Crystal structure and novel solid-state fluorescence behavior of the model benzoxazine monomer: 3,4-Dihydro-3,6-dimethyl-1,3,2H-benzoxazine

Solid-state fluorescent behavior of the model benzoxazine monomer, 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine denoted as MM, was firstly reported. Three characteristic fluorescence peaks were observed, i.e., (1) the ultraviolet peak at 317 nm with the excitation wavelength of 258 nm, (2) the blue emission peak at 450 nm with the excitation wavelength of 324 nm and (3) the other blue fluorescent peak at 454 nm with the excitation wavelength of 360 nm. It was proved that the benzoxazine core was responsible for the fluorescence of blue light due to the absence of the emission peak in the ring-opening benzoxazine dimer with the same moieties as the benzoxazine monomer MM. The single crystal X-ray structure of the compound MM recorded at 100 K showed that the irregular chair conformation of the ring-closure benzoxazine MM gave rise to the fluorescent property since all the members of the oxazine ring other than nitrogen atom arranged themselves at the plane of the benzene ring.