New High‐Power LEDs Open Photochemistry for Near‐Infrared‐Sensitized Radical and Cationic Photopolymerization

Abstract

Cyanines derived from heptamethines were investigated in combination with iodonium salts as initiators of the radical polymerization of tripropylene glycol diacrylate and epoxides derived from bisphenol‐A‐diglycidylether. A new near‐infrared (NIR) LED prototype emitting at 805 nm with an exposure intensity of 1.2 W cm−2 facilitated initiation of both radical and cationic polymerization using sensitizers derived from cyanines. This new light‐emitting device has brought new insight into the photochemistry of cyanines with the general structure 1 because a combination of photonic and thermal processes strongly influences reaction pathways. In particular, cationic cyanines comprising a cyclopentene moiety and diphenylamino group in the center initiated the cationic polymerization of epoxides. Selective oxidation of this unit explains why specifically these derivatives may function as initiators for cationic polymerization. In contrast, when the diphenylamino group was replaced by a barbital group at the meso‐position cationic polymerization of epoxides was not initiated.

Photochemistry with Cyanines in the Near Infrared: A Step to Chemistry 4.0 Technologies

“…High‐intensity NIR LEDs emitting at 805 nm depict an alternative radiation source in the NIR. Recently, cyanines derived heptamethines, comprising indolenine as terminal group, demonstrated the feasible generation of radicals and the first cationic photopolymerization . The LED provided a power density of 1.2 W cm −2 from 3.5 cm.…”

“…The NIR radiation absorbed leads to both the release of heat and initiation of photochemical reactions as a result of non‐radiative deactivation and photoinduced electron transfer (PET), respectively . Particular generation of heat by light absorption overcomes the necessary internal activation barrier of PET in cyanine‐based systems . A main advantage is the handling of such NIR sensitive systems at ambient light conditions.…”

“…Bridging of R 5 and R 5 ′ in A1 led to the examples 1 and 5 . NIR exposure in the presence of 13 resulted in oxidation of the central moiety by PET and yielded a photoproduct exhibiting a hypsochromic shift of about 100 nm, Scheme . This might surprise since 11 possesses an additional double bond in the conjugated backbone compared to 10 .…”

“…It demonstrates the necessity to introduce additional energy such as heat also under conditions in which Δ G et <0. NIR‐sensitive systems comprising sensitizers selected from either A1 , A2 or A3 and an acceptor ( 13 or 14 ) possess Δ G et values being either slightly positive, neutral or slightly negative; that is +/−0.5 eV . Outer sphere (dielectric constant of the surrounding) and inner sphere coordinates contribute to λ .…”

“…On the other hand, positively charged sensitizers ( 3 – 6 ) showed no reactivity with the aforementioned low intensity LED . Switching the LED source to a device providing significant higher exposure intensity resulted in a remarkable reactivity of even positively charged sensitizers . This helped the system to travel over an internal activation barrier.…”