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.

Rational Selection of Cyanines to Generate Conjugate Acid and Free Radicals for Photopolymerization upon Exposure at 860 nm

“…Preliminary studies indicated the possibility to overcome internal activation barrier of systems comprising 3 b and 3 c in combination with the cation of 5 [16, 38] and a high‐intensity NIR‐LED. Structures 1 , 2 , 3 a , and 4 appear new in this application field.…”

“…However, this differs in the case of a real‐time FTIR setup where heat formed in the reaction does not efficiently leave the system, rather resulting in conditions that reside between an adiabatic and isothermal system in the time frame of the reaction. This has often facilitated NIR‐sensitized radical photopolymerization needing both photons and heat to proceed PET according to an activated scheme [5, 16, 37, 38] …”

“…Big efforts have been made in both radical [5, 16, 37, 38, 49] and cationic [5, 16, 37, 49, 50] polymerization using NIR‐LEDs. Photochemical generation of initiating radicals and conjugate acid based on activated PET explain the use of strong emissive sources such as NIR lasers [18, 40, 41, 44, 47] or more sophisticated high‐power NIR‐LEDs [5, 16, 37, 38, 49, 50] . Heptamethines comprising either an indolium or benzoindolium moiety fit well in this scheme [5] .…”

Methods to Evaluate Near‐Infrared Photoinitiating Systems for Photopolymerisation Reactions Assisted By Upconversion Materials

“…Compared with a limited number of NIR PIs, lanthanide‐doped upconversion materials (UCms) in UCAP can convert NIR light into ultraviolet‐visible light as internally secondary light source to induce available PIs initiating photopolymerization [2b,3] . Since there is unnecessary to design and synthesize extra initiators, UCAP possesses excellent versatility and effectiveness in deep photopolymerization, living/controlled radical polymerization, chiral macromolecules synthesis, biomedical materials and 3D printing [1e,4] . However, the complicated and vague photoinitiation process of UCAP results in the absence of evaluations, which makes it incapable to accurately select or quickly develop an optimal photoinitiation systems to absorb the adequate amount of photons for polymerization.…”

Coumarin Ketoxime Ester with Electron-Donating Substituents as Photoinitiators and Photosensitizers for Photopolymerization upon UV-Vis LED Irradiation

“…Oxime esters are efficient photoinitiators because the N-O bond of the structure can be used to generate free radicals quickly and efficiently upon light exposure [ 28 , 29 , 30 ]. Oxime esters with long wavelength absorption have attracted general interest [ 20 , 31 , 32 , 33 , 34 , 35 ].…”