The heavy-atom effect on xanthene dyes for photopolymerization by visible light†
We investigated the influence of heavy halogen atoms (Br and I) on xanthene dyes for polymerization based on visible-light photoredox initiation. Since the heavy atoms directly affect intersystem crossing (ISC), which can act as a gatekeeper in the photoredox cycle and which was expected to also affect intermolecular photoinduced electron transfer (PET), we attempted to quantify the influence of the halogens. Six different xanthene dyes were chosen based on the number and types of heavy atoms on the xanthene ring. Thus, the photopolymerization degree clearly increased in the following order: fluorescein < 4′,5′-dibromofluorescein ≤ 2′,4′,5′,7′-tetrabromofluorescein < 2′,4′,5′,7′-tetraiodofluorescein. Furthermore, 4′,5′-dibromorhodamine 6G showed a drastic enhancement in the photopolymerization degree, compared with rhodamine 6G. Therefore, we concluded that the presence of halogens on the xanthene ring increases the photoredox initiating performance due to the enhanced ISC efficiency and PET rate.