A comparison of structures and optoelectronic properties of oxygen- and sulfur-containing heterocycles: Conjugated nonylbisoxazole and nonylbithiazole oligomers

The synthesis and characterization of 4,4'-dinonyl-2,2'-bisoxazole (NBO) an d its oligomers are described. The optical, electronic, and structural prop erties of these oligomers, NBO, NBO2, and NBO3, are compared to their sulfu r analogues (NBT, NBT2, and NBT3), In the series (NBO)(n) and (NBT)(n), the re is a red shift in the lambda(max) as "n" increases. This effect is obser ved in both the solid and solution states. The bithiazole oligomers have lo nger wavelength absorbances than those of the bisoxazole oligomers; however , the observed red shift is greater as "n" increases far the bisoxazole oli gomers leading to a merging of the absorption maxima between the two system s. The larger red shift for the bisoxazole series is explained by the large r atomic orbital coefficients at the coupling sites of the bisoxazole oligo mers versus those of the bithiazole derivatives. Both systems exhibit therm ochromic behavior corresponding to a transition from an ordered to disorder ed state. Upon heating, the bisoxazole system shows a gradual conformationa l change indicated by continual blue shifts of the absorption maximum. Conv ersely, the less rigid bithiazole system exhibits an immediate order-disord er transition upon heating evidenced by the appearance of an isosbestic poi nt in the absorption spectrum; The solid-state packing of the n = 2 and 3 o ligomers is very similar; however, the solid-state structures of NBO and NE T are different as determined by single-crystal X-ray diffraction. NBO pack s in a two-layered "ribbonlike" structure, whereas NET forms molecular stac ks. The reduction potentials of(NBO), increase (become more positive) from -2.97 to -2.60 to -2.46 V for n = 1, 2, and 3, respectively, and the number of reductions observed corresponds to the number of NBO units.