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Plasmon Enhanced Spectroscopy of N,N0 -Dialkylquinacridones Used as Codopants in OLEDs Elena del Puerto,*,† Concepcion Domingo,† Santiago Sanchez-Cortes,† Jose V. García-Ramos,† and Ricardo F. Aroca‡ † ‡
Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4
bS Supporting Information ABSTRACT: Plasmon-enhanced Raman scattering and plasmon-enhanced fluorescence of the two molecules N,N0 -dimethylquinacridone (DMQA) and N,N0 -diisoamylquinacridone (DIQA) used as codopants in OLEDs (with improved stability) are reported. The work includes a complete vibrational characterization using Raman and infrared spectra supported by DFT calculations. Plasmon enhancement is achieved using silver and mixed Ag/Au nanostructures: colloids and metal island films, supporting localized surface plasmon resonances (LSPR). However, fine-tuning of the experimental conditions is necessary to attain surface-enhanced (resonance) Raman scattering (SERS/SERRS) of the two molecules with similar structures. SERRS spectra of DMQA are obtained on silver colloids and from a LangmuirBlodgett (LB) monolayer deposited onto silver island films (SIF). High quality SERRS spectra of DIQA were only obtained from LB layers on mixed Ag/Au island films. Using spacer LB layers of arachidic acid (AA) to separate the fluorophore from the metal nanostructure, surface-enhanced fluorescence (SEF) spectra for both quinacridones were obtained on silver island films. The spectral data provided information on the subject of the molecular aggregation in the LB layers.
’ INTRODUCTION The first efficient organic light-emitting diode (OLED) based on a double layer of small organic molecules sandwiched between two injecting electrodes was reported in 1987.1 Since then there has been a continuous and widespread development of new strategies to improve the electroluminescent efficiency and stability of OLEDs. One of these strategies is related to the doping of the emission layer with fluorescent molecules.2 In particular, N,N0 -dimethylquinacridone (DMQA)24 and N,N0 diisoamylquinacridone (DIQA)5 are among the most efficient dopants of aluminum quinolate Alq3, one of the electron transport layer materials normally employed in the OLED industry. Another strategy to improve light extraction efficiency in OLEDs (and LEDs) consists of incorporating metallic nanoparticles (NPs) in the electrode/emission layer interface,69 this being an emerging field of research and development. The reason for the emission enhancement resides in the resonant excitation of the localized surface plasmons in NPs, increasing the local electromagnetic field which leads to luminescence enhancement. Recent results on LEDs incorporating plasmonic materials use either silver island films9 or colloidal gold nanocrystals,10 in both cases proceeding from colloidal solutions. The fabrication of multilayer OLED heterostructures is basically made by consecutive vapor phase deposition of thin films of the organic materials constituting the hole transport layer (HLT)—for r 2011 American Chemical Society
instance aromatic diamines1 or biphenyls (NPB)11—and the electron transport layer (ETL)—doped or undoped Alq3—followed by a thin film of the cathode material (largely Mg4 or Al12). The deposition is made on a substrate acting as the anode (ITO glass, for example). Such a scheme is usually followed both in macro13 and nano11 OLED fabrication. Spin-coating from solution is sometimes employed as deposition method for one or more of the organic layers involved in OLEDs.5,14 A prerequisite to understanding the functionality of such devices is to investigate the molecular characteristics and organization of the corresponding organic thin films, as well as their interactions with the substrates. In particular, LangmuirBlodgett (LB) films are model structures that permit the detailed study of the two-dimensional properties of molecular stacking and spectral properties. Referring to quinacridones, Langmuir Blodgett15 and other kinds of monolayers deposited on Ag(100)16 or HPOG17 have been experimentally studied employing STM and LEEDS techniques, as well as theoretically by density functional theory (DFT) methods.18 Vibrational spectroscopy has not been extensively applied to the characterization of OLED materials. Fourier transform Received: May 10, 2011 Revised: July 1, 2011 Published: July 01, 2011 16838
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The Journal of Physical Chemistry C infrared (FTIR) and/or Raman spectra of Alq319,20 and some derivatives,14 as well as Raman investigations about their interactions with different metals,2123 have been published, along with the infrared assignment of the hole transport material NPB.24 On the other side, plasmon-enhanced techniques such as surface-enhanced Raman scattering (SERS) and surfaceenhanced fluorescence (SEF), have proved to be adequate for the molecular characterization of LB films of electroactive dyes, such as perylenes.25,26 In addition, the high sensitivity of such techniques should allow the detection of the minute quantities of codopant dyes used in OLEDs, thus representing a possible diagnosis technique in real OLEDs. However, SERS studies of OLED materials are rare.27 The present report focuses on the molecular spectra of quinacridones DMQA and DIQA close to silver and silver/ gold nanostructures. The objective is to contribute to the understanding of metalmolecule interactions, analogous to conditions found in improved OLEDs. Further, the LB structures help to identify spectral changes due to aggregation effects that may modify the properties of materials; for instance, the color of a pigment and the performance of an electroluminescent device. In the case of quinacridones, ππ stacking of some quinacridone derivatives has been studied in solution by 1H NMR28 and also by UVvis absorption and steady state fluorescence.29 Here, we provide new data using SEF for DMQA and DIQA LB films.
’ EXPERIMENTAL SECTION DMQA was purchased from Aldrich with the highest purity. DIQA was synthesized following a published procedure,5 with high purity reactants quinacridone (TCI Europe, >99%), sodium hydride (Aldrich), N,N-dimethylacetamide (Aldrich), and 1-bromo-3-methylbutane (Aldrich). Silver nitrate (Merck) and hydroxylamine chloride (Aldrich) employed for the fabrication of silver colloids had analytical purity, as did the potassium nitrate (Merck) employed as aggregating agent. Arachidic acid (Fluka) used for getting LB mixed monolayers had a >98% purity, while silver and gold (Sigma) shots employed for making the island films had 99.999% purity. Silver colloids for SERS measurements were synthesized by reduction of silver nitrate with hydroxylamine chloride at room temperature.30 After aggregation with potassium nitrate solution up to a final concentration of 3 102 M, an aliquot of each quinacridone derivative solved in absolute ethanol was added to the silver colloids. Silver and silver/gold mixed island films (SIF, SGIF) were prepared under high vacuum (104 Pa) by thermal evaporation onto Corning 7059 glass slides maintained at 200 °C. Film mass thickness of approximately 9 nm was monitored by an XTC Inficon quartz crystal oscillator. LangmuirBlodgett (LB) films of DMQA and DIQA mixed with arachidic acid (AA) in 1:1 ratio were transferred from airwater interface onto SIF and SGIF, respectively. The monolayers were obtained by spreading an aliquot of the quinacridone derivatives mixed with AA in dichloromethane to a final concentration of 1.5 104 M for both, onto a water subphase with cadmium chloride (3 104 M) which increases the stability of the monolayer. AA was employed to provide flexibility and facilitate the transfer onto the SIF (SGIF) substrate. LB films were fabricated by Z-deposition at a constant pressure of 20 mN/m. The deposition rate was 2 cm2/min, and the transfer ratio in both cases was closer to 1.
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Figure 1. Molecular structures of DMQA and DIQA.
Figure 2. Absorption spectra of 103 M solutions of (a) DMQA and (b) DIQA in CH2Cl2. Dashed lines indicate the wavelength of the lasers available to get inelastic scattering spectra. Fluorescence spectra of the same solutions excited at 514.5 nm: (a0 ) DMQA; (b0 ) DIQA.
A Nima 302 M LB trough was used. The corresponding isotherms are given in Figure 1 of the Supporting Information. SEF measurements for LB films of both quinacridones were carried out after placing spacer LB layers of AA between the target LB layer containing the fluorophore and the metal island film. The spacer layers of the fatty acid permit the reduction of quenching and unveil the SEF signal of DMQA and DIQA. Here, one and three spacer layers of AA were deposited first on the SIF (SGIF) followed by one mixed quinacridone/AA LB monolayer. For SERS/surface-enhanced resonance Raman scattering (SERRS) experiments the mixed quinacridone/AA monolayer was transferred directly onto the SIF/SDIF. FTIR spectra of solid DMQA and DIQA dispersed in KBr were recorded with a Bruker IFS66 spectrometer with a DGTS detector. The spectral resolution was 8 cm1, and 500 scans were acquired. Raman spectra of solids (exciting at 785 nm) as well as SERS/ SERRS spectra in colloidal solutions were recorded with a Renishaw RM2000 Raman microscope. SEF and SERS/SERRS spectra of LB layer over SIF (SGIF) films were recorded with a Renishaw inVia Raman microscope. Both systems are equipped with a Leica microscope, a CCD camera, and lasers at 514.5, 632.8, and 785 nm. Raman spectra of solids DMQA and DIQA were also recorded with an FT-Raman Bruker MultiRam spectrometer having a liquid N2 cooled Ge detector and a Nd:YAG laser at 1064 nm.
’ RESULTS AND DISCUSSION Molecular structures of DMQA and DIQA are shown in Figure 1. The parent molecule, quinacridone (QA), also employed as a codopant in OLED devices, has the same basic 16839
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Figure 3. Raman and infrared spectra of solids DMQA and DIQA in the region below 1800 cm1. The complete list of frequencies is given in the Supporting Information.
structure consisting of five condensed rings with amine and carbonyl groups in two of the rings. Such a structure allows intermolecular hydrogen bonding which brings about strong aggregation effects, not desirable when stable devices are needed. In the case of the methyl (DMQA) and isoamyl (DIQA) derivatives, the H atoms of the amine groups are substituted by alkyl groups, preventing strong aggregation and making possible their use in OLEDs with improved stability.4,5 Figure 2 shows the absorption spectra of DMQA and DIQA in CH2Cl2 solutions. The same spectral profile is obtained for DMQA solved in ethanol, although the overall intensity is much lower. The absorption spectra agree with those reported for DMQA in CHCl331 and DIQA in dimethylformamide.5 The typical absorption structure of condensed aromatic molecules with maxima at 520, 485, and 455 nm25 is observed. Therefore, resonance Raman and SERRS are likely for excitation with the 514.5 nm laser source. The fluorescence spectra of the solutions, excited at 514.5 nm, are also given in Figure 2. Vibrational Spectra. The infrared and normal Raman spectra of solid DMQA (at 1064 nm) and DIQA (at 785 nm) are shown in Figure 3. The strong fluorescence of these solids prevents the
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observation of Raman spectra for excitation at 514.5 nm. Since the vibrational assignment of these molecules has not been reported, we proposed a complete interpretation of the fundamental vibrational modes supported by DFT quantum mechanical calculations using B3LYP and 6-31G(d,p) as a basis set in the Gaussian 09 program.32 The results of the vibrational analysis, experimental wavenumbers of DMQA and DIQA and calculated spectra, are given in the Supporting Information (Tables 1 and 2). Briefly, the strongest vibrational bands of DMQA correspond to CdO stretching modes (symmetric one at 1645 cm1 in Raman spectrum and antisymmetric one at 1626 cm1 in infrared spectrum) and CdC stretching vibrational bands (16101580 cm1). The methyl group deformations (15001400 cm1) lead to strong IR and medium intensity Raman bands, while ν(CN) vibrations, with contributions from ν(CC), δ(CH3), and δ(CHar), give rise to strong Raman (medium IR) bands around 1340 cm1. Characteristic aromatic out-of-plane (and in-plane) vibrations are associated with strong and medium infrared (and Raman) bands in the 700850 cm1 region. Assignment of aromatic and aliphatic CH stretchings observed in the 30503110 and 28002950 cm1 regions, and also overtone and combination bands, can be found in the Supporting Information. The vibrational spectra of DMQA and DIQA are very similar; however, for DIQA a small blue shift of ν(CdO) and ν(CdC) Raman bands is observed, which can be attributed to the stronger inductive effect of the isoamyl group. Besides, due to the presence of the CH2 and CH groups in the alkyl substituents, mediumweak Raman bands appear at 1388, 1284, and 1215 cm1 assigned to δ(CH3), t(CH2), and ω(CH2) vibrational modes. Slight changes of relative intensities are observed in the IR spectrum of solid DIQA when compared with the DMQA spectrum, also associated with the contributions of CH2 and CH groups. SERS/SERRS Spectra. Two SERS active substrates are used to obtain SERS or SERRS of DMQA and DIQA: metal colloid solutions and metal island films. The LB technique is used to deposit a single monomolecular layer onto the silver island film or silvergold mixed film. The LB work could provide insight into the characterization of codopants DMQA and DIQA as thin films, resembling what is found in real OLEDs. First, reproducible SERS/SERRS spectra of DMQA are obtained with both substrates, silver colloids as well as for one LB monolayer over silver island films (SIF). However, DIQA only gives reproducible SERS/SERRS spectra for the LB monolayer transferred to a mixed Ag/Au island films (SGIF). The mixed Ag/Au system keeps the desired plasmonic properties of Ag, but the Au avoids the perverse catalytic effect that the Ag in SIF seems to exert on DIQA, preventing attainment of reproducible SERS spectra. Notably, no SERS/SERRS of DIQA was obtained using Ag or Au colloids. However, the objective was achieved since good SERS/SERRS spectra of both molecules as LB layers over island films are obtained, which is closer to the real situation in OLEDs. Figure 4 shows the SERS/SERRS spectra of DMQA on silver colloids, excited with laser lines at 785 (SERS) and 514.5 nm (SERRS), together with the SERRS of LangmuirBlodgett monolayer over silver island film, excited at 514.5 nm. For comparison the normal Raman spectrum of solid DMQA excited at 785 nm is also shown. As can be seen in Figure 2, the 514.5 nm laser line is in resonance with the molecular electronic absorption and the plasmon absorption, the double resonance characteristic of SERRS. The 785 nm laser line is out of the molecular electronic absorption, and the observed spectrum benefits only from the plasmonic resonance or SERS. Comparing SERS and 16840
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Figure 5. SERRS spectra of DMQA in silver colloidal system at different concentrations: (a) 103, (b) 104, (c) 105, (d) 106, and (e) 107 M. The intensity of the ν(AgCl) band of the colloid at 240 cm1 was used for normalization purposes.
Figure 4. (a) Raman spectrum of DMQA solid at 785 nm, (b) SERS spectrum of DMQA solid on colloidal silver at 785 nm, and (c) SERRS of DMQA solid on Ag colloids at 514.5 nm. (d) SERRS of DMQA/AA LB monolayer over silver island film at 514.5 nm. The asterisks (/) indicate bands for arachidic acid used in the mixed LB films. Bottom: Outline of the proposed Ag/DMQA interaction.
normal Raman spectra given in Figure 4, the pronounced intensity decrease of the band corresponding to the CdO symmetric stretching (1644 cm1) in the Raman spectrum of solid DMQA becomes evident. The latter could be rationalized assuming that the DMQA adsorption on Ag takes place through an oxygen atom. The small shift of aromatic CdC stretchings in the 15001600 cm1 region could also indicate reorganization of the molecular charge due to metalmolecule interaction. The SERS spectrum excited at 785 nm seems to provide information about the relative orientation of the molecule and the Ag surface. The large intensity enhancement of typical in-plane modes, especially the 701 cm1 band which becomes the strongest band of the spectrum, supports an edge-on orientation (of the molecular plane) of DMQA with respect to the Ag surface, as outlined in the lower part of Figure 4. The spectral profile of the SERRS spectra of DMQA excited at 514.5 nm is reproducibly the same for either Ag colloids or mixed DMQA/AA LB monolayers. The molecular resonant effect is additionally confirmed by the large enhancement of overtones and combination bands in the 24002700 cm1 region (Figure 2 of the Supporting Information), a unique property previously reported for SERRS of perylenes.26 In addition, the bands assigned to fundamental vibrations of the chromophore (π-system of DMQA responsible for its large absorbance around 520 nm), centered around 1340 cm1, are specifically enhanced. Apart from the presence of AA bands in the SERRS spectrum of LB, marked with asterisks (/) in Figure 4 d, the small band shifts observed between colloidal SERRS and LB-SERRS can be attributed to ππ stacking33 of DMQA in the case of LB, since the monolayer has been fabricated by a compression mechanism.
Figure 6. (bd) SERS spectra of DIQA/AA LB monolayer over Ag/Au island films, excited at the laser wavelength indicated, compared with (a) the Raman spectrum of solid at 785 nm. The asterisks (/) indicate bands for arachidic acid used in the mixed LB films. Bottom: Outline of the proposed Au/DIQA interaction.
SERRS spectra of DMQA were obtained at different concentrations (Figure 5). As a consequence of the additional enhancement due to resonance, we were able to detect DMQA at very low concentrations in colloidal solutions (107 M), as can be seen in Figure 5e. In the case of LB-SERRS spectra the number of molecules of both DMQA and DIQA may be estimated as follows. When the DMQA monolayer is transferred at a fixed pressure of 20 mN/m, which corresponds to 39 Å2 area per molecule, the number of molecules in the 1 μm2 surface area of the LB monolayer is 16841
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Figure 7. SEF spectra of a LB monolayer of DMQA/AA and DIQA/AA employing (a) one spacer monolayer of AA and (b) three spacer layers. (c) Reference fluorescence of the LB monolayer on glass. (d) Reference fluorescence of DMQA and DIQA in solid state. All the spectra were acquired excited at 514.5 nm.
ca. 2.5 106 molecules. With the 50 objective the area illuminated by the laser (laser spot) is about 1 μm2, and therefore, the inelastic scattering detected comes from ca. 4 amol, which is the total number of molecules in the scattering area including the matrix of arachidic acid. A similar estimation was obtained for DIQA, where the area per molecule was 51 Å2, when the monolayer was deposited at 20 mN/m. Consequently, LBSERRS permits attomole detection of the target molecules. The SERS spectra of LB monolayer of DIQA/AA deposited on mixed AgAu island films, excited at three laser wavelengths, are shown in Figure 6, where the Raman spectrum of solid at 785 nm is also given for comparison. These SERS/ SERRS spectra are considerably less intense than those observed for DMQA on Ag, in particular for excitation at 785 nm (Figure 6d). This is probably due to the fact that DIQA is here adsorbed on Au in mixed SGIF, instead of Ag. As in the case of DMQA, the band corresponding to the CdO stretching decreases substantially, indicating interaction with the metal through the oxygen atom. Besides, there is a lower enhancement of typical in-plane modes (691694 cm1) which should indicate that there is no preferred relative orientation of DIQA toward the Au surface. Nevertheless, the close packing in the LB monolayer could explain minor band shifts with respect to the spectrum of solid. In addition, SERRS mappings of the LB sample were also recorded (not shown here) using the streamline capability of the inVia Renishaw Raman microscope to confirm the homogeneity of the LB signal at micrometer spatial resolution. SEF Spectra. The LB-SERRS of DMQA and DIQA monolayers deposited directly onto the metal nanostructure show very small fluorescence background due to effective quenching in close proximity of the monolayer to the metal.34 For fluorescent molecules, the first layer effect on Ag or Au nanostructures amounts to observing strong SERRS signals while quenching fluorescence. However, as the metal molecule distance increases, the fluorescence not only is seen, but is enhanced by the localized surface plasmon resonances (LSPR) of the metal nanostructure and detected as SEF.35 To observe SEF, it is necessary to deposit one or more spacer layers (such as arachidic acid) over the island films prior to the deposition of the corresponding fluorophore
DMQA/AA (DIQA/AA) LB monolayer. In the context of this work, SEF could provide complementary information on the aggregation state of the molecules in the monolayer. Taking into account that the aggregation of codopants used in OLEDs may change the performance of the luminescent devices, it is of great interest to investigate the SEF spectra of the monolayers. In order to achieve fluorescence enhancement from LB monolayers of the quinacridones, spacer layers of the fatty acid were used to separate the fluorophore from the metal surface.36 One and three monolayers of arachidic acid were transferred to metal island films to separate the metal nanostructure from the LB monolayer containing the fluorophore. Since the total thickness of the arachidic acid monolayer is ca. 2.5 nm, the maximum separation would be 7.5 nm. At these distances both the enhanced fluorescence and SERRS are observed, as can be seen in Figure 7. The reference fluorescence is that of the same LB monolayer of the fluorophore deposited on glass. We observe, as expected, that when the number of AA space layers increases from one to three, the SERRS intensity decreases to about 1/5 of the original value, results consistent with other observations of the distance dependence of SEF and SERRS signals.25 To consider the molecular aggregation in the deposited LB monolayers of DMQA and DIQA, we can compare the SEF spectra corresponding to one AA spacer layer shown in Figure 7a with both (a) the fluorescence spectra of the respective molecules in solution shown in Figure 2 and (b) the fluorescence spectra of the compounds in the solid state (Figure 7d). The SEF spectrum of the LB monolayer follows the spectral signature of the fluorescence in the solid state, broad and red-shifted with respect to the solution. The SEF spectra of the LB monolayers indicate a degree of aggregation, but the spectral features of the LB fluorescence do not have the effect of the three-dimensional packing of the solid state (Figure 7d). For DMQA there is an increase of the band around 630 nm, and similarly the SEF of DIQA presents increased emission below 650 nm. This fact seems to agree with partial two-dimensional aggregation extracted from the isotherms acquired during fabrication of the LB monolayer, given in Figure 1 of the Supporting Information. The latter is important because the existence of aggregates reduces their emission, then diminishing the device’s efficiency.5 16842
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’ CONCLUSIONS Trace analysis of dialkylquinacridones (DMQA and DIQA) used as codopants in the emission layer of OLEDs is demonstrated using SERRS and SEF. From the LB-SERRS and SERRS spectra on silver colloids, it is possible to extract information about the interaction between the molecules and the metallic substrate, as well as the molecular orientation. Despite the similarity of both molecular structures, their photophysics is very different with respect to metallic substrates employed in plasmonic enhancement. Thus, while DMQA gives SERRS spectra in Ag colloidal solutions and LB monolayers deposited on Ag island films, it was only possible to get SERS/SERRS of DIQA as LB monolayers deposited on Au/Ag island films. LB-SERRS may allow attomole detection of such codopants in real OLED devices. The SEF spectra gave complementary information about the aggregation of the corresponding molecules in the monolayers, important data related to the efficiency of the electroluminescent devices where the quinacridone derivatives are employed. ’ ASSOCIATED CONTENT
bS
Supporting Information. Tables with assignments of IR and Raman frequencies of DMQA and DIQA, supported by DFT calculations. Isotherms corresponding to the fabrication of LB monolayers of mixed DMQA/AA and DIQA/AA. CH stretching and overtone/combination spectral region of DMQA. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has been financially supported by the Ministerio de Ciencia e Innovacion of Spain (Projects FIS2007-63065, FIS2010-15405, and CONSOLIDER CSD2007-0058/TCP) and the Comunidad de Madrid (MICROSERES II Project S2009/TIC1476). E.P. acknowledges CSIC and FSE 20072013 for a JAECSIC predoctoral grant. ’ REFERENCES (1) Tang, C. W.; Van Slike, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Tang, C. W.; Vanslyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (3) Liu, H.; Yan, F.; Li, W.; Chu, B.; Su, W.; Su, Z.; Wang, J.; Hu, Z.; Zhang, Z. Appl. Phys. Lett. 2010, 96, 083301. (4) Shi, J.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. (5) Shaheen, S. E.; Kippelen, B.; Peyghambarian, N.; Wang, J. F.; Anderson, J. D.; Mash, E. A.; Lee, P. A.; Armstrong, N. R.; Kawabe, Y. J. Appl. Phys. 1999, 85, 7939. (6) Choulis, S. A.; Mathai, M. K.; Choong, V. E. Appl. Phys. Lett. 2006, 88, 213503. (7) Kwon, M. K.; Kim, J. Y.; Kim, B. H.; Park, I. K.; Cho, C. Y.; Byeon, C. C.; Park, S. J. Adv. Mater. 2008, 20, 1253. (8) Liu, D.; Fina, M.; Ren, L.; Mao, S. S. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 353. (9) Park, S. J.; Sohee, J.; Park, H. H.; Lee, S. W.; Jeon, S.; Lee, J. H.; Choi, D. G.; Jeong, J. H.; Choi, J. H. J. Nanosci. Nanotechnol. 2011, 11, 422.
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