Steady-State Measurements. Steady-state absorption and emission were measured at room temperature using a Carry Bio 300 spectrophotometer (Varian, Inc.) and a Cary Eclipse fluorescence spectrophotometer (Varian, Inc.), respectively. The molar extinction coefficients were determined from the slope of an absorbance versus concentration plot using a series of solutions of known concentration. Luminescence data for air-and N2-saturated aqueous solutions was collected every 0.5 nm with a 2 s averaging time, excitation and emission slit widths set to 5 nm, and the PMT voltage set to 800 V.