Conjugates of 5-(4-carboxyphenyl)-10,15,20-tris(4-methylpyridinium-4-yl)porphyrin (P-H) are promising photoactive agents for medical applications. As their ultimate efficacy will depend oil the behavior of initial excited states, photophysical parameters have been determined with conventional steady-state absorption and florescence as well as time-resolved femto-and nanosecond spectroscopies. The fluorescence quantum yield of P-H and P-H conjugated to uncharged groups increases from similar to 0.03 in pH 7 buffer to similar to 0.05 in Triton X100 micelles (TX100) and in ethanol and to 0.12 in sodium dodecyl sulfate (SDS) micelles, Corresponding S-1(1) lifetimes are similar to 5-10 ns. In buffer, an equilibrium between P-H monomers and small-size aggregates is observed. Conjugation with poly-S-lysine (P-(Lys)(n)) results in fluorescence quenching in all solvents. Structural reorganization of conjugates bearing a Di-O-isopropylidene-alpha-D-galactopyranosyl or a alpha/beta-D-galactopyrallosyl group Occurs in ethanol (k similar to 0.15 ps(-1)) after S-1(1) state solvation (similar to 700 fs). Relaxation of bulky P-(Lys)(n) polypeptide chains takes place on a longer time scale in all solvents (k <= 0.01 ps(-1)) with enhanced internal conversion. Triplet, State (T-3(1)) transient spectra of all derivatives in PBS, SDS, TX100, and ethanol exhibit a strong absorbance with a broad maximum in the 460-475 nm region and minor maxima at similar to 540, 630, and 690 nm. In ethanol, energy transfer from the P-H T-3(1) state to beta-carotene provides an estimate of epsilon similar to 40 000 M-1 cm(-1) at 460 nm for the P-H T-3(1) state. Using triplet meso-tetraphenylporphyrin as an actinometer, the P-H triplet quantum yield (Phi(T)) is estimated to be similar to 0.50 in all solvents. This high Phi(T) leads to effective singlet oxygen production in buffered solutions.