Abstract：Bilirubin dimethyl ester (BRE) is a linear tetrapyrrole which is an analogue of bilirubin (BR)[1-3]/ There is no significant difference betweenthe UV, IR and MS spectra of BR and BRE. BR/BRE can form complexes with a variety of metal ions. The coordination sites of BR and metal ions are mainly the pyrrole nitrogen, and propionic acid side chains, and the hydroxyl groups of the BR propionic acid side chains couldbe substituted with methyl groups to obtain BRE so that the metal ions only coordinate with the nitrogen on the pyrrole ring. Therefore, we have used esterified bilirubin BRE to study its interaction with metal ions, which has the advantages of reducing the coordination point with the metal, as well as the complexity of the product, and contributing to the spectral analysis. Metal ions have an effect on the fluorescence characteristics of BR/BRE. Unlike the fluorescence enhancement of Zn2+, Cu2+ has an obviousfluorescence quenching on BRE. Studies have shown that hyperbilirubinemia and gallstones are related to BR and copper ions[4-7]. Therefore, studying the interaction of bilirubin and its analogues with Cu2+ is of great significance for the prevention and treatment of these diseases. When linear tetrapyrrole molecules coordinate with metal ions, they generally transform from the original chain structure to a porphyrin-like ring structure to suit the coordination with the central ion, a copper ion and the four central pyrrole nitrogens on a BR/BRE molecule coordinate to form a metal-centre square planar structure (Fig.1). BRE itself has a relatively low fluorescence quantum yield because the vibration of the pyrrole ring causes some excited molecules to decay to the ground state in a non-radiative manner[8]. Coordination of Cu2+ with BRE may cause their energy levels to couple to generate a new non-radiative de-excitation pathway leading to a decrease in fluorescence, but further research is needed on the quenching mechanism of Cu2+ on BRE fluorescence. The de-excitation processes of the excited state molecules generally occur on the ultra-fast time scale, which cannot be observed by the traditional steady-state spectroscopy. The visible and infrared-band femtosecond transient absorption spectroscopy techniques were used to study the excited state spectral properties of BRE-Cu2+ complex in this paper. A new quenching mechanism of Cu2+ on the BRE flurescnece was discovered by the transient absorption spectrain the visible and infrared region. We have proposed and confirmed the Ligand-to-metal charge transfer (LMCT) as a result of the fluorescence quenching. Compared with the spectral properties of BRE, the charge transfer state would provide a non-radiative decay pathway to BRE-Cu2+ complex, reducing the fluorescence quantum yield of BRE to 5% of its original value. Moreover, two lifetimes (11 ps and 186 ps) were obtained by fitting the kinetic curves of the transient absorption spectra of the complex. We assign the 11 ps component to the build-up process of the LMCT state, while the 186 ps is the lifetime of the charge transfer state.
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