Development of Intramolecular Charge Transfer-Type NIR Fluorescent Dyes Bearing a Pyrazino[2,3-b]quinoxaline-Based Electron Acceptor.

Near-infrared (NIR) fluorescent dyes are promising materials for bio-imaging and security applications. However, achieving efficient photoluminescence (PL) over 1000 nm is still challenging due to the energy gap law. Herein, we report new intramolecular charge transfer-type fluorescent dyes 1-H and 1-t-Bu bearing pyrazino[2,3-b]quinoxaline-2,3-dicarbonitrile as an electron acceptor unit. In toluene, these dyes exhibited NIR emission at 947 and 979 nm, with PL quantum yields of 1.04% and 0.12%, respectively. Remarkably, crystallization led to substantial red shifts of their PL spectra to 1076 and 1268 nm, respectively. X-ray crystallographic analysis revealed the formation of dimeric species involving intermolecular π-π stacking between the donor and acceptor units for 1-H and between the acceptor units for 1-t-Bu. Furthermore, TD-DFT calculations indicated that the red shifts of PL are caused by the intermolecular charge transfer interaction. These combined experimental and theoretical results highlighted the crucial roles of molecular packing in tuning solid-state photophysical properties of ICT dyes exhibiting PL over 1000 nm.