How Does the Polymer Main Chain Influence the Side-Chain Mobility? A Fluorescence Probe Study by Means of Twisted Intramolecular Charge-Transfer Phenomena

ω-[[4-(N,N-Dimethylamino)benzoyl]oxy]alkyl methacrylates where alkyl is dodecyl, octyl, butyl, and ethyl were prepared and copolymerized with a large excess of methyl methacrylate. The content of the 4-(dimethylamino)benzoate (DMAB) as a fluorescence probe is less than 0.04 wt %. Fluorescence spectra of the samples were studied in dilute ethyl acetate (a good solvent) or n-butyl chloride (a poor solvent) solution. When rotation of the dimethylamino group is possible within the excited state lifetime, fluorescence from DMAB group is in part converted to twisted intramolecular charge-transfer (TICT) fluorescence. This fluorophore can therefore probe local segment mobility in the polymer. It was found that the ratio (R) of TICT emission intensity to the emission intensity from the local excited state of DMAB decreased with decreasing the alkyl spacer length. Together with the results of red edge excitation effects it was concluded that the local segment motion of side group was subjected to restriction by the polymer main chain as a function of separation from the main chain. Solvent and molecular weight effects were also discussed.