Barrierless Heptazine-Driven Excited State Proton-Coupled Electron Transfer: Implications for Controlling Photochemistry of Carbon Nitrides and Aza-Arenes

Abstract

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.

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

“…25,[87][88][89][90][91][92] For the Hz-phenol system in toluene solution, we have found clear spectroscopic evidence for the formation of hydrogen bonds and for the relevance of hydrogen bonding for the intra-complex PCET reaction. 50 The essential role of hydrogen bonding for excited-state PCET is also confirmed by the ab initio calculations. On the other hand, the results for functionalized phenols reveal that the strength of the ground-state hydrogen bond is not the decisive factor controlling the PCET reactivity.…”

“…The effect of hydrogen bonding can be observed in the absorption spectrum of TAHz, as is shown in Fig.9. Addition of 100 mM PhOH to TAHz in toluene causes a distinct red-shift of the absorption threshold of TAHz near 400 nm 50. On the other hand, weak peaks arising from nπ* states are blue-shifted due to hydrogen bonding and gain intensity, see the inset in Fig.9.…”

“…On the other hand, weak peaks arising from nπ* states are blue-shifted due to hydrogen bonding and gain intensity, see the inset in Fig.9. From the shift of the absorption spectrum as a function of PhOH concentration, the association constant of TAHz and PhOH can be determined 50.…”

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

“…The electronic population in the S1 state survives for tens or hundreds of nanoseconds due to the low fluorescence rate and the absence of S1 quenching by ISC. 10,34,72 A substantial fraction of the energy of the absorbed photon can thus be stored in the S1 state and is available for excited-state PCET reactions with the substrate.…”

Towards Metal-free Photocatalysis: Photochemical Regeneration of Organic Hydride Donors Using Phenazine-based Photosensitizers

“…While thermodynamic arguments (Scheme 2) suggest the proton-coupled electron transfer PC1(S1) + BIM + + PhOH → PC1 •+ + BIMH •+ + PhO − is expected to be a spontaneous process, the likely diffusion-limited charge separation and recombination dynamics obscure the role of PCET in this system. [42][43][44][45] The nsTA kinetics of PC1 in the presence of SD1 are shown in Figures S12 and S13. The global analysis of the data indicates that only S1 and T1 states are detected as transients.…”