Eur. Phys. J. D (2021) 75: 214
https://doi.org/10.1140/epjd/s10053-021-00227-z

Regular Article – Optical Phenomena and Photonics

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

Division of Physics, School of Advanced Sciences, Vellore Institute of Technology, 600 127, Chennai, India

^b vinitha.g@vit.ac.in

Received: 19 May 2021
Accepted: 10 July 2021
Published online: 26 July 2021

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 ${[{\text{CoCl}}_4]}^{2-}$ and cationic ${({\text{C}}_4{\text{H}}_{12}{\text{N}}_2)}^{2+}$ 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 ($P{2}_1/c$) 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$\text{N}-\text{H}\cdots \text{Cl}$, $\text{N}-\text{H}\cdots \text{O}$ and $\text{O}-\text{H}\cdots \text{Cl}$ 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 ($(1.44\pm 0.02)\times {10}^{12}{\text{Wm}}^{-2}$ and $(0.365\pm 0.02)\times {10}^3{\text{Wcm}}^{-2}$, respectively) promote the PCo cocrystal as a promising candidate for optical limiting application.