This contribution discusses photoinitiated crosslinking of multifunctional acrylic esters in polymeric binders based on digital imaging using the Computer to Plate (CtP) technology applying laser exposure at 830 nm in the near infrared (NIR). All coating components (NIR photoinitiator system, monomers, binder, adhesion promoter, contrast dye, and oxygen barrier material) were applied as thin double layer film (1-2 μm dry film thickness) on Al-plates with an anodized surface. Materials exposed exhibit a sensitivity between 30-200 mJ/cm 2 depending on the NIR-photoinitiator composition. This was processed in a weak aqueous alkaline bath to obtain the image. Generation of initiating radicals occurs by electron transfer from the excited state of the NIR-sensitizer to the radical generator; that is either an onium salt or neutral electron deficient compound. Redox potentials were determined for the NIR-sensitizers and the radical generator. These data allow a rough estimate regarding the free energy of electron transfer of the excited state of the sensitizer and radical generator in NIR-photosensitive imaging material. Photoinduced electron transfer plays a major function to generate initiating radicals by a sensitized mechanism but thermal events also influence sensitivity of the coating. Particular the non-radiative deactivation of the NIR-sensitizer possesses a major function to release selectively the heat. Absorption data of many NIR-sensitizers used exhibit molar extinction coefficient of more than 200 000 M −1 cm −1 .

Chlorine substituted hexaarylbisimidazole (o-Cl-HABI) efficiently initiates radical polymerization of multifunctional acrylic esters in the presence of a heterocyclic mercapto compound if the latter can form its tautomeric thione. Exposure of o-Cl-HABI results in lophyl radicals, which efficiently add to the thione in the first step while the second step releases a highly reactive thiyl radical from this intermediate. LC-MS and CID-MS measurements support this reaction scheme. Furthermore, photo-DSC experiments applying UV light between 320 and 380 nm showed that mercaptotriazole and phenylmercaptotriazole exhibited the best reactivity in the monomer 1,6-hexanediol diacrylate (HDDA) while alkyl substituted mercaptotriazoles showed less reactivity. Change of the triazole heterocycle by mercaptoimidazole resulted in a significant decrease of photoinitiation efficiency. This heterocycle does not form the corresponding thione in HDDA as shown by NMR measurements. Replacement of mercaptotriazole by an alkylthiol leads to a system showing the lowest photoinitiation efficiency in this series. Formation of thione structure in the case of heterocyclic mercapto compounds may cause higher reactivity of the heterocyclic mercapto compounds with the lophyl radical in the monomer chosen.

A series of new acidochromic 9‐(4‐dialkylaminostyryl)‐acridines (1) were synthesized and characterized by means of elemental analysis, m.p., mass and 1H‐NMR‐spectra. The colour change in the presence of perchloric acid and the pKa was investigated. Their photosensitizing ability was tested by oxygen consumption measurements.

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