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Dyeing KDPJ. Anand Subramony a , Sei-Hum Jang a & Bart Kahr aa Department of Chemistry, Purdue University, West Lafayette,IN, 47907-1393, USAVersion of record first published: 26 Oct 2011.

To cite this article: J. Anand Subramony , Sei-Hum Jang & Bart Kahr (1997): Dyeing KDP,Ferroelectrics, 191:1, 293-300

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Ferroelectric, 1997, Vol. 191, pp. 293-300 Reprints available directly from the publisher Photocopying permitted by l i s e only

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DYEING KDP

J. ANAND SUBRAMONY, SEI-HUM JANG, BART KAHR*

D e p a m n t of Chemistry, Purdue University, West Lclfayette, IN 47907-1393 USA

(Received March 26, 19%)

We dc8cribe the growfb and spectnwcopy of we l lde fd mixed crysrals of KHzpOg (KDP) ctmtaining botb natPral and syutbdk mganic dyes in the (010) and (1011 growth sectors. Our mmarynevaluates histohl dye inclusions, mveaisothers discoveredin a directed scraaring. and ulthtCly describe8 dyes ratiollally synthesized for the recognition of KDP surfaces. Absorption and emission spec- with polatiaed light are used to charstaize these mixed crystals. A model that accounts for the facial selectivity of the dyes is propod. We show that a chiral chnm@ore is selective enough to distinguish enaa?bmplms faces of KDP. Tailor-made dye iaClasions in KDP Buggest the possibility of developing new photo& materials by coupling the opt ica l~of tbehostandguest .

1. INTRODUCI’ION

We previously described the action of the first crystalline dye lasers1 made from K2S04 that was stemregularly doped in particular growth sectors with a variety of sulfonated fluorophores.2 Here, we seek to extend this work by demonstrating the growth of KH2Po4 (KDP) crystals containing sector specific inclusions of organic chromophores motivated by the possibility that fluorescent dyes ordered by host crystals with large second order nonlinear susceptibilities might ultimately function as compact, self frequency-doubled single crystal dye lasers.3 Our summary reevaluates historical dye inclusions, reveals others discovered in a directed screening process, and ultimately describes fluorophores rationally synthesized for the facially specific incorporation into KDP. Polarized electronic spectroscopy of dyes in KDP combined with INDO/S calculations suggest structures for these organic/inorganic hybrid crystals4 and should serve as a basis for the design of future dye-doped KDP crystals with prescribed optical properties.

Colored plant extracts that stained inorganic crystals during growths were troublesome to crystallographic theorists who could not place such strange solid solutions within the framework of Mitschcrlich’s Law of Isomorphism. That “law” presumes a constitutional similarity between host and guest. With these problems in mind Retgers, in 1893, tried to systematically stain inorganic salts with dyes. He attempted nearly 1000 such cocrystallizations with natural or synthetic organic colorants and a variety of simple water soluble salts.6 Of course, the vast majority of the organic dyes eschewed the inorganic lattices. For example, no dye - of some 30 screened - stained growing KDP crystals. KDP has since become one of the most well- studied crystalline substances, prized for its ferroelectric response and nonlinear optical

(501y293

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proPerties.7 However, modem researchers have struggled to identify and remove trace organic impurities such as carboxylic acids, alcohols, phenolss, and even microbesg, from KDP crystals, because they can reduce the laser power damage threshold. While mixed KDP crystals with inorganic dopants have received systematic attention in the femoeleclrics literature,lO KDP crystals with purposeful, oriented organic dopants have not previously been conceived.

2. HISTORICAL INCLUSION

The only contraindication in the literature to Retgers's failure to stain KDP crystals with dyes was found in a note by Blattner, Matthias, and Men stating that hematein (1) solutions stained KDP crystals and depressed the ferroelectric transition temperature by 2 OC.11 Since these workers were principally concerned with the effects of impurities on the KDP phase transition temperature they did not describe the morphological or optical properties of their mixed crystals. We naturally repeated their experiment and report here two new observations: (1) The inclusions of 1 are strongly dichroic; absorption is pronounced for light polarized along Q or b but absent for light polarized along c. INDOE calculations12 indicate that the electric dipole moment for the visible transition at 440 nm runs along the vector connecting the ispo-phenyl carbons thus suggesting that molecules of 1 are oriented such that this vector is parallel to [ 1 101; (2) Hematein, a naturally occuring compound with an absolute S configuration,l3 stains only one pair of the four prismatic faces; it stains the (010) sectors, but not the {lm] s tors. The a and b faces of KDP are related by diagonal glide planes in space group &d and are thus enantiomorphous (Figure 1). Therefore, 1 must recognize the KDP faces enantiospecfically.14 Nevertheless, the acid-base and redox chemistries of the polyhydroxybenzoquinone (1) are quite complex.lS It may well be that more than one chromophore resides in the KDP crystals. We therefore set out to find analogous recognition processes using redox stable dye guests with simpler pH dependencies.

A

FIGURE 1. Relative dimensions of the KDP phosphate lattice and 1. a) Absolute stereochemistry of the (010) face of KDP. K+ ions have been omitted for clarity. Long connections repsent H-bonds, b) Absolute (S) configuration of 1 with geometry optimization based on the AM1 Hamiltonian.16

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DYEING KDP [503)/295

FIGURE 2. 'Ihe idealized habit of KDP bounded by the dyes that slain the (101) and (OIO] growth sectors.

3. AZO-DYE LNCLUSIONS

We screened about 100 dyes - some purchased and others synthesized in our laboratories - for their activity with respect to KDP crystals growing from aqueous solutions (0.5 M KDP, 10-3-104 M dye) in insulated containers at room tempture. We simply inspected the large 1 cm3 crystals for regiospecific colorations that were

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29hdSo4] J.A. SUBRAMONY PI d.

plainly visible to the eye. Of the many types of dyes tested we found that the sulfonated azo dyes Chicago Sky Blue 6B (2), Amaranth (3), Direct Blue 15 (4);’ and Trypan Blue (5)18 stained the eight pyramidal { 101 1 faces.19 We have subsequently found 10 other sulfonated dyes that stain the (101) faces. Crysta ls containing 2-5 are representative of the group. Figure 2 shows the idealized habit of KDP with the dyes that stain the (101) faces while Figure 3 shows a photograph of real KDP crystals containing dyes 2 and 3.

FIGURE 3. Photograph of 2 (left) and 3 (right) inclusions in KDP crystals viewed along [loo]. Scale divisions = 1 mm. vertical axis [Ool]. Color Plate XIII

We measured the solid state absorption spectra of dye inclusions 2-5. In each case h m a was blue shifted for dyes in the crystal relative to neutral water solutions or saturated aqueous KDP solutions (Table 1). Several processes, even excepting interactions between the host and guest, may account for the blue shift. In basic solution (pH=12) orfho-hydroxy 820 dyes show a blue shift on the order of 20 nm which is attributed to the conjugate base. This mechanism is not likely however because saturated solutions of KDP are acidic (pH-5). Dye dimerization in some geometries can produce blue shifts according to simple exciton theory.20 Alternatively, the persistent blue shift could result from the rotation of one aryl substitutent with repect to another thereby reducing delocalization in the It-system in the crystal structure compared with the solution ground state structure. Such a conformational change might result from constraints imposed by the host.

Cursory inspection using linearly polarized light indicated that the inclusions are at best weakly dichroic. This observation was confirmed and quantified by measuring the polarized visible absorption spectra of cut, polished, and oriented single growth sectors with light incident in and polarized along the principal directions.21 Figure 3 shows the polarized absorption spectra of oriented 3KDP crystals and the corresponding absorption speetra in water. The dyes exhibited surprising thermal and photochemical stability in the KDP crystals. The color was not bleached up to the

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DYEING KDP Posy297

melting point of the crystals (253 "C) nor undertxposure for one hour to the output of a 500 W Hg arc lamp at room tempcram.

400 450 500 550 600 650 wavelength (nm)

FIGURE 4. Po&uized absorption spectra of oriented 3KDP crystals for light pnpagahg alw the ( l f J @ d ~ p & e l to 6 and c ~ x t & Ihe fine line is the ah@on specbum of3 in wata, pH=7.

TABLE I Absorption maxima for dyes 2-5 in aqueous solutions and in KDP crystals and corresponding emission maxima for dyes 6-7.

Dye Ci NO.^ b l a x h a x b a x H20 aq.KDPsoln. KDPcrystal

2 24410 617 617 592 3 16185 520 519 506 4 24400 596 594 584 5 23850 595 591 580 6 .......... 394,439 385,614 3%,42rl 7 .......... 448 448 422

4. LASER DYE INCLUSIONS

None of the dyes 2-5 have large quantum efficiencies for fluorescence and are not suited to laser applications. We therefore synthesized coumarin derivatives 6 and 7 carrying phosphate groups. Dye 6 recognizes the { 101) faces while dye 7 recognizes the (100) faces. This result is in kee ing with the fact that the number of free

respectively) is the same as the number of simultaneous lattice H-bonds an incipient phosphate group makes upon adsorption. Emission maxima are listed in Table I. Molecule 6 has two distinct emission maxima in solution. The low energy emission of similar coumarins has previously been attributed to the exciplex emission from the

hydrogen bonding substituents in the p E osphate groups of 6 and 7 (three and two

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protonated fluorophore.23 Unfortunatley , the luminescence from these dyes was not stable in solution nor in KDP crystals with respect to UV-irradiation (330 nm). We have previously found that molecules with ends of distinctly different hydrophilicities enter ionic crystals as dimers.24 Such an arrangement places the charge functionalities on the surface of the ionic crystals for the purpose of attachment and pointing away from the interface to encourage the nucleation and continued growth of the host crystal around an otherwise offensive impurity. The [2+2] photodimerization of coumarins is a well-studied reaction both in solution and in the solid state.25 Pairwise aggregation followed by photodimerization would lead to the observed decay in luminescence. Attempts to isolate the photoproducts are underway.

5. SELECTIVITY FOR { 100) OR ( 101) SECTORS

What is the mechanism or mechanisms of dye incorporation in KDP? Given the disparate structures 1-7 in Figure 2 there may not necessarily be a single mechanism. However, the following correlation between molecular structure and preference for the {loo) or { 101) faces has not been contradicted by experiment: the dyes 2,3,4,5,6 that have tetrahedral anions in which one and only one ligand is connected to the chromophore select the { 101) faces while only dyes 1 and 7 that do not have such functional groups select { 100). In keeping with this correlation, ten other sulfonated azo dyes show selectivity for { 101 ) as mentioned in section 2.

The distinct topological features of the { 101) surfaces of KDP are the emergent phosphate oxygens that point nearly along the body diagonal of the unit cell [vp-0: ~ 0 ~ 9 ~ 4 . 5 9 , cos6b=O.59, c0sf3~=0.55] (Figure 5) . Tetrahedral anion substitution for a phosphate on the { 101 ) surface would direct the dye molecule away from the surface along vp-0. Such a model is shown for the substitution of 3 upon the { 101) surface of KDP. The reciprocal recognition motif, guest phosphonate substituents substituing for crystalline sulfates, has been well-illustrated by Davey and coworkers in their studies of BaSO4 growth inhibition2

W

FIGURE 5. (101) face of KDP viewed edge on along [OlO] showing rows of oxygens in black emerging from the surface. A molecule of 3 in a geometry optimized using the AM1 Hamiltonian is shown subtituting on this surface.16 K+ ions have been omitted for clarity. Thick black line represents the electric transition dipole moment calculated according to the INDo/S method.

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DYEING KDP [507]/2?Y

Non-specific sulfonate substitution of the polysulfonate dyes on the { 101 ) faces would scramble the polarization and lead to a reduced dichroism. Moreover, as shown in Figure 5, the electric transition dipole moment of the azo dye in the orientation illustrated would point nearly along the body diagonal vector. As such the transition moment would have a large component along each of the principal axes. Here, we would expect a small linear dichroism.

Many structural and spectroscopic features of these dye doped KDP crystals must be defined in order to reach of level of detail that was recently illustrated by Cunningham and coworkers for Cr3 + doped NH4H 2P04 (ADP) crystals27 Nevertheless, we have shown that a variety of chromophores can be selectively oriented by growing KDP lattices. The synthesis of optical materials from such KDP crystals containing molecular guests is now a matter of design.

ACKNOWLEDGEMENTS

We thank the US National Science Foundation for support of this research through a Young Investigator Grant to BK.

Keywords: KDP, KH2PO4, enantiospecific, dye inclusion, polarization spectroscopy

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