Synthesis, optical and electrochemical properties of (D-π)₂-type and (D-π)₂Ph-type fluorescent dyes

The (D-π)₂-type fluorescent dye OTT-2 with two (diphenylamino)carbazole-thiophene units as D (electron-donating group)-π (π-conjugated bridge) moiety and the (D-π)₂Ph-type fluorescent dye OTK-2 with the two D-π moieties connected through a phenyl ring were derived by oxidative homocoupling of a stannyl D-π unit and Stille coupling of a stannyl D-π unit with 1,3-diiodobenzene, respectively. Their optical and electrochemical properties were investigated by photoabsorption and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, cyclic voltammetry (CV) and molecular orbital (MO) calculations. In toluene the photoabsorption and fluorescence maximum wavelengths (λ_max,abs and λ_max,fl) of OTT-2 appear in a longer wavelength region than those of OTK-2. The fluorescence quantum yield (Φ_fl) of OTT-2 is 0.41, which is higher than that (Φ_fl = 0.36) of OTK-2. In the solid state OTT-2 shows relatively intense fluorescence properties (Φ_fl-solid = 0.24 nm), compared with OTK-2 (Φ_fl-solid = 0.15 nm). CV results demonstrated that OTT-2 and OTK-2 exhibit a reversible oxidation wave. Based on photoabsorption, fluorescence spectroscopy and CV for the two dyes, it was found that the lowest unoccupied molecular orbital (LUMO) energy level of OTT-2 is lower than that of OTK-2, but OTT-2 and OTK-2 have comparable highest occupied molecular orbital (HOMO) energy levels. Consequently, this work reveals that compared to the (D-π)₂Ph-type structure, the (D-π)₂-type structure exhibits not only a bathochromic shift of the photoabsorption band, but also intense fluorescence emission both in solution and the solid state.