Excited state electronic polarization and reappraisal of the n <- pi* emission of acetone in water

Electronic polarization of the acetone molecule in the excited n -> pi* state is considered and its influence on the solvent shift in the emission spectrum is analyzed. Using an iterative procedure the electronic polarizations of both the ground and the excited states are included and compared with previous results obtained with Car-Parrinello dynamics. Analysis of the emission transition obtained using CIS(D)/aug-cc-pVDZ on statistically uncorrelated solute-solvent structures, composed of acetone and twelve explicit water molecules embedded in the electrostatic field of remaining 263 water molecules, corroborates that the solvent effect is mild, calculated here between 80 and 380 cm (1). (c) 2010 Published by Elsevier B.V.

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.

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.

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.

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.

Investigation of Ground- and Excited-State Photophysical Properties of 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphyrin with Ruthenium Outlying Complexes

The present work employs a set of complementary techniques to investigate the influence of outlying Ru(II) groups on the ground- and excited-state photophysical properties of free-base tetrapyridyl porphyrin (H(2)TPyP). Single pulse and, pulse train Z-scan techniques used M association with laser flash photolysis, absorbance and fluorescence spectroscopy, and fluorescence decay measurements, allowed us to conclude that the presence of outlying Ru(II) groups causes significant changes on both electronic structure and vibrational properties of porphyrin. Such modifications take place mainly due to the activation of. nonradiative decay channels responsible for the emission, quenching, as well as by favoring some vibrational modes in the light absorption process, It is also observed that, differently from what happens when the Ru(II) is placed at the center of the macrocycle, the peripheral groups cause an increase of the intersystem crossing processes, probably due to the structural distortion of the ring that implies a worse spin orbit coupling, responsible for the intersystem crossing mechanism.

Excited-state dynamics of meso-tetrakis(sulfonatophenyl) porphyrin J-aggregates

The excited-state dynamics of free-base meso-tetrakis(sulfonatophenyl) porphyrin J-aggregates obtained by the Z-scan technique in femto- and picosecond time scales, along with UV-Vis spectroscopy and flash photolysis is reported. Besides obtaining the S-1 state lifetime, the discrimination between internal conversion and intersystem crossing nonradiative processes from that state was also possible, and their rates and respective quantum yields were found. The aggregates present reverse saturable absorption at 532 nm for both singlet and triplet excited states. The data shown is important for several applications such as optical limiting, photodynamic therapy and others. (C) 2011 Elsevier B.V. All rights reserved.

Supersymmetric Isospectral Formalism for the Calculation of Near-Zero Energy States: Application to the Very Weakly Bound He-4 Trimer Excited State

We propose a novel mathematical approach for the calculation of near-zero energy states by solving potentials which are isospectral with the original one. For any potential, families of strictly isospectral potentials (with very different shape) having desirable and adjustable features are generated by supersymmetric isospectral formalism. The near-zero energy Efimov state in the original potential is effectively trapped in the deep well of the isospectral family and facilitates more accurate calculation of the Efimov state. Application to the first excited state in He-4 trimer is presented.

On the Mechanisms of Triplet Excited State Population in 8-Azaadenine

The photophysics of 8-azaadenine (8-AA) has been studied with the CASPT2//CASSCF protocol and ANO-L double-zeta basis sets. Stationary equilibrium structures, surface crossings, minimum energy paths, and linear interpolations have been used to study possible mechanisms to populate the lowest triplet state, T-1 (3)(pi pi*), capable of sensitizing molecular oxygen. Our results show that two main mechanisms can occur after photoexcitation to the S-2 (1)(pi pi*) state. The first one is through the S-2/S-1 conical intersection (((1)pi pi*/(1)n pi*)(Cl)), leading to the S-1 ((1)n pi*) state minimum, (S-1 ((1)n pi*))(min), where a singlet-triplet crossing, ((1)n pi*/(3)pi pi*)(STC), is accessible. The second one starts with the ((1)pi pi*/(3)n pi*)(STC) at the (S-2((1)pi pi*))(min), from which the system can evolve to the (T-2 ((3)n pi*))(min), with subsequent population of the T-1 excited electronic state, due to the ((3)n pi*/(3)pi pi*)(Cl) conical intersection.