The pulse radiolysis method is a pump–probe method that can measure the dynamics of intermediates produced when a sample is irradiated with high-energy beams (e.g., electron, ion, X-ray). In addition, by examining the optical absorption spectra, events related to transient chemical species and electronic transitions could be identified. By using 26 MeV EB pulses, even centimeter-sized optical cells containing condensed phase samples could be irradiated, delivering nearly uniform energy along their path. The kinetic information of intermediate species could be observed by measuring the intensity of analytical white light synchronized with EB pulses. In this study, kinetic traces after EB irradiation were observed over time with approximately 10 ns time resolution. The experiments were performed on the sample polymer solutions, namely PHS, P(TBOS-co-HS), and PTBOS solutions. The primary process of the radiation-induced chemistry in the resist polymer solutions in CHN under Ar saturation is considered to be as follows19:
(6)
$${\text{c}} – {\text{hex}}^{ \cdot + } + {\text{Polymer}} \to {\text{Polymer}}^{ \cdot + } ,$$
(7)
$${\text{Polymer}}^{ \cdot + } + {\text{e}}^{ – } \to {\text{Polymer}}^{*} ,$$
(8)
$${\text{Polymer}}^{ \cdot + } \to {\text{Polymer}}^{ \cdot } + {\text{H}}^{ + } ,$$
(9)
where c-hex, c-hex·+, and c-hex* represent CHN molecules, radical cations of CHN, and the excited states of CHN, respectively. In the EB-irradiated solution, first, the solvent molecules are mainly ionized (forming solvent radical cations and secondary electrons) and/or electronically excited. Then, positive holes migrate to the polymer with lower ionization energy or redox potential. Although the polymer in the solid film is directly ionized (Eq. 1), the radical cations of the solute polymers in the solution are mainly formed indirectly.
The transient absorption spectra of PHS-based polymers in CHN obtained by pulse radiolysis after 8 ns of EB irradiation are shown in Fig. 2. The absorption maxima around 400 nm can be identified as a phenoxy radical based on the previous results of pulse radiolysis of PHS solutions19,20,25. The peak intensities are similar, as shown in Fig. 2. The formation of phenoxy radicals through the deprotonation of the radical cation of phenolic molecules in nonpolar media has two proposed mechanisms. The first is an ultrafast path (< 1 ps) that occurs immediately after formation of the radical cation. The second is a slower path that occurs in the nano- to microsecond range17,18. The absorption of phenoxy radicals produced during the 8 ns EB pulse irradiation is induced by ultrafast deprotonation immediately after ionization, which is thought to occur in a similar manner, regardless of the copolymerization ratio of the polymer. This consistency may be attributed to the spread of charge resonance (CR) to the phenyl ring moiety of multiple monomer units in the polymer radical cations formed after ionization, with the vinylphenol unit acting as a proton source26. CR formation associated with conformational relaxation in PS radical cations occurs sufficiently fast (< 40 ps27) compared to the nanosecond timescale. Deprotonation from the vinylphenol unit was confirmed in the radical cation of poly(4-hydroxystyrene-co-styrene) copolymers26. Assuming that the styrene moiety, which has the lower deprotonation efficiency in the copolymer, is ionized first, efficient deprotonation from the hydroxyl group is induced by CR when the vinylphenol moiety is within 2.5 units. The broad absorption bands in the near-infrared (NIR) region with absorption maxima at 1200 nm shown in Fig. 2 are identified as CR bands of dimer or multimer radical cations28,29, where positive holes are delocalized across multiple aromatic monomer units19,30,31,32. The copolymer with a t-BOC protection ratio of n = 0.76 displays an additional absorption in the wavelength region of 450–700 nm in Fig. 2a. This absorption is attributed to the short-lived species associated with TBOS units, as it is observed in polymers with a high percentage of TBOS. More details will be discussed later, alongside the results of pulse radiolysis of PTBOS solutions.Figure 2Transient absorption spectra obtained by pulse radiolysis of 500 mM (unit conc.) polymers in Ar-saturated CHN: (a, b, c) P(TBOS-co-HS) and (d) PHS. Here, n is the percentage of OH groups of P(TBOS-co-HS) protected by t-BOC groups: (a) 0.76, (b) 0.55, and (c) 0.29.Next, the transient absorption spectra obtained by pulse radiolysis of the PTBOS solution are shown in Fig. 3. In the CHN solvent [Fig. 3A], absorption of phenoxy radicals is not observed. However, significant absorption in the ultraviolet–visible region (A-band) and the CR band in the NIR region is observed. Halocarbon solvents have been widely used in radiation chemistry to produce radical cations in solutes31,33,34. PHS has low solubility in halocarbons, whereas PTBOS has both low polarity and high solubility in halocarbons. The transient absorption spectra in the visible region in DCE [Fig. 3B] differ significantly from those in CHN, indicating a clear shoulder at approximately 450 nm. This absorption around 450 nm can be attributed to the local excitation (LE) band of the multimer radical cations35,36 or the charge transfer (CT) complex or ion pair formed by the recombination of the PTBOS radical cations with the Cl− produced by the dissociative electron attachment of DCE, as observed in the pulse radiolysis results of PS derivatives31,33. In DCE, electrons react immediately with the DCE molecule, and the electrons are replaced by anions with lower mobility. As a result, electron–hole recombination reactions between the solvent and/or polymer radical cations and electrons are suppressed. This suppression leads to a greater yield of radical cations in the time region (> 10 ns), as observed by the nanosecond pulse radiolysis shown in Fig. 3. The decay of PTBOS radical cations in CHN is slower than in polymers containing vinylphenol units, with a half-life longer than 400 ns. This suggests that the deprotonation of PTBOS radical cations is slower than that of the vinylphenol-containing OH group as a result of substitution by the t-BOC group. The decrease of the A-band observed in CHN is accelerated by the addition of an electron scavenger (TPS-nf) (Fig. S1), and the lifetime is longer than the emission lifetime from the singlet excited state (~ 10 ns). Therefore, the A-band can be identified as a radical anion of the polymer unit protected by t-BOC. Previous pulse radiolysis results of the PTBOS solution in air-saturated CHN using a variable temperature system suggested the A-band is due to cation absorption21. However, considering its slight reactivity with cation scavengers, the present results indicate that the A-band is unlikely to be a cation species.Figure 3Transient absorption spectra obtained by pulse radiolysis of 100 mM (unit conc.) PTBOS in Ar-saturated (A) CHN and (B) DCE.The relationship between the decay rate constant of the CR band absorption at 1200 nm in CHN and the t-BOC protection ratio of PHS is also shown in Fig. 4. The decay rate constants tend to decrease with increasing t-BOC protection ratio but are specifically enhanced around a 50% protection ratio. Next, the acid (deprotonation) yield produced after 50 kV EB irradiation in the polymer film was also measured, and the results are shown in Fig. 5. The acid yield correlates well with the rate constant for deprotonation of radical cations in pulse radiolysis and tends to decrease with increasing protection ratio, showing a local maximum around a 50% protection ratio. Additionally, the acid yield in the PTBOS film is approximately 80% that in the PHS film. The t-BOC protection ratio of the polymer before EB irradiation was confirmed by Fourier-transform infrared spectroscopy (FT-IR) measurement (Fig. S2).Figure 4Relationship between the protection ratio (n) and decay rate constant of the CR band at 1200 nm.Figure 5Relationship between the protection ratio (n) and relative acid yield in the polymer film obtained by the C6 titration method.Next, the effects of t-BOC and OH group substituents on radical cations of styrene derivatives modeled as monomer units were analyzed by quantum chemical calculations using DFT. As a model for radical cations formed in polymers, 2,4-diphenylstyrene derivatives as a diad (dimer) model were also investigated, including interactions between phenyl rings. For simplicity, isotactic (m) tacticity was applied in the dimer model. For the monomer and dimer models, the ionization energies and electron-donating behavior of p-substitution by t-BOC(oxy) or OH groups were evaluated. The polarity of the PHS thin films is relatively low [relative permittivity (ε) = 4.137], which may be further decreased with the addition of the t-BOC groups. Therefore, the stabilizing effect of radical cations by solvation was to be small and calculations were performed under vacuum conditions for simplicity. Since the Hamett parameter, which has been widely used as a descriptor of electron-donating behavior, is not applicable to radical cationic molecules, the pEDA descriptor proposed by Dobrowolski et al. was used instead38. The values of the pEDA descriptor represent the charge difference between substituted and unsubstituted molecules in a π-valence electron system using the NBO charge determined by quantum chemical calculations. The calculation results are shown in Table 1. In the monomer model, both t-BOC(oxy) and OH substituents for the neutral molecule and radical cation exhibit electron-donating behavior (positive pEDA value). Here, pEDA was calculated relative to ethylbenzene (pEDA = 0). In addition, the ionization energy, i.e., the difference between the Gibbs free energy of the radical cation and the neutral molecule, reduces by the presence of electron-donating substituents. The calculated ionization energy of ethylbenzene agrees well with the experimental value for ethylbenzene (8.6–8.8 eV)39.Table 1 Ionization energy (IP) and pEDA of the monomer and dimer models, energy difference (ΔG0) between conformations (anti, syn), distances between phenyl rings, CR band wavelength, and oscillator strength (f) of the dimer cation model in PHS, PTBOS, P(TBOS-co-HS), and polystyrene (PS) obtained by DFT and TD-DFT calculations.The intramolecular dimer radical cation models (p-substituted 2,4-diphenylstyrene derivatives) optimized using DFT calculations are shown in Fig. 6. NBO and spin charge maps are also shown in Figs. S3–S7. Three combinations of substituents (X and X′) on the two phenyl rings of the derivatives were analyzed: (X, X′) = (OH, OH), [t-BOC(oxy), t-BOC(oxy)], and [t-BOC(oxy), OH]. For the (OH, OH) model in Fig. 6A, two stable conformations were obtained depending on whether H atoms of the OH group were oriented in the same (syn) or opposite (anti) direction. Similarly, for the [t-BOC(oxy), t-BOC(oxy)] model in Fig. 6B, stable conformations were found based on whether the O atoms of the carbonyl group were oriented in the same (syn) or opposite (anti) direction. The two stable structures were obtained from the initial configuration of the substituent orientation. The energy differences between the respective syn and anti conformations in the dimer model and the interplanar distances between the benzene rings are presented in Table 1. The pEDA values were determined from the difference in the sum of the charges of the π electrons of the NBO charges on the carbon atoms of the benzene ring in 2,4-diphenylbenzene and its derivatives. These results are also presented in Table 1. The energy differences between the syn and anti conformations are less than kT (0.026 eV at 298 K), indicating a free conformational change between the two. Differences in the distances between benzene rings are negligible for (OH, OH) (0.325–0.326 nm). Conversely, for [t-BOC(oxy), t-BOC(oxy)], the anti-structure is well packed with substituents, showing a shorter distance between benzene rings (0.327 nm) compared to that for the syn-structure (0.338 nm). Despite the predicted decrease in π-π interactions with increasing distance between phenyl rings in the syn-structure, the Gibbs free energy for the syn-structure (ΔG0 =  − 0.008 eV) is similar to that for the anti-structure. The pEDA calculations for the dimer cation model of (OH, OH) reveal a small electron-donating behavior, with values of 0.024 and 0.03 for both conformations. By contrast, [t-BOC(oxy), t-BOC(oxy)] displayed an electron-accepting behavior with pEDA of − 1.284 (anti) and − 0.256 (syn). Similar results were observed for the neutral model, where (OH, OH) displayed an electron-donating behavior and [t-BOC(oxy), t-BOC(oxy)] displayed electron-accepting behavior. The lower ionization potential of the (OH, OH) model compared to that of the styrene dimer model can be explained by the electron-donating behavior of OH groups. However, [t-BOC(oxy), t-BOC(oxy)] has a lower ionization potential despite its electron-accepting behavior. This cannot be explained solely by substituent effects on π– π interactions, suggesting that bulky substituent interactions contribute to the stabilization of the cation formation. Contrary to the monomer cation model, the conversion of the substituent effect to electron-accepting behavior may be due to the increased electron density to the substituent with the increased interaction between the t-BOC(oxy) groups, as shown in Figs. S5 and S6. The total NBO charges of the t-BOC(oxy) groups in the dimer cation model obtained are [− 0.129 (anti)] and [− 0.100 (syn)], which are smaller than the value for the monomer cation (0.066).Figure 6Optimized structures of dimer cation model compounds [2,4-diphenyl pentane derivatives (m, m)] with SOMO (isovalue = 0.05). The three combinations of substituents of the phenyl ring (X, Xʹ) in the dimeric model are (A) (OH, OH), (B) (t-BOC, t-BOC ), and (C) (OH, t-BOC). In (OH, OH), the direction of the hydroxyl group and, in (t-BOC, t-BOC), the direction of the carbonyl oxygen are indicated as syn for the same direction and anti for the opposite direction. All DFT calculations are performed using the ωB97X-D/def2-TZVP level.Next, a model of a heterodimer radical cation substituted with t-BOC(oxy) and hydroxyl groups [t-BOC(oxy), OH] is discussed. The converged structure is shown in Fig. 6C. This structure exhibits a clear hydrogen bond between the H atom of the OH group and the carbonyl oxygen of the t-BOC group, which may contribute to the stabilization of the cation. The [t-BOC(oxy), OH] model has a negative pEDA value of − 0.148, indicating an electron-accepting behavior, similar to the [t-BOC(oxy), t-BOC(oxy)] group. The positive charge in the heterodimer radical cation is mainly localized to the vinylphenol unit, as indicated by the NBO charge and electron density maps. This suggests that deprotonation from the vinylphenol unit is likely to occur after charge equilibrium in the copolymer radical cation. Due to the stabilization of the cation by hydrogen bonding, the ionization energy of the heterodimer model is lower than that of the PS model and also the lowest compared to the PTBOS and PHS dimer models (Table 1). This implies that the localization of holes to the heterodimer moiety in the copolymer is more likely than to the homodimer moiety. In contrast, the probability of the presence of heterodimers in the diad in polymers reaches a maximum at 50% protection (Figure S8), corresponding to an extremum in the acid yield and a deprotonation rate constant around 50% protection. The presence of intramolecular hydrogen bonds as well as the reduction of π–π interactions, may contribute to the instability of the cation, causing an effect that promotes the deprotonation reaction. Therefore, it is suggested that the enhancement of the deprotonation via intramolecular hydrogen bonding is a factor contributing to the high acid yield and decay rate constant at 50% protection (Figs. 4, 5). As mentioned earlier, the acid yield in PTBOS film is approximately 80% of that of the PHS film, which is relatively high despite the absence of proton-releasing OH groups40. The low deprotonation efficiency in the PS film41 suggests that the main deprotonation source is the t-BOC(oxy) substituent. Thus, the high deprotonation efficiency of PTBOS may primarily be caused by intra- and intermolecular hydrogen bonding based on interactions between substituents. In addition, interactions between the bulky t-BOC groups may act as an electron-accepting substituents, thereby reducing π-electronic interactions and triggering deprotonations due to the reduced stability of the radical cation.Finally, the CR band transitions calculated by TD-DFT indicated that the transitions for PHS and PTBOS dimer cations are similarly predicted to be around 1100 nm. This observation is consistent with the pulse radiolysis results. Furthermore, the oscillator strength is greater for the (OH, OH) dimer model than for the [t-BOC(oxy), t-BOC(oxy)] model, which is consistent with the pulse radiolysis result that the absorption intensity of the CR band increases with the percentage of the vinylphenol unit as shown in Fig. 2c and d. However, [t-BOC(oxy), OH] does not show a clear CR band in the NIR region, owing to hole localization in the vinylphenol unit. Pulse radiolysis showed the lowest CR band absorption is for the n = 0.55 polymer as shown in Fig. 2b, suggesting a significant contribution of heterodimer radical cations in the copolymerization ratio polymer close to 50%.
Dynamics of ionized poly(4-hydroxystyrene)-type resist polymers with tert-butoxycarbonyl-protecting group
