Synthesis and spectral properties of tetrapyrazinoporphyrazinato metal complexes derived from a sterically hindered phenol

The synthesis of metal-containing tetrapyrazinoporphyrazines (azaphthalocyanines) bearing eight bulky substituents at the peripheral positions is described. The key precursor is prepared using a nucleophilic aromatic substitution reaction between a sterically hindered phenol and 5,6-dichloropyrazine-2,3-dicarbonitrile. Thus, azaphthalocyanine (AzaPc) derived from 2,6-isopropylphenol possesses substituents that are forced by steric constraints to adopt a non-planar conformation which drastically enhances the solubility of the macrocycles and inhibits efficient cofacial interaction of the macrocycles even in the solid state as evaluated by UV-Vis and 1 H NMR spectroscopic techniques. The intense absorption in the red visible region and photostability of these highly soluble and non-aggregated complexes enhance the dynamic activity and make these complexes potentially suitable for PDT.


Introduction
Phthalocyanine (Pc) and its derivatives are of interest due to their outstanding electronic and optical properties.In addition to their extensive role as colorants they have found application in nonlinear optics, as liquid crystalline electronic charge carriers, photoconductors, optical data storage, solar energy conversion, photodynamic cancer therapy, catalysis and as the active component of gas sensors. 1,2The extended planar, four-fold symmetry and stability also makes the phthalocyanine macrocycle an excellent building block for use in supramolecular 3 and polymer chemistry. 4nalogues of phthalocyanine in which nitrogen replaces some of the carbons within the four fused benzo-substituents of the phthalocyanine, possess significantly different physical properties including color, oxidation potential and stability, which may be beneficial for some of An aromatic nucleophilic substitution reaction 25 between 5,6-dichloro-pyrazine-2,3dicarbonitrile and 2,6-diisopropyl-4-nitrophenol in dry dioxane and NaH at 100 o C for 24 h gave the key intermediate 5,6-bis(2,6-diisopropyl-4-nitrophenoxy)pyrazine-2,3-dicarbonitrile 1.A milder base catalyzed-reaction (K 2 CO 3 ) failed to provide the target compound 1 in spite of the strong electron-withdrawing ability of the pyrazine-2,3-dicarbonitrile moiety which makes the chlorine atoms labile.Analytical and spectroscopic analysis of 1 were consistent with the structure.In the IR spectrum, the absorption at 2220 cm -1 is characteristic of the -CN groups.
The syntheses of AzaPc 2 and AzaPc 3 depended on metal ion template reactions of the dinitrile derivative 1 in quinoline at 160 o C using anhydrous zinc acetate, or cobalt chloride respectively. 26Column chromatography over silica gel was employed using chloroform as eluent to obtain the appropriate metallated AzaPcs (2, 3) in good isolated yields (50-55%) as compared with other methods. 27The high solubility of the intensely green products (2, 3) in most organic solvents (e.g.DCM, THF, CHCl 3 ) due to the effect of the eight phenoxy groups, probably contributes significantly to the observed good yields.The structure and purity of AzaPc (2, 3) were confirmed by H NMR, UV/Visible, matrix-assisted laser desorption ionization (MALDI) mass spectroscopy, together with elemental analysis.Analysis using MALDI-MS gave, in each case, a cluster of molecular ion peaks consistent with their molecular formulas.The UV/Visble absorption spectra of (2, 3) complexes in solution show the characteristic metal ion containing the octaazaphthalocyanine Q-band at 610-630 nm.The percentage weight changes as a function of temperature are shown in Figure 1.From the thermograms it was found that both complexes are highly stable in the temperature range of 300-320 o C and that above this, extensive decomposition occurs at temperatures between 350-420 o C. Therefore, the stability of the prepared Aza-complexes (2, 3) is similar to other aza-analogues of phthalocyanines which generally decompose without melting upon heating at high temperature (> 300 o C).

Material properties
As mentioned in the introduction, the self-association of azaphthalocyanine molecules is apparent even in dilute solution in which the macrocyclic molecules tend to form dimers, trimers and higher oligomers. 28This aggregation behavior can considerably affect their properties and limit their uses in many optical and electronic applications.For example, cofacial exciton interactions result in the quenching of the photochemically excited state of AzaPc and interfere with the formation of singlet oxygen. 29Thus, our aim was successfully achieved, to avoid the cofacial self-association by introducing eight bulky phenoxy substituents onto the azaphthalocyanine ring.
UV/Vis 30 and 1 H NMR 31 spectroscopy are powerful techniques used to study aggregation in solution.The UV/Vis absorption spectra of metal ion-containing azaphthalocyanines 2 and 3 (Zn +2 , Co +2 ) in THF solution exhibit a single Q-band accompanied by the typical vibrational satellite band on the blue side, suggesting a D 4h symmetry for the porphyrazine core (Table 1).The characteristic Q band has been efficiently used as a probe in assessing the self-assembly features of azaphthalocyanine in solution.3][34][35] As predicted, the spectra showed monomeric behavior evidenced by a single (narrow) Q-band, typical of metallated azaphthalocyanine complexes for both AzaPc 2 and 3 in THF.For example, the absorption spectrum of AzaPc 2 showed a signal sharp Q-band at λ max = 630 nm (Table 1, Figure 2) which is typical of non-aggregated species.For further assessment, we recorded the absorption spectra for each derivative at different concentrations ranging from 4.6x10 -6 to 11x10 -6 M as depicted in Figure 2. The position and appearance of the Q-band absorption maxima of AzaPc 2 remained unchanged and consistent with reported spectra of non-aggregated AzaPcs 36 as the concentration was increased which also results in the increase of the intensity.Similar behavior was detected for AzaPc 3 as outlined in Figure 3.In addition, the apparent molar extinction coefficient (absorptivity) remains constant (Figure 4); this further emphasizes nonaggregation in this concentration range.
(3) Controlling the molecular packing within the solid state of an azaphthalocyanine compound is necessary so that the electronic and optical properties (colour, molecular association, optical absorption) can be tailored to the requirements of the intended applications, such as optical limiting devices.Therefore, thin films of complexes 2 and 3 were prepared to investigate the nature of the azaphthalocyanine derivatives in the solid state.Spin-coated films derived from 2 and 3 were deposited onto glass substrates from chloroform solution.Both complexes gave highly uniform and visually clear films.The UV/visible spectra of spin-coated films (Figure 5, Table 1) of both complexes are almost identical to those obtained from dilute solution spectra, indicating that the AzaPc chromophores are not perturbed by intermolecular exciton effects even in the solid state.The high quality of these films and their intense fluorescence might be useful for device fabrication (e.g.sensors or organic light emitting diodes).This suggests an approach reducing the adverse effects of aggregation when using AzaPc derivatives as photosensitizers in PDT. 9 A commonly observed feature of the 1 H NMR spectra of the phthalocyanines and related disc-like molecules such as azaphthalocyanines, is the broadening and upfield shifted peaks due to the self-association of the macrocycle units. 37Thus, a good 1 H NMR spectrum can be obtained by running the NMR experiment either at high temperature (e.g.C 6 D 6 , 60 ºC) or using d 5pyridine as solvent to reduce the aggregation of the macrocycle.Therefore, we investigated the aggregation behavior of complexes 2 and 3 by NMR spectroscopy in chloroform over the concentration range between 1x10 -4 and 1x10 -2 M (Figure 6), at room temperature.Both complexes gave very clean spectra with well-resolved peaks in both the aromatic and aliphatic regions compared to those of reported substituted AzaPc derivatives.In addition, the chemical shifts of the aromatic and aliphatic protons were essentially independent of the concentration as shown in Figure 6.This is most likely attributable to the steric hindrance imposed by the bulky phenoxy substituents that prevent efficient π-π stacking.Based on the above findings, we emphasise that the position adopted by the bulky substituents relative to the AzaPc core is the most important factor influencing the molecular packing of the AzaPcs in the solid state.

Conclusions
In conclusion, we have successfully synthesized and characterized novel metal-containing octaazaphthalocyanine derivatives using metal-template cyclotetramerisation reactions.Our results showed that the eight bulky substituents represent an efficient way to prevent the aggregation and consequently obtain azaphthalocyanine material with intrinsically true solid solution properties, as inferred by UV/visible and 1 H NMR spectroscopy.Due to their interesting properties: the non-aggregation behavior, high solubility, short synthesis and good photostability, these new AzaPc derivatives could find applications in organic electronic devices and might be potentially suitable as photosensitizers in PDT.

Experimental Section
General Procedures.UV-vis spectra were recorded on a Cary 5 spectrophotometer.IR spectra were recorded as KBr pellets using a Perkin Elmer System 2000 FT-IR spectrophotometer. 1 H NMR spectra were recorded on a Bruker DPX 400, 400 MHz super-conducting NMR spectrometer.Mass spectra were recorded on a VG Auto-spec-Q (high resolution, high performance, tri-sector GC/MS/MS).The C, H, N analysis of the compounds was performed on a LECO CHNS-932 Elemental Analyzer.

Figure 4 .
Figure 4. Molar absorption coefficients at the Q-band maxima as a function of concentration for AzaPc complexes (2 and 3) in THF.

Figure 5 .
Figure 5. UV/Visible absorption spectrum of AzaPc 2 (a) in THF and (b) from a solvent cast film.

Figure 6 . 1 H
Figure 6. 1 H NMR spectra of AzaPc 2 in CDCl 3 at room temperature at different concentrations.

Table 1 .
Comparison of the Q-band position (nm) displayed by AzaPc 2 and AzaPc 3 in THF with that of its spin coated film