Crystal structure and physicochemical properties of a new optofunctional metal-organic cocrystal delivering intermolecular charge-transfer-enhanced nonlinear optical and optical limiting properties

Abstract

Crystal structure, physicochemical and intermolecular charge-transfer-enhanced third order nonlinear optical properties of a new metal-organic cocrystal $\{[\hbox {NiCl}_2(\hbox {C}_4\hbox {H}_{11}\hbox {N}_2)_2(\hbox {OH}_2)_2](\hbox {Cl})_2 (\hbox {OH}_2)_2\hbox {n}\}$ (PNi) is reported. An intermediate piperazinium chloride created during the pH optimization reacts with the metal centre (Ni), forming mono-substituted piperazine metal-organic cocrystal. The PNi adopts octahedral coordination geometry with triclinic ($P\overline{1}$) crystal system at its single crystalline phase. The crystal packing stabilization via $\hbox {O}-\hbox {H}\cdots \hbox {Cl}$, $\hbox {C}-\hbox {H}\cdots \hbox {Cl}$ and $\hbox {O}-\hbox {H}\cdots \hbox {O}$ noncovalent charge-transfer interactions is observed from single-crystal X-ray diffraction analysis. The PNi single crystal is thermally stable up to 87 $^\circ$C. The $\hbox {H}_2\hbox {O}$ molecules tend to evaporate at lower temperatures bringing moderate, rather sufficient thermal stability for using the title material in lasing applications. Vickers microhardness study revealed that the title crystal belongs to the soft material category. Investigation on dielectric properties showed polar dielectric nature of the material with consistent electronic polarizability ($\alpha =1.11\times 10^{22}\ \hbox {cm}^3$). The optical bandgap of 5.17 eV from UV-DRS spectroscopic analysis shows PNi belongs to the wide bandgap material category. The third order nonlinear optical absorption ($\beta$), nonlinear refractive index ($n_2$) and third order nonlinear optical susceptibility ($\chi ^{(3)}$) of PNi cocrystal using 532 nm CW DPSS laser are measured to be $(0.067\pm 0.001)\times 10^{-4}\ \hbox {cmW}^{-1}$, $(0.211\pm 0.0057)\times 10^{-8}\ \hbox {cm}^2\hbox {W}^{-1}$ and $(0.511\pm 0.09)\times 10^{-6}$ esu, respectively. Intermolecular charge-transfer interactions and electronic polarizability well influence nonlinear optical properties and are proved possible by calculating the intermolecular charge-transfer interactions at the B3LYP/6-31G(d,p) level of calculations. Further, the nonlinear absorption coefficient ($\beta$) was found to vary with the laser intensities, explicitly indicating the presence of excited state absorption assisted sequential 2PA in PNi cocrystal. Low onset limiting threshold for nanosecond pulsed and continuous-wave laser irradiance ($(1.66\pm 0.02)\times 10^{12}\ \hbox {Wm}^{-2}$ and $(0.804\pm 0.02)\times 10^{03}$ $\hbox {Wcm}^{-2}$, respectively) promotes the title material as an efficient candidate for optical limiting application.