E. E. Tseplin

Anomalously long-lived molecular negative ions of duroquinone.

The problem under investigation here is establishment of mechanisms of the resonant electron capture by molecules, using the example of duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone). A solution is important because it will provide new insights into the fundamental physical laws and widespread applications in various fields like molecular nanoelectronics, touched upon herein too. Resonant electron capture (REC) in duroquinone was studied with negative ion mass spectrometry of the REC as the main method, and UV absorption and the photoelectron spectroscopy as the auxiliary ones. The latter were used to study the electronic structures of the various neutral molecular states that are the parent ones for the negative molecular ions formed by electron attachment to the molecules. B3LYP/6-311 + G(d,p) calculations were widely used throughout the study. As a result, an intensive peak of the negative molecular ions with anomalously high lifetime (200 microseconds) was registered at the attached electron energy of 1.8 eV. The ions were determined to be quartets delaying the electron autodetachment because of spin prohibition and appearing via inter-system crossing from the negative molecular ion doublets produced in the core-excited Feshbach resonances. Finally, the pattern of the REC in duroquinone was obtained for the energy region of 1-4 eV which is presented by shape resonances, core-excited Feshbach resonances and by mechanisms little-known for molecules of inter-shell resonances and the formation of ion quartets. The latter were proposed to be related to the negative differential resistance in molecular nanoelectronics.

Identification of singlet excited electronic states of hydroxybenzoic acid isomers

The photoelectron spectra of benzoic and para-, meta-, and ortho-hydroxybenzoic acids and their UV absorption spectra in ethanol are obtained. The photoelectron spectra in the energy region up to 15.5 eV are interpreted based on the B3LYP/6-311+G(d, p) calculations. Based on the TDDFT B3LYP/6-311+G(d, p) calculations and taking into account the obtained interpretation of photoelectron spectra, the UV absorption bands are assigned to particular singlet transitions. For each transition, the electron configuration that makes the main contribution to the transition is determined. It is shown that the transition energy depends on the energy gap between the corresponding occupied and unoccupied molecular orbitals.

Identification of excited singlet states of chlorophenol isomers

The UV spectra of optical absorption of para-, meta-, and ortho-chlorophenol are recorded in the gas phase. The bands of UV spectra are assigned to the electronic transitions of molecules to definite excited singlet states on the basis of calculations by the TDDFT B3LYP/6-311++G(d, p) method. In each case the electron configuration making the predominant contribution to the particular singlet state is determined. The energies of singlet electronic transitions are shown to depend on the energy spacing between the molecular orbitals involved in these transitions.

Identification of singlet excited electronic states of toluidine isomers

The photoelectron and UV absorption spectra of para-, meta-, and ortho-toluidine in the gas phase are obtained. The photoelectron spectra in the energy region up to 15.5 eV are interpreted based on the B3LYP/6-311+G(d, p) calculations. Based on the TDDFT B3LYP/6-311+G(d, p) calculations and taking into account the obtained interpretation of photoelectron spectra, the UV absorption bands are assigned to particular singlet transitions. For each transition, the electron configuration that makes the dominant contribution to the transition is determined. It is shown that the transition energy for toluidine isomers depends on the energy gap between the corresponding occupied and unoccupied molecular orbitals. It is noted that this dependence is similar for different benzene derivatives.

The Lowest Triplet of Tetracyanoquinodimethane via UV–vis Absorption Spectroscopy with Br-Containing Solvents

This study was undertaken to find the previously unknown lowest triplet of the isolated molecule of tetracyanoquinodimethane (TCNQ), which is a widely used organic semiconductor. The problem is topical because the triplet excitation of this compound is involved in some processes which occur in electronic devices incorporating TCNQ and its derivatives, and information on the TCNQ triplet is needed for better understanding of these processes. The lowest triplet of TCNQ was obtained at 1.96 eV using UV-vis absorption spectroscopy with Br-containing solvents. Production of the triplet band with sufficient intensity in the spectra was provided by the capacity of the Br atom to augment the triplet excitation and through using a 100 mm cuvette. The assignment of the corresponding spectral band to the triplet transition was made by observation that this band appeared only in the spectra recorded in Br-containing solvents but not in spectra recorded in other solvents. Additional support for the triplet assignment came from the overall UV-vis absorption spectra of TCNQ recorded in various solvents, using a 10 mm cuvette, in the 1.38-6.5 eV energy range. Singlet transitions of the neutral TCNQo molecule and doublet transitions of the TCNQ¯ negative ion were identified in these overall spectra and were assigned with TD B3LYP/6-31G calculations. Determination of the lowest triplet of TCNQ attained in this work may be useful for theoretical studies and practical applications of this important compound.

Mechanisms of specific effects of polar solvent in optical absorption spectra

We measured the optical absorption spectra of para-benzoquinone and duroquinone in polar (methanol) and nonpolar (n-hexane) solvents. We find that the specific effect of the polar solvent, which manifests itself here as a bathochromic shift of one of π-π* bands, is caused by the formation of hydrogen bonds between solvent molecules and the molecule under study and, as a consequence, by a decrease in the energy gap between the corresponding occupied (π) and unoccupied (π*) molecular orbitals. This result is obtained by TDDFT B3LYP/6-311+G(d, p)-calculations of electronic spectra, which, in the case of isolated para-benzoquinone and duroquinone molecules, reproduce experimental optical absorption spectra of the corresponding compounds in n-hexane and, in the case where these molecules form complexes with methanol molecules by means of hydrogen bonds, reproduce spectra measured in methanol.

Specific effects of a polar solvent in optical absorption spectra of 1,2-naphthoquinone

The optical absorption spectra of 1,2-naphthoquinone in polar (methanol) and nonpolar (n-hexane) solvents are recorded. It is found that the specific effect of a polar solvent, which manifests itself in a hypsochromic shift of the first nπ* band and in a bathochromic shift of the second and third ππ* bands, is caused by the formation of hydrogen bonds between solvent molecules and the molecule under study and, as a result, by a change in the energy gap between the corresponding occupied and unoccupied molecular orbitals. This result is obtained by TDDFT B3LYP/6-311+G(d, p) calculations of electronic spectra, which, in the case of an isolated 1,2-naphthoquinone molecule, reproduce its experimental optical absorption spectra in n-hexane and, in the case of the same molecule forming a complex with methanol molecules by means of hydrogen bonds, reproduce the spectrum of 1,2-naphthoquinone in methanol.

Specific effects of a polar solvent in optical absorption spectra of 5,12-tetracenequinone

We have measured the optical absorption spectra of 5,12-tetracenequinone (5,12-naphthacenequinone) in polar protic (methanol) and nonpolar (n-hexane) solvents. It has been shown that shifts of some bands in the polar solvent compared to the nonpolar one are caused by the formation of high hydrogen bonds between methanol molecules and the molecules under study. The occurrence of hydrogen bonds leads to changes in the energy gaps between occupied and unoccupied molecular orbitals of 5,12-tetracenequinone and, as a consequence, to corresponding changes in energies of electronic transitions. Based on analysis of the absorption spectra in different solvents in combination with the data of calculations of electronic spectra and taking into account changes in the orbital pattern under the action of the solvent, we have determined exact electronic configurations of electronically excited singlet states of 5,12-tetracenequinone.