Excitation-wavelength dependent fluorescence of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

Abstract

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.

Fig. 1

a Ground-state rotational potential and the corresponding rotamer distribution function n(φ), b Dipole moments in the ground and first excited states as a function of the dihedral angle

Fig. 2

Fluorescence excitation spectra (a) and fluorescence spectra of EAADCy in THF at room temperature as a function of λ _obs (a) and λ _exc (b)

Fig. 3

The ratio I _exc(SW)/I _exc(LW) (a) and I _em(LW)/I _em(SW) (b) as a function of λ _obs and λ _exc, respectively

Fig. 4

Time-resolved emission spectra of EAADCy in THF at room temperature at various times after excitation (λ _exc = 400 nm)

Fig. 5

Time-dependence of the frequency of the short-wavelength (SW) and long-wavelength (LW) emission band maximum of EAADCy in THF

Fig. 6

Decay of the solvent correlation function c(t) of LE (b) and ICT (a) form of EAADCy in THF at room temperature