Photophysical properties of lyotropic liquid crystalline polybenzazoles and evidence of aggregate formation for polybenzazoles in methanesulfonic acid

The photophysical properties of the lyotropic liquid crystalline polybenzazoles [poly(pphenylene benzobis(oxazole)) (PBO), poly(2,5-benzoxazole) (ABPBO), and their random copolymer PBOABPBO] and their model compounds in solutions with methanesulfonic acid (MSA) were investigated in detail using UV absorption and emission spectroscopy. It was found that the absorption wavelength and emission wavelength can be well modulated by the composition of PBO in the copolymer from the lowest value for pure ABPBO to the highest for PBO. The photoluminescence spectra at various concentrations were also studied in detail. Both excitation spectra and emission spectra show significant changes in shape and position of the peaks. With the increase of the concentration, the intensity of emission is depressed, and the highly structured emission spectra gradually change to featureless, red-shifted, and broad spectra. The peak position in excitation spectra also shows a slight red shift. Another dramatic change in excitation spectra with concentration is a second broad peak that shows up in the intermediate concentration range. The change in emission spectra can be attributed to the existence of the interchain aggregates, which can be excited from the ground state to the excited-state directly. This explanation is further supported from the evidence in a poor solvent, selective excitation, and time-resolved photoluminescence spectroscopy.