β-Pyrrole Functionalized Push or Pull Porphyrins: Excited Charge Transfer Promoted Singlet Oxygen Generation

Abstract

Singlet oxygen ( 1 O 2 ) producing photosensitizers are highly sought for developing new photodynamic therapy agents and facilitating 1 O 2involved chemical reactions. Often singlet oxygen is produced by the reaction of triplet-excited photosensitizers with dioxygen via an energy transfer mechanism. In the present study, we demonstrate a charge transfer mechanism to produce singlet oxygen involving push or pull functionalized porphyrins. For this, 20 β-pyrrole functionalized porphyrins carrying either an electron-rich push or electron-deficient pull group have been newly synthesized. Photoexcitation of these push−pull porphyrins has been shown to produce high-energy MP δ+ -A δ− or MP δd − -D δ+ charge transfer states. Subsequent charge recombination results in populating the triplet excited states of extended lifetimes in the case of the push group containing porphyrins that eventually react with dioxygen to produce the reactive singlet oxygen of relatively higher quantum yields. The effect of the push and pull groups on the porphyrin periphery in governing initial charge transfer, the population of triplet excited states and their lifetimes, and resulting in improved singlet oxygen quantum yields are systematically probed. The improved performance of 1 O 2 generation by porphyrins carrying push groups is borne out from this study.

Porphyrin(2.1.2.1) organopalladium complexes as efficient singlet oxygen sensitizers

“…11 The meso -electron withdrawing substituents, such as pentafluorophenyl, induced red-shifts in the main absorption bands from 499 to 512 nm for Pd1–Pd4 , rather than the induced blue-shifted absorptions of regular porphyrin(1.1.1.1) Pd( ii ) complexes. 2 The optical absorptions of Pd1–Pd4 were similar to that of dipyrrin and dialkynylanthracene-bridged palladium complexes. 9 a , d This optical absorption character of Pd1–Pd4 was caused by the non-aromatic property, as opposed to the aromatic property of porphyrin(1.1.1.1) Pd( ii ) complexes.…”

“…This is highly relative to the planar or approximate planar geometry of porphyrin(1.1.1.1) and dipyrrin with dialkynylanthracene-bridged palladium complexes, as well as the X-shaped geometry of dipyrrin palladium complexes. 2,8,9…”

“…9 a , d This optical absorption character of Pd1–Pd4 was caused by the non-aromatic property, as opposed to the aromatic property of porphyrin(1.1.1.1) Pd( ii ) complexes. 2,10…”

“…10 c The measured HOMO–LUMO gaps of Pd1–Pd4 were 2.16, 2.18, 2.09, and 2.10 V, respectively; these measured HOMO–LUMO gaps were smaller than those of Pd( ii ) meso -tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin and meso -tetraphenyl porphyrin complexes. 2…”

“…Ideal photodynamic therapy (PDT) sensitizers employ a significant formation of singlet-state molecular oxygen ( 1 O 2 ). 1 Pyrrolic molecules and their organopalladium complexes are an important series of photosensitizers, and to date, molecules such as porphyrins (Chart 1a), 2 corroles, 3 phthalocyanines, 2 c ,4 porphycenes, 5 and N-confused calix[6]phyrin 6 have been well studied as chromophores for 1 O 2 generation due to their involvement of light absorption. Among these, porphyriniod complexes as potent photosensitizers have been clinically implemented in PDT for cancer treatment.…”