11780
1. Phys. Chem. 1994,98, 11780-11785
Electronic Structure of Oligorylenes in Thin Solid Films A. Schmidt* and N. R. Armstrong Department of Chemistry, University of Arizona, Tucson, Arizona 85721
C. Goeltner and K. Muellen Mar-Planck-Institut fuer Polyme8orschung, Posgach 3148, 0-55021 Mainz, Germany Received: April 11, 1994; In Final Form: July 21, 1994@
A series of oligorylenes with 2, 3, and 4 naphthalene units, symmetrically substituted by terr-butyl groups, was investigated in thin solid films by ultraviolet photoelectron spectroscopy, UV/vis absorbance, and fluorescence spectroscopy. The electronic structure of these oligorylenes was determined from these measurements and compared to theoretical and experimental data available for this class of materials. The ionization energy decreases with increasing length of the aromatic system from 5.46 to 4.59 eV, while the apparent electron affinity increases, from 2.77 to 3.01 eV. We predicted from this series the values for the polymer poly(peri-naphthalene) (PPN) and obtained an ionization energy of 3.69 eV. The fit for the predicted band gap of PPN was ca. 0.45 eV; therefore, the electron affinity should reach 3.24 eV. The polarization energy is 1.5 eV for all oligorylenes. There is a linear relation between the solid-state ionization potential and the electrochemical first oxidation potential. The slope is larger than 1, however, which may be explainable as arising from a solution solvation effect.
Introduction Polymers such as poly(peri-naphthalene) (PPN, see Figure l),with its ladder-like n-conjugated electron system, have been predicted to be good intrinsic conductors.' First theoretical investigations predicted a band gap of 3.5 eV,2a while new calculations gave band gaps ranging from 0 to 0.56 eV.2b-d Attempts to synthesize this polymer were not completely satisfactory. Most attempts have given materials with a high population of defects and partially graphitized material^.^ Subsequent purification of these systems has been hampered by the high melting point and low solubility of such ladderlike polymers. A chemically well defined series of oligomers, called tetratert-butyl(TTB)oligorylenes, has been recently synthesized with four symmetrically attached tert-butyl groups4 (see Figure 1). These functionalities give the PPN prototype materials good solubility, allowing the separation and chemical purification of the synthesis products. Oligorylenes, without tert-butyl groups, have also been synthesized by Clar.5 Because of the extreme insolubility of the molecules beyond perylene, the first and well investigated material in this rylene series, only a few investigations about the physical properties of these higher homologues exist.6 With the homologue oligorylene series, the optical "band gap" and electrochemical oxidation and reduction potential data in solution have been detem~ined.~ These experiments indicate that the "band gap" for these materials in the solid state will be below 0.9 eV. Linear and nonlinear optical properties have also been investigated in solution and thin solid films4-' The above-mentioned methods allow the determination of differences in the electronic states of the molecules, while the absolute values (against vacuum) cannot be obtained. In the present investigation we have determined the electronic structure of occupied states in this series of molecules, with the help of ultraviolet photoelectron spectroscopy (UPS) of ultrathin films, and have approximated, with the help of optical absorption 'Abstract published in Advance ACS Abstracts, October 1, 1994.
TTBBP
m 0 0 0
B a
lTBQ 0
PPN
0 0 0
@@El
Figure 1. Chemical structures of tetra-tert-butyl("l3)oligorylenes "Bperylene (TI'BP), "Bterrylene ("Tl3T), "Bquaterrylene (WQ), and "Bbiperylene ('"BBP) and the polymer poly(peri-naphthalene) (PPN).
spectroscopy,the electron affinity of these materials. The results suggest that the optical "band gap" of the PPN-like higher homologues will converge on a value of 0.45 eV. These results also have significant implications for the use of these materials in organiclorganic heterojunctions, where we seek optimization of such phenomena as photoconductivity or electroluminescence. The operation of electroluminescent devices is understood to be critically dependent upon the matching of ionization potentials and electron affinities of the luminescent material with those of the hole and electron transport agents, which are used to bracket the luminescent layer and which produce the necessary electrical rectification behavior.8 The study of the electronic structure of the oligorylenes is therefore significant to upcoming studies with these and related materials.
0022-365419412098-11780$04.50/0 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 45, 1994 11781
Electronic Structure of Oligorylenes
Experimental Section Material and Sample Preparation. We used the homologue series tetra-terf-butyl(TTB)oligorylenes, with four tert-butyl groups (Figure 1) symmetrically attached. The series consists of oligorylenes with two (TTBperylene, TTBP), three (TTBterrylene, TTBT), and four (TTBquaterrylene, TTBQ) naphthalene units in the aromatic system. The molecule TTBbiperylene (TTBBP) was also investigated. The synthesis and purification of these materials is described in ref 4. Thin solid films of these materials were deposited in high vacuum (base pressure Torr) atop atomically clean, polycrystalline gold and silver foils, which were argon-sputtered immediately before the evaporation process. Knudsen cell sources were used with sublimation temperatures of 203 "C for TTBBP, 103 "C for TTBP, 150 "C for TTBT, and 220 "C for TTBQ. Film thicknesses and growth rates were monitored by a quartz crystal microbalance (QCM, 10 MHz) mounted within the deposition chamber. The deposition rate was typically 0.5 k m i n . The material was held in the Knudsen cell at the abovementioned temperatures for at least half an hour before beginning the deposition on the clean substrates. In this way final outgassing and purification of each substance preceded each deposition. For the UV/vis and fluorescence spectroscopy a film of ca. 430 8, was deposited on sapphire. The sapphire substrate was cleaned in an ultrasonic bath with water (Milli-Q quality) and methanol and subsequently dried with nitrogen. Photoelectron Spectroscopy. The X-ray and UV photoelectron spectroscopy (UPS) data were acquired at room temperature with a VG ESCALAB MKII spectrometer. We used Mg K a radiation (1.2536 keV, 240 W) to determine Au4f or Ag-3d intensities before and after thin film deposition, to aid in the determination of the film thickne~s.~The ratio between equivalent monolayer thickness and QCM frequency change was determined by measuring the exponential decrease in the X-ray photoelectron signal of substrate peaks (Au-4f712, Ag-3d5,~)as a function of film thickness. These measurements gave the thickness of the film in units of the photoelectron escape depth. By assuming a typical escape depth of about 15 f 5 .&lo we approximated a QCM frequency shift of 5 f 2 Hz per angstrom of oligorylene film. The repeatability of the thickness of an ca. 100 A thick film is better than 2 A, while the accuracy is about &20 A. The peak intensities of the Au4f and Ag-3d peaks used for these measurements were obtained by subtracting the background from the raw data and integrating the remaining area under the peak.9 The electronic structure of the oligorylenes was measured with He I(21.2 eV) photons. In this mode the U P S spectrum (number of electrons with kinetic energy KE) is proportional to the joint density of states between the initial and final states of the electrons, superimposed on a background of secondary electrons. For survey scans (like in Figure 2), the signal was measured in steps of 0.05 eV, while, for the determination of peaks and onset of secondary electrons, steps of 0.01 eV were used. The electron analyzer was set to a constant retardation ratio (CRR) of 4. Typically the binding energies in nonmetallic solids are referred against the Fermi energy of a metal evaporated atop the films." The binding energy can then be calculated from the binding energy vs Fermi level by adding the work function @ of the sample. The work function itself is obtained from the photon energy minus the width of the spectra, i.e. the difference between the energy for the onset of secondary electrons KEonSet and the Fermi energy KEmaxll
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Figure 2. (a) UPS raw spectrum (He I) of ca. 120 8,TTBBP on gold. At -5.00 V bias at the sample the whole spectrum is shifted to higher kinetic energies and the onset of secondary electrons from the sample is visible. (b) U P S raw spectrum (He I) of ca. 90 8, TTBP on silver. The binding energy vs vacuum is detetmined from the difference between the onset of the secondary electrons and the peak position (here shown for the HOMO 'peak).
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We did not follow this procedure here. Instead, we used an equivalent way to obtain the binding energy vs a true vacuum level without using the Fermi level. The difference between the energy for the onset of the secondary electrons KEonSet and the experimentally measured kinetic energy KE of a state within the spectrum is equal to the difference between the photon energy hv and the binding energy vs vacuum BE,,, of this state."
The correction factor KEonSet to the law of energy conservation is necessary to account for spectrometer effects (Le. differences in the vacuum level of sample and spectrometer)." The equivalence between eqs l a and l b is shown by the definition of BE, as & , - KE. Unless the work function of the material is measured independently, both methods (eqs l a and lb) have the same experimental errors, but eq l b does not need the evaporation of a metal atop the organic material with possible destruction of the film. We have chosen this approach because the low electrical conductivity of these organic materials leads to charging effects, which shift with increasing thickness the position of the apparent Fermi level of the organic material vs the metal. Furthermore the Fermi level, which is determined by the charge carrier concentration, or consequently the work function of the organic
Schmidt et al.
11782 J. Phys. Chem., Vol. 98, No. 45, 1994
material depends strongly on dopants, which can have been intentionally applied or accidentally added as impurities. Oxygen especially plays a major role here. The threshold (ionization potential IP)and the positions of molecular orbitals, which appear as peaks in the UV photoelectron spectra, are more important to the discussion of physical properties of organic materials in the solid sate. We biased the sample with -5.00 f 0.01 V to obtain a well defined onset of secondary electrons (EonSet) with minimum kinetic energy. The detection sensitivity of the spectrometer is low for the lowest kinetic energy electrons from the samples, which have just enough kinetic energy to reach the analyzer. Therefore the bias is necessary to bring these lowest kinetic energy electrons into an energy region with a high enough detection probability. Fi ure 2a shows the difference between raw spectra of ca. 120 TTBBP on gold with and without bias. At a bias of -5 V the Poole-Frenkel effect lowers the work function12 (and ionization thresholds) only a few millielectronvolts, such that the two spectra are basically only shifted by 5 eV relative to each other. In the biased spectrum the secondary-electron onset is visible and the low kinetic energy side is ramped due to the secondary electrons. The transmission function of the analyzer does not change significantly in the whole region and the ramp does not change the peak positions at the high kinetic energies significantly; therefore, the only practical difference-and advantage-of the biased sample is the visibility of the secondary electron onset. UPS data analysis was carried out by determining the onset of the secondary electrons by linear extrapolation of the low kinetic energy spectral region down to zero intensity. The well defined peaks in the spectra were fit with a Gaussian peak shape after subtraction of a linear background. The threshold was determined as the linear extrapolation of the slope at the point of inflection of the first peak down to zero intensity. The UV/vis absorbance spectra were measured with a HITACHI U-2000 two-beam specaophotometer. An identical clean sapphire substrate was used as reference. The optical “band gap” was approximated from these data as the energy at 1/10 of the absorption peak with lowest energy.“ With the optical “band gap” and the ionization potential IP the threshold energy of the unoccupied states was approximated. This threshold is assumed to be a good approximation of the electron affinity (EA) of the material, assuming that the optical excitations occur to energy levels which would be occupied by electrons from an external source. The fluorescence spectra were obtained with a SPEX Fluorolog-2 spectrophotometer. The angle between excitation and detection is 30“ for the solid films. The fluorescence spectra of an identical sapphire substrate were measured independently and subtracted from the TTBP spectra.
1
Results and Discussion In Figure 2a and b typical U P S raw spectra are shown for the example of ca. 120 8, TTBBP on gold and ca. 90 8, TTBT on silver. Clearly recognizable are two peaks at the high kinetic energy side and the onset of the secondary electrons at the low energy side. As explained above the binding energy vs vacuum BE,, is determined from eq l b using the extrapolation of the onset to zero intensity as reference (Figure 2b shows this for the HOMO peak). Neither the bias voltage nor possible charging of the organic material vs the metal substrate plays a role in this determination, because only the difference between such shifted peaks within the organic spectrum is used. In general UPS spectra were recorded for these materials from coverages at monolayer levels and up-to c o n f i i that the shape of these ionization bands is independent of ~0verage.I~
0
e
A
A
Threshold HOMO 2ndHOMO
20
40
60
80
v v
V
0
A
100
120
TTBBiPerylenyle coverge [A]
Figure 3. Binding energies of the threshold (valence band edge; the HOMO peak, and the second HOMO peak ionization energy P), in the U P S spectra as a function of the TTBBP coverage on gold (filled symbols) and silver (open symbols). Changes in the polarization energy close to the metal surface bend the state levels. TABLE 1: Electronic Data for the Oligorylenes TTBBP, TTBP, TTBT, and TTBQ in Thin Solid Films 2nd ionization optical electron HOMO“ HOMOb potential band gap‘ affinity (eV) (eV) IP(eV) (eV) EA(eV) TTBP 7 . 3 8 f O . l 5 . 8 5 f 0 . 1 5 . 4 6 f O . l 2 . 6 9 f 0 . 0 3 2 . 7 7 f 0 . 1 TTBT 7.18 5.33 4.84 2.02d 2.82 1.58d 3.01 TTBQ 7.06 5.00 4.59 TTBBP 7.58 5.96 5.48 2.71‘ 2.77 fwhm: 0.80 f 0.04 eV for all oligorylenes. fwhm: 0.50 f 0.04 of the eV for all oligorylenes. Determined as the energy at absorbance of the lowest energy peak. Reference 4b. e Reference 7.
UPS spectra do not give a real picture of the occupied density of states but rather a convolution of occupied and unoccupied states. It is recognized that the electronic states in different oligorylenes have the same origin; specifically the symmetry and atomic orbital contributions of analog molecular orbitals are equivalent.2d It is therefore reasonable to compare UPS data within this series of oligomers, without significant error introduced in ignoring the convolution of these occupied and unoccupied densities of states. In Figure 3 the ionization potential IP and the presumed energies of the highest occupied molecular orbital (HOMO) and the second highest occupied molecular orbital (2nd-HOMO) are plotted for an example oligorylene (TTBBP) as a function of film thickness on gold and silver. We have chosen two different metals as substrates to rule out possible influences from the substrate on the binding energy of the organic molecules. The actual size of the plotted symbols reflects the experimental errors in these measurements. Above a coverage of ca. 30 8, the binding energies are constant and independent of the substrate. With decreasing film thickness below 30 8,the binding energies showed a systematic increase. In Table 1 the binding energies and related energies for all four oligorylenes are given. These binding energies are determined from analog experiments as for TTBBP (Figure 3), and the values are calculated for thick films (230 8,) by averaging over different films on gold and silver. All binding energies decrease with increasing size of the aromatic system. Table 1 also contains the optical band gap determined from the UV/vis absorbance spectra. Figure 4 shows the absorbance and the fluorescence as a function of wavelength for an ca. 430 A thick TTBP film on sapphire. With the help of the optical “band gap” the threshold for the unoccupied states (apparent electron affinity EA) can be approximated as the difference between the threshold (IP) and the optical “band gap”. We define the optical
J. Phys. Chem., Vol. 98, No. 45, I994 11783
Electronic Structure of Oligorylenes 430 A TTBpetylene on sapphire 35
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Threshold (IP) HOMO 2nd HOMO
m m
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TABLE 2: Polarization Energies and Relaxation Energies of the Oligowlenes TTBP, TTBT, and TTBO theoretical relaxation IP(gas)" P+ HOMOb energy' (eV) (eV) R(eV) (eV) (eV) 7.19 0.22f0.14 6.97 1.51f0.1 1.12f0.1 TTBP TTBT 6.42 1.58 1.09 6.78 0.36 1.11 6.57 0.46 TTBQ 6.11 1.52 Unsubstituted oligorylenes.6a Calculated for unsubstituted, single oligorylene molecules.3d Relaxation energy approximated by IP(gas) - theoretical HOMO. in a decrease of the binding energy of the valence states, while the threshold of the unoccupied states (EA) moves further away from the vacuum level. This behavior was also found in the electrochemical data4 of these substances in solution and theoretical calculations3 of the electronic states of single rylene molecules (without tert-butyl groups). As visible in Figure 5 the electronic states of I T B P and TTBBP are identical (as expected) within experimental error. Theoretical calculations show that the orbital contribution of the carbon atoms, at the position of the attached tert-butyl groups, to the highest occupied and lowest unoccupied states in these aromatic systems, is very smallzd Therefore we do not expect the electronic states to be much different between rylenes with and without tert-butyl groups. The equivalence of energies in TTBBP and TTBP confirms this hypothesis. It is therefore possible to compare the UPS data of unsubstituted perylene in the literature with our TTBP results. While the apparent HOMO position of ~ e r y l e n eagrees ' ~ ~ with our results from TTBP and TTBBP, we find that the separation between HOMO and ionization threshold is 0.6 eV in the literature14 but 0.4 eV in our experiments. This difference is due to the different methods used to obtain these values. While we determined the threshold position from the width of the photoelectron spectrum at a fixed photon energy, Sato et al.14 used different photon energies to extrapolate the threshold to the low-energy limit. Our own investigations with unsubstituted perylene showed a separation in HOMO and threshold of 0.38 f 0.05 eV, consistent with our results from TTBP. Additional experiments with the perylene derivative PTCDA (perylene3,4:9,10-tetracarboxylicdianhydride) showed a difference in threshold positions of 0.25 & 0.1 eV between our results and literature values.'4b We assume therefore that at least for perylene derivatives the difference between the low-energy ionization threshold and our results for IP is about 0.2 eV. This difference will then of course also influence the position of the apparent electron affinity which we determine with the help of the ionization threshold. The position of the ionization potentials in the solid state determined by UPS can only be approximated by theoretical calculations of the single-electron states of single molecules. Deviations are caused by the polarization energy, describing screening of the charged molecule after ionization, and the relaxation energies, already present in a single molecule. Another factor comes from the U P S experiment, which measures a convolution of occupied and unoccupied states. The polarization energies for the oligorylenes are given in Table 2. The values were obtained as differences between the gas-phase ionization potential IP, of the oligorylenes without tert-butyl and the valence threshold (polarization energy P+) or the HOMO position (polarization energy R ) . Both polarization energies do not show a significant difference for all three oligorylenes and are 1.54 & 0.1 eV for P+ and 1.11 f 0.1 eV for R. For most organic materials values of 1.7 f 0.4 and 1.1 f 0.2 eV are found.14a The difference in P+ is a result of our method of measuring IP (see above).
4
naphthalene units
Figure 5. Electronic structures of TTBP, TTBT, TTBQ, and TTBBP (open symbol). The structures of the occupied states is determined from the U P S measurements. The threshold for the unoccupied states (electron affinity EA) is determined by the ionization potential and the optical band gap. "band gap" as the energy where the absorbance is 1/10 of the highest wavelength absorbance peak. Another possibility is to determine this optical "gap" from the intersection of the fluorescence emission and excitation spectra. In the case of " l B P this gap energy, determined by both measurements, agrees within 0.02 eV. The peak positions and width of the absorbance spectra are nearly identical with those of the solution spectra (not The maximum absorption at the high-wavelength side is at 440 nm in the solid films and 439 nm in solution. This observation is consistent with the strong localized nature of these transitions, even in closest packed assemblies. The fluorescence spectra in thin solid films are broadened-instead of two peaks only a peak with a shoulder is recognizablecompared to solution and the Stokes shift is 45 nm instead of 6 nm4bfor the solution. It is noteworthy to compare the UV/vis absorbance in these slowly grown films in high vacuum (lo-* Torr) with spectra of fast evaporated TTBP films on glass at Torr.7 In the latter case the peaks are much broader and the intensity ratios between peaks are changed. We conclude that our slowly grown films have a much better defined structure. Figure 5 summarizes the collected U P S and optical results. The electronic structures for the four molecules TT'BBP, TTBP, TTBT, and TTBQ are given as a function of naphthalene units in the oligorylenes. This number of naphthalene units is a linear measure for the size of the aromatic system; therefore, TTBBP and TTBP have the same size of two naphthalene units. As expected, increasing the extension of the aromatic core results
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11784 J. Phys. Chem., Vol. 98, No. 45, I994
Schmidt et al.
TABLE 3: Fit Parameters of HOMO and IP for the Olieorvlenes in Solid State An) = opt band IP - e(Ox)b A 4- B/n HOMO (eV) Tp (eV) gap“ (eV) (eV) A 4.18f0.13 3.69f0.12 0.45f0.09 3.97f0.06 B 3.36 f 0 . 2 2 3.51 f0.21 4.54 f0.27 1.44f0.16 a For this fit data for T’I’B~entarylene~~ were used also. Data from ref 4a were used. ~
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The polarization energies for very thin films (
