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Fullerene-doped π-conjugated organic systems under infrared laser irradiation
15 July 2001
Optics Communications 194 (2001) 367±372
www.elsevier.com/locate/optcom
Fullerene-doped p-conjugated organic systems under infrared laser irradiation N.V. Kamanina a,*, I.V. Bagrov b, I.M. Belousova b, S.O. Kognovitskii c, A.P. Zhevlakov b a
c
Vavilov State Optical Institute, 12 Birzhevaya Line, St. Petersburg 199034, Russia b Institute for Laser Physics, 14 Birzhevaya Line, St. Petersburg 199034, Russia Ioe Physico-Technical Institute, 26 Polytekhnicheskaya Str., St. Petersburg 194021, Russia Received 15 January 2001; accepted 11 May 2001
Abstract An optical limiting (OL) eect of the laser radiation over IR range in organic compounds based on polyimide has been studied. The non-linear transmission at a wavelength of 1315 nm as well as spectral properties of the compounds have been investigated. The results obtained have been explained by the donor±acceptor interaction mechanism that aects non-linear-optical properties of organic molecules. The fullerene-doped polyimide structures have been determined to be eective OL materials for attenuating a power density of more than 1 J cm 2 in the IR range. Ó 2001 Published by Elsevier Science B.V. PACS: 42.65; 42.70 Keywords: Optical limiting; Infrared laser irradiation; Polyimide; Fullerene
1. Introduction It is well known, a thermal eect, reverse saturable absorption (RSA), two-photon and freecarrier absorption, non-linear refraction and laser induced scattering are applied in the fullerenedoped structures to explain the optical limiting (OL) eect in them [1±3]. Specially, the RSA eect is considered as the basic mechanism that is included in theoretical and experimental OL inves-
* Corresponding author. Tel.: +7-812-328-0231; fax: +7-812247-1017. E-mail address: kamanin@m.ioe.rssi.ru (N.V. Kamanina).
tigations over the visible spectral range. It is caused by the following fact [4,5]. The OL properties in the visible spectral range are determined by the ecient population of an excited state with a higher absorption cross-section than that of the ground state. For the pulse width of less than the lifetime of the triplet state, the triplet state will act as an accumulation site. For example, the limiting action of a C60 solution will be most eective for pulses shorter than the triplet state lifetime of 40 ls [5]. In this case, the population of the triplet state T1 increases as the incident energy increases. RSA, and therefore OL are realized due to the transition from Tn to T1 . Both kinetics of population and destruction of the
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excitation levels, which take place in OL, are well described by the six-level system [3,6]. The OL eect in the organic systems over the visible spectral range was revealed in polymethyl methacrylate doped with C60 [5,7], in bicyanovinylpyridine-C60 compounds [8], in polysilane-C60 structures [9]. Enhancement of photoconductivity in the fullerene-doped systems based on polyvinylcarbazole was observed in Refs. [10,11]. The eect of fullerene doping on the spectral properties of 2-cyclooctylamino-5-nitropyridine was shown in Ref. [12]. The ®rst OL results for these compounds were received in Ref. [13]. Peculiarities of the OL eect over the visible spectral range in polyimide systems doped with fullerenes C60 and C70 were investigated in Refs. [14,15]. It was shown that the F orster mechanism could be included to explain the OL eect for multicomponent systems consisted of fullerenes and dyes [15] and it was underlined the reinforcement of donor±acceptor interaction in them. In the present paper the OL eect in the IR spectral range have been studied both in the fullerene-doped polyimide solution and in thin ®lms. The polyimide compounds have been considered as eective systems for eyes and sensors protection over the broad spectral range, including the infrared one.
a matrix with a high laser strength for OL applications. The traditional experimental setup was applied for the OL investigations in the IR spectral range. A dependence of the transmission on an input energy was measured with a use of a photodissociation iodine laser with a wavelength of 1315 nm. The laser was pumped by a non-magnetic coaxial Xe lamp, which an interior quartz tube was ®lled in by components of a laser mixture: n-C3 F7 I (RI) and SF6 . Partial pressure of n-C3 F7 I (RI) and SF6 was 35 and 500 mm of Hg, respectively. The diameter and length of the active zone were 0.8 and 50 cm, respectively. The pumping and laser pulse width was 8 ls and 50 ns, respectively. A spot on the sample surface was 2 mm. The input energy was measured with a calorimeter. The energy transmitted through a set of ®lters and the sample was measured with a pyroelectric photometer. The low-power transmission for photosensitive polyimide 6B was about 0.85 at wavelength of 1315 nm, while the one for non-photosensitive polyimide 81A was about 0.75. Spectroscopic measurements were carried out with a Perkin± Elmer Lambda 9 instrument in the wavelength range 200±3000 nm.
2. Experiment
The dependence of the output energy density (Wout ) on the input energy density (Win ) is shown in Fig. 1 for the 1% solutions of photosensitive polyimide 6B in chloroform. The non-linear transmission was observed for all sensitized samples. In polyimide doped with 1 wt.% of C60 , the 2.5-fold attenuation of the incident beam was measured at Win of more than 1.0 J cm 2 . The polyimide system doped with 5 wt.% of C60 showed the near-linear transmission up to Win of 0.65±0.75 J cm 2 and the transmission saturation above Win of 1.1±1.2 J cm 2 . The attenuation of the incident beam for this compound was observed at 1.25 J cm 2 and exceeded at least by the factor of 3±4. The less OL eect was observed in the 0.5% solutions of polyimide in chloroform with 5 wt.% of C60 . No peculiarities were found in the polyimide±chloroform system with 1 wt.% of C60 . For
In our experiments, two types of structures were investigated. 0.5±1% solutions of photosensitive polyimide 6B, which chemical formula was described in Ref. [16] in chloroform were used. Fullerene C60 was applied as a sensitizer. Malachite green was used as an additional impurity. The fullerene concentration was varied from 0.5 to 5 wt.%, the dye concentration was 1 wt.%. A 1 mm thick quartz cell was used. The 2±3 lm thick fullerene-doped polyimide ®lms were prepared by spin coating of the 5±6.5% polyimide solution in 1,1,2,2-tetrachloroethane on a glass substrate. In this case, fullerene C70 was used as the sensitizer with the concentration of 0.1±0.5 wt.% in this case. Fullerene-doped nonphotosensitive polyimide 81A was investigated as
3. Results and discussion
N.V. Kamanina et al. / Optics Communications 194 (2001) 367±372
Fig. 1. Dependence of Wout on Win for the polyimide 6B±chloroform solutions with the C60 concentration: (1) ± 0 wt.%; (2) ± 1 wt.%; (3) ± 5 wt.%; and (4) ± 1 wt.% of C60 and 1 wt.% of malachite green.
comparison, the large OL eect was found in the polyimide±chloroform solution simultaneously doped with C60 and the malachite green dye. In this case the 5.5±6-fold attenuation of the incident beam was observed at Win of 1.5 J cm 2 . The system kept the essential laser strength up to the input energy density of 2.5 J cm 2 . It should be noticed that weak scattering was observed in the doped polyimide±chloroform solutions. This result was determined by the cluster formation causing ¯uctuations of the solution density. The ¯uctuations resulted in irregular irradiation absorption along the beam diameter. Indeed, scattering in¯uences the OL level in the polyimide solution. The dependence of the output energy density (Wout ) on the input energy density (Win ) is shown in Fig. 2 for the thin C70 -doped polyimide 6B ®lms. The OL eect was observed for all fullerene-doped ®lms at Win of more than 0.3±0.4 J cm 2 , corresponding to the attenuation of the laser energy density by the factor of 1.3±3.0 that depended on the fullerene concentration in the photosensitive polyimide matrix. It should be mentioned, that the maximum value of the output ``optically limited'' energy density was 0.27 J cm 2 (Fig. 2, curve 1) and 0.05 J cm 2 (Fig. 2, curve 2) at input energy density of 0.8 J cm 2 for the samples with 0.2 and 0.5 wt.% C70 , respectively. Thus, the 3- and 16-fold
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Fig. 2. Dependence of Wout on Win in ®lms: (1) polyimide 6B with the 0.2 wt.% of fullerene C70 and (2) polyimide 6B with 0.5 wt.% of fullerene C70 .
attenuation of the laser energy density was obtained for these samples. Therefore, the dierence in transmission between samples 1 and 2 was determined by the fullerene concentration. However, the result was caused not only by a higher fullerene concentration, but by a possible complex formation between a donor fragment of the polyimide molecule and fullerene as well. Really, the drastic attenuation of the laser energy density by the factor of 9±12 for the polyimide ®lms with 0.5 wt.% of C70 at Win of 0.6±0.8 J cm 2 was caused when the new complex was activated. The following evidences can be used. Electron anity of fullerene is about 2.65 eV, that is more than the one for acceptor fragments of most organic molecules. It is well known that the monomeric links of polyimides are intramolecular donor±acceptor complexes with the charge transfer between the donor (triphenylamine) and acceptor (diimide) molecular fragments, which can be changed using various dopant molecules. The acceptor diimide fragment is of electron anity of about 1.12±1.46 eV that is twice less than the one of fullerene. Interest in the investigation on the physical± chemical properties of the fullerene-doped systems is generated, among other things, by unique ability of fullerenes to in¯uence the initial donor±acceptor interaction. The high electron anity of fullerene suggests that ones are able to sensitize eciently the organic molecules creating new complexes with
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their donor fragments. It should be noticed that the simple model for the intramolecular transfer of an injected electron into C60 and C70 was proposed from the concept of orbital interaction [17]. In our case, the additional condition for the transfer is the arrangement of the molecular planes in parallel, that provides the largest overlapping the electron densities of the molecular orbitals. The C60 and C70 molecules are spherical or rugbyball in shape, respectively. On exciting, triphenylamine molecule experiences a conformational transformation, changing from the neutral tetrahedral form to the ionized planar one [18]. This eect along with less dimensions of the triphenylamine molecule (0.5 nm) than those of the fullerene molecule (0.7±0.8 nm) allow the arrangement of their molecular planes to be expected in parallel. From the previous results [14,15,18] one can say that fullerenes provoke the creation of RSA materials based on polyimide with the high absorption cross-section. The absorption cross-section of donor±acceptor complex of fullerene with donor polyimide 6B fragment (triphenylamine) was recently estimated in Ref. [18]. It is really about 300 times more than the one of the intramolecular polyimide complexes. Therefore, the fullerenes are more eective acceptors for the system studied. The spectral dependence of photoconductivity is presented in Fig. 3 for the C70 -polyimide and fullerene-free polyimide ®lms. It is known that,
Fig. 3. Spectral dependence of photoconductivity for (1) polyimide 6B ®lm with the 0.2 wt.% of fullerene C70 and (2) pure polyimide 6B ®lm.
when charge transfer complexes are formed, conductivity of individual components is always less than that of the complexes. In our case, an increase in conductivity was observed to be larger for the C70 -polyimide structure (Fig. 3, curve 1) than that for the fullerene-free polyimide ®lms (Fig. 3, curve 2). Using the fact that for fullerenes C60 and C70 , the photoconductivity is constant in the wavelength range 400±650 nm and it signi®cantly decreases at k > 700 nm [19], the results obtained testify the complex formation in the system studied. Moreover, the carriers become free after the charge transfer to the fullerene molecules, where the surface charge is delocalized [9]. Thus, the reinforcement of the donor±acceptor interaction in the ®lms investigated because of the free-carrier absorption in¯uence the OL eect in the IR spectral range for fullerene-doped structures. It should be noticed that the peculiarities of the OL eect in the fullerene±dye-doped polyimide± chloroform solution do not also contradict with the evidence for the complex formation mentioned above. Because the malachite green electron af®nity is 1.6 eV [20], it can be possible to create the complex with the polyimide donor fragment and to be the eective donor for fullerenes as well. The most OL eect observed in the polyimide± chloroform solution doped with fullerene and dyes simultaneously (Fig. 1, curve 4) presents this case. Recently, it has been shown that since the absorption spectrum of the fullerene±polyimide system is overlapped with the ¯uorescence spectrum of malachite green [15] the resonance conditions are ful®lled in the polyimide±dye±fullerene structure over the visible spectral range. Overlapping the electron shells of the dye and the fullerene molecules provides the favorable conditions for the charge transfer complex formation as the result of the free electron exchange between the donor (dye) and the acceptor (fullerene). It causes the spectral changes in the IR range and explains the OL peculiarities in that. It should be noticed that a new IR band with a peak at 1405 nm was found [18]. Moreover, the dye introduction makes the process complicated. The OL investigation allows the multistep interaction to be revealed in the multicomponent system. The interaction involves,
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4. Conclusion
Fig. 4. Dependence of Wout on Win in ®lms: (1) polyimide 81A with the 1 wt.% of fullerene C60 and (2) polyimide 81A with 1 wt.% of fullerene C70 . (3) A low-power transmission for nonphotosensitive polyimide 81A.
among other processes, the intramolecular complex formation both in polyimide and between dye and triphenylamine as well as the complex formation both between fullerene and triphenylamine and between fullerene (as acceptor) and dye (as donor). The dependence of Wout on Win for fullerenedoped non-photosensitive polyimide 81A at k 1315 nm is shown in Fig. 4. As seen from this ®gure, there are no OL peculiarities for the fullerene-doped structure, while the dopant concentration is 2±5 times more than that for the fullerene-doped polyimide 6B (Fig. 2, curves 1 and 2). Therefore, the processes observed in the fullerene-doped non-photosensitive polyimide ®lms are not associated with the reinforcement of the intramolecular donor±acceptor interaction, which is caused by the fullerene introduction in the photosensitive polyimide 6B. Thus, to reveal non-linear properties in organic materials, the carrier transfer mechanism is to manifest itself in the molecules sensitized by fullerenes. Therefore, the OL eect has not been found for non-photosensitive polyimide 81A in the IR spectral range, because the complex formation does not take place there. However, the OL eect is likely to be observed at more fullerene concentration than that is applied in current experiments or is to be revealed at larger intense laser beams, when IR-active vibrational modes of fullerene are activated [21].
In conclusion, the OL eect in IR spectral range in both fullerene-doped polyimide 6B solutions and thin ®lms as well as in the multicomponent systems consisted of fullerenes and dyes has been detected. The peculiarities observed have been explained both by the charge transfer complexes and by the free carriers. The absorption of the complexes and the free carriers in¯uences the attenuation of the laser energy in the OL eect. The results obtained have testi®ed that the fullerenedoped polyimide 6B structures could be applied as eective OL materials for attenuating the power density of more than 1 J cm 2 in the IR spectral range. No OL eect has been found in the fullerenedoped non-photosensitive polyimide 81A structures in the IR spectral range for laser beam power densities and fullerene concentration, which were used in the experiments. These results do not contradict with the idea about importance of the donor±acceptor interaction mechanism for nonlinear-optical properties existence, namely, the OL eect, in the p-conjugated organic compounds. Acknowledgements The authors would like to thank Prof. B.V. Kotov, Dr. V.I. Berendyaev and Dr. N.A. Vasilenko (Karpov Research Physical±Chemical Institute, Moscow, Russia) for their help in the work. This work was supported by Russian National Program ``Optoelectronic and Laser Technologies'' and International Grant ISTC Project 1454 ``Optical barrier''. References [1] L.W. Tutt, T.F. Boggess, Prog. Quant. Electron. 17 (1993) 299. [2] L.W. Tutt, A. Kost, Nature 356 (1992) 225. [3] S. Couris, E. Koudoumas, A.A. Ruth, S. Leach, J. Phys. B: At. Mol. Opt. Phys. 28 (1995) 4537. [4] A.V. Eletskii, B.M. Smirnov, Usp. Fiz. Nauk 165 (1995) 977 (in Russian). [5] V.P. Belousov, I.M. Belousova, V.P. Budtov, V.V. Danilov, O.B. Danilov, A.G. Kalintsev, A.A. Mak, J. Opt. Technol. 64 (1997) 1081.
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[6] H. Hoshi, N. Nakamura, Y. Maruyama, T. Nakagawa, S. Suzuki, H. Shiromaru, Y. Achiba, Jpn. J. Appl. Phys., Part 2 30 (1991) L1397. [7] A. Kost, L. Tutt, M.B. Klein, T.K. Dougherty, W.E. Elias, Opt. Lett. 18 (1993) 334. [8] M. Ouyang, K.Z. Wang, H.X. Zhang, Z.Q. Xue, C.H. Huang, D. Qiang, Appl. Phys. Lett. 68 (1996) 2441. [9] K. Hosoda, K. Tada, M. Ishikawa, K. Yoshino, Jpn. J. Appl. Phys., Part 2 36 (1997) L372. [10] Y. Wang, N. Herron, J. Casper, Mater. Sci. Eng. B 19 (1993) 61. [11] A. Itaya, I. Sizzuki, Y. Tsuboi, H. Miyasaaka, J. Phys. Chem. B 101 (1997) 5118. [12] N. Kamanina, A. Barrientos, A. Leyderman, Y. Cui, V. Vikhnin, M. Vlasse, Mol. Mater. 13 (2000) 275. [13] N.V. Kamanina, L.N. Kaporskii, A. Leyderman, A. Barrientos, Tech. Phys. Lett. 26 (2000) 279.
[14] N.V. Kamanina, L.N. Kaporskii, B.V. Kotov, Opt. Commun. 152 (1998) 280. [15] N.V. Kamanina, Opt. Commun. 162 (1999) 228. [16] P.I. Dubenskov, T.S. Zhuravleva, A.V. Vannikov, N.A. Vasilenko, E.V. Lamskaya, V.I. Berendyaev, Vysokomol. Soedin. A 30 (1988) 1211 (in Russian). [17] M. Okada, K. Okahara, K. Tanaka, T. Yamabe, Fuller. Sci. Technol. 4 (1996) 167. [18] Y.A. Cherkasov, N.V. Kamanina, E.L. Alexandrova, V.I. Berendyaev, N.A. Vasilenko, B.V. Kotov, Proc. SPIE 3471 (1998) 254. [19] M. Hosoya, K. Ichimura, Z.H. Wang, G. Dresselhaus, M.S. Dresselhaus, P.C. Eklund, Phys. Rev. B 49 (1994) 4981. [20] F. Gutman, L.E. Lyons, Organic Semiconductors, Wiley, New York, 1967. [21] K. Lee, R.A.J. Janssen, N.S. Sariciftci, A.J. Heeger, Phys. Rev. B 49 (1994) 5781.
Optics Communications 194 (2001) 367±372
www.elsevier.com/locate/optcom
Fullerene-doped p-conjugated organic systems under infrared laser irradiation N.V. Kamanina a,*, I.V. Bagrov b, I.M. Belousova b, S.O. Kognovitskii c, A.P. Zhevlakov b a
c
Vavilov State Optical Institute, 12 Birzhevaya Line, St. Petersburg 199034, Russia b Institute for Laser Physics, 14 Birzhevaya Line, St. Petersburg 199034, Russia Ioe Physico-Technical Institute, 26 Polytekhnicheskaya Str., St. Petersburg 194021, Russia Received 15 January 2001; accepted 11 May 2001
Abstract An optical limiting (OL) eect of the laser radiation over IR range in organic compounds based on polyimide has been studied. The non-linear transmission at a wavelength of 1315 nm as well as spectral properties of the compounds have been investigated. The results obtained have been explained by the donor±acceptor interaction mechanism that aects non-linear-optical properties of organic molecules. The fullerene-doped polyimide structures have been determined to be eective OL materials for attenuating a power density of more than 1 J cm 2 in the IR range. Ó 2001 Published by Elsevier Science B.V. PACS: 42.65; 42.70 Keywords: Optical limiting; Infrared laser irradiation; Polyimide; Fullerene
1. Introduction It is well known, a thermal eect, reverse saturable absorption (RSA), two-photon and freecarrier absorption, non-linear refraction and laser induced scattering are applied in the fullerenedoped structures to explain the optical limiting (OL) eect in them [1±3]. Specially, the RSA eect is considered as the basic mechanism that is included in theoretical and experimental OL inves-
* Corresponding author. Tel.: +7-812-328-0231; fax: +7-812247-1017. E-mail address: kamanin@m.ioe.rssi.ru (N.V. Kamanina).
tigations over the visible spectral range. It is caused by the following fact [4,5]. The OL properties in the visible spectral range are determined by the ecient population of an excited state with a higher absorption cross-section than that of the ground state. For the pulse width of less than the lifetime of the triplet state, the triplet state will act as an accumulation site. For example, the limiting action of a C60 solution will be most eective for pulses shorter than the triplet state lifetime of 40 ls [5]. In this case, the population of the triplet state T1 increases as the incident energy increases. RSA, and therefore OL are realized due to the transition from Tn to T1 . Both kinetics of population and destruction of the
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 3 2 2 - 0
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excitation levels, which take place in OL, are well described by the six-level system [3,6]. The OL eect in the organic systems over the visible spectral range was revealed in polymethyl methacrylate doped with C60 [5,7], in bicyanovinylpyridine-C60 compounds [8], in polysilane-C60 structures [9]. Enhancement of photoconductivity in the fullerene-doped systems based on polyvinylcarbazole was observed in Refs. [10,11]. The eect of fullerene doping on the spectral properties of 2-cyclooctylamino-5-nitropyridine was shown in Ref. [12]. The ®rst OL results for these compounds were received in Ref. [13]. Peculiarities of the OL eect over the visible spectral range in polyimide systems doped with fullerenes C60 and C70 were investigated in Refs. [14,15]. It was shown that the F orster mechanism could be included to explain the OL eect for multicomponent systems consisted of fullerenes and dyes [15] and it was underlined the reinforcement of donor±acceptor interaction in them. In the present paper the OL eect in the IR spectral range have been studied both in the fullerene-doped polyimide solution and in thin ®lms. The polyimide compounds have been considered as eective systems for eyes and sensors protection over the broad spectral range, including the infrared one.
a matrix with a high laser strength for OL applications. The traditional experimental setup was applied for the OL investigations in the IR spectral range. A dependence of the transmission on an input energy was measured with a use of a photodissociation iodine laser with a wavelength of 1315 nm. The laser was pumped by a non-magnetic coaxial Xe lamp, which an interior quartz tube was ®lled in by components of a laser mixture: n-C3 F7 I (RI) and SF6 . Partial pressure of n-C3 F7 I (RI) and SF6 was 35 and 500 mm of Hg, respectively. The diameter and length of the active zone were 0.8 and 50 cm, respectively. The pumping and laser pulse width was 8 ls and 50 ns, respectively. A spot on the sample surface was 2 mm. The input energy was measured with a calorimeter. The energy transmitted through a set of ®lters and the sample was measured with a pyroelectric photometer. The low-power transmission for photosensitive polyimide 6B was about 0.85 at wavelength of 1315 nm, while the one for non-photosensitive polyimide 81A was about 0.75. Spectroscopic measurements were carried out with a Perkin± Elmer Lambda 9 instrument in the wavelength range 200±3000 nm.
2. Experiment
The dependence of the output energy density (Wout ) on the input energy density (Win ) is shown in Fig. 1 for the 1% solutions of photosensitive polyimide 6B in chloroform. The non-linear transmission was observed for all sensitized samples. In polyimide doped with 1 wt.% of C60 , the 2.5-fold attenuation of the incident beam was measured at Win of more than 1.0 J cm 2 . The polyimide system doped with 5 wt.% of C60 showed the near-linear transmission up to Win of 0.65±0.75 J cm 2 and the transmission saturation above Win of 1.1±1.2 J cm 2 . The attenuation of the incident beam for this compound was observed at 1.25 J cm 2 and exceeded at least by the factor of 3±4. The less OL eect was observed in the 0.5% solutions of polyimide in chloroform with 5 wt.% of C60 . No peculiarities were found in the polyimide±chloroform system with 1 wt.% of C60 . For
In our experiments, two types of structures were investigated. 0.5±1% solutions of photosensitive polyimide 6B, which chemical formula was described in Ref. [16] in chloroform were used. Fullerene C60 was applied as a sensitizer. Malachite green was used as an additional impurity. The fullerene concentration was varied from 0.5 to 5 wt.%, the dye concentration was 1 wt.%. A 1 mm thick quartz cell was used. The 2±3 lm thick fullerene-doped polyimide ®lms were prepared by spin coating of the 5±6.5% polyimide solution in 1,1,2,2-tetrachloroethane on a glass substrate. In this case, fullerene C70 was used as the sensitizer with the concentration of 0.1±0.5 wt.% in this case. Fullerene-doped nonphotosensitive polyimide 81A was investigated as
3. Results and discussion
N.V. Kamanina et al. / Optics Communications 194 (2001) 367±372
Fig. 1. Dependence of Wout on Win for the polyimide 6B±chloroform solutions with the C60 concentration: (1) ± 0 wt.%; (2) ± 1 wt.%; (3) ± 5 wt.%; and (4) ± 1 wt.% of C60 and 1 wt.% of malachite green.
comparison, the large OL eect was found in the polyimide±chloroform solution simultaneously doped with C60 and the malachite green dye. In this case the 5.5±6-fold attenuation of the incident beam was observed at Win of 1.5 J cm 2 . The system kept the essential laser strength up to the input energy density of 2.5 J cm 2 . It should be noticed that weak scattering was observed in the doped polyimide±chloroform solutions. This result was determined by the cluster formation causing ¯uctuations of the solution density. The ¯uctuations resulted in irregular irradiation absorption along the beam diameter. Indeed, scattering in¯uences the OL level in the polyimide solution. The dependence of the output energy density (Wout ) on the input energy density (Win ) is shown in Fig. 2 for the thin C70 -doped polyimide 6B ®lms. The OL eect was observed for all fullerene-doped ®lms at Win of more than 0.3±0.4 J cm 2 , corresponding to the attenuation of the laser energy density by the factor of 1.3±3.0 that depended on the fullerene concentration in the photosensitive polyimide matrix. It should be mentioned, that the maximum value of the output ``optically limited'' energy density was 0.27 J cm 2 (Fig. 2, curve 1) and 0.05 J cm 2 (Fig. 2, curve 2) at input energy density of 0.8 J cm 2 for the samples with 0.2 and 0.5 wt.% C70 , respectively. Thus, the 3- and 16-fold
369
Fig. 2. Dependence of Wout on Win in ®lms: (1) polyimide 6B with the 0.2 wt.% of fullerene C70 and (2) polyimide 6B with 0.5 wt.% of fullerene C70 .
attenuation of the laser energy density was obtained for these samples. Therefore, the dierence in transmission between samples 1 and 2 was determined by the fullerene concentration. However, the result was caused not only by a higher fullerene concentration, but by a possible complex formation between a donor fragment of the polyimide molecule and fullerene as well. Really, the drastic attenuation of the laser energy density by the factor of 9±12 for the polyimide ®lms with 0.5 wt.% of C70 at Win of 0.6±0.8 J cm 2 was caused when the new complex was activated. The following evidences can be used. Electron anity of fullerene is about 2.65 eV, that is more than the one for acceptor fragments of most organic molecules. It is well known that the monomeric links of polyimides are intramolecular donor±acceptor complexes with the charge transfer between the donor (triphenylamine) and acceptor (diimide) molecular fragments, which can be changed using various dopant molecules. The acceptor diimide fragment is of electron anity of about 1.12±1.46 eV that is twice less than the one of fullerene. Interest in the investigation on the physical± chemical properties of the fullerene-doped systems is generated, among other things, by unique ability of fullerenes to in¯uence the initial donor±acceptor interaction. The high electron anity of fullerene suggests that ones are able to sensitize eciently the organic molecules creating new complexes with
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their donor fragments. It should be noticed that the simple model for the intramolecular transfer of an injected electron into C60 and C70 was proposed from the concept of orbital interaction [17]. In our case, the additional condition for the transfer is the arrangement of the molecular planes in parallel, that provides the largest overlapping the electron densities of the molecular orbitals. The C60 and C70 molecules are spherical or rugbyball in shape, respectively. On exciting, triphenylamine molecule experiences a conformational transformation, changing from the neutral tetrahedral form to the ionized planar one [18]. This eect along with less dimensions of the triphenylamine molecule (0.5 nm) than those of the fullerene molecule (0.7±0.8 nm) allow the arrangement of their molecular planes to be expected in parallel. From the previous results [14,15,18] one can say that fullerenes provoke the creation of RSA materials based on polyimide with the high absorption cross-section. The absorption cross-section of donor±acceptor complex of fullerene with donor polyimide 6B fragment (triphenylamine) was recently estimated in Ref. [18]. It is really about 300 times more than the one of the intramolecular polyimide complexes. Therefore, the fullerenes are more eective acceptors for the system studied. The spectral dependence of photoconductivity is presented in Fig. 3 for the C70 -polyimide and fullerene-free polyimide ®lms. It is known that,
Fig. 3. Spectral dependence of photoconductivity for (1) polyimide 6B ®lm with the 0.2 wt.% of fullerene C70 and (2) pure polyimide 6B ®lm.
when charge transfer complexes are formed, conductivity of individual components is always less than that of the complexes. In our case, an increase in conductivity was observed to be larger for the C70 -polyimide structure (Fig. 3, curve 1) than that for the fullerene-free polyimide ®lms (Fig. 3, curve 2). Using the fact that for fullerenes C60 and C70 , the photoconductivity is constant in the wavelength range 400±650 nm and it signi®cantly decreases at k > 700 nm [19], the results obtained testify the complex formation in the system studied. Moreover, the carriers become free after the charge transfer to the fullerene molecules, where the surface charge is delocalized [9]. Thus, the reinforcement of the donor±acceptor interaction in the ®lms investigated because of the free-carrier absorption in¯uence the OL eect in the IR spectral range for fullerene-doped structures. It should be noticed that the peculiarities of the OL eect in the fullerene±dye-doped polyimide± chloroform solution do not also contradict with the evidence for the complex formation mentioned above. Because the malachite green electron af®nity is 1.6 eV [20], it can be possible to create the complex with the polyimide donor fragment and to be the eective donor for fullerenes as well. The most OL eect observed in the polyimide± chloroform solution doped with fullerene and dyes simultaneously (Fig. 1, curve 4) presents this case. Recently, it has been shown that since the absorption spectrum of the fullerene±polyimide system is overlapped with the ¯uorescence spectrum of malachite green [15] the resonance conditions are ful®lled in the polyimide±dye±fullerene structure over the visible spectral range. Overlapping the electron shells of the dye and the fullerene molecules provides the favorable conditions for the charge transfer complex formation as the result of the free electron exchange between the donor (dye) and the acceptor (fullerene). It causes the spectral changes in the IR range and explains the OL peculiarities in that. It should be noticed that a new IR band with a peak at 1405 nm was found [18]. Moreover, the dye introduction makes the process complicated. The OL investigation allows the multistep interaction to be revealed in the multicomponent system. The interaction involves,
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4. Conclusion
Fig. 4. Dependence of Wout on Win in ®lms: (1) polyimide 81A with the 1 wt.% of fullerene C60 and (2) polyimide 81A with 1 wt.% of fullerene C70 . (3) A low-power transmission for nonphotosensitive polyimide 81A.
among other processes, the intramolecular complex formation both in polyimide and between dye and triphenylamine as well as the complex formation both between fullerene and triphenylamine and between fullerene (as acceptor) and dye (as donor). The dependence of Wout on Win for fullerenedoped non-photosensitive polyimide 81A at k 1315 nm is shown in Fig. 4. As seen from this ®gure, there are no OL peculiarities for the fullerene-doped structure, while the dopant concentration is 2±5 times more than that for the fullerene-doped polyimide 6B (Fig. 2, curves 1 and 2). Therefore, the processes observed in the fullerene-doped non-photosensitive polyimide ®lms are not associated with the reinforcement of the intramolecular donor±acceptor interaction, which is caused by the fullerene introduction in the photosensitive polyimide 6B. Thus, to reveal non-linear properties in organic materials, the carrier transfer mechanism is to manifest itself in the molecules sensitized by fullerenes. Therefore, the OL eect has not been found for non-photosensitive polyimide 81A in the IR spectral range, because the complex formation does not take place there. However, the OL eect is likely to be observed at more fullerene concentration than that is applied in current experiments or is to be revealed at larger intense laser beams, when IR-active vibrational modes of fullerene are activated [21].
In conclusion, the OL eect in IR spectral range in both fullerene-doped polyimide 6B solutions and thin ®lms as well as in the multicomponent systems consisted of fullerenes and dyes has been detected. The peculiarities observed have been explained both by the charge transfer complexes and by the free carriers. The absorption of the complexes and the free carriers in¯uences the attenuation of the laser energy in the OL eect. The results obtained have testi®ed that the fullerenedoped polyimide 6B structures could be applied as eective OL materials for attenuating the power density of more than 1 J cm 2 in the IR spectral range. No OL eect has been found in the fullerenedoped non-photosensitive polyimide 81A structures in the IR spectral range for laser beam power densities and fullerene concentration, which were used in the experiments. These results do not contradict with the idea about importance of the donor±acceptor interaction mechanism for nonlinear-optical properties existence, namely, the OL eect, in the p-conjugated organic compounds. Acknowledgements The authors would like to thank Prof. B.V. Kotov, Dr. V.I. Berendyaev and Dr. N.A. Vasilenko (Karpov Research Physical±Chemical Institute, Moscow, Russia) for their help in the work. This work was supported by Russian National Program ``Optoelectronic and Laser Technologies'' and International Grant ISTC Project 1454 ``Optical barrier''. References [1] L.W. Tutt, T.F. Boggess, Prog. Quant. Electron. 17 (1993) 299. [2] L.W. Tutt, A. Kost, Nature 356 (1992) 225. [3] S. Couris, E. Koudoumas, A.A. Ruth, S. Leach, J. Phys. B: At. Mol. Opt. Phys. 28 (1995) 4537. [4] A.V. Eletskii, B.M. Smirnov, Usp. Fiz. Nauk 165 (1995) 977 (in Russian). [5] V.P. Belousov, I.M. Belousova, V.P. Budtov, V.V. Danilov, O.B. Danilov, A.G. Kalintsev, A.A. Mak, J. Opt. Technol. 64 (1997) 1081.
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