Macrocyclic cis-Indolenine Squaraine Dyes as Efficient Near Infrared Emitters

The absorption and emission properties of macrocyclic cis-indolenine squaraine dyes are studied experimentally and theoretically. As compared to the monomeric species, large Stokes shifts of the emission are observed for cyclic trimers and tetramers. The theoretical analysis predicts that these shifts are caused by deviations of the excited state geometries from the strictly symmetric configurations. Breaking the symmetry makes fluorescence from lower excited states possible which otherwise is forbidden. The line shapes of the spectra are dominated by vibronic structures which can be identified using a vibronic coupling model. From the results, the important fact emerges that macrocyclic squaraine aggregates can be effectively used as building blocks for near-infrared (NIR) emitters.