Gas-Phase Pseudorotaxane Formation of Protonated Dibenzylamine with 21-Crown-7 and Vibrational Energy Redistribution Revealed by Cold Ion Spectroscopy

Pseudorotaxane formation is governed by a delicate balance among host–guest interactions, steric constraints, and solvent effects. Here, we investigate the complex of protonated dibenzylamine (dBAMH+) with 21-crown-7 (21C7), hereafter denoted as dBAMH+(21C7) in this study, using cryogenic UVPD spectroscopy, IR–UV DR spectroscopy, and quantum chemical calculations in the gas phase. The results of dBAMH+(21C7) were compared with those of the complexes with 15-crown-5 (15C5), 18-crown-6 (18C6), and 24-crown-8 (24C8). Although no stable dBAMH+(21C7) complex is detected by NMR spectroscopy in methanol, the complex is readily generated and isolated in the gas phase. The UVPD spectrum of dBAMH+(21C7) exhibits a congested vibronic structure resembling that of the threaded dBAMH+(24C8) pseudorotaxane rather than the simple spectrum of the nonthreaded dBAMH+(18C6) complex. Quantum chemical calculations show that a threaded form is more stable by more than 40 kJ mol–1 than a nonthreaded one for dBAMH+(21C7) in the gas phase. These results suggest that the dBAMH+(21C7) complex can adopt a threaded pseudorotaxane form. The unusually low photodissociation efficiency of dBAMH+(21C7) is also consistent with the smaller calculated oscillator strength and a substantial barrier along the dethreading pathway, providing further support for the formation of a pseudorotaxane. The IR–UV DR spectra reveal a striking difference in the signal polarity: the 15C5 and 18C6 complexes, assigned as nonthreaded, exhibit depletion signals, whereas the 21C7 and 24C8 complexes, assigned as threaded, show gain signals despite similar vibrational band positions. Vibrational density-of-states calculations indicate that this reversal cannot be explained by the number of accepting states. These findings suggest that molecular threading can influence the vibrational response and photodissociation dynamics of isolated host–guest complexes. Differences in vibrational coupling, rather than vibrational state density alone, may contribute to the distinct relaxation behavior. In the threaded form, dissociation requires passage through the transition state (TS) associated with the unthreading process. The complexes that absorb both IR and UV photons possess additional internal energy corresponding to the absorbed IR photon compared with those excited by UV irradiation alone. This additional vibrational energy may facilitate passage through the TS, thereby significantly enhancing the UVPD efficiency of the threaded form upon IR excitation. These mechanisms provide plausible explanations for the reversal of the IR–UV DR signal polarity between the nonthreaded and threaded complexes.