The Curious Case of Low-lying States in Non-linear Polyaromatic Hydrocarbons

The prediction of accurate singlet triplet (ST) gaps of polyaromatic hydrocarbons has been challenging due to the differential multireference character of the two states. The ST gaps of linear polyacenes have shown an exponential decay with system size due to the decreasing stability and increasing multireference nature of the singlet state. These low ST gaps can ideally be leveraged towards energy applications but is hindered by the decreasing stability of the system. While non-linear or kinked polyacenes are characterised by higher stability, multireference calculations on these systems are limited. In our work, we show that while the singlet states of kinked polyacenes are markedly less multireference, the triplet states are highly multireference in these systems and therefore, the correct trend of ST gap in the kinked polyacenes requires high-level multireference calculations. We show that unlike linear polyacenes, in the non-linear systems the ST gaps increase marginally with system size. The ST gaps also show absolutely no correlation with HOMO-LUMO gaps. These surprising trends are a combined effect of the non-linear connections (topology) and the geometrical factors. These results are in stark contrast to the observations in linear polyacenes.