Electronic Photodepletion Spectroscopy of Dibenzo-18-crown-6 with a Potassium Ion

Electronic photodepletion spectrum of dibenzo-18-crown-6 with a potassium ion (K+-DB18C6) was obtained in the gas phase using electrospray ionization and quadrupole ion-trap reflectron time-of-flight mass spectrometry. The spectrum exhibited rather a broad absorption band at 36350 cm(-1), which was tentatively assigned as the origin of the S-1 band. The photodepletion spectrum of Cs+-DB18C6 was also obtained to elaborate the effects of metal cations on electronic and geometric structures of metal cation-DB18C6 complexes. We found that the S-1 band of Cs+-DB18C6 was red-shifted by 180 cm(-1) from that of K+-DB18C6. With the results of quantum theoretical calculations using the density functional theory, we suggested that the red-shift arose mainly from weaker binding of Cs+ to DB18C6 than that of K+, which resulted from a larger size of Cs+ than that of the cavity in DB18C6.