP), a versatile organic photoinitiator and a well-known DNA photosensitizer. Benchmarks of various multiconfigurational/multireference (RASSCF/PT2) and time-dependent density functional theory (TD-DFT) approaches allowed assignments of experimental linear absorption signals of BP in the ultraviolet (UV) region, with unprecedented characterization of ground state absorptions in the far UV. Experimental transient absorption spectra obtained by UV-vis pump-probe spectroscopy at very short time delays are shown to be directly comparable to theoretical estimates of excited state absorptions (from the low-lying nOπ∗ and π π∗ singlet states) in the Franck-Condon region. Multireference computations provided reliable interpretation of the PP spectra, with TD-DFT results yielding a fair agreement as long as electronic transitions featuring double excitations contributions are not involved. These results lay the groundwork for further computational studies and interpretation of experimental nonlinear electronic spectra of benzophenone in more complex systems, such as BP/DNA adducts.

Benzophenone (BP) despite its relatively simple molecular structure is a paradigmatic sensitizer, featuring both photocatalytic and photobiological effects due to its rather complex photophysical properties. In this contribution we report an original theoretical approach to model realistic, ultra-fast spectroscopy data, which requires describing intra- and intermolecular energy and structural relaxation. In particular we explicitly simulate time-resolved pump-probe spectra using a combination of state-of-the art hybrid quantum mechanics/molecular mechanics dynamics to treat relaxation and vibrational effects. The comparison with experimental transient absorption data demonstrates the efficiency and accuracy of our approach. Furthermore the explicit inclusion of the solvent, water for simulation and methanol for experiment, allows us, despite the inherent different behavior of the two, to underline the role played by the H-bonding relaxation in the first hundreds of femtoseconds after optical excitation. Finally we predict for the first time the two-dimensional electronic spectrum (2DES) of BP taking into account the vibrational effects and hence modelling partially symmetric and asymmetric ultrafast broadening.

International audience; Anabaena Sensory Rhodopsin is a particular microbial retinal protein for which light-adaptation leads to the ability to bind both the all-trans, 15-anti (AT) and the 13-cis, 15-syn (13C) isomers of the protonated Schiff base of retinal (PSBR). In the context of obtaining insight into the mechanisms by which retinal proteins catalyse the PSBR photo-isomerization reaction, ASR is a model system allowing to study, within the same protein, the protein-PSBR interactions for two different PSBR conformers at the same time. A detailed analysis of the vibrational spectra of AT and 13C, and their photo-products in wild-type ASR obtained through femtosecond (pump-) four-wave-mixing is reported for the first time, and compared to bacterio-and channelrhodopsin. As part of an extensive study of ASR mutants with blue-shifted absorption spectra, we present here a detailed computational analysis of the origin of the mutation-induced blue-shift of the absorption spetra, and identify electrostatic interactions as dominating steric effects that would entail a red-shift. The excited state lifetimes and isomerization reaction times (IRT) for the three mutants V112N, W76F, and L83Q are studied experimentally by femtosecond broadband transient absorption spectroscopy. Interestingly, in all three mutants, isomerization is accelerated for AT with respect to wild-type ASR, and this the more, the shorter the wavelength of maximum absorption. On the contrary, the 13C photo-reaction is slightly slowed down, leading to an inversion of the ESLs of AT and 13C, with respect to wt-ASR, in the blue-most absorbing mutant L83Q. Possible mechanisms for these mutation effects, and their steric and electrostatic origins are discussed.

During the last two decades, 2D optical techniques have been extended to the visible range, targeting electronic transitions. Since the report of the very first 2D electronic measurement (Hybl et al. in J Chem Phys 115:6606-6622, [2001]), two-dimensional electronic spectroscopy (2DES) has allowed fundamentally new insights into the structure and dynamics of condensed-phase systems (Ginsberg et al. in Acc Chem Res 42:1352-1363, 2009; Jonas in Annu Rev Phys Chem 54:425-463, 2003), producing experiments that measure correlations among electronic states of an absorbing species within complex systems. 2DES is used to investigate photo-physical phenomena involving electronic or vibrational couplings in multi-chromophoric systems [energy transfer in photosynthesis is one great example of how 2DES can disentangle various energy transfer pathways (Brixner et al. in Nature 625-628, 2005; Engel et al. in Nature 446:782-786, 2007; Collini et al. in Nature 463:644-647, 2010)], but also ultrafast photochemical processes in which the tracked molecules change permanently or are heterogeneous (Ruetzel et al. in Proc Natl Acad Sci 111:4764-4769, 2014; Consani et al. in Science 339:1586-1589, 2013). We divide this chapter according to some of the major areas that have been established thanks to 2DES in the following fields: heterogeneity of systems, excitation energy transfer mechanisms, photo-induced coherent oscillations associated with electronic and vibrational couplings, and complex chemical reactions (Fig. 1). Fig. 1 Main fields impacted by two-dimensional electronic spectroscopy (2DES) in condensed phase. The major discoveries of each field will be described in different paragraphs.

A 40-nm broad pulse centred at 320nm is produced from an amplified Yb-doped fiber laser operated at 50kHz, and used in a BOXCARS geometry setup for 2DES, with shot-to-shot monitoring of the relative optical phase stability.

In contrast to the detailed understanding of inorganic materials, researchers lack a comprehensive view of how the properties of bulk organic materials arise from their individual components. For conjugated polymers to eventually serve as low cost semiconductor layers in electronic devices, researchers need to better understand their functionality. For organics, traditional materials science measurements tend to destroy the species of interest, especially at low concentrations. However, fluorescence continues to be a remarkably flexible, relatively noninvasive tool for probing the properties of individual molecules and allows researchers to carry out a broad range of experiments based on a relatively simple concept. In addition, the sensitivity of single-molecule spectroscopy allows researchers to see the properties of an individual component that would be masked in the bulk phase. In this Account, we examine several photophysical properties of different conjugated polymers using single-molecule spectroscopy. In these experiments, we probed the relationship between the conformation of single conjugated polymer chains and the distance scale and efficiency of energy transfer within the polymer. Recent studies used polarization anisotropy measurements on single polymer chains to study chain folding following spin-casting from solution. This Account summarizes the effects of monomer regioregularity and backbone rigidity, by comparing a regiorandom phenylene vinylene (MEH-PPV) with both a regiorandom and regioregular thiophene (P3HT). Synthesis of novel polymers allowed us to explore the role of different conformation-directing inclusions in a PPV backbone. We showed that these inclusions control the conformation of individual chains and that molecular dynamics can predict these structural effects. In situ solvent vapor annealing studies explored the dynamics of polymer chains as well as the effect of solvent evaporation on the structural equilibrium of the polymer. We observed that a slower rate of solvent evaporation results in a narrow population of highly ordered polymer chains. These highly ordered single chains serve as a model system to probe the effect of conformation on energy transfer following excitation in single MEH-PPV polymer chains in two distinct experiments. In the first, we correlated the anisotropy of the fluorescence emission of individual chains with the anisotropy of their fluorescence excitation. Using this data, we derived a model for energy transfer in a conjugated polymer, simulating chromophores along a chain, coupled via Förster energy transfer. In the second experiment, super-resolution measurements demonstrated the ability of single-molecule spectroscopy to directly visualize energy transfer along a polymer chain embedded in a model device environment. A capacitive device allowed for controlled localization of hole polarons onto the polymer chain. These positive charges subsequently quenched local excitations, providing insight into the range of energy transfer in these single polymer molecules. As researchers continue to characterize conjugated polymer films and develop methods for creating multichain systems, single-molecule techniques will provide a greater understanding of how polymer morphology influences interchain interactions and will lead to a richer description of the electronic properties of bulk conjugated polymer films.

The regioregularity of a conjugated polymer can greatly affect bulk film morphologies and properties. However, it remains unclear how regioregularity affects the conformation of isolated individual chains where interchain interactions are absent. Here, the effect of the regioregularity on the conformations adopted by regioregular (rr-) and regiorandom (rra-) poly(3-hexylthiophene) (P3HT) chains was studied using single molecule fluorescence excitation polarization spectroscopy. While every rr-P3HT chain within an ensemble was found to fold into a highly ordered conformation, single rra-P3HT chains adopted a wide variety of conformations, ranging from highly ordered to isotropic. This distribution is likely due to variations in the positions of nonhead-to-tail linkages of the side-chains along the backbone of the different polymer chains. Molecular dynamics simulation on atomistic models of rr- and rra-P3HT chains supports the effect of regioregularity on the collapsed conformations. These results demonstr...

By using single-molecule spectroscopy (SMS) several effects of solvent vapor induced annealing (SVA) were studied directly on single conjugated polymers, e.g.: SVA-induced translocations, folding/unfolding dynamics, and changes in the morphological order. It is shown that single chains can be trapped by spin-coating in a disordered conformation and subsequent SVA leads to an equilibrated, highly ordered conformation.

We have reexamined the fluorescence polarization anisotropy of single polymer chains of the prototypical conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) isolated in a poly(methyl methacrylate) (PMMA) matrix employing improved synthetic samples that contain a much smaller number of tetrahedral chemical defects per chain. The new measurements reveal a much larger fraction of highly anisotropic MEH-PPV chains, with >70% of the chains exhibiting polarization anisotropy values falling in the range of 0.6-0.9. High anisotropy is strong evidence for a rod-shaped conformation. A comparison of the experimental results with coarse grain, beads on a chain simulations reveals that simulations with the usual bead-bead pairwise additive potentials cannot reproduce the observed large fraction of high polarization values. Apparently, this type of potential lacks some yet to be identified molecular feature that is necessary to accurately simulate the experimental results.

Cryptochromes (Crys) are blue light receptors believed to have evolved from the DNA photolyase protein family, implying that light control and light protection share a common ancient origin. In this paper, we report the identification of five genes of the Cry/photolyase family (CPF) in two green algae of the Ostreococcus genus. Phylogenetic analyses were used to confidently assign three of these sequences to cyclobutane pyrimidine dimer (CPD) photolyases, one of them to a DASH-type Cry, and a third CPF gene has high homology with the recently described diatom CPF1 that displays a bifunctional activity. Both purified OtCPF1 and OtCPF2 proteins show non-covalent binding to flavin adenine dinucleotide (FAD), and additionally to 5,10-methenyl-tetrahydrofolate (MTHF) for OtCPF2. Expression analyses revealed that all five CPF members of Ostreococcus tauri are regulated by light. Furthermore, we show that OtCPF1 and OtCPF2 display photolyase activity and that OtCPF1 is able to interact with the CLOCK:BMAL heterodimer, transcription factors regulating circadian clock function in other organisms. Finally, we provide evidence for the involvement of OtCPF1 in the maintenance of the Ostreococcus circadian clock. This work improves our understanding of the evolutionary transition between photolyases and Crys.

We present a picosecond transient circular dichroism study of OBIP, the putative photoreceptor protein involved in the photophobic response of Blepharisma japonicum. The probe wavelength was chosen at 230 nm. The results are compared to those of the isolated chromophore, OxyBP, in solution. The CD changes in OBIP and OxyBP do not show the same dynamics: OBIP’s signal relaxes in a few ps whereas no such decay is obtained for OxyBP. This observation brings support to the formerly evoked existence of a fast photoinduced reaction in the chromoprotein, and demonstrates the implication of local geometrical changes that accompany this process. 2009 Published by Elsevier B.V.

International audience; The photoreactive pathways that may lead to DNA damage depend crucially upon the nature of the excited electronic states. The study of alternating guanine–cytosine duplexes by fluorescence spectroscopy and quantum mechanical calculations identifies a novel type of excited states that can be populated following UVB excitation. These states, denoted High-energy Emitting Long-lived Mixed (HELM) states, extend across both strands and arise from mixing between cytosine Frenkel excitons and guanine-to-cytosine charge transfer states. They emit at energies higher than ππ* states localized on single bases, survive for several nanoseconds, are sensitive to the ionic strength of the solution, and are strongly affected by the structural transition from the B form to the Z form. Their impact on the formation of lesions of the genetic code needs to be assessed.

[EN] Guanine-cytosine hairpins, containing a hexaethylene glycol bridge, are studied by steady-state fluorescence spectroscopy and time-correlated single photon counting; their properties are compared to those of duplexes with the same sequence. It is shown that, both in hairpins and in duplexes, base pairing induces quenching of the pi pi(star) fluorescence, the quantum yield decreasing by at least two orders of magnitude. When the size of the systems increases from two to ten base pairs, a fluorescent component decaying on the nanosecond time-scale appears at energy higher than that stemming from the bright states of non-interacting mono-nucleotides (ca. 330 nm). For ten base pairs, this new fluorescence forms a well-defined band peaking at 305 nm. Its intensity is about 20% higher for the hairpin compared to the duplex. Its position (red-shifted by 1600 cm(-1)) and width (broader by 1800 cm(-1) FWHM) differ from those observed for large duplexes containing 1000 base pairs, suggesting the involvement of electronic coupling. Fluorescence anisotropy reveals that the excited states responsible for high energy emission are not populated directly upon photon absorption but are reached during a relaxation process. They are assigned to charge transfer states. According to the emerging picture, the amplitude of conformational motions determines whether instantaneous deactivation to the ground state or emission from charge transfer states will take place, while pi pi(star) fluorescence is associated to imperfect base-pairing., The French Agency for Research (ANR-10-BLAN-0809-01), the Conselleria de Educacion-Generalitat Valenciana (VALi+D program to I.V., N° 2010033) and the US Department of Energy, Office of Basic Energy Sciences (grant no. DE-FG02-96ER14604 to F.D.L.) are acknowledged for financial support.

[EN] DNA double-stranded oligomers are studied by steady-state and time-resolved fluorescence spectroscopy from the femtosecond to the nanosecond time-scale, following excitation at 267 nm. It is shown that emission arises from three types of excited states. (i) Bright pi pi* states emitting around 330 nm and decaying on the sub-picosecond time-scale with an average lifetime of ca. 0.4 ps and a quantum yield lower than 4 x 10(-6). (ii) Excimers/exciplexes emitting around 430 nm and decaying on the sub-nanosecond time-scale. (iii) Excited states emitting mainly at short wavelengths (lambda < 330 nm) and decaying on the nanosecond time-scale, possibly correlated to GC pairs. The properties of the examined duplexes, exhibiting significant disorder with respect to the nearest neighbour base sequence, are radically different than those of the much longer and disordered calf thymus DNA. Such behaviour suggests that long range and/or sequence effects play a key role in the fate of excitation energy., The French Agency for Research (ANR-10-BLAN-0809-01) and the Conselleria de Educacion-Generalitat Valenciana (VALi+D program to I.V., no. 2010033) are acknowledged for financial support.

Conjugated polymers (CPs) are promising materials for use in electronic applications, such as low-cost, easily processed organic photovoltaic (OPV) devices. Improving OPV efficiencies is hindered by a lack of a fundamental understanding of the photophysics in CP-based thin films that is complicated by their heterogeneous nanoscale morphologies. Here, we report on a poly(3-hexylthiophene)-block-poly(tert-butyl acrylate)-block-poly(3-hexylthiophene) rod-coil-rod triblock copolymer. In good solvents, this polymer resembles solutions of P3HT; however, upon the addition of a poor solvent, the two P3HT chains within the triblock copolymer collapse, affording a material with electronic spectra identical to those of a thin film of P3HT. Using this new system as a model for thin films of P3HT, we can attribute the low fluorescence quantum yield of films to the presence of a charge-transfer state, providing fundamental insights into the condensed phase photophysics that will help to guide the development of the next generation of materials for OPVs.

International audience; Cryptochromes and photolyases are homologous flavoproteins either involved in photosensory functions or in the photorepair of UV-damaged DNA. We report on the presence, nature and ultrafast photoinduced dynamics of a photoantenna in two recently discovered cryptochrome-photolyase proteins (OtCPF1, a (6-4) photolyase, and OtCPF2, a cryptochrome-DASH), coming from the green alga Ostreococcus tauri. Whereas OtCPF1 does not show any apparent photoantenna after purification, OtCPF2 is found to bind the folate MTHF. Resonance energy transfer, from MTHF to the flavin cofactor (FAD) of OtCPF2, was studied by femtosecond transient absorption spectroscopy. It takes place in 15 ps when FAD is oxidized and 100 ps when it is fully reduced. The intrinsic photophysics of MTHF in acidic solution was studied for comparison. Two decays in the sub-100 ps regime were tentatively attributed to the presence of two conformers or to a quenching mechanism involving large-amplitude motions of the 1,2,3,4-tetrahydropyraxine ring. On the other hand, the nature of the missing photoantenna of OtCFP1 was questioned by means of sequence alignment and homology modeling. We propose that the deazaflavin 8-HDF could be the photoantenna of OtCFP1 in vivo.

We have previously shown that regioregularity (i.e., the percentage of head-to-tail linkages between monomer units in the polymer backbone) is a critical factor which determines the morphology of poly(3- hexylthiophene) (P3HT) even at the single chain level. Here, we examined the excitation wavelength dependence on the fluorescence polarization anisotropy of single regioregular ( rr- ) and regiorandom ( rra- ) P3HT. For both rr- and rra- P3HT chains, the observed anisotropy is higher as the longer wavelength is used to excite single polymer chains. This indicates that chromophores with longer conjugation length are predominantly located in more highly ordered regions of the chain. However, at any excitation wavelength, the difference between rr- and rra- P3HT is observed. Single rr- P3HT chains fold into highly ordered conformation while rra- P3HT chains assume a wide variety of conformations from isotropic to anisotropic. These new results further support that regioregularity is important for P3HT even at the single chain level and give deeper insights of polymer morphology and its effect on optical property.

International audience; The ultrafast photophysics of oxidized and reduced flavin adenine dinucleotide (FAD) in aqueous solution was studied by broadband UV-Vis femtosecond transient absorption spectroscopy. We observed that oxidized FAD (FADox) in solution readily aggregates at sub-millimolar concentration. Upon excitation of FADox, three excited-state lifetimes were found and assigned to three different species: the closed (stacked) conformation of the monomer (~5.4 ps), the open (extended) conformation of the monomer (~2.8 ns), and the dimer (~27 ps). In the case of the stacked conformation of the monomer we show that intramolecular electron transfer from the adenine to the isoalloxazine ring occurs with a time constant of 5.4 ps and is followed by charge recombination on a faster timescale, namely 390 fs. We additionally demonstrate that deprotonated reduced flavin (FADHminus) undergoes biphotonic ionization under high excitation fluence, and dissociates into a hydrated electron and the neutral semiquinone radical FADHdot.

The photoactivation dynamics of two new flavoproteins (OtCPF1 and OtCPF2) of the cryptochrome photolyase family (CPF), belonging to the green alga Ostreococcus tauri , was studied by broadband UV-vis femtosecond absorption spectroscopy. Upon excitation of the protein chromophoric cofactor, flavin adenine dinucleotide in its oxidized form (FAD(ox)), we observed in both cases the ultrafast photoreduction of FAD(ox): in 390 fs for OtCPF1 and 590 fs for OtCPF2. Although such ultrafast electron transfer has already been reported for other flavoproteins and CPF members, the present result is the first demonstration with full spectral characterization of the mechanism. Analysis of the photoproduct spectra allowed identifying tryptophan as the primary electron donor. This residue is found to be oxidized to its protonated radical cation form (WH(*+)), while FAD(ox) is reduced to FAD(*-). Subsequent kinetics were observed in the picosecond and subnanosecond regime, mostly described by a biexponential partial decay of the photoproduct transient signal (9 and 81 ps for OtCPF1, and 13 and 340 ps for OtCPF2), with reduced spectral changes, while a long-lived photoproduct remains in the nanosecond time scale. We interpret these observations within the model proposed by the groups of Brettel and Vos, which describes the photoreduction of FADH(*) within E. coli CPD photolyase (EcCPD) as a sequential electron transfer along a chain of three tryptophan residues, although in that case the rate limiting step was the primary photoreduction in 30 ps. In the present study, excitation of FAD(ox) permitted to reveal the following steps and spectroscopically assign them to the hole-hopping process along the tryptophan chain, accompanied by partial charge recombination at each step. In addition, structural analysis performed by homology modeling allowed us to propose a tentative structure of the relative orientations of FAD and the conserved tryptophan triad. The results of preliminary transient anisotropy measurements performed on OtCPF2 finally showed good compatibility with the oxidation of the distal tryptophan residue (WH(351)) in 340 ps, hence, with the overall Brettel-Vos mechanism.

International audience; Femtosecond measurements of the transient dichroism and near-IR time-resolved spectra revealed the ultrafast delocalization of the cationic state in poly(N-vinylcarbazole), leading to carrier photogeneration.

The cofactor content of OtCPF1, a (6-4) photolyase isolated from the green marine alga Ostreococcus tauri , was characterized by steady-state absorption and fluorescence spectroscopy. The heterologously expressed, GST-fused, purified protein (MW: 89 kDa) is non-covalently bound to flavin adenine dinucleotide (FAD), with a flavin to apoprotein molecular ratio of 64%. No light-harvesting chromophore was found in this protein. In freshly purified OtCPF1, FAD is present in three different redox states: the fully oxidized form (FAD ox , 82%), the neutral semiquinone (FADH , 14%) and the fully reduced anion (FADH − , 4%). Keeping the sample in the dark, at 5 °C, yields oxidation of FADH and FADH − , partial release of FAD to the solution and slow degradation of the protein. Upon steady-state blue-light irradiation of OtCPF1 at 450 nm, photoreduction processes leading to an accumulation of stable FADH and FADH − species are observed. We demonstrate that this accumulation is due to the presence of an external electron donor agent in the purification buffer. Composition changes observed under steady-state photoexcitation are interpreted in terms of photoinduced reductions of FAD ox and FADH states and competitive back reactions. Specific irradiation by red light at 620 nm shows both photoreduction of FADH to FADH − and irreversible oxidation of FADH to FAD ox . The photoinduced oxidation reaction is believed to be indirectly caused by the external donor agent present in the buffer. Photoexcitation is also shown to stabilize the binding of FAD to the protein. We suggest this effect to be due to slight changes in the protein conformation, possibly strengthening the hydrogen-bonding network surrounding FAD.

The interaction of blepharismin (BP) and oxyblepharismin (OxyBP) with bovine alpha-crystallin (BAC) has been studied both by steady-state and femtosecond spectroscopy, with the aim of assessing the possible phototoxicity of these compounds toward the eye tissues. We showed that these pigments form with BAC potentially harmful ground-state complexes, the dissociation constants of which have been estimated to be 6 +/- 2 micromol L(-1) for OxyBP and 9 +/- 4 micromol L(-1) for BP. Irradiation with steady-state visible light of solutions of blepharismins in the presence of BAC proved to induce a quenching of both the pigment and the intrinsic protein fluorescences. These effects were tentatively rationalized in terms of structural changes of alpha-crystallin. On the other hand, femtosecond transient absorption spectroscopy was used to check the occurrence of any type I photoactivity of oxyblepharismin bound to alpha-crystallin. The existence of a particular type of fast photoinduced reaction, not observed in former studies with human serum albumin but present in the natural oxyblepharismin-binding protein, could here be evidenced but no specific reaction was observed during the first few nanoseconds after excitation. Partial denaturation of alpha-crystallin was however found to alter the excited-state behaviour of its complex with oxyblepharismin, making it partly resemble that of free oxyblepharismin in solution.

The hypothesis of a photoinduced intramolecular H-atom transfer in hypericin is reexamined by comparison with O -hexamethoxyhypericin, a derivative bearing no labile proton. In both cases, femtosecond transient absorption spectroscopy reveals an additional picosecond rising component in the kinetics under high excitation fluences. For the hexamethoxy derivative this effect is mixed with solvation dynamics but we succeeded in decoupling it. The delayed rise is ascribed to bi/multiphotonic excitation to upper excited states, followed by internal conversion to a hot S 1 state that subsequently cools down. The present observations are not consistent with the intramolecular H-atom transfer interpretation.

Coherent multidimensional optical spectroscopy is an emerging technique for resolving structure and ultrafast dynamics of molecules, proteins, semiconductors, and other materials. A current challenge is the quality of kinetics that are examined as a function of waiting time. Inspired by noise-suppression methods of transient absorption, here we incorporate shot-by-shot acquisitions and balanced detection into coherent multidimensional optical spectroscopy. We demonstrate that implementing noise-suppression methods in two-dimensional electronic spectroscopy not only improves the quality of features in individual spectra but also increases the sensitivity to ultrafast time-dependent changes in the spectral features. Measurements on cresyl violet perchlorate are consistent with the vibronic pattern predicted by theoretical models of a highly displaced harmonic oscillator. The noise-suppression methods should benefit research into coherent electronic dynamics, and they can be adapted to multidimensional spectroscopies across the infrared and ultraviolet frequency ranges.

Transient-absorption spectroscopy is a common and well-developed technique for measuring time-dependent optical phenomena. One important aspect, especially for measurements using pulsed lasers, is how to average multiple data acquisition events. Here, we use a mathematical analysis method based on covariance to evaluate various averaging schemes. The analysis reveals that the baseline and the signal converge to incorrect values without balanced detection of the probe, shot-by-shot detection, and a specific method of averaging. Experiments performed with sub-7 fs pulses confirm the analytic results and reveal insights into molecular excited-state vibrational dynamics.