NIR fluorescence of A–D–A type functional dyes modulated by terminal Lewis basic groups

Near-infrared (NIR) emitting functional dyes have been attracting considerable attention because they are useful in the fields of cellular imaging, security application, energy conversion, and so on. In this study, to develop NIR fluorescent dyes with high photoluminescence (PL) quantum yields (ΦPL), novel acceptor–donor–acceptor (A–D–A) type π-conjugated molecules have been prepared, where dimeric dithienosilole (DTS) and freebase or quaternized 1-pyridylacryronitrile are involved as the D and A moieties, respectively. The freebase dye exhibited red fluorescent emission at 654 nm with ΦPL of 0.10 in dichloromethane at rt, whereas the methylation of the pyridine moieties at the molecular termini led to a significant red shift of the emission to the NIR region (PL maximum λPL; 822 nm), accompanied by improvement of a high ΦPL of 0.20 in dichloromethane at rt. The results of density functional theory (DFT) and time-dependent DFT calculations indicated that the considerable red shift of the emission band of the quaternized dye originates from effective intramolecular charge transfer from the DTS donor to the terminal pyridiniums and that the high value of ΦPL is brought about by minimal structural relaxation upon photoexcitation. NIR emission was also obtained when the freebase dye was placed under acidic conditions: protonation at the Lewis basic pyridine moieties brought about halochromic response of PL to yield red-shifted emission (λPL; 755 nm), when trifluoroacetic acid was added to the dichloromethane solution. [Display omitted] •A–D–A type fluorescent dithienosilole dimers with terminal pyridines were prepared.•Quaternization of the pyridine moieties led to red shifts of UV–vis and PL spectra.•The quaternized A–D–A dye exhibited intense near-infrared fluorescence.•The quaternized A–D–A dye adopted a highly planar structure at the excited state.•The freebase A–D–A dye showed near-infrared fluorescence under acidic conditions.