Curious Case of Singlet Triplet Gaps in Nonlinear Polyaromatic Hydrocarbons

Abstract

The singlet triplet (ST) gap of linear polyacenes decays exponentially with the system size as a result of extended conjugation and reducing highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps. These low ST gaps can ideally be leveraged toward energy applications but are hindered by the decreasing stability of the systems. Thus, there is the need to understand the ST gap of nonlinear polyacenes, which are markedly more stable than their linear counterparts. Here, we show that the ST gaps of the nonlinear polyacenes do not decrease with the system size and have no correlation with the HOMO–LUMO gaps or increased conjugation. The reason behind this is identified as the high multireference character of the triplet high-spin state. These unprecedented results are in stark contrast to the observations in linear polyacenes and are due to the combined effects of topology and geometrical factors.

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“…It could be because of the stronger multireference nature present in helicene. It is worth stressing that OBMP2 and O2BMP2 can reproduce DMRG prediction on the less dependence of ST gaps on the system size for helicene, which has not been observed by single-reference methods like DFT. , In Figure , we plot the spin densities of helicene[3] and helicene[4]. While MP2 spin densities are delocalized over the structures, OBMP2 ones are localized along the preferentially stable double bonds, entirely consistent with DMRG prediction …”

“…The last set we used to test the OBMP2 and O2BMP2 predictions of the ST gap is polyaromatic hydrocarbons (PAHs). The prediction of accurate ST gaps for polyaromatic hydrocarbons has been challenging for computational methods. − While the ST gaps of linear PAHs have shown an exponential decay with system size, those of the nonlinear PAHs are marginally sensitive to system size . Unfortunately, the latter has not been observed by single-reference methods like DFT .…”

“…ST gaps of linear polyacenes (left) and helicene (right) from different methods. The DMRG references are taken from ref . The basis set cc-pVDZ was used.…”

“…49−53 While the ST gaps of linear PAHs have shown an exponential decay with system size, those of the nonlinear PAHs are marginally sensitive to system size. 53 Unfortunately, the latter has not been observed by singlereference methods like DFT. 54 Dey and Ghosh have attributed the failure of DFT to the multireference nature of each state of nonlinear PAHs.…”

“…While MP2 spin densities are delocalized over the structures, OBMP2 ones are localized along the preferentially stable double bonds, entirely consistent with DMRG prediction. 53 We now move on to assess the performance of our methods in the prediction of molecular IPs. Here we use the Koopmans' approximation, in which IPs are derived from a direct difference between the energy expressions of the neutral and ionized states.…”