Chloridocobaltate(II) metal–organic cocrystal delivering intermolecular-charge transfer-enhanced passive optical limiting: A comprehensive study on structure–property relation

Abstract

The structure–property relation of a novel metal–organic cocrystal piperazine-1,4-diium tetrachloridocobaltate(II) monohydrate (abbreviated as PCo) for third-order nonlinear optical applications is reported. The acid hydrolysis during pH optimization yielded anionic $[\hbox {CoCl}_4]^{2-}$ and cationic $(\hbox {C}_{4}\hbox {H}_{12}\hbox {N}_{2})^{2+}$ which together formed a stable chemical structure. The bulk single crystals were grown by slow solvent evaporation method with optimized solution pH of 3.5. Its response to the single-crystal X-ray diffraction confirmed PCo belongs to the monoclinic ($P2_1/c$) crystal system. Investigated thermal, mechanical, linear absorption and emission properties show the suitability of PCo for optoelectronic device applications. The calculated molecular interaction energy at B3LYP/6-31G(p,d) level reveals the possibility of noncovalent intermolecular charge transfer via ${\hbox {N}- \hbox {H}\cdots \hbox {Cl}}$, ${\hbox {N}-\hbox {H}\cdots \hbox {O}}$ and ${\hbox {O}-\hbox {H}\cdots \hbox {Cl}}$ types of interactions. Rich availability of polarizable electronegative interactions between radicals upon laser stimuli enhanced nonlinear optical property in PCo cocrystal. The magnitude of third-order nonlinear optical susceptibility ($\chi ^{(3)}$), nonlinear refractive index ($n_2$) and nonlinear absorption coefficient ($\beta$) of PCo cocrystal under continuous-wave laser excitation were found to be $(9.32\pm 0.01)\times 10^{-6}$ esu, $(1.22\pm 0.006)\times 10^{-8}\,\mathrm{cm}^2\mathrm{W}^{-1}$ and $(1.43\pm 0.001)\times 10^{-4}\,\mathrm{cmW}^{-1}$, respectively. The excited state-assisted sequential two-photon absorption responsible for optical limiting is demonstrated by measuring the $\beta$ at different intensities of nanosecond pulsed laser excitations. Superior physicochemical properties with a low optical limiting threshold for both nanosecond pulsed and CW laser irradiance ($(1.44\pm 0.02)\times 10^{12} \hbox { Wm}^{-2}$ and $(0.365\pm 0.02)\times 10^{3} \hbox { Wcm}^{-2}$, respectively) promote the PCo cocrystal as a promising candidate for optical limiting application.

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