Two boron complexes of 5-phenyldipyrromethenes bearing isothiocyanate groups on the phenyl ring have been synthesized for the first time. The utility of these new fluorescence probes for labeling biologically relevant proteins is demonstrated on two monoclonal antibodies that bind to antigens overexpressed on cancer cells. Spectral comparison of the two structures reveals significant photophysical differences, including bathochromically shifted excitation and emission bands, increased molar absorptivity and a large increase in fluorescence quantum yield of approximately 10 times. Differences in photophysical parameters are linked to hindered rotation of the phenyl ring in one of the probes.