The role of the phenyl and biphenyl chromophores in the blue luminescent liquid crystalline polyacetylenes

We have studied the photoluminescence (PL) of liquid crystalline polyacetylenes {[HCC(CH2)9OCOBiphO(CH2)6CH3]n (1), [HCC(CH2)3OC6H4CO2C6H4O (CH2)5CH3]n (2), and [HCCC6H4CO2(CH2)6OCOBiphO(CH2)6CH3]n (3)}. The textures of the polymers are characterized by polarized optical microscope. In dilute tetrahydrofuran (THF) solution, 1 emits strong deep-blue PL with a single PL peak at ∼380 nm, while 2 emits faint blue PL with a single peak locating at ∼450 nm. Interestingly, the PL of 3 is composed of two peaks, one of which locates at 380 nm and the other at 450 nm. Using extended Hückel tight-binding method, we have calculated their electronic structures and the results show that the electronic states of the polymers are essentially an ensemble of the extended states characteristic of the backbone and the localized states characteristic of the pendant. Our experimental and calculated results prove that both the absorption and blue emissions take place in the phenyl or biphenyl mesogens in the pendants and the types of chromophores determine the emission colors of the polymers.