Theoretical Design Of Novel Diimine Cu(?)-based Dye Sensitizers

The global energy shortage and environmental pollution issues are challenging the current economy and social sustainable development,and stimulating people to seek green and efficient renewable energy to replace the current energy sources.Solar energy exhibits its great potential to satisfy the global need for energy in the future because of it's huge amount,extensive distrabution,no pollution,inexhaustible and sustainable development.Dye-sensitized solar cells?DSSC?,due to its low-cost fabrication,relatively high efficiency,abundant raw materials,less dependent on incident light,low demand on temperature and optical flux,large flexibility in shape,color and transparency,and the large-scale production,are gaining widespread attention as potential solar energy conversion devices.As the core material of photon-to-electron,dye sensitizer has a crucial influence on the overall performance of DSSC.Developing and utilizing the low-cost and pollution free dye sensitizers and enhancing their photon-to-electron efficencies and interfacial electron transfer properties would have great scientific value and realistic significance for DSSC study and acceleration of industrialisation process.In this thesis,we employed density functional theory?DFT?and time-dependent DFT methods to optimize dye sensitizers by using molecular engineering on electron donor and acceptor groups,to design and screen a series of novel diimine Cu-based dye sensitizers,and to explore their molecular configurations,electronic structures,and spectral properties so as to understand the photon-to-electron conversion mechanism and interfacial electron transfer mechanism.Results of this thesis would provide theoretical prediction and scientific evidence for design and screening of novel Cu-based dye sensitizers for DSSC.A series of heteroleptic Cu???-based dye sensitizers featuring 6,6?-dimethyl-4,4?-dicarboxylate-2,2?-bipyridine?dmdcbpy?withfunctionalized2,9-dimethyl-1,10-phenanthroline?dmp?ligands were investigated by DFT and TD-DFT.Molecular geometry,electronic structure,and absorption spectral property are predicted in dichloromethane?CH₂Cl₂?solution.Our results showed that increasing?-conjugation and heteroaromatic groups to ancillary dmp ligand would decrease HOMO-LUMO gap,broaden spectral cover range,boost absorption intensity,enhance light-harvesting efficiency?LHE?,and thus improve the absorption spectral properties.Structural optimization on dmp ligand had comparable influence on the electronic structure,spectral property,and LHE of Cu???-based complexes featuring dmp and bpy ligands relative to those featuring double bpy ligands.This work highlighted that heteroleptic Cu???-based dye sensitizers with more efficient functionalized groups would provide potential application prospect in DSSC.A series of heteroleptic Cu???-based dye sensitizers integrating dicarboxylic acid dimethyl bipyridine/phenanthroline with functionalized chromophores were investigated by DFT and TD-DFT.The molecular geometry,electronic structure,electronic excitation and absorption spectrum,light-harvesting efficiency,and intramolecular electron transfer?IET?were analyzed in dichloromethane solution.Results showed that,?1?the four-coordinated Cu???-based dye sensitizers exhibited distorted trigonal-pyramidal geometries,and the structural optimizations along the longitudinal direction had larger effects on geometrical distortion than those along the transverse direction.?2?Introducing the functionalized chromophoric ligands could not only disperse the electron distribution of HOMOs over the Cu t₂ and e orbitals and?-conjugated donors,but also downshifted the HOMO levels,and thus reduced the HOMO-LUMO gaps and enhanced the donative quantity and transition mode from the single metal-to-ligand chargetransfer?MLCT?to the mixed MLCT and ligand-to-ligand charge transfer?LLCT?.The Cu???-based dye sensitizers with dmp ligand had the smaller HOMO-LUMO gaps than those with dmbpy ligand due to the low LUMO level of dmp.?3?Increasing longitudinal?-conjugation would be more effective to improve the absorption intensity at the short wavelength region and broaden the spectral coverage towards the long wavelength region relative to those with transverse?-conjugation.The electronic excitations of all Cu???-based dye sensitizers at 300.0³75.0 and 475.0⁵75.0 nm region were more favorable for the direct electron injection into semiconductor than those at 400.0⁴50.0 nm region.?4?The structural optimizations along the transverse direction of the Cu???-based dye sensitizers exhibited the larger transferred electron quantities,the longer electron transfer distances,the smaller IET rates,and the less orbital overlaps than those along the longitudinal direction.The heteroatom N added to the donor subunit had a positive effect on the IET rate and LHE but with a high risk of electron recombination.The intrinsic optoelectronic properties of heteroleptic bipyridyl Cu???-based dye sensitizers complexes bearing functionalized acceptor subunits were investigated in DCM solution by DFT and TD-DFT methods.The four-coordinated Cu???-based dye sensitizers exhibited distorted trigonal-pyramidal geometries and typical MLET characteristics at the long wavelength region.Heteroaromatic groups and cyanoacrylic acid in the acceptor subunits pulled the electron distribution of LUMO closer to the anchoring group,lowered the HOMO-LUMO energy gap,and enhanced the light-harvesting capability of the complexes relative to those of C3 at the long wavelength region.More importantly,the presence of cyanoacrylic acid allowed favorable donor-to-acceptor IET processes with more electrons,longer distances,lower orbital overlaps,less electron recombination,and moderate transfer rates.Our results highlighted the effect of functionalized acceptors on optoelectronic properties and expanded the current understanding of photo-induced IET process of heteroleptic Cu???-based dye sensitizers for DSSC.The interfacial electron transfer and regeneration of donor-modified and acceptor-modified Cu???-based dye sensitizers were investigated by DFT/TD-DFT method.Our results showed that,different electron donors and electron acceptors had different effects on interfacial electron injection driving force,regeneration driving force,and coupling strength between dye sensitizers and semiconductor surfaces.As for the donor-modified Cu???-based dye sensitizers,the introduction of heteroatomic groups exhibited the better interfacial electron injection performaces with respect to those pure?conjugation.Among them,the Cu???-based dye sensitizer with the dithiafulvenyl group owned the largest electron injection ratio.As for the acceptor-modified Cu???-based dye sensitizers,the substitution of carboxylic acid with cyanoacrylic acid and phosphoric acid could greatly enhance the coupling strength between dye sensitizers and semiconductor surfaces.The sensitizer with phosphoric acid as the anchoring group owned the largest largest electron injection ratio,and therefore the phosphoric acid group would be a potential alternative to the current widely-used carboxylic acid.