Synthesis and spectroscopic properties of phthalocyanine-[60]fullerene conjugates connected directly by means of a four-membered ring

Takamitsu Fukuda, Yuu Kikukawa, Shiori Takaishi, Nagao Kobayashi

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

New covalently C 60-conjugated phthalocyanine (Pc) analogues in which the Pc and C 60 components are connected by means of a four-membered ring have been synthesized by taking advantage of a [2+2] cycloaddition reaction of C 60 with benzyne units generated from either a phthalocyanine derivative (8) or its precursor (1). The reaction of 1 with PhI(OAc) 2 and trifluoromethanesulfonic acid (TfOH) followed by the [2+2] cycloaddition of C 60 in the presence of tetra-n-butylammonium fluoride (TBAF) yielded the C 60-substituted Pc precursor (3). Mixed condensation of 3 and 4,5-dibutylsulfonylphthalonitrile (4) in a thermally promoted template reaction using a nickel salt successfully gave the Pc-C 60 conjugate (5). Results of mass spectrometry and 1H and 13C NMR spectroscopy clearly indicate the formation of the anticipated Pc-C 60 conjugate. Direct coupling of C 60 with the Pc analogue that contained eight peripheral trimethylsilyl (TMS) groups (8) also proceeded successfully, such that mono and bis C 60-adducts were detected by their mass, although the isolation of each derivative was difficult. The absorption and magnetic circular dichroism (MCD) spectra of 5 and the reference compound (7) differ from each other in the Q-band region, thereby suggesting that the presence of the C 60 moiety affects the electronic structure of the conjugate. The reduction and oxidation potentials of 5 and 7 obtained by cyclic voltammetry are comparative, except for the C 60-centered reduction couple at -1.53 V versus Fc +/Fc in o-dichlorobenzene (o-DCB). A one-electron reduction of 5 and 7 in tetrahydrofuran (THF) by using the sodium mirror technique results in the loss of band intensity in the Q-band region, whereas the characteristic marker bands for Pc-ring-centered reduction appear at around 430, 600, and 900 nm for both compounds. The final spectral shapes of 5 and 7 upon the reduction resemble each other, thus indicating that no significant molecular orbital (MO) interactions between the C 60 and Pc units are present for the reduced species of 5. In contrast, the oxidized species of 5 and 7 generated by the addition of NOBF 4 in CH 2Cl 2 show significantly different absorption spectra from each other. Whereas the broad bands at approximately 400-550 nm of 7 + are indicative of the cationic π-radical species of metallo-Pcs and can be assigned to a transition from a low-lying MO to the half-filled MO, no corresponding bands were observed for 5 +. These spectral characteristics have been tentatively assigned to the delocalized occupied frontier MOs for 5 +. The experimental results are broadly supported by DFT calculations.

Original languageEnglish
Pages (from-to)751-758
Number of pages8
JournalChemistry - An Asian Journal
Volume7
Issue number4
DOIs
Publication statusPublished - 2012 Apr 1

Keywords

  • benzyne
  • cycloaddition
  • electronic structure
  • fullerenes
  • phthalocyanines

ASJC Scopus subject areas

  • Biochemistry
  • Organic Chemistry

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