Linear Dichroism of Cyanine Dyes in Stretched Polyvinyl Alcohol Films: A Physical Chemistry Laboratory Experiment L. V. Nataraian, Margaret Robinson, and Robert E. Blankenship' Amherst College, Amherst, MA 01002 An imnortant and often ooorlv asnect of the . annreciated .. ahsorption of light by molecules is t h a t the absorption has a directional character. If a collection of molecules is not randomly oriented, t h e absorption of plane polarized light will not he the same for all orientations of the electric vector of the light. If the molecular transition dipole moment is parallel to t h e ulane of nolarization of the liaht. t h e absorntion will be ma&al, while if the transition &pole moment is perpendicular t o the plane of polarization of the light, no absorption will occur. This differential absorption of light with respect t o two perpendicular planes of polarization is known as linear dichroism. Some degree of molecular orientation is required before any linear dichroism can he detected. This orientation can he perfect, a s in a single crystal, h u t much useful information about the directional character of the electronic or infrared transitions of a molecule may he obtained with a much lower degree of order. In addition, many molecules, especially those of hiolo~icalinterest, cannot he satisfactorily crystallized, and even many single crystals are not suitahle for polarization measurements due t o the possibility of several molecules of differing orientation making up t h e unit cell or excitonic interactions. In recent years the stretched polymer film linear dichroism technique has been successfully used to establish transition moment directions in a large number of orsanic molecules I - , j ,. Thi- pr,,, d u r e L . , ~ * I stI>. < t ~ ~ ~ t h ~ rthe l d im n ;< h . ~ ~d ItI I I I C ~ C > II I I .I ptflvtiicr ni:ttrlx n h i ~ d ti ? t h m >trt 1111eii.11 IIIC < L I C + I1~1cd:rt11eI.. ~n,mqlI~t.ri~~:,l il will :11ix11 i1:eIi uitli i t , IOIIL ~ i i ~ ~ l e t . ~ ~ .h,11k l : ~ r : t~h\ .i stret,h . (11rt.ct i 8 t t 1 !see Fig. I,. :\ .urA
similar results are a liquid crystal matrix, hydrodynamic shear, and electric and magnetic fields. The stretched film technique described here is a very simple method and can be applied to a wide variety of molecules with proper choice of polymer. T h e stretching technique is t h e basis by which Polaroid@sheets and sunglasses are made. The solute in this case is a polymeric form of iodine. In the present study we report linerar dichroism of cyanine dves as an undereraduate nhvsical chemistrv lahoratorv expkriment. cyanin: dyes ha;e 6een chosen sin& they are quite anisotronic in shane and therefore orient well. Thev are also of t h e Goper symmetry (CZ,), so t h a t the data analysis is straightforward. Finally, they are often used in an experiment illustrating the free-electron "particle-in-a-box" model (6).
These experiments easily could he used in conjunction, giving students a deeper appreciation of the absorption phenomenon.
Experimental Materials. The cyanine dyes, namely 1,l'-diethyl-2,2'-cyanineiodide, I',l'~diethyl-2,2'-c~rbocy~nine iodide and 1,l'-dietbyl-2,2'dicarbocyanine iodide were obtained in pure form from Eastman Organic Chemicals and ICN Pharmaceuticals, Inc. and used as such. The purity of these dyes was confirmed by measuring their absorption spectra in ethanol. The powdered polyvinyl alcohol (PVA) was o b ~ tained from Polysciences, Inc., Warringtan, PA 18976 (cat. no. 15129 of mol wt. 78,000).Sheet-type polarizers were obtained from Edmund Scientific, (cat. no. P-41,168). Preparation of Films.A stock aqueous PVA solution is prepared by heating 9.1 g of PVA in 100 ml of water over a steam bath for 20 mi", stirring occasionally to keep from dehydrating at surface and forming a skin. Whatever does not rehydrate is separated later by centrifugation. The solution is then allowed to cool at room temperature in a moist environment (a desiccator with water instead of Drierite works well). To the stock clear aqueous PVA solution (25 ml), 2.5 rnl of the dye solution in ethyl alcnhol(1 X 10-3M) is added and stirred vigorously with a glass stirring rod far several minutes. The solution is evacuated in the vacuum desiccator described above for 1 hr so that bubbles can form. The PVA-dye solution is then poured into small flexible centrifuge tubes and centrifuged fur 10 min at 15,000rpm to remove the bubbles. The solution spectrum of PVA-dye with PVA solution as reference is taken to compare with the film spectrum. Always wear rubber gloves when handling the dyes since
out a e l l w c I' d:tr..~d
sheets give results identical to those obtained using prisms.
sorption spect;a. T h e student should v&fythis statement by performing the matrix multiplication in eqn. (1). For the rod-shaped molecules studied in this report, a single polarized absorption is sufficient to establish the values of the K's, although this is not in general true. If the valid assump-
Figure 3. Polarized absorption spectra of 1,l'diethyl-2.2'-carbocyanine iodide stretched in oolwinvl alcohol film.
light.
242
Journal of Chemical Education
STRETCH
RATIO
Figure 4. Dichroism of cyanine dyes as a function of stretch ratio l.l'-dieth~i-2.2'-~vanine iodide 1.1'-diethyi-2.2'-carbocyanine iodide A 1.3'-dielhyl-2.2'-dicarbocyanine iodide
To use the LD technique for determining the shapes of highly symmetric molecules, two transitions with different polarizations must be measured. If the transitions do not bverlap, eqn. (2) can be used directly to calculate the K's (5). In order to use the TEM method to determine the direction of transition moments in molecules of general symmetry, i t is essential to obtain independent values of two K's and also the direction of the molecular orientation axis (5). The simplicity and validity of the TEM method in utilizing linear results to oredict orientation and shaoe of - - - - ~dichroism ~ ~ molecules are illustrated in the present study of dichroiim of cvanine dves. The dichroic soectra of unstretched films were &e same,khereas stretching caused appreciable orientation of the dye molecules and hence a large dichroism. This indicates that the solute dye molecules are embedded fairly rigidly in cavities between the polymer chains. On stretching, previously unoriented regions in the polymer become partially aliened and hence solute dve molecules in all these regions acquire similar partial oiientation with respect to the stretchine direction. Further stretching hardly aligns the polymer chains in the amorphous regions any m o r e . ~ e n c e , i t does not affect the orientation of the cavities or the solute dye molecules if they are rigidly embedded. This reasoning mav explain the increase of dichroism (Fig. - 4) up to a certain stretch-ratio, and then the leveling off. K,, K, values can be deduced for the cyanine dyes by the TEM procedure and described and plotted in the orientation triangle (Fig. 5). For example, (Fig. 2) the d for l,l1-diethyl2,2'-carbocyanine is calculated as 2.5 so that A - dA, = 0. K, = dl(d 2) then corresponds to 0.56 andK, is 0.22. Given the assumptions, this point falls on the line for rod-shaped molecules (Fig. 5). Similar results were also obtained for the other cyanine dyes. ~~
Figure 5. The orientation triangle. a plot of K, against K,. The veriical distance fram the line K, = 1 - K, gives K,. Adapted from reference (5).The x indicates the values of K, and K, calculated fram Figure 3.
tion is made that A, = A, = 0, and that the observed absorbance arises solely from A,, eqn. (1) then reduces to K, - d (I - Kz)12= 0
(2)
whered = A/A,, the dichroic ratio. It is then the case that K, = dl(d 2). The dichroic ratio should he constant across an absorption hand, if the transition is purely polarized. For rod-shaued, hiehlv svmmetrical molecules (such as the cyanine dyes studied thi;report) the x and y axes are nearly equivalent, hence
+
and the reduced absorption along the z axis of the molecule is found from eqns. (1) and (3) to be A,(A)
=
[(l+K,)A(A) - 2(1 -K,)AI(A)I
(3Kz- 1)
~
~
+
Conclusion -
(4)
in which the orientation distribution is represented by a point with coordinates K,, K,. Perfect alignment of the z axis of all molecules with the stretching direction is represented by the point K, = 1, K,(=K,) = 0. Rod-like molecules with equivalent x and y axes correspond to the points on the line K, = (1 - K,)/2. Disk-shaped molecules with equivalent y and z axes correspond to the points on the line K, = K,. Any point inside the triangle corresponds to a more general type of orientation distribution. The totally random distribution is given by the point K, = K, = K, = '/3.
"
"
onstration of the concept of linear dichro&m and the validity of the TEM method in deducing orientations and also shapes of molecules from linear dichroism measurements. This experiment may be extended to ascertain shapes of disc, square, and other odd-shaped molecules using the "orientation triangle." Literature Cited (1) Vopev,A., M a ~ d i e sL. . . ~ n kD..Maiur. Y., J. Amar Chem Soc ,9l,4618 (1969). (2) Thu1atrup.E. W.. Mieh1.J.. andEggers. J. N . . J Phyr. Chrm., 74,%8fi8 (1970). 131 Nnrden. B.. A o o l Soec. R e u . 14.157 119781.
Volume 60 Number 3
March 1983
243
