J. Phys. Chem. 1993,97, 1421-1434
7427
Solid-state Absorption Spectroscopy of Alkyl-Substituted Oligothiophenes Shu Hotta' and Katsunori Waragai Quantum Wave Project, ERA TO, Research Development Corporation of Japan, 3-10-1 Higashimita, Tama-ku, Kawasaki 214, Japan Received: December 28, 1992
We have investigated the absorption spectra of thin solid films of alkyl-substituted oligothiophenes in both the neutral and doped states over a photon energy range of 0.05-5.0 eV. In the neutral state these oligomers display the U-T* transition bands in the near-UV to visible region with fine structures on the low-energy side. We found two different oxidized states for the doped species; the lower state is characterized by two subgap bands in the near-IR region that are induced upon doping and the higher state by only one band. In the two oxidized species, the T-U* band exhibits increasing intensity on the lower energy side at increasing doping levels and, finally, the absorption maximum is shifted to a low-energy region at the highest doping levels. This is an outstanding feature of the solid films and in sharp contrast with the spectroscopic characteristics observed for the materials in solution. The origin of this feature is related to the presence of molecular associates as charged species such as dimer radical cations. Of the two subgap modes for the lower oxidized state, the peak position of the lower energy modevaries inversely proportional to the number of the thiophene rings (Le,, the polymerization degrees of the oligomers) and is extrapolated into zero with increasing polymerization degrees approaching infinity. This feature characterizes the charge-resonance band of the dimer radical cations. We interpret these spectroscopic features observed for the neutral and doped forms on the basis of molecular orbital symmetry. Physicochemical implications are discussed in connection with the charge transport results that were obtained in FET (field-effect transistor) configurations. As a result, the charged species are associated with polarons in the solid-state physics terms. Doping-induced absorption bands are observed in the mid-IR region and these results are presented as well.
I. Introduction Semiconductingoligomers are currently being investigated as model compounds of semiconducting polymers because these oligomers have a uniform polymerization degree and a u-conjugated system is well-developed over the relatively long molecu1e.l-3 Many of these oligomers have large molecular extinctioncoefficient in visible to near-UV region. Furthermore, the semiconducting oligomers can be readily doped (oxidized) with acceptors. Optical absorption bands are induced by doping at positions specific to each oligomer species independent of differencein the chemicalspeciesof thedopants. These absorption bands are usually observed in visible to mid-IR regions similarly to thecase of the semiconductingpolymers.3-6 These spectroscopic features enable researches into electronic structure of the semiconducting oligomers through absorption spectroscopy. These studies have been carried out mostly in solution except for a few examples.' In the solution, especially at low concentrations of theoligomers, individualmolecules are separated pretty largely, and so interaction among them is likely to be weak. It is well-known, however, that once charges are introduced into an aromatic planar molecule in an appropriate fashion, such charged species easily form molecular associates such as dimer radical cations as a consequenceof interaction with the neutral molecule(s) in the vicinity. This can be observed even in relatively dilute solution or in organic glasses in which the molecules are dilutely dispersed.8 This is similarly the case with the semiconducting oligomers because those oligomers in most cases comprise the aromatic planar molecules.6 The charged species can be introduced, e.g., via y-ray irradiation in the frozen glasses, UV-laser irradiation in solution, and addition of an acceptor in the solution.s-'2 In a crystal or in solid film, on the other hand, the molecules are so closely packed that the interaction between them would be strong. In the solid state, the aromatic planar molecules form regular layered stacks in each of which the molecules are in densely
packed herringbone array.laJ3 In this molecular array the molecules are in nearly face-to-face arrangement so that the generated cation species can interact with surrounding neutral molecules to form the molecular associates such as dimerized species. Therefore, these ,charged molecular associates are probably generated even more easily and stand even more stable in the crystal or in the solid film than in the solution or glasses. Thus, we expect to observe spectral features in the solid state different from those for the solution. Bearing these circumstances in mind, we have studied absorption spectroscopy of the thin solid films of alkyl-substituted oligothiophenes over near-UV to mid-IR regions. The alkylsubstituted compounds are particularly suited to investigate the electronic structure in the doped state because both the terminal positionsare blocked with theinert alkyl groups;lZnonsubstituted compounds easily polymerizeto be polythiopheneand the spectral profiles specific to the oligomer species could be smeared.IZ In this paper we search into the absorption spectra of these alkylsubstitutedoligothiopheneswith a seriesof polymerizationdegrees (see Figure 1 for the chemical species and chemical structures) which were taken on their thin solid films in both neutral and doped states and characterize both the states. The most outstanding feature that distinguishes the solid-statespectra from those taken in solution is that the solid spectra display a strong u-r* absorption band in the oxidized form, whereas the corresponding band in the oxidized species in solution is inconspicuouslyweak. On the basis of these results, we infer that the charged species of molecular associatesplay a significantrole in the oxidized states of the solid films, especially at the light doping regime. In the previous papers we presented charge transport results of the alkyl-substitutedoligothiophenethin films in FET (field-effect transistor) configuration~.~~J~ There the charge transport is described as thermally activated hopping of polarons and the activation energy of this process is associated with binding energies of the polarons. In the present studies, we discuss physicochemical implicationsof the spectroscopicdata in
0022-3654/93/2091-1427%04.QQ/~0 1993 American Chemical Society
Hotta and Waragai
7428 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 1.o
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X Figure 1. Chemical structures of the alkyl-substitutedoligothiophenes.
The structures denote DMTT (dimethylterthiophene), DMQtT (dimethylquaterthiophene), DMQqT (dimethylquinquethiophene), and DMSxT (dimethylsexithiophene)from the top. The x- and y-axis define the directions of the molecular long axis and short axis, respectively.
Figure 2. Electronic spectra of the neutral oligothiophenes of DMQtT (dot-dashedcurve), DMQqT (broken curve), and DMSxT (solid curve). The spectrum of DMTT is shown as an inset. The peaks indicated with arrows are assigned to the 0-4 transition (see Discussion section). 1.6 I
connection with those charge transport results and show that the charged species may be regarded as an equivalent of the polaron in the solid-state physics terms.
II. Experimental Section Synthesis and purification methods of trimer to pentamer of the alkyl-substituted oligothiophenes can be seen elsewhere. 12 Hexamers were synthesized via coupling reaction of a'-monobrominated a-alkylterthiophenecompounds using mixed catalysts of zinc, tetrabutylammoniumiodide, and bis(tripheny1phosphine)nickel(I1) chloride.16 The hexamers were purified by recrystallization from chlorobenzene until the material showed only one spot on a TLC (thin layer chromatogram). Electronic spectra were recorded using a Hitachi U-4000 spectrophotometer under ambient environment over photon energiesof 0.5-5.0 eV on thin films of the oligothiopheneswhich were vacuum evaporated onto quartz glass substrates under a reduced pressure of 2 X 1 V Torr. A 2.0-mg portion of the material was fed for measurements of the electronic spectra. The distance between a quartz glass substrate and a Knudsen-type tungsten boat in which thematerial was accommodatedwas fixed at 5 cm. On this condition we obtained thin films of thicknesses ranging 94 to 143 nm (1 15 nm on average) for different five samples of DMSxT. We infer from these data that for other oligomers the film thickness changes around the averagethickness of 115 nm at most by 25% as well; the absorbance is expected to change accordingly. To determine the film thickness, the oligothiophenethin films were deposited so that the film covered half of the substrate and gold was subsequently deposited on the wholesubstrate. The film thickness was measured with a multiple interferometer (Nikon Surface Finish Microscope) using a 55 1.4nm-wavelengthlight. IR transmission measurementswerecarried out with a Hitachi Nicolet 1-5040 FT-IR spectrometer under dry nitrogen over probe energies of 400-4800 cm-'(0.05-0.6 eV) on the thin films of the oligomers evaporated on single crystal KBr disks. A 10.0-mg portion of the material was fed for the IR measurements and the film thickness was about 500 nm; the film thickness was proportional to the material fed. Since the evaporated thin films are optically uniform and transparent both on the quartz and KBr substrates, this enables us to obtain the transmission spectra without significant scattering from irregular structure. Theelectronicand IRspectra werecollectedboth on the neutral (as-evaporated) and subsequently doped films. The doping was performed either in a gaseous phase or in liquid. The liquidphase doping was carried out by soakingthe thin films in a solution containing2-20 mM iodineor a nitrosyl salt (NOW6 or NOBS) in which the neutral oligothiophenesare insoluble. The spectra
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Figure 3. Electronic spectra of DMSxT doped with iodine at different doping levels relative to the neutral species. An enlarged profile (X4) is that for the higher doping level. Two subgap modes are indicated with
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were reexamined about the materials which were reundoped through a methanol wash to ensure that original spectral profiles are reversibly recovered after the course of the doping and reundoping. Themethanol wash techniquewas utilized to prepare intermediate doping levels.
III. Results Figure 2 shows electronic spectra of the dimethyl-substituted compounds of trimer to hexamer (DMTT, dimethylterthiophene; DMQtT, dimethylquaterthiophene; DMQqT, dimethylquinquethiophene; and DMSxT, dimethylsexithiophene) in the neutral state. Two major absorption bands are observed for all the oligomers; one is located in the UV region (around 4.5 eV) and another is located in the near-UV to visible region having a broad feature around 4-2 eV and fine structures on the low-energy side that are due to the r-r* transition. The spectrum of DMTT is shown as an inset of Figure 2. The absorbance of the absorption maximum of the 7r** band was 0.32 (for DMTT), 0.35 (DMQtT), 0.20 (DMQqT), and 1.19 (DMSxT); the value is an average for different four to six samples. Subgap transitions are induced upon doping; Figures 3 and 4 show examples for DMSxT doped with iodine and NOPF6, respectively, at different doping levels relative to the neutral species. While iodine doping could yield only lower oxidized species that are characterized by the two subgap peaks (Figure 3), stronger acceptors such as the nitrosyl salts produce higher oxidized speciescharacterizedby only one peak occurringbetween the two subgap peaks (Figure 4). These subgap features correspond to the different oxidized states and these states are reversibly interchangeable. Figure 5 shows more precisely how these subgap features evolve depending on the doping levels. The subgap features are illustrated in Figure 6 for DMQtT, DMQqT, and DMSxT; the peaks of these bands arise at lower energy region
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7429 0.12 0.10
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Figure 4. Electronicspectra of DMSxT doped with NOPF6 at different doping levels relative to the neutral species. The broken and dot-dashed curves correspond to the lower and higher oxidized species, respectively. Two subgap modes for the lower oxidized species are indicated with arrows. The higher oxidized speciesis characterizedby a peak occurring between the two subgap peaks. h
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Figure 7. The near to mid-IR feature of DMSxT doped with iodine. Two different spectra taken with different apparatuswerejointed at an energy of 0.6 eV (see text). The two subgap bands are observed in the near-IR region and doping-induced bands in the mid-IR can be noticed below 0.2 eV. 0.20
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Figure 5. Evolution of the subgap featuresfor DMSxT dependingon the dopinglevels. DMSxT was doped with NOPF6. The spectra labeled a d denote the higher doping levels in this order.
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Figun6. ThesubgapfeaturesforDMQtT(a),DMQqT(b),andDMSxT
(c). The materialsweredoped withNOPF6. Asterkksdenotethesubgap band due to the higher oxidized species. The peaks of the subgap modes arise at lower energy region with increasing polymerization degrees. with increasing polymerization degrees. For DMTT, however, these well-resolved modes were not observed but only a broad featureless band was induced around 2.5 eV upon doping. Figure 7 depicts the near to mid-IR feature of DMSxT doped with iodine. Twodifferent spectra taken with different apparatus (the Hitachi U-4000 and the Hitachi Nicolet 1-5040) were joined at an energy of 0.6 eV. Since the thickness of the film for the IR measurements was 5 times as large as that for the measurements of the electronic spectra, the IR spectrum was suppressed by 5 times. The two subgap modes are observed in the near-IR region and doping-induced mid-IR modes can be noticed as well. Enlarged profiles of the mid-IR region for the iodine doped species are shown in Figure 8 relative to the neutral species.
0.15
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Figure 8. Enlarged profiles of the doping-induced mid-IR modcs for the iodine doped species (top) relative to the neutral species (bottom): (a) DMQtT, (b) DMQqT, and (c) DMSxT.
IV. Discussion 1. Characterizationof the NeutralSpecies. Both in the neutral and oxidized forms of the oligothiophene solid films, the molecules take trans zigzag and nearly planar conformation.1hJ7 Furthermore, the molecules tend to take this conformation even in solution in spite of being perturbed from its environment.18 This situation allows us to classify the oligothiophene molecules according to their symmetry. The molecules of an even number belong to CZh and those of an odd number to Cb (see Figure 1). Consideration of the MO (molecular orbital) symmetry based on
I430 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 Hiickel MO (HMO) calculations gives us a powerful clue to interpretations of the spectroscopic results. The absorption edges of the broad band due to the u-r* transition are red-shifted with increasing polymerizationdegrees of the oligomers. The absorbance of the T-T* band is increased suddenly when the polymerization degree of the oligomers reaches six.3 These are consistent with results by Fichou et ala3for nonsubstitutedoligothiophenes. The extinctioncoefficient of the absorptionmaximumof about lOS/cm for the hexamer (DMSxT) is comparable to that of polythiophenes. This implies that transition momentum of the r-r* transition becomes large and that r-conjugated system is well extended over the molecule of the hexamer. However, since the r-r* band is expected to reflect symmetry of themolecule,this must be taken into account. MOcalculations using the HMO method with parametrization listed in literature19 show that the one-electronexcitation configuration for the lowest energy optical transition (r+* transition) is bgauin the former case and a2b for the latter, ignoringlower energy electronicstates each of which accommodatestwo electronswith the up and down spin.20 These excitations are attributed to A, B, in the former case and Ai B1 for the latter. The elementary group theory teaches us that for the C2h molecules the transition is possible for the light that is polarized along the molecular long axis (x-axis in Figure 1) and for that polarized along the molecular short axis (y-axis).zo For theCbmolecules, on theother hand, the transition is allowed only for the light polarized along the molecular long axis. Therefore,if the CZ,molecules stand exactlyupright relative to the substrate plane, the absorbance will be zero. The crystallographicanalysis of DMQtTlk shows that the molecular length of the dimethyl-substituted oligothiophenesincreases by ca. 3.9 A for every increasing thiophene ring unit. We infer from the 8-28 measurementslk that the angle between the normal of the substrate and the molecular long axis is 40' (for DMTT), 26O (DMQtT), 23" (DMQqT), and 25" (DMSxT). Consequently, the absorbance of the r-r* transition for the CZ, molecules which is smaller than that for the C2h molecules may be interpreted in terms of the molecular symmetry;the absorbance of DMQqT is smaller than that for DMTT by the almost upright configuration of the DMQqT molecules. The absorption maxima of the r-r* bands in the near-UV to visible region were located at 3.50-3.52 (for DMTT), 3.67-3.69 (DMQtT), 3.52 (DMQqT), and 3.35-3.39 eV (DMSxT). The maximum are red-shiftedwith increasing polymerization degrees for the tetramer to hexamer but that of DMTT is close to the absorption maximum of DMQqT. The possible explanation is that the r-r* transition comprises several modes with different oscillator strengths. For the tetramer to hexamer the highestenergy mode appears to have the largest oscillator strength, whereas for the trimer a mid-energy mode within the r-r* transition seems to exhibit the largest oscillator strength (see the inset of Figure 2). Of these modes, the lowest-energy mode that is indicated with an arrow in Figure 2 is resolved as a real peak and the peak location is always observed at exactly the same position for different samples: 3.04 (for DMTT), 2.71 (DMQtT), 2.51 (DMQqT), and 2.39 eV (DMSxT). These characteristics reflect the regular molecular stacks in the thin solid films of the oligothiophenes. These energies agree well with the results by Fichou et al.21 calculated for the first one-photon-allowed transitions. The energy separation between this lowest energy peak and the second lowest energy mode appearing as a shoulder in the spectrum is 0.20 eV (ca. 1600 cm-l) that is common to DMTT, DMQtT, DMQqT, and DMSxT. These features may well be attributed to the vibronic coupling where the ringstretching modes are involved3 and related characteristics are noticeable for the parent polymer of polythiopheneand its alkylsubstitutedderivatives.22 Consequently,the lowest- and the second lowest-energy modes can be assigned to the 0-0 and 0-1
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Hotta and Waragai TABLE I: Characteristic Band Positions for Various Dimeth~l-Sth&hted o~gothiophenes.0-0 Transitions Arise on the Low-Energy Side of the r-r* Band as a Real Peak (See Figure 2). band wsitions (eV) higher neutral oxidation lower oxidation state State SDeciCS T-r' 0 4 DMTT (solid) 3.51 3.04 2.5 3.41 2.14 1.40 (so1ution)b DMQtT (solid) 3.68 2.71 1.78 (1.83) 1.08 (1.22) 1.47 3.12 1.85 1.11 1.71 (1.86) (solution)* 0.86 (1.00) 1.25 DMQqT (solid) 3.52 2.51 1.57 2.94 1.66 0.92 1.42 (1 S6) (solution)* DMSxT (solid) 3.37 2.39 1.45 (1.66) 0.71 (0.84) 1.15 2.83 1.55 0.80 1.51 (solution) a Values in parentheses are for secondary peaks. Reference 12a. 2-LUMO bg
( ~ 2 )
LUMO au(b2)
SOMO b g ( ~ 2 ) Figure 9. Schematic energy diagram of electronic structure and optical transitions in the radicalcations. HOMO,SOMO,LUMO, and 2-LUMO denote the highest occupied MO (molecular orbital),and singly (or spin) occupied MO, lowest unoccupied MO, and second-lowest unoccupied MO, respectively. Symmetry species of these MOs are shown on the right. a, and b, are for the molecules of the CU symmetry (Le., DMQtT and DMSxT); bz and a2 for the molecules of CZ,(DMTT and DMQqT). The transitions of E1 and E2 are assigned to the subgap modes.
transitions, respectively. The fine structures that can be noticed in the higher energy region are attributable to the 0-n (n = 2, 3, 4, ...) transitions. The IR transmission spectra are virtually the same for the trimer to hexamer (see Figure 8 for the mid-IR profiles of the tetramer to hexamer). The band assignment can be seen elsewhere. 2. Characterization of the Oxidized Species. 2.1. Lower Oxidized Species. Table I compares the characteristic band positions of the thin solid films with those recorded for the materials in dichloromethane solution.lk For theoxidized states of the tetramer to hexamer, both the cases follow basically the same trend; two new absorption bands are induced in the subgap region at related positions for the lower oxidized species, except that the doping-induced bands for the solid films are slightly red-shifted relative to the corresponding modes observed in the solution. At higher doping levels another band resulting from a higher oxidized species arises between the two modes and this band dominates in the subgap region at further higher doping levels both in the solid and solutiong7J2(Figure 5). Furthermore, the absorbance at the absorption maximum of the r-r* band is bleached upon doping (Figures 3 and 4). In Figure 9, we depict the energy diagram of a single isolated molecule with a positivecharge (a radical cation). Thesymmetry species of a, and b, for the C2h molecules (b2 and a2 for the CZ, molecules) appear alternatively in this order from the bottom level.7 Figure 9 shows the energy levels of importance with the optical transitions. When a positive charge is injected into a molecule, in other words an electron is removed from a molecule, the SOMO level is upshifted relative to the HOMO level of the neutral molecule and the LUMO level is downshifted relative to the LUMO of the neutral molecule because of the lattice deformation caused by the charge injection.23 In this system the transitions of E1 and E2 are expected to be observed in the subgap region because the SOMO and LUMO levels come closer compared to the neutral molecule. Both the transitions are
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Spectra of Alkyl-Substituted Oligothiophenes
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assigned to B, A, for the C2h molecules and A2 B2 for the Cb molecules. Since these transitions are allowed for the light polarized along the molecular long axis, strong absorbance resulting from their large transition momentum is anticipated. On the other hand, the HOMO-LUMO and SOMO to 2-LUMO (the second-lowest unoccupied MO) transitions (both assigned to B, B,) are forbiddenfor the CU,molecules;20these transitions in the Ca molecules (A2 A2) are only allowed for the light polarized along the molecular short axis for which weak absorbance due to smaller transition momentum is expected. For the isolated molecule of dication (where another electron is removed from SOMO of Figure 9) only the E1 transition (either A, B, or A1 B1 according to the symmetry) exhibits a strong absorbanceas discussed in section IV. 1. In this case the transition energy would become greater than that for the radical cation because of larger lattice deformationbrought about by introducing the two charges. A secondary peak or shoulder that sometimes accompanies the subgapbands on its high-energy side is assignable to thevibronictransitions even though the origin of such a sideband is unclear up to the present (Figure 6).3J2J4 Thus, for both the CU,and Cb molecules, the absorption bands that exhibit significant intensity are expected to arise only in the subgap region; E1 and E2 for the radical cation and E1 for the dication. These discussions based upon the molecular symmetry interpret appreciably well the experimental observations in the solution of us and other researchers.3J2J4 These discussions, moreover, seem to be generally applicable to interpretation of spectroscopic features of isolated long molecules having the ?r-conjugatedsystem with well-defined molecular symmetry.25 The spectroscopic features of the solid films, however, differ from those for the solution in that the T-T* band for the solid is given increasing intensity on the low-energy side at increasing doping levels, retaining the well-resolved fine structures (Figures 3 and 4). This is a striking feature of the solid film and in sharp contrast with the solution spectra; in the solution the T-T* band is almost completely bleached at higher doping levels.3 4 6 12.24 This outstanding feature is difficult to explain on the basis of the above model of the isolated molecule. One of most likely explanations is that the spectroscopic features of the solid film result from molecular associates. We have already reported the structural characteristics of the doped oligothiophenethin solid films.'% The results demonstrate the presence of the periodical layered structure in which the molecular long axis is nearly perpendicular against the substrate. Iodine doped DMQtT, for instance, has a periodic spacing of 2 1.6 A. This is roughly identical to the sum of spacing (18.1 A)128 for the neutral film and two times the van der Waals radius (2.0 %I) of iodine;26 we assume here that linear polyiodide anions such as I,- and 15-are laid down on top of the layer of the DMQtT m o l e c ~ l e s . 2Thus, ~ ~ ~ ~the face-to-face DMQtT molecular configuration is retained and in this configurationthe molecular associatescan be advantageously generated. Such structural characteristics may be responsible for the anisotropic charge transport characterized by high conductivity along thelateral direction.lh The term"?r-?r* band" is somewhat confusing for the oxidized species, but this terminology will be used in this paper as against the "subgap bands". For the solution spectra, a clear isosbestic point is observed at low doping levels, which indicates that only two chromophores that are mutually interchangeable are involved during the course of the doping; the two chromophoresare due to the neutral species and the radical cation.3~6.12We did not observe, however, a clear isosbesticpoint for the solid-state spectra even at the lowest doping levels; the intersection between the spectra of the neutral and doped species is shifted to the higher energy side with increasing doping levels (Figures 3 and 4). This implies that more than two chromophores are present during the doping. The molecular associates are expected to be involved among these chromophores; the simplest species is thedimer radical cation. Morecomplicated
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Figure 10. Schematic diagram of electronic structure and optical transitions in the dimer radical cations. The energy El corresponds to the peak energy of the lower energy subgap mode for the lower oxidized species. This band is attributed to the charge-resonance band. E2 corresponds to the higher energy subgap mode and El and E4 are due to locally excited bands. The charge-resonancestabilization energy is defined as AE.
species such as r-stacks6 of trimers: etc. might be involved as well. Nevertheless, the presence of the monomer species (e.g., radical cations) should not be excluded; the T-T* transition of this species would be negligible but the subgap transitions may well overlap with those occurring from the molecular associates. We show in Figure 10 schematically probable electronic structure and optical transitions of the dimer radical cation. The band splitting takes place because of interaction between the MOs. The El transition corresponds to the peak location of the lower energy subgap band for the lower oxidized species and is defined as the chargeresonance band29and E2 to the higher energy subgap mode. The transitions E3 and E4 are termed "locally excited" bands.*9 The dimer radical cation is stabilized by AE relative to the isolated system of the radical cation and neutral molecule, and so hE is termed the charge-resonancestabilization energy.29 If the widths of band splitting from the interaction between the SOMO and HOMO and between the two LUMOs are comparable, E3and E4 are roughly the same. These energies are close to the energy of the T-T* transition for the neutral species. Note that the E3 and E4 transitions are not forbidden; the energy levels result from splitting of the two sets of bands (i.e., SOMO/HOMO and LUMO/LUMO) that originally belong to the same symmetry. The excitations are, therefore, expected to retain the A, B, or AI BI character according to the molecular symmetry and, hence, to show strong absorptions (see discussion in section 1). It may be natural, moreover, that apart from the cause of MO symmetry such transition is given a large transition momentum resulting from a large spatial extent of the dimer radical cation compared to the monomer. These seem to explain the enhanced absorbance of the ?r-u* transition on the low-energyside for thedopedspecies. Thevibronicfeatures most likely due to involvement of the ring-stretching modes are observed as well (Figures 3 and 4). Despite the apparent consistency, an observed deviation from a simple relation of E3 (or E4) = E1 + E2 anticipated from the diagram in Figure 10 must be interpreted appropriately.3~24This could be done probably by taking electron correlation and CI (configuration interaction) into account.11.19 Of the two subgap bands observed for the lower oxidized species, the lower energy mode was peaked at 1.08 (for DMQtT), 0.86 (DMQqT), and 0.71 eV (DMSxT, indicated with a solid arrow in Figures 3 and 4). This band exhibits an Urbach-like taillo toward the mid-IR region and asymptotically approaches the base line (zero absorbance) around 0.2 eV (for DMSxT in Figure 7). The peak position is inversely proportional to the number of the thiophene rings included in the oligothiophene molecules. Similar relationship can be observed for the isolated molecules in solution as well.24 For the solution spectra, when the ring
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1432 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 number approaches infinity, the peak energy is extrapolated into a finite value of 0.2 eV (both in our previous studied2 and those of Guay et al.24). Although the peaks for the solid films arise at positions related to those of the apparently correspondingpeaks for the solution, we notice that the energy difference between the solid and solution spectra becomes larger with increasing polymerization degrees (Table I). Thus, when the ring number approaches infinity, the peak energy is extrapolated into zero for the solid films. Although this difference could be apparent and this peak possibly overlaps with the subgap modes from the monomer species, this feature noticed for the solid spectra represents characteristics of the charge-resonance bands of the dimer radical cations.” The peak positions of the higher energy mode were 1.78 (for DMQtT), 1.57 (DMQqT), and 1.45 eV (DMSxT, indicated with a broken arrow in Figures 3 and 4). The inversely proportional relationship was not observed for them. 2.2. Higher OxidizedSpecies. At increasingly higher doping levels, the u-r* band keeps growing on its low-energy side and the absorption maximum is shifted to a lower energy region at the highest doping levels. This band dominates in the spectrum and peaks at 2.76 (for DMQtT), 2.60 (DMQqT), and 2.45 eV (DMSxT); this peak position is closely related to that of the 0-0 transition observed for neutral species (Table I). The subgap peak locations of the higher oxidized species were 1.47 (for DMQtT), 1.25 (DMQqT), and 1.15 eV (DMSxT). Observation of only one subgap peak for the solid film is apparently related to that for the dication in the solution. Nevertheless, the band assignments are probably more complicated than in the case of the lower oxidized species. (i) The higher oxidized species is consistent with the dimer dication because in this species only one subgap transition is implied; as its electronic configuration is given by a diagram such that the highest energy electron is extracted from Figure 10, only the band corresponding to E1 would be observed in the subgap region. The peak position of the higher oxidized species larger than that of the lower oxidized ones (El) can be properly interpreted as follows: The interaction of the two SOMOs in the dimer dications will be stronger than the HOMOSOMO interaction in the dimer radical cations because the electron in the higher energy orbital is absent in the dimer dications. This stronger interaction results in the larger orbital splitting of El. (ii) Alternatively, the polaron-like wave functions mostly confined in individualmolecular associates like the dimer radical cations may well start interacting at increasingly higher doping levels. Such interaction probably takes place through the twodimensional herringbonearray of the oligothiophene molecules.lza In this case each energy level as depicted in Figure 10 is thought to be given a bandlike character as a consequenceof the interaction. Among the four energy levels, two intermediate levels play a major role (Figure 10);an intraband transition within the SOMO band and an interband transition between the SOMO-like and LUMO-like bands can be observed;32 the former transition is related to the unique subgap mode. If we assume the interaction between the dimer dications, the situation resembles that for bipolarons that are considered for conducting polymers such as polythiophenes.32J3 Id this case the lower three levels (having the bandlike character as well) are responsible for the optical transition; anticipated transitions are E1 and E3 in Figure 10. Note in the diagram that the higher energy electron is absent. Here the optical absorption edge is likely to arise at a nonzero energy. These descriptions may be associated with the polaron lattice and bipolaron lattice described by Stafstrbm and B r e d a ~ ) ~ except that they use a one-dimensional straight chain of polythiophene for their model calculations. The broad symmetric band in the subgap region without secondary structures (Figures 4 and 5) may well reflect the bandlike character of the energy levels. In the above discussion, the oxidized species are expected
Hotta and Waragai to have the oxidized states continuously changing between the two states of the dimer radical cation and dimer dication. At all events, we emphasize again that the intense T-T* band observed for the higher oxidized species is not accounted for on the basis of a model of the single isolated molecule. However, monomer species such as the dications that are more highly oxidized species should not be ruled out either, similarly as discussed in the previous subsection 2.1. Studies of the spin/ charge relationship will be obviously required in order to characterize the oxidized species in the solid more unambiguously. The assignment of the mid-IR bands induced by the doping is less clear than that of the visible to the near-IR bands. Nevertheless, the molecular symmetry may be again responsible for the Occurrence of these bands; for both DMQtT and DMSxT two modes are obviously induced upon doping at 1400-1 350 cm-l and several bands located at 1230-1040 cm-1 are given enhanced absorbance by the doping. In consequence, the resulting profiles of the mid-IR region resemble each other. To the contrary, the doped DMQqT exhibits an even more complicated profile. We must discern the modes arising from the charged molecular associates from those due to the single molecular moiety, which we could not do in the present studies; some of the latter modes may be raised because of breaking the molecular symmetry by introducing dopants. 3. Physicochemical Implicationsof the Doping-InducedBands. In a separate paper,15 we presented charge transport results of the oligomers of DMQtT, DMQqT, and DMSxT using configurations of FETs (field-effect transistors). We have pointed out there that the activation energies of the electrical conduction are associated with the doping-induced mode (that marked with a solid arrow in Figure 3, for example) in the near-IR region. We showed, furthermore, that the charge transport results are reasonably interpreted in terms of the thermally activated hopping of “polarons”. In an organic molecule, an excess charge injected, e.g., via doping or from the electrode, is expected to be self-trapped as a polaron to occupy a charged localized state as a consequence of the electron-lattice coupling. In other words, when the excess positive charge is injected into a molecule, the interatomic distances change depending on the chemical bond ~haracter3~ and the intermolecular distance between the said molecule and another molecule in the nearest vicinity becomes smaller in accordance with the self-trapping process of the ~ h a r g e . ~ ~ ~ 5 Following the polaron theory,30*35-37 the zero-field activation energy for the electrical conduction is equal to half of the polaron binding energy. The zero-field activation energy was found to be 0.24 (for DMQtT), 0.18 (DMQqT), and 0.16 eV (DMSxT) from the charge transport results investigated in the FET configurations.15 Therefore, the polaron binding energy will be 0.48 (for DMQtT), 0.36 (DMQqT), and 0.32 eV (DMSxT). The charge density estimated from the FET action analysis was about 1015/cm3;14 this is translated into the presence of a charge for every cube of about 1000 A. In this situation the charge will be accommodated as a polaron at the lowest oxidized state, typically in the radical cation or dimer radical cation; there is hardly a possibility that two charges are accommodated in a higher oxidized species such as a dication and a dimer dication. We define the energy upshift of the SOMO in the radical cation relative to the HOMO level of the neutral species as Ac according to Bredas and Street.23 Since A6 = &is + W,(where &ill is the distortion energy to be paid in the ground state so that the molecule can adopt the equilibrium geometry of the ionized state; Wp is the polaron binding energy) and Ac < El (where El is defined in Figure 9),23 Wp < El; this is for the case where the radical cation is assumed to be a polaron. When we think of the dimer radical cation as a polaron, the polaron binding energy Wp would be simply AE, i.e., the charge-resonance stabilization energy.29 This is because the energy level of the dimer radical cations should
Spectra of Alkyl-Substituted Oligothiophenes be referred to that of the isolated molecular pair of neutral molecule and radical cation. That is, charge transfer is expected to take place through the following series of reaction processes: D'+M MM'+M MD'+, where Do+,M'+, and M denote the dimer radical cation, radical cation, and neutral molecule, respectively. Here MM*+M is thought to be a transient state, and so the binding energy of the polaron D*+must be measured relative to MM*+(see Figure 10). In this case, therefore, Wp < El holds as well. Consequently, either model in which a polaron is accommodated in a radical cation or in a dimer radical cation is consistent with the experimental results in that the optical transition energy is larger than the polaron binding energy; compare the two sets of energies of 1.08,0.86, and 0.71 eV (see Table I) and 0.48,0.36, and 0.32 eV from the charge transport results. Interestingly, again, we find the inversely proportional relationship for the activation energies and polaron binding energies; these energies are extrapolated to zero with increasing polymerization degrees approaching infinity. On top of that, the estimated polaron binding energies are pretty close to the half of the energy of the subgap band with the smaller energy observed for the lower oxidized species, Le., 0.54 (for DMQtT), 0.43 (DMQqT), and 0.36 eV (DMSxT). If upshift of the SOMO orbital due to the lattice relaxation in the charged species is relatively sma11,23the resulting asymmetric splitting of the HOMO and SOMO orbitals (Figure 10) is mitigated, which leads to AE approaching this is likely the case with the present studies. Thus, from the spectroscopic and charge transport results as well as energetic consideration, we conclude that the molecular associates such as the dimer radical cations play an important role in the doped form of the oligothiophenesolid films, especially at the lower doping levels. It is worthwhile mentioning the early polaron model of Austin and Mott.3s By assuming a molecular pair, they predicted that the energy hw required for the optical transition which may take place in the molecular pair from one molecule to another without moving ions would be 2 times as large as the polaron binding energy according to the Franck-Condon principle. This is not necessarily true with our present case; we consider the optical transition within the dimer radical cation. In the dimer radical cation, however, the two molecular components are supposed to beat the same distortion state. In this sense, theoptical transition that takes place at that fixed distortion state and results in the charge-resonance band is thought to be an equivalent of that predicted by Austin and M ~ t t . ~ ~ The major doping-inducedmid-IR modes that arise at positions related or pretty close to the ring-associated modes are likely to play an important role in the electron-lattice coupling; since in the large molecules such as the oligothiophenes the ring-associated modes are comparable with or regarded as an equivalent of the optical ph0nons,30-~~ these doping-induced modes can be good candidates for those modulating the polaron motion. In fact, we obtained a hopping distance of the polarons of ca. 4 A for DMQtT by tentatively assuming that an averaged frequency of the dopinginduced mid-IR modes is identical to the optical lattice frequency that is relevant to the polaron hopping.l5 Thisdistance isconsistent with the presence of small polarons39~~ confined within molecular system, particularly like the dimer radical cations.
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V. Conclusion The spectroscopic results for the alkyl-substituted oligothiophenes both in the neutral and in the doped states have been presented and discussed. The neutral species are characterized by the A-r* absorption band in the near-UV to visible region with fine structures on the low-energy side. Of these structures, the lowest-energyfeature is resolved as a real peak, which reflects the regular layered stacks of the molecules. The prominent absorbance for the r--A*absorption maximum of DMSxT indicates that the r-conjugation is well extendedover the molecule.
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7433 In the oxidized states the r-u* band displays increasing intensity on the lower energy side at increasing doping levels. As a result, at the highest doping levels the absorption maximum is shifted to a low-energy region. This is a peculiar feature for the solid film and in sharp contrast with the solution spectra. We have discussed the origin of this feature on the basis of the molecular orbital symmetry. The results show that the origin of the intense a i * band observed for the oxidized species is difficult to explain on the basis of the single isolated molecule. Alternatively, the model based on the charged molecular associates such as the dimer radical cations successfully explains the spectroscopicresults in theoxidizedstates. We found twooxidized states for the doped species. The lower oxidized state is characterized by two subgap peaks that are induced upon doping in the near-IR region. Of these, the peak position of the lower energy mode changes inversely with the polymerization degrees of the oligomers and is extrapolated into zero with increasing polymerization degrees approaching infinity. This relationship characterizes the charge-resonance band of the dimer radical cations. The higher oxidized species is characterized by only one subgap band also observed in the near-IR region. In this form, the charged species can be the dimer dications or, in a more complicated case, the molecular associates that interact more widely through the two-dimensional herringbone array of the oligothiophene molecules. Careful comparisonof the spectroscopicresults with the charge transport data taken in the FET configurations also implies that the molecular associates such as the dimer radical cations play an important role in the low doping regime of the oligothiophene solid films. These charged species may be regarded as an equivalent of the polaron whose binding energy is associated with the optical transition that is observed in the subgap region for the lower oxidized state. Acknowledgment. We thank Dr. T. Inoshita, Dr. H. Noge, and Professor H. Sakaki for their helpful discussions and suggestions. We are indebted to Matsushita Research Institute Tokyo, Inc. for instrumental facilities. The present studies are supported by Research Development Corporation of Japan (JRDC) through the Exploratory Research for Advanced Technology (ERATO) funding for the Quantum Wave Project. References and Notes (1) Fichou, D.; Horowitz, G.; Nishikitani, Y.; Gamier, F. Synrh. Mer. 1989, 28, C723. (2) Jones, D.; Guerra, M.; Favaretto, L.; Modelli, A.; Fabrizio, M.; Distefano, G. J . Phvs. Chem. 1990. 94. 5761. (3) Fichou, D.iHorowitz, G.; Xu, B.; Gamier, F. Synrh. Mer. 1992,48, 167; 1990.39, 243. (4) Spangler, C. W.; Hall, T. J. Synrh. Mer. 1991, 44, 85. ( 5 ) Bally, T.; Roth, K.; Tang, W.; Schrock, R. R.; Knoll, K.; Park, L. Y. J. Am. Chem. Soc. 1992, 114, 2440. (6) Hill, M. G.; Mann, K. R.; Miller, L. L.; Penneau, J.-F. J. Am. Chem. Soc. 1992, 114, 2728. (7) Ehrenfreund, E.; Moses, D.; Heeger, A. J.; C o d , J.; BrMas, J.-L. Chem. Phys. Lett. 1992,196, 84. (8) Kira, A.; Imamura, M.J. Phys. Chem. 1979,83, 2267. (9) Tsuchida, A,; Tsuji, Y.; Ohoka, M.; Yamamoto, M. J . Chem. Soc. Jpn. Chem. Ind. Chem. 1989, 1285 (in Japanese). (10) Lewis, I. C.; Singer, L. S.J . Chem. Phys. 1965, 43, 2712. (1 1) Badger, B.; Brocklehurst, B. Trans. Faraday Soc. 1969, 65, 2582, 2588; 1970.66, 2939. (12) (a) Hotta,S.; Waragai,K.J. Mater. Chem. 1991, 1, 835. (b) Waragai, K.: Hotta. S.: Svnrh. Met. 1991. 41. 519. '(13) B e r n & &J.; Sarma, J. A . R.P.; Gavezzotti, A. Chem. Phys. Lerr. 1990, 174, 361. (14) Akimichi, H.; Waragai, K.;Hotta, S.;Kano, H.; Sakaki, H. Appl. Phys. Lerr. 1991, 58, 1500. (15) Waragai, K.; Akimichi, H.; Hotta, S.;Kano, H.; Sakaki, H. Synrh. Mer. 1993, 57, 4053. Otsubo, T.; Ogura, F. Bull. Chem. Soc. Jpn. 1989, (16) Yui, K.; Aso, Y.; 62, 1539. (17) Hotta, S.;Waragai, K. In The Physics and Chemistry of Organic
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