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.
To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H2O versus D2O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H2O.
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.
Recently, a derivative of the heptazine (tris-triazine) molecule, trianisole-heptazine (TAHz), was synthesized and was shown to catalyze the oxidation of water to hydroxyl radicals under 365 nm LED light in a homogeneous reaction (E. J. Rabe et al., J. Phys. Chem. Lett2018962576261). The possibility of water photo-oxidation with a precisely defined molecular catalyst in neat solvents opens new perspectives for clarifying the fundamental reaction mechanisms involved in water oxidation photocatalysis. In the present work, the effects of chemical substituents on the three CH positions of Hz on the photocatalytic reactivity were explored with wave function-based ab initio electronic-structure calculations for hydrogen-bonded complexes of Hz and three selected Hz derivatives (TAHz, trichloro-Hz, and tricyano-Hz) with a water molecule. While anisole is an electron-donating substituent, Cl is a weakly electron-withdrawing substituent and CN is a strongly electron-withdrawing substituent. It is shown that the barrier for the photoinduced abstraction of an H atom from the water molecule is raised (lowered) by electron-donating (electron-withdrawing) substituents. The highly mobile and reactive hydroxyl radicals generated by water oxidation can recombine with the reduced chromophore radicals to yield photohydrates. The effect of substituents on the thermodynamics of the photohydration reaction was computed. Among the four chromophores studied, TAHz stands out on account of the metastability of its photohydrate, which suggests self-healing of the photocatalyst after oxidation of TAHzH radicals by OH radicals. In addition, the effect of substituents on the H atom photodetachment reaction from the reduced chromophores, which closes the catalytic cycle, has been investigated. The energy of the repulsive 2πσ* state, which drives the photodetachment reaction is lowered (raised) by electron-donating (electron withdrawing) substituents. All four chromophores exhibit inverted S1/T1 gaps. This feature eliminates long-lived triplet states and thus avoids the activation of molecular oxygen to highly reactive singlet oxygen.
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