Large spectral shifts of electronic transitions in MoSI molecular wire dispersions as a function of bundle diameter

A systematic study of optical absorption spectra of Mo6S9−xIx (x = 6) molecular wire dispersions in ethanol, fractionated into different bundle diameter populations shows that electronic transitions shift significantly as a function of bundle diameter. Two electronic transitions show significant shifts: the Mo–S charge transfer peak shifts from 1.8 to 1.5 eV and the next inter-band transition shifts from 2.7 to 2.4 eV with increasing bundle diameter d, in the range 5–100 nm. This empirical observation hugely simplifies characterization of Mo6S9−xIx wire dispersions according to diameter, opening the way to rapid advances in processing of these materials. We discuss the possible origin of the shift, dismissing quantum size effects, impurities and solvatochromism as well as stoichiometric variations between x = 6 and x = 4.5.