Pseudo Jahn-Teller Distortion Research Articles

On the origin of some anomalous luminescence behavior of isomeric cyanopyridines

The results of a comparative study of the absorption and emission properties and the triplet yields of the isomeric cyanopyridines (CNP) and benzonitrile under various environmental conditions are reported. All the three CNP isomers are found to be nonfluorescent but exhibit a fairly strong phosphorescence emission of π, π character in rigid polar media at 77 K. ΦP values of the CNP are found to be sensitively dependent on the rigidity (or viscosity) and crystalline structure of the solvent medium. Whereas the 3-CNP molecule phosphoresces with moderately strong intensity in hydrocarbon solvents, the 2 and 4 isomers are completely nonluminescent in glassy or highly disordered crystalline phases of hydrocarbons, e.g., n-alkanes, cyclohexane (CH), and methylcyclohexane (MCH). A dramatic reappearance of the phosphorescence emission from these two isomers (with anomalously large red-shifted spectral positions) has, however, been observed in crystalline CH and MCH matrices. Unlike the 3 isomer, both the 2- and 4-CNP have been found to exhibit phosphorescence in pure crystalline form at 77 K. The n, π electronic states associated with the pyridinic nitrogen atom have been identified in the absorption (or excitation) spectra and shown to be responsible for the nonfluorescent nature and the high triplet yields (∼0.8) of CNP molecules. A tentative interpretation of all the diverse emissive characteristics of cyanopyridines has been offered in terms of the predominance of the effect of pseudo-Jahn–Teller distortion and the matrix potential in governing the radiative and radiationless deactivation of the lowest triplet state of these molecules. The distinctive differences observed in the emissive characteristics of the 3-CNP and other two isomers have been attributed to the differences in the distortability of the conformational mobility of the CNP isomers arising primarily as a result of inductive effects of the pyridinic nitrogen atom and the nitrile group.

Distant intramolecular interaction between identical chromophores: The n-π* excited states of p-benzoquinone

The structure and dynamics of the p-benzoquinone n-π* excited states have been investigated in solid Ne host. The two carbonyl excitations are essentially degenerate, with a B1g–Au splitting of +64 cm−1 between the singlets and −11 cm−1 between the triplets. There is no evidence that the lower 3B1g state has a double minimum potential via a pseudo Jahn–Teller distortion. The two triplets have very similar charge distributions and structures, despite their different formal symmetries. Excited singlet vibronic states show evidence for lifetime broadening due to intersystem crossing. Experiments in crystalline hosts are reviewed and reinterpreted. The structure in Ne host is closer to the intrinsic gas phase structure than are the structures in various single crystal environments. Valence bond theory, rather than molecular orbital theory, appears to be the natural description of the electronic structure.

Vibronic mixing and vibronic coupling of nearby electronic states. II: CNDO calculations of vibronic mixing in p-benzoquinone

The electronic structure of the lowest excited states of p-benzoquinone is given by calculations in the CNDO approximation. It is shown that for the description of the two nearly degenerate nπ* B1g and Au states the molecule can approximately be considered as a dimer. [A single p-benzoquinone molecule is considered as a dimer, the monomer unit being one-half of the molecule, i.e., a (CH)2C=O fragment.] The vibronic mixing of these two states is evaluated from calculations at displaced nuclear positions, which specify the relative importance of the different coordinates of b1u symmetry. The computed parameters are compared with parameters obtained, within the dimer model, from experimental data. The results of the calculations support the interpretation of Stark measurements of this compound which suggested a pseudo Jahn–Teller distortion of the lowest excited states.

Theory of natural optical activity in crystalline Cu2+:ZnSeO4.6H2O

The chiroptical properties associated with the d-d transitions of Cu2+ ions in crystalline Cu2+:ZnSeO4.6H2O are examined on a theoretical model which is based on a crystal-field representation of the Cu2+ d-d states, and which includes explicit consideration of pseudo Jahn-Teller distortions within the manifold of Cu2+ ligand field states. In Cu2+:ZnSeO4.6H2O, the Cu2+ ions are located at sites of C 2 symmetry. However, the six oxygen atoms (from H2O) coordinated directly to Cu2+ form a near perfect octahedron so that the ‘crystal field’ as felt by the Cu2+ 3d electrons deviates only slightly from cubic (Oh ) symmetry. This results in a ‘nearly degenerate’ electronic ground state which is subject to pseudo Jahn-Teller distortion. Calculations are carried out which incorporate both the crystal field and vibronic coupling (pseudo Jahn-Teller interactions) aspects of the fundamental theoretical model. The signs and magnitudes of the computed rotatory strengths are in qualitative agreement with experimental o...

Absorption and emission spectra of the lowest triplet state in hexachloroacetone

T 1 ← S 0 absorption and T 1 → S 0 phosphorescence spectra of neat cystalline hexachloroacetone have been analyzed at 4.2°K. From the lifetime and energy the upper state is assigned as 3nπ *. The spectra are sharp compared to other aliphatic ketones, with the 0-0 band at 26 248 ± 2 cm −1. The phosphorescence shows two strong progressions; one involving the CO stretching mode at 1784 cm −1 (x), the other a long progression of at least 8 bands involving a mode at 143 cm t-1 (a). The 143 cm −1 progression forming mode can best be asigned to the CO out-of-plane wagging vibration. The absorption shows the same two strong progressions, reduced in frequency to 1270 cm t-1 and 123 cm −1, respectively, but with the progression in mode a broadened with increasing n. The broadening is interpreted as arising from inversion doublets; the close harmonicity up to n = 5 allowing the potential barrier to inversion to be estimated as > 700 cm −1. A feature of the spectra is the absence of low frequency torsional modes suggesting lack of pseudo Jahn-Teller distortion of the triplet state potential surface. For comparison, the phosphorescence of crystalline hexafluoroacetone was also studied at 4.2°K. The spectrum exhibits broad bandedness with a 00 band tentatively assigned at 26 870 ± 20 cm −1.

Dual phosphorescence from rigid glass solutions of phenyl alkyl ketones

The study of solvent and substituent effects on the dual phosphorescence of propiophenone and butyrophenone suggests that the prerequisites for the observation of dual emission are the proximity of the lowest energy nπ* and ππ* triplet states and high viscosity of the glassy solvents. A model, which combines the pseudo Jahn—Teller distortion of the lowest triplet state and the viscosity-dependent matrix potential which opposes this conformational change, is proposed to rationalize these and other emission characteristics in phenyl alkyl ketones.

Reinterpretation of the non-fluorescent properties of the n,π * singlet state

An interpretation which has been given to the non-fluorescent properties of the nπ * singlet state is examined and is shown to be unsatisfactory in the light of recent experimental and theoretical works. A new interpretation is proposed, which considers the effect of pseudo Jahn-Teller distortion.

The zeeman effect in the benzene 3B 1u state. Evidence for a dynamic pseudo-jahn—teller distortion

The Zeeman effect in the phosphorescence photoexcitation spectrum of benzene is used to investigate the assignment of the intense vibronic doublet occurring in the 3B 1u ← 1A 1g absorption spectrum. The results provide further evidence for a dynamic pseudo-Jahn—Teller vibronic coupling between the 3B 1u and 3E 1u electronic states.

Pseudo Jahn-Teller effect and luminescence of nitrogen heterocyclic compounds

The correlation between the solvent dependence of the spectral shape of fluorescence and the solvent dependence of the radiationless transitions strongly suggests that it is the pseudo Jahn-Teller distortions of the lowest excited singlet state which leads to the strong solvent dependence of the fluorescence yield in nitrogen heterocyclic compounds.

Molecular distortions in the triplet state of benzene-polyhalo-alkane complexes

Benzene and polyhalo-alkanes containing weakly acidic protons, form intermolecular complexes in dilute solutions at low temperature. In these complexes the near u.-v. absorption and fluorescence spectra of the benzenes are shifted to higher frequencies, but there is no change in their vibrational structure. The ã1B2u→[graphic omitted]1A1g electronic transition remains symmetry forbidden in the complex and the band origin is very weak. In the phosphorescence spectrum, however, the intermolecular perturbation completely alters the vibrational structure and both the “true” origin and the progression in the totally symmetric breathing frequency ν1 based upon it, become intense. It is suggested that the intermolecular perturbation promotes a pseudo Jahn–Teller distortion of the ring in the ã3B1u state, which lowers its point symmetry to D2h. A Franck–Condon analysis of the band structure is used in an approximate partial reconstruction of the phosphorescence spectral profile and calculations of the degree of distortion are made from it.

Experimental Observation of Singlet–Triplet Absorption in Pure Crystalline Benzene

The B1u3 ← A1g1 intrinsic absorption spectra of C6H6 and C6D6 at 4.2°K have been obtained for the first time at high resolution using the phosphorescence–photoexcitation method. The spectrum exhibits very clear evidence of a pseudo-Jahn–Teller distortion of the normally hexagonal benzene molecule upon excitation to the triplet state. Factor-group splitting of the 0–0 and 0–0 + ν1 exciton bands have also been observed. The position of the mean of the 0–0 exciton band of C6H6 when compared to the phosphorescence origin of a C6H6 guest in a C6D6 crystal, indicates that the “static” intermolecular interactions between guest and host are different for C6H6 and C6D6 hosts. This result has important implications for the applicability of mixed crystal theories to benzene.