The photophysical properties of 2-phenylbenzothiazole and 2-phenylbenzoxazole were compared. At room temperature 2-phenylbenzothiazole in n-heptane exhibits a broad, structureless absorption and a low fluorescence yield of ≈0.005 (τf<200 ps). In contrast, 2-phenylbenzoxazole shows a structured absorption and a high fluorescence yield, Φf≈0.78 ± 0.05 (τf≈1.5 ns). These spectral differences arise from a drastic change in the dynamics of C1C1′ torsional motion, as evidenced in the temperature-dependent studies properties of their derivatives and AM1 calculations.