Synthesis and Properties of Boron Fluoride Complexes Using 2-( <i>N</i> -Pyridylamino)-1-azaazulene derivatives

Three new boron fluoride complexes were synthesized using azaazulene derivatives, and their optical properties were investigated. These complexes were identified using single-crystal X-ray structural analysis, nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy. We investigated the optical properties of these complexes in dichloromethane solution, poly(styrene) films, and the solid state. The addition of acid led to a hypsochromic shift of approximately 60-90 nm in the absorption and fluorescence wavelengths in the dichloromethane solution and poly(styrene) film. Moreover, two boron fluoride complexes exhibited up to 28.7-fold increases in fluorescence intensity upon protonation, while the remaining one exhibited fluorescence quenching. Density functional theory calculations revealed that the excited-state intramolecular proton transfer (ESIPT) involving the nitro group and subsequent donor excited photoinduced electron transfer (d-PeT) are responsible for these differences.