Physics

G. I. Kobyshev, G. N. Lyalin, Academician A. N. Terenin

Energy Transfer from the Uranyl Cation to Phthalocyanine in Solution and in the Adsorbed State

The long lifetime, of the order of \(10^{-4}\) sec, of the excited state of the uranyl cation \(\mathrm{UO}_2^{2+}\), manifested in its photochemical reactions and luminescence at low temperatures, should favor intermolecular transfer of excitation energy.

In the experiments described below, metal-free phthalocyanine \((\mathrm{H}_2\mathrm{Pc})\) and magnesium phthalocyanine \((\mathrm{MgPc})\) were taken as luminescent energy acceptors, since they possess discrete absorption bands in the red region that do not overlap the uranyl bands, which made possible, to some extent, separate excitation of the components*.

Fig. 1. Luminescence spectrum of a solution of magnesium phthalocyanine and uranyl nitrate in ethanol at \(290^\circ\mathrm{K}\) (1) and at \(77^\circ\mathrm{K}\) (2)

The investigation was carried out on the same photoelectric spectrophotometer of high dispersion and on the high-intensity monochromator with a diffraction grating that were used in work (¹), in which intramolecular transfer of excitation in a uranyl complex of phthalocyanine was studied.

Solutions of \(\mathrm{H}_2\mathrm{Pc}\) in dioxane and \(\mathrm{MgPc}\) in ethanol were used at concentrations \(10^{-4}\)—\(10^{-5}\ M\); the uranyl nitrate or acetate added to the solution had a concentration in it of \(10^{-3}\)—\(10^{-4}\ M\). The source of exciting light was an HBO-500 mercury lamp. For excitation with light of 366 mµ, a combined light filter UFS-4 + SZS-10 was used. The spectral width of the slit was \(30\ \mathrm{cm}^{-1}\) (\(60\ \mathrm{cm}^{-1}\) for the weaker uranyl luminescence).

Addition to solutions of \(\mathrm{H}_2\mathrm{Pc}\) in dioxane or \(\mathrm{MgPc}\) in ethanol of a uranyl salt (uranyl nitrate or uranyl acetate) substantially changes the luminescence spectrum. A photoelectric recording of the luminescence of an \(\mathrm{MgPc}\) solution in ethanol with uranyl nitrate at 290 and \(77^\circ\mathrm{K}\) is given in Fig. 1. A spectrum is observed with the main maximum of \(\mathrm{MgPc}\) luminescence at 673 mµ.

At \(290^\circ\mathrm{K}\) only \(\mathrm{MgPc}\) fluoresces; uranyl nitrate does not luminesce. Lowering the temperature to \(77^\circ\mathrm{K}\) reveals in the luminescence spectrum of the \(\mathrm{MgPc}\) solution, in addition to the maximum at 673 mµ, a strong band at 701 mµ, whose spectral position is close to the luminescence maximum of \(\mathrm{H}_2\mathrm{Pc}\).

* Preliminary experiments on this subject, carried out several years ago by photographic and spectrophotographic methods of lower dispersion in a limited spectral region, did not give unambiguous results. In addition, in the acetone solution employed, an irreversible photochemical reaction occurred between \(\mathrm{UO}_2^{2+}\) and \(\mathrm{MgPc}\).

Without uranyl, the luminescence of the pigments upon excitation at 436 mµ is very weak. Addition of a uranyl salt to the pigment solution causes, under the same excitation (436 mµ), a 10–20-fold increase in fluorescence intensity (Fig. 2). The sharp enhancement of pigment fluorescence in the presence of the uranyl ion is accompanied in both cases (\(\mathrm{H_2Phc}\) and \(\mathrm{MgPhc}\)), at low temperatures, by the appearance of the main maximum at 701 mµ upon excitation of luminescence in the region of the weak, longest-wavelength absorption band of uranyl (436 mµ). The addition, instead of uranyl salts, of magnesium or vanadium nitrates and acetates causes no changes, which rules out an explanation of the fluorescence enhancement by an ionic effect on the high levels of the pigment molecules. At the same time, along with the enhancement of luminescence in the presence

Fig. 2. Luminescence upon excitation at 436 mµ of a solution of \(\mathrm{H_2Phc}\) in the presence of uranyl acetate in dioxane at 290°K (1) and, under the same conditions, of a solution of \(\mathrm{H_2Phc}\) in dioxane without uranyl (2)

Fig. 3. Unusual temperature dependence of the luminescence of a solution of \(\mathrm{H_2Phc}\) and uranyl acetate in dioxane at 290°K (1) and at 77°K (2)

Fig. 4. Dependence of the luminescence spectrum on the wavelength of the exciting light for \(\mathrm{MgPhc}\) and uranyl nitrate in ethanol. 1 — \(\lambda_{\mathrm{ex}} 436\) mµ at 77°K; 2 — \(\lambda_{\mathrm{ex}} 405\) mµ at 77°K; 3 — \(\lambda_{\mathrm{ex}} 436\) mµ at 290°K; 4 — \(\lambda_{\mathrm{ex}} 405\) mµ at 290°K

of the uranyl ion, an unusual temperature dependence of the luminescence is also observed; it is most clearly expressed in a solution of \(\mathrm{H_2Phc}\) with uranyl acetate in dioxane (Fig. 3).

Lowering the temperature of pigment solutions from 290 to 77°K, in the absence of uranyl, causes a comparatively small increase in the intensity of the fluorescence spectrum (upon excitation at 366 mµ). But in the presence of uranyl, if illuminated at 436 mµ (and also at other wavelengths), such lowering of the temperature with freezing of the solution leads to a sharp decrease in the emission intensity of both phthalocyanines. As follows from Fig. 4, illumination at 436 mµ in the absorption minimum of \(\mathrm{MgPhc}\) gives, at 77 and 290°K, spectra comparable in intensity with those excited by light of 405 mµ, i.e., near the absorption maximum (390 mµ) of \(\mathrm{MgPhc}\).

The dependence of the spectra on the wavelength of the exciting light (405 and 436 mµ) is given in Fig. 4. At 77°K, alongside the pigment spectrum, weak fluorescence of the uranyl cation appears upon excitation at 366 mµ, with bands at 559, 534, 511, and 490 mµ (Fig. 1), which are considerably stronger in the absence of pigments, in agreement with energy transfer. The conditions for resonance

...of the resonance inductive mechanism of radiationless energy transfer in the system under consideration are not very favorable, since the emission bands of uranyl overlap only weakly with the absorption bands of the phthalocyanines. The situation here is analogous to the recently observed energy transfer from the triplet level of molecules to the excited singlet level of pigment molecules \({}^{(2)}\).

The fluorescence of the pigments observed under selective absorption of the exciting light by the uranyl cation indicates transfer of excitation energy from the uranyl ion to the pigment molecule; however, the fluorescence spectrum of the energy acceptor is the same for both phthalocyanines, i.e., at low temperature, under selective excitation of the uranyl cation (436 mµ), the most intense maximum at 701 mµ appears in both cases, and the maximum characteristic of metal-containing phthalocyanines (MgPhc) is absent from the spectrum. Excitation at room temperature with wavelengths of 405 and 366 mµ clearly reveals the existence of this maximum (Figs. 1 and 4).

The absence in the MgPhc spectrum of the 673 mµ maximum under selective excitation of the uranyl ion (436 mµ), and the similarity of the spectra observed for solutions of both pigments at low temperatures, indicate the manifestation in both cases of a luminescent carrier of the same nature, with preservation of the system of \(\pi\)-conjugated bonds of the phthalocyanine molecular ring.

To investigate energy transfer with the indicated systems in the adsorbed state, either H\(_2\)Phc or MgPhc was first introduced from solution into powdered magnesium oxide gel, and then uranyl nitrate was additionally precipitated from aqueous or ethanol solution.

Table 1

Position of maxima in the luminescence spectrum of MgPhc + uranyl nitrate/magnesium oxide (immersed in nonane). Wave numbers are given in cm\(^{-1}\)

Excitation with 366 mµ light at 77°K of the MgPhc + uranyl adsorbate reveals in the fluorescence spectrum an intense main pigment maximum at 685 mµ and side maxima at 720 and 755 mµ, shifted to the long-wavelength side by 10–20 mµ as compared with the solution, while the frequency intervals are preserved. Immersion of the adsorbate in nonane promotes manifestation of the spectral structure, greatly increasing the intensity of the green luminescence (of the surface crystal) \({}^{(3)}\), which should probably be explained by weakening of the action of the surface. Washing-out of the pigment is not observed in this case, i.e., the increase in luminescence is not connected with removal of concentration quenching. In addition, the spectrum contains weak maxima at 508 and 487 mµ of the hydrated uranyl cation in the adsorbed state \({}^{(3)}\), and a broad maximum at 560 mµ, with a half-width of 3000 cm\(^{-1}\), belonging to the dehydrated uranyl cation \({}^{(4)}\). Upon illumination at 436 mµ of an adsorbate of MgPhc/MgO alone, both at 77°K and at 290°K, no pigment luminescence is observed. In the presence of adsorbed

uranyl, and with the same excitation it appears only at 77°K. Immersion of the combined adsorbate in nonane and its freezing lead, under illumination at 436 mμ, to the appearance of a bright spectrum of adsorbed MgPc with distinct maxima at 685 (principal), 720, and 755 mμ. Their intensity is comparable with that produced by excitation at wavelengths of 579 and 405 mμ, absorbed directly only by MgPc. Bands are also present in the green region of the spectrum of hydrated uranyl. Thus, in the adsorbed state there is a selective spectral action of wavelengths absorbed by the hydrated uranyl cation, as a result of which phenomena are observed that are analogous to those in a binary solution or in a complex.

Leningrad State University
named after A. A. Zhdanov

Received
15 XI 1962

REFERENCES

1. G. N. Lyalin, G. I. Kobyshev, Optics and Spectroscopy, 14, No. 5 (1963).
2. V. N. Ermolaev, E. B. Sveshnikova, Izv. Acad. Sci. USSR, Phys. Ser., 26, 21 (1962).
3. G. I. Kobyshev, Izv. Acad. Sci. USSR, Phys. Ser., 25, 542 (1961).
4. G. I. Kobyshev, DAN, 127, 373 (1959).