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

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Abstract
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(B2u) ← S0(Ag) and S2(B1u) ← S0(Ag) band systems associated with excitation of the pyrene chromophore. The S1(B2u) ← S0(Ag) band systems of Na+-pyrene and K+-pyrene are dominated by progressions in Franck-Condon active ag vibrational modes, whereas transitions involving b3g 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 ag vibrational modes in the S1 ← S0 spectra of Na+-pyrene and K+-pyrene. The strong, broad S2(B1u) ← S0(Ag) vibronic bands of Na+-pyrene and K+-pyrene are also displaced to lower energy compared to the corresponding bands of the bare pyrene molecule.
Funding Information
  • Australian Research Council (DP160100474)

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