Long-lived excited-state dynamics of i-motif structures probed by time-resolved infrared spectroscopy

UV-generated excited states of cytosine (C) nucleobases are precursors to mutagenic photoproduct formation. The i-motif formed from C-rich sequences is known to exhibit high yields of long-lived excited states following UV absorption. Here the excited states of several i-motif structures have been characterized following 267 nm laser excitation using time-resolved infrared spectroscopy (TRIR). All structures possess a long-lived excited state of ~300 ps and notably in some cases decays greater than 1 ns are observed. These unusually long-lived lifetimes are attributed to the interdigitated DNA structure which prevents direct base stacking overlap.

Excited-state absorption spectroscopy in oxidized Cytochrome c

This work reports on the excited-state absorption spectrum of oxidized Cytochrome c (Fe(3+)) dissolved in water, measured with the Z-scan technique with femtosecond laser pulses. The excited-state absorption cross-sections between 460 and 560 nm were determined with the aid of a three-energy-level model. Reverse saturable absorption was observed below 520 nm, while a saturable absorption process occurs in the Q-band, located around 530 nm. Above 560 nm, a competition between saturable absorption and two-photon absorption was inferred. These results show that Cytochrome c presents distinct nonlinear behaviors, which may be useful to study electron transfer chemistry in proteins by one- and two-photon absorption. In addition, owing to these nonlinear optical features, this molecule may be employed in applications involving photodynamics therapy and saturable absorbers. (C) 2009 Elsevier B.V. All rights reserved.

Excited State Absorption of Doped and Undoped Polyanyline

Polyaniline is a conducting polymer with appealing electrical and optical properties, arising from the -conjugation along the polymer backbone. The understanding of its excited state absorption is of prime importance for designing and fabricating optical devices. Here, we report on the study of the excited state absorption of doped and undoped PANI by using femtosecond pulses in the spectral range from 450nm up to 850nm. For undoped PANI, we observed saturation of absorption as well as reverse saturable absorption, depending on the excitation wavelength. For doped PANI, however, only saturable absorption was observed.

Study of absorption spectrum and dynamics evaluation of the indocyanine-green first singlet excited state

Excited state absorption and excited state dynamics of indocyanine-green (ICG) dissolved in dymethyl sulfoxide were measured using white-light continuum Z-scan (WLCZScan) and white-light continuum pump-probe (WLCPP) techniques. The excited state absorption spectrum, obtained through Z-scan measurements, revealed saturable absorption (SA) for wavelengths longer than 630 nm, while reverse saturable absorption (RSA) appeared, as indicated by a band at approximately 570 nm. Both processes were modeled by a three-energy-level diagram, from which the excited state cross-section values were determined. SA and RSA were also observed in pump-probe experiments, with a recovery time in the hundreds of picoseconds time scale due to the long lifetime of the first excited state of ICG. Such results contribute to the understanding of ICG optical properties, allowing application in photonics and medicine. Copyright (C) 2010 John Wiley & Sons, Ltd.

A three-state model for the photophysics of guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated (1)(pi pi* L-a) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L-a)(CI). On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, (1)(pi pi* L-b) and, in particular, (1)(n(o)pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.

Excited-State Dynamics in fac-[Re(CO)(3)(Me(4)phen)(L)](+)

Excited-state dynamics in fac-[Re(CO)(3)(Me(4)phen)(cis-L)](+) (Me(4)phen = 3,4,7,8-tetramethyl-1,10-phenanthroline, L = 4-styrylpyridine (stpy) or 1,2-bis(4-pyridyl)ethylene (bpe)) were investigated by steady-state and time-resolved techniques. A complex equilibrium among three closely lying excited states, 3IL(cis-L), (3)MLCT(Re -> me4phen), and (3)IL(Me4phen), has been established. Under UV irradiation, cis-to-trans isomerization of coordinated cis-L is observed with a quantum yield of 0.15 in acetonitrile solutions. This photoreaction competes with radiative decay from (3)MLCT(Re -> Me4phen) and (3)IL(Me4phen) excited states, leading to a decrease in the emission quantum yield relative to the nonisomerizable complex fac-[Re(CO)(3)(Me(4)phen)(bpa)](+) (bpa = 1,2-bis(4-pyridyl)ethane). From temperature-dependent time-resolved emission measurements in solution and in poly(methyl methacrylate) (PMMA) films, energy barriers (Delta E(a)) for interconversion between (3)MLCT(Re -> me4Phen) and (3)IL(Me4phen) emitting states were determined. For L = cis-stpy, Delta E(a) = 11 (920 cm(-1)) and 15 kJ mol(-1) (1254 cm(-1)) in 5:4 propionitrile/butyronitrile and PMMA, respectively. For L = cis-bpe, Delta E(a) = 13 kJ mol(-1) (1087 cm(-1)) in 5:4 propionitrile/butyronitrile. These energy barriers are sufficient to decrease the rate constant for internal conversion from higher-lying (3)IL(me4phen) state to (3)MLCT(Re -> Me4phen), k(i) congruent to 10(6) s(-1). The decrease in rate allows for the observation of intraligand phosphorescence, even in fluid medium at room temperature. Our results provide additional insight into the role of energy gap and excited-state dynamics on the photochemical and photophysical properties of Re(I) polypyridyl complexes.

A CASSCF/MRCI Study of the Low-Lying Electronic States of the BeS Molecule

Accurate potential energy curves, dipole moment functions, dissociation energies, and molecular constants for several low-lying singlet and triplet electronic states of BeS were investigated using the CASSCF/MRCI methodology, and the cc-pV5Z basis set for beryllium, and the aug-cc-pV(5+d) Z set for sulfur. Besides presenting improved results for the three lowest lying states, this study presents the first theoretical characterization of another set of nine excited states so far unknown experimentally. Our results are sufficiently accurate to reliably guide the experimental search and characterization of these states, and also to confirm the experimental assignment of the B(1)Sigma(+) excited state. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 1694-1700, 2011

Excited-state dynamics of polyfluorene derivatives in solution

The excited-state dynamics of two polyfluorene copolymers, one fully conjugated containing phenylene vinylene units alternated with 9,9`-dihexylfluorenyl groups and the other segmented by -(CH2)(8)- spacer, were studied in dilute solution of different solvents using a picosecond single-photon timing technique. The excited-state dynamics of the segmented copolymer follows the Forster resonant energy-transfer model which describes intrachain energy-transfer kinetics among random oriented chromophores. Energy transfer is confirmed by analysis of fluorescence anisotropy relaxation with the measurement of a short decay component of about 60 ps. The fluorescence decay surface of the fully conjugated copolymer is biexponential with decay times of about 470 and 900 ps, ascribed to deactivation of chain moieties containing trans and cis isomers already in a photostationary condition. Thus, energy transfer is very fast due to the conjugated nature and rigid-rod-like structure of this copolymer chain.

Setting the preheating and steady-state operation of electronic ballasts, considering electrodes of hot-cathode fluorescent lamps

This paper presents a new methodology for the adjustment of the preheating process and steady-state operation of electronic ballasts intended for hot-cathode fluorescent lamps. The classical series-resonant parallel-loaded half-bridge inverter is the power stage analyzed in this paper. In addition, the preheating process is based on the imposition of a constant rms current through the electrodes, in order to provide a proper value of the R-h/R-c ratio before the lamp start. According to the proposed methodology, it is possible to set suitable operating points for, the electronic ballast, considering optimal conditions for the lamps electrodes. Therefore, the proposed methodology for setting the preheating and steady-state operation is a complete platform to the design of electronic ballasts for hot-cathode fluorescent lamps.

Prediction of a weakly bound excited state in the He-4(2)-Li-7 molecule

A scale-independent approach, valid for weakly bound three-body systems, is used to analyze the existence of excited Thomas-Efimov states in molecular systems with three atoms: a helium dimer together with isotopes of lithium (Li-6 and Li-7) and sodium (Na-23). With the present study and the available data, we can clearly predict that the He-4(2)-Li-7 system supports an excited state with binding energy close to 2.31 mK. (C) 2000 American Institute of Physics. [S0021-9606(00)30442-1].

Pump excited state absorption in holmium-doped fluoride glass

The primary excited state absorption processes relating to the (5)I(6) -> (5)I(7) 3 mu m laser transition in singly Ho(3+)-doped fluoride glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the (5)I(6) and (5)I(7) energy levels established the occurrence of two excited state absorption transitions from these energy levels that compete with previously described energy transfer upconversion processes. The (5)I(7) -> (5)I(4) excited state absorption transition has peak cross sections at 1216 nm (sigma(esa)=2.8x10(-21) cm(2)), 1174 nm (sigma(esa)=1x10(-21) cm(2)), and 1134 nm (sigma(esa)=7.4x10(-22) cm(2)) which have a strong overlap with the (5)I(8) -> (5)I(6) ground state absorption. on the other hand, it was established that the excited state absorption transition (5)I(6) -> (5)S(2) had a weak overlap with ground state absorption. Using numerical solution of the rate equations, we show that Ho(3+)-doped fluoride fiber lasers employing pumping at 1100 nm rely on excited state absorption from the lowest excited state of Ho(3+) to maintain a population inversion and that energy transfer upconversion processes compete detrimentally with the excited state absorption processes in concentrated Ho(3+)-doped fluoride glass. (c) 2008 American Institute of Physics.

Electron transfer by singlet excited state of porphyn and electron acceptor affinity in rigid medium: photoacoustic phase angle analysis

A espectroscopia fotoacústica obtém informações sobre amplitude e fase, da resposta de um sistema submetido a excitação por luz. Este artigo apresenta estudos do ângulo de fase no processo de transfereência de elétrons entre octaetilporfirina (OEP) e derivados de quinona ambos dispersos em uma matriz polimérica. Observou-se uma tendência no comportamento da fase para valores menores na região espectral próximo de 620 nm. Enquanto que para comprimentos de onda menores este efeito não foi apresentado. Estas medidas sugerem que a transferência de elétrons para o aceitador ocorreu com a participação do estado singleto excitado da octaetilporfirina.

Excited state dynamics of the Ho3+ ions in holmium singly doped and holmium, praseodymium-codoped fluoride glasses

The deactivation of the two lowest excited states of Ho3+ was investigated in Ho3+ singly doped and Ho3+, Pr3+-codoped fluoride (ZBLAN) glasses. We establish that 0.1-0.3 mol % Pr3+ can efficiently deactivate the first excited (I-5(7)) state of Ho3+ while causing a small reduction of similar to 40% of the initial population of the second excited (I-5(6)) state. The net effect introduced by the Pr3+ ion deactivation of the Ho3+ ion is the fast recovery of the ground state of Ho3+. The Burshstein model parameters relevant to the Ho3+-> Pr3+ energy transfer processes were determined using a least squares fit to the measured luminescence decay. The energy transfer upconversion and cross relaxation parameters for 1948, 1151, and 532 nm excitations of singly Ho3+-doped ZBLAN were determined. Using the energy transfer rate parameters we determine from the measured luminescence, a rate equation model for 650 nm excitation of Ho3+-doped and Ho3+, Pr3+-doped ZBLAN glasses was developed. The rate equations were solved numerically and the population inversion between the I-5(6) and the I-5(7) excited states of Ho3+ was calculated to examine the beneficial effects on the gain associated with Pr3+ codoping. (c) 2007 American Institute of Physics.

Energy of the Photosubstitutionally Reactive Excited State of Pentaammine(pyridine)ruthenium(II)

The photosensitized aquation of pentaammine(pyridine)ruthenium(II) by several dyes has been studied under conditions where only the sensitizers absorb light. The ratio of the quantum yields for ammine and pyridine substitution was the same as that for direct photoaquation. Sensitization was effective with singlet sensitizers Rhodamine-B (17 452 cm -1) and Safranine-T (17 690 cm -1), as well as the triplet sensitizer biacetyl (19 000 cm -1), but no reaction was observed with Neutral-Red (16 900 cm -1). The results indicate that the excited state precursor of the observed photosubstitution in the complex lies in the energy range between 17 000 and 17 700 cm -1.