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

The Na⁺-pyrene and K⁺-pyrene complexes are investigated using two-color resonance enhanced photodissociation action spectroscopy in a cryogenic ion trap and through complementary density functional theory calculations. Both complexes are predicted to have π-bound structures in which the metal cation lies above the plane of the pyrene molecule, with calculated binding energies of 10570 and 8150 cm^-1, respectively. Electronic spectra of Na⁺-pyrene and K⁺-pyrene over the 26,000-33,000 cm^-1 range exhibit S₁(B_2u) ← S₀(A_g) and S₂(B_1u) ← S₀(A_g) band systems associated with excitation of the pyrene chromophore. The S₁(B_2u) ← S₀(A_g) band systems of Na⁺-pyrene and K⁺-pyrene are dominated by progressions in Franck-Condon active a_g vibrational modes, whereas transitions involving b_3g vibrational modes, which are prominent in the spectrum of the bare pyrene molecule by virtue of Herzberg-Teller coupling, are relatively weak or absent altogether. The S₁ ← S₀ origin transitions are shifted to lower energy from the corresponding pyrene transitions by 273 cm^-1 for Na⁺-pyrene and 246 cm^-1 for K⁺-pyrene. Density functional theory calculations predict that an attached Na⁺ or K⁺ cation has a minor effect on the pyrene vibrational frequencies but enhances the intensity of the weak S₁ ← S₀ transition while reducing slightly the intensity of the S₂ ← S₀ transition, explaining the relative dominance of vibronic transitions involving a_g vibrational modes in the S₁ ← S₀ spectra of Na⁺-pyrene and K⁺-pyrene. The strong, broad S₂(B_1u) ← S₀(A_g) vibronic bands of Na⁺-pyrene and K⁺-pyrene are also displaced to lower energy compared to the corresponding bands of the bare pyrene molecule.