Boris F. Minaev

Response Theory and Calculations of Spin-Orbit Coupling Phenomena in Molecules

We review response theory and calculations of molecular properties involving spin-orbit interactions. The spin-orbit coupling is evaluated for reference states described by single- or multi-configuration self-consistent field wave functions. The calculations of spin-orbit related properties rest on the formalism of linear and quadratic response functions for singlet and triplet perturbations when no permutational symmetry in the two-electron operators is assumed and from which various triplet as well as singlet response properties are derived. The spin-orbit coupling matrix elements between singlet and triplet states are evaluated as residues of (multi-configuration) linear response functions, and are therefore automatically determined between orthogonal and non-interacting states. Spin-forbidden radiative transition intensities and lifetimes are determined from the spin-orbit coupling induced dipole transitions between two electronic states of different multiplicity and are obtained as residues of quadratic response functions. The potential of the theory and its range of applications is illustrated by a selection of recent investigations covering different molecular phenomena. The applications include second-order energy contributions, intensity rearrangement in electron spectra, calculation of predissociative lifetimes of dicationic states, assignment of triplet bands in absorption spectra, intersystem crossings and reactivity, external heavy atom effects on S-T transitions, phosphorescence spectra and radiative lifetimes of triplet states. We give an outlook on spin-orbit interaction induced phenomena in extended systems and on applications to general spin catalysis phenomena.

Theoretical study of the cyclometalated iridium(III) complexes used as chromophores for organic light-emitting diodes.

Time-dependent density functional theory with linear and quadratic response technology is used to calculate electronic structure, spectra, and spin-orbit coupling effects for analysis of the main mechanism for phosphorescence of the recently synthesized iridium complex [bis(2-phenylpyridine)(2-carboxy-4-dimethylaminopyridine)iridium(III)]. This compound exhibits strong green phosphorescence which is used in solution processable organic light-emitting diode devices (OLEDs) to overcome the efficiency limit imposed by the formation of triplet excitons. Attempting to foresee new structure-property relations that can guide an improved design of OLED devices based on phosphorescence of the lowest triplet state, we have conducted a theoretical analysis of the photophysical properties of a series of iridium cyclometalated complexes.

Spin uncoupling in surface chemisorption of unsaturated hydrocarbons

Unsaturated hydrocarbons, such as acetylene and ethylene, show strong geometrical distortions when coordinated to transition metals or to surfaces; the bonding is normally analysed in terms of a π-donation—π*-backdonation process. In the present work we use chemisorption of the unsaturated hydrocarbons (ethylene, acetylene, and benzene) on cluster models of the copper (100), (110), and (111) surfaces to demonstrate the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of the adsorbate at the surface. By comparison to the structures of the triplet excited states in the gas phase we demonstrate that these must be considered as the states actually involved in the bonding. This implies a spin-uncoupling in both adsorbate and substrate as part of the chemisorption process or bond formation. In particular, for benzene we identify the quinoid gas phase triplet state as the specific state that is most strongly bound to the Cu(110) substrate; the structure ...

Ab initio calculations of zero-field splitting parameters

We present calculations of electron spin-spin (SS) coupling strengths evaluated as expectation values over multi-configuration and restricted high-spin self-consistent field wave functions. Togethe ...

Ab initio calculations of electronic g-factors by means of multiconfiguration response theory

Abstract A response theory algorithm is derived for calculations of the second-order contribution to the electronic g -tensor, that is the combined perturbation of the orbital Zeeman effect and spin–orbit coupling, for single- and multiconfiguration reference states. Sample calculations are carried out for the NH and O 2 molecules, comparing with experimental data and testing different parametrizations of the wavefunctions.

Magnetic phosphorescence of molecular oxygen. A study of the b1Σg+-X3Σg− transition probability using multiconfiguration response theory

Abstract Multiconfigurational response function calculations of the magnetic dipole transition moment for the red atmospheric emission band b 1 Σ g + -X 3 Σ g − have been performed at different internuclear distance. Spin and orbital angular momentum contributions are calculated as residues of the linear and quadratic response functions using the full Breit-Pauli spin-orbit coupling operator. Vibronic band intensities are calculated by vibrational averaging of the magnetic dipole transition moments and compared with experimental data. The Einstein coefficient for the 0,0 transition probability (0.079 s − ) is obtained 10% smaller than the most recent experimental result of Ritter and Wilkerson (0.089 s − ) but in fair agreement with previous experimental data (0.077–0.0825 s −1 ), while an excellent agreement for all other known vibronic transition probabilities for the red atmospheric band is obtained.

The interpretation of the Wulf absorption band of ozone

Abstract Intensities and energies of the three lowest singlet—triplet transitions of the ozone molecule have been obtained by means of analytic response theory calculations based on multi-configuration wavefunctions. Contrary of the accepted interpretation of a 1 A 2 ←X 1 A 1 transition we find that the lowest electronic band in the ozone spectrum, the Wulf band, is due to a singlet—triplet 3 A 2 ←X 1 A 1 transition (oscillator strength 6 × 10 −7 ). The calculations also explain the absence of observable absorption to the lowest a 3 B 2 state because of negligible oscillator strength (8 × 10 −10 ).

Azatrioxa[8]circulenes: Planar Anti-Aromatic Cyclooctatetraenes

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes.

Diazadioxa(8)circulenes: Planar Antiaromatic Cyclooctatetraenes

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid-mediated oxidative dimerization of 3,6-dihydroxycarbazoles to yield the diazadioxa[8]circulenes in high yields. The synthetic protocol appears to be general, and is a one-pot transformation in which two C-C bonds and two C-O bonds are formed with the loss of two molecules of water. We also present a detailed characterization of the optical and electrochemical properties of this new class of stable planar cyclooctatetraenes. The properties of the diazadioxa[8]circulenes are compared with the properties of isoelectronic tetraoxa[8]circulenes and azatrioxa[8]circulenes. We discuss the antiaromatic nature of the planar central cyclooctatetraene moiety. The antiaromatic nature of the planar cyclooctatetraenes was studied by using computational methods (NICS calculations), and these calculations reveal that the central eight-membered ring has antiaromatic character.

The vibronically induced phosphorescence in benzene

Abstract The phosphorescence spectrum of benzene is particularly rich in vibronic structure and provides the best-known example of a spectrum of electronic transitions that by symmetry is both spin and orbital forbidden. Such transitions, only allowed through the coupling of nuclear and electronic motions, are notoriously difficult to analyze both on theoretical and experimental grounds. We investigate the vibronically induced phosphorescence by means of multi-configuration quadratic response theory calculations and explore vibronic intensities, polarization directions, transition moments for benzene phosphorescence and the radiative lifetime of triplet benzene. We find that the radiative decay of the 3 B 1u state takes place predominantly through vibronic coupling among the e 2g CC stretching modes ν 8 =1601 cm −1 and ν 9 =1178 cm −1 , with a close to complete out-of-plane polarization. The calculations predict relative intensities of different vibronic bands in good agreement with experiment. The oscillator strength for the 3 B 1u ← 1 A 1g absorption is predicted to 0.74×10 −10 , to be compared with the experimental value of ≈10 −10 . The computed averaged radiative lifetime falls in the interval of 22 to 96 seconds, depending on the quality of the basis set, with the best prediction being 64 seconds.