Intramolecular electronic communication in a dimethylaminoazobenzene-fullerene C60 dyad: an experimental and TD-DFT study.

An electronically push-pull type dimethylaminoazobenzene-fullerene C(60) hybrid was designed and synthesized by tailoring N,N-dimethylaniline as an electron donating auxochrome that intensified charge density on the beta-azonitrogen, and on N-methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4' positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor-bridge-acceptor dyad were studied experimentally and by performing TD-DFT calculations. The TD-DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV-vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the N=N group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n((N=N)) (nonbonding) and pi ((N=N)) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C(60) were analyzed by comparing the TD-DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)-N,N-dimethyl-4-(p-tolyldiazenyl)aniline (AZNME). The n((N=N)) --> pi*((N=N)) and pi((N=N)) --> pi*((N=N)) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of pi((N=N)) --> pi*((N=N)) excitation energy closer to the n --> pi*((N=N)) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid-like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the pi((N=N)) --> pi*((N=N)) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD-DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework.