Theoretical Studies On Structures And Properties Of N-Heterocyclic Carbene-Pyridine-based Ruthenium Sensitizers

As an inexhaustible,renewable and non-polluting natural energy,the utilization of solar energy is believed to be the most effective way to solve the world energy crisis and environmental pollution.DSSC is a type of novel photochemical solar cell with wide research and application prospect.Currently,how to improve the photoelectric conversion efficiency and stability is a key problem in dye-sensitized solar cell investigations.Dye could absorb sunlight and inject excited electrons into the conduction band of the semiconductor and become oxidized dyes.The oxidized dyes could be reduced and the original dyes could be regenerated by the REDOX in electrolyte.The properties of dye can directly influence the photoelectric conversion efficiency of DSSC,thus determining the photochemical solar cell efficiency.Therefore,it has become an important topic to design dye sensitizer with high stability,low cost,and high efficiency for commercialization in the development of DSSC research.It is well known that the material properties depend on its structure.In DSSC,the molecular structures of dyes could influence the capture efficiency of light,electrons injection,dyes regeneration and electronic recombination,through which affecting the photoelectric conversion efficiency of DSSC.The theoretical calculation assisted material design and performance analysis have become an indispensable research means for the experimental scientists.This paper aimed at the improvement of the photoelectric conversion efficiency and stability of the DSSC to design a series of novel dye molecules with ruthenium complex structures and investigate their electronic structural properties,electronic excited state properties and spectroscopic properties by the method of quantum chemistry calculations.N-heterocyclic carbene-pyridine auxiliary ligands modified by substituents are employed to replace a bipyridine ligand in N3 dye molecule in the design of a series of novel N-heterocyclic carbene-pyridyl bipyridine ruthenium complex dyes,whose geometric structures,electronic structures and spectroscopy properties are calculated by the method of DFT and TDDFT.The calculation results show that the HOMO obitals of this type of dyes are consist of d orbitals of ruthrium atoms and NCS ligands,LUMO orbitals are localized in the ?*orbitals of carboxylic terpyridine ligand.The contribution of NCS ligand to HOMO and that of carboxylic group to LUMO play important roles in the regeneration and electronic injection of DSSC.The HOMO and LUMO orbital energy levels in dyes 1-4 match well with the redox potential of iodide in electrolyte and the conduction band of TiO2 semiconductor.The dyes in CH3CN solution show good spectroscopy response in ultraviolet-visible spectrum region.With the increasing of the electron donating ability of substituents in N-heterocyclic carbene-pyridine ligands,the lowest energy absorption wavelength exhibits a regular redshift,owing to the d(Ru)??*(dcbpy)electronic transfer,(MLCT)electronic transition and NCS??*(dcbpy)electronic transfer.(LLCT)electronic transition.N749 dye have being adopted as parent structure and terpyridine as attached ligand,two thiocyanate ligands are replaced by bidentate N-heterocyclic carbene-pyridine ligands to design a series of bipyridyl ruthenium dye molecule containing both tridentate and bidentate ligands without thiocyanate groups.The geometric structures,electronic structures,excited state properties and spectroscopy properties of these novel dyes and N749 have investigated by DFT and TDDFT method.Compared with the parent molecule N749,the replacement of thiocyanate groups by N-heterocyclic carbene-pyridine could decrease both the HOMO and LUMO orbital energy and increase the HOMO-LUMO energy gap,which causes a better matching between redox potential of iodide in electrolyte and the HOMO orbital energy of the dye.Therefore,both the frontier orbital structure and the orbital energy of the dye molecules could meet the requirement of DSSC.This series of molecules exhibit good light absorption performance,with its minimum absorbing wavelength reaching nearly 800 nm and the absorbing transition properties identified as MLCT/LLCT mixed transition.The novel dye molecules have enough electron injection forces and regeneration ability to accomplish effective electronic injection and dyes regeneration,which could be potentially applied as environmentally friendly light-sensitive dyes.To further investigate the effect of protonation on the dye structure and spectroscopy performance of ruthenium photosensitive dye without thiocyanate ligands,the geometric structures,electronic structures and spectroscopy properties of the dye molecules[Ru(Hxtcterpy)(CF3-NHC-py)(NCS)]y(x=0,1,2,3;y=-2,-1,0,1)with different protonation extent are systematically studied by theoretical method.It is found that he protonation have certain effect on the electronic structures.The absorption spectra of the dyes in CH3CN solution shows that the HOMO and LUMO orbital energy decreased with the increasing of protonation extent and the orbital energy of LUMO decreased more than that of HOMO.Hence,with the increasing of protonation extent,the absorption wavelength in low energy region shows a redshift from the absorption spectra of the dye molecules.Meanwhile,the electronic injection forces of the dye molecules decreased regularly with the increasing of the protonation,however,the regeneration forces increased regularly at the same time.Collaboratively considering both the electronic injection forces and regeneration forces of the dye molecules,the double protonized dyes exhibit the best photosensitive properties in this series of ruthenium bipyridine photosensitive dye without thiocyanate ligands.