Tunable electronic properties of a proton-responsive N,N-dimethylaminoazobenzene fullerene (C60) dyad

A basic N,N-dimethylaminoazobenzene-fullerene (C(60)) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo- and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor-bridge-acceptor single-molecular skeleton, connecting the electron acceptor N-methylfulleropyrrolidine with an electron donor N,N-dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time-dependent density functional theory (TD-DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm (1)(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated β and α nitrogens, respectively, along with a finite weight of the C(60) pyrrolidinic nitrogen. Calculations reveal intermixing of n((N=N))→π*((N=N)) and charge transfer (CT) transitions in the neutral dyad, whereas, the n((N=N))→π*((N=N)) transition in the protonated dyad is buried under the dominant (1)(π →π*) feature and is red-shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N-dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α- and β-protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N-methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor-centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.