Ratiometric Fluorescence Depending on In/Out Isomerism of Host‐Guest Conjugate Having a Fluorosolvatochromic Dye

Abstract Host‐guest conjugate is a promising strategy for providing outstanding molecules. Herein, we designed host‐guest conjugate HG1 , which comprises a hydrophilic host unit, tetraaza[2.2.1.2.2.1]paracyclophane CPX 2 with a microhydrophobic cavity, and a guest unit, 2,3‐naphthalimide fused with two benzothiophenes G1 exhibiting fluorosolvatochromism, linked with an oxyethylene chain. HG1 initially showed a fluorescence band at 585 nm in MeOH/H 2 O (1/499 v/v); however, this band decreased with time, and instead, a new band at 430 nm appeared. The CIE chromaticity diagram showed that the fluorescent color changed from yellow‐orange to blue via white. NMR analysis of HG1 in methanol‐d 4 exhibited different signals of the G1 unit between the blue and yellow fluorescent structures. The blue fluorescent structures showed asymmetrical signals assigned as the G1 unit, affected by the cavity environment of the CPX 2 unit. Consequently, HG1 can form two conformations: In‐form and Out‐form. In In‐form, the G1 unit is inside the hydrophobic cavity of the CPX 2 unit and shows blue fluorescence. In Out‐form, the G1 unit is outside the CPX 2 unit and shows yellow fluorescence in an aqueous solvent. Therefore, HG1 performs as a ratiometric fluorophore depending on the in/out isomerism of the host‐guest conjugate.