Facile Access to Extremely Efficient Energy-Transfer Pairs via an Unexpected Reaction of Squaraines with Ketones

We have discovered that squaraine NCSQ easily
react
with ketones in the presence of ammonium acetate to form a novel type
of squaraine CCSQ. It is interesting to find that NCSQs exhibit unusual solid-state fluorescence, whereas CCSQs only exhibit fluorescence in solution. The quantum yield
of solid NCSQ-g is measured to be 0.36, which is the
highest among solid squaraines found so far. The large spectral overlap
between the emission of NCSQs and the absorption of CCSQs, and the structural similarity of these molecules make
them excellent energy-transfer (ET) pairs, as exemplified by the ET
pair of CCSQ-1/NCSQ-g. When a very small
amount (0.05 mol%) of CCSQ-g is doped into NCSQ-g, the ET efficiency reaches up to 96%. The Stern–Volmer quenching
constant KSV is calculated to be 65 800,
indicating that CCSQ-1/NCSQ-g forms an extremely
efficient ET pair. This study provides a novel skeleton and a facile
route to efficient ET pairs.