A note on second-order nonlinear optical response of high-spin bis-TEMPO diradicals with possible application

In this note, we have theoretically evaluated second-order nonlinear optical (NLO) responses of two structural isomeric forms of three different green fluorescent protein chromophores (viz. cyan fluorescent protein (CFP), orange variant of Discosoma coral (DsRed) and Zoanthus sp. (zFP)) coupled bis-TEMPO based photomagnetic high-spin diradicals by measuring the second-order effect of first molecular hyperpolarizabilities. The magnetic character and certain biological applicabilities of these diradicals are investigated theoretically in one of our recent papers (Bhattacharya et al., 2013). The second-order total molecular hyperpolarizabilities of these molecules are correlated with their respective HOMO-LUMO energy gaps (δE_H-L) and also with the extent of the π-network of the couplers. Relations between δE_H-L, chemical hardness, NLO response and exchange energy (Bhattacharya et al., 2013) are made. The applicability of these diradicals in the light of NLO properties are also highlighted in this note. Here these resulting fluoroprotein coupled diradicals can switch back and forth between two isomeric states of different colors upon irradiation by light of appropriate wavelength. Nonetheless, the magnitude of second-order NLO responses is different for different isomeric forms; hence we made a correlation between the color codes with the magnitude of second-order NLO responses. Thence with just simple visual inspection one can understand the extent of second-order of NLO response for these diradicals.