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R. Marshall Wilson, Edward J. Gardner,' and Richard H. Squire University of Cincinnati Cincinnati, Ohio 45221
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The Absorption of Light by oriented MOI~CUI~S A n experiment in o r g a n i c physical chemistry
This experiment is designed to expose the student to the classical techniaues of organic dve svnthesis, and a t the same time introduce him & a vaiiety of modem scientific knowledge and exotic physical phenomena which he might not encounter otheGse; The-experiment has been conducted in the Spring Quarter of our sophomore Organic Chemistrv Lahoratorv. " . and is well suited for students with some prior synthetic experience who would he stimulated more effectivelv hv a hvbrid svnthetic oreanic-nhvsical . . chemistry expeLimek. The absorption of light bv matter is a phenomenon that is so commonplace in our everyday experienre rhat we tend to forget that this is the mechanism through which we derive themaioritv of our information concernine the surrounding worfd. his fact is epitomized by thewwords of this sentence. Yet chemistry students seldom become aware of the basic nature of this process until their later years of college if even then. This experiment provides a vivid demonstration of the basic principles involved in the absorption of light by organic molecules, but in order t o understand the exotic phenomenon that the student can witness he will require the following background information. The Nature of Light Light is composed of an electric field and an in-phase magnetic field each rapidly oscillating in perpendicular planes as the entire array moves through space as shown in Figure l a ( I ) . Insofar as we are concerned the magnetic field can he disregarded, since i t does not affect the electrons of organic molecules to an appreciable extent (Figure lb). The index used to characterize a particular light wave may he the direction in which the electric field is oscillating. Thus, in Figure l b the electric field is oscillating in the x-direction, and is said to be x-plane polarized ( P J light. We will require plane polarized light for this experiment, but unfortunately the run-of-the-laboratory variety of light is composed of more or less equal parts of two mutually perpendicular plane polarized waves. Direction of light propagation Y X 1
Polarization components of normal light PI = Pz Py= Pz PY = PI
Consequently, it will he necessary for us to have a t our disposal a means of removing one or the other of these polarization components from normal light. The Generation of Plane Polarized Light The selective removal of one of the two perpendicular plane polarized components from normal light is an amazingly simple process. Consider a sheet of plastic such as polyvinyl alcohol (Fig. 2). The polymer molecules in this plastic sheet are interlacing and randomly oriented. When the sheet is stretched, the polymer molecules will unravel and extend to their maximum length in a more or 94
/ Journal of Chemical Education
field
field I$ n ~ . t ; o n Preparation of N-(2,4-dinitrophenyl) pyridinium chloride (11) To 4 g (57 mmol) af pyridine in a 50-ml Erlenmeyer flask is added 5 g (25 mmol) of 2.4-dinitrochlorobenaene. The mixture is swirled until the 2,4-dinitrochlorabenzene is dissolved. CAUTION: take care not to get any 2.4-dinitrochlorobenzene on your skin or in your eyes. Heat the reaction mixture on a water bath maintained at 60-70°C for 20 min in a hood. During this time the contents of the flask will solidify. Dissolve this solid in 15 ml of hot methanol, and then add 15 ml of benzene. Place the solution in a 50-ml round-bottom flask and concentrate it to about 20-25 ml under reduced pressure. Cool the concentrated solution in an ice hath, and if necessary induce crystallization by scratching. Continue coaling until the benzene freezes (-5°C). Thaw the benzene and immediately collect the salt by suction filtration. Wash the salt with cold benzene, and dissolve it in 5 ml of hot methanol. After the methanol solution has cooled to room temperature induce crystallization by the addition of 3 ml of benzene. Allow the crystallization to proceed slowly at room temperature to insure the formation of large crystals. When no more crystals are forming at room temperature cool the solution in an ice bath to increase your yield of material. Collect the crystals and dry them by suction filtration. The yield is about 4.93 g (72%). Evaporation of the mother liquor will afford an additional 1.24 g (18%) of the crude salt. Preparation of N-(5-pyrrolidinopenta-2,4-dieny1idene)2,4-dlnitroaniline (111) Dissolve 2.00 g (7.11 mmol) of N-(2.4-dinitraphenyl) pyridinium chloride (II)in 15 ml of absolute methanol in a 50-ml round-bottam flask, and cool the solution in an ice hath. Prepare a solution of 0.326 g (3.07 mmol) of sodium carbonate in 5 ml of water and a solution of 0.570 g (9.05 mmol) of pyrrolidine in 5 ml of absolute methanol. Mix the pyrrolidine and sodium carbonate solutions. A milky solution will result. Slowly add this milky solution over a period of 5-10 min with stirring to the by now cold solution of I1 in methanol. Upon the addition of the first drop of the milky salution a deep red color develops. Further addition produces a thick slurry of the crystalline dye 111, since the water coneentration has been adjusted to induce the precipitation of the dye 111 as soon as it is formed. Collect the dye by suction filtration, and wash the crystals with 15 ml af cold methanol. Dissolve the moist dye in 25 ml of methylene chloride in a 50-ml m u d b o t t o m flask. When the dye has dissolved completely add 10 ml of absolute methanol, and concentrate the resulting solution to about 15 ml under reduced pressure. Using suction filtration, collect the erystals that have been deposited during the concentration operation and wash them with 5-10 ml of cold methanol. Dry the crystals by sucking air t h r a u ~ hthe filter cake. The crystals have the appearance of fool's gold when packed in the filter cake. When they are removed from the filter they are a glistening green-black. The yield is about 1.44 g (66901, m.p. 164-168°C with decomposition. Spectral data were in accord with the assigned structure: ir (KBr pellet) 1620, 1600, 1580, 1520, and 1320 cm-I; uv(MeOH) A,,, 218 sh (e 14,9001, 263 sh (9000). 371 (18,200), and 486 nm (34,300); nmr (90 MHz, CDzCI2)6 2.00 (m, 4H), 3.37 (m, 4H), 5.30 (dd, IH, J = 12, 12Hz). 6.04 (dd, IH, J = 10, 14 Hz), 7.03 (m, 3H). 7.92 (d, lH, J = 10 Hz), 8.22 (dd, lH, J = 2.5, 9 Hz). and 8.55 (d, lH, J = 2.5 Hz); mass spectrum 70 eV m / e 316 (M+). Analysis Calculated for ClrHleN40n: C, 56.96; H, 5.10; N, 17.71. Found C, 56.89; H, 5.07; N, 11.71. Molecular Orientation Tape a piece of paper (8% x 11 in. notebook paper will do nicely) to a smooth surface. Firmly rub a microscope slide back and forth on the paper in a direction parallel to the long dimension of the slide. About 15-20 strokes is all that is necessary. Prepare two slides in this fashion, and remember which side of the slide you have rubbed. Add no more than two or three crystals of the dye I11 to 20-40 mg of BPC in a spot plate. Heat the spot plate over a low bunsen burner flame until the BPC melts (56'C). Mix the liquid crystal with the dye until a homogeneous solution results. Do not overheot this mixture or the dye will bleach. Place a single drop of this solution on the rubbed side of a microscope slide, and cwer the drop with the rubbed side of a second slide. Compress the solution between the slides and warm gently in order to pre-
Volume 50. Number 2. February 1973 / 97
vent the solution from crystallizing. View the liquid crystal solution as illustrated in Figure 8. Orient the Polaroid film to obtain the deepest red color. Rotation of the film from this position by 90' will result in the disappearance of the red color as s h o w in Figure 9.
We would like t o thank Norm Yunis for his contribution in the early stages of this work. Literature Cited 111 Shurclilf. W. A , and Bnllard. S . S.. "Polarized Li#hf."D. van Noatrand Co.. Ine. Plineelon, N.J.. 1984.
98 /Journal of Chemical Education
121 Do". ?., A n n u . Cham. Inlrmat. Ed.,Eng.. 5. 478 119661: Gins. F. and MaYer. G.. comptea R ~ M ' U . 258. 2039 119641; ~aillette.M.. c o m p m ~ ~ n ser. d B.~ 2628. . 264119661. (31 L ~ ~ R.~A . ~G . ~ H. ~ ~B.. ~ ~and . . cemar. G. P.. J. c h p m S O C . 92. 3653 119701: ceaaar. G. P., Levonson. R. A. and ray. H. B.. J ~ m e cr h s m Soc.. 91. ~ 2 1 1 9 6 8 1 ; ~ e a r ~ r P. . GandGray.H.B . ..I . ~ m . r .rhem SGC .91. IPI~19691. 14) Gray. G. W.. "Moiecular Structure and the Properties of Liquid Cly~tsls,"Academi e ~ ~ ~~ ~. w~ ~ ~oN.Y., r~ k ..1 9, 6 ~ . I51 Chatelain. P.. Comptes Rendus Amd. Sci. Po& 213. 875 1194Il; Maier. W. and Englert, G . ,2. ElarlmchPm.. 64.689 lL9601. 161 Sackmann. E. and Rehm. D.. Chem. Phvr. Lctrers, 4. 537 (19701: Sackmann. E.. Chem P ~ Y SI , . L ~ w8~. .263 119691: saekmann. E.. J mar them sot.. OD. 3569 1196RI; Saekmann. E.. Meibwm. S.. Snyder. L. C.. Meiiner. A. E.. and Dietz. R. E..J Amer Chem Soc.. 90,3567 119681.
