We studied the effect of acetonitrile-butanol-1 mixture composition on the photophysics (absorption, fluorescence emission spectra, the quantum yield as well as the relaxation of the excited state) of two BODIPY derivatives with an aryl and pyridine substituent in the meso position and methyl (ethyl) in α,β-position. The behavior of these photophysical properties (fluorescence quantum yields, fluorescence lifetimes, radiative and nonradiative constants) as a function of the mixture composition points to its correlation with the rotation of the phenyl group that is possible in BDP2 while it is hindered in BDP1 as a channel of the excited state energy relaxation. As a consequence, the photophysical properties of BDP1 are weakly dependent on the mixture composition while those of BPD2 undergo two stages of variation. In the low mole fraction of Butanol-1 (lower than 0.48), where the viscosity changes by 0.8 MPa.s, the BDP2 photophysical properties are slightly dependent on the mixture composition. We conjecture that the free rotation of the phenyl group allows both radiative and nonradiative excited state relaxation to occur. This is shown by a relatively high fluorescence quantum yield and nonradiative constant values that remain constant in this range of butanol-1 mol fraction. For its further increase, the photophysical properties become strongly dependent on the change of the mixture composition and this was associated with the gradual increase of the viscosity by a factor of 1.7 MPa.s that results in a hindrance of the rotation of the phenyl group. This makes 8-phenyl substituted BDP2 a sensitive molecular rotor for small changes in viscosity between 0.2 and 3 MPa.s.