Generation of stable carbocations in

Generation of stable carbocations in polydiphenylenesulfophthalide and orthosubstituted polytriarylcarbinols upon water desorption Nikolai M. Shishlov, Shamil S. Akhmetzyanov & Sergey L. Khursan

Journal of Polymer Research ISSN 1022-9760 Volume 22 Number 4 J Polym Res (2015) 22:1-12 DOI 10.1007/s10965-015-0698-2

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Author's personal copy J Polym Res (2015) 22: 58 DOI 10.1007/s10965-015-0698-2

ORIGINAL PAPER

Generation of stable carbocations in polydiphenylenesulfophthalide and ortho-substituted polytriarylcarbinols upon water desorption Nikolai M. Shishlov & Shamil S. Akhmetzyanov & Sergey L. Khursan

Received: 20 November 2014 / Accepted: 9 March 2015 / Published online: 24 March 2015 # Springer Science+Business Media Dordrecht 2015

Abstract Formation of Bgreen^ color centers (CCs) with two absorption bands (ABs) in the visible spectrum at 480 and 610 nm upon evacuation and/or weak heating (~100 °C) of f i l m s o f p o l y t r i a r y l c a r b i n o l ( P TA C ) a n d polydiphenylenesulfophthalide (PDSP) due to loss of sorbed water was observed. Electronic spectra (ES) of solutions of compounds that simulate polymers, PTAC and PDSP polymers in sulfuric acid were obtained. Isolated CCs having ABs in electronic spectra (ES) at 462 and 595 nm were generated by Bslight^ treatment of a PTAC film with sulfuric acid. The ES of a series of model carbocations and energies of ionization of the model triphenylcarbinols by proton, oxonium ion and Li+ ion were calculated in B3LYP/6-311G(d,p) approximation. Experimental and theoretical results are showed that the CCs in question are carbocation nature. The paper discusses the ionization mechanism in polymers and some aspects of heterogeneous (in water) and homogeneous (in organic polar solvent) methods of PTAC preparation.

were found to appear reversibly during evacuation and/or weak heating (~100 °C) of PTAC films [1–3]. These CCs have two absorption bands (ABs) in the visible spectrum at 480 and 610 nm which disappear (or weaken considerably) during storage of the polymer in the air. Since evacuation and heating result in polymer dehydration, it was assumed that CCs appear due to loss of sorption water, but they were not assigned to any particular type of species [3]. The emergence of green coloring in dehydrated PTAC films was later confirmed [4, 5]. The authors [4, 5] did not pinpoint the ABs responsible for this coloring and did not attribute them to any particular centers, though they attempted to relate the observed coloring with some processes involved in alkaline hydrolysis of PDSP, based on just one band at 566 nm. In this paper we describe some properties of the Bgreen^ coloring centers mentioned above. The main goal is to establish their chemical nature.

Experimental section Keywords Polytriarylcarbinol . Polydiphenylenesulfophthalide . Carbocations . Electronic spectra . TD DFT method

Introduction The preparation of polytriarylcarbinol (PTAC) by alkaline hydrolysis of polydiphenylenesulfophthalide (PDSP), see Scheme 1, has been reported [1]. Even in the first works on the preparation and study of PTAC, green color centers (CCs) N. M. Shishlov (*) : S. S. Akhmetzyanov : S. L. Khursan Ufa Institute of Chemistry of Russian Academy of Sciences, Pr. Oktyabrya 7 1, Ufa 450054, Russia e-mail: [email protected]

A synthesis of polydiphenylenesulfophthalide was reported in [6]. A preparation of salt-type polytriarylcarbinols by alkaline hydrolysis of PDSP was reported previously [1]. To prepare high quality films, the characteristic viscosity of the polymers should be no lower than 0.7–0.8 dL g−1. Strong transparent polymer films 5–100 μm thick were obtained by pouring solutions of the polymer in cyclohexanone (PDSP) or methanol (PTAC) onto cellophane or glass, followed by solvent evaporation at 25 °C and additional film drying at 100–150 °C. Electronic spectra of polymer films were recorded in an evacuated quartz cell. Electronic spectra of solutions of model compounds and polymers in 98 % sulfuric acid were recorded in standard quartz cells with a thickness of l=0.5 cm. The UV–VIS spectra were recorded at room temperature on Specord M-400 and Shimadzu UV-365 spectrophotometers, the IR spectra were obtained using Specord M-80

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J Polym Res (2015) 22: 58

Scheme 1 Alkaline hydrolysis of polydiphenylenesulfophthalide

spectrophotometer. The films were stored from 2 to 6 weeks at room temperature without controlling the air humidity. Calculations were carried out using Gaussian-09 program, Revision C1 [7]. The results were visualized using ChemCraft program [8]. Optimization of geometric parameters, calculations of energy characteristics and electronic spectra were carried out using the B3LYP method of density functional theory with the 6311G(d,p) basis set [9–11]. Conformity of the structures found to the minima on the potential energy surface was ascertained based on the absence of negative elements in the diagonalized Hessian matrix. Thermodynamic corrections were calculated for standard conditions. The heat effects of the reactions were calculated as the difference of absolute enthalpies of the final and initial states of a transformation of interest. Absolute enthalpies were found as the sum of the total energy as well as the zero-point energy and the thermal correction for enthalpy change from zero to 298 K. The latter values were determined by known equations of statistical thermodynamics using calculations of frequencies and moments of inertia.

Results and discussion

480 nm nearly disappears (Fig. 1, curve 5). A further increase in the heating temperature shifts λmax to 595 nm and increases the intensity of this AB considerably (Fig. 1, curves 6, 7, 8). It is also evident from Fig. 1 that heating a PTAC film results in the emergence and growth of a band at ~410 nm in its electronic spectrum. This AB was assigned to absorption of triarylmethyl-type radicals, while the AB at 595 nm was assigned to absorption of quinoid structures of Chichibabin’s hydrocarbon type [2, 3, 12]. Thus, apparently, at heating temperatures of 150–200 °C the Bgreen^ centers with ABs at 610 and 480 nm are replaced by Bblue^ quinoid structures with an AB at 595 nm, and the two CCs types can be represented in the spectra simultaneously, giving an additive band in the range of 595–610 nm. It was found that Bgreen^ coloring centers with ABs at ~610 and 480 nm exist in PDSP, too (Fig. 2, curve 1). The number of these centers increases abruptly upon evacuation and/or heating to 130–150 °C (Fig. 2, curves 2, 3). Heating a PDSP film at temperatures above 150 °C reduces the number of Bgreen^ CCs, up to total disappearance (Fig. 2, curves 4, 5). The band at ~410 nm that appears in the spectrum of PDSP, like that in the PTAC spectrum, corresponds to radicals [12, 13].

A spectral study of PTAC and PDSP Previously, the spectra of green CCs in PTAC were mostly characterized by the positions of absorption band maxima and were not studied in more detail. Therefore, let us first present the results of a spectral study of the green coloring phenomenon for PTAC. The lithium polymeric salt was mostly studied. However, it should be noted at once that green coloring is also observed in the corresponding sodium and potassium salts. Owing to the low concentration of Bgreen^ CCs, it is more convenient to study them in thick polymer films. The original PTAC-Li+ film 60 μm thick is already greenish, which was found to be due to a weak AB around 610 nm (Fig. 1, curve 1). Evacuation of the film results in a growth of the AB at 610 nm and emergence of a weak AB at ~480 nm (Fig. 1, curve 2). Warming an evacuated film at 100–150 °C results in some growth of the AB at 610 and 480 nm (Fig. 1, curves 3, 4). On heating at 184 °C, the intensity of the AB at 610 nm decreases and λmax shifts to ~600 nm, whereas the AB at

λ,

Fig. 1 Electronic absorption spectra of a PTAC-Li+ film (d=60 μm): original film (1); film evacuated for 1 h at a pressure of 10−4 Torr (2); film heated for 20 min at 130 °C (3), 146 °C (4), 184 °C (5), 200 °C (6), 210 °C (7), 246 °C (8)

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λ,

Fig. 2 Electronic absorption spectra of a PDSP film (d=100 μm): original film (1); film evacuated for 1 h at a pressure of 10−4 Torr (2); film heated for 20 min at 130 °C (3), 154 °C (4), 190 °C (5)

Heating of non-evacuated PTAC and PDSP films at 80– 150 °C results in intensification of their green coloring and gives ABs at 610 и 480 nm in the ES. Storage of polymer films in air, both after evacuation or after heating without evacuation, leads to a decrease in the ABs at 610 and 480 nm and hence a gradual weakening of the green coloring. If the films are evacuated and/or heated again, the green coloring and the corresponding ABs increase. As noted in [3], the intensity of the ABs at 610 and 480 nm in the case of PTAC films shows an antibate behavior to that of the ABs of deformation vibrations of sorbed water at 1630 cm-1 in the IR spectrum of PTAC, which allows us to attribute the origination and decay of Bgreen^ CCs to the water desorption and sorption processes, respectively. If one compares the near IR spectrum with the visible spectrum, as shown in Fig. 3, it is evident that the concentration of green CCs is related to the water content in a PTAC polymer. The absorption at ~1912 nm belongs to

λ,

Fig. 3 Electronic spectra of a PTAC Li film (d~30 μm): original film (1); after heating for 10 min at 150 °C (2); after storage of a pre-heated film for 1 h in air (3) -

+

the combination vibration band of water molecules δ(H-O-H) + ν(O-H) [14]. The intensity of this AB can be used as a measure of the water content in the polymer. One can see from Fig. 3 that the behavior of the AB at 610 nm is antibatic to that of the AB at 1912 nm. This observation is very useful from methodical point of view in further studies of the CCs in question, since it shows that one can monitor the content of both CCs and adsorbed water in the polymer while recording a single spectrum in the range of 200–2500 nm. Thus, evacuation and heating (to < 150 °C) of PTAC and PDSP films give rise to green CCs of apparently same structure, with absorption bands in the visible spectrum at 480 and 610 nm. These centers decay at Т>150 °C, and this decay is accompanied by a growth of the AB of triarylmethyl radicals with an absorption maximum at ~ 410 nm. One can see from Figs. 1 and 2 that the original PTAC and PDSP films already manifest weak ABs at ~ 610 nm, while the films themselves have a greenish tint. Naturally, a question arises concerning the nature of the observed green coloring centers. Below we present experimental and theoretical data that, in our opinion, allow us to obtain the most likely answer to the main question of our study.

Electronic spectra of model systems Based on the well-known ability of triphenylcarbinol to be ionized on Broensted and Lewis acidic sites of dried zeolites to give stable triphenylmethyl carbocations [15, 16], it is reasonable to assume that the Bgreen^ CCs observed in PTAC belong to triarylmethyl-type carbocations (hereinafter referred to as carbocations). To check our hypothesis, we might create carbocations in the polymers of interest by an established method and compare their optical spectra with those of Bgreen^ CCs. It is known that stable carbocations can be obtained by dissolution of starting compounds in strong inorganic acids. In particular, this method was used to obtain a series of triarylmethyl carbocations [17, 18]. First, the ionization capability in concentrated sulfuric acid was tested for compounds simulating PDSP and PTAC polymers, namely, diphenylsulfophthalide (DPSP) and the potassium and sodium salts of DPSP.


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Dissolution of these compounds in sulfuric acid produces yellow coloring. Furthermore, an intense AB at 452 nm with a shoulder around ~400 nm and a comparatively weak AB at 310 nm appear in the electronic spectra of the solutions (Fig. 4). The resulting spectra of these three compounds (Fig. 4, curves 2, 3, 4) nearly coincide and are generally similar to the spectrum of the triphenylmethyl carbocation. For example, the bands at 290, 400 and 425 nm were previously assigned to the absorption of this carbocation on kaolinite [16]. The spectrum of the triphenylmethyl carbocation that we obtained in sulfuric acid from triphenylcarbinol (Fig. 4,

curve 1) coincides with the reported spectra [17, 18]. The optical spectrum of a sulfuric acid solution of diphenylphthalide (DPP), which is a structural analogue of DPSP, also coincides in the long-wave region with the spectrum reported in [19], both in shape and in the absorption maximum position, i.e., 448 nm (446 nm in [19]). It is interesting to note that DPP ionization under our experimental conditions occurs incompletely, as evidenced by the partial retention of the AB at 260 nm (Fig. 3, curve 5). Apparently, an equilibrium is established in this case between the ionized and non-ionized DPP forms:

Thus, it follows from the results obtained that ionization of model compounds, each representing a monomeric unit of the polymers of interest, in concentrated sulfuric acid gives triphenylmethyl carbocations whose electronic spectra have an overall resemblance to the spectra of other known triphenylmethyl carbocations.

dissolution of PDSP and the sodium salt of polytriarylcarbinol (PTAC-Na+) in 98 % sulfuric acid, a broad intense AB with a maximum at ~690 nm and a distinct shoulder at ~465 nm appears in the ES of the solution (Fig. 5, curves 2, 3, 4). This absorption gives a bright green coloring to the polymer solutions. Let us note some significant details of the experiments that we performed:

Generation of carbocations in polymers

&

To expand the scope of experimental data, we generated carbocations directly in the polymers being studied. Upon

Ionization of PDSP occurs in the course of heterolysis of the sulfophthalide ring, whereas ionization of PTAC is accompanied by elimination of a hydroxide ion. At first

λ,

λ,

Fig. 4 Electronic spectra of solutions in 98 % sulfuric acid: 1 – triphenylcarbinol; 2 – TAC - Na + ; 3 – TAC - K + ; 4 – DPSP; 5 – diphenylphthalide

Fig. 5 Electronic spectra of solutions in 98 % sulfuric acid: 1 – polyterphenylenesulfophthalide; 2 – PTAC-Na+; PDSP solution with various viscosities: 3 – η= 0.49 dL g-1; 4 – η= 0.78; 5 – η = 0.78; storage for 48 h


& & &

&

glance, the carbocations formed in both cases should differ in the counter-ions at the sulfonate anion. However, this assumed difference is apparently not observed in sulfuric acid since the ES of carbocations in the compared polymers are nearly identical (Fig. 5). PDSP polymers with different characteristic viscosity, and hence with different molecular masses, give similar electronic spectra (cf. spectra 3 and 4 in Fig. 5). Prolonged storage (48 h) of a PTAC-Na+ solution in sulfuric acid results in accumulation of unidentified products with absorption in the range of 250–350 nm. Replacement of the biphenylene bridge in polyarylenesulfophthalide by a terphenylene one causes a long-wave shift of the observed ABs to 500 and 760 nm; a somewhat smaller bathochromic shift (to 742 nm) was observed in polyarylenephthalides [20]. A solution of polyterphenylenesulfophthalide (PTSP) in H2SO4 turns blue. There is a strong bathochromic shift of the long-wave AB of the carbocations in PDSP and PTAC in comparison with the positions of similar ABs both for monomeric carbocations with two para-biphenylene substituents (538 nm [17] and 581 nm [18]) and for carbocations with similar structures obtained by d is solution of polydiphenylenephthalide in sulfuric acid (590 nm [21]).

As concerns the main goal of this study, i.e., determination of the chemical nature of CCs, the following should be noted: the ES of polymeric carbocations that we recorded in sulfuric acid (Fig. 5) have two distinct ABs that, however, differ considerably in shape and position from the ABs of the desired Bgreen^ CCs. In addition to these obvious bands, the overall absorption definitely contains some other hidden bands: for example, the spectra contain an inflexion at 580–600 nm. If the green coloring centers are carbocations, then what is the reason for the considerable difference between the spectra of carbocations in polymer films and in solutions in sulfuric acid? It is believed that interactions between the adjacent carbocations in polydiphenylenephthalide may result in polyconjugation and bathochromic shift of their longwave AB [22]. The authors of Ref. [22] do not elaborate on their assumption. However, the most reasonable

Scheme 2 Expected quinoid conjugation in polydiphenylenephthalide

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mechanism of polyconjugation may be depicted as follows (Scheme 2): To evaluate the possibility of interaction between the green CCs let us estimate their concentration in the polymer. The extinction coefficient of triarylmethyl carbocations at the absorption maximum is generally 104–105 L mol−1 cm−1 [17]. Using these data and those derived from the spectra in Fig. 1, we obtain that the maximum concentration of Bgreen^ CCs in PTAC films is ~10−2–10−3 mol L−1, which corresponds to one coloring center per 400–4000 monomeric units of the polymer. Hence, Bgreen^ CCs are most likely isolated centers. Therefore, in order to identify the coloring centers reliably, we need to obtain isolated carbocations in the polymer being studied and compare their ES with those of Bgreen^ CCs. It was shown previously that the number of alcohol groups in PTAC decreased to the limit detectable by IR spectroscopy upon polymer heating at ~280 °C [3], though the exact structure of the polymer formed upon elimination of OH groups was not identified. Dissolution in sulfuric acid of PTAC-Na+ preheated at 278 °C gives a green coloring and two ABs at 460 and 634 nm in the ES (Fig. 6, curve 4). Thus, ionization of Bisolated^ carbinol groups in partially degraded PTAC with sulfuric acid gives carbocation centers whose ES is already more similar to the ES of the Bgreen^ CCs in PTAC and PDSP. It would be tempting to obtain isolated carbocations directly in a polymer film. It was found that if one cautiously applies a small amount of sulfuric acid on the surface of a PTAC-Na+ film, the film turns bluish-green and ABs at 462 and 595 nm are recorded in its ES (Fig. 6, curve 3). As one can see, a spectrum obtained in this case is even more similar to that of Bgreen^ CCs. The most natural conclusion from the analysis of the experimental results is that Bgreen^ CCs are isolated carbocations. It should be admitted that carbocations were mentioned among the broad list of potential coloring centers in PTAC [5]. Green coloring is also observed in polyfluorenylenesulfophthalide films whose electronic spectra manifest two absorption bands at ~470 and ~630 nm. The position, shape and intensity ratio of these ABs give us a reason to attribute them to the absorption of carbocations, i.e., the spontaneous ionization phenomenon is likely to be inherent in other polyarylenesulfophthalides as well.


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λ,

Fig. 6 Electronic absorption spectra: 1 – PDSP film (d= 100 μm) evacuated for 1 h at 10−4 torr; 2 – PTAC-Li+ film (d=60 μm) evacuated for 1 h at 10−4 torr and heated for 20 min at 130 °C; 3 – PTAC-Na+ film surface-treated with 98 % sulfuric acid; 4 – solution of PTAC-Na+ heated for 20 min at 278 °C in 98 % sulfuric acid

Theoretical simulation of the ES of carbocations Isolated carbocations in the polymers in question should have the structure

where the positive charge is mainly delocalized over two biphenylenes and one ortho-substituted phenyl group. To provide a theoretical substantiation of our assignment of green CCs to carbocations, we calculated the electronic spectra of a series of triarylmethyl carbocations that serve as model structures for the assumed carbocation species formed in PDSP and PTAC. Currently, the TD DFT method with various functionals and basis sets of wave functions is used rather

successfully in ES calculations [23]. For example, the theoretical spectra of over 100 dyes obtained by this method, mainly in B3LYP/6-31 + G(d,p) approximation, showed fairly good agreement with experimental data [24]. In [25], the spectra of a number of triarylmethyl cations simulated by TD DFT method reproduced the experimental spectra quite satisfactorily. In these simulations, the trend of variation in the positions of the most intense ABs is reproduced correctly, while the systematic deviation is taken into account by an empirical correction. In fact, the theoretical ES of carbocation 1 (Table 1) calculated in TD DFT B3LYP/6-311G(d,p) approximation well reproduces the intense ABs in the experimental spectrum [17, 18, 26], especially taking into account that the theoretical spectra were computed for isolated molecules, whereas the experimental spectra were obtained in condensed phase where various intermolecular interactions occur. The hypochromic shift of the longwave ABs in the experimental spectra is likely to be due to the effect of polar solvents [27], strong mineral acids in this case. The structures of the model carbocations for which ES have been computed are shown in Fig. 7. Replacement of biphenyl substituents by fluorenyl ones and addition of a salt group (SO3-Li+) do not strongly affect the overall spectra of structures 2 and 3: their theoretical electronic spectra are characterized by strong transitions in the range of 550–570 nm and medium intensity transitions at 470–490 nm. Addition of methyl and tert-butyl substituents at the para positions of the biphenyl moieties in the carbocations (structures 4 and 5) results in a bathochromic shift of the two most intense transitions in the theoretical ES but does not change the overall shapes of the spectra. Computations for structure 6, which is most similar in structure to the carbocation assumed in PTAC, show that the most intense electronic transition in the ES is at 617 nm. And while the most intense long-wave transitions in carbocations 1–5 mainly correspond to HOMO→LUMO transitions and the shorter-wave ones, that are next in intensity, correspond to HOMO-1→LUMO transitions, the main contribution to the computed absorption band of carbocation 6 at 617 nm is made by the HOMO-4→LUMO transition. The theoretical spectra

Table 1 Comparison of computed and experimental electronic spectra for structure 1.а Theoretical electronic spectra of structures 2–7.а TD DFT calculations in B3LYP approximation with basis set 6-311G(d,p) 1

2

3

4

5

6

7

λ

f

λb

εb

λc

λ

f

λ

f

λ

f

λ

f

λ

f

λ

f

562 491 362 348

0.8712 0.2748 0.1652 0.133

538 460 352

73,000 51,000 8800

540 461 353

551 469 353

1.1008 0.255 0.1108

568 498 477

0.9028 0.2335 0.0359

604 520 424

1.0505 0.2738 0.0308

617 527 424

1.1438 0.2784 0.0340

956 642 617 558

0.1209 0.0683 1.0381 0.0482

271

22,000

278

614 565 499 494 463

1.1095 0.136 0.7241 0.3545 0.1061

a

The structure of carbocations is shown in Fig. 7; wavelength λ, nm; f – oscillator strength; e – molar extinction, L mol 1 cm 1

b

[17]

c

[26]


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C

C CH2

CH2

1

2 H3C

C

C

CH3 - +

- +

SO3 Li

SO3 Li

3

4

C -

SO3 Li+ 5 OH

OH C -

-

-

SO3 Li+

+

SO3 Li

SO3 Li+

6 C

C -

+

SO3 Li

-

+

SO3 Li

7 Fig. 7 Model structures for which theoretical electronic spectra presented in Table 1 were computed

of model structures 1–5 qualitatively match the spectra of green CCs: they contain a strong transition in the region of ~570–620 nm and a much less intense transition at 480– 520 nm. Compared to monocations 1–3, the intense transition in the theoretical spectrum of dication 7 is shifted by almost 50 nm to longer waves; furthermore, the relative intensity of transitions near 500 nm increases. Thus, the mutual effect of just two cations localized at adjacent monomeric units causes both a considerable bathochromic shift of the most intense transition and emergence of new intense absorption bands. This result agrees with the aforementioned regularities of bathochromic shifts and AB broadening in polycations (see Fig. 5 and the corresponding explanations in the text). It should be noted that the angle between the phenyl rings in the biphenylene bridge linking the cationic centers in the optimized structure of dication 7 increases from 27 to 38° in comparison with the terminal biphenyls. An angle of ~27о is also characteristic of the biphenyls in monocations 1, 3–6. This fact makes us doubt that the conjugation effect is the reason behind the bathochromic shift of the long-wave band in the electronic spectrum of polycations in polymers. Apparently, mutual repulsion of similar charges decreases conjugation, while the bathochromic shift of bands in the electronic spectrum is

caused by the effect of the electric field of the adjacent atoms. It is well known that strong electrostatic field results in the decreasing of frontier molecular orbital energy gap [28]. Thus, the computation results for ES of the model carbocations do not contradict the assigning of green CCs to single carbocations in a polymer chain. Mechanism of carbocation formation Let us discuss the formation mechanism of carbocations in general terms. By analogy with triphenylcarbinol [15, 16], it may be assumed that ionization of carbinol units in the polymer on Broensted (H+, Hδ+) or Lewis (M+, catalyst residues) acidic centers occurs. The total thermal effects for model reactions I-VI of carbinol ionization (Scheme 3) calculated in B3LYP/6-311G(d,p) approximation are shown in Table 2. One can see from the data in Table 2 that spontaneous carbinol ionization is nearly impossible, whereas ionization upon interaction with a lithium ion becomes energetically favorable, and the reactions with a proton or oxonium ion give a very significant positive thermal effect. As shown in [29], hydration of PDSP produces a sulfonic acid. It is a strong acid: the pKa of toluenesulfonic acid is -2.8


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Scheme 3 R = Li, X = no reagent (I), X = H+ (II), X = H3O+ (II′), X = Li+ (III), X = AlCl3 (IV); R = H, X = H+ (V), X = H3O+ (V′); R = Na, X = Na+ (VI)

in aqueous media [30], so PDSP hydrolysis creates prerequisites for subsequent ionization of polymeric triphenylcarbinol o n B r o e n s t e d c e n t e r s . I n f a c t , i n t h e BN a f i o n triphenylcarbinol^ composite system suggested by Sadaoka et al. [31] as a humidity sensor with optical recording, at reduced humidities the triphenylcarbinol incorporated in the film is ionized to a carbocation on the protons generated by the sulfonic acid that is part of Nafion structure. As the air humidity and hence the water content in the polymer increase, the acidity in the film decreases, so the number of carbocations decreases, as shown by the absorption band of the triphenylmethyl carbocation at 450 nm. The following scheme was suggested for the assumed reactions [31]:

A similar mechanism of ionization of carbinol polymer units on Broensted sulfonic acid centers probably occurs in PDSP, where hydration and subsequent hydrolysis of sulfophthalide rings give rise to alcohol and sulfonic acid groups [29]. The ionization process in PDSP can be presented as simplified Scheme 4. A carbinol group and ionization center (H+) that are genetically related interact in this scheme. Clearly, one can imagine that carbinol groups interact with ionization centers belonging to other units or even to other macromolecules (Scheme 5). Possibly, the stability of ions will be higher in this case than if they were located in the same unit. It is implied in the schemes shown here that some obstacles to ion recombination exist,

Table 2

e.g., hydration or molecular conformational effects in the polymer. It is well known that alkali metal ions correspond to Lewis acids [32] and can actively participate in ionization reactions to give carbocations, and their ionizing capability decreases with an increase in the ionic radius [33, 34]. The thermal effect in the reactions I-VI shown above characterizes the relative ionizing capability of the ions in question and, in this context, can serve as a measure of their Lewis acidity. These results qualitatively agree with the experimental data obtained in [35], where it was shown that alkali metal salts generate trityl cations from trityl chloride and tritylcarbinol. In this case, the ionizing capability of lithium ions toward trityl chloride is comparable to that of oxonium ion, whereas the activity of lithium ions toward tritylcarbinol is much lower than the activity of oxonium ions but higher than that of sodium ions (cf. the results for the ionizing capability of lithium and sodium ions, Table 2). It has also been shown in [35] that hydrated salts of alkali metals have a lower ionizing activity than anhydrous salts, i.e., water decreases the Lewis acidity of Li+ and Na+. Our calculations and literature data [35] indicate that ionization of the carbinol moieties in PTAC on lithium ions is possible. The ionization mechanism is not so obvious in this case as in hydrated PDSP. If ionization on lithium cations is theoretically possible in lithium salts, then how are green CCs formed in sodium and potassium salts? Perhaps, ionization occurs on Broensted centers, since there is direct experimental evidence that hydroxonium ions exist in salt-type PTAC. On the low-frequency side of the broad AB (3700−2750 cm−1) in the IR spectra of polymeric salts (Fig. 8) there is a not-toointense but well pronounced shoulder in the range of 2900 −2750 cm−1. Previously, we distinguished a separate band in the spectrum of hydrated PDSP in this region, which we assigned to hydroxonium ions [29]. This band disappears upon heating a PTAC-Li+ film at 250 °С, while absorption at 1350 cm−1 appears (compare spectra 1 and 2 in Fig. 9). The emergence of an AB at 1350 cm − 1 indicates that sulfophthalide rings are reduced from acid groups (a detailed

Heat effects of ionization in reactions I – VI

Reaction

I

II

II′

III

IV

V

V′

VI

ΔH°298, kJ mol-1

843.9

−915.8

−237.1

−88.0

190.0

−880.2

−169.8

51.5

The reaction numbers match those in Scheme 3


Scheme 4 The expected intramolecular ionization in PDSP

analysis of changes in the IR spectra of salts is presented in [36]). The natural conclusion is that a considerable amount of ionized acid groups are formed and stabilized upon alkaline hydrolysis of polyarylenesulfophthalides. This important conclusion allows us to suggest that the ionization mechanism of polytriarylcarbinols is common with that of hydrated polyarylenesulfophthalides, to give carbocations on Broensted centers, i.e., hydrogen and/or hydroxonium ions. It should also be remembered that in lithium salts there is a theoretical possibility of ionization of carbinol groups on lithium ions. The small concentration of carbocations, namely, less than one per 400 monomeric units, shows the negligible probability of coinciding of two effects, namely, ionization of carbinol groups and stabilization of the resulting ions. It makes more difficult to unambiguously determine the formation mechanism of these ions. For example, one should not rule out the possibility of preferential ionization of some defective carbinol groups (terminal ones, those in anthrone units [12], etc.) or ionization on catalyst residues. In a solid polymer, the existence of kinetically non-equivalent functional groups should also be taken into account. As concerns the possibility to observe green CCs, it is appropriate to compare two methods of PTAC preparation. The heterogeneous PTAC preparation by alkaline hydrolysis of polyarylenesulfophthalides in an aqueous medium facilitates the detection of CCs formed upon water desorption [1]. A homogeneous preparation of PTAC in dimethylacetamide (DMAA) [4, 5], as well as in DMSO or DMF, creates some objective difficulties for such detection. Alkaline hydrolysis of PDSP in polar organic solvents involves an electron transfer reaction [37] to give triarylmethyl radicals and quinoid Scheme 5 The expected intermolecular ionization of carbinol moieties in hydrolyzed PDSP

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structures of Chichibabin’s hydrocarbon type [38], which have a strong absorption band around 560–590 nm, i.e., in the region close to the ABs of carbocations. Oxidation of quinoid structures gives Bred^ peroxides with an AB at 480 nm [38, 39]. The presence of these peroxides most probably accounts for the brown color of solutions and films of polymeric salts obtained in [4, 5]. Apparently, the ABs of carbocations can overlap with the ABs of quinoid structures and peroxides, similarly to the overlapping of the bands of carbocations and quinoid structures obtained in PTAC thermolysis (see Fig. 1). The polymeric salts obtained by heterogeneous hydrolysis in water are not brown and form transparent films that can be slightly colored due to carbocationrelated CCs, as shown above. It should also be taken into consideration that the structure of PTAC obtained in an organic solvent can potentially be modified due to addition of hydroxy radicals to benzene rings in the polymer. This possibility is discussed in [37] but it requires experimental proof. Analysis of the results in [4, 5] shows yet another obvious drawback of the preparation of polymeric salts by alkaline hydrolysis in an organic solvent, namely, it is difficult to remove the solvent from the polymer. The IR spectra of polymeric salts reported in [4, 5] contain a noticeable band in the region of 2800–2900 nm−1, which is missing in the spectrum of the original PDSP and obviously belongs to the stretching vibrations of C-H bonds in alkyl groups. The most reasonable explanation why this AB appears in the IR spectrum of the polymer is to attribute it to the vibrations of C-H bonds in DMAA methyl groups. The IR spectrum of Bfree^ DMAA (solution in CCl4) contains, along with the AB of the alkyl group, also strong absorption of the carbonyl group at 1660 cm−1 that is shifted to the 1640−1610 cm−1 region in DMAA complexes with organic molecules [40]. It should be deemed that the AB at 1641 cm−1 observed in [4, 5] at least partially corresponds to the carbonyl group of DMAA. It is concluded from the fact that the IR spectrum of the polymeric salt contains an AB at 1641 cm−1 that the polymer contains conjugated double bonds that appear due to conversion of the sulfophthalide benzene ring [5]. Though the existence of double bonds as remainders of quinoid structures cannot be completely ruled out in this method of salt preparation, we believe that one can reasonably speak of assignment of the


J Polym Res (2015) 22: 58

1350 cm−1 appears, which is not observed in the spectrum of the hydrated salt. This fact shows that a fraction of polymer units are hydrolyzed to give acidic groups in the PTAC prepared by authors of [4, 5], too. Decay of carbocationic CCs For completeness of the picture, it is also necessary to discuss the processes of carbocation disappearance in the polymers concerned. Here one should distinguish reactions of ions with water and decay of ions at elevated temperatures. According to [41], the reactions of carbocations with water occurs by the following scheme: ν,

Fig. 8 IR spectra of films of sodium (1) and lithium (2) salts of polydiphenylenesulfophthalide, 3 – IR spectrum of a film of the lithium salt immediately after heating at 250 °C (20 min). The film was heated and cooled in vacuo (without removal of thermolysis products). Spectra were recorded in air at room temperature

AB at 1641 cm−1 to a certain structural moiety of the polymer only provided that the solvent (DMAA) and water have been totally removed from the polymer. It is noted that removal of water from PTAC is also quite difficult [1]. Complete water removal in our experiments on PTAC thermolysis was achieved at temperatures above 200 °C where polymer destruction had already started. It is important that the homogeneous method of PTAC preparation fails to guarantee the absence of either an organic solvent or water in the polymer. What is more, the alkyl AB in the region of 2800– 2900 cm −1 partially overlaps the absorption region of hydroxonium, though it is evident in the IR spectra of the original polymeric salts that noticeable absorption below 2600 cm−1 is observed, which decreases noticeably in the dried salt [4, 5]. It should also be noted that in the IR spectrum of dried PDSP potassium salt reported in [4, 5], an AB around

ν,

Fig. 9 IR spectra of PTAC Li films: 1 – original; 2 – heated at 250 °C -

+

Rþ þ H2 O→ROH2 þ →ROH þ Hþ ;

i.e., this reaction generates a hydrogen ion that can participate in re-ionization of carbinol groups in both polymers (PTAC and hydrated PDSP) in the course of water desorption. A number of questions need to be solved in order to understand the regularities of thermal decay of carbocations. One question is whether recombination of ions (carbocations and SO3−) occurs in ion pairs shown in Schemes 4 and 5. As one can see from Figs. 1 and 2, disappearance of ions with an increase in temperature is accompanied by an increase in the number of radicals since the intensity of the AB at 410 nm increases. The next question is whether carbocations may be direct precursors of radicals. Carbocations are known to be strong electron acceptors (see [37, 42] and the references therein). Therefore, as the temperature is increased (and molecular mobility grows), suitable reagents can approach each other, and as a result, electron transfer from some donor (e.g., a hydroxide ion) to a carbocation occurs to give the corresponding radical. A small amount of radicals already appear during evacuation of PTAC samples. It cannot be ruled out that this effect is also due to transformations of carbocations. It was noted previously that radicals, or at least some of them, are formed upon PTAC photolysis, even under diffuse daylight [1, 3]. As it was found above, visible light is absorbed by carbocations in the polymers studied. Now, the third question arises: what groups, i.e., carbinol, carbocationic or both, are precursors of radicals of triarylmethyl type during photolysis of these polymers? According to literature data, excited states of carbocations in solutions [43] and in zeolites [44] manifest strong electron-withdrawing properties and are commonly converted to the respective radicals in electron transfer reactions. The mechanism of coupled formation of radicals and carbocations is also of more general interest, since other cases of parallel formation of these particles have also been reported [16, 45]. Thus, owing to the fact that both radicals and carbocations manifest themselves as absorption bands in electronic spectra, PTAC and PDSP films provide a convenient


system for studying the a reaction of possible electron transfer to carbocations to give radicals during photolysis and thermolysis of polymers. Answers to the questions raised above should be searched for in further studies.

Conclusions This paper provides a detailed description of the phenomenon of green coloring and emergence of absorption bands in electronic spectra at 480 and 610 nm for polydiphenylenesulfophthalide and its derivative, polytriarylcarbinol, in the course of water desorption. To identify green CCs, electronic spectra of solutions of the polymers in question and of two model compounds in concentrated sulfuric acid were obtained. It the case of polymer solutions, spectra of polycations are observed, which differ from the spectra of single carbocations. Single carbocations in polytriarylcarbinol were obtained by treatment of the polymeric film surface with sulfuric acid. Comparison of electronic spectra of green CCs with experimental spectra of isolated carbocations makes it possible to assign these spectra to carbocations. TD DFT calculations of electronic spectra of model carbocations confirm the assignment of green CCs to isolated carbocations and explain the difference in electronic spectra of single carbocations and cations formed on adjacent monomeric polymer units. A probable mechanism of ionization of carbinol groups both in hydrated PDSP and in polymeric salts on Broensted centers (H+) has been suggested. The low concentration of carbocations (less than one per 400 polymer units) complicates an unambiguous determination of the ionization mechanism. Since CCs concentration is related to the water content in polymers, coloring of a particular sample will be determined by its history (synthesis, storage conditions, preparation) and environment humidity. The combination of experimental and theoretical results shows that polyarylenesulfophthalides and their derivatives have a liability for spontaneous ionization to give relatively stable carbocations responsible for the observed green coloring of polymer films.

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