Solvatochromic effect on photophysical properties of Quinoxalin-2(1H)-one and 3-Benzylquinoxalin-2(1H)-one

Quinoxalin-2(1H)-one and its derived 3-Benzylquinoxalin-2(1H)-one were synthesized and characterized by UV–visible spectroscopy. The changes displayed by the photophysical properties of these molecules in different solvents can be explained in terms of a sum of dielectric polarity and hydrogen bonding effects taking part in the stabilisation of the structure. 3-Benzylquinoxalin-2(1H)-one exhibits two fluorescence emission bands (F _a and F _n) in very polar solvents and one band (F _n) in low polar solvents. These bands are assigned on the basis of the absorption and emission solvent effect. The abnormal fluorescence (F _a) observed in very polar solvents is attributed to an intermolecular interaction between solute and solvent molecules in the excited state (exciplex formation).     

Experimental Determination of Ground and Excited State Dipole Moments of N,N-Bis (2, 5-di-tert-butylphenyl)-3, 4:9, 10-perylenebis (dicarboximide) (DBPI) A Photostable Laser Dye

In the present work, the absorption, emission spectra and dipole moments(μ_g, μ_e) of N, N-bis (2, 5-di-tert-butylphenyl)-3, 4:9, 10- perylenebis (dicarboximide) (DBPI) have been studied in solvents of various polarities at room temperature. Using the methods of solvatochromism, the difference between the first excited singlet state (μ_e) and ground state (μ_g) dipole moments was estimated from Lippert – Mataga,, Bakhshiev, Kawski – Chamma – Viallet equations. The change in dipole moment (Δμ) was also calculated using the variation of the Stokes shift with microscopic solvent polarity parameter (E _T ^N ). It was observed that the value of excited singlet state dipole moment is higher (3.53 Debye) than the ground state one (1.92Debye), showing that the excited state of DBPI is more polar than the ground state.     

Excited State Lactim to Lactam Type Tautomerization Reaction in 5-(4-Fluorophenyl)-2-Hydroxypyridine: Spectroscopic Study and Quantum Chemical Calculation

The photophysical properties of 5-(4-fluorophenyl)-2-hydroxypyridine (FP2HP) have been studied by steady state and time resolved spectroscopy in combination with quantum chemical calculations. The molecule FP2HP exists as lactim and lactam form in the ground state due to small stability difference but does not undergo lactim to lactam isomerisation due to high barrier energy. Whereas in the excited state the lactim form undergoes tautomerization producing red shifted emission of the lactam tautomer along with the local emission of the lactim form. In polar protic solvents, the barrier for lactim-lactam tautomerization rapidly decreases forming the lactam tautomer only. Temperature has pronounced effect on the excited state tautomerization equilibrium and is clearly reflected in the measured equilibrium constant (K _tau ⁰ ) and free energy change (??G⁰). Structural calculations at Hartree Fock and Density Functional Theory levels, calculated stability of the isomers in different solvents using Polarized Continuum Model and the theoretical potential energy surfaces for the ground and excited states along the proton transfer coordinate are reported for the tautomeric equilibrium of FP2HP.     

Excited-state inter- and intramolecular proton transfer in methyl 3-hydroxy-2-quinoxalinate: effects of solvent and acid or base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical (AM1) and density functional theory using Gaussian 98 program calculations have been used to study the effects of solvent and acid or base concentration on the spectral characteristics of methyl 3-hydroxy-2-quinoxalinate (M3HQ). M3HQ is present as enol in less polar solvents and as keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to the phototautomer, formed by the excited-state intramolecular proton transfer, whereas fluorescence is only observed from keto in the polar solvents. In aqueous and polar solvents the monocation (MC5/MC6) is formed by protonating the carbonyl oxygen atom in the ground (S₀) and the first excited singlet states (S₁). Dication is formed by protonating one of N- atom of MC5/MC6. Monoanion is formed by deprotonating the phenolic proton of enol in the basic solution. pK_a values for different prototropic equilibriums were determined in S₀ and S₁ states and discussed.     

Preferential Solvation of 1,4-Dimethoxy-2,3-Dimethyl-9,10-Anthraquinone-A Spectrophotometric and Fluorometric Study

Electronic absorption and fluorescence emission spectra of DMDMAQ (1,4-dimethoxy-2,3-dimethyl-9,10-anthraquinone) have been studied as a function of solvent composition in some binary mixtures and in different neat solvents. The binary mixtures consist CCl₄ (Carbon tetrachloride)-DMSO (Dimethylsulfoxide), EtOH (Ethanol)-DMSO, and CCl₄-EtOH combination of single solvents. The wavelength maxima of the absorption band for DMDMAQ are quite solvent sensitive in aprotic solvents. But, in protic solvent, there is no marked shift in absorption and emission maximum which shows the absence of specific interaction. Excited state shows increasing shift with increasing solvent polarity compared to ground state. The ratio of dipole moment in the excited state to that in the ground state was calculated. Different criteria were considered to analyse preferential solvation characteristics in different binary mixtures, viz., local mole fraction (), solvation index (δ_S2) and exchange constant (K₁₂).     

Specific solvent effects of hydroxylic solvents on the emission properties of ruthenium(ii)tris(2,2′-bipyridyl) chloride

The excited state of Ru(II)[bpy]₃ ²⁺ dissolved in hydroxylic solvents is subject to specific solvent effects, which were hitherto not understood on a quantitative basis. We determined the solvent effects of linear monovalent alcohols on the energy gap law of internal conversion with the help of lifetime and intensity measurements. An on-line method for measurement of the temperature dependence of quantum efficiencies was introduced. A modified Franck-Condon analysis of emission spectra by taking into account the shape of a Morse potential of the involved states was applied to compute excited-state energies.Abbreviations used Ru(II)[bpy]₃ ²⁺ ruthenium(II)tris(2,2-bipyridyl) chloride - MLCT metal-to-ligand charge transfer - PMT photomultiplier tube     

Quenching of fluorescence by light: A new method to control the excited-state lifetimes and orientations of fluorophores

We report steady-state and time-resolved studies of quenching of fluorescence by light i.e. light quenching. The dyes rhodamine B (RhB) and 4-dicyanomethylene-2-methyl-6(p-dimethamino)-4H-pyrane (DCM) were excited in the anti-Stokes region from 560 to 615 nm. At a high illumination power the intensities of DCM and RhB were sublinear with incident power, an effect we believe is due to stimulated emission, andnot ground-state depopulation. The extent of light quenching was proportional to the amplitude of the emission spectrum at the incident wavelength, as expected for light-stimulated decay from the excited state. Control measurements at a decreased average illumination power, and in solvents of various viscosities, indicated that the effect was not due to undesired photochemical processes. Importantly, the frequency-domain intensity decays remained single exponentials, and the lifetimes were unchanged with light quenching, which suggests that the effect was not due to heating or other photochemical effects. These results are consistent with a quenching process which occurs within the quenching pulse. Importantly, as expected for light quenching with a single pulsed laser beam, the time 0 anisotropies of RhB and DCM were decreased due to orientation-dependent quenching of the excited-state population. In closing we discuss some possible future applications of light quenching to studies of dynamic processes.This report is partially based on the experimental data published previously [1,2].     

Solvent and Temperature Effects on Polymer-Coated Glass Fibers. Fluorescence of the Dansyl Moiety

E-type glass fibers were coated with poly(-aminopropyltriethoxysilane), treating them with a 1% (v/v) monomer aqueous solution. The fibers were labeled with a dansyl-sulfonamide conjugate by reaction of acetonitrile solutions of dansyl chloride with the amine groups immobilized on the glass fiber surface. Interactions of the labeled coating polymer with solvents of different polarities were estimated by measurements of the fluorescence band shifts of the label. It was found that for aprotic solvents, the solvent dipolar coupling relaxation mechanism is dominated by thermodynamic interactions of the solvent with the polymer matrix, while for protic solvents this mechanism is dominated by specific interactions between solvent molecules and the excited state of the chromophore. Different experimental excited-state dipole moments were obtained for nonpolar and polar solvents (^* _NP = 7.2 ± 1.6 D, ^* _P = 11.9 ± 1.5 D). Using the AM1 method, excited-state dipole moments for the first and second singlets were calculated and it was concluded that ^* _NP ^*2 ₁^1/2 and ^* _P ^*2 ₂^1/2. Accordingly, neither the glass support nor the coating polar influence the excited-state properties of dansyl. The temperature dependence of dansyl emission allows the determination of the relaxation temperature of the coating polymer, which was estimated as 175 K for the coating used.     

Self‐assembly of poly(3‐hexylthiophene) nanowire networks by a mixed‐solvent approach for organic field‐effect transistors

Organic field‐effect transistors (OFETs) based on interconnected nanowire networks of P3HT have been successfully fabricated by using a mixed‐solvent method. The nanowire network density can be tuned by controlling the anisole/chlorobenzene ratio of mixed solvents. The obtained field‐effect mobility, threshold voltage and the ratio of on‐state current and off‐state current (I_on/I_off) was 0.0435 cm²/V s, –10 V and 1.75 × 10⁴, respectively. The three‐dimensional and interconnected nanowire structure of the networks can enhance the charge transport in P3HT.

     

7-Hydroxy-4-methyl-8-(4′-methylpiperazine-1′-yl)methyl coumarin: An efficient probe for fluorescence resonance energy transfer to a bioactive indoloquinolizine system

Solvatochromic effects on the fluorescence behavior of 7-hydroxy-4-methyl-8-(4′-methyl-piperazine-1′ yl)methylcoumarin (HMMC) was studied in different solvents. The fluorescence of HMMC was found to be highly sensitive to both the polarity and the protic character of the solvent. Exploiting the polarity-sensitive fluorescence property of HMMC, its excited-state dipole moment has been determined. Fluorescence (Förster) resonance energy transfer (FRET) process from HMMC to a potent bioactive molecule 3-acetyl-4-oxo-6,7-dihydro-12 H indolo-[2,3-a] quinolizine (AODIQ) was studied. From the determined K_SV and R₀ values, it is argued that a long-range dipole-dipole interaction is operating for the energy transfer mechanism. The energy transfer efficiency (E) and the distance between the acceptor and the donor (r₀) have been determined.     

Luminescence-Based Oxygen Sensors: ReL(CO)3Cl and ReL(CO)3CN Complexes on Copolymer Supports

A new class of luminescent rhenium complexes has been tested as oxygen sensors based on luminescent quenching. ReL(CO)₃Cl and ReL(CO)₃CN (L = 2,2-bipyridine or 1,10-phenanthroline and substituted analogues) have several features that seem to indicate suitability as oxygen sensors. These include simple synthesis, long excited-state lifetimes, and high luminescence quantum yields. Intensity and lifetime oxygen quenching measurements were used to study the complexes in various polymer supports including homopolymers of PDMS (polydimethylsiloxane), a methacryloxy containing PDMS (Gp-163), and trimethylsilylmethylmethacrylate (T3642), and copolymers containing Gp-163 and T3642. In contrast to previous studies utilizing [Ru(4,7-diphenyl-1,10-phenanthroline)₃]²⁺ as an oxygen sensor, quenching of the Re complexes proved much more sensitive to the polymer support. With suitable supports, the rhenium chloro complexes demonstrated significant quenching; but the cyano complexes, in spite of being robust in solution, exhibited severe photochemical instability in polymers. The potential of this class of complexes as oxygen sensors and as molecular probes as well as the ramifications in the design of new and different types of sensors is discussed.     

Ionic equilibrium in mixtures of protophobic and protophilic polar non‐hydrogen bond donor solvents: acids,salts, and indicators in acetone containing 5 mol% DMSO

Properties of a protophobic polar non‐HBD solvent can be strongly modified by introduction of a small amount of a protophilic polar non‐HBD solvent. In this paper, acetone (AC) with 5 mol% additive of DMSO, a solvent with , was considered as a media for acid–base reactions. Conductance was used for determination of dissociation constants of a set of salts, hydrogen chloride, and picric acid. The last‐named was also studied by UV‐vis spectroscopy. The introduction of 5 mol% of DMSO results in suppressing, to some extent, the homoconjugation processes in AC media as well as of proton hydration by (possible) traces of water. The dissociation of salicylic acid and 2,4‐dinitrophenol was examined utilizing quinhydrone electrodes in a cell with liquid junction. The pK_a values of buffer acids and values of buffer solutions were calculated by taking into account the incomplete dissociation of salts. The response of the glass electrode appeared to be satisfactory, which allowed the estimation of the pK_a value of benzoic acid. The apparent ionization constants of 22 acid–base indicators in buffer mixtures and perchloric acid solutions were determined in (AC + 5 mol% DMSO) using the spectrophotometric procedure. Copyright © 2009 John Wiley & Sons, Ltd.     

Spectroscopic Studies of Fluorescent Dye <Emphasis Type="Italic">N</Emphasis>, <Emphasis Type="Italic">N</Emphasis>′- bis(2-hydroxy-5-methyl-benzylidene)-1,2-ethanediamine and its DNA Complex in Solution

Absorption and fluorescence emission properties of an N-salicylideneamine fluorescent dye molecule N, N-bis(2-hydroxy-5-methylbenzylidene)-1,2-ethanediamine (1) have been studied in three typical solvents—2-methylbutane, ethanol, dimethyl sulphoxide (DMSO), and its DNA complex in methanol/H₂O mixed solvent. The normal absorption band of 1 is observed in both aprotic and protic solvents and has been assigned to the l a transition in the enol form of 1. The long-wavelength absorption band of 1, which is caused by the formation of a cis-keto species in the ground state, is absent in aprotic solvents, but is observable in protic ones. Normal fluorescence emission from the excited enol state of 1 is obtained only when the normal absorption band is excited, while the excited-state intramolecular proton transfer (ESIPT) emissions from both cis- and trans-keto species are recorded in all cases, being acceptable for the variation of the relative emission intensities. A preliminary spectroscopic study of the 1–DNA complex indicates an intercalation-binding mode, the convincing supporting evidence being the enhanced ESIPT fluorescence intensity of 1 when complexed with DNA. Finally, a universal energy-state diagram is given to interpret the experimental results.     

The Fluorosolvatochromism of Brooker's Merocyanine in Pure and in Mixed Solvents

The fluorescence-based solvatochromism (fluorosolvatochromism) of 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (Brooker's merocyanine) was studied. The results revealed that the fluorescence emission band of the dye was dependent on the medium ( nm in water and nm in DMF). The fluorescence quantum yields (φ _f) were calculated for the dye in the solvents investigated. Low φ _f values ( < 10%) were obtained for the dye and in order to better comprehend the radiative and nonradiative decay processes of this dye, its fluorescence lifetime in methanol was measured and was found to be very short (230 ps). The results suggest that the dye in the excited state decays rapidly through nonradiative processes. The behavior of the probe in binary mixtures including a hydrogen-bond accepting solvent (acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide) and a hydroxylic solvent (water, methanol, ethanol, propan-2-ol, and butan-1-ol) was also investigated. All data were successfully fitted to a model based on solvent exchange equilibria, which allowed the separation of the different contributions of the solvent species in the solvation shell of the dye. The data obtained for the mixed solvents were explained based on solute–solvent and solvent–solvent interactions.     

Depletion of singlet and triplet states of 1-amino-5, 6, 7, 8-tetrahydronaphthalene due to external heavy-atom perturbation

Summary Quenching of luminescence of 1-amino-5, 6, 7, 8-tetrahydronaphthalene (ATHN) in nonpolar and polar solvents at 300 K and 77 K by the chloroalkanes carbon tetrachloride (CCl₄), methyl chloroform (CH₃CCl₃), chloroform (CHCl₃) and methylene chloride (CH₂Cl₂) is reported. Different photophysical parameters at 77 K of unperturbed and perturbed fluorescer ATHN in ternary solutions are determined. The bimolecular dynamic fluorescence quenching rate constant (K _q) increases with the electron affinity (EA) of the quenchers and a linear correlation exists between lnK _q and EA. The quenching depends on polarity of solvents. The quenching is ascribed to contact CT exciplex formed between chloroalkanes and excited singlet ATHN.

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Riassunto Si riporta lo spegnimento di luminescenza dell' l-amino-5, 6, 7, 8-tetraidronaftalene (ATHN) in solventi non polari e polari a 300 K e 77 K mediante i cloroalcani tetracloruro di metilene (CH₂Cl₂). Si determinano diversi parametri fotofisici a 77 K di ATHN fluorescente perturbato e non perturbato in soluzioni ternarie. La constante (K _q) del valore di smorzamento di fluorescenza dinamico bimolecolare aumenta con l'affinità elettronica (EA) degli spegnitori ed esiste una correlazione lineare tra lnK _q ed EA. Lo spegnimento dipende dalla polarità dei solventi. Lo spegnimento è attribuito all'exciplex CT di contatto formato tra cloroalcani e ATHN a singoletti eccitati.

     

Femtosecond time-resolved difference absorption spectroscopy of all-<Emphasis Type="Italic">trans</Emphasis>-β-Apo-8′-carotenal

The femtosecond time-resolved difference absorption spectra of all-trans-β-Apo-8′-carotenal have been recorded and analyzed by the singular-value decomposition (SVD) method followed by global fitting using a sequential model for the excited-state energy relaxation. With this model, we have obtained the excited-state absorption spectra and the lifetimes of the corresponding excited states both in nonpolar solvent n-hexane and polar solvent methanol. Three excited states, namely S₃(170fs), S₂(2.32ps) and S₁(26ps) in n-hexane, and two excited states S₂(190fs) and S₁(9.4ps) in methanol have been observed. The excited-state absorption spectra of all-trans-β-Apo-8′-carotenal in methanol display a red shift and broadeness, while the lifetime of S₁ state becomes shorter. It is proposed that these effects are related to the presence of a carbonyl functional group that leads to the solvent effect on the excited-state energy level. At the same time, it is shown that the SVD method is a useful tool in resolving the time-resolved absorption spectra.     

Fluorimetric Determination of Thyroxine Hormone with Eu(III)-(Pyridine-2,6-Dicarboxylate) Tris Complex

We describe a method for the determination of thyroxine (Thy) using its quenching effect on Eu(PDA)₃ ^3- tris complex fluorescence. The relative fluorescence intensities are measured at fixed _exc = 282 nm, _em = 615 nm by titrating the metal complex with Thy in piperazine buffer solution at pH 6.5. Data indicated an associative type of reaction of two molecules valid between 0.0 < R < 1.0, R being the mole ratio of Eu(PDA)₃ ^3- to Thy. Over this ratio and up to (R 1.0) collisional quenching of Eu(PDA)₃ ^3- complex ion emission is seen as a result of heavy atom effect, intermolecular energy transfer playing the main role. This is also confirmed by the Stern-Volmer equation. In optimized experimental conditions, the L- form of Thy is determined in a range of 15.5–551.6 g/ml (2.0 × 10^–5 – 7.1 × 10^–4 M) with relative error of ±1.17%.     

Solvent and pH Effects on the Fluorescence of 7-(Dimethylamino)-2-Fluorenesulfonate

A novel water-soluble solvatochromic molecule, 7-(dimethylamino)-2-fluorenesulfonate (2,7-DAFS), was prepared by a three-step reaction from 2-nitrofluorene in good overall yield. The pH and solvent effects on the UV-VIS absorption and fluorescence spectra of 2,7-DAFS have been studied. Protonation of the dimethylamino group switches the absorption from intramolecular charge-transfer (ICT) to π → π* transition. The ground state pKa value of 2,7-DAFS was determined as 4.51. The fluorescence spectrum of the excited basic form, *(DAFS), shows a structureless single band with a large Stokes shift, whereas that of the acidic form, *(⁺HDAFS), exhibits a structured band with a small Stokes shift. The emission intensities of the basic and acidic forms versus pH/Ho plots show stretched sigmoidal curves and indicate that (1) the rate of deprotonation of *(⁺HDAFS) is comparable to the fluorescence decay of the species, and (2) the efficient proton-induced quenching of *(DAFS) fluorescence occurs. The pKa* was estimated as −1.7 from the fluorescence titration curve. The fluorescence maximum of *(DAFS) is blue-shifted as the polarity of solvent decreases. However, no clear dependency of the emission intensity and spectral half width, and thus fluorescence quantum yield, on the solvent polarity was revealed. It appears that the fluorescence sensitivity of 2,7-DAFS is 15 ∼ 25 times greater than the sensitivity of a widely utilized fluorescent probe, 5-(dimethylamino)-1-naphthalenesulfonate. This higher sensitivity, together with the ease of derivatization, would provide the fluorene-based fluorescent molecules significant advantages for a variety of applications.     

Synthesis,Spectral, Solvent Dependent Linear and Nonlinear Optical Characteristics of (E)-N-(3-(3-(4(dimethylamino)phenyl)acryloyl) phenyl)quinolone-2-carboxamide

This paper presents the synthesis of novel organic compound (E)-N-(3-(3-(4(dimethylamino)phenyl)acryloyl)phenyl)quinolone-2-carboxamide, also known as Quinolinecarboxamide Chalcone (QCC) using aldol condensation and carboxamide formation method. The organic sample QCC was examined by FT-IR, ¹H NMR, ¹³C NMR and mass spectroscopic techniques, respectively. Linear and third-order nonlinear optical (TNLO) properties of QCC dissolved in polar solvents such as DMSO, DMF and Ethanol have also been studied. The order of nonlinear refractive index and nonlinear absorption coefficient of QCC was measured to be 10^?11 m²/W and 10^?5 m/W. The nonlinear refractive index (n₂) of QCC was attributed to negative nonlinearity due to self-defocusing effect, and nonlinear absorption coefficient (β) indicates the behaviors of saturable absorption (SA) and reverse saturable absorption (RSA). The real and imaginary features of the TNLO susceptibility (χ⁽³⁾) of QCC in polar solvents were calculated to be the order of 10^─7 esu. The spectral characteristics of solvent on TNLO susceptibility of QCC were discussed. The results divulged that the synthesized organic compound is a novel nonlinear optical (NLO) material for applications in photonics and optoelectronics.

     

Synthesis,properties, and solvatochromism of 1,3‐dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl) phenyl]methylidene}‐(1H,3H)‐pyrimidine‐2,4,6‐trione

A new merocyanine dye, 1,3‐Dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl)phenyl]methylidene}‐ (1H, 3H)‐pyrimidine‐2,4,6‐trione 3 , has been synthesized by condensation of 2‐[4‐(piperidyl)benzoyl]thiophene 1 with N,N′‐dimethyl barbituric acid 2 . The solvatochromic response of 3 dissolved in 26 solvents of different polarity has been measured. The solvent‐dependent long‐wavelength UV/Vis spectroscopic absorption maxima, v_max, are analyzed using the empirical Kamlet–Taft solvent parameters π* (dipolarity/polarizability), α (hydrogen‐bond donating capacity), and β (hydrogen‐bond accepting ability) in terms of the well‐established linear solvation energy relationship (LSER): (1) The solvent independent coefficients s , a , and b and (v_max)₀ have been determined. The McRae equation and the empirical solvent polarity index, E_T(30) have been also used to study the solvatochromism of 3 . Copyright © 2007 John Wiley & Sons, Ltd.