Fluorene studies

Y. Koizumi, S. Seki, A. Acharya, A. Saeki, and S. Tagawa, Delocalization of positive and negative charge carriers on oligo- and poly-fluorenes studied by low-temperature matrix isolation technique, Chem. Lett., 33 1290-1291, 2004. 

Recently, polyimines include the synthesis of long alkoxy (Cg-Cig) side chain derivatives [188,189], which are presumably soluble to some extent in organic solvents and derivatives containing fluorene cardo unit [190]. Trifluoromethyl groups [191] in the polymer backbone provide solubility in organic solvents. Studies of the electrical conductivity of doped conjugated aromatic polyimines and alkoxy derivatives have been reported [188], and the values are in the range of 10 to 10 S/ cm. 

Facial selectivities of spiro[cyclopentane-l,9 -fluorene]-2-ones 30a-30e were studied by Ohwada [96, 97]. The carbonyl tz orbital can interact with the aromatic % orbital of the fluorene in a similar manner to spiro conjugation [98-102]. The ketones 30 were reduced to alcohols by the action of sodium borohydride in methanol at -43 °C. The anti-alcohol, i.e., the syn addition product of the reducing reagent with respect to the substituent, is favored in all cases, irrespective of the substituent at C-2 or C-4 of the fluorene ring (2-nitro 30b syn anti = 68 32), 4-nitro... 

Epoxidation of substituted spiro[cyclopentane-l,9 -fluorene]-2-enes 68 with a peroxidic reagent was studied [98], The spiro olefins react with m-chloroperbenzoic acid (mCPBA) in chloroform at 3 °C to give a mixture of the epoxides. In all cases (2-nitro (68b), 4-nitro (68c), 2-fluoro (68d) and 2-methoxyl (68e) groups), the iyn-epoxides, i.e., the syn addition of the peroxidic reagent with respect to the substituent, is favored. For example, for 6 nsyn anti = 63 31 for 68c syn anti = 65 35. Thus, a similar bias is observed in both the reduction of the carbonyl derivatives of 30 and the epoxidation of the derivatives of 68. 

In the Spiro systems 30, the aromatic orbitals unsymmetrize the carbonyl orbital. Simultaneously, the carbonyl group can perturb the orthogonal aromatic ring. Nitration of the fluorene derivatives (30) bearing a spiro substituent was studied (Fig. 17) [96, 97]. 

FIGURE 7.23 Prototype diazobenzo[6]fluorene-based natural products kinamycin A and prekinamycin. Compounds prepared for this study are shown in the inset. 

Compounds shown in Scheme 11 are based on the fluorene system with different spacer units connecting the two benzene rings. The unit X in the 9-position strongly influences the optical properties of these alkynylgold(i) emitters. Complementary electrochemical studies have been carried out for the fluorenone species.79,80... 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

Time to depurate or biotransform 50% of accumulated PAHs (Tb 1/2) varied widely. Tb 1/2 values for Daphnia pulex and all PAH compounds studied ranged between 0.4 and 0.5 h (Southworth et al. 1978). In rainbow trout given an intraarterial injection of 10 mg/kg BW of 2-methylnaphtha-lene, fluorene, or pyrene, the Tb 1/2 values ranged between 9.6 and 12.8 h when route of exposure was intragastric and doses were 50 mg/kg BW, there was negligible uptake (Kennedy and Law 1990). For marine copepods and naphthalene, a Tb 1/2 of about 36 h was recorded (Neff 1982a). 

An important extension to phenyl-substituted PPVs was first reported by Tsutsui and coworkers [115], who used Gilch polymerization to synthesize fluorenyl-substituted PPVs (57,58,59) and studied their performance in PLEDs (Chart 2.11). Due to bulky but rigid fluorene substituents, these polymers have excellent solubility and yet are thermally stable up to 320°C and have a... 

In the effort to make pure blue-emitting materials Shim and coworkers [146] synthesized a series of PPV-based copolymers containing carbazole (polymers 95 and 96) and fluorene (polymers 97 and 98) units via Wittig polycondensation. The use of trimethylsilyl substituents, instead of alkoxy groups, eliminates the electron donor influence of the latter and leads to chain distortion that bathochromically shifts the emission (Amax = 480 nm for 95 and 495 nm for 97). In addition, a very high PLQY was found for these polymers in the solid state (64 and 81%, respectively). Single-layer PLEDs fabricated with 95 and 97 (ITO/polymer/Al) showed EL efficiencies of 13 and 32 times higher than MEH-PPV, respectively (see also Ref. [147] for synthesis and PLED studies of polymers 99 and 100) (Chart 2.20). 

Parent (unsubstituted) PF was first synthesized electrochemically by anodic oxidation of fluorene in 1985 [266] and electrochemical polymerization of various 9-substituted fluorenes was studied in detail later [220,267]. Cyclic voltammogram of fluorene ( r1ed= 1.33 V, Eox = 1.75 V vs. Ag/Ag+ in acetonitrile [267]) with repetitive scanning between 0 and 1.35 V showed the growth of electroactive PF film on the electrode with an onset of the p-doping process at 0.5 V (vs. Ag/Ag+). The unsubstituted PF was an insoluble and infusible material and was only studied as a possible material for modification of electrochemical electrodes. For this reason, it is of little interest for electronic or optical applications, limiting the discussion below to the chemically prepared 9-substituted PFs. 

Although the exact mechanism of the fluorenone formation is not known, it is believed that the monoalkylated fluorene moieties, present as impurities in poly(dialkylfluorenes), are the sites most sensitive to oxidation. The deprotonation of rather acidic C(9)—H protons by residue on Ni(0) catalyst, routinely used in polymerization or by metal (e.g., calcium) cathode in LED devices form a very reactive anion, which can easily react with oxygen to form peroxides (Scheme 2.26) [293], The latter are unstable species and can decompose to give the fluorenone moiety. It should also be noted that the interaction of low work-function metals with films of conjugated polymers in PLED is a more complex phenomenon and the mechanisms of the quenching of PF luminescence by a calcium cathode was studied by Stoessel et al. [300],... 

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

In addition to 195-200, many other alkyl substituents and their derivatives have been introduced at position 9 of the fluorene nucleus in order to create a processible stable blue-emitting PF material, e.g., 203a-h [273-275,305], Chiral-substituted PFs 200 and 203g,h have been synthesized to study their chiroptical properties [306], particularly interesting due to polarized emission in such materials (see Chapter 5 in this book) (Chart 2.47). 

While retaining much of the substituted PT character (e.g., good hole-transport properties and stability), these materials exhibit significantly improved fluorescence efficiency in the solid state (cl>Pi up to 29%) that leads to (hllof UP to 0.1% for ITO/453/Ca PLED (Table 2.6). Other widely studied thiophene copolymers with aromatic 9,9-disubstituted fluorene units were already described above in Section 2.3. 

G. Zeng, W.-L. Yu, S.-J. Chua, and W. Huang, Spectral and thermal stability study for fluorene-based conjugated polymers, Macromolecules, 35 6907-6914, 2002. 

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