Proton-Coupled Electron Transfer from Water to a Model Heptazine-Based Molecular Photocatalyst

Abstract

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.

A straightforward synthesis of a new family of molecules: 2,5,8-trialkoxyheptazines. Application to photoredox catalyzed transformations

“…Especially, their fluorescence emission 3 has been exploited in liquid crystalline materials 4 and OLEDs devices. 5 Finally, even more attracting, heptazine polymers have demonstrated outstanding photocatalytic properties, 6,7 and the heptazine core might well be one of the rare molecular platforms which could give birth to molecular catalysts able to perform water splitting [8][9][10] , the holy Grail of all photocatalytic processes. Two recent reviews have made a point on this highly interesting family of heterocyclic compounds 11,12 .…”

Photoinduced water oxidation in pyrimidine–water clusters: a combined experimental and theoretical study

“…7,8 To gain insight into the fundamental photophysics of heptazine-based materials, a heptazine-derived molecular photocatalyst, tri-anisole-heptazine (TAHz), was recently synthesized by Schlenker and coworkers. 9 It was shown that photoexcited TAHz can oxidize water via an H-atom transfer reaction, resulting in the quenching of the excited state of TAHz and the liberation of OH radicals that were detected using terephthalic acid. 9 These findings provide evidence that photooxidation of water with heptazine does not inherently require transport of charge carriers in polymeric or crystalline materials, but rather may occur as a molecular photochemical reaction in homogeneous solution.…”

Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

“…Pyridine and its larger analogs, such as quinoline or acridine, are weak bases that can be protonated at the nitrogen atom in the excited state, suggesting that such photocatalytically active CDs contain photobasic moieties in their structure. Interestingly, heptazine (tri- s -triazine), the building block of graphitic carbon nitrides (g-CN), also contains multiple N atoms at the edge sites of the aromatic structure and exhibits photobasic behavior 24 , 25 . Recent calculations show that in the excited state heptazine molecules induce proton and electron transfer from hydrogen-bonded water molecules leading to heptazinyl radicals 26 , 27 .…”