Stimulated resonance Raman scattering of Rhodamine B

Stimulated resonance Raman scattering (SRRS) of the Rhodamine B (RhB) in the methanol (CH₃OH) or ethanol (C₂H₅OH) solutions have been observed. SRRS from RhB can be distinguished from the amplified spontaneous emission (ASE) of RhB with increasing the input laser energy. When the RhB is dissolved in the methanol or ethanol (10^?3 mol/L), the RhB have the different peak wavelengths of SRRS, ASE, fluorescence and absorption, respectively. SRRS can be amplified by the ASE gain when the SRRS is near the peak of the ASE, and the peak wavelength of SRRS coincides with the maximal intensity wavelength of ASE.     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

The stimulated Raman scattering competition between solute and solvent in Rhodamine B solution

The competition between the stimulated resonance Raman scattering (SRRS) of Rhodamine B (RhB) and the stimulated Raman scattering (SRS) of ethanol (C₂H₅OH) is observed at the RhB in C₂H₅OH solution. For different concentrations of the solution, the peak wavelengths of the SRRS, the amplified spontaneous emission (ASE), the fluorescence and the absorption of RhB are different. The SRRS of RhB and the SRS of C₂H₅OH are simultaneously generated when the concentration of the solution is 10^-5 mol/L and the energy of the excitation laser is 20.4 mJ. Otherwise, only either the SRRS of RhB or the SRS of C₂H₅OH is generated. The SRRS can be amplified by the ASE gain when the SRRS is near the peak of the ASE, and the peak wavelength of the SRRS coincides with the wavelength of the maximal intensity ASE.     

Effect of pH on the fluorescence and phosphorescence spectra of indolecarboxylic acids. Evidence for the existence of a singlet excited-state indole acid dianion

Room-temperature electronic absorption, fluorescence, and low-temperature (77 K) phosphorescence spectra of a series of indolecarboxylic acids have been measured in predominantly aqueous solutions at pH 1.2, 7.0, 12.5, and in 5 N NaOH. The ¹L_a and ¹L_b π, π ¹ states have been assigned to the electronic transitions for all the compounds under study. The quenching of fluorescence, the appearance of a new fluorescence emission band, red-shifted by 40 nm, in very alkaline media, and the disappearance of the indole carboxylate short-wavelength band have been attributed to the formation of an excited singlet-state indolate carboxylate (dianion) stabilized by hydrogen bonding with the solvent. The existence of two successive excited singlet-state prototropic equilibria is discussed.     

Kamlet-Taft and Catalan Studies of Some Novel Y-Shaped Imidazole Derivatives

Some novel Y-shaped imidazole derivatives were developed and characterized by NMR and mass spectral techniques. The photophysical properties of these imidazole derivatives were studied in several solvents. The Kamlet-Taft and Catalan’s solvent scales were found to be the most suitable for describing the solvatochromic shifts of the absorption and fluorescence emission. The adjusted coefficient representing the electron releasing ability or basicity of the solvent, C _β or C _SB has a negative value, suggesting that the absorption and fluorescence bands shift to lower energies with the increasing electron-donating ability of the solvent. This effect can be interpreted in terms of the stabilization of the resonance structures of the chromophore. The observed lower fluorescence quantum yield may be due to an increase in the non-radiative deactivation rate constant. This is attributed to the loss of planarity in the excited state provided by the non co-planarity of the cinnamaldehyde ring attached to C(2) atom of the imidazole ring. Such a geometrical change in the excited state leads to an important Stokes shift, reducing the reabsorption and reemission effects in the detected emission in highly concentrated solutions.     

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).     

Absorption and Fluorescence Emission Attributes of a Fluorescent dye: 2,3,5,6-Tetracyano-p-Hydroquinone

Four cyano groups have been substituted on the aromatic ring of p-hydroquinone (2,3,5,6-tetracyanohydroquinone) in order to study the enhanced photoacidity of this molecule. The acid-base equilibria have been studied using absorption (for ground state pK_a) and fluorescence (excited state pK_a) spectra. Three distinct species (neutral, anionic and dianionic forms) were observed in the ground state and only two species (anionic and dianionic forms) were found in the excited state when studied at different pH/H_o in water. Absorption and emission characteristics were studied in various organic solvents, including protic and aprotic solvents. Deprotonation was also investigated using binary mixtures. It has been revealed that absorption and emission spectra are considerably changed with change in media. Proton transfer to the solvent has been observed in various solvents.     

Photophysical properties of a series of 4‐aryl substituted 1,4‐dihydropyridines

In this article, a series of Hantzsch 1,4‐dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absorption located around 350 nm and fluorescence emission in the blue–green region. A higher Stokes’ shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited state from the dimethylaniline to the dihydropyridine chromophores, which was corroborated by a linear relation of the fluorescence maxima (ν_max) versus the solvent polarity function (Δf) from the Lippert–Mataga correlation. A comparison between the experimental data and time‐dependent density functional theory‐polarizable continuum model calculations of the vertical transitions was performed to help on the elucidation of the photophysics of these compounds. For these calculations, the S₀ and S₁ states were optimized using Becke, three‐parameter, Lee–Yang–Parr/6‐31 G* and Configuration Interaction Singles/6‐31 G*, respectively. The predicted absorption maxima are in good agreement with the experimental; however, the theoretical fluorescence emission maxima do not match the experimental, which means that the excited specie cannot be related to neither a locally excited state nor to an aromatized structure. Copyright © 2012 John Wiley & Sons, Ltd.     

芴类荧光染料掺杂DNA-CTMA薄膜的放大自发辐射特性

钯催化Suzuki反应合成得到一种9,9-二乙基-2,7-二-(4-吡啶)芴（DPFP）荧光染料,研究了该染料的吸收和荧光光学特性,以及DPFP掺杂DNA-CTMA薄膜的荧光光谱特性和放大自发辐射特性。实验结果表明:DPFP的吸收峰位于333 nm,DPFP的荧光光谱在370 nm和386 nm出现荧光峰,在408 nm出现肩峰,存在从激发态S1能级到基态S0能级的S10-S00,S10-S01和S10-S02三种振动带的电子跃迁;同时,在Nd:YAG纳秒激光器355 nm输出光的泵浦下,DPFP掺杂DNA-CTMA薄膜在波长390 nm和406 nm处实现了放大自发辐射,其阈值能量密度分别为3.24和3.40 mJ/cm2;此外,通过调节DPFP掺杂DNA-CTMA的质量比可以实现特定波长的放大自发辐射。     

Spectral behavior study of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone

The photophysical properties such as singlet absorption, molar absorptivity, fluorescence spectra, fluorescence quantum yield (?_f) and transition dipole moment (μ₁₂) of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone (DMAPAPP) have been studied in different media. DMAPAPP exhibits a large red-shift in both absorption and emission spectra as the solvent polarity increases, indicating a large change in dipole moment of molecule upon excitation. The fluorescence quantum yield depends on the nature of the solvent. The absorption and emission spectra of DMAPAPP in dioxane–water mixture are also studied. The effect of different type of surfactants to determine their critical micellar concentration (CMC) and the microemulsion effect on the electronic absorption and emission spectra of DMAPAPP are recorded. The effect of acidity on the electronic absorption and emission spectra of DMAPAPP is studied to determine the pK_a and pK_a^? values.     

Fluorescence studies of retinol and retinyl acetate

The polarized emission study of all-trans retinol and retinyl acetate in solid films at room temperature has been carried out. The shape of the absorption and fluorescence is typical of Franck-Condon forbidden bands. The high positive polarization of the fluorescence excitation and the emission (when excited into main absorption band due to 1_Bu ← 1_Ag transition) suggests that the lowest excited singlet state of retinols has a large amount of 1_Bu character. The explanation for the high positive polarization involving strong vibronic interactions suggesting 1_Ag - (a very weakly allowed state) being the lowest excited singlet has also been considered.     

Photophysical parameters and laser performance of 3-(4′-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP): A new laser dye

The spectral properties such as singlet absorption, molar absorptivity, emission spectra, fluorescence quantum yield and excited state lifetime of 3-(4′-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP) have been determined in different solvents. DMAFP dye exhibits a large red shift in both electronic absorption and emission spectra as the solvent polarity increases, indicating a large change in the dipole moment of molecules upon excitation. A crystalline solid of DMAFP gives an excimer like emission at 566 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B-type class of Steven's classification. The ground and excited state protonation constants of DMAFP are calculated and amounted to 1.71 and 8.3, respectively. DMAFP acts as a good laser dye upon pumping with nitrogen laser (λ_ex=337.1 nm) in chloroform, methylene chloride and dioxane and gives laser emission in the range 460–590 nm. The laser parameters such as the tuning range, gain coefficient (α), emission cross section (σ_e) and half-life energy (E_1/2) are calculated. The photoreactivity and net photochemical quantum yield of DMAFP in chloromethane solvents are also studied.     

Dual-Fluorescence Probe of Environment Basicity (Hydrogen Bond Accepting Ability) Displaying no Sensitivity to Polarity

3-Hydroxyquinolones (3HQs) are a new class of water soluble dual fluorescence probes that can monitor both polarity and basicity (H-bond accepting ability) parameters. Both parameters play an important role in proteins and lipid membranes. Nevertheless, no method exists actually to measure the basicity parameter separately from the polarity. To achieve this aim, we synthesized 2-benzofuryl-3-hydroxy-4(1H)-quinolone (3HQ-Bf) and characterized its photophysical properties by UV, steady-state and time-resolved fluorescence spectroscopy. Due to its extended conjugation and totally planar conformation, 3HQ-Bf is characterized by a high fluorescence quantum yield. In solution, this dye shows an excited state intramolecular proton transfer (ESIPT) reaction resulting in two tautomer bands in the emission spectra. The ESIPT reaction can be considered as irreversible and is governed by rate constants from 0.6 to 8 × 10⁹ s⁻¹, depending on the solvent. The analysis of the spectral properties of 3HQ-Bf in a series of organic solvents revealed a marginal sensitivity to the solvent polarity, but an exquisite sensitivity to solvent basicity, as shown by the linear dependence of the logarithm of the emission bands intensity ratio, log(I_N*/I_T*), as well as the absorption or emission maxima wavenumbers as a function of the solvent basicity parameter. This probe may find useful applications through coupling to a protein ligand, for characterizing the H-bond acceptor ability at the ligand binding site as well as for studying the basicity changes of lipid membranes during their chemo- and thermotropic conversions.     

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).     

Photophysical properties, laser activity and photoreactivity of a heteroaryl chalcone: A model of solvatochromic fluorophore

The absorption and fluorescence characteristics of 3-(4′-dimethylaminophenyl)-1-(2-thienyl)prop-2-en-1-one (DMATP) have been investigated in different solvents. DMATP dye exhibits a large red shift in both absorption and emission spectra as solvent polarity increases, indicating a large change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMATP gave an excimer like emission at 570 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B-type class of Steven's classification. A dye solution ca. 10⁻³ mol dm⁻³ in CHCl₃ gave a good laser emission in the range 480–560 nm with emission maximum at 530 nm upon pumping by nitrogen laser (λ_ex=337.1 nm). The excitation energy transfer from 7-dimethylamino-4-methyl coumarine (DMC) to DMATP has been also studied in CHCl₃ and the values of energy transfer rate constant and critical transfer distance indicate a Főrster-type mechanism. The photoreactivity and net photochemical quantum yield of DMATP in chloromethane solvents are also determined. We applied semiempirical MO calculation using AMI and ZINO/S calculation to understand the geometric and electronic structure of DMATP molecule in both ground and excited states.     

Excited-state hydrogen bonding effect on dynamic fluorescence of coumarin 102 chromophore in solution: A time-resolved fluorescence and theoretical study

The steady-state absorption and emission as well as the time-resolved fluorescence spectra of coumarin 102 (C102) in both aprotic and alcoholic solvents have been used to study the effect of excited-state hydrogen bond on the dynamic fluorescence of C102 chromophore in various solutions. The dual fluorescence of C102 in alcohols, which is dependent on the hydrogen-bonded donation ability of the solvent, has been assigned to the distribution of free C102 and a hydrogen-bonded complex. Furthermore, a shift of the fluorescence spectra induced by excited-state hydrogen bond has been demonstrated to take place within hundreds of picoseconds by the performance of the time-resolved fluorescence spectra with the time-correlated single-photon-counting (TCSPC) technique. Moreover, the time-dependent density functional theory (TDDFT) has been used to calculate the hydrogen-bonded equilibrium constant pK_HB in different electronic states. It has been demonstrated for the first time that the hydrogen bond strengthening in electronic excited states could decrease the free energy of the hydrogen-bonded complex due to its stronger binding energy. Therefore, the hydrogen-bonded equilibrium will become markedly in favor of the hydrogen-bonded forms in electronic excited states by comparison with the case in the ground state.     

Fluorescence,Photophysical Behaviour and DFT Investigation of E,E-2,5-bis[2-(3-pyridyl)ethenyl]pyrazine (BPEP)

E,E-2,5-bis[2-(3-pyridyl)ethenyl]pyrazine (BPEP) has been prepared by aldol condensation between 2,5-dimethylpyrazine and pyridine-3-carboxaldehyde. It is characterized by IR, ¹H NMR, and ¹³C NMR. The electronic absorption and emission properties of BPEP were studied in different solvents. BPEP displays a slight solvatochromic effect of the absorption and emission spectrum, indicating a small change in dipole moment of BPEP upon excitation. The dye solutions (1 × 10^?4 M) in CHCl₃, EtOH and dioxane give laser emission in blue region upon excitation by a 337.1 nm nitrogen pulse (λ = 337 nm). The tuning range, gain coefficient (α) and emission cross – section (σ_e) have been determined. Ground and excited states electronic geometric optimizations were performed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), respectively. A DFT natural bond analysis complemented the ICT. The simulated maximum absorption and emission wavelengths are in line the observed ones in trend, and are proportionally red-shifted with the increase of the solvent polarity. The stability, hardness and electrophilicity of BPEP in different solvents were correlated with the polarity of the elected solvents. BPEP dye displays fluorescence quenching by colloidal silver nanoparticles (AgNPs). The fluorescence data reveal that radiative and non-radiative energy transfer play a major role in the fluorescence quenching mechanism.     

Stilbene-Like Molecules as Fluorescent Probes Applied for Monitoring of Polymerization Process

2-(p-N,N-dimethylaminostyryl)benzoxazole (OS), 2-(p-N,N-dimethylaminostyryl)-benzothiazole (SS) and 2-(p-N,N-dimethylaminostyryl)naphtiazole (PS) were prepared and their absorption and fluorescence spectra were measured in various solvents at room temperature. On the basis of the solvatochromic behavior the ground state (μ_g) and excited state (μ_e) dipole moments of these pN,N-dimethylaminostyryl derivatives were evaluated. The dipole moments (μ_g and μ_e) were estimated from solvatochromic shifts of absorption and fluorescence spectra as function of the dielectric constant (ɛ) and refractive index (n) of applied solvents. The absorption spectra only slightly are affected by the solvent polarity in contrast to the fluorescence spectra that are highly solvatochromic and display a large Stokes shift. The analysis of the solvatochromic behavior of the fluorescence spectra as function of Δf (ɛ, n) revealed that the emission occurs from a high polarity excited state. The large dipole moment change along with the strongly red-shifted fluorescence, as the solvent polarity is increased, demonstrate the formation of an intramolecular charge transfer state (ICT). Compounds under the study were used as fluorescence probes for monitoring the kinetics of polymerization. The study on the changes in fluorescence intensity and spectroscopic shifts of studied compounds were carried out during thermally initiated polymerization of methyl methacrylate (MMA) and during photoinitiated polymerization of 2-ethyl-2-(hydroxymethyl)propane-1,3-diol triacrylate (TMPTA).     

Spectroscopic and theoretical evidence for the photoinduced twisted intramolecular charge transfer state formation in N,N-dimethylaminonaphthyl-(acrylo)-nitrile

The phenomenon of excited state twisted intramolecular charge transfer (TICT) process in N,N-dimethylaminonaphthyl-(acrylo)-nitrile (DMANAN) has been reported on the basis of steady-state absorption and fluorescence spectroscopy in combination with quantum chemical calculations. The absorption and fluorescence characteristics of DMANAN in solvents of different polarity reveal the presence of a single species in the ground state which forms the intramolecular charge transfer state upon photoexcitation. The observed dual fluorescence is assigned to a high-energy emission from the locally excited or the Franck-Condon state and the red-shifted emission from the charge transfer (CT) state. In polar protic solvents, hydrogen-bonding interaction on CT emission has been established from the linear dependency of the position of the low-energy emission maxima on hydrogen-bonding parameter (α). The experimental findings have been correlated with the theoretical results based on TICT model obtained at density functional theory (DFT) level. The theoretical potential energy surface for the first excited state along both the donor and acceptor twist coordinates in the gas phase obtained by time dependent density functional theory (TDDFT) method and in polar solvent by time dependent density functional theory-polarized continuum model (TDDFT-PCM) method predicts well the experimental spectral properties.     

Rapid internal conversion in a symmetric molecule LD 700 studied by means of femtosecond fluorescence depletion

The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50fs laser pulse at 400nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S₅ of LD 700.