The Z-scan instrument was used to study the nonlinear optical (NLO) properties of the derivatives of 3,3′-biindole 2,2′(1H,1′H)-dione, with different substitutions such as bromine, methyl, and phenyl groups. Specifically, the nonlinear absorption coefficient, β, nonlinear refraction index, n2 , have been extracted and reverse saturation absorption and selfdefocusing phenomena were investigated. It is observed that the presence of bromine in the compound structure can reduce the NLO properties. The reduction of dipole moment with bromine substitutions in the compound structure was confirmed with the quantum calculation. The NLO properties of the biindole derivatives related to the thermo-optic coefficient and dipole moment of the compound. Overall, our findings indicate that the biindole derivatives are promising candidates for NLO absorption materials.

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