Search results for "Triplet"
showing 10 items of 159 documents
Ab initio study on the low-lying excited states of retinal
1997
Ab initio results for the electronic spectrum of all-trans-retinal and its truncated model 3-methyl-all-trans (10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. The study includes geometry determination of the ground state. Vertical excitation energies have been computed using multiconfigurational second-order perturbation theory through the CASPT2 formalism. The lowest singlet excited state in gas phase is predicted to be of nπ∗ character. The lowest triplet state corresponds, however, to a ππ∗ state. The most intense feature of the spectrum is due to the strongly dipole-allowed ππ∗ transition, in accordance with the observed maximum in the one-photon spectra. The vertical excitation …
Intramolecular Interactions in the Triplet Excited States of Benzophenone–Thymine Dyads
2005
Time-resolved and product studies on the synthesized dyads 1 and 2 have provided evidence that the benzophenone-to-thymine orientation strongly influences intramolecular photophysical and photochemical processes. The prevailing reaction mechanism has been established as a Paterno-Büchi cycloaddition to give oxetanes 3-6; however, the ability of benzophenone to achieve a formal hydrogen abstraction from the methyl group of thymidine has also been evidenced by the formation of photoproducts 7 and 8. These processes have been observed only in the case of the cisoid dyad 1. Adiabatic photochemical cycloreversion of the oxetane ring is achieved upon direct photolysis to give the starting dyad 1 …
Ultrafast Relaxation Dynamics of Osmium−Polypyridine Complexes in Solution
2013
We present steady-state absorption and emission spectroscopy and femtosecond broadband photoluminescence up-conversion spectroscopy studies of the electronic relaxation of Os(dmbp)(3) (Os1) and Os(bpy)(2)(dpp) (Os2) in ethanol, where dmbp is 4,4'-dimethyl-2,2'-biypridine, bpy is 2,2'-biypridine, and dpp is 2,3-dipyridyl pyrazine. In both cases, the steady-state phosphorescence is due to the lowest (MLCT)-M-3 state, whose quantum yield we estimate to be <= 5.0 x 10(-3). For Os1, the steady-state phosphorescence lifetime is 25 ns. In both complexes, the photoluminescence excitation spectra map the absorption spectrum, pointing to an excitation wavelength-independent quantum yield. The ultrafa…
Luminescence-detected EPR of oxygen-fluorine vacancy complexes in CaF2
2009
In several CaF2 single crystals grown by the Bridgman method and doped with CaO or in addition with SrF2 or NaF a luminescence band between 470 nm and 600 nm could be excited at 212 nm, its peak wavelength depending on the doping. With photo-luminescence (PL) detected EPR five spin triplet centres were identified. Their axial fine structure constants D varied from 87 mT to 690 mT whereby the most intense spectra had the smallest D value. Theoretical calculations of the fine structure tensors and superhyperfine interactions show that the most intense and probable triplet centre consists of a pair of an OF– on a F– site next to a nearest neighbour F– vacancy which, compared to the well-known …
Optically detected magnetic resonance investigation of oxygen luminescence centres in BaF2
2002
The structure of two oxygen-related luminescence centres in oxygen-doped BaF2 was investigated by means of photoluminescence (PL) and photoluminescence-detected electron paramagnetic resonance (PL-EPR). One of the oxygen-related luminescences peaking at 2.83 eV is associated with an excited triplet state (S = 1) of an oxygen–vacancy complex with the z-axis of the fine-structure tensor parallel to the 110 direction. This complex can be described as an oxygen on a fluorine lattice site with a next-nearest fluorine vacancy along the 110 direction. The luminescence at 2.25 eV is also associated with a triplet state. Its PL-EPR spectrum is probably due to oxygen–vacancy complexes with a nearest …
Optically detected magnetic resonance investigation of a luminescent oxygen–vacancy complex in Mn-doped LiBaF3
2006
The structure of an oxygen-related luminescence centre in manganese-doped LiBaF 3 was investigated by means of photoluminescence (PL) and PL-detected electron paramagnetic resonance. At 20 K an oxygen-related complex shows two luminescence bands peaking at about 430 and 475 nm, when excited at 220 nm. These bands can be attributed to an excited triplet state (S = 1) of an oxygen-vacancy complex with the z axis of the fine structure tensor parallel to the (110) direction. This complex is believed to be next to a Mn 2+ impurity on a Ba 2+ site and can be described as an oxygen on a fluorine lattice site with a nearest fluorine vacancy along the (110) direction.
Role of diffusing molecular hydrogen on relaxation processes in Ge-doped glass
2007
Temperature dependencies of steady-state and time-resolved photoluminescence (PL) from triplet state at 3.1 eV and singlet state at 4.2 eV ascribed to the twofold-coordinated Ge have been measured in unloaded and H2-loaded Ge-doped silica samples under 5.0 eV excitation in the 10–310 K range. Experimental evidences indicate that diffusing molecular hydrogen (H2) depopulates by a collisional mechanism the triplet state, decreasing both its lifetime of about 14% and the associated triplet PL intensity, whereas those of the singlet are insensitive to the presence of H2.
Temperature dependence of luminescence decay in Sn-doped silica
2005
We report an experimental study on the temperature dependence, in the range 18-300 K, of the decay kinetics of the emission at 4.1 eV from the first excited electronic state of oxygen deficient centers in a 2000 ppm Sn-doped sol-gel silica. At low temperature, this luminescence decays exponentially with a lifetime of 8.4 ns, whereas, on increasing the temperature, the time decay decreases and cannot be fitted with an exponential function. These results are expected if there is a competition between the radiative and the thermally activated intersystem-crossing decay channels toward the associated triplet state. The comparison with previous data in pure oxygen-deficient and Ge-doped silica g…
Bright and stable light-emitting electrochemical cells based on an intramolecularly π-stacked, 2-naphthyl-substituted iridium complex
2014
The synthesis and characterization of a new cationic bis-cyclometallated iridium(III) complex and its use in solid-state light-emitting electrochemical cells (LECs) are described. The complex [Ir(ppy)2(Naphbpy)][PF6], where Hppy = 2-phenylpyridine and Naphbpy = 6-(2-naphthyl)-2,2′-bipyridine, incorporates a pendant 2-naphthyl unit that π-stacks face-to-face with the adjacent ppy− ligand and acts as a peripheral bulky group. The complex presents a structureless emission centred around 595–600 nm both in solution and in thin film with relatively low photoluminescence quantum yields compared with analogous systems. Density functional theory calculations support the charge transfer character of…
Photoluminescence spectral dispersion as a probe of structural inhomogeneity in silica
2011
We perform time-resolved photoluminescence measurements on point defects in amorphous silicon dioxide (silica). In particular, we report data on the decay kinetics of the emission signals of extrinsic oxygen deficient centres of the second type from singlet and directly excited triplet states, and we use them as a probe of structural inhomogeneity. Luminescence activity in sapphire (alpha-Al(2)O(3)) is studied as well and used as a model system to compare the optical properties of defects in silica with those of defects embedded in a crystalline matrix. Only for defects in silica did we observe a variation of the decay lifetimes with emission energy and a time dependence of the first moment…