Introduction to the Symposium?in?Print: Photodynamic Therapy

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

Singlet oxygen (1O2) producing photosensitizers are highly sought for developing new photodynamic therapy agents and facilitating 1O2-involved 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 MPδ+-Aδ- or MPδ--Dδ+ 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 1O2 generation by porphyrins carrying push groups is borne out from this study.

Singlet Oxygen Generation in Peripherally Modified Platinum and Palladium Porphyrins: Effect of Triplet Excited State Lifetimes and meso-Substituents on 1 O2 Quantum Yields

A series of meso-substituted with aromatic (=tolyl, pyrenyl, fluorenyl, naphthyl, and triphenylamine) substituents, platinum (Pt), and palladium (Pd) porphyrins have been synthesized and characterized by spectroscopic and single-crystal X-ray diffraction studies to probe structure-reactivity aspects on the electrochemical redox potentials, and phosphorescence quantum yields and lifetimes. In the X-ray structures, the aromatic meso-substituents were rotated to some extent from the planarity of the porphyrin ring to minimize steric hindrance. Both Pt and Pd porphyrins revealed higher electrochemical redox gaps as compared to their free-base porphyrin analogs as a result of the harder oxidation and reduction processes. The ability of both Pt and Pd porphyrins to generate singlet oxygen was probed by monitoring the photoluminescence of 1 O2 at 1270 nm. Higher quantum yields for both triplet sensitizers compared to their free-base analogs were witnessed. Singlet oxygen quantum yields close to unity were possible to achieve in the case of Pt and Pd porphyrins bearing triphenylamine substituents at the meso-position. The present study brings out the importance of different meso-substituents on the triplet porphyrin sensitizers in governing singlet oxygen quantum yields; a key property of photosensitizers needed for photodynamic therapy, chemical synthesis, and other pertinent applications.

Superhydrophobic Photosensitizers: Airborne 1O2 Killing of an in Vitro Oral Biofilm at the Plastron Interface

Singlet oxygen is a potent agent for the selective killing of a wide range of harmful cells; however, current delivery methods pose significant obstacles to its widespread use as a treatment agent. Limitations include the need for photosensitizer proximity to tissue because of the short (3.5 μs) lifetime of singlet oxygen in contact with water; the strong optical absorption of the photosensitizer, which limits the penetration depth; and hypoxic environments that restrict the concentration of available oxygen. In this article, we describe a novel superhydrophobic singlet oxygen delivery device for the selective inactivation of bacterial biofilms. The device addresses the current limitations by: immobilizing photosensitizer molecules onto inert silica particles; embedding the photosensitizer-containing particles into the plastron (i.e. the fluid-free space within a superhydrophobic surface between the solid substrate and fluid layer); distributing the particles along an optically transparent substrate such that they can be uniformly illuminated; enabling the penetration of oxygen via the contiguous vapor space defined by the plastron; and stabilizing the superhydrophobic state while avoiding the direct contact of the sensitizer to biomaterials. In this way, singlet oxygen generated on the sensitizer-containing particles can diffuse across the plastron and kill bacteria even deep within the hypoxic periodontal pockets. For the first time, we demonstrate complete biofilm inactivation (>5 log killing) of Porphyromonas gingivalis, a bacterium implicated in periodontal disease using the superhydrophobic singlet oxygen delivery device. The biofilms were cultured on hydroxyapatite disks and exposed to active and control surfaces to assess the killing efficiency as monitored by colony counting and confocal microscopy. Two sensitizer particle types, a silicon phthalocyanine sol-gel and a chlorin e6 derivative covalently bound to fluorinated silica, were evaluated; the biofilm killing efficiency was found to correlate with the amount of singlet oxygen detected in separate trapping studies. Finally, we discuss the applications of such devices in the treatment of periodontitis.

Fluorinated Photodynamic Therapy Device Tips and their Resistance to Fouling for In Vivo Sensitizer Release

We describe progress on a one-step photodynamic therapy (PDT) technique that is simple: device tip delivery of sensitizer, oxygen and light simultaneously. Control is essential for their delivery to target sites to generate singlet oxygen. One potential problem is the silica device tip may suffer from biomaterial fouling and the pace of sensitizer photorelease is slowed. Here, we have used biomaterial (e.g. proteins, cells, etc.) from SQ20B head and neck tumors and whole blood for an assessment of fouling of the silica tips by adsorption. It was shown that by exchanging the native silica tip for a fluorinated tip, a better nonstick property led to an increased sensitizer output by ~10%. The fluorinated tip gave a sigmoidal photorelease where singlet oxygen is stabilized to physical quenching, whereas the native silica tip with unprotected SiO-H groups gave a slower (pseudolinear) photorelease. A further benefit from fluorinated silica is that 15% less biomaterial adheres to its surface compared to native silica based on a bicinchoninic acid assay (BCA) and X-ray photoelectron spectroscopy (XPS) measurements. We discuss how the fluorination of the device tip increases biofouling resistance and can contribute to a new pointsource PDT tool.

Singlet oxygen generation by cyclometalated complexes and applications

While cyclometalated complexes have been extensively studied for optoelectronic applications, these compounds also represent a relatively new class of photosensitizers for the production of singlet oxygen. Thus far, singlet oxygen generation from cyclometalated Ir and Pt complexes has been studied in detail. In this review, photophysical data for singlet oxygen generation from these complexes are presented, and the mechanism of (1) O2 generation is discussed, including evidence for singlet oxygen generation via an electron-transfer mechanism for some of cyclometalated Ir complexes. The period from the first report of singlet oxygen generation by a cyclometalated Ir complex in 2002 through August 2013 is covered in this review. This new class of singlet oxygen photosensitizers may prove to be rather versatile due to the ease of substitution of ancillary ligands without loss of activity. Several cyclometalated complexes have been tethered to zeolites, polystyrene, or quantum dots. Applications for photooxygenation of organic molecules, including "traditional" singlet oxygen reactions (ene reaction, [4 + 2] and [2 + 2] cycloadditions) as well as oxidative coupling of amines are presented. Potential biomedical applications are also reviewed.

Fluorine end-capped optical fibers for photosensitizer release and singlet oxygen production

The usefulness of a fiber optic technique for generating singlet oxygen and releasing the pheophorbide photosensitizer has been increased by the fluorination of the porous Vycor glass tip. Singlet oxygen emerges through the fiber tip with 669-nm light and oxygen, releasing the sensitizer molecules upon a [2 + 2] addition of singlet oxygen with the ethene spacer and scission of a dioxetane intermediate. Switching from a nonfluorinated to a fluorinated glass tip led to a clear reduction of the adsorbtive affinity of the departing sensitizer with improved release into homogeneous toluene solution and bovine tissue, but no difference was found in water since the sensitizer was insoluble. High surface coverage of the nonafluorohexylsilane enhanced the cleavage efficiency by 15% at the ethene site. The fluorosilane groups also caused crowding and seemed to reduce access of (1)O(2) to the ethene site, which attenuated the total quenching rate constant k(T), although there was less wasted (1)O(2) (from surface physical quenching) at the fluorosilane-coated than the native SiOH silica. The observations support a quenching mechanism that the replacement of the SiOH groups for the fluorosilane C-H and C-F groups enhanced the (1)O(2) lifetime at the fiber tip interface due to less efficient electronic-to-vibronic energy transfer.

A hand-held fiber-optic implement for the site-specific delivery of photosensitizer and singlet oxygen

We have constructed a fiber optic device that internally flows triplet oxygen and externally produces singlet oxygen, causing a reaction at the (Z)-1,2-dialkoxyethene spacer group, freeing a pheophorbide sensitizer upon the fragmentation of a reactive dioxetane intermediate. The device can be operated and sensitizer photorelease observed using absorption and fluorescence spectroscopy. We demonstrate the preference of sensitizer photorelease when the probe tip is in contact with octanol or lipophilic media. A first-order photocleavage rate constant of 1.13 h(-1) was measured in octanol where dye desorption was not accompanied by readsorption. When the probe tip contacts aqueous solution, the photorelease was inefficient because most of the dye adsorbed on the probe tip, even after the covalent ethene spacer bonds have been broken. The observed stability of the free sensitizer in lipophilic media is reasonable even though it is a pyropheophorbide-a derivative that carries a p-formylbenzylic alcohol substituent at the carboxylic acid group. In octanol or lipid systems, we found that the dye was not susceptible to hydrolysis to pyropheophorbide-a, otherwise a pH effect was observed in a binary methanol-water system (9:1) at pH below 2 or above 8.

Photosensitizer drug delivery via an optical fiber

An optical fiber has been developed with a maneuverable mini-probe tip that sparges O(2) gas and photodetaches pheophorbide (sensitizer) molecules. Singlet oxygen is produced at the probe tip surface which reacts with an alkene spacer group releasing sensitizer upon fragmentation of a dioxetane intermediate. Optimal sensitizer photorelease occurred when the probe tip was loaded with 60 nmol sensitizer, where crowding of the pheophorbide molecules and self-quenching were kept to a minimum. The fiber optic tip delivered pheophorbide molecules and singlet oxygen to discrete locations. The 60 nmol sensitizer was delivered into petrolatum; however, sensitizer release was less efficient in toluene-d(8) (3.6 nmol) where most had remained adsorbed on the probe tip, even after the covalent alkene spacer bond had been broken. The results open the door to a new area of fiber optic-guided sensitizer delivery for the potential photodynamic therapy of hypoxic structures requiring cytotoxic control.