Synthesis and photophysical properties of phosphole-cored dendrimers

Phosphole-cored dendrimers having poly(benzyl ether) units through the third generation have been synthesized. The dendrimers display intense blue photoluminescence, the quantum yield increasing with the increasing generation of the dendron units. The optical properties are easily tuned by oxidation of the phosphorus atom of the phosphole ring.     

新型树枝功能化萘酰亚胺类发光材料的合成及其光放大效应

在树枝(dendron)上引入特定的功能单元,将多个具有特定功能团(咔唑)连接起来．形成功能化树枝状周边分子团簇,然后将它们分别与核心色素萘酰亚胺相连接,合成新型周边树枝状功能化的有机发光材料(共三代)。稳态荧光研究发现周边功能团吸收的能量能以较高的效率传递给中心核色素萘酰亚胺。其传输效率与化合物的树枝代数密切相关,具有特殊的光采集、光放大效应。瞬态荧光研究表明树枝化合物中的咔唑单元都呈双指数衰减特征,其中较短荧光寿命成分是咔唑和萘酰亚胺单元之间的相互作用。     

Cyclam‐Cored Dendrimers Appended with Four Dendrons of Two Different Types: Intradendrimer Energy Transfer

We have synthesized two cyclam‐cored dendrimers appended with dendrons of two different types by proper protection/deprotection of the cyclam unit. The resulting dendrimers contain six naphthyl and two dansyl units ( N6 D2 ) or two dansyl and six naphthyl units ( N2 D6 ) at the periphery. Their photophysical properties have been compared to those of a dendrimer containing 8 dansyl units ( D8 ) and a previously investigated dendrimer containing 8 naphthyl units ( N8 ). The absorption spectra are those expected on the basis of the number of chromophores, demonstrating that no ground state interaction takes place. The emission spectra of N2 D6 and N6 D2 show naphthalene localized and naphthalene excimer emission similar to those observed in the case of N8 , together with a much stronger dansyl emission with maximum at 525 nm. Addition of CF₃SO₃H to dendrimer solutions in CH₃CN/CH₂Cl₂ 1:1 (v/v) leads to protonation of the aliphatic amine units of the cyclam core at first and then of the aromatic amine of each dansyl chromophores. Cyclam can be diprotonated and this affects dansyl absorption and, most significantly, emission bands by a charge perturbation effect. Each dansyl unit is independently protonated in both dendrimers. The most interesting photophysical feature of these heterofunctionalized cyclam‐cored dendrimers is the occurrence of an intradendrimer photoinduced energy transfer from naphthyl to dansyl chromophores of two different dendrons (interdendron mechanism). The efficiency of this process is 50 % for N6 D2 and it can be increased up to 75 % upon protonation of the cyclam core and formation of N6 D2 (2H⁺). This arises from the fact that protonation of the amine units of the cyclam prevents formation of exciplexes upon naphthyl excitation, thus shutting down one of the deactivation processes of the fluorescent naphthyl excited state.     

Synthesis and characterization of fluorescent poly(aromatic amide) dendrimers

The synthesis of a series of poly(aromatic amide) dendrimers up to the second generation is described herein. The AB(2) building block used throughout the synthesis of the dendrimers was the allyl ester of 3,5-diaminocinnamic acid, which has been synthesized from 3,5-dinitrobenzoic acid in good yield with use of a four-step procedure. Dendron synthesis was achieved via a convergent approach with use of a sequence of deprotection/coupling steps. Two commercially available alcohols, L-menthol and citronellol, were coupled to the AB(2) monomer by using an alkyl diacid spacer and two core units; 1,7-diaminoheptane and tris(2-aminoethyl)amine have been used to produce the final dendrimers. Characterization was carried out by NMR and IR spectroscopies, MALDI-TOF mass spectrometry, GPC, and DSC. The novel monomer and dendritic derivatives exhibited a strong fluorescence emission in the visible region (lambda approximately 500 nm) of the spectrum and a weak emission in the near-infrared (lambda approximately 850 nm) upon excitation in the near-UV region. The fluorescence emission characteristics were found to be solvent and dendrimer generation dependent.     

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.     

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.     

Redox active, hybrid dendrimers containing Fréchet- and Newkome-type blocks

A new series of dendrimers was prepared by covalently attaching a Newkome dendron, a Fréchet dendron, and a redox active, aminoferrocene group to a central triazine core. Growth of the Newkome dendron has a more pronounced effect on the half-wave potential for the one-electron oxidation of the ferrocene residue than growth of the Fréchet dendron. All dendrimers show reversible or quasireversible voltammetric behavior at scan rates in the range 0.10-2.0 V s-1.     

A tetraphenylmethane based dendritic tolan-anthracene dyad: synthesis and energy transfer properties

In order to synthesize a peripherally rigid dendritic donor-acceptor dyad for energy transfer studies, a tritolan dendron based on a tetraphenylmethane scaffold was prepared from New Fuchsin. The dendron showed a small degree of homoconjugation but a large hypochromic effect. Coupling of two such dendrons with an anthracene core led to a dendritic tolan-anthracene dyad whose steady state photophysical studies (UV, PL, PLE) showed vectorial transfer of excitation energy from the surface tolan units to the anthracene core.     

Photochemistry and photophysics of stilbene dendrimers and related compounds

The syntheses and reactions of photoresponsive dendrimers are described. Dendrimers with photoreversible stilbene cores undergo mutual cis–trans isomerization in organic solvents to give photostationary state mixtures of cis- and trans-isomers. Even stilbene dendrimers with molecular weights as high as 6500 underwent mutual cis–trans isomerization within the lifetime of the excited singlet state. The large dendron group surrounding the photoreactive core may affect the excited state properties of the core to induce the efficiency of photoisomerization and/or reduce the fluorescence efficiency. The photochemistry of stilbene dendrimers, with various types of dendron groups, azobenzene dendrimers and other photoresposive dendrimers is discussed.     

Antenna-functionalized dendritic β-diketonates and europium complexes: synthetic approaches to generation growth

Six dendritic β-diketonates and their corresponding europium complexes were synthesized. These dendritic β-diketonate ligands consist of dibenzoylmethane cores, Fréchet-type poly(aryl ether) dendrons, and the carbazole-grafted peripheral functional groups. The designs of dendrimers are on the basis of high light-harvesting capability and dendron functionalization in virtue of the high extinction coefficient and carrier-injection adjustment of carbazole units. Different approaches to generation growth were utilized: the first generation europium complexes through etheral connectivity were developed via convergent synthetic approach; the second and third generation dendrons through esteral connectivity were developed by a hyperbranch core approach containing the advantages of convergent and divergent approaches. Their chemical structures were well characterized. Preliminary results show that the dendron-functionalized carbazole units not only tune the carrier-transporting capability, but also exhibit strong light-harvesting potential, resulting in a strong intense emission from the central Eu(III) ion via sensitization.     

Vibrational spectra study of fluorescent dendrimers built from the cyclotriphosphazene core with terminal dansyl and carbamate groups

The FTIR and FT Raman spectra of the "Janus"-type dendrimers, possessing five carbamate groups on one side and five fluorescent dansyl derivatives on the other side, with amide G(1) and hydrazone G(2) central linkages were studied. These surface-block dendrimers are obtained by the coupling of two different dendrons. The FTIR and FT-Raman spectra of the zero generation dendrons, built from the hexafunctional cyclotriphosphazene core, with five dansyl terminal groups and one carbamate G(0v) and one oxybenzaldehyde function G(0v)' have been recorded. The structural optimization and normal mode analysis were performed for dendron G(0v)' on the basis of the density functional theory (DFT). The calculated geometrical parameters and harmonic vibrational frequencies are predicted in a good agreement with the experimental data. It was found that dendron molecule G(0v)' has a concave lens structure with planar -O-C(6)H(4)-CHO fragments and slightly non-planar cyclotriphosphazene core. The experimental IR and Raman spectra of dendron G(0v)' were interpreted by means of potential energy distributions. Relying on DFT calculations a complete vibrational assignment is proposed. The strong band 1597 cm(-1) show marked changes of the optical density in dependence of substituents in the aromatic ring. The frequencies of ν(N-H) bands in the IR spectra reveal the presence of the different types of H-bonds in the dendrimers.     

Core molecule dependence of energy migration in phenylacetylene nanostar dendrimers: Ab initio molecular orbital-configuration interaction based quantum master equation study

The core molecule dependence of energy (exciton) migration in phenylacetylene nanostar dendrimers is investigated using the ab initio molecular orbital (MO)-configuration interaction based quantum master equation approach. We examine three kinds of core molecular species, i.e., benzene, anthracene, and pentacene, with different highest occupied MO-lowest unoccupied MO (HOMO-LUMO) gaps, which lead to different orbital interactions between the dendron parts and the core molecule. The nanostars bearing anthracene and pentacene cores are characterized by multistep exciton states with spatially well-segmented distributions: The exciton distributions of high-lying exciton states are spatially localized well in the periphery region, whereas those of low-lying exciton states are done in the core region. On the other hand, for the nanostar bearing benzene core, which also has multistep exciton states, the spatial exciton distributions of low-lying exciton states are delocalized over the dendron and the core regions. It is found that the former nanostars exhibit nearly complete exciton migration from the periphery to the core molecule in contrast to the latter one, in which significant exciton distribution remains in the dendron parts attached to the core after the exciton relaxation, although all these dendrimers exhibit fast exciton relaxation from the initially populated states. It is predicted from the analysis based on the MO correlation diagrams and the relative relaxation factor that the complete exciton migration to the core occurs not only when the HOMO-LUMO gap of the core molecule is nearly equal to that of the dendron parts attached to the core (anthracene case) but also when fairly smaller than that (pentacene case), whereas the complete migration is not achieved when the HOMO-LUMO gap of the core is larger than that of the dendron parts (benzene case). These results suggest that the fast and complete exciton migration of real dendrimers could be realized by adjusting the HOMO-LUMO gap of the core molecule to be smaller than that of dendron parts, although there exist more complicated relaxation processes as compared to simple dendritic aggregate models studied so far.     

Silole‐core phenylacetylene dendrimers and their application in detecting picric acid

Silole‐core phenylacetylene dendrimers were designed and synthesized, among them, the model compound (n = 0) and the first generation of the dendrimer (n = 1) were obtained by the reaction of 2,5‐dibromosilole with corresponding terminal alkynes, the second generation of the dendrimers (n = 2) was synthesized from 2,5‐diiodosilole. These compounds indicated the absorptions of both phenylacetylene dendrons (250–350 nm) and silole core (400–500 nm). The first generation displayed efficient energy transfer from phenylacetylene dendrons to silole core, whose energy transfer efficiency was as high as 80%. These compounds were used as chemical sensors to probe explosive, for picric acid (PA), the Stern–Volmer constants of model compound and the first generation are 7120 and 5490M^?1, respectively. J. Heterocyclic Chem., (2012).     

Novel first-generation dendrimers on calix[4]resorcinol core equipped with multiple triazole units

Novel first-generation dendrimers on the calix[4]resorcinol core with four branches each containing multiple 1,2,3-triazole units have been synthesized in one-step by acid catalyzed condensation of resorcinols with a new aldehyde dendron, namely, 4-{3,5-bis[(1-benzyl-1H-1,2,3-triazol-4-yl)- methoxy]benzyloxy}benzaldehyde (obtained by alkyne–azide cycloaddition). The reaction proceeds stereoselectively to form rccc-diastereoisomers in high yields.     

Metal-assembling dendrimers with a triarylamine core and their application to a dye-sensitized solar cell

A series of charge-separable and hole-transporting phenylazomethine dendrimers with a triarylamine core are prepared and evaluated for use as a charge separator in dye-sensitized solar cells (DSSCs). Triphenylamine with dendric phenylazomethine (TPA-DPA) is prepared by synthesizing up to five generations of dendrons using a convergent method. The resultant dendrimer has a rigid sphere structure similar to globular protein, with a hydrodynamic radius of 2.43 nm. Electrochemical oxidation of the TPA core reveals that the dendron units in the dendrimer have 0.35 of the attenuation factor (beta) in the electron transfer. Complexation of TPA-DPA with SnCl2 proceeds in stepwise fashion from the core to the terminal imine following the basicity gradient among imine groups in each dendron shell. DSSCs prepared by casting these dendrimers onto dye-sensitized TiO2 film exhibited a higher open-circuit voltage than the bare film through the suppression of back electron transfer. The generational growth of dendrons increases the radius of the dendrimer, resulting in a stronger association with I3- and higher open-circuit voltage with an increasing number of generations. Complexation with SnCl2 reduces the resistance of TPA-DPA and improves the fill factor. The energy conversion efficiency of the DSSC prepared using fifth-generation TPA-DPA is 21% higher than that for the bare film and, when complexed with SnCl2, provides a 34% improvement.     

Synthesis, aggregation, and chiroptical properties of chiral, amphiphilic dendrimers

The syntheses of amphiphilic dendrimers based on 3,5-dihydroxybenzyl alcohol containing tri- or tetrafunctional chiral central cores and allyl ester termini are described. Water solubility is imparted to the dendrimers via a palladium-catalyzed deprotection of the peripheral allyl esters. This method affords complete deprotection of the carboxylate surface because, in contrast to the basic hydrolysis of methyl ester termini, the solubility of partially hydrolyzed intermediates is maintained throughout the course of the deprotection, thereby avoiding precipitation during the reaction. Chiroptical analysis indicates that the structure of the dendrimers collapses in water, resulting in an increased steric effect upon the central core that is manifested by lower optical rotatory power. However, contributions to the chiroptical properties from the dendron branch segments were not evident in water or organic media, suggesting that chiral substructures were not developing in the branch segments of the dendrimers. Multiangle light scattering studies revealed that the dendrimers experienced significant aggregation in aqueous media that decreased at higher generations. This behavior could be rationalized by a change in conformational preference from a disklike conformation at low generations to a more globular conformation at higher generations.     

Preparation, characterization, and electrochemical properties of a new series of hybrid dendrimers containing a viologen core and Frechet and Newkome dendrons

Five new 4,4'-bipyridinium (viologen) core dendrimers containing a Frechet (Fn, n = 1-3, first to third generation) and a Newkome (Nn, n = 1-3) dendron linked to each of the termini of the viologen residue were prepared and characterized. These macromolecules (FnNn) were prepared according to synthetic methodology already developed by our group. The electrochemical behavior of these dendrimers is characterized by the stepwise reduction of the viologen nucleus (V(2+)/V(+) and V(+)/V). The recorded half-wave potentials are affected by dendron growth in the three surveyed solvent media (dichloromethane, tetrahydrofuran, and acetonitrile). The size of the Newkome dendron has a more pronounced effect on the half-wave potentials than the size of the Frechet dendron. However, increasing the size of the Frechet dendron diminishes the magnitude of the cathodic potential shifts resulting from Newkome dendron growth. The largest dendrimers investigated (F1N3 and F2N3) exhibit quasi-reversible voltammetric behavior. The diffusion coefficients of these molecules were also determined using pulse gradient stimulated echo NMR techniques.     

Molecular batteries: ferrocenylsilylation of dendrons, dendritic cores, and dendrimers: new convergent and divergent routes to ferrocenyl dendrimers with stable redox activity

The ferrocenylsilylation of the phenol triallyl dendron 2, of the phenol nonaallyl dendron 4, and of the 9-, 27-, 81-, and 243-allyl dendrimers 7-10 (monitored by the disappearance of the signals of the olefinic protons in 1H NMR spectra) has been achieved using ferrocenyldimethylsilane 1 and Karstedt's catalyst in diethyl ether at 40 degrees C, yielding the corresponding ferrocenyl dendrons and dendrimers. An alternative convergent synthesis of the nonaferrocenyl dendron 5 was carried out by reaction of the triferrocenyl dendron 2 with a protected triododendron followed by deprotection. Reaction of the nonaferrocenyl dendron 5 with hexakis(bromomethyl)benzene gave the 54-ferrocenyl dendron 6. All the ferrocenyl dendron and dendrimers produce a chemically and electrochemically reversible ferrocenyl oxidation wave at seemingly the same potential. Stable platinum electrodes modified with the high ferrocenyl dendrimers were fabricated. The soluble orange-red ferrocenyl dendrimers can also be oxidized in CH2Cl2 by [NO][PF6] to the insoluble deep blue polyferrocenium dendrimers. For instance, the 243-ferrocenium dendrimer has been characterized by its Mossbauer spectrum, which is of the same type as that of ferrocenium itself. The ferrocenium dendrimers can be reduced without any decomposition back to the ferrocenyl dendrimer, indicating that these multielectronic redoxstable dendrimers behave as molecular batteries.     

Theoretical studies on conjugated phenyl-cored thiophene dendrimers for photovoltaic applications

Pi-conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of pi-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the core and the arms that are detrimental to pi-electron system delocalization over the structure.     

Intramolecular directional förster resonance energy transfer at the single-molecule level in a dendritic system

We report on the directional F?rster resonance energy transfer (FRET) process taking place in single molecules of a first (T1P4) and a second (T2P8) generation of a perylenemonoimide (P)-terrylenediimide (T)-based dendrimer in which the chromophores are separated by rigid polyphenylene arms. At low excitation powers, single-molecule detection and spectroscopy of T1P4 and T2P8 dendrimers point to a highly efficient directional FRET from P donors to the central T acceptor, optical excitation at 488 nm resulting in exclusively acceptor emission in the beginning of the detected fluorescence intensity. Donor emission is seen only upon the bleaching of the acceptor. High-resolution time-resolved single-molecule fluorescence data measured with a microchannel plate photomultiplier reveal, for T2P8, a broad range of FRET rates as a result of a broad range of distances and orientations experienced by the donor-acceptor dendrimers when immobilized in a polymer matrix. Single-molecule data from T2P8 on 488 nm excitation are indicative for the presence, after terrylenediimide bleaching, of a P-P excited dimer characterized by a broad emission spectrum peaking around 600 nm and by fluctuating fluorescence decay times. At high excitation powers, single T1P4 and T2P8 molecules display simultaneous emission from both donor and acceptor chromophores. The effect, called "exciton blockade", occurs due to the presence of multiple excitations in a single molecule.