Can Heteroatoms and Heteroareno Annelations Make Pentalenes Suitable as Singlet Fission Chromophores?

Abstract

Pentalene derivatives were earlier hypothesised to have potential as singlet fission (SF) chromophores, but their function is hampered by both the symmetry-forbidden transition to the first singlet excited state (S₁) and the thermal instability due to antiaromatic character in their ground state (S₀). A possible benefit is a Baird-aromatic character in the lowest ππ* excited states which may provide for higher photochemical stability than present SF chromophores. In this computational work, we explored different heteroatomic replacements on the pentalene skeleton as well as heteroareno fusions with the aim to identify derivatives with (i) allowed transitions to S₁, (ii) reduced antiaromatic character in S₀, (iii) highly Baird-aromatic pentalene cores in T₁ and S₁, and (iv) the SF energy criteria (2 ≤ E(S₁)/E(T₁) and E(S₁) < E(T₂)) fulfilled. The results show that the symmetry-forbidden nature of the transition to S₁ can be lifted, whilst maintaining excited state Baird-aromatic character providing for photostability. However, there are other drawbacks that impede usage of pentalenes as SF chromophores: (i) the E(S₁)/E(T₁) ratios when adiabatic S₁ states are considered are in nearly all cases well below 2 as the pentalene cores exhibit large relaxation energies in this state (thus, both adiabatic E(S₁) and E(T₁) must be used for Baird-aromatic SF chromophore candidates), and (ii) competing decay pathways exist due to accessible S₁/S₀ conical intersections. Hence, the design of pentalene derivatives that are suitable as SF chromophores will be challenging. Still, our findings may pave the way to other related species for which all drawbacks can be avoided.