Polymers for Microelectronics - American Chemical Society

Chapter 21

Charged Species inσ-ConjugatedPolysilanes as Studied by Absorption Spectroscopy with Low-Temperature Matrices 1,3

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K. Ushida , A. Kira , S. Tagawa , Y. Yoshida , and H. Shibata

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Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch021

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Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako-shi, Saitama 351-01, Japan University of Tokyo, 2-22 Shirakata-Shirane, Tokai-mura, Ibaraki 319-11, Japan Optical absorption spectra of the radical cation and anion of polysilanes were measured by aγ-irradiatedmatrix isolation technique. For both radical cation and anion, a sharp UV band and a broad IR band were observed. Although the UV bands are compatible with those reported in previous steady-state and pulse radiolysis studies, the broad band in the IR was observed even for polymethylpropylsilane (PMPrS) without any aryl pendant groups. The nega tive ions also exhibit similar bands in the IR which have not been reported in the previous matrix studies. In addition to the UV bands, which is due to the well-known charge delocalization along theσ-conjugatedmain chain, the IR bands indicate the existence of the charge-resonance (CR) interaction among the σ-conjugated segments. (Inter-segment charge resonance: ISCR) In the present radical ions, the charges are delocalized over more than one σ-con jugated segments in the polymer chain through ISCR.

Recently, considerable attention has been paid to polysilanes not only because of their potential usefulness as resists but also because of their unusual photochemi cal and photophysical properties (2). For example, the measurements of photo conductivity indicate the existence of very high mobile positive carriers (2). However, the electronic states of charged carriers have not been revealed yet. From this point of view, it is useful to know the electronic properties of the charged species (the radical cations and anions) of polysilanes. In these species, a single electron or hole is trapped on the polymer and useful information about the charge distribution can be obtained by, for example, optical absorption spectroscopy (3). As for the experiments with the radiation chemical technique, one of the present authors (Tagawa) presented the optical absorption spectra of both radical cation and anion of polysilanes and their pairing properties of both radical ions in his earliest work on pulse radiolysis (4). Tagawa and his co-workers reported that strong U V absorption bands due to the delocalized charge in the 3

Corresponding authors 0097-6156/94/0537-0323$06.00/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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or-conjugated main chain were observed for both polysilane radical cation (5) and anion (5, 6). Irie et al also published their results of pulse and steady-state radiolyses reporting that absorption bands in the IR region were observed only for the radical cations of polysilanes with aryl pendant groups (7, 8). They concluded that this IR band should be attributed to the charge resonance (CR) between two aryl pendant groups. This is incompatible with our earliest conclusion because their assignment for the IR band indicates that the positive charges are not delocalized along the main chain but localized on the pendant aryl groups. Moreover, their results for the radical anion, which showed no IR band, are very different from ours and inconsistent with the pairing properties which should be observed for the radical cations and anions. The present article describes optical absorption spectra of positive and negative ions of polysilane observed for -^irradiated low temperature matrices. Our results agree with the previous conclusion that the charge is delocalized along the cr-conjugated main-chain. In addition, we proposed that the IR absorption should be attributed to the inter-segment charge resonance (ISCR) interaction. EXPERIMENTAL In the present study, we use freon mixture (FM: mixture of CFC1 and CFBr CFBr in a 1:1 volume ratio) (5, 10) and sec-butyl chloride (sBuCl) for radical cation and MTHF for radical anion as matrices. Purification methods are the same as those described elsewhere (5,11,12). Polymethylpropylsilane (PMPrS) and Polymethylphenylsilane (PMPS) were synthesized according to the conventional method (1, 13). Absorption spectra measured in 1.5 mm quartz cells at 77K were recorded on a Cary 14RI spectrophotometer. A l l the samples were sealed in the cell and irradiated with 'y-ray from a Co source at The Institute of Physical and Chemical Research. Photo-bleaching was performed with a tungsten lamp incorporated in the spectrophotometer through Toshiba glass filters (3, 11, 12, 14). All the polymers were dissolved in a matrix for 0.1-20mM (monomer units). Only PMPS, however, could not be successfully dissolved in F M or sBuCl at sufficient concentration to produce PMPS radical cation successfully. 3

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RESULTS Charge Transfer Reaction From The Matrix To Polysilanes The method of preparation of radical ions used in this work is widely known in the field of radiation chemistry (3, 11, 12). The radical ions of the solute are produced by a charge- (hole- or electron-) transfer process from ^irradiated low temperature matrices. This method is applicable for the formation of radical ions of polymers including polysilanes (7, 8), and a number of reports about polymer ion radicals have been published so far (22). However, some problems exist in formation of polymer radical ions because of their low solubility and the tendency to be aggregated in low-temperature matrices. Under such conditions,

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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the solvent and solute systems are sometimes mixed inhomogeneouly and some difficulties occur: (i) Since part of the polymers cannot be mixed homogeneously with solvent molecules, the yield of the charge scavenging process is lower than expected from the solute concentration characterized by the monomer units, (ii) Since the solubility of polymers are quite low in general, the effective concentration of the polymer is sometimes insufficient for the charge-scavenging process from the solvent to the polymer, (iii) If the charge can travel through the inside of polymers by some sequential charge-transfer processes or charge derealization, additional charge recombination process will be feasible and the yield of the ions will decrease: Therefore, assuring the occurrence of charge-scavenging process with no side reactions, careful examination should be indispensable for assignment of absorption spectra obtained for polymer systems. Actually, residual holes or electrons were sometimes observed and the dose dependence check and the stoichiometric consideration were performed in each case. The irradiation dose was carefully restricted within a magnitude where all the spectral lineshape was not changed and the intensity growth was linearly dependent on the irradiation dose. Positive Charge Trapped on PMPrS Figure 1 indicates absorption spectra taken for y-irradiated F M glassy solution of PMPrS which should be attributed to the radical cation of PMPrS. Immediately after irradiation, there exist several peaks in the spectrum marked as (a). After photobleaching with IR light (> 900 nm), the spectral lineshape changed gradually to become spectrum (b), which consists of two band peaks at 350 nm and 1600 nm. Similar two bands, which decayed with the same lifetime, were observed for the cationic species of the same compounds in the pulse radiolysis study (4-6, 9). Photobleaching with monochromatic light of 350 nm slowly reduced the intensity of both bands. This indicates that both bands should be attributed to the same cationic species which decomposes with a slow photochemical reaction. Since the spectral change on the IR photobleaching progresses irreversibly, the last lineshape (b) should be attributed to the fully relaxed radical cation. The strong peak at 350nm in the spectrum (b) is narrow and was unchanged during photobleaching; therefore, we assigned these U V peaks to a a-cr * transition within a single cr-conjugated segment (2, 15). In contrast to the ISCR transition discussed later, here we term this transition intra-segment excitation (ISE). This assignment is essentially the same as those reported in previous reports (4-6). The broad band observed in visible-IR region (400-2000nm) in the initial spectrum (a) is similar to the spectrum for the residual hole trapped in the F M matrix (10) but the position of the spectral peak is different from those obtained for y-irradiated neat F M glass. On the spectral change from (a) to (b), no radical cation seems to be newly produced by photobleaching because the intensity of the U V peak was constant. Therefore we concluded that the whole absorption bands from the visible to the IR region should be attributed to the charge


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resonance (CR) interaction between two cr-conjugated segments each of which acts as a trap for positive holes, (see Appendix)In this article, we term this CR interaction Inter-Segment Charge Resonance (ISCR). The whole band in seems to consist of a number of CR bands having various transition energy gaps depending on the distribution of the o--conjugated segments. A detailed explanation will be presented in the Discussion section. Essentially the same result was obtained for the sBuCl matrix.


Negative Charge Trapped on PMPS The absorption spectrum for the radical anion of PMPS was obtained in MTHF matrix as is indicated in Figure 2, although no valid data was obtained for positive charges of PMPS. Initially, the strong absorption of the trapped electron was observed (a) but disappeared on photobleaching. (see insertion) The residual spectrum also has two main peaks, an ISE peak at 365 nm and a ISCR peak (> 2000 nm). Structural relaxation could not be distinguished because the unrelaxed anion may be bleached together with trapped electrons. However, spectral conversion of the ISCR band seems fairly small or non-existent because the spectrum displayed in the insertion is almost the same as the spectrum for trapped electrons reported previously (3). Accordingly, the final spectrum should be assigned to the fully-relaxed radical anion. The IR peak position seems identical with those for PMPrS(-), which indicate that only a small amount of negative charge would be distributed on the side-chain phenyl groups in PMPS. Negative Charge Trapped on PMPrS For P M P r S / M T H F system, the strong absorption of the trapped electrons was observed immediately after irradiation and disappeared on photobleaching, which is compatible with the results for PMPS in MTHF. (These results are not shown.) Finally a weak absorption band remained in the IR region, of which the peak seemed to exist at > 2000 nm. Considering the results for PMPS in MTHF described below, the band should also be assigned to the ISCR band of the PMPrS radical anion. The low yield of the radi cal anion should reflect upon the small electron affinity of PMPrS. However, the U V (ISE) peak could not be measured because the self-absorption of PMPrS obscures it in such a highly concentrate solutions. For diluted solution, the scavenging process was not completed and no absorption spectra for the radical anion could be observed. Comparative Analysis with Pulse Radiolysis (4-6, 9) In the pulse radiolysis study for the liquid phase, similar U V bands have been reported previously (4-8). Our recent pulse radiolysis study also indicates that all the radical ions studied here exhibit the ISCR band in the IR region (9). Moreover, the slow (several microseconds) spectral relaxation of the IR band was also observed in the pulse radiolysis study (9). This result is compatible with the present matrix study especially for PMPrS( + ). However, several differences were observed for peak positions and intensities. In the liquid phase, the


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Charged Species in a-Conjugated Polysilanes

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Figure 2. Optical Absorption Spectra Observed for y-irradiated PMPS/MTHF(20mM). (a) Immediately After Irradiation (b) After Photobleaching With IR Light. (> 900nm): The Differential Spectrum in the Insertion is Attributed to the Trapped Electron.


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polymer main chain should cause fast fluctuations, and the structural dependence of absorption spectra should be averaged out. DISCUSSION


The Intra-Segment Excitation (ISE) Band Observed in UV Region The sharp U V band was observed for both PMPrS( +) and PMPS(-) in our experiment. Similar bands have also been reported for both radical cations and anions of various polysilane derivatives previously (4-8). This band is extremely sharp and was unchanged by any photobleaching by IR light. This result indicates that the transition energy of this band is insensitive to the geometry of the main chain in contrast to the IR band. Therefore this band should be attributed to the a- cr * excitation within a single Si-Si conjugated segment (ISE). Although we could not observe this band for PMPrS(-) and PMPS( +) in the present study, well-resolved absorption spectra were obtained in the previous pulse radiolysis experiment for liquid phase (4-9). The radical cation and anion of the same polysilane have the U V band at almost the same wavelength. This result indicates that the Pairing Theorem is valid for the polysilane radical ions and that the interaction with other o--orbital is quite small, (see Appendix) The Inter-Segment Charge Resonance (ISCR) Band Observed in the IR Region For PMPrS( + ), PMPrS(-) and PMPS(-), strong, broad bands were observed in the visible-IR region. We concluded that all these bands should be attributed to the charge resonance among delocalized positive or negative charges in main chain conjugated segments, (ISCR) for the following reason. Irie et al (7, 8) have reported that the radical cation of phenyl-substituted polysilanes exhibit a similar band in the IR region and assigned it to the CR band between two phenyl groups. They also reported that no IR absorption was observed for the radical anion. In the present study, however, we observed this IR band also for PMPrS(+) and for two polysilane radical anions. Since PMPrS has no aromatic groups, the optical transition should be related with the delocalized positive charge on the o-conjugated polymer main chain. We consider that the whole spectrum observed in the visible-IR region (400-2000 nm) at the first stage (Figure la) should consist of a number of overlapping bands which were finally relaxed into a single band centered at 1300 nm. (Figure lb). Since photobleaching is thought to realign the geometry of the Si-Si main chain, the experimental results indicate that these optical transitions in the IR should depend delicately on the geometry of the polymer main chain. Based on these results, we finally assigned these strong, broad IR bands to the ISCR bands. Another choice of the interpretation of this IR band could be a phonon-side band of the trapped charge based on a polaron model (26, 27). However, we suppose that the ISCR model is more appropriate to explain our present results in which the spectrum indicates a variety of transition energies including rela-


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tively large values such as 2 eV at the first stage before photobleaching. (Figure la) The magnitude of the electron-phonon coupling in this system is known to be very small. Moreover, no spectral shifts were observed for the U V band which correspond to a wide spectral shift of the IR band. Essentially the same interpretation is applicable to the IR bands observed for PMPrS(-) and PMPS(-).


The Dynamic Behavior on Photo-Bleaching Observed for PMPrS( +) It is well-known that polysilanes have large cr-conjugated system on the main chain. Under the present experimental conditions the polymers are frozen in randomly folded structures and some gaps are formed in the cr-conjugated system where Si-Si bonds are fixed in some irregular conformation. Accordingly, the main chain is separated into a number of cr-conjugated segments. Each of these segments should act as the traps for either positive holes and electrons, which have various sizes and depths (18). Immediately after irradiation, positive charges are trapped randomly at one of the nearest-neighbor traps (which include shallow and unstable ones), and accordingly the complex spectral lineshape as in Figure l a reflects the existence of various types of positive hole traps. On photobleaching of the next stage, these holes are excited again and obtain enough energy to migrate towards a more stable trap. Moreover, an amount of thermal energy sufficient to change the folded structure (maybe on local annealing of the surrounding matrices) should be ejected on the non-radiative decay of the excited radical cation. Although a similar spectral shift on annealing the solution has previously been reported by Irie et al for polysilastyrene, they did not find any shift with photobleaching (7). Finally the bulk system should attain the most stable hole distribution on the cation polysilane main chain. As is referred above, the spectrum indicates a variety of ISCR transition energies including large values up to 2 eV. Accordingly, if there are various segment pairs in various geometries, there will exist a variety of CR-transition energies. This explanation based on ISCR is displayed in schematic diagram of Figure 3c. After photobleaching, the IR band was converged into a single peak band (Figure lb) and the average value of the CR transition energy seemed to be lowered. This may indicate the enhancement of the CR area. The irreversible relaxation process on photobleaching should be interpreted as a charge transfer from one short cr-conjugated segment to another longer one. Since each segments are thought to be sufficiently long, the splitting of ISE energy gap is not sensitive to the length of the segment. However, the magnitude of ISCR interaction will be smaller when the charges are more delocalized in one segment, (see Appendix) In addition, this relaxation may include the reorientation of the Si-Si main chain itself on annealing due to local heating of the matrix. The converged value of the ISCR energy should reflect the most stable geometry of CR interacting cr-conjugated segments which corresponds to the most preferred conformation of the bridging part of the main chain.


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d) biter-Segment CR Model


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UV vis Figure 3. Electronic Energy Diagram of Polysilane Radical Ions (a) Radical Cation (b) Radical Anion (c) Energy Diagram of Charge Resonance Absorption Band in The Case of A Pair of Cationic and Neutral Segments, (d) Two Centered Charge Resonance (CR) Model Applied For Two Segments Taking Part In the Interaction. The CR Band Appears Extremely Broad Because of Number of Overlapping CR Bands Corresponding to a Variety of CR Transition Energies.

For radical anions, almost no photochemical relaxation was observed and fully relaxed anions seemed to have existed immediately after irradiation. It is well-known for the present matrix method that the stabilization energy of negative charge is always smaller than for the positive charge. Therefore the negative charge can be delocalized along the polysilane main chain from the beginning to make a most stable distribution of the negative charges. CONCLUSION Based on our measurements of optical absorption spectroscopy, we obtained several conclusions as follows. (1) Both positive and negative radical ions of the two types of polysilanes exhibit two prominent absorption bands at the U V (350-365nm) and IR (1300-2000nm) regions. These results are compatible with those obtained by recent pulse radiolysis studies (9). (2) The U V band is the cr-cr* transition band within a single conjugated segment. (ISE band) (3) The IR band is attributed to the charge-resonance interaction among a number of conjugated segments of the polysilane main chain.(ISCR band) (4) The structural relaxation of the main chain was observed on photobleaching. (5) Our results are incompatible with those reported by Irie et al (7, 8) which includes following remarkable difference from ours: Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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a) The IR absorption bands were observed even for the radical ions of polymethylpropylsilane (PMPrS), which has no aromatic groups on the sidechain. b) We also observed the same IR bands for radical anions of polysilane. c) Our results support that the charges of radical ions are delocalized on polymer main chain, however Irie et al. assumed that they are localized on the aromatic side chain. APPENDIX


Background and Definition of the Terms Used for the Assignments in This Article Once radical cations are produced, characteristic absorption often emerges in visible-IR region because new transitions involving the SOMO (singly occupied molecular orbital) become possible; this is illustrated schematically in Figure 3a and 3b for the case of radical cation and anion of polysilanes, respectively. The energy diagram is based on the case of the Sandorfy model C (la, 15). When the parent neutral molecule has well-conjugated electronic orbitals without any interactions with orbitals in other symmetries, we note that the observed absorption transition for the radical cation and anion should be identical. This nature is widely known as "the Pairing Theorem". Concerning the o--conjugation of the polysilane main chain, this theorem should be closely related to the Sandorfy C and H orbital models (la, 15,19). In the case that this theorem is approximately valid, the electronic property of the main chain can be explained by the model C, which is a more simple approximation than the other. When a molecule has equivalent or near equivalent groups strong and broad charge resonance (CR) bands are frequently observed (20), for which a diagram is also indicated in Figure 3c. The CR band has a transition energy of E , as is indicated in Figure 3c. In the text we applied this interpretation to the present case, regarding each segment of the polysilane as an interacting groups. When the segments are sufficiently long, each electronic states should converge into one level and should give almost no dependence on the length of the segments. Consequently, we can regard each segments approximately equivalent. In the CR interaction, we can include both static (time-independent) and dynamical (time-dependent) perturbation such as direct overlap of the orbitals (static perturbation), intramolecular vibrational coupling and dynamical exchange interaction induced by surrounding systems (dynamic perturbation). This optical transition is always symmetrically allowed and has a fairly large transition dipole moment which lies along the line connecting the two centers. Since ECR should be corresponding to the magnitude of the interaction energy between two chromophores, it depends delicately on the geometry (e.g. distance and orientation) and dynamic property of the two chromophores. The CR band is well-known as an evidence for formation of dimer radical ions (11, 12, 21). C R

REFERENCES 1. a) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359 b) Wallraff, G. M.; Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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3. 4. 5. 6. 7. 8. 9. 10) 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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Miller, R. D.; Baier, M.; Ginsburg, E. J.; Kurz, R. R. J. Photopolym. Sci. Technol. 1992, 5, 111. a) Kepler, R. G.; Zeigler, J. M.; Harrah, L. Α.; Kurtz, S. R. Phys. Rev. 1987, B35, 2818 b) Fujino, M . Chem. Phys. Lett. 1987, 138, 451 c) Abkowitz, Μ. Α.; Stolka, M . ; Weagley, R. J.; McGrane, K. M . ; Knier, F. E., 1990 p467 "Silicon-based Polymer Science" ACS Adv. Chem. Ser. 224, edited by Zeigler, J. M.; Fearonin, F. W. G. Shida, T., Electronic Absorption Spectra of Radical Ions ; Elsevier 1988 Tagawa, S. Abstract of IBM Polymer Colloquium '87, May, 1987, Fuji, Japan and Abstract of Intern. Topical Workshop, "Advances in Silicon-based Polymer Science", 1987, Hawaii, 20. a) Tagawa, S.; Washio, M.; Tabata, Y.; Ban, H.; Imamura, S. J. Photopolymer Sci. Technol. 1988, 1, 323 b) Tagawa, S. Polymer Preprints, ACS, Washington, 1990, 31, 242. a) Ban, H.; Sukegawa, K.; Tagawa, S. Macromolecules, 1987, 20, 1775, ibid, 1988, 21, 45 b) Ban, H.; Tanaka, Α.; Hayashi, N.; Tagawa, S.; Tabata, Y . Radiat. Phys. Chem., 1989, 34, 587. Irie, S.; Oka, K.; Irie, M . Macromolecules, 1988, 21, 110 Irie, S.; Irie, M . Macromolecules, 1992, 25, 1766 Tagawa, S; Yoshida, Y; Ushida, K.; Kira, A . in preparation. Grimson, Α.; Simpson, G.A. J. Phys. Chem., 1968, 72, 1776 Shida, T.; Haselbach, E.; Bally, T. Acc. Chem. Res. 1984, 17, 180 Kira, A . Chapt, VIIIA in Handbook of Radiation Chemistry edited by Tabata, Y.; Ito, Y.; Tagawa, S. CRC Press 1991 Trefonas, P.; Djurovich, P. I.; Zhang, X.; West, R.; Miller, D.; Hofer, D. J. Polym. Sci., Polym. Lett. 1983, 21, 819 Ushida, K.; Shida, T.; Shimokoshi, K. J. Phys. Chem. 1989, 93, 5388 Sandorfy, C. Can. J. Chem. 1955, 33, 1337 Rice, M . J.; Phillpot, S. R. Phys. Rev. Lett. 1987, 58, 937 Tagawa, S. J. Photopolym. Sci. Technol. 1991, 4, 231 Klingensmith, Κ. Α.; Downing, J. W.; Miller, R. D.; Michl, J. J. Am. Chem. Soc. 1986, 108, 7438 Herman, Α.; Dreczewski, B.; Wojnowski, W Chem. Phys. 1985, 98, 475 Ishitani, Α.; Nagakura, S. Mol. Phys. 1967, 12, 1 Ushida, K.; Shida, T. Chem. Phys. Lett. 1984, 108, 200

Received January 28, 1993


Polymers for Microelectronics - American Chemical Society

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