Investigating Charge Transfer Complexes in Brown Carbon Aerosols

Aerosols are suspensions of particles in the air, commonly seen as dust or fog in the atmosphere. Brown carbon is a particular classification of carbonaceous atmospheric aerosol that increases in absorption from the visible to ultraviolet region, making it important for radiative forcing models. Elucidating the structures of brown carbon chromophores has been difficult as brown carbon is a broad category and the chromophore type can change depending on emission source, temperature, humidity, and season. Twisted intramolecular charge transfer (TICT) molecules have been identified as potential brown carbon chromophores. TICT molecules are those that allow charge transfer to occur between portions of the same molecule when that molecule is in a particular twisted conformation upon photoexcitation. Therefore, 1-phenylpyrrole (1PhPy) was examined using both resonant two-photon ionization (R2PI) spectroscopy and computational methods as a potential TICT molecule and brown carbon chromophore. These methods were carried out for both 1PhPy as the bare chromophore in the gas phase and with one water-molecule complexation, 1PhPy + 1H2O. Additional calculations were carried out for 1PhPy + 2H2O and 1PhPy + 3H2O. Both the bare chromophore and water clusters were shown to become more twisted in their excited state compared to their ground state analogue. Furthermore, the addition of water solvent decreased the energetic barrier to twisting, facilitating charge transfer. The effect of the water solvation on charge transfer is important for considering how chromophores will behave in the aerosol condensed phase. The presence of a TICT state shifts the absorbance of brown carbon chromophores to overlap more strongly with the solar flux. Developing a molecular-level understanding of the photophysics of brown carbon chromophores with and without water solvent will lead to greater understanding of the outcomes from aerosol solar absorption