N-Monoalkylated 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrroles as effective one- and two-photon fluorescence chemosensors for fluoride anions

We have synthesized three N-monoalkylated DPP (mDPP) derivatives, 3,6-bis(4-chloro-/methyl-/bromophenyl)-2-octyl-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-diones (mDPP-Cl/mDPP-Me/mDPP-Br), by facile one-step monoalkylation of commercially available or easy-prepared corresponding DPP derivatives. All the three compounds have high fluorescence quantum yields (Phi) and exhibit usable two-photon absorption cross-sections (delta = 58-87 GM). They are not only effective one-photon but also more effective two-photon fluorescence chemosensors selectively for fluoride anions. Moreover, the substituents at the benzene ring, even if simple methyl, chloride and bromide, could affect the intramolecular charge transfer from the lactam N to the DPP core, and the sensing performance of mDPP to fluoride ions is in the order, mDPP-Br >= mDPP-Cl > mDPP-Me. Proton NMR titrations indicate that the intermolecular proton transfer between the hydrogen atom on the lactam N and the fluoride anion is the sensing origin. The stable chemical structure, small molecular size and usable Phi delta in terms of femto-second laser pulses make mDPP moieties suitable for the construction of new and effective fully aromatic two-photon materials applicable in two-photon science and technology.