According to Hund's rule, the lowest triplet state (T 1 ) is lower in energy than the lowest excited singlet state (S 1 ) 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 (1,3,4,6,7,9,9bheptaazaphenalene or tri-s-triazine) exhibits an inverted S 1 /T 1 energy gap (Δ ST ≈ −0.25 eV). This appears to be the first example of a stable closedshell organic molecule exhibiting S 1 /T 1 inversion at its equilibrium geometry. The origins of this phenomenon are the nearly pure HOMO− LUMO excitation character of the S 1 and T 1 states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S 1 /T 1 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)-1,3,4,6,7,9,9bheptaazaphenalene (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 S 1 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 heptazinebased polymers (e.g., graphitic carbon nitride) which have yet to be systematically explored and exploited.

Impressive progress has recently been achieved in photocatalytic hydrogen evolution with polymeric carbon nitride materials consisting of heptazine building blocks. However, the fundamental mechanistic principles of the catalytic cycle are as yet poorly understood. Here, we provide first-principles computational evidence that water splitting with heptazine-based materials can be understood as a molecular excited-state reaction taking place in hydrogen-bonded heptazine-water complexes. The oxidation of water occurs homolytically via an electron/proton transfer from water to heptazine, resulting in ground-state heptazinyl and OH radicals. It is shown that the excess hydrogen atom of the heptazinyl radical can be photodetached by a second photon, which regenerates the heptazine molecule. Alternatively to the photodetachment reaction, two heptazinyl radicals can recombine in a dark reaction to form H, thereby regenerating two heptazine molecules. The proposed molecular photochemical reaction scheme within hydrogen-bonded chromophore-water complexes is complementary to the traditional paradigm of photocatalytic water splitting, which assumes the separation of electrons and holes over substantial time scales and distances.

To inform prospective design rules for controlling aza-arene photochemistry, we studied hydrogen-bonded complexes of 2,5,8-tris­(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst chemically related to graphitic carbon nitride, with a variety of phenol derivatives. We have focused on excited state proton-coupled electron transfer (ES-PCET) reactions of heptazines because the excited state properties governing this process remain conceptually opaque compared to proton reduction reactions for these materials. We used ground-state absorption, time-resolved photoluminescence, and ab initio quantum chemical calculations to analyze TAHz reactivity toward a series of six para-substituted phenol derivatives. We determined association constants (K A), excited-state quenching rate constants (k Q), kinetic isotope effects, and transition-state barriers (ΔE ⧧). From this data, we provide a generalizable picture of hydrogen bond formation and excited state reactivity of heptazine-based materials with hydrogen-atom donating solvents. These results provide important insights into strategies to tune charge transfer state energies and increase ES-PCET rates.

Polymeric carbon-nitride materials consisting of triazine or heptazine units have recently attracted vast interest as photocatalysts for water splitting with visible light. Adopting the hydrogen-bonded triazine-water complex as a model system, we explored the photochemical reaction mechanisms involved in the water splitting reaction in this system, using wavefunction-based ab initio electronic-structure methods. It is shown that photoexcited triazine can abstract a hydrogen atom from the water molecule by the sequential transfer of an electron and a proton from water to triazine, resulting in the triazinyl-hydroxyl biradical in the electronic ground state. It is furthermore shown that the excess hydrogen atom of the triazinyl radical can be photodetached by a second photon, which regenerates the triazine molecule. The hydrogen-bonded water molecule is thus decomposed into hydrogen and hydroxyl radicals in a biphotonic photochemical reaction. These results shed light on the molecular mechanisms of the water-oxidation reaction catalyzed by triazine-based organic polymers.