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

Abstract

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 (CF₃) were bonded to 2PUA to form OMe–2PUA and CF₃–2PUA, 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 S₁ state for the OMe–2PUA:AcO⁻ hydrogen-bonded complex. On the contrary, weak CT due to the electron-withdrawing CF₃ group led to H2 transfer in the S₁ state for CF₃–2PUA. 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. CF₃–2PUA⁻ could keep its nearly planar structure in the S₁ state and emit “abnormal” fluorescence. On the other hand, the anionic OMe–2PUA⁻ underwent a twisting motion to form a twisted structure in the S₁ state with very low energy, and this led to a rapid internal conversion (IC) to quench long-wave fluorescence in the emission spectra.