Synthesis and Electronic Properties of 1,2‐Hemisquarimines and Their Encapsulation in a Cucurbit[7]uril Host

Abstract

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2-hemisquarimines (1,2-HSQiMs), through the microwave-assisted condensation of aniline derivatives with the 1,2-squaraine core is reported. In CH3CN, 1,2-HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2-HSQiMs indicate that charge injection into TiO2 is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion.

Comparison of Complexation-Induced pKa Shifts in the Ground and Excited States of Dyes as Well as Different Macrocyclic Hosts and Their Manifestation in Host-Retarded Excited-Dye Deprotonation

“…The study of the interactions of fluorescent dyes with water-soluble macrocyclic host molecules has developed into an important area of supramolecular chemistry. − Upon complexation, the fluorescent dye frequently changes significantly, rarely dramatically, its fluorescence properties, such that both its complexation as well as its decomplexation can be followed with high sensitivity. This fluorescence response provides a robust method to (i) measure and compare the binding propensities of different macrocycles at low dye concentrations, − (ii) explore the physical properties of the inner cavities of macrocycles by means of solvatochromic effects, ,− (iii) develop refined fluorescent switches for advanced supramolecular architectures ,,,,− or molecular logic gates, ,− and (iv) monitor the reversible complexation of analytes to the macrocycles through indicator displacement strategies. − The latter method, because it can be both miniaturized and scaled up to high-throughput screening format, has enormous application potential in the environmental, biological, and food sciences.…”