We consider quantum spin systems with dimerization, which at strong coupling have singlet ground states. To account for strong correlations, the S"1 elementary excitations are described as dilute Bose gas with infinite on-site repulsion. This approach is applied to the two-layer Heisenberg antiferromagnet. Our analytic results for the triplet gap, the excitation spectrum and the location of the...

The electronic structure of an isolated oxygen vacancy in SrTiO3 has been investigated with a variety of ab initio quantum mechanical approaches. In particular we compared pure density functional theory ~DFT! approaches with the Hartree-Fock method, and with hybrid methods where the exchange term is treated in a mixed way. Both local cluster models and periodic calculations with large supercell...

Factors influencing the rate of reverse intersystem crossing (krISC ) in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third-generation heavy-metal-free organic light-emitting diodes (OLEDs). However, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and ...

A density functional theory (DFT) study of {CoNO}(8) cobalt nitrosyl complexes containing the [n,n]tropocoronand ligand (TC-n,n) has revealed a sharp reduction of singlet-triplet gaps as the structures change from near-square-pyramidal (for n = 3) to trigonal-bipyramidal with an equatorial NO (for n = 5, 6). An experimental reinvestigation of [Co(TC-3,3)(NO)] has confirmed that it is not parama...

Different porphyrin conformations are believed to play a role in controlling the cofactor properties in natural tetrapyrrole-protein complexes. In order to study the correlation between macrocycle nonplanarity and physicochemical properties in detail, a series of six porphyrins with graded degree of macrocycle distortion was investigated. These conformationally designed porphyrins are based on ...

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanosecon...

Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ-DBTO2, showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ-DBTO2 has minimal singlet-triplet energy splitting due to its near-rigid molecular geometry. However, the electronic coupling between the local triplet (3...

The electronic structure of CO3 is characterized by equation-of-motion and coupled-cluster methods. C(2v) and D(3h) isomers are considered. Vertical excitation energies, transition dipoles, and the molecular orbital character of the excited states are presented for singlet and triplet manifolds. Ground-state equilibrium structures and frequencies are strongly affected by vibronic interactions w...

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 nonr...

Two different approaches to increase intersystem crossing rates in polymethine-like molecules are presented: traditional heavy-atom substitution and molecular levels engineering. Linear and nonlinear optical properties of a series of polymethine dyes with Br- and Se-atom substitution, and a series of new squaraine molecules, where one or two oxygen atoms in a squaraine bridge are replaced with ...