Density Functional Theory Study Of Electronic Structure And Spectra Of Transition Metal Complexes Of N-confused Porphyrins

Porphyrin derivatives are a kind of macrocyclic compounds in which four pyrrole rings arelinked to each other in cyclic fashion with meso-carbon bridges and have been extensivelystudied as they play important roles in many applications, for example, nonlinear optical (NLO),photodynamic therapy (PDT), organic light-emitting device (OLED) and so on. In the pastdecades, considerable advances have been made in porphyrin derivatives, containing porphyrinanalogs, homologues and isomers, which possess attractive electronic and photophysicalstructures as well as coordination properties. In this thesis, we carried out theoreticalinvestigations of electronic structures and spectra of a series of novel porphyrin derivatives andtheir transition metal complexes.1. Density functional theory (DFT) is used on studying cis-doubly N-confused porphyrin(cis-N₂CP), hexaphyrin (HP) and their d¹⁰ transition metal (Cu³⁺, Ag³⁺ and Au³⁺) complexes. Theelectronic structures and bonding situations of these molecules are investigated by using NaturalBond Orbital (NBO) procedure and the topological analysis of Electron Localization Function(ELF) method. The electronic spectra of these complexes are studied by using time-dependentDFT (TDDFT) method. Additionally, the relativistic TDDFT is also carried out to check thespin-orbit coupling effects on the excitation energies of these complexes. Based on theseanalyses, the following conclusions can be achieved: (1) NBO and ELF analyses show that allthe metal-ligand bonds show covalent characters and large bond polarities. All these bondsexhibit variable polarities depending on the different metals. Moreover, the higher polarities ofthese bonds are entirely toward ligands; (2) The introduction of Group 11 transition metals leadsto blue shifts in electronic spectra of M-N₂CP with respect to that of cis-N₂CP. However, themetalation leads to red shifts in electronic spectra of bis-metal and mixed bis-metal HP comparedwith HP. It is mainly derived from the stabilizations of the HOMOs of M-N₂CP anddestabilizations of the HOMOs of bis-metal and mixed bis-metal HP. Energy gap (E_gap) betweenHOMO and LUMO of M-N₂CP becomes larger than that of free base N₂CP, whereas, E_gap ofmetal HP decreases compared with that of HP; (3) The electronic excitations are differentbetween these complexes. Firstly, absorption spectra of Cu-N₂CP show some weak Q bands thatmainly arise from combination of ligand-to-metal charge transfer (LMCT) and ligand-to-ligandcharge transfer (LLCT). Moreover, it is noteworthy that the spectra of copper containedcomplexes of HP are mainly derived from combination of LMCT and LLCT transitions. Forother complexes, the spectra mainly arise from LLCT; (4) Spin-orbit coupling has a significanteffect on the electronic spectra of Au-N₂CP. It results in some new bands that are composed ofthe mix of singlet and triplet states. However, spin-orbit coupling has little effects on the spectra of all the other complexes.2. TDDFT is applied to study seven asymmetric n-conjugated porphyrins with extendedbenzoheterocycles: quinoline, indole, benzoimidazole, benzothiazole, benzooxazole,2,1,3-benzothiadiazole and 2,1,3-benzoxadiazole. Solvation effects on the excitation energies forthese porphyrin derivatives in chloroform are taken into account by using the conductor-likepolarizable continuum medium combined with TDDFT method. The results show that thismethod makes a closer agreement with the experimental values, especially for the locations of Qand B bands of these compounds. It also corrects the oscillator strengths of the two splitting Bbands. From our results, it is clear that Gouterman four-orbital model works well for Q bands,however, breaks down for B bands of these Complexes. It is worth while noting that the effects ofthese benzoheterocycles on the spectra of these porphyrin derivatives differ from each other veryslightly.