Fluorene chromophores

Micellar media created by sodium dodecyl sulfate or cetyltrimethylammon-ium bromide have also a positive effect on the ortho selectivity [262,269-271]. In a recent article, the PFR of 1-naphthyl acetate in aqueous solutions of an antenna polyelectrolyte like poly(sodium styrenesulfonate-co-2-vinylfluorene) has been reported, which, in addition to the micellar effect, also provides singlet sensitization by means of fluorene chromophores. In this particular case, the occurrence of in-cage versus out-of-cage processes can be clearly correlated with the different packing of the microdomains, which depends on the molar fraction of monomers in the antenna copolymer [272]. 

We tried to optimize the polycoupling conditions by varying such parameters as polymerization time, monomer concentration and monomer addition mode, in an effort to control the polymer formation and to render the polymers soluble and processable. The optimization worked well and our A2 + B3 approach offered ready access to a soluble hb-PAE containing luminescent anthracene and fluorene chromophores (Scheme 5) [27]. Similarly, soluble azo-functionalized polymers hb-P13 and hb-P15 were obtained from the palladium-catalyzed polycoupling of triiodoarenes (12 and 14) with a di-ethynylazobenzene (11) [28]. 

Furthermore, TPA of dipolar compounds can be tuned by the introduction of heterocyclic moieties with electron-withdrawing (A) properties resulting in 43b-f [230, 415, 417-419, 421], For this purpose, fluorene chromophores bearing a distinct D-rc-A substitution pattern were investigated for TPA. Although 43b-f comprise different acceptor units with distinct acceptor strengths, there is no... 

Figure 2. Typical Fmoc deprotection profiles (measured at 275 nm) observed during continuous flow SPPS. (a) Unhindered release of fluorene chromophore from a fully solvated peptide-polymer matrix, (b) Release from a partially aggregated peptide-polymer. (c) Release from a fully aggregated peptide-polymer matrix.

Aryl substituents also activate benzyHc C-H bonds adjacent to a nitrene center. An interesting study on this topic has dealt with the atropoisomeric 3,5-dimethyl-2-(9-fluorenyl)phenylazides 49 and 52. ° The low-temperature photochemistry (77 K) of these rotameric azides proved to be fundamentally different Photolysis of 49 resulted in the formation of nitrene 50 in addition to azanorcaradiene 51, while photolysis of 52 gave nitrene 53 in addition to iminoquinone methide 54. This subsequently underwent intramolecular cycloaddition to yield dihydroindenocridine 55. Laser flash photolysis showed that the formation of iminoquinone methide 54 from azide 52 occurred in less than 10 ns. Deuteration of the 9-position of the fluorene chromophore resulted in a significantly diminished yield of 55. LFP of the rotameric azide 49 yielded the rate constant for the formation of azanorcaradiene 51 which was determined as k = 7.1 x 10 ... 

In the absence of capping ligands, polymeric alkynylmercury derivatives can be readily prepared. Polymer 57 is formed by reaction of HgC with 9,9-bis(4-ethynylphenyl)-fluorene in a basic methanolic solution. This polymer, whose average molecular weight (/I7W) is only 8,900, shows a strong triplet emission of the organic chromophore enhanced by the presence of the mercury heavy atom.69... 

W. Wang, J. Xu, and Y.-H. Lai, Alternating conjugated and transannular chromophores tunable property of fluorene-paracyclophane copolymers via transannular tt-tt interaction, Org. Lett., 5 2765-2768, 2003. 

Exciton coupling between the two styryl chromophores in each of the conformers (A)-(C) leads to a negative Cotton effect at around 270 nm, as shown by theoretical analysis using the 71-electron SCF Cl dipole velocity MO method. Therefore, the tram-2,.1-disubstituted spirocyclo-propane-1,9 -fluorene derivative 6, from which the distyryl derivative was obtained, must have accordingly the 2S3S absolute configuration122. 

Another interesting system that was studied for two-photon 3D data storage in our group is based on the modulation of fluorescent properties of 2PA chromophores by protonation [95]. For example, due to differences in basicity (pKb), fluorene 3 undergoes selective, stepwise protonation, first by proto-... 

As for Boc amino acids, all proteinogenic and several non-natural Fmoc amino acids esterified with Wang or similar resins are commercially available. Deprotection with 20% piperidine in DMF is usually complete within a few minutes, and the release of the fluorene derivative is easily monitored spectrophotometrically at 300-320 nm (see, e.g., [33]). The peak area can be used to determine the amount of chromophore released, which is proportional to the efficiency of the preceding coupling reaction. At the end of an automated peptide synthesis, inspection of the chromatogram of all Fmoc releases enables rapid assessment of the quality of the resin-bound peptide and quick location of positions in the peptide where coupling was unsuccessful or difficult. 

However, one needs to take into account that all the calculations are done assuming a temperature of 0 K, and hence at room temperature these couplings will be minimal. Furthermore, in collaboration with Beljonne and co-workers, transition densities were calculated [25] for excitation transfer between two per-yleneimide chromophores coupled by a fluorene trimer (separation 3.4 nm) and found to be in line with the Forster approximations. 

Since PDAFs are wide bandgap materials, incorporation of other smaller bandgap chromophores as comonomers or substituents enables tuning of the emission colour due to efficient energy transfer from the fluorene to the other units. Random copolymers of fluorenes with other chromophores are readily prepared by the copolymerisation of the dihalochromophore with a dihalofluorene as exemplified in Scheme 9 for the synthesis of copolymers of dioctylfluorene with perylene dyes (1-5 mol%) [46,47]. 

Scheme 10 Copolymers of fluorene with other stable chromophores...

If a very low mole fraction of a red or orange-emitting chromophore is incorporated into a PDAF, then energy transfer is incomplete so that emission from both the fluorene and the comonomer is observed. By carefully balancing the amounts of emission form the fluorene and comonomer, white emission can be obtained. This approach has been used very successfully by Wang and coworkers to obtain white-emitting copolymers [55]. The highest efficiency for white emission (9 cd/A), was obtained from copolymer... 

These approaches have been adopted more recently to incorporate phosphorescent chromophores into PF in order to make use of the fact that a large proportion (up to 75%) of all excitons formed in LEDs are triplet states, whose energy can only be harvested by using phosphorescent units. The first fluorene copolymers with phosphorescent units 34-35 were made by Holmes and coworkers who added monobrominated red- or green-emitting iridium complexes to an AA-BB Suzuki polycondensation [57]. With short fluorene chains, only emission from the iridium complexes are observed, but with longer fluorene chains some blue emission is also seen. Other groups have since incorporated different phosphorescent units such as platinum [58] or zinc salen [59] units or porphyrins [60,61 ]. 

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