NIR‐Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free‐Radical Photopolymerization

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 805 nm, 860 nm, or 870 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.

Photoinitiation mechanisms and photogelation kinetics of blue light induced polymerization of acrylamide with bicomponent photoinitiators

“…[1][2][3] There has been a trend of replacing ultraviolet (UV) with visible and near-infrared light. [4][5][6] Longer wavelengths have greater depth of penetration into both biological and synthetic materials, making them particularly attractive for biomedical applications. [7][8] Visible photopolymerization is greatly facilitated by the invention of blue LED that provides clean and efficient energy transformation.…”

“…Photopolymerization is a facile approach to manufacturing of polymer materials 1–3 . There has been a trend of replacing ultraviolet (UV) with visible and near‐infrared light 4–6 . Longer wavelengths have greater depth of penetration into both biological and synthetic materials, making them particularly attractive for biomedical applications 7–8 .…”

NIR Organic Dyes as Innovative Tools for Reprocessing/Recycling of Plastics: Benefits of the Photothermal Activation in the Near‐Infrared Range

“…NIR wavelengths are known to enable a greater light penetration into the photocurable resin and to allow a better light‐to‐heat conversion than the UV–visible wavelengths. [ 21,22 ] The mechanisms involved in the production of heat are already well‐described in the literature, especially in the medical field through the photothermal therapy. [ 23–25 ] These wavelengths are also used to initiate crosslinking processes using the heat generation.…”

NIR‐sensibilisierte kationische und hybride radikalische/kationische Polymerisation und Vernetzung

“…Die erste Publikation, welche über die erfolgreiche kationische Vernetzung von Epoxiden in diesem Gebiet berichtete, erschien 2019, wobei ein neues NIR‐LED‐Gerät mit hoher Strahlungsintensität den entsprechenden Fortschritt in dieses Gebiet brachte [9] . Dieses Gerät war erforderlich, um die interne Aktivierungsbarriere des kationischen NIR‐Sensibilisators in Kombination mit einem Iodoniumsalz zu überwinden [26, 27] . Ein spezielles Substitutionsmuster in der zentralen Position des Sensibilisators verhinderte die Bildung nukleophiler Photoprodukte, welche typischerweise die kationische Polymerisation von Epoxiden inhibieren [9, 50] .…”

“…Solche Untersuchungen zur kationischen Polymerisation können erweitert werden, um die Bildung interpenetrierender Polymernetzwerke zu untersuchen, was unter Verwendung von UV‐Bestrahlung durchaus fehlschlagen kann [52, 53] . Die NIR‐sensibilisierte Polymerisation von hybriden Systemen basierend auf einem radikalischen und kationischen Mechanismus kann neue Einblicke in dieses Gebiet bringen, weil diese Systeme zusätzlich Wärme zur Verfügung stellen, welche durch interne Konversion des Sensibilisators generiert wurde [11, 26] . Das ist bei solchen Systemen durchaus hilfreich, um interne Diffusionsbarrieren zu überwinden.…”