Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

Abstract

We present a conspectus of recent joint spectroscopic and computational studies which provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario which unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Triangular boron carbon nitrides: an unexplored family of chromophores with unique properties for photocatalysis and optoelectronics

“…should also be noted that the magnitude of ΔST increases with increasing size of the triangular BCNs, whereas the magnitude of ΔST deceases when Hz is substituted with pendants and the S1/T1 inversion can be lost for somewhat larger aromatic substituents at the CH positions. 41 Comparing For the functionality of triangular BCNs in organic optoelectronics, only the S1 and T1 states are of relevance. By the robust inversion of the energies of the S1 and T1 states, all BCNs are predestinated as chromophores for improved OLEDs.…”

“…38 Tuning the energy of the bright 1 ππ* state closer to the maximum of the solar spectrum and the energy of the S1 state closer to the thermodynamic limits of the water-splitting reactions could result in a significant boost of the quantum efficiency of water splitting beyond the current value of ≈ 1%. 39 While modifications of the optical properties of the Hz chromophore by substitutions at the three CH groups have been explored in computational 40,41 and spectroscopic 10,21 studies, the range of tuning of the excitation energies is limited if the highly desirable S1/T1 inversion is to be preserved. In this communication, we propose a novel scenario for developing chromophores with tailored properties for optoelectronics as well as for photocatalysis.…”

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.…”

“…For example, the excitation energy of the lowest bright state of TAHz is lower than the excitation energy of the lowest bright state of Hz by 0.8 eV. [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.…”