Journal of Porphyrins and Phthalocyanines

Anionic, neutral and cationic amino porphyrins were synthesized as precursors of photodynamic antimicrobial agents with an aim to functionalize cotton surface through 1,3,5-triazine link. Structures of porphyrin-triazine derivatives were characterized by ¹H NMR, MS and UV-vis confirming the feasibility of this novel concept. Porphyrinic cotton fabrics have been developed from these derivatives, and tested in vitro against Staphylococcus aureus. These novel photodynamic surfaces showed strong and varied antimicrobial activity.

New peripherally (β) and non-peripherally (α) substituted metal octakis(hexylthio)phthalocyanines (β- and α-MOcHexTPc) containing cobalt and manganese as metal centers were synthesized. Their characterization using electrochemical methods showed that these complexes exhibit several redox processes at E_1/2 (mV vs. Ag∣AgCl) = 380 (212) (I), 1140 (864) (II), -450 (-460) (III) and -1170 (-1304) (IV) for β- (α-) CoOcHexTPc. These redox processes were assigned to Co^IIIPc^-2/Co^IIPc^-2 (I), Co^IIIPc^-1/Co^IIIPc^-2 (II), Co^IIPc^-2/Co^IPc^-2 (III) and Co^IPc^-2/Co^IPc^-3 (IV) using spectroelectrochemistry. For the β- (α-) MnOcHexTPc complex the redox processes were observed at E_1/2 (mV vs. Ag∣AgCl) = -20 (5) (I), -530 (-640) (II) and -1270 (-1380) (III) and were assigned to Mn^IIIPc^-2/Mn^IIPc^-2 (I), Mn^IIPc^-2/Mn^IIPc^-3 (II) and Mn^IIPc^-3/Mn^IIPc^-4 (III). Electrochemical and microscopic characterization using AFM showed that the self-assembled monolayers (SAMs) are formed on the gold surface using these complexes. The electrochemical characterization showed the blocking of the Faradaic processes at SAMs modified electrodes and these reactions are well-known to easily occur at unmodified gold electrodes. The AFM characterization showed an increase in surface roughness upon modifying the gold surface with MOcHexTPc SAMs, further confirming the presence of the monolayers on the gold surface. The MOcHexTPc SAMs were investigated for their electrocatalytic application towards H₂O₂ detection. The MOcHexTPc SAMs modified gold electrodes gave excellent currents for H₂O₂ detection. The observed H2O₂ electrocatalytic reduction peaks were close to where the metal redox processes from the MOcHexTPc occurred, showing the involvement of the metal redox processes in the electrocatalytic mediation reactions.

Singlet excitation transfer in self-assembled dyads formed via axial coordination of imidazole appended free-base tetraphenylporphyrin, H₂PIm, to either a magnesium phthalocyanine, MgPc, or a magnesium naphthalocyanine, MgNc is investigated in non-coordinating solvents using spectroscopic (optical and mass), electrochemical, and time-resolved transient absorption techniques. The newly assembled dyads are fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral data are found to be in the range of 10³–10⁴ M^-1 for the 1:1 dyads suggesting fairly stable complex formation. However, the anticipated 2:1 complex for Mg coordination is not observed under the present solution conditions. Electrochemical and computational studies suggested that photoinduced electron transfer to be a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. However, selective excitation of H₂PIm entity in these dyads resulted in rapid excitation transfer and the position of the imidazole linkage on the H₂P entity seem to direct the overall efficiency of excited energy transfer. Kinetics of energy transfer is monitored by transient absorption measurements using pump-probe technique and is compared with the earlier reported H₂PIm:ZnPc and H₂PIm:ZnNc dyads. The time constants are in the order of 0.2–18 ps depending upon the type and relative orientation of the donor and acceptor entities of the dyad indicating ultrafast excitation transfer, and agree fairly well with theoretically estimated values assuming Förster energy transfer mechanism.

In this contribution, the influence of lipids on excitation energy transfer from lipophilic photosensitizer tetraphenylporphyrin to oxygen was investigated in chloroform solutions of phosphatidylcholine as well as in bulk lipid. The excited states kinetics were examined in a wide range of lipid concentrations (from zero to the saturated concentration) by direct time- and spectral-resolved detection of weak near infrared phosphorescence of the photosensitizer (around 840 nm) and singlet oxygen (about 1278 nm). While photosensitizer triplet kinetics follows single-exponential decay with lifetime of 0.52 μs in pure chloroform, two distinct components with lifetimes of approximately 0.4 and 1 μs appear after phosphatidylcholine addition. Both the lifetimes exhibit shortening tendency with increasing lipid concentration. Relative weights of the two components depend on the lipid concentration. Singlet oxygen kinetics exhibit single-exponential rise with lifetimes roughly corresponding to the shorter components of photosensitizer decays while their decays require two exponentials. The lifetime of the longer component decreases with increasing concentration of lipid from (77.6 ± 1.3) μs at pure chloroform to (14.3 ± 1.1) μs at the saturated lipid concentration. The time-constants obtained in bulk lipid sample follow the above-mentioned trends. Tetraphenylporphyrin photoproduct formation under pulsed excitation in chloroform solutions was demonstrated. The quantum yield of singlet oxygen production of the photoproduct is lower than that of the tetraphenylporphyrin. It was shown that lipids prevent the singlet-oxygen mediated formation of TPP photoproduct, probably by efficient quenching of singlet oxygen. This quenching is justified by shortening of the longer component of singlet oxygen luminescence decays with increasing concentration of the lipid. Moreover, the lipids also quench triplet states of the photosensitizer.

The photodegradation of a series of water-soluble Zn(II) and Al(III) phthalocyanines has been studied in aerobic aqueous solutions. The photobleaching quantum yields of zinc phthalocyanine derivatives range from 2.2 × 10^-4 for octacarboxy- to 1.5 × 10^-5 for ZnPc, bearing 16 positively charged groups on periphery. Their aluminum counterparts are more photostable and photobleach with quantum yields in the narrow range (0.75 ÷ 2 × 10^-6). The pH dependences of the photobleaching quantum yields for aluminum phthalocyanines show enhanced photodegradation for molecules with deprotonated axial H₂O ligand. Some aspects of the mechanism of dyes photodegradation were studied and discussed. It was found that contribution to photooxidation of type II pathways depends upon a particular dye structure. Thus, no contribution of singlet oxygen to the photooxidation of cationic phthalocyanines has been found, even though singlet oxygen is involved in the photodegradation of negatively charged phthalocyanines.

This work presents the synthesis and characterization of a series of substituted pyridylpentafluroporphyrins, including the separation of the cis- and trans-isomers, the latter being characterized by X-ray crystallography. The spectroscopic and electrochemical properties of the series are dependent on the number of electron withdrawing pentafluorophenyl substituent, but they do not depend on the symmetry of the molecule. Ongoing from the monosubstituted to the more substituted pentafluorophenyl porphyrin H₂(MPyTFPP) derivative, the Soret bands are slightly red-shifted and their quantum fluorescence yields range from 0.035 to 0.046, consistent with the value of 0.045 for the fully substituted 5,10,15,20-tetrapentafluorophenylporphyrin (dichloromethane solutions). The redox potentials of the reductive processes of monoanion and dianion formation are also sensitive to the number of pentafluoro substituents, shifting 180 mV to more positive values for the P⁰/P^-1 process ongoing from the monopentafluoro to the tris-pentafluorophenyl substituted derivative.

The photoelectroectrochemical studies of water soluble octacarboxylated oxotitanium (OTiOCPc), zinc (ZnOCPC), hydroxyaluminium ((OH)AlOCPc), dihydroxysilicon ((OH)₂SiOCPc), hydroxygallium (OHGaOCPc) and low symmetry zinc monocarboxy (ZnMCPc) phthalocyanines were performed. The dyes were adsorbed to nanoporous ZnO electrodeposited in the presence of eosin Y as structure directing agent (SDA) on FTO substrates by refluxing or soaking the films in a solution containing the dye of interest such that a full surface coverage was achieved. High external (IPCE) and internal (APCE) quantum efficiencies of up to 50.6% and 96.7% were achieved for the OTiOCPc complex. There was a lower overall cell efficiency for cells sensitized with phthalocyanines containing hydroxyl as axial ligand ZnO/(OH)₂SiOCPc, ZnO/(OH)GaOCPc and (OH)AlOCPc because of strong aggregation on the surface of the electrodes. To further suppress dye aggregation, the zinc complex of a new monocarboxylated phthalocyanine sensitizer with bulky naphtho side groups (ZnMCPc) was employed. Among the studied sensitizers, ZnMCPc gave the highest overall cell efficiency of phthalocyanine electrodeposited on ZnO of η = 0.48%.

TiO₂/PS, Fe(III) tetraphenylporphyrin (FeTPP)/PS and TiO₂/FeTPP/PS nanofibers were prepared by electrospinning a mixture solution of corresponding compounds in N, N-dimethylformamide (DMF). The absorbance of FeTPP was characterized by UV-vis spectroscopy. The surface morphologies of these electrospun mats were evaluated with scanning electron microscope (SEM). Their photocatalytic activities were examined with the photodegradation reaction of methyl orange in aqueous solution under the irradiation of visible light. The result shows that the photocatalytic activity of TiO₂/FeTPP/PS mat is higher than that of FeTPP/PS mats, which in turn is better than that of TiO₂/PS mats in the photodegradation of methyl orange. The degradation percent of methyl orange could be up to 97.8% in 3 h by the FeTPP/TiO₂/PS mat photocatalyst.