Effects of absorption overlap on the modulation of the polarized fluorescence of Eu(DBM)₃Phen-doped azobenzene polymer films

Two azopolymers, poly(4-(4′-nitrophenylazo)phenoxy)hexyl methacrylate (PNAzoPOH) and poly(6-(N-methyl-N-(4-(4′-nitrophenylazo)phenyl)amino)hexyl methacrylate) (PNAzoPAH), bearing strong push–pull azobenzene chromophores, have been designed and synthesized. Azopolymer films doped with a europium complex, tris(dibenzoylmethanido)(o-phenanthroline)europium(III) (Eu(DBM)₃Phen), were prepared by a drop-casting method. UV-vis spectra indicated that the absorption peak of Eu(DBM)₃Phen at 365 nm overlaps with that of PNAzoPOH, rather than that of PNAzoPAH. Polarized fluorescence from both of the oriented films was observed to depend on the absorption overlap between the complex and the azopolymer. When excited by unpolarized light at 365 nm, the polarization ratio R of Eu(DBM)₃Phen-doped PNAzoPOH film shows an obvious decreasing trend with increasing irradiation time, and the ratio R of Eu(DBM)₃Phen-doped PNAzoPAH film has a relative stable value of about 1.47. When excited by polarized light at 365 nm, the florescence intensities from the two films show different dependences upon the angle between the direction of orientation of the film and the direction of polarization of the excitation light: for Eu(DBM)₃Phen-doped PNAzoPOH film, the strength of the polarized fluorescence depended on the direction of polarization of the excitation light; for Eu(DBM)₃Phen-doped PNAzoPAH film, no such dependence was observed. These different results arise due to differences between the azopolymers in terms of the allocation of energy between the oriented azobenzene groups and the complex.