The Effects of Substituents and Solvents on the Ground‐State π‐Electronic Structure and Electronic Absorption Spectra of a Series of Model Merocyanine Dyes and Their Theoretical Interpretation

Two series of new merocyanine dyes have been synthesised and the dependence of their electronic structure on substituents and solvents has been studied by NMR spectroscopy, by using both the NMR ¹³C chemical shifts between adjacent C atoms in the polymethine chain and the ³J(H,H) coupling constants for trans‐vicinal protons. The widely used valence bond (VB) model based on two contributing structures cannot account theoretically for the observed alternating π‐electron density in the polymethine chain. In addition, the prediction of zero‐π‐bond order alternation (or zero‐bond length alternation) by this model is also incorrect. However, the results are consistent with the predictions of a qualitative VB model which considers the resonance of a positive charge throughout the whole polymethine chain. Based on this model and the Franck‐Condon principle the effect of substituents and solvents on the fine structure of the electronic spectra of these dyes can be explained as vibronic transitions from the vibrational state v=0 to v′, where v is the vibrational quantum number of the totally symmetric C?C valence vibration of the polymethine chain in the electronic ground state and v′ is that in the electronic excited state. In contrast, neither the effects of substituents or solvents on the electronic structure of merocyanines and their electronic spectra can be accounted for by the simple two state VB model.     

Interactions Intramoleculaires—X: Etude par RMN des Équilibres Conformationnels de Cyclohexènes Monosubstitutés en 4 et du Dicyano-1,2 Cyclohexane trans

The conformational free energies of CN, Cl and COOC₂H₅ substituents have been evaluated from the PMR spectra of 4-substituted 3,3,6,6-d₄-cyclohexene. The PMR spectra of trans-1,2-dicyano-3,3,6,6-d₄-cyclohexane permit the conformational equilibrium (in different solvents) and the gauche diequatorial interaction energy between the two CN substituents to be estimated.     

STUDIES ON THE CHEMILUMINESCENCE OF LUMINOL IN DIMETHYLSULFOXIDE AND DIMETHYLSULFOXIDE-WATER MIXTURES*

Abstract— The absorption, fluorescence, and chemiluminescence (CL) characteristics of luminol have been studied in basic dimethylsulfoxide (DMSO) and various basic DMSO — water mixed solvents. It has been shown that the luminol dianion can be produced quantitatively in carefully deoxygenated ‘dry’ DMSO using potassium t-butyl alcoholate (BTO) as the base. A direct correlation has been found between the intensity of CL and the concentration of luminol dianion, Indicating that the dianion is the reactive species in the chemiluminescent reaction in DMSO. Increasing concentrations of water in the mixed solvents greatly reduced the CL intensity because of the decrease in luminol dianion concentration. Solvent effects on the fluorescence of 3-aminophthalic acid samples was noted and identified. Chemiluminescent spectra of luminol were run as a function of solvent composition, and compared with fluorescence spectra in the same media. The need for correcting these spectra for comparison was noted. Stopped flow kinetic studies were run in DMSO and DMSO-water mixtures. These data can be interpreted on the basis of two second-order steps and one first-order step in the reaction. The first-order step is probably last and probably arises from decomposition of a ‘peroxy’ intermediate. The rate constant for this step is k= 1·2±0·3×10^-1 sec^-1.     

UV-visible absorption spectra and luminescence of the pesticide fenarimol

The UV-visible absorption and emission spectra have been measured of the pesticide fenarimol ((±)-2,4′-dichloro-α-(pyrimidin-5-yl)-benzhydryl alcohol) in solution. From comparison with the spectra of chlorotoluenes and pyrimidine, and from the effect of solvent polarity on the absorption spectrum, it is shown that the lowest excited singlet state is localized on the pyrimidine ring, and has n,π* character. Higher excited π,π* states are localized on both chlorotoluene and pyrimidine rings. Fenarimol shows a weak, fluorescence from the n,π* state, with a quantum yield which is strongly dependent on solvent. It is shown that this is due to changes in the nonradiative decay rate, particularly in protic solvents, due to increased intersystem crossing. Phosphorescence is observed in low temperature glasses. Although this shows two decay components, it is suggested that only one triplet state is involved, and that this has predominantly π,π* character.     

Spectroscopic study of rigid-rod polymers. I. Structures of model compounds

Poly(p-phenylene benzbisoxazole) and poly(p-phenylene benzbisthiazole) belong to the class of extended-chain, rigid-rod polymers possessing high modulus, high strength, and good thermal and oxidative resistance. Fibers and films of these polymers are processed from anisotropic solutions in strong acids such as methane sulfonic acid or polyphosphoric acid. The electronic absorption and vibrational spectra of the model compounds have been investigated in order to characterize the structures in the solid state and in nonprotonic solvents. The dramatic intensity differences in the spectra obtained have been interpreted by variations in the dihedral angle between the plane of the phenyl group and the plane of the central heterocyclic ring which affect the resonance configuration between the two rings.     

A Combined Theoretical and Experimental Approach to the Study of the Structural and Electronic Properties of Curcumin as a Function of the Solvent

Curcumin, a chemical compound present in the well-known Indian spice turmeric, has uses in many different fields ranging from medicinal chemistry to the dye industry. Its poor water solubility, though, makes Curcumin difficult to handle, making it less appealing for potential uses. The principal aim of this work is to perform a computational study of the structural and electronic properties of Curcumin {IUPAC name: 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione} in several solvents, and a comparison with experimental data. Rotameric equilibria, vibrational and thermochemical analysis, and electronic absorption spectra (with ab initio and semi-empirical methodologies) have been studied, both in vacuum and in three selected solvents. Different computational techniques have been applied and the results compared. Combined approaches resulted in very satisfactory results. Interesting results have emerged, which suggest subsequent investigations about the nature of the excited states and potential derivatives of Curcumin that possibly have non-linear optical applications, as a π-core for innovative materials in laser engineering and photonics.     

Enhancement of photoinduced intramolecular cross-cycloaddition of α-(9-anthryl)-ω-(1-naphthoyl) end-labeled poly(ethylene glycol) via lipophobic interactions and complexation with metal cations

The fluorescence spectra and photocycloadditions of poly(ethylene glycol) labeled at the chain termini with one 9-anthryl and one 1-naphthoyl group (N-P_n-A) both in non-polar and polar solvents in the presence of alkali-metal cations have been investigated. Lipophobic interactions in non-polar solvents and complexation of the polyether with cations in polar solvents force the two terminal groups of N-P_n-A into proximity, and irradiation of the solutions produces intramolecular photocyclomers to the exclusion of intermolecular products.     

Study of fluorescent aggregates of polymethine dyes

Absorption, fluorescence, and fluorescence excitation spectra have been studied for a number of anionic, cationic, and cationic-anionic polymethine dyes in low-polar and nonpolar solvents, as well as in binary mixtures of solvents differing in polarity. For most of the dyes studied, fluorescent aggregates have been found to form. Their broad fluorescence bands are located in the long-wave region with respect to those of the initial dyes. The quantum yield of the aggregate fluorescence is normally higher than that of the initial dyes. Fluorescence excitation spectra of some cationic-anionic dyes in nonpolar solvents disagree with their absorption spectra because of contact and solvent-separated ion pairs simultaneously present in the solution.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 69–75, January, 1993.     

Photophysics of benzazole derived push–pull butadienes: A highly sensitive fluorescence probes

Two new homologues of 1,4-diphenylbutadiene, namely, 1-(2-benzazolyl)-4-(p-dimethylaminophenyl)buta-1,3-diene have been synthesized and their absorption and fluorescence properties have been investigated in different organic solvents. The absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra, which show dual emission and high solvatochromic effect in polar solvents. Using an efficient solvatochromic method, a large excited state dipole moment parallel to the smaller ground state dipole moment was calculated. Other properties of the lowest excited state such as the planar ICT B_u nature, fluorescence quantum yield and the basicity of the two nitrogen atoms (of the benzoxazole or benzothiazole ring as well as the amino group) were studied by spectroscopic techniques and semiempirical AM1 quantum chemical calculations. The findings have been presented and discussed along with the promising fluorescence probing and pH-sensing properties of these two dienes. The main spectroscopic properties of the two derivatives have been also compared.     

Carbon-13 NMR studies of benzoquinones

The assignment of all ring carbons of p-benzoquinones derived from perezone and from thymoquinone was completed using gated decoupled spectra. The long range proton-carbon couplings are discussed in terms of the degree of substitution of the quinone ring. The tautomeric interconversion of the two energetically equivalent forms of 2,5-dihydroxy-1,4-benzoquinones has been studied in various solvents and at several temperatures.     

Optical properties of inherently dissymmetric polyamides

A study of the optical rotatory dispersion (ORD), circular dichroism (CD), and ultraviolet spectra (UV) of polyamides derived from optically active biphenyl acid chlorides, and aromatic, and aliphatic diamines, was made. The optically active monomers were (–)-(S)-2,2′-dinitro-6,6′-dimethylbiphenyl-4,4′-dicarbonyl chloride and (–)-(S)-2,2′-dichloro-6,6′-dimethylbiphenyl-4,4′-dicarbonyl chloride. The diamines were o-, m-, and p-phenylenediamine, piperazine, trans-2,5-dimethylpiperazine, and 1,2-pyrazolidine. The ORD spectra of the o-phenylenediaminepolyamide taken in different solvents indicated the existence of some ordered structure in the least polar solvent. All other polyamides existed in a random coil conformation in the solvents employed.     

The rα structure of allene: a study of solvent effects in NMR of oriented molecules

NMR spectra of partially oriented allene in 14 different liquid crystal solvents have been analyzed. The resulting direct couplings corrected for harmonic vibrations were used to determine the r_α-structures. Considerable solvent effects were detected which disappeared, if the data were corrected also for the correlated molecule deformation. A solvent independent r_α-structure which agreed well with IR results, and the interaction parameters of the CH and CC bonds of allene for all the solvents were determined.     

Synthesis,spectrophotometric, voltammetric,and density functional theory studies of tetrahydro[3,2-b]indolocarbazoles for sensing small molecules

Absorption spectra of tetrahydro[3,2-b]indolocarbazoles (THICZs) with respective molecular property and alkyl substituents have been prerecorded in diverse solvents in the scope between 200 and 600 nm for very fewer times in the literary study. The photo-physical behavior of liquified THICZs belonging to the existence surroundings. The solvatochromic behaviors of THICZs and solvent substance physical phenomenon can be analyzed by implementation of linear solvating energy state conception. Compound 4 shows excellent properties for sensing small molecules. The electrochemical behavior of some THICZs was investigated at carbon paste electrode where two electrode reactions were involved, irreversible oxidation-one electron transfer and quasi-reversible redox reactions forming phenolic followed by quinolone moiety electro active species. The density functional theory which means calculated molecular orbital energies (B3LYP/6-31G) and HOMO-LUMO gap/space for some presented indolocarbazoles have been performed.     

Photochromic polypeptides

Two photochromic polypeptides were synthesized by reaction of 1-(4-iodobutyl)-3,3- dimethylindolindolino-6′-nitrobenzospiropyran with poly-L -tyrosine; their molar contents on photochromic units were 27.3 and 44.7%. The spectra of the photo-induced merocyanines and their decoloration kinetics were compared with these of the monomeric model compound, obtained by reaction of the same N-(4-iodobutyl)-indolinospiropyran derivative with N-acetyltyrosine methyl ester. Different types of solvents have been examined, mainly dimethylformamide and pyridine, acetone and tetrahydrofuran, and methanol and ethylene glycol. The polypeptides showed a much less pronounced solvatochromism than their model; on the other hand, their absorption spectra presented two absorption maxima instead of one for the model. These differences in photochromic behavior were interpreted on the basis of the solvatation of the polymeric chain. Inverse photochromism was observed for polypeptide P₂ as well as for the model in ethylene glycol solution; this effect is due to a higher merocyanine content at the thermal equilibrium spiropyran ? merocyanine in high polar solvent.     

Spectrophotometric and spectroscopic studies of complexation of 8-hydroxyquinoline with π acceptor metadinitrobenzene in different polar solvents

The complexation of electron donor–acceptor complexes of 8-hydroxyquinoline (8HQ) and metadinitrobenzene (MNB) have been studied spectrophotometrically and thermodynamically in different polar solvent at room temperature. A new absorption band due to charge transfer (CT) transition is observed in the visible region. A new theoretical model has been developed which take into account the interaction between electronic subsystem of 8HQ and MNB. The results indicate the extent of charge transfer complexes (CTCs) formation to be more in less polar solvents. Stoichiometry of the complex was found to be 1:1 by straight line method and ¹H NMR between donor and acceptor at the maximum absorption bands. Ionization potential (I_D) and resonance energy (R_N) were determined from the CT transition energy in different solvents. The formation constants of the complexes were determined in different polar solvents from which ΔG° formation of the complexes was estimated and also extinction coefficient of the charge transfer complex (CTC) was calculated. Oscillator strength, transition dipole strengths and maximum wavelength of the CTC (λ_CT) in various solvents and IR spectra of the CTC have also been discussed. It has been observed that all parameters described above changed with change in polarity and concentration of donor.     

Dual fluorescence of omeprazole: effect of solvents and pH

Absorption, steady state fluorescence and time-resolved fluorescence spectra of omeprazole (OMP) have been studied in solvents of different polarity and pH. With an increase in the polarity of the solvents, blue shift is observed in the longer wavelength whereas red shift is noticed in the shorter wavelength band. The dual emission observed in non-polar solvents suggests that the energy of the twisted intramolecular charge transfer (TICT) state is lower than that of the locally excited (LE) state. The normal Stokes-shifted band originates from the LE state, and the large Stokes-shifted band is due to the emission from a TICT state. The Stokes shift of OMP is correlated with various solvent polarity scales like E_T(30) and f?(D,n).     

Studies on mesoionic compounds,II. The solvent effect on the electronic absorption spectra of mesoionic 2,3-diaryl-2H-tetrazolium-5-thiolate derivatives

The electronic absorption spectra of 2,3-di(2-fluorophenyl)-, 2,3-di(4-fluorophenyl)-, 2,3-di-(2-chlorophenyl)-, 2,3-di(2-methylphenyl)-, 2,3-di(4-methylphenyl)- and 2,3-di(2-methoxyphenyl)-2H-tetrazolium-5-thiolates have been measured in pure and mixed solvents. They were found to exhibit three characteristic absorption bands at 480-380 nm, near 250 and near 210; the longest wavenlength band of which was assigned to an n → π* transition.     

PHOTOPHYSICAL PROPERTIES AND CONFORMATIONAL EQUILIBRIUM OF trans-6-STYRYLQUINOXALINE

The wavelength dependent fluorescence spectra and bi-exponential fluorescence decays of trans-6-styrylquinoxaline (6-StQx) indicate that two conformers exist in equilibrium in solution. When comparing the conformationally restricted analogues (5Me-6-StQx and 7Me-6-StQx) with 6-StQx, the short-wavelength absorbing conformer of 6-StQx can be assigned to conformer(A) and the long-wavelength absorbing component to conformer(B). The temperature effects on the fluorescence lifetime indicate that the short-lived conformer(A) is the more stable conformer in non-polar solvents, while the long-lived conformer(B) is the stable component in polar solvents. The results of semiempir-ical AMI and CNDO/S CI calculations on both the ground and excited states are consistent with the conclusions drawn from the spectral data.     

Polarity-Induced Excited-State Intramolecular Proton Transfer (ESIPT) in a Pair of Supramolecular Isomeric Multifunctional Dynamic Metal–Organic Frameworks

A pair of supramolecular isomers of Cd^II-based MOF have been synthesized by utilizing a flexible N,N′-donor linker and a dicarboxylate with ESIPT (excited-state intramolecular proton transfer) fluorophore by varying the reaction media. One of the MOFs has a 3D four-fold interpenetrating framework with guest solvent in the structure that undergoes a solvent-dependent crystalline-to-crystalline structural transformation, which has been extensively studied by powder XRD and IR spectroscopy. The other MOF is structurally rigid in nature and has a two-fold interpenetrating structure without any guest molecules. Both the compounds show moderate CO₂ adsorption and one of them, the MOF with the four-fold interpenetrating structure, also shows moderately high H₂ adsorption. Furthermore, both the compounds show interesting luminescence behavior. In the solid state, the two compounds show single-peak spectra, whereas upon suspension of these compounds in polar solvents, the maxima split into two peaks with a large Stokes shift. On the other hand, in nonpolar solvents, only one emission maximum is observed. This solvatochromic dual-emission phenomenon is due to ESIPT, which has been extensively studied.