Origin of Chiroptic Amplification in Perylene-Diimide Helicenes

Molecules with a large chiroptic response in the visible range are key in the design of new efficient chiral optoelectronic devices. It has recently been shown that NP3H, a helicene-based nanoribbon with three perylene diimides connected by two naphthalene units, exhibits a huge amplification of the electronic circular dichroism response at 400 nm with respect to NPDH, its analogue with only two perylene diimides bridged by a single naphthalene. Here we show that time-dependent density functional theory can reproduce this experimental feature, attributing it to a strong increase in the magnetic transition dipole moments of the involved electronic states in NP3H. A more qualitative understanding of the phenomenon is obtained with a number of readily accessible computational tools. An analysis of the difference in the electronic densities of the excited and ground states indicates that the key units in explaining the enhancement in electronic circular dichroism are not the perylene diimides but the idealized double[6]helicene substructure of NP3H, formed by the naphthalenes and the adjacent rings within the perylene diimides. Such an analysis further suggests that the amplification arises from an exciton-like coupling, not possible in NPDH where only one [6]helicene subunit exists. Computations on different fragments of NP3H and the application of simple exciton models provide additional support for this mechanism. These results show that this double[6]helicene unit could represent a key core in the design of new chiral conjugated materials with enhanced chiroptical properties in the visible range.