Solvent-Polarity-Tunable Dimeric Association of a Fullerene (C60)–N,N-Dimethylaminoazobenzene Dyad: Modulated Electronic Coupling of the Azo Chromophore with a Substituted 3D Fullerene

Authors

  • K. Senthil Kumar,

    1. Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 (India), Fax: (+91) 44-2257-4202
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  • Prof. Archita Patnaik

    Corresponding author
    1. Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 (India), Fax: (+91) 44-2257-4202
    • Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 (India), Fax: (+91) 44-2257-4202
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    • Currently Fulbright–Nehru Visiting Research Professor at the Lawrence Berkeley National Laboratory, Berkeley, CA (USA).


Abstract

The tunable self-assembly of a fullerene (C60)–N,N-dimethylaminoazobenzene (DPNME) molecular system as a function of solvent polarity in THF/water binary solvent is reported. Gradual increase of the volume fraction of the nonsolvent water in a 1×10−5M THF solution of DPNME at a mixed dielectric constant εmix≈42 resulted in initial redshifting of the 1(π→π*) absorption band, which signified the 1D head-to-tail or J-type arrangement of the DPNME molecular system. Further increase in the solvent polarity to εmix≈66 evidenced formation of an antiparallel head-to-tail or H-type molecular arrangement in conjunction with the J-aggregates, thereby establishing a solvent-polarity-dependent dynamic equilibrium between the monomer ↔ J-aggregate ↔ H-aggregate. The controlled aggregation was governed by the synergetic effect of intermolecular donor–acceptor interaction between the electron-deficient fullerene ring and the electron-rich N,N-dimethylamino-substituted aromatic ring; typically, van der Waals and π–π interactions between the molecules constituting a pair of dimers were envisaged. An agreement between the semiempirically calculated drastically reduced oscillator strength of the DPNME H-dimer in the antiparallel configuration (0.69 vs. 1.29 in the monomeric DPNME) and the experimental electronic absorption spectra beyond εmix=66 further strengthened this assignment to the hitherto forbidden antiparallel H-dimer. Complementing the above, the periodicity of molecular self-assembly dictated a monoclinic unit cell in the single-crystal XRD packing pattern with a C2/c space group; the molecules packed laterally with mutual interdigitation with the donor (E)-N,N-dimethyl-4-(p-tolyldiazenyl)aniline (AZNME) parts in an antiparallel fashion (contrary to the usual expectation for H-aggregates) with strong inter- and intrapair van der Waals and π–π interactions between the constituent fullerene moieties. Unlike those of porphyrin/phthalocyanine bowl-like donor-initiated architectures, a rare class of DPNME dyadic supramolecular self-assemblies was realized with π-extended 2D fullerene networks, in which the linear geometry of the AZNME donor and the conformational rigidity of the fullerene acceptor played crucial roles.

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