Solvent effect on absorption and fluorescence spectra of coumarin laser dyes: evaluation of ground and excited state dipole moments

The effect of solvents on absorption and fluorescence spectra and dipole moments of coumarin 307 (C307) and coumarin 522B (C522B) have been studied extensively in various solvents, viz., general solvents, alcohols and binary mixtures (acetonitrile-benzene) at 298K. The bathchromic shift observed in absorption and fluorescence spectra of C307 and C522B with increasing solvent polarity indicates that transition involved are pi-->pi*. Solvatochromic correlations were used to obtain the ground and excited state dipole moments. The excited state dipole moments are observed to be greater than their ground state counterparts in all the solvents studied. Further, the experimentally obtained Deltamu were compared with those using normalized polarity terms E(T)(N) from Reichardt equation.     

Solvent effects on absorption and fluorescence spectra of diphenyltriafulvenes. Three modes of dipole moment variation upon excitation

Three representative diphenyltriafulvenes, 4,4′-dicyano-1,2-diphenyltriafulvene (I), hexaphenyltriapentafulvalene (II) and 10-(2,3-diphenylcyclopropenyliden)-anthrone (III) have been investigated. On excitation, the dipole moment of I decreases and reverses sign, i.e., from a value of 7.9 D in the ground state to a value of about ?5.3 D in the first excited electronic state. That of II remains unchanged at 6.3 D, and that of III increases from 8.5 D to about 26 D. The Onsager radius of 1 was found to be about 8.7 Å and that of IIi, about 12 Å.     

Solvent effects on the absorption and fluorescence spectra of some laser dyes: estimation of ground and excited-state dipole moments

The absorption and fluorescence spectra of three extensively used laser dyes namely 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), 2-(4'-t-butylphenyl)-5-(4'-biphenylyl)-1-oxa-3,4-diazole (BPBD), 1,4-bis[2-(2-methylphenyl)ethenyl]-benzene (Bis-MSB) have been recorded at room temperature (300K) in solvents of different polarities. The effects of the solvents upon the spectral properties are discussed. The ground-state dipole moments (mu(g)) were determined experimentally by Guggenheim and Higasi method separately and were compared with theoretical values obtained using quantum chemical method. The ground-state dipole moments obtained by using Guggenheim method were then used in the estimation of excited-state dipole moments (mu(e)) by using Lippert's, Bakhshiev's and Kawski-Chamma-Viallet's equations. In all the above three equations the variation of the Stokes shift with the solvent dielectric constant and refractive index was made use of. It was observed that dipole moments of excited state were higher than those of the ground state for all the dyes.     

Comments on the determination of excited state dipole moment of molecules using the method of solvatochromism

The present note comments on several publications which appeared in different journals containing many inaccurate statements and lacking honest citations of basic papers dealing with the application of solvatochromism to determine excited state dipole moments.     

The electronic spectra and excited state dipole moment of 2-fluoropyridine

The electronic absorption spectrum in the vapour state and in solution in different solvents in the region 3000–1900 Å and the fluorescence and phosphorescence emission spectra in ethanol or cyclohexane at 77 K have been studied for 2-fluoropyridine and analysed. Two systems of absorption band corresponding to the π→π* transition II and π→π* transition III have been observed and the excited state dipole moments have been determined from the solvent-induced shifts of the electronic absorption bands. The half-life of phosphorescence in cyclohexane at 77 K is found to be 3.5 s.     

Electronic absorption spectrum and excited state dipole moment of 2,6-difluoropyridine

The u.v. absorption spectrum of 2,6-difluoropyridine in the region 41 000-34 000 cm⁻¹ in the vapour state and in solution has been recorded and a vibronic analysis made. Only one system of bands arising from the π → π* transition has been observed and the 0,0 band is located at 37 840 cm⁻¹ in the vapour-phase spectrum. The oscillator strength of the band system in solution and the dipole moment in the excited state associated with the transition were determined.     

Effects of solvent on the electronic absorption and fluorescence spectra of quinazolines,and determination of their ground and excited singlet-state dipole moments

The electronic absorption, and fluorescence excitation and emission spectra of 11 quinazolines have been measured at room temperature (298 K) in several solvents of different polarities (cyclohexane, dioxane, ethylether, chloroform, ethylacetate, 1-butanol, 2-propanol, ethanol, methanol, acetonitrile, dimethylformamide and dimethyl sulfoxide). The effects of the solvent upon the spectral properties are discussed. Experimental ground-state dipole moments were measured for quinazolines and were used in combination with the spectral results to evaluate their first excited singlet-state dipole moments by means of the solvatochromic shift method. The theoretical ground and excited singlet-state dipole moments for selected quinazolines were calculated as a vector sum of the π-component (obtained by the PPP method) and the σ-component (obtained from σ-bond moments). A reasonable agreement was observed between the experimental and the theoretical values. Excited singlet-state dipole moments are higher than the ground-state values for most quinazolines.     

Study of the electronic spectra of 2-fluoro-5-chloropyridine and determination of the excited state dipole moment

The ultraviolet absorption spectrum in the region 300-190 nm in the vapour phase and in solution in different solvents, and the luminescence emission spectra in ethanol and cyclohexane at 77 K have been measured for 2-fluoro-5-chloropyridine and analysed. The molecule shows two systems of absorption bands corresponding to the π→π* transition II and π→π* transition III. The oscillator strength of the two systems of absorption bands in solutions and the excited state dipole moment in the ¹π, π* state have been determined. The half-life of phosphorescence in cyclohexane is measured and found to be 3·6 s.     

The electronic absorption spectrum and excited state dipole moment of 3-fluoropyridine

The electronic absorption spectrum of 3-fluoropyridine in the vapour state and in solutions in different solvents in the region 3000-1900 Å has been measured and analysed. Three systems of absorption bands; n→π* transition I, π→π* transition II and π→π* transition III are identified. The oscillator strength of the absorption band systems due to the π→π* transition II and π→π* transition III and the excited state dipole moments associated with these transitions have been determined by the solvent-shift method.     

Solvent- andpH-dependence of the absorption and fluorescence spectra of harman: Detection of three ground state and four excited state species

ThepH-dependence of the absorption and fluorescence spectra of the alkaloid harman has been investigated. Three species, namely the cation, the neutral molecule and the anion have been found in absorption, whilst four species, namely the cation, the neutral molecule, the anion and the zwitterion were detected by fluorimetry. The zwitterion must be formed by a double proton transfer during lifetime of the excited state. Fluorescence quantum yields are entirely different for the various species, being highest for the cation ( _f in 1N sulfuric acid 0.89).Unlike quinine sulfate the fluorescence of harman cation is not quenched by chloride ion, which suggests its use as a fluorescence standard superior to quinine.The ground statepK _a 's of harman are 7.37 and 14.6, the excieted statepK _a 's, as calculated from theFörster-Weller-equation, are 12.0 and 8.65. Thus the observed zwitterion fluorescence is predicted from the calculations.Dedicated to Prof. Dr. Dr. h. c.O. Kratky on the occasion of his 80th birthday.     

Simulation of x-ray absorption near-edge spectra and x-ray fluorescence spectra of optically excited molecules

The x-ray absorption near-edge spectra (XANES) and fluorescence spectra of molecules in the ground state and optically excited states are computed using time-dependent density functional theory and time-dependent Hartree-Fock theory. The calculated XANES spectra of optically excited methanol, benzonitrile, hydrogen sulphide, and titanium tetrachloride and the fluorescence spectra of optically excited methanol can be used to simulate ultrafast optical pump/x-ray probe experiments.     

Electronic absorption and fluorescence spectra and excited singlet-state dipole moments of biologically important pyrimidines

Electronic absorption and fluorescence emission spectra of several biologically important pyrimidines were measured at room temperature (298 K) in the following solvents: dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol, 2-propanol, methanol, dimethylformamide, acetonitrile, and dimethyl sulfoxide. The compounds studied were uracil, thymine, cytosine, 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, 2-thiouracil, barbituric acid, and orotic acid. In combination with the ground-state dipole moments of the above compounds, these spectral data were used to determine their lowest excited singlet-state dipole moments using the soivatochromic method. The effects of the solvent upon the spectral properties and of the structure upon the ground and excited singlet-state dipole moments are discussed. For most of the compounds, the excited singlet-state dipole moments are higher than their ground-state counterparts.The theoretical dipole moments for all the pyrimidines listed above, as well as for pyrimidine, alloxan, and uracil-5-carboxylic acid, were calculated by two methods. One approach involved a combination of the PPP (-LCI-SCF-MO) method for the -contribution to the overall dipole moment and the -contribution obtained as a vector sum of the -bond moments and group moments. The second set of theoretical values was obtained by the CNDO/2 method. The results were compared with the experimental dipole moments.Presented, in part, at the XIth IUPAC Symposium on Photochemistry, Lisbon, Portugal, July 27–August 1, 1986, and at the 192nd National Meeting of the American Chemical Society, Anaheim, CA, September 7–12, 1986. Part of this work was carried out at the University of Texas at El Paso, El Paso, TX, during the tenure of one of the authors there (C. P.). This paper is dedicated to Dr. Rudolf Zahradník, the teacher of one of the authors (C. P.) and the authors' friend and colleague.     

Effect of substituents on the fluorescence and absorption spectra of coumarins

The absorption and fluorescence emission of substituted coumarins have been studied in different solvents. It is observed that the substituents alter both the absorption and fluorescence emission maxima. These shifts are discussed in terms of the change in mobility of π-electrons caused by the nature and position of the substituent group in the parent compound.     

A quantitative study of the effect of solvent on the electronic absorption and fluorescence spectra of substituted phenothiazines: evaluation of their ground and excited singlet-state dipole moments

Electronic absorption and fluorescence excitation and emission spectra of five phenothiazines (phenothiazine, promethazine, thionine, methylene blue and Azure A) were determined at room temperature (293 K) in several solvents of various polarities (cyclohexane, dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol, 2-propanol, ethanol, methanol, acetonitrile, dimethylformamide and dimethyl sulfoxide). The effect of the solvents upon the spectral characteristics was studied. In combination with the ground state dipole moments of these phenothiazines, the spectral data were used to evaluate their first excited singlet-state dipole moments by means of the solvatochromic shift method (Bakhshiev's and Kawski—Chamma—Viallet correlations). The theoretical ground and excited singlet-state dipole moments for phenothiazines were calculated as a vector sum of the π component (obtained by the Pariser—Parr—Pople method) and the σ component (obtained from σ-bond moments). A reasonable agreement was found with the experimental values. For most phenothiazines under study, excited singlet-state dipole moments were found to be significantly higher than their ground-state counterparts. The application of the Kamlet—Abboud—Taft solvatochromic parameters to the solvent effect on spectral properties of phenothiazines is discussed.     

The experimental studies on the determination of the ground and excited state dipole moments of some hemicyanine dyes

The ground state (mu(g)) and excited state (mu(e)) dipole moments of 15 hemicyanine dyes were studied at room temperature. They were estimated from solvatochromic shifts of the absorption and the fluorescence spectra as function of the solvent dielectric constant (varepsilon) and refractive index (n). In this paper we applied the Stokes shift phenomena not only for the determination of the difference in the dipole moment of excited state and ground state, but to determine the value of polarizability alpha as well. The obtained results show that excited state dipole moments of hemicyanine dyes are in the range from 5 to 15 Debye, and the difference between the excited and ground state dipole moments vary from 1 to 7 Debye. The excited and ground state dipole moments difference (mu(e)-mu(g)) obtained for selected dyes are positive. It means that the excited states of the dyes under the study are more polar than the ground state ones. Additionally, the value of both polarizability (alpha) and the Onsager radius (a) of each investigated hemicyanine dye molecule are determined, and the ratio of alpha/a(3) for each dye were calculated, which oscillate from 0.29 to 5.21. The increase in dipole moment has been explained in terms of the nature of excited state and its resonance structure.     

Determination of dipole moment in the ground and excited state by experimental and theoretical methods of N-nonyl acridine orange

The absorption and fluorescence spectra of N-nonyl acridine orange are determined at room temperature (298 K) in cyclohexane, benzene, carbon tetrachloride, chloroform, chlorobenzene and dichloromethane. The ground state of dipole moment was obtained by impedance measurements using Guggenheim-Debeye's method. The experimental excited state dipole moment of N-nonyl acridine orange was determined using Bakhshiev's and Kawski-Chamma-Viallet's formulae and solvent polarity parameter proposed by Reichardt. These experimental results were completed with theoretical results using quantum chemical methods. The experimental (muexp=10.76 D) and theoretical (mucal=9.9 D) dipole moments in the ground and excited state (muexp*=14.56 D) were compared.     

Vibronic absorption spectra of acenaphthene in solution and its excited state electric dipole moments and polarizabilities

The vibronic spectra of acenaphthene in solution have been studied in detail in the region 27778–50000 cm⁻¹. A vibronic analysis of the two longest-wavelength absorption bands was made to reveal the vibrational modes that contribute to the enhancement of the intensities of these bands. The oscillator strengths of the various electronic transitions and the electric dipole moments and polarizabilities of several excited states were determined, the latter two by the solvent spectral frequency shift method.     

A genetic algorithm based determination of the ground and excited (1Lb) state structure and the orientation of the transition dipole moment of benzimidazole

The structure of benzimidazole has been determined in the electronic ground and excited states using rotationally resolved electronic spectroscopy. The rovibronic spectra of four isotopomers and subsequently the structure of benzimidazole have been automatically assigned and fitted using a genetic algorithm based fitting strategy. The lifetimes of the deuterated isotopomers have been shown to depend on the position of deuteration. The angle of the transition dipole moment with the inertial a-axis could be determined to be -30 degrees. Structures and transition dipole moment orientation have been calculated at various levels of theory and were compared to the experimental results.     

Solvent effects on the steady-state absorption and fluorescence spectra of uracil, thymine and 5-fluorouracil

We report a comparison of the steady-state absorption and fluorescence spectra of three representative uracil derivatives (uracil, thymine and 5-fluorouracil) in alcoholic solutions. The present results are compared with those from our previous experimental and computational studies of the same compounds in water and acetonitrile. The effects of solvent polarity and hydrogen bonding on the spectra are discussed in the light of theoretical predictions. This comparative analysis provides a more complete picture of the solvent effects on the absorption and fluorescence properties of pyrimidine nucleobases, with special emphasis on the mechanism of the excited state deactivation.     

A new method for estimating excited state dipole moments of molecules from the solvent effect on their electronic spectra

Some comments are made on the paper entitled “Excited state dipole moments of some monosubstituted benzenes from the solvent effect on electronic absorption spectra” by Prabhumirashi et al. (Spectrochim. Acta 39A, 663,1983) and a new method, which has been tested on some molecules and found satisfactory, is suggested for estimating the excited state dipole moment of molecules from the solvent effect on their electronic spectra.