Effects of substituents on the photophysical properties of symmetrical porphyrins

Porphyrin compounds having groups that mimic the phenolic moiety of m- and p-isomers of 5,10,15,20-tetrakis(hydroxyphenyl) porphyrin (THPP) have been synthesized along with 5,10,15,20-tetrakis(heteroaryl) porphyrins bearing 2-thienyl and 5-thiazolyl groups. Absorption and fluorescence spectroscopy, including fluorescence lifetime (τf) and quantum yield (Φf) measurements, were employed to characterize the singlet excited state of all compounds, using 5,10,15,20-tetraphenylporphyrin (H2TPP) as a standard (Φf = 0.12 in DMF). The generation of singlet oxygen by each porphyrin photosensitizer was measured as the singlet oxygen quantum yield (ΦΔ), using H2TPP as a standard (ΦΔ = 0.64 in DMF). Partition coefficients were determined using 2-octanol as the organic phase and PBS solution as the aqueous phase. Fluorescence quantum yields ranged from 0.01 to 0.18 for all compounds, with heteroaryl porphyrins having the lowest values. Singlet oxygen quantum yields ranged from 0.40 to 0.65, with heteroaryl porphyrins having the highest values, showing them to be better sensitizers than m- and p-THPP. Log P values were all >1 showing higher solubility in the 2-octanol layer.

► Ten symmetrical porphyrins were synthesized and characterized.
► Fluorescence quantum yields (Φf) of phenyl-substituted porphyrins were higher than Φf of heteroaryl-substituted porphyrins.
► On average, heteroary-substituted porphyrins generated higher singlet oxygen yields.
►  Log P values of all 10 porphyrins were >1 showing higher solubility in 2-octanol layer than PBS.