Efficient enhancement of second order nonlinear optical response by complexing metal cations in conjugated 7-substituted coumarin

Most of the coumarins have been found useful as non-linear optical chromophores. The four novel water-soluble coumarin-based compounds (OC6, NC6, OC7, and NC7) and the metallic compounds of NC7 with different metal cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe²⁺, and Zn²⁺) have been investigated by carrying out density functional theory (DFT). Our DFT calculations revealed that the second-order nonlinear optical properties have a pronounced enhancement by means of the introduction of π-conjugatd electron donor (dimethylamino phenyl alkynyl) in 7-position of the coumarin ring and metal cations, especially for transition metals. The further investigations of the larger first hyperpolarizability (β_tot) reveal that the NC7*Fe²⁺ and NC7*Zn²⁺ present the larger values as 1.151 × 10^?27 and 1.083 × 10^?27 esu owing to the lower transition energies and larger oscillator strengths of crucial electronic transitions. Moreover, time-dependent DFT results show that the large intramolecular charge transfers exist in the NC7*Fe²⁺ and NC7*Zn²⁺. In addition, the natural bond orbital analysis demonstrated that the second-order stabilization energies is from the lone pair (LP) orbital on O atom to the LP* orbital of metal cations interaction correlate with the O-Mⁿ⁺ atomics distance. On the other hand, the atoms in molecules analysis showed that the O-Mⁿ⁺ interactions can be characterized by the presence of a bond critical point (BCP) and the O-Fe²⁺ and O-Zn²⁺ interactions have partially ionic and partially covalent bonds rather than an electrostatic character for O-Mⁿ⁺ (Na⁺, K⁺, Mg²⁺ and Ca²⁺). In addition, the delocalization indices of O-Mⁿ⁺ bonds correlate reasonably well with electron density, kinetic and potential energy densities in these complexes. Thus, we hope this research will introduce a new relation between the structure and the property of chromophore nonlinear optical activity.