To design novel dyes with a controllable fluorescence on-off switching mechanism, understanding the dark state at the atomic level should be a key focus. In this study, we focused on the radiative and nonradiative mechanisms of 4'-N,N-diethylamino-3-hydroxyflavone (DEAHF) based on theoretical and experimental viewpoints. In the excited state, an excited-state intramolecular proton transfer (ESIPT) reaction of DEAHF occurs, and both the normal (N*) and tautomer (T*) forms exist in solution. To discuss the electronic structure changes through ESIPT, we mainly focused on two structural changes: the rotation of the diethylamino group and the bending motion of DEAHF in the excited state. The potential energy surfaces (PESs) passing through the rotation of the diethylamino group indicated that rotation may occur by thermal fluctuation during each phase. When the diethylamino group is rotated by 90 degrees in the N* form, the oscillator strength becomes zero, which may be critical in nonradiative decay pathways. For the bending motion, we found a conical intersection, which could be a key pathway of nonradiative decay. By employing molecular orbital analysis, we concluded that the electronic structure changes induced by ESIPT play a key role in determining the decay pathways. Additionally, we compared the fluorescence quantum yield in acetonitrile with that in cyclohexane and showed that the solvent polarity also affects the radiative and nonradiative mechanisms of DEAHF.