Large Nonlinear Refraction and Higher Order Nonlinear Optical

J. Phys. Chem. B 2000, 104, 179-188

179

ARTICLES Large Nonlinear Refraction and Higher Order Nonlinear Optical Effects in a Novel Organic Dendrimer O. Varnavski,† A. Leanov,‡ L. Liu,‡ J. Takacs,‡ and T. Goodson, III*,† Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202, and Department of Chemistry, UniVersity of NebraskasLincoln, Lincoln, Nebraska 68588 ReceiVed: June 7, 1999; In Final Form: September 15, 1999

The fast electronic nonlinear optical (NLO) properties of a novel synthetic organic dendrimer are reported. The complete synthesis of the organic dendrimer is provided. Both the z-scan and self-phase modulation techniques were used to measure the nonlinear refractive index. The magnitude of the nonlinear refractive index (n2) was measured to be 1.1 × 10-4 cm2/GW. The presence of two-photon absorption was observed and the coefficient (β) was found to be 1.2 × 10-2 cm/GW. The nonlinear figure of merit of the dendrimer system is found to be 0.02 for intensities used in our experiments. The NLO responses of the individual structural components of the dendrimer molecule are provided to probe the structure-function relationships. The presence of a higher order, χ(5), NLO effect was also detected and analyzed. The fast electronic processes of the dendrimer molecule and its components where also investigated by up-conversion luminescence spectroscopy. These studies provide the first probe into the ultrafast electronic NLO effects in organic dendrimers.

Introduction Organic macromolecules have been the subject of great attention due to their potential applications in nonlinear optics (NLO), optical switching, photorefractive effects, optical limiters, and light emitting diodes.1-3 The large NLO susceptibilities exhibited in macromolecules such as conjugated polymers,4 molecular crystals,5 and fullerenes,6 as well as organic dendrimers,7 are closely connected with their extended π-electronic structures and in some cases their charge transfer properties.4 Indeed, the potential use of organic device materials in optoelectronics is now a very serious matter. The material’s suitability for device fabrication can in most cases be estimated by considering a figure of merit (FOM) detailing the ratio of the magnitude of the imaginary part or optical loss effect to that of the real part of the NLO effect.8 The FOM is usually given as (T ) 2βλ/n2) < 1).8 Organic polymers such as PTSPDA have been shown to have a FOM of ∼0.04 when measured at ∼1.6 µm, a wavelength far from the linear resonance.9 Other synthetic architectures such as novel organometallic systems have been shown to have a FOM of 0.07.10 While the FOM is a qualitative estimate of a material’s potential use in device fabrication, it can also be used as a comparison of the NLO effects between different materials at a specified wavelength. It should be noted that there have been no FOM results reported for organic dendrimer systems. Organic dendrimers are an example of a class of macromolecules constituting a group known as functional macromolecules. These systems have rather unusual physical as well as chemical * To whom all correspondence should be addressed. † Wayne State University. ‡ University of NebraskasLincoln.

properties, which are mostly due to their morphology.11-14 The multifunctional and amorphous nature of synthetic NLO dendrimers, the large chromophore density, and their relative ease of synthesis affords the opportunity to tailor make dendrimers with large dipole moments and molecular polarizabilities. However, the number of reports devoted to the NLO and photorefractive properties of dendrimers is a far fewer than the number of studies devoted to conjugated polymers.15,16 Sasabe et al.15 investigated a carbazole-containing NLO dendrimer for photorefractive effects and phase conjugation by four-wave mixing (a χ(3) dominated process).15 Carbazole-containing molecules are generally incorporated into photorefractive systems due to their relatively efficient charge transporting properties. The NLO carbazole-containing dendrimer in the work of Sasabe et al.15 showed large second NLO properties and good image reconstruction capabilities at low laser powers. This is an intriguing finding since it is an induced third-order NLO process (exhibited in the dendrimer) that actually governs the mechanism resulting in the reconstructed image seen by Sasabe et al.15 Although there have been other reports of NLO and photorefractive effects in organic carbazole-containing dendrimers, there have been no reports of the third-order fast electronic NLO properties using femtosecond pulses. Particularly, the role of carbazole has not been determined in the overall fast electronic NLO response of the dendrimer system. A better understanding of the role of the different dendrimer components in relation to the fast electronic NLO processes would identify critical interactions of NLO chromophores and charge transporting molecules within the same amorphous macromolecule. Detailed studies of a novel dendrimer system could be of great importance in broadening our understanding of structure-

10.1021/jp9918353 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/16/1999

180 J. Phys. Chem. B, Vol. 104, No. 2, 2000 function relationships with regard to enhancing nonlinear optical responses. Toward this goal, we have investigated the NLO properties of a simple first generation organic dendrimer containing nitroaminostilbene and carbazole groups utilizing femtosecond intense light fields. The results of nonlinear refraction and multiphoton absorption measurements utilizing the z-scan, nonlinear transmission, and self-phase modulation (spectral pulse broadening) techniques are reported. The results of two-photon luminescence are presented for the dendrimer molecule excited at 790 nm. The contribution from a higher order (χ(5)) susceptibility is discussed in regards to the effective NLO response of the dendrimer molecule. The contributions from the different structural components in the dendrimer macromolecule to the magnitude of the NLO effect are given. The fast electronic processes observed in the dendrimer molecule and its components where also investigated by up-conversion luminescence spectroscopy. These studies provide the first probe into the ultrafast electronic nonlinear optical effects in organic dendrimers. Experimental Section A. Synthesis. All chemicals and reagents used in the synthesis, with the exception of Bisphenol A diglycidyl ether (DER Resin 332, SIGMA), benzyl alcohol (Baker Chemical Co.), and 5% palladium on activated carbon (Strem Chemicals), were purchased from Aldrich Chemical Co. and used without further purification. All chromatography was performed on flash-grade silica (230-400 mesh, 60 Å pore size, Scientific Adsorbents Inc.) unless specified otherwise. The NMR spectra were obtained on 300 and 500 MHz GE Omega NMR spectrometers. The NMR data are reported as follows: chemical shift (δ) in ppm referenced to the residual chloroform peak of CDCl3 (7.26 ppm) or the residual DMSO peak of DMSO-d6 (2.49 ppm); multiplicity; coupling constant, J, in Hz; and number of hydrogens. The IR spectra were obtained on a Nicolet Avatar 360 FTIR spectrophotometer with a resolution of 4.0 cm-1. The IR data are reported as follows: band frequency in cm-1, structural assignment, and the band intensity. Samples for the IR measurements were prepared by casting film on a NaCl plate using tetrahydrofuran as the solvent. The mass spectra were obtained on a Kratos MS-50 high-resolution mass spectrometer and a Macromass high-resolution mass spectrometer using highresolution FAB and EI techniques, respectively. Melting points were determined in open end capillary tubes on a Mel-Temp apparatus. Size exclusion chromatography (SEC) was performed using a ISCO model 2350 pump, a ISCO V4 absorbance detector (λ ) 254 nm), and a Jordi Gel DVB mm 500 Å column (250 × 10 mm, Alltech) with tetrahydrofuran as solvent at a 1.0 mL/ min flow rate. Polystyrene standards (Mw ) 760, 2360, 3700; Aldrich Chemical Co.) were used for calibration. 4-Nitro-4′-acetamidostilbene was obtained according to the method by Tsibouklis.17 Benzyl 3,5-dibromobenzoate was obtained from commercial 3,5-dibromobenzoic acid and benzyl alcohol via N,N-dicyclohexylcarbodiimide-mediated coupling in the presence of DPTS (4-(dimethylamino)pyridinium 4-toluenesulfonate).18 [N-Propargyl carbazole was synthesized according to literature procedure.19 Commercially available ammonium formate was recrystallized from 100% ethanol and dried in vacuo over P2O5, as suggested by the literature procedure.20 Unless otherwise noted, all reactions were run under a nitrogen atmosphere. (N-Methyl)-4-nitro-4′-acetamidostilbene. To a solution of 4-nitro-4′-acetamidostilbene (3.803 g, 13.47 mmol) in DMSO (100 mL) was added NaH (dry powder, 0.505 g, 21.04 mmol)

Varnavski et al. portionwise over 2 min. The mixture turned dark-purple upon addition of the NaH. A solution of methyl iodide (2.6 mL, 5.93 g, 41.8 mmol) in DMSO (6 mL) was added dropwise via syringe over 20 min; then, the reaction mixture was stirred (4 h, rt). By the end of the stirring period, the reaction mixture had turned to a dark-orange color. Excess NaH was quenched by the addition of a few drops of cold methanol. Then water (100 mL) was added to the reaction mixture to precipitate the product. The crystals were separated via vacuum filtration and washed with water (400 mL). The crude material was dissolved in CH2Cl2 (600 mL) and dried over sodium sulfate. Concentration in vacuo afforded the crude product (3.600 g, 90%), which was used in the next step without further purification. A portion of the product was purified for spectroscopic characterization via recrystallization from ethyl acetate (orange needles): TLC analysis (1:3 hexane/ethyl acetate v/v) Rf 0.48; mp 179.5 °C; 1H NMR (CDCl , 300 MHz) δ 8.19 (d, J ) 8.1 Hz, 2H), 7.643 7.57 (overlapping peaks, 4H), 7.28-7.10 (overlapping peaks, 4H), 3.27 (s, 3H), 1.90 (s, 3H); 13C NMR (CDCl3, 80 MHz) 171.0, 147.5, 145.4, 144.1, 136.3, 132.6, 128.8, 128.1, 127.9, 127.6, 124.8, 37.7, 23.1; FTIR (cm-1) 1660 (CdO, s), 1589 (CdC, s), 1509 (NO2, s), 1419 (aromatic CdC, w), 1336 (NO2, m), 849 (C-NO2, s); HRMS analysis (EI, C17H16N2O3 ) 296.1161), found m/z 296.1169. (N-Methyl)-4-nitro-4′-aminostilbene (CAS Registry no. 10257032-1). A solution of crude (N-methyl)-4-nitro-4′-acetamidostilbene (3.600 g, 12.2 mmol) in a mixture of absolute ethanol (90 mL) and concentrated HCl was refluxed (13 h). The reaction mixture was then cooled (ice bath), and the resulting orangebrown crystals were isolated via filtration. The crystals were suspended in water (400 mL), at which point they turned dark purple. The suspension was basified to pH 10 with saturated aq Na2CO3 solution, and the product was isolated via vacuum filtration. The crystals were air-dried to afford the title compound (2.395 g, 69.2% overall from 4-nitro-4′-acetamidostilbene) as fine dark-purple crystals: TLC analysis (1:3 hexane/ethyl acetate v/v) Rf 0.75; mp 229-230 °C; 1H NMR (DMSO-d6, 300 MHz) 8.14 (d, J ) 8.6 Hz, 2H), 7.70 (d, J ) 8.6 Hz, 2H), 7.42-7.33 (overlapping peaks, 3H), 7.03 (d, J ) 16.2 Hz, 1H), 6.55 (d, J ) 8.4 Hz, 2H), 6.23 (br. s, 1H), 2.69 (s, 3H); 13C NMR (DMSOd6, 80 MHz) 151.4, 146.4, 146.2, 135.4, 129.8, 127.3, 125.1, 121.7, 113.1, 30.7; FTIR (cm-1) 3405 (N-H, m), 1610 (CdC, m), 1588 (aromatic CdC, m), 1508 (NO2, s), 1336 (NO2, s), 837 (C-NO2, s); HRMS analysis (EI, C15H14N2O2 ) 254.1055), found m/z 254.1055. Benzyl 3,5-Bis(3-carbazolylpropynyl)benzoate. In a flamedried, nitrogen-purged flask, a mixture of propargyl carbazole (2.668 g, 13.00 mmol), benzyl 3,5-dibromobenzoate (2.221 g, 6.04 mmol), copper iodide (0.229 g, 1.20 mmol), and triphenyl phosphine (0.885 g, 3.37 mmol) in triethylamine (200 mL) was stirred until the organics dissolved (rt, 0.5 h). Dichlorobis(triphenylphosphine)palladium(II) (0.219 g, 0.31 mmol) was added, and the reaction mixture refluxed (3 h). By the end of the reflux period, the reaction mixture developed precipitate. The solids were separated via vacuum filtration, and the residual triethylamine was removed under reduced pressure. The product was purified by column chromatography on silica (starting with a 1:1 mixture of hexane/methylene chloride, then gradually changing the eluent to 1:2 hexane/methylene chloride) to afford the title compound (2.188 g, 58.6%) as a white fluffy solid: TLC analysis (1:2 hexane/methylene chloride v/v) Rf 0.53; mp 180-182 °C; 1H NMR (CDCl3, 500 MHz) δ 8.16, (d, J ) 7.6 Hz, 4H), 7.79 (d, J ) 1.4 Hz, 2H), 7.58-7.50 (overlapping peaks, 8H), 7.48-7.43 (overlapping peaks, 6H), 7.34 (overlap-

NLO Properties of a Novel Organic Dendrimer ping peaks, 4H), 5.34 (s, 2H), 5.18 (s, 4H); 13C NMR (CDCl3, 125 MHz) δ 165.5, 140.6, 139.5, 136.3, 133.3, 131.3, 129.4, 129.2, 129.1, 126.7, 124.0, 123.8, 121.2, 120.3, 109.5, 85.8, 82.7, 67.8, 33.6; FTIR (cm-1) 1721 (CdO, s), 1595, 1485 (aromatic CdC, s), 1454 (CH2, s), 1331 (C-N, s), 1228 (C(O)O, s), 1120 (O-C, m); HRMS analysis (FAB, C44H30N2O3 ) 618.2307), found m/z 618.2306. 3,5-Bis(3-carbazolylpropyl)benzoic acid. To a solution of benzyl 3,5-bis(3-carbazolylpropynyl)benzoate (1.479 g, 2.40 mmol) in THF/ethanol (1:1, 70 mL) was added 5% palladium on carbon (1.7 g) and dry ammonium formate (4.500 g, 71.4 mmol). The resulting mixture was refluxed (3.5 h) and then cooled and filtered. The filtrate was concentrated in vacuo to afford the title compound (1.015 g, 79.7%) as a white solid, which was used without further pruification: TLC analysis (5% methanol in methylene chloride v/v) Rf 0.6; mp 200-202 °C; 1H NMR (DMSO-d , 500 MHz) δ 8.06 (d, J ) 7.6 Hz, 4H), 6 7.60 (s, 2H), 7.43 (d, J ) 8.1 Hz, 4H), 7.31 (br. dd, J1 ) 8.1 Hz, J2 ) 7.3 Hz, 4H), 7.10 (br. dd, J1 ) 7.6 Hz, J2 ) 7.3 Hz, 4H), 7.01 (s, 1H), 4.29 (t, J ) 6.7 Hz, 4H), 2.57 (m, 4H), 1.98 (m, 4H); 13C NMR (DMSO-d6, 125 MHz) δ 168.8, 142.7, 141.0, 133.4, 132.6, 127.9, 126.7, 123.2, 121.3, 119.8, 110.2, 42.9, 33.2, 31.0; FTIR (cm-1) 3530-2350 (O-H, s), 1715, 1687 (Cd O, s), 1598, 1484 (aromatic CdC, s), 1453 (CH2, s), 1326 (CN, s), 1214 (C(O)-O, s); HRMS analysis (FAB, C37H32N2O2 ) 536.2464), found m/z 536.2463. Formation of the Dendrimer Core (DNSC2). A mixture of (N-methyl)-4-nitro-4′-aminostilbene (0.863 g, 3.35 mmol) and bisphenol A diglycidyl ether (0.552 g, 1.62 mmol) in 1-methoxy2-propanol (PM, 2 mL) was refluxed for 48 h. The reaction mixture was concentrated under reduced pressure. Chromatography on silica (Davisil, Aldrich Chemical Co., 100-200 mesh, 150 Å pore size; 100:0.5 methylene chloride/methanol (v/v)) afforded the title comnpound (0.720 g, 51.6%) as a red solid: TLC analysis (0.5% methanol in methylene chloride, v/v) Rf 0.22; mp 164-167 °C; 1H NMR (DMSO-d6, 300 MHz) 8.14 (d, J ) 8.6 Hz, 4H), 7.71 (d, J ) 8.6 Hz, 4H), 7.44 (d, J ) 8.6 Hz, 4H), 7.38 (d, J ) 16.5 Hz, 2H), 7.09-7.03 (overlapping peaks, 6H), 6.82 (d, J ) 8.6 Hz, 4H), 6.72 (d, J ) 8.6 Hz, 4H), 5.21 (s, 2H), 4.03 (s, 2H), 3.86 (d, J ) 4.3 Hz, 4H), 3.58 (br. dd, J1 ) 4.7 Hz, J2 ) 14.9 Hz, 2H), 3.36 (br. dd, J1 ) 6.8 Hz, J2 ) 14.9 Hz, 2H), 2.97 (s, 6H), 1.55 (s, 6H); 13C NMR (DMSOd6, 80 MHz) δ 156.2, 149.6, 145.2, 145.1, 142.7, 134.0, 128.5, 127.4, 126.2, 124.0, 123.5, 120.8, 113.9, 111.6, 69.9, 67.0, 65.0, 41.1, 30.7; FTIR (cm-1) 3398 (O-H, w), 1607 (CdC, m), 1583 (aromatic CdC, s), 1508 (NO2, s), 1336 (NO2, s), 833 (CNO2, m); HRMS analysis (FAB, C51H52N4O8 ) 848.3785), found 848.3745 m/z. Formation of the Dendrimer (CzD4NSC2). To a mixture of 3,5-bis(3-carbazolylpropyl)benzoic acid (0.913 g, 1.70 mmol), DNSC2 (0.600 g, 0.71 mmol), and 4-(dimethylamino)pyridinium 4-toluenesulfonate (DPTS, 0.500 g, 1.70 mmol) in methylene chloride (60 mL) was added 1,3-dicyclohexylcarbodiimide (DCC, 0.60 g, 2.9 mmol). The resulting mixture was stirred (rt) and the progress of the reaction monitored by TLC. After 23 h, additional portions of 3,5-bis(3-carbazolylpropyl)benzoic acid (0.160 g, 0.30 mmol) and DCC (0.060 g, 0.30 mmol) were added to the reaction mixure. TLC analysis indicated the reaction to be complete after a total of 38 h. After removing the urea precipitate by filtration, the reaction mixture was concentrated under reduced pressure. Chromatography on silica (Davisil, Aldrich Chemical Co., 100-200 mesh, 150 Å pore size), starting with 1:4 (v/v) hexane/methylene chloride and gradually changing the eluent to 100% methylene chloride) afforded the title

J. Phys. Chem. B, Vol. 104, No. 2, 2000 181 compound (1.188 g, 88.8%) as a red solid: TLC analysis (1:4 (v/v) hexane/methylene chloride) Rf 0.46; SEC (Jordi Gel DVB mm 500 Å column (250 × 10 mm, Alltech) with THF as the eluting solvent at 1.0 mL/min flow rate) elution volume 9.20 mL (corresponds to Mpeak 1820); mp 119-123 °C; 1H NMR (CDCl3, 300 MHz) δ 8.12-8.08 (overlapping peaks, 12H), 7.59 (s, 4H), 7.43-7.15 (overlapping peaks, 36H (observed integration is 8% higher than the expected value)), 7.06-6.73 (overlapping peaks, 14H (observed integration is 7% higher than the expected value)), 5.61 (m, 2H), 4.30-4.20 (overlapping peaks, 12H), 3.97 (br dd, J1 ) 7.3 Hz, J2 ) 15.2 Hz, 2H), 3.81 (br dd, J1 ) 6.3 Hz, J2 ) 15.2 Hz, 2H), 3.04 (s, 6H), 2.63 (t, J ) 6.8, 8H), 2.17 (m, 8H), 1.65 (s, 6H); 13C NMR (DMSOd6, 80 MHz) δ 167.1, 157.2, 150.4, 146.5, 145.5, 144.7, 142.4, 141.2, 134.2, 130.8, 129.5, 128.8, 128.3, 126.8, 126.6, 125.4, 124.9, 123.8, 122.4, 121.4, 119.9, 115.0, 113.1, 109.6, 71.5, 67.7, 52.8, 43.1, 42.7, 39.9, 33.8, 32.0, 30.7; FTIR (cm-1) 1717 (CdO, m), 1607 (CdC, m), 1584 (aromatic CdC, s), 1510 (NO2, s), 1484 (aromatic CdC, m), 1335 (NO2, s), 834 (C- NO2, w); HRMS analysis (FAB, C125H112N8O10 + H+ ) 1885.8580), found 1885.8599 m/z. The structure of the dendrimer system is shown in Figure 1 along with the functional groups: nitroaminostilbene derivative NS1 (B), carbazole group CZD4MA2. B. NLO Measurements. The steady-state absorption was measured with a fiber optical spectrometer. The emission measurements were taken on a spectrofluorometer (Spex). The absorption and emission spectra of dendrimer CZD4NS2 are depicted in Figure 2. The NLO experiments were performed using the fundamental laser emission from a Tsunami Ti:sapphire (Spectra Physics) femtosecond laser system pumped by a Millennia 5 W CW Nd: YVO4 laser (Spectra Physics). This system delivered stable ultrashort light pulse sequences with individual pulse durations of less than 100 fs, and at a repetition rate of 82 MHz. The output spectrum of the laser radiation was monitored with a spectrum analyzer (REESE Instruments), which was used as a probe to ensure pure mode-locking regime of the femtosecond laser. The maximum average output laser power obtainable at 790 nm was about 700 mW. The experimental setup for z-scan measurements is shown in Figure 3. The z-scan technique is performed when a guassian beam is focused inside the material to induce a nonlinear lens. The presence of this lens influences the beam propagation and distorts its wave front. While the input power is kept constant, the sample experiences a different light field (amplitude and phase) at different z-positions. The wave front distortion emerges in beam expansion or contraction in the far field, thus changing the fraction of light passing through the aperture as the sample position is changed. The transmission through the aperture shows a characteristic peak/valley dependence on the z-position of the sample.21 From this dependence one can easily estimate the nonlinear refraction which is related to the nonlinear susceptibility of the material. In the case of relatively small nonlinear phase shifts (and relatively weak nonlinear absorption), it is sufficient to know the difference between normalized peak and valley transmissions {Tp - Tv ) ∆Tpv}.21-23 By removal of the aperture (in other words collecting all of the light transmitted) one is able to measure the nonlinear transmission (transmission intensity dependence). This case is referred to as open-aperture z-scan, as opposed to closed-aperture z-scan with the aperture inserted in the far field. In the case of strong nonlinear absorption, when nonlinear variation of normalized transmission in open-aperture z-scan is comparable to the closed aperture

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Figure 1. Structure of dendrimer system CZD4NS2 (A) and functional groups nitroaminostilbene derivative NS1 (B) and carbazole group CZD4MA2 (C) used in the study.

scheme, the signal of the nonlinear absorption suppresses the peak and enhances the value in closed-aperture z-scan.21,22 If the normalized transmission change caused by nonlinear absorption is small, the nonlinear refraction can be estimated from closed-aperture z-scan without additional corrections.21 An important parameter in closed-aperture z-scan is the aperture nonlinear transmission, S ) 1 - exp(-2ra2/wa2), where ra is

the aperture radius and wa is the beam radius at the aperture in the linear regime. This parameter was in our case equal to 0.05. With a circular variable neutral density filter (see Figure 2) we were able to vary the input beam intensity by more than 2 orders of magnitude, allowing the measurement of the intensity dependence of the z-scan effect as well as direct measurement of nonlinear transmission in a wide intensity range. In most

NLO Properties of a Novel Organic Dendrimer

J. Phys. Chem. B, Vol. 104, No. 2, 2000 183 frequency generation with a delayed reference pulse within a nonlinear crystal of β-barium borate (BBO). The sum-frequency signal was dispersed in a monochromator and detected with a single photon counting unit. The fwhm of the cross-correlation function at 790/395 nm was estimated to be 190 fs. The rotating sample cell and a holder (1 mm thickness) were used to avoid the thermal and photochemical accumulative effects. Excitation average power was kept at the level of ∼1 mW. Results and Discussion

Figure 2. UV-vis absorption and fluorescence spectra of the dendrimer system CZD4NS2 in chloroform.

experiments, we used solutions with concentration 3 × 10-3 mol/L (any exceptions are noted in the text) in the fused quartz cell of 1 mm length. Also shown in Figure 3 is the microscope objective coupled to a fiber for spectral and excitation intensity measurements of two-photon excited luminescence that was observed in our dendrimer sample. A remarkable feature of this z-scan technique is the possibility of the estimation of higher order nonlinearities in a similar manner as that used for the third-order susceptibility.21 Also, due to the linear dependence of the nonlinear phase distortions on ∆Tp-v, one can in some cases analyze the complicated situations of simultaneous presence of comparable nonlinearities of different order.21,24 In these experiments, where the peak intensity is large, it is possible to observe these higher order effects to an appreciable degree. We also performed ultrafast time-resolved fluorescence measurements using the femtosecond fluorescence up-conversion technique.25 The sample was excited with laser pulses delivered by a frequency doubled output of the Ti:sapphire laser. The temporal profile of the fluorescence was monitored by sum-

We have measured the nonlinear optical effects arising from large incident peak intensities by the z-scan technique. Shown in Figure 4 is the result of the closed appeture z-scan measurement for the dendrimer CZD4NS2 with 790 nm as the fundamental source. As stated above in the z-scan technique, optical effects are measured by translating the sample in and out of the focal region. This consequently increases and decreases the maximum intensity incident on the sample, producing wave front distortions created by nonlinear optical effects. It is clearly seen that there is a large effect in the dendrimer sample, where at z-positions close to focus, the change in transmission is large. The peak-valley configuration of this z-scan shows a negative (self-defocusing) nonlinearity. One notes that the curve is not symmetric, where a large change is observed on the positive side of the focal path. We were able to switch the femtosecond system to CW mode while keeping the average incident power at the exact same level. The CW mode result is also shown in Figure 4. A very large difference in closed z-scan results due to operating in mode-locked or CW modes can be seen. The large peak intensity from the femtosecond pulse system gives rise to this large nonlinear response. This experiment shows a negligible contribution of the steady state thermal lens effect caused by linear absorption. There was no detectable (by various characterization techniques) sample damage or photochemical destruction of the dendrimer system when analyzed after the z-scan experiments were performed. We have calculated the third-order NLO (real part) due to

Figure 3. Experimental setup: CVNDF, circle variable neutral density filter; BS, beam splitter; PD1, silicon phptodiode; L1, lens, L2; micro objective; sample cuvette length is 1 mm.

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Figure 4. Close-aperture z-scans for dendrimer CZD4NS2 in chloroform at different incident intensities. Laser emission wavelength, 790 nm. The CW-mode result at average power 560 mW is also shown (open squares). Insert: open-aperture z-scan data for dendrimer system at 790 nm.

nonlinear refraction as outlined by Van Stryland et al.,21 and the result is n2 ) 1.1 × 10-4 cm2/GW. The asymmetry in the closed-aperture z-scan curve mentioned above might arise from nonlinear absorption. To test this possible mechanism, we performed the open-appetrure z-scan measurement, and the result is shown in the insert of Figure 4. The open z-scan curves where measured over a wide range of intensities (0.3-9 GW/cm2) and only a relatively small magnitude of nonlinear absorption at higher intensity was detected. At the largest peak intensities, approximately 1% (shown in the insert) of the loss was attributed to nonlinear absorption. From the open z-scan result (as well as from direct measurement of nonlinear transmission), the nonlinear absorption coefficient (β) was found to be 1.2 × 10-2 cm/GW. At an incident wavelength of 790 nm, we are far from the one-photon resonance (see Figure 2) and the figure of merit for the case of nonlinear absorption as a limitation of all-optical switching and interferometer nonlinear etalons can be given as FOM ) 2βλ/ n2.8 Our estimation of the dendrimer’s FOM showed an attractive value of 0.02, which is smaller when compared to the organic polymers and organometallic systems described above.9,10 It is interesting to note that while our value for the nonlinear refraction is not as big as a number of other reports for organic macromolecular materials, the figure of merit is still small. For example, the n2 value obtained by Woodruff et al.10 for poly(phenylenevinylene) (PPV) at 800 nm with much higher peak intensities was -2 × 10-3 cm2/GW. However, the figure of merit for the PPV system was offset by a substantial twophoton absorption (β ≈ 30 cm/GW).10 As one notes from Figure 4, we detected an extremely large nonlinear refraction effect from the closed z-scan using a peak intensity of 3 GW/cm2, approximately three times smaller than that used to get an appreciable nonlinear absorption result from the open z-scan measurement. Thus, it does not appear that

nonlinear absorption can be the origin of the asymmetry or distortion seen in the closed z-scan result. In certain cases an improper setting or alignment of the z-scan apparatus can cause an asymmetry in the z-scan curve.22 However, the curves measured with our apparatus for the CHCl3 solvent in the modelocked and CW modes were symmetric, verifying that our systems alignment was correct. Another reason for asymmetry could be a rather large nonlinear phase distortion.21 In this case, however, estimation of this shift via ∆Tp-v remains valid and we find the nonlinear phase shift |∆φo| is between π/2 and π at 3 GW/cm2 of incident intensity. It is also interesting to note that the asymmetry did not decrease at lower incident intensities. To further illustrate the fact that we have detected a strong nonlinear refraction we measured the spectrum of the transmitted beam. The result of the measurement for the incident laser beam only, and that of the transmitted beam through the dendrimer sample is shown in Figure 5. It can be seen from Figure 5 that the spectrum of the transmitted beam is broadened and has structure. We assign these features as being due to spectral broadening of short laser pulses undergoing strong time-dependent nonlinear changes in refractive index.26 The fast time-dependent change of refractive index leads in turn to the fast phase-modulation of light wave resulting in spectral broadening. This well-known phenomenon, the self-phase modulation (SPM) of short powerful light pulses in the nonlinear media, has been extensively investigated in optical fibers and waveguide structures.27 In general, the spatial beam profile that is observed can be rather complicated at high intensities, due to several factors involving self-focusing effects. It leads to a complicated SPM spectrum, which is the result of interference between components with different phase shifts corresponding to different points in the beam profile. From the degree of spectral broadening, we can estimate a value of χ(3) equal to 9 × 10-13 esu due to SPM. The observed broadening

NLO Properties of a Novel Organic Dendrimer

J. Phys. Chem. B, Vol. 104, No. 2, 2000 185 Although the nonlinear absorption is relatively weak in our case (