Search results for "Singlet"

Singlet and triplet excitons in conjugated polymers.

1992

Exciton states in conjugated polymers are theoretically studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions. The relationship between exciton energies and basic interaction parameters is clarified, demonstrating the special nature of one-dimensional excitons. The binding energies of the lowest singlet and triplet excitons depend sensitively upon the on-site Coulomb energy. Relevant experiments in polydiacetylene can be explained by the present model using moderate interaction strength.

Spin dependence of low energy charge exchange between H 2 + and Na

1987

The difference in charge exchange rate in collisions between spin oriented sodium atoms and H 2 + ions has been measured at an energy of about 1 eV. H 2 + was stored in a Penning trap and polarized by spin exchange with Na beam atoms from a hexapole magnet. The ion loss from the trap due to charge exchange was different as we depolarized the atomic beam. From the data we obtain a ratio of cross sections for singlet and triplet collisionsQ 1/Q 3=1.5±0.2 andQ 3=1.2·10−15 cm2.

Ultrafast decay of the excited singlet states of thioxanthone by internal conversion and intersystem crossing.

2010

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high-level ab initio CASPT2 calculations of the singlet- and triplet-state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin-orbit coupling terms. The initially populated singlet pi pi* state is shown to decay through internal conversion and intersystem crossing processes via intermediate n pi* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversio…

A Two-State Computational Investigation of Methane C-H and Ethane C-C Oxidative Addition to [CpM(PH3)]n+ (M=Co, Rh, Ir;n=0, 1)

2006

Reductive elimination of methane from methyl hydride half-sandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH(3))(CH(3))(H)] (M = Co, Rh, Ir). For each metal, the unsaturated product has a triplet ground state; thus, spin crossover occurs during the reaction. All relevant stationary points on the two potential energy surfaces (PES) and the minimum energy crossing point (MECP) were optimized. Spin crossover occurs very near the sigma-CH(4) complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH(4) molecule is almost completely expelled from the metal coordinati…

Decoupling the artificial special pair to slow down the rate of singlet energy transfer

2012

Trimer 2, composed of a cofacial heterobismacrocycle, octamethyl-porphyrin zinc(II) and bisarylporphyrin zinc(II) held by an anthracenyl spacer, and a flanking acceptor, bisarylporphyrin free-base ( Ar = -3,5-(t Bu )2 C 6 H 3), has been studied by means of absorption spectroscopy, "steady state and time-resolved fluorescence" and fs transient absorption spectroscopy, and density functional theory (DFT) in order to assess the effect of decoupling the chromophores' low energy MOs on the rate of the singlet, S1, energy transfer, k ET , compared to a recently reported work on a heavily coupled trimeric system, Trimer 1, [biphenylenebis(n-nonyl)porphyrin zinc(II)]-bisarylporphyrin free-base ( A…

Theoretical Determination of the Singlet → Singlet and Singlet → Triplet Electronic Spectra, Lowest Ionization Potentials, and Electron Affinity of C…

2003

The singlet → singlet and singlet → triplet electronic spectra of cycloocta-1,3,5,7-tetraene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANOs) basis sets. The observed dipole-allowed features at 4.43, 6.02, and 6.42 eV and the spin-forbidden singlet → triplet bands with maxima at 3.05, 4.05, and 4.84 eV (Frueholz, R. P.; Kuppermann, A. J. Chem. Phys. 1978, 69, 3614) are assigned as the transitions 1 1 A 1 → 1 1 A 2 , 1 1 A 1 → 2 1 B 2 (3p z ), 1 1 A 1 → 3 1 E, and 1 1 A 1 - 1 3 A 2 , 1 1 A 1 → 1 3 E, 1 1 A 1 → 1 3 B 1 , respectively. The lowest (3s) Rydberg singlet and triplet states are placed at 5.58 (2 1 A 1 ) and…

DNA damage profiles induced by oxidizing agents

1995

High nuclearity mixed-valence magnetic clusters: theoretical study of the spin coupling in the C602− fulleride ion

1998

Abstract The problem of delocalization of a pair of electrons over the fullerene C 60 is considered, with the aim of elucidating the nature of the ground spin state, as well as the structure of the low-lying energy levels in this mixed-valence molecule C 60 2− . A model that considers the Coulomb interactions between the two extra electrons, as well as the two single-electron transfer processes involved in the electron delocalization, is developed. The influence of these electronic parameters on the spectrum of the low-lying energy levels is discussed. We find that the ground state is always a spin singlet, whatever the relative values of these parameters are.

Mixed-valence polyoxometalate clusters. II. Delocalization of electronic pairs in 18-site heteropoly blues with Wells-Dawson structure

1995

Abstract The problem of delocalization of two electrons in the 18-site Wells-Dawson polyoxometalate is examined from a general approach that takes into account both single- and double-transfer processes, as well as the Coulomb interactions between the two delocalized electrons. The electronic energy levels of this mixed-valence cluster are calculated and the conditions giving rise to the stabilization of a singlet ground spin state for the electronic pair are elucidated. It is shown that the spin pairing results from the simultaneous effects of single- and double-electron transfer processes, which are operative even when the two delocalized electrons are fairly widely separated in the Wells…

Design and photophysical properties of zinc(II) porphyrin-containing dendrons linked to a central artificial special pair.

2011

The click chemistry synthesis and photophysical properties, notably photo-induced energy and electron transfers between the central core and the peripheral chromophores of a series of artificial special pair-dendron systems (dendron = G1, G2, G3; Gx = zinc(II) tetra-meso-arylporphyrin-containing polyimides) built upon a central core of dimethylxanthenebis(metal(II) porphyrin) (metal = zinc, copper), are reported. The dendrons act as singlet and triplet energy acceptors or donors, depending on the dendrimeric systems. The presence of the paramagnetic d(9) copper(II) in the dendrimers promotes singlet-triplet energy transfer from the zinc(II) tetra-meso-arylporphyrin to the bis(copper(II) por…