Fluorescence quenching of deprotonated phenylurea through twisting motion induced by an electron-donating substituent group.

The excited-state proton transfer (ESPT) reaction between anthracen-2-yl-3-phenylurea (PUA) derivatives and tetrabutylammonium acetate (TBAAc) in dimethyl sulfoxide (DMSO) solvent was theoretically investigated using time-dependent density functional theory. The electron-donating methoxy group (OMe) and electron-withdrawing trifluoromethyl group (CF) were bonded to 2PUA to form OMe-2PUA and CF-2PUA, respectively. Two hydrogen bonds formed in the 1 : 1 hydrogen-bonded complexes between the 2PUA derivative and acetate ion (AcO), namely N1-H1⋯O1 and N2-H2⋯O2. Strong charge transfer (CT) due to the electron-donating OMe group led to H1 transfer in the S state for the OMe-2PUA:AcO hydrogen-bonded complex. On the contrary, weak CT due to the electron-withdrawing CF group led to H2 transfer in the S state for CF-2PUA. After the ESPT reaction, the binding energies of the hydrogen-bonded complexes strongly decreased in both cases, and this promoted the separation of contact-ion pairs (CIPs*) and formed different types of anionic species. CF-2PUA could keep its nearly planar structure in the S state and emit "abnormal" fluorescence. On the other hand, the anionic OMe-2PUA underwent a twisting motion to form a twisted structure in the S state with very low energy, and this led to a rapid internal conversion (IC) to quench long-wave fluorescence in the emission spectra.