π-Stacked poly(vinyl ketone)s with accumulated push–pull triphenylamine moieties in the side chain

A novel triphenylamine containing vinyl ketone monomer, isopropenyl 4-[bis(4-methylphenyl)amino]phenyl ketone (IMAPK), was prepared from bis[(4-methylphenyl)amino]benzene and methacryloyl chloride through the Friedel–Crafts acylation. t-Butyl 4-[bis(4-methylphenyl)amino]phenyl ketone (BMAPK) was also synthesized as a monomeric unit model. IMAPK was homopolymerized and copolymerized with styrene under free-radical conditions. While homopolymerizability of IMAPK was rather low, copolymers with styrene were obtained at fair monomer conversions. The obtained polymers were proposed to possess a rather rigid, restricted conformation in which the side-chain chromophores are π-stacked on the basis of NMR spectra, and they indicated remarkable hypochromicity and red shifts in absorbance spectra and red shifts in emission spectra compared with BMAPK. In addition, the polymers exhibited red shifts in the emission spectra of which the extent was higher in a more polar solvent among cyclohexane, tetrahydrofuran, chloroform, and acetonitrile; this observation was ascribed to the conformational relaxation of the side-chain 4-[bis(4-methylphenyl)amino]phenyl group in excited states which is facilitated in a polar solvent. The conformational relaxation may be facilitated also by close interactions between the side-chain chromophore groups along the polymer chain. In addition, polymer aggregation was found to occur in solution where the size of aggregates was greater in cyclohexane than in tetrahydrofuran. Further, the polymers showed reduced oxidation potentials compared with BMAPK in cyclic voltammetry analysis, supporting that they have a π-stacked conformation.