Octakis(dodecyl)phthalocyanines: Influence of Peripheral versus Non-Peripheral Substitution on Synthetic Routes, Spectroscopy and Electrochemical Behaviour

Abstract

Non-peripherally octakis-substituted phthalocyanines (npPc’s), MPc(C12H25)8 with M = 2H (3) or Zn (4), as well as peripherally octakis-substituted phthalocyanines (pPc’s) with M = Zn (6), Mg (7) and 2H (8), were synthesized by cyclotetramerization of 3,6- (2) or 4,5-bis(dodecyl)phthalonitrile (5), template cyclotetramerization of precursor phthalonitriles in the presence of Zn or Mg, metal insertion into metal-free phthalocyanines, and removal of Mg or Zn from the phthalocyaninato coordination cavity. The more effective synthetic route towards pPc 8 was demetalation of 7. npPc’s were more soluble than pPc’s. The Q-band λmax of npPc’s was red-shifted with ca. 18 nm, compared to that of pPc’s. X-ray photoelectron spectroscopy (XPS) differentiated between N–H, Nmeso and Ncore nitrogen atoms for metal-free phthalocyanines. Binding energies were ca. 399.6, 398.2 and 397.7 eV respectively. X-ray photoelectron spectroscopy (XPS) also showed zinc phthalocyanines 4 and 6 have four equivalent Nmeso and four equivalent N–Zn core nitrogens. In contrast, the Mg phthalocyanine 7 has two sets of core N atoms. One set involves two Ncore atoms strongly coordinated to Mg, while the other encompasses the two remaining Ncore atoms that are weakly associated with Mg. pPc’s 6, 7, and 8 have cyclic voltammetry features consistent with dimerization to form [Pc][Pc+] intermediates upon oxidation but npPc’s 3 and 4 do not. Metalation of metal-free pPc’s and npPc’s shifted all redox potentials to lower values.

Ferrocene-Bearing Dodecylphthalocyanines: Synthesis, Spectroscopic and Electrochemical Behavior

“…Recently, we reported the homo-cyclotetramerization of 3,6- as well as 4,5-bis­(dodecyl)­phthalonitrile, 2 and 3 , to generate the nonperipherally substituted octadodecylated phthalocyanine, 6a (see Scheme for 6a structure), and its peripherally substituted isomer, 6f (not shown in Scheme ). With the exclusion of short-chain alcohols, phthalocyanine 6a was more soluble in common organic solvents than the peripherally substituted isomer, 6f . Here, phthalocyanine 6a was obtained in a statistical condensation reaction, Scheme .…”

“…Spectrochemical-grade dichloromethane (DCM) and tetrahydrofuran (THF) for use in electrochemistry were stored under argon. 4,5-Bis­(dodecyl)­phthalonitrile, 2 , and 3,6-bis­(dodecyl)­phthalonitrile, 3 , were synthesized as described previously . The synthesis of the new compounds 4-(4′-ferrocenylbutoxy)­phthalonitrile, 1 , and 3-(4′-ferrocenylbutoxy)­phthalonitrile, 4 , is described in the Supporting Information.…”

“…Complex 7a was previously synthesized from 6a and characterization thereof has been documented . HR-MS calcd for [C 128 H 208 N 8 O 8 Zn + H] + : 1921.5913; found, 1921.5919.…”

“…PDT applications are especially feasible if photosensitization and subsequent reactive oxygen species generation can occur between 650 and 780 nm. , In particular, the light excitation wavelength of Photofrin (630 nm) allows for tissue penetration depths of around 5 mm, while the light excitation wavelength of some metallophthalocyanine complexes have been found to be 675 nm, which can penetrate up to 10 mm of bodily tissue . Nonperipheral substituents have been shown to red-shift the wavelength of maximum absorption (Q-band) of a series of octa-alkyl zinc phthalocyanines by as much as 20 nm from 680 to 700 nm compared to their peripherally substituted derivatives . This is beneficial in PDT as photosensitization can then occur at longer wavelengths where light penetrates body tissue much deeper.…”

“…Electrochemical studies of the dodecylated metal-free derivative revealed four one-electron ring-based redox processes in dichloromethane. Two of these were reductions, while the remainder were oxidations …”