Synthesis, photochemistry and cross-linking of visibly sensitised photopolymers of PVA based on (E)-2-(4-Formylstyryl)-3,4-dimethylthiazolium Methylsulfate

Abstract

The synthesis and cross-linking of new, visibly sensitive high-speed water-soluble photoresists are reported. Attempts to sensitise our existing poly(vinyl alcohol)(PVA)-based photopolymer into the visible region with various additives at between 1 and 5 mol% have been unsuccessful. The additives may disrupt the formation or configuration of the dimeric aggregates previously found to be a prerequisite for cross-linking or have a screening effect absorbing some or all of the photonic energy, thereby preventing excitation of any aggregated chromophore groups.

Synthetic modification of the original (E)-2-(4-formylstyryl)-3,4-dimethylthiazolium methylsulfate produces chromophores which, when grafted onto PVA, generate photopolymers absorbing in the visible region. (E)-2-[4-(2-Ethylbutoxy)-styryl]-3-methylbenzothiazolium methylsulfate forms a photopolymer with λ_max at 400 nm; while (E)-2-[2-(2,2-diethoxyethoxy)-4-(N,N-diethylamino)styryl]-3,4-dimethylthiazolium methylsulfate forms a photopolymer with λ_max at 495 nm. Step-wedge analysis, UV absorption and FT-NMR spectroscopy indicate that the thiazolium and benzothiazolium photopolymers cross-link by a [2 + 2]cycloaddition reaction generating interchain cross-links via cyclobutane units. However, the amine photopolymer does not show any evidence of cross-linking, with the stencil being completely washed out during step-wedge analysis. Molecular modelling indicates that the diethoxyethoxy group prevents formation of the aggregates normally formed by electrostatic attractions.

This theoretical prediction is confirmed by fluorescence spectroscopy. In high-concentration solutions the thiazolium and benzothiazolium models show significant emission bands relating to excimer or higher-order aggregate species. Upon dilution these bands are blue shifted to wavelengths comparable to those observed for photopolymer films. At very dilute concentrations they are lost and monomer bands appear while for thin films they are lost upon dimerisation. The amine photopolymer shows very weak emission bands unaffected upon exposure. This confirms that very few aggregates are formed, suggesting they are in non-reactive configurations.