Effect of proton acceptors on the excited-state intermolecular proton transfer reaction of fluoranthene–urea compounds

Abstract

To investigate the effect of proton acceptors on the excited-state intermolecular proton transfer (ESPT) reaction of fluoranthene–urea compounds, three fluoranthene–urea compounds with different substituents were prepared. Photoexcitation of complexes composed of these compounds and proton acceptors in the ground state produced a normal form (N∗), and subsequent ESPT generated a tautomer form (T∗). Spectroscopic measurements of these complexes in the presence of tetrabutylammonium acetate (TBAAc) as a source of CH₃COO⁻ and tetrabutylammonium phosphate (TBAP) as a H₂PO₄⁻ source were performed. ¹H NMR spectra showed that the 1-(fluoranthene-3-yl)-3-phenylurea (3FU) derivatives formed complexes with CH₃COO⁻ and H₂PO₄⁻ in the ground state through hydrogen bonding interactions. Time-resolved fluorescence spectra showed that the maximum fluorescence wavelength of the T∗ species containing CH₃COO⁻ did not change with time, whereas that of the T∗ species containing H₂PO₄⁻ shifted to a longer wavelength. These results indicated that in the excited state, the structure of T∗ with H₂PO₄⁻ might be different from that of T∗ with CH₃COO⁻. Furthermore, the distribution of electron density of T∗ might be delocalized from the fluorescent moiety to the proton acceptor rather than being localized on the fluorescent moiety.