Synthesis and Properties of Novel Dual-Color Fluorescent Molecular Switch with 1,8-Naphthalimide Unit

Spiropyran compound is widely used as an important class of photochromic compounds. Three new types of 1,8-naphthalimide compounds with spiropyran unit were synthesized and the structures were characterized. The photochromic characteristics research shows that their response time of the ring opening process is long even difficult to be detected comparing the classical spiropyran. And the reason of their prolonged ring opening reaction is analyzed theoretically and structurally. The photochromic properties of SP3 are more obvious both in solid medium and organic solvents due to the presence of the strong electro-withdraw substituent. It turned into orange red from yellow in polyethylene glycol(200) and from yellow to grey purple in acetone. The investigation of the UV-Vis absorption spectra also showed its negative solvent effect in different organic solvents. There is no significant difference in the absorption wavelength of the spiropyran unit ring opened after irradiation compared with the standard one. The fluorescence properties of the three compounds were investigated to find that SP1 and SP2 showed obvious fluorescence color changes prior and after the UV irradiation in the silica gel, and no fluorescence color changes were detected in polymethylmethacrylate (PMMA) and organic solvents. Fluorescence changes of compound SP3 in acetone were not detected after the UV irradiation. The compound SP3 emitted green fluorescence and became orange yellow fluorescence after ultrasonic irradiation in polyethylene glycol (200). The changing process was detected by fluorescence spectrum. The fluorescence color change of SP3 is more obvious in the thin layer silica gel before and after irradiation. As time elapses, the green fluorescence converted to red one and turned into red from orange yellow eventually. The fluorescence spectrum of the compounds in PMMA film showed that the compound SP3 played the role of a dual-color fluorescent molecular switch. Because of the amino-group in its molecular structure, this dual-color fluorescent molecular switch is suitable for application in labeling biological system.