What is the meaning of lifetime measurement?

The lifetime measurement of molecular excited state has been the subject of many papers and experiments. Very often the experimental data are fitted by single or bi exponential decays which in many case is the best fit that can be done owing the signal to noise ratio. The times constants obtained from these fit are often discussed in term of one species associated with one lifetime: depending on the studied system, the species can be one type of molecule, one isomer from a given molecule or local environment. How justified is this assumption

Excited state of protonated benzene and toluene

International audienceWe present photo-fragmentation electronic spectra of the simplest protonated aromatic molecules, protonated benzene and toluene, recorded under medium resolution conditions and compared with the photo-fragmentation spectrum of protonated pyridine. Despite the resolution and cold temperature achieved in the experiment, the electronic spectra of protonated benzene and toluene are structure-less, thus intrinsically broadened. This is in agreement with the large geometrical changes and the fast dynamic toward internal conversion predicted by ab-initio calculations for protonated benzene (M. F. Rode, A. L. Sobolewski, C. Dedonder, C. Jouvet, and O. Dopfer, J. Phys. Chem. A 113, 5865–5873 (2009)

Photoinduced dynamics in protonated aromatic amino acid

UV photoinduced fragmentation of protonated aromatics amino acids have emerged the last few years, coming from a situation where nothing was known to what we think a good understanding of the optical properties. We will mainly focus this review on the tryptophan case. Three groups have mostly done spectroscopic studies and one has mainly been involved in dynamics studies of the excited states in the femtosecond/picosecond range and also in the fragmentation kinetics from nanosecond to millisecond. All these data, along with high level ab initio calculations, have shed light on the role of the different electronic states of the protonated molecules upon the fragmentation mechanisms

Photoinduced water splitting in pyridine water clusters

International audienceAb-initio calculations predict that pyridine (Py) can act as a photo-catalyst to split water by absorption of a UV photon following the reaction Py-H 2 O + hν → PyH● + OH●. To test this prediction, we performed two types of experiment: in the first, we characterize the electronic spectroscopy of the PyH● radical in the gas phase. In the second, we evidence the reaction through UV excitation of molecular Py-(H2O) n clusters obtained in a supersonic expansion and monitoring the PyH● reaction product. The results show unambigu-ously that PyH● is produced, thus that water is split using pyridine as photo-catalyst. In this paper we show that pyridine can act as a photo-catalytic molecular system, which can dissociate the water covalent bond with UV C light. Water molecule is a ubiquitous system to produce H2 , however VUV light is needed to photo-dissociate the H-OH covalent bond (186 nm, 6.66 eV). Since most of the sunlight reaching the earth is in the visible spectral region, we cannot dissociate the H-OH bond under normal sunlight conditions and it is necessary to design a system that could break the H-OH bond with visible light

Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine

International audienceThe excited state properties of protonated ortho (2-), meta (3-) and para (4-) aminopyridine molecules have been investigated through UV photo fragmentation spectroscopy and excited state couple cluster CC2 calculations. Cryogenic ion spectroscopy allows recording well-resolved vibronic spectroscopy that can be nicely reproduced through Franck Condon simulations of the pp* local minimum of the excited state potential energy surface. The excited state lifetimes have also been measured through a pump-probe excitation scheme and compared to the estimated radiative lifetimes. Although protonated aminopyridines are rather simple aromatic molecules, their deactivation mechanisms are indeed quite complex with unexpected results. In protonated 3-and 4-aminopyridine, the fragmentation yield is negligible around the band origin, which implies the absence of internal conversion to the ground state. Besides, a twisted intramolecular charge transfer reaction is evidenced in protonated 4-aminopyridine around the band origin, while excited state proton transfer from the pyridinic nitrogen to the adjacent carbon atom opens with roughly 500 cm-1 of excess energy

Hydrogen bonds vs. π-stacking interactions in the p-aminophenol⋯p-cresol dimer: an experimental and theoretical study

International audienceThe gas phase structure and excited state lifetime of the p-aminophenol?? ?p-cresol heterodimer have been investigated by REMPI and LIF spectroscopy with nanosecond laser pulses and pump–probe experiments with picosecond laser pulses as a model system to study the competition between p–p and H-bonding interactions in aromatic dimers. The excitation is a broad and unstructured band. The excitedstate of the heterodimer is long lived (2.5 ? 0.5) ns with a very broad fluorescence spectrum red-shifted by 4000 cm?1 with respect to the excitation spectrum. Calculations at the MP2/RI-CC2 and DFT-oB97X-D levels indicate that hydrogen-bonded (HB) and p-stacked isomers are almost isoenergetic in the ground state while in the excited state only the p-stacked isomer exists. This suggests that the HB isomer cannot be excited due to negligible Franck–Condon factors and therefore the excitation spectrum is associated with the p-stacked isomer that reaches vibrationally excited states in the S1 state upon vertical excitation. The excited state structure is an exciplex responsible for the fluorescence of the complex. Finally,a comparison was performed between the p-stacked structure observed for the p-aminophenol?? ?p-cresol heterodimer and the HB structure reported for the (p-cresol)2 homodimer indicating that the differences are due to different optical properties (oscillator strengths and Franck–Condon factors) of the isomers of both dimers and not to the interactions involved in the ground stat

Communication: UV photoionization of cytosine catalyzed by Ag+

The photo-induced damages of DNA in interaction with metal cations, which are found in various environments, still remain to be characterized. In this paper, we show how the complexation of a DNA base (cytosine (Cyt)) with a metal cation (Ag+) changes its electronic properties. By means of UV photofragment spectroscopy of cold ions, it was found that the photoexcitation of the CytAg+ complex at low energy (315-282) nm efficiently leads to ionized cytosine (Cyt+) as the single product. This occurs through a charge transfer state in which an electron from the p orbital of Cyt is promoted to Ag+, as confirmed by ab initio calculations at the TD-DFT/B3LYP and RI-ADC(2) theory level using the SV(P) basis set. The low ionization energy of Cyt in the presence of Ag+ could have important implications as point mutation of DNA upon sunlight exposition.Fil: Taccone, Martín Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Féraud, Geraldine. Aix Marseille Université. Physique des Interactions Ioniques et Moléculaires; FranciaFil: Berdakin, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Dedonder Lardeux, Claude. Aix Marseille Université. Physique des Interactions Ioniques et Moléculaires; FranciaFil: Jouvet, Christophe. Physique des Interactions Ioniques et Moléculaires; FranciaFil: Pino, Gustavo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentin

Photo-fragmentation spectroscopy of benzylium and 1-phenylethyl cations

The electronic spectra of cold benzylium (C6H5-CH2+) and 1-phenylethyl (C6H5-CH-CH3+)cations have been recorded via photofragment spectroscopy. Benzylium and 1-phenylethyl cations produced from electrosprayed benzylamine and phenylethylamine solutions, respectively, were stored in a cryogenically cooled quadrupole ion trap and photodissociated by an OPO laser, scanned in parts of the UV and visible regions (600-225 nm). The electronic states and active vibrational modes of the benzylium and 1-phenylethyl cations as well as those of their tropylium or methyl tropylium isomers have been calculated with ab initio methods for comparison with the spectra observed. Sharp vibrational progressions are observed in the visible region while the absorption features are much broader in the UV. The visible spectrum of the benzylium cation is similar to that obtained in an argon tagging experiment [V. Dryza, N. Chalyavi, J.A. Sanelli, and E.J. Bieske, J. Chem. Phys. 137, 204304 (2012)], with an additional splitting assigned to Fermi resonances. The visible spectrum of the 1-phenylethyl cation also shows vibrational progressions. For both cations, the second electronic transition is observed in the UV, around 33 000 cm-1 (4.1 eV), and shows a broadened vibrational progression. In both cases the S2 optimized geometry is non planar. The third electronic transition observed around 40 000 cm-1 (5.0 eV) is even broader with no apparent vibrational structures, which is indicative of either a fast non-radiative process or a very large change in geometry between the excited and the ground states. The oscillator strengths calculated for tropylium and methyl tropylium are weak. Therefore, these isomeric structures are most likely not responsible for these absorption features. Finally, the fragmentation pattern changes in the second and third electronic states: C2H2 loss becomes predominant at higher excitation energies, for both cations

Excited States of Proton-bound DNA/RNA Base Homo-dimers: Pyrimidines

We are presenting the electronic photo fragment spectra of the protonated pyrimidine DNA bases homo-dimers. Only the thymine dimer exhibits a well structured vibrational progression, while protonated monomer shows broad vibrational bands. This shows that proton bonding can block some non radiative processes present in the monomer.Comment: We acknowledge the use of the computing facility cluster GMPCS of the LUMAT federation (FR LUMAT 2764

Excited states of protonated DNA/RNA bases

International audienceThe very fast relaxation of the excited states to the ground state in DNA/RNA bases is a necessary process to ensure the photostability of DNA and its rate is highly sensitive to the tautomeric form of the bases. Protonation of the bases plays a crucial role in many biochemical and mutagenic processes and it can result in alternative tautomeric structures, thus making important the knowledge of the properties of protonated DNA/RNA bases. We report here the photofragmentation spectra of the five protonated DNA/RNA bases. In most of the cases, the spectra exhibit well resolved vibrational structures, with broad bands associated with very short excited state lifetimes. The similarity between the electronic properties e.g. excitation energy and very short excited state lifetimes for the canonical tautomers of protonated and neutral DNA bases, suggests that the former could also play an important role in the photostability mechanism of DNA