The control of radiative and nonradiative decay is important in the design of bioimaging molecules. Dimethylaminobenzonitrile (DMABN) is a suitable model molecule to study radiative and nonradiative decay processes and has been investigated by theoretical and experimental methods. However, an atomistic understanding of the nonradiative decay in solutions remains to be achieved. In this study, we investigated the potential -energy surfaces in excited states along the rotation of the dimethylamino group and found that the degeneration between S-1 and T-1 states is one of the key factors in the nonradiative decay in polar solvents. In addition, we found that the degeneration is precisely controlled by a fundamental physical property, exchange integral. Although DMABN is a simple molecule, the understanding of the nonradiative decay process on the basis of physical properties should be useful in the design of more complicated imaging molecules.