Quaterphenyls

Table 4. Physical Properties of a Commercial Terphenyl—Quaterphenyl Mixture...

Since the thermal dehydrocondensation proceeds by a free-radical mechanism (37), various radical-forrning promoters like acetone, ethanol, or methanol have been found useful in improving conversion of ben2ene to condensed polyphenyls. In the commercial dehydrocondensation process, ben2ene and some biphenyl are separated by distillation and recycled back to the dehydrocondensation step. Pure biphenyl is then collected leaving a polyphenyl residue consisting of approximately 4% o-terphenyl, 44% y -terphenyl, 25% -terphenyl, 1.5% triphenylene, and 22—27% higher polyphenyl and tars. Distillation of this residue at reduced pressure affords the mixed terphenyl isomers accompanied by a portion of the quaterphenyls present. 

Depending on intended use, more or less of the quaterphenyl fraction is included with the terphenyl cut. 

Batchwise fractional distillation can be used to adjust the ratio of isomers in the mixture. For example, the heat-transfer composition of Table 4 is obtained by collecting all of the ortho- some of the meta- and excluding most of the i ra-terphenyl present in the natural mixture. Economics and considerations of melting point depression favor inclusion of lower melting quaterphenyl isomers. 

The terphenyl—quaterphenyl heat-transfer medium (Table 4), sold as Therminol 75 heat-transfer fluid, is shipped in dmms, tank car, or tank tmck lots. Its U.S. freight classification is Heat-Transfer Media, NOIBN. The material does not requite a DOT ha2ardous material label, but does fall under the ha2ardous chemical criteria of the OSHA Ha2ards Communications Standard (19 CFR 1910.1200). 

As in the case of biphenyl, current worldwide production figures for terphenyls are not readily obtainable, but the volume is probably around 6.8—8.2 million kg/yr. Currently, most of the terphenyl produced is converted to a partially hydrogenated form. U.S. production of terphenyls has remained steady at several thousand metric tons per year over the past decade. The 1991 small lot price for mixed terphenyls was about 3.89/kg whereas the specially fractionated heat-transfer-grade terphenyl—quaterphenyl mixture sold as Therminol 75 heat-transfer fluid was priced around 6.93/kg. Partially hydrogenated mixed terphenyls were priced in the 6.05—7.48/kg range depending on quantity and grade. 

Hexa(oligophenyl)benzenes (e. g. 31 or 33) present one possible approach to the realization of this aim. Two efficient synthetic routes have been elaborated for the preparation of hexa(terphenyl)- and hexa(quaterphenyl)benzene. The first, involving palladium-catalyzed trimerization of diarylacetylenes [54] as the key step, was demonstrated by the synthesis of a hexakis-alkylated hexa(terphenyl)benzene derivative 31 from the corresponding bis(terphenyl) acetylene (32). The peripheral tert-alkyl substituents serve to solubilize the molecule. 

The second synthetic route consists of the coupling of hexa(4-iodophenyl)ben-zene (34) with an alkylated oligophenylboronic acid to produce a hexa(oligo-phenyl)benzene by extending the aromatic chain [52]. This route is illustrated by the reaction of hexa(4-iodophenyl)benzene (34) with an alkylated terphenyl boronic acid with formation of the hexa(quaterphenyl)benzene derivative 33. Once again, the aliphatic substituents serve to guarantee sufficient solubility. 

M.A. Keegstra, V. Cimrova, D. Neher, and U. Scherf, Synthesis and electroluminescent properties of quaterphenyl and sexiphenyl containing copolymers, Macromol. Chem. Phys., 197 2511-2519, 1996. 

Figure 3.20. The CIE color diagram for electroluminescent devices, consisting of anode/Spiro-TAD/Spiro-quaterphenyl Rubrene/Alq3/cathode with different concentrations of rubrene. The spectrum shifts from blue to yellow, crossing the white point (E).

Biphenyl, terphenyl, and quaterphenyl all have room-temperature structures that behave similarly, and also all have low-temperature phases in which only one conformer is present. The torsion angle about the central C-C bond in biphenyl is estimated to be about 10° in the low-temperature form, which is appreciably less than that in the gas phase. In the case of terphenyl, it has been established (41) that the room-temperature structure is disordered each molecule librates in a double-well potential, with the barrier height being about 0.6 kcal/ mol. In the low-temperature form the molecule is stabilized in one of the two minima of the well, and has its terminal rings rotated in the same sense, so that the molecule conserves its center of symmetry. This alternation of rotations between adjacent rings is found (42) also in the low-temperature form of quaterphenyl and results in the molecule being noncentric (even though the crystal structure has a center of symmetry). 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

The 3,7-disulfonyl chloride of dibenzothiophene 5,5-dioxide has been isolated from the reaction of biphenyl with chlorosulfonic acid. The reaction proceeds via the 2,4,4 -trisulfonyl chloride of biphenyl. This reaction has now been extended to give sulfonic acid derivatives of 3-phenyl- and 3-biphenylyldibenzothiophene 5,5-dioxide. Treatment of p-terphenyl with oleum or chlorosulfonic acid at 100° yields (141a) (46%), and similarly p-quaterphenyl yields 141b (47%). A later... 

Fig. 2. Effect of silica pore structure on separation. Stationary phases are 10 LiChrospher SI 100, SI 00, SI 1000, and SI 4000 having 100, 500, 1000. and 4000 A mean pore diameter, respectively. Rowrate and inlet pressure are 5 ml/min and 125 bar, respectively. Sample components I, benzene 2. diphenyl 3, m-terphenyl 4, m-quaterphenyl 5, m ipiliiqiicphcnyl 6, m-sexiphenyl. (Cotiuesy of Merck AO.)...

Although many hydrocarbons exhibit fluorescence, often they are colourless and the fluorescence is only just into the blue, e.g. the linear terphenyl and quaterphenyl. Polycyclic ring systems such as terrylene and rubicene are used in analytical methods and OLEDs but a large number are ruled out of commercial use because of their potential to act as carcinogens. However, as mentioned in section 3.5.1.3, pery-lene is a useful fluorophore in a variety of outlets and pyrene is another polycyclic compound of interest. One derivative of pyrene (3.27) is a yellow-green fluorescent dye used in textile applications (Cl Solvent Green 7) and in analysis and sensors (see section 3.5.6.4). 

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