Optical and Photonic Functions of Conjugated Polymer Superlattices

Chapter 31

Downloaded by UNIV MASSACHUSETTS AMHERST on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch031

Optical and Photonic Functions of Conjugated Polymer Superlattices and Porphyrin Arrays Connected with Molecular Wires 1

Takeo Shimidzu

Division of Molecular Engineering, Kyoto University, Kyoto 606-01, Japan

Conjugated polymer superlattices and porphyrin arrays connected with molecular wires are superstructured materials, which exhibit unique optical and photonic functions. The former shows a shift in photoluminescence to higher energy which is interpreted as a quantum size effect. The latter class of materials exhibits photoconductivity by a hole carrier mechanism and photo– information storage by a localized excitation mechanism. The syntheses of these two classes of materials are described.

Both quantum functional materials and molecular devices are considered to be the ultimate functional materials. The former shows a novel property which is specific to the structure and the latter represents the smallest possible functional material. Their properties are closely related to optical and photonic functions. The former shows a quantum size effect, a photoluminescence shift to higher energy which depends on layer thickness, which gives us an idea of a nonlinear optical system. Porphyrin arrays connected with molecular wires show a hole carrier photoconductivity or a photos witching and a photo-information storage, which suggests an idea of a photoactive neuron model. In this paper, conjugated polymer superlattices and porphyrin arrays connected with molecular wires are described. Conjugated Polymer Superlattices Since almost all conjugated polymers are organic semiconductors, structural control, such as compositional control of a copolymer thin film, corresponds Current address: Kansai Research Institute, Kyoto Research Park, 17 Chudoji-Minami-Machi, Shimogyo-ku, Kyoto 600, Japan

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© 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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Conjugated Polymer Superlattices & Porphyrin Arrays

to the manipulation of the electronic band structure of the thin film. Structural control of semiconductor thin films in the l-10nm scale range can give rise to novel optical and photonic functions which come from carrier confinement due to quantum-size effects. In the past few decades, inorganic semiconductor superlattice and multiple quantum wells have been extremely active research subjects in semiconductor physics and materials science ever since the proposal by Esaki and Tsu in 1970 (7). Many new physical phenomena, such as negative differential resistance, have been discovered, and many novel device concepts, such as high electron mobility transistor, have been developed based on these ultra-thin layered structures of semiconductor materials constructed by molecular beam epitaxy and metalorganic chemical vapor deposition. The most important feature, which has produced this great activity, is energy quantization of electronic structure by mesoscopic modulation in materials (2). This not only changes the energy band structure but also alters the density of the states and restricts electron motion within the layer planes, leading to a lower dimensional electron system. This feature gives rise to new charge transport, optical, and magnetic properties. Furthermore, because one can control and largely prescribe all these features by adjusting parameters of the heterostructures, periodicities of the layers, and band discontinuities by composition of the constituent layers, new materials with desired physical properties can be essentially designed. A function which is specific to the overall structure of a heterostrctured material such as superlattice period or a quantum function due to ultrasmall size can be regarded as an ultimate function of the material which can only be creaeted by proper fabrication methods. The design and fabrication of such ultimate function materials has developed into a new field of advanced materials called wave function engineering, quantum technology, etc. Super lattices of inorganic materials are, in general, fabricated by ultrahigh vacuum systems such as molecular beam epitaxy (MBE). However, in order to realize "wave function engineering" in conjugated polymer thin films, and prepare superlattices, a novel method of compositional modulation in conjgated polymer thin films is required. Polymerization followed by the insitu deposition of the resulting polymer onto a substrate makes the fabrication of superlattice structures possible. Electropolymerization is one of the most interesting methods to control the copolymer composition in molecular or chain sequence. Accordingly, in the case that the electropolymerized material is electroconductive and insoluble, a heterolayered structure and/or a sloped structure with conducting polymers can be constructed on the electrode. The potential-programmed electropolymerization method (PPEP) is utilized for modulating the composition of conducting polymer thin films in the depth direction (3 4). By this method, nanometer scale compositional control of the thin films of conducting polymers are obtained, permitting the fabrication of alternate layered and graded heterostructures. Monomers such as pyrrole, thiophene, and their derivatives, can be electropolymerized, so that the corresponding conducting polymer thin films are obtained on the surface of a working y


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PHOTONIC AND OPTOELECTRONIC P O L Y M E R S

electrode, if they are insoluble. The growth rate of the film thickness is proportional to current i. In general, the PPEP method consists of the electropolymerization of a mixture of monomers under potentiostatic control in accordance with an appropriate potential sweep function. The function is programmed in advance from the current fraction curves for each monomer, which leads to a definite copolymer composition and control of layer thickness. The resulting conducting polymer film has a compositionally modulated depth structure corresponding to the applied potential sweep function, for example a layered structure for a rectangular (square wave) function and a graded structure if using a varying function, such as a sawtooth wave. By applying the PPEP method to the copolymerization of pyrrole and 3methylthiophene, various kinds of conducting polymer heteromultilayers were fabricated. Figure 1 shows the resulting transmission electron microscopy (TEM) cross section and electron-probe microanalysis (ΕΡΜΑ) line analysis on sulfur reflecting thiophene content. The layer on the electrode side, from which the layer grew, showed a clear and flat interface. The depth profile of the resulting conducting polymer multilayers also was evaluated by SIMS, AES, TEM and ΕΡΜΑ (5). Alternate layered structures were fabricated by a rectangular potential wave and stair-like step sweep function, and triangular sloped structures by a triangular sweep function and triangularly sloped structures resulted from sawtooth potential waves (4). Figure 2 shows the band structures of several homopolymers and pyrrole-bithiophene copolymers estimated by electrochemical and optical methods as examples. A combination of these homopolymers and/or copolymers implies various kinds of superlattice structures. The electrochemical preparation of both homopolymer multiheterolayers and/or copolymer multiheterolayers results in a superlattices. The electrochemical copolymerization method as used to prepare heterolayers was easier than in the homopolymer heterolayers. The copolymer multiheterolayers are prepared by simply changing the applied electrode potential. On the contrary, the latter needs exchange of the mother solutions. The present electrocopolymerization method which makes compositionally modulated copolymer heterolayers possible is considered to be one of the most fascinating methods to fabricate organic superlattices. The fabrication of copolymer multiheterolayers was carried out on a rotating highly oriented pyrolytic graphite (HOPG) disk electrode (working electrode; 1000/r.p.m.) which leads to flat and sharp interfaces having a resolution of order lnm. The electrocopolymerization of a mixture of pyrrole (2.5xl0 M) and bithiopehene (2.5xl0 M) by a rectangular potential sweep having limits of 1.0V and 1.4V, gave superlattice multilayers whose dedoped layers were expected to be a type II superlattice structure whose band structures of well layer and barrier layer are alternative (i). The barrier layer was composed of 33% bithiophene and 67% pyrrole while the well layer was composed of 87% bithiophene and 13% pyrrole. In this superlattice, the conduction band edge difference AE is 0.58V and the _4

-2

C


SHIMIDZU



31.

CH3CN

Fig. 1 Ultrathin conducting polymer heterolayers by the potential sweep programmed electropolymerization of pyrrole (25mM) and 3methylthiophene (50mM) in containing lOOmM L1CIO4.: Potential sweep programs and TEM pictures of their cross sections.


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4*.

SHEVirozu


valence band edge difference Δ Ε is 0.41V, as shown in Figure 3. Both layers can be controlled by copolymer conposition, as shown in Figure 4 (c) in which the barrier layer is composed of 54% bithiophene an 46% pyrrole, giving AE =0.20eV and AEv=0.18eV. ν


c

The photoluminescence spectra of the dedoped copolymer (pyrrole/bithiophene) films whose thiophene content was higher than 50%, consisted of three peaks around 2.0, 1.8, and 1.7eV corresponding to phonon side bands at 10K. These peaks correspond to the radiative relaxation of self-trapped excitons. The peak at the highest energy reflects the copolymer's band gap. Actually, the peak positions observed in the spectra of copolymer films shifted to higher energy as thiophene content in the film decreased and peak positions showed good agreement with Eg as estimated in Figure 3. The copolymer containing a thiophene fraction less than 50% did not, however, show photoluminescence. The photoluminescence of the multilayers having 6.0nm of 87% thiophene layer and lO.Onm of 33% thiophene layer (10 layers) [(87)LW(33)L ]IO (Lb=10- ) shifted to higher energy as compared with that of the bulk (87% thiophene content) copolymer film. (Figure 4) (6). The photoluminescence of the above-mentioned multilayers shifted to higher energy as the thickness of the well layer (Lw) became smaller than 12.0nm, even when the barrier thickness (Lb) remained constant (lO.Onm) and when the ratio, Lw/Lb was constant (0.6). On the other hand, the bulk thin layer did not show a significant energy shift. Such a shift to higher energies is considered to be the result of the confinement of excited electrons in the quantum well layer. We have also found a good fit of experimental results with the Kronig-Penney model, which derives the energy-wave number vector relationship in rectangular type potential profile by assuming that m*=0.6 m where m is the electron mass (depicted as the solid line in Figure 4). The Kronig-Penney model is given as : 0nm

D

e

cosk (Lw + Lb) = cos

^

2

f

e

E

Lwcosh ^ ™ * ( V o - E ) Lb 1/2

+

(S-

)

l

______

*> -=^~

Lwsinh ^ψ ^ 7

Lb

where Vo and m* are barrier height and effective mass, respectively. It is noteworthy that other multilayers have also shown a similar phenomenon. These observed photoluminescence spectral properties of conjugated polymer heterolayers fabricated by the present PPEP method appear to be due to quantum size effects. However, additional studies of these materials will be necessary to establish the true origin of their properties. In any case, these results also suggest that many other novel structures of functional materials and devices can be fabricated by this method. Porphyrin Arrays Connected With Molecular Wires Nanofabrication of molecular photoelectronic, optical and photonic devices is important (7-11). Incorporation of multiple redox centers into conducting


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ι 0

i t 20

t ι 40 60 S/(SIN) / %

i 00

l

l 100

Fig. 3 Fabrication of type II conducting polymer heterolayer superlattice by the electro-copolymerization of pyrrole (2.5x10 M) and bithiophene (2.5xlO- M) LiC10 (l.OxlO^M) CH3CN solution. Sweep potentials, copolymer compositions and AE , ΔΕ of the resulting heterolayers. _4

2

4

C

ν


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(a)

(b)

Lw/A Fig. 4 Structure of type II heterolayer superlattice and emission peak shift as a function of layer thickness, (solid line is estimated from Kronig-Penney model). (a) [(87) w(33) ]io Lw/Lb=0.6 (b) [(87) w(33) ]io Lb=10.0nm const. (c) [(87)LW(54)L ] 10-20 Lb=10.0nm const. L

Lb

L

Lb D

L and L denote well layer and barrier layer, and ( ) denotes bithiophene content in pyrrole-bithiophene copolymer. w

b


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molecular systems is a useful approach for trial construction of molecular devices. For example, the incorporation of a photosensitizer and a suitable electron donor and/or an acceptor into a polymeric chain has been proposed as a molecular electronic device system based on photoinduced electron transfer (11). However, the production of such polymers containing a number of large aromatic moieties or metal complexes is difficult because of the lack of the solubility and flexibility, which also limit the possibility for controlled fabrication. To overcome these difficulties, electrochemical polymerization is used, since the polymer is deposited directly on the terminal electrode. With this in mind, we have synthesized a series of oneor two-dimensional porphyrin arrays connected with conjugated wires which can be polymerized by normal electrochemical oxidation. On the other hand, porphyrin arrays connected with insulating wires have also been synthesized by esterification. Construction of intramolecular systems whose photoactive molecules are linked with conducting and insulating molecular wires is an important objective towards the realization of molecular electronic or photonic devices. For such an objective, systematization of donor-photosensitizer-acceptor triad molecules into large molecular systems is one of the feasible approaches to realize a simple model. This is because the incorporation of a photoactive moiety and a suitable electron donor and/or acceptor into a conducting polymeric chain is useful for various molecular electronic systems based on photoinduced electron transfer (11). A symmetrical donor-acceptor-donor triad molecule was polymerized by normal electrochemical oxidation which led to one-dimensional donor-acceptor polymers with porphyrin moieties separated by an ordered oligothienyl molecular wire (1 D porphyrin array) (12).

The oligothiophenes, which are easily polymerized by electrochemical oxidation, were used not only as molecular wires but also as the coupling elements for connecting the phorphyrin moieties. Phosphorus(V)porphyrins (P(V)TPP) which have strong oxidizing powers and are stable to the electrochemical oxidation were used as the photoactive moieties (13, 14). Since P(V)TPP can form two stable axial bonds on the central phosphorus atom, P(V)TPP triads having two oligothiophene moieties in the axial direction can be synthesized easily (15). Three different P(V)TPP derivatives (Figure 5) containing two thienylalkoxy or oligothienylalkoxy groups at the axial positions of the central phosphorus atom were synthesized by the reaction of the dichlorophosphorus-(V)tetraphenylporphyrin and the corresponding thienyl or oligothienyl alcohols (12). The resulting triad molecules give normal P(V)TPP absorption spectra as well as characteristic thienyl or oligothienyl absorption peaks. All the porphyrin derivatives have similar fluorescence originating from P(V)TPP moiety, but their lifetimes and the relative quantum yields of fluorescence depended on the axial substituents. In particular, the fluorescence was strongly quenched in the latter two cases of molecules (a), (b), and (c) in Figure 5 as compared with diethoxyphosphorus(V)tetraphenylporphyrin which was free of any thienyl



I

P.

I

+

porphyrin

bithiophene p o r p h y r i n

poφhyrin

Fig. 5 Schematic representation of relationships between photoinduced electron transfer and corresponding energy levels of phosphorus(V)porphyrin and oligothienyl axial groups.

thiophene

terthiophene

- Λ Ϊ


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moieties. Taking into account the energy levels of the P(V)TPP and the oligothienyl moieties, the fluorescence quenching can be attributed to the photoinduced electron transfer from the oligothienyl moieties to the P(V)TPP (12). Iffluorescenceis quenched, the oxidation potential of the oligothienyl moieties is sufficiently low as compared with the reduction potential of the singlet excited state of the P(V)TPP. These results suggest that the reductive electron transfer occurs in (b) and (c), as depicted in Figure 5. An important point is that the P(V)TPP is able to act as a good photoinduced hole generator in the donor-acceptor molecules with oligothienyl moieties and is therefore expected to play a similar role in donor-acceptor polymers. Both of the P(V)TPP derivatives (b) and (c) (Figure 5) were polymerized by electrochemical oxidation to give polymers, whereas (a) was scarcely polymerized. (Figure 6 (A)). Consequently, poly-(b) and poly-(c), 1 D porphyrin arrays, were electrochemically deposited on the ITO electrode at potentials >1.2V and 0.9V (vs. SCE), respectively. The peak current observed around -0.4V, which was assigned to the redox reaction of the P(V)TPP moieties, increased and thus signaled the deposition of the porphyrin polymer onto the electrode. The photoconductivity of the polymers was measured by using a sandwich cell composed of ITO / polymer / Au. Both current-potential (i-E) curves of poly-(b) and poly-(c) show that each contact between the polymer and the electrode is ohmic. In these polymers, it is confirmed that a Schottky junction was not formed at the interface with either the ITO or Au. The d.c. conductivity of the polymers in the dark was 1.2x10 S cm and 5.1xl0~ S cm" for poly-(b) and poly-(c), respectively. Interestingly, the conductivity of both poly-(b) and poly-(c) were strongly enhanced upon photoirradiation, and was strongly dependent on light intensity. A little oriented polymer which was prepared in a micropore film, showed significantly greater photoconductivity. This implies that intramolecular photoinduced carrier formation occurs efficiently in these donor-acceptor polymers (16, 17). 2 D porphyrin array was also prepared by electropolymerization of phosphorus(V)porphyrin derivatives containing four oligothienyl groups at meso-position of porphyrin ring. (Figure 6 (C)). These 2 D porphyrin arrays also showed similar functions as 1 D porphyrin arrays. STM image supports the 2D array nature of these materials (Figure 7). -9

8

-1

1

ID porphyrin arrays connected with insulating molecular wires were recently synthesized (Figure 6 (B)) (18, 19). The polymers were synthesized by esterification of dichloro-P(V)) porphyrin with glycols. The porphyrin arrays connected with shorter insulating molecular wires showed that both singlet and triplet excited states are localized, but the arrays connected with longer insulating wires did not show a localized excitation. This was proved by transient absorption spectroscopy by Triplet-Triplet and Singlet-Singlet fs laser pulse excitations. Figure 8 shows a schematic picture of the localized and delocalized excitations of the porphyrin arrays. The results suggest possible photo-information storing capability at certain porphyrin rings of the porphyrin arrays in the porphyrin arrays connected with short insulating



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YOCH CH OH 2

2

N Fig. 6 (A) (Β) (C) (D)

ID porphyrin arrays connected with conjugating molecular wire ID porphyrin array connected with insulating molecular wire 2D porphyrin array connected with conjugating molecular wire A proposed 3D porphyrin array connected with molecular wire

Continued on next page




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Fig. 7 S T M image of 2D porphyrin array (Fig. 6(C)) electrochemically synthesized on the Au(l 11) substrate. The image was processed with Fourier filter to remove noise. Periodicity of surface level is ca. 2.9nm which is similar to an estimated porphyrin-porphurin distance (ca. 3nm) by C P K model.

Fig. 8 Singlet and triplet photoexcited states of 1-D porphyrin arrays connected with insulating molecular wires of short and long chains.


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wires such porphyrin arrays are thus possible information transduction and storing molecular device systems even if the lifespans of transition status are not sufficiently long. The present study on photoactive ID and 2D porphyrin arrays connected with molecular wires together with their syntheses open the way to 3 D porphyrin array which is expected to be a proto-type molecular device and an artificial photo-neuron.


Conclusion Both conjugated polymer superlattices and porphyrin arrays connected with molecular wires device which are described here represent powerful candidates for optical and photonic materials. Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research on New Program from the Ministry of Education, Science and Culture of Japan. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

Esaki, L.; Tsu, R. IBM J. Res. Develop. 1970, 14, 61. Chang, L.L.; Esaki, L. Physics Today 1992, 45, 36. Shimidzu,T. Reactive Polymers 1989, 11, 177. Iyoda T.„Toyoda H., Fujitsuka M., Nakahara R., Tsuchiya H., Honda K.; Shimidzu T., J. Phys. Chem., 1991, 95, 5215. Iyoda, T.; Toyoda, H.; Fujitsuka, M.; Nakahara, R.; Honda, K.; Shimidzu, T.; Tomita, S.; Hatano. Y.; Soeda, F.; Ishitani, Α.; Tsuchiya, Η Thin Solid Films, 1991, 205, 258. Fujitsuka, M.; Nakahara, R.; Iyoda, T.; Shimidzu, T. Synth. Metals 1993, 55-57, 966. Aviram, Α.; Ratner, M.A. Chem. Phys. Lett. 1974, 29, 281. Wrighton, M.S. Science 1986, 231, 32. Chidsey, C.E.D.; Murray, R.W. Science 1986, 231, 25. Simon. J.; Tournilhac, F.; Andre, J.-J. New J. Chem. 1987, 11, 383 Hopfield, J.J.; Onuchic, J.N.; Beratan, D.N. Science 1988, 241, 817 Segawa, H.; Nakayama, N.; Shimidzu, T. J. Chem. Soc., Chem. Commun. 1992; 784. Sayer, P.; Gouterman, M.; Connell, C.R. J. Am. Chem. Soc. 1977, 99, 1082. Carrano, C.J.; Tsutsui, M. J. Coord. Chem., 1977, 7, 79. Segawa, H.; Kunimoto, K.; Nakamoto, Α.; Shimidzu T. J. Chem. Soc. Perkin Trans. 1992, 1,939. Segawa, H.; Nakahara, R.; Iyoda, T.; Shimidzu, T. J. Appl. Phys. 1993, 74, 1283. Shimidzu, T.; Segawa, H.; Wu, F.; Nakayama, N.J.Photochem. Photobiol., A. Chem. 1995, 92, 121. Segawa, H.; Kunimoto, K.; Susumu, K.; Taniguchi, M.; Shimidzu T. J. Amer. Chem. Soc. 1994, 116, 11193. Susumu, K.; Kunimoto, K.; Segawa, H.; Shimidzu, T. J. Phys. Chem. 1995, 99, 29.


Optical and Photonic Functions of Conjugated Polymer Superlattices

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