An unusual mechanism for HOMO–LUMO gap narrowing in a minimal near-IR dye generated by the deprotonation of bis(dicyanomethylene)indan

A minimal near-IR (NIR) dye with the lower molecular-weight than 250 is produced by the deprotonation of 1,3-bis(dicyanomethylene)indan. Despite the small π-conjugation system, the anionic dye shows light absorption and fluorescence in the red to near infra-red region. This Letter reports on the unusual mechanism for HOMO–LUMO gap narrowing for the NIR dye. Density functional theory (DFT) analyses show the molecular-orbital transition from the LUMO+1 to HOMO by the deprotonation, which significantly reduces the HOMO–LUMO gap. Time-dependent DFT and group-theory analyses well explain the electronic-excitation properties. This is a new mechanism for reducing a HOMO–LUMO gap without extending π-conjugation systems.