Stabilization of excited 2,3-bis(2-methyl[b]benzothiophen-3-yl)maleic anhydride in a poly(ethylene glycol)-g-polysiloxane

Fluorescence studies on 2,3-bis(2-methyl[b]benzothiophen-3-yl)maleic anhydride (BTMA)-doped polymer films were investigated. Poly(ethylene glycol) grafted polysiloxane (PEG-g-polysiloxane) films containing 5 wt % of BTMA were prepared by using a poly(ethylene glycol) modified sol-gel precursor and tetraethoxysilane (TEOS) in an acidic condition. The BTMA-doped film was transparent and showed a water contact angle of 40°. The fluorescence spectrum of BTMA in the PEG-g-polysiloxane film showed a large Stokes shift of 0.81 eV, compared to that of BTMA-doped polyethylene film at 0.65 eV. The Stokes shifts in the emission spectra of BTMA molecules in other polymeric hosts such as polystyrene, polycarbonate, and PMMA showed strong correlations with the solubility parameter and dielectric constant of the hosts and coincidentally the contact angle of the doped films. Thus it seems entirely plausible that there exist strong interactions between the excited BTMA molecules with a polar polymeric host. The more polar becomes the host, the stronger would be the interactions. The Stokes shift of 0.81 eV in PEG-g-polysiloxane is attributed to a stabilization of excited BTMA molecules in the polar PEG-g-polysiloxane system. From the plot of Stokes shift (v_a - v_f) against [(ε - 1)/(2ε + 1) - (n² - 1)/(2n² + 1)] for different polymeric media, it is proposed that the excited state of BTMA molecules have a larger dipole moment than those of the ground state.