Synthesis, Growth, Structural, Optical, and Thermal Properties of a New Semiorganic Crystal: 4-Dimethylamino Pyridinium Dihydrogen Phosphate S. Manivannan and S. Dhanuskodi*
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 845-850
Department of Physics, Bharathidasan University, Tiruchirappalli - 620 024, India Received February 1, 2004;
Revised Manuscript Received April 21, 2004
ABSTRACT: Single crystals of 4-dimethylamino pyridinium dihydrogen phosphate (DMAPDP) (C7H13N2PO4) were successfully grown for the first time by the slow evaporation method at constant temperature (30 °C) at a pH of 4.45 with dimensions 15 × 13 × 2 mm3. The solubility was found to be high in water and so the crystallization of DMAPDP was performed from its aqueous solution. FT-IR and FT-Raman spectral studies have been performed to identify the functional groups. 1H and 13C NMR were recorded to elucidate the molecular structure. The grown crystals were characterized by chemical analysis and density measurements. The cell dimensions were determined from single crystal X-ray diffraction studies. Morphological analysis reveals that the crystal has six developed faces with major faces (0 1 h 1) (0 1 2) and (1 0 0). Thermogravimetric analysis and differential thermal analysis have also been carried out, and the thermal behavior of DMAPDP crystals has been studied. This study does not show any structural phase transition and its thermal stability has been found. The optical transmittance window and the lower cutoff wavelength of the DMAPDP have been identified by UV-Vis-NIR studies. 1. Introduction Compared to the extensive amount of research conducted on the synthesis and characterization of new molecular structures for second-order nonlinear optical applications, the study of third-order nonlinear processes on molecular materials has received relatively limited attention. Recently, the scope for the synthesis and characterization of third-order materials has expanded considerably. The impetus for this increased activity has been a quest for fundamental understanding of the structure property relationship and the strong technological interest in all optical signal processing provided by the third-order processes.1 The microscopic theoretical models predict large nonresonant third-order optical nonlinearity associated with delocalized π electron systems. Therefore, no particular bulk symmetry is required for nonzero χ(3). A crystalline medium in which the molecules align in the same direction has a larger χ(3) value.2-4 Development of novel molecular and crystal design techniques for assembling such materials is of great current interest. In particular, semiorganic systems provide many interesting structure and bonding schemes for the molecular engineering of new materials. Furthermore, insertion of an inorganic unit in a conjugated structure significantly influences the π-electron behavior that can have important manifestations in optical nonlinearity.5 Semiorganic structures containing inorganic (particularly PO4 units) molecules offer new properties derived from the richness of the various other features toward stability in these systems. Classical examples are potassium dihydrogen orthophosphate (KDP) and 2-amino 5-nitropyridinium dihydrogen phosphate (2A5NPDP). In all these compounds, there is an * Corresponding author: S. Dhanuskodi, Professor, Department of Physics, Bharathidasan University, Tiruchirappalli - 620 024, Tamil Nadu, India. Phone: +91-431-2407057. Fax: +91-431-2407045. Email:
[email protected].
Scheme 1. Reaction of DMAP with Orthophosphoric Acid
inorganic subnetwork formed by the dihydrogen phosphate anions (H2PO4)n. When the organic species are strongly dipolar, the anion sublattice is organized in a polar structure; examples are L-arginine dihydrogen orthophosphate monohydrate (LAP), 2A5NPDP. Moreover, they offer the prospect of the tailorability of inorganic-organic molecular interaction. For these reasons, semiorganic hybrid structures have received reasonable attention recently.6,7
Figure 1. Solubility graph of DMAPDP in water.
10.1021/cg049950c CCC: $27.50 © 2004 American Chemical Society Published on Web 06/03/2004
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Crystal Growth & Design, Vol. 4, No. 4, 2004
Manivannan and Dhanuskodi
4-dimethylamino pyridine (DMAP) onto inorganic frameworks will lead to several hybrid compounds that can exhibit high nonlinear response. Following this, synthesis of a few salts of DMAP (dihydrogen phosphate, tartrate, chloride, and bromide) have been carried out. In this paper, a report on synthesis, crystal growth, structural, thermal, and optical properties of 4-dimethylamino pyridinium dihydrogen phosphate (DMAPDP) is presented as a potential material for third-order nonlinear optics for the first time. The three-dimensional crystal structure by single crystal X-ray diffraction has been solved; also the thermal and dielectric properties of DMAPDP have been studied and will be reported in future communications. 2. Experimental Procedures Figure 2. As-grown single crystal of DMAPDP.
Figure 3. Morphology of the as-grown DMAPDP crystal.
In this context, a new family of organic-inorganic crystals for nonlinear optics which can combine the high nonlinear response of organic chromophores enhanced with thermal, chemical, and mechanical properties through inorganic subnetworks is proposed. The anchorage of charge-transfer organic molecules such as
Figure 4.
1
H FT-NMR of DMAPDP.
2.1. Material Preparation. A polarizable organic molecule, DMAP, selected as a guest, can be anchored onto various inorganic or organic anionic matrixes. The formation of DMAPDP on chemical reaction with orthophosphoric acid is indicated with the molecular structures in Scheme 1. The hydrogen bond networks that will be formed in these new organic inorganic structures provide crystalline materials with improved thermal, chemical, and mechanical stabilities compared with those of pure organic molecular crystals built with the similar chromophore. The commercially available (LOBA Chemie, 99%) organic compound DMAP is a weak Bronsted base that gains a proton in strong acidic aqueous solutions at lower pH (∼ < 3), leading to the formation of salts with the conjugate bases of the involved acids. Thus, the DMAPDP salt is obtained by dissolving DMAP in a orthophosphoric acid solution at 65 °C in the molar ratio 1:1:20 for DMAP, H3PO4, and triple distilled water, respectively. On cooling, the salt crystallized at low temperature (
