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. 2023 May 30;145(23):12737–12744. doi: 10.1021/jacs.3c02832

Two-Photon Excitation Photoredox Catalysis Enabled Atom Transfer Radical Polymerization

Yu-Ying Yang 1, Pengfei Zhang 1, Nikos Hadjichristidis 1,*
PMCID: PMC10273230  PMID: 37253455

Abstract

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In recent years, metal-free photoredox-catalyzed atom transfer radical polymerization (O-ATRP) has gained wide attention because of its advantages (e.g., no metal contamination and mild reaction conditions). However, this traditional one-photon excitation catalysis has thermodynamic limits. Most photocatalysts cannot effectively reduce the initiators and drive the polymerization under visible light. Herein, we investigate the two-photon excitation-catalyzed O-ATRP, in which the catalyst can absorb two photons to accumulate energy. Compared to one-photon excitation catalysis, this method not only has distinct advantages in the controllability, reaction rate, and catalyst loading but also can chemically reduce the various initiators (e.g., aryl halides) to initiate the polymerization. Density functional theory (DFT) calculation reveals that the two-photon excitation process reached a higher energy end state with stronger reduced ability via a thermodynamically more stable intermediate. We believe that this work will provide a new strategy for photoredox-catalyzed O-ATRP.

Introductrion

Since the groundbreaking studies of atom transfer radical polymerization (ATRP) were reported,1−3 this polymerization has been proven one of the most efficient methods of controlled free radical polymerization.4−9 With the development of this polymerization, over the past decade, the photoredox ATRP has received fast-growing interest in the polymer community with the advantages of low-temperature operation, high response, and optical control.10−16 In earlier research, most photocatalysts (PCs) are complexes with a metal center, like Cu(I) or Ir(III) complex.10−13 Those PCs will inevitably contaminate the polymer and restrict its application.8,17,18 In recent years, some elegant reports have shown that some organics, like the polycyclic aromatic hydrocarbons,19,20 phenothiazines,21,22 dihydrophenazine,23,24 carbazoles,25 and dyes,26,27 have great potential applications in metal-free photoredox-catalyzed ATRP (also called O-ATRP: organocatalyzed ATRP) with controlled molecular weight and low polydispersity (Đ).

The mechanisms of O-ATRP are similar to the other photoredox catalyses, which depend on the ability of PCs, like metal complexes and organic dyes, to engage in single-electron-transfer (SET) processes (Figure 1a, using the oxidative quenching as the example) with an initiator upon photoexcitation.14,28 During the photoredox catalysis process, redox transformations of the excited state PC (PC*) can proceed either by oxidative or reductive quenching.14,28−30 For the oxidative quenching pathway, PC* is a reductant, reducing the alkyl halide initiator (R-X) by a single electron. The products of this SET process are the radical R·X– and the oxidized form of the PC (PC+). The PC+ is a strong oxidant that accepts an electron from the donor and goes back to the ground state, completing the photocatalytic cycle. Alternatively, in the reductive quenching pathway, PC* functions as an oxidant, accepting an electron from the donor to give the reduced radical anion (PC·–). This PC·– is a good reductant and can donate an electron to the initiator to afford the ground-state species PC.

Figure 1.

Figure 1

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(a) Single-electron-transfer process excited by one photon; (b) consecutive photoelectron transfer process excited by two photons; (c) process of biological photosynthesis; (d) two-photon excitation mechanism. The blue ovals represent the active species to reduce the initiator.

For the O-ATRP, some specific needs should be considered.14,31,32 On the one hand, the initiation of O-ATRP needs a strong reductant, and the polymerization can only drive when the reduction potential of PC* E1/2(PC+/PC*) or PC·– E1/2(PC·–/PC) is lower than −0.8 V vs the saturated calomel electrode (SCE).31 On the other hand, the PC should be excited by a low-energy photon (like the visible-light photon), as the high-energy photon may cause uncontrolled polymerization and other side reactions. However, the reduction ability of the PCs* or PC·– is positively correlated with the energy of the absorbed photon. Furthermore, there is inherent thermodynamic excitation-energy loss of PCs caused by internal conversion and intersystem crossing. All those limitations create challenges for O-ATRP, in which the range of redox potentials of the PCs* is rather narrow. Hence, there are not many alternative initiators for O-ATRP, and some functional molecules with a very negative reduction potential (for example, the fluorescent aryl halides) are usually not in the list. Currently, the functionalization of polymers is gaining increasing attention in chemistry and material communities.33 Therefore, overcoming the thermodynamic limits and introducing the functional end group directly will be very interesting.

With those problems in mind, we want to find a way to break those limitations of O-ATRP. It is widely known that, during photosynthesis, the water-oxidizing complex of photosystem II absorbs four photons from the low-frequency light to accumulate enough energy to generate oxygen from water (Figure S2).34 This multi-photon excitation may be a potentially valuable strategy for designing the PC with a strong ability in reduction. In 2014, the two-photon excitation photoredox catalysis in the reduction of aryl halides was reported.35 For the two-photon excitation process (Figure 1b), the PCs can absorb two (or more) photons to accumulate enough energy. In this way, the radical excited state (PC·–*) has a low reduction potential and functions as a strong reductant, even absorbing long-wavelength light.36,37 This new strategy can break the thermodynamic limitations of SET photoredox catalysis, which may broaden the range of initiators and have great potential in O-ATRP. Furthermore, it is very important to choose a suitable catalyst to perform the one-photon and two-photon catalysis and compare the two mechanisms.

Based on the above analysis, we tried to introduce the two-photon excitation photoredox catalysis in ATRP. Herein, we report our recent effort using the N,N′-bis(2,6-diisopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide (PTDI) as the PC to drive ATRP. PTDI is a commercially available fluorescent dye molecule, which is used extensively in electronic devices and fluorescence imaging. The photophysical process of PTDI has been studied clearly, and its secondary excitation is proven to form PC·–*.38−40 Previous studies have shown that PTDI and its derivatives have applications in photoredox catalysis.41,42 So, we used the PTDI as the photocatalyst in the polymerization of methyl methacrylate (MMA) using ethyl 2-bromopropionate (EBrP) as the initiator. First, we used the PTDI to catalyze ATRP by one-photon excitation. Then, based on our hypothesis and previous research in photo- and electrochemistry, we introduced the electron donor into the reaction system and achieved the two-photon photoredox catalysis ATRP. Unlike the mechanism of the one-photon excitation-catalyzed ATRP, in this mechanism, the PC, similar to the photoredox core (Mn4CaO5) in photosynthesis (Figures 1c and S2), can accumulate energy and becomes a strong reductant after absorbing two photons (Figure 1d). The aryl halides can be reduced into the radical to initiate the polymerization, and introduce the functional end group directly. Compared to the one-photon excitation catalysis of PTDI, two-photon excitation catalysis has distinct advantages in the efficiency of the initiation, molecular weight dispersity, and reaction rate. We also conducted density functional theory (DFT) calculation to further explore the reaction details of the two-photon catalytic process. The experimental and calculated results support the two-photon excitation catalysis mechanism with the presence of an electron donor.

Results and Discussion

We first conducted photoredox ATRP with traditional protocol, which is thought to be a one-photon excitation process. Irradiation of the N,N-dimethylformamide (DMF) solution of MMA, EBrP, and 100 ppm PTDI with a 10 W blue LED for 6 h afforded poly(methyl methacrylate) (PMMA) in 45.5% conversion (Table 1, entry 1). The resulting polymer has a high number-average molecular weight (Mn) of 47.7 kg mol–1 and a high dispersity Đ of 1.69. This one-photon excitation photoredox catalysis (SET process) also gave a low initiator efficiency (I*) of only 9.8%. As the electron donor was added, the polymerization became more controllable and the initiator efficiency was also increased (Table S1, entries S2–8). We evaluated the results in different solvents and electron donors (Table S1, entries S9–15). The polymerization in DMF with five equivalents of triethylamine (Et3N) offered the best result of this polymerization with the lowest dispersity (Đ = 1.22) and the highest initiator efficiency I* = 66.7%. The addition of Et3N can efficiently improve the conversion and controllability even in different monomers (Table S2). We next adjust the ratio of initiator and monomer to investigate the effects of initiator concentration on the polymerization (Table 1, entries 2–7). As the concentration of EBrP decreases, the conversion of MMA gradually goes down. The polymerization showed an efficient controllable result, in which Mn changes systematically with the change of initiator-to-monomer ratio. For the high Mn polymerization (entries 4–7), a high I* result (up to 91.8%) was achieved. When the concentration of PTDI decreases to 5 ppm (entry 9), after 6 h, a high conversion of 78.0% determined by 1H NMR with Mn = 17.1 kg mol–1 was observed, which indicates the extremely efficient polymerization with the presence of Et3N.

Table 1. Results of the Polymerization of Methyl Methacrylate Using PTDI as the Photocatalysta.

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entry [MMA]:[ EBrP]:[Et3N]:[PTDI] conversion (%)b Mn,SEC (kg mol–1)c Đ (Mw/Mn)c I* [Mn(theo)/Mn(exp)]d
1 100:1:0:0.01 45.5 47.7 1.69 9.9
2 50:1:5:0.01 88.4 6.7 1.33 68.6
3 100:1:5:0.01 88.0 13.5 1.22 66.5
4 200:1:5:0.01 89.6 23.2 1.32 78.0
5 500:1:5:0.01 82.9 47.5 1.47 87.6
6 750:1:5:0.01 76.9 63.0 1.44 91.8
7 1000:1:5:0.01 75.2 87.7 1.75 85.9
8 100:1:5:0.001 87.5 16.4 1.51 54.4
9 100:1:5:0.0005 78.0 15.6 1.71 51.2
10 100:1:5:0.0001 55.9 128.5 1.72 4.5
11 100:1:5:0.00001 36.3 161.2 1.83 2.4
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a

See the Methods part for experimental details.

b

Calculated by 1H NMR.

c

Measured by SEC in DMF (PMMA standard, 45 °C).

d

I* = theoretical Mn/experimentally measured Mn × 100. Mn(theo) calculated using the equivalents of monomer based on [MMA]:[ EBrP].

Control experiments revealed that the omission of the light source, PTDI, or initiator resulted in no polymeric product (Table S3, entries S24–26). Another control experiment with Et3N but no photocatalyst PTDI or light source also shows a result without polymerization (Table S3, entries S27–28). Therefore, the driving force of the polymerization without Et3N is the oxidative quenching of PTDI* with low reduction potential (E1/2 (PC+/PC*) = −1.08 V vs SCE, based on the λ = 456 nm) (Figure 2a).43 For the polymerization with Et3N, the PTDI can obtain one electron from Et3N and produces a radical anion PTDI·–. There may be two pathways for PTDI·– to initiate the polymerization. One is reducing the initiator directly and initiating the polymerization (reductive quenching pathway, as shown in Figure 1a). However, the potential of PTDI·– is −0.44 V vs SCE. It is impossible for PTDI·– to reduce the initiator, EBrP, which has a lower reduction potential (lower than −0.8 V vs SCE).23 The control experiment using the 2.4,6-tris(4-methoxyphenyl)pyrylium (p-OMeTPT+) as the PC in the presence of Et3N gives no polymerization (Table S3, entries S29–30). Previous reports have shown that the excited state of p-OMeTPT+ can only be reduced to PC·– and the E1/2(PC·–/PC) is −0.50 V vs SCE, lower than −0.44 V vs SCE,43 which means that the reductive quenching mechanism is impossible. As the results of spectroscopy showed, a high concentration of donor (500 equivalent triethylamine compared to PTDI) only has little effect on the absorption of PTDI (Figures S6 and S7). Upon irradiation with blue light in an inert atmosphere, the PTDI can be reduced into the radical anion PTDI·–. It has been proven that the radical anion PTDI·– can absorb one photon and become the excited state PTDI·–*.35,40 Based on the results of control experimental and previous reports, we proposed a two-photon excitation pathway for the ATRP using PTDI as a photocatalyst with blue light (Figure 2a). First, the photoexcitation of PTDI results in an excited state PTDI*, then it accepts an electron from the electron donor Et3N to form the PTDI·–. This radical species will absorb the photon again and become the radical anion excited state PTDI·–*. As the previous report shows that the reduction potential of the PTDI·–* (E1/2 (PC/PC·–*) = −1.87 V vs SCE) is strong enough to reduce the initiator, and it will undergo an efficient photoinduced electron transfer process as the initiators have a reduction potential >−1.7 V.39

Figure 2.

Figure 2

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(a) Proposed photocatalytic mechanisms of the one-photon and two-photon excitation process. The “X” represents the transferable halogen; (b) Calculated energy profile of the PTDI photo-excitation process and Gibbs free energy of the reaction between excited PTDI (PTDI*) and excited PTDI anion (PTDI·–*) with two different initiators and Et3N. (c) Optimized structures of all species in the calculation.

To further elucidate this mechanism, we performed DFT calculations (Figure 2b,c). We calculated the Gibbs free energies for each species proposed to participate in the photoredox process. The thermodynamic stability of each molecule was then compared with the ground state of PTDI as a reference. The results show that there are two pathways to quench the excited state PTDI*. For the primary excitation, the excited state PTDI* can undergo an oxidative quenching process via SET with an initiator to produce a radical anion [ini1]·– and PTDI+. However, this process has high end-state energy (2.30 eV vs PTDI), which is unfavorable and results in low catalytic efficiency. Most of the PTDI* molecules will go back to the ground state by luminescence or other pathways. When the initiator has a more stable radical state (e.g., [ini2]·–), the end-state energy will decrease and the one-photon catalytic efficiency will increase. This is in good agreement with the experimental results, in which the polymerization has the highest conversion rate using the ethyl α-bromophenylacetate as the initiator without the donor (Table S1, entry S16–17). Compared to the one-photon process oxidizing by ini1 or ini2, the two-photon process of PTDI is more likely to occur in the presence of Et3N. When PTDI is reduced into the radical anion PTDI·–, the end-state energy of the reductive quenching process (1.08 eV) is significantly lower than that of the oxidative quenching pathway (2.30 eV). Furthermore, a large excess of Et3N gives more chances for the reductive quenching pathway. For the second photoexcitation, the excited state PTDI·–* will undergo an oxidative quenching process with the initiator and back to the ground state (PTDI) rather than be reduced into the PTDI2–, which the latter is thermodynamically unstable with high end-state energy of 3.04 eV. Those calculated results indicate that the polymerization of the two-photon process takes place more easily. However, there is no evidence that the one-photon (SET) process can completely be prevented even if the Et3N is in large excess, which possibly leads to a relatively high Đ. Thus, it can be seen that the initiators, which are easily reduced into a stable radical, are not a good choice (e.g., ethyl α-bromophenylacetate) because these initiators will more easily undergo a one-photon process. The Gibbs free energy change for the bromide dissociation reaction ([Pn-Br]− + Et3N·+ → Et3N + Br·– + [Pn]·, where Pn stands for polymer chain-end) was calculated to be 6.0 kcal mol–1, which corresponds to an equilibrium constant of 4.17 × 10–5 at 298.15 K. This small value shows a strong deactivation effect and is comparable to the KATRP that was experimentally determined for conventional Cu-based ATRP (usually 10–5–10–8).5,44,45

To confirm the polymerization process could be instantly activated/deactivated by light, the on–off switch experiments for one- and two-photon excitation catalyzed ATRP were performed and compared, using the same ratio of [MMA]:[EBrP]:[PTDI] = 100:1:0.01. These two reactions were subjected to repeated cycles of 1 h of illumination followed by 1 h of dark time (for more detail, see the general polymerization procedure in SI). As the results show, both polymerization processes were strictly controlled by light. Polymerization exclusively occurs during the irradiation period, while no monomer is consumed during “dark” periods (Figure 3a). With the same illuminating time, the conversion of the two-photon mechanism is significantly higher than the one-photon mechanism, also Mn is closer to the target molecular weight and Đ is lower. As Figure 3b shows, the monomer conversion of both polymerization processes follows first-order kinetics during irradiation, and the two-photon excitation pathway makes the reaction faster (k1 = 0.426 h–1, two-photon; k2 = 0.175 h–1, one-photon). As shown in Figure 3c, there is a positive correlation between Mn and the monomer conversion, although it is not a linear increase. This might be because the PTDI can drive polymerization under blue light by absorbing one photon although the electron donors are large in excess. Another reason is the low PC loading. The initiator efficiency is low at the beginning of the polymerization. As the polymerization proceeds, the diffusion rate of the polymer chain (macroinitiator) will become slow. The PC will more easily reduce the initiator EBrP, and the initiation efficiency will gradually increase. Mn increases almost linearly with increasing monomer conversion. Moreover, the polymerization of one-photon excitation is more uncontrollable, the Mn has reached about 30 kg mol–1 even at the beginning of the polymerization (Figure S8). As the monomer conversion increases, the polymerization by two-photon excitation catalysis becomes more controllable and Đ becomes lower.

Figure 3.

Figure 3

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(a) Plot of monomer conversion vs time demonstrating the controllability during the on–off switch experiment; (b) first-order kinetic plot of monomer conversion vs irradiation time; (c) plot of the Mn and Đ as a function of monomer conversion for the two-photon excitation catalyzed polymerization.

The PMMA synthesized with a monomer-to-initiator ratio of 50:1 was analyzed by MALDI-TOF MS (matrix-assisted laser desorption ionization time-of-flight mass spectrometry) and 1H NMR (Figure 4a,b), confirming the molecular weight and chain end fidelity. It can be observed that there are two populations. Both populations have the same mass separation of one monomer unit (100.1 Da). By further analysis of the main peak, we found the molecular weight is in accordance with the expected values, including the initiating unit and the bromine end group, which indicates the polymer has high chain-end fidelity. The minor population showing the polymer without a -Br group is probably due to the high-energy laser used during the MALDI-TOF MS test. The result of 1H NMR agrees well with the MALDI-TOF MS. Not only initiating unit was observed, but also the calculated molecular weight (3.9 kg mol–1) is in full agreement with the MALDI-TOF MS result. Furthermore, we performed chain-extension experiments to demonstrate the livingness and used the isolated PMMA sample as the macroinitiator to investigate the activity of the bromine end group. The macroinitiator PMMA was dissolved in DMF and reintroduced to O-ATRP conditions ([monomer]:[MMA]:[Et3N]:[PTDI] = 100:1:5:0.01). The chain extension was successful with the newly added MMA monomer (Figure 4c). The molecular weight was increased as expected and shows no tailing or bimodality on SEC, which again confirms the majority of the polymer have the -Br group at the chain end. We further expanded the monomer scope to butyl acrylate and acrylonitrile and successfully obtained PMMA-b-PBA, PMMA-b-PAN diblock copolymers. The characteristic peaks corresponding to the monomer units were clearly observed in the 1H NMR spectra of the copolymers (Figures S9 and S10) as well as the shift on SEC traces (Figure 4c).

Figure 4.

Figure 4

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(a) MALDI-TOF MS spectra and (b) 1H NMR spectra of the PMMA oligomer; (c) chain-extension polymerizations from PMMA macroinitiator and overlayed SEC traces.

Thanks to the two-photon excitation process, the PTDI can accumulate energy and become a very strong reductant PTDI·–*. In order to further investigate the advantages of this catalyzed mechanism, we performed the O-ATRP with some common (Table 2, initiator 1–6) and unusual (Table 2, initiator 7–10) initiators. For unusual initiators, we choose some aryl halides which are difficult to reduce into the radical with a potential E1/2(Ar·X–/ArX) < −1.8 V vs SCE. As shown in Table 2, all the initiators showed the polymerization results with the two-photon excitation mechanism. Interestingly, the most active initiator 1 shows a low conversion and initiator efficiency. This initiator has a more positive reduction potential compared to others. Furthermore, the PTDI* has an oxidative quenching pathway (the one-photon O-ATRP in this work). As a result, even though there are excessive donors in the two-photon excitation O-ATRP system, the PTDI* is more easily directly quenched by initiator 1 than by other initiators. That is consistent with the calculated results, in which the energy of 1·– is lower than EBrP·– (Figure 2b). As shown in Table 2 entries 18–21, the polymerization of those unusual initiators is controllable, and some initiators show a high conversion and initiation efficiency. To validate the initiation of the aryl halides, the solid power of the PMMA (Table 2 entry 21) and the initiator 10 were irradiated by UV light (Figure S11). The PMMA emits light-cyan light which is a red shift compared to the blue-emitting initiator 10. That indicates that initiator 10 was reduced and initiated the polymerization, and the electron-withdrawing groups Br went away, which is consistent with the result of 1H NMR spectrum (Figure S12).

Table 2. Results of the One and Two-Photon Excitation Polymerization of Methyl Methacrylate Using Different Initiatorsa,b.

graphic file with name ja3c02832_0007.jpg

entry initiator conversion (%)c Mn,SEC (kg mol–1)d Đ (Mw/Mn)d I* [Mn(theo)/Mn(exp)]e
12 1 62.5 21.0 1.49 30.6
13 2 76.0 15.3 1.43 50.8
14 3 85.6 15.3 1.42 57.1
15 4 77.3 16.9 1.39 46.8
16 5 76.5 21.9 1.41 35.8
17 6 90.4 14.1 1.32 65.8
18 7 58.5 23.2 1.52 26.4
19 8 80.6 14.1 1.72 58.6
20 9 69.4 7.0 2.09 102.9
21 10 68.9 15.1 1.61 50.0
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a

See the Methods part for experimental details.

b

The donor is 5 equiv Et3N.

c

Calculated by 1H NMR.

d

Obtained by SEC in DMF (PMMA standard, 45 °C).

e

I* = theoretical Mn/experimentally measured Mn × 100. Mn(theo) calculated using the equivalents of monomer based on [MMA]:[EBrP].

Conclusions

In this work, we have successfully introduced the two-photon excitation catalysis mechanism into O-ATRP. In this mechanism, the PC can absorb two photons to accumulate energy, thus breaking the thermodynamic limitations of one-photon excitation O-ATRP. Experimental results and DFT calculation reveal that it is more favorable for PTDI to undergo a two-photon excitation pathway in the presence of a tertiary amine. Compared to the one-photon excitation O-ATRP, the two-photon excitation O-ATRP shows more controlled molecular weight and higher initiation efficiency, especially for the initiator with a low activation rate constant. Furthermore, this catalysis mechanism shows great advantage in the range of initiators, even the aryl halides can be reduced into a radical and initiate the controllable polymerization. The results in this PTDI-mediated two-photon excitation O-ATRP are not as good as the best results of the reported one-photon studies (very low polydispersity and near 100% initiator efficiency), which may be due to the coexistence of a one-photon excitation pathway. However, this work shows a promising future (low catalyst loading and very negative reduction potential) of the two-photon catalysis, which is similar to the biological photosynthesis system. Choosing the suitable PC with theoretically no one-photon pathway will be the key point on which our future research of two-photon excitation O-ATRP will focus. We also anticipate that this research can provide a new mentality for O-ATRP’s further development.

Acknowledgments

KAUST baseline funding (BAS/1/1342-01-01) and KAUST Supercomputing Laboratory (KSL) are gratefully acknowledged for financial support and computing resources. This work is dedicated to the memory of the late Professor Yusuf Yagci, a giant in photopolymerization.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c02832.

  • Experimental details, synthetic procedures, calculation details, NMR, SEC, UV–vis–NIR, and other characterization results (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja3c02832_si_001.pdf (1MB, pdf)

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