59th Annual Report on Research 2014

Figure 1. Structures of monomeric phthalocyanine octacarboxylic acid CuPc and cyclic tetrameric phthalocyanine hexadecacarboxylic acid (CuPc*)₄. Our first round of experiments revealed that the material reported in the literature as cyclic tetrameric copper phthalocyanine (CuPc*)₄ is actually NOT a tetramer, as reported in 1982, but simply the monomeric copper phthalocyanine octacarboxylic acid CuPc. Our mass spectrometric and chromatographic analysis of the reported material unambiguously demonstrated for the first time that the phthalocyanine obtained from pyromellitic dianhydride is CuPc and not (CuPc*)₄. Our results indicate that dozens of papers published over the years in a variety of journals, including the prestigious Nature and Physical Review Letters, have erroneously attributed the colossal dielectric constant of the phthalocyanine material derived from pyromellitic dianhydride to (CuPc*)₄, referencing the 1982 paper for the synthesis of the material. Different reaction conditions (starting materials, molar ratios, temperature, reaction time, catalyst) only yielded the same monomeric phthalocyanine CuPc.  Therefore, we devised new synthetic pathways for the preparation of the cyclic tetrameric phthalocyanine (CuPc*)₄. Our first attempt involved the condensation of dipentyl-4,5-dicyanophthalate and naphthalene-2,3,6,7-tetracarbonitrile.  1,2-Dibromo-4,5-bis(dibromomethyl)benzene was converted to 6,7-dibromonapthalene-2,3-dicarbonitrile by reaction with fumaronitrile and further converted to naphthalene-2,3,6,7-tetracarbonitrile with copper cyanide. The reaction of naphthalene-2,3,6,7-tetracarbonitrile with dipentyl-4,5-dicyanophthalate, however, did not yield the expected cyclic tetrameric phthalocyanine.  Next, we proposed a new route for the preparation of a similar tetrameric phthalocyanine hexadecacarboxylic acid, based on the following reaction scheme (Fig. 2):   Figure 2: Condensation of differently functionalized phthalocyanine starting materials to yield a cyclic tetrameric phthalocyanine.   This approach is based on the condensation of two pairs of asymmetrically functionalized phthalocyanines, one with carbonyl functionalities (B) on two adjacent sides, and another with amine functionalities (C).  Both molecules have carboxylic acid (protected) functionalities (A) on the other two sides, providing the peripheral haxedecacarboxylic acid substitution for the final product. Carbonyl and amine groups react readily to form a Schiff base, which can be reduced for increased stability of the product. In parallel with the organic synthesis described above, we have carried out the tasks described in our proposal using the monomeric CuPc. To prepare panel-like building blocks for metal-organic frameworks, we installed various donor groups on the four sides of the phthalocyanine core. Similarly, we attached various chromophores onto the phthalocyanine core, to prepare new light-harvesting materials for improved organic solar cells. So far, our improved synthetic methodology allowed the successful synthesis of the following pure tetra-functionalized phthalocyanines:

1.      Tetrakis-(N-((4-carboxy)phenyl)phthalimide) copper phthalocyanine

2.      Tetrakis-(N-((4-ethoxycarboxy)phenyl)phthalimide) copper phthalocyanine

3.      Tetrakis-(N-((3,5-dicarboxy)phenyl)phthalimide) copper phthalocyanine

4.      Tetrakis-(N-(4-pyridyl)phthalimide) copper chthalocyanine

5.      Tetrakis-(N-(p-tolyl)phthalimide) copper phthalocyanine

6.      Tetrakis-(N-(pyrazole-4-yl)phthalimide) copper phthalocyanine

7.      Tetrakis-(N-(anthracene-2-yl)phthalimide) copper phthalocyanine

8.      Tetrakis-(N-(pyrene-1-yl)phthalimide) copper phthalocyanine

9.      Tetrakis-(N-(4-(trifluoromethyl)-2H-chromene-2-one-7-yl)phthalimide) copper phthalocyanine The chromophore functionalized materials (7, 8 and 9) were studied by femtosecond fluorescent measurements to understand the interaction of the chromophores with the phtahlocyanine core, as well as inter-Pc interactions. Also, two-photon absorption measurements were carried out to study their non-linear optical properties. All new materials were characterized by electrospray ionization mass spectrometry (ESI-MS) and UV-vis spectroscopy (examples shown in Figures 3 and 4).  ESI-MS confirms that the products are fully substituted on all four sides.  Crystal growing attempts are in progress to obtain single crystals for X-ray diffraction experiments.

Figure 3. Electrospray ionization mass spectrum of tetrakis-(N-(pyrazole-4-yl)phthalimide) copper phthalocyanine.

Figure 4. UV-vis spectra of variously substituted copper phthalocyanine building blocks. To prepare MOFs based on the CuPc* building-blocks shown above, we employed solvothermal synthesis using a variety of metal nodes (Fe, Ni, Cu, Zn, Pb). The microcrystalline materials obtained so far are being characterized by powder X-ray diffraction, and gas sorption measurements. The catalytic activity of copper phthalocyanine octacarboxylic acid (CuPc) in oxidizing organic sulfides (dibutylsulfide, diphenylsulfide, methylphenylsulfide) to sulfoxides and sulfones has been investigated. The eight carboxylic acid groups on the periphery impart solubility in highly polar solvents, such as water, dimethylsulfoxide and dimethylformamide, offering an advantage over simple copper phthalocyanine, which is insoluble in all solvents. We are monitoring the catalytic oxidation of the sulfide by aqueous hydrogen peroxide in dimethylformamide using Nuclear Magnetic Resonance (NMR), and gas chromatography with mass spectrometric detection (GC-MS). An example is given in Figure 5, which shows a much improved conversion (87%) of dibutylsulfide to dibutylsulfoxide, compared to the uncatalyzed reaction (55%).

Figure 5. GC-MS analysis of the reaction mixture obtained by the uncatalyzed (bottom) and 0.5 mol% CuPc-catalyzed (top) oxidation of dibutylsulfide (at 3.25) to dibutylsulfoxide (at 5.41) by hydrogen peroxide after 2 hours.             This ACS PRF grant supports a full-time graduate research assistant, who started working in my lab when the PRF project started. He is greatly benefiting by working on this project: in only a year and a half, he learned new synthesis and purification techniques, as well as instrumentation methods, including nuclear magnetic resonance, UV-vis and fluorescence spectroscopy, mass spectrometry and GC-MS analysis. He already presented his results at two conferences, the 44th ACS Central Regional Meeting, in Mt. Pleasant, MI, and the 15th Annual Chemistry Graduate Research Symposium, at Wayne State University, Detroit, MI.             Working on this project has also impacted my own career in many ways. For the first time in my research group, we are developing catalysts and are studying catalyzed reactions. Also for the first time, we are preparing metal-organic frameworks, and are developing novel materials for gas storage. These new avenues of research introduced me to previously unexploited techniques, such as GC-MS analysis, gas sorption measurements and powder X-ray diffraction.