Steady state and time-resolved luminescence experiments and calorimetric studies, as well as time-dependent density functional theory calculations performed on [ClRe(CO)(3)(Bathocuproinedisulfonate)](2), show that the photophysical properties of the Re(I) anionic complex are determined by the balance between intraligand (1IL) and metalligand-to-ligand charge transfer ((MLLCT)-M-3) excited states. In organic solvents, (MLLCT)-M-3 states prevail and the usual expected behavior is observed: bathochromic shift of the emission maximum, a reduced luminescence quantum yield and the shortening of the excited-state lifetime upon increasing the polarity of the solvent. In addition, singlet oxygen (O-1(2)) is generated with high quantum yields (Phi(Delta) approximate to 0.5 in CH3CN) due to the quenching of the (MLLCT)-M-3 luminescence by O-3(2). The total quenching rate constant of triplet state by oxygen, kq, reach values between 2.2 and 2.4 x 10(9) M-1 s(1) for the organic solvents studied. In CH3CN, the fraction of triplet states quenched by O2 which yield O-1(2), fO(2)(T), is nearly unity. In aqueous solution, where no singlet oxygen is generated, the luminescence of the Re(I) complex is of (IL)-I-1 character with a emission quantum yield (Phi(em)) strongly pH dependent: Phi(em,(pH=2))/Phi(em,(pH=10)) approximate to 5.6. The variation of the pH of the solution tunes the photophysical properties of the Re(I) complex by changing the relative amount of the different species existing in aqueous solutions: [ClRe(CO)(3)(BCS)](2), [(OH)Re(CO)(3)(BCS)](2-) and [(H2O)Re(CO)(3)(BCS)](-). TD-DFT calculations show that the percentage of charge transfer character of the electronic transitions is substantially higher in the organic solvents than in aqueous solutions, in agreement with the increase of 1IL character of HOMO in [(H2O)Re(CO)(3)(BCS)](-) relative to [ClRe(CO)(3)(BCS)](2-).