Incorporation of a Photosensitizer Core within Hyperbranched Polyether Polyols: Effect of the Branched Shell on the Core Properties

Abstract

A photosensitizer core has been covalently incorporated into a hyperbranched polyether polyol structure, using a one-step protocol. Partially deprotonated 2,2‘,4,4‘-tetrahydroxybenzophenone was used as initiator in the anionic polymerization of glycidol, yielding hyperbranched polyglycerols with benzophenone core in every macromolecule formed. Molecular weights in the range of 1500 to 5800 g/mol and moderate polydispersity (1.3 < Mw/Mn < 2) were obtained. Incorporation of the functional core was unequivocally evidenced by MALDI−TOF mass spectroscopy. Further modification via acetylation of the polyglycerol hydroxyl groups afforded polymers of lowered polarity. The photochemical and photophysical behavior of the obtained benzophenone core-containing hyperbranched polyglycerols has been investigated in a variety of solvents and compared with that of the model compound 2,2‘,4,4‘-tetramethoxybenzophenone. The hyperbranched polymer periphery induced molecular weight-dependent properties in the photoactive core. Although absorption spectra of the polymers show the typical bands of a tetra(alkoxy)benzophenone unit, a dramatic hyperchromic effect of the band assigned to the 2,4-dialkoxybenzoyl chromophore was observed with increasing molecular weight. Steady-state and time-resolved fluorescence measurements evidenced an increase of both the fluorescence quantum yield and the singlet lifetime with the increase of molecular weight. On the other hand, laser flash photolysis (LFP) experiments showed that, as in the case of the reference compound, the lowest-lying triplet of the hyperbranched derivatives in methanol is mainly of the π,π* nature, while both triplets n,π* and π,π* are populated in acetonitrile, which is indicative of easy access of the solvent to the core. However, the quenching rate constant of the aromatic ketone triplet excited-state by naphthalene decreased with the polymer molecular weight, which agrees with the role of the branched shell as a barrier for external quenching. The polymers showed good photostability, i.e., degradation was insignificant upon 1 h of irradiation in methanol.