Overtone spectroscopy of chlorine substituted benzene derivatives

The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400-20,000 cm(-1). A large number of bands associated with the fundamental, the overtones and the combination frequencies of C-H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C-H stretch vibrations for the six molecules have been determined using local mode model. The C-H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6-31+G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C-H stretching mode and the C-H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms.     

The synthesis of electron donor-acceptor substituted pyrazoles

A variety of 1,3- and 1,5-donor-acceptor substituted pyrazole derivatives have been synthesized by the cyclocondensation of α,β-ethynyl ketones with substituted phenyl hydrazines. The regioselectivity of the cyclization depends on the reaction conditions in a manner consistent with competitive 1,2- and 1,4-addition followed by ring closure. 1,4-Disubstituted derivatives can be prepared from the corresponding 4-iodopyrazole using palladium catalyzed carbon-carbon bond forming reactions. The pyrazole chromophores are expected to show interesting nonlinear optical properties.     

Correlation of ESCA shifts and Hammett substituent constants in substituted benzene derivatives

ESCA shifts of substituted benzene derivatives are shown to correlate with Hammett σ constants. An explanation of this correlation is given in terms of the thermodynamic model for core electron binding energy shifts, and it is shown that ESCA shifts in benzene derivatives can be treated as a linear free energy relationship.     

A systematic shift in the electronic spectra of substituted benzene molecules trapped in helium nanodroplets

Electronic spectra (S1<--S0) have been recorded from five separate substituted benzene derivatives trapped in helium nanodroplets. Each member of the series is found to exhibit a blueshift with respect to the equivalent transition in the gas phase. Taken together with previous results for benzene, the observed shifts show a remarkably good correlation with changes in electron density that occur within each of the aromatic rings as a result of electronic excitation.     

The electronic spectrum of benzene

Assignment of the benzene spectrum is discussed using a new approach to the semi-empirical π-electron theory. The ordering ¹B_1u <¹E_1u <¹E_2g is shown to be theoretically preferred.     

The electronic spectra of the alkyl,fluorine and chlorine substituted derivatives of silane

The main characteristics of the ultraviolet absorption spectra of simple alkyl, fluorine and chlorine derivatives of silane are described. The interpretation of these spectra is discussed in relation to the respective photoelectron spectra, the Rydberg-valence-shell distinction and involvement of 3d atomic orbitals.Dedicated to Professor Dr. Hermann Hartmann on the occasion of his 65th birthday.     

Monitoring DNA structures by dual fluorescence of pyrene derivatives

We have developed a nucleotide modified by a pyrene derivative with dual fluorescence. The dual fluorescence of the fluorophore, which was incorporated into DNA, was effectively controlled at ambient temperature according to DNA structural status. Our nucleoside with dual fluorescence is effective as a conceptually new probe for monitoring DNA hybridization by the color change without multilabeling with fluorescent dyes.     

pH-induced changes in electronic absorption and fluorescence spectra of phenazine derivatives

The visible electronic absorption and fluorescence spectra as well as fluorescence polarization degrees of imidazo-[4,5-d]-phenazine (F1), 2-methylimidazo-[4,5-d]-phenazine (F2), 2-trifluoridemethylimidazo-[4,5-d]-phenazine (F3), 1,2,3-triazole-[4,5-d]-phenazine (F4) and their glycosides, imidazo-[4,5-d]-phenazine-N1-beta-D-ribofuranoside (F1rib), 1,2,3-triazole-[4,5-d]-phenazine-N1-beta-D-glucopyranoside (F4gl), were investigated in aqueous buffered solutions over the pH range of 0-12, where the spectral transformations were found to be reversible. The effects of protonation and deprotonation on spectral properties of these dyes were studied. We have determined the ranges of pH, where individual ionic species are predominant. In aqueous buffered solutions the fluorescence was found only for neutral species of F1, F1rib, F2, and F4gl dyes, whereas for the ionic forms of these dyes, as well as for F3 and F4 ones, the fluorescence has not been detected. The concentrational deprotonation pKa values were evaluated from experimental data. It was shown that donor-acceptor properties of the substituent group in the second position of the pentagonal ring substantially affect the values of the deprotonation constants and the character of protonation for chromophore. The substitution of a hydrogen atom in the NH-group by the sugar residue blocks the formation of the anionic species, and results in enhancement of the dye emission intensity. The steep emission dependence for F1 and F1rib over pH range of 0-7 with intensities ratio of IpH 7/IpH 1=60 allows us to propose them as possible indicator dyes in luminescence based pH sensors for investigation of processes accompanied by acidification, e.g. as gastric pH-sensors. A comparative analysis of the studied dyes has shown that F4gl is the most promising compound to be used as a fluorescent probe for investigation of molecular hybridization of nucleic acids.     

The electronic states of polyfluorene copolymers with alternating donor-acceptor units

We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-ylbenzo[1,2,5]thiadiazole (TBT) units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical (ZINDO) method we can assign the characteristic bands of the "camel-back" absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the pi-conjugated electron system. Additional "dark" charge transfer states in the gap between these bands have been revealed. Calculations are also made on the red light emitting polyfluorene-based copolymer poly(fluorene-co-benzothiadiazole) (F8BT), which contains benzo[1,2,5]thiadiazole instead of TBT. The nature of the electronic states in F8BT and DiO-PFDTBT are found to be qualitatively the same.     

Lewis-acid-promoted additions of carbonyl compounds to donor-acceptor substituted cyclopropanes: a new synthesis of 2,3-dihydrofurane derivatives

Methyl 2,3-dihydrofurane-3-carboxylates are synthesized from siloxycyclopropaneesters and ketones in the presence of TiCl₄ with good yields.     

Synthesis of donor-acceptor substituted oligothiophenes by Stille coupling

A synthesis of donor-acceptor-substituted oligothiophenes by Stille coupling is described. The 5′-estanyl derivatives, readily prepared from 5-alkoxy- and 5-amino-2,2′-bithiophenes 7 were coupled with the appropriate aryl or heteroaryl bromides to give the title compounds.     

Synthesis and electronic absorption and fluorescence of 2-arylbenzothiazole derivatives

A series of new 2-arylbenzothiazoles have been prepared in high yields by Jacobson's cyclization condensation of 2-aminobenzenethiol with benzoyl chloride or benzaldehyde derivatives under three different routes. These compounds have been fully characterized by EA, IR, NMR and MS. The electronic absorption and fluorescence of these compounds have been systematically investigated for the first time. The relationships between their photophysical properties and structures have been discussed. The alteration of absorption and emission wavelengths can be elucidated by Hammett's substituent constants.     

Study on the relationship between retention behavior and molecular structure parameters of substituted benzene derivatives in RPLC

Summary A quantitative structure-retention model derived from the concept of hydrophobic substituent constant () and the total solubility parameter (_T) has been established. It has been used to predict successfully the retention behavior of a number of solutes on various columns in several different eluents. The experimental results indicate that the model can be employed for the prediction of retention of substituted benzenes in a large volume fraction range of mobile phase. In addition, the physical meaning of the sign and the influence of the coefficients of the model on the retention process had been discussed together with the dependence of these coefficients on the composition of mobile phase.     

Assignment of the electronic transitions in benzene

The problem of the assignment of π-electronic transitions in benzene is discussed using all criteria presently available. It is shown that based on this information a few different assignments are impossible to exclude. The correct assignment ³B_1u < ³E_1u < ¹B_2u < ³B_2u < ¹B_1u < ³E⁺_2g < ¹E_1u < ¹E^?_2g < ³E^?_2g has been selected as result of theoretical considerations based on a new approach to the semi-empirical π-electron theory. The results confirm the adequacy of the π-electron model for energy level calculations, and emphasise the fundamental importance of multi-excited configurations.     

Solvatochromic effects on the photoinduced charge-transfer states in donor-acceptor substituted polydioctylfluorenes

Polymer morphology affects quantum efficiency. The influence of polymer morphology on the emission from charge transfer states within donor-acceptor (D-A) polydioctylfluorene derivatives is investigated. Two D-A copolymers, comprising one- and two-electron-donating triphenylamines substituted into the electron-accepting dioctylfluorene repeat unit were studied. Time-resolved emission spectra (with a resolution of 330 fs) in both liquid and glass phase isolate nuclear relaxation to the large-amplitude motion of the triphenylamine moiety about the single bond, analogous to the twisted intramolecular charge-transfer (TICT) model. The charge separation in the polymers' emitting states is therefore increased, suggesting a potential approach to enhance quantum efficiency in devices otherwise limited by exciton recombination.     

Gas-phase electronic spectra of two substituted benzene cations: phenylacetylene+ and 4-fluorostyrene+

The visible spectra of phenylacetylene+ and 4-fluorostyrene+ have been measured by laser photodissociation spectroscopy. The observed vibronic systems were assigned to the B2A' <-- X2A' and C2B1 <-- X2B1 electronic transition in the 4-fluorostyrene+ and phenylacetylene+ cations, respectively. Two methods were employed and compared: a resonant multiphoton dissociation scheme of the bare cations and a resonant photodissociation technique applied to the chromophore+-argon n=1,2 ionic complexes. The latter approach allowed the intrinsic profile to be resolved, revealing different intramolecular dynamical behavior. Their electronic relaxation has been rationalized in terms of an apparent energy gap law for the benzene derivative cations.     

The electronic absorption spectra of the alkyl-substituted derivatives of bis(benzene)chromium(0) in the vapour phase

The UV and visible absorption spectra of (arene)₂chromium(0) (arene = benzene (I), toluene (II), ethylbenzene (III), cumene (IV), tert-butylbenzene (V), mesitylene (VI) in the vapour phase have been investigated. Four band systems A,B,C and D are revealed in the spectra. The bands of the system with the shortest wavelengths, D, represent the Rydberg series. The first ionisation potentials IP_a1g, 5.18 and 5.01 eV respectively. The Rydberg bands correspond to the allowed electrodipole transitions from the highest occupied molecular orbital (MO) a_1g to the vacant MO of either the a_2u or e_1u type.System C corresponds to the intense band of the solution spectra. The electronic transition e_2g → e_2g obviously makes a great contribution to this system. System B is assigned to the transition from a_1g to vacant a_2u or e_1u MO, which can be Rydberg orbitals. System A can be assigned to the a_1g → e_2u transition or to the Rydberg transition, which is forbidden in the D_6h point group but becomes allowed upon reduction of symmetry.     

The mechanism of the vapor-phase chlorination of benzene derivatives

The title reaction, displaying peculiar characteristics as to relative rates and isomer distributions, has been studied in detail. Prior to this study, different mechanisms had been advanced by several groups. Kinetic features (isomer patterns, relative and absolute rates, reaction orders, influences of additives, H/D isotope effects) strongly point to a free-radical (chain) process, in which (1) is a crucial step. This abstraction reaction, endothermal by about 6 kcal/mol, apparently proceeds via a transition state closely resembling the free aryl radical. Relative rates and isomer distributions therefore reflect differences in stabilization energies, or in DH°(Ar – H). With high arene–Cl₂ intake ratios or, more pronounced, with CCl₄ as the reagent, aryl radicals also lead to biaryl, where arene successfully competes with the halogenating agent. This interpretation is quantitatively supported by our observation that “added,?” recognizable aryl radicals yield the same chlorination–arylation product ratio, and by the results of competitive chlorination of benzene and chloroform over a temperature range of 200°C, where the latter study substantiates the value DH⁰(C₆H₅ – H) ≈ 109 kcal/mol.     

Engaging the pyridine-DMAD zwitterion in a novel strategy for the selective synthesis of highly substituted benzene and cyclopentenedione derivatives

[reaction: see text] The pyridine-mediated reaction of dimethyl acetylenedicarboxylate and cyclobutene-1,2-diones affords selective access to either hexasubstituted benzene derivatives or cyclopentenedione derivatives depending on the concentration of pyridine.