Density functional theory studies on the excited-state properties of Bilirubin molecule

Abstract

Bilirubin is the main pigment in human bile, which is closely related to human health. Bilirubin combining with fluorescent protein represents a new type of fluorescent chromophore and has important applications in the field of biological imaging and biosensor. Due to the lack of efficient and accurate electronic structure methods, the electronic structure and excited-state properties of bilirubin molecule are not characterized quantitatively and accurately. Firstly, the vertical absorption energy, oscillator strength and vertical emission energy of the lowest singlet excited state of bilirubin molecule are calculated by combining the implicit solvent model and the linear response time-dependent density functional theory (TDDFT) method. Compared to the experimental data and high-level RI-ADC(2) calculation, the prediction performance of a series of density functional methods is systematically investigated. The results show that the optimally-tuned range separated density functional method has the best overall performance and the minimum absolute and relative errors. This is obviously due to the fact that the suitable proportion of exact exchange included in density functionals can produce neither delocalized nor localized electronic structures. Based on the produced wavefunction by the optimally-tuned method, the excited-state characteristics of the S₁ state of bilirubin molecule indicate a hybrid local and charge transfer excitation, based on the quantitative characterization using hole-electron analysis and interfragment charge transfer method. This work can provide a theoretical basis for the study of excited-state dynamics and spectral properties of bilirubin molecules and the optimally tuned range-separated DFT method also provide a reliable and efficient theoretical tool to study the excited-state properties of other biomolecular systems in the future.