2159
COMMUINICATIONS TO THE EDITOR
Photochemicid and Fluorescence Properties of Anthracene Radical Cation’ Publication costs assisted by the Ford Foundation
Sir. When anthracene is dissolved in concentrated sulfuric acid the anthracene radical cation A * + is formed by removal of an electron from neutral anthracene, presumably with H2SO4 acting as an electron acceptor (for review see, for example, ref 2). Another species which is formed is the anthracene carbonium ion AH+. The sulfuric acid solution exhibits a complex absorption spectrum in the visible and ultraviolet regions which changes in the dark over a period of hours.3 The absorption spectrum of A * + has been identified3 by comparison with the spectrum of anthracene anion radical and exhibits many peaks, prominent among which are those a t 315, 352, and 720 nm. The radical cation A * + has also been characterized by esr spectroscopy.4 Radical ions have also been produced by y-irradiation of anthracene in rigid organic media at low temperature.5 Recently6 it was found that A * + can be formed in boric acid glass at room temperature both by X-ray radiation and by far-ultraviolet light, while 365and 436-nm irradiation are ineffective. We wish to report that both near-ultraviolet and blue light produce additional radical cations from anthracene dissolved in concentrated sulfuric acid, and that these radicals slowly decay in the dark. Furthermore, anthracene dissolved in a 1:2 glacial acetic acid-sulfuric acid mixture shows no A * + formation, but on short irradiation with 365-nm light exhibits a transient A * +production. A 10-4 M solution of anthracene in concentrated (98%) HzS04 was prepared by dissolving anthracene in cyclohexane and then shaking this solution with concentrated sulfuric acid to generate the anthracene radical cation in the HzS04 fraction7 The absorption spectrum of this yellow HzS04 solution exhibits a strong peak a t 415 nm, and smaller peaks at 315, 352, and 720 nm of which we identify the last three with the antracene radital cation. The spectrum also !shows a very strong band with maxima at 246 and 255 nm. The absorption spectrum changes slowly in the dark over a period of hours, the 415-nm peak decreases, and a general increase occurs in the ultraviolet region. When the solution in a 0.2-cm pathlength cell was irradiated for ‘15 sec with 46-nm light (intensity 3.45 X einsteins sec-1 cm-2) the 315- and 352-nm peaks doubled in absorption while the 415-nm peak decreased by a factor of about 0.5. Increases also occur in the 250nm band and in the visible region, particularly at 720 nm. When the 15-sec irradiated sample is left in the dark for 30 min, the heights of the 315-, 352-, and 720-nm peaks diminish to their original values. There is also a small increase in the 400-nm region and a general increased background in the ultraviolet. Similar changes are observed when such a sample is irradiated with 365-nm light (in-
tensity 6 x 10-8 einsteins sec-1 cm-2). We therefore conclude that blue and near-ultraviolet light produces additional A*+species. The photochemical production of the antracene radical cation was also followed by esr. The anthracene-sulfuric acid sample exhibits the characteristic signal of A * + . On irradiation for 15 sec with 436- or 365-nm light of the same intensity as before, this signal increased and then slowly decreased in the dark to its initial value over a period of 0.5 hr. The anthracene-sulfuric acid solution shows very complex fluorescence properties. On excitation between 300 and 430 nm a blue fluorescence is observed with a maximum emission a t 440 nm and a tail extending toward the red. The excitation spectrum of the 440-nm band exhibits three peaks, namely, at 360, at 378, and at 395 nm. With excitation between 395 and 430 nm an emission peak at 525 nm in addition to the 440-nm peak is observed, which becomes the sole emission peak when the sample is excited between 440 and 480 nm. Throughout there is a much weaker tail extending into the red. At excitation wavelengths exceeding 500 nm the fluorescence spectra are remarkable in that the emission spectrum depends on the excitation wavelength. Thus on excitation with 500 nm the emission maximum is a t 570 nm, with 568 nm at 610 nm, and with 600-nm excitation a fluorescence maximum at 645 nm is observed. Excitation beyond 600 nm only produces a red fluorescence tail in which no maximum could be detected, possibly due to limitations of the instrument. In the 500-650-nm region the optical density of the sample in a 1-cm pathlength fluorescence cell did not exceed 0.04. It is therefore certain that no trivial photometric errors, such as reabsorption of the emitted light, account for the effects described. The photochemically produced anthracene radical cation can be more clearly demonstrated when anthracene is dissolved in a mixture of glacial acetic acid and concentrated sulfuric acid, with a CH&OOH:H2S04 ratio of 1:2 by volume (10-4 M in anthracene). The absorption spectrum of the solution shows no prominent peaks at 315, 352, and 720 nm, but on 10-sec irradiation with 365-nm light (intensity as above) shows an immediate appearance of these bands (Figure 1). Similarly, the esr spectrum of the solution (prepared in the dark) shows only a weak structureless signal. But on irradiation of the sample in the cavity with 365 nm light, the characteristic esr signal of the A n + species immediately appears (Figure 2). It is clear, therefore, that in an acetic acid-sulfuric acid mixture no anthracene radical cation is formed in the dark, but is produced on irradiation with 365-nm light. The structured esr signal and the absorption bands characteristic of A * + disappear with a half-life of about 30 sec, when the irradiation is terminated (Figures 1 and 2). In this system illumination with blue light does not produce A * + since no esr signal nor the characteristic bands in the ultraviolet and far red are observed with 436-nm irThe Journal of Physical Chemistry, Vol. 77, No. 7 7, 7973
21 60
Communications to the Editor ground-state interaction with anthracene molecule. A weaker electron acceptor such as boric acid glass6 would then require more energetic photons. In the acetic acidsulfuric acid system the rapid decay may be attributed to the instability of the cation in this medium once it is formed. The fact that blue light irradiation of the concentrated sulfuric acid system produces A * + could conceivably due to the reaction A H A * + H.. The wavelength dependence of the fluorescence of the anthracene radical cation is reminiscent of the luminescence behavior of azulenes and of Malachite Green in the bound state.g In the present work there seems to be an overlapping of two or more absorption bands in the region of 500 nm and greater. Results of Raman scattering experiments on the radical cation system show that the fluorescence phenomenon is not an artifact due to scattering. The shifting of the emission maxima might be due to the presence of more than one emission wherein the net emission maximum depends on the cross section of the lower-lying absorption relative to that of the higher one. In view of the fact that the anthracene radical cation is characterized by a band at 720 nm, one might expect that if this populates the lowest excited state, and if this state is emissive, emission from this state should lie a t wavelengths greater than 720 nm.10
+
I
I
I
I
I
I
320
350
380
410
44
WAVELENGTH (nm) Figure 1. Absorption spectra of (a) (---) anthracene, M in i : 2 glacial acetic acid-sulfuric acid mixture; (b) (- - - - - - -) case a irradiated with 365 nm (6 x l o - * einstein sec-I ) case b 1 min in dark. Absorption for 10 sec; (c) (-.-.-.---.
spectrum and irradiation are performed in a 0.2-cm pathlength cell.
-+
+
References and Notes (1) This work is supported by Ford Foundation Grant NO. 690-0106:, (2) M. C. R. Symons in "Advances in Physical Organic Chemistry, VOl. 1, V. Gold, Ed., Academic Press, New York, N. 1963. (3) W. lj. Aaibersberg, G. J. Hoijtink, E. L. Mackor, and W. P. Weijland, J. Chem. SOC.,3049 (1959). (4) S. I. Weissmann, E. de Boer, and T. T. Conradi, J. Chem. Phys., 25, 190 (1957). (5) T. Shida and W. H. Hamill. J. Chem. Phys., 44,4372 (1966). (6) 2. H. Zaidi and B. N. Khanna, J. Chem. Phys., 50,3291 (1969). (7) V. Gold and F. L. Tye, J. Chem. SOC.,2172 (1952). (8) M. Beer and H. C . Longuet-Higgins, J. Chem. Phys., 23, 1390 (1955); G. Viswanath and M. Kasha, ibid., 24,574 (1956). (9) 6 ,Oster and G. K. Oster in "Luminiscence of Organic and Inorganic Materials," H. P. Kallmann and G. M. Spruch, Ed., Wiley, New York. N. Y., 1962. (10) Acknowledgment is made to a reviewer for interpretational comments. (11) Deceased July 15, 1972.
e.,
H
___y
Figure 2. Esr spectra for cases a and b as in Figure 1. Modulation amplitude at 0.8 G . radiation. When the ratio of acetic and sulfuric acid is increased to 2:1, no A*+ is produced on illumination with ultraviolet light. The solution of anthracene in acetic acid-sulfuric acid (1:2) exhibits a blue fluorescence when excited with ultraviolet light and only an extremely weak fluorescence in the red when excited with 500-nm light. When the sample is irradiated for 10 sec with 365-nm light (intensity as above), the 550-nm emission excited by 500 nm is drastically increased but disappears within 45 sec. The same result is obtained when the 600-nm emission (excited by 500 nm) is observed immediately after 365-nm irradiation has stopped. We therefore conclude that the long wavelength fluorescence phenomena excited by light above 500 nm is due to the radical cation. The formation of the radical cation is influenced by a t least two factors: the presence of a sufficiently powerful electron acceptor and the stability of the radical cation in its medium. Near-ultraviolet light obviously cannot remove an electron from a neutral anthracene molecule, but such process becomes possible in the presence of a powerful electron acceptor such as HzS04, which may show The Journal of Physicai Chemistry, Vol. 77, No. 17, 1973
Department of Gynecology and Obstetrics and Department of Physics Mt. Sinai School of Medicine of the City University of New York New York, New York 10029 Division of Pure and Applied Sciences Richmond College of the Cify University of New York Sfaten Island. New York 10301
Gisela K. OsterIi
Nan-Loh Yang"
Received May 16, 1973
Aquaphotochromism
Sir: Chemical compounds exhibiting a reversible change of visible color on exposure to light are photochromic1* and the first wholly organic structures possessing this characteristic were described just before the turn of this century.2 More recently, many different types of chemi-
