Refractive index modulation in polymers bearing photoreactive phenyl and naphthyl ester units using different UV wavelengths

Abstract

Tuning and patterning of the refractive index in polymeric films is of importance for modern optical applications such as wave guiding and optical data storage. In this contribution, we report on the synthesis of new photoreactive polymers bearing aryl esters which undergo the photo-Fries rearrangement to give aromatic hydroxyketones. Poly(norbornene)-based polymers bearing naphthyl ester groups (poly-2) and copolymers bearing both phenyl and naphthyl ester groups (poly(1-co-2)) were prepared by ring opening metathesis polymerization (ROMP). The photochemistry of these polymers under irradiation with UV light of either 254 nm or 313 nm was investigated. The photoreaction causes an increase of the refractive index in poly-2 between Δn₅₈₉ = +0.022 and +0.048 depending on the irradiation wavelength. The copolymer poly(1-co-2) offers the possibilities of selectively exciting either the naphthyl ester with UV light of 313 nm causing an increase in the refractive index of Δn₅₈₉ = 0.010, or both the naphthyl and phenyl chromophores with UV light of 254 nm (Δn₅₈₉ = 0.036). When thin films of poly(1-co-2) already irradiated with 313 nm light are subjected to a second illumination step with 254 nm light, a significant decrease of the refractive index (Δn₅₈₉ = −0.043) is observed. The reason for this behaviour seems to arise from the formation of different photoproducts depending on the irradiation sequence. Thus the final refractive index can be adjusted to values between n₅₈₉ = 1.564 to n₅₈₉ = 1.633. The two step illumination procedure provides the possibility of erasing and even inverting the index contrast generated during the first illumination step.