Investigation into the Oxygen-Involved Electrochemiluminescence of Porphyrins and Its Regulation by Peripheral Substituents/Central Metals.

We provide evidence of oxygen-involved electrochemiluminescence (ECL) of metal-free porphyrins and metalloporphyrins first. O and OH, which are oxygen intermediates, are indispensable for the formation of excited porphyrins, which has been proven by trapping free radical strategies. The wide differences regarding emission location and mechanism between metal-free porphyrins [including meso-tetra(4-methoxyphenyl)porphine (HTMPP), meso-tetraphenylporphyrin (HTPP), and meso-tetra(4-carboxyphenyl)porphine (HTCPP)] and metalloporphyrins (MTPP) depend on whether protons are present in the center of the porphin ring. Besides, the oxygen-involved ECL of porphyrins can be controlled regularly by peripheral substituents with different polarities. Because of the stretched molecular structure and the decrease in electron density around the protons located at porphin ring, electron-withdrawing groups are more conducive to protons being attacked by O, as well as the enhancement of porphyrins ECL. The ECL efficiency [Φ, which is normalized with respect to Ru(bpy)(PF) (taking Φ of Ru(bpy)(PF) = 1)] is gradually improved from HTMPP (Φ = 0.16), to HTPP (Φ = 2.20), to HTCPP (Φ = 3.83); the Φ = 4.21 of Zn(II)TPP is significantly higher than those of other MTPPs [e.g., Co(II)TPP and Cu(II)TPP]. A deeper understanding regarding the improvement of porphyrins ECL efficiency and new application toward porphyrins-related devices can be achieved from this work.