Fluorescent star ATRP initiators and fluorescent star poly(methyl methacrylate)s: Synthesis and photophysical properties

摘要

Fluorescent star ATRP initiators containing four and six initiating sites, comprised with stilbene and alpha-cynostilbene units were synthesized and their photophysical properties were studied thoroughly. These multifunctional ATRP initiators showed solvatochromism. The dipole moment difference between ground state and excited state of the molecules were calculated using Lippert-Mataga equation and found that the molecules were more polar at excited state. The absorption coefficient, absorption and emission wavelength of the initiators prepared with alpha-cyanostilbene unit were found high compared to that prepared without -CN substituent. On the other hand, fluorescent lifetime of alpha-cyanostilbene based initiators was found low. The optical band gap energy calculated for the initiators were considerably low in the range of 2.56 eV-2.83 eV. Based on the photophysical properties, the hexa functional ATRP initiator containing alpha-cyanostilbene unit was used for ATRP of methyl methacrylate monomer to prepare fluorescent star PMMA; CuBr/PMDETA and CuBr/2,2'-bpy were used as catalysts. The molecular weight of star PMMAs ranged between 1.79x10(4) g/mol and 8.42x10(4) g/mol and the polydispersity index were in the range of 1.12-1.48. The linear increase of molecular weight against the monomer conversion indicated the living nature of polymerization. The absorption and emission spectra of polymer solution and film, and observation of polymer in bulk under UV light confirmed the polymer as green fluorescent; the average fluorescence lifetime of the polymer was found to be 0.92 ns.