Photophysical comparisons of PEGylated porphyrins, chlorins and bacteriochlorins in water

Abstract

Comparison of the photophysics of analogous members of the porphyrin, chlorin, and bacteriochlorin family has rarely been carried out, and most studies of individual members have been performed in non-aqueous media. Advances in synthesis and molecular design now enable such comparative studies in aqueous solution. A water-soluble bioconjugatable free base porphyrin (FbP1) has been synthesized, complementing recently prepared chlorin and bacteriochlorin analogues. The porphyrin (a trans-AB architecture; i.e., 5,15-substitution pattern) is equipped with one meso-aryl group that bears three short polyethylene glycol (PEG) substituents at the 2,4,6-positions for aqueous solubility, and one meso-aryl group that bears an oxyacetic acid group at the p-position for bioconjugation. Spectroscopic examination of porphyrin FbP1 gave the following quantum yields in DMF versus water: fluorescence (Φ_f = 0.077 vs. 0.066), intersystem crossing (Φ_isc = 0.90 vs. 0.84), and internal conversion (Φ_ic = 0.023 vs. 0.094). The increase in Φ_ic upon moving from DMF into water reflects the 4-fold increase in rate constant for internal conversion, from (600 ns)⁻¹ to (140 ns)⁻¹. Photophysical comparisons (τ_S, k_f, k_ic, k_isc, Φ_f, Φ_ic, Φ_isc) were made with analogous chlorins and bacteriochlorins in DMF and water. The value of k_ic in both media increases in the order (visible-absorbing) porphyrin < (red-absorbing) chlorin < (near-infrared-absorbing) bacteriochlorin, consistent with the general S₁-energy dependence of k_ic expected on the basis of the energy-gap law for non-radiative decay. In summary, there are now satisfactory molecular designs, viable synthetic routes, and thoroughly characterized photophysical properties that permit aqueous solution studies of a complement of porphyrins, chlorins, and bacteriochlorins.