Experimental and Theoretical Investigation on Excited State Intramolecular Proton Transfer Coupled Charge Transfer Reaction of Baicalein

The excited state intramolecular proton transfer （ESIPT） coupled charge transfer of baicalein has been investigated using steady-state spectroscopic experiment and quantum chemistry calculations. The absence of the absorption peak from S1 excited state both in the experi-mental and calculated absorption spectra indicates that S1 is a dark state. The dark excited state S1 results in the very weak fluorescence of solid baicalein in the experiment. The fron- tier molecular orbital and the charge difference densities of baicalein show clearly that the S1 state is a charge-transfer state whereas the S2 state is a locally excited state. The only one stationary point on the potential energy profile of excited state suggests that the ESIPT reaction of baicalein is a barrierless process.

The substituent effect on the excited state intramolecular proton transfer of 3-hydroxychromone

The excited state intramolecular proton transfer of four derivatives(FM, BFM, BFBC, CCM) of 3-hydroxychromone is investigated.The geometries of different substituents are optimized to study the substituent effects on proton transfer.The mechanism of hydrogen bond enhancement is qualitatively elucidated by comparing the infrared spectra, the reduced density gradient, and the frontier molecular orbitals.The calculated electronic spectra are consistent with the experimental results.To quantify the proton transfer, the potential energy curves(PECs) of the four derivatives in S0 and S1 states are scanned.It is concluded that the ability of proton transfer follows the order: FM > BFM > BFBC > CCM.

Exploring the effect of aggregation-induced emission on the excited state intramolecular proton transfer for a bis-imine derivative by quantum mechanics and our own n-layered integrated molecular orbital and molecular mechanics calculations

We theoretically investigate the excited state intramolecular proton transfer(ESIPT) behavior of the novel fluorophore bis-imine derivative molecule HNP which was designed based on the intersection of 1-(hydrazonomethyl)-naphthalene-2-ol and 1-pyrenecarboxaldehyde. Especially, the density functional theory(DFT) and time-dependent density functional theory(TDDFT) methods for HNP monomer are introduced. Moreover, the "our own n-layered integrated molecular orbital and molecular mechanics"(ONIOM) method(TDDFT:universal force field(UFF)) is used to reveal the aggregation-induced emission(AIE) effect on the ESIPT process for HNP in crystal. Our results confirm that the ESIPT process happens upon the photoexcitation for the HNP monomer and HNP in crystal, which is distinctly monitored by the optimized geometric structures and the potential energy curves. In addition, the results of potential energy curves reveal that the ESIPT process in HNP will be promoted by the AIE effect. Furthermore, the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) for the HNP monomer and HNP in crystal have been calculated. The calculation demonstrates that the electron density decrease of proton donor caused by excitation promotes the ESIPT process. In addition, we find that the variation of atomic dipole moment corrected Hirshfeld population(ADCH) charge for proton acceptor induced by the AIE effect facilitates the ESIPT process. The results will be expected to deepen the understanding of ESIPT dynamics for luminophore under the AIE effect and provide insight into future design of high-efficient AIE compounds.

Theoretical investigation on the excited state intramolecular proton transfer in Me_(2)N substituted flavonoid by the time-dependent density functional theory method

Time-dependent density functional theory(TDDFT)method is used to investigate the details of the excited state intramolecular proton transfer(ESIPT)process and the mechanism for temperature effect on the Enol^(*)/Keto^(*)emission ratio for the Me_(2)N-substited flavonoid(MNF)compound.The geometric structures of the S_(0) and S_(1) states are denoted as the Enol,Enol^(*),and Keto*.In addition,the absorption and fluorescence peaks are also calculated.It is noted that the calculated large Stokes shift is in good agreement with the experimental result.Furthermore,our results confirm that the ESIPT process happens upon photoexcitation,which is distinctly monitored by the formation and disappearance of the characteristic peaks of infrared(IR)spectra involved in the proton transfer and in the potential energy curves.Besides,the calculations of highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)reveal that the electronegativity change of proton acceptor due to the intramolecular charge redistribution in the S_(1) state induces the ESIPT.Moreover,the thermodynamic calculation for the MNF shows that the Enol^(*)/Keto^(*)emission ratio decreasing with temperature increasing arises from the barrier lowering of ESIPT.

近红外ESIPT型有机荧光探针的设计合成及其应用

荧光探针检测因其在活体样品中的高灵敏度、高分辨率和实时检测成像等优势备受关注,广泛应用于化学检测、临床和生物成像等领域。近红外小分子荧光探针具有低背景干扰、高灵敏度和深组织穿透性等优势,不仅能够克服传统荧光探针的局限性,还能在生物成像中展现其特有的优势,因此受到越来越多领域的关注。与传统荧光染料相比,具有激发态分子内质子转移(ESIPT)机制的化合物具有较大的Stokes位移,可以有效消除内滤光作用和荧光自吸收效应,降低自身荧光的干扰,从而提高分子探针检测的灵敏性。此外,这类化合物具有较强的位点可修饰性,可以通过分子对接等方法更好地优化光学性能。因此,本文结合了近红外和ESIPT等优点,设计并合成了一种近红外ESIPT型有机小分子荧光探针,并研究了其在水合肼检测中的可行性。

Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer

Mechanochromic polymers based on non-covalent changes have attracted much attention recently.Herein,we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process.PhMz-NH2-OH and PhMz=2A are designed and obtained by simple and high-yield synthesis,and are connected into polyurethane and poly(methyl acrylate-co-2-ethylhexyl acrylate),respectively.In the initial state,the PhMz-NH2-OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons,owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA-2-EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission,implying that the intramolecular hydrogen bonds matter.Furthermore,under stretching,external force can tune the emission of the PhMz=2A@PMA-2-EA sample from K* to E* state.Though external force can putatively still promote a bond rotation,ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen-bond donating environment.DFT calculation confirms the force-induced increase in dihedral angle for the transition of ESIPT-on/off.Thus,PhMz-NH2-OH@PU and PhMz=2A@PMA-2-EA showed disparate initial ESIPT states and further different responses/sensitivity to force.This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.

A Versatile BenzimidazoleSubstituted Spirolactam Mechanophore:When Excited State Intramolecular Proton Transfer Couples with Rhodamine

Exploration of multicolor mechanochromic bulk polymers based on a single mechanophore is a big challenge to date.Herein,we report a versatile benzimidazole-substituted spirolactam mechanophore where excited state intramolecular proton transfer(ESIPT)coupled with rhodamine.The mechanophore was facilely synthesized and then covalently linked to polyurethane(PU)chains.The PU film containing the mechanophore(1@PU)showed cooperative photochromism upon irradiation involving simultaneous enhancement of normalized enol and rhodol emissions based on a cooperative ESIPT process and the ring-opening reaction of spirolactam.Moreover,the film exhibited dual-mode multicolor mechanochromism upon stretching and compression.The normalized intensity of enol emission increased and the fluorescence turned from light green to cyan after stretching,then red coloration appeared from colorless after compressing.Control experiments and density functional theory calculations confirmed that the stretch-induced increase of enol emission was attributed to torsion of the dihedral angle between xanthene and benzimidazole in the mechanophore via force-induced disaggregation and direct force action on the isolated mechanophore.Torsion of the dihedral angle and the ring-opening reaction of spirolactam in a single mechanophore occurred sequentially during compression,resulting in an observed red coloration.This study might provide a glimpse into the design of novel multicolor mechanochromic mechanophores.

Ab initio investigation of excited state dual hydrogen bonding interactions and proton transfer mechanism for novel oxazoline compound

Owing to the importance of excited state dynamical relaxation, the excited state intramolecular proton transfer(ESIPT) mechanism for a novel compound containing dual hydrogen bond(abbreviated as "1-enol") is studied in this work.Using density functional theory(DFT) and time-dependent density functional theory(TDDFT) method, the experimental electronic spectra can be reproduced for 1-enol compound. We first verify the formation of dual intramolecular hydrogen bonds, and then confirm that the dual hydrogen bond should be strengthened in the first excited state. The photo-excitation process is analyzed by using frontier molecular orbital(HOMO and LUMO) for 1-enol compound. The obvious intramolecular charge transfer(ICT) provides the driving force to effectively facilitate the ESIPT process in the S1 state. Exploration of the constructed S0-state and S1-state potential energy surface(PES) reveals that only the excited state intramolecular single proton transfer occurs for 1-enol system, which makes up for the deficiencies in previous experiment.

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer(ESDPT)in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol(HYDRAVH_(2))ligand was studied by the density functional theory and time-dependent density functional theory method.The analysis of frontier molecular orbitals,infrared spectra,and non-covalent interactions have crossvalidated that the asymmetric structure has an influence on the proton transfer,which makes the proton transfer ability of the two hydrogen protons different.The potential energy surfaces in both S_(0)and S_1 states were scanned with varying O-H bond lengths.The results of potential energy surface analysis adequately proved that the HYDRAVH_(2)can undergo the ESDPT process in the S_1 state and the double proton transfer process is a stepwise proton transfer mechanism.Our work can pave the way towards the design and synthesis of new molecules.

Effect of intramolecular and intermolecular hydrogen bonding on the ESIPT process in DEAHB molecule

Density functional theory（DFT） and time-dependent density functional theory（TDDFT） methods are used to investigate the influences of intramolecular and intermolecular hydrogen bonding on excited-state intramolecular proton transfer（ESIPT） for the 4-N,N-（diethylamino）-2-hydroxybenzaldehyde（DEAHB）. The structures of DEAHB and its hydrogenbonded complex in the ground-state and the excited-state are optimized. In addition, the detailed descriptions of frontier molecular orbitals of the DEAHB monomer and DEAHB-DMSO complex are presented. Moreover, the transition density matrix is worked out to gain deeper insight into the orbitals change. It is hoped that the present work not only elaborates different influence mechanisms between intramolecular and intermolecular hydrogen bonding interactions on the ESIPT process for DEAHB, but also may be helpful to design and develop new materials and applications involved DEAHB systems in the future.

具有AIE和ESIPT双重性质的荧光分子的合成及光物理性质初探

化学实验训练是化学专业教学中不可或缺的一环,但陈旧的实验方案在一定程度上限制了学生的探索和创新精神。本实验方案深入结合科学前沿的热门领域,利用羰基的亲核加成反应,制备了一种具有聚集诱导发光(AIE)和激发态分子内质子转移(ESIPT)性质的荧光小分子。实验研究了该分子的基本光物理性质及酸碱刺激响应能力。整个反应过程不使用有毒或危险性试剂,反应物只需在乙醇中回流0.5 h即可完成,充分体现绿色有机合成的策略。实验过程紧凑有序,现象明显且荧光色彩变化丰富,能够更好地激发学生的实验热情。同时,本实验充分融合了有机合成基本操作及核磁共振、红外光谱、紫外可见吸收光谱、荧光光谱等多种现代分析和表征方法,是一个直观有效的开放性实验,可有效提升学生的专业综合素质、培养学生的创新性研究能力。

基于ESIPT机理检测Al^3+荧光探针分子结构和电子光谱的理论研究

本文应用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)方法,研究了具有激发态分子内质子转移(ESIPT)特性的3-羟基黄酮(3HF)及其两种氰基和氨基取代衍生物(3HF-CY和3HF-AM)作为水溶液中Al^3+离子检测的荧光探针分子结构和电子光谱性质.计算得到了与ESIPT过程相关的键长、键角以及势能曲线,模拟计算了单独分子和分子@Al3+复合物的吸收和荧光光谱.结果表明,氰基或氨基的引入均会抑制3HF的质子在基态(S0)或激发态(S1)的转移.而从得到的吸收光谱可以看出,在3HF中引入吸电子基团氰基可以引起其吸收光谱的红移,而给电子基团氨基的引入则出现相反现象.此外,与3HF-AM的荧光光谱相比,3HF-AM@Al^3+复合物发生了75.88 nm的蓝移,由此推测3HF-AM与水中的Al^3+反应后,在光激发下溶液荧光会由绿色转变为紫色,表明3HF-AM分子可以作为有效检测水中Al3+的荧光探针.

基于AIE+ESIPT原理的荧光纳米探针构建及其在环境水中硫化氢的检测研究

环境水中有机物厌氧腐烂产生的硫化氢(H_(2)S)是一种剧毒的气体,严重威胁人体健康。因此设计一种高效的用于检测环境水中H_(2)S的方法尤为重要。基于此,本文基于聚集诱导发光(AIE)和激发态分子内质子转移(ESIPT)原理,构建了一种“关-开”型的H_(2)S荧光纳米探针(NP-1)。实验结果表明,NP-1具有较好的水分散性、选择性好、检测限低(-38 nM)、线性范围广(0-40μM)等优点。此外,NP-1被成功应用于环境水中H_(2)S的荧光检测。

Relationship between ESIPT properties and antioxidant activities of 5-hydroxyflavone derivates

It is of great significance to study the relationship between the excited state intramolecular proton transfer(ESIPT)properties and antioxidant activities of compounds in the field of life sciences.In this work,two novel compounds 5HF-OMe and 5HF-NH2 are designed through introducing a methoxy-and amino-group into the structure of 5-hydroxyflavone(5HF)respectively.The relationship between the ESIPT reaction and antioxidant activities of the three compounds is studied via the density functional theory(DFT)and time-dependent DFT(TD-DFT)methods.The calculated potential energy curves suggest that the rate of ESIPT reaction will gradually slow down from 5HF to 5HF-OMe and 5HF-NH2.In addition,the antioxidant activities of the three compounds gradually enhance from 5HF to 5HF-OMe and 5HF-NH2,which can be seen from the calculated energy gaps and ionization potential values.Interestingly,the above results imply that the rate of ESIPT reaction has a negative relationship with the antioxidant activities of the compounds,i.e.,the slower rate of ESIPT reaction will reflect the higher antioxidant activity of the compound,which will provide valuable reference for detecting the antioxidant activity of compound via the photophysical method.

Excited state intramolecular proton transfer fluorescence emission of o-hydroxyphenyl-triazine derivatives

o-hydroxyphenyl-triazine derivatives with formyl substituents were surveyed for the excited state intramolecular proton transfer (ESIPT).The occurrence of ESIPT was confirmed by well-separated emission bands for the derivatives.A low energy change from enol to keto in the excited state explains ESIPT for the derivatives.

Theoretical study on the relationship between the position of the substituent and the ESIPT fluorescence characteristic of HPIP

The influences of the substituent base position on the excited state intramolecular proton transfer fluorescence properties were explored in 2-(2'-hydroxyphenyl)imidazo[1,2-a]-pyridine(HPIP)and HPIP's derivatives(5'Br-HPIP and 6'BrHPIP).And the density functional theory(DFT)and time-dependent DFT(TD-DFT)methods were used to calculate the molecule structures.The calculated results showed that the influence of 5'Br-HPIP on the fluorescence intensity is stronger than that of 6'Br-HPIP.The fluorescence emission peak of 5'Br-HPIP occurred a blue shift compared with HPIP,and 6'BrHPIP exhibited an opposite red shift.The change of the fluorescence emission peak was attributed to the decrease of the energy gap from 6'Br-HPIP to 5'Br-HPIP.Our work on the substituent position influence could be helpful to design and develop new materials.

Theoretical investigation of fluorescence changes caused by methanol bridge based on ESIPT reaction

The different fluorescence behavior caused by the excited state proton transfer in 3-hydroxy-4-pyridylisoquinoline(2a)compound has been theoretically investigated.Our calculation results illustrate that the 2a monomer in tetrahydrofuran solvent would not occur proton transfer spontaneously,while the 2a complex in methanol(MeOH)solvent can undergo an asynchronous excited state intramolecular proton transfer(ESIPT)process.The result was confirmed by analyzing the related structural parameters,infrared vibration spectrum and reduced density gradient isosurfaces.Moreover,the potential curves revealed that with the bridging of single MeOH molecular the energy barrier of ESIPT was modulated effectively.It was distinctly reduced to 4.80 kcal/mol in 2a-MeOH complex from 25.01 kcal/mol in 2a monomer.Accordingly,the ESIPT process induced a fluorochromic phenomenon with the assistant of proton-bridge.The elucidation of the mechanism of solvent discoloration will contribute to the design and synthesis of fluorogenic dyes as environment-sensitive probes.

Influence of intramolecular hydrogen bond formation sites on fluorescence mechanism

The fluorescence mechanism of HBT-HBZ is investigated in this work. A fluorescent probe is used to detect HClO content in living cells and tap water, and its structure after oxidation by HCl O(HBT-ClO) is discussed based on the density functional theory(DFT) and time-dependent density functional theory(TDDFT). At the same time, the influence of the probe conformation and the proton transfer site within the excited state molecule on the fluorescence mechanism are revealed. Combined with infrared vibrational spectra and atoms-in-molecules theory, the strength of intramolecular hydrogen bonds in HBT-HBZ and HBT-ClO and their isomers are demonstrated qualitatively. The relationship between the strength of intramolecular hydrogen bonds and dipole moments is discussed. The potential energy curves demonstrate the feasibility of intramolecular proton transfer. The weak fluorescence phenomenon of HBT-HBZ in solution is quantitatively explained by analyzing the frontier molecular orbital and hole electron caused by charge separation. Moreover, when strong cyan fluorescence occurs in solution, the corresponding molecular structure should be HBT-ClO(T). The influence of the intramolecular hydrogen bond formation site on the molecule as a whole is also investigated by electrostatic potential analysis.

A DFT/TD-DFT study of effect of different substituent on ESIPT fluorescence features of 2-(2’-hydroxyphenyl)-4-chloromethylthiazole derivatives

Based on density functional theory(DFT) and time-dependent density functional theory(TD-DFT), the effects of substituent on the excited-state intramolecular proton transfer(ESIPT) process and photophysical properties of 2-(2’-hydroxyphenyl)-4-chloromethylthiazole(HCT) are studied. The electron-donating group(CH_(3), OH) and electronwithdrawing group(CF_(3), CHO) are introduced to analyze the changes of intramolecular H-bond, the frontier molecular orbitals, the absorption/fluorescence spectra, and the energy barrier of ESIPT process. The calculation results indicate that electron-donating group strengthens the intramolecular H-bond in the S_(1) state, and leads to an easier ESIPT process. The electron-withdrawing group weakens the corresponding H-bond and makes ESIPT process a little harder. Different substituents also affect the photophysical properties of HCT. The electron-withdrawing group(CF_(3), CHO) has a little effect on electronic spectra. The electron-donating group(CH_(3), OH) red-shifts both the absorption and fluorescence emission peaks of HCT, respectively, which causes the Stokes shift to increase.

基于激发态分子内质子转移过程的HBT-OMe分子检测HClO的荧光增强机理

采用密度泛函理论(DFT)和含时密度泛函理论(TDDFT)方法,基于连续介质模型,对HBT-OMe分子及其与HClO反应的产物基态和激发态结构及电荷分布等性质进行研究,优化构型结果表明,HBT-OMe分子不能发生质子转移过程而其产物分子则能发生.红外振动光谱与分子共价作用分析进一步证实产物分子能发生质子转移过程.计算的前线分子轨道结果表明:HBT-OMe分子存在扭曲电荷转移(TICT)过程,最高已占据分子轨道(HOMO)和最低未占据分子轨道(LUMO)电荷密度重叠小导致HBT-OMe分子荧光强度弱,结合产物分子势能曲线发现,产物分子质子转移过程能抑制分子的TICT过程,从而使其荧光强度增强.