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

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. Be square! 1,2‐Hemiquarimines (see figure), with one unsubstituted or derivatized phenylimine group, can be synthesized from the corresponding 1,2‐squaraine. The interesting photophysical and redox characteristics of these dyes make them useful candidates for charge transfer and sensitization studies.