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

We have studied the photoluminescence (PL) of liquid crystalline polyacetyl enes {-[HC=C(CH2)(9)-OCO-Biph-O-(CH2)(6)CH3](n)-(1), -[HC=C(CH2)(3)-O-CH6H4 -CO2-C6H4-O -(CH2)(5)CH3](n)- (2), and -[HC=C-C6H4-CO2-(CH2)(6)-OCO-Biph-O( CH2)(6)CH3](n)-(3)}. The textures of the polymers are characterized by pola rized optical microscope. In dilute tetrahydrofuran (THF) solution, I emits strong deep-blue PL with a single PL peak at similar to 380 nm, while 2 em its faint blue PL with a single peak locating at similar to 450 nm. Interes tingly, the PL of 3 is composed of two peaks, one of which locates at 380 n m and the other at 450 nm. Using extended Huckel tight-binding method, we h ave calculated their electronic structures and the results show that the el ectronic states of the polymers are essentially an ensemble of the extended states characteristic of the backbone and the localized states characteris tic of the pendant. Our experimental and calculated results prove that both the absorption and blue emissions take place in the phenyl or biphenyl mes ogens in the pendants and the types of chromophores determine the emission colors of the polymers. (C) 2001 Elsevier Science B.V. All rights reserved.