Search

The triplet excited state properties of two BODIPY phenothiazine dyads (BDP-1 and BDP-2) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the BODIPY chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34–40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (kCS = 2.2 × 1011 s–1 and 2 × 1012 s–1 for BDP-1 and BDP-2, respectively; kCR = 4.5 × 1010 and 1.5 × 1011 s–1 for BDP-1 and BDP-2, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexci...

Carbon monoxide recombination dynamics in a mutant of the truncated hemoglobin from Thermobida fusca (3F-Tf-trHb) has been analyzed by means of ultrafast Visible-pump/MidIR-probe spectroscopy and compared with that of the wildtype protein. In 3F-Tf-trHb, three topologically relevant amino acids, responsible for the ligand stabilization through the formation of a H-bond network (TyrB10 TyrCD1 and TrpG8), have been replaced by Phe residues. X-ray diffraction data show that Phe residues in positions B10 and G8 maintain the same rotameric arrangements as Tyr and Trp in the wild-type protein, while Phe in position CD1 displays significant rotameric heterogeneity. Photodissociation of the ligand has been induced by exciting the sample with 550 run pump pulses and the CO rebinding has been monitored in two mid-IR regions respectively corresponding to the nu(CO) stretching vibration of the iron-bound CO (1880-1980 cm(-1)) and of the dissociated free CO (2050-2200 cm(-1)). In both the mutant and wild-type protein, a significant amount of geminate CO rebinding is observed on a subnanosecond time scale. Despite the absence of the distal pocket hydrogen-bonding network, the kinetics of geminate rebinding in 3F-Tf-trHb is very similar to the wild-type, showing how the reactivity of dissociated CO toward the heme is primarily regulated by the effective volume and flexibility of the distal pocket and by caging effects exerted on the free CO on the analyzed time scale.

The possibility to monitor in real-time the emission of dioxins produced by incineration of waste or by industrial processes is nowadays a necessity considering the high toxicity of these compounds, their persistence in the environment and their ability to bio-accumulate in the food chain. Recently it has been demonstrated the potentiality of detecting dioxins in carbon tetrachloride via MIR Quantum Cascade Lasers. A fundamental step in real time monitoring of dioxins emission is the possibility to recognize the most toxic congeners within complex mixtures and at low concentrations. Taking into account the lack of spectroscopic data about these very toxic environmental pollutants and the necessity to monitor their emissions we have recorded infrared spectra of 13 of the 17 most toxic congeners of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs) dissolved in carbon tetrachloride. In this way we have obtained a small database that we have used to test the ability of a linear regression algorithm to recognize each congener and its relative concentration in complex mixtures of these compounds. (C) 2014 Elsevier B.V. All rights reserved.

A relevant number of experiments on short peptides has been performed in recent years. One of the major problems rises from the simultaneous presence of slightly different conformers at equilibrium in solution. In the present paper, the conformational characteristics of the Gly-l-Ala-Methyl amide dipeptide in D2O and DMSO solutions are investigated by nonlinear IR spectroscopy. The pump-probe scheme with ultrashort mid-infrared pulses, in the Amide I region, is used to determine the mutual orientation of the two C═O bonds and the dynamics due to solute-solvent interactions. The coupling between Amide I modes is evaluated from both linear and 2D spectra. The interconversion between the different conformations occurs on time scales longer than the vibrational lifetime, and the spectral diffusion observed in 2D spectra is attributed to the solvent dynamics. Quantum mechanical calculations and molecular dynamics simulations are performed to identify the most stable geometries. By comparing the experimental and the theoretical data, we establish the prevalence of β-like polar conformers in both water and DMSO solvents.

We describe the theory, some experimental details and the data analysis procedures of two-dimensional infrared (2D-IR) spectroscopy. A brief description of an application of the technique to the study of a dipeptide in solution is also reported. Like multi-dimensional NMR spectroscopy, 2D-IR can provide additional pieces of information hidden in the inhomogenously broadened bands observed in linear IR spectra. In addition, the presence of off-diagonal peaks allows a direct estimate of the couplings between vibrational modes. By means of this technique, making use of ultrashort mid-infrared pulses, exploration of the structure and of the dynamics of molecular systems in the condensed phase and on very short time scales becomes now achievable. The effects of solvent dynamics on Glycine-L-Alanine-Methylamide by 2D-IR are discussed. Copyright © 2013 John Wiley & Sons, Ltd.

Dye-sensitized solar cells (DSSCs) have attracted significant interest in the last few years as effective low-cost devices for solar energy conversion. We have analyzed the excited state dynamics of several organic dyes bearing both cyanoacrylic acid and siloxane anchoring groups. The spectroscopic properties of the dyes have been studied both in solution and when adsorbed on a TiO2 film using stationary and time-resolved techniques, probing the sub-picosecond to nanosecond time interval. The comparison between the spectra registered in solution and on the solid substrate evidences different pathways for energy and electron relaxation. The transient spectra of the TiO2-adsorbed dyes show the appearance of a long wavelength excited state absorption band, attributed to the cationic dye species, which is absent in the spectra measured in solution. Furthermore, the kinetic traces of the samples adsorbed on the TiO2 film show a long decay component not present in solution which constitutes indirect evidence of electron transfer between the dye and the semiconductor. The interpretation of the experimental results has been supported by theoretical DFT calculations of the excited state energies and by the analysis of molecular orbitals of the analyzed dye molecules.

On 1 February 2013, a marketing authorisation valid throughout the European Union was issued for aflibercept (Zaltrap) in combination with irinotecan/5-fluorouracil/folinic acid chemotherapy for the treatment of adults with metastatic colorectal cancer resistant to or progressive after an oxaliplatin-containing regimen. Aflibercept is a recombinant fusion protein which blocks the activation of vascular endothelial growth factor (VEGF) receptors and the proliferation of endothelial cells, acting as a soluble decoy receptor that binds to VEGF-A with higher affinity than its native receptors, as well as placental growth factor and VEGF-B. The use of aflibercept was studied in a randomised, double-blind, placebo-controlled phase III study, in patients with metastatic colorectal cancer (mCRC) who had previously been treated with an oxaliplatin-based treatment with or without prior bevacizumab. Aflibercept (n=612) was compared with placebo (n=614), both in combination with FOLFIRI (infusional fluorouracil, leucovorin and irinotecan). The primary endpoint of the study was overall survival (OS). The median OS in the intent-to-treat population was 13.5 months in subjects treated with aflibercept compared with 12.1 months for subjects in the control arm (stratified HR=0.817, 95% CI 0.714 to 0.935, stratified pvalue=0.0032). The frequency of adverse events was higher in the aflibercept arm compared with the placebo arm, reflecting the toxicity profile of anti-VEGF agents in combination with chemotherapy. This paper is based on the scientific review of the application leading to approval of aflibercept in the EU. The detailed scientific assessment report and product information for this product are available on the European Medicines Agency website (http://www.ema.europa.eu). Trial registration number NCT00561470, Results.

The relationships between the photophysics and structural properties of 4,7-dithien-2-yl-2,1,3-benzothiadiazole as a function of solvent polarity are investigated both experimentally and by computational methods. Stationary fluorescence measurements are consistent with a model envisaging the presence of three types of conformers in equilibrium in the ground state. They are characterized by different relative orientations of the thiophene rings. Due to a low rotational barrier, the sample in solution is characterized by a distribution of relative internal orientations. By applying the Kawski method, we evaluate the average dipole moment of ground and excited states of the three types of conformers. The ground state dipole moments are small and similar for the three types of conformers. On the contrary, dipole moments differ substantially in the excited state. X-ray diffraction of a single crystal confirms the presence of an orientational disorder of thiophene rings. Transient absorption UV-visible spectroscopy experiments allows the identification of the main mechanisms responsible for the large Stokes shift observed in this push-pull molecule. Time dependent spectra provide a picture of the relaxation processes occurring after excitation: the primary step is an internal charge transfer assisted by thiophene ring planarization which occurs on a time scale ranging from 0.88 to 1.3 picoseconds depending on solvent polarity. Moreover, time-resolved fluorescence measurements are consistent with a mechanism involving planarization accompanied by a stabilization of the charge transfer state as observed in polar solvents. In the latter, longer fluorescence lifetimes are observed along with a quantum yield decrease due to the activation of specific non-radiative relaxation channels. The photophysical behavior of 4,7-dithien-2-yl-2,1,3-benzothiadiazole in a solid matrix of polymethyl methacrylate is similar to that observed in solution, but the overall non-radiative process rate is slow with respect to that in the liquid phase. As a consequence, the radiative processes are enhanced giving rise to a fluorescence quantum yield of 90%. Such behavior is consistent with the proposed relaxation model.

How does a crystal melt? How long does it take for melt nuclei to grow? The melting mechanisms have been addressed by several theoretical and experimental works, covering a subnanosecond time window with sample sizes of tens of nanometers and thus suitable to determine the onset of the process but unable to unveil the following dynamics. On the other hand, macroscopic observations of phase transitions, with millisecond or longer time resolution, account for processes occurring at surfaces and time limited by thermal contact with the environment. Here, we fill the gap between these two extremes, investigating the melting of ice in the entire mesoscopic regime. A bulk ice I-h or ice VI sample is homogeneously heated by a picosecond infrared pulse, which delivers all of the energy necessary for complete melting. The evolution of melt/ice interfaces thereafter is monitored by Mie scattering with nanosecond resolution, for all of the time needed for the sample to reequilibrate. The growth of the liquid domains, over distances of micrometers, takes hundreds of nanoseconds, a time orders of magnitude larger than expected from simple H-bond dynamics.

A series of organic D-π-A dyes, endowed with different silicon-based anchoring groups, has been prepared to assess the stability of such anchoring moieties on nanocrystalline TiO2 in dye-sensitized solar cells. Due to the difficulties encountered in finding a reliable and robust preparation protocol to obtain pure trialkoxysilanes, replacement with a silatrane moiety was evaluated. It was found that the silatrane group could be easily introduced on three different molecular scaffolds by using a simple amide coupling reaction mediated by EDC-Cl. Furthermore, the spectroscopic properties and anchoring mode on nanocrystalline TiO2 of the silatrane dyes were found to be nearly identical to those of the trialkoxysilane compounds, and both gave a much more stable attachment to the semiconductor compared with their cyano­acrylic acid counterpart, as shown by desorption experiments.

In this work we present, to our knowledge for the first time, the results of a transient infrared spectroscopic study of the photoinduced valence tautomerism process in cobalt–dioxolene complexes with sub-picosecond time resolution. The molecular systems investigated were [Co(tpa)(diox)]PF6 (1) and [Co(Me3tpa)(diox)]PF6 (2), where diox = 3,5-di-tert-butyl-1,2-dioxolene; tpa = tris(2-pyridylmethyl)amine and Me3tpa its 6-methylated analogue. Complex (1) is present in solution as ls-CoIII(catecholate) (1-CAT), while (2) as hs-CoII(semiquinonate) (2-SQ). DFT calculation of the harmonic frequencies for (1) and (2) allowed us to identify the vibrational markers of catecholate and semiquinonate redox isomers. Irradiation with 405 and 810 nm pulses (∼35 fs) of (1-CAT) induces the formation of an intermediate excited species from which the ground state population is recovered with a time constant of 1.5 ± 0.3 ns. Comparing the 1 ns transient infrared spectrum with the experimental difference spectrum FTIR(2-SQ2-SQ)–FTIR(1-CAT1-CAT) and with the calculated difference spectrum IRc(1-SQ1-SQ)–IRc(1-CAT1-CAT) we are able to unequivocally identify the long lived species as the semiquinonate redox isomer of (1). On the other hand, no evidence of photoconversion is observed upon irradiation of (2) with 405 nm. Temporal evolution of transient spectra was analyzed with the combined approach consisting of singular values decomposition and global fitting (global analysis). After 405 and 810 nm excitation of (1-CAT), the semiquinonate excited species is formed on an ultrafast time scale (

The choice of a donor and an acceptor with suitable optical and self-assembly properties is essential in the design of organogel-based light harvesting systems. Organogels can provide supramolecular structures capable of enhancing energy transfer processes. In this work, we present the characterization of N -(naphthalene-1-carboxyamide)-(3 S ,4 S )-pyrrolidin-(3,4)-bisdodecyl-carbamoyldiester ( 1 ) and N -(4-nitrobenzofurazan-7-amino)-(3 S ,4 S )-pyrrolidin-(3,4)-bisdodecyl-carbamoyldiester ( 2 ) which are used as donor and acceptor moieties, respectively. The donor molecule is hardly capable to form a gelon its own but it can be assembled at reasonable concentrations with the acceptor gelator to form a two-component donor–acceptor organogels in cyclohexane. Stable organogels are formed from cyclohexane for gelator concentrations as low as ≈10 −3 M. UV–vis and steady-state fluorescence spectroscopies were used to provide a characterization of their molecular interactions. The optical changes observed during the cooling of two-component solutions of these systems are indicative of typical sol–gel transitions. The occurrence of excitation energy transfer processes in the gels is confirmed by comparison of their excitation and absorption spectra.

International audience; We have generated continuous supersonic beams of dicarbon (C2) and cyano (CN) radicals by a high-pressure radio-frequency discharge beam source starting from dilute mixtures in rare gases of suitable precursor molecules. We have subsequently characterized their internal quantum state distributions by laser-induced-fluorescence (LIF) in a new crossed molecular beam-laser apparatus. We have used these supersonic beams to study the reactive scattering of C2 and CN radicals with unsaturated hydrocarbons. We report here on the C2 and CN radical beam characterization by LIF and on dynamics studies of the reactions CN + C2H2 (acetylene) and CN + CH3CCH (methylacetylene) by the crossed molecular beam scattering technique with universal mass spectrometric detection and time-of-flight analysis. The role of CN rovibrational excitation on the dynamics of the CN + C2H2 reaction is discussed with reference to previous dynamics and kinetics studies. These reactions are of interest in the chemistry of planetary atmospheres (Titan) and the interstellar medium as well as in combustion.

We investigated a novel bichromophoric antenna system, characterized by energy transfer between a naphthalene group acting as the donor and a benzofurazane group acting as the acceptor. We studied the spectroscopic properties (infrared, Raman, UV–vis and fluorescence) of self-assembled monolayers of this molecular antenna on Ag nanoisland films and the energy-transfer process upon irradiation at 300 nm.

S1 --S(n) spectra of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication have been measured in the energy range 2-3 eV above S1 at room temperature in solution by means of transient absorption spectroscopy exciting with femtosecond pulses. Highly excited pi pi* states not active in the conventional S0 --S(n) spectrum have been observed. The experimental data are discussed on the basis of the time dependent density functional theory taking advantage of large scale calculations of configuration interaction between singly excited configurations (DF/SCI). The DF/SCI calculation on porphyrin has allowed to assign g states active in the S1 --S(n) spectrum. Applying the same calculation method to tetraoxaporphyrin dication the S0 --S(n) spectrum is reproduced relatively to the Q and B (Soret) bands as well as to the weaker E(u) bands at higher energy. According to our calculation the S1 --S(n) transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate pi and pi* orbitals such as 2e(g) --4e(g). In the case of diprotonated porphyrin it is shown that the complex of the macrocycle with two trifluoroacetate anions plays a significant role for absorption. Charge transfer excitations from the anions to the macrocycle contribute to absorption above the Soret band, justifying the intensity enhancement of the S0 --S(n) spectrum with respect to the other two macrocyclic systems.

As one of the most promising classes of third-generation PV technologies, dye-sensitized solar cells (DSSC) have been the subject of constant attention in recent years thanks to the enhancement in their efficiency and stability. Metal-free D-?-A dyes (characterized by donor (D) and acceptor (A) moieties joined by a conjugated unit) are especially interesting, since their particular architecture allows a fine adjustment of their photo- and electrochemical properties. In this work, four new D-?-A dyes for DSSCs were synthesized. First, we introduced an electronrich ProDOT moiety in the conjugated unit of a traditional DSSC dye (named D5) to increase the light-harvesting efficiency of the structure (1). Then, we explored the possibility to improve the stability of the dye on the semiconductor by using the alkoxysilane group, recently awarded to be one of the most stable anchoring units for DSSC (2). We changed the corresponding cyanoacrylic acid (3 and 4 are obtained from the acid DF15) and the length between the dyes and the semiconductor surface in order to verify the difference between them. Finally, we analyzed the interfacial charge transfer processes taking place between the dyes and a nanocrystalline semiconductor (TiO2), by meanso of UV-Vis and IR transient absorption spectroscopy. The results obtained with the various anchoring groups were compared in order to study the different dynamics occurring in each case.

Dye-sensitized solar cells (DSSC) have been the subject of constant attention in recent years for their potential as novel PV technology, which led to the enhancement in their efficiency and stability. Among the sensitizers, metal-free D-?-A dyes are especially interesting, since their particular architecture allows a fine adjustment of their photo- and electrochemical properties. Our work focused on developing new synthetic methods to insert the alkoxysilane anchoring group, recently recognized as one of the most stable and high-performing anchoring units for DSSC. First, we studied the insertion of two different spacers for the silicon-based moiety on the structure of already known dye DF15. Then, we altered the dye conjugate unit by replacing the central thiophene ring with a more electronrich ProDOT moiety with the aim to improve the dye light harvesting properties and explored a different way to insert the trialkoxyphenylsilyl anchor. The corresponding cyanoacrylic acid dye was also prepared for comparison. Finally, we analyzed the photochemical properties of the dyes in different solvents and the interfacial charge transfer processes taking place between them and a nanocrystalline semiconductor (TiO2), by means of transient absorption spectroscopy. The results obtained with the various anchoring groups were compared in order to study the different dynamics occurring in each case.

The irradiation of colchicone 5 led to the formation of lumicolchicone 7. The same reaction cannot be obtained by using thiocolchicone 6 as substrate. Transient absorption spectroscopy of colchicone and beta-lumicolchicone showed that probably the photoisomerization occurred on colchicone in its first excited singlet state. The spectroscopic data are in agreement with the hypothesis that lumicolchicone was generated in the ground state from the S1 state of colchicone without the presence of any intermediate. Semiempirical calculations on colchicone and thiocolchicone showed that the highest single occupied molecular orbital and the lowest unoccupied molecular orbital of the singlet excited colchicone can give a disrotatory ring closure to 7, while thiocolchicone cannot give the same type of process.

This work presents an experimental and computational study of the intramolecular electronic energy transfer process occurring in two newly synthesized bichromophoric species: N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino-bis-ethyl-2-[(4-chloro-1-naphthyl)oxy]acetate ( f-Bi ) and N-(7-nitrobenzo[c][1,2,5]oxadiazole-4-yl)-(3S, 4S)-pyrrolidin-3,4-bis-yl-2-[(4-chloro-1-naphthyl)oxy]acetate ( r-Bi ). In both f-Bi and r-Bi the donor chromophore is the [(4-chloro-1-naphthyl)oxy]acetate moiety, whereas the acceptor units belong to the family of the 4-dialkylaminonitrobenzoxadiazoles, well-known fluorescent probes. The two bichromophores differ in the structural flexibility. In f-Bi , acceptor and donors are linked by a diethanolamine moiety, whereas in r-Bi through a (3S, 4S)3,4-dihydroxypyrrolidine ring. By means of steady-state and time-resolved UV–vis spectroscopies we carried out a detailed analysis of the photo-response of donor and acceptor chromophores as individual molecules and when covalently linked in f-Bi and r-Bi . The intramolecular energy transfer process occurs very efficiently in both the bichromophores. The rate constant and the quantum efficiency of the process are k ET = (2.86 ± 0.16) × 10 11 s −1 and Q = 0.998 in f-Bi , and k ET = (1.25 ± 0.08) × 10 11 s −1 and Q = 0.996 in r-Bi . Semiempirical calculations were utilized to identify the energy and the nature of the electronic states in the isolated chromophores. Molecular mechanics calculations have been performed to identify the most stable structures of the bichromophoric compounds. The predictions of Forster theory are consistent with the experimental results and provide a suitable way to evaluate the structural differences between the two compounds.

Antibacterial treatments based on photosensitized production of reactive oxygen species is a promising approach to address local microbial infections. Given the small size of bacterial cells, identification of the sites of binding of the photosensitizing molecules is a difficult issue to address with conventional microscopy. We show that the excited state properties of the naturally occurring photosensitizer hypericin can be exploited to perform STED microscopy on bacteria incubated with the complex between hypericin and apomyoglobin, a self-assembled nanostructure that confers very good bioavailability to the photosensitizer. Hypericin fluorescence is mostly localized at the bacterial wall and accumulates at the polar regions of the cell and at sites of cell wall growth. While these features are shared by Gram-negative and Gram-positive bacteria, only the latter are effectively photoinactivated by light exposure.

We have analyzed the excited state dynamics of the heteroleptic [(NCS)2Ru(bpy-(COOH)2)(bpy-(C6H13)2)] Z907 solar cell sensitizer in solution and when adsorbed onto thin TiO2 films, by combining transient visible and infrared (IR) spectroscopies with ab initio Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations. Upon excitation with ultra-short pulses in ethanol and dimethyl-sulphoxide solutions, the visible spectra show the appearance of a positive signal around 650 nm, within the instrumental time resolution (2) charge transfer (LML'CT). Vibrational cooling is observed in both solvents; in ethanol it is overtaken by the hydrogen bond dynamics. On the basis of DFT/TDDFT calculations, explicitly modeling the interaction of the NCS and COOH groups with solvent (ethanol) molecules, we rationalize the observed IR and visible spectral evolution as arising from the change in the hydrogen-bond network, which accompanies the transition to the lowest-energy triplet state. This interpretation provides a consistent explanation of what is also observed in the transient visible spectra. Transient IR measurements repeated for molecules adsorbed on TiO2 and ZrO2 films, allow us to identify the structural changes signaling the dye triplet excited state formation and evidence multiexponential electron injection rates into the semiconductor TiO2 film.

Transient spectra for 4‘-(N,N-dimethylamino)-3-hydroxy- (1a) and 4‘-(N,N-dimethylamino)-3-methoxyflavone (2a) in several solvents at room temperature are reported following excitation with pulses o...

The transient spectra of azulene in solution have been measured in the spectral region 600−350 nm at room temperature, pumping into the S1 and S2 states with femtosecond pulses and probing with a delayed femtosecond white light continumm. The spectra are a combination of ground-state bleaching, stimulated emission, and excited-state absorption. The latter component gives a direct information on excited states which may be not active in the ground-state absorption. S1 → Sn and S2 → Sn absorptions have been discussed with the help of ab initio calculations of the MCSCF/CAS type with the 6-31G* basis set and including perturbative corrections. The calculated S0 → Sn, S1,eq → Sn, and S2,eq → Sn vertical excitation energies and oscillator strengths are in satisfactory agreement with the experimental results. It has been found that electronic states, weakly active in the ground-state absorption, occur with high intensity in the femtosecond transient spectra, in particular in the energy range 36000−44000 cm-1 ab...

The photophysics of 9(19),16(17),23(24)-tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)phenyl] phthalocyaninatozinc(II) (ZnPc) in solution and adsorbed on TiO2 and ZrO2 nanoparticle films is characterized by stationary and time-resolved spectroscopies in the subpicosecond to nanosecond time interval. The comparison between the solution and the solid substrate data allows us to identify different pathways of the energy and electron relaxation. On the solid substrate, the presence of H-aggregates adds a further nonradiative deactivation channel competing with the charge injection into the Ti-2-conducting band, thus providing an explanation of the reduced efficiency of the charge transfer processes. The comparison between the kinetics recorded after excitation of the S-0-S-2 transition and those recorded after excitation of the S-0-S-1 transition provides an estimate of the internal conversion between S-2 and S-1 which occurs very efficiently and on an ultrafast (

By means of one- and two-dimensional transient infrared spectroscopy and femtosecond stimulated Raman spectroscopy, we investigated the excited state dynamics of peridinin, a carbonyl carotenoid occurring in natural light harvesting complexes. The presence of singly and doubly excited states, as well as of an intramolecular charge transfer (ICT) state, makes the behavior of carbonyl carotenoids in the excited state very complex. In this work, we investigated by time resolved spectroscopy the relaxation of photo-excited peridinin in solvents of different polarities and as a function of the excitation wavelength. Our experimental results show that a characteristic pattern of one- and two-dimensional infrared bands in the C=C stretching region allows monitoring the relaxation pathway. In polar solvents, moderate distortions of the molecular geometry cause a variation of the single/double carbon bond character, so that the partially ionic ICT state is largely stabilized by the solvent reorganization. After vertical photoexcitation at 400 nm of the S2 state, the off-equilibrium population moves to the S1 state with ca. 175 fs time constant; from there, in less than 5 ps, the non-Franck Condon ICT state is reached, and finally, the ground state is recovered in 70 ps. That the relevant excited state dynamics takes place far from the Franck Condon region is demonstrated by its noticeable dependence on the excitation wavelength.

Intramolecular electron transfer in the excited state of a [(bpy)ReI(CO)3(L)]+ complex (bpy = 2,2‘-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L = N-[4...

The relaxation dynamics of pure acetonitrile isotopomers has been investigated in the temperature range 8 to 75 ° C. The overall response of the liquid is measured either recording directly the decay of the optical Kerr signal with heterodyne detection (OHD-OKE) and Fourier transforming the depolarized Rayleigh scattering spectra (DRS). The OHD-OKE signals show a decay that can be described by a bi-exponential law. At some temperatures, stressing to a maximum level the sensitivity of the OHD-OKE experimental set-up, a damped oscillation is observed on top of the fast decay component. The two techniques provide same results with a high level of reproducibility, as far as the slow component is concerned. This latter is described by an exponential law with the time constants ranging in the interval 2.0 to 0.85 ps in the light and approximately in the same interval in the deuterated molecule. The decays are, at all temperatures, well reproduced by the extended diffusion J-model. The fast component, better observed with the OHD-OKE experiments in a restricted temperature range, has time constants ranging from 550 to 350 fs. After the subtraction of the curve due to the slower decay component, the data have also been analyzed by Fourier transforming the fast part of the decay. The spectrum then consists of a broad (approximately 80 cm-1 wide) band centered at 50 cm-1. This band is interpreted as the manifestation of intermolecular vibrational motions.

The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitive to small variations in dimensions, shape and surface capping. In the present work CdTe quantum dots are synthesized in water using 3-mercaptopropionic acid and 1-thioglycerol as stabilizers. Nanocrystals with an average dimension of 4.0 ± 1.0 and 3.7 ± 0.9 nm were obtained, when 3-mercaptopropionic acid or 1-thioglycerol, respectively, was used as a capping agent. The steady-state characterization shows that the two types of colloids have different luminescence behavior. In order to investigate the electronic structure and the dynamics of the exciton state, a combined study in the time domain has been carried out by using fluorescence time-correlated single photon counting and femtosecond transient absorption techniques. The electron–hole radiative recombination follows the non-exponential decay law for both colloids, which results in different average decay time values (of the order of tens of nanoseconds) for the two samples. The data demonstrate that the process is slower for 1-thioglycerol-stabilized colloids. The ultrafast transient absorption measurements are performed at two different excitation wavelengths (at the band gap and at higher energies). The spectra are dominated in both types of samples by the negative band-gap bleaching signals although transient positive absorption bands due to the electrons in the conduction band are observable. The analysis of the signals is affected by the different interactions with the defect states, due to ligand capping capacities. In particular, the data indicate that in 1-thioglycerol-stabilized colloids the non-radiative recombination processes are kinetically more competitive than the radiative recombination. Therefore the comparison of the data obtained from the two samples is interpreted in terms of the effects of the capping agents on the electronic relaxation of the colloids.

S1 excited state relaxation processes of [2,2′-bipyridyl]-3,3′-diol are investigated at room temperature in solution. After excitation at 355–360 nm, the transient UV–visible spectra are investigated in the 350–650 nm region where both transient absorption and stimulated emission occur. The signal is measured at various probe wavelengths as a function of time up to delays of 150 ps. The spectral features appear with times shorter than the instrumental resolution (

UV pulses of 100 fs at 340 nm are utilised to excite pyrene to the vibrationally cold S2 state. Population and depopulation processes occurring in the S2 state are investigated by transient absorption spectroscopy with a white continuum probe. In the first 400 fs after excitation it is possible to measure the growth and disappearance of a band at 585 nm which is assigned to the S14←S2 transition. As a consequence of the internal conversion process from S2 to S1 state, the disappearance of the 585 nm band and the growth of the transient spectrum due to Sn←S1 transitions can be observed. Kinetic measurements of the S2 decay and S1 growing signals provide an estimate of the rate of internal conversion which occurs with a time constant of 150±50 fs.

Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the photochemistry of trans-azobenzene in n-hexane. Excitation to the S-1(n pi*) state results in transient absorption bands at ca. 400 nm (strong) and 550 nm (weaker) which decay with a lifetime 2.5 +/- 0.2 ps on excitation at 503 nm, close to the S-1 origin, and with an additional fast component of ca. 0.6 ps on excitation at 390 or 420 nm, both well above the S-1 origin. Excitation to the S-2(pi pi*) state results in transient absorption at 400 nm which decays with a dominant component of ca. 0.9 ps and a weaker component of ca. 15 ps; this 400 nm band itself is observed to rise synchronously as a transient band at 475 nm decays with a lifetime of < 200 fs. These results are discussed in terms of the dual mechanism proposed for azobenzene photoisomerization.

Internal water molecules play an active role in ligand uptake regulation, since displacement of retained water molecules from protein surfaces or cavities by incoming ligands can promote favorable or disfavorable effects over the global binding process. Detection of these water molecules by X-ray crystallography is difficult given their positional disorder and low occupancy. In this work, we employ a combination of molecular dynamics simulations and ligand rebinding over a broad time range to shed light into the role of water molecules in ligand migration and binding. Computational studies on the unliganded structure of the thermostable truncated hemoglobin from Thermobifida fusca (Tf-trHbO) show that a water molecule is in the vicinity of the iron heme, stabilized by WG8 with the assistance of YCD1, exerting a steric hindrance for binding of an exogenous ligand. Mutation of WG8 to F results in a significantly lower stabilization of this water molecule and in subtle dynamical structural changes that favor ligand binding, as observed experimentally. Water is absent from the fully hydrophobic distal cavity of the triple mutant YB10F-YCD1F-WG8F (3F), due to the lack of residues capable of stabilizing it nearby the heme. In agreement with these effects on the barriers for ligand rebinding, over 97% of the photodissociated ligands are rebound within a few nanoseconds in the 3F mutant case. Our results demonstrate the specific involvement of water molecules in shaping the energetic barriers for ligand migration and binding. Fil: Bustamante, Juan Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Abbruzzetti, Stefania. Università di Parma; Italia Fil: Marcelli, Agnese. European Laboratory for Non-linear Spectroscopy; Italia Fil: Gauto, Diego Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Boechi, Leonardo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Cálculo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bonamore, Alessandra. Instituto de Investigaciones Universitarias Roma la Sapienza; Italia Fil: Boffi, Alberto. Instituto de Investigaciones Universitarias Roma la Sapienza; Italia Fil: Bruno, Stefano. Università di Parma; Italia Fil: Feis, Alessandro. Universita Degli Studi Di Firenze; Italia Fil: Foggi, Paolo. Università di Perugia; Italia. INO-CNR; Italia Fil: Estrin, Dario Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Viappiani, Cristiano. Università di Parma; Italia

An optical parametric generator and amplifier producing 15 ps pulses at wavelengths tunable around 2 μm, with energies up to 15 mJ/pulse, has been realized and characterized. The output wavelength is chosen to match a vibrational combination band of water. By measuring the induced birefringence changes we prove that a single pulse is able to completely melt samples of ice in the 10⁻⁶ cm³ volume range, both at room pressure (263 K) and at high pressure (298 K, 1 GPa) in a sapphire anvil cell. This source opens the possibility of studying melting and freezing processes by spectroscopic probes in water or water solutions in a wide range of conditions as found in natural environments.

We have studied the effect of transient vibrational inversion of population in trans-beta-apo-8'-carotenal on the time-resolved femtosecond stimulated Raman scattering (TR-FSRS) signal. The experimental data are interpreted by applying a quantum mechanical approach, using the formalism of projection operators for constructing the theoretical model of TR-FSRS. Within this theoretical frame we explain the presence of transient Raman losses on the Stokes side of the TR-FSRS spectrum as the effect of vibrational inversion of population. In view of the obtained experimental and theoretical results, we conclude that the excited S-2 electronic level of trans-beta-apo-8'-carotenal relaxes towards the S-0 ground state through a set of four vibrational sublevels of S-1 state. (C) 2014 AIP Publishing LLC.

Applied static pressure can largely modify the structure and dynamics of molecular systems, with consequences on their optical properties and chemical stability. When photochemical effects are exploited in conjunction with the structural and dynamical conditions attained at high density, chemical reactivity may become highly selective and efficient, yielding technologically attractive products. Non-linear optical spectroscopies are a powerful tool to investigate molecular energetics and dynamics, and thus unveil key aspects of the chemical reactivity at a molecular level. Their application to high-pressure samples is experimentally challenging, mainly because of the small sample dimensions and the non-linear effects generated in the anvil materials. In this paper we review the main results on the behavior of electronic states at high pressure, obtained by non-linear optical techniques, discussing the relationship between pressure-induced structural modifications and chemical reactivity, and the state of the art of ongoing research.

We have characterized the dynamics and the efficiency of electronic energy transfer (EET) in a newly synthesized molecular dyad, composed of a styryl-pyridinium donor and a BODIPY acceptor. The kinetics of the process has been studied with femtosecond transient absorption spectroscopy in different solvents. In all the analyzed media EET is quantitative and very fast, as we find that almost 70% of the overall excitation energy is transferred from the donor to the acceptor on a subpicosecond time scale. The experimental measurements have been supported by a theoretical analysis; the electronic couplings between the donor and acceptor moieties have been calculated at the (TD)DFT level and complemented by a conformational analysis of the full dyad. The computed energy transfer times are in good agreement with the experimental values; this allowed us to verify the correctness of the Forster equation, demonstrating that, although EET in the examined system occurs on an ultrafast time scale, the approximations i...

Fast relaxation processes in water and HCl-water solutions are measured by time-resolved heterodyne detected optical kerr effect. The decay of the signal, observed after 500 fs, is well fitted by a biexponential. The temperature dependence of the two decay time constants and the relative weights of the two exponentials, for water and for samples of different HCl concentration, are consistent with a picture in which the hydrogen bonds dominate the dynamics and the optical response of the system: the fast decay time is closely related to the mean lifetime of individual H bonds, while the slow time constant is attributed to collective relaxation of structured regions of the liquid.

The fast dynamics of the population build-up of S 2 state of azulene in ethylene glycol and the following relaxation processes are investigated by transient absorption spectroscopy. The transient spectrum shows bands at 469, 413 and 366 nm. The spectral data are compared to the results of semi-empirical calculations based on the QCFF/PI method. The build-up of S 2 population is measured by following the growth in intensity of the main peak (366 nm) as the function of the delay between the pump (340 nm) and the probe (broad continuum). The experimental points are fitted by a single exponential curve with time constant of 300±50 fs. The process is dominated by the fast motions of the solvent cage. The decay of the state occurs with a single exponential mechanism with time constant of 1.06±0.03 ns.

We have studied the temperature and pressure dependence of the vibrational energy levels of orthorhombic sulfur. Spectroscopic measurements were performed using standard Raman equipment; high pressure experiments made use of a diamond anvil cell (DAC). In general all external modes are sensitive to temperature variation in the range 50−300 K; internal modes exhibit a different behavior. It was found that mode energies were more influenced by pressure (0−5 GPa) than by temperature (12−300 K). In addition it was found that the intensities of prominent features of the Raman spectrum were pressure dependent.

We demonstrate the possibility of a quantitative analysis of the concentration of several dioxins and furans, among the most toxic ones, by only using infrared absorption laser spectroscopy. Two broadly tunable quantum cascade lasers, emitting in the mid-infrared, have been used to measure the absorption spectra of dioxins and furans, dissolved in CCl4, in direct absorption mode. The minimum detectable concentrations are inferred by analyzing diluted samples. A comparison between this technique and standard Fourier transform spectroscopy has been carried out and an analysis of future perspectives is reported.

We describe the quantitative analysis of the most toxic dioxins and furans by means of direct absorption spectroscopy with broadly tunable quantum cascade lasers in the middle infrared.

The matching of group velocities achieved in non-collinear three-wave interaction is shown to yield efficient parametric conversion of femtosecond pulses from a frequency-doubled amplified Ti:sapphire laser. Tunability in the whole visible range, pulse duration of 90 fs and single-pass energy conversion efficiencies as large as 40% have been obtained in b-BaB 2 O 4 .

The orientational dynamics of β‐carotene in n‐alkane solutions is investigated by resonance enhanced optical Kerr effect. By use of this spectroscopic technique, it is possible to selectively investigate the relaxation of a probe molecule at a concentration level low enough to allow the observation of the averaged single‐molecule dynamics. For delay times longer than ∼20 ps all solutions show a single exponential decay, with a time constant depending on the viscosity, that is ascribed to the β‐carotene orientational relaxation. The dependence on viscosity of the measured relaxation times is compared with the predictions of different models. The purely hydrodynamic theories overestimate, by far, the solute effective volume and hence its orientational relaxation time; a much better agreement is obtained from two quasihydrodynamic models.

The dynamics of nitromethane in the liquid phase is investigated by steady‐state (Raman and Rayleigh) and time‐resolved (optical Kerr effect) spectroscopic experiments performed at variable temperature. Both experiments show that the entire relaxation process is completed in a few picoseconds and that the dynamics can be described by assuming a biexponential function for the molecular response. The prominent dynamical feature at longer times appears to be that of a diffusionally reorienting symmetric top. The time dependence for orientational correlation at the shorter times observed in the coherent optical processes (Rayleigh and OKE) suggests that perturbative phenomena in the subpicosecond time scale (collisional and cage effects) are operative prior to the onset of the diffusional regime. Reorientational times from steady‐state coherent and incoherent light scattering experiments enable us to exclude that pair‐particle orientational correlation is effective. The same orientational activation energy wa...

International audience; Carbon monoxide recombination dynamics upon photodissociation with visible light has been characterized by means of ultrafast visible-pump/MidIR probe spectroscopy for the truncated hemoglobins from Thermobifida fusca and Bacillus subtilis. Photodissociation has been induced by exciting the sample at two different wavelengths: 400 nm, corresponding to the heme absorption in the B-band, and 550 nm, in the Q-bands. The bleached iron-CO coordination band located at 1850-1950 cm(-1) and the free CO absorption band in the region 2050-2200 cm(-1) have been observed by probe pulses tuned in the appropriate infrared region. The kinetic traces measured at 1850-1950 cm(-1) reveal multiexponential subnanosecond dynamics that have been interpreted as arising from fast geminate recombination of the photolyzed CO. A compared analysis of the crystal structure of the two proteins reveals a similar structure of their distal heme pocket, which contains conserved polar and aromatic amino acid residues closely interacting with the iron ligand. Although fast geminate recombination is observed in both proteins, several kinetic differences can be evidenced, which can be interpreted in terms of a different structural flexibility of the corresponding heme distal pockets. The analysis of the free CO band-shape and of its dynamic evolution brings out novel features about the nature of the docking site inside the protein cavity.

CO recombination kinetics has been investigated in the type II truncated hemoglobin from Thermobifida fusca (Tf-trHb) over more than 10 time decades (from 1 ps to 100 ms) by combining femtosecond transient absorption, nanosecond laser flash photolysis and optoacoustic spectroscopy. Photolysis is followed by a rapid geminate recombination with a time constant of 2 ns representing almost 60% of the overall reaction. An additional, small amplitude geminate recombination was identified at 100 ns. Finally, CO pressure dependent measurements brought out the presence of two transient species in the second order rebinding phase, with time constants ranging from 3 to 100 ms. The available experimental evidence suggests that the two transients are due to the presence of two conformations which do not interconvert within the time frame of the experiment. Computational studies revealed that the plasticity of protein structure is able to define a branched pathway connecting the ligand binding site and the solvent. This allowed to build a kinetic model capable of describing the complete time course of the CO rebinding kinetics to Tf-trHb. Fil: Marcelli, Agnese. European Laboratory for Non-Linear Spectroscopy; Italia Fil: Abbruzzetti, Stefania. Università di Parma; Italia Fil: Bustamante, Juan Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Feis, Alessandro. University of Florence; Italia Fil: Bonamore, Alessandra. Università degli studi di Roma "La Sapienza"; Italia Fil: Boffi, Alberto. Università degli studi di Roma "La Sapienza"; Italia Fil: Gellini, Cristina. University of Florence; Italia Fil: Salvi, Pier R.. University of Florence; Italia Fil: Estrin, Dario Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Bruno, Stefano. Università di Parma; Italia Fil: Viappiani, Cristiano. Università di Parma; Italia Fil: Foggi, Paolo. European Laboratory for Non-Linear Spectroscopy; Italia. University of Perugia; Italia

In this work, we present the characterization of an enantiomeric pair of fluorescent dye organogelators and the properties of their stable gel at low concentration in organic solvents. The gels of both enantiomers and of their mixtures were analyzed by differential scanning calorimetry, circular dichroism (CD), atomic force microscopy, UV–vis absorption, and fluorescence. The acquired data were supported by molecular modeling of the helical assembly of the gelators and by the simulation of their CD spectra by means of DeVoe method, and suggested the occurrence of an enantiomeric discrimination process during the formation of the gels. Chirality, 2011. © 2011 Wiley-Liss, Inc.