The photophysics of rhodamine B

Abstract The effects of solvent viscosity and dielectric constant on the photophysics of rhodamine B in its protonated and zwitterion forms were studied. The internal conversion rate is dependent on the S 1 -S 0 energy separation and increases when this gap is lowered through rotation of the diethylamino groups. Comparison is made with rhodamine 640 in which torsional motion about the C—N bond is not possible.