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[6] T. Ikeda, O. Tsutsumi Science 1995, 268, 1873.[7] P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 1995, 66, 136.[8] D. Y. Kim, S. K. Tripathy, L. Li, J. Kumar, Appl. Phys. Lett. 1995, 66, 1166.[9] P. S. Ramanujam, N. C. R. Holme, S. Hvilsted, Appl. Phys. Lett. 1996, 68,

1329.[10] a) J. Kumar, L. Li, X. L. Jiang, D. Y. Kim, T. S. Lee, S. K. Tripathy, Appl.

Phys. Lett. 1998, 72, 2096. b) S. Bian, L. Li, J. Kumar, D. Y. Kim, J.Williams, S. K. Tripathy, Appl. Phys. Lett. 1998, 73, 1817.

[11] C. J. Barrett, P. Rochon, A. Natansohn, J. Chem. Phys. 1998, 109, 1505.[12] T. G. Pedersen, P. M. Johansen, N. C. R. Holme, S. Hvilsted, Phys. Rev.

Lett. 1998, 80, 89.[13] P. S. Ramanujam, M. Pedersen, S. Hvilsted, Appl. Phys. Lett. 1999, 74,

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Ichimura, Langmuir 1993, 9, 211.

Fluorophores Related to the Green FluorescentProtein and Their Use in OptoelectronicDevices**

By Yujian You, Yingke He, Paul E. Burrows,Stephen R. Forrest, Nicos A. Petasis, andMark E. Thompson*

Bioluminescent organisms range in size from bacteria tolarger organisms, such as insects, fish, squid, jellyfish, etc.[1] Inthese organisms, an emissive molecule is excited in an enzy-matic chemiluminescent reaction. The emission efficiencyfrom the excited molecules is typically very high and the ob-served emission colors cover the entire visible spectrum.[1]

One interesting bioluminescent organism is the jellyfishAequorea victoria. The bioluminescence from this jellyfishwould be in the blue part of the spectrum (with a peak emis-sion intensity at wavelength kmax = 469 nm); however, ratherthan emitting blue light, the energy is transferred to an acces-sory protein, that fluoresces in the green.[2] Extraction andstabilization of the fluorescent center in this green emissiveprotein could give an efficient fluorescent dye. In this paperwe discuss our approach to the utilization of such a biologi-

cally inspired fluorescent dye. We have prepared related fluo-rophores with a wider tunable color range than observed forrelated naturally occurring organisms and mutants.

The green fluorescent protein (GFP) from Aequorea showsintense luminescence from a narrow emission band, centeredat 508 nm (with a full width at half maximum of 50 nm),[3]

The chromophore responsible for the fluorescence is formedin an autocatalytic, post-translational cyclization and oxida-tion of a tripeptide unit in the protein.[4] The fluorescent chro-mophore, or fluorophore, in GFP is a deprotonated p-hy-droxybenzylideneimidazolidinone group (Fig. 1), which iscompletely protected from bulk solvent by the protein shell.

The fluorophore is held in a planar conformation in the interi-or of the protein.[5±8] GFP has been used extensively as a bio-logical fluorescent marker.[9] The fluorophore in GFP as wellas several mutants exhibits highly efficient fluorescence on ex-citation at the primary absorption wavelength, with quantumyields of 0.6±0.8.[1,10,11] GFP's photophysical properties sug-gest that fluorophores similar to those found in GFP could bevery useful in organic light-emitting diodes (OLEDs), whichbenefit from highly efficient, narrow linewidth emission.

OLEDs have received a great deal of attention recently,due to their promising applications to emissive displays.[12]

OLEDs consist of a thin amorphous organic film or multilayer(ca. 1000 �) sandwiched between anode and cathode con-tacts. The anode is typically a film of transparent indium tinoxide (ITO) precoated on a transparent substrate (e.g., glass).The different materials used in adjoining layers in an OLEDconsist of one that provides the predominant path for hole mi-gration (the hole-transporting layer, HTL) and a second thatforms the path for electron migration (the electron-transport-ing layer, ETL). When a voltage is applied across the twoelectrodes, carriers are injected into the organic film. Ascharged carriers migrate toward the opposite electrode (i.e.,holes toward the cathode, electrons toward the anode), theyrecombine within the organic layer promoting one of the or-ganic molecules into its excited state. This excited state, or ex-citon, is also mobile in the molecular thin film. The exciton

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±[*] Prof. M. E. Thompson, Dr. Y. You, Y. He, Prof. N. A. Petasis

Department of Chemistry, University of Southern CaliforniaLos Angeles, CA 90089 (USA)E-mail: met@usc.edu

Dr. P. E. Burrows, Prof. S. R. ForrestDepartment of Electrical Engineering, Princeton UniversityPrinceton, NJ 08544 (USA)

[**] We thank Universal Display Corporation, the Air Force Office of Scien-tific Research, and the National Science Foundation for financial supportof this work. The authors thank Dr. Haiping Hong for help with some ofthe photophysical measurements.

Fig. 1. Fluorophore structures for GFP as well as imidazolidinone and oxazo-lone compounds examined here.

migrates by diffusive hopping between adjacent molecules un-til it either radiatively or non-radiatively relaxes back to theground state.[12±15]

A common technique used to tune the emission color inOLEDs is to use a low concentration (ca. 1 wt.-%) of an effi-cient fluorescent[16,17] or phosphorescent[18] dye, doped intothe transport material to act as the emissive center. In thisway, the carrier conduction and emissive functions are de-coupled. In dye-doped OLEDs, the emission color and effi-ciency can readily be tuned by modification of the dopantwithout affecting the electrical properties of the device. A sec-ond benefit of doping is that the low concentration of dyemolecules acts to inhibit film crystallization or carrier trap-ping in the conducting host film, significantly increasing theoperational lifetime of the device.[19,20]

While the photophysical properties of the GFP fluorophoreare ideal for its application in optoelectronic devices, e.g.,OLEDs, the protein scaffold and phenoxide anion donormake it impractical for incorporation into such devices. Theprotein scaffold greatly increases the molecular weight andadds a thick insulating layer around the fluorophore, therebyhindering energy or electron transfer between the fluoro-phore and the surrounding matrix. To make a GFP-like fluo-rophore for general use in optoelectronic applications, it istherefore important to eliminate this protein scaffold. Thefluorophore in GFP is an imidazolidinone group, Figure 1.[4]

An imidazolidone closely related to GFP has been reported,where the two points of attachment of the protein have beenreplaced with methyl groups.[21] The acidic phenol group ofthis complex makes its use in optoelectronic applicationsproblematic. We have prepared both imidazolidinone (Fig. 1,R = R¢ = H, X = NC4H9) and oxazolone (Fig. 1, R = R¢ = H,X = O) derivatives, related to the GFP fluorophore, which donot have reactive functional groups. For the imidazolidinonederivatives reported here the peptide is replaced with alkyland aryl groups, while in the oxazolone one of the peptidechains is replaced with an oxygen lone pair, and the other withan aryl group. The syntheses of these complexes involves anErlenmeyer coupling of benzaldehyde and hipuric acid deriv-atives. Compounds 1 and 2 have very similar absorption andfluorescence spectra, as shown in Figure 2. The amide deriva-tive, 1, is slightly red shifted relative to the ester, 2, in both ab-sorption and emission. The fluorescence quantum yields forthe two complexes in solution are also similar, at 0.01 and0.005, respectively. The dominant transition for both absorp-tion and emission is a charge-transfer transition between thephenyl group and the carbonyl and imine groups of the imida-zolidinone or oxazolone groups.[4]

In the GFP fluorophore, a phenoxide donor is coupled tothe imidazolidinone acceptor. This charge-transfer band emitslight in the green spectral region. In compounds 1 and 2 thedonor is a phenyl group, which is a comparatively poor donor.If a stronger electron-donating group is used in place of phen-yl, the emission spectrum should red shift, approaching theGFP fluorescence band. In compound 3 (Fig. 1, X = O, R =N(CH3)2, R¢ = H) a dimethylamino group has been substi-

tuted into the para position of the phenyl group. This donorhas an electron-donating strength similar to that of the phen-oxide ion, such that to first order the emission spectrum is ex-pected to be near that observed for GFP. The absorption andfluorescence spectra of 3 are shown in Figure 3, where thefluorescence spectrum of GFP is also shown for comparison.While the linewidth observed for 3 is broader than that ob-served for GFP (full width at half maximum = 80 nm), thefluorescence energy is very close that that of GFP. The quan-tum yield for fluorescence for 3 is 0.01, which is significantlyless than the 0.8 observed for GFP. The increased linewidth

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Fig. 2. Absorption and fluorescence spectra of imidazolidinone, 1, and oxazo-lone, 2, compounds in CH2Cl2.

Fig. 3. Absorption and fluorescence spectra of spectra of compounds 3 and 4 inCH2Cl2. The spectra for compound 3 are shown at the top and those of 4 at thebottom. The fluorescence spectrum of the GFP protein is shown as a dashedline in the top spectrum.

and decreased quantum efficiency for 3 are not surprising fora flexible fluorophore in a fluid matrix. The GFP fluorophoreis held in a protein scaffold, which prevents aggregation orself quenching, and holds the fluorophore in a planar confor-mation, preventing excited-state isomerization or aggregationprocesses, which can lead to complete quenching of fluores-cence.[22]

Compound 4 was made by condensing para-dimethylamino-benzaldehyde with para-cyanohipuric acid (Fig. 1, R =N(CH3)2, R¢ = CN, X = O). In this compound, an acceptinggroup has been added to the oxazolone system, leading to amarked red shift in emission and absorption relative to 3, asshown in Figure 3. The emission is deep orange, with the samequantum yield as observed for 3.

Compounds 1±4 are small molecules with good thermal sta-bility. They can be processed in solution or sublimed, makingthem potentially useful for incorporation into optoelectronicdevices. Two of these materials, 3 and 4, have been used as do-pants in OLEDs. The oxazolone compounds were incorpo-rated into the OLEDs as dopants in the tris(8-hydroxyquino-line) aluminum (Alq3) layer. Both 3 and 4 have absorptionenergies that overlap strongly with the emission spectrum ofAlq3, a necessary prerequisite for efficient energy transfer andtrapping of the singlet excitons generated near the HTL/ETLinterface. The layers were grown on transparent and conduc-tive ITO-coated glass substrate, which acts as the anode. Theorganic layers are deposited by thermal evaporation in a vacu-um of <10±6 torr onto the precleaned ITO-coated substrate.[12]

The device structure consisted of ITO/TPD (300 �)/Alq3:do-pant(450 �)/Mg:Ag(500 �)/Ag(1000 �) (TPD = N,N¢-diphe-nyl-N,N¢-di-m-tolylbenzidine hole transporter). In the controldevice where no dopant is used, direct green emission fromAlq3 is observed with kmax = 520 nm and a linewidth of110 nm. The addition of 0.8 wt.-% 3 into the Alq3 layer shiftedthe emission spectrum slightly to 535 nm, and decreased thelinewidth to 80 nm. In a second device, an orange-emittingOLED was achieved by doping 4 into the Alq3 layer. No emis-sion from Alq3 is observed, and the orange luminescence isproduced solely by the dopant with the maximum at 590 nmand linewidth of 80 nm. The electroluminescence (EL) spectraof undoped, as well as 3- and 4-doped OLEDs are shown inFigure 4. The doping levels for 3 and 4 (0.8 and 1.5 %, respec-tively) were optimized for complete quenching of the Alq3

emission line and achieving the highest OLED quantum effi-ciency. The current±voltage characteristics of these dopedOLEDs are very similar to other doped Alq3 devices,[16,17] withturn-on voltages for both devices of 4±5 V.

The external quantum efficiencies, gext, for 3- and 4-dopedOLEDs were 0.3 % and 0.2 %, respectively, which are roughlyone half and one third of the efficiency of an undoped Alq3 de-vice (gext = 0.6 %), respectively. The photoluminescent effi-ciencies of 3 and 4 in solution are ca. 0.01 in comparison to 0.3for Alq3.[23] The gext value for a doped device is expected tohave an efficiency weighted by the ratio of the photolumines-cence yield of the dye to that of Alq3.[16] Thus, if the lumines-cence yield of the dye is greater than that of Alq3, gext should

be greater than that of the undoped reference. With the lowfluorescence yields of the oxazolone dyes, the expected quan-tum yields for the doped Alq3 device should be 1/30 of thatseen for the undoped device. The observed values of 0.3 % and0.2 % are significantly greater than the 0.02 % that would havebeen expected based on the relative fluorescent yields, suggest-ing that the oxazolones have significantly improved fluores-cence quantum yields in the solid Alq3 matrix. The proteinshells of GFP and GFP mutants have high fluorescence quan-tum yields for several reasons. The protein shell around theimidazolidinone fluorophore restricts intra-fluorophore ringrotations or isomerizations and prevents the formation of dyeaggregates or excimers, all of which can lead to non-radiativerelaxation of the excited state.[22,24] The Alq3 matrix has a simi-lar effect on 3 and 4. The fluorophores are held in rigid confor-mations and are prevented from diffusing or isomerizing in thesolid film, leading to higher fluorescence quantum yields in anAlq3 matrix than observed in fluid solution.

In this study, we have examined a number of oxazolone andimidazolidinone compounds as tailorable fluorophores. Thefluorophores designed here were successfully incorporatedinto OLEDs and give a wider color tunable range than is ac-cessible in the biological system, however, the luminance effi-ciencies of the oxazolones were significantly lower than thoseof the natural systems in solution. Significant improvementsin luminescence yields were observed in the solid state(OLEDs). There are an enormous variety of fluorescent andbioluminescent organisms, all of which utilize specific emis-sion centers that nature has tailored to have a high efficiencyand color specificity. Our oxazolone and imidazolidinonefluorophores have photophysical properties similar to thoseof the GFP. In a similar manner, other materials can be pre-pared, using the insight provided by the naturally occurringsystems, which may be useful in electronic and optoelectronicapplications. The materials presented here are useful due totheir fluorescent properties; however, biological systems existthat give efficient phosphorescence, carrier transport, andother functions that can also be incorporated in a variety ofapplications.

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Fig. 4. EL spectra of 3- (0.8 wt.-%) and 4- (1.5 wt.-%) doped OLEDs. Thestructure used for these devices was ITO/TPD(300 �)/Alq3:dopant(450 �)/Mg:Ag(500 �)/Ag(1000 �).

Experimental

The Erlenmeyer condensation procedures have been modified and appliedto synthesize the oxazolone compounds [25]. One equivalent of hipuric acid orits derivative, one equivalent of benzaldehyde or its derivative, one equivalentof sodium acetate, and ten equivalents of acetic anhydride were mixed andwell-stirred under Ar for over 2 days. The resulting solutions were eitherpoured into ice water and the precipitates collected by filtration, or were ex-tracted by organic solvent and the solvent was evaporated to obtain the solid.The solid was re-dissolved in a minimum amount of acetone and was precipi-tated by addition of a minimum amount of water. Crystalline materials are ob-tained after sublimation. The yields for compounds 1±4 are over 80 %. The solu-tion absorption and emission spectra were measured using CH2Cl2 solutions.The kmax value for the absorption spectrum was used as the excitation wave-lengths for the emission spectra.

2-Phenyl-4-benzylidene-5(4H)-oxazolone (2): m.p.: 162 �C. Anal.: calc. C,77.1; H, 4.45; N, 5.62; found C, 77.1; H, 4.38; N, 5.74. 1H NMR (CDCl3): d 8.30±8.10 (m, 4H), 7.70±7.30 (m, 7H) ppm; MS (EI, 70 eV): 249 (p), 105 (C6H5CO),77 (C6H5). UV-vis, kmax [nm] (loge): 350 (4.5), 365 (4.6), 386 (4.4). Fluorescence,kmax [nm] (u): 415 (0.005).

2-Phenyl-4-(4¢-dimethylaminobenzylidene)-5(4H)-oxazolone (3): m.p.: 212 �C.Anal.: calc. C, 74.0; H, 5.52; N, 9.58; found C, 73.9; H, 5.51; N, 9.54. 1H NMR(CDCl3): d 8.15 (2d, 4H, 8 Hz), 7.60±7.40 (m, 3H), 7.22 (s, 1H), 6.82 (d, 2H,9.25 Hz), 3.12 (s, 6H) ppm; MS (EI, 70 eV): 292 (p), 159 (Me2NC6H4CHCN),105 (C6H5CO), 77 (C6H5). UV-vis, kmax [nm] (loge): 472 (4.7). Fluorescence, kmax

[nm] (u): 520 (0.01).2-(4¢-Cyanobenzyl)-4-(4¢-dimethylaminobenzylidene)-5(4H)-oxazolone (4):

m.p.: 278 �C. Anal.: calc. C, 71.9, H, 4.76; N, 13.2; found C, 71.2; H, 4.70; N, 13.1.1H NMR (CDCl3): d 8.23 (d, 2H, 8.25 Hz), 8.13 (d, 2H, 8.75 Hz), 7.78 (d, 2H,9 Hz), 7.29 (s, 1H), 6.75 (d, 2H, 9 Hz), 3.13 (s, 6H) ppm. MS (EI, 70 eV): 317(p), 159 (Me2NC6H4CHCN). UV-vis, kmax [nm] (loge): 497 (4.7). Fluorescence,kmax [nm] (u): 570 (0.01).

2-Phenyl-4-benzylidene-5(4H)-N¢-n-butyl-imidazolidinone (1): Compound 2(1.40 g, 5 mmol), n-butylamine (0.365 g, 5 mmol), and sodium acetate anhy-dride were mixed with glacial acetic acid (20 mL) and stirred while bringing toreflux. After 0.5 h, all the reactants were dissolved. After a further 4 h stirring,the solvent was removed under reduced pressure. The red residue was trituratedwith ethanol, and isolated by suction. The product was obtained by ethanol re-crystallization. The pure product was obtained by sublimation at 180 �C. m.p.:94±96 �C, yield: 0.86 g 56 %. Anal.: calc. C, 78.92, H, 6.62; N, 9.20; found C,79.16; H, 6.54; N, 9.20. 1H NMR (CHCl3): 8.20 (dd, 2H), 7.78 (dd, 2H), 7.47 (m,6H), 7.23 (s, 1H), 3.76 (t, 2H), 1.54 (m, 2H), 1.25 (m, 2H), 0.83 (t, 3H) ppm; MS:303 (M ± 1), 260 (M ± 1 M-> C3H7). UV-vis, kmax [nm] (loge): 377 (4.3). Fluores-cence, kmax [nm] (u): 424 (0.01).

Device Fabrication and Characterization: All the chemicals were vacuum de-posited (10±5±10±6 torr) from tantalum boats. TPD was first deposited at a rateof 1±4 �/s (300 � total) onto an ITO-coated glass substrate. The electron-trans-porting layer (Alq3) was doped with an oxazolone dye (3 or 4). The depositionrate of dopant was 0.1±0.2 �/s and the rate for Alq3 was 10±20 �/s. The thick-ness of this doped Alq3 layer was 450 �.

A stainless steel mask was placed directly on the TPD/Alq3-coated sub-strates in the air. Magnesium and silver (10:1 ratio) were then co-deposited, asdescribed above for the organic films, onto the coated substrate. The thicknessof this Mg±Ag layer was 500 �. Finally, the devices were capped with 1000 �Ag at a deposition rate of 1±4 �/s.

Received: March 20, 2000Final version: August 3, 2000

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Synthesis of Mesoporous Aluminosilicates withEnhanced Stability and Ion-Exchange Capacityvia a Secondary Crystallization Route**

By Robert Mokaya*

The recent synthesis of the M41S family of mesoporous in-organic solids, which possess well-ordered pores of diameter

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±[*] Dr. R. Mokaya

School of Chemistry, University of NottinghamUniversity Park, Nottingham NG7 2RD (UK)Email: r.mokaya@nottingham.ac.uk

[**] The author is grateful to the EPSRC for an Advanced Fellowship. The as-sistance of Y. Khimyak with NMR measurements is greatly appreciated.