Optical and Magnetic Properties of Rigid Rod Conjugated Molecules
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313 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
the molecule is a rigid and conjugated rod, where the central and the end groups could be as follows: X = phenyl or thienyl and Ar-R = phenoxy, phenyl-galvinoxyl, phenyl-nitroxide, phenylnitronyl-nitroxide. The 4-bromo-2,6-di-t-butyl phenyl acetate (1) was prepared by a phase transfer catalysed acylation with tetra-n-butyl ammonium hydrogenosulfate [18] from the 4-bromo-2,6-di-t-butyl phenol with a yield of 97 %. The intermediate derivative bis-(2,6-di-t-butyl phenyl acetate) diethynyl-2,5-didodecanoxy benzene was obtained by a Heck coupling reaction of (1) with the 1,4-diethynyl-2,5-didodecanoxy benzene using a palladium complex catalyst as previouly described [19,20]. Thus we prepared the dihydroxy derivative, bis-(2,6-di-t-butylphenol)diethynyl-2,5-didodecanoxy benzene, by reduction with LiAIH 4 in THF [21] as a yellow product in 85 % yield. The biradical (2) was subsequently obtained by oxidation with fresh PbO 2 in benzene as a dark red product in 95 % yield. 0c
.-
12
S
00
C12
0
4)
(2
Elemental analysis calcd % for C58 H 860 4 : C, 82.27; H, 10.16, 0, 7.5 found: C, 81.49; H, 10.03; 0, 8.15. IR spectra indicated clearly the disapperance of the OH stretching band at 3625cm-1 and showed one strong peak at 1607 cm- 1 which could be attributed to phenoxy radicals.
(5)
0
i
0C 12
)PdCl2,TPPCuOAc, Br-
2) PbO
2
Br /
Scheme I: synthesis of the bis-galvinoxyl-diethynylaryl biradical
314
The thienyl bisphenol derivative (3) and the bisphenoxy radical (4) were synthetized by the same synthetic route. The (4) was produced as a dark brown powder soluble in organic solvents as THF, CHCI3 , MeOH, etc... The synthetic route of the galvinoxyl mono (5') and biradical (6) is outlined in the scheme I. The {4-[(3,5-di-t-butyl-4-hydroxyphenyl)(3.5-di-t-butyl-4-oxocyclohexa-2,5-dienylidene) methyl]-phenyl)acetylene (5) was synthesised by an organometallic route followed by the elimination of the trimethyl protecting groups according to ref. [ 14]. The monoradical species (5') was obtained by oxidation of (5) by PbO 2 in benzene. The condensation of the non oxidized molecule (5) with the 1,4-dibromo-2,5-didodecanoxy benzene was performed by Heck coupling using the procedure previouly described, followed by the classical oxidation process with PbO 2. Elemental analysis was conform to predictions and IR spectroscopy shows the disappearance of the OH band at 3621 cm-1 and the presence of a strong band at 1575 cm-1 attributed to 0. radical. Optical spectroscopic characterizations: The UV-Vis. spectra of phenoxy derivative (2) and (4), and the galvinoxyl molecules (5') and (6) are given respectively in Figure la and b. On Figure la the spectrum of (2) shows a broad band with a strong absorption (s = 3.5 x 10 4 1 mole- 1) at the maximum wavelength 421nm, while (4), which have a quite lower molar absorption at his maximum 462nm (s = 2.5 x 104 1 mole-1) shows a very broad absorption tail up to 750nm. This absorption tail of the thienyl derivative a
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