A series of hyperbranched poly(etherketone)-b-poly(benzobisthiazole)-b-hyperbranched poly(etherketone) triblock copolymers were synthesized. Two different types of hyperbranched poly(etherketone)s (PEKs) were utilized as dendritic A-block units and carboxylic acid-terminated polybenzobisthiazoles (PBZTs) with two different molecular weights as a conjugated, optically responsive, rigid-rod B-block units to afford dumbbell-shaped ABA triblock copolymers. Although all dumbbell-shaped samples are soluble in strong acids such as trifluoroacetic acid (TFA), methanesulfonic acid (MSA), trifluoromethanesulfonic acid (TFMSA) and sulfuric acid, only phenoxy-terminated dumbbell shaped triblock copolymers (3a and 3b) were soluble in common polar aprotic solvents such as NMP, DMF, DMAc and DMSO. Their solubility in common organic solvents was apparently a consequence arising from the obstruction of the close packing of PBZT components by the bulky dendritic PEK ends. A very surprising and interesting observation was made when the 3a and 3b solutions in NMP were placed under room light for 24 h: all major absorption peaks became featureless and blue-shifted. This photochromism was investigated by static and dynamic UV-absorption and fluorescence spectroscopy under various conditions. Although the response time was quite long, it could be controlled as a function of excitation wavelength, light intensity, and solvent viscosity and/or polarity. We propose that the origin of observed spectral changes is caused by the change in conjugation lengths of PBZT in the dumbbell-shaped ABA triblock copolymers.
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