Noncentrosymmetric Rb₃(COOH)₃(H₃BO₃)₂ vs Centrosymmetric Cs₃(COOH)₃(H₃BO₃)₂

Abstract

Two new alkali metal formic-borates named Rb3(COOH)3(H3BO3)2 and Cs3(COOH)3(H3BO3)2 have been successfully synthesized under mild conditions. Influenced by the different cation sizes, these two stoichiometrically equivalent compounds reveal discrepant symmetries: Rb3(COOH)3(H3BO3)2 is noncentrosymmetric (NCS) while Cs3(COOH)3(H3BO3)2 is centrosymmetric (CS). Especially, these two title compounds exhibit unique planar [(COOH)3(H3BO3)2]3– layers constructed from two kinds of planar π-conjugated asymmetric BO3 and COOH units, which induce the large birefringence, 0.09@1064 nm for Rb3(COOH)3(H3BO3)2 and 0.10@1064 nm for Cs3(COOH)3(H3BO3)2, respectively. Optical property measurements indicate that these two compounds display short UV absorption edges, suggesting that they are promising short-wave UV birefringent materials. A detailed theoretical analysis confirmed that the excellent linear optical property was derived from the synergistic effect of the two kinds of π-conjugated planar groups.

Two Carboxylate-Cyanurates with Strong Optical Anisotropy and Large Band Gaps

“…Moreover, the existence of the coordination water in compound II is confirmed by the existence of the band at 3575 cm –1 . The vibrations are essentially in good agreement with the results reported previously, demonstrating the presence of the cyanurate 6-MRs and carboxylate groups in these two compounds. ,,, The UV–vis–NIR diffuse reflectance spectra of I and II were measured with powder samples, and their experimental cutoff edges can extend to 208 and 218 nm, corresponding to the band gaps of 5.43 and 5.20 eV, respectively (Figure ). The UV cutoff edges of them are both shorter than those of A­(H 3 C 3 N 3 O 3 )­(NO 3 ) (A = K, Rb) (about 230 nm) and Ba 2 (C 3 N 3 O 3 )­(CNO) (240 nm), and their band gaps are close to those of most metal cyanurates as well as formic borates containing two types of π-conjugated groups, including K 2 (HC 3 N 3 O 3 ) (4.94 eV), Ba 2 M­(C 3 N 3 O 3 ) 2 (M = Mg, Ca) (5.20, 5.10 eV), Zn­(H 2 C 3 N 3 O 3 ) 2 ·3H 2 O (5.24 eV), K 2 M­(H 2 C 3 N 3 O 3 ) 4 ·4H 2 O (M = Zn, Cd) (5.23, 5.15 eV), A 3 (COOH) 3 (H 3 BO 3 ) 2 (A = Rb, Cs) (5.43, 5.38 eV), and (NH 4 ) 3 [B­(OH) 3 ] 2 (COOH) 3 (5.29 eV), but much larger than those of mono-carboxylate A 3 Pb 2 (CH 3 COO) 2 X 5 (A = Rb, Cs; X = Cl, Br, I) (2.55–3.64 eV). , This shows that the introduction of planar π-conjugated cyanurate groups can help increase the band gaps of carboxylate-containing compounds, obtaining a potential material for short-wave UV optics.…”

“…The vibrations are essentially in good agreement with the results reported previously, demonstrating the presence of the cyanurate 6-MRs and carboxylate groups in these two compounds. 22,33,45,46 The UV−vis−NIR diffuse reflectance spectra of I and II were measured with powder samples, and their experimental cutoff edges can extend to 208 and 218 nm, corresponding to the band gaps of 5.43 and 5.20 eV, respectively (Figure 3). The UV cutoff edges of them are both shorter than those of A(H 3 C 3 N 3 O 3 )(NO 3 ) (A = K, Rb) 29 (about 230 nm) and Ba 2 (C 3 N 3 O 3 )(CNO) (240 nm), 30 and their band gaps are close to those of most metal cyanurates as well as formic borates containing two types of π-conjugated groups, including K 2 (HC 3 N 3 O 3 ) (4.94 eV), 46 32,33 This shows that the introduction of planar π-conjugated cyanurate groups can help increase the band gaps of carboxylate-containing compounds, obtaining a potential material for short-wave UV optics.…”

“…As we all know that a desirable pathway to design novel functional materials and improve the optoelectronic properties of compounds is to combine two types of well-aligned planar π-conjugated groups into one compound, since the synergetic effect of these active groups is beneficial to the superposition of their microscopic linear and nonlinear properties. − To date, this strategy has afforded several compounds with excellent performance, for example, Pb 2 (BO 3 )­(NO 3 ) containing both π-conjugated nitrate and borate anions exhibits a significant strong second-harmonic generation (SHG) response of about 9.0 times that of potassium dihydrogen phosphate (KDP) and a large birefringence of 0.174@1064 nm. , Pb 7 O­(OH) 3 (CO 3 ) 3 (BO 3 ) with π-conjugated carbonate and borate anions shows a large SHG response of approximately 4.5 times that of KDP . Pb 6 O 4 (BO 3 )­(NO 3 ) containing π-conjugated nitrate and borate anions displays a large birefringence of 0.276@546 nm .…”

Halogen regulation triggers structural transformation from centrosymmetric to noncentrosymmetric switches in tin phosphate halides Sn2PO4X (X = F, Cl)

“…Subsequently, a new compound Sn 2 PO 4 F was synthesized, but Sn 2 PO 4 F showed non-NLO activity since its NLO-active units were arranged symmetrically. To the best of our knowledge, a large number of examples show that the substitution of homologous cations with different ionic radii can promote structural transformation from CS to NCS, [37][38][39] such as K 2 SO 4 •(SbF 3 ) 2 (CS) to Rb 2 SO 4 •(SbF 3 ) 2 40 (NCS) and Cs 3 (COOH) 3 (H 3 BO 3 ) 2 (CS) to Rb 3 (COOH) 3 (H 3 BO 3 ) 2 41 (NCS).…”