Excited and ground state conformations of p-dimethylamino-benzaldehyde and p-dimethylaminoacetophenone

p-dimethylaminobenzaldehyde (DMABA) and p-dimethylaminoacetophenone (DMAA) were investigated in nonpolar and polar protic and aprotic solvents over a wide temperature range. The sequence of the lower lying excited states was established. The temperature dependence of the fluorescence yields was interpreted within the irreversible reaction kinetics scheme b¹→TICT (twisted internal charge transfer) state. The activation energies and the rate constants were evaluated. Differences between the low-temperature fluorescence or phosphorescence excitation spectra and the absorption spectrum were found to be due to the existence of different rotamers in the ground state. Quantum chemical (INDO) calculations of the energies of electronic transitions, oscillator strengths and dipole moments were performed for flat and twisted conformations of DMABA.     

Solvent Effect on Cocaine and Methyl Benzoate Phosphorescence

In polar, nonhydrogen-bonding solvents, cocaine's phosphorescence differs from that of its model compound, methyl benzoate, in what can be attributed to an amine-benzoate charge transfer interaction and both compounds exhibit long wavelength bands not present in nonpolar media.     

Solvent Polarity and Excitation Wavelength Dependence of the Dual Fluorescence in N,N-Diethyl-4-Nitrosoaniline

Dual fluorescence in N,N-Diethyl-4-nitrosoaniline (DENA) has been studied employing absorption, excitation and emission spectroscopic techniques and computational methods. The absorption and fluorescence spectra of DENA were measured in solvents of various polarities at room temperature. The emission spectra of DENA were found to exhibit a single emission band in non polar solvent (cyclohexane) and in a highly polar solvent (acetonitrile). In the contrary, two emission bands were observed in medium polar solvents (tetrahydrofuran, 1,2-dichloroethane and dichloromethane) whereby the short (local excited; LE) and long (charge transfer; CT) emission maxima correspond to the emission maxima of the compound observed in cyclohexane and acetonitrile solutions, respectively. Moreover, the two emission bands have shown strong excitation wavelength dependence, and area normalization resulted in an iso-emissive point. The two emission maxima were in addition found to correspond to two excitation maxima in 3D fluorescence spectra. Further, two minima were obtained in potential energy surface calculation of DENA. From the experimental and computational results it was concluded that the dual fluorescence may be attributed to the presence of two different ground state structural conformers of DENA in equilibrium that are stabilized through solute-solvent interaction.     

4-四氢喹啉酮的合成及发光性能

对喹吖啶酮结构改造合成了新型结构荧光体4-四氢喹啉酮,对其吸收光谱及荧光光谱进行了研究,发现该化合物的吸收峰及荧光发射峰随溶剂极性的增加而不断红移,表明荧光来源于分子内激发态电荷转移发光.该化合物在固态即显示荧光,表明经结构改造后,消除了喹吖啶酮类的固态荧光猝灭效应.4-四氢喹啉酮结构简单,易于合成和结构修饰.这对于寻找新的主体荧光发射材料具有重大意义.     

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.     

Electronic structure and spectral fluorescence properties of umbelliferone and herniarin

We have studied the characteristic features of the spectral fluorescence properties of natural compounds (umbelliferone and herniarin) in aprotic and proton-donor solvents of different polarities. Based on quantum chemical analysis by the AM1 semiempirical method of the changes in the electronic charge and bond orders in the ground state and the first excited state, we have interpreted the anomalously high Stokes shifts and mirror symmetry of the absorption and fluorescence bands of these compounds in aprotic solvents. We have established that the fluorescence spectra of umbelliferone in water, ethanol, alkaline and acidic solutions differ considerably from the analogous spectra of herniarin. This is connected with detachment and transfer of a proton in the neutral form of umbelliferone upon photoexcitation, with possible formation (depending on the pH of the medium) of an anion, cation, or tautomer. Good agreement was achieved between the calculated and experimental values of the maxima for the electronic bands of these forms. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 569–576, September–October, 2007.     

利用光谱法和分子对接技术研究4-乙基-2-甲氧基苯酚与人血清白蛋白之间的相互作用

4-乙基-2-甲氧基苯酚是一种被广泛应用的食品添加剂,但它也具有一定的毒性,为了探究4-乙基-2-甲氧基苯酚对人体的影响,将多种光谱技术与分子模拟等技术结合起来对4-乙基-2-甲氧基苯酚与人血清白蛋白Human Serum Albumin(HSA)之间的相互作用进行了较全面的研究。紫外吸收光谱的结果说明,4-乙基-2-甲氧基苯酚与人血清白蛋白之间形成了新的复合物。荧光光谱的结果说明了4-乙基-2-甲氧基苯酚的存在,可以增加人血清白蛋白的荧光强度。通过15 nm的同步荧光光谱同时结合荧光增强效应方程可以计算出4-乙基-2-甲氧基苯酚与人血清白蛋白之间的结合常数,且它们之间的结合常数随着温度的升高而减小。热力学参数表明,4-乙基-2-甲氧基苯酚主要靠氢键和疏水作用力与人血清白蛋白结合在一起。同步荧光光谱、三维荧光光谱和圆二色光谱的结果说明人血清白蛋白的构象会随着4-乙基-2-甲氧基苯酚的作用而发生一定的变化。分子对接得到结果说明4-乙基-2-甲氧基苯酚键合在人血清白蛋白的IB区域。     

Preparation and Spectral Characterization of Fluorescence Probes Based on 4-N,N-Dimethylamino Benzoic Acid and Sterically Hindered Amines

The adducts of simple chromophore 4-N,N-dimethylamino benzoic acid with 2,2,6,6-tetrametyl-4-hydroxy- or 4-amino-piperidine were examined as fluorescence probes (spin double sensors) to monitor radical processes. The links in the adducts were either an ester or amide group, and the sterically hindered amines were in the form of -NH, -NO? and -NOR. The spectral properties of the three related derivatives (esters or amides) were quite similar. The maxima of the absorption spectra were in the range of 295-315?nm, and the maximum of fluorescence was located in the range of 330-360?nm, depending on the polarity of the solvent. In polar solvents, a red-shifted fluorescence band at 460-475?nm was observed. The fluorescence of these derivatives was rather weak as compared to anthracene under the same conditions. The Stokes shift was large, as high as 6,000?cm(-1), indicating the formation of a twisted intra-molecular charge transfer (TICT) state. No large differences in Stokes shifts were observed in polymer matrices of poly(methyl methacrylate), polystyrene and poly(vinyl chloride). The extent of intramolecular quenching was expressed as Φ(NX)/Φ(NO) (X?=?H, NOR) and was in the range of 1-3 in solution and as high as 8 in polymer matrices. The low efficiency of intramolecular quenching limits the application of these new adducts as fluorescence probes for the monitoring of radical processes in solution but favors their application in polymer matrices.     

香豆素—1的荧光介质效应

详细研究了香豆素－１在环己烷－醇、１，４－二氧六环－水二元溶剂及环糊精，表面活性剂溶液体系中的荧光光谱。通过溶剂－溶质分子间氢键作用及对其非荧光性的ＴＩＣＴ态形成的影响。讨论了这种荧光探针的荧光光谱对环境极性非常敏感的原因，测定了它与环糊精的包结常数和与表面活性剂胶束的结合常数。     

有机添加物对7-胺基香豆素分子发光的增强效应

测定了在有卤代苯等存在时,一些7-胺基香豆素在不同溶剂中的吸收和发射谱及荧光寿命。所得结果表明,具有给电子能力的有机添加物可对非刚性7-胺基香豆素衍生物在极性溶剂中的荧光起增强作用。基于荧光衰变动力学的分析及添加物的荧光增强能力与其氧化电位的线性关系,提出荧光增强作用的机理是添加物通过分子间电荷转移作用使香豆素分子激发态不发光的TICT态向发光的ICT态转化所致。     

Specific effects of a polar solvent in optical absorption spectra of 5,12-tetracenequinone

We have measured the optical absorption spectra of 5,12-tetracenequinone (5,12-naphthacenequinone) in polar protic (methanol) and nonpolar (n-hexane) solvents. It has been shown that shifts of some bands in the polar solvent compared to the nonpolar one are caused by the formation of high hydrogen bonds between methanol molecules and the molecules under study. The occurrence of hydrogen bonds leads to changes in the energy gaps between occupied and unoccupied molecular orbitals of 5,12-tetracenequinone and, as a consequence, to corresponding changes in energies of electronic transitions. Based on analysis of the absorption spectra in different solvents in combination with the data of calculations of electronic spectra and taking into account changes in the orbital pattern under the action of the solvent, we have determined exact electronic configurations of electronically excited singlet states of 5,12-tetracenequinone.     

The role of environment in the nonradiative relaxation of the triplet state of naphthalene derivatives in solid solutions at 77 K

The nonradiative relaxation of the triplet states of oxy-and amino-derivatives of naphthalene and conjugated ions is studied by luminescent and kinetic methods (measurements of the phosphorescence decay and of phosphorescence excitation and phosphorescence spectra) in solid solutions of ethanol-h ₆, ethanol-d ₆, and mixtures of toluene-h ₈ with piperidine-h ₁₁ at 77 K. It is found that, along with intramolecular factors, a microsolvate surrounding a molecule or an ion substantially affects the nonradiative relaxation. The contribution of this factor in ethanol increases in the series cation-polar molecule-anion and in mixtures of toluene with piperidine—with increasing piperidine concentration. The results are interpreted assuming the inductive-resonance dipole-dipole transfer of the triplet energy to the dipole acceptors of intramolecular bonds and bonds with molecules of the environment. The relative arrangement of hydrogen atoms of OH groups of ethanol molecules in microsolvates of cations and anions estimated using the inductive-resonance model agrees with the difference in the structure of solvates of oppositely charged ions, which is caused by the electrostatic charge-dipole interaction and the distribution of the electron density in the ground state of the corresponding emitting center. The inductive-resonance model was used for studying the features of solvation processes involving polar molecules. It is shown that the difference between the structures of microsolvates of 2-oxy-naphthalene molecules in solvents with close dipole moments (ethanol and piperidine) is mainly caused by the different ability of these solvent molecules to form associates. The structure of microsolvates of oxy-derivatives of naphthalene in the associated amphoteric solvent is found to depend on the number and position of substituents. In oxy-derivatives of naphthalene with spatially separated OH groups in ethano-d ₆, deuteroexchange occurs in both substituents, whereas in the naphthalene derivative with adjacent OH groups this occurs only in one of the groups. Comparison of the phosphorescence spectra of hydrogen-bond complexes and proton-transfer complexes in nonpolar solvents at 77 K revealed the existence of molecular naphthol entities in the triplet state that were formed from ionized entities in the singlet state.     

2019年3期 电子期刊

通过Debus-Radziszewski、Suzuki-Miyaura和Knoevenagel缩合反应,设计合成了一种新型的咪唑衍生物L,并使用傅里叶变换红外光谱和核磁共振氢谱对合成化合物进行了结构表征。不同极性溶剂中光物理性质和密度泛函理论计算结果表明该系列化合物具有典型的分子内电荷转移（Intramolecular charge transfer,ICT）效应;紫外-可见吸收和荧光发射光谱证实它表现出聚集诱导猝灭（Aggregation-caused quenching,ACQ）性质。进一步的光物理性质测试表明化合物L在THF/H₂O混合溶液中可实现对2,4,6-三硝基苯酚（苦味酸,picric acid,PA）的检测,检测限为3.7×10^-6 mol/L。     

Quenching of excited states of xylenols due to hydrogen bonding with triethylamine in different solvents

Hydrogen bonding between xylenols in the ground state and triethylamine (TEA) brings about red shift and increase of intensity of the absorption bands of the xylenol molecules. TEA quenches fluorescence of the xylenols at 300 K in varying degrees, possibly through a very short-lived CT species formed by strong hydrogen bonding interaction between excited xylenols and TEA, which leads to large charge transfer. At 77 K, restricted solvent relaxation or orientation impedes formation of such CT complex and consequently impedes the quenching process. Quenching is weak and absent in polar aprotic and alcoholic solvents, respectively. The monoexponential nature of phosphorescence decay of xylenols indicates establishment of very rapid equilibrium between hydrogen-bonded complexes in the triplet state and free molecules.     

TDDFT Investigation of the Electronic Structures and Photophysical Properties of Fluorescent Extended Styryl Push-Pull Chromophores Containing Carbazole Unit

Push-pull chromophores attached to carbazole based π-conjugating spacers bearing N-alkylamino donors, cyanovinyl and carbethoxy acceptors have been studied by the means of UV-Visible measurements. The intramolecular charge transfer (ICT) of these π-conjugated systems has also been tested by investigating the ability of the solute molecules to undergo shifts in their fluorescence emission maxima with increasing solvent polarity. Density Functional Theory [B3LYP/6-31G(d)] and Time Dependent Density Functional Theory [TD-B3LYP/6-31G(d)] computations have been used to have more understanding of the structural, molecular, electronic and photophysical parameters of push-pull dyes. The largest wavelength difference between the experimental and computed electronic absorption maxima was 45 nm. For emission, a largest difference of 61 nm was observed. The ground state and excited state dipole moments in different solvents were determined using experimental solvatochromic data and computed Onsager radii. The dipole moments of the molecules in the excited state were observed to be higher than in the ground state.     

Excitation-wavelength Dependent Fluorescence of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.     

Selective nonspecific solvation under dielectric saturation and fluorescence spectra of dye solutions in binary solvents

The influence of selective nonspecific solvation on the fluorescence spectra of three substitutedN-methylphthalimides in a binary solvent system consisting of a nonpolar (n-heptane) and a polar (pyridine) component has been studied under conditions close to dielectric saturation. The substantially nonlinearity of the effect is confirmation that the spectral shifts of fluorescence bands depend on the number of polar solvent molecules involved in solvating the dye molecule. The measured fluorescence spectral shifts determined by substituting one nonpolar solvent molecula with a polar one in the proximity of the dye molecule agree quantitatively with the forecasts of the previously proposed semiempirical theory which describes this nonlinear solvation phenomenon.     

Theoretical study on twisted intramolecular charge transfer of 1-aminoanthraquinone in different solvents

The twisted intramolecular charge transfer and the excited state relaxation of 1-aminoanthraquinone(1-NH_2-AQ)in different solvents are investigated using quantum chemical calculations in this paper.The geometries of the ground state are optimized both in gas and solvents based on the high-level ab initio calculations,the lowest excited singlet state geometry is optimized only in gas for simplicity.An intramolecular charge transfer property is substantiated by the large change of dipole moments between the S_0 and S_1 states.The mechanism of twisted intramolecular charge transfer is proposed by the conformational relaxation on the potential surface of the S_1state.Quantum chemical calculations present that internal conversion and intersystem crossing are important approaches to the ultrafast deactivation of the S_1 state via the twisting of the amino group.The smaller energy difference between the S_0 and S_1 state shows that the internal conversion process is much faster in a polar solvent than in a nonpolar solvent.Energy intersections between the T_2 and S_1 state in cyclohexane and dioxane indicate a faster intersystem crossing process in them than in ethanol.These theoretical results agree well with the previous experimental results.Energy barriers are predicted on the potential surface of the S_1 state,and they have a positive correlation to solvent viscosity,and the timescale of twisted intra-molecular charge transfer in dioxane is predicted to be longer than in cyclohexane and ethanol.     

Emissive Properties and Intramolecular Charge Transfer of Pyrazoloquinoline Derivatives

The fluorescence properties of several pyrazoloquinoline derivatives were performed by stationary as well as by time-resolved spectrosopy. Non-donor-substituted compounds show a high quantum yield; transition dipole moments of absorption and fluorescence as well as experimental and calculated lifetimes are in excellent agreement. The donor-substituted compound DMA-DPPQ exhibits a charge transfer fluorescence in polar solvents. Additionally, dual fluorescence appears in polar protic solvents. The nature of the charge transfer state is discussed with respect to the the TICT model.     

Singlet and Triplet State Properties and Singlet Oxygen Yield of an Amino-Acyl-Quinoxalinone Yellow Dye

Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (Φ_F?≈?0.002–0.004), and short singlet state lifetime (τ_expt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ_??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M^?1 cm^?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λ_max?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency.