Pi-conjugated organic semiconductors form a highly versatile class of materials with tremendous promise for the development of cheap, flexible and non-toxic opto-electronic devices such as photovoltaics, sensors and solid state lighting. However, bulk organic materials are often highly amorphous and suffer from strong electronic disorder that limits their performance as light-harvesting materials.

Recently, a striking solution for this has appeared in the form of ’on-surface’ synthesis techniques which allow molecular building blocks and their emergent solid state properties to be rationally designed, fabricated and characterized for functional material applications.

Poly-acenes polymers

Recently, a controllable topological (Z2) phase transition has been demonstrated by varying the monomer size and chain length in a series of ethynylene-bridged poly-acene polymers deposed on Au(111). However the impact of topology on the excitonic properties of acene-based light harvesting structures has, to date, remained unexplored and unmeasured.

By using state-of-the-art manybody perturbation theory techniques (DFT, self-consistent GW and the Bethe-Salpeter equation), I have shown that the topological Z2 phase transition occurring in these systems is accompanied by a topological excitonic phase transition: the band inversion in the non-trivial phase yields real-space exciton wave functions in which electrons and holes exchange orbital characters with respect to the trivial phase. The topological excitonic phase transition results in a broad tunability of the singlet-triplet splittings, opening appealing perspectives for the occurrence of singlet fission. Finally, the flatness of the single-particle electronic structure in the topological non trivial phase leads to negatively dispersing triplet excitons in a large portion of the Brillouin zone, opening a route for spontaneously coherent energy transport at room temperature.