Interplay between Intra‐ and Interligand Charge Transfer with Variation of the Axial N‐Heterocyclic Ligand in Osmium(II) Pyridylpyrazolate Complexes: Extensive Color Tuning by Phosphorescent Solvatochromism

Abstract

The rational design and syntheses of a new series of Os(II) complexes with formula [Os(fppz)(2)(CO)(L)] (1: L=4-dimethylaminopyridine; 2: L = pyridine; 3: L = 4,4'-bipyridine; 4: L = pyridazine; 5: L = 4-cyanopyridine), bearing two (2-pyridyl)pyrazolate ligands (fppz) together with one carbonyl and one N-heterocyclic ligand at the axial positions are reported. Single-crystal X-ray diffraction studies of, for example, 2 reveal a distorted octahedral geometry in which both fppz ligands reside in the equatorial plane with a trans configuration and adopt a bent arrangement at the metal center with a dihedral angle of approximately 23 degrees , while the carbonyl and pyridine ligands are located at the axial positions. Variation of the axial N-heterocyclic ligand leads to remarkable changes in the photophysical properties as the energy gap and hence the phosphorescence peak wavelength can be tuned. For complexes 1 and 2 the solvent-polarity-independent phosphorescence originates from a combination of intraligand (3)pi-pi* ((3)ILCT) and metal-to-ligand charge transfer transitions ((3)MLCT). In sharp contrast, as supported by cyclic voltammetry measurements and theoretical calculations, complexes 3--5 exhibit mainly ligand-to-ligand charge transfer (LLCT) transitions, resulting in a large dipolar change. The phosphorescence of complexes 3--5 thus exhibits a strong dependence on the polarity of the solvent, being shifted for example, from 560 (in C(6)H(12)) to 665 nm (in CH(3)CN) and from 603 (in C(6)H(12)) to 710 nm (in CH(3)CN) for complexes 3 and 5, respectively. The results clearly demonstrate that a simple, straightforward derivatization of the axial N-heterocyclic ligand drastically alters the excitation properties per se from intraligand charge transfer (ILCT) to LLCT transitions. The latter exhibit remarkable LLCT phosphorescence solvatochromism so that a broad range of color tunability can be achieved.

Theoretical studies on Ru(fppz)2(CO)L (L = N-heterocyclic ligand): Electronic structure, absorption, phosphorescence, and solvatochromism

“…Thus, this emission can be described as originating from 3 {[d yz (Ru) + π(fppz)] [π*(L)]} excited state with the 3 MLCT/ 3 LLCT character (Figure 4(b)). Our calculated results agree with the variation trend of the corresponding Os complexes obtained by Chou et al [21] .…”

“…The frontier molecular orbital energy levels of 1−3 in vacuo, C 6 H 12 , and CH 3 CN media are given in Table 9. For complexes 1 and 2, Tables 8 and 9 show that there is little solvent effect on the energy levels of frontier molecular orbital (∆E < 0.25 eV), the excitation energy of the lowest-lying absorption (∆E < 0.23 eV) and the emission (∆E < 0.08 eV), which is consistent with the experimental trends obtained by Chou et al [21] . The calculation results also showed that there is a little blue-shift in the absorption and the emission spectra of 1 and 2 along with the increase of the solvent polarity in vacuo, C 6 H 12 , and CH 3 CN media.…”

“…Chou and co-workers [21] have synthesized a new series of Os(II) complexes with formula [Os(fppz) 2 (CO) (L)] (L = N-heterocyclic ligand), and they found that the phosphorescence properties could be changed from 3 MLCT/ 3 ILCT (intraligand charge transfer) to 3 MLCT/ 3 LLCT (ligand-to-ligand charge transfer) by tuning the substituent on the L ligand, and the emission with 3 MLCT/ 3 LLCT character had a strong dependence on the polarity of the solvent. According to the relationship between Ru and Os complexes discussed above, the corresponding Ru complexes are expected to have good phosphorescence properties similar to Os complexes.…”

Osmium‐ and Ruthenium‐Based Phosphorescent Materials: Design, Photophysics, and Utilization in OLED Fabrication

“…Moreover, treatment of complex 20 with a nitrogen donor ligand led to the formation of a new series of Os II complexes with formula [Os(fppz) 2 (CO)(L)] ( 23 , L = 4‐dimethylaminopyridine; 24 , L = pyridine; 25 , L = 4,4′‐bipyridine; 26 , L = pyridazine; 27 , L = 4‐cyanopyridine); the remaining carbonyl ligands and the N‐heterocyclic ligand are located at the axial positions 34. Variation in the axial N‐heterocyclic ligand leads to remarkable changes in photophysical properties such that the energy gap and the phosphorescence peak wavelength can be fine‐tuned.…”

Stimuli-Responsive Near-Infrared Emissive Os(II)–Terpyridine Complexes with a Sense of Logic

“…In order to accomplish our goal, we have designed two Os­(II)–terpyridine complexes based on a bifunctional terpyridine–bipyridine ligand, dipy-Hbzim-tpy, and thoroughly studied their stimuli-responsive photophysical properties (Chart ). Although a sizeable number of reports on polypyridyl Os­(II) complexes exhibiting NIR luminescence are available in the literature from our laboratory − as well as from other research groups, − but to our knowledge, there is no report wherein the optical as well as NIR luminescence properties of bis-terpyridine Os­(II) complexes were tuned and/or switched simultaneously upon interaction with anions, cations, pH, protons, and solvents.…”