Singlet-Triplet Inversion in Heptazine and in Polymeric Carbon Nitrides

According to Hund's rule, the lowest triplet state (T₁) is lower in energy than the lowest excited singlet state (S₁) in closed-shell molecules. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a positive singlet-triplet energy gap (Δ_ST). Exceptions are known for biradicals and charge-transfer excited states of large molecules in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are spatially separated, resulting in a small exchange integral. In the present work, we discovered with ADC(2), CC2, EOM-CCSD, and CASPT2 calculations that heptazine (1,3,4,6,7,9,9b-heptaazaphenalene or tri-s-triazine) exhibits an inverted S₁/T₁ energy gap (Δ_ST ≈ -0.25 eV). This appears to be the first example of a stable closed-shell organic molecule exhibiting S₁/T₁ inversion at its equilibrium geometry. The origins of this phenomenon are the nearly pure HOMO-LUMO excitation character of the S₁ and T₁ states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S₁/T₁ inversion is found to be extremely robust, being affected neither by substitution of heptazine nor by oligomerization of heptazine units. Using time-resolved photoluminescence and transient absorption spectroscopy, we investigated the excited-state dynamics of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a chemically stable heptazine derivative, in the presence of external heavy atom sources as well as triplet-quenching oxygen. These spectroscopic data are consistent with TAHz singlet excited state decay in the absence of a low-energy triplet loss channel. The absence of intersystem crossing and an exceptionally low radiative rate result in unusually long S₁ lifetimes (of the order of hundreds of nanoseconds in nonaqueous solvents). These features of the heptazine chromophore have profound implications for organic optoelectronics as well as for water-splitting photocatalysis with heptazine-based polymers (e.g., graphitic carbon nitride) which have yet to be systematically explored and exploited.