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.

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.

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.

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.

Ground and excited state charmonium production in p plus p collisions at root s=200 GeV

We report on charmonium measurements [J/psi (1S), psi' (2S), and chi(c) (1P)] in p + p collisions at root s = 200 GeV. We find that the fraction of J/psi coming from the feed-down decay of psi' and chi(c) in the midrapidity region (vertical bar y vertical bar < 0: 35) is 9.6 +/- 2.4% and 32 +/- 9%, respectively. We also present the p(T) and rapidity dependencies of the J/psi yield measured via dielectron decay at midrapidity (vertical bar y vertical bar < 0.35) and via dimuon decay at forward rapidity (1.2 < vertical bar y vertical bar < 2.2). The statistical precision greatly exceeds that reported in our previous publication [Phys. Rev. Lett. 98, 232002 (2007)]. The new results are compared with other experiments and discussed in the context of current charmonium production models.

Quantum deformation of the angular distributions of synchrotron radiation. Emission from particles in the first excited state

The exact expressions for the characteristics of synchrotron radiation of charged particles in the first excited state are obtained in analytical form using quantum theory methods. We performed a detailed analysis of the angular distribution structure of radiation power and its polarization for particles with spin 0 and 1/2. It is shown that the exact quantum calculations lead to results that differ substantially from the predictions of classical theory.

Study of singlet excited state absorption spectrum of lutetium bisphthalocyanine using the femtosecond Z-scan technique

In this study we investigate the singlet excited state absorption of lutetium bisphthalocyanine (LuPc2) over a wide spectral range. It was observed distinct nonlinear absorption behaviors; saturable (SA) and reverse saturable absorption (RSA). The RSA effect was observed below 640 and above 680 nm, while SA occurs around the Q-band region, located around 660 nm. To describe the main singlet-singlet transitions, we employed the rate equation model considering the simplified three-energy level diagram. Our results reveal a ratio between excited and ground state absorption smaller than 0.05 at the Q-band region, and of approximately 4 for the other regions. (C) 2012 Elsevier B.V. All rights reserved.

Effect of interaction with micelles on the excited-state optical properties of zinc porphyrins and J-aggregates formation

This work reports on the photophysical properties of zinc porphyrins meso-tetrakis methylpyridiniumyl (Zn2+TMPyP) and meso-tetrakis sulfonatophenyl (Zn2+TPPS) in homogeneous aqueous solutions and in the presence of sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide (CTAB) micelles. The excited-state dynamic was investigated with the Z-scan technique, UV-Vis absorption, and fluorescence spectroscopy. Photophysical parameters were obtained by analyzing the experimental data with a conventional five-energy-level diagram. The interaction of the charged side porphyrin groups with oppositely charged surfactants can reduce the electrostatic repulsion between porphyrin molecules leading to aggregation, which affected the porphyrin characteristics such as absorption cross-sections, lifetimes and quantum yields. The interaction between anionic ZnTPPS with cationic CTAB micelles induced the formation of porphyrin J-aggregates, while this effect was not observed in the interaction of ZnTMPyP with SDS micelles. This difference is, probably, due to the difference in electrostatic repulsion between the porphyrin molecules. The insights obtained by these results are important for the understanding of the photophysical behavior of porphyrins, regarding potential applications in pharmacokinetics as encapsulation of photosensitizer for drug delivery systems and in its interaction with cellular membrane.

On the Deactivation Mechanisms of Adenine-Thymine Base Pair

In this contribution, the multiconfigurational second-order perturbation theory method based on a complete active space reference wave function (CASSCF/CASPT2) is applied to study all possible single and double proton/hydrogen transfers between the nucleobases in the adenine-thymine (AT) base pair, analyzing the role of excited states with different nature [localized (LE) and charge transfer (CT)] and considering concerted as well as step-wise mechanisms. According to the findings, once the lowest excited states, localized in adenine, are populated during UV irradiation of the Watson-Crick base pair, the proton transfer in the N-O bridge does not require high energy in order to populate a CT state. The latter state will immediately relax toward a crossing with the ground state, which will funnel the system to either the canonical structure or the imino-enol tautomer. The base pair is also capable of repairing itself easily since the imino-enol species is unstable to thermal conversion.

Photoisomerization for a model protonated Schiff base in solution: Sloped/peaked conical intersection perspective

The topographical character of conical intersections (CIs)-either sloped or peaked-has played a fundamental and important role in the discussion of the efficiency of CIs as photochemical "funnels." Here this perspective is employed in connection with a recent study of a model protonated Schiff base (PSB) cis to trans photoisomerization in solution [Malhado et al., J. Phys. Chem. A 115, 3720 (2011)]. In that study, the calculated reduced photochemical quantum yield for the successful production of trans product versus cis reactant in acetonitrile solvent compared to water was interpreted in terms of a dynamical solvent effect related to the dominance, for the acetonitrile case, of S-1 to S-0 nonadiabatic transitions prior to the reaching the seam of CIs. The solvent influence on the quantum yield is here re-examined in the sloped/peaked CI topographical perspective via conversion of the model's two PSB internal coordinates and a nonequilibrium solvent coordinate into an effective branching space description, which is then used to re-analyze the generalized Langevin equation/surface hopping results. The present study supports the original interpretation and enriches it in terms of topographical detail. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754505]

Molecular Dynamics Investigations of PRODAN in a DLPC Bilayer

Molecular dynamics computer simulations have been performed to identify preferred positions of the fluorescent probe PRODAN in a fully hydrated DLPC bilayer in the fluid phase. In addition to the intramolecular charge-transfer first vertical excited state, we considered different charge distributions for the electronic ground state of the PRODAN molecule by distinct atomic charge models corresponding to the probe molecule in vacuum as well as polarized in a weak and a strong dielectric solvent (cyclohexane and water). Independent on the charge distribution model of PRODAN, we observed a preferential orientation of this molecule in the bilayer with the dimethylamino group pointing toward the membrane's center and the carbonyl oxygen toward the membrane's interface. However, changing the charge distribution model of PRODAN, independent of its initial position in the equilibrated DLPC membrane, we observed different preferential positions. For the ground state representation without polarization and the in-cyclohexane polarization, the probe maintains its position close to the membrane's center. Considering the in-water polarization model, the probe approaches more of the polar headgroup region of the bilayer, with a strong structural correlation with the choline group, exposing its oxygen atom to water molecules. PRODAN's representation of the first vertical excited state with the in-water polarization also approaches the polar region of the membrane with the oxygen atom exposed to the bilayer's hydration shell. However, this model presents a stronger structural correlation with the phosphate groups than the ground state. Therefore, we conclude that the orientation of the PRODAN molecule inside the DLPC membrane is well-defined, but its position is very sensitive to the effect of the medium polarization included here by different models for the atomic charge distribution of the probe.

Effect of resonant tunneling on exciton dynamics in coupled dot-well nanostructures

Excitonic dynamics in a hybrid dot-well system composed of InAs quantum dots (QDs) and an InGaAs quantum well (QW) is studied by means of femtosecond pump-probe reflection and continuous wave (cw) photoluminescence (PL) spectroscopy. The system is engineered to bring the QW ground exciton state into resonance with the third QD excited state. The resonant tunneling rate is varied by changing the effective barrier thickness between the QD and QW layers. This strongly affects the exciton dynamics in these hybrid structures as compared to isolated QW or QD systems. Optically measured decay times of the coupled system demonstrate dramatically different response to temperature change depending on the strength of the resonant tunneling or coupling strength. This reflects a competition between purely quantum mechanical and thermodynamical processes.

Evidence of the participation of electronic excited states in the mechanism of positronium formation in substitutional Tb1-xEux(dpm)(3) solid solutions studied by optical and positron annihilation spectroscopies

Positronium formation in the bimary molecular solid solutions Tb1-xEux (dpm)(3) (dpm = dipivaloylmethanate) has been investigated. A strong linear correlation between the D-5(4) Tb(III) energy level excited state lifetime and the positronium formation probability has been observed. This correlation indicates that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronium formation inhibition, as previously proposed. A kinetic mechanism is proposed to explain this correlation and shows that excited electronic states have a very important role in the positronium formation mechanism.

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field//configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T-2 electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T-1 -> T-3 and T-1 -> T-5 transitions, supporting that the intermediate triplet state (T-2) decays by internal conversion to T-1. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4738757]

Luminous efficiency enhancement of PVK based OLEDs with fac-[ClRe(CO)(3)(bpy)]

The luminous efficiency of organic light-emitting diodes based on poly(N-vinylcarbazole), PVK, was improved by adding fac-[ClRe(CO)(3)(bpy)], bpy = 2,2`-bipyridine, to PVK host. Emissive layers with various Re(I) complex/host ratio were employed and optoelectronic properties were compared with the single PVK device. The single PVK device exhibits a characteristic electroluminescence with blue emission, lambda(max) 420 nm, assigned to the PVK excimer. On the other hand, the intense and broad band at lambda(max) 580 nm of the Re(I) complex/PVK OLEDs is ascribed to the metal-to-ligand charge transfer excited state emission of fac-[ClRe(CO)(3)(bpy)]. At 30 V, the device luminous efficiency increased from 16 mcd/A for the single PVK device to 211 mcd/A for the 11% (w/w) Re(I) complex/PVK OLED, in which fac-[ClRe(CO)(3)(bpy)] acts as an electron-trap in PVK films. The device current is space-charge limited and exhibits typical emissive layer thickness dependence. (C) 2011 Elsevier B.V. All rights reserved.