Microscopic Study on Molecular Recognition of Host–Guest Complexes Between Crown Ethers and Aromatic Molecules

Abstract

Crown ethers (CEs) are the typical molecules in the host–guest chemistry. In this chapter, laser spectroscopy and quantum chemical calculations of the cold gas-phase neutral host–guest complexes of crown ethers (CEs) are described. Here, we chose 3n-crown-n (3nCn, n = 4, 5, 6, 8) as the host and substituted benzenes as the guest. The cold gas-phase complexes are produced by a supersonic expansion technique. Electronic spectra of the complexes are observed by laser-induced fluorescence (LIF) spectroscopy and discrimination of different isomers is performed by ultraviolet–ultraviolet hole-burning (UV–UV HB) spectroscopy. Conformer specific vibrational spectra are measured by IR-UV double-resonance (IR-UV DR) spectroscopy. The structures of the complexes are determined by comparing the obtained IR spectra with those of the energy-optimized structures obtained by quantum chemical calculations. We will discuss how the host and guest species change their flexible structures to form a best-fitted stable complex via “key and lock” or “induced-fitting” mechanism, and what kinds of interactions are operated for the stabilization of the complexes. We also will show the effect of complexation on the photophysics of the guest species.