Rotaxane Formation Increases Squaraine Fluorescence Brightness beyond 900 nm by 25-fold

A squaraine dye with two flanking benzo[cd]indoles absorbs strongly at 881 nm, but the fluorescence quantum yield is very low due to dye flexibility. Rigidification of the squaraine structure by encapsulation within a rotaxane produces a 25-fold increase in fluorescence brightness with an SWIR emission band that peaks at 915 nm and extends beyond 1100 nm. Excitation of the rotaxane's anthrylene sidewalls with 375 nm light produces a very large pseudo-Stokes shift of 540 nm.