- Email: [email protected]
Large order–disorder transition in polydihexylsilane films as studied by second-harmonic generation spectroscopy
4 February 2000
Chemical Physics Letters 317 Ž2000. 260–263 www.elsevier.nlrlocatercplett
Large order–disorder transition in polydihexylsilane films as studied by second-harmonic generation spectroscopy T. Manaka
a,)
, H. Hoshi a , K. Ishikawa a , H. Takezoe a , S. Koshihara M. Kira d,e, T. Miyazawa c
b,c
,
a
c
Department of Organic and Polymeric Materials, Faculty of Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan b Department of Physics, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Kanagawa Academy of Science and Technology (KAST) KSP East, 3-2-1, Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan d Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN) 19-1399, Koeji, Nagamachi, Aoba-ku, Sendai 980-0952, Japan e Department of Chemistry, Graduate School of Science, Tohoku UniÕersity, Aoba-ku, Sendai 980-8578, Japan Received 6 July 1999; in final form 14 September 1999
Abstract The one-photon forbidden excited state in an one-dimensional s-electron-conjugated polymer, polysilane, was investigated by means of second-harmonic ŽSH. generation spectroscopy. It was found that resonant SH peak attributable to a forbidden excited state was observed on the high-energy side of the linear absorption peak and that the SH intensity exhibits a drastic change accompanied with the backbone order–disorder phase transition in alkylpolysilane. This behavior was discussed on the basis of the electron delocalization in their backbone of one-dimensional conjugated polymers and the resonance condition of non-linear optical processes. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction For the media with inversion symmetry, the second-harmonic generation ŽSHG., being forbidden under the electric-dipole approximation, can actually be observed due to the electric-quadrupole process, if resonance transition is associated with it. Since the electric-quadrupole transition occurs only between
) Corresponding author. Fax: q81-3-5734-2876; e-mail: [email protected]
the same parity states, the SHG due to the higherorder mechanism can be used to study the dipole-forbidden excited states. We have observed the dipoleforbidden excited states in p-conjugated systems such as polydiacetylene w1x and phthalocyanine w2x. In particular, we could observe the lowest forbidden excited state located below the optical gap in polydiacetylene. Polysilane is a one-dimensional s-electron-conjugated polymer containing only silicon atoms in the backbone. Because of their widely delocalized selectron along the backbone, polysilane has attracted
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 3 8 2 - 2
T. Manaka et al.r Chemical Physics Letters 317 (2000) 260–263
considerable attention with respect to their interesting optical and electrical properties and their potential technological applications. It is well known that polydihexylsilane ŽPDHS. adopts a crystalline structure at room temperature and undergoes a transition to a hexagonal columnar liquid-crystalline phase at higher temperatures w3x. Due to the order-to-disorder transition in side-chain alkyl groups, defects are introduced into the main-chain structure at the phase transition to liquid-crystalline phase. Generally, polysilane has a wide gap in comparison with p-conjugated polymer so that a strong peak due to the lowest one-photon allowed exciton is observed at the UV region in the absorption spectrum. The electronic structure Žand hence optical properties. of polysilanes should depend significantly on the conformation structures of the backbone w4x. When the phase transition from the ordered to disordered phases occurs, the lowest singlet exciton band shifts to a higher-energy side, suggesting that the delocalization of s-electrons along the polymer backbone is decreased by the conformational change. In this Letter, we report a drastic change in the SH spectra in polysilane, ascribing it to a phase transition with temperature.
261
to remove the fundamental light. In order to certify that the SH signal from the PDHS film was actually due to a higher-order SH mechanism instead of fluorescence, the inactiveness of SHG under the normal incidence and p–s geometry was confirmed. Thin film of PDHS was prepared by spin-coating the material in toluene solution on the quartz substrate. The sample was held in a cryostat to control the temperature ranging from 173 to 373 K.
3. Results and discussion Absorption spectra at elevated temperatures for PDHS are shown in Fig. 1. A strong peak observed at about 3.3 eV is attributed to the above-mentioned lowest one-photon allowed exciton along the backbone of the ordered phase. At 315 K where order– disorder phase transition occurs in PDHS, the absorption peak shifts to the high-energy side and the spectral shape becomes broader. Fig. 2 shows the temperature dependence of SH spectra for PDHS films. An SH peak was observed at about 4.2 eV for the ordered crystalline phase at 307 K. The location is coincident with that by other
2. Experiment The light source ranging from 460 nm Ž2.7 eV. to 630 nm Ž2.0 eV. was obtained using an optical parametric oscillator ŽOPO BMI OP-901-355. pumped by the third-harmonic light of a Q-switched Nd:YAG laser. The s-polarized fundamental light from the OPO was focused on the sample using a convex lens Ž f s 200 mm. with an incidence angle of 458 after passing through a long-pass filter to eliminate SH light from various optical components. The p-polarized SH light transmitted through a UV quartz-glass substrate was filtered by a fundamentalcut filter to remove the strong fundamental light, and was detected by a photomultiplier tube after passing through a monochromator. To avoid film damage, the power of the fundamental light was controlled so as not to exceed 4 mJrpulse. An obstacle signal was observed under the p–p geometry, even if the fundamental-cut filter and the monochromator were used
Fig. 1. Temperature dependence of the absorption spectra for PDHS thin film. The base lines are properly shifted.
262
T. Manaka et al.r Chemical Physics Letters 317 (2000) 260–263
Fig. 2. Temperature dependence of the SH spectra for PDHS thin film. The base lines are properly shifted.
non-linear optical measurements such as two-photon absorption w5,6x and electro-absorption w6,7x. Soos and Kepler w5x measured two-photon absorption at 11 and 295 K and showed that a two-photon absorption peak shifts to the lower-energy side with increasing temperature. These results are consistent with our SHG data, although the difference exists that they claimed a continuous shift of the absorption peak and the present result suggests the change of the resonance states from a higher-energy peak to a lower one with increasing temperature. Consequently, the observed peaks in SH spectra can be attributed to the one-photon forbidden exciton. It is also found that the SH intensity gradually decreased with the increase in temperature, and completely vanished when the backbone order–disorder phase transition occurred. Miller et al. w8x measured the temperature-dependent third-harmonic generation ŽTHG.. They found a decrease in the TH intensity only by factor of two and six at 1.9 mm Ž0.65 eV. and 1.06 mm Ž1.17 eV., respectively, when the order–disorder transition takes place, instead of a drastic extinguishment as in the present SHG. Let us discuss the different behaviors observed in SHG and THG. Two reasons should be examined for the change in SHG and THG intensity
in the order–disorder phase transition of PDHS: Ž1. decrease in conjugation length and Ž2. resonance condition. It is well known that non-linear polarizabilities of conjugated polymers strongly depend on their chain length Ži.e. conjugation length. and SHG and THG intensities decrease with decreasing conjugation length. From simple consideration for a freeelectron model, polarizabilities increase with conjugation length in the same manner for THG and higher-order SHG processes, although the influence of the conjugation length is smaller for the normal SHG process compared with that in THG and higher-order SHG processes. Therefore, the effect of the decrease in conjugation length, which may occur at the order–disorder phase transition, on change in SHG and THG intensity is equivalent for SHG and THG. Let us consider the alternative reason, i.e, the resonance condition in SHG and THG processes. Resonance enhancement occurs in both THG and SHG if one of the processes coincides with transitions. However, there is a crucial difference in the resonance condition between THG and the higherorder SHG. In THG, resonance occurs mainly in one-photon-allowed transitions. In higher-order SHG, however, resonance mainly occurs in one-photonforbidden transitions. Because the TH wavelength at 355 nm used by Miller et al. w8x is located at the tail of the absorption peak in the ordered phase, the large decrease in TH intensity by 1.06 mm excitation was attributed to the off-resonance condition in the disordered phase. According to the theoretical calculation using Pariser–Parr–Pople ŽPPP. models by Soos and Hayden w9x, the forbidden excited state is expected to locate at 1.2–1.3 Eg, where Eg means the location of the absorption peak. Since the forbidden excited state is located at 1.27 Eg Ž Eg s 3.3 eV. at 307 K Žorder phase. in this experiment, the forbidden excited state in the disorder phase is expected to locate at about 5 eV, judging from the linear absorption peak at about 4 eV. However, we could not observe any SHG resonance in this high-energy region. The higher-order SHG processes contain three matrix elements; ² g < m < n:, ² n < m < m: and ² m < q < g :, where ² g < means ground state, ² n < and ² m < mean excited states and m and q are dipole and quadrupole transition moment operators, respectively. For THG and electro-absorption, ² g < m < n: and ² n < m < m: play an important role in the spectral feature. According to
T. Manaka et al.r Chemical Physics Letters 317 (2000) 260–263
the results of electro-absorption w7x, change in matrix elements, ² g < m < n: and ² n < m < m:, seems to be small. Hence, a drastic change in the SH spectra is attributed to the change of quadrupole matrix elements. Thus, the different signal change in SHG and THG at the order–disorder transition is attributed to the different resonance conditions, although the details of the resonance condition Žforbidden band. in the disordered phase is not clear at present. The higher-order SHG is influenced by the excited states where the electron delocalization is much pronounced compared with the ground state. It was also found that polymethylphenylsilane ŽPMPS., whose backbone conformation is disordered and the crystallinity is considerably low Ž 10%. w10x, gives no SH peaks at the high-energy side of the one-photon gap. This is coincident with the result of disorder PDHS, in which the introduction of disorder in the backbone extinguishes the SHG activity. As mentioned above, there are two SH peaks: the higher-energy peak is dominant at lower temperatures while the lower-energy peak is dominant at higher temperature. An additional peak which is generally assigned to remnants of a disordered phase in the absorption spectra at about 3.9 eV appears at the higher-energy side of the main peak with increasing temperature. They imply that SHG spectroscopy may provide some additional information in the structural change which cannot be detected by means of other spectroscopy methods. Detailed experiments and analyses are now in progress in order to reveal the spectral feature.
263
4. Conclusion In conclusion, we reported the first observation of the SHG including the electric-quadrupole process in the s-conjugated system, polysilane and observed the drastic change in the SH signal associated with the order-to-disorder phase transition in PDHS thin films. Two causes were examined. Ž1. The decrease in the s-electron delocalization is by the introduction of defects in the backbone, resulting in the decrease in SH intensity as in the TH intensity. Ž2. The resonance condition, such as the transition probability of the electric quadrupole transition, may change. References w1x T. Manaka, T. Yamada, H. Hoshi, K. Ishikawa, H. Takezoe, Synth. Met. 95 Ž1998. 155. w2x T. Yamada, H. Hoshi, T. Manaka, K. Ishikawa, H. Takezoe, A. Fukuda, Phys. Rev. B 53 Ž1996. 13314. w3x P. Waber, D. Guillon, A. Skoulious, R.D. Miller, J. Phys. France 50 Ž1989. 793. w4x L.A. Harrah, J.M. Zeigler, J. Polym. Sci. 23 Ž1985. 209. w5x Z.G. Soos, R.G. Kepler, Phys. Rev. B 43 Ž1991. 11908. w6x H. Tachibana, M. Matsumoto, Y. Tokura, Y. Morimoto, A. Yamaguchi, S. Koshihara, R.D. Miller, S. Abe, Phys. Rev. B 47 Ž1993. 4363. w7x H. Tachibana, Y. Kawabata, S. Koshihara, Y. Tokura, Solid State Commun. 75 Ž1990. 5. w8x R.D. Miller, F.M. Schellenberg, J.C. Baumert, H. Looser, P. Shukla, W. Torreullas, G.C. Bjorklund, S. Kano, Y. Takahashi, ACS Symp. Ser. 455 Ž1991. 636. w9x Z.G. Soos, G.W. Hayden, Chem. Phys. 143 Ž1990. 199. w10x S. Demoustier-Champagne, A. Jonas, J. Devaux, J. Polym. Sci. B 35 Ž1997. 1727.
Chemical Physics Letters 317 Ž2000. 260–263 www.elsevier.nlrlocatercplett
Large order–disorder transition in polydihexylsilane films as studied by second-harmonic generation spectroscopy T. Manaka
a,)
, H. Hoshi a , K. Ishikawa a , H. Takezoe a , S. Koshihara M. Kira d,e, T. Miyazawa c
b,c
,
a
c
Department of Organic and Polymeric Materials, Faculty of Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan b Department of Physics, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Kanagawa Academy of Science and Technology (KAST) KSP East, 3-2-1, Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan d Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN) 19-1399, Koeji, Nagamachi, Aoba-ku, Sendai 980-0952, Japan e Department of Chemistry, Graduate School of Science, Tohoku UniÕersity, Aoba-ku, Sendai 980-8578, Japan Received 6 July 1999; in final form 14 September 1999
Abstract The one-photon forbidden excited state in an one-dimensional s-electron-conjugated polymer, polysilane, was investigated by means of second-harmonic ŽSH. generation spectroscopy. It was found that resonant SH peak attributable to a forbidden excited state was observed on the high-energy side of the linear absorption peak and that the SH intensity exhibits a drastic change accompanied with the backbone order–disorder phase transition in alkylpolysilane. This behavior was discussed on the basis of the electron delocalization in their backbone of one-dimensional conjugated polymers and the resonance condition of non-linear optical processes. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction For the media with inversion symmetry, the second-harmonic generation ŽSHG., being forbidden under the electric-dipole approximation, can actually be observed due to the electric-quadrupole process, if resonance transition is associated with it. Since the electric-quadrupole transition occurs only between
) Corresponding author. Fax: q81-3-5734-2876; e-mail: [email protected]
the same parity states, the SHG due to the higherorder mechanism can be used to study the dipole-forbidden excited states. We have observed the dipoleforbidden excited states in p-conjugated systems such as polydiacetylene w1x and phthalocyanine w2x. In particular, we could observe the lowest forbidden excited state located below the optical gap in polydiacetylene. Polysilane is a one-dimensional s-electron-conjugated polymer containing only silicon atoms in the backbone. Because of their widely delocalized selectron along the backbone, polysilane has attracted
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 3 8 2 - 2
T. Manaka et al.r Chemical Physics Letters 317 (2000) 260–263
considerable attention with respect to their interesting optical and electrical properties and their potential technological applications. It is well known that polydihexylsilane ŽPDHS. adopts a crystalline structure at room temperature and undergoes a transition to a hexagonal columnar liquid-crystalline phase at higher temperatures w3x. Due to the order-to-disorder transition in side-chain alkyl groups, defects are introduced into the main-chain structure at the phase transition to liquid-crystalline phase. Generally, polysilane has a wide gap in comparison with p-conjugated polymer so that a strong peak due to the lowest one-photon allowed exciton is observed at the UV region in the absorption spectrum. The electronic structure Žand hence optical properties. of polysilanes should depend significantly on the conformation structures of the backbone w4x. When the phase transition from the ordered to disordered phases occurs, the lowest singlet exciton band shifts to a higher-energy side, suggesting that the delocalization of s-electrons along the polymer backbone is decreased by the conformational change. In this Letter, we report a drastic change in the SH spectra in polysilane, ascribing it to a phase transition with temperature.
261
to remove the fundamental light. In order to certify that the SH signal from the PDHS film was actually due to a higher-order SH mechanism instead of fluorescence, the inactiveness of SHG under the normal incidence and p–s geometry was confirmed. Thin film of PDHS was prepared by spin-coating the material in toluene solution on the quartz substrate. The sample was held in a cryostat to control the temperature ranging from 173 to 373 K.
3. Results and discussion Absorption spectra at elevated temperatures for PDHS are shown in Fig. 1. A strong peak observed at about 3.3 eV is attributed to the above-mentioned lowest one-photon allowed exciton along the backbone of the ordered phase. At 315 K where order– disorder phase transition occurs in PDHS, the absorption peak shifts to the high-energy side and the spectral shape becomes broader. Fig. 2 shows the temperature dependence of SH spectra for PDHS films. An SH peak was observed at about 4.2 eV for the ordered crystalline phase at 307 K. The location is coincident with that by other
2. Experiment The light source ranging from 460 nm Ž2.7 eV. to 630 nm Ž2.0 eV. was obtained using an optical parametric oscillator ŽOPO BMI OP-901-355. pumped by the third-harmonic light of a Q-switched Nd:YAG laser. The s-polarized fundamental light from the OPO was focused on the sample using a convex lens Ž f s 200 mm. with an incidence angle of 458 after passing through a long-pass filter to eliminate SH light from various optical components. The p-polarized SH light transmitted through a UV quartz-glass substrate was filtered by a fundamentalcut filter to remove the strong fundamental light, and was detected by a photomultiplier tube after passing through a monochromator. To avoid film damage, the power of the fundamental light was controlled so as not to exceed 4 mJrpulse. An obstacle signal was observed under the p–p geometry, even if the fundamental-cut filter and the monochromator were used
Fig. 1. Temperature dependence of the absorption spectra for PDHS thin film. The base lines are properly shifted.
262
T. Manaka et al.r Chemical Physics Letters 317 (2000) 260–263
Fig. 2. Temperature dependence of the SH spectra for PDHS thin film. The base lines are properly shifted.
non-linear optical measurements such as two-photon absorption w5,6x and electro-absorption w6,7x. Soos and Kepler w5x measured two-photon absorption at 11 and 295 K and showed that a two-photon absorption peak shifts to the lower-energy side with increasing temperature. These results are consistent with our SHG data, although the difference exists that they claimed a continuous shift of the absorption peak and the present result suggests the change of the resonance states from a higher-energy peak to a lower one with increasing temperature. Consequently, the observed peaks in SH spectra can be attributed to the one-photon forbidden exciton. It is also found that the SH intensity gradually decreased with the increase in temperature, and completely vanished when the backbone order–disorder phase transition occurred. Miller et al. w8x measured the temperature-dependent third-harmonic generation ŽTHG.. They found a decrease in the TH intensity only by factor of two and six at 1.9 mm Ž0.65 eV. and 1.06 mm Ž1.17 eV., respectively, when the order–disorder transition takes place, instead of a drastic extinguishment as in the present SHG. Let us discuss the different behaviors observed in SHG and THG. Two reasons should be examined for the change in SHG and THG intensity
in the order–disorder phase transition of PDHS: Ž1. decrease in conjugation length and Ž2. resonance condition. It is well known that non-linear polarizabilities of conjugated polymers strongly depend on their chain length Ži.e. conjugation length. and SHG and THG intensities decrease with decreasing conjugation length. From simple consideration for a freeelectron model, polarizabilities increase with conjugation length in the same manner for THG and higher-order SHG processes, although the influence of the conjugation length is smaller for the normal SHG process compared with that in THG and higher-order SHG processes. Therefore, the effect of the decrease in conjugation length, which may occur at the order–disorder phase transition, on change in SHG and THG intensity is equivalent for SHG and THG. Let us consider the alternative reason, i.e, the resonance condition in SHG and THG processes. Resonance enhancement occurs in both THG and SHG if one of the processes coincides with transitions. However, there is a crucial difference in the resonance condition between THG and the higherorder SHG. In THG, resonance occurs mainly in one-photon-allowed transitions. In higher-order SHG, however, resonance mainly occurs in one-photonforbidden transitions. Because the TH wavelength at 355 nm used by Miller et al. w8x is located at the tail of the absorption peak in the ordered phase, the large decrease in TH intensity by 1.06 mm excitation was attributed to the off-resonance condition in the disordered phase. According to the theoretical calculation using Pariser–Parr–Pople ŽPPP. models by Soos and Hayden w9x, the forbidden excited state is expected to locate at 1.2–1.3 Eg, where Eg means the location of the absorption peak. Since the forbidden excited state is located at 1.27 Eg Ž Eg s 3.3 eV. at 307 K Žorder phase. in this experiment, the forbidden excited state in the disorder phase is expected to locate at about 5 eV, judging from the linear absorption peak at about 4 eV. However, we could not observe any SHG resonance in this high-energy region. The higher-order SHG processes contain three matrix elements; ² g < m < n:, ² n < m < m: and ² m < q < g :, where ² g < means ground state, ² n < and ² m < mean excited states and m and q are dipole and quadrupole transition moment operators, respectively. For THG and electro-absorption, ² g < m < n: and ² n < m < m: play an important role in the spectral feature. According to
T. Manaka et al.r Chemical Physics Letters 317 (2000) 260–263
the results of electro-absorption w7x, change in matrix elements, ² g < m < n: and ² n < m < m:, seems to be small. Hence, a drastic change in the SH spectra is attributed to the change of quadrupole matrix elements. Thus, the different signal change in SHG and THG at the order–disorder transition is attributed to the different resonance conditions, although the details of the resonance condition Žforbidden band. in the disordered phase is not clear at present. The higher-order SHG is influenced by the excited states where the electron delocalization is much pronounced compared with the ground state. It was also found that polymethylphenylsilane ŽPMPS., whose backbone conformation is disordered and the crystallinity is considerably low Ž 10%. w10x, gives no SH peaks at the high-energy side of the one-photon gap. This is coincident with the result of disorder PDHS, in which the introduction of disorder in the backbone extinguishes the SHG activity. As mentioned above, there are two SH peaks: the higher-energy peak is dominant at lower temperatures while the lower-energy peak is dominant at higher temperature. An additional peak which is generally assigned to remnants of a disordered phase in the absorption spectra at about 3.9 eV appears at the higher-energy side of the main peak with increasing temperature. They imply that SHG spectroscopy may provide some additional information in the structural change which cannot be detected by means of other spectroscopy methods. Detailed experiments and analyses are now in progress in order to reveal the spectral feature.
263
4. Conclusion In conclusion, we reported the first observation of the SHG including the electric-quadrupole process in the s-conjugated system, polysilane and observed the drastic change in the SH signal associated with the order-to-disorder phase transition in PDHS thin films. Two causes were examined. Ž1. The decrease in the s-electron delocalization is by the introduction of defects in the backbone, resulting in the decrease in SH intensity as in the TH intensity. Ž2. The resonance condition, such as the transition probability of the electric quadrupole transition, may change. References w1x T. Manaka, T. Yamada, H. Hoshi, K. Ishikawa, H. Takezoe, Synth. Met. 95 Ž1998. 155. w2x T. Yamada, H. Hoshi, T. Manaka, K. Ishikawa, H. Takezoe, A. Fukuda, Phys. Rev. B 53 Ž1996. 13314. w3x P. Waber, D. Guillon, A. Skoulious, R.D. Miller, J. Phys. France 50 Ž1989. 793. w4x L.A. Harrah, J.M. Zeigler, J. Polym. Sci. 23 Ž1985. 209. w5x Z.G. Soos, R.G. Kepler, Phys. Rev. B 43 Ž1991. 11908. w6x H. Tachibana, M. Matsumoto, Y. Tokura, Y. Morimoto, A. Yamaguchi, S. Koshihara, R.D. Miller, S. Abe, Phys. Rev. B 47 Ž1993. 4363. w7x H. Tachibana, Y. Kawabata, S. Koshihara, Y. Tokura, Solid State Commun. 75 Ž1990. 5. w8x R.D. Miller, F.M. Schellenberg, J.C. Baumert, H. Looser, P. Shukla, W. Torreullas, G.C. Bjorklund, S. Kano, Y. Takahashi, ACS Symp. Ser. 455 Ž1991. 636. w9x Z.G. Soos, G.W. Hayden, Chem. Phys. 143 Ž1990. 199. w10x S. Demoustier-Champagne, A. Jonas, J. Devaux, J. Polym. Sci. B 35 Ž1997. 1727.