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Substituents para-to-ortho positioning effect driving the photoreactivity of a dibenzothiophene-based oxalate series used as LEDs excitable free radical photoinitiators

Abstract

Four dibenzothiophene-based (DBT) methyl oxalates, synthesized through a one-step Friedel-Crafts acylation reaction, were developed as high-performance photoinitiators (PIs) for free radical polymerization of acrylate resins excitable using LED sources which emit in the near-UV and visible region (i.e. 365-425 nm). These type I PIs show good optical absorption properties, (ε365nm=1000~5300 M-1•cm-1). Their respective photophysical properties as well as the sequential photocleavage mechanism were investigated using a large set of theoretical and experimental methods. A relevant structure/reactivity relationship opening the way to enhance the PIs performances is highlighted. Indeed, a para-to-ortho positioning effect of the oxalate-based substituent both regulates the absorption and photocleavage abilities within this DBT series. Para-isomers consequently exhibit a photoinitiating reactivity whose efficiency is more than one order of magnitude higher than their ortho homologues. Associated to a H-donor alkylamine co-reactant, these photoinitiating efficiency can be even more amplified due to an additional reactional pathway that judiciously assigns a new role to the aryloyl radicals by-products. These whole results correlate the PIs photopolymerization performances with the photoinduced processes occurring in the excited states.

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Publication details

The article was received on 14 Jan 2019, accepted on 12 Feb 2019 and first published on 12 Feb 2019


Article type: Paper
DOI: 10.1039/C9PY00052F
Citation: Polym. Chem., 2019, Accepted Manuscript

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    Substituents para-to-ortho positioning effect driving the photoreactivity of a dibenzothiophene-based oxalate series used as LEDs excitable free radical photoinitiators

    H. Pan, S. Chen, M. Jin, J. Malval, D. Wan and F. Morlet-Savary, Polym. Chem., 2019, Accepted Manuscript , DOI: 10.1039/C9PY00052F

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