Theoretical study on proton tautomerism of chryosphanol and its analogues in the ground and excited states

Proton tautomerism of 1,8-dihydroxy-3-methyl-anthraquinone and its analogues were studied using HF and CIS methods with 6-31g(d,p) basis set for the ground and singlet excited states. The calculations indicate that the compound exists two strong intramolecular hydrogen bonds (IHB), and shows similar characters in its proton transfer processes considering the geometries and Mulliken charge population. Calculation results further show that intramolecular proton transfer (IPT) is not favored in view of the energy trend for chryosphanol, which has two hydrogens of hydroxyl groups bond with a common oxygen of carbonyl group and exists two IHBs in the peri region. However, it exhibits normal intramolecular proton transfer for the derivatives of chryosphanol, which have only one pair of adjacent hydroxyl group and carbonyl group existing in the peri region. Hereby, it can be conjectured from a theoretical point of view that IPT is absent in the similar structure such as hypericin’s peri region. Calculation results on the photophysical process show that the isomerization process is competitive with the intersystem crossing process, which facilitates the increase of triplet state quantum efficiency and photosensitive activity.     

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.     

用于监控过氧化氮的近红外荧光探针的光物理性质及PET机理

含有机硒的七甲川菁染料是基于光诱导电子转移(PET)的近红外(IR)荧光探针, 能在生理条件下高灵敏、高选择性地监控过氧化氮. 本文应用含时密度泛函理论(TD-DFT)计算方法研究其光物理性质和PET机理.结果表明, 在激发态, 荧光母体发生最高占有分子轨道(HOMO)到最低非占有分子轨道(LUMO)的电子跃迁, 识别基团上的HOMO轨道能级提高到荧光母体的单电子占据的HOMO轨道能级之上, 并向其转移一个电子, 使激发态电子回落过程受阻而导致荧光部分淬灭. 硒被氧化后, 识别基团上的HOMO轨道能级降低, PET过程被阻断, 荧光发射恢复. 研究进一步证明, PET效应来自于识别基团上苯胺N原子的p电子, 它的电子转移能力受到其对位苯硒基的氧化-还原状态的影响, 产生了荧光信号的“开-关”作用.     

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.     

Design and synthesis of intramolecular hydrogen bonding systems. Their application in metal cation sensing based on excited-state proton transfer reaction

We reported the design and synthesis of a new type of metal-cation probes, 3-hydroxy-4-(1,4,7,10-tetraoxa-13-azacyclopentadec-13-ylmethyl)naphthalene-2-carbaldehyde (1a) and its single hydrogen-bond analogue 1-(1,4,7,10-tetraoxa-13-azacylopentadec-13-ylmethyl)-2-naphthol (2a), in which 1-aza-15-crown-5 ether in combination with the naphthol oxygen acts as a receptor, while the mechanism of excited-state intramolecular proton transfer (ESIPT) is exploited as a signal transducer. The association constant of (2.5±0.5)×10⁴, (3.8±0.4)×10⁴, (5.5±0.5)×10³ and (1.2±0.3)×10⁴ M⁻¹ for the formation of 1a/Na⁺, 1a/Ca²⁺, 2a/Na⁺ and 2a/Ca²⁺ complexes, respectively, in CH₃CN plus drastic fluorescence changes due to the fine-tuning of ESIPT reaction upon complexation, lead 1a and 2a to be highly sensitive fluorescent sensors. The results add a new class into the category of metal-cation probes, with the perspective of designing ESIPT systems capable of sensing bio-analytes.     

黄芩素激发态分子内质予转移耦合电荷转移反应的实验和理论研究

通过稳态光谱实验和量子化学计算相结合,研究了黄芩素激发态质子转移耦合电荷转移的反应. 实验和计算中S1态吸收峰的缺失表明S1态是暗态. S1暗态导致在实验中观察不到黄芩素在乙醇溶液中的荧光峰,且固体的荧光峰很弱. 黄芩素分子的前线分子轨道和电荷差异密度表明S1态是电荷转移态,然而S2态是局域激发态. 计算的黄芩素分子的势能曲线在激发态只有一个稳定点,这表明了黄芩素激发态分子内质子转移的过程是一个无能垒的过程.     

Photophysics, photochemistry and photobiology of curcumin: Studies from organic solutions, bio-mimetics and living cells

Curcumin, with its recent success as an anti-tumor agent, has been attracting researchers from wide ranging fields of physics, chemistry, biology and medicine. The chemical structure of curcumin has two o-methoxy phenols attached symmetrically through α,β-unsaturated β-diketone linker, which also induces keto–enol tautomerism. Due to this, curcumin exhibits many interesting photophysical and photochemical properties. The absorption maximum of curcumin is 408–430 nm in most of the organic solvents, while the emission maximum is very sensitive to the surrounding solvent medium (460–560 nm) and the Stokes’ shift varied from 2000 to 6000 cm⁻¹. The fluorescence quantum yield in most of the solvents is low and reduced significantly in presence of water. The fluorescence lifetime is short (<1 ns) and displayed multi-exponential decay profile. The singlet excited states of curcumin decay by non-radiative processes contributed mainly by intra- and intermolecular proton transfer with very low intersystem crossing efficiency. Polarity, π-bonding nature, hydrogen bond donating and accepting properties of the solvent influence the excited state photophysics of curcumin in a complex manner. The triplet excited states of curcumin absorb at 720 nm and react with oxygen to produce singlet molecular oxygen. The photodegradation of curcumin produces smaller phenols and the photobiological activity of curcumin is due to the generation of reactive oxygen species.Being lipophilic in nature, the water solubility of curcumin could be enhanced upon the addition of surfactants, polymers, cyclodextrins, lipids and proteins. Changes in the absorption and fluorescence properties of curcumin have been found useful to follow its interaction and site of binding in these systems. Curcumin fluorescence could be employed to follow the unfolding pattern and structural changes in proteins. The intracellular curcumin showed more fluorescence in tumor cells than in normal cells and fluorescence spectroscopy could be used to monitor its preferential localization in the membrane of tumor cells. This review, presents the current status of research on the photophysical, photochemical and photobiological processes of curcumin in homogeneous solutions, bio-mimetics and living cells. Based on these studies, the possibility of developing curcumin, as a bimolecular sensitive fluorescent probe is also discussed.     

Proton- and metal cation-enhanced excited state intramolecular proton transfers of 2-(2-hydroxyfluorophenyl)benzoxazole having imidazole moiety

2-(2-Hydroxyfluorophenyl)benzoxazole having an imidazole moiety 1 was synthesized by the two step reactions starting from 2-(pentafluorophenyl)benzoxazole. Protonation at the 3-imidazole nitrogen atom of 1 enhances the green emission around 500 nm, where the positive character caused by the protonation is inductively communicated to the hydroxy group, to recover its intramolecular hydrogen bonding, leading to the ESIPT process. Addition of Al³⁺ or Zn²⁺ to 1 enhances both the green emission and the blue emission around 450 nm in chloroform-acetonitrile.     

A study on multifunctional protein fibre with UV protection,moth repellency and antibacterial properties using ESIPT core containing benzimidazole and benzothiazole based functional acid azo dyes

Water soluble fluorescent acid azo dyes with benzimidazole and benzothiazole component having excited state intramolecular proton transfer (ESIPT) core were synthesised by diazo coupling. The structure of the dyes were analysed and confirmed by Fourier Transform Infrared – spectroscopy (FT-IR), elemental analysis, ¹H NMR and ¹³C NMR analysis. Absorption and emission characteristics of the dyes were studied in different polarity solvents shown bathochromic (red) shift as solvent polarity increases. Wool and silk fabric dyed with synthesised dyes displayed high exhaustion, uniform dyeing and good wet fastness properties. The dyed substrate showed green and purple fluorescence under UV light (366 ​nm) along with UV protection. Dyed wool fabric was also assessed for antibacterial activity by calculation of bacterial reduction Staphylococcus Auerus (Gram positive), Klebsiella Pneumonia (Gram negative) bacteria. The dyed wool were also examined for the resistivity against insect pests Anthrenus Flavipies (Le Conte) by calculation of the fabric weight loss, Mortality of moth and visible assessment of fabric attacked by moth after 14 days in given conditions. Consequently, it was demonstrated that wool fabric dyed with Benzimidazole and benzothiazole based acid dyes had various functionalities, such as UV protection, antibacterial activity and mild moth repellency.     

Syntheses,Crystal Structures and Photophysical Properties of a Series of Cadmium(II) Coordination Polymers

Three cadmium(II) coordination polymers [Cd(NA)₂(H₂O)₂]_n ( 1 ), {[Cd(NA)(phen)(NO₃)]·(H₂O)_1/2}_n ( 2 ), {[Cd(NA)(CH₃C₆H₄COO)(H₂O)₂]·(CH₃C₆H₄COOH)}_n ( 3 ) (HNA = nicotinic acid, phen = 1, 10‐phenanthroline) have been synthesized by hydrothermal method. Their single‐crystal structures were determined by X‐ray diffractometry. The absorption, excitation and emission spectra were investigated and all the complexes emit strong fluorescence: λ^em_max = 544 nm (λ^ex = 492 nm), 1 ; λ^em_max = 466 nm (λ^ex = 393 nm), 2 ; λ^em_max = 430 nm (λ^ex = 313 nm), 3 . At room temperature in the solid state the fluorescence lifetimes of the complexes were investigated and the relationships between the spectra were discussed as well as the connections of luminescence and crystal structures.     

2-氨基苯并噻唑的结构及激发态质子转移动力学

通过傅里叶变换红外光谱（FTIR）、傅里叶变换拉曼（FT-Raman）和488 nm拉曼光谱,结合密度泛函理论（DFT）计算研究了2-氨基苯并噻唑（ABT）在晶态和溶剂中的二聚体结构,并解释了质子性溶剂中ABT二聚体与溶剂分子间的氢键作用.电子光谱实验揭示了ABT二聚体的光物理和光化学反应;紫外吸收和荧光发射光谱结果表明,溶剂、激发波长和pH值对ABT二聚件激发态衰变具有调控作用;含时密度泛函理论（TD-DFT）解释了ABT二聚体双荧光现象,提出了高激发态的质子转移机理.     

2-(benzimidazol-2-yl)-3-hydroxychromone derivatives: spectroscopic properties and a possibile alternative intramolecular proton phototransfer

Three 2-(benzimidazol-2-yl)-3-hydroxychromone derivatives were synthesized. Their spectroscopic and fluorescent properties, due to excited state intramolecular proton transfer (ESIPT) from OH to carbonyl, were studied. Theoretical possibility of an alternative intramolecular H-bonding and experimental evidence for such behavior are discussed.      

6-甲基-4-羟基嘧啶单体及二聚体激发态质子转移异构化反应及光谱的理论研究

采用ab initio HF理论的组态相关CIS方法和连续溶剂模型PCM, 分别在6-311+G*和6-31G水平上研究了6-甲基-4-羟基嘧啶单体及二聚体激发态质子转移的异构化反应; 对其反应势能面的研究发现, 单体基态和激发态的异构化反应一起可以形成四能级的分子电子体系, 而二聚体的却不能, 由此解释了单体和二聚体的紫外吸收光谱和荧光发射光谱均对应于酮式构型的原因. 利用混合含时密度泛函TD/MPW1PW91理论方法在溶剂存在下计算了标题物质的紫外吸收光谱和荧光发射光谱.     

Excited-state intramolecular proton transfer in 2-(2′,6′-dihydroxyphenyl)benzoxazole: effect of dual hydrogen bonding on the optical properties

2-(2′,6′-Dihydroxyphenyl)benzoxazole (DHBO) has been synthesized by using palladium-catalyzed oxidative cyclization. The compound utilizes both O-H···N and O-H···O bonds to ensure a coplanar structure between the benzoxazole and phenol fragments. Optical comparison with the parent compound 2-(2′-hydroxyphenyl)benzoxazole (HBO) reveals that the dual hydrogen bonding in DHBO plays an essential role in raising the desirable keto emission for ESIPT and tuning the polarity sensitivity toward the molecular environment. DHBO also exhibits a higher quantum yield (?_fl = 0.108 in methanol) than HBO (?_fl = 0.0025) in the same solvent.     

8-羟基喹啉单体及二聚体质子转移反应的理论研究

Density functional theory （DFT） of quantum chemistry method was employed to investigate proton transfer reactions of 8-hydroxyquinoline （8-HQ） monomers and dimers. By studying the potential energy curves of the isomerization, the most possible reaction pathway was found. The total energy of 8-hydroxyquinoline was lower than that of quinolin-8（1H）-one, whereas the order was reversed in dimers. The findings explained the contrary experimental phenomena. The minimum reaction barrier of intramolecular proton transfer was 47.3 kJ/mol while that in dimer was only 25.7 kJ/mol. Hence it is obvious that proton transfer reactions of 8-HQ monomer have a considerable rate but it is easier to proceed for 8-HQ dimer than monomers. It implied that the hydrogen bond played an important role in depressing the activation energy of reaction. The mechanism of the tautomerization was discussed on the basis of theoretical results.     

Photophysical and electrochemical properties of meso, meso-linked oligoporphyrin rods with appended fullerene terminals.

The electrochemical and photophysical properties of molecular architectures consisting of oligomeric meso,meso-linked oligoporphyrin rods linked at both extremities to methanofullerene moieties are presented in comparison to those of model systems. Cyclic voltammetry data evidence the presence of a strong intramolecular electronic coupling along the porphyrin oligomers that varies slightly with their length. This interaction affects the redox potentials of both fullerene and porphyrin moieties. The electronic coupling between the two chromophores is confirmed by comparing the redox potentials of porphyrin arrays before and after attachment of the carbon sphere. Electronic absorption, fluorescence, and phosphorescence spectra of the porphyrin oligomers in toluene are reported, which provide the energy of the lowest singlet and triplet electronic excited states. In the fullerene-porphyrin conjugates, ground-state charge-transfer (CT) interactions are evidenced by low-energy absorption features above 750 nm. These systems also exhibit near-infrared (NIR) CT luminescence in toluene with lifetimes shorter than 1000 ps. On increasing the solvent polarity (from toluene to Et2O and THF), CT emissions become progressively weaker, red-shifted, and shorter lived, which reflects the energy-gap law and Marcus inverted region effects. Luminescence is not detected in benzonitrile. Picosecond transient absorption spectroscopy of the porphyrin-fullerene conjugates allows detection of the porphyrin cation as a clear fingerprint for electron transfer. The rate of charge recombination is in agreement with CT luminescence lifetimes, which confirms the occurrence of NIR radiative back-electron transfer.     

Excited state properties, fluorescence energies, and lifetime of a poly(fluorene-pyridine) copolymer, based on TD-DFT investigation

The structural and electronic properties of the fluorene-pyridine copolymer (FPy)(n), (n = 1-4) were investigated theoretically by means of quantum mechanical calculations based on density functional theory (DFT) and time-dependent DFT (TD-DFT) using the B3LYP functional. Geometry optimizations of these oligomers were performed for the ground state and the lowest excited state. It was found that (FPy)(n) is nonplanar in its ground state, whereas a more pronounced trend toward planarity is observed in the S(1) state. Absorption and fluorescence energies have been extrapolated to infinite chain length making use of their good linearity with respect to 1/n. An extrapolated value of 2.64 eV is obtained for vertical excitation energy. The S(1)<--S(0) electronic excitation is characterized as a highest occupied molecular orbital to lowest unoccupied molecular orbital transition and is dominating in terms of oscillator strength. Fluorescence energies and radiative lifetime were calculated as well. The obtained results indicate that the fluorescence energy and radiative lifetime of (FPy)(n) are 2.16 eV and 0.38 ns, respectively. The decrease of fluorescence energy and radiative lifetime with the increase in the chain length is discussed.     

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 properties,fluorescence energies,and lifetimes of a poly(fluorene‐phenylene), based on TD‐DFT investigation

The structural and electronic properties of fluorene‐phenylene copolymer (FP)_n, n = 1–4 were studied by means of quantum chemical calculations based on density functional theory (DFT) and time dependent density functional theory (TD‐DFT) using B3LYP functional. Geometry optimizations of these oligomers were performed for the ground state and the lowest singlet excited state. It was found that (FP)_n is nonplanar in its ground state while the electronic excitations lead to planarity in its S₁ state. Absorption and fluorescence energies were calculated using TD‐B3LYP/SVP and TD‐B3LYP/SVP+ methods. Vertical excitation energies and fluorescence energies were obtained by extrapolating these values to infinite chain length, resulting in extrapolated values for vertical excitation energy of 2.89 and 2.87 eV, respectively. The S₁ ← S₀ electronic excitation is characterized as a highest occupied molecular orbital to lowest unoccupied molecular orbital transition and is distinguishing in terms of oscillator strength. Fluorescence energies of (FP)_n calculated from TD‐B3LYP/SVP and TD‐B3LYP/SVP+ methods are 2.27 and 2.26 eV, respectively. Radiative lifetimes are predicted to be 0.55 and 0.51 ns for TD‐B3LYP/SVP and TD‐B3LYP/SVP+ calculations, respectively. These fundamental information are valuable data in designing and making of promising materials for LED materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010