Journal of Porphyrins and Phthalocyanines

Porphyrins may serve as the active charge-storage medium in memory chips. A series of 14 porphyrins (trans-A₂B₂, trans-AB₂C, or A₄ type; free base, copper or zinc chelate) has been synthesized for investigation of metal deposition on porphyrin monolayers. Each trans-AB₂C porphyrin is equipped with a surface attachment group and a distal functional group (or flanking functional groups). The surface attachment groups include S-acetylthiomethyl, hydroxymethyl, TMS-ethynyl, allyl, and triallylmethyl; the functional groups include amino, cyano, dipyrrin-5-yl, formyl, and nitro; all of which are attached to the p-position of a porphyrin meso-phenyl group. Two non-redox-active triallylmethyl-substituted arenes were also prepared for use as control compounds.

Nickel phthalocyanine is synthesized in an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate) giving a high yield of one-dimensional structures. The morphology of the synthesized material is detected by a high-resolution scanning electron microscope, a high-resolution transmission electron microscope, characterized by powder X-ray diffraction, and a CHN analyzer. The nonlinear optical properties of the synthesized phthalocyanine are also investigated.

The porphyrin-ferrocene conjugates (P-nFc; n = 1, 2 and 4), which are simple examples of a donor-acceptor charge separation system, were synthesized. Their photoindued intramolecular and intermolecular processes have been investigated by time-resolved emission and nanosecond transient absorption techniques. Upon excitation of the porphyrins (H₂P and ZnP) moieties, an efficient fluorescence quenching of the excited singlet porphyrin is observed. It was found that the quenching efficiency increases with increasing number of attached ferrocene moieties and solvent polarity. The main quenching pathway involves (i) electron transfer from ferrocene to the singlet excited porphyrin and (ii) the heavy-atom effect.

A donor-σ-acceptor compound incorporating tetrathiafulvalene and tetraphenylporphyrin units was synthesized. The structures of the target compound and its intermediates have been characterized by ¹H NMR, ESI MS, UV-vis, IR and mp. The results of UV-vis and cyclic voltammetry of the compound indicated negligible intramolecular charge transfer interaction in its ground state. The fluorescence and fluorescence lifetime of the compound were reduced, compared to tetraphenylporphyrin, which evidently indicated that photoinduced electron transfer occurred from the tetrathiafulvalene (TTF) unit to the tetraphenylporphyrin unit in the excited state. The fluorescence intensity of the compound could be reversibly modulated by sequential oxidation and reduction of the TTF unit using electrochemical and chemical methods. Therefore, a new fluorescence molecular switch based on TTF and porphyrin units was established.

Several clinical studies, as well as investigations performed on tissue cultures and murine tumor models, have demonstrated the in vitro and in vivo efficacy of Photofrin II and hypericin as radiosensitizing agents. The mechanisms involved in the radiosensitizing action of Photofrin II and hypericin are partially understood; the recognition of the major role performed by oxygen regarding the modulation of cellular radiosensitivity has prompted the present investigations on the relevance of oxygenation for the success of Photofrin II or hypericin-based radiation therapy of tumors. RT4 human bladder carcinoma cell lines were seeded and incubated with various concentrations of Photofrin II or hypericin under ambient and 5% oxygen levels. The cells were irradiated with ionizing radiation between 1 and 6 Gy. The same experiments were repeated with Photofrin II and hypericin alone, without radiation. The cell survival was evaluated. The results demonstrated an increase of radiation-induced cell damage in the presence of Photofrin II and hypericin, respectively, when sufficient oxygen was available. Low levels of oxygen reduced the activity of Photofrin II as well as of hypericin as a radiosensitizer, with minimal tumor damage (p < 0.05 in a Student t-test). The mechanism of action of Photofrin II and hypericin as radiosensitizers requires the presence of sufficiently high oxygen concentrations.

The interaction of meso-tetraarylporphyrins, H₂T(4-X)PP, with sulfur trioxide pyridine complex (SO₃.py) in chloroform only produced 1:2 molecular complexes, [H₂T(4-X)PP(SO₃)₂]. UV-vis, ¹H NMR, and FT-IR spectra, as well as elemental analysis, suggested pseudo tetrahedrally distorted porphyrin core structures with σ-bonding from two pyrrolenine nitrogen donors to the d orbital of sulfur in two SO₃s and the occurrence of two hydrogen bonds between two pyrrolic NH and oxygens of sulfur trioxides from above and below the mean plane of the porphyrins.

A straightforward procedure for supporting porphyrin-derivatives onto resin beads has been developed by reacting the commercially available 4,6-dichloro-1,3,5-triazene polymer-bound, with 5-(4-aminophenyl)-10,15,20-triphenylporphyrins as free base and as Zn(II) and Co(II) complexes. The supported porphyrins have been characterized by on-resin UV-vis spectroscopy. Kinetic and Hammett LFER studies have shown that the S_NAr reaction with the triazine-bound reagent occurs at a similar rate for free and metalated complexes, with a nucleophilic strength comparable to that of the unsubstituted aniline (σ_p⁺ = 0.07-0.08).

A α,ω-dimethyldipyrrin with and without a meso-situated phenyl substituent has been investigated as the substrate in the oxidation reaction with the Mn^II/O₂ system. While for the meso-phenyldipyrrin, the expected, though impure and sensitive diformyl derivative is formed, the reaction of the meso unsubstituted dipyrrin leads to a novel tripyrrolic species, the α-(vinylpyrrolyl)dipyrrin, as the only isolable product. The new tripyrrole displays positive solvatochromic behavior and has been investigated with respect to this property.

Three non-peripherally substituted tin(IV) macrocylic compounds, octahexylphthalocyaninato dichlorotin(IV) (3a), octahexyltetrabenzo-5,10,15- triazaporphyrinato dichlorotin(IV) (3b) and octadecylphthalocyaninato dichlorotin(IV) (3c) were synthesized and their photophysical and electrochemical behavior studied. Complex 3b, containing a CH group in place of one of the aza nitrogen atoms of the phthalocyanine core, shows a split Q-band due to its lower symmetry. The triplet state quantum yields were found to be lower than would be expected on the basis of the heavy atom effect of tin as the central metal for phthalocyanine derivatives (3a and 3c). In contrast, 3b shows a triplet quantum yield Φ_T = 0.78. The triplet state lifetimes were solvent dependent, and were higher in tetrahydrofuran than in toluene. Cyclic voltammetry and spectroelectrochemistry of the complexes revealed only ring-based redox processes.