ABSTRACT
Different cyanines absorbing in the NIR between 750 and 930â nm were applied to study the efficiency of both radical and cationic polymerization in combination with diaryliodonium salt. Variation of the connecting methine chain and structure of the terminal indolium moiety provided a deeper insight in the structure of the cyanine NIR-sensitizer and the efficiency to generate initiating radicals and conjugate acid. Photophysical studies were pursued by fluorescence spectroscopy providing a deeper understanding regarding the lifetime of the excited state and contribution of nonradiative deactivation resulting in generation of additional heat in the polymerization process. Furthermore, electrochemical experiments demonstrated connection to oxidation and reduction capability as influenced by the structural pattern of the sensitizer. LC-MS measurements provided a deeper pattern about the photoproducts formed. A nonamethine-based cyanine showed the best performance regarding bleaching in combination with an iodonium salt at 860â nm.
ABSTRACT
Cyanines comprising either a benzo[e]- or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR-LED prototypes emitting at 805â nm, 860â nm, or 870â nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso-position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.
