Tetrapyrazinoporphyrazine (TPz or TPyzPz), also known as azaphthalocyanine (AzaPc), is a planar, aromatic, macrocyclic, organic compound that is viewed as an aza-analogue of phthalocyanine (Pc). It was first discovered and reported in 1937 by R. P. Linstead, the scientist who was able to configure the structures of porphyrazines and phthalocyanines as well. The structure of TPz is similar to that of Pc with eight nitrogen atoms substituting the carbons at the α-positions.
Electronic and optical properties of TPz Similar to its analogues, the central core of TPz can be present as a free base (2H), which yields a D2h symmetry molecule, or metalated, in general, with a first-row transition metal ion (M) to yield a molecule with D4h symmetry. TPz molecules possess characteristic UV-Vis absorption spectra, observed as Soret bands (also known as B-bands) and Q-bands, similar to their analogous macrocycles. These bands result from the extended aromatic π-conjugation, allowing TPz molecules to absorb in the visible region. In the range of 620-720 nm, an intense and narrow Q-band is observed, whereas a less intense and broader B-band is observed at around 350 nm, both of which are attributed to be resulting from π-π* transitions. For metalated cores that are generally more symmetrical with D4h symmetry, only one band is observed in the Q-band region of UV-Vis, whereas the less symmetrical D2h non-metalated cores show two peaks in that region. The location of wavelength maxima in these spectra greatly depends on the electron-donating or withdrawing nature of the peripheral substituents, which can shift the peak positions bathochromically or hypsochromically depending on their electron-withdrawing or donating properties. TPz molecules possess both acidic and basic properties depending on the type of solvents they are dissolved in. Upon protonation in acidic conditions, both azomethine nitrogen atoms within the porphyrazine core as well as the pyrazine nitrogen atoms at the α-positions of TPz can get protonated. In basic conditions, however, central pyrrole nitrogen atoms of non-metalated cores are deprotonated into H2TPz•- or H2TPz2-, with the acidity of these protons depending heavily on the peripheral substituents attached to the TPz molecule.
Comparison of TPz to related compounds Structurally, TPz is related to other well-known macrocycles formed of tetrapyrrole subunits, such as porphyrin, porphyrazine, and phthalocyanine. Due to the negative inductive effect of the eight extra electron-withdrawing nitrogen atoms in the TPz structure, TPz molecules show greater electron deficiency than Pc analogous structures. Additionally, TPz molecules generally manifest increased intramolecular charge transport, lower reduction potentials, enhanced conductivity, but less effective π-conjugated systems. Free-base TPz molecules possess similar optical, magnetic, and structural properties to these analogues when each is reduced to its anionic states: radical anion (H2TPz•-) or dianion (H2TPz2-), yet with higher stability, allowing more air-stable functional compounds based on anionic TPz. This enhanced stability results from the more positive reduction potentials of TPz derivatives upon their aza-substitution compared to the reduction potentials of their Pc-based analogs.
Upon increasing the number of aza-substitutions of the Pc macrocycle, the HOMO–LUMO gap as well as the excitation energies increase, as shown through both cyclic voltammetry (CV) experimental studies and density functional theory (DFT) theoretical calculations. This increase is also confirmed through a shift to higher energy of the Q-band maximum wavelength position of upon isosteric aza-substitution. In addition, TPz derivatives often show possess strong fluorescence band in the visible region, permitting their use as red fluorophores, unlike most Pc derivatives whose fluorescence bands appear in the near-infrared region and are thus not within the visible region. Other similar characteristics between these analogues include poor solubility of the unsubstituted core in various solvents, which limits their utilization in different applications. Thus, researchers have resorted into adding substituents that can provide both: enhanced solubility and additional functionalization sites. Some studies have reported the synthesis of Pc and TPz derivatives that are even water-soluble, like the example in which click chemistry was utilized to add polyethylene glycol (PEG) functionalities as periphery substituents, rendering them soluble in aqueous media as well as organic solvents.
Synthesis
TPzs are usually synthesized upon the cyclotetramerization of pyrazine-2,3-dicarbonitriles, which itself can be synthesized either from the condensation of diaminomaleonitrile (DAMN) and substituted α-diketone derivatives or from the substitution of chloro pyrazine-2,3-dicarbonitriles. To achieve TPz synthesis, pyrazine-2,3-dicarbonitriles can be reacted either through alkoxide-initiation using lithium butoxide or through templated cyclization pathways by adding a metal ion template. Greatly electron-deficient macrocycle formation reactions, as is the case of TPz, require the use of templated method by utilizing a cyclotetramerization agent to avoid the risk of alkoxide initiator exchanging the positions of peripheral substituents. Often, a mild cyclotetramerization agent is utilized to accompany the metal ion template, such as magnesium butoxide, to attain the MgTPz macrocycle. The magnesium-templated cyclotetramerization reaction can be followed by a central metal removal to attain non-metalated core or a metal center exchange reaction to replace the central metal with a transition metal. This option is particularly advantageous since Mg2+ ions are easily demetalated form the central TPz core in acidic conditions. 1H-NMR (Nuclear Magnetic Resonance) spectroscopy can be utilized to characterize TPz molecules and their derivatives. Yet, it is critical to find a suitable solvent in which the TPz derivative is soluble. 1H-NMR peaks arising from TPz derivatives can appear for i) peripheral substituents attached to the -positions of TPz, or ii) from central core protons for the non-metalated counterparts which appear at around -0.5 to -1.3 ppm (lower field shifted compared to those of metal-free Pc derivatives that appear at around -3 to -5 ppm).
Potential Applications
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