Triaryl boron-based A-pi-A vs triaryl nitrogen-based D-pi-D quadrupolar compounds for single- and two-photon excited fluorescence

Three new A-pi-A-type compounds with trivalent boron, protected by two mesityl groups, as electron acceptor have been synthesized and investigated together with their two diphenylamino-ended D-pi-D analogues. These boranes exhibit large two-photon absorption cross sections and high fluorescence quantum yields.     

Trivalent boron as acceptor in D-pi-A chromophores: synthesis,structure and fluorescence following single- and two-photon excitation

A series of new donor-pi-acceptor type compounds with trivalent boron as acceptor which show strong two-photon excited up-conversion fluorescence have been synthesized and one crystal structure described.     

One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2'-dipyridylnitrogen functional group

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.     

Boron-containing two-photon-absorbing chromophores. 3. One- and two-photon photophysical properties of p-carborane-containing fluorescent bioprobes

Boron-containing two-photon-absorbing fluorophores have been prepared as new bifunctional molecules, potentially useful in two-photon excited microscopy (TPEM) and boron neutron capture therapy. They are based on a one-dimensional conjugated system containing a p-carborane entity at one end of the molecule and various electron-donating groups containing oxygen or nitrogen atoms at the other end. We investigated their one- and two-photon photophysical properties. They showed efficient fluorescence in an organic solvent, as well as in water for two of them, allowing microscopy on cell cultures. High two-photon absorption cross sections were determined in the 700-900 nm range. TPEM images were obtained with these new p-carborane-containing fluorophores, with laser intensities in the submilliwatt range.     

Highly selective two-photon chemosensors for fluoride derived from organic boranes

[reaction: see text] Three organoboron compounds are shown to be two-photon fluorescent sensors for fluoride anion with high sensitivity and selectivity. The recognition mechanism is attributed to the unique steric structure of the bulky dimesitylboryl group and the Lewis acid-base interaction between trivalent boron atom and fluoride anion.     

Diaroyl(methanato)boron difluoride compounds as medium-sensitive two-photon fluorescent probes

This paper evaluates the use of diaroyl(methanato)boron difluoride compounds for designing efficient fluorescent probes through two-photon absorption. Three different pathways allowing for the syntheses of symmetrical and dissymmetrical molecules are reported. The stable diaroyl(methanato)boron difluoride derivatives can be easily obtained in good yields. They exhibit a large one-photon absorption that is easily tuned in the near-UV range. Their strong fluorescence emission covers the whole visible domain. In addition to these attractive linear properties, several diaroyl(methanato)boron difluoride derivatives possess significant cross sections for two-photon absorption. The derived structure-property relationships are promising for designing new generations of molecules relying on the diaroyl(methanato)boron difluoride backbone.     

Disclosing the Structure/Activity Correlation in Trivalent Boron‐Containing Compounds: A Tendency Map

Most trivalent boron reagents are electrophiles owing to the vacancy for two electrons to fill the outer orbital of boron; however, interestingly, trivalent boron compounds can change their electrophilic character to a nucleophilic character by only changing the nature of the substituents on the boron atoms. With the help of computational tools, we have analyzed the structural‐ and electronic properties of boryl fragments that were either bonded to main‐group metals or coordinated to transition‐metals/rare‐earth‐metals and we have designed a map that might help to identify certain trends. This trend map will be useful for selecting an appropriate trivalent boron compound, depending on the sought reactivity.     

V形咔唑衍生物的合成及荧光性质

以N-烷基咔唑作为电子给体和共轭桥中心, 二米基硼作为端基电子受体, 合成了两个V形A-π-D-π-A型新化合物: 3,6-二{[(E)-2-(5-二米基硼)噻吩]乙烯基}-N-丁基-咔唑 {N-butyl-3,6-bis{(E)-2-[5-(dimesitylboryl)thiophen-2-yl]-vinyl}-carbazole, BBTC}和3,6-二[(E)-(4-二米基硼)苯乙烯基]-N-己基-咔唑, {N-hexyl-3,6-bis[(E)-4-(dimesitylboryl)-styryl]-carbazole, BBSC}. 这两个化合物在蓝绿光波段都有较强的荧光发射. 光谱数据表明, 扩大共轭体系并在端基引入含硼基团导致吸收谱和发射谱显著红移, 并增大分子内电荷转移.     

Conformationally restricted dipyrromethene boron difluoride (BODIPY) dyes: highly fluorescent, multicolored probes for cellular imaging

Novel fluorescent, conformationally restricted dipyrromethene boron difluoride (BODIPY) dyes have been prepared by introducing a naphthalenyl group at the meso position of the BODIPY core. These BODIPY dyes exhibit increased fluorescence quantum yields compared with dyes that have a meso-position phenyl group with internal rotation. The absorption and emission wavelengths of such conformationally restricted BODIPY dyes can be easily tuned to the near-IR range by derivatization through a condensation reaction with benzaldehyde derivatives. The two-photon absorption properties of these BODIPY dyes were also investigated and the results show that they exhibit increased two-photon excited fluorescence compared to analogue dyes that contain a phenyl group. The one- and two-photon fluorescence imaging of living cells by using selected BODIPY dyes has been successfully demonstrated.     

D-π-A型三联吡啶衍生物的合成及光学性质

以4'-(4-甲苯基)-2,2'∶6',2″-三联吡啶为电子受体(A)基元, 通过Wittig反应引入电子给体(D)基元, 合成了2个新的D-π-A型三联吡啶衍生物; 研究了其单、 双光子吸收荧光特性, 并通过量子化学计算从结构角度进行了解释. 目标产物的荧光量子产率(Ф)最高达到0.45, 双光子吸收截面(δ₂)最大达406 GM.     

Trivalent boron as an acceptor in donor-pi-acceptor-type compounds for single- and two-photon excited fluorescence

The synthesis, structure, and fluorescence properties of a series of new donor-pi-acceptor (D-pi-A) type compounds, with a trivalent boron, protected by two mesityl groups, as acceptor, and with various typical donors and different pi-conjugated bridges, are reported. All these stable organoboron compounds show intense single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (TPEF) in a wide spectral range from blue to green, with the spectral peak position of the SPEF being basically the same as that of the TPEF. The remarkably strong Cbond;B(mesityl)(2) bonding, and the well-conjugated pi-system, shown in X-ray crystal structures of two compounds, indicate some charge transfer features of the ground state. Meanwhile, spectral data indicate that the charge transfer from donor to acceptor is greatly enhanced in the excited states. Based on typical structural data and comprehensive spectral data, the following structure-property relationships can be drawn: 1) the moderate arylamino donor can more effectively enhance the SPEF and TPEF intensities than can the strong alkylamino donor; 2) stilbene is a better pi-bridge than styrylthiophene for its capability of enhancing and blue-shifting the SPEF and TPEF of the corresponding D-pi-A compounds; and 3) when compared to its boron-free precursors and other analogues, -B(mesityl)(2) invariably and consistently acts as an effective SPEF and TPEF fluorophore in all this series of organoboron compounds, which may result from its strong pi-electron-withdrawing and charge transfer-inducing nature in the ground-state and, more dominantly, in the excited-state. Combining all the above positive structure factors, trans-4'-N,N-diphenylamino-4-dimesitylborylstilbene (compound 3) stands out as the optimized green SPEF and TPEF emitter. This compound exhibits an SPEF quantum yield Phi of 0.91 at 522 nm in THF, a TPEF cross-section sigma' that is an order of magnitude larger than that of its boron-free precursor upon excitation by 800 nm femto-second laser pulses, and a two-photon absorption section sigma of 3.0 x 10(-48) cm(4) s. In the blue light region, trans-4'-N-carbazolyl-4-dimesitylboryl-stilbene (compound 4) shows significant SPEF and TPEF properties, with Phi=0.79 at 464 nm in THF and a large sigma' value, which is five times that of fluorescein upon excitation by 740 nm femto-second laser pulses.     

Strong two-photon absorption and its saturation of a self-organized dimer of an ethynylene-linked porphyrin tandem

The two-photon absorption properties of a self-organized dimer of a free-base and zinc(II) porphyrins tandem linked with an ethynylene group and terminated by imidazolyl and phenylethynyl groups were investigated. The self-organized dimer was found to exhibit strong two-photon absorption and furthermore the saturation of the two-photon absorption owing to the intense transition.     

末端基团对芴类衍生物分子非线性光学性质的影响

通过采用预估矫正的时域有限差分方法数值求解速率方程-光场强度方程,研究了纳秒激光脉冲在具有不同末端基团的对称性芴类衍生物分子2,7-双(4′-(二甲基氨基)苯乙烯基)-9-氢-芴(F1分子)和2,7-双(4′-硝基苯乙烯基)-9-氢-芴(F2分子)中的动力学传播过程以及光限幅效应,分析了两种分子的光限幅特性随传播距离(z)、粒子数密度(N)以及脉冲宽度(τ)的变化情况,并且拟合了两种分子的动态双光子吸收(TPA)截面。计算结果表明,该系列分子具有较大的双光子吸收截面以及较好的光限幅效应。此外,F2分子的末端基团―NO2与F1分子的末端基团―N(CH3)2相比具有更强的得电子能力,因而使得F2分子具有更大的跃迁偶极矩,双光子吸收截面增大,光限幅效应更为明显。     

Spectroscopic Characteristics of Hyperbranched Conjugated Polymers Studied by One- and Two-Photon Excitations

Spectroscopic properties of new hyperbranched conjugated polymers functionalized in periphery with N,N-dimethylaniline and tert-butyl-benzene as terminal groups are investigated by one- and two-photon excitations. The absorption, °uorescence excitation and emission spectra are examined in chloroform and N,N-dimethylformamide. The two-photon excitation measurements show that the new hyperbranched conjugated polymer possesses large two-photon excitation cross section which makes it a very attractive candidate for the potential application as nonlinear optical materials. As an example, the two-photon induced three-dimensional data storage is also demonstrated.     

香豆素肟酯类双光子引发剂的合成及性能研究

开发高效的双光子引发剂是提升双光子聚合速度的关键。本文基于光致脱羧机制,设计并合成了两种以共轭香豆素作为生色团、肟酯作为引发基团的双光子引发剂,并通过实验测试结合模拟计算对该类引发剂的光物理和光化学行为进行了研究。结果表明,该类引发剂在400~500 nm区域具有较强吸收,在LED可见光辐照下发生分解,具有光漂白特性,光解后释放的活性种可引发丙烯酸酯类单体聚合。利用双光子三维微纳成型技术,该类香豆素肟酯化合物可有效用于构建高分辨率的三维微纳结构。并通过量子化学计算,对该类引发剂的引发机理进行了探讨。     

o-nitrobenzyl photolabile protecting groups with red-shifted absorption: syntheses and uncaging cross-sections for one- and two-photon excitation

We evaluated the o-nitrobenzyl platform for designing photolabile protecting groups with red-shifted absorption that could be photolyzed upon one- and two-photon excitation. Several synthetic pathways to build different conjugated o-nitrobenzyl backbones, as well as to vary the benzylic position, are reported. Relative to the reference 4,5-dimethoxy-2-nitrobenzyl group, several o-nitrobenzyl derivatives exhibit a large and red-shifted one-photon absorption within the near-UV range. Uncaging after one-photon excitation was studied by measuring UV-visible absorption and steady-state fluorescence emission on model caged ethers and esters. In the whole series investigated, the caged substrates were released cleanly upon photolysis. Quantum yields of uncaging after one-photon absorption lie within the 0.1-1 % range. We observed that these drop as the maximum wavelength absorption of the o-nitrobenzyl protecting group is increased. A new method based on fluorescence correlation spectroscopy (FCS) after two-photon excitation was used to measure the action uncaging cross section for two-photon excitation. The series of o-nitrobenzyl caged fluorescent coumarins investigated exhibit values within the 0.1-0.01 Goeppert-Mayer (GM) range. Such results are in line with the low quantum yields of uncaging associated with cross-sections of 1-50 GM for two-photon absorption. Although the cross-sections for one- and two-photon absorption of o-nitrobenzyl photolabile protecting groups can be readily improved, we emphasize the difficulty in enlarging the corresponding action uncaging cross-sections in view of the observed trend of their quantum yield of uncaging.     

Tautomerism of anthraquinones: X. Quinizarin boron complex

Products of reactions of hydroxyanthraquinones with boric acid exist as equilibrium mixtures of tautomeric boron complexes and boric acid esters in which one or two boron atoms are not coordinated to carbonyl groups. Tautomerism is responsible for the appearance of several π_l,π bands in the electronic absorption spectra and considerable differences in the data obtained by different authors. Boron-containing quinizarin derivatives have mostly 1,10-quinoid structures. The use of quinizarin as an analytical reagent for the determination of boron is based on replacement of tautomeric equilibria due to complex formation rather than on coordination-induced red shift of the absorption maximum.     

Two-photon absorption dependence on the temperature for azoaromatic compounds: effect of molecular conformation

This work reports on the effect of temperature on the two-photon absorption cross section of azoaromatic chromophores. A linear decrease in the two-photon absorption cross section with the temperature was observed for several azochromophores. This process was characterized by introducing a two-photon absorption thermal coefficient (ddelta/dT), whose typical values are approximately 2GM/degrees C for all the azochromophores studied here. Such an effect was attributed to thermal induced molecular conformation changes, described by the sum-over-states model and semiempirical calculations, which affect the molecular dipole moments. The characterization of the phenomenon reported here for other nonlinear materials can help in the design of specific applications using two-photon absorption.     

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.     

Modulation of the photochromic property in an organoboron-based diarylethene by a fluoride ion

Organoboron-based diarylethene was successfully synthesized and exhibited photochromic property. Furthermore, its photochromic property can be modulated by a fluoride ion. The absorption maximum of the photostationary state shifted from 655 to 490 nm upon addition of a fluoride ion. The modulation mechanism is attributed to the special Lewis acid-base interaction between a trivalent boron atom and a fluoride ion.