Theoretical design of durable and strong polycarbonates against photodegradation

Abstract

The photodegradation mechanism of polycarbonate (PC) was investigated by quantum chemistry, and a novel antidegradation molecular design using substituents was proposed. It was demonstrated that electron-withdrawing substituents in the phenyl moiety controlled bond alternation, leading to inhibition of the O–C bond cleavage in the carbonate moiety. These results provide a promising alternative for durable PC synthesis.

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Bisphenol-A polycarbonate (BPA PC) is a widely used material in our daily life owing to its high transparency, high toughness, and other good performances.¹ It loses its physical properties notably if it is overexposed to sunlight or ultraviolet (UV) light.^2,3 To improve PC performance, a common strategy in experiment^4–12 is to modify its chemical structure. Replacing PC phenyl hydrogen atoms with electron-donating or -withdrawing groups would make it possible to achieve the above goal by suppressing PC photodegradation.

A few computational mechanistic studies have focused on the photodegradation of PC¹³ and aromatic esters.^14,15 Recently, our group reported the mechanism of PhO–COO bond scission using a simplified PC model bisphenol-A hydrogen carbonate (BPAHC, Fig. 1) based on quantum chemical methods.¹³ As shown in Fig. 1, two transitions are involved in the O–C cleavage: and . The former leads to C–C elongations by strengthening the C–C out-of-phase overlap within the phenyl groups and enhances the two C=C double bonds by C–C in-phase overlap. The two C=C bonds induce the formation of the Ph=O double bond owing to geometric constraints, facilitating quinoid-like structure formation and breaking the PhO–COO bond. The latter further enhances PhO–COO elongation via the reinforced out-of-phase overlap. These two transitions based on the ground state (GS) S₀ geometry jointly cleave the PhO–COO bond in the excited state (ES) S₁₃ geometry.

Fig. 1 The PhO–COO bond cleavage of BPAHC.

Given the lack of theoretical studies on the photodegradation of PC, our goal was to provide a detailed investigation of the mechanism affected by substituents, with the aim of designing light-resistant PC materials. Therefore, based on our previous study,¹³ we applied the density functional theory (DFT) and time-dependent DFT (TDDFT¹⁶) methods to study the substituent effect on the photodegradation of two PC models with electron-donating (–NH₂) and -withdrawing (–NO₂) groups as phenyl substituents.

The GS and ES geometries of m(NH₂)-BPAHC and m(NO₂)-BPAHC (Fig. 2) were fully optimized using B3LYP^17–19/6-31G(d) and TD-B3LYP/6-31G(d), respectively, as B3LYP can generate reasonable results (Fig. S1–S5 and Tables S1–S3, ESI†) for these organic systems.^20–22 The S₀ and S_n geometries in this study are the optimized geometries of the singlet GS and singlet nth ES, respectively. The computational details are provided in the ESI.†

Fig. 2 Molecular structures of the BPAHC and substituted BPAHC at the –C(CH₃)₂-meta positions (denoted as m(R)-BPAHC, R = NH₂, NO₂) (bold green: atomic number).

To assess the substituent effect on the carbonate O–C bond, Fig. 3 shows the transition behaviors of the models studied. As reported in our previous study on BPAHC,¹³ the electronic transition (corresponding to S₀ → S₁₃ excitation in the current study) attributed to and plays a key role in breaking the PhO–COO bond. Hence, S₀ → S₂₁ (m(NH₂)-BPAHC) and S₀ → S₄₀ (m(NO₂)-BPAHC) transitions are discussed because their major contributions also focus on the excitations to and as in the S₀ → S₁₃ transition (Fig. 3). Relative to the oscillator strength of 0.0294 in the S₀ → S₁₃ transition, those of S₀ → S₂₁ and S₀ → S₄₀ transitions have similar values (0.0364 and 0.0272, respectively), implying a similar impact on the absorption spectra. The absorption spectra and parameters of the vertical excitation are shown in Fig. S6 and Table S4 (ESI†), respectively.

Fig. 3 Calculated molecular orbitals (MOs) (isovalue = 0.03) involved in vertical excitation of BPAHC, m(NH₂)-BPAHC, and m(NO₂)-BPAHC in the S₀ geometry (percentage values: transition contribution, black color values: orbital energy in eV).

The effect of substituents on the O–C bond was determined by examining the variation in the transition contributions compared to those of BPAHC. As shown in Fig. 3 (left), the large contribution (41.2 → 36.5%) maintained and the remarkable increased contribution (17.5 → 33.8%) indicate the promotion of O2–C1 bond scission under the –NH₂ effect. As shown in Fig. 3 (right), the contributions excited to Ph2 π* orbitals remain substantial (41.2 → 42.6%), while those excited to orbitals are nearly eliminated (17.5 → 2.1%), implying the suppression of O2–C1 bond cleavage influenced by the –NO₂ group.

To further examine the impact of substituents on the O–C bond, the GS and ES geometries of the models studied were investigated (Fig. 4 and Fig. S7, ESI†). Upon excitation, the S₁₃ geometry (Fig. 4, center) of BPAHC with a distorted Ph2 group relative to S₀ geometry is prone to be a quinoid-like structure along the C6–C3–O2 line with C4=C5, C7=C8, and C3=O2 bonds, finally cleaving the O2–C1 bond.

Fig. 4 GS and ES geometries of BPAHC, m(NH₂)-BPAHC, and m(NO₂)-BPAHC, including their main bond lengths (Å).

Similarly, upon excitation of , the S₂₁ geometry (Fig. 4, top) of m(NH₂)-BPAHC tends to have a different quinoid-like structure with C4=C5, C6=C7, and C3=O2 bonds from that of BPAHC. This might be because the in-phase interaction on C7=C8 in BPAHC is destroyed on C7–C8 at the L orbital due to the phase-node along the C7–C4–NH₂ line (Fig. 3, left) in m(NH₂)-BPAHC. The single-bond nature of C4–NH₂ maintained in the S₂₁ geometry relative to the S₀ geometry enables the bond alternation along the O2–C3–C4–C5 line, leading to the C3=O2 double bond, finally breaking the O2–C1 bond, which is the same behavior observed in the S₁₃ geometry of BPAHC.¹³

Distinct from the results of BPAHC,¹³ for m(NO₂)-BPAHC (Fig. 4, bottom), the O2–C1 bond maintains the single-bond nature in the S₄₀ geometry, as in S₀ geometry, implying that it is not easily broken by excitation under the –NO₂ effect. The C4–NO₂ bond (1.362 Å) in the S₄₀ geometry is much shorter than that (1.472 Å) in the S₀ geometry, revealing C4=NO₂ bond formation in the S₄₀ geometry. Relative to the S₁₃ geometry of BPAHC, the S₄₀ geometry tends to be a new quinoid-like structure with C3=C8, C5=C6, and C4=NO₂ bonds upon excitation. It can be considered that the in-phase interaction on C7=C8 in BPAHC is destroyed on C7–C8 at the L + 2 orbital due to –NO₂ substitution (Fig. 3, right) as in m(NH₂)-BPAHC. The C4=NO₂ bond formation allows bond alternation along the C7–C4–NO₂ line, maintaining the single-bond nature of C3–O2, and finally suppressing the O2–C1 bond scission. For the ES geometry of m(NO₂)-BPAHC, there may be another possible alternated structure with C3=O2 and C7=C8 bonds that was not obtained via calculations (Fig. S8, ESI†). However, such an ES structure similar to S₄₀ geometry may not exist in BPAHC and m(NH₂)-BPAHC. As no substituent is located at the –C(CH₃)₂-meta positions in BPAHC, and the C4–NH₂ bond maintains the single-bond nature in the S₂₁ geometry of m(NH₂)-BPAHC. To exclude the possibility of such an ES structure in m(NO₂)-BPAHC, Fig. S9 (ESI†) shows geometric comparisons of the different starting structures and . The results indicate that this ES structure is not favored, even for different starting structures.

To explore the influence of substituents on the above geometric changes, Fig. 5 displays MO comparisons in the S₀ geometry for the models studied. In BPAHC (Fig. 5, center) by excitation to , the C3–C4, C3–C8, C5–C6, and C6–C7 bonds are elongated by enhancing the out-of-phase overlaps (blue boxes of ). These C–C elongations lead to a distorted Ph2 in the S₁₃ geometry relative to the S₀ geometry, with the in-phase C4=C5 and C7=C8 bonds enhanced by C–C in-phase overlaps (magenta boxes of ), forming a quinoid-like structure in the S₁₃ geometry. Upon excitation to , the reinforced O2–C1 out-of-phase overlap (blue box of ) resulted in O2–C1 bond elongation. These two excitations cause O2–C1 bond breakage in the S₁₃ geometry.

Fig. 5 Transition of BPAHC, m(NH₂)-BPAHC, and m(NO₂)-BPAHC in S₀ geometry, and corresponding ES geometries (percentage values: transition contribution).

In m(NH₂)-BPAHC, the O2–C1 bond cleavage in the S₂₁ geometry relative to the S₀ geometry, occurs as in BPAHC. As shown in Fig. 5 (top), the C3–C8 and C5–C6 bonds are stretched owing to the enhanced out-of-phase overlaps (blue boxes of ) upon excitation to . These two C–C elongations result in a distorted Ph2 group in the S₂₁ geometry relative to S₀ geometry, forming C4=C5 and C6=C7 bonds due to geometric constraints. Hence, another quinoid-like structure different from that of BPAHC is formed in the S₂₁ geometry, breaking the O2–C1 bonds. Upon excitation to , the O2–C1 elongation occurs because of the enhanced out-of-phase overlap (blue box of ). These two transitions cause O2–C1 bond scission in the S₂₁ geometry as in BPAHC. Besides, the presence of a stronger driving force elongates the O2–C1 bond owing to a higher transition contribution in m(NH₂)-BPAHC upon excitation to (33.8%) relative to that of BPAHC (17.5%), implying that the –NH₂ group can promote O2–C1 bond scission.

Conversely, in m(NO₂)-BPAHC, the O2–C1 bond maintains a single-bond nature in the S₄₀ geometry as in the S₀ geometry. There are two factors to explain this situation (Fig. 5, bottom): (1) the enhanced C4–NO₂ in-phase overlap (magenta box of ), and (2) the out-of-phase overlap at C3–C8 and C5–C6 (blue boxes of ). The former leads to much shorter C4=NO₂ formation as well as shorter C3–C8 and C5–C6, while the latter is longer in all lengths between carbons in the phenyl ring. Factor (2) has a larger weight than (1) by excited state calculations. Nevertheless, the effect of (2) does not appear in excited geometries. From factors (1) and (2), it is suggested that the resulting C3=C8 and C5=C6 imply a very strong geometric effect of bond alternation caused by (1) C4=NO₂ compared to the out-of-phase effect in (2). Finally, a new quinoid-like structure of the S₄₀ geometry along the C7–C4–NO₂ line is strongly affected by the very short C4=NO₂, suppressing O2–C1 bond cleavage under the –NO₂ effect.

To analyze the O–C bond scission affected by substituents, Fig. 6 shows the potential energy surfaces (PESs) obtained by scanning the O2–C1 bond length and the C4–C3–O2–C1 dihedral angle for the models studied, and Fig. S10–S12 (ESI†) display the other three views of the PESs. In Fig. 6 (center) and Fig. S10 (ESI†), the points f1, e1, and g1 correspond to the local minima on the S₁₃ PES of BPAHC.¹³ The point f1 corresponds to the S₁₃ geometry where the O2–C1 bond is broken. It has a lower energy of about 2.9 kcal mol⁻¹ relative to point b1 (the ES state via vertical excitation from a1 (S₀ geometry)). The energy barrier (a1 → f1) for breaking the O2–C1 bond is about 18.1 kcal mol⁻¹, implying that this barrier can be easily overcome to break the O2–C1 bond. Points e1 and g1 are the intersections between the S₀ and S₁₃ PESs, and the recombination (e1 → a1) or separation (e1 → h1 or g1 → h1) of the two radicals (Ph=O˙ and ˙COOH) may occur on the S₀ PES from these two points. Besides, possible intersystem crossing exists from the excited singlet to the triplet via a crossing point (Fig. S13, ESI†).

Fig. 6 PESs of the GS and ES states for BPAHC, m(NH₂)-BPAHC, and m(NO₂)-BPAHC as functions of O2–C1 bond lengths and C4–C3–O2–C1 dihedral angles.

On the PESs of m(NH₂)-BPAHC, shown in Fig. 6 (left) and Fig. S11 (ESI†), the O2–C1 bond lengths at points a2 and c2 are 1.348 Å for the S₀ geometry and 1.742 Å for the S₂₁ geometry. The O2–C1 bond is cleaved on the S₂₁ PES with an energy barrier of about 26.2 kcal mol⁻¹ (b2 → c2), which is slightly higher than that in BPAHC. It is difficult to judge the substituent effect on O2–C1 bond scission from this barrier via current coarse grid calculations. Relative to the energy at point f1 (−2.9 kcal mol⁻¹) of BPAHC, the energy of the local minimum point c2 is much lower (−23.8 kcal mol⁻¹) than that at point b2, revealing that it is more likely to reach point c2. In contrast to the PES result of BPAHC, there is no intersection between the S₀ and S₂₁ PESs in the current calculation, implying that recombining the broken O2⋯C1 pathway is blocked to some extent and the O2–C1 bond is more easily cleaved. This indicates that the –NH₂ group can promote O2–C1 bond scission due to the much lower energy of the local minimum point c2 than that of BPAHC and the blockade to some degree of the damaged O2⋯C1 bond recombination.

For m(NO₂)-BPAHC, as shown in Fig. 6 (right) and Fig. S12 (ESI†), points a3 and b3 correspond to S₀ and S₄₀ geometries, respectively. Unlike BPAHC, O2–C1 bond cleavage does not occur in m(NO₂)-BPAHC because the O2–C1 bond lengths are similar in S₀ (1.360 Å) and S₄₀ (1.361 Å) geometries. On the S₄₀ PES, a local minimum point c3 has a lower energy (−17.2 kcal mol⁻¹) relative to point b3. The energy barrier (b3 → c3) is about 33.5 kcal mol⁻¹, which is much higher than that of BPAHC, indicating that it is difficult to overcome this barrier to reach point c3. Besides, relative to BPAHC, it is difficult to recombine the broken O2⋯C1 bond as there is no intersection between S₀ and S₄₀ PESs. Hence, point b3 may be a better location than c3 (still 1.40 Å) on the S₄₀ PES, maintaining the single-bond nature of O2–C1. This indicates that –NO₂ on the phenyl rings can suppress O2–C1 bond breakage.

In a word, the excitation to the phenyl π* orbital is responsible for forming the quinoid-like structure and the carbonate π* orbital is responsible for elongating the carbonate O–C bond. These two excitations conjointly cleave the O–C bond in the ES geometry. Relative to the results of BPAHC, the maintained high transition facilitates quinoid-like structure formation with the Ph=O bond, and the enhanced transition facilitates O–C bond extension, finally promoting O–C bond cleavage in m(NH₂)-BPAHC. This promotion is further confirmed by the fact that the broken O⋯C bond recombination is somewhat hindered because there is no intersection between the GS and ES PESs. It is difficult to overcome the much higher energy barrier of the S₄₀ geometry with the single-bond nature of O–C than that of BPAHC, and the energy minimum at the S₄₀ energy surface is still short, maintaining a bonding with 1.40 Å. Even if it is broken, it is difficult to recombine the O⋯C bond because of the separation between the S₀ and S₄₀ PESs. These results provide a comprehensive awareness and feasible direction for designing UV-resistant PC material.

This work was financially supported by a Grant-in-Aid for JSPS Fellows DC1 (KAKENHI: 202321990) from the Japan Society for the Promotion of Science (JSPS), the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) (KAKENHI: JP23245005, JP16KT0059, JP25810103, JP15KT0146, JP16K08321, and JP20H00588), and the Japan Science and Technology Agency (JST), CREST. The computations were carried out using Linux systems in our research group and high-performance computing systems at the Research Institute for Information Technology at Kyushu University.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp03533f

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