Executive Summary : | Spin crossover properties of the transition metal complexes make them suitable candidates for molecular sensors, optical switches and other applications in data storage devices. Spin crossover processes induced via external perturbations, such as, temperature, pressure, light, or magnetic field, have been widely studied both experimentally and computationally. Most of these studies focus on d4-d7 transition metal systems. Ni(II), a d8 system, is an unlikely candidate for spin crossover. However, Ni(II) complexes can show spin crossover when an alternative way of inducing spin-crossover is considered, i.e., when its coordination number is altered, also known as coordination induced spin state switch (CISSS). A four coordinated square planar Ni(II) complex prefers a low-spin state, while an octahedral Ni(II) prefers a high-spin configuration, whereas a square pyramidal Ni(II) can alternate between low and high-spin states based on the ligand field. In recent times there have been a few reports of CISSS with Ni(II) system using phenazine based ligands. Computationally, CISSS is a challenging process to model. The presence of unpaired electrons and ligand fields, makes the electronic structure of the problem quite complex. The involvement of the excited states of different coordination environment adds further complexity to their photophysics. The current project proposes a computational modelling of CISSS in Ni(II) complexes by employing multi-configurational electronic structure calculations and quantum wave packet dynamics. The electronic structure calculations will characterize the ground and excited electronic states of the square planar, square pyramidal, and octahedral complexes. They will also characterize the internal degrees of freedom that describe the association/dissociation of the axial ligands. A diabatic vibronic Hamiltonian within the well-known linear vibronic coupling (LVC) model, taking into account the electronic states and nuclear degrees of freedom, will be constructed. The coupling parameters will be obtained from the lower-order fitting of the adiabatic electronic states. Quantum wave packet dynamics on these coupled electronic states will be employed to unravel the excited state photophysics underlying the CISSS process in Ni(II) systems. |