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     

Self‐immolative dendrimers as novel drug delivery platforms

Self‐immolative dendrimers were recently developed and introduced as a potential platform for a single‐triggered multi‐prodrug. These unique structural dendrimers can release all of their tail units through domino‐like chain fragmentation, which is initiated by a single cleavage at the dendrimer core. The incorporation of drug molecules as the tail units and an enzyme substrate as the trigger generates a multi‐prodrug unit that is activated with a single enzymatic cleavage. We have demonstrated several examples of self‐immolative dendritic prodrug systems and have shown significant advantages with respect to the appropriate monomeric prodrug. We anticipate that single‐triggered, dendritic prodrugs will be exploited to further improve selective chemotherapeutic approaches in cancer therapy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1569–1578, 2006     

Dendrimers based on [1,3,5]‐triazines

A comprehensive and chronological account of dendrimers based on [1,3,5]‐triazines is provided. Synthetic strategies to install the triazine through cycloaddition, cyclotrimerization, and nucleophilic aromatic substitution of cyanuric chloride are discussed. Motivations and applications of these architectures are surveyed, including the preparation of supramolecular assemblies in the solution and solid states and their use in medicines, advanced materials, and separations when anchored to solid supports. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3411–3433, 2006     

Triazine dendrimers by divergent and convergent methods

Two types of dendrimers with a 1,3,5‐triazine ring at the branch point were synthesized by divergent and convergent methods. The divergent method began with 2,4,6‐tris(p‐nitroanilino)‐1,3,5‐triazine as a trifunctional core. Each cycle involved the reduction of the peripheral NO₂ group followed by a reaction with 2‐chloro‐4,6‐bis(p‐nitroanilino)‐1,3,5‐triazine. The synthetic cycle was completed by the coupling with 2,4‐dianilino‐6‐chloro‐1,3,5‐triazine (DACT) to eventually accomplish second‐generation dendrimers ([G2]₃‐C) bearing 12 benzene rings at their edge. The convergent approach started with the reaction of DACT with p‐nitrophenol to give rise to 2,4‐dianilino‐6‐(p‐nitrophenoxy)‐1,3,5‐triazine. The synthetic cycle consisted of reduction of the NO₂ group and coupling with 2,4‐dichloro‐6‐(p‐nitrophenoxy)‐1,3,5‐triazine. The final step was the connection of each monodendron with cyanuric chloride to produce tridendron; in this way, the second‐generation dendrimer ([EG2]₃‐C) was obtained. Gel permeation chromatography analyses indicated the aggregation of dendrimers in solution. Ultraviolet spectroscopic analyses revealed that the larger dendrimer had a more conjugated electron system from the core to the periphery. The thermal properties were evaluated by thermogravimetric analysis (TGA); excellent heat resistance was indicated, especially in [G1]₃‐C, which included alternately imine‐like nitrogen‐linked 1,3,5‐triazine and benzene rings. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4385–4395, 2000     

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.     

Efficient energy transfer between amphiphilic dendrimers with oligo(p‐phenylenevinylene) core branches and oligo(ethylene oxide) termini in micelles

The efficient fluorescence resonance energy transfer (FRET) between amphiphilic dendrimers with oligo(p‐phenylenevinylene) core branches and oligo(ethylene oxide) termini have been observed in micelles. All dendrimers show the critical micelle concentration and lower critical solution temperature as well as fluorescent emission. Tailoring electronic structures of the conjugated amphiphiles for FRET have been conveniently achieved by varying the branch number and/or the conjugated core structure. The Stern‐Volmer constants (K_SV) for FRET were found to be 4.51 × 10^?5 and 8.78 × 10^?5 M for Den 30–40 and Den 50–40, respectively. The effects external stimuli such as solvent and temperature on FRET have been also investigated. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Light harvesting and energy transfer within coumarin‐labeled polymers

Linear copolymers that have pendant coumarin‐2 and coumarin‐343 chromophores were prepared as analogues to previously synthesized light‐harvesting dendrimers. The chromophore ratios within these polymers were maintained similar to those of the various generation dendrimers to investigate the effect of polymer architecture on the energy‐transfer efficiency between the coumarin‐2 donors and coumarin‐343 acceptors. Both physical and photophysical properties of these polymers were analyzed and compared to those of the analogous dendrimers. Energy‐transfer efficiencies were relatively high in the polymers; however, deleterious excimer formation between the coumarin‐343 chromophores diminished the quantum yield of fluorescence of the polymers when compared to the analogous dendrimers. Overall, it was found that the ultimate performance of the dendritic light‐harvesting antennae was superior to that of the polymeric analogues, but the polymers were more practical in terms of synthetic accessibility. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1366–1373, 2001     

Luminescent copolymers for applications in multicolor‐light‐emitting devices

Light‐emitting diodes based on organic materials [organic light‐emitting diodes (OLEDs)] have attracted much interest over the past decade. Several different attempts have been made to realize multicolor OLEDs. This article describes a new approach based on energy transfer in a donor/acceptor system. A copolymer containing both donor and acceptor compounds as comonomer units is prepared. The polymer consists of a derivative of a luminescent dye [4‐dicyanmethylene‐2‐methyl‐6‐4H‐pyran (DCM); acceptor compound], which is copolymerized with fluorene (donor compound) to combine the properties of an electroactive polymer with a highly luminescent dye. Photochemical processing is achieved by UV irradiation of this copolymer in the presence of gaseous trialkylsilanes. This reagent selectively saturates the C?C bonds in the DCM comonomer units while leaving the fluorene units essentially unaffected. As a result of the photochemical process, the red electroluminescence of the acceptor compound vanishes, and the blue‐green electroluminescence from the polyfluorene units is recovered. Compared with previous approaches based on polymer blends, this copolymer approach avoids problems associated with phase‐separation phenomena in the active layer of OLEDs. © 2006Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4317–4327, 2006     

Effects of benzyl ether type dendrons as hole-harvesting antennas, and shielding for the neutralization of stilbene core radical cations with chloride ion during two-photon ionization of stilbene dendrimers having stilbene core and benzyl ether type dendrons

The two-photon ionization (TPI) process (308 and 266 nm) of stilbene dendrimers having a stilbene core and benzyl ether type dendrons has been investigated in an acetonitrile and 1,2-dichloroethane mixture (3:1) in order to elucidate the dendrimer effects. The quantum yield of the formation of stilbene core radical cation during the 308-nm TPI was independent of the dendron generation of the dendrimers, whereas a generation dependence of the quantum yield of the radical cation was observed during the 266-nm TPI, where both the stilbene core and benzyl ether type dendron were ionized, suggesting that the subsequent hole transfer occurs from the dendron to the stilbene core, and that the dendron acts as a hole-harvesting antenna. The neutralization rate of the stilbene core radical cation with the chloride ion, generated from the dissociative electron capture by 1,2-dichloroethane, decreased with the increase in the dendrimer generation, suggesting that the dendron is an effective shield of the stilbene core radical cation against the chloride ion.     

White electroluminescent single‐polymer achieved by incorporating three polyfluorene blue arms into a star‐shaped orange core

A series of star‐like dopant/host single‐polymer systems with a D‐A type star‐shaped orange core and three blue polyfluorene arms were designed and synthesized. Through tuning the doping concentration of the orange core and thermal annealing treatment of white polymer light‐emitting diodes based on them, highly efficient white electroluminescence has been achieved. A typical single‐layer device (ITO/PEDOT:PSS/polymer/Ca/Al) realized pure white emission with a luminous efficiency of 16.62 cd A^?1, an external quantum efficiency of 6.28% and CIE coordinates of (0.33, 0.36) for S‐WP‐002TPB3 containing 0.02 mol % orange core. The high efficiency of the devices could be mainly attributed to the suppressed concentration quenching of the dopant units, more efficient energy transfer from polymer host to orange dopant and thermal annealing‐induced α‐phase polyfluorene (PF) self‐dopant in amorphous PF host. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

PAMAMOS: The first commercial silicon‐containing dendrimers and their applications

The world of dendrimers presently encompasses several thousands of publications, including scientific papers and patents, several hundreds of different compositions, but only a few commercially available families of products (i.e. polyamidoamines (PAMAMs), until recently polypropyleneimines, and phosphorus‐containing dendrimers) This is the story of PAMAMOS, which recently became the first commercially available silicon‐containing dendrimers, and how we journeyed from our initial idea to the point of their commercialization. It focuses on three unique features of these new dendrimers: their ability to form precisely defined nano‐structured, honeycomb‐like, 2D or 3D networks; the amazing usefulness of these networks for the preparation of a variety of nanocomplexes and nancomposites; and some of the new vistas that these materials open up for engineering nanotechnology applications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2755–2773, 2006     

Synthesis and characterization of poly(fluorene‐co‐alt‐phenylene) containing 1,3,4‐oxadiazole dendritic pendants

A series of poly(fluorene‐co‐alt‐phenylene)s containing various generations of dendritic oxadiazole (OXD) pendent wedges were synthesized by the Suzuki polycondensation of OXD‐functionalized 1,4‐dibromophenylene with 9,9‐dihexylfluorene‐2,7‐diboronic ester. The obtained polymers possessed excellent solubility in common solvents and good thermal stability. Photophysical studies showed that the dendronized polymers appended with higher generations of OXD dendrons exhibited enhanced photoluminescence efficiencies and narrower values of the full width at half‐maximum. This was attributed to the shielding effect induced by the bulky dendritic OXD side chains, which prevented self‐quenching and suppressed the formation of aggregates/excimers. The energy transfer from the OXD dendrons to the polymer backbones was very efficient when excitation of the peripheral OXD dendrons resulted mainly in the polymer backbone emission alone. In particular, the photoluminescence emission intensities by the sensitized excitations of OXD dendrons in solid films of the polymers were all stronger than those by the direct excitations of their polymer conjugated backbones. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6765–6774, 2006     

Multifunctional Supramolecular Dendrimers with an s‐Triazine Ring as the Central Core: Liquid Crystalline,Fluorescence and Photoconductive Properties

Novel liquid crystal (LC) dendrimers have been synthesised by hydrogen bonding between an s‐triazine as the central core and three peripheral dendrons derived from bis(hydroxymethyl)propionic acid. Symmetric acid dendrons bearing achiral promesogenic units have been synthesised to obtain 3:1 complexes with triazine that exhibit LC properties. Asymmetric dendrons that combine the achiral promesogenic unit and an active moiety derived from coumarin or pyrene structures have been synthesised in order to obtain dendrimers with photophysical and electrochemical properties. The formation of the complexes was confirmed by IR and NMR spectroscopy data. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X‐ray diffractometry. All complexes displayed mesogenic properties, which were smectic in the case of symmetric dendrons and their complexes and nematic in the case of asymmetric dendrons and their dendrimers. A supramolecular model for the lamellar mesophase, based mainly on X‐ray diffraction studies, is proposed. The electrochemical behaviour of dendritic complexes was investigated by cyclic voltammetry. The UV/Vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers were also investigated     

Self‐assembly of a new class of amphiphilic poly(amidoamine) dendrimers and their electrochemical properties

Amphiphilic poly(amidoamine) (PAMAM) dendrimers consisting of a hydrophilic dendrimer core and hydrophobic aromatic dansyl or 1‐(naphthalenyl)‐2‐phenyldiazene (NPD) shells have been synthesized. These amphiphilic dendrimers from the zero generation to the third generation self‐assemble into vesicular aggregates in water. The self‐assembly behavior of these dendrimers strongly depends on their generations. The generation dependence has been further investigated by an exploration of their electrochemical properties. For the PAMAM–NPD aggregates, the photoisomerization process leads to a change in the aggregate size. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5512–5519, 2005     

Luminescence properties of soluble 2,2′,7,7′‐Tetrakis(2‐(9‐hexyl‐9H‐carbazol‐3‐yl)‐vinyl)‐9,9′‐spirobifluorene‐labeled dendrimers: Photoluminescence and electroluminescence

New light emitting dendrimers were synthesized by reacting 3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzoic acid or 3,5‐bis‐[3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzyloxy]‐benzoic acid with a carbazolyl vinyl spirobifluorene moiety. A blue‐emitting core dye was encapsulated by multibenzyloxy dendrons, and two dendrimers having different densities of dendrons were prepared. Photoluminescence (PL) studies of the dendrimers demonstrated that at the higher density of benzyloxy dendrons, the featureless vibronic transitions were improved, causing lesser excimer emission. The similarity of the solution and solid emission spectra of the larger dendrimer, 10 , revealed the suppression of molecular aggregation in the solid film, which is attributed to the presence of the bulky benzyloxy dendrons. The electroluminescence spectra of multilayered devices made using 10 predominantly exhibited blue emissions; similar emission was observed in the PL spectra of its thin film. The multilayered devices made using 3 , 9 , and 10 showed luminances of 1021 cd m^?2 at 5 V, 916 cd m^?2 at 6 V, and 851 cd m^?2 at 6.5 V, respectively. The largest dendrimer, 10 , bearing a greater number of benzyloxy dendrons, exhibited a blue‐like emission with CIE 1931 chromaticity coordinates of x = 0.16 and y = 0.13, which is due to the influence of a higher shielding effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 501–514, 2008     

Blue electroluminescence from 3,6‐silafluorene‐based copolymers

Poly(3,6‐silafluorene) is a typical wide band‐gap conjugated polymer with ultraviolet light emission. The blue electroluminescence from the 3,6‐silafluorene‐based copolymers via intrachain energy transfer was reported in this study. The monomer containing vinylene, anthracene, and tri‐arylamine moieties incorporated into the poly(3,6‐silafluorene) backbone can form efficient deep‐blue emitting copolymers with EL efficiency of 1.1–1.9%. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3286–3295, 2009     

New π‐conjugated polyaryleneethynylenes containing a 1,3,5‐triazine unit in the main chain: Synthesis and optical and electrochemical properties

A series of new π‐conjugated poly(aryleneethynylene)s containing a 1,3,5‐triazine unit in the main chain were synthesized in yields higher than 70% by polycondensation between dibromophenyl‐1,3,5‐triazine monomers and alkyl‐substituted diethynylbenzene (or diethynylfluorene) comonomers with Pd(PPh₃)₄ and CuI as catalysts in the presence of triethylamine. The polymers had a number‐average molecular weight in the range of 5000–10,000 and showed good solubility in common organic solvents. The polymers were photoluminescent both in solutions and in the solid state. X‐ray diffraction patterns of the powders of the polymers revealed that the polymers were semicrystalline. Electrochemically, the polymers appeared to be reversible under reduction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3797–3806, 2006     

White electroluminescence from a single polymer: A blue‐emitting polyfluorene incorporating orange‐emitting benzoselenadiazole segments on its main chain

We have developed efficient white‐light‐emitting polymers through the incorporation of low‐bandgap orange‐light‐emitting benzoselenadiazole ( BSeD ) moieties into the backbone of a blue‐light‐emitting bipolar polyfluorene (PF) copolymer, which contains hole‐transporting triphenylamine and electron‐transporting oxadiazole pendent groups. By carefully controlling the concentrations of the low‐energy‐emitting species in the resulting copolymers, partial energy transfer from the blue‐fluorescent PF backbone to the orange‐fluorescent segments led to a single polymer emitting white light and exhibiting two balanced blue and orange emissions simultaneously. Efficient polymer light‐emitting devices prepared using this copolymer exhibited luminance efficiencies as high as 4.1 cd/A with color coordinates (0.30, 0.36) located in the white‐light region. Moreover, the color coordinates remained almost unchanged over a range of operating potentials. A mechanistic study revealed that energy transfer from the PF backbone to the low‐bandgap segments, rather than charge trapping, was the main operating process involved in the electroluminescence process. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2938–2946, 2007     

The first observation of the effect of dendritic structure to produce the triplet excited state of the core stilbene by dendron excitation

We report that both singlet and triplet energy transfers in stilbene-cored benzophenone dendrimers (trans-BPST) took place quite efficiently. On excitation (290 nm) of stilbene group, the intramolecular singlet energy transfer from the excited core stilbene to the benzophenone part (99.7%) was confirmed by quenching of the fluorescence from the core stilbene. The benzophenone in the excited singlet state is known to undergo intersystem crossing to give its excited triplet state quantitatively. However, the very weak phosphorescence from benzophenone part in trans-BPST was observed even at 77 K. The phosphorescence intensity of trans-BPST is only 1% of that of model compound (4-methylbenzophenone) at 77 K. During the irradiation, the absorption spectra also changed due to the trans-cis isomerization. This is probably due to the ultrafast triplet energy transfer from the benzophenone to produce the triplet state stilbene.     

Thermal and X‐ray powder diffraction studies of aliphatic polyester dendrimers

The syntheses and thermal and X‐ray powder diffraction analyses of three sets of aliphatic polyester dendrimers based on 2,2‐bis(hydroxymethyl)propionic acid as a repeating unit and 2,2‐dimethyl‐1,3‐propanediol, 1,5‐pentanediol, and 1,1,1‐tris(hydroxymethyl)ethane as core molecules are reported. These dendritic polyesters were prepared in high yields with the divergent method. The thermal properties of these biodendrimers were evaluated with thermogravimetric analysis and differential scanning calorimetry. The thermal decomposition of the compounds occurred around 250 °C for the hydroxyl‐ended dendrimers and around 150 °C for the acetonide‐protected dendrimers. In addition, the crystallinity of the lower generation dendrimers was evaluated with X‐ray powder diffraction. The highest crystallinity and the highest melting points were observed for the first‐generation dendritic compounds. The higher generation dendrimers showed weaker melting transitions during the first heating scan. Only the glass‐transition temperatures were observed in subsequent heating scans. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5574–5586, 2004