Electronic Interaction between Aromatic Chromophores in Dibenzo-Crown Ether Complexes with Alkali Metal Ions Investigated via Cold Gas-Phase Spectroscopy.

This study investigated the geometric and electronic structures of dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) complexes with alkali metal ions, identified as M + (DB21C7) and M + (DB24C8) (M = Na, K, Rb, and Cs), respectively. We observed the ultraviolet photodissociation (UVPD) spectra of these complexes under cold (∼10 K) gas-phase conditions. The conformations of the M + (DB21C7) and M + (DB24C8) complexes were determined by comparing the UVPD spectra with the calculated electronic transitions of the local-minimum forms. The interactions between the electronic excited states of the two benzene chromophores in the M + (DB21C7) and M + (DB24C8) complexes were examined and compared with those of previously studied complexes (dibenzo-15-crown-5 (DB15C5) and dibenzo-18-crown-6 (DB18C6)). The S 1 -S 0 and S 2 -S 0 electronic excitations of the M + (DB21C7) complexes were almost localized in one of the benzene rings. In contrast, the closed conformers of the M + (DB24C8) (M = K, Rb, and Cs) complexes were delocalized over the two chromophores for electronic excitations, exhibiting strong electronic interactions between the benzene rings. For the M + (DB24C8) complexes (M = K, Rb, and Cs), the short distance between the benzene rings (∼3.9 Å) led to a strong interaction between the benzene chromophores. We conclude that this strong interaction in the M + (DB24C8) complexes correlates strongly with the broad absorption in the UVPD spectra, suggesting the presence of an intramolecular excimer for the K + (DB24C8), Rb + (DB24C8), and Cs + (DB24C8) complexes.