Organic Hourglass Inclusions A Review of Past and Recent Work and a Student Experiment Bart Kahr, Jason K. Chow, and Matthew L. Peterson
Purdue University, West Lafayette, IN 47907 The descriptive crystallographic literature of the 19th centurv is an untamed database replete with materials that address contemporary scientific problems ( 1 , . The socalled orzanrc hour~lossinclusions (Hlsl are cwstals that deserve modem examination. These simple saits contain ormnic dves included throueh specific faces that color growth sectors in patterns &m&imes reminiscent of a bow-tie or hourglass. First observed in 1854 (21, HIS were last studied comprehensively in the 1930's by Buckley (3) and France (4) and the 1950's by Whetstone (5).Nevertheless, HIS are relevent to contemporary scientists because they have characteristics that make them promising objects for crystal growth, spectrompic, and photonic studies.' Organic HIS are odd (what are large organic molecules doing in simple ionic lattices?), but they can be easy to prepare. HIS can be used to illustrate important ideas in solidstate chemistry including Miller planes, crystal habit and its modification by impurities, molecular recognition by surfaces, anisotropy of light absorption, and solid solution structure. Herein, we present a brief discussion of the history and utility of organic hourglass inclusions and deL scribe a recipe for svnthesizine a particular svstem (. K- 8 0 . containing acid fuchsin), the &antifieation Gf the dye content, and the observation of its dichroism. This experiment is conducted easily in a general chemistry laboratory from which students can take away "jewels" ?
a
Historical
The regular coloration of simple salt crystals by dyes was first observed by de SBnarmont (2) while growing SrtN03)~ 4Hz0 in a water solution containing logwood (a common component in children's chemistry sets of many years ajio, rich in a red dye called hematox;lin~. 1.ehmann realized that only some crystal faces "attracted" the dye, thus staining only the portion? that had grown through particular hces 161.A handful of analogous observations wcrc made hctwccn 1879 and 1909 171. .. Most of these observations were isolated reports. except for those of Gaubert (8).Some researchers delieved t h i t these mixed cwstals resembled ordinary solid solutions. Others invoked the tenuous idea of s~ncrystallization, where microcrystals of the dye were presumed to be deposited a s discrete units. The HIS were not studied again until Buckles revived the subiect some 20 years later. ~ a r o l dBuckley, a cry&lographer&orking in Manchester in the 1930's. was one of the first 20th centurv scienusts t o d ~ , s e r i ~ , u s s t u oncrystill d~ habit modification. After a series of papers in which he discussed the influence of inorganic i&hties on the relative areas of the faces exl~ourglass inclusions are known in the mineral kingdom where wlored ions differentially bind to a pair of faces that are not related by symmetry. We use the term HI to refer only to mixed crystals where the guest is an organic dye. 'The experiment described herein was incorporated successfully into a sophomore organic chemistry laboratory at Purdue University by Professor Jean A. Chmielewski. 584
Journal of Chemical Education
pressed by simple salt crystals (9), Buckley suddenly shilled his focus bv usine oreanic dves a s his habit-modifving agents (7).~&che&r\as-aid still is-a textile a i d dyeing center. Buckley had access to many commercial dyes from the ICI corporation and elsewhere. Many dyes had little effect on the habits ofthe ioniccrystals. Occasionally there was a marked change that Bucklev intemreted as a close interaction between the uolar sibstituents on the dye molecules and the growing &stal surfaces (10). Rare were those dyelsalt systems where the dye molecules were included in the salt lattic+Bnckley tried 16.000 wmbinations(l1). Bucklev was unable to b lace the recoenition urocesses on a structGa1 basis despite his tremendius phenomenological database. A comparison of nine isomeric dyes showed a helter-skelter range of action upon the host crystals. Bucklev concluded. "apart from a general tendencv of all the c'nfigurations to- 'do something it is evident that the effects are highly specific to the dye molecule and the crystal surface" (3b). Conformational analysis was insufliciently developed a t this time to permit accurate inferences of dye interaitions with crystil surfaces. Wesley France conductrd parallel studies at The Ohio State University, but hc suffered from the same limitations as did Buckley (4,. Among the general features of HIS is their change in color in plane polarized light. Dichmism is a natural consequence of directional crystal growth. Presumably, order is imposed on the chromophores because of a structural homology between host surface and bwest. Moreover, the mixed crystal usually are excellent "single crystals" of the hosts. Lauc photob~aphsoften do not distinguish between purc salt crystals and those containing dye. This implies that despite the perturbation of insertine a larm or~anic molecule in a grdwing ionic lattice, single crystal register is regained quickly. Scattered post-Bucklev HI researchers invoked e~itaxial relationships or isomo&hisms between host anb guest (121, which sometimes lacked the detail to be convincing. Neuhaus' approach involved matching molecular dimensions of dyes with crystallographic lattice constants (13). This procedure is compelling when it works, but impressive fits can be found for noninteracting dyelsalt systems.
-
- -
New Uses for Hourglass Inclusions
Recently, we began to study, once again, HIS (14). As scientists of the sixth successive generation to study these crystals, we see in HIS scientific relevance and potential applications. HIS reveal hiehlv specific interactions between host and guest. How do&& Helectively recognize discrete sets of symmetry-related faces of salt crystals? Chemists recently have shown that habit modification studies are an important step in isolating the relevant chemical interactions in the crystal growth process (15).Intermolecular interactions of acid fuchsin, the dye illustrated in subsequent sections, may take on greater importance because recently it was shown to inhibit HNreplication in uitro (16).
Hourglass Inclusions Grown
Dye
Color Index NOS.1st ed. ( 18)/3rd(3
Host Salt
-
Active Growth Sector
Ref
Methylene blue
922152015
( 8)
Methylene blue
922152015
(s)
Naphthol green
5/10020
(3a)
Acid fuchsin
692142685
Acid fuchsin
692142685
(34 new
Quinoline yellow
801147005
(3a)
Quinoline yellow
801147005
(11)
Amaranth
184116185
Amaranth
184116185
(36) newa
Chromotrope 26
4511 6575
(36)
Fluorescein
766145350
(Qd
Metanil Yellow
138113065
newa
Bismarck Brown
331121000
(24
P-P-OH-6
synthetic
(24
synthetic 0-D-OH-6 These dyes were known to modify alum-habit (36).
and it is the easiest to Drenare. An undereraduate experiment based on t k s system f o l l o w ~ : ~ ~ ~ r o x l matelv 20 e of KaO, is dissolved in 200 mL of d ~ s dye .soiution l ~ is prepared from 30 mg tilled ~ ~ 0 acid fuchsin (Na' salt was purchased from Aldrich, $16.35125 g) in 30 mL HzO. A 60-mL solution of a KzS04 is placed in each of three Petri dishes, to which 4, 8, and 12 mL of the dye solution are added. The dishes are placed on the benchtop for one week. ~ f r ethis r time, well-formed transparent crystals with fuschia coloring are selected from the respective dishes and examined (Fig. 1). Caution: Treat acid fuchsin with care. As with most dyes, avoid contact with skin and avoid inhalation. The solid can be an irritant and should be handled only with gloves in a fume hood (21).
Notice the marked differences in the habits of the three batches of crystals (Fig. 2). The 1021) faces are predominant in the batch containing the least amount of dye. This habit is identified easily, because the edge [1001bisecting the including sectors is not parallel to these planes. In the intermediate concentration range the 1010) faces are predominant. Now the bisecting direction is in the plane of the largest face. In the high-wncentration
(24
Aromatic molecules trapped in ordered ionic matrices are unique and may be expected to exhibit unusual spec-
troscopic properties resulting from the "salting" of chromoOptical spectroscopic studies of single crystals phores (17). of dyes usually are complicated by the broad bands resulting from the formation of absorption complexes, excitons, or charge-transfer absorption. Isolation of dyes in glassy matrices or polymers invariably preclude single crystal orientation. HIS are, therefore, promising spectroscopic objects. HIS contain oriented, isolated organic chromophores sealed in inorganic matrices and may be robust materials Acenwith second-order nonlinear optical properties (18). tricity, a requisite condition for second-order nonlinear optics, can be a natural consequence of the facially selective growth mechanisms. Additionally, HIS may serve a s solidstate dye laser gain media.
Acid Fuchsin
Crystal Survey We recentlv have worked out conditions for remaking a number of Hf crystals, listed in the table, by surveying &vera1 orders of maenitude of relative dve and salt concentrations. Other caniidates, known to earlier chemists, are in preparation. However, they are not always easy to reproduce. Structural assignments in the first edition of the Color Index must be accepted optimistically (19).Failure to reproduce some of the HI crystals described previously is undoubtedly due to the f a d that the active species was a n impurity in the commercial dyes not present in similarly labeled materials manufactured today. Experimental The most appealing HI system that we have grown, K>SO, lllOlIacld fuchsiq3 was discovered by Buckley 1201,
3The Miller indices (110) refer to the reciprocals of the intercepts along the three crystallographic axes. A succinct desniption of this notation is provided in ref. 21.
Figure 1 . Selected K2S041acidfuchsin crystals displaying various habits and colorations. Volume 71 Number 7 July 1994
585
comparing the strength of the color in the crystals with the standard solutions of equal path length. It is easy to observe dichroism with a sheet of polaroid or a polarizing mimoseope. A crystal can be mounted in various orientations with a small piece of clay. By rotating the crystal with respect to the incident polarization it is possible to see pronounced color changes in some orientations and not in others. The best orientation for this purpose is not presented when looking through the largest flat faces of the crystals ((0211and {0101).The dichroism is stronger when viewed through the a and c directions. The crystals can be cut with a razor blade to expose these planes. The dichroism of acid fuchsin in the crystal host is distinct from that of a water solution and demonstrates that the dye molecules are oriented in the lattice. A detailed structure determination is in progress in our laboratories (25).
Figure 2. Morphology of a representative K,SOdacid fuchsincrystal. Shading marks regions mntaining dye. regime the including surfaces oRen are poisoned. Growth stops through I1101 and the crystals become dramatically elongated in the c direction. Variations on these themes are common and subtly dependent on the temperature and rate of evaporation. The molar absorptivity (E) of acid fuehsin in water is calculated from standard solutions and Beer's Law, A = dc. Absorbance at 542 nm is measured on a spectrophotometer and is linear for solutions up to M. Our slope gave E = 22000 L mol-'em-'. Several crystals (-200 mg) are selected from each batch, weighed and dissolved in 3 mL HzO. The absorbance is measured and the concentration calculated with the recently determined E. ARer carrying out these operations, a student should be able to calculate the number of moles of acid fuchsin and the dyekalt mole ratio. We found that for the three batches of crvstals mown from solutions containing 4,8, and 12 mL ofthe dyesolution, the number of moles of salt per mole of dye were 2.1 x lo5, 5.4 x lo4, 3.3 x lo4. These values can be factored by measuring the fraction of the individual crystal volumes that are colored by the I1101 p w t h sectors. Usually it falls somewhere between 15-40%. We have found that the limiting quantity of acid fuchsin in KzSOa is about 1molecule per 5,000 K2S04 units or 1molecule for every 1250 unit cells. K2S04crystallizes in an orthorhombic lattice w$h a = 5.772 A.b = 10.072 A.c = 7.483 A. Volume = 435.0 A3. Z
d~efliter is obtained' We t ~ ~ i c afound l l ~ eoncentrations in the millimolar range. This result can be checked by
586
Journal of Chemical Education
Acknowledgment Many thanks to Bill Robinson and Bill Gleason for encouraging the preparation of this report. J. C. thanks Scherling-Plough for a QUEST Fellowship. We also thank Michael Kelly, Steven Antonelli, Deborah Parkins, and Jean Chmielewski for their assistance. We gratefully acknowledge the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research (ACS-PRF #24331-G6). Literature Cited 1. Grmh, P. ChrmischaKv8Loll0808phie. W. Engelm-: Leip~ig,19061919; Kahr,B. McB"de, J. M.Angow.Chem. I n t Ed Engl. 1992,31,1. 167.491. 2. S4nmor.t. H. Ann. Phvr Cham.18M. ~~,~ , ~ ~ 3. is1 Buckley, H. E. &it K%f. ISM, 88,243. ibl Buckle~,H. E. Crystd Gmmlh;John W~leu:New York. 1951: 00 377378. ~
7. There examples were fimt collected byGsubert,P.iBull. S a Ii- Mi*. 1800,23,2111 and laterby Buckley, H. E. (ZeiL f i s t . 1938, 85,581. 8. Gaubert, P BuU. Soc. Min. *. 1894.17.123: lSW.23.211; 1805,25.223,1904.27, 233,298: 1 W . 28,130,286; Comple3 h n d u s 1910,151,1134. 9. See for example: Buckley, H. E. &it KIist. 19%. 76, 147; 1932,81,157. lo. Buckelx H. E & i f & i s L IS34,88,381. 11. Buckdy, H. E. Memoirs and P d i n g a o f t h e MancksterlitemlyondPhilosophi. mlSaiety, 1939.31; 1851,77. 12, (a1 Kh1opin.V G.;lklstaya, M. A. J.Phya. Chem. (USSR), 1940,12,941; lb)Lindanberg, W Z. Notuforsch, 1866,llb. 177: 1 4 Slavnwa. E. N. Dakldy A k d Nauk SSSR, 1954, 107, 693:(d) Vedeneeva, N. E.; Slsvnwa, E. N. X d y Imt. K ~ i s t , A k d . Nouk SSSR, 1952,7,135: (el I&*, E. M. Zhuc Neog. Khim 1968,3,29;itl Hartmann, H. Zur Physik Chem. &~tollphasphon: Tagvng Phyaik. Gea.: Greif. m d d . 1959: pp 136:igl Kleber,W Freihrsr Forschungsh. 1959,B57.11. 13. Neuhaus,A.Ang~u.C k m . 1941,54,523:Z& Krisl. 1941, la?, 21. 14. Kahr B.: KeUes M. Chow J. unoubliahed results. 15. Welssbuch, I.; Addadi, L.. M.; lalsemwitz, L. Scioneo 1991,253,637: and refer en^ therein. 13. BabaM.:Sehok.D.;Pauwek,R.;Balzarini. J.;DeCleqE.Biochem.Biophys.%. Commun. lM8, 155.14M. 17. Kirkor,E.S.;Gebicld,J.;Phillip,D.R.;Michl.J.J.Am.Chem.Sac.l888,IOB,7106( Kirkor, E. S.; David, D. E.; M i a , J. J. Am. Ckm. 9oc. 19W,112, 139. 1s. Prasad. P. N.; Williams, D.J.I"lmduction fo Nonlinror OptimIEffe~cfsin Mdaa'ie8 and Palymprs: John Wiley: New York,1991. 19. &we F. M., Ed. Colaur Index, l i t ed. Society d Dyers and Colnuiate: Bradford,
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1930.73.443. 2, Rigterink, M D.;Franee, W G. J Phys. Chem. 1858,42,1079. 25. K&X M P; B. d m i t * d f ~ ~ ~ ~ b ~ ~ ~
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