INTRODUCTION

Most scientific reports on the study of bisphenol A (BPA) concern its synthesis [1], obtaining polycarbonate from BPA [2], its toxicity to living organisms [3, 4], and methods of its utilization [5–12]. Moreover, the mechanism of photochemical transformation of BPA is unconfirmed. In [13], the authors used the semiempirical PM3 method to find agreement with the experimental data on the position of the first absorption band and the photoluminescence maxima of synthesized BPA-based dyes. According to the authors, the results obtained may be of considerable interest for the development of new effective organic phosphors technology for electroluminescent applications. In [14], an attempt to interpret the peaks in the BPA spectrum in the region of 400–4000 сm–1 using the density functional theory (DFT) with the B3LYP 6-311G++(3df 3pd) configuration was presented.

BPA is a well-known endocrine disruptor with devastating estrogen-like side effects. It causes undesirable effects on humans and wildlife. According to available data on BPA toxicity, its concentration level is 1 μg/L in fresh water [15] but can reach 17.2 μg/L in the landfill filtrate. In this regard, the question of finding a way to effectively remove BPA from wastewater before it is discharged into natural waters remains topical. The literature most often contains publications on BPA photodegradation under the action of various sources using UV/H2O2 and photocatalysis technologies [16–19]. However, photocatalytic oxidation often does not result in BPA reduction, and conventional biological treatment at wastewater treatment plants is not effective in removing BPA from wastewater. In this regard, further study of the BPA phototransformation mechanism remains topical.

Studying the influence of the molecular structure on the reactivity of neutral and ionized molecules is of considerable scientific and practical interest [20–23]. The basic idea of studies of the spectra of organic molecules is based on the fact that optical spectra are related in a certain way to the spatial and electronic structure of molecules the combination of which just determines their various properties. One way to study physicochemical properties and their changes is through the spectra of electronic absorption and fluorescence of the systems under study. However, in a series of cases, the solution to this problem cannot be obtained only on an experimental basis, and the dependence of the electronic structure of a molecule on its structure can only be correctly obtained using quantum chemistry methods. Present-day quantum chemical methods are effectively used for studying the geometrical and electronic structures of organic molecules, as well as the influence of these characteristics on their reactivity. The goal of our research is the ways for effective BPA disruption under the action of sunlight in order to reduce the harmful BPA effect on the environment and humans.

RESEARCH TECHNIQUES

Experimental Methods

Absorption spectra of neutral and charged BPA forms were recorded on a Cary-500 Scan UV-Vis-NIR spectrophotometer (Varian, USA) [24] in distilled water, as well as in the presence of acid HCl and alkali NaOH at a concentration of 10–4 mol/L at room temperature in the range of 200–600 nm. All measurements were performed according to the standard technique in a quartz cuvette with an optical path length of 10 mm. The compound under study is moderately soluble in water (6 g of the substance per 100 g of water).

Using derivative spectrophotometry, bands that appear only as hidden maxima and fuzzy kinks in the absorption spectrum were isolated [25]. The quantum yield of fluorescence (γ) was determined by the relative method. As a reference, BPA in water was chosen (γ = 0.001) [26].

Quantum-Chemical Calculations, Comparison with the Experiment

Spectral-luminescent characteristics of molecules are a reflection of peculiarities of their electron shell structure determined by the chemical structure of the compound. The main objective of studies of spectral-luminescent properties of molecules is to establish the relationship between these properties and the chemical and electronic structure of the objects under study. In this investigation, the semiempirical method of intermediate neglect of differential overlap (INDO) and the package of quantum-chemical programs created in the department of photonics of the Siberian Physical Technical Institute at Tomsk State University based on the method are used. The software package we use, together with the original parametrization [27], is oriented to the study of photonics of organic polyatomic molecules. This software package calculates the energy of molecular orbitals (MOs), energy, and intensity of electronic transitions. The analysis of coefficients of the expansion of molecular orbitals in atomic orbitals allows one to determine the orbital nature and localization of electronic transitions, i.e., to interpret absorption spectra of organic molecules. The software system calculates the electron density distribution on atoms and chemical bonds of the molecule under study, as well as its dipole moments in the ground and excited states.

The main difference between the described software package based on the INDO method and similar ones is the possibility to calculate the rate constants of emissive (kr) and nonradiative (constants of internal kic and singlet-triplet kST conversion) processes in polyatomic molecules. The rate constants of nonradiative processes were calculated according to the technique [28], they are of an estimative character, and allow one to establish not only the trend of the fluorescence quantum yield γ = kr/(kr + kic + kST) in series of similarly constructed molecules but also to establish the cause of the absence of the fluorescence process. This software system calculates spectra of singlet and triplet electronically excited states both of neutral and of charged forms of molecules.

Correctness of the calculation of absorption spectra is estimated by comparison with experimental spectra of solutions by the position of the maximum of absorption spectrum bands and by the value of the molar extinction coefficient at the maximum of these bands; correctness of the electronic charge distribution is estimated by comparison with the dipole moment of the molecule in the ground state if experimental data are available. The calculated spectrum of an isolated molecule is compared with the spectrum in a nonpolar solvent. Correctness of conclusions regarding the fluorescence spectra is controlled by the experimentally measured band positions and the quantum yield of the process. The geometry of the studied structures was calculated using averaged values of chemical bond lengths and valence angles according to [29]. To calculate charged forms and complexes, the package used includes a program for calculating the molecular electrostatic potential (MEP) [30, 31], which makes it possible to determine the most electron-acceptor center of the molecule and the most probable site of proton attachment. In the case of the BPA molecule, these centers are the oxygen atoms of the hydroxyl groups. The results of MEP calculation were used in modeling both the BPA + 2Н2О complex and the BPA + 2Н+Н2О double-charged cation (Fig. 1b). The BPA cation was modeled by a complex with a hydrogen bond between the BPA molecule and a proton solvated by a water molecule.

Fig. 1.
figure 1

Structure formulas of the (a) neutral BPA complex with water with composition of 1 : 2, (b) BPA cation (q = +2е), and (c) +2Н2О anion (q = –1е).

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The anionic form of BPA is formed when one or two protons are detached from hydroxyl groups. This results in the formation of a single- or double-charged ion. We considered the single-charged ion (Fig. 1c) under the assumption that its formation more likely because it requires less energy for breaking one of the O–H bonds. The better agreement of the calculated absorption spectrum of the single-charged ion with the experimental spectrum corroborated our assumption. Calculation of the ion structure shows that about 80% of the negative charge of the anion is concentrated at the oxygen atom of the hydroxyl group with the detached proton (О7 or О17, Fig. 1). The two unseparated pairs of electrons of the oxygen atom remaining after the O–H bond is broken form two MEP minima, which allows two neutral water molecules to be attached to the oxygen atom (Fig. 1c).

To correctly calculate the fluorescence spectrum, we took into account the change in the chemical bond lengths of the molecule in the fluorescent state with respect to the ground state which is determined from the change in the electronic density (population) of the chemical bond according to Mulliken [32] by the dependence

$$\Delta R\text{*}{\kern 1pt} \left( {{\text{AB}}} \right) = k\Delta P\text{*}{\kern 1pt} \left( {{\text{AB}}} \right),$$

where ΔP*(AB) is the change in the population of the bond AB when passing to the fluorescent state; the value of coefficient k = 0.46 was obtained from the change in the length of the С–С bond of benzene in the transition S0 → S1 [33].

The approach we proposed earlier in [34, 35] to the study of chemical bond photobreaking in the electronically excited states is as follows. Chemical bond breaking occurs in the πσ*, σπ*, and σσ* states because the σ–σ bonds constituting the structure core of the molecule can be broken only in states of this type. The possibility of bond breaking in any electronic state of the abovementioned type is evidenced by the decrease in the strength of the bond under study.

The potential curve of the ground state of the molecule was modeled by the Morse potential [36]:

$$E\left( R \right) = D({{e}^{{ - 2}}}^{\alpha }-2{{e}^{\alpha }}),$$

where \(\alpha = {v}{{R}_{0}}\sqrt {M{\text{/}}2D} \). The bond dissociation energy (D), the equilibrium length of the bond under study (R0), the oscillation frequency (ω), and the reduced mass of the oscillator (M) are taken from the experiment. The length of the bond investigated for breaking is varied and, at each step, the energy of the excited state localized at the bond under study is calculated. Then, the potential curve of the excited state under study is plotted. The shape of the potential curve is used to judge whether the bond is broken or, on the contrary, whether it is stable.

RESULTS AND DISCUSSION

Electronic Absorption Spectra of Charged BPA Forms

The influence of different solvents (water, methanol, diethyl ether, n-hexane, cyclohexane, and mixtures) and excitation energy on the fluorescence quantum yield of compounds (phenol, anisole, p-cresol, p-methylanisole, 3,5-dimethylphenol, and 2,4,6-trimethylphenol) was studied by the authors of [37, 38]. It was found that the fluorescence quantum yield of the compounds in nonpolar solvents and in water decreased markedly. The authors suppose that one of the causes for this behavior is the participation of OH-groups. The formation of hydrogen bonds between the molecules of the abovementioned compounds and water leads to an increase in the nonradiative transitions. The proton–acceptor equilibrium of hydroxy compounds was studied in [39–42]. The authors note that photoexcitation of molecules leads to significant changes in their electronic structure and reactivity associated with proton addition or detachment. It has been shown that at neutral pH values, hydroxy compounds in water exist in ionic forms in the ground and excited states. In this study, we assume that BPA in water can also exist in ionic forms.

Figure 2 presents the absorption spectra; Fig. 3, the fluorescence spectra of BPA and its charged forms in water—cation and anion—in the region from 200 nm. The absorption spectrum of BPA in water was modeled in calculations by a hydrogen-bonded complex of the composition 1 : 2 with the distance between oxygen atoms of BPA molecules and water equal to 2.7 Å, the average value of this parameter in complexes with a hydrogen bond of the –НО…Н–ОН type bond [43].

Fig. 2.
figure 2

BPA absorption spectra in the aqueous solution at pH (a) 5.6, (b) 1, and (c) 10.11.

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Fig. 3.
figure 3

BPA fluorescence spectra in the aqueous solution at pH (a) 5.6, (b) 1, and (c) 10.11.

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As follows from the form of the BPA spectrum in water at pH 5.6 (Fig. 2, curve a), it contains two absorption bands. Processing of the absorption spectrum using the second derivative method gives two electronic transitions of 36 200 and 35 090 сm–1 for the longwave band and one for the band in the region of 43 700 сm–1. Comparison performed earlier [44] for the absorption spectra in hexane and water showed that the formation of the hydrogen-bonded complex in an aqueous solution of BPA (BPA + 2H2O) in fact did not change the position of the longwave band and its intensity but shifted the band with a maximum at 43 700 сm–1 towards low energies. In addition, it was noted that the longwave band includes an electronic transition S0 → S3 (πσ*) the σ*-orbital of which is localized at the С5–С8–С11 bond (Fig. 1). Earlier, it was shown [45] that it is the electronic transition that is responsible for the break of one of BPA bonds (С5‒С8 or С8–С11).

Comparison of the absorption spectra of the BPA + 2Н2О complex and the BPA + 2Н+Н2О cation (q = +2e) reveals their almost complete identity: two absorption bands in the region of 36 200 and 43 700 сm–1 (Fig. 2). Analysis of the calculation results shows a significant difference only in the populations (strengths) of ordinary BPA bonds. In particular, the formation of a cation carrying a charge +2е in the state S6 (πσ5–8–11) strengths the С5–С8 and С8–С11 bonds and weakens the С8–С9 and С8–С10 bonds. However, in S3 (πσ*) state, the С5–С8 and С8–С11 bonds remain weaker. The population of these bonds in the BPA + 2Н+Н2О cation in the ground and first singlet excited states insignificantly differs from the population of the same bonds in the complex with water (Table 1).

Table 1. Populations (PAB) of ordinary bonds in the complex with water and in charged forms of bisphenol A
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As for the strength of the О–Н bond, it increases in the cation as compared to the BPA + 2Н2О complex and is almost unchanged in all states presented in Table 1. This fact indirectly corroborates that this group does not take part in the formation of absorption spectrum bands of the BPA cation in the spectral range of >200 nm.

Considerably greater differences with respect to the spectrum of the BPA + 2Н2О complex take place in the absorption spectrum of the BPA anion (Table 2). It is known that the isolated BPA molecule is nonplanar [44, 45], and its complex with water and its charged forms are also nonplanar (Fig. 1a). The core of the BPA molecule consists of two identical phenol fragments connected by the С(СН3)2 group; the planes of benzene rings are at an angle to each other. The middle fragment of the С(СН3)2 molecule prevents the appearance of π-conjugation between the phenyl rings, which leads to the existence of two isolated π‑systems in the BPA molecule. The nonplanar structure of BPA also leads to a strong “mixing” of atomic wave functions of π- and σ-types. Therefore, referring an electronic state to a particular orbital type (ππ*, πσ*, or σσ*) is conventional depending on the contribution of the corresponding MOs in the configurations forming one or another electronic state.

Table 2. Experimental and theoretical characteristics of absorption spectra of the complex with water and charged BPA forms in water
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Orbital mixing of ππ*- and πσ*-states manifests itself in the orbital nature of excited states of the singly charged BPA + 2Н2О anion. In contrast to two previous structures, the S1 state of which is classified as the ππ*-type state, the orbital nature of the S1 state of the BPA anion is “mixed” (Table 2). In addition, more than 50% belong to the configuration of the πσ*-type the atomic orbital σ* of which is localized at the С5‒С8 and С8–С11 bonds.

The absorption spectrum of the anion is bathochromically shifted (Fig. 2, curve c) relative to the BPA spectra in water at pH 5.6 and 1 (Table 2). The absorption spectrum of the BPA anion in the region of >280 nm overlaps with the spectrum of solar radiation (>300 nm). Probably, this leads to an increase in the absorption of solar radiation by the BPA anion and to a higher probability of the break of one of the single C–C bonds (С5–С8 or С8–С11), i.e., the bond from which the proton is detached during the formation of the singly charged anion. As for the strength of the broken bond in the anion, according to calculations it changes little in comparison with the BPA + 2Н2О complex in the ground and ππ*-states but is less strong in comparison with the BPA + 2Н+Н2О cation in the πσ\(_{{5 - 8}}^{*}\)-state. The strength of the С–С(СН3) bonds also decreases (Table 2) but remains noticeably stronger than the С5–С8 and С8–С11 bonds in the πσ* state localized at these bonds.

According to the experimental data, the longwave absorption band of the BPA + 2Н+Н2О cation is shifted to the longwave region by only ~100 сm–1 (Fig. 2). Taking into account that the strength of the broken С5–С8 bond in the cation is higher than in the anion (Table 2), one should not expect an increase in the efficiency of С5–С8 bond breaking in the cationic BPA form. Comparison of the calculation results with the experiment (Table 2) for the recorded absorption bands of the charged BPA forms demonstrates a satisfactory agreement of the calculation with the experiment. According to the calculations, the longwave absorption band at ~36 200 nm in all structures is formed by several electronic transitions, mainly of the ππ*-type, which form the band intensity. Similarly, the ~43 700 nm band is formed (Table 2). Analysis of the orbital nature of the electronic transitions in the region of 200–600 nm shows that the electronic transitions forming the absorption of these two bands in the absorption spectrum are localized on phenyl rings. The hydroxyl and CH3 groups of the central fragment of the molecule do not participate in the formation of electronic transitions in the region of bands of 36 200 and 43 700 сm–1.

Fluorescence Spectra of Charged BPA Forms

BPA weakly fluoresces both in hexane and in water. Since the absorption spectra of BPA and solar radiation have little overlap, fluorescence of the aqueous BPA solution was excited at the edge of the longwave absorption band of the molecule. The fluorescence band maximum of the isolated molecule (solution in hexane) in the experiment lies at ~300 nm (33 300 сm–1) [34, 36]. The fluorescence band maximum of the aqueous BPA solution (BPA + 2Н2О complex) is located at ~306 nm (32 300 сm–1). Table 3 presents the calculated fluorescence characteristics for aqueous solutions of BPA and its charged forms in comparison with the experiment. It follows from the tables that the calculation results for the charged forms of BPA and experiment coincide satisfactorily in the position of the fluorescence band and worse in the fluorescence quantum yield. The latter may be related to the failure to take into account some of decay channels of the fluorescent state of the molecules. According to calculations, the main reason for the low fluorescence quantum yield of the BPA + 2Н2О complex (Table 3) is the effective singlet–triplet conversion process in the S1(ππ*) ⤳ Tn(πσ*) channel.

Table 3. Calculated and experimental characteristics of BPA fluorescence, BPA complex with water, and BPA charged forms
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It follows from data of Table 3 that in all the structures considered, the efficiencies of the nontradiative channels of the fluorescent state decay are close, while the efficiencies of the radiation channel differ more significantly. The calculated anionic form has the lowest fluorescence quantum yield (Table 3). This is caused by the change in the orbital nature of the fluorescent state of the anion form as compared to other structures studied. The fact is that the orbital nature of the S1 state of the anion is mixed and consists of ππ*‑and πσ*-type configurations, where the contribution of the πσ*-type configuration exceeds that of the ππ*-type configuration. The small oscillator strength of the S0 → S1 transition (Table 2) leads to a small rate constant of radiative decay, which, along with the high rate constant for singlet–triplet conversion, just gives an anion quantum yield equal to ~10–5.

Photolysis of Charged BPA Forms

In quantum chemical calculations, the electron density is distributed between atoms of the molecule and bonds between them. The value of the electron density on the bond and the population of the bond indicate the strength of the chemical bond. An increase in the bond population indicates its strengthening (an increase in bonding energy); a decrease, on the contrary, indicates its weakening (a decrease in its energy). As follows from comparison of population of bonds between atoms of the same type, such as carbon atoms, single C–C bonds are less strong. It follows from data in Table 1 that the С–(СН3) and OH bonds in the ground state are the weakest in the BPA molecule; in the πσ*-type states, however, the С5–С8 and С8–С11 bonds turn out to be weaker. Moreover, as mentioned above, the С–(СН3) and О–Н bonds do not take part in the formation of electronically excited states forming the absorption in the spectral range of >200 nm. It also follows from data in Table 1 that single bonds in this molecule under excitation change differently depending on the orbital nature of the excited state. The greatest decrease in bond strength is characteristic of the С5–С8 and С8–С11 bonds in the states localized on these bonds. In the positively charged BPA form, the С5–С8 and С8–С11 bonds are somewhat strengthened; in the anion, populations of bonds in the OH group from which the proton is detached (С5–С8 or С8–С11) do not change as compared to populations of these bonds in the analogous excited state of the BPA + 2Н2О complex. Table 1 presents the bond populations only in the singlet states. A similar trend also holds for triplet states of similar orbital nature and localization.

The Morse potential curve for the С5–С8 (or С8‒С11) bond was constructed using the following experimental values: the equilibrium bond length R0 = 1.5 Å [44], the dissociation energy of the ordinary С–С bond D = 346 kJ/mol [46], and ω = 1000 сm–1 [47]. The calculated potential curves of singlet excited states of different forms of the BPA molecule are presented in Fig. 4, where the length of the С8–С11 (or С5–С8) bond is taken as the reaction coordinate; it varied from 1.5 to 3.0 Å.

Fig. 4.
figure 4

Potential curves of the ground and excited photodissociative states of the (a) BPA + 2Н2О complex, (b) cation, and (c) BPA + 2H2O anion. ΔE is the potential barrier of the photolysis.

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Earlier studies of the photolysis of polyatomic organic molecules showed that the molecule was stable in various compounds in the ππ*-type states [34, 35]. The same is true for the isolated BPA molecule, its BPA + 2Н2О complex [44], and charged BPA forms. The character of the potential curves of ππ*‑type states of the complex and the charged BPA forms is similar to the character of the curves of this type in the previously studied molecules [34, 35]; for this reason, the potential curves of the ππ*-type structures under study are not shown in order to avoid overloading of the figure. The potential curves of πσ*-type states localized at one of the breakable bonds (С5–С8 or С8–С11) of BPA have a significant potential barrier to photolysis. Nevertheless, the authors [48] experimentally discovered by liquid chromatography in deionized water a product of BPA degradation under sunlight irradiation the formation of which is possible only when one of BPA bonds (С5–С8 or С8–С11) is broken.

Potential curves of the photodissociative states of charged forms have the following potential barriers: ~12 100 сm–1 for the BPA + 2Н+Н2О cation, which is higher by ~1000 сm–1 than for the BPA + 2Н2О complex, and ~9700 сm–1 for the anion, which is lower by ~3000 сm–1 than the potential barrier for photolysis of the BPA + 2Н2О complex. The shape of the potential curve of the S1 state (ππ* + πσ\(_{{5 - 8}}^{*}\)) of the anion differs from similar curves of the BPA + 2Н+Н2О cation and the BPA + 2H2O complex by the absence of the drop of the potential curve of the photodissociative state with the growth of the length of the broken bond. However, such facts as (i) a significant increase in the overlap of the absorption spectra of the BPA anion and solar radiation, which increases the amount of absorbed energy, (ii) decrease in the strength of the broken bond in the photodissociative state, and (iii) decrease in the potential barrier of the photodissociative state in comparison with the potential barrier of the BPA + 2Н2О complex (Fig. 4) suggest that the probability of photodetachment of the С8–С11 (or С5‒С8) bond in ionic BPA forms fits into the following series: anion > isolated molecule > BPA + 2Н2О complex > BPA + 2Н+Н2О cation. Note that the authors [49] demonstrated that ozonation of the BPA solution facilitated the process of its photolysis when the solution is alkalized to pH 10.8, i.e., with the formation of the BPA anion.

CONCLUSIONS

It has been established experimentally that absorption spectra of charged forms contain no new absorption bands in the region of 200–600 nm as compared to the neutral form. Absorption spectra of ionic BPA forms are shifted to the longwave region in relation to the neutral form: insignificantly in the case of the cation and noticeably longer for the anion with an increase in the intensity of absorption bands.

The low fluorescence quantum yield of BPA and its ionic forms in water is caused by the significant predominance of the singlet–triplet conversion channel over the channel of radiation decay of the fluorescent state. The nonplanar structure of BPA and its charged forms leading to mixing of ππ*- and πσ*-type orbitals, which is typical of the fluorescent anion state, leads to a noticeable decrease in the radiation decay efficiency of the anion form and, together with the singlet–triplet conversion, to the lowest fluorescence quantum yield among the considered structures.

Quantum chemical calculations demonstrate that the efficiency of BPA cation photolysis under the action of solar radiation is lower than in the case of the BPA + 2H2O complex, as a consequence of an increase in the potential barrier. The efficiency of anion photolysis should be expected to be higher than that of the cation and complex with water due to a decrease in the potential barrier, as well as an increase in the overlap of the absorption spectra of the anion and the solar radiation.