Enhanced photoinduced birefringence in hydrogen-bonded polymer-dye complexes

摘要

Photoinduced optical anisotropy of azobenzene-containing polymers has emerged as a source of exciting optical phenomena with a range of potential applications. In the simplest form of amorphous photoactive polymers, the azo-dyes are dissolved in a passive polymer matrix. Such guest-host systems are flexible and cost-effective as they only require mixing of the constituents to produce the desired compound. However, their applicability is restricted by dye aggregation and by poor temporal and thermal stability of the induced anisotropy. These drawbacks can be addressed by covalently bonding the photoactive moieties to the polymer backbone to form side-chain systems. However, the sample preparation in the latter case is slower and more expensive as organic synthesis is required for each combination of a polymer and an active molecule. Here we show that the benefits of the guest-host and covalently-linked systems can be combined by exploiting specific non-covalent interactions between the dye molecules and the polymer host. More precisely, hydrogen bonding between the dye and the polymer host enables to use high dye concentrations compared to conventional guest-host polymers and also improves the temporal stability of the anisotropic alignment of the dyes. We studied the photoinduced anisotropy of a common azo-dye, Disperse Red 1 (DR1), doped in polystyrene (PS) and poly(4-vinylphenol) (PVPh) [Fig. 1 (a)]. PS is a nonpolar reference polymer with no significant interactions with the polar DR1 molecules. PVPh is a polar polymer containing a phenol group, which can form hydrogen bond with the nitro group of DR1. The samples were spin-coated thin films with DR1 doping ranging from 5 wt.% to 30 wt.%. The thickness of each sample was adjusted to maintain constant optical density at the writing wavelength (532 nm). The light from an 850 nm diode laser was used to probe the photoinduced birefringence.