New group of inorganic salts of 1,3-diaminoguanidinium(1+) cation – crystal structures, vibrational spectra, linear and nonlinear optical properties

Abstract

In the search for new molecular crystals for nonlinear optics (NLO), nine inorganic salts containing the 1,3-diaminoguanidinium(1+) cation as a promising organic “Y-aromatic” carrier of NLO properties were prepared and studied. Crystal structures of eight new salts, namely 1,3-diaminoguanidinium(1+) chloride (P21/c), hypophosphite (P21/c), hydrogen phosphite (P-1), dihydrogen phosphate (P21/c), hydrogen phosphate (P21/m), nitrite (P-1), sulfite (Pbcm) and chlorate (P21/c) were solved by the single crystal X-ray diffraction. The crystal structures contain centrosymmetric arrangements of pairs of counterions assembled into hydrogen-bonded undulated sheets. The charge-assisted hydrogen bonds in the resulting hydrogen bond networks are accommodated by the conformation of the 1,3-diaminoguanidinium(1+) cation as a hydrogen bond donor. All salts were also characterized by powder X-ray diffraction. The vibrational spectra (FTIR and Raman) were studied for all newly prepared salts and the previously published non-centrosymmetric sulfate (P6122). The assignment of the vibrational bands of these hydrogen-bonded crystals is derived from quantum-chemical computations (B3LYP/6-311G+(d,p) along with the Potential Energy Distribution analysis) concerning isolated 1,3-diaminoguanidinium(1+) cation and the results of correlation analysis involving present inorganic anions. The linear and nonlinear optical properties of the only non-centrosymmetric salt – 1,3-diaminoguanidinium sulfate – were studied. This uniaxial positive crystal is optically transparent down to 230 nm, and refractive indices (ne = 1.65 and no = 1.56) have been determined at 532 nm. The dispersion curves of the refractive indices were also calculated by the Q(LFT) procedure in the 532–1907 nm region. The experimentally determined (using 800 and 1000 nm laser irradiation) very low efficiency of second harmonic generation (i.e., ∼1% of KDP), which is consistent with the Kleinman symmetry assumption for the P6122 space group, limits further use of this crystal for second-order NLO applications.