Physical Chemistry

Yu. D. Pimenov, V. E. Kholmogorov, Academician A. N. Terenin

Spectral Detection of Molecular Anion-Radicals upon Adsorption of Vapors of Electron-Acceptor Molecules on Oxides

Upon adsorption from solutions of chingidron on Ba(OH)\(_2\)·8H\(_2\)O, (\(^1,\,^2\)) and on Al\(_2\)O\(_3\) (\(^3\)), an EPR signal without hyperfine structure was detected, attributed to the anion-semiquinone of \(n\)-benzoquinone stabilized by the alkaline surface. In an earlier work of our laboratory (\(^4\)), adsorption of \(n\)-benzoquinone, chloranil, and anthraquinone from vapors on ZnO and MgO was carried out. The absorption spectra obtained and the structureless EPR signal indicate the formation of anion-semiquinones as a result of electron capture from the adsorbent. Recently, upon adsorption of \(n\)-benzoquinone vapors on active Al\(_2\)O\(_3\) under vacuum conditions, absorption bands (415 and 430 nm) and an EPR signal without hyperfine structure, evidently belonging to the anion-semiquinone, were also detected (\(^5\)).

In the present work, absorption spectra and EPR signals were obtained upon adsorption of vapors of \(n\)-benzoquinone and its halogen-tetrasubstituted derivatives (fluoranil, chloranil, and bromanil) on ZnO, TiO\(_2\), and MgO under high-vacuum conditions. Since F nuclei have a larger magnetic moment than H nuclei, it was expected that the fluoro-substituted quinone (fluoranil) would, upon adsorption, give an anion-radical having an EPR signal with a characteristic hyperfine structure. In addition to quinones, adsorption from vapors of electron-acceptor molecules of another type—trinitrobenzene and tetracyanoethylene—was carried out on the same adsorbents.

Procedure. Zinc oxide was used, obtained by thermal decomposition of ZnCO\(_3\), and a commercial grade M-1, obtained by burning the metal in a muffle furnace. Magnesium oxide was obtained by thermal decomposition of MgCO\(_3\). The powdered oxides were preliminarily calcined in air at 600° for 6–8 hours to burn off possible organic contaminants. The adsorbents were then placed in specially shaped glass vessels, in which they were subjected to vacuum treatment at 500° (2–3 hours). The residual pressure was \(10^{-5}\) torr. The vessel was sealed off from the vacuum apparatus, and adsorbate vapors were admitted by breaking, with an internal striker, an ampoule containing the adsorbate previously sublimed in vacuum. Adsorption of vapors was carried out at 20° for \(n\)-benzoquinone and fluoranil, or by heating the entire vessel to 80–200° in the case of the other adsorbates (chloranil, bromanil, trinitrobenzene, and tetracyanoethylene). The contact time of the vapors varied from minutes to hours, depending on the objects.

After completion of adsorption, the adsorbent powder was transferred in vacuum into round, flat cuvettes soldered to the vessel (35 mm in diameter, 3 mm thick), equipped with a thin calibrated side arm for insertion into the resonator of an RE 13-01 radiospectrometer. The absorption spectra of the surface colorations obtained on the adsorbents were recorded on an SF-10 (or SF-2M) photoelectric spectrophotometer in diffusely reflected light with the aid of an integrating sphere. A differential method of simultaneous recording relative to the spectrum was used.

reflection cuvettes with the original adsorbent similarly pretreated in vacuum, which excluded the spectrum of the latter.

Results and discussion. When vapors of p-benzoquinone were admitted to ZnO, MgO (from carbonates)* within several seconds a bright bluish-green coloration appeared, with spectral absorption maxima (diffuse-reflection minima) given in Table 1.

Table 1

Maxima of the absorption bands (in nm) of adsorbates on various adsorbents

At the same time an EPR signal arises in the form of an intense line with half-width \(\Delta H = 7\) Oe and \(g = 2.004 \pm 0.001\). The absorption spectra of the four adsorbates on ZnO are shown in Fig. 1a, and the corresponding EPR signals in Fig. 1b. Upon adsorption of fluoranil on ZnO and MgO the EPR signal,

Fig. 1. Absorption spectra (a) and EPR (b) of quinones adsorbed on ZnO: 1 — p-benzoquinone (40 min, 20°); 2 — fluoranil (10 min, 20°); 3 — chloranil (30 min, 85°); 4 — bromanil (70 h, 200°). In parentheses are given the duration and temperature of the vapor adsorption process.

Fig. 2. EPR spectrum of fluoranil adsorbed on MgO: 1 — duration of vapor adsorption 10 min at 20°; 2 — the same sealed sample, heated for 3 h at 200° and cooled to 20°.

showing hyperfine structure consisting of 5 lines, in accordance with the magnitude of the moment of the F nucleus, leaves no doubt that an anion-radical with an unpaired electron interacting with 4 equivalent F nuclei is present.** With increasing adsorption time, i.e., with increasing surface concentration, the hyperfine structure of the EPR signal of fluoranil becomes blurred. If pump-

* ZnO of grade M-1 has a small specific surface area (7 m²/g), and therefore an uncharacteristic optical spectrum of the adsorbate and a low-intensity EPR signal.

** An analogous EPR signal with hyperfine structure was observed in the chemical preparation of the anion-semiquinone of fluoranil and p-benzoquinone by a chemical reaction in solution (6, 7).

If the cuvette with fluoranil adsorbed at 20° on ZnO or MgO is heated to 200° for 2 h, the EPR signal increases, but an additional intense narrow signal with a similar value of the \(g\)-factor is superimposed on the hfs. (Fig. 2). In the optical spectrum, the long-wavelength band at 510 nm (Table 1) is shifted by 15 nm toward the short-wavelength side.

When air was admitted into the cuvette, the colored samples rapidly acquired a gray coloration, caused by blurring of the absorption bands. At the same time a decrease in the EPR signal was observed.

Adsorbed on silica gel, vapors of benzoquinone give a broad absorption band in the region of 450 nm, similar to the spectrum of benzoquinone in inert solvents \((^8)\), belonging to the \(n-\pi\) transition of the molecule. The absorption band, which, depending on the adsorbent, has maxima at 435 (ZnO), 440 (TiO\(_2\)), and 430 nm (MgO) (Table 1), accompanied by an EPR signal, should be compared with the absorption maximum at 430 nm of the benzoquinone anion-radical obtained by a chemical reaction in alcoholic solutions \((^{9,10})\). For the chloranil anion-radical an absorption maximum at 450 nm was obtained in solution \((^{11})\), close to the maxima 455 and 470 nm established in the present work upon adsorption of chloranil vapors on ZnO and MgO, respectively (Table 1). In contrast to the results of \((^4)\), we did not detect a maximum at 488 nm, attributed to the hydroxyquinone anion in solutions \((^9)\). The bands of anion semiquinones are regularly shifted to the long-wavelength side with increasing electron affinity of the molecule (Table 1).

Fig. 3. \(a\)—absorption spectra of symm.-trinitrobenzene adsorbed from vapors: 1—on ZnO (15 min, 85°); 2—on ZnO (10 min, 120°); 3—the same as sample 2, heated in a sealed cuvette (3 h, 150°) and cooled to 20°; 4—on MgO (10 min, 120°). \(b\)—EPR spectra of symm.-trinitrobenzene of samples 2 and 4 in Fig. 3a.

On ZnO and TiO\(_2\) semiconductors of the \(n\)-type, the local levels known for them, filled with electrons, can be electron donors \((^{13})\). For MgO, which is not a photosemiconductor, the donor function can nevertheless be performed by oxygen vacancies occupied by electrons \((^{15,16})\). Such oxygen vacancies can also form for ZnO obtained by decomposition of salts \((^{14})\).

The broad absorption bands in the 500–700 nm region (Table 1 and Fig. 1a), which impart a blue coloration to the adsorbents, can be interpreted in two ways. These bands appear and increase with increasing surface concentration, accompanied by broadening and intensification of the EPR signal. It is possible that these bands are charge-transfer bands in binary associates formed from the anion-radical and the neutral quinone molecule. On the basis of the Briegleb–Czekalla equation \((^{18})\), for such a system an approximately constant absorption band corresponding to charge transfer is obtained for partners with varying values of electron affinity. The asymmetry of the EPR signal, arising with increasing concentration, also indicates the appearance of a new surface formation.

On the other hand, the broad band producing the blue coloration, observed also in other works, can be attributed to dimers composed of two anion-radicals, which do not possess paramagnetism \((^2)\).

Trinitrobenzene (TNB) adsorbed on silica gel absorbs in the UV region \((^{17})\). Adsorption of TNB vapors on ZnO and MgO leads to the appearance of surface coloration with the absorption bands shown ...

…in Fig. 3a. The spectra of TNF coincide with the spectrum reported in paper (18), obtained in air by grinding a powder of this compound with MgO. Our experiments, carried out under pure vacuum conditions with the complete absence of traces of water, refute the interpretation given in the cited paper, which attributes the visible coloration of adsorbed TNF to interaction with OH\(^{-}\) ions of the alkaline MgO surface. From our point of view, the presence of an e.p.r. signal associated with the appearance of the short-wavelength band at 460 nm (Fig. 3) leaves no doubt that it belongs to the TNF anion radical. The presence of indications of three lines in the structure of its e.p.r. spectrum confirms the interaction of the unpaired electron with the nucleus of the N atom of the nitro group. The broad band at 520 nm may apparently be assigned to intermolecular charge transfer.

Fig. 4. Absorption spectra (a) and e.p.r. spectra (b) of tetracyanoethylene adsorbed from vapor: 1 — on ZnO (30 min, 80°); 2 — on MgO (30 min, 80°).

The 440-nm absorption band of tetracyanoethylene (TCNE) vapors adsorbed on ZnO and MgO (Fig. 4a) should evidently be assigned to the anion radical of this compound, since an analogous band was observed for the tetracyanoethylene—tetramethyl-p-phenylenediamine system with an absorption maximum at 435 nm (19). The appearance of the band is accompanied by an e.p.r. signal (Fig. 4b). The broad long-wavelength band of adsorbed TCNE at 550 nm exhibits behavior analogous to the broad bands of quinones. It also, apparently, belongs to the charge-transfer spectrum or may be assigned to nonparamagnetic dimers.

Received
22 III 1965

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