73 research outputs found
First-Principles Semiclassical Initial Value Representation Molecular Dynamics
A method for carrying out semiclassical initial value representation
calculations using first-principles molecular dynamics (FP-SC-IVR) is
presented. This method can extract the full vibrational power spectrum of
carbon dioxide from a single trajectory providing numerical results that agree
with experiment even for Fermi resonant states. The computational demands of
the method are comparable to those of classical single-trajectory calculations,
while describing uniquely quantum features such as the zero-point energy and
Fermi resonances. By propagating the nuclear degrees of freedom using
first-principles Born-Oppenheimer molecular dynamics, the stability of the
method presented is improved considerably when compared to dynamics carried out
using fitted potential energy surfaces and numerical derivatives.Comment: 5 pages, 2 figures, made stylistic and clarity change
Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials
Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin-orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. Here we present a general design rule and a method to effectively reduce radiationless transitions and hence greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via Diels-Alder click chemistry is devised as a method. A sharp increase in phosphorescence quantum efficiency is observed in a variety of polymer matrices with this method, which is ca. two to five times higher than that of phosphor-doped polymer systems having no such covalent linkage.ope
Kinetic model for UV/H2O2 degradation of 8-methoxypsoralen
The influence of Н2О2 on the degradation of 8-methoxypsoralen (8-MOP) in water-ethanol solutions under the action of KrCl and XeBr excilamp radiation in a photoreactor is investigated. A kinematic model of photodegradation of the investigated molecule is constructed. In water-ethanol solutions the addition of Н2О2 altered the mechanism of decay of 8-MOP under the action of a KrCl excilamp in comparison with irradiation by a XeBr excilamp. This behavior is explained by the fact that the action of 283 nm radiation leads to accumulation of a stable photoproduct. In order to establish the toxicity of this product further research is needed
Electronic excitation and singlet-triplet coupling in uracil tautomers and uracil-water complexes
Electronic spectra of uracil in its diketo (lactam) form and five
enol (lactim) tautomeric forms have been investigated by means of combined
density functional and configuration interaction methods. We have
simulated the effects of hydrogen bonding with a protic solvent by
recomputing the spectrum of uracil in the presence of two, four, or six
water molecules. Geometries of the electronic ground state and several
low-lying excited states have been optimized. Spin-orbit coupling has been
determined for correlated wavefunctions employing a non-empirical spin-orbit
mean-field approach.
In accord with experiment, we find the diketo tautomer to be the most stable
one. The calculations confirm that the first absorption band arises from the
1() excitation. The experimentally observed
vibrational structure in this band originates from a breathing mode of the
six ring. Complexation with water molecules is seen to cause a significant
blue shift of excitations while leaving excitations
nearly uninfluenced. Computed radiative lifetimes are presented for the
experimentally known weak phosphorescence from the excited T1
state. Among the uracil lactim tautomers, one is particularly interesting
from a spectroscopic point of view. In this tautomer, the
excitation gives rise to the S1 state
Protonation-State-Driven Photophysics in Phenothiazinium Dyes: Intersystem Crossing and Singlet-Oxygen Production
The impact of altering the solvent pH value on the photodynamic activity of thionine has been studied computationally by means of density functional theory and multi‐reference interaction methods. To this end, we have investigated the electronic structure of the ground and excited states of diprotonated (TH22+) and neutral imine (T) forms of thionine (TH+). It is well known experimentally that the T1 state of TH+ undergoes acid–base equilibrium reactions resulting in a pronounced pH effect for the efficiency of singlet‐oxygen (1O2) production. Our results show that the energy‐transfer reactions from the T1 state of TH22+ and T to 3O2 correspond to reversible equilibrium processes, whereas in TH+ this process is very exothermic in a vacuum (−0.66 eV) and in aqueous solution (−0.49 eV). These facts explain the experimental observation of a much smaller efficiency of 1O2 production for TH22+ than for TH+. Moreover, we found that the pH value significantly effected the intersystem crossing (ISC) kinetics impacting the concentration of triplet‐state species available for energy transfer. In very acidic aqueous solution (pH22+ is the prevailing species, the ISC proceeds with a rate constant of ≈108 s−1. In a basic medium where T is the dominant species, ISC decay occurs by means of a thermally activated channel (≈108 s−1) which competes with fluorescence (5.32×107 s−1). According to these results, maximum ISC efficiency is expected for intermediate acidic pH values (TH+, ≈109 s−1)
Rotationally resolved electronic spectroscopy of 2,3-bridged indole derivatives: Tetrahydrocarbazole
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Vibronic coupling in indole: II. Investigation of the l-1(a)-l-1(b) interaction using rotationally resolved electronic spectroscopy
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