Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-o-carboranyl compounds, in which hydrogen (15CH and 26CH) or trimethysilyl groups (15CS and 26CS) were appended at the o-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while 15CS and 26CS showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH. These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo-π-interaction, resulting in a higher quantum efficiency (Φ_em) for ICT-based radiative decay than those of 15CH and 26CH. Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φ_em and lowest k_nr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o-carboranyl luminophores could be achieved by the formation of a specific geometry between the o-carborane cage and the aromatic plane.     

Structural Insights Into 9‐Styrylanthracene‐Based Luminophores: Geometry Control Versus Mechanofluorochromism and Sensing Properties

Organic luminophores have been increasingly applied in various fields and the corresponding structural studies are emerging as hot topics, which supply essential information on the relationships between molecular structures and photophysical properties. Herein, a series of 9‐styrylanthracene‐based luminophores with different geometries (symmetrical DTVA , asymmetrical TVA and CPVA ) were facilely synthesized for a systematical photophysical invesitigation. The targets display reversible and diverse mechanofluorochromism in the solid state. In particular, DTVA and TVA exhibit an obviously higher solid‐state fluorescence quantum efficiency and distinctive AIE‐active behavior, with contrary emission shifts under external force. Moreover, DTVA and TVA showed high sensitivity for detection of picric acid (PA) in aggregated states. This study may be helpful to reveal the structural essence of luminesecent materials and pave the way to the development of novel functional luminophores.     

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.     

Novel chemosensor for alkaline earth metal ion based on 9-anthryl aromatic amide using a naphthalene as a TICT control site and intramolecular energy transfer donor

Novel fluorescent chemosensors 1 and 2 with two different fluorophores (naphthalene and anthracene) at the both ends of polyether was synthesized. These compounds based on 9-anthryl aromatic amide adopted a naphthalene as a TICT controller and an intramolecular energy transfer source. Compound 1 shows high fluorescence efficiency upon complexation with metal ion, and the fluorescence efficiency of 2 is regulated by metal ionic size.     

Electronic absorption spectra of pyrylium and benzodihydrochromenylium salts

The electronic absorption spectra of benzodihydrochromenylium, tetrahydrochromenylium, and dichloropyrylium salts were studied for the first time. It was noticed that intramolecular charge transfer takes place mainly between the substituent at position 4 of the pyrylium ring and the π-system of the cation as a whole, while the bathochromic shift depends substantially on the geometry of the molecule. It was shown that the chlorine atoms have an effect on the form of the spectra when they are introduced into the ring of the pyrylium cation and into the aryl substituents. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1152–1159, August, 2008.     

Solid‐State Emission of the Anthracene‐o‐Carborane Dyad from the Twisted‐Intramolecular Charge Transfer in the Crystalline State

The emission process of the o ‐carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene‐o ‐carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation‐ and crystallization‐induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual‐emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o ‐carborane would allow for rotation even in the condensed state.     

交叉共轭型烯酮化合物分子内的电荷转移

近年来有关分子内电行转移化合物的设计、合成及其光谱、光物理并为的研究引起＾们极大的兴趣[1－4]．这类化合物在激发态时常表现出一些特殊的性能，如出现扭曲的分子内电行转移（TICT）现象，并在荧光光谱中出现奇特的K重发光和在荧光强度与温度的依赖关系上出现反常等[5]．TICT态的形成强烈的依赖于化合物分子本身的结构，屯依赖于化合物分子所处环境的极性、温度和粘度等因素同．带有强给电子基（CH3卜N烁酮类化合物是一类有效的光敏剂．它广泛的应用于如光引发聚合、全息成像[7-8]等不同领域．同时其光谱及光物理行为的研究也已兑…     

Influence of carbazol-9-yl substitution in polysilanes on charge carrier trapping and recombination

A series of poly{[3-(carbazol-9-yl)propyl]silane-co-methylphenylsilane}s were investigated by optical absorption, photoluminescence and thermoluminescence measurements. It was found that the optical absorption bands of the carbazolyl side groups superimpose on those of the Si backbones in the ultraviolet range. This feature reduces photodegradability of Si-Si bonds during UV irradiation. The TL spectra recorded in the 15 - 325 K temperature range after photoexcitation at 15 K show that the carbazolyl side groups act as trapping sites in polysilanes. Increasing density of carbazolyl groups results in increasing population of deeper (ca. 150 meV) traps. Spectral analyses of the thermoluminescence at different temperatures are discussed and compared with analogous results for poly(9-vinylcarbazole) (PVK). It is concluded that the monomeric mechanism of luminescence dominates at low temperatures while the excimeric mechanism prevails at higher temperatures, similarly to PVK.     

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger.     

Iteration Method for Solving the Radiative Transfer Equation of an Atmosphere-Ocean System

Taking the atmosphere and ocean water as homogeneous respectively and regarding the ocean surface as a plane, a new method, the successive iteration method, for solving the radiative transfer equation (RTE) of the atmosphere-ocean system is presented. As a result, an expression of the azimuth integrated radiance in this system is developed.     

Theoretical insights into the fluoride anion response mechanism of a fluorescence chemosensor 4‐((tert‐butyldiphenylsilyl)oxy)isophthalaldehyde

It is well known that ions play important roles in our life sciences, and the detection of ions has attracted more and more attention. In this work, we focus on the sensing response mechanism of a novel fluoride chemosensor, 4‐((tert‐butyldiphenylsilyl)oxy)isophthalaldehyde (BIPA). Based on density functional theory and time‐dependent density functional theory methods, we clarify that fluoride anions could trigger the cleavage reaction of the Si‐O bond of BIPA in the ground state. And, the potential energy curve of desilylation process reveals the rapid response to fluoride anions. Comparing the binding energies between fluoride anions and other anions, we confirm that only the fluoride anions could be detected using the BIPA chemosensor in ethanol solvent. Considering the photo‐excitation process, we find the strong intramolecular charge transfer process for the S₀ → S₁ transition could explain the red shift of the absorption spectra of the BIPA system. This work not only clarifies the specific fluoride‐sensing mechanism, but also plays a role in facilitating designing and synthesizing of novel fluorescent sensors in future.     

Photophysical Property of Photoactive Molecules with Multibranched Push-Pull Structures

The structure-property characteristics of a series of newly synthesized intramolecular charge-transfer (ICT) compounds, single-branch monomer with triphenylmethane as electron donor and 2,1,3-benzothiadiazole as acceptor, the corresponding two-branch dimer and three-branch trimer, have been investigated by means of steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) techniques in different polar solvents. The TD-DFT calculations are further performed to explain the observed ICT properties. The interpretation of the experimental results is based on the comparative stud-ies of the series of compounds which have increased amount of identical branch moiety. The similarity of the absorption and fluorescence spectra as well as strong solvent-dependence of the spectral properties for the three compounds reveal that the excited state of the dimer and trimer are nearly the same with that of the monomer, which may localize on one branch. It is found that polar excited state emerged through multidimensional intramolecular charge transfer from the donating moiety to the acceptor upon excitation, and quickly relaxed to one branch before emission. Even so, the red-shift in the absorption and emission spectra and decreased fluorescence radiative lifetime with respect to their monomer counterpart still suggest some extent delocalization of excited state in the dimer and trimer upon excitation. The similar behavior of their excited ICT state is demonstrated by FS TR-SEP FD mea-surements, and shows that the trimer has the largest charge-separate extent in all studied three samples. Finally, steady-state excitation anisotropy measurements has further been carried out to estimate the nature of the optical excitation and the mechanism of energy redistribution among the branches, where no plateau through the ICT band suggests the intramolecular excitation transfer process between the branches in dimer and trimer.     

环己烷中苯甲酰苯胺的双重荧光--分子内电荷转移的直接光谱证据

自从O Connell等［1］报道苯甲酰苯胺(BA)在EPA玻璃体中异常的长波长荧光发射特性以来, 已有众多学者尝试解释该"异常"荧光的发射态性质［2～12］. Kasha等［2～7］认为长波长荧光是质子转移(PT)和电荷转移(CT)两种激发态发射的叠加, 而Azumaya等［8］则认为发射态只包含分子内扭转电荷转移(TICT)态. 应该指出的是, 在上述研究中, 电荷转移态的指认并无有力的实验事实, 而主要是依据与具有CT双重荧光的对二甲氨基苯甲氰［13,14］的类比. 显然, 有关苯甲酰苯胺的长波长荧光发射态的准确性质仍待实验阐明     

A theoretical study on the excited‐state intramolecular proton transfer mechanism of 4′‐dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S₀ and S₁ states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S₁ state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S₁ state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics.     

Solvent effect on excited state potential energy surfaces of Thioflavin T. Qualitatively different results by TDDFT and SA-2-CASSCF methods

Thioflavin T (ThT) is a viscosity-sensitive fluorescent dye and its emission intensity undergoes a significant enhancement upon binding to DNA or amyloid fibrils. This fluorescence light-up feature has been attributed earlier to restriction of structural rearrangements in the excited state that are coupled to an intramolecular charge transfer (ICT) reaction. In this work TDDFT (using B3LYP and CAM-B3LYP functionals) and SA-2-CASSCF calculations were carried out to obtain relaxed excited-state potential energy surfaces (PES) along twisting φ and wagging δ angles that describe mutual orientation of benzothiazole (BTZ) and dimethylaniline (DMA) fragments in ThT. For isolated ThT molecule both methods predict that during structural rearrangements of the initially excited Franck-Condon state, besides twisting along C C bond which connects BTZ and DMA fragments, a considerable wagging motion is expected to occur. Account for solvent effect using polarized continuum model showed qualitative differences in the excited state PES features calculated by SA-2-CASSCF and TDDFT methods. Single-reference TDDFT calculations failed to describe solvation of TICT state and predicted increase of its energy in more polar media.     

具有扭曲的分子内电荷转移的芳香酮衍生物的结构与光谱性能研究

设计合成了一系列基于芳香酮的具有分子内扭曲态电荷转移(TICT)特性的化合物,通过线性光物理性质与双光子吸收性质的表征,研究了分子结构中不同共轭基团和不同取代基位置对化合物光谱性能的影响,同时通过溶剂效应研究了化合物的分子内电荷转移性质。结合理论计算结果表明分子的共轭骨架和取代基的位置都能显著影响分子内电荷转移特征。其中芴酮系列的化合物表现出了较强的双光子吸收与聚集诱导荧光增强效应,在生物荧光成像领域有着潜在的应用价值。     

马来腈二硫纶·邻菲咯啉镍(II)配合物的电子光谱和理论研究

测量了标题配合物Ni(mnt)(phen)在多种介质中的电子吸收光谱和发射光谱, 使用密度泛函理论的B3LYP方法和分子轨道理论的PM3方法研究了其气态分子几何构型、电子结构和成键, 用ZINDO/S方法通过多组态的组态相互作用(CI)计算解释了实验光谱. 结果表明: 该配合物分子为平面结构, 对称性属于点群C_2v, 基态为自旋三重态, 配位键Ni—N和Ni—S为典型的共价结合, Ni的3d电子反馈效应较显著; 可见区的吸收带和发射带(对应于基态电子组态到较低能量激发态组态的跃迁)本质上属于配体phen到mnt^2－的荷移跃迁(LL'CT), 紫外区的吸收带本质上属于配体的π→π*跃迁.     

基于对二烷基氨基苯甲酰苯胺分子内电荷转移的单糖比值法荧光受体

Two dual fluorescent receptors (1 and 2) for monosaccharides based on 4-dialky(alkyl=methyl and n-butyl) containing boronic acid group at the amido aniline were synthesized and their spectral properties were investigated. These receptors exhibited dual fluorescence with the long-wavelength band displaying strong solvent-polarity dependence, indicating the occurrence of the excited-state intramolecular charge transfer （ICT).With increasing pH value in aqueous solutions, the hybridization of the boron atom changed from sp^2 to sp^3, inducing a decrease in the total fluorescence quantum yield. The experimental results indicated that the anionic form of the boronate group acted as an electron donor and the benzanilide-like charge transfer was promoted upon hybridization change. In the presence of monosaccharides, the boronic acid in 1 and 2 changed from neutral to anionic form. The intensity of the locally excited (LE) state emission decreased in the presence of sugars while a slight increase in the intensity at the charge transfer (CT) emission occurred. Based on the change in the CT to LE intensity ratios of 1 and 2 due to sugar binding, ratiometric fluorescent assays for monosaccharide sensing were established.