Photochemical Relaxation Pathways in Dinitropyrene Isomer Pollutants

Area of Science:

• Environmental Chemistry
• Photochemistry
• Physical Chemistry

Background:

• Dinitropyrenes are common polycyclic aromatic pollutants.
• Their environmental photodegradation is known, but substitution effects on relaxation pathways are understudied.

Purpose of the Study:

• Investigate excited-state dynamics of 1,3- and 1,8-dinitropyrene isomers.
• Compare relaxation pathways with the 1,6-dinitropyrene isomer.
• Understand the impact of nitro-group substitution patterns.

Main Methods:

• Femtosecond-to-microsecond spectroscopy.
• Steady-state measurements.
• Ground- and excited-state density functional calculations.

Main Results:

• Excitation at 425 nm leads to rapid S1 state branching.
• High yield (~90%) of triplet state formation and low yield (<10%) of nitropyrenoxy radical.
• Nitro-group position affects spectra and various yields (fluorescence, triplet, photodegradation, radical formation) but not the core relaxation mechanism.

Conclusions:

• Nitro-group position significantly influences dinitropyrene photodegradation pathways and properties.
• Radical formation is key to photodegradation, with direct triplet state reactions occurring in the presence of hydrogen donors.