Molecular Design of Heptazine-Based Photocatalysts: Effect of Substituents on Photocatalytic Efficiency and Photostability

Abstract

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.

Organic molecules with inverted gaps between first excited singlet and triplet states and appreciable fluorescence rates

“…The observation that systematic substitution of C-H by N in 1 and 18 leads to many new INVEST molecules partly supports the prediction made in a very recent paper that the inverted singlet-triplet gap is an intrinsic property of isoelectronic compounds of 2. 164 However, the statement proves to be too optimistic as a significant fraction of these derivatives lose the inverted singlet-triplet gap as demonstrated in Figure 1 and Figure 5. We also realized that the introduction of an increasing number of nitrogen atoms at appropriate positions leads to a gradual increase of the vertical excitation energy of the first excited singlet.…”

Singlet‐Triplet Excited‐State Inversion in Heptazine and Related Molecules: Assessment of TD‐DFT and ab initio Methods

“…These properties are driven by short‐range charge‐transfer, [11] which spatially separate the hole and the electron densities on neighbouring atomic sites while ensuring a high polarizability [12] . Oligomerization of heptazine is also a promising and versatile route towards catalysis for hydrogen production and water splitting [13–15] and CO 2 uptake, [16] as well as substitution at the corners to improve the photocatalytic performance [17] . This scenario allows to foresee a blooming of studies for energy conversion and storage, as well as for industrial and environmental applications of heptazine derivatives and oligomers [18,19] …”

Can Hydrated Electrons be Produced from Water with Visible Light?

“…The feasibility of this reaction depends primarily on the barrier of the H-atom transfer reaction in the long-lived S 1 (ππ*) state of Hz, as discussed in detail in recent publications. [43,45,46] In the second photoreaction, the excess hydrogen atom of the HzH radical is transferred to the water environment as discussed in the present work. In this reaction, the electron in the π-type SOMO of the HzH radical is photoexcited to a σ*-type orbital which drives a barrierless transfer of the excess hydrogen atom of HzH to a hydrogen-bonded water molecule, generating a hydrated H 3 O radical.…”

“…There are various possibilities of shifting the absorption maximum of the lowest bright 1 ππ* state of Hzbased chromophores towards the visible spectrum as well as of increasing the oscillator strength of this state. [45,46] The reduced Hz chromophore (HzH) inherently absorbs in the visible. Exploratory computational screening studies are underway in our laboratory to identify Hz-based chromophores with optimal properties for driving PCET reactions in pure water with visible light.…”

“…[45] The absorption spectra of Hz as well as of the HzH radical can be tuned into the visible by suitable substituents. [46] These considerations suggest that the formation of hydrated electrons from water with visible light may tentatively be possible.…”