J. Phys. Ckem. 1994,98, 12270-12277
12270
Molecular-Length Dependence of Third-Order Nonlinear Optical Properties in Conjugated Organic Materials Hirohisa Kanbara; Hideki Kobayashi, Toshikuni Kaino, Naoki Ooba? and Takashi Kuriharat N I T Opto-electronics Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-01, Japan Received: May 20, 1994; In Final Form: July 21, 1994@
Molecular-length dependence of the third-order nonlinear optical properties in conjugated organic materials is systematically investigated by third-harmonic generation measurement and optical Ken; shutter measurement. The molecular length represents a very important factor in the third-order nonlinear optical properties. The third-harmonic generation measurement shows that the nonlinear refractive index related to the electronic polarization effect increases in proportion to the powers of a conjugated length or an absorption edge. The optical Kerr shutter measurement indicates that the application of longer molecules causes the nonlinearity of the solution medium to be dominated by the electronic polarization effect, reducing the contribution of the molecular orientation effect.
I. Introduction Various kinds of third-order nonlinear optical materials have been studied to achieve all-optical signal processing. Among them, conjugated organic materials have exhibited advantages of strong laser resistance and easy chemical design and modification, accentuating their difference from the other candidates. As a result, these materials have been regarded as promising media for ultrafast switching devices. They are categorized into roughly two groups: polymers and lowmolecular-weight compounds. Polymers, for instance, polydiacetylenes (PDAs), have yielded very large nonlinearity 2 orders of magnitude larger than that of carbon disulfide, the conventionally used material.' PDAs have also given ultrafast response times of less than a subpico~econd.~-~ Most PDAs, however, have problems with processability. Therefore, many trials, such as chemical conversion to other polymer series, have been carried out to overcome these On the other hand, low-molecular-weight compounds have exhibited the advantage of good processability, which makes them applicable as media for an optical fiber or film waveguide when dissolved into organic solvents or matrix In spite of these advantages, the nonlinearity of the lowmolecular-weight compounds is rather small compared to that of polymer^.^^,'^ Moreover, most solution media cannot give a subpicosecond response time because their nonlinearities are generally dominated by the molecular orientation effect.14 These problems must be overcome in order to fabricate highly efficient ultrafast optical devices. For this purpose, obtaining a guideline for a chemical design which will enhance the nonlinearity and supressing the molecular orientation effect for solutions of low-molecular-weight compounds are considered to be the primary objectives. This paper presents the conjugated-length dependence of third-order nonlinearity. The guideline for a chemical design achieving a more efficient material is acquired through thirdharmonic generation (THG) measurement. In this paper, the molecular-length dependence of the molecular orientation effect is also described by means of optical Kerr shutter measurement. It is shown that the solutions of low-molecular-weight cam-
' 162 Shirakata, Tokai, Ibaraki 319-11, Japan. @
Abstract published in Advance ACS Absrracfs, November 1, 1994.
0022-3654/94/2098- 12270$04.50/0
pounds can provide subpicosecond response nonlinearity dominated by the electronic polarization effect, thereby excluding the molecular orientation effect.
11. Experimental Section The THG measurements were conducted by using lithium niobate difference-frequency generation between a Q-switched Nd:YAG laser and a tunable dye laser. The pump pulse duration was 6 ns, and the repetition rate was 10 Hz. The peak pump power density ranged from 50 to 100 MW/cm2. All the measurements were made in the air. A condenser lens with a short focal length of 50 mm reasonably eliminated the thirdharmonic effect of the air.15 In the optical Kerr shutter measurements, a Nd:YAG nanosecond laser ( 6 4 s duration; 10-Hz repetition) and a Nd:YAG laser-pumped nanosecond dye laser (6-ns duration; 10-Hz repetition) were used to generate gate beams. A laser diode pulse with a wavelength of 0.81 p m was used as a probe beam. The duration was 50 ns. A photomultiplier tube with a 2 4 s response was employed as a detector. The polarization of the gate beam was set at n/4 with respect to that of the probe beam. The laser beams were focused using a condenser lens with a 100-mm focal length. The peak pump power density was around 50 MW/cm2.
111. Results and Discussion A. Molecular Design to Obtain Efficient Materials. ( I ) Conjugated-Length Dependence of Third-Order Nonlinearity. The third-order nonlinearity of one-dimensional conjugated systems has been theoretically predicted to exhibit a powerlaw dependence on the conjugated length in the powers of four to five.16-21 In this section, these properties are investigated experimentally to obtain highly efficient materials, using the solutions of quasi-one-dimensional low-molecular-weight compounds. The nonlinear refractive index value n2 is calculated by converting a third-order susceptibility value ~ ( obtained ~ 1 from THG measurement:22 this n2 value is defined in self-phase modulation (SPM). The n2 value obtained by THG measurement (n2[THG] value) originates almost entirely from the electronic polarization effect even for solution media, since the other kinds of nonlinearities, such as the molecular orientation effect, do not contribute to the THG measurement. The n20 1994 American Chemical Society
Molecular-Length Dependence of Nonlinearities 1010
J. Phys. Chem., Vol. 98, No. 47, 1994 12271
1
-STAD
1016
r
I
1
10
Conjugated Length
102
(A)
Figure 1. Conjugated-length dependence of the n2[THG] value for solutions of quasi-one-dimensional low-molecular-weight compounds. The dotted line represents the /3-carotene series. The solid line shows the benzene-ring series. The n*[THG] value originating from the electronic polarization effect rapidly increases as the conjugated chain is lengthened.
[THG] value of the solution was evaluated according to the following equation by comparing the third-harmonic intensity with the standard of fused silica g l a s ~ : ' ~ ~ ~ ~
Here, n is the refractive index for the pump light, lc is the Benzene-ring series
coherence length, and Z30 is the third-harmonic peak intensity. The suffix S means standard fused silica glass. Figure 1 shows the conjugated-length dependence of the nz[THG] value for the solutions of quasi-one-dimensional lowmolecular-weight compounds. These compounds were all saturated in N,N-dimethylformamide (DMF) and then poured into a glass cell having a 1-mm path length. Carbon disulfide, nitrobenzene, and /?-ionone, which are liquids at room temperature, were used without being dissolved in DMF. The n2[THG] value in the figure was calculated as that for a solution concentration of 1 M. The proportional increase in the n,[THG] values relative to the solution concentration was ascertained in this experimental region. To estimate a nonresonant nz[THG] value, the fundamental light wavelength was set at 2.1 pm for carbon disulfide and the benzene-ring series and at 1.75 pm for the /?-carotene series. The refractive index was regarded as being the same value, 1.5, as that of the standard fused silica glass, except for carbon disulfide, which is 1.62. The molecular structures are illustrated in Figure 2. The conjugated length was calculated by summing up every bond length reported in ref 24. The conjugated region was regarded to be from the nitrogen atom of a diethylamino group to the oxygen atom of a nitro group in the case of 4-(N,N-diethylamino)-p-nitrostyrene (DEANST). The nz[THG] values of the /?-carotene series increased in proportion to the fifth power of the conjugated length, which shows that a longer conjugated chain effectively produces larger nonlinearity. The exponent of 5 properly coincides with the
j C S ,~P-carotene series
!Figure 2. Molecular structures of quasi-one-dimensional low-molecular-weight compounds. The molecular length changes by degrees.
12272 J. Phys. Chem., Vol. 98, No. 47, 1994
Kanbara et al.
Figure 4. Molecular structure of SBAC. This material has a structure in an absorption spectrum if it keeps a relatively high molecular planarity.
by the interaction with DMF. This interpretation is confirmed by a similar experiment with 2,5-dichloroterephthalalbis-(40.3 0.4 0.5 0.6 0.7 (N,N-diethy1amino)aniline) (SBAC), whose molecular structure Wavelength (pm) is similar to that of HH-SBA, as illustrated in Figure 4. The Figure 3. Absorption spectra of the quasi-one-dimensional lowvibronic structure of SBAC is coupled with the electronic molecular-weight compounds. The solid line shows the spectrum of transition only when its molecular planarity is relatively high.29 the B-carotene solution. The dotted line represents that of HH-SBA The structure observed in the spectrum of the p-carotene M. solution. Two compounds were dissolved in DMF at 5 x solution indicates that p-carotene can have a relatively high planarity even in DMF. This is supported by the spectra for predictions calculated using the one-dimensional free-electron p-ionone and retinal, belonging to the p-carotene series, which model and the self-consistent field configuration interaction show a clearer vibronic structure under a high molecular this result is very close to other t h e ~ r y . ~Furthermore, ~,~~ planarity.31 This high molecular planarity of the p-carotene experimental results for polyene material^.^^^^^ series in DMF provided a linear n2[THG] increase to a For the benzene-ring series, the n2[THG] value was found to conjugated length of at least 20 A, which is the conjugated increase in approximately the same way as in the P-carotene length of P-carotene. The neighboring peak distance in the series. A solution of 4,4’-bis[p-(N-ethyl-N-(((((butoxycarbonyl)P-carotene spectrum was estimated to be about 1100 cm-’, methyl)carbamoyl)oxy)ethyl)amino)-o-(methylphenyl)azo]awhich can be assigned to C-C stretching vibration, C-H zobenzene (BCMU-STAD) exhibited an n2[THG] value 3 orders deformation vibration, or C-CH3 stretching vibration in the of magnitude larger than that of nitrobenzene. excited state. The benzene-ring series is synthesized following different Lengthening the conjugated chain of the /?-carotene series chemical designs. DEANST and 4-(N,N-diethylamino)-4‘having a polyene structure is a promising possibility for a nitrostilbene (DEANS) are composed in donor-acceptormolecular design to obtain larger n2[THG] materials. The substituted intramolecular charge-transfer formation^.^^,^* These p-carotene series yields a relatively large n2[THG] value and is compounds are assumed to yield large nonlinearity originating expected to increase its nz[THGI value according to the fifthfrom a large transition dipole moment between the ground state power law because it maintains its good molecular planarity in and the excited state. On the contrary, compounds like solution. This molecular design is justified by the previous terephthalalbis(4-(NJV-dihexylamino)aniline)(HH-SBA) or BCreport that polyene materials exhibit no saturation of the MU-STAD consist of symmetrically substituted donor group^?^,^^ nonlinearity, at least up to a conjugated length of around 40 It is expected that, with this non-donor-acceptor formation, the A.17,32,33 nonlinear electronic polarization effect can be increased by However, the p-carotene series exhibits poor solubility in controlling the transition moments between the excited states. organic solvents, resulting in a solution n2[THG] value of Consequently, this experimental result, in which all the comp-carotene 2 orders of magnitude smaller than that of HH-SBA pounds of the benzene-ring series behaved as one group, or BCMU-STAD, as shown in Table 1. The solubility can be suggests that lengthening the conjugated chain plays a very improved by modifying the molecular structure. Notable important role in obtaining larger nonlinearity, even for the examples are illustrated in Figure 5 . Compounds of terephbenzene-ring series. thalalbis(4-(N,N-diethylamino)aniline)(SBA) and HH-SBA or In a closer view, the nz[THG]-value change in the benzene4-(N,N-dimethylamino)-4’-nitrostilbene(DANS) and DEANS ring series was different from that in the /?-carotene series. The are varied only in the amino groups. Nevertheless, they have solutions of 2-methyl-4-nitroaniline (MNA) and DEANST remarkably different solubilities in organic solvents. SBA exhibited a steeper n2[THG] increase beyond the fifth-power exhibits a solubility of less than 0.1 wt % in methylmethacrylate law, whereas HH-SBA or BCMU-STAD solution showed (MMA), while HH-SBA exhibits a high solubility of over 20 gradual saturation of the n2[THG] value. The n2[THG] increase wt % in MMA, having the dihexylamino group, which is larger beyond the fifth-powder law is thought to be a result of the than the diethylamino group of SBA. Similarly, DEANS attains enhancement effect brought about by intramolecular chargeapproximately 3 times larger solubility in DMF than that of transfer formation besides the effect of lengthening the conjuDANS. In short, a chemical modification to enlarge an alkyl gated chain. The n2[THG] saturation is possibly caused by the part of the amino group effectively increases the solubility in restraint of the effective conjugated length due to the increase solvents, resulting in a large n2[THG] value for the solution. A in molecular nonplanarity . This conjugated length restraint steric hindrance of the larger alkyl amino group is considered might exceed the benefits of the chemical design used to to reduce the cohesive force between the conjugated molecules, enhance the nonlinearity. which increases the solubility. For this reason, the benzeneThe molecular nonplanarity can be explained by investigating ring series solutions could become large nz[THG] materials, as absorption spectra. Absorption spectra of 0-carotene and HHshown in Table 1. SBA solutions of DMF ( 5 x M) are shown in Figure 3. Their conjugated lengths are almost the same as each other. The same type of modification is important in the P-carotene HH-SBA has no structure in the spectrum, being different from series to obtain efficient materials. Recently, a new compound, the case of p-carotene. This implies that HH-SBA decreases in which one side of a @-carotenemolecule was substituted with its planarity in DMF and its vibronic coupling is diminished porphyrin, was reported to improve the solubility in chloroform
Molecular-Length Dependence of Nonlinearities
J. Phys. Chem., Vol. 98, No. 47, 1994 12273
TABLE 1: Solubilities and Nonresonant n2[THG] Values for the Solutions of Quasi-One-Dimensional Low-Molecular-Weight CompoundP benzene-ring series DEANS HH-SBA BCMU-STAD nitrobenzene MNA DEANST solubility (M) 9.8 2.9 2.0 9.8 x 10-2 4.5 x 10-2 3.2 x lo-* solution n2[THG] (cm2/W) 5.1 x 3.5 x 10-15 2.4 x 10-14 9.1 x 10-15 1.1 10-14 1.5 x 10-14 1 M n$l"G] (cm2/W) 5.2 x 1.2 x 10-15 1.2 10-14 9.3 x 10-14 2.5 x 10-13 4.7 x 10-13 ~
~~
~~
~
CS2, /?-caroteneseries
solubility (M) solution nz[THG](cm2/W) 1 M n2[THG] (cm2/W)
cs2
/?-ionone
retinol
retinal
/?-carotene
1.6 x 10' 1.1 10-14 6.7 x
4.9 7.4 x 10-15 1.5 x 10-15
8.0 x 10-1 1.3 x 10-14 1.6 x 10-14
8.0 x lo-' 2.6 x 10-14 3.3 x 10-14
5.0 x 10-5 2.2 x 10-16 4.4 x 10-12
The /?-caroteneseries provides a large 1 M n*[THG]value. The benzene-ring series yields a large solution n2[THG] value because of its high solubility.
Figure 5. Molecular structures giving a greatly different solubility in organic solvents. The solubility is increased through chemical modification in an alkyl part of the amino group. Molecules having a larger amino group provide a higher solubility. up to 1.2 x M and to increase the solution nonlinearity by more than 1 order of magnitude above the previous value for the p-carotene solution.34 The key to improving the solubility of the p-carotene series was shown even in our experiment. The solubility of the p-carotene series depends on the molecular size and the presence of a polar group. For example, retinal or retinol exhibits a solubility in DMF more than 4 orders of magnitude larger than that of p-carotene, as shown in Table 1. This is because the molecular length of retinal or retinol is only half that of p-carotene and because a polar group on the side of retinal or retinol effectively increases solvation with the polar solvent DMF. If retinal and retinol, which possess almost the same molecular lengths, are compared to each other, retinal has the advantage of giving a longer conjugated chain by its aldehyde group. Chemical modification can raise the material nonlinearity as well as the solubility. A powder of DEANS yielded several times larger third-harmonic intensity than that of DANS. Since DEANS has a larger donor amino group than DANS, it might give rise to a stronger donor effect, thus enhancing the thirdorder nonlinearity more than the case for DANS.
(2) Absorption-Edge Dependence of the Third-Order Nonlinearity. Lengthening the conjugated chain is generally accompanied by a red-shift of the absorption edge. Consequently, the third-order nonlinearity of one-dimensional conjugated systems can be predicted by measuring the absorption edge.35 Here, the dependence of the n2[THG] value on the absorption edge is examined as follows. The 1 M nz[THG] values for solutions of quasi-onedimensional low-molecular-weight compounds are plotted as functions of the absorption edge in Figure 6. The absorption edge was estimated by using 5 x loF5M DMF solution poured into a 1-mm-path glass cell. The n2[THG] values for the ,&carotene series and the benzene-ring series both increased rapidly as the absorption edge red-shifted. This result points out that the red-shift of the absorption edge is beneficial to increase the n2[THG] value. The n2[THG] value was found to increase approximately in proportion to the tenth power of the absorption edge. This is somewhat steeper than previously reported.35 Our experimental results may be supported by the power-law dependence; the absorption edge red-shifts simply in proportion to the square root of the conjugated length.36.37
Kanbara et al.
12274 J. Phys. Chem., Vol. 98, No. 47, 1994
Wavelength (pm) .\on
1.0 0.8
pcarotene 0
0.6
,‘
0.4
M0:PPV BCMU-STAD
retinal retinol p-ionone
,t‘
DEANS
,d
,tb/.
DEANST
MNA Nitrobenzene
0.1
0.2
0.5
1
2
Absorption Edge (p.m) Figure 6. Absorption-edge dependence of the nz[THG] value for solutions of quasi-one-dimensionallow-molecular-weightcompounds. Both nz[THG] values increased approximately in proportion to the tenth power of the absorption edge. The P-carotene series yielded an nz[THG] value about 30 times larger than that of the benzene-ring series having the same absorption edge.
1 .o
2.0
3.0
4.0
Photon Energy (eV) Figure 8. Absorption spectra of the PAV thin films. They exhibit differences in the absorption edge corresponding to activation of the conjugated systems.
I
~ C H = C H & 4PV
MO-PPV
I
0.2
CH=CH& CH
m -(&H=cH~
Figure 7. Molecular structure of the PAV series, which possesses
similar conjugated lengths to one another. This dependence law has been experimentally verified for polyene material^.^^,^^ In our experiment, this dependence law was ascertained to be extended to the benzene-ring series as well as the @-caroteneseries. Therefore, it is credible that the nz[THG] value for both series increases in proportion to the tenth power of the absorption edge, in cases in which the 112[THG] values change proportionally to the fifth power of the conjugated length. Contrary to the results in Figure 1, the benzene-ring series exhibited a linear absorption-edge dependence of the nz[THG] value. The absorption edge directly reflects the effective conjugated length. Figure 6 indicates the merits of the @-carotene series. At the same absorption edge, the @-caroteneseries provides an n2[THG] value almost 30 times larger than that of the benzenering series. This means that synthesizing a material having a polyene structure, such as the @-caroteneseries, is useful to attain a larger n$lXG] value than synthesizing the benzene-ring series. Examining the absorption-edge dependence of the third-order nonlinearity is especially available for molecular systems having almost the same conjugated lengths. The poly(aryleneviny1ene) (PAV) series is a case in point. The molecular structures are illustrated in Figure 7 . The PAV series has been developed to improve the optical properties of PDAs? This series possesses high stability and sufficient processability, exhibiting a large nonlinearity comparable to PDAs. Absorption spectra of the PAV thin films are shown in Figure 8. In spite of the almost equivalent conjugated lengths, the PAV series exhibited gradu-
0.4
0.6
0.8 1.0
Absorption Edge (pm) Figure 9. ni[THG] value of the PAV thin films as a function of the absorption edge. The PAV thin films showed almost the tenth-powerlaw dependence.
ally shifted absorption edges. This results from the difference in the activation of the conjugated systems; that is, a poly(2,5dimethoxy-p-phenylenevinylene)(MO-PPV) substitutes hydrogen atoms of poly@-phenylenevinylene) (PPV) with methoxy groups, and poly(2,5-thienylenevinylene)(PTV) substitutes a phenylene group of PPV with a thienylene group. The thin films were used to estimate the n*[THG] values of the PAV series. The film thickness was varied from 0.1 to 0.5 pm. In calculating these values, the following equation was used instead of eq 1:15
where 1 is the thickness of the thin film. The refractive indices of the PPV, MO-PPV, and PTV films were 2.0, 1.92, and 2.66, respectively. These differences in the refractive index, which varied quite a bit, were taken into consideration. If the difference in the refractive index n is considered, the ni[THG] = n2 x n2[THG] value exhibits a power dependence on the absorption edge.22 The dependence of the n*’[THG] value for PAV thin films is plotted in Figure 9. The n
