Zero-field splitting of methyl-substituted phosphorescent aromatic molecules in their lowest triplet states

The effect of methyl substitution on the ZFS of phosphorescent aromatic compounds in their lowest triplet states has been investigated for both homocyclic and heterocyclic molecules. The experimental results of ESR measurements on fifteen molecules are given. The ZFS parameters have been calculated theoretically applying the semiempirical WM method. The experimental and theoretical D and E values are briefly discussed.     

The lowest singlet and triplet excited states of pyrazine

The phosphoresence, fluorescence and absorption spectra of pyrazine-h₄ and -d₄ have been obtained at 4°K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine-h₄ in d₄ were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure.The coincidence of origins of absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as ${}^1\text{B}{}_{\mathrm{3u}}\text{←}{}^1\text{A}{}_{\mathrm{1g}}$. The forbidden component ¹B_2g, predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain.The phosphorescence spectrum, when compared with the singlet-triplet excitation spectrum, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and the coincidence of origins of absorption and emission. In accordance with previous work, the triplet state is designated as ³B_3u.The vibronic structure of the phosphorescence spectrum is very complicated. The first work on the analysis of this spectrum concluded that a simple progression of ν_6a exists. Under the high resolution attainable in the present experiments, the supposed ν_6a progression proves to have a composite structure, starting from the second member of the progression. Not only is the ν_9a hydrogen-bending mode present as shown by the appearance of the CD bending mode in the d₄ spectrum, but a band assigned as 2ν_6b was also identified. The latter's anomalous intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with 2ν_6a and ν_9a bands. To help resolve the present controversy over the origin of the phosphoresence spectrum observed in crystalline pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.     

Fine-structure phosphorescence and radiative deactivation of the lowest triplet states in the most toxic dioxins

The transition dipole moments for the transition T₁(ππ*) → S₀ to vibrational energy levels of the nontotally symmetric vibrational modes of 2,3,7,8-tetrachloro-and 1,2,3,7,8-pentachlorodibenzo-p-dioxins are calculated. The interpretation of the fine-structure phosphorescence spectrum of the first of these compounds is refined, and the radiative deactivation rate constants for the s sublevels of the lowest triplet state T₁ are estimated. For a number of polychlorinated compounds, the effect of chlorine atoms occupying the α and β positions in a molecule on the T^s → S₀ transition dipole moments is discussed.     

Bimolecular gas-phase processes with the participation of vibrationally excited triplet molecules of aromatic ketones

Time-resolved moderated luminescence of aromatic ketones (benzophenone, aceptophenone) in the presence of neutral and reactive hydrogeneous foreign gases (ethylene, pentane, triethylamine) is investigated. It is shown that the addition of hydrogeneous foreign gases leads to strong quenching of the triplet molecules of aromatic ketones. Effectivenesses of such bimolecular processes as the collision detachment of a hydrogen atom and establishment of vibrational and thermal equilibrium are compared. It is concluded that the vibrational degrees of freedom of the acceptors, the aromatic ketones, are inactive in the photochemical process of collision detachment of a hydrogen atom that takes place after the establishment of vibrational equilibrium. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 73–77, January–February, 1999.     

Polarization of phosphorescence transitions and probabilities of intramolecular nonradiative deactivation of the lowest triplet state of aromatic molecules

This study continues the experimental testing of the validity of the inductive resonance theory of dipole-dipole energy transfer from the T ₁→S ₀ transition dipole to stretching vibrations of intramolecular CH bonds of naphthalene and its hydroxy derivatives. To this end, in the series of compounds under study, the range of variation of the geometrical parameter [Φ(CH)]² of the Förster theory, which accounts for the mutual orientation of the energy donor and acceptor, is estimated. Preliminarily, the angles between the transition dipole moments of the radiative and absorptive electronic transitions (T ₁→S ₀ and S ₀→S ₁; T ₁→S ₀ and S ₀→S ₂; S ₁→S ₀ and S ₀ →S ₁; and S ₁→S ₀ and S ₀→S ₂) are measured at 77 K by the method of polarization photoselection. From the polarization measurements, the angles between the phosphorescence transition dipole moment and the plane of a molecule are determined. It was found that, upon passage from naphthalene to its β derivatives, the orientation of the dipole moment of the radiative T ₁→S ₀ transition relative to the plane of a molecule markedly changes, with the in-plane component of the dipole moment being increased by an order of magnitude. The experimentally determined rate constants of nonradiative deactivation of the T ₁ state averaged over the CH groups of the naphthalene ring system, k _nr(CH), are compared with the rate constants [Φ(CH)]² of the inductive resonance energy transfer from the dipole of the T ₁→S ₀ transition to the dipole of the CH vibrations polarized in the plane of a molecule, calculated with regard to the orientational factor [Φ(CH)]². This comparison showed that, in the series of compounds under study, a change in the orientation of the dipole moment of the radiative T ₁→S ₀ transition relative to the plane of a molecule does not affect the rate of the nonradiative T ₁?S ₀ transition. This inference is confirmed by the absence of a correlation between the rate constants k _dd(CH) calculated by us (with regard to [Φ(CH)]²) and the well-known rate constants k _nr(CH) of individual sublevels of the T ₁ state measured at T≤1.35 K for a number of organic molecules. The possible sources of discrepancy between the experimental data that k _nr(CH) is independent of [Φ(CH)]² and the predictions of the theory are considered. A conclusion is made that the electronic-vibrational energy transfer between electric dipoles is the most probable mechanism of the T ₁?S ₀ transitions, but the rate constant of the dipole-dipole energy transfer upon interaction of the electronic and vibrational dipoles in a molecule does not depend on their orientations.     

The lowest triplet state of Zn porphin

The dynamics of populating and depopulating the phosphorescent triplet state of Zn porphin in an n-octane matrix is investigated by microwave induced delayed phosphorescence experiments at 1·2K. Values for the absolute rates of decay of the three spin levels are given and also the relative steady-state populations, radiative decay rates and probabilities for entry through intersystem crossing. Further, it is established that the phosphorescence is inplane polarized. The results are compatible with the decay modes expected for a lowest vibronic state that deviates from D ₄ symmetry by combined action of the Jahn-Teller effect and crystal field anisotropy.     

Nonradiative deactivation of the lowest triplet state of tetrachlorodibenzo-p-dioxin

In the nonadiabatic approximation, we have studied how the shape of promoting out-of-plane vibrational modes and vibronically induced spin-orbit interactions in structural elements of the 2,3,7,8-tetrachlorodibenzo-p-dioxin molecule affect the energy degradation rate constants K _dg ^s of triplet T ₁ ^s sublevels.     

Studies of hyperfine splittings and spin distributions of the lowest excited triplet states of substituted benzenes and pyridines

Hyperfine splittings and linewidths of the E.P.R. spectra of the lowest excited triplet states of several cyano-substituted benzenes, pyridines and 1,2,4,5-tetrachlorobenzene were studied, and spin distributions were estimated. These spin distributions as well as the zero field parameters were discussed in terms of the modified benzene π electron MO. It is shown that one excited configuration dominates in the studied systems, but a small amount of configuration mixing is necessary in order to explain ZFS. The experimentally determined spin densities were compared with those calculated from UHF methods in the case of benzonitrile and phenylacetylene. The calculated spin densities agree well with those estimated experimentally when proper choice of integral is made. It is also shown that the spin distributions in triplet state and radical anions are similar in the systems studied here. The geometries of the excited triplet states of substituted benzenes are also investigated from UHF calculation.     

Semi-empirical calculations of the magnetic properties of condensed hydrocarbons

Coupled Hartree-Fock perturbation theory is used to calculate the non-local contribution to ¹H chemical shifts and magnetic susceptibilities of alternant benzenoid, alternant non-benzenoid and non-alternant conjugated hydrocarbons. It is shown that the agreement between theory and experiment is satisfactory. For the calculation of the coupled first-order orbitals a simplified iterative procedure is proposed.     

Semi-empirical calculations of hydrogen defects in silicon

Calculations for hydrogen defect(s) in a monovacancy silicon cluster yield a stable position for this defect which: (a) does not saturate any of the silicon dangling bonds; and (b) contributes defect level(s) in the gap whose implications remain to be understood.     

The lowest triplet state of the biexciton may be bound

Using Haken's hamiltonian and a simple variational wavefunction, the (1_σg, 1_σu)³Σ_u state of the biexciton in a polar crystal is found to be stable with respect to dissociation into two ground-state excitons, for ceratain range of (realistec) material parameters, unlike the corresponding state in the H₂ molecule.     

Semi-empirical calculations for the ranges of fast ions in silicon

A semi-empirical method is proposed to calculate the ion ranges in energy region E?=?0.025–10?MeV/nucleon. The dependence of ion ranges on the projectile nuclear charge, mass and velocity is analysed. The calculations presented for ranges of ions with nuclear charges Z?=?2–10 in silicon are compared with SRIM results and experimental data.     

Excited singlet and triplet states of aromatic hydrocarbons determined by a laser photolysis technique

A kinetic spectrophotometric technique, employing a Q-switched, frequency-doubled ruby laser as the excitation source, has been used to determine the short-lived (S₁ → S_p), and long-lived (T₁ → T_q) transient absorption spectra of several benzenoid aromatic hydrocarbons. The excited singlet states determined in this way are compared with the singlet states observed by absorption from the ground state (S₀ → S_n). Some previously assigned triplet states are reassigned as singlet levels.     

New triplet states of SiO

Two new triplet states of SiO (³Σ^? and ³Π_r) which were recently predicted by ab initio calculations (J. M. Robbe, J. Schamps, H. Lefebvre-Brion, and G. Raseev, J. Mol. Spectrosc.74, 375 (1979)) were observed in emission. Fragmentary rotational analyses were performed for two transitions: ³Σ-b³Π_r and ³Π_r-b³Π_r. The following molecular constants (in cm^?1) were derived for the two new electronic states: ³Σ: T₀ ≤ 53 937, B = 0.619; ${}^3\text{Π}{}_{\mathrm{r}}\text{: T}{}_0\text{≤ 52 513, B(}\text{Π}{}_0\text{) = 0.634, A ? +23,}\text{Δ}\text{G(}\text{1}\text{2}\text{) = 589}$.     

Symmetries in the triplet superconducting states

Depending upon the point-group symmetry of the system several phases occur within the superconducting state. The gap parameter for four different point group symmetries is calculated.     

Mechanism of the electron spin polarization of excited triplet states caused by the mutual annihilation of triplet states

A new mechanism of the triplet spin polarization is considered qualitatively and quantitatively. The spin polarization arises due to the spin-selective triplet-triplet annihilation and the subsequent spin dynamics in the spin-correlated pairs of triplets. With the time-dependent perturbation theory the simple rules are found which specify the polarization pattern of triplets.