The effect of alkoxyl groups on the photoproperties of meta-octasubstituted tetraphenyl porphyrins

Abstract

• . Installing substituent groups with different lengths/sizes and conformations at the meta-octa-substituted position of meso-tetraphenyl porphyrin. • . H 2 T(OHE) 2 PP with longer and branched alkoxyl chains effectively suppressed aggregation-caused quenching effect. • . H 2 T(OH) 2 PP in solid state shows the naked-eye visible fluorescence, making its practical applications possible. The emission of most organic fluorescent molecules in aggregation states decays or fades, as interpreted by the aggregation-caused quenching (ACQ) effect, because the excited state energy is depleted via nonradiative channels when chromophores stack together. However, most fluorescent materials are utilized in the solid state, and hence it is vital to tune the chromophore stacking mode for modulating the photoluminescence properties. In this work, the meso-tetraphenyl porphyrin was taken as the parent chromophore unit to investigate the substituent-group effect on the luminescence properties of porphyrin-based complexes. The selected substituent groups (-OH, -OMe and 2-ethylhexyloxy) with different lengths/sizes and conformations were installed at the meta-octa-substituted position of meso-tetraphenyl porphyrin. The distance of porphyrin chromophores was regulated by the substituent chain. It was found that in the solid or aggregate state, porphyrin with longer and branched alkoxyl chains effectively suppressed the π-π stacking interaction, maintained the photoluminescence property of the porphyrin chromophore as in its dilute solution and overcame the ACQ effect. In contrast, porphyrin with a shorter alkoxyl chain, for example, the methoxyl group, affords a weak emission property in the solid state, and the substituent hydroxyl group leads to the complete quenching of the emission in the solid-state due to the close stacking of porphyrin rings.