Photochemical Hydrogen Storage with Hexaazatrinaphthylene

Area of Science:

• Photochemistry
• Organic Chemistry
• Spectroscopy

Background:

• Hexaazatrinaphthylene (HATN) is a polycyclic aromatic hydrocarbon with unique electronic properties.
• Understanding the photochemical behavior of HATN is crucial for developing new light-driven reactions.

Purpose of the Study:

• To investigate the photochemical hydrogenation of HATN in various alcohol solvents.
• To elucidate the mechanism of hydrogen atom transfer and identify photoproducts.

Main Methods:

• UV-Vis irradiation of HATN in alcohols (methanol, ethanol, isopropanol, water).
• Fluorescence decay time measurements and kinetic isotope effect analysis.
• Electron paramagnetic resonance (EPR) and Nuclear Magnetic Resonance (NMR) spectroscopy.
• Mass spectrometry for photoproduct identification.

Main Results:

• HATN efficiently extracts hydrogen atoms from alcohols, forming long-lived hydrogenated species.
• A kinetic isotope effect of 1.56 in deuterated methanol suggests the involvement of the lowest singlet excited state in hydrogen transfer.
• Photochemical hydrogenation was observed in methanol, ethanol, and isopropanol, but not in water.
• Hydrogenated HATN was detected optically (1.78 eV absorption band) and via EPR/NMR.
• Mass spectrometry revealed di-hydrogenated HATN, methoxylated, and methylated HATN, indicating radical pathways.

Conclusions:

• The excited state of the HATN-solvent complex facilitates barrierless hydrogen transfer from methanol.
• The photochemical reaction is solvent-dependent, with water being unreactive.
• Theoretical calculations support the experimental findings regarding hydrogen transfer barriers.