An OFF-ON-OFF type of pH fluorescent sensor: Benzo[c,d]indole-based dimethine cyanine dye-synthesis, spectral properties and density functional theory studies

We synthesized a novel OFF-ON-OFF type of pH-dependent fluorescent sensor: benzo[c,d]indole-based dimethine cyanine dye D1, with donor-π-acceptor (D-π-A) structure based on intramolecular charge transfer system (ICT), which employed dimethine cyanine dye as a fluorophore and pentavalent nitrogen NH+ group as a pH modulator, respectively. The product was identified by 1H NMR, 13C NMR, IR, UV-vis and HRMS. The investigation of spectral properties found that dye D1 showed excellent spectroscopic properties and its absorption maxima and fluorescence quantum yield were basically larger in protic solvent than in aprotic solvent. Meanwhile, the absorption spectra of D1 were revealed to hypochromatic-shift and the absorption intensity was gradually decreased along with the increase of pH value. Interestingly, dye D1 showed remarkable fluorescence when pH value was in the range of 6.00-9.80 with the peak at 8.21, which was defined as an OFF-ON-OFF type of pH-dependent fluorescent sensors based on ICT. In addition, dye D1 exhibited a high selectivity for H+ over other common ions, such as Cl−, K+, Fe2+ etc. Theoretical calculations based on density functional theory (DFT) were employed to provide a better understanding of this particular dye sensor. These results indicated that D1 would be able to act as an efficient pH-sensor and had a potential to play an important role in biological and medical study.