Photomagnetic and nonlinear optical properties in cis-trans green fluoroprotein chromophore coupled Bis-imino nitroxide diradicals

The current study extends an earlier investigation (Bhattacharya, et al., Phys. Chem. Chem. Phys. 2012, 14, 6905) to further explore various photomagnetic and optical properties of bis-imino nitroxide, that is, (IN)₂-based green fluorescent protein (GFP) chromophore coupled diradicals revealing new significant features. The conversion mechanisms of selected trans-isomers into their corresponding cis-conformers are discussed in detailed using a number of recently-developed density functional theory (DFT) functionals based on the Minnesota suite of DFT-models as well as using some other DFT functionals developed earlier. To provide a more in-depth analysis of variations in magnetic properties as trans-conformers (singlet ground-state) convert into their cis-analogues (triplet ground-state), the changes in exchange magnetic coupling constants J are compared with the variation of the selected aromaticity indices. The aromaticity indices include the nuclear independent chemical shift [NICS(0)] values calculated at the center of ring structures and the harmonic oscillator model of aromaticity. Furthermore, the investigation of static nonlinear optical response properties in the (IN)₂-based GFP chromophore coupled diradicals reveal unusually large static first hyperpolarizabilities for these systems which is highly significant for practical applications in optics and optoelectronics.