Electronic Spectra of Cryogenically Cooled Na⁺-Pyrene and K⁺-Pyrene Complexes

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Pyrene is a polycyclic aromatic hydrocarbon with well-defined electronic and vibrational spectra.
• Metal-cation interactions with aromatic systems can significantly alter their photophysical properties.
• Understanding these interactions is crucial for designing new materials and understanding molecular recognition.

Purpose of the Study:

• To investigate the structure and electronic properties of sodium-pyrene (Na⁺-pyrene) and potassium-pyrene (K⁺-pyrene) complexes.
• To elucidate the influence of metal cations on the vibrational and electronic transitions of pyrene.
• To compare experimental spectroscopic data with theoretical density functional theory (DFT) calculations.

Main Methods:

• Two-color resonance enhanced photodissociation action spectroscopy was performed on mass-selected ions in a cryogenic ion trap.
• Density functional theory (DFT) calculations were employed to predict structures, binding energies, and spectral properties.
• Analysis focused on the electronic transitions S₁(B_2u) ← S₀(A_g) and S₂(B_1u) ← S₀(A_g) of the pyrene chromophore.

Main Results:

• Both Na⁺-pyrene and K⁺-pyrene complexes adopt predicted π-bound structures with the cation above the pyrene plane.
• Calculated binding energies are 10570 cm^-1 for Na⁺-pyrene and 8150 cm^-1 for K⁺-pyrene.
• Electronic spectra show shifts in S₁ ← S₀ and S₂ ← S₀ transitions, with altered dominance of specific vibrational modes (a_g vs. b_3g) compared to bare pyrene.

Conclusions:

• Metal cations (Na⁺, K⁺) minimally affect pyrene vibrational frequencies but significantly influence electronic transition intensities and spectral band positions.
• The observed spectral changes are attributed to modifications in Franck-Condon factors and Herzberg-Teller coupling upon cation complexation.
• DFT calculations successfully rationalize the experimental observations, providing insights into the cation-π interactions in these complexes.