J . Phys. Chem. 1993,97, 10515-1 05 17
10515
Third-Order Nonlinear Optical Properties of Polymorphs of Oxotitanium Phthalocyanine Hari Singh Nalwa,' Toshiro Saito, Atsushi Kakuta, and Takao Iwayanagi Hitachi Research Laboratory, Hitachi Ltd., 7-1 -1 Ohmika-cho, Hitachi City, Ibaraki 31 9-12, Japan Received: May 19, 1993; I n Final Form: August 23, 1993'
Third-order nonlinear optical susceptibilities x ( ~of) amorphous, a, @, and Y polymorphs of oxotitanium phthalocyanine (TiOPc) measured by third-harmonic generation technique are reported. The x(~)(-~w;w,w,o) values of amorphous, a,@, and Y polymorphs of TiOPc varied significantly depending upon the crystal structures and measurement wavelengths. The x(~)(-~w;w,w,o) of a-TiOPc was found to be 5 times as large as that of @-TiOPc and more than 2 times as large as that of Y-TiOPc a t 2.1 pm. The a-TiOPc showed the largest X(~)(-~W;W,W,O) of 1.59 X 1O-Io esu a t 2.43 pm. The origin of third-order optical nonlinearity in different polymorphs is discussed in terms of molecular packing of the TiOPc molecules that causes variations in the intermolecular interactions and hence affects optical nonlinearity.
n
Introduction Recently, organometallic materials are attracting a great deal of attention in the field of nonlinear optics. In particular, metallophthalocyanines, metalloporphyrins, metallocenes, metallopolyynes, and polysilanes have been studied for third-order nonlinear optic~.I-~ Metallophthalocyanines have been considered valuable materials for the development of nonlinear optical devices because of their versatility, architectural flexibility, exceptionally high environmental stability, and ease of processing and fabrication. Furthermore, physical properties of metallophthalocyanines can be tailored over a wide range either by inclusion of a variety of central metal atoms or by substituting peripheral functionalities at the ring. The phthalocyanine ring has a twodimensional conjugated *-electron system, and the magnitude of third-order nonlinear optical susceptibility x ( ~varies ) by several orders depending on the metal atom substitution.&I2 Recently, we have reported that the *-conjugated system of the phthalocyanine ring can be further extended by introducing an additional benzene ring to isoindole units. As a consequence, a naphthalocyanine ring comprising benzoisoindole basic units offers additional *-electron delocalization and hence shows a large thirdorder optical n ~ n l i n e a r i t y . ~ ~InJ ~particular, a processable vanadylnaphthalocyanine (VONc) derivative showed the largest x(')of 8.6 X esu. It is well-known thevanadylphthalocyanine (VOPc) shows polymorphism, and the x 0 )of VOPc increases by a factor of 2 on thermal annealing.10 Recently, polymorphs of oxotitanium phthalocyanine (TiOPc) have been utilized in developing photoreceptor devices,l5-I7 and their photoconductivities were found to vary remarkably depending on the crystal structures. With the same view, we selected to investigate the third-order nonlinear optical properties of different polymorphs of TiOPc to establish a structure-property relationship. Though there has been some reports on the third-order nonlinear optical susceptibility x ( ~of) the a form (phase 11) and @ form (phase I) of TiOPc,l0the x ( ~of)Y form and amorphous TiOPc are discussed for the first time. This Letter discusses a systematic study of the x ( ~of) amorphous, a, 0, and Y polymorphs of oxotitanium phthalocyanine (TiOPc) over the wavelength region 1.2-2.43 wn.
Experimental Methods The chemical structure of oxotitanium phthalocyanine is shown in Figure 1. In this study, we prepared polymorphs of TiOPc by the vacuum deposition method. TiOPc was first evaporated in
* To whom correspondence should be addressed.
* Abstract published in Aduance ACS Absrracrs, October 1, 1993.
-
Figure 1. Chemicalstructureof theoxotitanium phthalocyanine (TiOPc).
a-form-
Tetrahydrofuran
-!i
Xylene
Amorphous -p-form
1
Chlorobenzene water mixture
Y-form
Figure 2. Processes of treatment to transform TiOPc into the respective polymorphs.
a vacuum of 10-5-104 Torr, and then thin films were deposited on fused silica substrate at room temperature. These vacuumdeposited films were treated by different organic solvents to prepare the respective TiOPc polymorphs. Figure 2 shows the processes of treatment to transform TiOPc into different polymorphs. The a-TiOPc was prepared by exposing amorphous TiOPc to tetrahydrofuran vapors. The fl-TiOPc was obtained by treating amorphous TiOPc to xylene liquid. To obtain Y-TiOPc, amorphous TiOPc was exposed to the vapors of a chlorobenzenewater (1O:l) mixture. In all cases, solvent treatment was carried out for 13-15 h at room temperature. The structures of all polymorphs were confirmed by thin film X-ray diffraction patterns. The details of different polymorphs and their analysis have been reported elsewhere.18 Absorption spectra of vacuumdeposited TiOPc thin films were recorded from a UV-vis spectrophotometer (Model 330, Hitachi Ltd.) in the range from 200 to 1200 nm at room temperature. Third-order nonlinear optical susceptibilities of TiOPc polymorphs were evaluated by frequency tunable third-harmonic generation (THG) measure-
0022-365419312097-10515$04.00/0 0 1993 American Chemical Society
10516 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
1.6
Letters
a-Form P-Form A Y-Form
.1.6
b b b
: 1.2 2
1.5
S
0 Amorphous
v) Q)
0
r
0 1.0.
.
0.0
200
400
v 7
I
1
600
800
0.0 1000
12M)
cx
0.5
Wavelenath (rm)
Figure 3. Optical absorption spectrum and wavelength dependence of the x(3)(-30;w,w,w) of a-TiOPc thin films.
TABLE I: Film Thickness, Optical Absorption Peaks, and Third-Order NLO Susceptibilities of TiOPc Polymorphs Measured by the THC Technique at Different Wavelengths’ film
TiOPc thickness (nm) Amax (nm) polymorph a form (phase 11) 62 350,650 m, 835 fl form (phase I) 107 370,675 m, 772 Y form 150 382,720 s, 842 amorphous 106 350,660 s, 722 0 s = shoulder, m = medium-intensity peak.
(10-1’ esu) at 2.43 Fm 15.9 4.86 9.8 1.99
ments using an optical parametric oscillator (OPO, GWU Umweltechnik, Germany). The measurements on TiOPc thin films were carried by using a Q-switched Nd:YAG laser (SpectraPhysics GCR-3)and a frequency OPO with a tuning range of 0.42-2.5 pm. The detailsof the OPO system have been described by Fix et aLI9 The samples on fused silica glass substrate were mounted on a rotation stage. Maker fringes were generated by rotating the sample through the range from -30’ to +30° to the normal. Further details of this experimental system are published elsew heresz0
Results and Discussion The absorption spectral data of all polymorphs are listed in Table I. Figure 3 shows the optical absorption spectrum of the a-TiOPc polymorph. Phthalocyanines exhibit two absorption bands: the Soret band in the near-UV (300-400 nm) and the Q-band in the visible region (600-800 nm) arising from the electronic transitions to the n-s* states of E, symmetry.21 The absorption spectra of TiOPc polymorphs show the characteristic absorptions for the Soret band and Q-band transitions. The Q-band varies from 644 to 842 nm for the different polymorphs. Similarly, the position of the Soret band varied from 350 to 380 nm. Theabsorption in Q-band region appears due to the transition from the ground state to two excited states. The ~(~)(-3w;w,w,w)values of TiOPc polymorphs were determined using the following equation22
13wj
= 13wms[(als/2)/1 - e x ~ ( - a 1 s / 2 ) I ~
(2)
where ~ ( ~ and ) ~ lrcf ~ fare the third-order nonlinear optical susceptibility and the coherence length of the reference silica substrate, respectively, Z3,, is the measured third-harmonic intensity from the fringes of the sample, and Z3, is the absorptioncorrected THG signal from the sample. The 1, and al, are the sample thickness and absorbance, respectively, where a = absorbance/thickness, and 1 3 w r c f ithe ~ THG intensity of the fused silica reference measured under identical conditions to the sample. The x ( ~value ) of the TiOPc samples was evaluated by comparing the THG signal of fused silica substrate. The ~ ( 3 of ) 3.1 X 10-14
0.0
Wavelength (nm)
Figure 4. Wavelength dependence of the ~(~)(-3w;w,w,w)of a,fl, Y, and amorphous TiOPc thin films.
TABLE 11: THG Measured Third-Order Nonlinear Optical Susceptibility of TiOPc Polymorphs measurement ~(~)(-3w;w,w,w) wavelength material phase (10-11 esu) (pm) ref TiOPc a form (phase 11) 15.9 2.43 this work fi form (phase I) 4.86 2.43 this work Y form 9.8 2.43 this work 2.94 1.80 this work amorphous TiOPc fi form (phase I) 1.o 1.907 10 a form (phase 11) 4.6 1.907 10 TiOPc (unknown) 5.3 2.1 26 to 2.8 X esu for fused silica was used as a reference in the 1.0-2.43-pm range.23 The refractive index of TiOPc thin films was assumed to be the same as that of the fused silica substrate. The wavelength dependence of the x ( ~of) a-TiOPc thin films measured by the THG technique in wavelength region 1.2-2.5 pm is shown in Figure 3 along with the electronic spectrum. The x(3)(-3w;w,w,w) values of a,@, Y, and amorphous TiOPc were found to vary noticeably at different wavelengths depending on the crystal structure (Figure 4). The ~(~)(-3w;o,w,0) of a-TiOPc was found to be 5 times as large as that of j3-TiOPc and more than 2 times as large as that of Y-TiOPc at 2.1 pm. The a-TiOPc shows the largest x ( ~of) 1.59 X esu at 2.43 pm. The magnitude of valuevari varied in the order a > Y > @ > amorphous where the x(3)of the a form was enhanced by a factor of 3.27 compared to the @ form. The value of ~ ( 3 is) maximum at 2.43 pm because it is influenced by the three-photon (3w) resonance effect and decreases toward lower wavelengths. At lower wavelengths, the resonance effect is smaller and hence partially affects the ~ ( 3 values. ) Therefore, the large difference noticed in x ( ~values ) of a-TiOPc occurs due to the dispersion of the ~ ( 3 ) in resonant and nonresonant regions. The x(’)apparently changes with absorption intensity and almost follows the absorption spectrum. The a form and Y form show the largest x(3)(-3w;w,w,w) of 1.59 X 10-10 and 9.8 X 10-Il esu, respectively, at 2.43 pm. Table I1 compares the THG measured third-order nonlinear optical susceptibility of TiOPc. Interestingly, the ~(~)(-3w;w,w,w) values of the a and B polymorphs of TiOPc were found to be several times larger that those reported by Hosoda et a1.10 The ~ ( 3values ) of Y form and amorphous TiOPc have not been investigated so far, and these are the first results ever reported. Our THG measurements demonstrate that the new polymorph Y-TiOPc has a very large x(3)(-3w;w,w,w) almost similar in magnitude to that of a-TiOPc. At 2.43 pm, the ~(~)(-3w;w,o,w) of the a and Y polymorphs of TiOPc were found to be the largest. A better orientation film obtained during crystal formation of TiOPc may be responsible for the large magnitude of ~(~)(-3w;o,w,o) in this
Letters study. The magnitude of $3) values of TiOPc polymorphs could be explained by evaluating the orientation of different crystal structures. The crystal structures of TiOPc polymorphs have been analyzed by using X-ray diffraction, infrared, and electron spin resonance (ESR)s p e c t r ~ s c o p y . The ~ ~ ~Y-TiOPc ~~ shows strong X-ray diffraction peaks indicating the state of stacked TiOPc molecules, while on the other hand, the amorphous TiOPc did not show any definite peaks demonstrating the irregular crystal orientation. The a form has a triclinic structure, and the Ti-0 in the molecules are aligned in same direction along the axis.25 The order of Ti-0 arrangement in TiOPc molecules is stronger in the a form compared with the other forms. Contrary to this, the molecules in the fl form are aligned in an alternate up-anddown arrangement affected by the direction of the Ti=O bonds, and due to this molecular interactions are significantly reduced. The alternate opposite alignment of the Ti-0 bonds in the /3 form and disordered crystal structure in amorphous TiOPc may be responsible for the lower magnitudeof the optical nonlinearity. If the orientation of T i 4 is taken into account, then the &form should have a lower x ( 9 than the amorphous form, but this is not the case here. The x ( ~value ) of the B form is larger than the amorphous form over the measured wavelengths. As a consequence, it is concluded that the orientation of T i = O may not be the sole factor for the enhancement of x 0 ) in TiOPc. Hor and P ~ p o v i creported ~~ humidity-dependent photoconductive and fluorescence properties of Y-TiOPc, showing that water plays an important role in the photogeneration processes. The water molecules induces a special molecular packing arrangement in Y-TiOPc that assists the formation of charge-transfer states. The photoconductivity of Y-TiOPc decreases remarkably by removing water molecules, 98% after being heated at 140 OC in vacuum, and recovers by exposure to water vapors.28 In the present study, THG measurements were done under ambient conditions where the Y-TiOPc contains water molecules, and this may affect optical nonlinearity. The effect of water molecule on photoconductivity and fluorescence of TiOPc is established, but it is difficult to visualize its effect on x ( ~ ) Such . an effect of water molecules on third-order optical nonlinearity is of considerable interest and will be studied for Y-TiOPc samples in future. This study demonstrates that third-order nonlinear optical susceptibility of TiOPc polymorphs varied significantlywith molecular packing and can be tailored by crystal transformations. Strong intermolecular interactions in TiOPc molecules in the a form and Y form may be responsible for large optical nonlinearity though it seems difficult to establish a clearcut relationship between crystal structure and mechanism of optical nonlinearity in different TiOPc polymorphs.
Conclusions A systematic study of third-order nonlinear optical susceptibilities of a,8, Y, and amorphous TiOPc was reported. The a-TiOPc shows the largest of 1.59 X esu at 2.43 pm, and the magnitude of the x ( ~value ) varied in the order a > Y > B > amorphous. The Y-TiOPc also showed large ~ ( 3 ) (-3w;w,w,w) almost similar in magnitude to that of a-TiOPc.
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10517
The value of x ( 9 is maximum at 2.43 pm because it is influenced by the three-photon (3w) resonance effect and decreases toward lower wavelengths.
Acknowledgment. The authors gratefully acknowledge the kind technical assistance from Dr. Anthony Ticktin and Dr. K.H. Haas of BASF AG Advanced Polymer Research at Tsukuba for OPO-THG measurements. This work was performed by Hitachi Ltd. under the management of Japan High Polymer Center as a part of Industrial Science and Technology Frontier Program supported by New Energy and Industrial Technology Development Organization. References and Notes (1) Bredas, J. L., Chance, R. R., Eds. Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics, and Molecular Electronics; Kluwer: Dordrecht, 1991. (2) For example,see the following review: Nalwa, H. S.Appl. Organomet. Chem. 1991, 5, 349. (3) For example, see the following review: Nalwa, H. S.Adv. Mater. 1993, 5, 341. (4) Nalwa, H. S.;Kakuta, A.; Miyata, S.Nonlinear Optics of Organic Molecular and Polymeric Materials; CRC Press: Boca Raton, FL, in press. (5) Ho, Z. Z.; Ju, C. Y.; Heterrington 111, W. M. J . Appl. Phys. 1987, 62, 716. (6) Ho, Z. 2.;Peyghambariam, N. Chem. Phys. Lett. 1988,148, 107. (7) Wang, N. Q.;Cai, Y. M.; Helfin, J. R.; Garito, A. F. Mol. Cryst. Liq. Cryst. 1990, 189, 39. (8) Shirk, J. S.;Lindle, J. R.; Bartoli, F. J.; Hoffman, C. A.; Kafafi, Z. H.; Snow, A. W. Appl. Phys. Lett. 1989.55, 1287. (9) Shirk, J. S.;Lindle, J. R.; Bartoli, F. J.; Boyle, M.F. J . Phys. Chem. 1992, 96, 5847. (10) Hosoda, H.; Wada, T.; Yamada, A,; Garito, A. F.; Sasabe, H. Nonlinear Opt. 1992, 3, 183. (11) Norwood, R. A.; Sounik, J. R. Appl. Phys. k t t . 1992,60, 295. (12) Grund, A,; Kaltbeitzel, A.; Mathy, A.; Schwarz, R.; Bubeck, C.; Vermehren, P.; Hanack, M. J. Phys. Chem. 1992, 96, 7450. (13) Nalwa, H. S.;Kakuta. A.; Mukoh, A. J . Phys. Chem. 1993,97,1097. (14) Nalwa, H. S.;Kakuta, A.; Mukoh, A. Chem. Phys. Lett. 1993.203, 109. (15) Enokida, T.; Kurata, R.; Seta, T.; Otsuka, H. Electrophotography 1988, 27, 533 (in Japanese). (16) Ohaku, K.; Nakano, H.; Aizawa, M. U S . Patent 4,728,592, 1988. (17) Suzuki, T.; Murayama, T.; Ono,H.; Otsuka,S.;Nozomi, M. US. Patent 4,664,997, 1987. Kobayashi, T.; Suzuki,S.;Iwayanagi, T. J . Phys. (18) Saito, T.; Sisk, W.; Chem. 1993, 97, 8026. (19) Fix, A.; Schroder, T.; Wallenstein, R. Laser Optoelektronik 1991, 23, 106. (20) Gierulski, A.; Naarmann, H.; Schrof, W.; Ticktin, A. Proc. SPIE 1991, 1560, 172. (21) Stillman, M. J.; Nyokong, T. In Phthalocyanines: Properties and Applications;Lemoff,C.C.,Lever,A. B. P., Eds.; VCHPublishers: Weinhcim, 1989: Vol. 1. Chauter 3. D 133. (22) Tomaru, S.; K u u e r a , K.; Zembutsu, S.;Takeda, K.; Hasegawa, M. Electron. Lett. 1981, 23, 595. (23) Kajzar, F.;Messier, J. Phys. Rev. A 1985, 32, 2352. (24) Enokida, T.; Hirohashi, R.; Nakamura, T. J . Imag. Sci. 1990, 31. 234. (25) Hiller, W.; Strahle, J.; Kobel, W.; Hanack, M. Z . Kristallogr. 1982, 159, 173 (in German). (26) Matsuda, H.; Okada, S.;Masaki, A.; Nakanishi, H.; Suda, Y.; Shigehara, K.; Yamada, A. SPIE Proc. 1990, 1337, 105. (27) Hor, A.; Popovic, Z . D. Proceedings ofIS& Ts Eight International Congress on Advances in Non-Impact Printing Technologies; The Society of Imaging Science and Technology, 1992, Vol. 1, pp 247-249. (28) Fujimaki, Y. Proc. IS&Ts Seventh International Congress on Advances in Non-impact Printing Technologies, Portland, OR, The Society of Imaging Science and Technology, 1991; Vol. I, p 269.
