Tailoring the Emission of Fluorinated Bipyridine-Chelated Iridium

6 mins ago - New functionalized tris(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(III) ((dfpypy)3Irs) complexes, including small molecules a...
0 downloads 0 Views 2MB Size
Download PDF
This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article Cite This: ACS Omega 2018, 3, 13808−13816

http://pubs.acs.org/journal/acsodf

Tailoring the Emission of Fluorinated Bipyridine-Chelated Iridium Complexes Guang Zhang,†,§,∥ Martin Baumgarten,*,† Dieter Schollmeyer,‡ and Klaus Müllen*,† †

Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz D-55128, Germany Institute für Organische Chemie, Johannes-Gutenberg Universität, Mainz D-55128, Germany § Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China ‡

ACS Omega 2018.3:13808-13816. Downloaded from pubs.acs.org by 185.251.14.213 on 10/24/18. For personal use only.

S Supporting Information *

ABSTRACT: New functionalized tris(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(III) ((dfpypy)3Irs) complexes, including small molecules and their dendrimer embedded analogoues, were synthesized and characterized. It is demonstrated that both the fac-(dfpypy) 3 Ir-based polyphenylene dendrimers and (triisopropylsilyl)ethynyl (TIPSE)-substituted (dfpypy)3Ir complexes induce large bathochromic shifts (∼50 nm) of emission bands compared with fac-(dfpypy)3Ir. This is due to the pronounced 3π−π* character of emissive excited states and the extended conjugation. A further remarkable feature is the small bathochromic shift of the emissions of fac-tris(2-phenylpyridine) iridium ( fac-(ppy)3Ir)-based polyphenylene dendrimers when compared to those of the iridium (Ir) complex core. Obviously, the triplet metal-to-ligand charge transfer makes emission less sensitive to extended conjugation than the 3π−π* transition. This finding suggests new concepts for designing blue phosphorescent dendrimer emitters. Both the dendrimers and the TIPSE-substituted (dfpypy)3Ir complexes represent new green and the trimethylsilyl-functionalized (dfpypy)3Ir new blue phosphorescent emitters. Incorporation of TIPSE moieties into the ligands of iridium complex gives rise to enhanced phosphorescence.

■

INTRODUCTION Phosphorescent emitters (PEs) such as iridium (Ir) and platinum (Pt) complexes have been intensively investigated since the first report on PE-based organic light emitting diodes (OLED) by Baldo et al. in 1998.1−5 PEs could provide much higher efficiencies than conventional organic singlet fluorophores because the former generates both singlet and triplet excitons in devices (corresponding to 100% internal quantum efficiency and around 20% external quantum efficiency (EQE) theoretically).1−8 Significant improvements of red,9 green,10 blue,11−13 and white14 OLEDs based on small-molecule PEs have been achieved so far and all have reached or passed 20% EQEs. PEs have also received attention in applications such as photocatalysis,15−17 chemical sensors,18,19 solar cells,20,21 and biolabeling,22 due to their intriguing photophysical properties. The neat films of PEs are prone to undergo severe selfquenching.23−27 Therefore, the emitters (also called dopants) are usually incorporated into a matrix to ensure high photoluminescence quantum yields (PLQYs). The matrix composed of host materials, e.g., 4,4′-bis(N-carbazolyl)-1,1′biphenyl, serves as medium for charge transport and energy transfer to the dopants.6,28−30 However, these devices based on small molecules still suffer from expensive ultra high vacuum © 2018 American Chemical Society

fabrication and inhomogeneous distribution of the dopants in the matrix. Dendrimers can overcome these problems because of their good film formation and generation-by-generation sitespecific functionalization.31−35 In a rigid and structurally welldefined dendritic molecule, the core can be a phosphorescent emitter while the surface is functionalized with host moieties. Thus, the ratio between the hosts and dopants and their positions are accurately controlled in a dendritic structure.32,33 There have been many studies on dendrimer-based phosphorescent emitters, especially green and red ones, since the pioneering work of Samuel and Burn et al. in 2001.35−43 So far, very few blue PEs were reported and they all suffered from poor color purity and/or low efficiencies.44−48 Accordingly, it is worthwhile to develop high-performance, dendrimer-based blue PEs. Shape-persistent polyphenylene dendrimers (PPDs) as emitters have been widely studied.49−54 Core and surfacefunctionalized, first-generation PPDs were shown to improve OLED performance through the shielding effect and Received: August 8, 2018 Accepted: September 25, 2018 Published: October 22, 2018 13808

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Article

Figure 1. Molecular structures of fac-(ppy)3Ir, G1, fac-(dfpypy)3Ir, and the targeted iridium complexes.

Scheme 1. Synthetic Routes for TMS-Functionalized (dfpypy)3Ira

(a) (1) n-BuLi, Et2O, −78 °C, 1 h, (2) TMSCl, room temperature (rt), 12 h, 82%; (b) (2,6-difluoro-3-pyridinyl)boronic acid 9, Pd(PPh3)4, K2CO3, tetrahydrofuran (THF), water, 85 °C, 24 h, 50%; (c) IrCl3·nH2O, 2-ethoxyethanol, 140 °C, 24 h, 48%; (d) compound 7, AgSO3CF3, K2CO3, mesitylene, 170 °C, 24 h, 53%; (e) UV light, THF, 12 h, 90%; (f) (1) isopropylamine, THF, n-BuLi, 0 °C, 30 min for making fresh lithiumdiisopropylamide (LDA), (2) LDA, THF, −78 °C, 1 h, (3) triisopropylborate, rt, 12 h, 1 M HCl (aq), 100%; (g) ICl, CCl4, 80 °C, 12 h; (h) Ir(acac)3, ethylene glycol, 200 °C. a

PPD, en route to unprecedented blue phosphorescent dendrimer emitters. In this contribution, as depicted in Figure 1, several (dfpypy)3Ir-based small molecules and a firstgeneration PPD with carbazoles in the periphery (1) are introduced, whereby carbazoles act as charge transport and energy transfer moieties.

appropriate choice of surface moieties. The latter were thought to bring about efficient surface-to-core energy transfer and charge transport.50,51 PPDs were also employed as phosphorescent green and red emitters.49,53 Interestingly, the peak emission of fac-(ppy)3Ir-based PPDs (G1−G4) (Figure 1) in solution was located at 516 nm, i.e., very close to that of fac(ppy)3Ir (λem: ∼508 nm).55 It follows that the attachment of polyphenylene dendrons on fac-(ppy)3Ir does not change the emission wavelength notably. fac-Tris(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(III) (fac-(dfpypy)3Ir) (Figure 1) was described as a pure blue PE (λmax: 438 and 463 nm) with high PLQY (∼0.71) and thermal stability (5% loss at 452 °C from thermogravimetric analysis).56 We thus chose this chromophore as the core of a

■

RESULTS AND DISCUSSION Synthesis. A trimethylsilyl (TMS)-substituted Ir complex 4 was considered as the initial candidate because TMS groups could be transformed into iodo substituents later.57 As depicted in Scheme 1, 2-bromo-5-trimethylsilylpyridine (8) was synthesized from 2,5-dibromo-pyridine (10) and trimethylsilylchloride (TMSCl) in high yield (82%). Then, 13809

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Article

functionalized ligands (14 and 15). The first step involved generating the bromo-functionalized bipyridines (16 and 17) from 2,6-difluoro-pyridine-5-boronic acid (9) and 2-iodo-4bromopyridine (19) or 2-iodo-5-bromopyridine (18) by Suzuki coupling in which the iodo reacted much faster than the bromo groups. Then, (triisoprypylsilyl)acetylene was allowed to react with compounds 16 and 17 by Sonogashira coupling to afford the TIPSE-functionalized bipyridine ligands 14 or 15 in high yields (85 and 96%). For iridium complex formation, the functionalized bipyridines 14 or 15 were treated with IrCl3·nH2O to furnish the dichloro-bridged dimers 12 and 13 in moderate yields (57 and 24%) (Scheme 2). Thereafter, TIPSE-functionalized Ir complexes 2 and 3 were obtained by a reaction between the dimer and TIPSE-substituted bipyridine 14 or 15 under basic conditions. The low yields (25 and 20%) are similar to those commonly reported for Ir complexes.47 The collected iridium complexes 2 and 3 were in meridional configurations, as confirmed by 1H and 19F NMR measurements (Supporting Information). Compound 2 could not be transformed into its facial isomer by photochemical treatment, probably due to decomposition upon prolonged UV light irradiation. Finally, dendrimers 20 and 1 were obtained starting from the TIPSE-substituted molecule 2. As depicted in Scheme 3, first, the TIPS fragments were removed by treatment with tetrabutylammonium fluoride (TBAF).12 With the ethynyl-substituted Ir complex 22 available, dendrimer 20 was obtained by a Diels−Alder reaction between 22 and molecule 21 in moderate yield (55%). The resulting product is meridional, as confirmed by the 1H NMR and 19F NMR spectra (Supporting Information). As the facial isomer usually displays a higher PLQY than that of the meridional one,60,63 the former was successfully prepared by photolysis in 83% yield. The facial dendrimer 1 was comprehensively characterized by 1H and 19F NMR spectroscopy, matrix-assisted laser desorption ionization timeof-flight (MALDI-TOF) mass, and high-resolution mass spectrometry (HRMS). In the 1H NMR spectra of dendrimer 1 (Figure 3) proton Ha ortho to the fluorine atom appears as a singlet peak at around 5.90 ppm. Protons Hb and Hc occur at 8.13 and 7.52 ppm, respectively. The carbazole protons at 4,5-positions (Hf) are found at 8.05 ppm, and the tert-butyl group protons at 1.33 ppm are clearly assigned. Moreover, there are only two peaks present in the 19F NMR spectra, corresponding to the two fluorine atoms in one ligand of the molecule. The MALDI-TOF mass spectra of dendrimer 1 exhibits a single peak of the molecular ion, in good agreement with the molecular mass of the dendrimer (Figure S3). The HRMS spectra show clear isotope patterns of the molecular ion, which is consistent with the calculated result (Figure S3). Photophysical Characterization. As depicted in Figure 4a, both TMS-substituted Ir complexes 4 and 5 exhibit strong absorptions between 250 and 300 nm due to the ligand-based π−π* transitions.56 The relatively weak bands between 350− 400 nm are attributed to the singlet metal-to-ligand charge transfer (1MLCT).56,60 In addition, as shown in the inset of Figure 4a, a shoulder between 400 and 450 nm is tentatively assigned to a mixture of spin−orbital coupling-enhanced ligand (L)-centered 3π−π* (3LC) and triplet MLCT (3MLCT) transitions, which is based on a comparison with fac(dfpypy)3Ir.56 This is consistent with the two absorption bands of molecule 4, in which one centered at 412 nm is

compound 8 was reacted with 2,6-difluoropyridinyl-3-boronic acid (9) obtained from commercial 2,6-difluoropyridine (11) to furnish the functionalized bipyridine ligand (7) by Suzuki coupling in moderate yield (50%). Subsequently, the one-step reaction between bipyridine 7 and iridium(III) acetylacetonate (Ir(acac)3) to the iridium complex (4 and 5) failed.56,58−60 A two-step method was therefore applied by refluxing the bipyridine ligand 7 with IrCl3, providing a dichloro-bridged dimer (6) as a yellow solid in moderate yield (48%). Then, the dimer (6) and the ligand (7) were heated under basic conditions,47 leading to a yellow solid (5). Attempted substitution of the TMS group with iodine remained unsuccessful. Tri-cyclometalated iridium(III) complexes usually exist in two configurations, i.e., a facial and meridional, with C3 and C1 symmetry, respectively.60−62 The obtained TMS-functionalized Ir complex 5 possesses meridional configuration, as demonstrated by its 1H and 19F NMR spectra (Supporting Information). The facial isomer 4 was easily obtained by stirring a THF solution of 5 under UV light for 12 h in high yield (90%) (Scheme 1e).60−62 For Ir complex 4, only 4 proton signals are present in the low-field region of its 1H NMR spectra, corresponding to the 4 protons of the bipyridine ligand; in addition, two peaks in the 19F NMR spectra represent the two fluorine atoms in one ligand, thus demonstrating a C3 symmetry of complex 4. On the other hand, the number of corresponding NMR signals for molecule 5 was tripled in comparison. Single crystals of both complexes 4 and 5, suitable for X-ray diffraction, were obtained by slow addition of methanol to a dichloromethane (DCM) solution. As shown in Table S2 and Figure 2, the facial configuration possesses nearly identical Ir−

Figure 2. Crystal structure views of Ir complexes 4 and 5 with ellipsoids at 50% probability (proton atoms were hidden for clarity).

N bonds (∼2.12 Å) and Ir−C bonds (∼2.00Å), similar to those in fac-(dfpypy)3Ir56 and consistent with its C3 symmetry. For the meridional one, however, the Ir−N2 bond has a notably longer length (∼0.1 Å) than that of others. This suggests that these bonds can break and reorganize to form the facial isomer 4, which is in accord with the successful meridional-to-facial transformation upon photochemical treatment.60−62 Moreover, several short intermolecular contacts exist for both materials (e.g., F···H−C, F···C, C···C, N···H−C, and face-to-face π···π interactions) (Supporting Information), similar to those in fac-(dfpypy)3Ir.56 With the successful synthesis of the TMS-substituted Ir complexes, the preparation of (dfpypy)3Ir-based PPDs became possible. In view of the Diels−Alder reaction with carbazolefunctionalized tetraphenylcyclopentadienone (21), ethynyl units had to be introduced (Scheme 2), yielding TIPSE13810

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Article

Scheme 2. Synthetic Routes for TIPSE-Functionalized Ir Complexesa

a (a) Pd(PPh3)4, K2CO3, THF, water, 80 °C, 24 h, 39% for 16, 34% for 17; (b) Pd(PPh3)2Cl2, PPh3, CuI, triethylamine, (triisopropylsilyl)acetylene, 80 °C, 24 h, 85% for 14 and 96% for 15; (c) IrCl3·nH2O, 2-ethoxylethanol, 140 °C, 24 h, 57% for 12, 24% for 13; (d) TIPSE-substituted bipyridine 14 or 15, K2CO3, AgSO3CF3, 1,3,5-trimethylbenzene, 170 °C, 24 h, 25% for 2, 20% for 3.

Scheme 3. Synthetic Routes for Iridium Complex-Based Dendrimersa

(a) TBAF, THF, rt, 46%; (b) o-xylene, 150 °C, 48 h, 55%; (c) UV light, THF, 12 h, rt, 83%.

a

presumably ascribed to 3LC and the other (λmax: 438 nm) to 3 MLCT according to the proposed orbital diagram of the Ir complexes (Figure 6, top left). The facial one appears as a stronger and deeper blue emitter than the meridional one (Figure 4b), which is similar to the situation prevailing in other reported Ir complexes,60,63 such as fac-(ppy)3Ir and mer(ppy)3Ir.60 Both emission spectra are structured, resembling fac-(dfpypy)3Ir.56 The small bathochromic shift of the emission of compound 4 against that of fac-(dfpypy)3Ir56 is due to the electron-donating effect of the TMS groups (Table 1). In line

with that, the emission is essentially attributed to ligand-based 3 π−π* transitions rather than 3MLCT because electrondonating moieties lead to a blue-shifted emission for a 3 MLCT-type phosphorescence.64 As indicated in Figure 6, TMS-substituted bipyridine (7) has a higher highest occupied molecular orbital (HOMO) but about the same lowest unoccupied molecular orbital (LUMO) level compared to those of difluoro-substituted bipyridine 26. With the new orbital formation between Ir and ligand, it shows that the transition energy of MLCT and LC of material 4 increases and 13811

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Figure 3. 1H NMR and (solvent: CD2Cl2).

Article

19

F NMR (inset) spectra of dendrimer 1

Figure 5. UV−vis absorption (a) and emission spectra (b) of compounds 3, 2, 20, and 1 (10−5 M in DCM, ex: 385 nm for 3 and 2 and 348 nm for 20 and 1, measured at room temperature).

enhanced 3π−π* and 3MLCT, referring to fac-(dfpypy)3Ir.56 Both are green emitters with pronounced bathochromic shifts (∼45 nm for compound 3) in contrast to fac-(dfpypy)3Ir (Figure 5b), which should relate to the extended conjugations. This also suggests that the emissive excited states exhibit dominant 3π−π* rather than 3MLCT nature because the former induces bigger bathochromic shift than the latter when the conjugation is extended. As shown in Figure 6, for Ir complex 2, due to the smaller energy gap of ligand 14 than that of difluoro-substituted bipyridine (26), the reduction of transition energy is considerable for LL but is much less notable for MLCT, compared to that for fac-(dfpypy)3Ir. This is consistent with the 3LC nature of the emissive excited states of materials 4 and 5. Additionally, both the bathochromically shifted absorption and emission of Ir complex 2 compared with molecule 3 is ascribed to the bigger band gap of 4-TIPSE-substituted bipyridine 15 than that of ligand 14, which indicates that conjugation at 5 position is more effective (Figure 6). Moreover, Ir complex 3 appears as a stronger emitter than phosphore 2 (Figure 5b), owing to very high extinction coefficient between 400 and 500 nm (ε ≤ 6000 M/cm) of lumiphore 3, which corresponds to 3LC and 3MLCT transitions. This is comparable to that of highly emissive fac(ppy)3Ir (ε ≤ 6000 M/cm) in the same region.65 This is also consistent with the higher PLQY of Ir complex 3 than that of 2 (Table 1). The Ir complex 3 is a rare case of a strongly emitting cyclometalated Ir complex with alkynyl groups in the ligand.49,53 As to the absorption of dendrimers, a strong band at around 297 nm is characteristic for carbazole absorptions.66 The band between 320 and 350 nm and the one ranging from 370 to 400 nm are due to electronic transitions of the ligands50 and

Figure 4. UV−vis absorption (a) and photoluminescence spectra (b) of compound 4 and 5 in 10−4 M DCM solution (excited at 330 nm).

decreases respectively, compared with those of fac-(dfpypy)3Ir and consequently causes blue- and red-shifted emissions individually. This is consistent with the conclusion from Lee et al., that the emission of fac-(dfpypy)3Ir is mainly attributed to a 3LC nature of the emissive excited states, even though their calculated results support 3MLCT.56 For the absorption of Ir complexes 2 and 3 (Figure 5), the first two bands between 270 and 350 nm are due to the π−π* transitions of the ligands. The weaker one (370−400 nm) is likely attributed to the 1MLCTs.56,60 The little shoulders above 400 nm are assigned to a mixed state of spin−orbital coupling13812

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Article

Table 1. Photophysical and Electrochemical Properties of Ir Complexes λab (nm) Sol (rt) fac-(dfpypy)3Ir 5 4 3 2 1

375e 259, 370, 262, 334, 270, 329, 289, 349, 297, 334,

436, 441 370, 411, 438 376 383 348

λem (nm) Sol (rt) 438, 448, 444, 508 489, 515

463e 475 472 525

Sol (77 K)

ET b

PLQYc

HOMO (ev)d

LUMO (ev)d

Ega

−6.03 −6.01

−2.60 −2.57

3.43 3.44

−5.68 −5.55

−2.83 −2.11

2.85

0.71e 446, 440, 479, 486, 486,

475 469 509 521 517

2.78 2.82 2.59 2.55 2.55

0.58 0.73 0.30 0.31

a

Calculated from the energy difference between HOMO and LUMO. bCalculated from the highest energy peak of emission spectra (77 K) (Figure S5), which is 1240/λem. cMeasured in toluene solution with quinine sulfate in 0.5 M H2SO4 solution as the standard. dCalculated from cyclic voltammetry (CV) by comparing the first redox onset of the compounds and the oxidation onset of ferrocene. eObtained from ref 56.

Figure 6. Molecular orbital distributions and energy levels of Frontier molecular orbitals (FMOs) of the ligands (calculated using DFT, B3LYP, 6.31G method). 1

stronger emission than that of the meridional one, similar to other Ir complexes, e.g., materials 5 and 4, but the emission intensity of both are much lower than that of compound 3. This is consistent with the much weaker absorptions of 3LC and 3MLCT of the dendrimers than those of compound 3. Extending the conjugation of the ligand is an effective way to tune the color of emission of metal−organic complexes,68 which has been reported for dendrimer-based blue PEs as well.44 Electrochemistry. The redox properties of some of the prepared Ir complexes were studied by cyclic voltammetry (CV). Compound 4 and 5 exhibit two oxidation processes, starting above 1.5 V (Figure S7), which indicates their relatively low HOMOs (∼−6.00 eV) (Table 1), similar to that in fac-(dfpypy)3Ir.56 Both of them present three reversible reduction couples (LUMO: ∼−2.60 eV). Going to Ir complex 2, due to the extended conjugations, the HOMO (−5.68 eV) is raised and the LUMO (−2.83 eV) is decreased compared with molecules 4 and 5. As to dendrimer 1, the HOMO and LUMO were calculated to be −5.55 and −2.11 eV, respectively. This is

MLCTs, respectively. The less notable shoulders (400−450 nm) stem from a mixed state of spin−orbit coupling-enhanced 3 π−π* and 3MLCT.56 Both dendrimers are green emitters (Table 1), similar to materials 2 and 3. Consequently, dendrimer 1 exhibits a bathochromically shifted emission of 50 nm compared to that of fac-(dfpypy)3Ir. This strong shift is correlated to 3π−π* rather than 3MLCT, similar to complex 2. This feature is quite different from fac-(ppy)3Ir-based PPDs bearing very small redshifts (∼8 nm) in emissions as compared with parent fac-(ppy)3Ir.53 Lo et al. argued that this is due to the lack of conjugation between dendron and ligand.67 We rather propose that this is due to the dominant 3MLCT nature of emissions of fac-(ppy)3Ir-based PPDs (Figure S6). As shown in Figure 6, for molecule 25, polyphenylene dendrons indeed significantly decrease the band gap of the ligand compared with bipyridine 26 (4.25 vs 5.05 eV calculated). The close band gaps between TIPSE-substituted bipyridine 14 and ligand 25 and the similar emission colors of Ir complexes 2 and 1 further manifest that their luminescence has primary 3LC nature. The facial dendrimer displays slightly 13813

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Article

consistent with the FMOs of the peripheral carbazoles,66 probably owing to the big ratio between the number of carbazole groups and Ir complex segment within one molecule (6:1).

mean the onset oxidation potential of the targeted molecule, the onset oxidation potential of ferrocene, and the onset reduction potential of the targeted molecule compared with the reference electrode. The calculated isotope distributions of the molecular mass of the dendrimer were conducted with mMass software. The X-ray intensity data for 4 was measured on a STOE IPDS-2T X-ray diffractometer system equipped with a Motarget X-ray tube. The data frames were collected using the program X-Area and processed using the program Integrate routine within X-Area. The reflection intensities were corrected for absorption on the basis of the crystal faces using XRED-32. The X-ray intensity data for 5 was measured on a Bruker SMART APEX2 CCD-based X-ray diffractometer system equipped with a Mo-target X-ray tube. The data frames were collected using the program APEX2 and processed using the program SAINT routine within APEX2. The data were corrected for absorption on the basis of the multiscan technique, as implemented in SADABS. Structure solution and refinement were performed using SHELT-2014. Crystal of compound 4 contains one molecule of CH2Cl2. The density functional theory calculations for ground-state geometry optimizations were performed using PC controlled Gaussian software.

■

CONCLUSIONS We find that the TMS-functionalized and TIPSE-substituted (dfpypy)3Irs and (dfpypy)3Ir-based polyphenylene dendrimers exhibit bathochromically shifted emissions, when compared with parent fac-(dfpypy)3Irs due to the dominant 3LC nature of their emissive excited states. This demonstrates that strong red-shifted emission occurs when both extended conjugations and significant 3LC character are involved in a phosphore. It is also found that when both extended conjugations and primary 3 MLCT nature are present in an Ir complex, the bathochromic shift is less notable or much weaker, as seen, for example, in fac-(ppy)3Ir-based polyphenylene dendrimers. Therefore, this work provides useful insights into the design of new blue phosphorescent dendrimer emitters besides the most commonly utilized method of breaking the conjugation between the PE core and the dendron with alkyl segments.45 Appropriate steps are: (i) to select the core of a dendrimer with a blue PE with dominant 3MLCT nature of its emissive excited state or (ii) to utilize a UV PE with major 3LC nature of its emissive excited states as the core of the dendrimer to push the emission of the dendrimer into pure blue region by extended conjugations.

■

ASSOCIATED CONTENT

* Supporting Information S

■

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01942.

EXPERIMENTAL SECTION Materials and Methods. All chemicals and solvents were purchased from commercial sources and used as received except where noted. Reactions were all conducted under argon atmosphere. 1H NMR and 13C NMR spectra were recorded on Bruker AMX 250, AC 300, and AMX 500 NMR spectrometers. The solvents for NMRs were CD2Cl2 with the reference peak at 5.32 ppm (1H) and 53.84 ppm (13C) and DMSO-d6 with the reference peak at 2.50 ppm (1H). Field desorption mass spectra were recorded on a VG-Instruments ZAB 2-SE-FDP using 8 kV accelerating voltage. MALDI-TOF mass spectra were recorded by a Bruker Reflex II spectrometer with fullerene as the reference and dithranol as the matrix. HRMALDI-TOF mass spectra were recorded by a SYNAPT G2-Si time-of-flight instrument equipped with a MALDI source (Waters Corp. Manchester, U.K.). UV−vis absorption spectra were measured by a Varian Cary 4000 UV−vis spectrometer (Varian Inc. Palo Alto) using quartz cells with path length of 1 cm. Fluorescence spectra were recorded with a SPEX Fluorolog 2 spectrometer. Cyclic voltammetry (CV) measurements were conducted on a computer-controlled GSTAT12 workstation using a three-electrode system in which a Pt wire, a silver wire (or a saturated calomel electrode), and a glassy carbon electrode were used as counter, reference, and working electrode, respectively. The measurements were conducted in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) solution under argon environment with a scan rate of 100 mV/ s at room temperature. The solvents were DCM for the oxidation part and acetonitrile for the reduction part. Ferrocene/ferrocenium (Fc/Fc+) worked as the internal reference throughout the measurements. The calculations of HOMO and LUMO using CV were based on the following equations, HOMO (eV) = −Eonset + Eonset − 4.80 and LUMO ox Fc onset onset onset (eV) = −Ered + EFc − 4.80, where Eoxonset, Eonset Fc/Fc+, and Ered

Synthetic procedures and characterization data of the targeted molecules: 1H NMR, 13C NMR, X-ray singlecrystal diffraction, low-temperature photoluminescence, and cyclic voltammetry (PDF) Crystallographic data (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.B.). *E-mail: [email protected] (K.M.). ORCID

Martin Baumgarten: 0000-0002-9564-4559 Klaus Müllen: 0000-0001-6630-8786 Present Address ∥

Institute of Functional Nano & Soft Materials, Soochow University, Suzhou, 215123, China (G.Z.).

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

The authors gratefully thank the Deutsche Forschungsgemeinschaft (SFB625) for providing financial support for this research. The authors also gratefully thank Dr Wen Zhang for MALDI-TOF mass characterizations. G.Z. also thanks the financial support from Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 Project. 13814

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

■

Article

matrix as solid-state oxygen sensors. Anal. Chem. 1998, 70, 5019− 5023. (20) Kalyanasundaram, K.; Gratzel, M. Applications of functionalized transition metal complexes in photonic and optoelectronic devices. Coord. Chem. Rev. 1998, 177, 347−414. (21) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gratzel, M. A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nat. Mater. 2003, 2, 402−407. (22) Lai, C. W.; Wang, Y. H.; Lai, C. H.; Yang, M. J.; Chen, C. Y.; Chou, P. T.; Chan, C. S.; Chi, Y.; Chen, Y. C.; Hsiao, J. K. Iridiumcomplex-functionalized Fe(3)O(4)/SiO(2) core/shell nanoparticles: A facile three-in-one system in magnetic resonance imaging, luminescence imaging, and photodynamic therapy. Small 2008, 4, 218−224. (23) Dexter, D. L.; Schulman, J. H. Theory of Concentration Quenching in Inorganic Phosphors. J. Chem. Phys. 1954, 22, 1063− 1070. (24) Xie, H. Z.; Liu, M. W.; Wang, O. Y.; Zhang, X. H.; Lee, C. S.; Hung, L. S.; Lee, S. T.; Teng, P. F.; Kwong, H. L.; Zheng, H.; Che, C. M. Reduction of self-quenching effect in organic electrophosphorescence emitting devices via the use of sterically hindered spacers in phosphorescence molecules. Adv. Mater. 2001, 13, 1245−1248. (25) Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir(III) complexes in a solid film. Phys. Rev. Lett. 2006, 96, No. 017404. (26) Kalinowski, J.; Stampor, W.; Mezyk, J.; Cocchi, M.; Virgili, D.; Fattori, V.; Di Marco, P. Quenching effects in organic electrophosphorescence. Phys. Rev. B 2002, 66, No. 235321. (27) Reineke, S.; Walzer, K.; Leo, K. Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters. Phys. Rev. B 2007, 75, No. 125328. (28) Wu, F. I.; Shih, P. I.; Tseng, Y. H.; Chen, G. Y.; Chien, C. H.; Shu, C. F.; Tung, Y. L.; Chi, Y.; Jen, A. K. Y. Highly efficient redelectrophosphorescent devices based on polyfluorene copolymers containing charge-transporting pendant units. J. Phys. Chem. B 2005, 109, 14000−14005. (29) Ikai, M.; Tokito, S.; Sakamoto, Y.; Suzuki, T.; Taga, Y. Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer. Appl. Phys. Lett. 2001, 79, 156−158. (30) He, G. F.; Pfeiffer, M.; Leo, K.; Hofmann, M.; Birnstock, J.; Pudzich, R.; Salbeck, J. High-efficiency and low-voltage p-i-n electrophosphorescent organic light-emitting diodes with doubleemission layers. Appl. Phys. Lett. 2004, 85, 3911−3913. (31) Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Electroluminescent diodes from a single-component emitting layer of dendritic macromolecules. Adv. Mater. 1996, 8, 237−241. (32) Burn, P. L.; Lo, S. C.; Samuel, I. D. W. The development of light-emitting dendrimers for displays. Adv. Mater. 2007, 19, 1675− 1688. (33) Lo, S. C.; Burn, P. L. Development of dendrimers: Macromolecules for use in organic light-emitting diodes and solar cells. Chem. Rev. 2007, 107, 1097−1116. (34) Hwang, S. H.; Moorefield, C. N.; Newkome, G. R. Dendritic macromolecules for organic light-emitting diodes. Chem. Soc. Rev. 2008, 37, 2543−2557. (35) Xu, X. B.; Yang, X. L.; Zhao, J.; Zhou, G. J.; Wong, W. Y. Recent Advances in Solution-Processable Dendrimers for Highly Efficient Phosphorescent Organic Light-Emitting Diodes (PHOLEDs). Asian J. Org. Chem. 2015, 4, 394−429. (36) Lupton, J. M.; Samuel, I. D. W.; Frampton, M. J.; Beavington, R.; Burn, P. L. Control of electrophosphorescence in conjugated dendrimer light-emitting diodes. Adv. Funct. Mater. 2001, 11, 287− 294. (37) Markham, J. P. J.; Lo, S. C.; Magennis, S. W.; Burn, P. L.; Samuel, I. D. W. High-efficiency green phosphorescence from spincoated single-layer dendrimer light-emitting diodes. Appl. Phys. Lett. 2002, 80, 2645−2647.

REFERENCES

(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998, 395, 151−154. (2) Xiao, L.; Chen, Z. J.; Qu, B.; Luo, J. X.; Kong, S.; Gong, Q. H.; Kido, J. J. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926−952. (3) Wong, W. Y.; Ho, C. L. Functional metallophosphors for effective charge carrier injection/transport: new robust OLED materials with emerging applications. J. Mater. Chem. 2009, 19, 4457−4482. (4) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2012, 24, 3169−3190. (5) Sasabe, H.; Kido, J. Multifunctional Materials in HighPerformance OLEDs: Challenges for Solid-State Lighting. Chem. Mater. 2011, 23, 621−630. (6) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Highefficiency organic electrophosphorescent devices with tris(2phenylpyridine)iridium doped into electron-transporting materials. Appl. Phys. Lett. 2000, 77, 904−906. (7) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J. Appl. Phys. 2001, 90, 5048−5051. (8) Greenham, N. C.; Friend, R. H.; Bradley, D. D. C. AngularDependence of the Emission from a Conjugated Polymer LightEmitting Diode - Implications for Efficiency Calculations. Adv. Mater. 1994, 6, 491−494. (9) Meerheim, R.; Nitsche, R.; Leo, K. High-efficiency monochrome organic light emitting diodes employing enhanced microcavities. Appl. Phys. Lett. 2008, 93, No. 043310. (10) Su, S.-J.; Chiba, T.; Takeda, T.; Kido, J. Pyridine-containing triphenylbenzene derivatives with high electron mobility for highly efficient phosphorescent OLEDs. Adv. Mater. 2008, 20, 2125−2130. (11) Su, S.-J.; Gonmori, E.; Sasabe, H.; Kido, J. Highly Efficient Organic Blue-and White-Light-Emitting Devices Having a Carrierand Exciton-Confining Structure for Reduced Efficiency Roll-Off. Adv. Mater. 2008, 20, 4189−4194. (12) Jeon, S. O.; Jang, S. E.; Son, H. S.; Lee, J. Y. External Quantum Efficiency Above 20% in Deep Blue Phosphorescent Organic LightEmitting Diodes. Adv. Mater. 2011, 23, 1436−1441. (13) Hang, X. C.; Fleetham, T.; Turner, E.; Brooks, J.; Li, J. Highly Efficient Blue-Emitting Cyclometalated Platinum(II) Complexes by Judicious Molecular Design. Angew. Chem., Int. Ed. 2013, 52, 6753− 6756. (14) Du, M.; Feng, Y. S.; Zhu, D. X.; Peng, T.; Liu, Y.; Wang, Y.; Bryce, M. R. Novel Emitting System Based on a Multifunctional Bipolar Phosphor: An Effective Approach for Highly Efficient WarmWhite Light-Emitting Devices with High Color-Rendering Index at High Luminance. Adv. Mater. 2016, 28, 5963−5968. (15) Huo, H.; Shen, X. D.; Wang, C. Y.; Zhang, L. L.; Rose, P.; Chen, L. A.; Harms, K.; Marsch, M.; Hilt, G.; Meggers, E. Asymmetric photoredox transition-metal catalysis activated by visible light. Nature 2014, 515, 100−103. (16) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363. (17) Koike, T.; Akita, M. Visible-light radical reaction designed by Ru- and Ir-based photoredox catalysis. Inorg. Chem. Front. 2014, 1, 562−576. (18) DeRosa, M. C.; Hodgson, D. J.; Enright, G. D.; Dawson, B.; Evans, C. E. B.; Crutchley, R. J. Iridium luminophore complexes for unimolecular oxygen sensors. J. Am. Chem. Soc. 2004, 126, 7619− 7626. (19) Di Marco, G.; Lanza, M.; Mamo, A.; Stefio, I.; Di Pietro, C.; Romeo, G.; Campagna, S. Luminescent mononuclear and dinuclear iridium(III) cyclometalated complexes immobilized in a polymeric 13815

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816

ACS Omega

Article

(38) Lo, S. C.; Anthopoulos, T. D.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. Encapsulated cores: Host-free organic light-emitting diodes based on solution-processible electrophosphorescent dendrimers. Adv. Mater. 2005, 17, 1945−1948. (39) Bera, R. N.; Cumpstey, N.; Burn, P. L.; Samuel, I. D. W. Highly branched phosphorescent dendrimers for efficient solution-processed organic light-emitting diodes. Adv. Funct. Mater. 2007, 17, 1149− 1152. (40) Anthopoulos, T. D.; Frampton, M. J.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. Solution-processable red phosphorescent dendrimers for light-emitting device applications. Adv. Mater. 2004, 16, 557−560. (41) Ding, J. Q.; Gao, J.; Cheng, Y. X.; Xie, Z. Y.; Wang, L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Highly efficient green-emitting phosphorescent iridium dendrimers based on carbazole dendrons. Adv. Funct. Mater. 2006, 16, 575−581. (42) Tian, W. W.; Yi, C.; Song, B.; Qi, Q.; Jiang, W.; Zheng, Y. P.; Qi, Z. J.; Sun, Y. M. Self-host homoleptic green iridium dendrimers based on diphenylamine dendrons for highly efficient single-layer PhOLEDs. J. Mater. Chem. C 2014, 2, 1104−1115. (43) Zhou, G. J.; Wong, W. Y.; Yao, B.; Xie, Z. Y.; Wang, L. X. Triphenylamine-dendronized pure red iridium phosphors with superior OLED efficiency/color purity trade-offs. Angew. Chem., Int. Ed. 2007, 46, 1149−1151. (44) Lo, S. C.; Bera, R. N.; Harding, R. E.; Burn, P. L.; Samuel, I. D. W. Solution-Processible Phosphorescent Blue Dendrimers Based on Biphenyl-Dendrons and Fac-tris(phenyltriazolyl)iridium(III) Cores. Adv. Funct. Mater. 2008, 18, 3080−3090. (45) Lo, S. C.; Harding, R. E.; Shipley, C. P.; Stevenson, S. G.; Burn, P. L.; Samuel, I. D. W. High-Triplet-Energy Dendrons: Enhancing the Luminescence of Deep Blue Phosphorescent Iridium(III) Complexes. J. Am. Chem. Soc. 2009, 131, 16681−16688. (46) Lo, S. C.; Richards, G. J.; Markham, J. P. J.; Namdas, E. B.; Sharma, S.; Burn, P. L.; Samuel, I. D. W. A light-blue phosphorescent dendrimer for efficient solution-processed light-emitting diodes. Adv. Funct. Mater. 2005, 15, 1451−1458. (47) Xia, D.; Wang, B.; Chen, B.; Wang, S. M.; Zhang, B. H.; Ding, J. Q.; Wang, L. X.; Jing, X. B.; Wang, F. S. Self-Host Blue-Emitting Iridium Dendrimer with Carbazole Dendrons: Nondoped Phosphorescent Organic Light-Emitting Diodes. Angew. Chem., Int. Ed. 2014, 53, 1048−1052. (48) Lo, S. C.; Harding, R. E.; Brightman, E.; Burn, P. L.; Samuel, I. D. W. The development of phenylethylene dendrons for blue phosphorescent emitters. J. Mater. Chem. 2009, 19, 3213−3227. (49) Qin, T.; Ding, J. Q.; Baumgarten, M.; Wang, L. X.; Mullen, K. Red-Emitting Dendritic Iridium(III) Complexes for Solution Processable Phosphorescent Organic Light-Emitting Diodes. Macromol. Rapid Commun. 2012, 33, 1036−1041. (50) Zhang, G.; Baumgarten, M.; Auer, M.; Trattnig, R.; ListKratochvil, E. J. W.; Mullen, K. Core-and-Surface-Functionalized Polyphenylene Dendrimers for Solution-Processed, Pure-Blue LightEmitting Diodes Through Surface-to-Core Energy Transfer. Macromol. Rapid Commun. 2014, 35, 1931−1936. (51) Zhang, G.; Auer-Berger, M.; Gehrig, D. W.; Blom, P. W. M.; Baumgarten, M.; Schollmeyer, D.; List-Kratochvil, E. J. W.; Muellen, K. Blue Light Emitting Polyphenylene Dendrimers with Bipolar Charge Transport Moieties. Molecules 2016, 21, 1400−1414. (52) Qin, T.; Zhou, G.; Scheiber, H.; Bauer, R. E.; Baunigarten, M.; Anson, C. E.; List, E. J. W.; Mullen, K. Polytriphenylene Dendrimers: A Unique Design for Blue-Light-Emitting Materials. Angew. Chem., Int. Ed. 2008, 47, 8292−8296. (53) Qin, T.; Ding, J. Q.; Wang, L. X.; Baumgarten, M.; Zhou, G.; Mullen, K. A Divergent Synthesis of Very Large Polyphenylene Dendrimers with Iridium(III) Cores: Molecular Size Effect on the Performance of Phosphorescent Organic Light-Emitting Diodes. J. Am. Chem. Soc. 2009, 131, 14329−14336. (54) Qin, T.; Wiedemair, W.; Nau, S.; Trattnig, R.; Sax, S.; Winkler, S.; Vollmer, A.; Koch, N.; Baumgarten, M.; List, E. J. W.; Mullen, K.

Core, Shell, and Surface-Optimized Dendrimers for Blue LightEmitting Diodes. J. Am. Chem. Soc. 2011, 133, 1301−1303. (55) Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M. E. Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes. J. Am. Chem. Soc. 2009, 131, 9813−9822. (56) Lee, S. J.; Park, K. M.; Yang, K.; Kang, Y. Blue Phosphorescent Ir(III) Complex with High Color Purity: fac-Tris(2′,6′-difluoro-2,3′bipyridinato-N,C-4′)iridium(III). Inorg. Chem. 2009, 48, 1030−1037. (57) Schlosser, M.; Rausis, T. The structural proliferation of 2,6difluoropyridine through organometallic intermediates. Eur. J. Org. Chem. 2004, 1018−1024. (58) Zhu, M.; Li, Y. H.; Hu, S. J.; Li, C. G.; Yang, C. L.; Wu, H. B.; Qin, J. G.; Cao, Y. Iridium phosphors with peripheral triphenylamine encapsulation: highly efficient solution-processed red electrophosphorescence. Chem. Commun. 2012, 48, 2695−2697. (59) Zhu, M.; Zou, J. H.; He, X.; Yang, C. L.; Wu, H. B.; Zhong, C.; Qin, J. G.; Cao, Y. Triphenylamine Dendronized Iridium(III) Complexes: Robust Synthesis, Highly Efficient Nondoped Orange Electrophosphorescence and the Structure-Property Relationship. Chem. Mater. 2012, 24, 174−180. (60) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. Synthesis and characterization of facial and meridional tris-cyclometalated iridium(III) complexes. J. Am. Chem. Soc. 2003, 125, 7377−7387. (61) McDonald, A. R.; Lutz, M.; von Chrzanowski, L. S.; van Klink, G. P. M.; Spek, A. L.; van Koten, G. Probing the mer- to facisomerization of tris-cyclometallated homo- and heteroleptic (C,N) (3) iridium(III) complexes. Inorg. Chem. 2008, 47, 6681−6691. (62) Karatsu, T.; Nakamura, T.; Yagai, S.; Kitamura, A.; Yamaguchi, K.; Matsushima, Y.; Iwata, T.; Hori, Y.; Hagiwara, T. Photochemical mer -> fac one-way isomerization of phosphorescent material. Studies by timeresolved spectroscopy for Tris[2-(4′,6′-difluorophenyl)pyridine]iridium(III) in solution. Chem. Lett. 2003, 32, 886−887. (63) Ragni, R.; Plummer, E. A.; Brunner, K.; Hofstraat, J. W.; Babudri, F.; Farinola, G. M.; Naso, F.; De Cola, L. Blue emitting iridium complexes: synthesis, photophysics and phosphorescent devices. J. Mater. Chem. 2006, 16, 1161−1170. (64) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Che, C. M.; Zhu, N. Y.; Lee, S. T. Light-emitting tridentate cyclometalated platinum(II) complexes containing sigma-alkynyl auxiliaries: Tuning of photoand electrophosphorescence. J. Am. Chem. Soc. 2004, 126, 4958− 4971. (65) King, K. A.; Spellane, P. J.; Watts, R. J. Excited-State Properties of a Triply Ortho-Metalated Iridium(Iii) Complex. J. Am. Chem. Soc. 1985, 107, 1431−1432. (66) Ding, J.; Zhang, B. H.; Lu, J. H.; Xie, Z. Y.; Wang, L. X.; Jing, X. B.; Wang, F. S. Solution-Processable Carbazole-Based Conjugated Dendritic Hosts for Power-Efficient Blue-Electrophosphorescent Devices. Adv. Mater. 2009, 21, 4983−4986. (67) Lo, S. C.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. Synthesis and properties of highly efficient electroluminescent green phosphorescent iridium cored dendrimers. Macromolecules 2003, 36, 9721− 9730. (68) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Blue and near-UV phosphorescence from iridium complexes with cyclometalated pyrazolyl or N-heterocyclic carbene ligands. Inorg. Chem. 2005, 44, 7992−8003.

13816

DOI: 10.1021/acsomega.8b01942 ACS Omega 2018, 3, 13808−13816