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-
21 61
Communications to the Editor cals, photochromic in the ultraviolet, visible, or infrared regions, have been the subject of increasing attenti0nla.b because of their potential utility as components of optical fibers, data storage and display equipment, and temperature control devices.1193.4 We now wish to report a new type of reversibly lightsensitive compound, based on the 1-hydroxyimidazole skeleton.5.6 This substituted heterocycle can be readily synthesized either directly by the condensation of a-dione monoximes with aldehydes and ammonia, or by the NaBH4 reduction of 1-hydroxyimidazole 3-oxides obtained by the reaction of (1)a-diketones with aldehydes and hydroxylamine, ( 2 ) , a-diketones with hydroxylamine and aldoximes, or (3) a-dioximes and aldoximes.7 The most photochromic member of this group thus far encountered was prepared by the room temperature condensation of 6-methylpyridine-2-carboxaldehyde(1.21 g, 10 mmol) with butan-2,3-dioqe monoxime (1.01 g, 10 mmol) in ethanol (95%, 30 ml) containing aqueous ammonia (d, 0.88, 2 ml). This product, the hydrate of l-hydroxy-2-(picolin-6yl)-4,5-dimethylimidazole (Ia), crystallized from acetone
N
C
d
e f
9
01H CiAc 01H 0H 0H H 0H
Me Me Me Ph Ph Me Me
I1
I11
sensitivity of the hydroxyimidazole (Ia) diminished on purification by repetitive crystallization from anhydrous mixtures of acetone and hexane. All the foregoing facts suggested that only the hydrate of Ia is photoactive and this was confirmed by comparison of the water content of the original photochromic hydrate (9.1% HzO) and of the recrystallized, feebly photochromic material (0.5% HzO, mp 150-152"), determined using the Karl Fischer titration procedure.8 The occurrence of this new aquaphotochromic effect can be satisfyingly explained in terms of structures Va and Vb in which the light-actuated transfer of hydro-
N,
Rl
a b
Me
Me
Me Me Me H Ph Me Me
-.
N N N
N N N CH
Me Me H Me Me Me H
as white prisms (810 mg): mp 149-153", A,, a t 280 (C 8080) and 314 nm (c 17120). Anal. Calcd fohCllH13N30aH20: N, 20.0%. Found: N, 20.4%. This compound showed no photochromic properties whatsoever when dissolved in a variety of protic and aprotic monomeric or polymeric solvents. However, the white hydroxyimidazole (Ia) in the solid state turned a deep purple when exposed to sunlight. This color change is indicative of an absorption maximum between yellow and red (600-700 nm) for the excited state. Color reversion from purple to white occurred after storage in the dark, during prolonged heating a t 105", or extremely rapidly after exposure in a conventional KBr pellet (1 mg of Ia/400 mg of KBr) to the infrared beam of a Beckman IR-10 spectrophotometer. The photoinduced color change of the white hydroxyimidazole was recorded by photography of a thin ethanol-applied coating of Ia on polyethylene terephthalate sheeting after exposure to xenon light for various periods of time (0-1200 sec). The relative optical densities of the photographs, measured with a double-beam recording automatic integrating microdensitometer (Model MK IIIC, Joyce, Loeble & Co., Ltd.) showed that the photochromic reaction followed first-ordler kinetics with a rate constant of 1.2 x sec-1 and a half-life of 57.8sec. The photochromicity of Ia is apparently quite structure specific and the closely related compounds (Ib-g, 11-IV) were without comparable activity. Moreover, the light
Va (colorless)
9-H
Vb (purple)
gen from the o-methyl group of the pyridine ring is temporarily stabilized by the cyclic hydrogen bonding complex formed from the hydroxyl groups of both the imidazole ring and the hydrate water and by the electron-donating characteristics of the methyl substituents on the imidazole heterocycle. Support for this interpretation is provided by the observation that photochromic behavior is exhibited only in the presence of molar or less than molar quantities of water and not in the presence of methanol, ethanol, acetone, dioxane, or dimethyl sulfoxide. Likewise, photochromic activity was also absent from solid solutions of Ia in poly(methyl methacrylate), polyvinylpyrrolidone, polyetbylenimine, or poly( bisphenol-A carbonate). The mechanism proposed implies that an analogous 1hydroxyimidazole having a hydrogen atom already attached to the nitrogenous substituent would be strongly colored. This prediction was verified by the synthesis9 of l-hydroxy-2-(pyrr-2-yl)-4,5-dimethylimidazole (VI) which crystallized from acetone-dimethylformamide as deep purple plates: mp 231-232". Anal. Calcd for C9HllN30: The Journal of Physical Chemistry, Vol. 77, No. 1 7 . 1973
21 62
Communications to the Editor
N, N, /H
&-:
(2) (3) (4)
(5) (6) (7) (8)
D. L. Ross and J. Blanc, p 471; (e) J. D. Margerum and L. J. Miller, p 557; (f) S. K. Deb and L. J. Forrestal, p 633; (g) R. J. Araujo, p 667; (h) L. P. Vernon and 8. Ke, p 687; (i) R. C. Bertelson. p 733. W. Marckwald, 2. Phys. Chem., 30, 140 (1899). G. H. Dorion and A. F. Wiebe, "Photochromism, Optical and Photographic Applications," Focal Press, New York, N. Y., 1970. G. Smets, Pure Appl. Chem., 30, 1 (1972). F. J. Allan and G. G. Allan, Chem. Ind. (London), 1837 (1964). K. Akagane, F. J. Aiian, G. G. Allan, T. Friberg, S. 0 Muircheartaigh, and J. E. Thomson, Bull. Chem. SOC. Jap. 42, 3204 (1969). K. Akagane,and G. G. Allan, unpublished research. L. Meites, Handbook of Analytical Chemistry," 1st ed, McGraw-Hili, New York, N. Y., 1963, pp 4-8. K. Akagane, G. G. Allan, C. S.Chopra, T. Friberg, T. Mattiia. S. 0 Muircheartaigh, and J. B. Thomson, Suomen Kemistilehti, 45, 223 (1972).
VI 23.71%. Found: N, 23.70%. The related furan and thiophen analogs without the hydrogen on the heteroatom are essentially color1ess.g
(9)
References and Notes
Department of Chemical Engineering and College of Forest Resources University of Washington Seattle, Washington 98 195
(1) "Technique of Organic Chemistry," Voi. I1 I , "Photochromism," G. H. Brown, Ed., Wiley-interscience, New York, N. Y., 1971: (a) G. H . Brown, p 1; (b) R. C. Bertelson, p 45; (c) G. Eigemann, p 433; (d)
Received April 3, 7973
K. Akagane G . G . Allan* J. S. Bindra T. Friberg A. N. Neogi
