Improved Visible Light Absorption of Potent Iridium(III) Photo-oxidants for Excited-State Electron Transfer Chemistry

Abstract

Three iridium photosensitizers, [Ir(dCF 3 ppy) 2 (N−N)] + , where N−N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino[2,3-a]phenazine (pzph), or benzo[a]pyrazino[2,3-h]phenazine (bpph) and dCF 3 ppy is 2-(3,5-bis(trifluoromethylphenyl)pyridine), were found to be remarkably strong photo-oxidants with enhanced light absorption in the visible region. In particular, judicious ligand design provided access to Ir-bpph, with a molar absorption coefficient, ε = 9800 M −1 cm −1 , at 450 nm and an excited-state reduction potential, E(Ir + * /0 ) = 1.76 V vs NHE. These complexes were successful in performing light-driven charge separation and energy storage, where all complexes photo-oxidized seven different electron donors with rate constants (0.089−3.06) × 10 10 M −1 s −1 . A Marcus analysis provided a total reorganization energy of 0.7 ± 0.1 eV for excited-state electron transfer.

A Roadmap Towards Visible Light Mediated Electron Transfer Chemistry with Iridium(III) Complexes

“…This was due to the stabilization of the π* C−N orbital (Figure 2B), and resulted in i) a vibronically structured photoluminescence spectrum, ii) a long‐lived excited‐state, and iii) a larger molar absorption coefficient in the visible range. For the diimine (N−N) ligand, [8,37] extended π‐conjugation similarly led to a red‐shift of the spectroscopic properties, here caused by the stabilization of π* N−N orbital (Figure 2B). In addition, when the degree of conjugation was high enough, the reduction of the π–π* gap promoted the mixing between LC N−N and CT transitions, as indicated again by i) structured photoluminescence spectra, ii) longer excited‐state lifetimes, and iii) larger molar absorption coefficients.…”

“…Finally, perhaps the most developed class of iridium(III) complexes for electron transfer chemistry is the cationic [Ir(C−N) 2 (N−N)] + type complexes (Figure 3), which form the IrC 2 N 4 family [3a–j,8,27] . These heteroleptic complexes encompass a much broader range of electrochemical behavior than the other classes.…”

“…The addition of fused aromatic rings has been extensively investigated in the literature, with the development of a wide variety of C−N [25c,36] and N−N ligands [8,37] . Benzo‐, naphtho‐, quinoline‐, quinoxaline‐ and other derivatives have been reported and are illustrated in figure 6A.…”

“…This property has been exploited to adjust the HOMO‐LUMO gap in order to achieve photoluminescence spanning a wide range of wavelengths [7] . Tuning this energy gap has allowed access to photocatalysts with interesting (photo)redox properties for electron transfer chemistry [3,8] . Furthermore, the range of (photo)redox potentials available for Ir(III) complexes is far greater than for their Ru(II), Os(II) or Cu(I) counterparts (Table 1 and Figure 1).…”

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA‐UC Performance

“…Of course, previous studies have investigated a variety of different aryl groups as components of cyclometallating C^N ligands, including early, seminal [21] studies on luminescent Ir III complexes [26] . Other recent examples of red phosphorescence from Ir III species have been achieved through the use of conjugated triazole, [27] conjugated phenazine, [28] and cyclometallating phenylquinazoline [29] ligands. Teets and co‐workers have also described a systematic study of a series of anionic ancillary ligands which facilitate excellent control over the emitting state energies of their Ir III complexes [30] .…”