POSS基树枝状大分子的合成与应用

树枝状大分子(dendrimer)是一种高度支化、纳米尺度的人工合成大分子,具有独特的物理化学性能和重要的应用前景。利用具有8个可官能化顶点的多面体低聚倍半硅氧烷(POSS)作为树枝状大分子的核心,可在一定程度上简化树枝状大分子繁琐的合成与分离过程,在低代数时就可获得较大的表面官能团密度,并使树枝状分子呈现球形对称结构。POSS基树枝状大分子结合了POSS和树枝状分子结构与性能的优势,是一类极具潜力的有机-无机纳米杂化材料。本文综述了近年来POSS基树枝状大分子的最新研究成果,介绍了具有代表性的POSS基树枝状大分子的合成方法以及它们在催化剂、生物材料、液晶材料和发光材料等领域的应用研究进展,并对该新型材料的发展趋势做了展望。     

Optically active dendrimers with a binaphthyl core and phenylene dendrons: light harvesting and enantioselective fluorescent sensing

Optically active dendrimers containing a 1,1'-binaphthyl core and cross-conjugated phenylene dendrons were synthesized and characterized. The chiral optical properties of these phenylene-based dendrimers are different from the previously reported phenyleneethynylene-based dendrimers probably because of the increased steric interaction between the adjacent phenylene units. UV and fluorescence spectroscopic studies demonstrate that the energy harvested by the periphery of the dendrimers can be efficiently transferred to the more conjugated core, generating much enhanced fluorescence signal at higher generation. The fluorescence of these dendrimers can be quenched both efficiently and enantioselectively by chiral amino alcohols. The energy migration and light-harvesting effects of the dendrimers make the higher generation dendrimer more sensitive to fluorescent quenchers than the lower ones. Thus, the dendritic structure provides a signal amplification mechanism. These materials are potentially useful in the enantioselective recognition of chiral organic molecules.     

Synthesis of Binaphthol Based Poly(arylene) Dendrimers and Their Use as Enantioselective Fluorescent Scensors

New optically active dendrimers (1-3) containing rigid and cross-conjugated units have been synthesized and characterized. UV and fluorescence spectroscopic studies demonstrate that the energy harvested by the periphery of the dendrimers can be efficiently transferred to the core. The fluorescence of the dendrimers can be quenched by amino alcohols (4-6) both efficiently and enantioselectively. The energy migration and light harvesting effects of the dendrimers make the higher generation dendrimers more sensitive fluorescent quencher than the lower ones.     

Donor-acceptor interactions in red-emitting thienylbenzene-branched dendrimers with benzothiadiazole core

Synthesis and characterization of dendrimers containing thienylbenzene repeating units, red-emitting benzothiadiazole core, and triarylamine peripheries that bear naphthyl units are reported. The relevant dendrimers of different generations are classified as G(nb) (n=1-3), while the tert-butyl dendrimers G(na) with the acceptor alone were also synthesized to serve as control chromophores that avoid donor-acceptor interactions. The resulting dendrimers are capable of harvesting photon energy through efficient energy transfer among donor-acceptor moieties, so that highly luminescent red fluorophores result. Transient fluorescence studies suggest that the energy transfer and its efficiency are approximately unity in all G(a) dendrimers, whereas the rate of energy transfer for the G(b) dendrimers is suppressed, that is, charge transfer from the core to the periphery is a significant quenching pathway. These dendrimers are amorphous in nature with high glass transition temperatures (176-201 degrees C). Electroluminescent devices were fabricated by using the dendrimers as hole-transporting emitters, and the devices exhibit promising red emission parameters.     

Energy and electron transfer in bifunctional non-conjugated dendrimers

Nonconjugated dendrimers, which are capable of funneling energy from the periphery to the core followed by a charge-transfer process from the core to the periphery, have been synthesized. The energy and electron donors involve a diarylaminopyrene unit and are incorporated at the periphery of these dendrimers. The energy and electron acceptor is at the core of the dendrimer, which involves a chromophore based on a benzthiadiazole moiety. The backbone of the dendrimers is benzyl ether based. A direct electron-transfer quenching of the excited state of the periphery or a sequential energy transfer-electron-transfer pathway are the two limiting mechanisms of the observed photophysical properties. We find that the latter mechanism is prevalent in these dendrimers. The energy transfer occurs on a picosecond time scale, while the charge-transfer process occurs on a nanosecond time scale. The lifetime of the charge separated species was found to be in the range of microseconds. Energy transfer efficiencies ranging from 80% to 90% were determined using both steady-state and time-resolved measurements, while charge-transfer efficiencies ranging from 70% to 80% were deduced from fluorescence quenching of the core chromophore. The dependence of the energy and charge-transfer processes on dendrimer generation is analyzed in terms of the backfolding of the flexible benzyl ether backbone, which leads to a weaker dependence of the energy and charge-transfer efficiencies on dendrimer size than would be expected for a rigid system.     

Conjugated dendrimers with triazine peripheries and a distyrylanthracene core

We describe the synthesis and luminescence characterization of conjugated dendrimers with triazine peripheries and a distyrylanthracene core that are suitable for electroluminescence applications. The dendrimers consist of dendritic triazine wedges with high electron affinity, stilbene branches, and a distyrylanthracene core as an emitting moiety. The dendrimers have lowest unoccupied molecular orbital values of about ?2.7 eV. Photoluminescence studies have indicated that a cascade energy transfer occurs from the triazine wedges to the stilbene bridges and finally to the distyrylanthracene core. Thus, the emission wavelength is determined by the distyrylanthracene core unit. The energy‐transfer efficiency of the distyrylanthracene‐cored dendrimers is about 47 and 20% for the first and second generations, respectively. A preliminary electroluminescence property investigation has also been conducted. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5855–5862, 2006     

An alternative convergent synthesis of dendrimers with 2,5-diarylsilole at the core

Dendrimers with 2,5-diarylsilole at the core are readily synthesized by the Ni-catalyzed reaction of 1,1,2,2-tetramethyldisilane and 1,6-diynes having poly(benzyl ether)-dendron units. The dendrimers display, upon excitation of the silole ring, an emission at about 500 nm. The fluorescence quantum yield of the dendrimers increases with increasing the generation of the dendron units. In addition, upon excitation of dendron units in the periphery, the dendrimers also display an emission from the silole ring at the core through the energy transfer from the dendron units to the silole core within the dendrimers.     

Dendrimer analogues of linear molecules to evaluate energy and charge-transfer properties

[reaction: see text] We have designed and synthesized difunctionalized dendrimers containing two donors in the periphery and an acceptor at the core to serve as scaffolds for comparison with linear analogues to investigate the advantage of dendritic scaffolds for energy and charge transfer. Comparison of these dendrimers with the fully decorated dendrimers provides information on the advantage of chromophore density in energy/charge transfer from periphery to the core.     

The effect of macromolecular architecture in nanomaterials: a comparison of site isolation in porphyrin core dendrimers and their isomeric linear analogues

The influence of macromolecular architecture on the physical properties of polymeric materials has been studied by comparing poly(benzyl ether) dendrons with their exact linear analogues. The results clearly confirm the anticipation that dendrimers are unique when compared to other architectures. Physical properties, from hydrodynamic volume to crystallinity, were shown to be different, and in a comparative study of core encapsulation in macromolecules of different architecture, energy transduction from the polymer backbone to a porphyrin core was shown to be different for dendrimers as compared to that of isomeric four- or eight-arm star polymers. Fluorescence excitation revealed strong, morphology dependent intramolecular energy transfer in the three macromolecular isomers investigated. Even at high generations, the dendrimers exhibited the most efficient energy transfer, thereby indicating that the dendritic architecture affords superior site isolation to the central porphyrin it surrounds.     

光捕获树枝形聚合物体系能量传递和电子转移研究进展

树枝形聚合物是一类围绕着中心核,外围链段和官能团呈指数增长的支化高分子.合成方法的发展使发色团可被精确地置于树枝形聚合物的核心、外围甚至支化节点处.树枝形聚合物的特殊结构使其作为模拟光捕获体系被广泛研究.光诱导电子转移和能量传递是光合作用中的重要过程,研究树枝形聚合物体系中的电子转移和能量传递对未来树枝形聚合物在光电器件中的应用有着重要意义.本文综述了近年来光捕获树枝形聚合物体系的研究进展,并重点介绍光捕获树枝形聚合物体系中的能量传递和电子转移过程研究.     

Single-component films of different generations of dendrimers bearing a diphenylanthracene core

Different generations of carboxyl-terminated poly(aryl ether) dendrimers bearing a diphenylanthracene core were designed and synthesized. It is interesting to see that not only two-generation dendrimers but also one-generation dendrimers can be fabricated into thin films by self-deposition. Fluorescence spectra indicate that increasing the generation number of a dendrimer can effectively control the quenching of the fluorescence. Moreover, the fluorescence property of the diphenylanthracene core of the dendrimers in a solid film is quite similar to that of one in a solution, which is important for designing light-emitting materials.     

Nanosized gradient pi-conjugated thienylethynylene dendrimers for light harvesting: synthesis and properties

[reaction: see text] A family of pi-conjugated dendrimers based on truxene and thienylethynylene units are synthesized via a mixed divergent/convergent growth approach. These dendrimers possess an intrinsic energy gradient from the periphery to the core through branches and thus show a broad absorption in the UV-vis range and an efficient energy transfer to the lower-energy center. The molecules hence have the potential to be used as light harvesting materials.     

Relationship between incoherent excitation energy migration processes and molecular structures in zinc(II) porphyrin dendrimers

Multiporphyrin dendrimers are among the most promising architectures to mimic the oxygenic light-harvesting complex because of their structural similarities and synthetic convenience. The overall geometries of dendrimers are determined by the core structure, the type of dendron, and the number of generations of interior repeating units. The rigid core and bulky volume of exterior porphyrin units in multiporphyrin dendrimers give rise to well-ordered three-dimensional structures. As the number of generations of interior repeating units increases, however, the overall structures of dendrimers become disordered and randomized due to the flexibility of the repeating units. To reveal the relationship between molecular structure and processes of excitation-energy migration in multiporphyrin dendrimers, we calculated the molecular structure and measured the time-resolved transient absorption and fluorescence anisotropy decays for various hexaarylbenzene-anchored polyester zinc(II) porphyrin dendrimers along with three types of porphyrin dendrons as references. We found that the congested two-branched type dendrimers exhibit more efficient energy migration processes than one- or three-branched type dendrimers because of multiple energy migration pathways, and the three-dimensional packing efficiency of dendrimers strongly depends on the type of dendrons.     

Designing systems for a multiple use of light signals.

The photochemical and photophysical behavior of two dendrimers consisting of a benzophenone core and branches that contain dimethoxybenzene units has been investigated. Such dendrimers can undergo a variety of photochemical and photophysical processes, namely: 1) quenching of the fluorescence and phosphorescence of the dimethoxybenzene units by energy transfer to the benzophenone core (antenna effect), 2) direct and sensitized phosphorescence (and delayed fluorescence) of the benzophenone core, 3) hydrogen abstraction by the triplet excited state of the benzophenone core from solvent molecules, 4) intramolecular hydrogen abstraction by the triplet excited state of the benzophenone core from the dendrimer branches, 5) quenching of the phosphorescence and hydrogen abstraction reaction of the benzophenone core by energy transfer to terbium ions and dioxygen; 6) conversion of the UV light absorbed by the dendrimer branches into visible (Tb3+) or near infrared (O2) emission via the sequence of processes 1) and 5). The results obtained emphasize the great potential of suitably designed dendrimers for a multiple use of light signals.     

Energy and charge transfer dynamics in fully decorated benzyl ether dendrimers and their disubstituted analogues

We examine the photophysics of a series of molecules consisting of a benzthiadiazole core surrounded by a network of benzyl ether arms terminated by aminopyrene chromophores, which function as both energy and electron donors. Three classes of molecules are studied: dendrimers whose peripheries are fully decorated with aminopyrene donors (F), disubstituted dendrimers whose peripheries contain only two donors (D), and linear analogues in which a pair of benzyl ether arms link two donors to the central core (L). The electronic energy transfer (EET) and charge transfer (CT) rates are determined by fluorescence lifetime measurements on the energy donors and electron acceptors, respectively. In all three types of molecules, the EET time scales as the square root of the generation number G, consistent with the flexible nature of the benzyl ether framework. Transient anisotropy measurements confirm that donor-donor energy hopping does not play a major role in determining the EET times. The CT dynamics occur on the nanosecond time scale and lead to stretched exponential decays, probably due to conformational disorder. Measurements at 100 degrees C confirm that conformational fluctuations play a role in the CT dynamics. The average CT time increases with G in the L and D molecules but decreases for the F dendrimers. This divergent behavior as G increases is attributed to the competing effects of larger donor-acceptor distances (which lengthen the CT time) versus a larger number of donors (which shorten the average CT time). This work illustrates two important points about light-harvesting and charge-separation dendrimers. First, the use of a flexible dendrimer framework can lead to a more favorable scaling of the EET time (and thus the light-harvesting efficiency) with dendrimer size, relative to what would be expected for a fully extended dendrimer. Second, fully decorated dendrimers can compensate for the distance-dependent slowdown in CT rate as G increases by providing additional pathways for the CT reaction to occur.     

Synthesis and luminescence characteristics of conjugated dendrimers with 2,4,6‐triaryl‐1,3,5‐triazine periphery

We have synthesized conjugated dendrimer with triazine peripheries, and their luminescence properties were investigated. The dendrimers consist of dendritic triazine wedges for electron transport, distyrylbenzene core as an emitting moiety, and t‐butyl peripheral groups for good processing properties. The dendrimers have LUMO values of about ?2.7 eV possibly because of the triazine moiety with high electron affinity. Photoluminescence study indicates that energy transfer occurs from the triazine wedges to the stilbene bridge, and finally to the core chromophore units due to a cascade decrease of bandgap from the peripheral wedge to core moiety. Therefore, the emission wavelength was determined by the structure of the core unit. The energy transfer efficiency of distyrylbenzene‐cored dendrimers was about 75 and 55% for Trz‐1GD‐DSB and Trz‐2GD‐DSB, respectively. A preliminary electroluminescence property also was investigated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 254–263, 2006     

树枝状有机电致发光材料*

高度有序、三维结构的树枝状大分子(Dendrimers)作为功能有机材料越来越引起人们的兴趣。与传统的小分子和高分子发光材料相比,树枝状化合物在发光材料方面的应用具有无可比拟的优势。树枝状发光材料的发光特性可以方便地由中心核的调换不同的荧光染料来实现,另外大量的表面功能团和不同的代数可供选择来得到一些有趣的性质,如载流子传输功能、区域隔离效应、溶解性和天线效应等。该类型的发光材料已被认为是第三类电致发光材料。本文简要介绍近期树枝状分子在有机电致发光材料领域中的研究进展,评述树枝状分子在该研究领域所特有的优势,重点介绍了树枝状化合物的设计及其对应的性质,并进一步展望树枝状分子未来在有机电致发光领域的研究前景。     

Light-harvesting in carbonyl-terminated phenylacetylene dendrimers: The role of delocalized excited States and the scaling of light-harvesting efficiency with dendrimer size

The photophysics of a family of conjugated phenylacetylene (PA) light-harvesting dendrimers are studied using steady-state and time-resolved optical spectroscopy. The dendrimers consist of a substituted PA core surrounded by meta-branched PA arms. The total number of PA moieties ranges from 3 (first generation) to 63 (fifth generation). By using an alcohol/ketone substituent at the dendrimer core, we avoid through-space Forster transfer from the peripheral PA donors to the core acceptor (in this case, the carbonyl group), which simplifies the analysis of these molecules relative to the perylene-terminated molecules studied previously. The delocalized excited states previously identified in smaller dendrons are seen in these larger dendrimers as well, and their influence on the intersite electronic energy transfer (EET) is analyzed in terms of a point-dipole Forster model. We find that these new delocalized states can both enhance EET (by decreasing the spatial separation between donor and acceptor) and degrade it (by lowering the emission cross section and shifting the energy, resulting in poorer spectral overlap between donor and acceptor). The combination of these two effects leads to a calculated intersite transfer time of 6 ps, in reasonable agreement with the 5-17 ps range obtained from experiment. In addition to characterizing the electronic states and intersite energy transfer times, we also examine how the overall light-harvesting efficiency scales with dendrimer size. After taking the size dependence of other nonradiative processes, such as excimer formation, into account, the overall dendrimer quenching rate k(Q) is found to decrease exponentially with dendrimer size over the first four generations. This exponential decrease is predicted by simple theoretical considerations and by kinetic models, but the dependence on generation is steeper than expected based on those models, probably due to increased disorder in the larger dendrimers. We discuss the implications of these results for dendrimeric light-harvesting structures based on PA and other chemical motifs.     

A facile convergent procedure for the preparation of triphenylamine-based dendrimers with truxene cores

A simple convergent procedure has been developed for the preparation of triphenylamine dendrons containing an alkene at the center, which can be coupled in a single step to give dendrimers that contain truxene for the core without any protection-deprotection chemistry. These conjugated dendrimers exhibit similar absorption and emission behaviors in solutions and in thin films, which are indicative of the high isolation effect of well-organized three-dimensional dendrimers. They also have high fluorescence quantum yields and high glass transition temperatures, which indicate that these dendrimers are candidates for the application in OLED as light emitting materials.     

Chromophore‐labeled dendrimers for use in single‐layer light‐emitting diodes

Two novel chromophore‐labeled dendrimers are presented and their incorporation into two‐component, single layer organic light emitting diodes (OLEDs) is described. The photoluminescence (PL) spectra, both in solution and in the solid state, demonstrate that Forster energy transfer from the donor chromophores on the dendrimers periphery to the acceptors located at the core is highly efficient, and affords emission exclusively from the core dyes, either coumarin 343 or a benzene‐capped pentathiophene. When these dendrimers are doped into single layer OLEDs, the electroluminescence (EL) is nearly identical to the photoluminescence described above, indicating once again exclusive emission from the core chromophores.