Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the 1H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 μs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet–triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.


■ INTRODUCTION
Energy upconversion, which converts low-energy photons into high-energy photons, is a photophysical phenomenon that has important applications in, and implications for, several critical technological areas.These include light harvesting and solar materials, photocatalysis, bioimaging, security, and communications technologies.One of our ongoing interests is the development of light-activated, sensitizer molecules that can be used in triplet−triplet annihilation energy upconversion (TTA-UC). 1TTA-UC relies upon two photoactive molecular components, a sensitizer and an annihilator.Light absorption is therefore dictated by the sensitizer species and attractive properties for maximizing TTA-UC efficiency include good molar absorption and a long triplet excited state lifetime.The triplet level (T 1 ) of the annihilator species, commonly a polyaromatic molecule such as 9,10-diphenylanthracene (DPA), must allow forward triplet−triplet energy transfer (TTET) from a compatible photosensitizer.Following annihilation, upconverted delayed fluorescence then occurs from the S 1 level of the annihilator molecule.
Photoactive transition metal complexes have proven very attractive candidates as photosensitizers for TTA-UC, as they can marry good molar absorption in the visible region with extended triplet excited state lifetimes.Ru(II), 2 Re(I), 3 and Ir(III) 4 complexes have been successfully utilized to produce TTA-UC efficiencies up to 39.3%. 5 Pt(II) species have also been investigated in TTA-UC, 6 including organometallic complexes 7 and Schiff base ligand complexes 8 and the wellknown Pt(II) octaethylporphyrin, 9 which combines very efficient visible light absorption with a microsecond duration lifetime of the triplet state. 10,11Interestingly, a phosphorescent Cr(III) complex has recently been reported in TTA-UC, demonstrating an alternative sensitizing mechanism utilizing Cr-centered excited states. 12n this context, perhaps the most valuable attribute of photoactive organometallic Ir(III) complexes is the ability to finely tune the excited triplet state properties, especially through different ligand combinations 13 and ligand substituents: 14 there is a substantial body of work based on a detailed understanding of the principles that underpin color tuning in such systems, including on the nature of the lowest excited triplet state, 15 which is essential for applications such as lightemitting devices. 16In TTA-UC, the importance of this has been demonstrated through studies which show that even very subtle changes to the excited state properties (achieved through relatively trivial changes in ligand structure) can result in profound variations in TTA-UC behavior. 5,17uilding on these advances, in this paper, we investigated the TTA-UC characteristics of a series of structurally analogous cyclometalated Ir(III) complexes.This study was underpinned by a detailed description of the photophysical properties of the complexes, which reveal how a single point of functional variation on a chelating ligand can result in a fundamental change in triplet excited state character and thus modulation of the resultant TTA-UC behavior.The study reveals that for a given ligand system, structural changes can be rationally invoked to optimize Ir(III) photosensitizers for TTA-UC.
■ EXPERIMENTAL SECTION 1 H, 19 F{ 1 H}, and 13 C{ 1 H} NMR spectra were recorded in d6-DMSO on an NMR-FT Bruker 400 or 500 MHz spectrometer. 1 H and 13 C{ 1 H} NMR chemical shifts (δ) were determined relative to residual solvent peaks with digital locking and are given in ppm.Coupling constants are quoted in Hz.Mass spectra were obtained by the staff at Cardiff University.The UV/vis absorption spectra were recorded with the UV2550 spectrophotometer (Shimadzu Ltd., Japan).The emission spectra were recorded with an FS5 spectrofluorometer (Edinburgh Instruments, UK).The experimental lifetimes were recorded with an OB920 luminescence lifetime spectrometer (Edinburgh Instruments, UK), and timecorrelated single photon counting (TCSPC) method was used.Quantum yield measurements were obtained on aerated MeCN solutions of the complexes using [Ru(bipy) 3 ](PF 6 ) 2 in aerated MeCN as a standard (Φ = 0.018). 18he nanosecond transient absorption spectra were acquired on LP980 laser flash photolysis spectrometer (Edinburgh Instruments, UK).The signal was digitized with a Tektronix TDS 3012B oscilloscope.The samples were excited with a nanosecond pulsed laser (Surelite I-10, USA; the wavelength is tunable in the range of 410−2400 nm).The typical laser energy is 5 mJ per pulse.The samples were deaerated with N 2 for 15 min prior to investigations.The data were processed by L900 software.

■ CYCLIC VOLTAMMETRY
Cyclic voltammetry was conducted employing a PalmSens4 potentiostat.HPLC grade MeCN served as the solvent with an analyte concentration of 1 mM at 293 K, utilizing triply recrystallized [ n Bu 4 N][PF 6 ] as the supporting electrolyte at a concentration of 0.1 M. The experimental setup included a three-electrode system comprising a platinum disc working electrode, a platinum wire counter-electrode, and a silver wire pseudoreference electrode.Prior to experimentation, solutions were purged for 10 min with a MeCN-saturated stream of nitrogen gas.Voltammograms were referenced to the ferrocene/ferrocenium redox couple measured under the same conditions.■ X-RAY CRYSTALLOGRAPHY Data Collection and Processing.Suitable crystals of Ir− OMe, Ir−H, and Ir−Me were selected and data collected following a standard method. 19For each compound, the selected crystal was mounted on a MITIGEN holder in oil on a Rigaku FRE+ diffractometer with Arc)Sec VHF Varimax confocal mirrors, a UG2 goniometer, and HyPix 6000HE detector.Each crystal was kept at a steady T = 100(2) K during data collection.The structures were solved with the ShelXT 20 structure solution program using the intrinsic phasing solution method and by using Olex2 21 as the graphical interface.The model was refined with version 2018/3 of ShelXL 22 using least squares minimization.CCDC 2336520−2336522 contains supplementary X-ray crystallographic data for XXXX.This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, Union Road, Cambridge, CB2 1EZ; fax(+44) 1223-336-033 or email: deposit@ccdc.cam.ac.uk.
Computational Methods.Electronic structure calculations were all performed using density fitted-density functional theory within the Gaussian 09 computational chemistry suite. 23ll calculations were performed using the Stuttgart-Dresden (SDD) effective core potential and basis set in the treatment of the iridium, 24 in combination with a 6-31G* basis set for all other light atoms. 25Full geometry optimizations were performed for the cationic complexes utilizing the selfconsistent reaction field (SCRF) modelwhich treats the solvent implicitly as a dielectric continuum.In all cases, the solvent chosen was dichloromethane.
All geometry optimizations were performed using an ultrafine grid and very tight convergence criteria, and the minima were confirmed as stationary points through the computation of harmonic vibrational frequencies, each of which showed no imaginary components.These stationary points were used in single point TD-DFT calculations to compute vertical excitation energies.All TD-DFT calculations were undertaken using a linear response approach.All TD-DFT calculations were also performed with a long-range corrected hybrid functional (CAM-B3LYP).
Phosphorescence and spin-forbidden absorption bands were investigated using unrestricted density functional theory to compute parameters associated with the first triplet state (T 1 ), using an identical methodology as for the singlet states.Decomposition of the molecular orbital character was performed using the GaussSum software package. 26Crystal structure overlays with optimized computational structures has been performed using the Chimera software package, which has also been used to calculate root mean squared deviation (RMSD) values for these comparative structures. 27SYNTHESIS General Procedure for Iridium(III) Complexes Synthesis.IrCl 3 •xH 2 O (assumed trihydrate) (1.0 equiv) and imidazole-naphthalene ligand (2.0 equiv) were added to a Schlenk flask under N 2 with the addition of 2-ethoxyethanol/ H 2 O (3:1).The reaction was heated to reflux for 74 h, then cooled, and water was added.After filtering the reaction mixture, the crude solid was washed with water and dried under vacuum to obtain the iridium dimer which was then used without further purification.The iridium(III) dimer (1.0 equiv) and 2,2′-bipyridine (2.0 equiv) were added to a Schlenk flask along with 2-ethoxyethanol under a N 2 atmosphere and heated for 36 h.The reaction was then cooled and 0.1 M NH 4 PF 6 was added.The resultant precipitate was filtered, rinsed with water, and dried in vacuo.Purification of the crude product was achieved using column chromatography (SiO 2 ; DCM/MeOH; solvent gradient 100:0 → 90:10).
[Ir(L1)  ■ RESULTS AND DISCUSSION Synthesis of the Ligands and Complexes.In this study, we were interested in a conjugated cyclometalating ligand architecture, with ease of functionality, which could promote triplet excited states of both ligand-centered (LC) and metalto-ligand charge transfer (MLCT) character in heteroleptic cationic Ir(III) species.The overall synthetic pathway to the complexes, via the ligands, is therefore shown in Scheme 1.
−24 The different benzaldehydes conveniently introduce a range of substituents of differing electronic classifications to the phenyl component of the ligand.The formation of the ligands was confirmed using 1 H NMR spectroscopic studies with the signature NH resonance noted at around 12.8 ppm; this resonance was subtly shifted downfield for the electron-withdrawing Cl and CF 3 substituted variants.For completion and reference, all relevant spectral data for L1−L5 are presented in Figures S1−S10.This series of ligands was then utilized as cyclometalating agents for Ir(III) using the traditional approach described by Nonoyama 28 to yield, first, the μ-dichloro bridged dimer species, [(C^N) 2 Ir-μ-Cl 2 −Ir(C^N) 2 ].Subsequent addition of 2,2′-bipyridine in 2ethoxyethanol and anion exchange using aqueous NH 4 PF 6 yielded the crude monometallic cationic complexes, [Ir-(L) 2 (bipy)]PF 6 .Further purification using column chromatography gave the five complexes as colored, air-stable solids.
The proposed molecular structures of the isolated complexes investigated in this study are shown in Scheme 2. Again, 1 H NMR spectroscopy helped provide categorical evidence for the formation of the desired species.First, the two C^N ligands are equivalent and present one set of 1 H resonances which were easily resolved.In all cases, the NH signal was shifted downfield to 14−15 ppm, indicative of coordination to Ir(III).Both Ir−Me and Ir−OMe possess an additional aliphatic signal associated with the ligand substituent; these were noted at 1.98 and 3.47 ppm, respectively, representing an upfield shift in each case (cf 2.40 and 3.86 ppm for L2 and L3) induced by coordination to the iridium center.A proton resonance associated with the 3-position of the naphthyl ring was observed as a singlet (integral of 2H) at ca. 5.8−5.9ppm.This highly shielded aromatic resonance is attributed to the orientation and spatial proximity of this proton to the aromatic ring of the coordinated 2,2′-bipyridine ligand (see later discussion of the X-ray crystal structure details) as noted in other benzimidazole systems. 29 13C{ 1 H} NMR data was also obtained for each complex and revealed a furthest downfield resonance ca.168 ppm, assigned to the (naphthyl) imidazolyl carbon.The anticipated C−F coupling arising from the trifluoromethane group in Ir−CF 3 was identified via both 1 J CF and 3 J CF in aromatic resonances between 120 and 140 ppm.For Ir−CF 3 , 19 F{ 1 H} NMR spectroscopy revealed a singlet at −61.16 ppm, which was consistent with an aryl CF 3 substituent, 30 and coincidentally close to the value of the free ligand, L5 (−61.29 ppm).All spectra are available in Figures S10−S21.
IR spectroscopy data for the complexes showed a NH vibrational stretch around 3392−3433 cm −1 , indicating that the NH group is retained in the complexes, consistent with the proposed binding mode (i.e. the ligand is not dianionic).The Ir(III) complexes gave excellent HRMS data (Figure S22) with m/z values that were consistent with the cationic complex fragment in each case.
Single-Crystal X-Ray Crystallographic Studies of Ir− OMe, Ir−H, and Ir−Me.Suitable crystals were obtained for X-ray diffraction studies on three of the complexes using either vapor diffusion of i Pr 2 O into concentrated solutions of the complex in MeCN (Ir−OMe and Ir−Me) or a mixture of methanol and hexane (Ir−H).The data collection parameters are shown in Table S1.In summary, the diffraction data confirmed the proposed formulations and geometries for the complexes (Figure 1).The 2-phenyl-1H-naphtho[2,3-d]-imidazole cyclometalating ligands coordinate in the expected fashion with a mutually cis-C,C arrangement to the Ir−C bonds, inducing a modestly distorted octahedral geometry.There is very little intraligand distortion noted for the conjugated ligands.The principal bond lengths (Table 1; for bond angles, see Table S2) that describe the coordination spheres are consistent with a recent report on related complexes, 31 as well as earlier work on chromophore conjugated benzimidazole derivatives. 32Each of the structures shows that the 3-position naphthyl ring proton is oriented toward the plane of the 2,2′-bipyridine ligand.The resultant H − centroid distances are 2.6363(12) and 2.9334(14) Å (for Ir−H), 2.955(5) and 2.896(5) Å (for Ir−Me), and 2.886(6), 2.847(6), and 3.122(6) Å (for Ir−OMe) which may help explain the pronounced shielding observed for this proton in the 1 H NMR spectra of the complexes.
Spectroscopic, Redox, and Electronic Properties.The electrochemical properties of the series of iridium complexes were explored using cyclic voltammetry in degassed MeCN solution at a 1 mM concentration, using [ n Bu 4 N][PF 6 ] as the supporting electrolyte (0.1 M) and the Fc/Fc + redox couple as a reference.The complexes generally showed very poor electrochemical stability (Figure S23), but each complex showed an irreversible oxidation above +1.0V which was tentatively assigned to the Ir 3+/4+ couple.The complexes with electron-withdrawing substituents, Ir−Cl and Ir−CF 3 , appeared to be the hardest to oxidize which is consistent with a metal-based process.Both Ir−Cl and Ir−CF 3 also showed an  additional irreversible feature ca.+ 0.5 V which is tentatively assigned to oxidation of the secondary amine group within the ligand. 33Each complex showed several reduction features which were typically irreversible suggesting poor electrochemical stability.
The UV−vis absorption data was obtained in aerated methanol and presented in Table 2 and Figure 2. The complexes absorb strongly (ε > 5000 M −1 cm −1 ) below 425 nm, which is primarily attributed to a combination of spinallowed, ligand-centered transitions associated with the different aromatic constituents within the ligands.Quite pronounced vibronic features are superimposed upon the different absorption features, which is consistent with the highly planar naphthyl-based structures.A weaker shoulder feature at 400−450 nm is evident across the series of complexes and is attributed to a band comprising a metal-toligand charge transfer (MLCT) contribution.A much weaker (ε < 1000 M −1 cm −1 ) feature at 450−550 nm, which is most pronounced for Ir−H, is possibly due to a spin forbidden S 0 → T 1 transition mediated by the heavy Ir atom (which possesses a large spin−orbit coupling constant). 34Across the series of complexes, the absorption spectra are broadly comparable suggesting the type of substituent present in the cyclometalated ligand produces a very subtle effect in terms of the absorption character of the complexes.Placed in context, the absorption features of these complexes share some similarities with the ubiquitous [Ir(ppy) 2 (bipy)] + (where ppy = 2phenylpyridine), 35 but with more efficient molar absorption in the 350−400 nm range, which is likely due to the more conjugated 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands.
The photoluminescence properties of the complexes were assessed under a range of conditions (using excitation at 410 nm). Figure 3 shows a comparison of the spectra obtained from the complexes in degassed solutions (toluene, dichloromethane, acetonitrile, and methanol) at room temperature.Photoluminescence was found to be strongly quenched under aerated conditions (Figure S24).While the emission maxima for the complexes lie in the range of 554−598 nm, implying these are green-to-orange emitters (note that [Ir-    (ppy) 2 (bipy)] + emits with a broad featureless peak at 602 nm assigned to an admixture of 3 MLCT/ 3 LLCT states), the appearance of the spectral profile varies profoundly across the series.Both Ir−Me and Ir−OMe display emission bands which are broad and featureless, whereas Ir−Cl and Ir−CF 3 possess highly structured emission spectra.Ir−H was perhaps intermediate between the two, with somewhat dampened vibronic features which were also shown to be solvent sensitive.
Time-resolved luminescence studies (Figure S25) also revealed some interesting differences.First, Ir−Cl and Ir− CF 3 possessed very similar aerated lifetimes of 604 and 638 ns which extend dramatically upon deoxygenation to ca. 7.4 and 10.0 μs, respectively.Together with the structured spectral appearance of the emission band, it appears that a ligandcentered triplet ( 3 LC) emission most likely dominates for these two complexes.The decay profiles for aerated solutions of Ir−Me and Ir−OMe were apparently biexponential and typically <300 ns.Upon deoxygenation, the fitted decay profile again yielded two lifetime values which were both extended, but to a much lesser degree than Ir−Cl and Ir−CF 3 .Ir−H showed decay kinetics that lie between these two extremes.This data showed that the nature of the ligand substituent strongly influences the excited state character of the emitting state.In the case of Ir−Cl and Ir−CF 3 , the emission is thus assigned to a 3 LC excited state localized on the cyclometalating ligand, while Ir−Me and Ir−OMe are probably dominated by 3 MLCT behavior, and Ir−H probably has a relatively increased admixture of 3 LC character to the emitting state.
Supporting computational studies (DFT) were deployed to further rationalize the experimental observations regarding the electronic properties of the complexes.All calculations were undertaken using the B3LYP functional with a 6-31G* basis set and an SDD ECP for the central iridium atom.The validity of the models were corroborated by comparing the minimum energy optimized geometries (Figure S26) with the crystallographic data; the DFT optimizations for Ir−H, Ir−Me, and Ir−OMe are in excellent agreement with the experimental crystal structures (Figure 4).
Molecular orbital decomposition analyses were performed in each case and predicted that the occupied orbitals have varying extents of Ir d-orbital and ligand-centered π character while the unoccupied orbitals are almost exclusively ligand-centered.Pictorial representations of the important frontier orbitals are shown in Figure 5 and S27−S29.
Although the calculations predict that the LUMO is located on the bipyridine ligand, it is not strongly represented in the transitions with the highest predicted oscillator strengths.In all cases, the calculations predict varying mixtures of MLCT (Ir → L C^N ) and C^N ligand-centered orbital contributions to the excited state.The pattern of orbital contributions is very similar for Ir−H and Ir−Me where the key LUMO+1 is localized on the C^N ligands.Interestingly, for Ir−OMe, the calculations predict a loss of degeneracy across the two C^N ligands (Figure 5) and thus a mixture of both MLCT and LLCT may contribute to the excited state.DFT predicts that the introduction of an electron-withdrawing group (Ir−Cl and Ir−CF 3 ) reduces the HOMO and HOMO−1 density at the metal, implying an overall weaker MLCT contribution, as noted in the experimental data.To estimate the vertical excitation energies of the low-lying singlet and triplet excited states of the complexes, TD-DFT calculations were carried out from the optimized ground-state geometries.The most important singlet transitions and their associated oscillator strengths are tabulated (Table 3 and Tables S3−S6) and suggest potential origins to the spectral features.The predicted spin forbidden S 0 → T 1 wavelengths also correlate well with the weak tail to the lowest energy absorption bands of the complexes.The calculations also performed quite well (Table 4) in the prediction of the T 1 → S 0 emission energies, with a clear correlation to the experimental data observed for both Ir−Cl and Ir−CF 3 versus the other complexes in the series.
Transient absorption spectra (degassed CH 2 Cl 2 ) were obtained (Figure 6) and corroborated the differences in the triplet character of the complexes.The data show the changes in optical density as a function of time following nanosecond pulsed laser irradiation at 420 nm.The spectra of Ir−CF 3 and Ir−Cl showed two main positive excited state absorption features ca.450 and 650 nm, whereas the other complexes look qualitatively different with several features across the 400−750 nm range.The decay kinetics (Figure S30) showed that both Ir−CF 3 and Ir−Cl possess much longer time scale triplet characteristics compared to the other complexes, which aligns with the observations of the photoluminescent measurements and may imply that the transient absorption spectra arise from closely related excited states. 37TA-UC studies were undertaken assessing each of the Ir(III) complexes as a prospective photosensitizer and 9,10diphenylanthracene (DPA) as the annihilator (the T 1 level of DPA is ca.700 nm and therefore lies below the triplet emitting level of all of the complexes herein).Time-resolved emission spectra were collected after sequential additions of DPA in deoxygenated CH 2 Cl 2 .In all cases, an excitation wavelength of 445 nm was used, which is selective for the visible absorption band of the complexes and avoids direct excitation of the DPA.Therefore, any emission from DPA in the 400−500 nm range can be solely attributed to an upconversion process via TTA resulting in delayed fluorescence.In summary, both Ir−CF 3 and Ir−Cl were demonstrated to be viable photosensitizers for TTA-UC (Figures 7 and S31−S34).For example, the data for Ir−CF 3 showed (Figure 9) that upon addition of DPA, the triplet emission at 550−750 nm was strongly quenched (and the lifetime reduced from 18.0 to 2.2 μs) and a new band was evident at 430 nm which is attributed to the delayed fluorescence of the DPA.The decay kinetics of this band showed two features: first, a grow-in phase which has a rise time of 2.2 μs, followed by a slow decay (τ = 11.1 μs).Thus, the rise time of the DPA emission corresponds to the accumulation of the DPA triplet state and the resultant decay of Ir−CF 3 in the presence of DPA.The reduction in lifetime of triplet emission from Ir−CF 3 is as expected if annihilation is assumed to be a diffusion-controlled process.In contrast, neither Ir−Me nor Ir−OMe was a viable photosensitizer for TTA-UC, suggesting that both complexes were clearly hindered by their comparatively short triplet lifetimes (Figures S31 and S32).In comparison, Ir−H produced weak, but definitive TTA-UC fluorescence, which we ascribe to the much shorter phosphorescence lifetime of the complex (2.3 μs, which is then quenched to <1 μs in the presence of DPA) (Figure S33).Finally, as predicted from the spectral data, Ir− Cl demonstrated broadly comparable behavior to Ir−CF 3 .The quenching of the photosensitizer triplet emission results in a substantive diminution of the lifetime from 24.0 to 2.7 μs (Figure S34); again, the rise time of the delayed fluorescence is comparable to the quenched lifetime of the photosensitizer.
From the time-resolved data, the rate constants for the triplet energy transfer were obtained directly from the fitted rise-times of the decay trace obtained at 430 nm (e.g., Figure 7d) giving k ET = 4.5 × 10 5 s −1 , 5.3 × 10 5 s −1 , and 6.7 × 10 5 s −1 for Ir−CF 3 , Ir−Cl, and Ir−H, respectively.Therefore, as Ir− CF 3 , Ir−Cl, and Ir−H each show a long triplet lifetime (>1.2 μs) this is beneficial to the triplet energy transfer to DPA and upconversion emission.Conversely, the relatively short triplet

■ CONCLUSIONS
This paper describes the synthesis and characterization of a series of luminescent iridium(III) complexes that incorporate cyclometalated imidazole ligands prepared from 2,3-diaminonaphthalene.The point of structural variance is a substituent on the metalated phenyl ring, providing either electrondonating or withdrawing capacity to this donor moiety.Three of the complexes were fully characterized in the solid state using single crystal X-ray diffraction.Both steady state and time-resolved photophysical studies unanimously revealed the phosphorescent nature of the complexes, but with significant variation noted within the series, which in turn depended upon the type of ligand substituent.This approach allowed rational and exquisite tuning between either a predominant 3 MLCT or 3 LC character to the emitting state of these complexes (which was further supported by DFT).The series of complexes was then assessed as photosensitizers in TTA-UC using DPA as the annihilator species.Our results showed that the Ir(III) complexes with the longest triplet lifetimes (those with a dominant 3 LC character) performed best as photosensitizers for TTA-UC, therefore suggesting that the use of electronwithdrawing substituents on the phenyl group in this ligand type is a convenient strategy for optimizing photosensitizer performance in TTA-UC.

Scheme 1 .
Scheme 1.General Synthetic Scheme for the Ligands and Complexes

Figure 2 .
Figure 2. (a) UV/vis absorption spectra of the Ir(III) complexes in methanol, c = 1.0 × 10 −5 M. (b) Phosphorescence emission spectra of the Ir(III) complexes in degassed dichloromethane (N 2 atmosphere).Optically matched solutions were used in each panel (each of the solutions gives the same absorbance at the excitation wavelength, A = 0.100), λ ex = 410 nm, 20 °C.

Figure 3 .
Figure 3. Phosphorescence emission spectra of the (a) Ir−Me, (b) Ir−H, (c) Ir−CF 3 , (d) Ir−Cl, and (e) Ir−OMe complexes in degassed solvents.Optically matched solutions were used in each panel (each of the solutions gives the same absorbance at the excitation wavelength, A = 0.100), λ ex = 410 nm, 20 °C.

Figure 5 .
Figure 5.A comparison of the calculated Kohn−Sham frontier molecular orbitals for Ir−H (left) and Ir−OMe (right).

Table 1 .
Iridium ligand Bond Lengths (Å) for the Three Crystal Structures

Table 4 .
Computed Values for the Absorption and Emission Maxima of the Experimentally Isolated Ir(III) Complexes a Experimentally determined wavelength maxima shown in parentheses.lifetimes of Ir−Me and Ir−OMe render triplet energy transfer less favorable because 3 MLCT→ S 0 competes very effectively. a