Multiple Near‐Infrared Chromisms of a Heteromerous Overcrowded Ethylene with Large Permanent Dipole Moment

An overcrowded ethylene composed of electron-donating anion, naphthoxide, and electron-accepting cation, acridinium, has been synthesized. It is in equilibrium between a folded conformer having a smaller permanent dipole moment with visible light absorption and a twisted conformer having a larger permanent dipole moment with NIR light absorption. The overcrowded ethylene shows multiple NIR chromisms, such as solvatochromism, thermochromism, mechanochromism, vapochromism, halochromism, and amphoteric electrochromisms, which are caused by the conformational change between folded and twisted conformers or by controlling the energy difference between the HOMO of the donor moiety and the LUMO of the acceptor moiety. The small difference between the HOMO energy level of naphthoxide moiety and the LUMO energy level of acridinium moiety, and their weak interaction are crucial for NIR absorption and large dipole moment in the twisted conformer. The drastic change in the ratio of folded and twisted conformers by changing the polarity of solvents, the detection of CO₂, and the disappearance of NIR absorption by electrochemical oxidation and reduction are noteworthy for this new type of organic dye.