Molecular structures, reorganization energies and charge transport matrix elements of coronene and its fluoro-, hydroxyl- and sulfhydryl-substituted derivatives have been studied at the B3LYP/6-31G** level. Based on the semi-classical model of electron transfer, charge transport rate constants of the title molecules have been calculated. The results indicate that the coronene molecule is helpful to the transport of negative charge, and the transport rate of positive charge is between those of hexaazatriphenylene and triphenylene.
Connecting two discotic mesogens via a spacer not only stabilizes the columnar mesophase but also leads to the formation of glass columnar phase, and therefore improves the physical properties of discotic liquid crystals as organic semiconductor. Here, we report the synthesis of eight diacetylene-bridged triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)4(OMe)O2C-C8H16-C≡≡ C-]2, 3(n), (n = 4-8), [C18H6(OC6H13)5O2C-C8H16-C≡≡ C-]2, 6 and [C18H6(OC6H13)5O-(CH2)m-C≡≡ C-]2, 8(m), (m = 1, 3) by Eglinton coupling reaction. The thermotropic liquid crystal properties were studied by differential scanning calorimetry and polarized optical microscopy. The results are showed as follows: the length of peripheral alkyl chains of triphenylene influenced properties of liquid crystal dimers; dimers 3(n) (n = 5-8) and 6 had glass columnar phase, and no crystallization was observed above -50℃ for all the triphenylene dimers; compared with 3(6), 6 showed higher molecular symmetry, more stable columnar mesophase and wider mesophase range. The connecting group, length and rigidity of spacer had important influence on the mesomorphism of diacetylene-bridged triphenylene discotic liquid crystal dimers.
The fluorophobic effect of the semi-fluorinated chain of discotic molecules efficiently improves the stability of columnar mesophase.Low symmetric discogens exhibit decreased melting points and wide mesophase ranges.A series of triphenylene derivatives 1,C18H6(OCnH2n+1)4(OMe)(O2CC2H4C6F13),and their hydrocarbon analogues 2,C18H6(OCnH2n+1)4(OMe)(O2CC8H17),n = 4―8,were synthesized for the investigation of the influence of semi-fluorinated peripheral chain and molecular symmetry on mesomorphism.The structures were characterized with 1H NMR and MS.The mesomorphism was studied with differential scanning calorimetry(DSC) and polarizing optical microscopy(POM).The results showed that all triphenylene derivatives exhibited enantiotropic columnar mesophase.The semifluorinated triphenylenes 1a―1e displayed higher melting points and clearing points than their hydrocarbon analogies 2a―2e.Most of them were in columnar mesophase at room temperature.
