Intermolecular Hydrogen Bonding Tunes Vibronic Coupling in Heptazine Complexes

Area of Science:

• Photochemistry and Photophysics
• Materials Science
• Supramolecular Chemistry

Background:

• Aza-aromatic materials, such as graphitic carbon nitride analogs, are crucial molecular photocatalysts.
• Understanding excited-state dynamics is key to optimizing photocatalytic efficiency.
• Hydrogen bonding plays a significant role in modulating molecular properties and reactivity.

Purpose of the Study:

• To investigate the influence of hydrogen bonding on the excited-state landscapes of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz).
• To correlate hydrogen bond strength with changes in vibronic emission spectra.
• To elucidate the molecular modes responsible for spectral changes in hydrogen-bonded TAHz complexes.

Main Methods:

• Synthesis and study of hydrogen-bonded complexes between TAHz and various phenol derivatives (R-PhOH).
• Time-resolved photoluminescence spectroscopy to analyze excited-state emission.
• Displaced-oscillator model fitting to extract vibrational mode parameters (frequencies, Huang-Rhys factors).
• Quantum chemical calculations to identify relevant vibrational modes.

Main Results:

• A spectral component attributed to the R-PhOH-TAHz hydrogen-bonded complex was identified.
• A striking change in the relative amplitude of vibronic peaks was observed with varying phenol substituents.
• The displaced-oscillator model revealed coupling to two dominant vibrational modes.
• Quantum chemical calculations identified heptazine ring-breathing and ring-puckering modes as responsible for the vibronic progression.
• Increasing hydrogen bond strength led to decreased molecular distortion in the excited state.

Conclusions:

• Intermolecular hydrogen bonding strength directly influences the excited-state molecular geometry of TAHz.
• The observed spectral changes are linked to specific vibrational modes of the heptazine core.
• These findings provide crucial insights for controlling excited-state proton-coupled electron transfer and proton transfer reactions in aza-aromatic systems.