Selective Probing of Potassium Ion in Solution by Intramolecular Excimer Fluorescence of Dibenzo‐Crown Ethers

Abstract

Observation of an excimer fluorescence in solution is proposed for detecting the encapsulation of potassium ion as opposed to other alkali ions by dibenzo-crown ethers. The scheme has been validated by ultraviolet photodissociation (UVPD) spectroscopy of dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with potassium ion, K ⋅ DB21C7 and K ⋅ DB24C8, performed under cold (∼10 K) conditions in the gas phase and by quantum chemical calculations of the geometry and electronic structures of the complexes. Calculations suggest the formation of a closely spaced excimer structure of benzene rings only for the K ⋅ DB24C8. Interaction of the rings may lead to lifetime broadening in UV absorption, which is experimentally observed in the gas phase, indeed, only for this cold complex. Consistently, intramolecular excimer fluorescence of DB24C8 in solution is observed only for K ⋅ DB24C8. The excimer fluorescence is not observed with other alkali metal ions. The detection of such intramolecular excimer fluorescence can, therefore, potentially serve as a simple, background-free, selective probe of potassium ion in solution.

Conformer Separation of Dibenzo-Crown-Ether Complexes with Na+ and K+ Ions Studied by Cryogenic Ion Mobility-Mass Spectrometry

“…The electronic transition energy of the open conformer was obtained by TD-DFT calculations at the M05-2X/6-311++G­(d,p) level (a scaling factor of 0.8369, which was determined by comparison between experimental and theoretical results of K + (DB18C6), was found to be 36795 cm –1 in the present calculations). This value agreed well with the band origin of experimental UVPD spectrum of K + (DB21C7) (36777 cm –1 ) . The weak peak at 156.6 Å 2 was assigned to the closed conformer from the comparison between experimental and theoretical CCSs, as shown in Table .…”

“…This value agreed well with the band origin of experimental UVPD spectrum of K + (DB21C7) (36777 cm −1 ). 50 The weak peak at 156.6 Å 2 was assigned to the closed conformer from the comparison between experimental and theoretical CCSs, as shown in Table 2.…”

Conformation of K+(Crown Ether) Complexes Revealed by Ion Mobility–Mass Spectrometry and Ultraviolet Spectroscopy

“…In these isomers, the relative positions of the benzene rings were different. We have previously reported that the K + (SymDB24C8) complex exhibited broad UV absorption even under cold, gas-phase conditions, and this was attributable to the formation of an intramolecular excimer upon irradiation by UV light . Conversely, the K + (SymDB24C8) complex has been suspected to exhibit another type of conformation because weak, sharp bands also appeared from the broad absorption .…”

“…The capacity of IM–MS to distinguish different conformations for host–guest systems has not been demonstrated. Dearden and coworkers also reported the MS–MS studies on host–guest complexes. , More recently, crown ether complexes with metal ions were investigated in the gas phase through spectroscopic techniques. − Ultraviolet photodissociation (UVPD) spectroscopy performed under cold, gas-phase conditions could highly distinguish several conformers. ,,− However, the UVPD spectra of larger systems tend to reveal broad spectral features, which limited the distinction of different conformers or the determination of their structures . Rizzo and coworkers reported pioneering works on the cold-ion spectroscopy of biomolecular ions selected by IM. , They elaborated on the relation between the conformation and spectroscopic features.…”

“…We have previously reported that the K + (SymDB24C8) complex exhibited broad UV absorption even under cold, gas-phase conditions, and this was attributable to the formation of an intramolecular excimer upon irradiation by UV light . Conversely, the K + (SymDB24C8) complex has been suspected to exhibit another type of conformation because weak, sharp bands also appeared from the broad absorption . However, these sharp bands were too weak to be identified as different conformers through double-resonance spectroscopies. ,,− The IM–MS experiment is free from the restriction originating from the optical transition such as the oscillator strength, and it can separate different conformers and determine the abundance ratio quantitatively.…”