Initiation of radical photopolymerization via the long-lived triplet charge-separated state of donor−acceptor thermally activated delayed fluorescence compounds

Abstract

Long-lived charge-separated (CS) states are observed in a series of organic electron donor–acceptor (D–A) thermally activated delayed fluorescence (TADF) emitters (PTZ-DTO, PSeZ-DTO, and DPTZ-DTO). We report, for the first time, the initiation of radical photopolymerization via the long-lived ³CS state. After adding a co-initiator, diphenyliodonium hexafluorophosphate (DPI), we monitored the fluorescence intensity and lifetime quenching of the TADF initiators. The lifetime quenching study showed that the quenching of the ³CS state is more significant than that of the ¹CS state. Using nanosecond transient absorption (ns-TA) spectroscopy, we analyzed the quenching of the ³CS and the triplet localized excited (³LE) states in detail, and the Stern–Volmer quenching constants (K_SV) were found to be K_SV(³CS) = 3.4 × 10⁴ M⁻¹, K_SV(³LE) = 2.9 × 10⁴ M⁻¹, and K_SV(¹CS) = 8.6 × 10² M⁻¹. These results demonstrated that the quenching of the ³CS state predominates in the intermolecular electron process with DPI as the electron acceptor. This concept is different from the use of conventional photoinitiators (PIs) that use ³LE state to initiate the intermolecular electron transfer with the co-initiator, and thus this work represents a paradigm shift of photoinitiated polymerization. Finally, the PIs PTZ-DTO was applied in lithography to obtain a series of high-resolution patterns.

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Introduction

Photopolymerization is important,^1–18 not only for its applications in photocuring adhesives,^19–21 photolithography and holo-graphy,^22–24 but also in fundamental photochemistry studies.^25–32 A typical photopolymerization includes using a photoacid or photobase, as well as radical photoinitiators (PIs) to initiate the photopolymerization. Radical PIs are usually based on a Norrish type I or Norrish type II photoinitiation mechanism, while typical commercialized PIs include oxime esters, or diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (TPO). For Norrish type II PIs, usually co-initiators, such as triethylamine, N-phenylglycine (NPG), tetrabutylammonium tris(3-chloro-4-methylphenyl)hexylborate (NB), diphenyliodonium hexafluoro-phosphate (DPI), are used. These photoinitiating molecular systems usually operate in the same way, i.e., an intermolecular electron transfer between the PIs and co-initiator occurs, generating a reactive radical, such as an alkyl radical, to initiate the radical polymerization of the alkene monomers (such as acrylates). Photoinitiation mechanisms based on photoredox processes are of particular of interest (Fig. 1). Normally the triplet state (T₁), or even the singlet excited state (S₁) of the PIs, populated by photoexcitation (S₀ → S₁) or by the subsequent intersystem crossing (ISC, S₁ → T₁), drives the intermolecular electron transfer, to produce the reactive radical.^33,34 The S₁ state has a higher excited state energy than the T₁ state and thus has a favorable excited state redox potential that drives the intermolecular electron transfer with the co-initiator. The thermodynamics of this process can be evaluated by the Gibbs free energy change (ΔG) of the electron transfer. However, the disadvantage of the S₁ state is its short excited state lifetime (ca. a few ns), which renders the intermolecular electron transfer inefficient in fluid solution at a reasonable concentration,³⁵ because it is a diffusion-controlled process that takes microseconds. In a solid film, this drawback is overcome to some extent by the high concentration in the solid matrix.

Fig. 1 Comparison of this work to previous works on radical photopolymerization.

Conversely, the T₁ state has the advantage of a long excited state lifetime (μs to ms), which makes the intermolecular electron transfer efficient.^36,37 The long-lived photoexcited state of the PIs can reduce the load of the PIs required to initiate the reaction.^38,39 However, for aromatic compounds that have a planar molecular structure, the electron exchange (J) is usually large, and thus, the T₁ state energy is much lower than that of the S₁ state, which makes the excited state redox potential of the T₁ state less favorable for intermolecular electron transfer than that of the S₁ state.³⁵ Additionally, the ISC ability is usually weak, and it is still a major challenge in photochemistry to design heavy atom-free triplet PIs.^40,41 Moreover, in these two scenarios, both the singlet and triplet localized excited (³LE) states drive the intermolecular electron transfer with the co-initiator, to produce the reactive radical to initiate the radical polymerization.

Both cases operate in the same way, i.e., the crucial step is the excited state redox reaction between the electronically excited state PIs molecules and the co-initiator.^15,24,42 However, one may wonder whether a long-lived charge-separated (CS) PIs state can be used directly to initiate the intermolecular electron transfer thermal redox reaction, thus addressing the above disadvantages. Unfortunately, no such examples have been reported to date. Notably, obtaining long-lived CS states in dyad compounds with simple molecular structures remains a challenge, and usually the CS state is short-lived (a few ns or shorter).

Recently, thermally activated delayed fluorescence (TADF) emitters based on electron donor–acceptor (D–A) dyads have attracted much attention, mainly due to their potential applications in organic light emitting diodes (OLEDs).^43–48 For these dyads, at least three states are involved in the TADF process, i.e., an emissive ¹CS state and a dark ³LE and dark ³CS state. This is the three-state model for TADF mechanism.^49–53 The three-state model has been experimentally confirmed using femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) spectroscopic methods.^54–56 TADF emitters are of particular interest when trying to access long-lived CS states because the ³CS state lifetimes of these compact electron D–A compounds can be up to a few μs. This long CS state lifetime is attributed to the electron spin-control effect, because the charge recombination (CR) of the ³CS state to the ground state is electron spin forbidden.

p-Type TADF emitters based on a Zn(II)–carbene dithiolate dimer complex were used for intermolecular triplet state energy transfer, as demonstrated by the sensitized cis–trans isomerization of stilbene.⁵⁷ TADF molecules have also been used as photocatalysts.^58–62 D–A dyads have been used as photocatalysts in photoredox catalytic organic reactions that proceed via an energy transfer activation mechanism.^58,63 However, in these examples, it was proposed that the photocatalysis does not proceed via electron transfer.⁵⁸ The triplet state of the photocatalysts has been used in intermolecular electronic transfer.^64,65 Although TADF compounds have been used in photocatalytic redox organic reactions,^57,59,66,67 the application of TADF molecules as PIs for photopolymerization is rare.^68,69 TADF molecules were used as PIs for the photoinitiation of the cationic polymerization of epoxides and the free radical polymerization of (meth)acrylates. The effect of the excited state lifetime on aspects of the photopolymerization, such as the monomer conversion ratio, was studied. It was stressed that the long lifetime of the singlet excited states facilitates the photoinitiation.^68,69 TADF emitters were used as PIs (energy donors) together with thermally-assisted fluorescence materials in site-initiated photo-induced electron transfer reversible addition–fragmentation chain transfer polymerization (RAFT), but the initiation mechanism was not elucidated.⁷⁰ TADF molecule-induced redox-relay-based electron transfer was used for the synthesis of precise polyfluoroalkene materials.³⁹ The quenching of prompt and delayed fluorescence by co-initiators was studied using luminescence measurements. However, the electronic configuration of the T₁ state was not discussed.

Previously, Cu(I) complex-based TADF emitters and some sulfoxide-based organic TADF emitters were used in the free radical polymerization of methacrylates, with the long excited state lifetimes proposed to improve the photopolymerization efficiency.⁶⁹ However, the role of the CS state in the initiation mechanism was not studied.⁶⁹ It should also be pointed out that no CS state is involved in the TADF mechanism involving the copper complexes.⁶⁹ The PIs mechanism was not studied with ns-TA spectroscopy. Some phosphorescent Ir(III) complexes have been used for photopolymerization;⁷¹ however, the high cost of Ir(III) complexes hinders the practical application of these precious metal complexes in photopolymerization. TADF emitters have been used as photocatalysts in photo atom-transfer radical polymerization.^72,73 However, the electronic configuration of the T₁ state (i.e., whether or not it corresponds to a ³LE state or ³CS state) was not elucidated, nor was the role of the ³CS state in the photo-induced electron transfer.

Here we propose to shift the paradigm of the fundamental step of radical photopolymerization from the conventional scenario of using a localized excited state (either the ¹LE or the ³LE states) to that of performing the photoinitiation based on the long-lived CS state of the electron D–A systems. First, the long-lived intramolecular CS state of the PIs molecule is obtained upon photoexcitation. Then, this long-lived CS state was used to drive the intermolecular electron transfer with the co-initiator.²⁹ This new photoinitiation mechanism is actually a thermal redox reaction in the ground state (Fig. 1), which removes the limitations of the intrinsically short lifetime of the S₁ state and that of the low energy of the T₁ state or the elusive ISC ability by driving the intermolecular electron transfer. For the new method, there are no such limitations. The intermolecular electron transfer efficiency is dependent on the ground state redox potentials, which can be feasibly tuned by selecting an appropriate electron donor or acceptor in the PIs molecular structure. Certainly, the efficiency of the new method is dependent on the CS state lifetime of the PIs. Thus, it is crucial to develop PIs with long-lived CS states, and by taking advantage of the recent developments in the areas of charge separation and the study of TADF emitters, this is no longer an unsurmountable challenge in photochemistry.

In traditional electron D–A dyads, it is challenging to access the long-lived CS state, especially in small organic molecules. Normally long distances between the donor and acceptor are required to reduce the coupling between the donor and acceptor, to thus prolong the CS state lifetime. However, this strategy makes the synthesis of the compounds challenging. As mentioned above, a novel approach has been developed recently to prolong the CS state lifetime, i.e., to use the electron spin-control approach by population of the ³CS, rather than the ordinary ¹CS state, because the CR of ³CS → S₀ is electron spin forbidden.⁷⁴

The electron D–A dyad-based TADF emitters are proposed as ideal candidates to exploit this new strategy. According to the recently proposed spin–vibronic coupling mechanism of the TADF process, at least three low-lying states are involved in TADF process of these compounds: the ¹CS state, ³LE state, and ³CS state. The first of these is bright but short-lived (∼ns), while the latter two are dark and long-lived (∼μs). Note that in these compounds with simple molecular structures, the ³CS state is long-lived, up to a few μs or even longer, which is sufficient for intermolecular electron transfer in fluid solution at a reasonably low concentration. Our new strategy is to use the long-lived ³CS state of the D–A-based TADF emitters for photoinitiation, i.e., to use a PIs (TADF molecule) with a long-lived CS state to initiate the intermolecular electron transfer with the co-initiators. The fundamental difference between the new approach and the previously used method is that the thermal electron transfer of the current method does not invoke the intrinsic disadvantages of the traditional PIs which use a short-lived S₁ state or a T₁ state with low energy and thus have a smaller driving force for the intermolecular electron transfer. Note the ¹CS and ³CS states share similar energy.

A few representative D–A TADF emitters based on dibenzothiophene-S,S-dioxide (DTO) as the electron acceptor and 10H-phenothiazine (PTZ) as the electron donor were selected (Scheme 1).^49,50,75,76 Various steady-state and transient spectroscopic methods were used to quantitatively characterize the photoinitiation mechanism of these TADF emitters, such as the quenching of the CS state of the TADF emitter PIs by a typical co-initiator with electron accepting ability, diphenyliodonium hexafluorophosphate (DPI).^{3,12,29,77–80} Based on the determined Stern–Volmer quenching constants (K_SV), we demonstrate that the ³CS state plays a major role in the quenching of the CS state of the PIs by DPI (not the ¹CS state and ³LE state) via intermolecular electron transfer. This new approach is also used for the development of a new photoresist for photolithography.

Scheme 1 The molecular structures of the TADF emitters PTZ-DTO, PSeZ-DTO and DPTZ-DTO used as photoinitiators (PIs). The molecular structures of the co-initiator diphenyliodonium hexafluorophosphate (DPI) and the monomer pentaerythritol triacrylate (PETA) are also presented.

Results and discussion

UV-Vis absorption spectroscopy and photobleaching

The UV-Vis absorption spectra of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO in toluene were studied (Fig. 2). The absorption bands of all compounds are located in the range of 300–375 nm. These compounds have a weak absorption at 365 nm, a typical wavelength of a mercury lamp, which is favorable for photopolymerization and i-line lithography because of the advantage of deep-layer photocuring ability in the films.^11,81

Fig. 2 UV-Vis absorption spectra of (a) PTZ-DTO, PSeZ-DTO, and DPTZ-DTO, c = 1.0 × 10⁻⁵ M in toluene, 25 °C. (b) PTZ-DTO (1.9 × 10⁻⁴ M) and (c) PSeZ-DTO (1.4 × 10⁻⁴ M) upon exposure to a 365 nm LED in the presence of DPI in deaerated toluene, c[DPI] = 1.0 × 10⁻³ M, the power density is 50 mW cm⁻², 25 °C.

The photobleaching of the compounds in toluene solution was studied (Fig. 2b and c). With the addition of ca. 10 eq. of DPI, the mixture was exposed to UV light irradiation.⁶⁹ The addition of DPI resulted in rapid photolysis of the PIs and the formation of a new photoproduct with an absorption band centered at 365 nm (Fig. S15†).⁶⁸ At the same time, an absorption band of PTZ⁺˙-centered at 520 nm was observed (Fig. S22†), and the cationic absorption band gradually disappeared again after an extended period of UV light exposure.

Fluorescence quenching: study of the photoinitiation mechanism

The quenching of the delayed fluorescence of the TADF emitters by DPI under an N₂ atmosphere was studied (Fig. 3).³ With incremental addition of DPI to the solution, the fluorescence of PTZ-DTO is quenched (Fig. 3a). Similar results were observed for PSeZ-DTO and DPTZ-DTO (Fig. S17†). The quenching of the fluorescence of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO was quantitatively studied using the Stern–Volmer equation (eqn (1), Fig. 3b).

Fig. 3 Fluorescence quenching of (a) PTZ-DTO upon incremental addition of DPI, in an N₂-saturated toluene solution. λ_ex = 310 nm, A = 0.10, c ≈ 1.0 × 10⁻⁵ M, 25 °C. (b) The corresponding Stern–Volmer plots for the fluorescence quenching studies using different PIs.

The Stern–Volmer quenching constants (K_SV) of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO with DPI as the quencher under N₂ atmosphere were determined to be K_SV = 6.2 × 10⁴ M⁻¹, 9.6 × 10⁴ M⁻¹, and 2.3 × 10⁴ M⁻¹, respectively. Based on the magnitude of theses quenching parameters, the weakened fluorescence should be predominantly due to the quenching of the triplet state (³LE and ³CS) by the DPI and not the quenching of the emissive ¹CS state. It should be pointed out that because of the short lifetime of the prompt fluorescence (ca. 4.3–31.1 ns), the K_SV values should be much smaller given the ¹CS state is responsible for the quenching.⁸² Indeed, under an air atmosphere, the ³LE/³CS states are significantly quenched by O₂, and the ¹CS state is kept intact to a large extent, the degree of quenching of the fluorescence of compounds by DPI was significantly reduced and K_SV decreased to 8.6 × 10² M⁻¹, 5.4 × 10² M⁻¹, 1.0 × 10³ M⁻¹, respectively (Fig. S18†). Therefore, considering the photophysics of the TADF emitters, it is proposed that the ³CS state and ³LE state are mainly quenched by DPI in fluid solution via intermolecular electron transfer. One finding to support this is that the K_SV trend does not agree with the magnitude of the prompt fluorescence lifetime, indicating that a state other than the ¹CS state is responsible for the quenching of the fluorescence of the TADF emitters by DPI. It should be noted that both static and dynamic quenching may contribute to the observed fluorescence quenching. However, considering the molecular structure, the reasonably low concentration and the variation of the luminescence lifetimes (see later section), we envision that dynamic quenching plays a dominant role.⁸²

To further understand the quenching mechanism, the variation of the fluorescence lifetimes of the TADF emitters in the presence of DPI was studied (Fig. 4). Biexponential decay character was observed for the TADF emitters, with prompt fluorescence lifetimes on the scale of ca. 20 ns, and delayed fluorescence lifetimes on the scale of ca. 2.0 μs. With the addition of the co-initiator DPI to the solution, significant quenching of the delayed fluorescence lifetime was observed, but quenching of the prompt fluorescence lifetime was not. For instance, the delayed fluorescence lifetime was shortened from 2.0 μs to 1.1 μs, then 0.5 μs, with the addition of 1.32 eq. and 4.80 eq. of DPI, respectively (Fig. 4a). However, for the prompt fluorescence lifetime, it is virtually not quenched with incremental addition of DPI to the solution. This agrees with the magnitude of the prompt fluorescence lifetime and the delayed fluorescence lifetimes, and the diffusion-controlled character of the quenching by DPI in fluid solution. Similar results were observed for other TADF emitters (Fig. S19†). The quenching of the delayed fluorescence lifetime of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO was studied using the Stern–Volmer equation (eqn (1), Fig. 4):⁸² the K_SV values of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO were calculated to be K_SV (DF) = 5.1 × 10⁴ M⁻¹, 6.1 × 10⁴ M⁻¹, 1.3 × 10⁴ M⁻¹, respectively. These values virtually follow the same trend as that determined using the delayed fluorescence intensity quenching (Fig. 3b). These results support our postulate that the fluorescence intensity quenching of the TADF emitters by DPI is caused by quenching of the ³LE/³CS state, not the bright ¹CS state, under the specific experimental conditions.³⁹ However, according to the recently proposed spin–vibronic coupling mechanism of the TADF process, the ³LE and ³CS states are in equilibrium, which is crucial for fast and efficient reverse ISC,^49,51,55 and therefore, at this point, we needed additional studies of the thermodynamics to clarify which state, either the ³CS state or the ³LE state, is quenched by DPIvia intermolecular electron transfer. Therefore, we further studied the thermodynamics of the electron transfer-induced quenching of the ³CS state and the ³LE state based on electrochemical data and optical spectral data (Tables 1 and 2).

Fig. 4 Fluorescence decay traces of (a) PTZ-DTO at 560 nm and (b) the corresponding Stern–Volmer plots for the compounds, c = 1.0 × 10⁻⁵ M, λ_ex = 340 nm, 25 °C.

Table 1 Photophysical data of the compounds

Compounds	λ abs  [nm]	ε ^b	λ em  [nm]	τ F  [ns]	τ T  [μs]
a Significant UV-Vis absorption wavelength, c = 1.0 × 10^–5 M, 20 °C. b Molar absorption coefficient, ε: 10^4 M^−1 cm^−1. c Fluorescence emission wavelength. d Fluorescence lifetime, λex = 340 nm. e Triplet state lifetime, λex = 355 nm.
PTZ-DTO	319	0.7	568	25.3 (96.2%)	3.5
				2000 (3.8%)
PSeZ-DTO	322	0.9	559	4.3 (93.3%)	4.3
				2800 (6.7%)
DPTZ-DTO	324	1.1	595	31.1 (96.2%)	1.1
				870.3 (3.8%)

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Table 2 Stern–Volmer constants (K_SV) of the compounds obtained by linear fitting of the quenching data^a

Compounds	Int.–N2 ^b	τ DF	τ PF	^1CS^e	^3CS^f	^3LE^g
a Stern–Volmer quenching constants, KSV: 10^4 M^−1. b Fitted from fluorescence intensity quenching under N2. c Fitted from delayed fluorescence lifetime quenching under N2. d Fitted from prompt fluorescence lifetime quenching under N2. e ^1CS state quenching: fitted from fluorescence intensity quenching under air. f ^3CS state quenching: fitted from DTO^−˙ lifetime quenching under N2. g ^3LE state quenching: fitted from ^3DTO lifetime quenching under N2. h Not observed.
PTZ-DTO	6.2	5.1	0.17	0.086	3.4	2.9
PSeZ-DTO	9.6	6.1	−^h	0.054	8.4	3.7
DPTZ-DTO	2.3	1.3	0.78	0.1	5.1	2.8

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Electrochemical properties

To obtain the thermodynamic data relating to the intermolecular electron transfer, the electrochemical properties of the compounds was studied by measuring their cyclic voltammograms (Fig. 5). The redox potentials of all compounds were determined with ferrocene as the internal standard (E_Fc/Fc⁺ = 0). For PTZ-DTO, a reversible reduction potential of −2.12 V (vs. Fc/Fc⁺) was observed, which is attributed to the reduction of the electron acceptor DTO unit (Fig. S21†). A reversible oxidation potential of +0.37 V (vs. Fc/Fc⁺) was observed, which is due to the oxidation of the electron donor PTZ unit.

Fig. 5 Cyclic voltammograms of PTZ-DTO, PSeZ-DTO in deaerated acetonitrile, and DPTZ-DTO in deaerated dichloromethane. Ferrocene (Fc) was used as internal reference (set as 0 V in the cyclic voltammograms). 0.10 M Bu₄NPF₆ was the supporting electrolyte. Scan rate: 50 mV s⁻¹, c = 1.0 × 10⁻³ M, 25 °C.

As DPTZ-DTO has similar electron acceptor and electron donors to PTZ-DTO, the redox potentials of DPTZ-DTO and PTZ-DTO are virtually the same. These results are similar to the previously reported redox potentials of the compounds.⁷⁵ The redox potentials of PSeZ-DTO were determined to be +0.35 V and −2.17 V. The oxidation potentials of the native PSeZ and PTZ units are different, and therefore the oxidation potential of PSeZ-DTO is different from that of PTZ-DTO.

The spectroelectrochemistry of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO was also studied to obtain the absorption bands of the radical anion and cation (Fig. S22†). The cation absorption band of PTZ-DTO and DPTZ-DTO is centered at 520 nm, while a broad absorption band in the range of 600–1000 nm is also observed. The absorption band of the cation of PSeZ-DTO is slightly different from PTZ-DTO. For PSeZ-DTO, the absorption range of the cation at a longer wavelength is 800–1000 nm. A negative potential of −2.1 V (vs. Ag/AgNO₃) was applied to obtain the absorption of the anions of the three compounds. The absorption spectra of the anions of the three compounds are similar, all centered at 420 nm, and no absorption band was observed at longer wavelengths (Fig. S22†). Upon photoexcitation, charge separation occurs for the TADF emitters, and the anion parts of the CS states of PTZ-DTO, PSeZ-DTO, and DPTZ-DTO act as electron donors in the intermolecular electron transfer process, whereas the diaryliodonium salts (DPI) act as electron acceptors (E_RED = –1.07 V, vs. Fc/Fc⁺).⁸³ Based on the discussions in the previous section, the potential electron donor includes the ³CS state (actually the radical anion, i.e., DTO⁻˙) and the ³LE state.

The ¹CS state only plays a minor role in the quenching due to its short excited state lifetime, although it has the same redox potential as the ³CS state. The oxidation potentials of the excited states of the compounds can be calculated from eqn (2). The free energy changes for the intermolecular electron transfer process can be calculated from eqn (3).^84,85 The data are collected in Table 3.

Table 3 Ground state redox potentials (E_RED and E_OX) and excited state oxidation potentials (E_OX^*), and Gibbs free energy changes between DPI and the triplet PIs, with the CS state (ΔG_CS) and the ³LE state (ΔG³_LE) as the precursors, for each compound^a

	E RED/V	E OX/V	E OX ^*(^3LE)/V	E OX(CS)/V	ΔGCS/eV	ΔG^3LE/eV
a A reduction potential of −1.07 V is used for DPI. b E 00 = 2.71 eV. c E 00 = 2.71 eV. d E 00 = 2.67 eV. E00 = 1240/λ. E00 is the triplet excited state energy, λ is the wavelength of the starting point of the luminescence of the compound at 77 K.
PTZ-DTO	−2.12	+0.37	−2.34	−2.12	−1.05	−1.27
PSeZ-DTO	−2.17	+0.45	−2.26	−2.17	−1.10	−1.19
DPTZ-DTO	−2.15	+0.41	−2.26	−2.15	−1.08	−1.19

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The driving force for the intermolecular electron transfer can be evaluated from the Gibbs free energy changes (ΔG) with the CS state or the ³LE state as the electron donor (ΔG_CS and ΔG³_LE, respectively. Table 3). When the ³LE state is the electron donor, the intermolecular electron transfer with DPI as the electron acceptor is slightly more exothermic. However, as the difference is small (ca. 0.1–0.2 eV), both the CS state and ³LE state may act as the electron donor to be quenched by DPI. To further clarify this situation, nanosecond transient absorption (ns-TA) spectroscopy were used to study the quenching mechanism.

Nanosecond transient absorption spectroscopy: selective quenching of the ³CS state by DPI

The ³CS and ³LE states of the electron donor–acceptor dyad-derived TADF emitters can be discriminated using ns-TA spectroscopy. To unambiguously understand the role of the ³CS state and ³LE state in the quenching by DPI, ns-TA spectroscopy was used to monitor the intermolecular electron transfer-induced quenching process (Fig. 6).³

Fig. 6 (a) Nanosecond transient absorption spectra of PTZ-DTO (6.0 × 10^–5 M) with DPI added at different delay times after laser flash, c (DPI) = 1.3 × 10^–4 M in toluene. (b) The decay traces of PTZ-DTO (2.0 × 10^–5 M) at 460 nm upon incremental addition of DPI and (c) the corresponding Stern–Volmer plots for quenching of the PTZ-DTO, PSeZ-DTO and DPTZ-DTO³CS states, λ_ex = 355 nm. All solutions used in measurements were deaerated with N₂, 25 °C.

The ns-TA spectrum of PTZ-DTO shows two distinct positive absorption bands centered at 350 nm and 450 nm, respectively, as well as a shoulder band centered at 520 nm (Fig. 6a). Based on the spectroelectrochemical measurements, these absorption bands are attributed to the ³DTO state (T₁ → T_n transitions), the DTO⁻˙ radical anions (D₀ → D_n transitions) and the PTZ⁺˙ radical cations (D₀ → D_n transitions), respectively (Fig. S22 and S28†). Although some TADF compounds have been used in photoredox organic reactions, and oxidative or reductive quenching of the excited states has been proposed, the respective roles of the ³CS and ³LE states were not studied in detail with spectroscopic methods.⁶¹ In the absence of DPI, all these transient absorption bands decay with the same kinetics, indicating that the ³DTO state and ³CS state are in equilibrium, and the interconversion of two states is fast (Fig. S23†). In the presence of DPI, however, an interesting variation of the kinetics of the above mentioned transient absorption bands was observed (Fig. 6a). The lifetimes of the species become drastically different. For the ³DTO state, which absorbs at 350 nm, the lifetime became shorter upon the addition of DPI. For instance, the lifetime monitored by the trace at 350 nm changes from 3.6 μs (0 eq. DPI added), to 2.6 μs (0.5 eq. DPI) and 2.0 μs (1.25 eq. DPI). Accordingly, K_SV is calculated to be 2.9 × 10⁴ M⁻¹. This result indicates that the ³DTO state is quenched by DPI most probably, via intermolecular electron transfer. Note that the T₁ state of DPI, calculated using TDDFT method, has an energy level of 3.65 eV, and thus quenching induced by intermolecular triplet energy transfer is excluded because the ³LE state energy level of PTZ-DTO is 2.71 eV.

Interestingly, the transient species lifetime obtained by monitoring the decay trace at 450 nm (where the DTO⁻˙ radical anion absorbs) shortened more significantly, it changes from 3.4 μs (0.17 eq. of DPI), to 2.4 μs (0.92 eq.) and 1.4 μs (2.5 eq.). K_SV is calculated to be 3.4 × 10⁴ M⁻¹. Conversely, the transient species lifetime obtained by monitoring the decay at 520 nm (where PTZ⁺˙ absorbs) showed longer lifetimes (Fig. 6a) as compared to those seen in the absence of DPI (3.5 μs). This is reasonable because the quencher, DPI, is an electron acceptor and only the radical anion DTO⁻˙ can be quenched by DPI, and thus the radical cation (PTZ⁺˙) is unable to be quenched by DPI and it remains intact. Moreover, the decay of the unquenched cation in the absence of DPI by intramolecular charge recombination is inhibited after the DTO⁻˙ is selectively quenched by DPI. The decay of PTZ⁺˙ can occur only by intermolecular electron transfer from the DTO⁻˙ anion, which is already quenched by DPI, thus the lifetime of PTZ⁺˙ becomes longer (239.8 μs) in the presence of DPI, whereas the lifetime of DTO⁻˙ becomes shorter. This strong experimental evidence suggests that the CS state of the TADF PIs was quenched by DPI. In other words, the long-lived ³CS state of the TADF emitter PTZ-DTO directly drives the intermolecular electron transfer with DPI as the electron acceptor.

Considering the similar K_SV values of the ³LE state and ³CS state, it is proposed that both states act as electron donors in the intermolecular electron transfer, with the ³CS state playing a slightly more major role as the electron donor. The K_SV values are of the same order of magnitude as those measured by quenching of the fluorescence intensity and fluorescence lifetime measured under an N₂ atmosphere and are much larger than the ¹CS state quenching constants. To the best of our knowledge, this is the first report of using the ³CS state of a TADF compound, in combination with a suitable electron acceptor such as DPI, as a Type II electron donating photoinitiator for radical photopolymerization. Although it is preliminarily proposed that both the singlet state and triplet state of the TADF emitter can initiate intermolecular electron transfer, no detailed transient spectroscopic methods were used for confirmation of the postulated mechanism.⁶⁸ Our ns-TA spectroscopic studies unambiguously show that the radical anion of the ³CS state was quenched by DPI, i.e., the ³CS state is quenched by intermolecular electron transfer, not by energy transfer.⁵⁸ Caution is required in interpreting the quenching of ³CS/³LE states via energy transfer because the ³CS and ³LE states may exist in equilibrium. As a result, quenching of the ³LE state by energy transfer may depopulate the ³CS state as well, potentially leading to misinterpretation of the quenching of the ³CS state by energy transfer.

Photopolymerization investigations

To study the efficiency of radical photopolymerization with these novel PIs that exhibit a long-lived ³CS state, the photopolymerization of pentaerythritol triacrylate (PETA) was studied using PTZ-DTO, PSeZ-DTO, and DPTZ-DTO as the PIs and DPI as a co-initiator (Fig. 7). First, it was found that all three compounds can rapidly initiate the photopolymerization of PETA monomers (Fig. 7a). Next, the effect of the concentrations of the PIs on the polymerization rate was studied, and the double bond conversion rate was monitored using real-time IR absorption spectroscopy. Although DPI alone can initiate the photopolymerization of PETA monomers under UV light without PIs, the conversion is slower and the rate is lower (Fig. S29†). As the concentration of PTZ-DTO increased from 0.02 wt% to 0.5 wt%, the double bond conversion rate increased from 45% to 83%. At a concentration of 0.5 wt% in the polymerization blend, the conversion rate was the highest and the fastest. The PIs demonstrates excellent solubility in the photopolymerization blend with photoinitiator loadings ranging from 0.02 to 0.5 wt%. A similar trend was observed for DPTZ-DTO, where an increase in PIs concentration leads to an increased conversion rate (Fig. S29†). In contrast, for PSeZ-DTO, the optimum conversion rate was achieved at 0.2 wt% concentration. The polymerization rate of PSeZ-DTO was faster than that of PTZ-DTO at 0.2 wt% concentration, which is consistent with the fact that the K_SV of PSeZ-DTO is larger than that of PTZ-DTO. The effects of different PIs on the polymerization were also compared (Fig. 7c). The double bond conversion of PTZ-DTO is the highest when the PIs content was 0.5 wt%.

Fig. 7 (a) The photopolymerization of PETA under N₂ before and after irradiation using 30 mW cm⁻² of light power density with a 365 nm LED in the presence of PTZ-DTO, PSeZ-DTO, DPTZ-DTO and DPI. PIs/DPI (0.5 wt%/1.0 wt%). Photopolymerization profiles of PETA at (b) different concentrations of PTZ-DTO, DPI (1 wt%) as the co-initiator. (c) Photopolymerization profiles in the presence of PIs and DPI. PIs/DPI (0.5 wt%/1.0 wt%). The light source is a 365 nm LED with 30 mW cm⁻² irradiation intensity. The arrows indicate that the photo-irradiation starts at t = 2 s.

To study the effect of oxygen on the efficiency of the polymerization, comparative experiments were carried out, which show that the polymerization rate and the final double bond conversion of the system were reduced under an air atmosphere as compared to oxygen-free conditions (Fig. S30†). However, this is common of ordinary radical photopolymerizations, and it can be alleviated to some extent by using a laminar polymerization method. We further explored the effect of other co-initiators. When NPG was used as a co-initiator, the polymerization of the PETA monomer could be induced, but the rate of polymerization was slower compared to that when DPI was used as the co-initiator (Fig. S31†). Conversely, when NB was used as a co-initiator, the polymerization of the PETA monomer could not be initiated even after a longer period of photoirradiation time. This may be due to the mismatch of redox potentials between NB and PTZ-DTO, resulting in the inability to achieve effective intermolecular electron transfer. In addition, the PIs exhibited time-controlled behavior with the UV light switched on and off, and the polymerization reaction initiated only under light irradiation. After switching the UV light off, the monomer conversion was negligible, which is similar to a phenomenon previously reported in the literature,⁸⁶ suggesting that the reaction stops when the light is switched off and that irradiation under a continuous light source is required for the photopolymerization to proceed.

Finally, we performed a comparative experiment between PTZ-DTO and the commercial photoinitiator TPO (Fig. S31†). Under identical conditions, including PIs loading and irradiation intensity (30 mW cm⁻²), PTZ-DTO efficiently initiated the rapid polymerization of the PETA monomers (13 s photoirradiation for solidification of the blends), whereas TPO required a substantially longer time (48 s) to attain a similar polymerization degree. The PTZ-DTO/DPI system maintains excellent thermal stability at high temperatures. No significant thermal electron transfer was observed even at temperatures up to 125 °C (Fig. S31†). The above result shows that the PTZ-DTO system exhibits superior photopolymerization performance than traditional photoinitiators. It is worth noting that the system requires only two components, PTZ-DTO and DPI, to achieve efficient polymerization without the need to introduce a third component, which greatly simplifies the formulation system and improves process stability. Moreover, the synthesis of the electron donor/acceptor dyads is feasible.

The photoinitiation mechanisms are summarized in Scheme 2. As TADF emitters, all the three compounds show charge separation upon photoexcitation, and three states are formed, i.e., the ¹CS, ³CS and ³LE states. Based on the lifetimes of the three states, the different driving forces (Gibbs free energy changes) for the intermolecular electron transfer with DPI as the electron acceptor, different quenching constants determined from the fluorescence intensity and lifetime quenching experiments, and the ³LE state and ³CS state quenching constants determined using ns-TA spectroscopy. It is mainly the ³LE and ³CS states that were quenched by the co-initiator DPI, and the ³CS state played a major role in initiating the intermolecular electron transfer with DPI as the electron acceptor. After electron transfer, DPI decomposes, and a highly reactive phenyl radical is produced, which initiates the polymerization of the monomer PETA. A preliminary lithography study was also carried out with the photoinitiation system (i-line aligner, Fig. 8), and the results were promising.

Scheme 2 Photochemical mechanisms of the PTZ-DTO/DPI photoinitiation systems upon photoexcitation. The ¹LE and ³CS state energies were calculated using TD-DFT at the B3LYP level of theory using Gaussian16. The ¹CS state energy was obtained using electrochemistry. The ³LE state energy was estimated from the vibrational 0–0 transition of T₁ → S₀ based on phosphorescence spectra.

Fig. 8 (a) Lithography pattern image of the PTZ-DTO/DPI/Poly(acrylic acid) (0.2 wt%/1 wt%/30 wt%) initiated PETA (35 wt%) photopolymerization. (b) The use of scanning electron microscopy and (c) metallographic microscopy for the preliminary study of the application of the system as a lithograph photoresist.

Conclusions

In conclusion, we developed novel electron donor–acceptor (D–A) dyad TADF emitters—PTZ-DTO, PSeZ-DTO, and DPTZ-DTO—with dibenzothiophene-S,S-dioxide as an electron acceptor and 10H-phenothiazine and 10H-phenoselenazine as electron donors to obtain long-lived triplet charge separated (CS) states. The triplet CS state is used to initiate the intermolecular electron transfer with a diphenyliodonium (DPI) co-initiator to initiate radical photopolymerization. This is different from conventional photoinitiators (PIs) that usually use localized excited states (singlet state, ¹LE, or triplet state, ³LE) to drive intermolecular electron transfer to initiate radical photopolymerizations. This work represents a paradigm shift for radical photopolymerization. The new method may address the problems associated with conventional methods, i.e., the short ¹LE state lifetimes and the low ³LE state energy level that are both detrimental to intermolecular electron transfer and thus the radical photopolymerization. The Stern–Volmer quenching constants (K_SV) of ³LE, ³CS, and ¹CS states with DPI as the quencher (electron acceptor) were determined from the fluorescence intensity quenching, fluorescence lifetime quenching, and quenching of the triple state lifetimes via nanosecond transient absorption (ns-TA) spectroscopy, are K_SV (³CS) = 3.4 × 10⁴ M⁻¹, K_SV (³LE) = 2.9 × 10⁴ M⁻¹, and K_SV (¹CS) = 8.6 × 10² M⁻¹. This trend is in agreement with the driving force of the intermolecular electron transfer, i.e., the Gibbs free energy charges (ΔG) of the CS and LE states are −1.05 eV and −1.27 eV, respectively. It has been successfully demonstrated that the radical photopolymerization is generated through direct electron transfer with the co-initiator DPI. This approach is different from the conventional method of using the ¹LE and ³LE states of the initiator to drive electron transfer, mitigate the limitations of the short lifetime of the singlet state (S₁) and the low energy level of the triplet state (T₁). The efficiency of the new approach depends only on the redox potential, and the lifetime of the CS state can be extended by the spin-control method. Finally, the PIs PTZ-DTO was applied in photolithography to obtain high-resolution patterns. Our findings not only provide new insights into the design of radical photoinitiation systems but also open up new avenues for the development of advanced radical photopolymerization techniques.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

J. Zhao conceived the research, directed the analysis of the results and acquired the funding; Y. Pei synthesized the compounds, performed the UV-Vis absorption, fluorescence, and nanosecond transient absorption studies, and the electrochemistry, photopolymerization and performed parts of the data analysis; Y. Pei and X. Chen performed the photopolymerization studies. Y. Hou did part of data analysis; Y. Li and S. Ji revised parts of the manuscript; all authors provided comments on writing and modification of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. Z. thanks the National Key Research and Development Program of China (2023YFE0197600), the NSFC (22473021, U2001222), the Research and Innovation Team Project of Dalian University of Technology (DUT2022TB10), the Fundamental Research Funds for the Central Universities (DUT22LAB610) and the State Key Laboratory of Fine Chemicals for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5py00351b

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