Journal of Porphyrins and Phthalocyanines

Phthalocyanines are an interesting class of aromatic macrocycles which possess unique physical and chemical properties that make them excellent building blocks for the construction of molecular materials. Among the different functional groups that can be incorporated into the phthalocyanine's skeleton, the alkynyl group is one of the most interesting; this is confirmed by the large number of organic synthetic materials constructed from acetylene-based scaffolds due its rigidity and linearity, allowing high exciton and electron coupling between chromophore units. Additionally, these systems are particularly important, considering the wide range of functional group interconversions that the triple bond may permit. This account represents a concise overview of the most important contributions on the synthesis and applications of mono- and poly-alkynyl-substituted phthalocyanine-based molecular systems, towards the development of new functional molecular materials.

In this review, we report on the newly developed area of research devoted to the formation of self-assembled monolayers of metallophthalocyanines by focusing on some significant examples dedicated to electroanalytical applications. We also summarize recent examples on the use of electropolymerized metallophthalocyanine films in electroanalysis. In both cases, activation and detection of thiols are the main targeted applications.

Photodynamic therapy is a method for treating several diseases, most notably cancer. Recent synthetic activity has created a number of phthalocyanines for potential use as photodynamic therapy photosensitizers. In this mini-review article, the background and the concepts in the development of new phthalocyanines are introduced.

A new spectroelectrochemical cell with a long optical path length has been fabricated for redox studies of phthalocyanines, where the propagation axis of the optical beam is parallel to the electrode/solution interface. It is composed of a homemade Teflon block that fits inside a standard 1 cm quartz optical-cell, equipped with a platinum-foil working electrode, a platinum-wire auxiliary electrode, and a AgCl-coated silver-wire reference electrode. This new cell makes it much easier to acquire spectral data for dilute solutions containing species, like phthalocyanines, that have limited solubility in common organic solvents or that strongly aggregate in solution. As examples, electrolyses of the [tetra(tert-butyl)phthalocyaninato]zinc(II) complex and its cobalt(II) analogue were performed to monitor spectral changes ascribable to phthalocyanine-ligand-centered oxidation and metal-centered reduction, respectively. Spectra of unaggregated electrogenerated species were successfully observed.

A series of novel bis[3,4,12,13,21,22,30,31-octa(dodecylthio)-2,3-naphthalocyaninato] rare earth(III) complexes M[Nc(SC₁₂H₂₅)₈]₂(M = Nd, Eu, Gd, Tb, Y, Ho, Er) have been prepared by tetramerization of 2,3-bis(dodecylthio)-6,7-dicyanonaphthalene with M(acac)₃·nH₂O in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing n-octanol. These sandwich double-decker complexes were characterized by elemental analysis and various spectroscopic methods including UV-vis, near IR, IR, and mass spectroscopies. Their liquid crystal behavior was studied by polarized optical microscope (POM), differential scanning calorimeter (DSC), and X-ray diffraction. These alkylthio-substituted bis(2,3-naphthalocyaninato) rare earth complexes yield a hexagonal columnar liquid crystal whose stable temperature range is between 42 and 243°C. The stable temperature domain of the mesophase shows a trend to be dependent on the central rare earth ionic size. In contrast, the molecular packing of these complexes as revealed by X-ray diffraction was found to be almost independent of the ionic size of the central rare earth.

Metal phthalocyanines containing an unsymmetrically substituted nitro group were synthesized from 4-nitrophthalonitrile and the corresponding anhydrous metal salts (Co(OAc)₂, Zn(OAc)₂) by the method of statistically mixed condensation reactions. The new type of metallo-bisphthalocyanine compounds connected by a N=N bridge was prepared from the reactions of respective nitro compounds with sodium hydroxide (NaOH) and activated zinc powder in a mixture of methanol (MeOH) and tetrahydrofuran (THF). The structures of these compounds were characterized by using ¹H NMR, IR, elemental analyses, UV-vis and magnetic circular dichroism spectroscopies.

Two series of very large discotic complexes of tetrakis (2,3,6,7-tetraalkoxy)triphenylenocyaninato copper(II) (abbreviated as (C_nO)₁₆TcCu; n = 8, 10, 12 and 14) and tetrakis(2,3,6,7-tetraalkoxy)-1,4-diazatriphenylenocyaninato copper(II) (abbreviated as (C_nO)₁₆TzCu; n = 8, 10, 12 and 14), were synthesized to investigate their mesomorphic properties by differential scanning calorimetry, thermogravimetry analysis, polarizing optical microscope observation and temperature-dependent X-ray studies. The results show that each of the (C_nO)₁₆TcCu derivatives exhibits two or three disordered rectangular columnar (Col_rd) mesophases from room temperature to the decomposition temperature at ca. 330-340°C under nitrogen gas. However, each of the (C_nO)₁₆TzCu complexes exhibits only one ordered tetragonal columnar (Col_tet.o) mesophase in an extremely wide temperature region from room temperature to the decomposition temperature at ca 300°C and ca. 340-350°C under the atmosphere and nitrogen gas, respectively. It is very interesting that the Col_tet.o mesophase of (C_nO)₁₆TzCu for n = 10, 12 and 14 shows remarkable spontaneous homeotropic alignment in a large area, without disclinations and polydomains, in the temperature range from room temperature to the decomposition temperature. As far as we know, these discotic liquid crystals are the first examples to show homeotropic alignment at room temperature. Furthermore, the difference between the mesomorphic properties of (C_nO)₁₆TcCu and (C_nO)₁₆TzCu originated only from the additional eight nitrogen atoms in the central core in (C_nO)₁₆TzCu.

Supramolecular triads composed of zinc naphthalocyanine (ZnNc) as an electron donor, pyridylfullerenes as primary electron acceptors, and either ferrocene (Fc) or N,N-dimethylaminophenyl (DMA) entities as second electron donors were constructed via metal-ligand axial coordination. The formation of the supramolecular triads was monitored using optical absorption methods and the measured binding constants revealed a moderately stable complex formation. The B3LYP/3-21G(*) optimized structures showed the disposition of the three entities of the triads to be in a linear fashion. The redox behavior of the different components was studied using cyclic voltammetry in o-dichlorobenzene containing (n-C₄H₉)₄NClO₄. The donor ability evaluated from the oxidation potentials of the electron donors followed the trend: ZnNc < Fc < DMA. Efficient electron transfer from the excited singlet state of zinc naphthalocyanine to the fullerene entity was observed in all of the studied triads in o-dichlorobenzene by fluorescence quenching measurements. Clear evidence, that the charge-separation process formed the radical ions in the triads, was obtained from a nanosecond transient absorption spectral technique, indicating the contribution of the second electron donor in prolonging the charge-separated states.

In our search for the definition of new molecular compounds combining low linear absorption with high nonlinear optical absorption in the same wavelength range, we have prepared a series of vanadyl, chloroindium and lead naphthalocyaninato complexes with a relatively low linear optical absorption within the visible range when compared to phthalocyanines, but with similar chemical stability. The transmission properties of these complexes for nanosecond pulses at 532 nm have been evaluated and confronted in both linear and nonlinear regimes. Halogenated naphthalocyanines displayed the best optical-limiting performances. It was also found that an increase in concentration generally results in a reduction of the limiting efficiency of the systems, due to the phenomena of intermolecular aggregation and consequent excited-state quenching.