V. A. Sazhnikov

Model of the formation of dibenzoylmethanatoboron difluoride exciplexes with aromatic hydrocarbons on silica surface

Dibenzoylmethanatoboron difluoride (DBMBF2) is a fluorophore capable of forming exciplexes with benzene and its derivatives in solutions [1] or in the adsorbed state on the silica gel surface by reacting with vapor of these substances [2, 3], a property that may be used for the fabrication of highly selective and responsive sensor devices. In this study, the dependence of the fluorescence spectra of silica-supported DBMBF2 samples on the vapor concentration of the aromatic compounds benzene, toluene, and p-xylene has been examined. A model describing the observed spectral changes has been proposed. The data on the concentration dependence of the fluorescence spectra of silica gel microspheres bearing adsorbed DBMBF2 are in good agreement with the proposed model.

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.

Fluorescence Spectra and Structure of the Difluoro(dibenzoylmethanato)boron Monomers and Dimers Absorbed on Silica Gel

It has been shown that difluoro(dibenzoylmethanato)boron ((dbm)BF2) can be absorbed on silica gel in the form of fluorescent monomers and dimers with the emission properties that change in the presence of vapors of volatile organic compounds, such as ethanol, acetone, toluene, and meta-xylene. Fluorescence quenching was observed for the (dbm)BF2 monomers and dimers in the case of ethanol and acetone, whereas the formation of fluorescent exciplexes with monomers and enhancement of the dimer fluorescence were observed in the case of toluene and meta-xylene. Results of the quantum-chemical calculations of the structure of the (dbm)BF2 monomer complex with the matrix and toluene and (dbm)BF2 dimers with matrix are presented.

Fluorescence properties and conformation of dibenzoylmethanatoboron difluoride in solutions

The electronic absorption and fluorescence spectra of dibenzoylmethanatoboron difluoride (DBMBF2) in a number of polar and nonpolar solvents have been studied; the quantum yields and fluorescence lifetimes have been measured, and the vibrational structure of the spectra has been analyzed. The equilibrium configuration parameters of DBMBF2 in the ground state have been determined by the DFT method. It has been found that there is only one stable conformation of DBMBF2 in the ground state.

Photodegradation of Boron Difluoride Dibenzoylmethanate in Solutions

The UV photostability of boron difluoride dibenzoylmethanate (DBMBF2) in the aprotic solvents acetonitrile, dimethylsulfoxide, acetone, methyl cyclohexane, dioxane, and tetrahydrofuran has been studied. The DBMBF2 half-life largely depends on the solvent and varies from 3 min in tetrahydrofuran to 9 h 40 min in acetonitrile. It has been found that the photodegradation of DBMBF2 involves the detachment of the BF2 molecule to yield dibenzoylmethane occurring in the ketone (absorption at 220–240 nm) and enol (absorption at 340–345 nm) forms. It has been shown that DBMBF2 can be stabilized by sterically hindered piperidines, thereby suggesting the participation of singlet oxygen in the photodegradation of DBMBF2.

Atomistic simulations of materials for optical chemical sensors: DFT-D calculations of molecular interactions between gas-phase analyte molecules and simple substrate models

The structures of complexes of some small molecules (formaldehyde, acetaldehyde, ammonia, methylamine, methanol, ethanol, acetone, benzene, acetonitrile, ethyl acetate, chloroform, and tetrahydrofuran, considered as possible analytes) with ethylbenzene and silanol (C6H5C2H5 and SiH3OH, considered as models of polystyrene and silica gel substrates) and with acridine (C13H9N, considered as a model of an indicator dye molecule of the acridine series) and the corresponding interaction energies have been calculated using the DFT-D approximation. The PBE exchange-correlation potential was used in the calculations. The structures of complexes between the analyte and the substrate were determined by optimizing their ground-state geometry using the SVP split-valence double-zeta plus polarization basis set. The complex formation energies were refined by single-point calculations at the calculated equilibrium geometries using the sufficiently large triple-zeta TZVPP basis set. The calculated interaction energies are used to assess the possibility of using dyes of the acridine series adsorbed on a polystyrene or silica substrate for detecting the small molecules listed above.

Preparation of chemosensor materials based on silica nanoparticles with covalently anchored fluorophores by inkjet printing

Samples of sensor layers containing a mixture of spherical silica gel microparticles and spherical macromolecular silica sol nanoparticles have been prepared by inkjet printing. The average diameter of microparticles is 5 μm; nanoparticles about 100 nm in diameter contain covalently anchored fluorophore, dibenzoylmethane boron difluoride (DBMBF2), on the surface. The microstructure of the layers is shown to considerably affect the availability of the fluorophore indicator for gas-phase analyte molecules of the methylbenzene group. The sensitivity of the sensor layers is shown to reach 0.5 ppm with a response time of about 100 s.

Assessment of TDDFT- and CIS-based methods for calculating fluorescence spectra of (dibenzoylmethanato)boron difluoride exciplexes with aromatic hydrocarbons

The applicabilities of various CIS- and TDDFT-based procedures to the calculation of the fluorescence spectra of DBMBF2 exciplexes with aromatic hydrocarbons are analyzed. It is shown that a reasonable agreement between the calculated and experimental fluorescence spectra is obtained when a combined procedure is used. In this combined procedure, the geometries of the exciplexes are optimized beforehand using the CIS method with an empirical dispersion correction, while the exciplex transition energies are calculated at the optimized geometries using the CIS(D) method or the TDDFT approach with double-hybrid B2PLYP and mPW2PLYP functionals.

Characteristic features of nile red fluorescence in transparent xerogels

A procedure for preparing transparent silica gels and xerogels has been developed and samples of these materials have been fabricated to investigate the effect of microenvironment on spectral and sensor properties of the Nile Red dye. Spectral properties of the dye in modified and unmodified gels and xerogels have been investigated using fluorescence spectroscopy. It has been found that these xerogels are permeable to volatile organic substances, and their potential use as fluorescent sensor material has been demonstrated.

Density functional calculations of 9-diphenylaminoacridine fluorescent indicator and its interactions with analyte molecules: I. Structures of complexes in the ground electronic states and absorption spectra

The interaction of 9-diphenylaminoacridine dye (indicator) with several small analyte molecules (methanol, acetonitrile, acetone, tetrahydrofuran, benzene, ammonia, formaldehyde, and acetaldehyde) has been theoretically studied in relation to the problem of the development of optical chemosensors based on organic dyes. The structures of the resulting complexes and the absorption spectra of 9-diphenylaminoacridine and its complexes with analytes were calculated using density functional theory (DFT) with the PBE0 functional and the 6-31G(d,p) basis set. It was demonstrated that complexes of two types with different mutual arrangements of molecules corresponding to the lateral and stacking structures can be formed for each analyte. The calculated absorption spectrum only weakly changes upon complex formation, which is in agreement with experimental data on the absorption spectra of 2,7-dimethyl-9-ditolylaminoacridine in solutions of corresponding solvents. The method for the calculation of excited states that was used in this work can be applied to the calculation of the fluorescence spectra of 9-diphenylaminoacridine complexes.