Stimuli-responsive poly- N -isopropylacrylamide: Phenylene vinylene oligomer conjugate

Phenylene vinylene trimer (OPV) and PNIPAM conjugate with stimuli-responsive optical properties has been synthesized through the formation of amide linkage between PNIPAM and carboxylic-acid-terminated OPV. This material exhibits thermoresponsive optical properties as temperature exceeds the lower critical solution temperature (LCST), which is 32 °C for PNIPAM and the conjugate. This PNIPAM-trimer conjugate is fully characterized by using NMR, FT-IR, temperature-dependent UV-vis, and fluorescence spectroscopy. We have found that the polymer conjugate solution turns opaque as temperature exceeds lower critical solution temperature and a five-fold increase in fluorescence intensity as temperature increases from 20 to 70 °C. Such distinct increase in fluorescence intensity is likely due to the rigidchromism, that is, the change in optical properties due to confinement of the chromophores resulting from restriction of polymer conformational structures. The PNIPAM-trimer conjugate also shows a decrease in decay lifetime with increasing temperature, whereas OPV trimer alone shows no change in decay lifetime as a function of temperature. These unique optical properties are not observed in the trimer and PNIPAM mixture, suggesting that the stimuli-responsive optical properties can occur only in PNIPAM-trimer conjugate linked through covalent bond.