Solvent-polarity-tunable dimeric association of a fullerene (C <inf>60</inf>)-N,N-dimethylaminoazobenzene dyad: Modulated electronic coupling of the Azo chromophore with a substituted 3D fullerene

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 × 10-5 M THF solution of DPNME at a mixed dielectric constant Emix ≈ 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 Emix ≈ 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 Emix=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. The 3D effect: Substitution of fullerene C60 with a J-aggregate-forming azobenzene-based chromophore yields an antiparallel H-type dimer, as evidenced from XRD and electronic structure calculations (see picture). The results reveal a novel C60-azobenzene chromophoric system for the construction of supramolecular assemblies with optoelectronically active constituents. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.