Assignments of Lowest Triplet State in Ir Complexes by Observation of Phosphorescence Excitation Spectra at 6 K

Abstract

We tried the assignment of the origin of phosphorescent bands in Ir complexes. It is important to elucidate the luminescent mechanism in order to design organic light-emitting devices (OLEDs) besed on new materials. The Stokes shift between the phosphorescence and phosphorescence excitation spectra of Ir complexes such as fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy) 3 ], fac-tris(2-(2-thienyl)pyridine) iridium(III) [Ir(thpy) 3 ], bis(2-phenylpyridine [Ir(bzq) 3 ] and bis [(4,6-difluorophenyl)pyridinato](picolinato) iridium(III) [FIrpic] was measured in a solution and (phenyl) 4 Sn at 6 K. The amount of Stokes shift corresponds to the nature of the lowest triplet state. We discovered that the amount of Stokes shift clearly differs depending on whether the lowest triplet state of each Ir complex is triplet metal-to-ligand charge transfer ( 3 MLCT) or 3 -Ã . Namely, the case of 3 MLCT shows a large shift, while the case of 3 -Ã shows a small shift. We also present the resolved phosphorescence and phosphorescence excitation spectra of Ir complexes in (phenyl) 4 Sn. The sharp bands were assigned to the 3 -Ã transition, and the broad bands were assigned to the 3 MLCT state. The nature of the lowest triplet state is also discussed on the basis of resolved spectra.

Syntheses, Spectroscopic, Electrochemical, and Third‐Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2‐phenylpyridine)}iridium Complex

“…In cyclometalated iridium complexes, the excitation processes can be both ligand‐centred and MLCT in nature 2. 14, 18, 20, 21b, 22 The presence of vibrational structure in the emission spectra of 6 – 10 (Figure S31–S36) indicates that the relevant excited state in these complexes may possess significant ligand‐centered character, and likely resulting from mixing the 3 MLCT states with π→π* states as previously demonstrated for 13 14. 20…”

“…The evolution of the linear absorption behaviour as the fac ‐tris(2‐phenylpyridine)iridium(III) core is functionalized and then complexed to three Ru II centers is also of interest, so UV/Vis/NIR data were obtained for all new complexes (Figure S23); band maxima are listed in Table 3, together with data for 12 – 15 . For the tris(2‐phenylpyridine)iridium(III) derivatives 6 – 10 , the intense absorption bands centered at about 35 000 cm −1 ( 6 ) or 26 000–30 000 ( 7 – 10 ) can be assigned as ligand‐centered (LC) spin‐allowed π→π* in nature 20. These bands undergo a red shift in proceeding from iodo‐functionalized 6 to ethynyl‐functionalized 7 – 10 , and a gain in intensity in proceeding from the complexes with the shorter alkynyl group ( 7 / 8 ) to those with the longer alkynyl unit ( 9 / 10 ).…”

“…These bands undergo a red shift in proceeding from iodo‐functionalized 6 to ethynyl‐functionalized 7 – 10 , and a gain in intensity in proceeding from the complexes with the shorter alkynyl group ( 7 / 8 ) to those with the longer alkynyl unit ( 9 / 10 ). The broad absorption bands at lower energies with maxima at about 24 000–27 000 cm −1 are typical of spin‐allowed 1 MLCT transitions20 and at even lower energies, the weaker bands which reach into the visible region are assigned as formally forbidden 3 MLCT transitions,20 with the former undergoing an increase in intensity in proceeding from 7 / 8 to 9 / 10 . In addition to these absorptions characteristic of a functionalized tris(2‐phenylpyridine)iridium(III), the ruthenium–iridium hybrid complex 11 also shows an intense band at 24 200 cm −1 , which (consistent with previous reports) is attributed to a 1 MLCT transition that is localized at the ruthenium alkynyl unit 19c…”

Electromodulation of photoluminescence in vacuum-evaporated films of fac-tris(2-phenylpyridine)iridium(III)

“…The observed PL emission is commonly identified as phosphorescence from the 3 MLCT state. This assignment originally based on PL lifetime measurements and solvent dependence of PL spectra [18] was later confirmed by quantum chemical calculations [19,30,33] and extensive experimental works [8,[24][25][26][34][35][36] including: (i) the temperature dependence of the emission band structure [34], (ii) the rigidochromic (blue) band shift of PL spectra on going from a room temperature fluid solution to a low temperature glass [24,26], (iii) the Stokes shift and band structure analysis for Ir(ppy) 3 placed in Shpol'skii crystals [35], (iv) the high magnetic field effects on low temperature PL spectra [34] and (v) the cyclic voltammetry measurements of reduction/oxidation potentials [25,36]. Additionally, at low temperatures a weak higher-energy PL band was observed (at 455 nm for Ir(ppy) 3 dispersed in PMMA [24] or at 396 nm for neat films [28]) which was attributed to emission from a 3 LC [24] or 1 MLCT state [28].…”

Quantitative analysis of an organic thin film by XPS, AFM and FT‐IR

“…The Firpic molecule (C 28 H 16 F 4 IrN 3 O 2 ; MW = 694.67, average molecular diameter of about 8 Å)6 displayed in Fig. 1 is a blue phosphorescent emitter characterized by π → π* transition 7. Firpic is widely used in the development of modern organic light‐emitting diodes 8, 9.…”