Tricationic Porphyrin Conjugates: Evidence for Chain-Structure-Dependent Relaxation of Excited Singlet and Triplet States

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 on the behavior of initial excited states, photophysical parameters have been determined with conventional steady-state absorption and fluorescence 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 ∼0.03 in pH 7 buffer to ∼0.05 in Triton X100 micelles (TX100) and in ethanol and to 0.12 in sodium dodecyl sulfate (SDS) micelles. Corresponding 1S1 lifetimes are ∼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-α-d-galactopyranosyl or a α/β-d-galactopyranosyl group occurs in ethanol (k ∼0.15 ps−1) after 1S1 state solvation (∼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 (3T1) 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 ∼540, 630, and 690 nm. In ethanol, energy transfer from the P−H 3T1 state to β-carotene provides an estimate of ε ∼40 000 M−1 cm−1 at 460 nm for the P−H 3T1 state. Using triplet meso-tetraphenylporphyrin as an actinometer, the P−H triplet quantum yield (ΦT) is estimated to be ∼0.50 in all solvents. This high ΦT leads to effective singlet oxygen production in buffered solutions.