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Ferrocenyl donor–organic acceptor complexes for second order nonlinear optics
Chemical Physics 265 (2001) 313±322
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Ferrocenyl donor±organic acceptor complexes for second order nonlinear optics A. Krishnan, S.K. Pal, P. Nandakumar, A.G. Samuelson 1, P.K. Das * Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India Received 14 November 2000; in ®nal form 17 February 2001
Abstract First hyperpolarizability (b) of a set of donor±acceptor complexes derived by functionalizing two organic acceptors with a donor such as ferrocene or an aromatic ring through a ±C@N± linkage has been measured by hyper-Rayleigh scattering in solution. Spectral analysis of the second harmonic scattered light reveals that there is a signi®cant twophoton ¯uorescence (TPF) contribution to the total signal in some compounds. In fact, the TPF contribution was as high as 25% in two compounds. The measured b values after correcting for TPF indicate that these molecules possess high second order nonlinearity, much larger than similar organic compounds or charge transfer complexes formed by ferrocenyl donors and organic acceptors. The dispersion free b values of these complexes have been calculated using two dierent methods depending on the nature of the complex. The conventional nonresonant two-state model which gives a good estimate of the intrinsic second order polarizability away from resonance, has been applied to compounds with no absorption around the second harmonic wavelength. Another model in which a damping parameter related to the inhomogeneous line width of the absorption band near the second harmonic frequency is used for obtaining reliable values of b0 , has been employed for the other compounds. In conjugation with anthraquinone as acceptor, ferrocene as a donor appears to be a better choice for quadratic nonlinear optics applications than conventional organic donors. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Organic donor±acceptor molecules with a pbridge in-between are widely recognized as potential materials for nonlinear optics (NLO) applications [1,2]. Like organic molecules, organometallic complexes [3,4] have also been intensely
*
Corresponding author. Fax: +91-80-360-1552/360-0683. E-mail address: [email protected] (P.K. Das). 1 Also corresponding author.
investigated in recent times for their NLO property since they can exhibit large NLO response, fast response time, and ease of fabrication and integration into composite materials. In order to investigate organometallic chromophores for their photonics application, their accurate molecular ®rst hyperpolarizability (b) needs to be measured. The recently developed hyper-Rayleigh scattering (HRS) [5±8] technique has been increasingly used for the determination of molecular b in solution. In HRS, light scattered at the second harmonic wavelength due to ¯uctuations in the molecular dipoles is monitored as a function of chromophore concentration. Although this technique is simple
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to use and has been employed to study the quadratic polarizability of a variety of molecules, it has recently been pointed out that the presence of absorption bands around the second harmonic wavelength region can hamper an accurate determination of b [9,10]. In such compounds, part of the second harmonic scattered light can originate from multiphoton and primarily two-photon ¯uorescence (TPF). This may result in an overestimation of b. Most of the HRS experiments make use of an interference ®lter to detect the second harmonic light from the background scattering. But ¯uorescence at the same wavelength of that of the second harmonic light cannot be completely eliminated by a ®lter. In such experiments measured b tends to be larger than actual. Flipse et al. [9] reported two-photon absorption followed by ¯uorescence in a series of donor±acceptor p-conjugated organic compounds including 4-dimethylamino-40 -nitrostilbene (DANS) and 4methoxy-40 -nitrostilbene (MONS) at the fundamental wavelength of 1064 nm which was used as the incident wavelength for second harmonic generation (SHG). They recommended that HRS at 1064 nm not be used to measure the ®rst hyperpolarizability of compounds which show TPF at this wavelength. The TPF contribution to the HRS signal can be eliminated in a variety of ways. Song et al. [11] have examined the TPF contribution to b of several well known organic NLO chromophores such as para-methoxy nitro stilbene, MONS and para-diethylaminob-nitro styrene (PDEANS) using HRS and have used a monochromator to discriminate TPF from scattering. Schmalzin et al. [13] have used a ¯uorescence quencher to suppress the TPF at the second harmonic wavelength. Using a femtosecond laser pulse for excitation the immediate two-photon scattering (HRS) has been separated from the time-delayed ¯uorescence by Noordman and van Hulst [14]. In the frequency domain Olbrechts et al. [15] have used a phase modulation technique to achieve separation of TPF from HRS. Using the same technique they have demonstrated that in subphthalocyanines approximately 90% signal at the second harmonic wavelength of the incident light beam (1300 nm) comes from multiphoton ¯uorescence [16]. In fact, at 1064 nm which is the most commonly used
wavelength for HRS measurements, TPF is observed in a variety of molecules/systems such as organic dyes [12], donor±acceptor organics [17], and aromatic amino acids [18]. Hence it is imperative that a careful analysis of the spectral characteristics and intensity dependence of the HRS signal is carried out ®rst to determine the TPF contribution to the measured second harmonic signal. Song et al. [12] have demonstrated that the second harmonic light as well as the TPF in Disperse Red 19 can be ®tted to Gaussian functions and the b corrected. In this paper, we have adopted a procedure similar to theirs except that we have ®tted the second harmonic signal to a Lorentzian function as suggested earlier [19,20] and the broad TPF signal to a Gaussian function [12]. This ®tting procedure is simple and has been discussed in the text. Here, we have synthesized a new series of ferrocene donor±organic acceptor complexes and studied their second harmonic response in solution. Recent powder SHG measurements on Schi-base complexes of ferrocene linked to substituted benzene acceptors [21] [e.g., (C5 H5 )Fe (C5 H4 )±CH@N±C6 H4 ±X (X NO2 , F, Br)] have shown that their SHG eciency is comparable to that of urea (0.33 U). Lehn et al. [22] reported very large quadratic hyperpolarizability in compounds where ferrocene has been used as an electrondonor and a dicyanovinyl group as an acceptor at the two ends of a polyene chain. Barlow et al. [23] have reported large b for metallocene-p-bridgeacceptor complexes and they have proposed a simple orbital model for explaining the observed nonlinearity. Several other donor±acceptor complexes have been prepared and their optical properties were studied either in solid or in solution. A series of heterobimetallic complexes with a ferrocenyl donor and a metal carbonyl moiety as an acceptor connected by a p-conjugated system were prepared [24] and their NLO responses have been measured by the HRS technique. All these complexes possess high second order nonlinearity. Among these the chromium carbonyl complex with the ferrocenyl ligand, Fc±(CH@CH)n ±C6 H5 [Cr(CO)3 ], with n 2 shows a b0 value of 164 10 30 esu.
A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
In the present study, two dierent organic acceptors, an anthraquinone or a dicyanomaleonitrile moiety have been linked through a Schi base to a ferrocene donor. This generates symmetrically substituted donor±acceptor±donor (D±A±D) or unsymmetrically substituted donor±acceptor (D± A) compounds. Symmetrically substituted D±A±D complexes, because of the symmetric nature of their charge distribution from the terminal positions of a conjugated system to the middle, are expected to have large two-photon absorption cross-section and consequently, substantial TPF quantum yield [17]. For comparison, we have synthesized similar organic compounds where the ferrocene donor has been replaced by a benzene ring. Using careful spectral analysis we have obtained TPF-free b of all the complexes and established the importance of such a correction in some of them. In addition, we have extracted the dispersion free hyperpolarizability, b0 of these complexes using a recently developed analysis procedure [25,26].
2. Experimental Fig. 1 shows the various compounds studied in this work. All the compounds were synthesized in nitrogen atmosphere using standard Schlenk techniques [27]. The compounds were puri®ed by chromatography and characterized by IR and NMR spectroscopy. Solvents were obtained commercially, and dried and degassed by standard methods before use. The UV±VIS spectra of the compounds were recorded with freshly prepared solutions in chloroform in a Hitachi U-3000 spectrometer. The emission spectra of the compounds were recorded in a Hitachi F-2000 spectrometer. 2.1. Hyper-Rayleigh measurements First hyperpolarizability of all the molecules was determined by the HRS technique using the external reference method [28]. Experiments were carried out in chloroform employing the fundamental wavelength (1064 nm) of a Q-switched Nd:YAG laser (Spectra Physics, 10 Hz, 8 ns). All
315
data were collected at laser powers 6 24 mJ/pulse. The experimental setup used for the HRS measurements is similar to our previous description [29]. The exciting beam is focussed by a biconvex lens (f.l. 10 cm) to a spot 5 cm away after passing through the glass cell containing the sample. Scattered light in the perpendicular direction is collected by a UV±VIS photomultiplier tube (PMT). A high through-put monochromator (Czerny Turner, 0.25 m) was used for wavelength dispersion and no other collection optics was employed. The resolution of the monochromator was 0.4 nm and both the entrance and exit slit-widths were 1.25 mm. The monochromator was scanned at 2 nm intervals and at each wavelength the signal output from the PMT was averaged over 400 laser shots. The input power was monitored using a power meter. First, para-nitroaniline (pNA) was studied in chloroform and the b value found is 17:4 0:6 10 30 esu by the external reference method [28] using pNA in dioxane (b 16:9 0:4 10 30 esu) as the reference. HRS measurements were then carried out ®rst on PDEANS and then on 1a±4b. The b value after correcting for TPF was obtained by the ®tting procedure discussed in the text. Concentrations were kept at 6 10 5 M and absorption of the second harmonic light was not a problem at such low concentrations [29]. 3. Results and discussion PDEANS has an absorption maximum around 451 nm and one photon excitation at this wavelength results in an emission band with a maximum at 574 nm (data not shown). The UV±VIS spectra of all the ferrocenyl compounds in chloroform are shown in Fig. 2. Two characteristic bands in the near ultraviolet and visible region: one around 350 nm and another at 550 nm are seen. Both bands originate from the ferrocene center. Another high energy band below 300 nm is also seen and it has its origin with the acceptors. The band around 350 nm is centered on the cyclopentadienyl moiety and corresponds to a p±p transition. The longer wavelength band at 550 nm is due to the metal-to-ligand charge transfer
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Fig. 1. Structures of compounds 1a±4b.
(MLCT). A red shift of this band is observed when stronger acceptors are attached to the ferrocene. For the dicyanoethylene acceptor, the MLCT band shifts to longer wavelength compared to the anthraquinone substituted compound. Further addition of a ferrocenyl or a phenyl group to 3 causes the MLCT band to shift about 80 nm to the red due to an increase in the conjugation length as in 4 (Table 1). No single photon ¯uorescence is detected from these compounds upon excitation
either at the maxima of the MLCT band or at 532 nm. The HRS experiments on PDEANS were carried out in methanol. The HRS spectrum of PDEANS recorded in the region 500±800 nm is shown in Fig. 3. The spectrum shows a sharp peak at 532 nm and a broad band at 575 nm. The broad band perhaps results from TPF excited by two photons at 1064 nm as reported previously by Song et al. [11]. This band is pronounced in solu-
A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
317
tions having larger number densities of the solute. We have analyzed the dierent bands seen in the spectra using a nonlinear curve-®tting software. Previously, Song et al. [12] have used Gaussians to ®t the HRS as well as TPF spectra, we have used a Lorentzian line shape function of the type L x
a0 1
f x
a1 =a2 g
2
;
1
where a0 is the amplitude, a1 is the peak center and a2 is the peak width at half maximum (FWHM), for the HRS peak. The basis for this choice is that the spectral line shape of elastic harmonic light scattering is best described by a Lorentzian [19,20]. The TPF spectrum is ®tted to a single or multiple Gaussians (wherever necessary) in our case. The spectra of PDEANS was ®tted to a Lorentzian and two Gaussian functions. The Gaussian line shape function used is of the form G x a0 expf 0:5 x
2
a1 =a2 g;
2
where a0 is the amplitude, a1 is the center, and a2 is the FWHM of the Gaussian function. The addition of the line shape functions reproduced the experimental spectrum. 2 For the ferrocenyl compounds, a single TPF was observed, and therefore, a Lorentzian and a Gaussian were adequate to ®t the experimental spectra (Fig. 4). All the compounds show a second harmonic Rayleigh peak at 532 nm followed by a broad ¯uorescence band centered around 550 nm. This ¯uorescence band, perhaps, has its origin in the two-photon absorption in the MLCT band of the ferrocenyl complex. After separation of the HRS and TPF spectra, the Lorentzian peak height at 532 nm was considered as the TPF-free HRS response. This was taken directly proportional to I2x . A plot of log(I2x ) vs. log(Ix ) was made, and a slope of 1.8 obtained for PDEANS (data not shown) indicating that the 2
The HRS peak from PDEANS were also ®t to a single Gaussian and the TPF spectra to two Gaussians using the procedure adopted by Song et al. The residuals obtained for the Lorentzian±Gaussian (LG) combination ®tting varied in between 0.01 and 0.02, and that of the Gaussian±Gaussian (GG) combination ®tting were 0.06±0.1. Thus, for the complexes we have chosen the LG ®tting procedure.
Fig. 2. UV±VIS spectra of compounds 1b±4b.
signal was due to a two-photon process. The quadratic dependence of the TPF band on the incident intensity was also veri®ed (slope 1:86). Then the ®rst hyperpolarizability of the ferrocenyl compounds were obtained in chloroform. The plot of I2x =Ix2 vs. concentration for the ferrocenyl complexes is shown in Fig. 5. The b values are listed in Table 1. These b values are corrected for TPF and are readily obtained from the analysis of the experimental spectra. The TPF corrected b for PDEANS using our method of analysis is 780 45 10 30 esu. The result is in good agreement with the value 744 10 30 esu obtained by Song et al. [11]. In Table 1 we have also listed the uncorrected b, by taking the total signal at the second harmonic wavelength 532 nm without accounting for TPF. For compounds 3b and 4b the TPF correction is 25% which is signi®cant. The static hyperpolarizability, b0 which is the intrinsic molecular property was also calculated for our experimental conditions using the two-state model by [30]. b=b0 x0 4 = x20
x2 x20
4x2 ;
3
where x0 is the single photon absorption maximum of the molecule in wavenumbers, and x is the laser fundamental. Eq. (3) is valid strictly in the o-resonance limit i.e., at 2x x0 . Application of this equation in other regions leads to over-correction of the dispersion eects. Therefore, in the resonance region a damping parameter (c) is
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Table 1 Experimental hyperpolarizability of compounds 1a±4b
1a 1b 2a 2b 3a 3b 4a 4b
kmax (nm)
buncorr 10
414 495 360 498 357 508 385 587
132 972 108 913 81 597 64 717
30
(esu)
b 10 132 939 108 889 81 476 64 576
Fig. 3. HRS spectrum of compound PDEANS in methanol: ( ) experimental data points; ( ) Lorentzian ®t to the HRS peak; (- - -) Gaussian ®t to the TPF peak; and (Ð) sum of the peaks (total intensity).
introduced and a more general expression of b=b0 valid in both the regions is obtained [26] b=b0 x20 =3f1= x0 ic 2x x0 ic x 1= x0 ic 2x x0 ic x 1= x0 ic x x0
ic
xg:
4
This parameter, c should be taken as the homogeneous line width of a single molecule absorption pro®le. However, traditionally it has been taken as the half width of the broad spectral transition in solution. First Otomo et al. [25] and later Berkovic et al. [26] have suggested that the absorption band
30
(esu)
% correction buncorr b=b 100%
b0 10
± 3.5 ± 2.7 ± 25 ± 24.4
44 90 52 45 39 40 26 52
30
(esu)
Fig. 4. HRS spectrum of compound 3b in chloroform: ( ) experimental data points; ( ) Lorentzian ®t to the HRS peak; (- - -) Gaussian ®t to the TPF peak; and (±±) sum of the peaks (total intensity).
in the second harmonic region could be treated as a inhomogeneously broadened line shape (Gaussian) arising from the absorption of a large number of molecules in solution. The solution absorption pro®le is thus a convolution of molecular peaks of individual half width c and b is given by a weighted summation of bÕs provided by individual molecules. For a Gaussian convolution, a peak at x0 y has a weight factor of exp y 2 =G2 , where G is the Gaussian width. Therefore, the ratio of b=b0 is written as [26] Z 1 2 b=b0 x0 =3 1=p1=2 G 1
exp y 2 =G2 F x0 y dy;
5
A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
Fig. 5. I2x =Ix2 vs. number density of 1±4b.
where F x0 1= x0 ic 2x x0 ic x 1= x0
ic
2x x0
1= x0 ic x x0
ic ic
x x:
6
319
symmetrically and unsymmetrically substituted compounds from the above analysis. Albota et al. [17] have reported that symmetric charge distribution within a molecule leads to larger TPA cross-section. TPF is proportional to TPA and the ¯uorescence quantum yield. The percent contribution of TPF in 2 and 4 has been listed in Table 1. Although the TPA cross-sections may be larger for the quasi-symmetric molecules, their ¯uorescence quantum yield at 532 nm appears to be much smaller. Furthermore, in solution compounds 2 and 4 may not exist in centro-symmetric conformers only. Their large b values clearly indicate the presence of noncentrosymmetric conformers in solution. To understand this, we have examined the crystallographically characterized Schi base complexes using the Cambridge Structural Data Base [31,32]. In ferrocenyl Schi bases derived from ferrocene carboxaldehyde, having no additional constraints, an average twist of 3° is observed about the azomethine double bond as measured by the torsion angle /1 (Fig. 6). The plane of the cyclopentadienyl ring of the ferrocene is at an angle of 9° (average) to the azomethine plane (/2 ).
The integration in Eq. (5) can be carried out numerically and is not very sensitive to the value of c. Evaluating Eq. (5) for maximum enhancement, according to Berkovic et al. [26] one obtains jb=b0 j2xx0 2=3 p ln21=2 x0 =HWHM:
7
The enhancement of b (relative to b0 ) at the twophoton resonance frequency is approximately equal to the ratio of the excited state energy (x0 ) divided by its half width at half maximum (HWHM). For the ferrocenyl compounds the b0 values calculated by Eq. (7) using a single Gaussian to ®t the absorption band at 550 nm and obtaining HWHM from the ®t, are very high. They are listed in Table 1. The corresponding organic compounds 1a±4a have no absorption in the two-photon region and hence their b0 values have been calculated by the two-state model using Eq. (3). We hoped to estimate the relative contribution of TPF in the second harmonic signal in
Fig. 6. Torsion angles used to calculate deviations from planarity around the imino bond.
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A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
However the average twist phenyl group from the plane of the double bond as measured by /3 is 54° (average). Nonbonded interactions between C±H on the azomethine bond, the lone pair of electrons on the azomethine N, and the Cp or the phenyl group lead to nonplanar structures and restricted rotation about the single bonds (C2 C3 and NC4 ). In the corresponding aromatic Schi bases, the average deviations of the aromatic rings from planarity appear to be about 17° for rings attached to the carbon on the imino bond and 34° for the rings attached to the nitrogen. While the solid state structures could be biased due to noncovalent interactions in the lattice, they are in general indicative of the conformations preferred by the molecule. The all aromatic compounds 1a±4a, are thus expected to be much more nonplanar than the ferrocene substituted compounds, 1b±4b. The TPF contribution to second harmonic scattering in these molecules is understandable based on the above ®ndings. The TPF correction is negligible in the all organic compounds 1a±4a since they are highly nonplanar and the charge distribution over the entire molecule is asymmetric. However, in systems containing a ferrocene center, 1b±4b, the opposite is expected and TPF is likely to be signi®cant. Among the ferrocene containing compounds, TPF is much larger when the central acceptor is dicyanomaleonitrile (25% of the measured SH signal comes from the TPF contribution) but is only 3% when anthraquinone is used as an acceptor. Based on TPF alone one can suggest that in the substituted anthraquinone, the twist across the azomethine bond is larger. This, in turn, would lead to more unsymmetric structures and consequently smaller TPA. The series 1b±4b has nearly 6±9 times larger b than the corresponding phenyl substituted systems 1a±4a. Consistent increase in the measured b is achieved through ferrocene substitution. The reason for large b in ferrocene containing systems is that the absorption band lies very close to 2x leading to dispersion. In fact, it has been suggested that polyene spacers are better than aromatic spacers for enhancing b [33]. Hence, considering the presence of the ferrocene donor and the polyene spacer, compounds 3b and 4b rather than 1b
should have large b. One possible reason for the smaller b observed in the case of 3b and 4b could rest with the symmetric charge distributions in these molecules in the ground state. Apart from the TPF correction to b, dispersion correction is also necessary to gauge the intrinsic molecular hyperpolarizability of a compound. Dispersion correction using the two-state model is satisfactory only if the absorption band is away from the second harmonic frequency. In spite of this limitation, a good estimate of b0 has often been obtained using this model. For the all organic compounds 1a±4a where the absorption band is far away from 532 nm, we have calculated b0 using this model. However, this model is not applicable to ferrocenyl compounds since they have the MLCT band in the second harmonic region. In 1b±4b we have corrected for dispersion using the recently proposed method of Berkovic et al. [26] described earlier. All the b0 values are listed in Table 1. Based on our earlier discussion, it is understandable that compound 1b has the largest value of b0 . In spite of having an aromatic spacer, compound 1b has the largest b0 in this series. Higher values might be achievable by replacement of the anthraquinone spacer with poly-ene-one linkers. It is also interesting to compare CT complexes formed by the quinone and cyano acceptors that have been used as linkers in these metallocene based systems. The b measured in the charge transfer complexes formed with quinones and tetracyano-ethylene is considerably less [34]. This clearly shows that a covalent linkage of acceptors to donors is more eective in inducing large hyperpolarizability. 4. Conclusion In this paper, two organic acceptors have been functionalized with a ferrocene donor or an organic donor and their microscopic second order nonlinearity studied by HRS in solution. TPF contribution to the second harmonic scattered light has been eliminated by introducing a monochromator in the signal collection pathway and by ®tting the data to a combination of Lorentzian
A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
and Gaussian line shape functions. Dispersion free b0 has been obtained using the analysis procedure suggested by Berkovic et al. The ferrocence substituted compounds show high values of b as well as b0 . The importance of TPF correction is emphasized in ferrocence donor dicyanomaleonitrile acceptor based systems where 25% contribution to signal collected at the second harmonic wavelength, that is, 532 nm in our experiments, comes from two-photon induced ¯uorescence. However, in the anthraquinone acceptor based systems this correction is not important and the ®rst hyperpolarizability can be obtained by using a narrow bandwidth cuto ®lter at the second harmonic wavelength. The corresponding all organic molecules have signi®cantly lower b0 than the organometallic complexes. The superiority of ferrocene appears to result from the greater degree of delocalisation it imposes in comparison with conventional aromatic donors. In comparison with CT complexes, formed by these ferrocence complexes with organic acceptors, the covalently linked systems are superior in their second order NLO property. Future eorts with ole®nic spacers should result in better materials. Acknowledgements The research described here were supported by grants from All India Council for Technical Education and the Council of Scienti®c and Industrial Research, Govt. of India. We thank G. Berkovic for many helpful discussions. We also thank the Chairman, Bioinformatics Center, Indian Institute of Science, Bangalore, India for access to the CSD. References [1] D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, vols. I and II, Academic Press, Orlando, FL, 1987. [2] S.R. Marder, G.D. Stucky (Eds.), Materials for Nonlinear Optics, Chemical Perspectives, Am. Chem. Soc., Washington, DC, 1991.
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Ferrocenyl donor±organic acceptor complexes for second order nonlinear optics A. Krishnan, S.K. Pal, P. Nandakumar, A.G. Samuelson 1, P.K. Das * Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India Received 14 November 2000; in ®nal form 17 February 2001
Abstract First hyperpolarizability (b) of a set of donor±acceptor complexes derived by functionalizing two organic acceptors with a donor such as ferrocene or an aromatic ring through a ±C@N± linkage has been measured by hyper-Rayleigh scattering in solution. Spectral analysis of the second harmonic scattered light reveals that there is a signi®cant twophoton ¯uorescence (TPF) contribution to the total signal in some compounds. In fact, the TPF contribution was as high as 25% in two compounds. The measured b values after correcting for TPF indicate that these molecules possess high second order nonlinearity, much larger than similar organic compounds or charge transfer complexes formed by ferrocenyl donors and organic acceptors. The dispersion free b values of these complexes have been calculated using two dierent methods depending on the nature of the complex. The conventional nonresonant two-state model which gives a good estimate of the intrinsic second order polarizability away from resonance, has been applied to compounds with no absorption around the second harmonic wavelength. Another model in which a damping parameter related to the inhomogeneous line width of the absorption band near the second harmonic frequency is used for obtaining reliable values of b0 , has been employed for the other compounds. In conjugation with anthraquinone as acceptor, ferrocene as a donor appears to be a better choice for quadratic nonlinear optics applications than conventional organic donors. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Organic donor±acceptor molecules with a pbridge in-between are widely recognized as potential materials for nonlinear optics (NLO) applications [1,2]. Like organic molecules, organometallic complexes [3,4] have also been intensely
*
Corresponding author. Fax: +91-80-360-1552/360-0683. E-mail address: [email protected] (P.K. Das). 1 Also corresponding author.
investigated in recent times for their NLO property since they can exhibit large NLO response, fast response time, and ease of fabrication and integration into composite materials. In order to investigate organometallic chromophores for their photonics application, their accurate molecular ®rst hyperpolarizability (b) needs to be measured. The recently developed hyper-Rayleigh scattering (HRS) [5±8] technique has been increasingly used for the determination of molecular b in solution. In HRS, light scattered at the second harmonic wavelength due to ¯uctuations in the molecular dipoles is monitored as a function of chromophore concentration. Although this technique is simple
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to use and has been employed to study the quadratic polarizability of a variety of molecules, it has recently been pointed out that the presence of absorption bands around the second harmonic wavelength region can hamper an accurate determination of b [9,10]. In such compounds, part of the second harmonic scattered light can originate from multiphoton and primarily two-photon ¯uorescence (TPF). This may result in an overestimation of b. Most of the HRS experiments make use of an interference ®lter to detect the second harmonic light from the background scattering. But ¯uorescence at the same wavelength of that of the second harmonic light cannot be completely eliminated by a ®lter. In such experiments measured b tends to be larger than actual. Flipse et al. [9] reported two-photon absorption followed by ¯uorescence in a series of donor±acceptor p-conjugated organic compounds including 4-dimethylamino-40 -nitrostilbene (DANS) and 4methoxy-40 -nitrostilbene (MONS) at the fundamental wavelength of 1064 nm which was used as the incident wavelength for second harmonic generation (SHG). They recommended that HRS at 1064 nm not be used to measure the ®rst hyperpolarizability of compounds which show TPF at this wavelength. The TPF contribution to the HRS signal can be eliminated in a variety of ways. Song et al. [11] have examined the TPF contribution to b of several well known organic NLO chromophores such as para-methoxy nitro stilbene, MONS and para-diethylaminob-nitro styrene (PDEANS) using HRS and have used a monochromator to discriminate TPF from scattering. Schmalzin et al. [13] have used a ¯uorescence quencher to suppress the TPF at the second harmonic wavelength. Using a femtosecond laser pulse for excitation the immediate two-photon scattering (HRS) has been separated from the time-delayed ¯uorescence by Noordman and van Hulst [14]. In the frequency domain Olbrechts et al. [15] have used a phase modulation technique to achieve separation of TPF from HRS. Using the same technique they have demonstrated that in subphthalocyanines approximately 90% signal at the second harmonic wavelength of the incident light beam (1300 nm) comes from multiphoton ¯uorescence [16]. In fact, at 1064 nm which is the most commonly used
wavelength for HRS measurements, TPF is observed in a variety of molecules/systems such as organic dyes [12], donor±acceptor organics [17], and aromatic amino acids [18]. Hence it is imperative that a careful analysis of the spectral characteristics and intensity dependence of the HRS signal is carried out ®rst to determine the TPF contribution to the measured second harmonic signal. Song et al. [12] have demonstrated that the second harmonic light as well as the TPF in Disperse Red 19 can be ®tted to Gaussian functions and the b corrected. In this paper, we have adopted a procedure similar to theirs except that we have ®tted the second harmonic signal to a Lorentzian function as suggested earlier [19,20] and the broad TPF signal to a Gaussian function [12]. This ®tting procedure is simple and has been discussed in the text. Here, we have synthesized a new series of ferrocene donor±organic acceptor complexes and studied their second harmonic response in solution. Recent powder SHG measurements on Schi-base complexes of ferrocene linked to substituted benzene acceptors [21] [e.g., (C5 H5 )Fe (C5 H4 )±CH@N±C6 H4 ±X (X NO2 , F, Br)] have shown that their SHG eciency is comparable to that of urea (0.33 U). Lehn et al. [22] reported very large quadratic hyperpolarizability in compounds where ferrocene has been used as an electrondonor and a dicyanovinyl group as an acceptor at the two ends of a polyene chain. Barlow et al. [23] have reported large b for metallocene-p-bridgeacceptor complexes and they have proposed a simple orbital model for explaining the observed nonlinearity. Several other donor±acceptor complexes have been prepared and their optical properties were studied either in solid or in solution. A series of heterobimetallic complexes with a ferrocenyl donor and a metal carbonyl moiety as an acceptor connected by a p-conjugated system were prepared [24] and their NLO responses have been measured by the HRS technique. All these complexes possess high second order nonlinearity. Among these the chromium carbonyl complex with the ferrocenyl ligand, Fc±(CH@CH)n ±C6 H5 [Cr(CO)3 ], with n 2 shows a b0 value of 164 10 30 esu.
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In the present study, two dierent organic acceptors, an anthraquinone or a dicyanomaleonitrile moiety have been linked through a Schi base to a ferrocene donor. This generates symmetrically substituted donor±acceptor±donor (D±A±D) or unsymmetrically substituted donor±acceptor (D± A) compounds. Symmetrically substituted D±A±D complexes, because of the symmetric nature of their charge distribution from the terminal positions of a conjugated system to the middle, are expected to have large two-photon absorption cross-section and consequently, substantial TPF quantum yield [17]. For comparison, we have synthesized similar organic compounds where the ferrocene donor has been replaced by a benzene ring. Using careful spectral analysis we have obtained TPF-free b of all the complexes and established the importance of such a correction in some of them. In addition, we have extracted the dispersion free hyperpolarizability, b0 of these complexes using a recently developed analysis procedure [25,26].
2. Experimental Fig. 1 shows the various compounds studied in this work. All the compounds were synthesized in nitrogen atmosphere using standard Schlenk techniques [27]. The compounds were puri®ed by chromatography and characterized by IR and NMR spectroscopy. Solvents were obtained commercially, and dried and degassed by standard methods before use. The UV±VIS spectra of the compounds were recorded with freshly prepared solutions in chloroform in a Hitachi U-3000 spectrometer. The emission spectra of the compounds were recorded in a Hitachi F-2000 spectrometer. 2.1. Hyper-Rayleigh measurements First hyperpolarizability of all the molecules was determined by the HRS technique using the external reference method [28]. Experiments were carried out in chloroform employing the fundamental wavelength (1064 nm) of a Q-switched Nd:YAG laser (Spectra Physics, 10 Hz, 8 ns). All
315
data were collected at laser powers 6 24 mJ/pulse. The experimental setup used for the HRS measurements is similar to our previous description [29]. The exciting beam is focussed by a biconvex lens (f.l. 10 cm) to a spot 5 cm away after passing through the glass cell containing the sample. Scattered light in the perpendicular direction is collected by a UV±VIS photomultiplier tube (PMT). A high through-put monochromator (Czerny Turner, 0.25 m) was used for wavelength dispersion and no other collection optics was employed. The resolution of the monochromator was 0.4 nm and both the entrance and exit slit-widths were 1.25 mm. The monochromator was scanned at 2 nm intervals and at each wavelength the signal output from the PMT was averaged over 400 laser shots. The input power was monitored using a power meter. First, para-nitroaniline (pNA) was studied in chloroform and the b value found is 17:4 0:6 10 30 esu by the external reference method [28] using pNA in dioxane (b 16:9 0:4 10 30 esu) as the reference. HRS measurements were then carried out ®rst on PDEANS and then on 1a±4b. The b value after correcting for TPF was obtained by the ®tting procedure discussed in the text. Concentrations were kept at 6 10 5 M and absorption of the second harmonic light was not a problem at such low concentrations [29]. 3. Results and discussion PDEANS has an absorption maximum around 451 nm and one photon excitation at this wavelength results in an emission band with a maximum at 574 nm (data not shown). The UV±VIS spectra of all the ferrocenyl compounds in chloroform are shown in Fig. 2. Two characteristic bands in the near ultraviolet and visible region: one around 350 nm and another at 550 nm are seen. Both bands originate from the ferrocene center. Another high energy band below 300 nm is also seen and it has its origin with the acceptors. The band around 350 nm is centered on the cyclopentadienyl moiety and corresponds to a p±p transition. The longer wavelength band at 550 nm is due to the metal-to-ligand charge transfer
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Fig. 1. Structures of compounds 1a±4b.
(MLCT). A red shift of this band is observed when stronger acceptors are attached to the ferrocene. For the dicyanoethylene acceptor, the MLCT band shifts to longer wavelength compared to the anthraquinone substituted compound. Further addition of a ferrocenyl or a phenyl group to 3 causes the MLCT band to shift about 80 nm to the red due to an increase in the conjugation length as in 4 (Table 1). No single photon ¯uorescence is detected from these compounds upon excitation
either at the maxima of the MLCT band or at 532 nm. The HRS experiments on PDEANS were carried out in methanol. The HRS spectrum of PDEANS recorded in the region 500±800 nm is shown in Fig. 3. The spectrum shows a sharp peak at 532 nm and a broad band at 575 nm. The broad band perhaps results from TPF excited by two photons at 1064 nm as reported previously by Song et al. [11]. This band is pronounced in solu-
A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
317
tions having larger number densities of the solute. We have analyzed the dierent bands seen in the spectra using a nonlinear curve-®tting software. Previously, Song et al. [12] have used Gaussians to ®t the HRS as well as TPF spectra, we have used a Lorentzian line shape function of the type L x
a0 1
f x
a1 =a2 g
2
;
1
where a0 is the amplitude, a1 is the peak center and a2 is the peak width at half maximum (FWHM), for the HRS peak. The basis for this choice is that the spectral line shape of elastic harmonic light scattering is best described by a Lorentzian [19,20]. The TPF spectrum is ®tted to a single or multiple Gaussians (wherever necessary) in our case. The spectra of PDEANS was ®tted to a Lorentzian and two Gaussian functions. The Gaussian line shape function used is of the form G x a0 expf 0:5 x
2
a1 =a2 g;
2
where a0 is the amplitude, a1 is the center, and a2 is the FWHM of the Gaussian function. The addition of the line shape functions reproduced the experimental spectrum. 2 For the ferrocenyl compounds, a single TPF was observed, and therefore, a Lorentzian and a Gaussian were adequate to ®t the experimental spectra (Fig. 4). All the compounds show a second harmonic Rayleigh peak at 532 nm followed by a broad ¯uorescence band centered around 550 nm. This ¯uorescence band, perhaps, has its origin in the two-photon absorption in the MLCT band of the ferrocenyl complex. After separation of the HRS and TPF spectra, the Lorentzian peak height at 532 nm was considered as the TPF-free HRS response. This was taken directly proportional to I2x . A plot of log(I2x ) vs. log(Ix ) was made, and a slope of 1.8 obtained for PDEANS (data not shown) indicating that the 2
The HRS peak from PDEANS were also ®t to a single Gaussian and the TPF spectra to two Gaussians using the procedure adopted by Song et al. The residuals obtained for the Lorentzian±Gaussian (LG) combination ®tting varied in between 0.01 and 0.02, and that of the Gaussian±Gaussian (GG) combination ®tting were 0.06±0.1. Thus, for the complexes we have chosen the LG ®tting procedure.
Fig. 2. UV±VIS spectra of compounds 1b±4b.
signal was due to a two-photon process. The quadratic dependence of the TPF band on the incident intensity was also veri®ed (slope 1:86). Then the ®rst hyperpolarizability of the ferrocenyl compounds were obtained in chloroform. The plot of I2x =Ix2 vs. concentration for the ferrocenyl complexes is shown in Fig. 5. The b values are listed in Table 1. These b values are corrected for TPF and are readily obtained from the analysis of the experimental spectra. The TPF corrected b for PDEANS using our method of analysis is 780 45 10 30 esu. The result is in good agreement with the value 744 10 30 esu obtained by Song et al. [11]. In Table 1 we have also listed the uncorrected b, by taking the total signal at the second harmonic wavelength 532 nm without accounting for TPF. For compounds 3b and 4b the TPF correction is 25% which is signi®cant. The static hyperpolarizability, b0 which is the intrinsic molecular property was also calculated for our experimental conditions using the two-state model by [30]. b=b0 x0 4 = x20
x2 x20
4x2 ;
3
where x0 is the single photon absorption maximum of the molecule in wavenumbers, and x is the laser fundamental. Eq. (3) is valid strictly in the o-resonance limit i.e., at 2x x0 . Application of this equation in other regions leads to over-correction of the dispersion eects. Therefore, in the resonance region a damping parameter (c) is
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Table 1 Experimental hyperpolarizability of compounds 1a±4b
1a 1b 2a 2b 3a 3b 4a 4b
kmax (nm)
buncorr 10
414 495 360 498 357 508 385 587
132 972 108 913 81 597 64 717
30
(esu)
b 10 132 939 108 889 81 476 64 576
Fig. 3. HRS spectrum of compound PDEANS in methanol: ( ) experimental data points; ( ) Lorentzian ®t to the HRS peak; (- - -) Gaussian ®t to the TPF peak; and (Ð) sum of the peaks (total intensity).
introduced and a more general expression of b=b0 valid in both the regions is obtained [26] b=b0 x20 =3f1= x0 ic 2x x0 ic x 1= x0 ic 2x x0 ic x 1= x0 ic x x0
ic
xg:
4
This parameter, c should be taken as the homogeneous line width of a single molecule absorption pro®le. However, traditionally it has been taken as the half width of the broad spectral transition in solution. First Otomo et al. [25] and later Berkovic et al. [26] have suggested that the absorption band
30
(esu)
% correction buncorr b=b 100%
b0 10
± 3.5 ± 2.7 ± 25 ± 24.4
44 90 52 45 39 40 26 52
30
(esu)
Fig. 4. HRS spectrum of compound 3b in chloroform: ( ) experimental data points; ( ) Lorentzian ®t to the HRS peak; (- - -) Gaussian ®t to the TPF peak; and (±±) sum of the peaks (total intensity).
in the second harmonic region could be treated as a inhomogeneously broadened line shape (Gaussian) arising from the absorption of a large number of molecules in solution. The solution absorption pro®le is thus a convolution of molecular peaks of individual half width c and b is given by a weighted summation of bÕs provided by individual molecules. For a Gaussian convolution, a peak at x0 y has a weight factor of exp y 2 =G2 , where G is the Gaussian width. Therefore, the ratio of b=b0 is written as [26] Z 1 2 b=b0 x0 =3 1=p1=2 G 1
exp y 2 =G2 F x0 y dy;
5
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Fig. 5. I2x =Ix2 vs. number density of 1±4b.
where F x0 1= x0 ic 2x x0 ic x 1= x0
ic
2x x0
1= x0 ic x x0
ic ic
x x:
6
319
symmetrically and unsymmetrically substituted compounds from the above analysis. Albota et al. [17] have reported that symmetric charge distribution within a molecule leads to larger TPA cross-section. TPF is proportional to TPA and the ¯uorescence quantum yield. The percent contribution of TPF in 2 and 4 has been listed in Table 1. Although the TPA cross-sections may be larger for the quasi-symmetric molecules, their ¯uorescence quantum yield at 532 nm appears to be much smaller. Furthermore, in solution compounds 2 and 4 may not exist in centro-symmetric conformers only. Their large b values clearly indicate the presence of noncentrosymmetric conformers in solution. To understand this, we have examined the crystallographically characterized Schi base complexes using the Cambridge Structural Data Base [31,32]. In ferrocenyl Schi bases derived from ferrocene carboxaldehyde, having no additional constraints, an average twist of 3° is observed about the azomethine double bond as measured by the torsion angle /1 (Fig. 6). The plane of the cyclopentadienyl ring of the ferrocene is at an angle of 9° (average) to the azomethine plane (/2 ).
The integration in Eq. (5) can be carried out numerically and is not very sensitive to the value of c. Evaluating Eq. (5) for maximum enhancement, according to Berkovic et al. [26] one obtains jb=b0 j2xx0 2=3 p ln21=2 x0 =HWHM:
7
The enhancement of b (relative to b0 ) at the twophoton resonance frequency is approximately equal to the ratio of the excited state energy (x0 ) divided by its half width at half maximum (HWHM). For the ferrocenyl compounds the b0 values calculated by Eq. (7) using a single Gaussian to ®t the absorption band at 550 nm and obtaining HWHM from the ®t, are very high. They are listed in Table 1. The corresponding organic compounds 1a±4a have no absorption in the two-photon region and hence their b0 values have been calculated by the two-state model using Eq. (3). We hoped to estimate the relative contribution of TPF in the second harmonic signal in
Fig. 6. Torsion angles used to calculate deviations from planarity around the imino bond.
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A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
However the average twist phenyl group from the plane of the double bond as measured by /3 is 54° (average). Nonbonded interactions between C±H on the azomethine bond, the lone pair of electrons on the azomethine N, and the Cp or the phenyl group lead to nonplanar structures and restricted rotation about the single bonds (C2 C3 and NC4 ). In the corresponding aromatic Schi bases, the average deviations of the aromatic rings from planarity appear to be about 17° for rings attached to the carbon on the imino bond and 34° for the rings attached to the nitrogen. While the solid state structures could be biased due to noncovalent interactions in the lattice, they are in general indicative of the conformations preferred by the molecule. The all aromatic compounds 1a±4a, are thus expected to be much more nonplanar than the ferrocene substituted compounds, 1b±4b. The TPF contribution to second harmonic scattering in these molecules is understandable based on the above ®ndings. The TPF correction is negligible in the all organic compounds 1a±4a since they are highly nonplanar and the charge distribution over the entire molecule is asymmetric. However, in systems containing a ferrocene center, 1b±4b, the opposite is expected and TPF is likely to be signi®cant. Among the ferrocene containing compounds, TPF is much larger when the central acceptor is dicyanomaleonitrile (25% of the measured SH signal comes from the TPF contribution) but is only 3% when anthraquinone is used as an acceptor. Based on TPF alone one can suggest that in the substituted anthraquinone, the twist across the azomethine bond is larger. This, in turn, would lead to more unsymmetric structures and consequently smaller TPA. The series 1b±4b has nearly 6±9 times larger b than the corresponding phenyl substituted systems 1a±4a. Consistent increase in the measured b is achieved through ferrocene substitution. The reason for large b in ferrocene containing systems is that the absorption band lies very close to 2x leading to dispersion. In fact, it has been suggested that polyene spacers are better than aromatic spacers for enhancing b [33]. Hence, considering the presence of the ferrocene donor and the polyene spacer, compounds 3b and 4b rather than 1b
should have large b. One possible reason for the smaller b observed in the case of 3b and 4b could rest with the symmetric charge distributions in these molecules in the ground state. Apart from the TPF correction to b, dispersion correction is also necessary to gauge the intrinsic molecular hyperpolarizability of a compound. Dispersion correction using the two-state model is satisfactory only if the absorption band is away from the second harmonic frequency. In spite of this limitation, a good estimate of b0 has often been obtained using this model. For the all organic compounds 1a±4a where the absorption band is far away from 532 nm, we have calculated b0 using this model. However, this model is not applicable to ferrocenyl compounds since they have the MLCT band in the second harmonic region. In 1b±4b we have corrected for dispersion using the recently proposed method of Berkovic et al. [26] described earlier. All the b0 values are listed in Table 1. Based on our earlier discussion, it is understandable that compound 1b has the largest value of b0 . In spite of having an aromatic spacer, compound 1b has the largest b0 in this series. Higher values might be achievable by replacement of the anthraquinone spacer with poly-ene-one linkers. It is also interesting to compare CT complexes formed by the quinone and cyano acceptors that have been used as linkers in these metallocene based systems. The b measured in the charge transfer complexes formed with quinones and tetracyano-ethylene is considerably less [34]. This clearly shows that a covalent linkage of acceptors to donors is more eective in inducing large hyperpolarizability. 4. Conclusion In this paper, two organic acceptors have been functionalized with a ferrocene donor or an organic donor and their microscopic second order nonlinearity studied by HRS in solution. TPF contribution to the second harmonic scattered light has been eliminated by introducing a monochromator in the signal collection pathway and by ®tting the data to a combination of Lorentzian
A. Krishnan et al. / Chemical Physics 265 (2001) 313±322
and Gaussian line shape functions. Dispersion free b0 has been obtained using the analysis procedure suggested by Berkovic et al. The ferrocence substituted compounds show high values of b as well as b0 . The importance of TPF correction is emphasized in ferrocence donor dicyanomaleonitrile acceptor based systems where 25% contribution to signal collected at the second harmonic wavelength, that is, 532 nm in our experiments, comes from two-photon induced ¯uorescence. However, in the anthraquinone acceptor based systems this correction is not important and the ®rst hyperpolarizability can be obtained by using a narrow bandwidth cuto ®lter at the second harmonic wavelength. The corresponding all organic molecules have signi®cantly lower b0 than the organometallic complexes. The superiority of ferrocene appears to result from the greater degree of delocalisation it imposes in comparison with conventional aromatic donors. In comparison with CT complexes, formed by these ferrocence complexes with organic acceptors, the covalently linked systems are superior in their second order NLO property. Future eorts with ole®nic spacers should result in better materials. Acknowledgements The research described here were supported by grants from All India Council for Technical Education and the Council of Scienti®c and Industrial Research, Govt. of India. We thank G. Berkovic for many helpful discussions. We also thank the Chairman, Bioinformatics Center, Indian Institute of Science, Bangalore, India for access to the CSD. References [1] D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, vols. I and II, Academic Press, Orlando, FL, 1987. [2] S.R. Marder, G.D. Stucky (Eds.), Materials for Nonlinear Optics, Chemical Perspectives, Am. Chem. Soc., Washington, DC, 1991.
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