Computational Characterization of "Dark" Intermediates in the Ultrafast Deactivation of Photoexcited Bilirubin

The characterization of various intermediates in the ultrafast deactivation of photoexcited ( Z, Z)-bilirubin-IXα was carried out using different computational methods. Various excited states of ( Z, Z)-bilirubin-IXα and their respective vertical excitation energies were calculated using time-dependent density functional theory (TD-DFT) employing the Coulomb-attenuating method (CAM) combined with the B3LYP functional, which is known to predict accurate results on the charge transfer excitation process. Optimized geometries and absorption spectra were determined in chloroform solvent using the polarizable continuum model incorporating the integral equation formalism. The optimized geometries of different conformers of bilirubin ( ZZ, ZE, EZ, and EE) along with their relative energies and vertical excitation energies were obtained. The geometry of the first excited state, S₁, for the ZZ conformer was optimized using TD-DFT. The computational study suggests that excited-state intramolecular proton transfer (ESIPT) plays a major role in the deactivation process of ( Z, Z)-bilirubin-IXα on a shorter time scale. The lactam-lactim tautomerism that arises from the ESIPT process gives rise to various intermediates of ( Z, Z)-bilirubin-IXα. The computational results nicely corroborate the experimental findings available in the literature.