Synthesis and spectral properties of new fluorescent alkoxy-substituted thieno[3,2-b]indole derivatives

• Synthesis of three new fluorescent substituted thieno[3,2-b]indoles (TIs).
• Electronic absorption and fluorescence spectra of TIs in DMSO.
• Theoretical electronic absorption and fluorescence spectra calculated by MO method.
• Red-shift of fluorescence emission wavelengths of TIs relative to unsubstituted TI.

The synthesis and optical properties of three new fluorescent alkoxy-substituted thieno[3,2-b]indole (TI) derivatives, including 7-methoxy thieno[3,2-b]indole (7-MeOTI), 6,7- methylenedioxythieno[3,2-b]indole (6,7-MDTI) and 6,7-dihexyloxythieno[3,2-b]indole, (6,7-DHTI), were investigated. Electronic absorption spectra, fluorescence excitation and emission spectra, fluorescence quantum yields (ΦF), lifetimes (τF), and other photophysical parameters of the three TI derivatives were measured in DMSO solutions at room temperature. Theoretical electronic absorption and fluorescence spectra were also calculated by means of a molecular orbital (MO) method. For all three alkoxy-TI derivatives, the fluorescence emission maximum wavelength was significantly red shifted relative to un-substituted TI, which was attributed to delocalization of the fused hetero-aromatic ring π electronic system by the electron-donating alkoxy group(s). ΦF values varied from 0.12 to 0.19, according to the compound. τF were short, in the range 0.56–1.13 ns.

Fluorescence excitation (ex) and emission (em) spectra of 7-MeOTI (a), 6,7-MDTI (b) and 6,7-DHTI (c) (1.0 × 10−6 M) in DMSO at room temperature. Frontier molecular orbitals (FMOs) involved in the electronic absorption processes of the 7-MeOTI molecule, with the calculated absorption band wavelengths and the experimental ones in parentheses.Figure optionsDownload as PowerPoint slide