Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolinolate) phenolate complexes were synthesized. A tunable, blue-to-green emission is achieved by attaching electron-withdrawing modulators to the emisssive quinaldinate ligand. The electronic nature of modulator substituents attached to the position of the highest HOMO (highest occupied molecular orbital) density is used to modulate ligand HOMO levels to achieve effective emission tuning to obtain blue-emitting materials. Optical and electrochemical properties of the resulting complexes were investigated and compared to the results of density functional theory (DFT/B3LYP/6-31G*) studies. The resulting materials may find application as organic light-emitting device materials

Anzenbacher, Pavel, Jr.

Montes, Victor A.

Perez-Bolivar, Cesar

English

Bowling Green State University: ScholarWorks@BGSU

Bowling Green State University ScholarWorks@BGSU Chemistry Faculty Publications Chemistry 11-2006 True Blue: Blue-emitting Aluminum(iii) Quinolinolate Complexes Cesar Perez-Bolivar Victor A. Montes Pavel Anzenbacher Jr. Bowling Green State University, pavel@bgsu.edu Follow this and additional works at: https://scholarworks.bgsu.edu/chem_pub  Part of the Chemistry Commons Repository Citation Perez-Bolivar, Cesar; Montes, Victor A.; and Anzenbacher, Pavel Jr., "True Blue: Blue-emitting Aluminum(iii) Quinolinolate Complexes" (2006). Chemistry Faculty Publications. 63. https://scholarworks.bgsu.edu/chem_pub/63 This Article is brought to you for free and open access by the Chemistry at ScholarWorks@BGSU. It has been accepted for inclusion in Chemistry Faculty Publications by an authorized administrator of ScholarWorks@BGSU. True Blue: Blue-Emitting Aluminum(III) Quinolinolate ComplexesCe´sar Pe´rez-Bolı´var, Victor A. Montes, and Pavel Anzenbacher, Jr.*Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,Bowling Green, Ohio 43403Received June 12, 2006Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolino-late)phenolate complexes were synthesized. A tunable, blue-to-green emission is achieved by attaching electron-withdrawingmodulators to the emisssive quinaldinate ligand. The electronicnature of modulator substituents attached to the position of thehighest HOMO (highest occupied molecular orbital) density is usedto modulate ligand HOMO levels to achieve effective emissiontuning to obtain blue-emitting materials. Optical and electrochemicalproperties of the resulting complexes were investigated andcompared to the results of density functional theory (DFT/B3LYP/6-31G*) studies. The resulting materials may find application asorganic light-emitting device materials.Electroluminescent organometallic complexes are valuedemitters for the fabrication of small-molecule-based organiclight-emitting devices (SMOLEDs).1 The unique electron-transport and emissive properties of tris(8-quinolinolate)alumin-um(III) (Alq3) and its derivatives resulted in extensive useof Alq3 in OLED fabrication (Figure 1). The advantageof aluminum(III) quinolinolate complexes is in their semi-conductor properties, namely, the ability to transport electronsand act as an electron-transport layer material and a host forvarious dopants. This, together with the ease of depositionby thermal vacuum evaporation, made aluminum(III) quino-linolate complexes widely used OLED materials. An obstaclein the fabrication of SMOLED-based full-color displays sofar appears to be the limited availability of a pure blue color.1Additionally, in the case of blue-emitting complexes, the blueemission attests to the fact that the HOMO-LUMO gap(energy difference between the highest occupied molecularorbital and lowest unoccupied molecular orbital) is 3.2-3.3eV regardless of the actual energy level of each molecularorbital. However, for successful device operation, the HOMOand LUMO energy levels of the electron-transporting mate-rial must also be aligned with the levels of the other materialsin the OLED, namely, the dopant, both hole-blocking andbuffer layers on the cathode side, and the hole-transport layeron the anode side. Few materials satisfy these conditions,which makes them highly desirable.1In the past, numerous research groups investigated alumin-um(III) quinolinolate complexes in studies focused on unrav-eling their photophysical and semiconductor properties.2 Boththe semiconductor and emissive properties are largely definedby HOMO-LUMO levels of the quinolinolate ligand andits lowest electronic ð-ð* transitions.3 A number of studiesaimed at HOMO-LUMO levels and emission color tuning inthe Alq3-type materials were attempted with a variable degreeof success. Theoretical studies using density functional theory(DFT) calculations provided an insight into the distributionof the HOMO-LUMO densities in the quinolinolate ligandin the mer isomer of Alq3 and suggested that the HOMOorbitals are located mostly on the phenoxide side of the ligand,C8 and C5 in particular, while the highest LUMO density isfound at the pyridine ring, namely, on C2 and C4 (Figure 1).The studies aimed at investigating the effect of quinoli-nolate substituents on photophysical properties revealed thatthe attachment of electron-donating substituents to thephenolate ring results in red-shifted emission, while theattachment of electron-donating substituents to the pyridinering results in blue-shifted emission from the complex. Whilethe parent Alq3 emits at 525 nm,2 a methyl group on C2,C3, C4, and C5 results in emission shifting to 495,4 520,5505,6 and 545 nm,5 respectively. Attachment of the ð-electron-* To whom correspondence should be addressed. E-mail: pavel@bgnet.bgsu.edu.(1) (a) Organic electroluminescence; Kafafi, Z. H., Ed.; CRC Press: BocaRaton, FL, 2005. (b) Shinar, J. Organic Light-Emitting DeVicessASurVey; Springer-Verlag: Berlin, 2003. (c) Kelly, S. M. In Flat PanelDisplays: AdVanced Organic Materials; Connor, J. A., Series Ed.;Royal Society of Chemistry: Cambridge, U.K., 2000.(2) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161-174.(3) Vogler, A.; Kunkely, H. Luminescent Metal Complexes: Diversityof Excited States. In Topics in Current Chemistry; Yersin, H., Ed.;Springer-Verlag: Berlin, 2001; Vol. 213.(4) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357-5360.(5) Sapochak, L. S.; Padmaperuma, A.; Washton, N.; Endrino, F.; Schmett,G. T.; Marshall, J.; Fogarty, D.; Burrows, P. E.; Forrest, S. R. J. Am.Chem. Soc. 2001, 123, 6300-6307.Figure 1. Structures of a mer isomer of Alq3 with numbered positions inthe ligand and 2Meq2AlOPh.Inorg. Chem. 2006, 45, 9610−96129610 Inorganic Chemistry, Vol. 45, No. 24, 2006 10.1021/ic061051l CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 10/24/2006rich substituents in the C5 position resulted mostly in red-shifted emission. For example, C5 phenyl, fluoro, chloro,and cyano substituents resulted in complexes with emissionmaxima at 550,7 535,8 540,2 and 520 nm,9 respectively. Anexample of effective blue emission from an Alq3 derivativeis the C5 piperidine sulfonamide emitting at 480 nm.10Recently, our group performed extensive studies aimed atemission color tuning in Alq3-type materials.11 We used sim-ple quinolinolate complexes with electron-donating or electron-withdrawing aromatic moieties connected either directly12aor via an acetylene spacer12b to the C5 of the ligand. Wehave demonstrated that electron-deficient moieties cause ablue shift in emission while the electron-donating substituentsresult in red-shifted emission. We also showed that thesematerials retain their semiconductor properties and may beused for OLED fabrication.13Unfortunately, none of the above efforts yielded a high-puri-ty blue emitter, one with emission centered around 450-470nm while possessing a high emission quantum yield. Such anemitter, however, is required for the fabrication of full-colordisplays to generate the red-green-blue signal.1 Also, re-cently discovered high-efficacy (18%) white OLEDs forinterior lighting require a good-quality blue emitter.14 We haverealized that it is unlikely that a single substituent on thequinolinolate ligand would exert such a strong effect to yielda blue emitter of sufficient quality and that we may need touse a combined effect of two substituents. The question was,how much the substituent effects that modulate the quinolino-late emission were additive, which is the subject of this study.Here we explore the properties of fluorescent quinolinolatecomplexes, in which the quinolinolate ligand bears two sub-stituents to modulate the emission. First, we decided to use the2-methyl substituent (2Meq), which is known to blue shift theemission for ca. 0.2 eV compared to the parent Alq3 througha combination of electronic and geometrical effects.2 The 2Meqligand, however, is too sterically demanding to form a tris-complex. Therefore, the 2-methylquinolinolate complexes ofAl(III) are complexes comprising two 2Meq ligands and oneancillary ligand. We used a phenolate as an ancillary ligandand used the reaction of aluminum(III) phenoxide15 with two2-methyl-8-hydroxyquinoline ligands and its C5-substitutedderivatives16 to prepare the complexes described in Figure 2.The C5 substituents are divided into two groups: electron-poor substituents were chosen to maximize the blue shift inthe emission, while the two electron-rich substituents wereincluded for the purpose of investigating the additivity ofthe substituent effects. The 2-methyl induces a blue shift,while the 5-phenyl and, namely, the 5-(4-dimethylaminophe-nyl) induce a red shift.12,13The substituent-mediated emission color tuning in com-plexes 1a-f can be observed by a naked eye. Upon excitationof CH2Cl2 solutions with black light (365 nm), complexes1a-f show bright photoluminescence (PL) that shifts fromblue to green and red, depending on the electronic nature ofthe aryl substituent (Figure 3).Table 1 summarizes the properties of complexes 1a-fdetermined from UV-visible and fluorescence measure-ments. The data for the parent complex 2Meq2AlOPh(substituent ) H) are also included for comparison.The emission maxima of complexes 1a-f span over 120nm between 455 and 575 nm, while showing strong emissionin the blue region of the visible light spectrum. In general,we found that the C5 substituents provide a strongeremission-tuning effect compared to 2-methyl. It appears thatthe substituents on the phenoxide and pyridine rings inducean additive effect on the emission wavelength. The combina-(6) Kido, J.; Iizumi, Y. Chem. Lett. 1997, 963-964.(7) Tokito, S.; Noda, K.; Tanaka, H.; Taga, Y.; Tsutsui, T. Synth. Met.2000, 111-112, 393-396.(8) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357-5360.(9) Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S.R.; Cronin, J. A.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991-8006.(10) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson, M. L.; Schmidt,A.; Kippelen, B.; Padias, A. B.; Hall, H. K., Jr.; Peyghambarian, N.;Armstrong, N. R. Chem. Mater. 1996, 8, 344-351.(11) Montes, V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.sEur.J. 2006, 17, 4523-4535.(12) (a) Pohl, R.; Montes, V. A.; Shinar, J.; Anzenbacher, P., Jr. J. Org.Chem. 2004, 69, 1723-1725. (b) Pohl, R.; Anzenbacher, P., Jr. Org.Lett. 2003, 5, 2769-2772.(13) Montes, V. A.; Pohl, R.; Li, G.; Shinar, J.; Anzenbacher, P., Jr. AdV.Mater. 2004, 16, 2001-2003.(14) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest,S. R. Nature 2006, 440, 908-912.(15) This procedure is an adaptation of the method described in: Bryan,P. S.; Lovecchio, F. V.; Van Slyke, S. A. U.S. Patent 5,141,671, 1992.This method uses aluminum(III) isopropoxide.(16) The synthesis of the C5-substituted 2-methyl-8-hydroxyquinolineligands used to synthesize 1a-f is reported in the SupportingInformation.Figure 2. Structure of C5-substituted derivatives and 2Meq2AlOPh.Figure 3. Top: Emission of 1a-f in CH2Cl2 solutions illuminated withblack light (365 nm). Bottom: Emission spectra of complexes 1a-f.COMMUNICATIONInorganic Chemistry, Vol. 45, No. 24, 2006 9611tion of both types of substituents may be used to achieve adesired blue emission. For example, when compared to Alq3(ìF  525 nm), 1a (455 nm) shows a 0.4 eV blue shift.This shift appears to be a combined effect of the 2-methyland 5-dimethoxytriazinyl substituents, both known to inducea 0.2 eV shift when individually attached to Alq3 sepa-rately.11 Also, in agreement with the energy gap law,17 wefound a decreasing fluorescence quantum yield with adecreasing emission energy (Table 1).While the photophysical data confirmed that the opticalHOMO-LUMO gap is sufficient for a blue emission, weperformed cyclic voltammetry (CV) experiments to deter-mine the absolute HOMO and LUMO levels (Table 2).18The data in Table 2 show that the reduction potentials andthe corresponding LUMO levels in 1a-f are not significantlychanged across the series. On the other hand, the oxidationpotentials and HOMO levels underwent systematic changes,reflecting the tuning of the HOMO-LUMO gap (Figure 4,left panel). We reason that this is due to the 2-methylsubstituent in the C2 position with the highest LUMO densityin all derivatives, while the varying substituents in the C5position affect predominantly the HOMO levels because C5is the site with high HOMO density. The magnitudes of theHOMO-LUMO gap for 1a-f correlate with the observedemission properties. Thus, the most blue complex 1a showsthe highest HOMO-LUMO gap of 3.26 eV, and the mostred-shifted emission of complex 1f corresponds to a gap of3.08 eV calculated from electrochemistry measurements.Figure 4 shows a systematic decrease in the HOMO-LUMOgaps across the series 1a-f.There is a good correlation between the photophysical dataand the data derived from electrochemistry (Figure 4, rightpanel). As mentioned before, this is partially due to the factthat the photophysical properties of these materials aredefined by their HOMO-LUMO gap, with theS0fS1 electronic transition having a large character ofHOMOfLUMO. The linear dependence of the HOMOlevels and the HOMO-LUMO gap on the S1-S0 emissionenergy attests to the uniform photophysical and semiconduc-tor behavior of the complexes within the series.Also, the above findings were supported by the DFT calcu-lations of the HOMO levels and the HOMO-LUMO gap atthe B3LYP/6-31G* level of theory in vacuum. The absolutevalues of the HOMO levels were uniformly overestimatedin the DFT calculations, presumably because of the fact thatthe calculations were carried out for 1a-f in vacuum.Otherwise, the DFT calculations showed the same decreasein the HOMO levels as that observed in electrochemistry.In conclusion, a new class of quinolinolate complexes1a-f with tunable properties was prepared. Complexes 1a-dshow strong blue emission (…FL ) 30-60%) and emissionmaxima at 455-504 nm, which makes them comparable withmost small-molecule blue emitters.1,2 We have shown thatthe electronic nature of the aryl substituent affects theemission color, predominantly via effective modification ofthe HOMO levels located on C5 of the quinolinolate ligand.The optical properties of the complexes 1a-f also correlatewell with the electrochemical properties. All of our observa-tions suggest that the electronic effects that modulate theemissive properties in quinolinolate complexes are additiveand the effects of individual substituents may be combinedto obtain emitters of desired properties. The charge-transportand electroluminescence properties of the complexes 1a-fare under investigation.Acknowledgment. This work was supported by the NSF(Grant DMR-0306117) and BGSU (TIE and FRC grants).Supporting Information Available: Experimental data for thepreparation of 1a-f, their UV-visible and PL spectra, CV curves,and DFT calculations. This material is available free of charge viathe Internet at http://pubs.acs.org.IC061051L(17) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164.(18) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R.; Ba¨ssler, H.;Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551-554.Table 1. Photophysical Properties of Complexes 1a-facomplex Amax ()b ìF (nm) …F c ìFSS (nm)optic bandgap (eV)H 357 (7.0  104) 495 0.40 492 3.471a 377 (1.04  105) 455 0.60 467 3.291b 364 (1.44  104) 468 0.40 487 3.401c 363 (1.37  104) 484 0.60 485 3.411d 368 (7.39  104) 504 0.30 504 3.371e 370 (1.04  104) 515 0.10 513 3.351f 353 (1.06  103) 574 >0.01 ND 3.51a Absorption maximum (Amax), fluorescence emission maximum (ìF),and quantum yield (…F) in a CH2Cl2 solution at room temperature. Solid-state fluorescence maximum (ìFSS) measured in 20% (w/w) poly(methylmethacrylate) in a CH2Cl2 suspension. Refer to the Supporting Informationfor emission spectra. The optical band gap was estimated from the UV-visible spectra. b Units: mol-1 cm-1. c Determined using quinine sulfatein 0.05 M H2SO4 as a standard.Table 2. Electrochemical Properties of Complexes 1a-facomplexE ox(V)E red(V)HOMO(eV)LUMO(eV)HOMO-LUMO(eV)H 1.70 -1.43 -6.27 -3.14 3.131a 1.90 -1.36 -6.47 -3.21 3.261b 1.86 -1.38 -6.43 -3.18 3.241c 1.85 -1.37 -6.42 -3.19 3.221d 1.57 -1.37 -6.14 -3.19 2.941e 1.57 -1.37 -6.14 -3.20 2.941f <1.69b -1.39 <-6.20b -3.18 <3.00ba E ox and E red were determined using CV of 1.0 mM solutions in CH2Cl2containing 0.1 M tetrabutylammonium perchlorate. See the SupportingInformation for more details. b The redox chemistry of the dimethylaminomoiety prevented the exact determination of the E ox, HOMO, and HOMO-LUMO gap values.Figure 4. Left: HOMO-LUMO band gap for 1a-f and 2Meq2AlOPh,estimated by CV. Right: Correlation between the HOMO level andHOMO-LUMO obtained from redox potentials as a function of thefluorescence emission energy. The asterisk in 1f indicates the potentialuncertainty due to the redox chemistry of the dimethylamino moiety.COMMUNICATION9612 Inorganic Chemistry, Vol. 45, No. 24, 2006

True Blue: Blue-emitting Aluminum(iii) Quinolinolate Complexes

https://scholarworks.bgsu.edu/cgi/viewcontent.cgi?article=1169&context=chem_pub

True Blue: Blue-emitting Aluminum(iii) Quinolinolate Complexes

Abstract

Similar works

Full text

Available Versions

Bowling Green State University: ScholarWorks@BGSU