10184
J. Phys. Chem. B 2005, 109, 10184-10188
Effect of Donor-Acceptor Substitution on the Nonlinear Optical Properties of Oligo(1,4-phenyleneethynylene)s Studied by Third Harmonic Generation Spectroscopy Kaloian Koynov,* Ayi Bahtiar,† and Christoph Bubeck Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
Bastian Mu1 hling and Herbert Meier Institute of Organic Chemistry, UniVersity of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany ReceiVed: February 21, 2005
Third-harmonic generation (THG) spectroscopy was performed for oligo(1,4-phenyleneethynylene)s (OPEs) with terminal donor-acceptor (DA) substitution and compared to the results of merely donor substituted OPEs and regular OPE chains with 2,5-dipropoxy benzene rings. Both, extension of the conjugation and push-pull effect enhance the molecular hyperpolarizability γ, even for the DAOPEs, which exhibit a hypsochromic shift of the long-wavelength absorption for increasing length L of the conjugated chain.
Introduction
SCHEME 1
Conjugated oligomers and polymers, such as the 1,4phenyleneethenylenes (OPVs and PPVs) and the corresponding 1,4-phenyleneethynylenes (OPEs and PPEs), attract a lot of attention in organic chemistry and materials science.1 It is wellknown that both linear and nonlinear optical properties of these types of materials are largely determined by the extent of electron delocalization. In particular conjugated oligomers usually exhibit a convergent bathochromic shift of both the absorption and fluorescence when the number of repeat units, n, is increased.2-7 Recently, however, we found that some donor-acceptor substituted OPVs8-11 as well as donoracceptor-substituted OPEs12 display the opposite behavior: a hypsochromic shift is induced upon extending the length of the chromophores. We attributed this effect to the dominant contribution of an intramolecular charge transfer (ICT) for the short oligomer molecules and a continuous decrease of the ICT and its influence on the transition energy S0 f S1 for increasing distance of donor D and acceptor A.8-12 Apparently the intramolecular charge transfer could influence not only the linear but also the nonlinear optical properties of DA substituted OPEs and OPVs. It is important to emphasize, at this point, that conjugated polymers are considered to be the most promising organic materials for high-speed photonic switching and signal processing concepts because of their high third-order optical nonlinearity, fast response times, and relative ease of waveguide preparation.13-15 That is why there has been a continuous effort to measure the third-order optical nonlinearities of a large number of conjugated polymers and oligomers and relate these nonlinearities to the chemical structure of the materials.16-18 In this context, it is very interesting to study the influence of the terminal donor-acceptor substitution on the third-order optical nonlinearities of conjugated oligomers. Earlier works18-21 have shown that such substitution may lead to a strong increase of the molecular hyperpolarizability of conju-
gated systems and could be used for molecular engineering20,21 of appropriate materials for nonlinear optical applications. In this paper, we present our study of the NLO properties of the donor-acceptor substituted OPEs 1 and the purely donor substituted systems 2 by means of third harmonic generation (THG) spectroscopy.16 The results are compared with our previous study5 of nonlinear optical properties of a series of oligo(2,5-dipropoxy-1,4-phenyleneethynylene)s 35 and conclusions on the influence of the donor-acceptor substitution are made.
* To whom correspondence should be addressed. E-mail: koynov@ mpip-mainz.mpg.de. † Present address: Department of Physics, University of Padjadjaran Bandung, Jl. Jatinangor km. 21 Sumedang, 45363 Indonesia.
Materials and Methods Materials. The preparation of the compounds 1 and 2 was described in a previous paper.12 Their chemical structure is shown in Scheme 1. Thin Film Preparation and Characterization. Our studies of nonlinear optical properties required thin oligomer films on fused silica substrates. Because of their crystallization tendency, we disclaimed on the formation of neat films and prepared thin films of 1a (n ) 1), 1b (n ) 2), 2a (n ) 1), 2b (n ) 2), and 2c (n ) 3) dispersed in polystyrene (PS) similarly to our earlier study.5 The compounds were mixed with polystyrene (MW ) 100 000) at (10.00 ( 0.03)% of concentration by weight, dissolved in toluene using a concentration by weight of the mixture of 2.7%, and spin-cast on fused silica substrates at spinning speeds of 500 and 9000 rpm. This resulted in final
10.1021/jp0508913 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005
Nonlinear Optical Properties of OPEs
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10185
TABLE 1: Linear and Nonlinear Optical Properties of the OPEs 1a,b and 2a-c in Highly Diluted CHCl3 Solutions and in Thin Films of Polystyrene (PS) with (10.00 ( 0.03)% (mass) of OPE compound
1a
repeat units n λmax [nm] (( 1 nm) in CHCl3 max [102 L mol-1 cm-1] (( 3%) λmax [nm] (( 3 nm) in PS films Rmax [104 cm-1] (( 3%) |χres(3)| [10-13 esu] (( 5%) φres [°] (( 10°)
1 301 256 298 0.95 19.9 195
1b 1 433 237 430 0.82 16 85
2 339 367 331 0.80 22 38
2a 2 399 301 398 0.53
1 341 250 345 1.52 6.4 22
2b 2 300 280 300 0.91 8.7 216
3b 2 374 449 376 1.49 13.4 77
3 327 469 327 1.05 11.8 103
3 380 578 378 0.98
film thicknesses d of the order of 200 and 50 nm, respectively, as measured with a Tencor model P10 step profiler. The dispersion of the absorption coefficients R(λ) and the refractive index n(λ) of ultrathin films (d ∼ 50 nm) were determined from the transmission and reflection spectra measured with a UVvis-NIR spectrometer (Lambda 900) as described in earlier work.16 The λmax and absorption coefficient Rmax data of the thin films are given in Table 1. They show spectroscopic data similar to those obtained for the CHCl3 solutions. Quantitative data concerning the chain length L in nm, defined as the distance between the terminal carbon atoms of the benzene rings, were obtained by the linear equation
L ) na + b
(1)
The parameters a ) 0.66 and b ) 0.28 represent average values derived from crystal structure analyses of molecules having tolane subunits.22 Third-Harmonic Generation Spectroscopy. Measurements of THG by using the Maker fringe technique were performed by means of the experimental setup which is shown in Figure 1. We used an Nd:YAG laser (EKSPLA model PL 2143B), the
Figure 2. Harmonic light intensity as function of the incidence angle for a thin film of 2c at the front side of a fused silica substrate (triangles) and the substrate after removal of the film (circles). The solid lines show the theoretical fits.
modulus |χ(3)| and phase angle φ of the complex value of χ(3)
χ(3) ) |χ(3)| exp(iφ)
(2)
The χ(3) values were determined with respect to one and the same reference value (|χ(3)| ) 3.11 × 10-14 esu) for the fused silica substrate24 for all laser wavelengths λL. Since this reference value is not accurately known, its weak spectral dependence was not taken into account. Results and Discussion
Figure 1. Experimental setup of Third Harmonic Generation.
second harmonic output of which pumped a parametric generator (EKSPLA model PG 501), which gave laser pulses with a duration of 20 ps, repetition rate 10 Hz, and a wavelength tuning range between 680 and 2000 nm. The laser beam was focused on the sample, which was placed in an evacuated chamber and mounted on a rotation stage. The harmonic signals of the sample and reference were measured simultaneously to compensate for the effect of laser intensity fluctuations, as shown in Figure 1. Typical results of Maker fringes measured for the film on substrate (triangles) and substrate alone after wiping off the film (circles) are shown in Figure 2. The Maker fringes were evaluated taking into account the measured data of the sample (thickness, refractive index, and absorption coefficients at the fundamental and harmonic wavelengths), the free and bound harmonic waves, and their reflections at the interfaces as described earlier.16,23 The only fitting parameters were the
Because of solubility problems, we were able to prepare films with good enough optical quality only from oligomers 1a, 1b, and 2a-c. By using the method for the THG spectroscopy described above, we evaluated modulus |χ(3)| and phase angle φ of the macroscopic, complex third-order susceptibility χ(3)(-3ω,ω,ω,ω) for each of these films. The results for the |χ(3)| of 2a-c measured at various laser wavelengths λL are shown in Figure 3 together with the UV/vis absorption spectra of diluted solutions of these oligomers in CHCl3. The scales of the wavelength of the absorption spectra λ and of the laser pulses λL are appropriately chosen to visualize a three-photon resonance of χ(3)(-3ω,ω,ω,ω). The THG spectrum of 2a (Figure 3a) shows only one pronounced three-photon resonance at λL ) 1140 nm, which corresponds to the linear absorption maximum but is redshifted. The oligomers 2b (Figure 3b) and 2c (Figure 3c) show two maxima in their linear absorption as described earlier.12 The corresponding |χ(3)| spectra show similar resonances which are again red-shifted. Figure 4 depicts our results for the DA OPEs 1a,b. Again we see a close correlation between the absorption spectra and the |χ(3)| spectra with some red shift of the former. The data of the modulus |χres(3)| and phase angles φres of films of oligomer series 1 and 2 at the peaks of three-photon resonances are summarized in Table 1. As can be seen from the table, the modulus |χres(3)| increases strongly with both chain
10186 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Koynov et al.
Figure 4. Modulus of the third-order optical susceptibility |χ(3)| measured by THG with variable laser wavelengths λL (squares) and linear absorption spectra of diluted CHCl3 solutions of 1a (a) and 1b (b).
Figure 3. Modulus of the third-order optical susceptibility |χ(3)| measured by THG with variable laser wavelengths λL (squares) and linear absorption spectra of diluted CHCl3 solutions of 2a (a), 2b (b), and 2c (c).
length and donor strength. When considering these results, we should emphasize that in general, the measured values of |χres(3)| may include the contributions of both the OPE oligomers and the PS matrix. In our previous paper5 on the nonlinear optical properties of oligomers 3, we have described a simple way for separation of these contributions. The procedure was based on the following considerations: (i) As |χres(3)| is measured at the peak of the three-photon resonance of the OPEs, the contribution χOPE(3) is purely imaginary16,17 and therefore φOPE ) 90°. (ii) χPS(3) is nonresonant and real, because λL is much larger than 3 λmax of PS, which implies φPS ) 0°. With this in mind we had calculated |χres,OPE(3)| using eq 3
|χres,OPE(3)| ) Im(|χres(3)|exp(iφres))
(3)
In the case of the donor-acceptor OPEs 1 and 2, however, the situation is more complicated. Because of the strong influence of the push-pull effect, these oligomers show rather complex absorption and χ(3)(-3ω,ω,ω,ω) spectra, which in some cases feature two distinct resonances. In this situation, validity of (i) is not evident. On the opposite, as can be seen in Table 1, the values of φres differ significantly for the different oligomers. Therefore, we have performed additional THG experiments with
pure PS films in order to determine its influence. We found that for the typical laser intensities used in our experiments, the third harmonic signal generated from the PS films is very small and could not be separated from those of the silica substrate. We conclude that for all studied oligomers from the series 1 and 2 the PS matrix contribution to |χres(3)| can be neglected. Such finding could appear surprising because in our previous THG experiments5 with the OPEs 3, we found an influence of the PS matrix on the measured |χres(3)| of the films from shorter oligomers 3a and 3b. This discrepancy, however, is a consequence of the fact (as we will show later) that the push-pull series 1 and 2 exhibit significantly bigger optical nonlinearities than unsubstituted 3. To understand the influence of chain length and donoracceptor substitution on the nonlinear optical properties, it is better to consider the second hyperpolarizability γ, which is a molecular parameter, instead of the macroscopic susceptibility χ(3). In the cgs system of units, these quantities are related by eq 4 where N is the number of oligomers per unit volume17
χ(3)(-3ω,ω,ω,ω) ) Nf(3ω)f(ω)3γ(-3ω,ω,ω,ω)
(4)
We have calculated N as in eq 5 where NA is the Avogadro constant, MOPE and cOPE are the molecular mass and the concentration by weight of the respective OPE compound, and F is the density of the film
N ) NAFcOPE/MOPE
(5)
The dimensionless Lorentz local field factors f(ω) depend on the refractive index n(ω) and are given by17
f(ω) ) (n2(ω) + 2)/3
(6)
Using eqs 4-6 we have calculated |γres| of the studied OPEs from the corresponding third-order susceptibilities |χres(3)|. The
Nonlinear Optical Properties of OPEs
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10187
TABLE 2: Hyperpolarizabilities |γres| Obtained for the OPEs 1a,b (n ) 1 and 2), 2a-c (n ) 1-3), and 3a-e (n ) 1-5) and Lengths L of the Corresponding Conjugated Chains n |γres| [10 esu] |γres| [10-32 esu] |γres| [10-32 esu] L [nm] -32
1 2 3 a
1 3.00 1.13 0.13 0.94
a
2
3
4
5
5.10 2.80 a 0.81 1.60
2.88 2.04 2.26
3.93 2.92
5.47 3.58
Corresponds to the longer wavelength resonance of |χres(3)| in Table
1.
ability |γres| saturate to values similar to those of the OPEs 3a-e without donor-acceptor contribution. Similar results showing strong enhancement of γ with the increase of DA strength were reported for the series of pushpull diphenylpolyenes.20 In the case of diphenylpolyenes however, no diminishment of the donor-acceptor contribution with the increase of the chain length was found up to 5 repeat units. This is not in contradiction with our findings as the repeat unit of the OPEs is significantly longer than that of the polyenes. Summary and Conclusions Nonlinear optical properties of the donor-acceptor substituted OPEs 1 and the purely donor substituted systems 2 were studied by means of third harmonic generation spectroscopy. The second molecular hyperpolarizability |γres| of the oligomers was evaluated and compared with that of (2,5-dipropoxy-1,4-phenyleneethynylene)s 3. The donor-acceptor substitution leads to significant enhancement of the |γres| of the short oligomeres (with n ) 1 and 2) caused by the push-pull effect. However, with the increase of oligomer length, the influence of this effect decreases and molecular susceptibility approaches that of the OPEs without donor-acceptor substitution.
Figure 5. Double logarithmic plot of molecular hyperpolarizability |γres| of oligomers 1 (triangles), 2 (open squares), and 3 (circles), versus chain length L. The dashed line represents an increase from 3a to 3b according to a power law γres ∼ L3‚3.
values of |γres| for the oligomer series 1, 2 are listed in Table 2, and their dependence on chain length L is plotted in Figure 5. For comparison, the values of |γres| for the 2,5-dipropoxy substituted OPEs5 (3) are also shown in the figure. These data reveal a superlinear increase of |γres| with L, which has similarly been observed in the cases of other conjugated oligomers such as oligophenylenes,25 oligothiophenes,26,27 and oligo-p-phenylenevinylenes.16 The dashed line shown in the figure represents an increase from 3a to 3b according to a power law γres ∼ L3.3. It is important to remember at this point that the chain length L used for plotting Figure 5 was defined as the distance between the terminal carbon atoms of the benzene rings (see eq 1). Thus, we have assumed the same values of L for oligomers with the same number of repeat units, even if they have different terminal substitution, i.e., belong to different series 1, 2, 3. Although some extension of the conjugation over the N-C and O-N-C bonds of the terminal groups is certainly possible, our assumption is justified from the fact that these bonds are rather short and their contribution to conjugation length is not precisely known. As can be seen from Figure 5, the donor-acceptor substitution has a major impact on the molecular hyperpolarizability, especially for the shorter oligomers. For example, the |γres| value grows with the increase of the D-A strength from 0.13 × 10-32 esu for 3a to 1.13 × 10-32 esu for 2a and 3.0 × 10-32 esu for 1a. For the longer oligomers, however, the growth of |γres| caused by donor-acceptor substitution is not so significant. This behavior can be interpreted in the following way: for small oligomer numbers n ) 1 and 2, where the distance between donor and acceptor group is also small, the -push-pull effect leads to a strong increase of the dipole strength and consequently to an increase of |γres|, even when λmax decreases! For higher oligomer numbers n however, the influence of the donoracceptor substitution decreases as the distance between these groups increases. Consequently, the molecular hyperpolariz-
Acknowledgment. We thank G. Herrmann and W. Scholdei for the technical help with film preparation and optical spectroscopy, respectively. K.K. acknowledges the financial support of the European Community through a Marie Curie Fellowship. References and Notes (1) Selected books and review articles: (a) Salaneck, W. R.; Lundstro¨m, I.; Rånby, B. R. Conjugated Polymers and Related Materials; Oxford University Press: Oxford, U.K., 1993. (b) Tour, J. M. Chem. ReV. 1996, 96, 537-553; c) Moore, J. S. Acc. Chem. Res. 1997, 30, 402-413. (d) Roncali, J. Chem. ReV. 1997, 97, 173-205. (e) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 110, 416-443; Angew. Chem. Int. Ed. 1998, 37, 403-428. (f) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: Weinheim, Germany, 1998. (g) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207. (h) Diederich, F.; Gobbi, L. Top. Curr. Chem. 1999, 201, 43-79. (i) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999, 99, 1863-1933. (j) Scherf, U. Top. Curr. Chem. 1999, 201, 163-222. (k) Martin, R. E.; Diederich, F. Angew. Chem. 1999, 111, 1440-1469; Angew. Chem., Int. Ed. Engl. 1999, 38, 1350-1377. (l) Bunz, U. H. F. Top. Curr. Chem. 1999, 201, 131-161. (m) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644. (n) Segura, J. L.; Martin, N. J. Mater. Chem. 2000, 10, 2403-2435. (o) Hadziioannou, G.; van Hutten, P. F. SemiconductiVity Polymers; Wiley-VCH: Weinheim, Germany, 2000. (p) Roncali, J. Acc. Chem. Res. 2000, 33, 147-156. (q) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804. (r) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B. Chem. ReV. 2000, 100, 1973-2011. (s) Bunz, U. H. F. Acc. Chem. Res. 2001, 34, 998-1010. (t) Szafert, S.; Gladysz, J. A. Chem. ReV. 2003, 103, 4175-4205. (u) Meier, H. Angew. Chem. 2005, in print; Angew. Chem., Int. Ed. 2005, in print. (2) See for example refs 1f and 1k. (3) Stalmach, U.; Kolshorn, H.; Brehm, I.; Meier, H. Liebigs Ann. 1996, 1449-1456. (4) Meier, H.; Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379384 and references therein. (5) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar, A.; Bubeck, C. Eur. J. Org. Chem. 2001, 4431-4443. (6) Meier, H.; Ickenroth, D. Eur. J. Org. Chem. 2002, 1745-1749. (7) Ickenroth, D.; Weissmann, S.; Rumpf, N.; Meier, H. Eur. J. Org. Chem. 2002, 2808-2814. (8) Meier, H.; Gerold, J.; Kolshorn, H.; Mu¨hling, B. Chem. Eur. J. 2004, 10, 360-370. (9) Meier, H.; Gerold, J.; Jacob, D. Tetrahedron Lett. 2003, 44, 19151918. (10) Meier, H.; Petermann, R.; Gerold, J. Chem. Commun. 1999, 977978.
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