Unusual Temperature-Dependent Photophysics of Oligofluorene

Dec 8, 2011 - Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry .... Journal of Organometallic Chemistry 2015 775, 5...
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Unusual Temperature-Dependent Photophysics of OligofluoreneSubstituted Tris-Cyclometalated Iridium Complexes Qifan Yan, Yuanpeng Fan, and Dahui Zhao* Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry and the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The photophysical properties of a series of tris-cyclometalated Ir(III) complexes bearing oligofluorene-substituted 2-phenylpyridine (ppy) and/ or 1-phenylisoquinoline (piq) ligands were studied at both room temperature and 77 K, for delineating the oligomer-substitution effects on the photophysics in such metal-complex-containing conjugated oligomers/polymers. Unique temperature dependence was observed with the triplet excited-state lifetime of the studied oligomers. Molecules having one of the three ppy ligands substituted with an oligofluorenyl group at varied positions exhibited two distinct types of phosphorescing behaviors. When the oligoflurene group was coupled to ppy in a conjugative fashion (i.e., at 5- or 4′- position), the complexes appeared to emit from a 3MLCT-dominated state perturbed by LC transition, as evidenced by the relatively short lifetimes of phosphorescence as well as hypsochromic shift upon lowering the temperature. Surprisingly, even shorter triplet lifetimes were detected at 77 K for such oligomers. When the oligofluorenyl was tethered to the phenyl ring of ppy meta to pyridine, emission properties were consistent with a 3LC-dominated state, mixed with a certain MLCT component. Uniquely, for these oligomers an evident bathochromic shift of emission with a significantly retarded radiative decay rate was observed at 77 K. Furthermore, when a piq ligand was incorporated, red phosphorescence characteristic of Ir-piq-based 3MLCT transition emerged, disregarding the substation position of the oligofluorene. All these different photophysical behaviors, particularly their unique temperature dependence, were explained by considering an energy transfer process between different triplet states, with dominant MLCT and LC characteristics. In complexes having all ppy-derived ligands, these two states were of similar but different energy. While one played a more important role than the other, both were contributing to the phosphorescence emission. The temperature dependence of the photophysics reflected the equilibrium shifting process. When the 3MLCT-dominated state was lower in energy, faster radiative decay and shorter lifetimes were manifested upon lowering the temperature, as a result of more favored 3MLCT-dominated state. Whereas if the 3LC-dominated state was more stable, slower radiative decay emerged at decreased temperature due to further a reduced MLCT contribution. The bathochromic shift was also a result of equilibrium shifting to the state of lower energy. When the piq ligand was engaged, the emission was governed by the 3MLCT state of the Ir-piq moiety, which had much lower energy compared to the triplet states localized in oligofluorenyl ppy. DFT calculations substantiated the above hypothesis by identifying separate molecular orbitals possessing mixed but imbalanced MLCT and LC components.

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INTRODUCTION Cyclometalated iridium(III) complexes have been extensively studied mainly for their exceptional phosphorescence properties, exhibited by virtue of the large spin−orbital coupling (SOC) constant and efficient intersystem crossing (ISC) imparted the heavy metal.1 Incorporation of such metal complex moieties into conjugated polymer scaffolds has proven an effective approach to accessing triplet excited states of polymer materials and inducing enhanced phosphorescence emission.2 In addition to light-emitting diodes (LEDs),3 iridium complexes have also found versatile applications in more diverse fields, including sensors,4 biolabeling,5 and photovoltaic devices.6 For attaining optimal performance, various applications actually demand much differed photophysical properties. With the phosphorescent materials harvesting the triplet © 2011 American Chemical Society

excitons, the internal efficiency of LEDs can be considerably improved.7 In addition to pure light color, phosphors in LEDs are generally desired to possess high quantum yields and short lifetimes, i.e., rapid radiative and slow nonradiative decay rates, in order to maximize the brightness while suppressing triplet− triplet annihilation.8 In contrast, long-lived excitons are advantageous for sensory applications; that is, spontaneous radiative and nonradiative rates should both be slow.9 Longlifetime emission is favorable for bioimaging applications, which helps minimizing the interference of background fluorescence.10 Moreover, the long lifetime of triplet excitons has Received: October 31, 2011 Revised: November 21, 2011 Published: December 8, 2011 133

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also been shown facilitating charge separation in photovoltaic devices.11 Apparently, to suit such a wide range of applications, the ability to tailor the materials properties by structure modifications is highly desirable, and a proper understanding of the correlation of photophysical properties with chemical structures is indispensable for rational molecular designs.2,12 To this end, along with investigations focused on exploring innovative applications,13 systematic research was performed to delineate the structure−property relationship in organometallic phosphors.14−16 The basic principles and common characteristics of triplet emission in small-molecule complexes have generally been established. Specifically for cyclometalated iridium complexes, Thompson and co-workers carefully studied a number of blue-emitting complexes and elucidate the origin of the temperature dependence of their radiative decay process.16 Tsuboyama and co-workers surveyed a series of homoleptic iridium complexes of red phosphorescence,15 showing that for typical metal-to-ligand charge transfer (MLCT) excited state the emission underwent hypsochromic shift upon freezing the solvent, resulting from subsided solvent relaxation around the more polar excited state.17 In contrast, phosphorescence from ligand-centered (LC) triplet state (3π−π*) was much less sensitive to temperature, with minimal change in both band shape and emission wavelength.15a,18 Moreover, a slower radiative decay rate was typically observed with the 3LC state compared to that of 3MLCT state due to a limited participation of heavy metal orbitals in the LC transition. Moreover, a number of unique systems featuring an intramolecular energy transfer process between different triplet states have been reported. Earlier examples were some Ru complexes covalently linked to pyrene units.19 More recently, Thompson et al. studied a benzoporphyrin−Pt complex attached with four BODIPY units exhibiting triplet energy transfer.20 Ceroni and Zhao groups independently investigated Ru complexes coupled with trithiophene oligomer and pyrene units, both of which displayed reversible triplet energy transfer.21 In addition to studies on small molecules, Ir complexes have also been incorporated into linear2,22 and dendritic23 polymer scaffolds. In these systems, the auxiliary structures not only facilitated photon and carrier collections but also suppressed triplet−triplet annihilation by acting as antenna and/or insulating layers. However, compared to small-molecule systems, the photophysical details of metal-complex-containing conjugated polymers is more obscure and less understood,18,24,25 but relevant information is useful to avail more precise structure−property prediction and efficient material designs. Among different iridium complexes, tris-cyclometalated systems are of particular interest because of their extraordinarily high phosphorescing quantum efficiency16b,26 as well as the noncharged nature and chemical stability. Following a previous investigation,25 herein we report a more in-depth study on the temperature dependence of the photophysics of a series of triscyclometalated Ir(III) complexes having oligofluorenyl 2phenylpyridine (ppy) and 1-phenylisoquinoline (piq) ligands (Charts 1 and 2). On the basis of the experimental results, we proposed that an energy transfer between different triplet states (MLCT- and LC-dominated, respectively) occurred and was responsible for the observed unique temperature-dependent photophysics exhibited by the studied molecules. On the other hand, the current study also demonstrated effective approaches

Chart 1. Chemical Structures of Oligofluorenyl Ir(ppy)3 and Reference Complex Ir(ppy)3

to tuning the triplet state properties of related materials by modulating the ligand structures.

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RESULTS AND DISCUSSION Materials. Reference compounds Ir(ppy)3 and Ir(piq)3 were obtained according to procedures reported in the literature.8e,15a Oligofluorene-substituted Ir(ppy)3 complexes (Chart 1) were synthesized using our previously reported methods.25 Similar protocols were used to obtain oligofluorenesubstituted complexes containing piq ligands (Chart 2). Specifically, 1-chloroisoquinoline was reacted with tert-butyl or bromine-substituted phenylboronic acids under Suzuki coupling conditions, yielding the corresponding substitutedpiq ligands. Various tris-cyclometalated Ir(III) complexes with brominated ppy and piq ligands were subsequently acquired via Nonoyama reactions followed by ligand-exchange reactions. The tert-butyl groups on the phenyl rings helped improve the solubility of bromine-substituted complexes and allowed the use of column chromatography for their purification and separation from byproducts generated in ligand-exchange reactions. These intermediate complexes were then subjected to Suzuki coupling conditions to react with oligofluorenylboronic acids of different chain lengths, offering the target oligomeric complexes (synthetic details are described in the 134

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Chart 2. Chemical Structures of Oligofluorenyl Ir(ppy)(piq)2 and Ir(ppy)2(piq) with Reference Complex Ir(piq)3

Figure 1. Absorption spectra of some oligofluorene-substituted Ir(III) complexes in comparison with those of Ir(piq)3 and Ir(ppy)3 (spectrum of Ir(m-TFppy) was reproduced from ref 25).

absorption peaks at ca. 370 nm (Figure 1b). These optical bands with large extinction coefficients were as well ascribed to the spin-allowed π−π* transition of the trifluorene groups. The major absorption band of complex Ir(pTFpy)2(piq) exhibited a nearly identical maximum wavelength with that of Ir(pTFpy), suggesting that the excitation transitions took place relatively independently in each ligand. Complex Ir(TFpiq)2 with a trifluorenyl directly linked to piq manifested a slightly redshifted absorption maximum relative to those of Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq), imaginably due to electronic coupling between the trifluorenyl group and the piq unit.15a In summary, the absorption spectra of all studied oligomeric Ir(III) complexes corresponded mainly to intraligand electronic transitions. Charge transfer between the iridium center and ligands played a minor role in the excitation process. Photoluminescence Properties. The photoluminescence properties of all oligomer complexes were investigated at both room temperature and 77 K. The spectra are shown in Figures 2−4 and Figure S1, with relevant data summarized in Table 1 in comparison with those of reference compounds Ir(ppy)3 and Ir(piq)3. The emission spectra were collected by exciting at the absorption maxima of respective oligomer complexes, corresponding to the 1π−π* transition of the oligofluorenylsubstituted ligands. The observed properties of sensitivity to oxygen, large Stokes shift, and microsecond-ordered lifetimes all suggested that the emissions originated from triplet excited states. The fact that no significant fluorescence was detected with any of the oligomers confirmed that intersystem crossing occurred very rapidly and efficiently in these oligomeric iridium complexes. Our study was first focused on the complexes having all ppyderived ligands. Compared with phosphorescence spectra collected at 298 K,25 an evident hypsochromic shift was

Supporting Information). The structures of all newly synthesized complexes were characterized and confirmed by 1 H NMR and mass spectroscopies. Absorption Spectra. For complex series having an oligofluorene chain attached to the phenyl ring of ppy meta to the pyridine unit, i.e., Ir(m-Fppy), Ir(m-DFppy), and Ir(mTFppy), both the absorption maxima and absorbability increased with the oligofluorene chain length (Figure 1a). Evidently, the absorption spectra were dominated by the spinallowed LC 1π−π* transition of the oligofluorene moiety. The minor band centered at about 450 nm, showing a lower extinction coefficient, was from MLCT transitions, which remained nearly unchanged with the oligofluorene chain extension. Generally, these absorption features were similar to those of complexes Ir(Fppy), Ir(DFppy), and Ir(TFppy).25 The three oligomer complexes having piq ligands and trifluorenyl groups at different positions also exhibited principal 135

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Figure 2. Steady-state photoluminescence spectra and time-resolved emission decay curves (inset) of (a) Ir(TFppy) and (b) Ir(pTFpy) at 77 K (black lines) and 298 K (green lines, the room-temperature data were reproduced from ref 25); reduced lifetimes were observed at the lower temperature (12 and 7.4 μs for Ir(TFppy), 4.2 and 3.6 μs for Ir(TFppy) at 298 and 77 K, respectively).

exhibited by the emissions of both Ir(TFppy) and Ir(pTFpy) at 77 K, with evident vibronic features shown at both high and low temperatures (Figure 2). These optical features clearly revealed the complex nature of the triplet excited state in these oligomers to be the admixture of MLCT and LC states. Namely, the appearance of resolved vibronic structures at room temperature testified the LC characteristic, while the hyposochromic shift upon temperature decrease evidenced the charge transfer feature, which entailed solvent relaxation for stabilizing the more polar excited state at higher temperatures. Such solvent reorganization motions were substantially suppressed upon freezing the solvent. Surprising results emerged when time-resolved emission data were obtained. Typically, the triplet-state lifetimes of smallmolecule metal complexes extend upon lowering the temperature. One of the reasons for longer lifetime is that nonradiative decay rate contains a temperature-dependent component, which decreases with temperature because excitons of reduced thermal energy are less likely to overcome the activation barrier and access the nonradiative decay pathways. Another origin of the temperature-sensitive lifetime for iridium complexes lies in the different radiative decay rates of the three sublevels of the triplet state. The situation is well presented by Ir(ppy)3, which features a very small nonradiative decay rate and a large zerofield splitting, as illustrated by Thompson et al.16b Thermal population of the higher triplet substates that are capable of faster radiative decay was proposed to account for the reduced lifetime of Ir(ppy)3 at increased temperatures. However, contrary to these previous observations of common small-molecule

Figure 3. Normalized photoluminescence spectra and time-resolved emission decay curves (inset) of (a) Ir(m-Fppy), (b) Ir(m-DFppy), and (c) Ir(m-TFppy) at 77 K (black line) and 298 K (green lines); extended lifetimes were detected at lowered temperature (1.5 and 3.5 μs for Ir(m-Fppy), 45 and 236 μs for Ir(m-DFppy), 114 and 645 μs for Ir(m-TFppy) at 298 and 77 K, respectively).

iridium complexes, oligomers Ir(TFppy) and Ir(pTFpy) both exhibited substantially reduced lifetimes at the cryogenic temperature (Figure 1 and Table 1). To seek explanation for this unusual temperature dependence of lifetimes, additional data were collected. First, the chain length effect of the oligofluorenyl on the triplet exciton lifetime was evaluated. All three complexes having oligofluorenyl positioned on the phenyl ring of ppy para to the pyridine unit, Ir(Fppy), Ir(DFppy), and Ir(TFppy), showed hypsochromically shifted phosphorescence spectra upon lowering the temperature (Figure S1). While Ir(ppy)3 showed lifetimes of 1.5 and 3.4 μs at 298 and 77 K, respectively, complex Ir(Fppy) with one fluorene unit attached exhibited nearly unchanged lifetime values of 4.1 and 4.0 μs at the same temperatures (Table 1). When the oligomer chain was further extended, 136

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extended lifetime and hypsochromic shift of emission were observed with Ir(m-Fppy) when the temperature was lowered to 77 K (Figure 3a and Table 1). In stark contrast, when more fluorene units were appended, dissimilar photophysical properties were displayed by Ir(m-DFppy) and Ir(m-TFppy). While the emission quantum yield remained a significant value of 0.82, the excited state lifetime at 298 K became substantially prolonged to 45 μs for Ir(m-DFppy). More remarkably, when the temperature was lowered to 77 K, a particularly longer lifetime of over 200 μs was detected. On the basis of the radiative and nonradiative decay rates (kr and knr, respectively) calculated using the quantum yield (Φ) and lifetime (τ) values at 298 K, it was evident that a decrease of knr could not solely be responsible for such a long lifetime at 77 K, considering its relatively small value even at room temperature. In other words, kr must have dropped significantly with temperature for this molecule. Moreover, the phosphorescence of Ir(m-DFppy) exhibited a noticeable bathochromic shift at lowered temperature (Figure 3b). This phenomenon was rather unusual and not commonly observed previously with either 3MLCT- or 3 LC-governed emissions.15 An even more significant bathochromic shift and further extended lifetime (>600 μs) of emission were observed with Ir(m-TFppy) at 77 K. By analyzing the data presented above, the following information was deduced. Consistent with the conclusion drawn earlier,25 the emission of oligomers Ir(Fppy), Ir(DFppy), and Ir(TFppy) originated from mixed MLCT and LC transitions. As the oligofluorene chain was extended, the contribution of MLCT to the excited state was reduced relative to that of the LC state. This was evidenced by the decreased kr and knr with increasing oligofluorenyl chain length. However, the MLCT state remained the more important contributor over the entire series, as evidenced by the hypsochromic shift of phosphorescence observed at lowered temperature. On the other hand, for complexes having oligofluorenyl and pyridyl groups placed at meta positions, the triplet excited state appeared to experience a transition from being MLCT-dominated state, in Ir(m-Fppy), to LC-prevailed one, as in Ir(m-DFppy) and Ir(mTFppy).27 Such disparate photophysical behaviors of the above two series at room temperature were reasonably well explained by the different electronic coupling effect of m- vs p-phenylene linkage. For Ir(ppy)3, the MLCT process primarily relies on the LUMO of pyridine moiety to accept the negative charge. When the oligofluorene was located at the para position of the pyridine unit, the negative change could be effectively delocalized and thus stabilized by the entire conjugated oligofluorenyl ppy ligand. In contrast, delocalization of the negative change to the oligofluorene chain was not viable for the meta linkage. Consequently, for oligomer series Ir(m-Fppy), Ir(m-DFppy), and Ir(m-TFppy), the energy level of MLCT-dominated state remained relatively constant, whereas the energy level of the LC-dominant state was continuously lowered with extended π-conjugation. When the appended chain was short, as in Ir(m-Fppy), the LC-dominated excited state was still higher in energy and the MLCT process prevailed. As the oligomer chain grew longer, the LC-dominated state became lower in energy than the MLCT state. Hence, the lowest triplet state turned from a typical MLCT-dominated state to one mostly dictated by the LC process as more fluorene units were appended. Nonetheless, the unique temperature-dependent phenomena still awaited explanation, namely the shortened lifetimes of Ir(DFppy), Ir(TFppy), and Ir(pTFpy) but significantly

Figure 4. Normalized photoluminescence spectra and time-resolved emission decay curves (inset) of (a) Ir(pTFpy)2(piq), (b) Ir(mTFppy)2(piq), and (c) Ir(TFpiq)2 at 298 (green lines) and 77 K (black lines); the lifetimes of the main emission peaks were determined to be 1.2 and 2.9 μs for Ir(pTFpy)2(piq), 1.4 and 2.4 μs for Ir(m-TFppy)2(piq), and 1.2 and 2.0 μs for Ir(TFpiq)2 at 298 and 77 K, respectively; the red decay curves were obtained from the minor emission peak at 563 nm at 77 K, with a lifetime of 7.0 μs and 2.3 ms for Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq), respectively.

i.e., for Ir(DFppy), shorter lifetimes were observed at 77 K, similar to the observation made with Ir(TFppy). In general, for this homologue series, the triplet-state lifetime and its dependence on temperature manifested a tendency to be less similar to that of Ir(ppy)3 as the oligomer chain grew longer. However, rather distinct results were obtained from another oligomer series, which had the oligofluorenyl group tethered to phenyl ring of ppy meta to pyridine. In this series, with a single fluorenyl group, complex Ir(m-Fppy) exhibited very similar properties to those of Ir(ppy)3, i.e., showing very short excitedstate lifetime and a near-unity quantum efficiency, reflecting a very rapid radiative decay and a negligible contribution of nonradiative pathways. Also similar to Ir(ppy)3, moderately 137

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Table 1. Photoluminescence Properties of Studied Oligofluorene-Substituted Iridium Complexesa complex Ir(ppy)3d d

Ir(Fppy) Ir(DFppy)d Ir(TFppy)d Ir(pTFpy)d Ir(m-Fppy) Ir(m-DFppy) Ir(m-TFppy)d Ir(piq)3 Ir(pTFpy)2(piq) Ir(m-TFppy)2(piq) Ir(TFpiq)2

λmax (nm) (298|77 K) 510|495 550|527 558|536 559|540 568|549 518|503 523|531 524|540 619|598 628|604 622|600 641|626

Φ(298 K) e

0.97 0.81 0.67 0.67 0.82 >0.99 0.82 0.50 0.60f 0.69 0.72 0.44

τ (μs)b (298|77 K)

kr|knrc (/105 s−1)

1.5|3.4 4.1|4.0 8.0|6.0 12|7.4 4.2|3.6 1.5|3.5 45|236 114|645 1.1|2.5 1.2|2.9 (7.0g) 1.4|2.4 (2.3 × 103 g) 1.2|2.0

6.5|0.2 2.0|0.46 0.84|0.41 0.56|0.27 2.0|0.43 6.6|0.07 0.18|0.04 0.04|0.04 5.5|3.6 5.8|2.6 5.1|2.0 3.7|4.7

Data separated by “|” were obtained at 298 and 77 K, respectively; Ir(ppy)3 and oligofluorenyl Ir(ppy)3 were measured in degassed toluene; Ir(piq)3 and oligofluorenyl Ir(ppy)2(piq)/Ir(ppy)(piq)2 were measured in degassed MeTHF. bLifetimes shorter than 20 μs were measured by the timecorrelated single-photon counting method using NanoLED of 339 or 369 nm as the excitation light source; lifetimes longer than 20 μs were measured using a pulsed Xe lamp as the excitation light source. ckr and knr (radiative and nonradiative decay rates) were calculated at 298 K using kr = Φp/τ and knr = (1 − Φp)/τ, respectively. dData at room temperature were from ref 25. eThis value from ref 16b was used as the standard for the quantum yield measurement of oligofluorene-substituted Ir(ppy)3 complexes. fThis value from ref 28 was used as the standard for the quantum yield measurement of oligofluorenyl Ir(ppy)2(piq) and Ir(ppy)(piq)2 complexes. gData measured at 77 K for the minor emission band with a maximum at 563 nm. a

became the more important contributor to the emission, as in Ir(m-DFppy) and Ir(m-TFppy), and the temperature dependence of photophysics readily revealed the equilibrium shifting of the energy-transfer process. The higher MLCT state became less populated at lower temperature, resulting in further retarded decay rates and extended lifetimes. Since the LCdominated state was of lower energy, bathochromic shift of emission emerged as the temperature dropped. It is noteworthy that the LC-dominated state in Ir(m-DFppy) and Ir(m-TFppy) was likely more emissive than those in Ir(DFppy), Ir(TFppy), and Ir(pTFpy), since the oligofluorene segment was electronically coupled to the metal center via a p-phenylene unit, and thus a more direct d-orbital contribution to the LC-dominated transition was viable. This was clearly viewed from the DFTcalculated molecular orbitals (Figure S3). The triplet energy transfer in previously reported systems typically took place between 3MLCT and LC states localized on separate chromophores,19−21 and the 3π−π* state mostly served as a “dark” energy reservoir, affecting the exciton lifetime. A distinct difference of our current system is that the MLCT- and LC-dominated triplet states might be both emissive. Because of a more integrated coupling of the oligofluorene with the metal center, substantial mixing of the transition metal d-orbital with the LC process occurred, enabling a pronounced radiative decay rate of the LC-dominated state.27 Furthermore, with a relatively small knr value typical of Ir complexes, the emission from the LC-dominant state was detectable. On the other hand, likely due to the large size and disorder of the oligomeric ligands, the MLCT- and LCdominated states were relatively localized and vibrational coupling between them was not particularly efficient. All these factors contributed to the observation of the energy transfer process. Subsequent investigations were conducted with the three complexes accommodating piq ligands (Figure 4 and Table 1). The reference complex Ir(piq)3 emitted red phosphorescence with a quantum yield of ca. 0.6. At room temperature, a very short excited-state lifetime of 1.1 μs was exhibited, characteristic of a typical 3MLCT state. Its significantly red-shifted emission

prolonged ones of Ir(m-DFppy) and Ir(m-TFppy) in companion with different chromic shift of emissions at lowered temperature. These observations led to a hypothesis of an energy-transfer process between two triplet states, with dominant MLCT and LC characteristics in these oligomers.27 In the above oligomer complexes, because these two states possessed similar energy, a reversible energy transfer might have occurred, and the emission appeared as a single decay process due to rapid equilibration. For the oligomer series featuring para-linked oligofluorenyl and pyridine, both the energy levels of the MLCT- and LC-dominated states were lowered upon elongation of the oligofluorene chain. Although the LC-dominated state was stabilized more effectively than the MLCT state at the same chain length, the latter remained lower in energy and being the major contributor to the emission over the entire series, which was evidenced by the hypsochromic shift of emission with decreased temperature. As the temperature decreased, the MLCT-dominated state of lower energy was favored even more strongly, thus giving rise to faster radiative decay and shortened lifetime. It should be noted that in these oligomers the MLCT-dominated state is probably significantly mixed with LC transition. On the other hand, the LC-dominated state might not possess a very significant radiative decay rate, due to the minimal involvement of the metal atom, but its presence affected the emission lifetime by entailing the energy-transfer process. For the meta-linked series, as aforementioned, while the LC state gained stabilizing energy from the oligomer chain extension, the MLCT state remained roughly unaffected due to the meta linkage between the pyridine unit and oligofluorene chain. Therefore, in Ir(m-Fppy) the MLCT-dominated state was much lower in energy compared to the LC-dominated state, and therefore the molecule exhibited very similar properties to those of Ir(ppy)3. The lifetime dependence on temperature of Ir(m-Fppy) likely followed a similar scenario to that of Ir(ppy)3. That is, increased thermal population of higher triplet state sublevels brought about faster radiative decay at increased temperature. However, when the LC-dominated state was stabilized and overrode the MLCT-dominated state, it 138

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magnitudes to those observed for Ir(pTFpy) and Ir(m-TFppy) at the same temperature, suggesting that these minor emission bands were from the MLCT- and LC-dominated triplet states of the oligofluorenyl ppy ligands, respectively. Upon the attachment of a trifluorenyl group directly to piq unit, complex Ir(TFpiq)2 exhibited a phosphorescence peak at ca. 640 nm, bathochromically shifted by >20 nm relative to that of Ir(piq)3. The quantum efficiency was lowered to 0.44 at room temperature, resulting from both decreased kr and increased knr compared to those of Ir(piq)3. The bathochromic shift was apparently due to the π-system extension via conjugative coupling of trifluorenyl with piq. While the increased knr was a reasonable consequence of the energy gap law, the slightly smaller kr perhaps implied a slight mixing of the LC state with MLCT transition in trifluorenyl piq.15

compared to that of Ir(ppy)3 was due to the enlargement of the π-system upon fusing a benzo group to the pyridine unit, which substantially enhanced the electron-accepting ability and lowered the MLCT transition energy. Notably, at both 298 and 77 K the photophysical properties exhibited by Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq) were akin to that of Ir(piq)3, but distinctly different from those of related oligomers Ir(pTFpy) and Ir(m-TFppy). Specifically, both Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq) emitted red phosphorescence with wavelength maxima at ca. 600 nm. The room-temperature lifetimes were slightly longer than 1 μs. At 77 K, the emission maxima were blue-shifted by >20 nm, exhibiting slightly extended lifetimes of 2−3 μs. All these results suggested that the two oligomer complexes were emitting from a triplet state similar to that of Ir(piq)3, that is, the 3MLCT of Ir-piq moiety. The property difference between Ir(pTFpy)2(piq)/Ir(mTFppy)2(piq) and Ir(pTFpy)/Ir(m-TFppy) was understandable considering the energy levels of relevant triplet states. The emission wavelengths clearly revealed that the 3MLCT state of Ir-piq was considerably lower in energy than both 3MLCT- and 3 LC-dominated states conferred by oligofluorenyl ppy.15a Notably, since the emissions of these oligomers were recorded by exciting at their absorption maxima (corresponding to π−π* transition of trifluorenyl ppy), an interligand energy transfer to the Ir-piq MLCT state presumably happened upon excitation. Imaginably, this energy-transfer process was nearly irreversible considering the large energy difference. Hence, phosphorescence from oligofluorenyl ppy-based MLCT- or LCdominated states was undetectable at room temperature for Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq). Nonetheless, at 77 K a trace of emission from these trifluorenyl ppy triplet states was detected, when the emission bands were narrowed and better resolved. A very small emission band was detected at ca. 560 nm, the lifetime of which was ca. 7 μs for Ir(pTFpy)2(piq) and over 2 ms for Ir(m-TFppy)2(piq). These values corresponded in

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CONCLUSION In the current study, the photophysical behaviors of a series of tris-cyclometalated Ir(III) complexes having oligofluorenesubstituted ppy and piq ligands were examined at both room temperature and 77 K, in comparison with those of the prototype molecules Ir(ppy)3 and Ir(piq)3. Basically, phosphorescent properties with unique temperature dependence were observed, which were explained by referring to an energytransfer process between different triplet states in the complexes. The involved states and relevant energy transfer processes in various molecules are schematically depicted in Figure 5. The respective energy levels were estimated based on the emissions of the oligomers at 77 K.29 Depending on the ligand structure and anchoring position of the oligomer, complexes displayed three distinct types of photophysical behaviors. In the first case, the two triplet states were of similar energy, but the energy level of the MLCTdominated state was slightly lower than that of the LCdominated state. This situation was presented by complexes Ir(DFppy), Ir(TFppy), and Ir(pTFpy). A reversible energy

Figure 5. Schematic representation of the energy transfer processes among different triplet states in varied trifluorene-substituted iridium complexes.29 139

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degassed by conducting three freeze−pump−thaw cycles before the cuvettes were sealed under vacuum. Lifetimes shorter than 20 μs were measured with time-correlated single-photon counting using a Horiba Jobin Yvon FluoroHub-B instrument and FluoroMax-4P spectrofluorometer and NanoLED of 339 or 369 nm as the excitation source. The full width at half-maximum (fwhm) of the instrument response function (IRF) obtained by an aqueous solution of Ludox colloidal silica was typically ∼1 ns in our system. Phosphorescence lifetimes were fitted with single-exponential decay functions without deconvolution. Single-exponential fits were performed using Horiba Jobin Yvon DAS6 software. Lifetimes longer than 20 μs were measured with FluoroMax-4P spectrofluorometer with program pulsed Xe lamp as the excitation source. fwhm of the Xe lamp pulse was typically ∼3 μs. Phosphorescence lifetime was fitted with single-exponential decay functions without deconvolution. Single-exponential fits were performed using the Origin software.

transfer thus took place between the two triplet states with the equilibrium favoring the LC-dominated state. As the temperature decreased, the equilibrium was more strongly biased, giving rise to faster radiative decay and shortened exciton lifetime. The emission wavelength change with temperature confirmed the greater contribution from the MLCT transition in these oligomers. A different energy level arrangement was illustrated by complexes Ir(m-DFppy), Ir(m-TFppy), in which the LC-dominated state was slightly more stable than MLCTdominated state. The much longer emission lifetime evidenced the dominant role of the LC transition. At reduced temperature, further prolonged lifetime and bathochromic shift of emission were manifested, resulting from a further shifted equilibrium toward the LC-dominated state. The third scenario emerged when the oligomer complexes incorporated piq as ligand. Since the 3MLCT state of Ir-piq moiety was much lower in energy compared to both MLCT- and LC-dominated states located in oligofluorenyl ppy ligand, the emission naturally originated from the former state. The fact that the photophysical properties of Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq), including the temperature dependence, were comparable to those of Ir(piq)3 served as unambiguous evidence for the similar nature of their lowest triplet states. Additionally, the properties of Ir(pTFpy)2(piq) and Ir(m-TFppy)2(piq) also proved that interligand energy transfer occurred efficiently, irrespective of the oligomer substitution position on ppy. The current investigation demonstrated that, by manipulating the relative energy levels of the 3MLCT and 3LC states through ligand structure modifications, photophysical properties (e.g., lifetime, kr, etc.) could be tuned in a wide range. By virtue of the rapid intersystem crossing and large spin−orbital coupling constant of the iridium center, optimal quantum yields were achieved in all different cases. Additionally, when large oligomer/polymer chains are covalently tethered to the complex, photophysical properties that are unconventional for small molecule systems may emerge. Our results delineated certain structure−property correlation and provided useful information for rational designs of relevant metal-complexcontaining conjugated polymers, while offer the possibility of tailoring the triplet state properties to suit various applications.

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ASSOCIATED CONTENT S Supporting Information * Synthetic procedures, additional absorption and emission spectra, and DFT calculated molecular orbitals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Projects 51073002 and 21174004) and the Fok Ying-Tung Educational Foundation (No. 114008).

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REFERENCES

(1) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Adv. Mater. 2009, 21, 4418. (2) (a) Carlise, J. R.; Wang, X.-Y.; Weck, M. Macromolecules 2005, 38, 9000. (b) Evans, N. R.; Devi, L. S.; Mak, C. S. K.; Watkins, S. E.; Pascu, S. I.; Köhler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. J. Am. Chem. Soc. 2006, 128, 6647. (c) Liang, B.; Wang, L.; Xu, Y.; Shi, H.; Cao, Y. Adv. Funct. Mater. 2007, 17, 3580. (d) Haldi, A.; Kimyonok, A.; Domercq, B.; Hayden, L. E.; Jones, S. C.; Marder, S. R.; Weck, M.; Kippelen, B. Adv. Funct. Mater. 2008, 18, 3056. (e) Powell, A. B.; Bielawski, C. W.; Cowley, A. H. J. Am. Chem. Soc. 2010, 132, 10184. (f) Ma, Z.; Ding, J.; Zhang, B.; Mei, C.; Cheng, Y.; Xie, Z.; Wang, L.; Jing, X.; Wang, F. Adv. Funct. Mater. 2010, 20, 138. (g) Lai, W.-Y.; Levell, J. W.; Jackson, A. C.; Lo, S.-C.; Bernhardt, P. V.; Samuel, I. D. W.; Burn, P. L. Macromolecules 2010, 43, 6986. (3) (a) Thompson, M. E.; Djurovich, P. I.; Barlow, S.; Marder, S. R. In Comprehensive Organometallic Chemistry; O’Hare, D., Ed.; Elsevier: Oxford, 2007; Vol. 12, pp 101−194. (b) Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H., Ed.; Wiley-VCH: Berlin, 2007. (c) Chou, P. T.; Chi, Y. Chem.Eur. J. 2007, 13, 380. (d) You, Y.; Park, S. Y. Dalton Trans. 2009, 1267. (e) Wong, W.-Y.; Ho, C.-L. J. Mater. Chem. 2009, 19, 4457. (f) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. Adv. Mater. 2010, 22, 572. (4) (a) Sax, S.; Fisslthaler, E.; Kappaun, S.; Konrad, C.; Waich, K.; Mayr, T.; Slugovc, C.; Klimant, I.; List, E. J. W. Adv. Mater. 2009, 21, 3483. (b) Zhao, Q.; Li, F.; Huang, C. Chem. Soc. Rev. 2010, 39, 3007. (c) Lo, K. K.-W.; Li, S. P.-Y.; Zhang, K. Y. New J. Chem. 2011, 35, 265. (5) (a) Lo, K. K.-W.; Tsang, K. H.-K.; Sze, K.-S.; Chung, C.-K.; Lee, T. K.-M.; Zhang, K. Y.; Hui, W.-K.; Li, C.-K.; Lau, J. S.-Y.; Ng, D. C.-M.; Zhu, N. Coord. Chem. Rev. 2007, 251, 2292. (b) FernándezMoreira, V.; Thorp-Greenwood, F. L.; Coogan, M. P. Chem. Commun. 2010, 46, 186. (c) Zhao, Q.; Huang, C.; Li, F. Chem. Soc. Rev. 2011, 40, 2508. (6) Schulz, G. L.; Holdcroft, S. Chem. Mater. 2008, 20, 5351.

EXPERIMENTAL SECTION

General Procedures. UV−vis absorption spectra were recorded on a Hitachi U-4100 spectrophotometer. Photoluminescence spectra were recorded on a Horiba Jobin Yvon FluoroMax-4P spectrofluorometer with a right-angle geometry, using 1 cm quartz cuvettes for solution samples. The emission spectra were corrected for the wavelength dependency of the detector sensitivity and monochromator gratings. For quantum yield measurements, the optical density of the solutions was approximately 0.05−0.1 (at concentrations of ca. 1.0 × 10−6 M) at the excitation wavelength. A quantum yield value of 0.97 for Ir(ppy)3 in toluene16b was used as the standard for oligofluorenyl Ir(ppy)3 complex series with an excitation wavelength of 340 nm. A quantum yield of 0.60 for Ir(piq)3 in dichloromethane28 was used as the standard value for oligofluorenyl Ir(ppy)2(piq) and Ir(ppy)(piq)2 with an excitation wavelength of 350 nm. The solutions used for quantum yield measurements were degassed by bubbling N2 for 8 min prior to experiments. The precision of quantum yield measurements was ±5% upon repetition. Samples for photoluminescence measurements at 77 K were contained in a quartz tube placed in a Dewar cooled by liquid nitrogen. Lifetime Measurements. Samples for phosphorescence lifetime measurement were dissolved in MeTHF (distilled from CaH2) or toluene (purified by Mbraun SPS-800 solvent purification system) at similar concentrations for quantum yield measurements. Samples were 140

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Macromolecules

Article

(7) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) O’Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 74, 442. (c) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (8) (a) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377. (b) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 7992. (c) Zhou, G.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L. Angew. Chem., Int. Ed. 2007, 46, 1149. (d) Ho, C.-L.; Wong, W.-Y.; Gao, Z.-Q.; Chen, C.-H.; Cheah, K.-W.; Yao, B.; Xie, Z.; Wang, Q.; Ma, D.; Wang, L.; Yu, X.-M.; Kwok, H.-S.; Lin, Z. Adv. Funct. Mater. 2008, 18, 319. (e) Lee, S. J.; Park, K.-M.; Yang, K.; Kang, Y. Inorg. Chem. 2009, 48, 1030. (9) (a) Huynh, L.; Wang, Z.; Yang, J.; Stoeva, V.; Lough, A.; Manners, I.; Winnik, M. A. Chem. Mater. 2005, 17, 4765. (b) Schmittel, M.; Lin, H. Inorg. Chem. 2007, 46, 9139. (c) Liu, Z.; Bian, Z.; Bian, J.; Li, Z.; Nie, D.; Huang, C. Inorg. Chem. 2008, 47, 8025. (d) Mak, C. S. K.; Pentlehner, D.; Stich, M.; Wolfbeis, O. S.; Chan, W. K.; Yersin, H. Chem. Mater. 2009, 21, 2173. (e) Ji, S.; Wu, W.; Wu, W.; Song, P.; Han, K.; Wang, Z.; Liu, S.; Guo, H.; Zhao, J. J. Mater. Chem. 2010, 20, 1953. (10) Lo, K. K.-W.; Hui, W.-K.; Chung, C.-K.; Tsang, K. H.-K.; Ng, D. C.-M.; Zhu, N.; Cheung, K.-K. Coord. Chem. Rev. 2005, 249, 1434. (11) (a) Shao, Y.; Yang, Y. Adv. Mater. 2005, 17, 2841. (b) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurišić, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nature Mater. 2007, 6, 521. (c) Li, Y.; Mastria, R.; Li, K.; Fiore, A.; Wang, Y.; Cingolani, R.; Manna, L.; Gigli, G. Appl. Phys. Lett. 2009, 95, 043101. (d) Ning, Z.; Zhang, Q.; Wu, W.; Tian, H. J. Organomet. Chem. 2009, 694, 2705. (e) Clem, T. A.; Kavulak, D. F. J.; Westling, E. J.; Fréchet, J. M. J. Chem. Mater. 2010, 22, 1977. (f) Wong, W.-Y.; Ho, C.- L. Acc. Chem. Res. 2010, 43, 1246. (12) (a) Yersin, H.; Donges, D. Top. Curr. Chem. 2001, 214, 81. (b) Yam, V. W.-W. Acc. Chem. Res. 2002, 35, 555. (c) Baranoff, E.; Scopelliti, I. J. R.; Solari, E.; Grätzel, M.; Nazeeruddin, M. K. Dalton Trans. 2011, 40, 6860. (13) For recent examples, see: (a) Zhao, Q.; Liu, S.; Li, F.; Yi, T.; Huang, C. Dalton Trans. 2008, 3836. (b) Chiu, Y.-C.; Chi, Y.; Hung, J.-Y.; Cheng, Y.-M.; Yu, Y.-C.; Chung, M.-W.; Lee, G.-H.; Chou, P.-T.; Chen, C.-C.; Wu, C.-C.; Hsieh, H.-Y. ACS Appl. Mater. Interfaces 2009, 1, 433. (c) Xie, Z.; Ma, L.; deKrafft, K. E.; Jin, A.; Lin, W. J. Am. Chem. Soc. 2010, 132, 922. (d) Palmer, J. H.; Durrell, A. C.; Gross, Z.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2010, 132, 9230−9231. (e) Chen, Z.-Q.; Bian, Z.-Q.; Huang, C.-H. Adv. Mater. 2010, 22, 1534. (f) Costa, R. D.; Orti, E.; Bolink, H. J.; Graber, S.; Housecroft, C. E.; Constable, E. C. J. Am. Chem. Soc. 2010, 132, 5978. (g) Tan, W.; Zhou, J.; Li, F.; Yi, T.; Tian, H. Chem.Asian J. 2011, 6, 1263. (14) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Top. Curr. Chem. 2007, 281, 143. (15) (a) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (b) Okada, S.; Okinaka, K.; Iwawaki, H.; Furugori, M.; Hashimoto, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Tsuboyama, A.; Takiguchi, T.; Ueno, K. Dalton Trans. 2005, 1583. (16) (a) Lamansky, S.; Djurovich, P. I.; Mruphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (b) Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A. III; Thompson, M. E. J. Am. Chem. Soc. 2009, 131, 9813. (17) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985, 107, 1431. (18) (a) Glusac, K. D.; Jiang, S. J.; Schanze, K. S. Chem Commun. 2002, 2504. (b) Cunningham, G. B.; Li, T.; Liu, S.; Schanze, K. S. J. Phys. Chem. B 2003, 107, 12569. (19) (a) McClenaghan, N. D.; Leydet, Y.; Maubert, B.; Indelli, M. T.; Campagna, S. Coord. Chem. Rev. 2005, 249, 1336. (b) Lavie-Cambot,

A.; Lincheneau, C.; Cantuel, M.; Leydet, Y.; McClenaghan, N. D. Chem. Soc. Rev. 2010, 39, 506. (20) Whited, M. T.; Djurovich, P. I.; Robert, S. T.; Durrell, A. C.; Schlenker, C. W.; Bradforth, S. E.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 88. (21) (a) Manca, P.; Pilo, M. I.; Sanna, G.; Zucca, A.; Bergamini, G.; Ceroni, P. Chem. Commun. 2011, 47, 3413. (b) Ceroni, P. Chem. Eur. J. 2011, 17, 9560. (c) Ji, S.; Wu, W.; Wu, W.; Guo, H.; Zhao, J. Angew. Chem., Int. Ed. 2011, 50, 1626. (22) (a) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby, C. E.; Köhler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004, 126, 7041. (b) Tavasli, M.; Bettington, S.; Bryce, M. R.; Al Attar, H. A.; Dias, F. B.; King, S.; Monkman, A. P. J. Mater. Chem. 2005, 15, 4963−4970. (c) Jiang, J. X.; Xu, Y. H.; Yang, W.; Guan, R.; Liu, Z. Q.; Zhen, H. Y.; Cao, Y. Adv. Mater. 2006, 18, 1769. (d) Schulz, G. L.; Chen, X.; Chen, S.-A.; Holdcroft, S. Macromolecules 2006, 39, 9157. (23) (a) Qin, T.; Ding, J.; Wang, L.; Baumgarten, M.; Zhou, G.; Mullen, K. J. Am. Chem. Soc. 2009, 131, 14329. (b) Chen, L.; Ding, J.; Cheng, Y.; Xie, Z.; Wang, L.; Jing, X.; Wang, F. Chem.Asian J. 2011, 6, 1372. (24) Kozhevnikov, D. N.; Kozhenikov, V. N.; Shafikov, M. Z.; Prokhorov, A. M.; Bruce, D. W.; Williams, J. A. G. Inorg. Chem. 2010, 50, 3804. (25) Yan, Q.; Yue, K.; Yu, C.; Zhao, D. Macromolecules 2010, 43, 8479. (26) (a) Tanaka, I.; Tabata, Y.; Tokito, S. Jpn. J. Appl. Phys., Part 2 2004, 43, L1601. (b) Kawamura, Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071104. (c) Holzer, W.; Penzkofer, A.; Tsuboi, T. Chem. Phys. 2005, 308, 93. (d) Endo, A.; Suzuki, K.; Yoshihara, T.; Tobita, S.; Yahiro, M.; Adachi, C. Chem. Phys. Lett. 2008, 460, 155. (27) It should be noted that in such organometallic complexes it is unlikely that electronic transitions of sheer MLCT or LC properties could exist exclusively. The “MLCT-dominated state” discussed herein refers to the excited state featuring a prevailing MLCT transition, but it usually incorporates certain contribution from the LC π−π* transition; similarly, the “LC-dominated state” is used to describe an excited state with a predominant LC π−π* transition property, which inevitably contains minor MLCT component. Such a complex nature of electronic transitions comprising imbalanced contributions from two types of processes was supported by DFT-calculated molecular orbitals, which showed simultaneous contributions, with differed composing ratios, from both d-orbital of the transition metal and π-orbital of the oligofluorenyl moiety (see the Supporting Information for calculated molecular orbitals). (28) Huang, Y.-T.; Chuang, T.-H.; Shu, Y.-L.; Kuo, Y.-C.; Wu, P.-L.; Yang, C.-H.; Sun, I.-W. Organometallics 2005, 24, 6230. (29) Estimated energies of oligofluorenyl-ppy based LC states from currently studied complexes were consistent with data of related structures from references: Hertel, D.; Setayesh, S.; Nothofer, H.-G.; Scherf, U.; Müllen, K.; Bässler, H. Adv. Mater. 2001, 13, 65 , and ref 16b.

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