Theoretical investigation of stokes shift of 3,4-diaryl-substituted maleimide fluorophores

According to the experimental data, a new type of fluorophores based on 3,4-diaryl substituted maleimide compounds was found to exhibit moderate to large Stokes shift. This large Stokes shift is assumed to be the consequence of a substantial change of the geometric structure from the ground state (S0) to the first excited state (S1). In this study, the hypothesis is examined by the computational methods. The semiempirical AM1 and the density functional computational methods were used to generate the ground and the excited state structures of maleimide and its derivatives. The absorption and emission energies of maleimide derivatives were obtained by the semiempirical ZINDO method. The calculated Stokes shift (ΔE) is related to the difference between the absorption and emission energies. The theoretical calculations confirmed that the magnitude of the Stokes shift is parallel to a change of the molecular structure from non-planar in the S0 ground state to more planar in the S1 excited state. The calculated absorption and emission energies and related Stokes shift agree well with the experimental results.