A series of hemicyanine derivatives are excellent fluorescent viscosity sensors in live cells and in imaging of living tissues due to their low quantum yields in solution but large fluorescence enhancements in viscous environments. Herein, three carbazole-based hemicyanine dyes with different heterocycles are studied. They have different background quantum yields, and hence different sensitivities to viscosity detection, large Stokes shifts, and high sensitivity. Better understanding of the structure–property relationships for viscosity sensitivity could benefit the design of improved dyes. Computational studies on these dyes reveal the mechanism of viscosity sensitivity of fluorescent molecular rotors and the nature of the difference in viscosity sensitivity of the three dyes. The results show that the greatly raised HOMO and greatly lowered LUMO in the S1 state compared with the S0 state are responsible for the large Stokes shift of the three dyes. The heterocyclic moieties have the primary influence on the LUMO levels of the three hemicyanine dyes. Rotation about the CC bond adjacent to the carbazole moiety of the three dyes drives the molecule toward a small energy gap between the ground state and the first excited state, which causes mainly nonradiative deactivation. The oscillator strengths in the lowest singlet excited state drop rapidly with increasing rotation between 0 and 95°, which leads to a dark state for these dyes when fully twisted at 95°. We draw a mechanistic picture at the molecular level to illustrate how these dyes work as viscosity-sensitive fluorescent probes. The activation barriers and energy gaps of CC bond rotation strongly depend on the choice of heterocycle, which plays a major role in reducing fluorescence quantum yield in the free state and provides high sensitivity to viscosity detection in viscous environments for the carbazole-based hemicyanine dyes.