Photochromic Sunglasses A Patent-Based Advanced Organic Synthesis Project and Demonstration Bruce Osterby, Ronald D. McKelvey, and Lisa Hill University of Wisconsin-La Crosse. La Crosse, WI 54801 A recent computerized search of Chemical Abstracts using the keyword "sunglasses" produced 43 references. Of these, all except four were patents. Most dealt with photochromic materials. All of those using organic photochromic compounds (other than organic ligands) were patents based on some variation of the spiropyrans ( I ) . Irradiation of these compounds produces a colored quinoid structure in an electrocyclic ring opening. Thermally, these revert back t o the colorless, cyclic form.
Wavelength
One of the patents (2)describes the preparation and photochromism of spiroindolinenaphthoxadinederivatives
for use in sunglasses, and is the basis of the present paper (3, 4).
Synthesis of a Photochromlc Compound Equations 1and 2 show the reactions used in the preparation of one of the photochromic compounds. This sequence was assigned t o a student (LH) as an individual synthetic project and has been repeated twice without difficulty. Details are given in the Experimental Section.
Figure 1. UV-vis spectra of Serengeti.
sunglasses. (A)
nm
B 8 L Ray Ban. IB) Corning
The starting materials, 1and 3, are commercially available (5).Although the reactions are slightly "off the beaten path" of first-year organic chemistry, nitrosation is a less common example of electrophilic aromatic substitution, and the coupling reaction poses some interesting mechanistic challenges for the student (and instructor?). Coating of Photochromlc Fllm An ideal pair of sunglasses should absorb all of the solar ultraviolet light (A < 400nm) and most but not all of the blue light (X 400-500 nm), which contributes more than other colors to haze. Figure 1shows the UV-vis spectra of two pair of sunglasses, one medium-priced (B & L Ray Ban) and one expensive (Corning Serengeti). The more expensive ones do a commendable job on the blue light, although they are less effective on UV. hesented in part at the 23rd Great Lakes Reglonal Meeting. ACS, DeKalb, IL. May 1990.
424
Journal of Chemical Education
match the ideal spectrum for sunglasses. Indeed, the photochromic film transmits strongly in the blue. Experimental
Figure 2. Vlsible spectra of photochromic "sunglasses" =Her substantial decay.
after irradktbn and
Photochromic sunglasses, in addition, should be essentially colorless under low-light conditions, he converted rapidly to a form with a n appropriate absorption spectrum under intense lighting, and revert hack quickly when returned to low light. All of this should happen over a wide temperature range, and the system must be capable of repeating the cycle a great number of times without fatigue. We have found that overhead oroiector transoarencies can he readily coated to make photoch;omic films that stimulate sunglasses. T h e techniques for coating have been described by one of us previously (6)and are summarized in the Experimental Section. Briefly, a solution of the compound is spread on the transparency using a draw-down bar, and the sample is heat-treated to vaporize the solvent quickly. DernoMtratlon ot Photochromlc Effect T h e coated transparencies are colorless in room light. ExDosure to midday, outdoor (not filtered through window glass) solar radiation, or a sunlamp produces a blue color. The color persists as long as the transparency is kept in the strong light. As soon as i t is removed to room light, the color begins to fade. This cycle has been repeated 20 times with no apparent fatigue. The coated films do not follow first-order decay and are not suitable for kinetic studies. The half-life of the pseudo-first-order decay is about 78 s a t room temperature. For lecture demonstrations, a photoflash can he used as the light source if its plastic UV filter is removed and the flash is held in contact with the transparency. An overhead projector can he used to show the phot&hro~ism. The thermal fading is readily seen, but can be speeded up using a heat gun or hair dryer as a n "eraser". Figure 2 shows the visible spectrum of a coated transparencv hefore and immediatelv after irradiation. The transparency base material ahsords in the ultraviolet. The magnitude of the nhotochromic effect can he varied bv chaneine .. the concentration and thickness of the coating. One can see from the figure that a considerable amount of visible lieht is absorbed a&er irradiation. However, the spectrum do& not ~
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2.7-Dihydroxy-i-nltrosonaphthalene (2)(7) To a 1-L three-necked flask equipped with a mechanical stirrer and addition funnel is added 13.9 g (0.087 mal) of 2.7-dihydroxynaphthalene (1) (5) and 150 mL of 0.60 M sodium hydroxide solution. The mixture is cooled to O°C with an ice-salt bath, and 6.0 g (0.087 mol) of sodium nitrite is added. With stirring, 10 mL (18 g, 0.18 mol) of conc. aulfurie acid is slowly added to the mixture such that the temperature is maintained at 0% During the addition, 2,7dihydroxy-l-nitrosonaphthaleneprecipitates (dark purple). After all the sulfuric acid is added, the mixture is allowed to stir for 1h a t Low temperature. The precipitate is suction-filtered, thoroughly washed with water, and allowed to air dry for 2-3 days. A total of 16.1 g (98%) of 2,7-dihydroxy-l-nitrosonaphthalene,m.p. 285'C, was obtained. 1,3,3-Trimethyi-~-hydroxyspirolndolinenaphthoxadine (4)(2) To a 1W-mL round-bottomed flask equipped with a condenser, addition funnel, and stirring bar is added 2.0 g (0.011 moll of 2 and 20 mL of absolute ethanol. The mixture is gently refluxed and stirred while a solution of 3.5 g (0.012 moll of l,2,3,3-tetramethyl3H-iudolinium iodide (3) (5),1.8 mL of triethylamine, and 12mL of absolute ethanol is added over a 30-min period. The mixture is refluaed for 2 h and rotary evaporated to approximately % of the original vdume. The dark, viscous rnaterinl is allowed ro arand overnight,suction-filtered,and rinsed with cold ethanol togive 3.1 g of crude 1.Two rrcr\,stalli7ationsfrom ethanol gave I .r p: (R"Ool uf 4.
Coating the FNm A solution suitable for coating on dear, heavy transparency film is made by dissolving 1.4 g of medium molecular weight cellulose propionate (5) and 0.1CM.20 g of 4 in 25 mL of 50:50 acetonel ethanol. The coating procedure has been described in detail previously (6). Briefly, a draw-down bar is prepared by wrapping a steel rod with thin wire. A sample of the solution ia poured onto the transparency and spread with one pass of the bar. The sample is warmed with a heat gun to provide the necessary rapid evaporation ofthe solvent. Thick transparency film (e.g., Scotch 501) is required to get a smooth uniform coating. Conclusions The material covered here makes an ideal individual project for an upper level student. It contains some interesting chemistry, and the student gets to appreciate the application hv makine h i s h e r own ~hotochromicfilm. The coated transparencies also make an effective demonstration for the oreanic classroom. An additional challenge can be added hv having the student work directly from i h e patent, rather than from this article. Llteralure Clted 1. Caluert, J. G.;Pit*, J. N. Photochemistry: Wiley: New Yorh, 1966; p 486. 2. H o s d a , M.. Eur. Pet. Appl. EP 186364 AZ, July 2, 1986 (Chem. Absf 1986,160 1201. L81587e. Patent svsilsbie from STN document delivery servieel. 3. Ansrticlethatdescribesinorganicphotochromiesunglasse~appearodpreviouslyinfhis Journol:Stookey,S. D. J. C h ~ m E d u r 1970.47.1i6. . I . Other e~smplesafsynfhesisor kineticsofphotochromic mmpounds havealsoappeared in this Journai: (a) Au1f.A.; Kaubs, C. J. Chem.Edue. 1974.51,395. ibl Zaczek. N. M.; Levy, W. D.;Jordan, M. L.; Nieminr, 3. A. J. Chem. Educ. 1982.59, 705 (cl Petersen, R. L.; Harris, G. L. J. Cham. Educ. 1985, 62, 802. (dl Hutton, A. T.; Blaehburn.D. J. Cham. Educ. 1986,63,688. (el Pichering, M. J. Cham. Educ. 1980,
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5. Aldrich Chemical Compsny. 6. Osterhy, B..J. Chem. Educ. 1989,66.1026. 7. Based on the preparation of l-nitrono-2-naphthol: Marvel, C. S.: Porter, P. K. Om. Synlh. Coll. Vol. 11941,411.
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