Synthesis, Characterization, and Spectroscopy of Donor-Acceptor Charge Transfer Complexes

Author

Dominic Kofi KersiFollow

Publication Date

12-15-2017

Abstract

A series of square-planar metal (diimine)(dichalcogenolene) chromophoric complexes have been synthesized, characterized and studied spectroscopically. Studies conducted on diimineplatinum(II) dichalcogenolene complexes (LPtL’) using time resolved spectroscopic techniques such as transient absorption and emission, reveal a charge-separated excited state of the type dichalcogenolene → diimine charge-transfer (LL’CT) [(dichalcogenolene^•+)Pt(diimine^•-) with an open shell donor-acceptor biradical character. This makes the low-energy ligand-to-ligand charge-transfer (LL’CT), or the mixed-metal ligand-to-ligand charge-transfer (MMLL’CT), transitions of diimineplatinum(II) dichalcogenolene complexes particularly interesting to us in facilitating our knowledge of biradical charge-separated states. Long-live excited state lifetimes were observed for these complexes upon visible photoexcitation of the LL’CT or MMLL’CT band. These complexes are abbreviated as (E,E’) in this dissertation, with Pt(dbbpy)(bdt), (S,S), Pt(dbbpy)(bds), (Se,Se), Pt(dbbpy)(bSO), (S,O), Pt(dbbpy)(bSSe), (S,Se), Pt(dbbpy)(bSeO), (Se,O), and Pt(dbbpy)(CAT), (O,O). The excited-state lifetimes observed for these d⁸ platinum (II) compounds shows no periodic dependence on the heteroatoms of the donor ligand, i.e. no direct relationship was observed between the principal quantum number of dichalcogenolene donor atoms and excited-state lifetime. The relationship between observed lifetimes and principal quantum number can be explained by using heteroatom-dependent singlet-triplet (S―T) energy gaps and anisotropic covalency contributions to the M―E (where the donor atom, E= O, S, Se and M=Pt) bonding scheme that direct the rates of intersystem crossing between the singlet (S) and the triplet (T) states. For O,O; no emissive behavior was observed due to the very rapid relaxation to the ground state within 0.63 ns. This behavior was ascribed to the fact that the energy of the T₂ state lies above that of the S₁ excited state [E(T₂) > E(S₁)] and hence a rapid nonradiative (nr) decay is observed from the S₁ excited state to the S₀ ground state. In contrast, long-lived photoluminescence was observed for S,S; S,Se; S,O; Se,O; and Se,Se due to the fact that E(T₂) ≤ E(S₁) and this helps to direct S→T intersystem crossing. Diimineplatinum(II) dichalcogenolene complexes (LPtL’) with mixed E donor atoms display excited state lifetimes values between that of O,O; and S,S. In the two-level approximation, spin-orbit coupling (SOC) controls the rate constant for intersystem crossing (k_ISC) in C_2v symmetry according to k_ISC ∝ n|L_n|T_n>² / ∆E_ST²

1|L_i|T₁> = 1|L_i|A_i> = 0……………………….…......……………………(1)

1|L_x|B₂> ≠ 0 ……………………………...……………………………….…(2)

where L_i are the orbital angular momentum operators, which transform as a₂ , b₁ , and b₂ in C_2v symmetry. Intersystem crossing (ISC) from S₁→ T₁ and T₁ →S₀ is symmetry-forbidden in C_2v symmetry (equation 1) and the observed rates of ground state recovery are often slow. Conversely, ISC from S₁→ T₂ is symmetry-allowed according to equation 2 and the observed rates are highest when the energies of T₂ and S₁ are very close. A direct linear relationship is observed to exist between photoluminescence rates, calculated SOC matrix elements, and ¹³C-NMR chemical shifts. These heteroatom effects have modulated the excited-state lifetimes by not less than three orders of magnitude. The non-radiative decay rates (k_nr) for Pt(diimine)(dichalcogenolene) complexes show a direct relationship between the differences in ¹³C-NMR chemical shifts. A closer inspection of the non-radiative decay rates (knr) for the series suggests that Se,Se; does not follow the linear trend exhibited by S,S; S,Se; Se,O; and S,O, as it possesses a shorter lifetime than S,S. The shorter lifetime (T₁→ S₀ ground-state recovery) is likely due to vibronic SOC contributing to T₁ depoulation. A linear correlation was found to exist between the squares of the differences in ¹³C-E-E’- ¹³C’ ligand ¹³C NMR chemical shifts (Δδ)² and the observed excited state lifetimes. This same relationship was found to exist between the square of the SOC matrix elements (1|L_i|S₀>²) and ground-state recovery lifetimes. The computed 1|L_i|S₀> matrix elements from DFT SOC calculations shows an exceptional linear relationship with the T₁→ S₀ lifetime. Anisotropic covalency in the E-Pt-E’ bonding scheme induces an orbital rotation in the HOMO (Pt d_xy and d_zx) and this increases the SOC. The Pt d-orbital rotation is therefore the dominant source of non-zero 1|L_i|S₀> matrix elements. For O,O; S,S; and Se,Se there is no d-orbital rotation and 1|L_i|S₀> = 0. Chapter 1 details the theory used in this dissertation. Chapter 2 describes the synthetic scheme, and the methods used to characterize the systems studied. Chapter 3 provides spectroscopic and computational studies of diimineplatinum(II) dichalcogenolene complexes. Finally, chapter 4 describes the solvatochromic shifts observed for the LL’CT band of Pt(II), Pd(II) and Ni(II) diimine dichalcogenolene complexes. A series of M(diimine)(dichalcogenolene) complexes were tested against the electric field of widely used organic solvents such as acetone, acetonitrile, dimethylsulfoxide (DMSO), dichloromethane (DCM), chloroform, tetrahydrofuran (THF), dichloroethane (DCE), dimethylformamide (DMF), toluene and benzene. First and foremost, it was observed that an increase of the polarity of the solvent remarkably stabilizes the occupied orbitals (HOMO) and destabilizes the virtual orbitals of the complexes. Thus, the HOMO-LUMO gap is observed to increase, and the LL’CT band is blue shifted. This trend is in total accordance with the experimental results. From TD-DFT calculations, it was determined that the HOMO orbitals are localized on both the metal and the dichalcogenolene atoms, and the LUMO is of diimine character. The solvent field stabilizes the orbitals that are localized on both the dichalcogenolene ligand and metal and destabilizes the orbitals which are mainly diimine in character. The electron density difference map (EDDM) plots of these model systems shows that electron density is always lost from the HOMO and gained by the LUMO during the vertical transition. The observed trend in the increasing energy of the LL’CT band as the solvent polarity increases can be explained based on the charge distribution in the ground state. A linear relationship was observed when the energy of the LL’CT band maxima was plotted versus the solvent parameter, E*_MLCT. This same trend was also observed when the energy of the LL’CT band maxima was plotted against the calculated dipole moment.

Project Sponsors

NIH, NSF

Language

English

Keywords

chromophore, diimineplatinum(II) dichalcogenolene, donor-acceptor biradical character, excited state lifetimes

Document Type

Dissertation

Degree Name

Chemistry and Chemical Biology

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Martin. L. Kirk

Second Committee Member

Yang Qin

Third Committee Member

Changjian Feng

Fourth Committee Member

Ramesh Giri

Recommended Citation

Kersi, Dominic Kofi. "Synthesis, Characterization, and Spectroscopy of Donor-Acceptor Charge Transfer Complexes." (2017). https://digitalrepository.unm.edu/chem_etds/86