Pyrene as chromophore and electrophore: encapsulation in a rigid polyphenylene shell

Starting from the fourfold ethynyl-substituted chromophore 1,3,6,8-tetraethynylpyrene as core, a series of polyphenylene dendrimers was prepared in high yield by combining divergent and convergent growth methods. The fluorescence quantum yields (Q(f)>0.92) of the encapsulated pyrene chromophore were independent of the size of the polyphenylene shell. Fluorescence quenching studies and temperature-dependent fluorescence spectroscopy were performed to investigate the site isolation of the core. They indicate that a second-generation dendrimer layer is needed to efficiently shield the encapsulated pyrene and prevent aggregate formation. Alkali-metal reduction of the encapsulated pyrene core was carried out to afford the corresponding pyrene radical anions, for which hampered electron transfer to the core was observed with increasing dendrimer generation, which is further proof of the site isolation due to the polyphenylene shell. To improve film formation and solubility of the material, solubilizing alkyl chains were introduced on the periphery of the spherical particles. Furthermore, highly transparent films obtained by a simple drop-casting method showed blue emission mainly from the unaggregated species. The materials presented herein combine high quantum efficiency, good solubility, and improved film-forming properties, which make them possible candidates for several applications in electronic devices.     

Role of solvent and dendritic architecture on the redox core encapsulation

Dendrimers with redox cores can accept, donate, and/or store electrons and are used in nanoscale devices like artificial receptors, magnetic resonance imaging, sensors, light harvesting antennae, and electrical switches. However, the dendrimer molecular architectures can significantly alter the encapsulation of the redox core and charge transfer pathways, thereby changing the electron transfer rates. In this study, we used molecular dynamics simulations to investigate the role of solvent and peripheral groups on molecular structure and core encapsulation of iron-sulfur G2-benzyl ether dendrimers in polar and nonpolar solvent. We found that the dendrimer branches collapse in water and swell in chloroform. The presence of the long hydrophobic alkyl groups at the periphery deters the encapsulation of the core in water which may cause an increase in electron transfer rate. However, in chloroform, the dendrimer branches remain in the extended form, which leads to an increased radius of gyration. Our results suggest that peripheral alkyl chains in dendrimers cause steric hindrance, which prevents branches from back folding in chloroform solvent, but in water it reverses the trend. Overall, the presence of a hydrophobic interior and hydrophilic periphery in a dendrimer improves core encapsulation in water while hindering encapsulation in chloroform.     

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 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.     

Probing stepwise complexation in phenylazomethine dendrimers by a metallo-porphyrin core

A series of dendritic phenylazomethines (DPA), which have a meso-substituted zinc porphyrin core (DPAGX-ZnP, X = 1-4), were synthesized. Structural studies of these dendrimers were carried out using Tri-SEC (triple detection after size exclusion chromatography), intrinsic viscosity analysis, TEM (tunneling electron microscopy), and molecular modeling calculations by AM1. As a result, a sphere-like structure within a single-nanometer scale (R(h) = 22 A for DPAG4-ZnP) was observed. In addition, encapsulating effects by the DPA shell in the larger dendrimers were confirmed as fundamental properties, based on the UV-vis abosorption spectra, cyclic voltammograms, and 1H NMR spin-lattice relaxation times (T1). The DPAGX-ZnP acts as a multi-metal ion reservoir for SnCl2 and FeCl3. The generation-4 dendrimer (DPAG4-ZnP) can take up to 60 molar amounts of metal complexes around the porphyrin core. A quantitative study of the metal assembling reaction by UV-vis titration revealed stepwise layer-by-layer complexations from the inner imines nearest to the core to the surface. The redox behavior and fluorescence of the zinc porphyrin in these metal-assembled dendrimers also support the stepwise complexation of the metal ion. These analyses suggest that the finely assembled metal complexes in a dendrimer architecture strongly affect the electronic status of the porphyrin core. Results from transient absorption measurements strongly indicate a very fast electron transfer on a subpicosecond time scale between the core and assembled metal complexes.     

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.     

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.     

Electrogenerated chemiluminescence. Effect of a magnetic field on the delayed fluorescence and ECL of several systems involving excimers or exciplexes

The use of magnetic field effects in delayed fluorescence (DF) and electrogenerated chemiluminescence (ECL) studies to obtain information about the involvement of triplet state species in reactions leading to the production of monomeric and dimeric (excimer or exciplex) excited states is described. In the room temperature DF of pyrene and 1,2-benzanthracene, identical field effects are observed for monomer and excimer emission, in agreement with a mechanism involving a common intermediate produced on triplet-triplet annihilation (TTA), and different than previous DF results for 1,2-benzanthracene solutions at low temperatures. The ECL of the pyrene/N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) system also shows monomer and excimer emissions identically affected by magnetic field and in agreement with an ECL mechanism involving TTA. In the ECL of the S-methylanthracene (MA)/tri-p-tolylamine (TPTA) system, a field effect is observed for both the ¹MA^* and longer wavelength emission. In this case, however, a smaller effect is observed for the longer wavelength emission. A mechanism based on TTA to form ¹MA^* and some direct formation of exciplex on electron transfer is proposed.     

Polysulfurated pyrene-cored dendrimers: luminescent and electrochromic properties

We have synthesized a novel class of dendrimers, consisting of a polysulfurated pyrene core with appended poly(thiophenylene) dendrons (PyG0, PyG1, and PyG2, see Scheme 1), which exhibit remarkable photophysical and redox properties. In dichloromethane or cyclohexane solution they show a strong, dendron-localized absorption band with a maximum at around 260 nm and a band in the visible region with a maximum at 435 nm, which can be assigned to the pyrene core strongly perturbed by the four sulfur substituents. The dendrimers exhibit a strong (Phi=0.6), short-lived (tau=2.5 ns) core-localized fluorescence band with maximum at approximately 460 nm in cyclohexane solution at 293 K. A strong fluorescence is also observed in dichloromethane solution at 293 K, in dichloromethane/chloroform rigid matrix at 77 K, and in the solid state (powder) at room temperature. The dendrimers undergo reversible chemical and electrochemical one-electron oxidation with formation of a strongly colored deep blue radical cation. A second, reversible one-electron oxidation is observed at more positive potential values. The photophysical and redox properties of the three dendrimers are finely tuned by the length of their branches. The strong blue fluorescence and the yellow to deep blue color change upon reversible one-electron oxidation can be exploited for optoelectronic devices.     

New cyclen-cored dendrimers functionalized with pyrene: Synthesis characterization,optical and photophysical properties

New cyclen (1,4,7,10-teraazacyclododecane) cored dendrimers up to the second generation, functionalized with 4, 8 and 16 pyrene units, respectively, were synthesized following a convergent procedure. All new compounds were characterized by NMR spectroscopies and ESI or MALDI TOF mass spectrometry. The optical and photophysical properties of the new dendrimers were studied in THF solution. Absorption spectra showed the typical absorption bands of pyrene moieties. In the fluorescence spectra, monomer as well as excimer emission were observed for all compounds. An increased proportion of excimer emission was observed in the dendrimer of the highest generation.     

Electrogenerated chemiluminescence. 81. Influence of donor and acceptor substituents on the ECL of a spirobifluorene-bridged bipolar system

The electrochemistry and radical ion annihilation electrogenerated chemiluminescence (ECL) of 9,9'-spirobifluorene-bridged bipolar systems containing 1,3,4-oxadiazole-conjugated oligoaryl and triarylamine substituents were investigated. The stability of the oxidized spirobifluorenes was improved by functionalization with triarylamine centers. These donor-acceptor (DA) compounds exhibited a good fluorescence efficiency with an emission maximum that correlated with the potential difference between radical anion and cation formation, suggesting a charge transfer (CT) emission band. An ECL mechanism based on the formation of the CT excited state by radical ion annihilation or production of the triplet state followed by triplet-triplet annihilation, with perhaps some excimer contribution, is proposed.     

Effect of dendrimer generation on electron self-exchange kinetics between metal tris(bipyridine) core dendrimers

Here we report the first measurement of homogeneous electron transfer between oxidized and reduced metal tris(bipyridine) core dendrimers by NMR line-broadening; the results indicated that, as the generation of the dendrimer increased, the rate of self-exchange decreased.     

Synthesis of phosphorus dendrimers bearing chromophoric end groups: toward organic blue light-emitting diodes

Several series of phosphorus dendrimers decorated by potential fluorescent end groups (naphthalene, anthracene, and pyrene) have been synthesized. Unexpectedly, we found that it is absolutely necessary to link the fluorophore to the dendrimer through an alkyl link, and not directly through heteroelements such as oxygen or nitrogen, in order to preserve the fluorescence. One series of dendrimers from generation 1 (6 pyrene end groups) to generation 4 (48 pyrene end groups) has been tested for the elaboration of organic light-emitting diodes (OLEDs). The threshold voltage for the emission of light is high (over 20 V), however, electroluminescence is observed in all cases.     

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。     

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.     

Application of Visible‐to‐UV Photon Upconversion to Photoredox Catalysis: The Activation of Aryl Bromides

The activation of aryl–Br bonds was achieved by sequential combination of a triplet–triplet annihilation process of the organic dyes, butane‐2,3‐dione and 2,5‐diphenyloxazole, with a single‐electron‐transfer activation of aryl bromides. The photophysical and chemical steps were studied by time‐resolved transient fluorescence and absorption spectroscopy with a pulsed laser, quenching experiments, and DFT calculations.     

Strong blue photoluminescence and ECL from OH-terminated PAMAM dendrimers in the absence of gold nanoparticles

A product showing strong blue photoluminescence was obtained by oxidation of OH-terminated PAMAM dendrimers, such as G4-OH, G2-OH, and G0-OH, with HAuCl4 or (NH4)2S2O8. The fluorescence emission spectrum peaked at 450 nm, while the excitation maximum was at 380 nm, independent of the generation of dendrimer. The product also shows two weak electrogenerated chemiluminescence (ECL) signals upon cycling the potential between about 1.2 and -1.7 V.     

Dendritic encapsulation-roles of cores and branches

Examples of new dendrimers are presented. In the first, the effect on electron transfer rate attenuation is investigated for two dendrimer isomers that differ only in the linkage (ortho- versus meta-linked) of the phenyl ether units within one generation of the structure. Second, the effect of encapsulation on electrochemical and luminescence behavior of a new type of rhenium selenide cluster core dendrimer is illustrated.     

Synthesis and electrochemical properties of dendrimers containing meta-terphenyl peripheral groups and a 4,4′-bipyridinium core

Fréchet-type poly(arylether) dendrons carrying m-terphenyl peripheral groups were synthesized up to second generation by convergent methodology. Simple quarternisation of 4,4′-bipyridine with the dendritic bromides afforded the corresponding dendrimers containing a 4,4′-bipyridine core. The electrochemical parameters were obtained for all the dendrimers and the half-wave potentials of both the first and second redox processes shift to less-negative values as the dendrimer generation increases.     

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