BODIPY photosensitisers for holographic patterning of photopolymerisable materials Aimee Sheehan 1 , Mikhail Filatov 1 , Tatsiana Mikulchyk 2 Izabela Naydenova 2 1 School of Chemical and BioPharmaceutical Sciences, TU Dublin, Grangegorman, Dublin. 2 Centre for Industrial and Engineering Optics, School of Physics and Clinical and Optometric Sciences, TU Dublin, Grangegorman, Dublin. Boron dipyrromethene dyes (BODIPYs) are a class of fluorophores that possess superior optical properties, 1 and can be chemically modified at any position. The aim of this work was to synthesise a series of new BODIPY dyes that can act as photosensitizers, materials which generate long living triplet states when irradiated with light, without relying on the heavy-atom effect (Fig. 1) . This is the introduction of halogens or heavy metals to the BODIPY core in order to promote intersystem crossing. 2 However, the use of heavy atoms is not feasible long term due to their high costs and negative environmental impact. 3
Figure 1: a) A heavy-atom-containing BODIPY; b) General structure of new BODIPYs synthesised.
An alternative strategy being employed here is the generation of triplet excited states through a mechanism called spin-orbit charge transfer intersystem crossing (SOCT-ISC), which does not require the introduction of any transition metals or halogens into the structure. Triplet states are instead generated from charge transfer (CT) states when these heavy-atom-free compounds are irradiated. 4 For this study BODIPYs containing ester substituents in the 2,6 positions were synthesised. Each compound had a different electron donating substituent in the meso position. The dyes were analysed using NMR, optical spectroscopy and their singlet oxygen quantum yields (Φ Δ ) were determined. New BODIPYs possessing good Φ Δ (>0.5 in toluene) were employed as photosensitisers for holographic recording in a recently developed cellulose acetate-polyethylene glycol (CA-PEG) based photopolymer. 5 The key advantages of the CA-PEG based photopolymer are 1) water resistance and 2) suitability for use as a self- standing hologram, as the material can easily be peeled off the substrate. CA-PEG based photopolymer contains acrylamide as a monomer and N,N -methylenebisacrylamide as a cross-linker which are polymerised under exposure to light (Fig. 2)
Figure 2: Reaction scheme for polymerisation of acrylamide and N,N- methylenebisacylamide.
A photoinitiating system consisting of a free-radical generator, N -phenylglycine (NPG), and a BODIPY photoinitiator was used to trigger the polymerisation reaction. As a result of holographic recording in transmission mode with the spatial frequency of 800 lines/mm, diffraction efficiency of up to 56% was obtained for the CA- PEG photopolymer sensitised with new diester-BODIPYs, compared to 22% for the one sensitised with the reference I 2 BDP. Thus, the new BODIPYs demonstrated the ability to initiate the free radical polymerisation of the monomers under 532 nm irradiation leading to the formation of a volume transmission grating. References 1. A. Loudet and K. Burgess, Chem. Rev. , 2007, 107 , 4891–4932.
2. J. Zhao, K. Xu, W. Yang, Z. Wang and F. Zhong, Chem. Soc. Rev. , 2015, 44 , 8904–8939. 3. L. Huang, X. Cui, B. Therrien and J. Zhao, Chem. – A Eur. J., 2015, 19 , 17472-17482. 4. M. A. Filatov, Org. Biomol. Chem. , 2020, 18 , 10–27. 5. Sabad-E-gul, J. Cassidy and I. Naydenova, Photonics , 2021, 8 , 329.
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