水杨酰苯胺衍生物分子内电荷/质子转移荧光

将苯甲酰苯胺(BA)类电荷转移(CT)反应基团耦合到具有激发态分子内质子转移(ESIPT)反应通道的水杨酸(SA)分子中,设计合成了苯胺对/间位取代的水杨酰苯胺及其羟基甲基化衍生物邻甲氧基苯甲酰苯胺,考察了环己烷、乙醚、乙腈和甲醇中两类水杨酰苯胺衍生物吸收光谱和荧光光谱的溶剂效应和取代基效应.结果表明,水杨酰苯胺的荧光为SA-型质子转移(PT)荧光,对取代基的依赖性较小;当电子给体苯胺基的给电子能力提高时,如N-甲基水杨酰苯胺分子,其荧光为BA-型CT荧光.而羟基甲基化后的衍生物邻甲氧基苯甲酰苯胺则表现出与BA类似的荧光光谱特性,具有明显的取代基效应.认为水杨酰苯胺衍生物的激发态存在着相互竞争的BA-型CT和SA-型PT通道,二者可经苯胺基上取代基的电子效应调控.     

2-(2-巯苯基)苯并噁唑分子内质子转移取代基电子效应的密度泛函理论研究

在B3LYP/6-31G(d,p)和TDB3LYP/6-31++G(d,p)//CIS/6-31G(d,p)水平上研究了2-(2-巯苯基)苯并噁唑及其衍生物基态和激发态分子内质子转移现象,并探讨取代基电子效应对分子内质子转移的影响,研究结果表明,在基态时,硫醇式异构体为优势构象,供电子取代基使基态分子内正向质子转移能垒(烯醇式→酮式)升高;而吸电子取代基则可降低能垒,有利于基态分子内质子转移并有助于硫酮式异构体的稳定.在激发态时,硫酮式结构为优势构象,所研究的2-(2-巯苯基)苯并噁唑化合物及衍生物均可以发生无能垒或低能垒(≤1.5kJ/mol)的激发态分子内质子转移.巯苯基部分是激发态失活的主要活性部分,供电子基团有利于激发态的质子转移,吸电子基团使激发态跃迁困难,不利于激发态的质子转移.     

双希夫碱-N,N’-二(2-羟基-1-萘甲醛)缩-1,4-苯二胺的电子光谱和光致变色机理研究

测定了双希夫碱,N,N＇-二(2-羟基-1-萘甲醛)缩-1,4-苯二胺(BNP)在环己烷和乙腈中的稳态和瞬态吸收光谱以及稳态荧光光谱,讨论了光致变色机理.在非极性溶剂中,BNP以烯醇式为主要存在形式,最大吸收位于紫外区.在极性溶剂中,既有烯醇式,又有质子转移产物的吸收,但以前者为主.无论在极性还是非极性溶剂中,其荧光发射都是来源于质子转移产物的激发态.     

侧链取代的芴苯共聚物的光电性质及其溶剂化效应研究

采用Suzuki偶联聚合的方法合成了一系列化学结构明确、侧基性质(长度、体积、给/吸电子性质)不同的Hairy-Rod型芴苯共聚物. 通过光谱、电化学和模拟计算等手段研究了苯环上不同性质的侧基取代芴苯聚合物的发光性质、电化学性质和溶剂化效应等, 同时研究了侧基性质的变化对这些物理性质的影响规律. 苯环上烷基侧链长度的改变对取代共聚物的光谱、电化学和发光效率等影响很小|而随着苯环上烷氧基侧链长度的增加, 聚合物的光谱稳定性逐渐增强, 荧光发射光谱中的0-1转变逐渐被抑制, 荧光发射半峰宽减小. 苯环上取代侧基的给/吸电子性质变化对聚合物的光电性能具有全面的影响, 改变取代侧基的给/吸电子性质可调节芴苯共聚物的发光颜色和HOMO, LUMO能级以及HOMO-LUMO能隙等, 因此, 通过引入不同性质的侧基可实现对此类聚合物光物理性能的调控. 溶剂的极性对聚合物溶液的光谱性质具有显著影响, 溶液光谱随溶剂极性的增大逐渐向长波移动. 当聚合物本身带有强极性基团时, 在强极性溶剂中将发生聚合物分子链与溶剂分子间的强极性相互作用, 从而会引起更复杂的结果.     

2-(2′-氨基苯基)苯并咪唑衍生物分子内质子转移理论研究:取代基效应

研究了2-(2′-氨基苯基)苯并咪唑(APBI)氨基中一个H被CH3(E-C),SiH3(E-OSi),NH2(E-N),COH(E-CO),NO2(E-NO2),CF3(E-F),CN(E-CN3),OMe(E-OMe),COCH3(E-CC),Ts(E-S),p-CH3C6H4CO(E-C=O)和p-CH3C6H4NHCO(E-NH)取代后,其基态及激发态分子内质子转移(ESIPT)性质的变化规律.结果表明各衍生物基态最稳定构型为烯醇式构型E,次稳定构型旋转异构体R,酮式构型K只有当取代基为E-CN3,E-F,E-NO2,E-N,E-OMe和E-S时才存在.基态各环的核独立化学位移(NICS)研究表明取代基的引入会影响APBI环电子离域性.所有APBI衍生物都能发生激发态分子内质子转移,当引入取代基为E-CN3,E-N或E-OMe时,所得的APBI衍生物S1态分子内质子转移是无能垒过程;引入取代基为E-C,E-C=O或E-OSi时,对APBI的ESIPT势能面基本无影响,而当取代基为E-CC,E-NH,E-CO,E-F,E-NO2和E-S时,使得S1态APBI的K*构型能量低于E*.     

2-(3-巯基-2-吡啶基)苯并咪唑分子内质子转移及溶剂化效应

在B3LYP/6-31G(d,p)和TD B3LYP/6-31++G(d,p)//CIS/6-31G(d,p)水平上,研究了2-(3-巯基-2-吡啶基)苯并咪唑(MPyBI)在气相和七种溶剂(环己烷、苯、三氯甲烷、乙醇、乙腈、二甲亚砜和水)中基态和激发态的分子内质子转移(GSIPT和ESIPT)过程.在基态势能面的研究中发现,该化合物存在分子内双质子转移,其中分步的双质子转移在动力学上具有优势.同时对激发态质子转移势能面及激发态转移过程中的光物理现象进行了研究,结果表明该化合物存在快速的无能垒的激发态分子内质子转移,随着溶剂极性的增强,可以降低基态过渡态的能垒,改变硫醇式与硫酮式互变异构体的比例,从而灵敏地控制荧光的强度.     

超分子作用对2-(2-氨基苯基)苯并噻唑分子内质子转移的影响

采用稳态荧光、瞬态荧光及量子化学计算等手段对2-(2-氨基苯基)苯并噻唑(APBT)在不同溶剂中的质子转移进行了研究。结果表明,溶剂的极性及质子化对APBT的质子转移有较大的影响,通过对超分子作用的考察,发现七元瓜环(CB[7])的加入对APBT质子转移起到了一定的抑制作用, APBT与CB[7]能形成化学计量比为1:1的主客体包合物,同时测定了包合物的结合常数等热力学参数。此外,核磁共振氢谱和包合物的理论计算表明APBT分子进入了CB[7]的疏水空腔。     

2-(2′-羟苯基)苯并噁唑氨基衍生物电子结构和光谱性质的理论研究

采用量子化学方法优化了间位和对位2-(2′-羟苯基)苯并噁唑氨基取代衍生物(4-AHBO和5-AHBO)基态的第一单重激发态所有可能的稳定构型, 分析了这些异构体在不同溶剂中的相对稳定性. 利用含时密度泛函理论(TDDFT)的不同泛函, 计算了4-AHBO和5-AHBO各异构体在溶剂中的吸收与发射光谱, 考察了它们的电子结构和光谱特征. 结果表明, 4-AHBO(5-AHBO)的双荧光不是由同一种异构体发射的, 而是来源于不同异构体的发射: 长波区的荧光由酮式构型发射, 短波区的发射则可能由四种醇式异构体共同产生. 另外, 也解释了5-AHBO在质子溶剂中光谱异常的原因, 分析了不同氨基取代位和溶剂极性的变化对各异构体构型、光谱性质及电子结构的影响. 理论预测的光谱与实验结果一致.     

水杨酸-2′-乙基己基酯在胶束中的增溶位点

考察了阳离子、非离子和阴离子表面活性剂存在下水杨酸-2′-乙基己基酯(EHS)的吸收光谱和激发态分子内质子转移(ESIPT)荧光光谱.结果表明,EHS可增溶在胶束中,2′-乙基己基碳链朝向胶束内核,而水杨酸基朝向胶束-水界面;胶柬环境有利于EHS分子对紫外光的吸收和分子内氢键的形成,从而使ESIPT荧光显著增强,激发态分子以发射可见光和非辐射去活化方式衰减;并根据EHS和表面活性剂分子的结构和大小,解释了EHS分子在胶束中的结合位点,荧光猝灭和酯水解的光谱测量进一步为此结合位点提供了佐证.     

水杨酸-2’-乙基己基酯在胶束中的增溶位点

考察了阳离子、非离子和阴离子表面活性剂存在下水杨酸-2’-乙基己基酯(EHS)的吸收光谱和激发态分子内质子转移(ESIPT)荧光光谱. 结果表明, EHS可增溶在胶束中, 2’-乙基己基碳链朝向胶束内核, 而水杨酸基朝向胶束-水界面; 胶束环境有利于EHS分子对紫外光的吸收和分子内氢键的形成, 从而使ESIPT 荧光显著增强, 激发态分子以发射可见光和非辐射去活化方式衰减; 并根据EHS和表面活性剂分子的结构和大小, 解释了EHS分子在胶束中的结合位点, 荧光猝灭和酯水解的光谱测量进一步为此结合位点提供了佐证.     

Excited-state intramolecular proton transfer in 2-(2'-arylsulfonamidophenyl)benzimidazole derivatives: the effect of donor and acceptor substituents

A series of water-soluble 2-(2'-arylsulfonamidophenyl)benzimidazole derivatives containing electron-donating and accepting groups attached to various positions of the fluorophore pi-system has been synthesized and characterized in aqueous solution at 0.1 M ionic strength. The measured pK(a)'s for deprotonation of the sulfonamide group of monosubstituted derivatives range between 6.75 and 9.33 and follow closely Hammett's free energy relationship. In neutral aqueous buffer, all compounds undergo efficient excited-state intramolecular proton transfer (ESIPT) to yield a strongly Stokes-shifted fluorescence emission from the phototautomer. Upon deprotonation of the sulfonamide nitrogen at high pH, ESIPT is interrupted to yield a new, blue-shifted emission band. The peak absorption and emission energies were strongly influenced by the nature of the substituents and their attachment positions on the fluorophore pi-system. The fluorescence quantum yield of the ESIPT tautomers revealed a significant correlation with the observed Stokes shifts. The study provides valuable information regarding substituent effects on the photophysical properties of this class of ESIPT fluorophores in an aqueous environment and may offer guidelines for designing emission ratiometric pH or metal-cation sensors for biological applications.     

The synthesis and the solvent and substituent effect on the spectroscopic characteristic of 3-ethyl-2-(p-substitued styryl)benzothiazolium iodides

The photophysical and photochemical properties of p-substitued 2-styryl-ethylbenzothiazolium iodides, possessing different electron-withdrawing or electron-donating groups are described. The dyes were prepared by the condensation of 3-ethyl-2-methylbenzothiazole salts with p-substituted benzaldehydes. The synthesis of suitable substrates is presented as well. We describe here the absorption, emission spectra and the luminescence quantum yield of hemicyanine dyes (SH) measured in 11 different organic solvents of varying polarity. Molecular structure of the synthesized dyes was established by (1)H NMR, electronic absorption and fluorescence spectrometry. The spectral data confirmed that all the compounds exist in E-configuration of their styryl residues. The planar molecular conformation is typical for the compounds with five-membered side aromatic moieties (for example benzothiazole). The compounds possessing N-alkyl substituent in phenyl ring, in contrast to the compounds with other substituents, exhibit low fluorescence quantum yield in THF solution. This indicates that for N-alkyl derivatives the non-radiative processes are much more effective than the radiative ones. The electronic absorption and fluorescence emission spectra of tested dyes demonstrate high sensitivity to the nature of substituent introduced into the aromatic ring.     

NEW FEATURES IN THE PHOTOPHYSICS and PHOTOCHEMISTRY OF 2-(2''-HYDROXY-PHENYL)BENZOTHIAZOLES INTRODUCED BY AMINE SUBSTITUTION

The photophysics and photochemistry of the 4'-diethylamino derivative of both 2-phenyl-benzothiazole and 2-(2'-hydroxyphenyl)benzothiazole have been studied by nanosecond and microsecond laser flash photolysis and picosecond emission spectroscopy. For the non-hydroxy substituted molecule, the singlet excited state was shown to relax primarily via fluorescence emission, and a very weak triplet transient was observed after laser flash excitation. The 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (AHBT) was shown to undergo excited state intramolecular proton transfer (ESIPT) in the picosecond timescale (k greater than 3 x 10(10) s-1) to form a colored zwitter-ion/keto form in solution at room temperature while the ground state back proton transfer was slower by a factor of approximately 10(5). However, in marked contrast with other derivatives of 2-(2'-hydroxyphenyl)benzothiazole and related molecules, the ESIPT was not the only deactivation process of the lowest singlet excited state of the enol form. Under steady-state excitation at room temperature (and low temperature), the fluorescence emission of the enol form was observed. The T-T absorption of the enol form was also observed and furthermore, the ESIPT was shown to have an activation energy which was estimated to be approximately 4 kJ. None of the foregoing, fluorescence and T-T absorption of the enol nor activation energy for proton transfer have been observed for the parent or derivatives of 2-(2'-hydroxyphenyl)benzothiazoles. The striking new features for the ESIPT photochemistry and photophysics for the 4'-diethylamino derivative of 2-(2'-hydroxyphenyl)benzothiazole are discussed and MO calculations are used to aid in the interpretation of some of the experimental results.     

Substitution effect,absorption, and fluorescence behaviors of 11,12-benzo-1,7,10,13-tetraoxa-4-aza- cyclopentadec-11-ene (benzoaza-15-crown-5) derivatives upon cation complexation in solvent extraction

Substitution effect, absorption, and fluorescence behaviors of some benzoaza-15-crown-5 derivatives upon cation complexation in solvent extraction were studied. The introduction of a substituent on the nitrogen atom in benzoaza-15-crown-5 enhanced extractabilities in the solvent extraction of aqueous alkali metal picrates. The nondonating substituents raised the cation selectivity for Na(+) over K(+), but the donating substituents reduced the cation selectivity. The absorption and fluorescence spectral behavior was different with the alkali metal cations.     

Excited State Intramolecular Proton Transfer in Ethynyl‐Extended Regioisomers of 2‐(2′‐Hydroxyphenyl)benzothiazole: Effects of the Position and Electronic Nature of Substituent Groups

Although the organic dyes based on excited state intramolecular proton transfer (ESIPT) mechanism have attracted significant attention, the structure‐property relationship of ESIPT dyes needs to be further exploited. In this paper, three series of ethynyl‐extended regioisomers of 2‐(2′‐hydroxyphenyl)benzothiazole (HBT), at the 3′‐, 4′‐ and 6‐positions, respectively, have been synthesized. Changes in the absorption and emission spectra were correlated with the position and electronic nature of the substituent groups. Although 4′‐ and 6‐substituted HBT derivatives exhibited absorption bands at longer wavelengths, the keto‐emission of 3′‐substituted HBT derivatives was found at a substantially longer wavelength. The gradual red‐shifted fluorescence emission was found for 3′‐substituted HBT derivatives where the electron‐donating nature of substituent group increased, which was opposite to what was observed for 4′‐ and 6‐substituted HBT derivatives. The results derived from the theoretical calculations were in conformity with the experimental observations. Our study could potentially provide experimental and theoretical basis for designing novel ESIPT dyes that possess unique fluorescent properties.     

3-Hydroxybenzo[g]quinolones: dyes with red-shifted absorption and highly resolved dual emission

New derivatives of 3-hydroxyquinolone (3HQ) with a fused benzene ring (3-hydroxybenzo[g]quinolones) have been synthesized. They display a remarkable red shift of their absorption spectrum in comparison with other 3HQ analogs allowing their excitation by common He/Cd and Ar-ion lasers. As a result of their irreversible excited-state intramolecular proton transfer (ESIPT) reaction, they display a dual fluorescence in a series of solvents of varying polarities, starting from toluene to methanol. The dual emission of these dyes correlates well with solvent H-bond basicity, which is connected with the effect of this solvent property on the kinetics of the ESIPT reaction. In addition to their red-shifted absorption and fluorescence, these new derivatives show a larger separation of their two emission bands and a more appropriate range of their intensity ratio than the previously synthesized 3HQs. These properties allow an improved ratiometric evaluation of the local H-bond basicity of unknown environments, which will favor future applications of the new dyes in polymer and biological sciences.     

Femtosecond dynamics on 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole: solvent polarity in the excited-state proton transfer.

Detailed insights into the excited-state enol(N*)-keto(T*) intramolecular proton transfer (ESIPT) reaction in 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (HABT) have been investigated via steady-state and femtosecond fluorescence upconversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2'-hydroxyphenyl)benzothiazole (>2.9+/-0.3 x 10(13) s(-1)), HABT undergoes a relatively slow rate (approximately 5.4+/-0.5 x 10(11) s(-1)) of ESIPT. In polar aprotic solvents competitive rate of proton transfer and rate of solvent relaxation were resolved in the early dynamics. After reaching the solvation equilibrium in the normal excited state (N(eq)*), ESIPT takes place with an appreciable barrier. The results also show N(eq)*(enol)<-->T(eq)*(keto) equilibrium, which shifts toward N(eq)* as the solvent polarity increases. Temperature-dependent relaxation dynamics further resolved a solvent-induced barrier of 2.12 kcal mol(-1) for the forward reaction in CH(2)Cl(2). The observed spectroscopy and dynamics are rationalized by a significant difference in dipole moment between N(eq)* and T(eq)*, while the dipolar vector for the enol form in the ground state (N) is in between that of N(eq)* and T(eq)*. Upon N-->N* Franck-Condon excitation, ESIPT is energetically favorable, and its rate is competitive with the solvation relaxation process. Upon reaching equilibrium configurations N(eq)* and T(eq)*, forward and/or backward ESIPT takes place with an appreciable solvent polarity induced barrier due to differences in polarization equilibrium between N(eq)* and T(eq)*.     

Crystal structure, spectroscopic, and theoretical investigations of excited-state proton transfer in the doubly hydrogen-bonded dimer of 2-butylamino-6-methyl-4-nitropyridine N-oxide

The crystal structure of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) was resolved on the basis of X-ray diffraction. Solid 2B6M occurs in the form of a doubly hydrogen-bonded dimer with squarelike hydrogen-bonding network composed of two intra- (2.556(2) A) and two intermolecular (2.891(2) A) N-H...O type hydrogen bonds. The molecule thus has both a protonable and a deprotonable group that led us to investigate the possibility of an excited-state proton transfer (ESIPT) reaction in different solvents by means of experimental absorption, steady state, and time-resolved emission spectroscopy. The results were correlated with quantum mechanical TD-DFT and PM3 calculations. Experimental and theoretical findings show the possibility of an ESIPT reaction in polar solvents. It is demonstrated that in particular the emission spectra of 2B6M are very sensitive to solvent properties, and a large value of the Stokes shift (about 8000 cm(-1)) in acetonitrile is indicative for an ESIPT process. This conclusion is further supported by time-resolved fluorescence decay measurents that show dual exponential decay in polar solvents. Vertical excitation energies calculated by TD-DFT reproduce the experimental absorption maxima in nonpolar solvents well. The majority of electronic transitions in 2B6M is of pi --> pi* character with a charge shift from the electron-donating to the electron-accepting groups. The calculations show that, due to the charge redistribution on excitation, the acidity of the amino group increases significantly, which facilitates the proton transfer from the amino to the N-oxide group in the excited state.     

Electrochromic modulation of excited-state intramolecular proton transfer: the new principle in design of fluorescence sensors

Internal Stark effect (or internal electrochromy) consists of the shift of light absorption and emission bands under the influence of electric field produced by proximal charges. In the studies of 3-hydroxyflavone (3HF) derivatives exhibiting the excited-state intramolecular proton transfer (ESIPT), we describe a new phenomenon - a very strong internal electrochromic modulation of this reaction. Fluorescence spectra of 3HF derivatives with charged groups attached to the chromophore from the opposite sides without pi-electronic conjugation, N-[(4'-diethylamino)-3-hydroxy-6-flavonyl]methyl-N,N-dimethyloctylammonium bromide and 4-[4-[4'-(3-hydroxyflavonyl)]piperazino]-1-(3-sulfopropyl)pyridinium, were compared with those of their neutral analogues in a series of representative solvents. The introduction of the proximal charge results in shifts of absorption spectrum and of both normal (N) and tautomer (T) emission bands, which correspond to initial and phototautomer states of the ESIPT reaction. The observed shifts are in accordance with the Stark effect theory. The direction of the shift depends on the position of the proximal charge with respect to the chromophore. The magnitude of the shift depends strongly on the solvent dielectric constant and on screening or unscreening produced by addition of the hydrophobic salts. In all of these cases, the spectral shifts are accompanied by extremely strong variations of relative intensities of N and T emission bands. This signifies a strong influence of internal electric field on the ESIPT reaction, which produces a dramatic change of emission color. Thus, the coupling of the initial electrochromic sensory signal with the ESIPT reaction allows for the breaking of the limit in magnitude of response inherent to common electrochromic dyes. This suggests a new principle of designing the ultrasensitive electrochromic two-wavelength fluorescence sensors and probes for analytical chemistry, macromolecular science, and cellular biology.     

Excited state intramolecular proton transfer in 2-(2'-arylsulfonamidophenyl)benzimidazole derivatives: insights into the origin of donor substituent-induced emission energy shifts

Donor-substituted 2-(2'-arylsulfonamidophenyl)benzimidazoles undergo efficient excited-state intramolecular proton transfer (ESIPT) upon photoexcitation. The tautomer emission energy depends strongly on the substituent attachment position on the fluorophore pi-system. While substitution with a donor group in the para-position relative to the sulfonamide moiety yields an emission energy that is red-shifted relative to the unsubstituted fluorophore, fluorescence of the meta-substituted derivative appears blue-shifted. To elucidate the origin of the surprisingly divergent emission shifts, we performed detailed photophysical and quantum chemical studies with a series of methoxy- and pyrrole-substituted derivatives. The nature and contribution of solvent-solute interactions on the emission properties were analyzed on the basis of solvatochromic shift data using Onsager's reaction field model, Reichardt's empirical solvent polarity scale ET(30), as well as Kamlet-Abboud-Taft's empirical solvent index. The studies revealed that all ESIPT tautomers emit from a moderately polarized excited-state whose dipole moment is not strongly influenced by the donor-attachment position. Furthermore, the negative solvatochromic shift behavior was most pronounced in protic solvents presumably due to specific hydrogen-bonding interactions. The extrapolated gas-phase emission energies correlated qualitatively well with the trends in Stokes shifts, suggesting that solute-solvent interactions do not play a significant role in explaining the divergent emission energy shifts. Detailed quantum chemical calculations not only confirmed the moderately polarized nature of the ESIPT tautomers but also provided a rational for the observed emission shifts based on the differential change in the HOMO and LUMO energies. The results gained from this study should provide guidelines for tuning the emission properties of this class of ESIPT fluorophores with potential applications in analytical chemistry, biochemistry, or materials science.