Simple imine linked colorimetric and fluorescent receptor for sensing Zn ions in aqueous medium based on inhibition of ESIPT mechanism

Herein, we report the highly selective binding of Zn²⁺ ion by the salicylaldimine based Schiff base chromogenic receptor 1 [(N,N′-bis (salicylidine)-o-phenylenediamine]. Receptor 1 senses Zn²⁺ ion in aqueous medium by colorimetric and fluorescent response in the presence of other metal ions like Pb²⁺, Hg²⁺, Sn²⁺, Cd²⁺. Receptor 1 on binding with Zn²⁺ ion exhibits fluorescence enhancement which is due to the inhibition of the (ESIPT) mechanism.     

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*.     

Role of structural flexibility in the fluorescence and photochromism of salicylideneaniline: the general scheme of the phototransformations

The mechanism of light-induced transformation in the salicylideneaniline molecule was studied by semiempirical PM3 calculations. The structures and energies of the minima and saddle points (transition states) on the S₀, S₁ and T₁ potential energy hypersurfaces (PESs) were obtained, together with the gradient lines on the PESs. The structure-energy scheme was compared with the experimental findings. According to the results obtained, the following principle processes are observed: fast S₁ excited state intramolecular proton transfer (ESIPT), followed by typical ESIPT fluorescence; the formation of two S₁ twisted intramolecular charge transfer (TICT) structures which quench the ESIPT fluorescence; the diabatic formation of two ground state metastable coloured “post-TICT” structures responsible for photochromism.     

Excited State Intramolecular Proton Transfer of New Diphenylethylene Derivatives Bearing Imino Group: A Combination of Experimental and Theoretical Investigation

In this paper, we described the synthesis and characterization of new diphenylethylene bearing imino group. We concentrated particularly on the investigation of the possibility of the excited state intramolecular charge transfer (ESIPT) of the new dyes experimentally and theoretically. The absorption and fluorescence spectroscopy of the dyes were determined in various solvents. The results showed that the maximal absorption wavelength of 2‐[(4′‐N,N‐dimethylamino‐diphenylethylene‐4‐ylimino)methyl]phenol ( C1 ) and 4‐[(4′‐N,N‐dimethylamino‐diphenylethylene‐4‐ylimino)methyl]phenol ( C2 ) exhibited almost independence on the solvent polarity. While as contrast, the maximal fluorescence wavelength of the dyes showed somewhat dependence on the solvent polarity. In particular, C1 displayed well‐separated dual fluorescence spectroscopy. The second fluorescence peak was characterized with an "abnormal" fluorescence emission wavelength in aprotic solvents with large Stokes shift (ca. 140 nm in THF), which was much more than normal Stokes shift (ca. 30 nm in THF). This emission spectroscopy could be assigned to ESIPT emission. On the other hand, the ESIPT fluorescence of C1 was much reduced or lost in the protic solvents. While, only normal fluorescence emission was detected in various solvents. Although the absorption maxima of C1 exhibited about 10 nm red‐shift with respect to those of C2 , the normal fluorescence maxima of C1 and C2 were almost identical in various solvents. These results suggested that C1 could undergo ESIPT, but C2 was not able to proceed ESIPT. The molecular geometry optimization of phototautomers in the ground electronic state (S₀) was carried out with HF method (Hartree‐Fock) and at DFT level (Density Functional Theory) using B3LYP both, while the CIS was employed to optimize the geometries of the first singlet excited state (S₁) of the phototautomers of C1 and C2 respectively. The properties of the ground state and the excited state of the phototautomers of C1 and C2 , including the geometrical parameter, the energy, the frontier orbits, the Mulliken charge and the dipole moment change were performed and compared completely. The data were analyzed further based on our experimental results. Furthermore, the absorption and fluorescence spectra were calculated in theory and compared with the measured ones. The rate constant of internal proton transfer (9.831×10¹¹ s^?1) of C1 was much lower than that of salicylidene methylamine ( C3 , 2.045×10¹⁵ s^?1), which was a typical Schiff base compound and was well demonstrated to undergo ESIPT easily under photoexcitation.     

All-optical switchings of 3-hydroxyflavone in different solvents

3-hydroxyflavone （3-HF） is an organic molecule with an excited-stated intramolecular proton transfer （ESIPT） effect. All-optical switchings and beam deflections of 3-HF in three kinds of solvents （cyclohexane, ethanol and dimethyl sulfoxide） have been investigated by using the third-harmonic generation （355 nm） of a mode-locked Nd：YAG laser as a pump beam and a continuous-wave （cw） He-Ne laser （632.8 nm） as a probe beam. The nonlinear refractive indices of 3-HF in the three different solvents are determined by using the Z-scan technique under an ultraviolet （UV） pump beam at a wavelength of 355 nm. It has been found that the optical switching and beam deflection effects result from the change in refractive index of 3-HF under the irradiation of the pump beam. On the basis of the analyses of absorption spectra and fluorescence spectra, we conclude that the change in refractive index of 3-HF is due to not the thermal effect but the ESIPT effect of 3-HF under the pump beam. As the ESIPT is exceedingly fast, 3-HF might be an excellent candidate for high-speed optical switching.     

Theoretical insights into excited-state intramolecular proton transfer in 1,8-dihydroxydibenzo[a,h]phenazine

1,8-Dihydroxydibenzo[a,h]phenazine (DHBP) is a new synthetic compound possessing two intramolecular hydrogen bonds; however, it has been found to exhibit the excited-state intramolecular single proton transfer (ESSPT) behaviour, in recent experiment. To explain the phenomenon reasonably, two combined methods of CASSCF/CASPT2 and DFT/TD-DFT have been employed to investigate the structural and spectral properties of its three tautomers, corresponding to the non-proton-transferred (E), the single-proton-transferred (SK) and the double-proton-transferred (DK) forms. These studies suggest that the E form is the global minimum in the S₀ state, while the SK form is the most stable in the S₁ state, both of which are responsible for the experimental absorption peak at 2.54 eV and emission band at 1.64 eV, respectively. Because of the relatively high energy barrier, the DK form will play no important role in the fluorescence emission of DHBP. The present results lend a good support to the experimental finding of single proton transfer (SPT).     

Computational prediction on photophysical properties of two excited state intramolecular proton transfer (ESIPT) fluorophores bearing the benzothiazole group

ABSTRACT

In this contribution, the photophysical properties of two excited state intramolecular proton transfer (ESIPT) fluorophores of 2,6-dibenzothiazolyl-4-methylphenol (I) and 2-benzothiazolyl-6-(2-(benzothiazolyl)vinyl)-4-methylphenol (II) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods at the PBE0 theoretical level. To probe into the origin of the absorption and emission bands observed experimentally, the absorption and emission spectra of I and II were simulated by the TD-PBE0/6-311?+?G(d,p) calculations. In addition, the photo-induced proton enol–keto tautomerization of the two targeted molecules was also explored. The present studies indicate that a good agreement is found between theoretical predictions and experimental data. Moreover, both of these molecules can undergo an ultrafast ESIPT process, which should be responsible for the single proton-transfer tautomer emission.     

生色团连接的苯骈三氮唑衍生物的激发态分子内质子转移

用从头算和密度泛函理论研究了对硝基二苯乙烯作为生色团连接的2-(2-羟基-苯基)-苯骈三氮唑的衍生物2-羟基-5-[对硝基-二苯乙烯基-氧亚甲基]-苯基-(2H-苯骈三氮唑)(C1)和4′-硝基-3,4-二[2-羟基-(2H-苯骈三氮唑)-苄氧基]-二苯乙烯(C2)发生激发态分子内质子转移(ESIPT)的可能性.系统研究了C1和C2发生ESIPT的互变异构体的基态与激发态的性质变化,包括相关的键长、键角等结构参数,Mulliken电荷和偶极矩,前线轨道以及势能曲线.计算结果表明,对于C1来讲,酮式(keto)的基态(K)不存在稳定结构,因此发生基态分子内质子转移(GSIPT)可能性很小.酮式的激发态(K*)的氢键强度要远强于烯醇式(enol)的激发态(E*)的氢键强度.分子在光致激发后,质子供体所带负电荷减小而质子受体所带负电荷增加.在K*,HOMO→LUMO的电子跃迁导致电子密度从"酚环"向质子化杂环转移.E*→K*跃迁只需要克服较小的能垒(约41 kJ.mol-1).计算结果表明C1发生ESIPT的可能性很大.C2由于具有高能量,其具有基态的单质子转移特征的异构体EK(同时含烯醇E与酮K结构)、具有基态的双质子转移特征的异构体2K(含有双酮结构),以及具有双酮结构特征的激发态2K*均无法获得它们的稳定结构,因此,基态分子内单或双质子转移和激发态分子内双重质子转移发生的可能性极小.然而,由于双烯醇式的激发态(2E*)和EK的激发态(EK*)存在稳定结构,且2E*→EK*跃迁具有低能垒,因此C2有可能发生激发态分子内单重质子转移.本文进一步计算了两个分子的紫外-可见吸收光谱与荧光发射光谱,获得了具有较大斯托克位移的ESIPT的荧光发射峰.     

Excited state proton transfer based two color fluorescence: Perspectives and some biophysical applications

Photoinduced excited state intramolecular proton transfer (ESIPT) reactions comprise an important and extensively explored class of reactions in photochemistry. Till date, plant flavonols are one of the most widely known class of naturally abundant organic molecules exemplifying ESIPT and ‘two color’ fluorescence. From a bio-medical perspective, flavonols and related polyphenols, which are powerful antioxidants, have attracted significant interest as novel drugs (of high potency and low cyto-toxicity) for the prophylaxis and therapy of free radical induced and other important diseases. This article presents perspectives on proton transfer in photoexcited organic molecules from a historical context, emphasizing ESIPT reactions, in particular. Highlights of representative research findings are discussed, exemplifying the promising potential of plant flavonols as their own ESIPT based ‘fluorescence sensors’ for exploring their interactions with proteins, DNA (possessing duplex as well as higher order structures), and biomembranes, which represent the targets underlying the various pharmacological actions of flavonols. The usefulness of such approach for studying the confinement of intrinsically fluorescent flavonols in nano-vehicles for drug encapsulation, is also demonstrated.     

Impact of Heteroatom Substitution on Dual-State Emissive Rigidified 2-(2’-hydroxyphenyl)benzazole Dyes: Towards Ultra-Bright ESIPT Fluorophores**

2-(2’-Hydroxyphenyl)benzazole (HBX) fluorophores are well-known excited-state intramolecular proton transfer (ESIPT) emitters largely studied for their synthetic versatility, photostability, strong solid-state fluorescence and ability to engineer dual emission, thus paving the way to applications as white emitters, ratiometric sensors, and cryptographic dyes. However, they are heavily quenched in solution, due to efficient non-radiative pathways taking place as a consequence of the proton transfer in the excited-state. In this contribution, the nature of the heteroring constitutive of these rigidified HBX dyes was modified and we demonstrate that this simple structural modification triggers major optical changes in terms of emission color, dual emission engineering, and importantly, fluorescent quantum yield. Investigation of the photophysical properties in solution and in the solid state of a series of ethynyl-TIPS extended HBX fluorophores, along with ab initio calculations demonstrate the very promising abilities of these dyes to act as bright dual-state emitters, in both solution (even in protic environments) and solid state.