Chromophores dendrimers

Mullen et al. presented a multi-chromophoric dendrimer which absorbs over the entire visible spectrum. Three different dyes are positioned in a rigid polyphenylene dendrimer scaffold in such a manner that an energy gradient is generated between the periphery and the centre of the core and efficient energy transfer to the central chromophore (A) takes place on excitation of the peripheral chromophore (Cl, C2) (Fig. 3.16) [77]. 

Alternatively, the dendrimer backbone itself can concurrently be used as the energy donor or acceptor. Several types of chromophoric dendrimer backbones such as poly(phenylacetylene) [3], poly(phenylene) [4], and poly(benzylether) [5] have been used as light absorbers, and the energy was efficiently transferred to the core acceptor. While most of these systems have high energy transfer efficiencies, they still suffer from a weak fluorescence or a low fluorescence quantum yield. However, polyphenylene dendrimers composed of tens or hundreds of out-of-plane twisted phenyl units can be used as chromophoric backbones [6] carrying highly luminescent dyes at the periphery. 

In addition to the antennae effect, dendrimers are also effective insulators and exhibit the shell effect. In providing a dense shell around the incorporated chromophores, dendrimers effectively prevent aggregation which leads to non-emissive excimers and self-quenching that occurs when chromophores with small Stokes shifts are within short distances of one another. This shell effect allows for increased photoluminescence efficiency of the enclosed chromophore, which is important for optoelectronic devices. 

The synthesis of a typical electro-optic chromophore is shown in Scheme 6.1. The synthesis of multi-chromophore dendrimers is shown in Scheme 6.2 and Scheme 6.3. Although the discussion of the synthesis of the multitude of chromophores that have been prepared to the present is beyond the scope of this chapter, it should be noted that optimization of reaction yields is an important requirement. In this... 

Dendrimer 1 + is a classical example of a dendrimer containing a luminescent metal complex core. In this dendrimer the 2,2 -bipyridine (bpy) ligands of the [Ru(bpy)3] +-type core carry branches containing 1,2-dimethoxybenzene- and 2-naphthyl-type chromophoric units [15]. 

The [Ru(bpy)3] + core has more recently been used to construct first-generation dendrimers containing coumarin-450 chromophoric groups. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

Dendrimers with a polyphenyl core around a central biphenyl unit decorated at the rim with peryleneimide chromophores have been investigated both in bulk and at the single-molecule level in order to understand their time and space-resolved behavior [28]. The results obtained have shown that the conformational distribution plays an important role in the dynamics of the photophysical processes. Energy transfer in a series of shape-persistent polyphenylene dendrimers substituted with peryleneimide and terryleneimide chro-mophoric units (4-7) has been investigated in toluene solution [29]. 

Energy hopping among the peryleneimide chromophores, revealed by anisotropy decay times [30], occurs with a rate constant of 4.6x10 s E When three peryleneimide and one terryleneimide chromophores are attached to the dendrimer rim, energy transfer from the former to the latter units takes place with... 

In dichloromethane solutions, excitation of the chromophoric groups of the dendrons causes singlet-singlet energy transfer processes that lead to the excitation of the porphyrin core. It was found that the dendrimer 17, which has a spherical morphology, exhibits a much higher energy transfer quantum yield (0.8) than the partially substituted species 13-16 (quantum yield <0.32). Fluo-... 

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

G2, to G3, and to G4, the effective enhancement was 10%, 36%, and 35% larger than the value estimated by the simple addition of monomeric values. The enhancement included the local field effect due to the screening electric field generated by neighboring molecules. Assuming the chromophore-solvent effect on the second-order susceptibility is independent of the number of chro-mophore units in the dendrimers, p enhancement can be attributed to the inter-molecular dipole-dipole interaction of the chromophore units. Hence, such an intermolecular coupling for the p enhancement should be more effective with the dendrimers composed of the NLO chromophore, whose dipole moment and the charge transfer are unidirectional parallel to the molecular axis. 

Solvatochromic probes have been used for a variety of applications like the study polarity of pure and mixed solvents [99], and the retention behavior in reverse-phase liquid chromatography [100] among other applications. Frechet et al. used 4-(N-methylamino)-l-nitrobenzene (p-MANB), as the chromophore, to probe the microenvironment of polyaromatic ether based dendrimers [101]. 

Dendritic molecules with electroactive units at either the focal point or core have been reported [92, 97]. There are, however, only a few examples of such moieties specifically pinned within cascade infrastructure. Our recent efforts in this direction [104-106] involve the incorporation of chromophoric 1,4-di-aminoanthraquinone (35) within the cascade infrastructure. Dendrimers based on a four-directional pentaerythritol core were synthesized using the extended 1 — 3 building block 36. A high dilution technique was applied to synthesize 36... 

A comparison of the thus calculated with the measured specific rotations of the 0th- to 4th-generation dendrimers of this kind gave a close resemblance, with a curve, approaching asymptotically a limiting value (Fig. 26). It was also shown that the shape of this curve was independent of solvent, concentration and temperature. This was not the case when CD spectra of these dendrimers were compared (Fig. 27) in solvents such as CH2C12 and f-butyl methyl ether a constant rise of the Cotton effect was observed, which correlates with the increasing amount of benzene chromophores in the dendrimers. However, in the... 

Photophysical studies have been performed on dendrimers 41 [49], built around a [Ru(bpm)3]2+ core (bpm=2,2 -bipyrimidine),and42 [59],built around a [Ru(QP)3]2+ core (QP = 2,2 3, 2" 6",2" -quaterpyridine). In both compounds energy transfer from the peripheral Re(I)-based chromophores to the central Ru(II)-based unit occurs with unitary efficiency. 

---