Understanding the Difference in Photophysical Properties of Cyclometalated Iridium(III) and Rhodium(III) Complexes by Detailed Time-Dependent Density Functional Theory and Frontier Molecular Orbital Supports

Abstract

Two sets of new mononuclear cyclometalated complexes of the types [MIII(ppy)2(L)] (Ir1 and Rh1) and [MIII(bzq)2(L)] (Ir2 and Rh2) [where M = Rh/Ir; L = N-(furan-2-ylmethyl)-2-pyridinecarboxamide; ppy = 2-phenylpyridine (1); bzq = benzo­[h]­quinoline (2)] are presented to understand the significant differences of emission behaviors between Ir­(III) and Rh­(III) complexes. The complexes were fully characterized by using physicochemical and spectroscopic tools along with the detailed structural studies of Ir1 and Rh2 by single-crystal X-ray diffraction. Structural analyses showed that in both complexes the metal exhibits a distorted octahedral geometry. The geometries, electronic structures, lowest-lying singlet–singlet absorptions, vertical singlet–triplet excitation, and triplet–singlet emission behaviors of all the complexes (Ir1–2 and Rh1–2) were examined by means of DFT and TDDFT approaches. In order to find out the considerable differences in experimentally observed emission properties of Ir­(III) (5d6) and Rh­(III) (4d6) complexes, the radiative and nonradiative processes were calculated and compared quantitatively and somewhat qualitatively. The results indicate that the smallest S1–T1 splitting energy (ΔE S1-T1), large transition dipole moment (μS1), and reverse order of energy states between the triplet metal-to-ligand charge transfer (3MLCT) and triplet metal-centered (3MC) states were responsible for the higher radiative process of Ir­(III) complexes than those of Rh­(III) complexes. Since the nonradiative decay process plays an important role in the photodeactivation process, the potential energy profile for the deactivation process from 3MLCT via 3MC was explored. In addition to the relative energies of 3MLCT and 3MC states, the position of minimum energy 3MC/1GS crossing point (MECP) was also computed.

Insights into the luminescent properties of anionic cyclometalated iridium(III) complexes with ligands derived from natural products

“…At this point, it is important to note that the non‐radiative T 1 → 3 MC→S 0 process must overcome two energy barriers, this is for the T 1 → 3 MC and 3 MC→S 0 pathways. Theoretical studies have shown that the T 1 → 3 MC barrier is ∼0.4 eV when the 3 MC state lies below the T 1 state [ E ( 3 MC)< E (T 1 )]; while, the barrier is higher in ∼1.0 eV when E ( 3 MC)> E (T 1 ). In all the cases, the barrier for the 3 MC→S 0 process is at least 80% lower than the T 1 → 3 MC path; thus, the T 1 → 3 MC process is the limiting step in the non‐radiative pathway .…”

“…Theoretical studies have shown that the T 1 → 3 MC barrier is ∼0.4 eV when the 3 MC state lies below the T 1 state [ E ( 3 MC)< E (T 1 )]; while, the barrier is higher in ∼1.0 eV when E ( 3 MC)> E (T 1 ). In all the cases, the barrier for the 3 MC→S 0 process is at least 80% lower than the T 1 → 3 MC path; thus, the T 1 → 3 MC process is the limiting step in the non‐radiative pathway . In this framework, the access to the 3 MC states can be avoided by the adequate assembly of the emitting device in a solid host, avoiding (or decreasing) the intramolecular motions associated with the stretching Ir‐ligand vibrations; hence, increasing the energy barriers of the limiting T 1 → 3 MC step.…”

Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of the Anthrol Molecule

“…In order to obtain correct molecular constructions, the implicit solvation model based on density (SMD) is considered with density functional theory (DFT) and its time-dependent (TD) method. The TD-DFT functional method has been confirmed by contemporary researchers to be efficient and accurate for investigating photochemical reactions. − The dielectric constant (ε) has been set as 38.8 to simulate the acetonitrile solution environment in experiment. All geometric constructions are fully optimized by Gaussian 09 software carried out at B3LYP-D3/6-31G­(d,p) calculation level, the transition-state constructions are calculated by the Berny arithmetic method .…”

Metal and Substituent Influence on the Cytostatic Activity of Cationic Bis‐cyclometallated Iridium and Rhodium Complexes with Substituted 1,10‐Phenanthrolines as Ancillary Ligands

“…While the photophysical properties of the Ir III compounds are superior to those of the corresponding Rh III complexes, and therefore make the iridium complexes more useful with regard to diagnostic and phototherapeutic studies, this is not the general case for the antiproliferative properties. For example, while the iridium complex [Ir(ptpy) 2 (4,4'‐dinonyl‐2,2'‐bipy)] + (ptpy = para ‐tolyl‐pyridinato) was much more effective towards transfected HEK293T cells than its rhodium congener, the rhodium complexes [Rh(phquin) 2 (4,4'‐R 2 ‐2,2'‐bipy)] + (phquin = phenylquinolinato) performed much better in MDA‐MB‐231 cells (R 2 = diphenyl) or RAW264.7 cells (R 2 = dinonyl) than their iridium counterparts.,, Besides the nature of the metal, the substituents on the cyclometallating ligands as well as on the ancillary ligand have a great influence on the cancerostatic performance of these complexes.…”

“…[3] It was also shown that modifications of the cyclometallating ligand "C ∧ N" and/or the ancillary ligand "N ∧ N" allowed both fine-tuning of the anticancer and imaging properties. [4] While the photophysical properties of the Ir III compounds are superior to those of the corresponding Rh III complexes, [5] and therefore make the iridium complexes more useful with regard to diagnostic and phototherapeutic studies, this is not the general case for the antiproliferative properties. For example, while the iridium complex [Ir(ptpy) 2 (4,4Ј-dinonyl-2,2Јbipy)] + (ptpy = para-tolyl-pyridinato) was much more effec-structures of compounds 1b and 4a in the solid state were determined by single-crystal X-ray diffraction.…”