Ratiometric fluorescent nanosensors for copper(II) based on bis(rhodamine)-derived PMOs with J-type aggregates

By using pentyl-linked bis(rhodamine)-derived tetra-siloxane (PRh-Si4 ) as the organosilica precursor, highly ordered PRh-bridged periodic mesoporous organosilicas (PRhPMOs) were prepared. When excited at λ=500 nm, the PRhPMO suspension that contained metal ions showed two separate emission peaks at λ=550 and 623 nm. The first peak, located at λ=550 nm, was due to ring-opening of the spiro structure in the rhodamine moiety and the second, located at λ=623 nm, originated from fluorescent aggregates of the PRh units embedded in the silica framework of the PRhPMO. By using the different intensity ratios of the two fluorescence signals (FI550/623), PRhPMOs could be used as turn-ON type fluorescent ratiometric chemosensors for Cu(2+). Furthermore, based on the single-exciton theory, it was deduced that the fluorescent aggregates formed were of the J-type and had a coplanar configuration. Consequently, PRhPMOs display a longer fluorescence lifetime and greater fluorescent quantum yield than the respective monomers dissolved in solution, which is consistent with the experimental results.