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.

Tackling the impossible: Photoinduced electron transfer between an NIR sensitizer and oxime esters results in unexpected initiation of radical photopolymerization using cyanines as sensitizers. This strategy facilitates unexpected chemistry with environmentally friendly devices.

Introduction

Oxime ester derivatives (R−CR′=N−OR′′) were developed as photoinitiators to generate initiating radicals for radical photopolymerization.1 Excitation typically results in cleavage of the N−OR bond, resulting in the generation of two radicals, that is, R−CR′=N^. and ^.OR′′. Both are electrophilic radicals, facilitating addition onto (meth)acrylic monomers for the initiation of radical polymerization. Very early it became clear that substantial research must be pursued to improve the performance, that is, to match the absorption with available radiation sources and to improve thermal stability.2 Consequently, efforts proceeded to improve the match of absorption by introduction of appropriate substitution patterns, resulting in a bathochromic shift of the absorption. The second point, that is, thermal stability, can still needs to be addressed. As a result, more research activities resulted in the commercialized oxime esters OXE‐01 and OXE‐02 (see Figure 1).3, 4, 5, 6 Photophysical investigations and time‐resolved spectroscopy helped to direct the development of such materials.7, 8

Figure 1.

Structures of the oxime esters used.

Recently, oxime esters comprising a coumarin structure, exhibiting a significant red bathochromic shift of absorption, enabled their use in combination with industrial UV‐Blue LED devices.9 This finding has received attention since new regulations require the use of such light sources in new green and energy saving technologies.10 Thus, the improvements in thermal stability and bathochromic shifts of the absorption make them reliable photoinitiators for photopolymers in LED applications,11 which also have attracted attention in related two‐photon excitations.12

Besides the aforementioned improvements with respect to absorption and thermal stability, these derivatives also possess a significantly better compatibility with many environments. In particular, their use as coinitiators13, 14 in digital imaging applications15, 16, 17, 18 as alternatives to iodonium salts19 has many benefits. Iodonium salts and their tetraphenyl borates exhibit cytotoxicity,20 however such issues are not known for OXE‐01 and OXE‐02. Iodonium salts also exhibit a certain thermal instability in combination with NIR sensitizers.21 In addition, they also exchange the anion with the NIR sensitizer in the case that these might be different, resulting in undesired crystallization of the NIR sensitizer in the light‐sensitive layer. Thus, it is recommended that the same anion shall be used for both the cyanine and the iodonium cation together.19 This aspect might explain why these derivatives can be found in printing applications,13, 14 applicable for development of on‐press technologies as well.16, 17 These technologies, based on excitation in the near‐infrared at 830 nm, apply diode lasers for digital imaging.16, 17, 18 Fundamental knowledge is more or less in its infancy since most of the technological developments occurred in industry.

Recently, a review showed the potential of photopolymerization for many applications.22 Complementary studies about 3D and 4D printing extended the potential of photopolymerization.23 Furthermore, NIR excitation based on up‐conversion nanoparticles enabled blue and UV light photoinitiators,24, 25 while excitation at 980 nm resulted in a nonlinear four‐photon absorption process in the generation of UV light.26 An extensive review discussed the potential of free‐radical and living radical polymerization.27 This potential is based mainly on UV and visible‐light excitation, but NIR excitation did was not addressed. Recently, an approach based on a photocatalytic system using Cu^II in combination with a cyanine comprising a barbiturate group also demonstrated applications in NIR‐sensitized photopolymerization.28 Excitation based on NIR radiation has the advantage of i) deeper penetration into matter because of a significantly lower scattering coefficient,29 ii) embedding additives with UV absorption,10, 30, 31, 32 and iii) the release of heat applicable for photothermal treatment of cancer cells.33 In particular, the introduction of new strongly emitting NIR‐LED devices has brought about progress in this field.34 NIR light enabled photoinitiating systems comprising cyanines as the sensitizer and iodonium salts as the coinitiator work in free‐radical polymerization systems.30 Photoinduced‐electron transfer (PET) in most of these systems are based on an internal barrier, that is, low intensity NIR sources, and did not lead to initiation of photopolymerization.21 Interestingly, a combination of NIR photoinitiator systems with special additives also resulted in systems showing a certain tolerance to oxygen inhibition.35

Cyanines with absorption in the NIR convert most of the absorbed light energy, as shown by the fluorescence quantum yield residing between 5 and 20 % for systems requiring 800 nm excitation.15, 21, 28, 36 This aspect has also received interest for industrial applications enabling thermally driven processes such as curing of blocked isocyanates10 or melting and crosslinking of powder coatings in just one step.37 This success has potential for replacing energy wasting furnace‐driven processes by more ecologically LED techniques to design a system generating heat on demand.31, 32 Although the development of LEDs has brought new impetus in this field,36 there still exists a demand for more alternative NIR‐LED devices. This research introduces for the first time a new device emitting at 860 nm with an intensity of 1.5 W cm⁻². Nowadays, more research is focused on the design of alternative devices whose emission also covers the region between 900 and 1000 nm. This focus will help to substitute lasers emitting at 980 nm with more user‐friendly LED devices.

The NIR‐sensitized photoinitiated electron transfer (PET) mechanism has not been well explored based on fundamental research. The free energy of photoinduced electron transfer (ΔG _el) resides at either around zero eV or is slightly negative for many cationic absorbers. The recently introduced model of internal activation of PET has brought new light in such systems.34, 36 It resulted, in the case of iodonium salts, in bond cleavage of the cyanine chain, reduction of the iodonium salt, and therefore release of initiating radicals.36, 38 Exposure of the absorber without any second component, except oxygen, also resulted in similar reaction products39, 40, 41, 42 compared to systems explored with the NIR sensitizer and iodonium salt.38 Singlet oxygen might have a major function in the cleavage of the polymethine chain of the cyanine, while H₂O₂ and O₂ ^−. were found to have minor reactivity with cyanines,42 although previous research showed that O₂ ^−. played a major role in the photodegradation of cyanines.43 However, our previous experiments with a singlet oxygen microscope showed that almost no singlet oxygen was found, as already mentioned in Reference [38] (compare in this Ref. note 6). There were also synthesized many different structural motifs to explain the reactivity at distinct positions in the cyanine.44

Shown herein for the first time is the NIR‐sensitized decomposition of oxime esters, resulting in photoinitiation of radical polymerization. New high‐power NIR‐LEDs facilitate the decomposition of oxime esters. The question as to whether decomposition follows an entire photonic mechanism or thermal decomposition might play a certain function as well. Different structural motifs of the cyanines were chosen to determine PET leading either to chain breaking or changes of the polymethine structure with hypsochromically shifted absorption.13, 14, 28, 36

Results and Discussion

Figure 1 depicts the structural motifs of the oxime esters studied. While OXE‐01 and OXE‐02 represent state of the art in this field, COXE‐15 45 and BTCF‐OXE depict new structures. COXE‐15 comprises a coumarin exhibiting hypsochromic‐shifted absorption compared to alternative coumarins.9 This feature improves yellow safe light stability. Moreover, BTCF‐OXE contains a new structural motif resulting in an improved thermal stability and comparable reactivity compared to OXE‐01 and OXE‐02. Thermal treatment in tri(propylene glycol) diacrylate (TPGDA) resulted in an increase of the temperature for thermal initiation, T _i (Table 1). This temperature was below 70 °C for both OXE‐01 and OXE‐02, while this quantity should only slightly increase for COXE‐15. Surprisingly, T _i significantly increased in the case of the thiophene comprising the initiator BTCF‐OXE, with an initial temperature for the start of thermal polymerization for TPGDA being close to that for the neat monomer without any other additive.21 Thus, one obtains the following stability of the oxime esters investigated:

Table 1.

Results obtained for optical data (λ _max: absorption maximum, ϵ: extinction coefficient), electrochemical data (E _ox: oxidation potential, E _red: reduction potential), initial temperature of intrinsic polymerization of TPGDA comprising NIR‐sensitizer and oxime ester OXE‐01 T _i applying a gradient heating program up to 200 °C with a heating rate of 10 K min⁻¹,21 and final conversion x _∞ obtained for photopolymerization of TPGDA followed by FTIR spectroscopy.

Sens	λ max [nm][b]	ϵ max [m −1 cm−1]	ϵ(805 nm) [m −1 cm−1]	ϵ(860 nm) [m −1 cm−1]	ϵ(395 nm) [m −1 cm−1][c]	E ox [V][c]	E red [V][c]	T i [°C][d]	x ∞ (805 nm) [%][e]	x ∞ (870 nm) [%][f]	x ∞ (395 nm) [%][g]
TR [a]	992	2.23×105	–	–		0.66	−0.34		0	0
1 bX1	820	1.22×105	1.05×105	1.71×104		0.78	−0.46	117	–[h]	60[i]
2 bX1	858	2.09×105	7.23×104	2.08×105		0.73	−0.27	142	0	0
2 bX6	[h]	[h]				[h]	[h]		0	0
3 bαZ4X1	792	2.41×105	1.94×105	3.37×103		0.57	−0.60	80	90	88
3 bαZ1X4	786	3.23×105	1.92×105	1.87×103		0.58	−0.56	107	81	69
4 aαZ2	757	2.62×105	7.89×103	<0.1		0.41	−0.86	92	78	68
4 cβZ3X7	813	1.62×105	1.25×105	1.01×104		0.77	−0.38	138	0	38
5 dZ4X5	835	2.16×105	1.52×105	1.27×105		0.54	−0.57	83	–	82[i]
6 cZ3X3	844	2.39×105	9.92×104	1.32×105		0.56	−0.57	80	0	90
6 aZ2	791	3.03×105	1.59×105	6.2×102		0.48	−0.97	91	91	93
OXE‐01	326	2.95×104	0	0	6.43×102	1.72	−1.36	65[k]			71
OXE‐02	337	2.30×104	0	0	2.48×102	1.51	−1.26	68[k]			70
COXE‐15	300	1.65×104	0	0	4.00×102	1.59	−1.30	78[k]			74
BTCF‐OXE	356	2.28×104	0	0	3.76×102	1.70	−1.26	173[k]			81

[a] From Ref. 21. [b] In methanol. [c] In acetonitrile. [d] Onset temperature of the polymerization of TPGDA. [e] Final conversion of photopolymerization of TPGDA in the RT‐FTIR measurement with 805 nm NIR‐LED irradiation. [f] Final conversion of photopolymerization of TPGDA in the RT‐FTIR measurement with 870 nm NIR‐LED irradiation. [g] Final conversion of photopolymerization of TPGDA for the Photo‐DSC measurement with 395 nm UV‐LED irradiation. [h] Not determined because of structural similarity with 2 bX₁. [i] Conversion of photopolymerization of TPGDA after 10 min with 860 nm NIR‐LED irradiation. [j] Not determined because of structural similarity with 3 bαZ₄X₁. [k] Onset temperature of the polymerization of TPGDA without NIR sensitizer.

BTCF‐OXE≫>COE‐15>OXE‐1≈OXE‐2.

Further addition of a sensitizer to these mixtures resulted in a general improvement of thermal stability. T _i increased in general by about 50 K after adding the NIR sensitizers 1 bX₁, 2 bX₁, 2 bX₆, 3 bαZ₁X₄, or 4 cβZ₃X₇ while 3 bαZ₄X₁, 4 aαZ₂, 5 dZ₄X₅, 6 cZ₃X₃, or 6 aZ₂ resulted in a smaller increase (Table 1). Scheme 1 shows their structures. Nevertheless, the overall thermal stability increased after addition of the NIR sensitizer, ruling out the thermal instability of OXE as the only source for the reactivity observed upon NIR exposure. The cyanines 1 and 2, which comprise no bridged structure, exhibited the best stability when combined with OXE‐01.

Scheme 1.

Structures of the cyanines 1–6 carrying different substituents, R₁, R₂, and Z.

Photoinitiated polymerization by LED exposure at 395 nm resulted in final double‐bond conversion (x _∞) of about 70 % in for OXE‐1, OXE‐2, and COXE‐15. Thus, the structure of these three derivatives were not really affected by x _∞ (compare the conversion—time profiles for the four derivatives of Figure 1 in Figure SI1 in the Supporting Information). The derivative BTCF‐OXE was the only representative showing a final conversion greater than 80 %, although its initial reactivity was lower compared to the other three oxime ester compounds. Thus, the lower reactivity resulted in a higher overall final double‐bond conversion because of higher selectivity.

The initiators OXE‐01, OXE‐02, and COXE‐15 were identified to initiate polymerization below 80 °C, bringing them to purposes requesting heat to generate radicals. Therefore, these initiators were tested in TPGDA comprising the cyanine TR, as shown in Figure 2, which nearly quantitatively generates heat upon excitation at 980 nm. Such a radiation source facilitated initiation of thermally driven physical and chemical processes such as melting of powder coatings37 or the reaction of blocked isocyanates.10 There was no polymerization activity although the temperature increased to 170 °C as monitored by a thermal sensitive camera (for details see the Supporting Information). This increase of temperature did not result in thermal initiation of polymerization of TPGDA comprising TR and OXE‐01. Thus, a mechanism requiring mainly heat to start TPGDA polymerization can be seen of minor importance. Previous investigations showed that TR contributes to the generation of heat.10, 37

Figure 2.

Structure of the sensitizer TR used for laser exposure at 980 nm.

This result shifted the strategy and the cyanines in Scheme 1 were selected for LED excitation at 805 nm, 860 nm, and 870 nm (new prototype). These devices exhibited intensities greater than 1 W cm⁻². Some of these cyanines were already found to work as sensitizers to generate initiating radicals for photopolymerization using an iodonium salt as the coinitiator in combination with a strongly emitting LED device.36 The oxime ester would function as the coinitiator.

According to the redox potentials shown in Table 1, the oxime esters may react according to a photoinitiated electron transfer (PET) based on a sensitized oxidation mechanism. A calculation of the reaction enthalpy of PET,46 $\Delta G_{\mathrm{e}\mathrm{t}}$ , yields an overall endothermic scenario requiring thermal energy for the reaction ($\Delta G_{\mathrm{e}\mathrm{t}}$ in eV: 6 aZ₂=0.17; 3 bαZ₄X₁=0.27; 5 dZ₄X₅=0.4). Furthermore, NIR‐photopolymers comprising a cyanine and an iodonium salt possess an inner activation barrier ($\Delta G_{\mathrm{i}\mathrm{n}}^{\ne }$ ), which can exceed $\Delta G_{\mathrm{e}\mathrm{t}}$ by several dozen kJ mol⁻¹,36 that is, $\Delta G_{\mathrm{i}\mathrm{n}}^{\ne }>\Delta G_{\mathrm{e}\mathrm{l}}$ . These energetic conditions give such systems a certain inner threshold. The reaction only occurred in combination with a strong radiation source generating initiating radicals for photopolymerization, thus explaining their use for imaging purposes with a laser excitation at 830 nm.13, 14, 16 In this contribution, the LED devices emitting at either 805 nm, 860 nm, or 870 nm result in efficient generation of excited density whose deactivation and therefore also internal release of heat helps to overcome $\Delta G_{\mathrm{i}\mathrm{n}}^{\ne }$ . To make it clear again, these systems work only in combination with strong emission sources. The use of such high‐intensity LED‐devices directs the system toward photopolymerization while low‐intensity LEDs do not facilitate photopolymerization.

In this study, the sensitizer structures 1–6 generate initiating radicals to photopolymerize TPGDA by using strong irradiation sources. They comprise unbridged (1,2) and bridged (3–6) methine chains. The five‐membered rings in the centers of 3 and 5 keep the conjugated system in the plain while the structure at the center of 4 and 6 distorts the planarity, thus improving the solubility in certain environments.38 The compounds 1–6 comprise, depending on substitution pattern, different alkyl groups (R₁). Substitution at the meso‐position also varies with change of the Z group. Furthermore, cyanine carries an anion X⁻ if it possesses a positive charge. Data compiled in Table 1 exhibit a larger batchochromic shift of absorption for NIR sensitizers comprising a benzo[c,d]indolium structure (2), in comparison with those having the same number of π electrons in the methine chain as for the benzo[e]indolium derivatives 3.28 This aspect might have an impact on the color of the photoproducts. Oxidation of long methine chains often results in colored photoproducts.38, 40, 41, 42 NIR‐sensitizers with shorter chains should therefore exhibit less colorization upon exposure and in the presence of a coinitiator.

Embedding of the sensitizer 6 aZ₂ in TPGDA together with the oxime esters shown in Figure 1 showed polymerization activity for OXE‐01, OXE‐2, and COXE‐15 upon using a strong emitting device at 805 nm, whereas BTCF‐OXE showed no activity. The temperature increased to over 100 °C (Figure 3). Since the thermal equilibrium is attained between the surroundings and the substrate, the sample temperature remains between 100 and 110 °C (Figure 3). The change to a LED device with emission at 860 nm resulted in a slight change in pattern. At this wavelength, the absorber possesses a lower absorption. Therefore, the excitation light can penetrate deeper into the sample, resulting in reaction with the oxime ester and therefore also loss of absorption, explaining the temperature decrease in the case of 6 aZ₂. A photoreaction connected with bleaching results in the consumption of available sensitizer molecules, generating heat by internal conversion. The higher the concentration of the excited absorber molecules in the respective excitation volume, the higher the temperature generated. Exposure experiments followed by UV‐VIS spectroscopy indicated a decrease of absorption. The lower the absorption of the NIR sensitizer, the lower the temperature generated (see Figure SI9).

Figure 3.

The temperature (T/°C) generated by sensitizers (0.05 wt %) in the samples (thickness: 160 μm) comprising the monomer TPGDA and the oxime ester OXE‐01 (2Wt%). a) The samples were irradiated with the LED device from Phoseon at 805 nm having an intensity of 1.2 W cm⁻². b) The samples were irradiated with the LED device from Easytech at 860 nm exhibiting an intensity of 1.5 W cm⁻².

Figure 4 shows conversion–time profiles obtained for different NIR‐sensitizers (Sens) in combination with the oxime esters of Figure 1. OXE‐01 showed the best performance in combination with 6αZ₂ (Figure 4 a) while OXE‐2, COXE‐15 and BTCF‐OXE showed lower reactivity. PET may explain the observed reactivity although the enthalpy (ΔG _el) of PET is positive. This behavior is possible under some circumstances.43, 46, 47 The fact that OXE‐01 showed the best reactivity was surprising since the remaining oxime esters OXE‐2, COXE‐15, and BTCF‐OXE exhibited similar reduction potentials but different thermal stability in TPGDA. Thus, heat generated by the NIR‐sensitizer can also facilitate endothermal events because it contributes to a general increase of entropy. This additional energy released also helps the system to react so that Sens + OXE resulted in formation of Sens ^+. and radicals. This reaction changes the number of molecules in the reaction and therefore the entropy of the system increases. The system investigated also possesses an internal activation energy differing for each type of sensitizer and OXE, explaining the different reactivities. The reaction between the sensitizer and the oxime ester also follows a bimolecular reaction as shown by the concentration variation of OXE‐1 in Figure 4 b. In addition, the structure of the sensitizer also results in a different reactivity (Figure 4 c). There was also no clear correlation between electrochemical data and reactivity with the oxime ester where 6 aZ₂ showed the best reactivity and 4 aαZ₂ showed the lowest efficiency. In this example, benzo[e]indolium derivatives exhibited higher reactivity compared to other indolium derivatives.

Figure 4.

a) Radical photopolymerization of TPGDA with the sensitizer 6 aZ₂ (0.05 wt %) and different oxime esters (2 wt %). b) Radical photopolymerization of TPGDA with the sensitizer 6 cZ₃X₃ (0.05 wt %) and OXE‐01, at different OXE‐01 concentrations (d). c) Radical photopolymerization of TPGDA with different sensitizers (0.05 wt %) and OXE‐01 (2 wt %). As excitation source served a NIR LED source emitting at 870 nm with an intensity of 1.2 W cm⁻². Data were taken with a Bruker Vertex 70 in ATR‐mode in real time (see the Supporting Information for more details).

These findings led to the model of NIR‐sensitized decomposition of oxime esters shown in Scheme 2. It is based on a mechanism where the combination of heat and chemical reactivity between the sensitizer Sens and the oxime ester OXE results in successful bleaching of Sens and initiating radicals as shown for the polymerization example in Figure 4. A mechanism based on thermal decomposition may be of minor importance because this cannot explain the bleaching of Sens observed. The heat generated may lead to decomposition as shown in part (a), but the optical density should remain constant. Laser exposure with a system generating mainly heat showed inefficient reaction based on route (a). More reasonable appears a mechanism where PET occurs over an activation barrier and the fact that heat additionally generated by thermal deactivation of Sens additionally results in an increase of entropy. Exposure of Sens and OXE connects with the chemical reactivity resulting in formation of the respective reaction of the sensitizer as well. They were identified by reference materials13, 14 synthesized and by LC‐MS measurements.36, 38 This can be cleavage of the polymethine chain resulting in the products bearing as terminal carbonyl group. These photoproducts exhibit yellow/brownish color as approved by the increased absorption between 400–600 nm38 (Figure 5 a). Exposure of sensitizers comprising a five‐membered ring at the center of the cyanine resulted in photoproducts exhibiting a 100 nm hypsochromic‐shifted absorption although they carry an additional bond. Figures 5 b and c depict the respective structures of the cyanine cations. Thus, the color changes to blue as previously confirmed for the sensitizer comprising the diphenyl amino group using an iodonium salt as the coinitiator.36 It turns out that a general a cyanine structure comprising either 3 or 5 favors formation of reaction products carrying an additional bond (see also the Supporting Information).

Scheme 2.

Proposal of possible reactions contributing to formation of radicals by NIR‐sensitized activated photoinduced electron transfer.

Figure 5.

Change of UV/Vis‐NIR spectra of 6 aZ₂ (a), 3 bαZ₄X₁ (a), and 3 bαZ₁X₄ (a) upon exposure to NIR light at 805 nm applying an intensity of 1.2 W cm⁻² and with OXE‐01 as coinitiator in PEGMA. The resulting polymer from PEGMA after LED exposure was dissolved in methanol and the measurement was done using a 1 cm×1 cm cuvette. Figures 5 b and c depict the absorption of the respective oxidized product (blue). Exposure was pursued under nitrogen. For a sake of simplicity, only the cyanine structure was included.

Figure 6 shows the kinetics of 3 bαZ₄X₁ and 3 bαZ₁X₄. Because there was no total absorption, data obtained followed first‐order kinetics. The sensitizer comprising the phenyl ring at the meso‐position (3 bαZ₄X₁) exhibited a higher reactivity in the matrix, laurylmethacrylate (LMA), while the the diphenylamino compound 3 bαZ₁X₄ showed lower reactivity upon exposure. This reactivity can differ by a change in the solvent, and also when oxygen is available during exposure (see the Supporting Information for more details).

Figure 6.

The kinetics of 3 bαZ₄X₁ (blue) and 3 bαZ₁X₄ (red). Samples comprising sensitizers (3.5×10⁻⁶ mol L⁻¹), OXE‐01 (2.0×10⁻⁵ mol L⁻¹), and LMA were irradiated with the 805 nm NIR LED device (1.2 W cm⁻²) at different exposure times(for details see the Supporting Information).

Quantum‐chemical calculations can explain the distinct electron distributions for 3 dαZ₁X₄ and its reaction product 3 dαZ₁X₄‐ox (Figure 7). This structure shows differences in the HOMO, where the oxidized product has a structure in the middle that is comparable with that of aromatic structures, an unsaturated moiety as with a cyanine (Figure 7). Conjugated aromatics typically depict an absorption located at shorter wavelength compared to cyanines.48 Calculations also confirmed the trend found in the experiments that reaction products exhibit lower absorption wavelength and absorption coefficient.

Figure 7.

Electron density in the HOMO and LUMO of 3 dαZ₁X₄ 3 dαZ₁X₄‐ox obtained after a DFT calculation based on the B3LYP//6‐31G* method implemented in Spartan 16 to optimize the ground state.

Conclusion

The results obtained showed NIR‐sensitized decomposition based on PET occurred. Heat released by nonradiative decomposition of the NIR‐sensitizer plays a major function in this system as it results in an increase of entropy, promoting the occurrence of reactions under endothermic conditions and helping such a system to work although it possesses an internal activation barrier. These are two different points—thermodynamics and kinetics—explaining together the overall observed reactivity. This finding might affect the development of future technologies since their use requests strong emissive radiation sources in connection with systems having a certain daylight stability. This aspect makes easier handling possible under certain conditions in practice.

The blue color formed exhibits a further interesting feature. Thus, the methine chain remains in the oxidation process with no formation of reaction products exhibiting nucleophilic properties. This feature brings these materials also to systems facilitating cationic photopolymerization. Furthermore, the color formed might depict a further feature for systems requiring read‐only in the exposed areas, which is the case in fullly automized CtP systems operating in on‐press procedures in modern printing shops.

Future work might focus on the development of systems exhibiting a better compatibility with the environment. Introduction of the FAP‐anion did not bring the expected effort. Potential exists to vary the alkyl substituent at the indolium ring since earlier investigations with similar materials showed promising results.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Y. Pang, S. Fan, Q. Wang, D. Oprych, A. Feilen, K. Reiner, D. Keil, Y. L. Slominsky, S. Popov, Y. Zou, B. Strehmel, Angew. Chem. Int. Ed. 2020, 59, 11440.