Electronic spectra and luminescence properties of 1,2-benzotetraphene in crystalline matrices at 77 K

Quasilinear absorption and luminescence spectra of 1,2-benzotetraphene were obtained in polycrystalline matrices at 77 K. Tne energies of successive excited singlet states as well as the energy of the lowest excited triplet state were found experimentally and compared with those calculated by the PPP CI method. The fluorescence lifetime and quantum yield were determined experimentally. Moreover, the radiationless transition probabilities, lifetime of triplet state and phosphorescence quantum yield were estimated employing the Siebrand-Williams model. The results obtained suggest that radiationless ISC processes are the main deactivation channel of the S₁ and T₁ states. The vibrational analysis of quasilinear absorption and luminescence spectra was performed and fundamental frequencies of ground and first excited singlet states were determined.     

The absorption,fluorescence and phosphorescence spectra of α and β-F,Cl, Br-naphthalenes in crystalline matrixes at 77 K

The paper studies the absorption, fluorescence and phosphorescence spectra of α and β-F, Cl, Br-naphtalenes (α, β-F, Cl,BrN) in different matrixes at 77 K and different concentrations. From these spectra one obtaines the vibrational frequences.     

Emission and absorption spectra of some bridged 1,5-benzodiazepines

Absorption spectra in neutral and acidic media are reported for a series of bridged 1,5-benzodiazepines, which are unable to tautomerize. Comparison is made with non-bridged 1,5-benzodiazepines capable of tautomeric rearrangement. Both bridged and non-bridged 1,5-benzodiazepines are essentially non-fluorescent due to the “proximity effect” of interaction between singlet ηπ* and ππ* states of similar energy, a phenomenon previously recognised in six-membered nitrogen heterocycles.     

Synthesis,absorption spectra,and luminescence properties of dihydrobenzoacridinone derivatives

9,9-Dimethyl-12-[(5-aryl-2-furyl) (or 5-aryl-2-thienyl, or 5-aryl-1-methyl-1H-pyrrolyl)]-7,8,9,10,11,12-hexahydrobenzo[a]acridin-11-ones were synthesized, and their absorption and luminescence spectrum characteristics in ethanol at room temperature and at 77 K were studied. The spectra suggest the existence of these compounds in a liquid solution as mixtures of conformers, each of which is characterized by its own absorption and fluorescence spectra.     

Intramolecular charge transfer and electronic absorption and luminescence spectra of fluoroquinolinones

TD-DFT study on the effect of donor and acceptor substituents on molecular orbital localization and charge distribution in fluoroquinolinone molecules showed that their photoexcitation is accompanied by electron density redistribution over particular fragments. Depending on the protolytic form, frontier molecular orbitals are localized on different fragments, whereas variation of substituents weakly affects localization of these orbitals.     

Vibrational and electronic absorption spectra of some trisubstituted benzenes

Despite our studies on the Π* ← Π systems in some trisubstituted benzenes, the available data are limited, especially concerning two heavy and one light or one heavy and two light substituents. We therefore report and interpret the vapour phase electronic absorption, infrared and Raman spectra of 1-bromo-2,4-difluoro-, 1-bromo-3,4-difluoro-, 1-bromo-3,5-difluoro-, 1,3-dichloro-2-fluoro- and 1,3-dichloro-4-fluoro-benzenes.     

Infrared and electronic absorption spectra of some trisubstituted benzenes

The electronic absorption spectra of 2,3-, 2,4-, 2,5-, 2,6- and 3,4-difluorobenzonitriles, 3,4-difluoroaniline and 3,4-difluoroanisole in the ultraviolet region in vapour phase have been recorded on medium quartz and Hilger large quartz spectrographs and on a Hitachi model 150-20 UV-VIS ratio recording spectrophotometer. All the molecules investigated have exhibited two π* ← π band systems corresponding to ¹B_2u ← ¹A_1g (λ2600 Å) and ¹B_1u ← ¹A_1g (λ2100 Å) systems of benzene. The infrared absorption spectra of all the molecules studied have also been recorded and analysed. These infrared data have been taken to help analyse the u.v. spectra of the molecules studied.     

Quasi-linear fluorescence and phosphorescence of some aromatic and heterocyclic compounds in tetrahydrofuran at 77 K

The fluorescence and phosphorescence emission spectra of 31 polynuclear aromatic hydrocarbons and a number of heterocyclic compounds have been studied in tetrahydrofuran and in n-paraffin solvents at 77 K. A comparison of the characteristics of the quasi-linear luminescence emission observed in these solvents has been made. The effect of the presence of peroxide impurities in the tetrahydrofuran solvent on the emission spectrum observed for coronene has been investigated.     

Ultraviolet absorption and luminescence properties of some cannabis constituents

The characteristics of the absorption, fluorescence and phosphorescence spectra of the cannabinols have been determined. The data provide a basis for the detection of these compounds at concentrations down to 10 ng ml .     

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.     

Some characteristic features of the electronic absorption spectra of 1,3,5-trimethoxy benzene in the solid phase and in rigid glass media at 77 K

The near ultraviolet absorption system of 1,3,5-trimethoxy benzene in the solid phase and in an isobutyl alcohol rigid glass at 77 K is characterised by the appearance of the 0,0 band and excitation of totally symmetric vibrations. In the spectrum of the solid, two out-of-plane vibrations are coupled with the electronic transition. The spectral features indicate that in the solid phase, the molecular symmetry is lower than D_3h and the molecule is probably not planar.     

Density matrix analysis, simulation, and measurements of electronic absorption and fluorescence spectra of spirobifluorenes

The linear absorption and fluorescence spectra as well as the oscillator strengths of 2,2',7,7'-tetraphenyl-9,9'-spirobifluorene (A), 2,2',7,7'-tetrakis(biphenyl-4-yl)-9,9'-spirobifluorene (B), and 2,2',7,7'-tetrakis(9,9'-spirobifluoren-2-yl)-9,9'-spirobifluorene (C) are calculated on the basis of the collective electronic oscillator (CEO) approach of Mukamel et al. (see, e.g., Chem. Rev. 2002, 102, 3171). The graphical visualization and quantitative characterization of CEO modes allows one to extract the real-space distribution of electronic excitations of the molecules under study. Effects of the lengthening and branching of the oligophenylene segments have been analyzed. The influence of the lowest excited (S1) vs ground-state (S0) geometry changes on the CEO modes is investigated and related to the geometry changes of the molecular parts. The obtained theoretical results are in good agreement with experimental trends observed in absorption and fluorescence data.     

S K and P K absorption spectra and electronic structures of layered thiophosphates MPS3

The S K and P K absorption spectra of layered thiophosphates MPS₃ (M = Mg, Mn, Fe, Ni, Zn, Cd, Sn) were measured. The general features of the S K absorption spectra resemble one another, but the intensity ratio of the first peak to a higher energy structure and the energy position of a shoulder vary, depending on the metal species. All the P K absorption spectra exhibit a prominent peak in the neighborhood of the threshold. It is found that (1) the spectra mainly reveal the p-like partial density of states of the unoccupied energy levels of a [P₂S₆]^4- cluster and (2) the first peak arises predominantly from the electronic transitions to the antibonding levels of the PS bonds. The electronic structures and the optical spectra are discussed.     

Electronic properties and ultraviolet absorption and fluorescence spectra of 2,6-pyridinediamine

The u.v. absorption and fluorescence spectra of 2,6-pyridinediamine and its carboxylate salts were measured in various solvents [isooctane (2,2,4-trimethylpentane), methylcyclohexane and isopentane mixed solvent, ethanol and isooctane mixed solvent, and pH adjusted solvent] at room temperature and 77 K. These spectral data were interpreted by the molecular orbital method. From these results it was found that the formation of 6-amino-2(1H)-pyridinimine through the 2,6-pyridinediamine hydrogen bonded complex with ethanol occurs only in the π,π* excited singlet state in the ethanol and isooctane mixed solvent. On the other hand, the fluorescence spectrum of 2,6-pyridinediamine in the pH > 13 controlled solution was assigned to the monoanion species, that is, the 6-amino-2-pyridylamide ion which was formed in the first π,π* excited singlet state.     

Electronic properties and ultraviolet absorption and fluorescence spectra of 2-pyridinamine

The u.v. absorption and fluorescence spectra of 2-pyridinamine, its methyl derivatives and its carboxylate salts were measured in various solvents (isooctane, methylcyclohexane and isopentane mixed solvent, ethanol and isooctane mixed solvent, acetic acid and isooctane mixed solvent, and pH adjusted solvent) at room temperature and 77 K. These spectral data were interpretated by the molecular orbital method. From these results it was found that the formation of 2(1H)-pyridinimine with the accompanying 2-pyridinium amine occurs only in the π,π* excited singlet state in the acetic acid and isooctane mixed solvent. On the other hand, the fluorescence spectrum of 2-pyridinamine in the pH ⪢ 11 controlled solution was assigned to the monoanion species, that is, the 2-pyridylamide ion which was formed in the first π,π* excited singlet state.     

Solvent effects on the electronic absorption spectra and acid strength of some substituted pyridinols.

The electronic absorption spectra of some substituted pyridinols in organic solvents of different polarities are studied. Also, the solvent effects on the intramolecular charge transfer bands are discussed using various solvent parameters. The acid-base equilibria of the compounds used are studied spectrophotometrically in various mixed aqueous solvents at 25 degrees C and 0.1 M ionic strength (NaClO4). Furthermore, the influence of the solvents on the dissociation constants and tautomeric equilibria of a pyridinol derivatives are discussed. The effect of molecular structure of the pyridinols on the pK's is also examined.     

Syntheses and electronic absorption spectra of some new penta- and di-methine cyanine dyes

The reaction of a ratio of 1 mole of 5-chloro-3-methyl-1-phenyl pyrazolo-4[2(4)]-dimethine cyanine ( 1a,b ) or (5-chloro-4-formyl-3-methyl-1-phenylpyrazole) 2 with 2 moles of an appropriate 2(4)-heterocyclic quaternary salt afforded the novel pentamethine cyanine dyes ( 3a–3e ). Other reactions of 2 with heterocyclic nitrogen bases, followed by condensation of the newly formed quaternary nitrogen salts ( 4a–4c ) with methoheterocyclic iodide salts provided the corresponding unsymmetrical dimethine cyanine dyes ( 5a–5g ). The new pentamethine and dimethine cyanine dyes were identified by elemental analyses and IR and ¹H-NMR spectral data. The visible absorption spectra of some selected dyes were investigated in pure and mixed solvents as well as in universal buffer solutions. The measure of the variation of absorbance with pH was utilized for the determination of the pK_a values for compounds 3b, 3d, 3e, 5e , and 5f , and these data are discussed. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8 : 259–266, 1997.     

Photophysical properties of some phenetidines in non-polar and hydrogen bonding solvents at room temperature and 77 K

The present study was undertaken to investigate the photophysical processes in o-, m- and p-phenetidines, when dissolved in nonpolar and hydrogen bonding solvents, in their ground state and excited electronic state S₁, both at 300 and 77 K. In the ground as well as in the S₁ state it is proposed that the o-phenetidine molecule possesses a structure in which NH₂ and OC₂H₅ groups are away from each other, both in nonpolar cyclohexane (CH) and H-bond acceptor solvent triethylamine (TEA). The formation of a transient or nonemissive charge transfer (CT) complex resulting from strong excited state hydrogen bonding interaction with TEA is found to be responsible for the observed fluorescence quenching of the proton donor phenetidines at 300 K. From the room as well as low (77 K) temperature electronic absorption and steady state fluorescence studies, it was deduced that nonplanarity in the structure of the molecules increases as one moves from aniline to the phenetidines. It is suggested that in the solvent stiffening temperature 77 K, triplet states of all the phenetidines (o-, m- and p-) acquire some nπ* character due to conformational changes, whereas ππ* character is retained in their S₁ state. This facilitates a larger intersystem crossing (ISC) rate in phenetidines relative to the situation in aniline where both S₁ and T₁ possess the same nπ* nature at 77 K due to its more planar structure. However, ISC efficiency in phenetidines at 77 K is found to be impeded, especially in the case of o- and m-isomers, in the presence of TEA as inferred from the lowering of φ_p/φ_⨍ values and the increment of τ_p. In p-phenetidine, rapid equilibrium between a triplet state hydrogen bonded species and free molecules during the triplet excited state lifetime is suggested.