J. Phys. Chem. B 2009, 113, 14565–14573
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Synthesis, Strong Two-Photon Absorption, and Optical Limiting Properties of Novel C70/C60 Derivatives Containing Various Carbazole Units Xinhua Ouyang,†,‡ Heping Zeng,*,‡ and Wei Ji*,† Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore, 117542, Republic of Singapore, and School of Chemistry and Chemical Engineering, South China UniVersity of Technology, Guangzhou, 510641, P. R. China ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: September 23, 2009
An approach was demonstrated toward the design and synthesis of a series of novel C70 and C60 derivatives for large two-photon absorption (TPA). The molecular structures of fullerene derivatives were confirmed by MALDI-MS, 1H NMR, and FT-IR. With femtosecond open-aperture Z-scans and frequency-degenerate pump-probe measurements at 780 nm, the TPA cross sections of up to 3.47 × 10-46, 1.64 × 10-46, 1.1 × 10-46, and 7.82 × 10-47 cm4 s photon-1 molecule-1 were determined for C70-TCTA, C60-TCTA, C70-BCzMB, and C70-MQEtCz in toluene with concentrations of 10-4 M, respectively. The normalized light transmittances of solutions of these molecules were attenuated to the range between 33% and 50% for C70-TCTA, C60TCTA, C60-BCzMB, and C70-MQEtCz as the input irradiance was increased to about 150 GW/cm2, showing that they are effective optical limiters. Both intensity-dependent Z-scans and pump-probe experiments confirmed that the reduction in the light transmittance results mainly from the TPA process. In addition, the molecule concentration dependence of the TPA cross sections was also investigated. It was found that the TPA cross sections are extremely sensitive to the concentration with the greatest TPA cross-section of 1.0 × 10-45 cm4 s photon-1 molecule-1 for C70-TCTA measured in the low concentration regime (∼10-5 M). Introduction Materials that exhibit large two-photon absorption (TPA) have attracted intensive research interest owing to their potential applications in a number of fields, including two-photon fluorescence imaging,1 three-dimensional optical data storage,2 optical power limiting,3 and photodynamic therapy.4 Especially, the advantages such as synthetic flexibility, high damage resistance, and large optical nonlinearities, have made organic molecules promising candidates for applications based on TPA. Over the past few years, there has been considerable effort in designing a diverse variety of chromophores with large TPA cross sections. The most frequently involved molecular structural motifs for TPA materials are π-conjugated bridge systems with strong electron donor (D) and acceptor (A) groups, containing symmetrical (D-π-D or A-π-A) or asymmetrical (D-π-A) arrangements.5-8 In these systems, the intramolecular chargetransfer (ICT) is responsible for both linear and nonlinear absorption features, with effectiveness depending on the charge transfer abilities of D and A groups. Meanwhile, the branching strategy has also been proposed and demonstrated for enhanced TPA.9-12 For multibranched systems, various features in the TPA cross-section have been reported, including co-operative enhancement, additive behavior, or weakening. Therefore, an understanding of the influences by D/A groups and/or branching symmetry is of direct relevance to the molecular design and synthesis of large two-photon absorbers. The exceptional electron accepting characteristics (e.g., acceptance of up to six electrons of C60 and seven electrons of C70) of fullerenes (C60 and C70) have been proven to be the most * To whom correspondence should be addressed. E-mail: phyjiwei@ nus.edu.sg;
[email protected]. † National University of Singapore. ‡ South China University of Technology.
desirable sources in altering TPA properties of fullerene derivatives.13 This is because fullerenes easily undergo chargetransfer interactions both in the ground and excited states when they are in contact with electron rich moieties via intramolecular or intermolecular charge transfer.14,15 In a comparison between C60 and C70, C70 is more amenable to accept electrons in the context of delocalization of surface electrons.16 In addition, pristine fullerenes (C60 and C70) themselves also show a sizable nonlinear-optical response owing to their large π-conjugated surface and extensive charge delocalization.17 To realize TPAbased applications, fullerenes (C60 and C70) have been used extensively as electron acceptor units and produced long charge transfer species with different electron donor units in organic moieties.14-17 The enhancement of charge delocalization over the fullerene cage in connection with charge transfer has been shown to be an efficient source to enlarge the TPA cross sections of fullerene derivatives and consequently improve their performance in applications such as optical limiting. Triphenylamine derivatives and carbazole derivatives, known as important chemical intermediates, are some of the typical aromaticaminesandpossesssuperiorelectron-donatingproperties.18,19 The electron transfer between aromatic amines derivatives and fullerenes has been well investigated in the context of intermolecules and intramolecules.20-22 Elim et al.23 reported a new class of C60 derivatives contained fluorene and diphenylamine units with large values of TPA cross-section and fast recovery of a few picoseconds. However, to the best of our knowledge, both TPA and TPA-based optical limiting of covalently functionalized C70 derivatives have not been investigated yet. We demonstrate herein for the first time TPA materials using a novel series of C70 derivatives (Figure 1) with multibranched tris(4-(9H-carbazol-9-yl) phenyl) amine (TCTA), planar structure of (E)-9-ethyl-3-(2-(8-methoxynaphthalen-2-yl) vinyl)-9Hcarbazole (MQEtCz) and nonconjugated structure of 1,4-
10.1021/jp905390q CCC: $40.75 2009 American Chemical Society Published on Web 10/13/2009
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Figure 1. Molecular structures of C70 and C60 derivatives.
TABLE 1: The Variation in the Synthetic Yields of TCTA-CHO with Different Proportions of DMF and POCl3 in Different Solvents
Figure 2. Linear absorption spectra of C70 and C60 derivatives.
bis((9H-carbazol-9-yl)methyl)benzene (BCzMB). Both intensitydependent Z-scans and pump-probe experiments eliminate the possibility of singlet and triplet excited-state absorption, respectively, in the demonstration. To gain insight into the relationships between the structures and the desired properties, we have also prepared the C60 derivatives with the same functionalized groups of TCTA, MQEtCz, and BCzMB. Together with the C70 derivatives, their TPA, concentration dependence and TPA-based optical limiting are characterized with femtosecond laser pulses at 780-nm wavelength. From such nonlinear-optical characterizations, we reveal some understanding in the structure-properties relationships of these molecules, confirming that the TPA cross-section is enhanced by increasing the degree of molecular π-conjunction and/or the number of branches. In addition, it is found that the TPA cross-section decreases as the molecule concentration increases, with the largest TPA cross-section of 1.0 × 10-45 cm4 s photon-1 molecule-1 measured at the lowest concentration (∼10-5 M) for the compound C70-TCTA. Our findings offer a new avenue to design a novel series of organic molecules with highly efficient TPA, which can be used in many nonlinear-optical applications as well as studies on their fundamental properties. Results and Discussions In the preparation of these dyads, the Vilsmeier and 1,3dipolar cycloaddition reactions were considered as key steps to synthesize the afore-said fullerene derivatives, especially monoad-
nDMF (mol)
nPOCl3 (mol)
0.015 0.01 0.01 0.01 0.005 0.002 0.001 0.001 0.001 0.001
0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.002 0.005 0.01
solvent
yield of monodehyde (%)
DCE DCE DMF DCM DCE DCE DCE DCE DCE DCE
38.3 45.3 24.3 40.2 39.3 28.3 20.4 17.4 10.3 6.5
TABLE 2: Summaries of the Linear Absorptive Coefficients (r0), TPA Coefficients (β), Concentrations ([C]), and TPA Cross Section Values (σ2) of These Samples in Toluene with Wavelength of 780 nm chromophore C70 C70-TCTA
C70-MQEtCz C70-BCzMB C60-TCTA C60-MQEtCz C60-BCzMB
[C] (M) 1.3 × 10-3 1.34 × 10-3 6.7 × 10-4 1.34 × 10-4 1.34 × 10-5 1.14 × 10-3 5.7 × 10-4 1.14 × 10-4 9.2 × 10-4 9.2 × 10-5 1.24 × 10-3 6.2 × 10-4 1.24 × 10-4 1.08 × 10-3 1.02 × 10-3
R0 β (/cm) (cm/GW)
σ2 (×10-48 cm sec photon-1 molecule-1 or GM)
0.53 0.79 0.3 0.22 0.11 0.7 0.28 0.19 0.44 0.25 0.7 0.43 0.21 0.65 0.5
0.81 (81) 3.5 (350) 30 (3000) 347 (34700) 1040 (104000) 10.8 (1080) 24 (2400) 110 (11000) 9.7 (970) 78.2 (7820) 10.2 (1020) 21.2 (2120) 164 (16400) 9.8 (980) 7.5 (750)
0.0025 0.011 0.048 0.11 0.033 0.029 0.033 0.03 0.021 0.017 0.03 0.031 0.048 0.025 0.018
4
ducts. In view of this, the Vilsmeier reaction to synthesize monoaldehydes became naturally the most important step during the whole preparation. The feasible strategy in the successful synthesis of these fullerene derivatives was on the controlling of added amounts of N, N-dimethylformamide (DMF) and phosphorus oxy-chloride (POCl3). By analyzing the results of many experiments (Table 1), we concluded that the amount of DMF and POCl3 with a ratio of about 10:1 should produce the highest yields of the monoaldehydes, which would be extremely
Properties of C70/C60 Derivatives with Carbazole
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Figure 3. (a) Open-aperture Z-scans on 1-mm-thick, 10-3 M toluene solutions of C70-TCTA, C60-TCTA, C70-MQEtCz, C60-MQEtCz, C70-BCzMB, and C60-BCzMB; (b) 10-4 M solutions of C70-TCTA, C60-TCTA, C70-MQEtCz, and C70-BCzMB; (c) 10-5 M solutions of C70-TCTA with different irradiances; (d) concentration dependence of TPA cross sections of C70-TCTA, C60-TCTA, and C70-MQEtCz.
helpful for purification of monoaldehydes as well. In Table 1, the first four data rows exhibit variations in the yield with respect to both stability and activity of Vilsmeier reagent. The Vilsmeier reagent was unstable with high temperature (150 °C) using DMF as solvent. In dichloromethane (DCM) with low temperature, the activity would be very low. Therefore, dichloroethane (DCE) was the best choice. The data rows 5-10 point out a decrease in the yield of monoaldehyde with a relative increase of POCl3 proportion in the same solvent (DCE), this might be due to the increase in the activity of Vilsmeier reagent, as the amount of POCl3 is increased. Multidehydes became major products and affect the yield of monodehydes. Then, the synthesis of pyrrolidinofullerene (C60 and C70) relied on the 1,3-dipolar cycloaddition reaction which generated in situ pyrrolidinofullerene derivatives. There were many active bonds which could react easily with monodeydes on the surfaces of fullerene (C60 and C70). To address this issue, the following two tactics were adopted: (i) injecting an excess of fullerene (C60 and C70), which was proportional to that of monodeydes; and (ii) adding monodeydes to fullerene solution dropwise to obtain monoadducts of fullerene derivatives. The reaction mixture was separated by flash chromatography using gradients of toluene/ cyclohexane as eluent, and the monoadducts were obtained following the first fraction of the surplus C60 or C70. The monodducts were clearly confirmed by MALDI-TOF-MS. As far as the pyrrolidino-C70 derivatives were concerned, they were
all mixtures of three isomeric monoadducts due to the known reactivity of C70.24 As shown in the 1H NMR spectrum of these C70 derivatives, three distinct N-methyl resonances presented from δ 2.45 to δ 2.61 ppm indicated a mixture of three isomers, which resulted from the addition to six, 6-ring fusions of C70 at the 1, 9-; 7, 8-; and 22, 23-bonds, as previously reported for the cycloaddition of N-methyl-azomethine ylide to C70 derivatives.24 The structures of all newly synthesized derivatives were confirmed by FT-IR, 1H NMR, MALDI-TOF MS, and elemental analysis. In addition, for the C60 derivatives, as shown in Figure 2, a weak but characteristic long-wavelength absorption band of the C60 cage centered at ∼705 nm was detected, which gave the convincing evidence of C60 derivatives with a similar energy gap between the ground and singlet excited-states. To characterize the TPA cross sections and nonlinear transmittance properties of the as-prepared molecules, both openaperture Z-scans and irradiance-dependent transmission measurements were performed with 450-fs laser pulses at the wavelength of 780 nm. Both linear and nonlinear optical properties of the fullerene derivatives in toluene are summarized in Table 2. Consistent with the previous reports,23 the TPA cross sections increase largely in a consistent trend, as the solution concentration is decreased from 10-3 to 10-5 M, (see Table 2). Such concentration dependence has been primarily attributed to molecule aggregation, though other more complicated factors contributing to this dependence cannot be ruled out entirely.
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Figure 4. Irradiance independence of the TPA cross-section profile of C70-TCTA in four concentrations indicated.
Interestingly, with the high concentration (10-3 M), the values of σ2 are relatively close to each other in the range of (7.5-10.8) × 10-48 cm4 s photon-1 molecule-1, except for C70-TCTA, which indicates that the degree of molecule aggregation depends strongly on the molecular structures, polarization-induced forces, elastic forces, electrostatic forces, Van der Waals forces, heating of particles, and surrounding medium. Therefore, the results at this concentration are not the intrinsic absorption properties of the molecules. Instead, they reflect the contributions from interactions among the molecules and/or between the molecules and surrounding medium.25 Since the TPA value of C70-TCTA is less than half of that of C70-MQEtCz or C70-BCzMB, it implies that the molecule C70-TCTA should be easier to aggregate and /or the above-said factors should play a more important role in toluene than the others. As the concentration decreases to 10-4 M, a noticeable increasing trend is observed in Figure 3d, and the values of σ2 change from 3.5, 10.8, 9.7, and 10.2 × 10-48 cm4 s photon-1 molecule-1 to 347, 110, 78.2, and 164 × 10-48 cm4 s photon-1 molecule-1, respectively. For the molecules C70-TCTA, C70-MQEtCz, and C60-TCTA, it should be singled out that, for the molecule C70-TCTA, the value of σ2 increases over 100 times to 347 × 10-48 cm4 s photon-1 molecule-1, compared to the value measured at the higher concentration. At this concentration, the interactions between the molecules and/or other the above-mentioned factors are diminished considerably. Thereby, the measured σ2-value results from the electronic states and conjugation factors within the molecule. The results in Table 2 also reveal the significant effects of branched fullerene derivatives in enhancing TPA cross sections. In the molecule C70-TCTA, the value of σ2 increases monotonically to 1040 × 10-48 cm4 s photon-1 molecule-1when the concentration of C70-TCTA is decreased to ∼10-5 M. This is the highest TPA cross-section measured so far on the femtosecond time scale for fullerenes and their derivatives. As thermally induced scattering effects and exited-states absorption caused by high irradiance may influence the accuracy of overall TPA cross sections measured, we undertook the Z-scans of C60-TCTA, C70-TCTA, C60-MQEtCz, C70-MQEtCz, C60-BCzMB and C70-BCzMB in toluene with different irradiances. As an example, the results of C70-TCTA are shown in Figure 4. The others exhibited similar behavior. We found that the measured σ2 values, obtained from the best fits between the TPA theory and Z-scan data, remain nearly constant in a straight line across the irradiance range from 66 to 256 GW cm-2 within the experimental errors, confirming the independence of the laser
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Figure 5. Transient transmission change (∆T/T) in solutions of C70TCTA, C60-TCTA, C70-MQEtCz, and C70-BCzMB.
Figure 6. Normalized transmittances of C70-TCTA, C60-TCTA, C70MQEtCz, C60-MQEtCz, C70-BCzMB, and C60-BCzMB in toluene.
intensity. This indicates the observed nonlinear absorption is induced from a pure TPA process. Other nonlinear mechanisms such as thermally induced scattering and excited-state absorption, in particular singlet excited-state absorption, are negligiable in our experiments. To study more details about the observed nonlinearities, we also conducted a degenerate pump-probe experiment with 450 fs, 780 nm laser pulses close to the TPA peak positions of C70-TCTA, C60-TCTA, C70-MQEtCz, and C70BCzMB in toluene solution. As shown in Figure 5, the relaxation process may be described quantitatively by using a doubleexponential fitting: ∆T/T ) A1 exp(-t/τ1) + A2 exp(-t/τ2), where the fastest component, τ1, about 0.45 ps, is found to be independent of the pump intensity and is interpreted as the autocorrelation between the pump and probe pulses.26 The slowest component, τ2, on the picosecond scale is attributed to radiative, band-to-band recombination.27 The maximum transmittance change (∆T/T) in these solutions were found to be negative, showing TPA or reverse saturation absorption. Meanwhile, we also found the magnitude of ∆T/T of these samples were consistent with the order of C70-TCTA > C60-TCTA > C70MQEtCz > C70-BCzMB, in agreement with that of the previously discussed Z-scans. Furthermore, the response time of these samples were evaluated to be 1.8, 1.4, 3.4, and 2.8 ps for C70BCzMB, C70-MQEtCz, C60-TCTA, and C70-TCTA, respectively. On this time scale, it would be impossible for intramolecular energy transfer from the side group moiety to the fullerene moiety and or excited state absorption. Normally, the decay time
Properties of C70/C60 Derivatives with Carbazole
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TABLE 3: Optical Limiting Response to Femtosecond or Picosecond Laser Pulses materials
wavelength (nm)
linear transmittance (%)
limiting threshold (GW/cm2)
BCADQ30 (fs) BCADQ60 (fs) BCADQ90 (fs) C60(>DPAF-C10) (ps) C60(>DPAF-C9) (ps) C60(>DPAF-C2M) (ps) CdSe QDs (fs) (TPA)2F(DPO)4(fs) C60-TCTA (fs) C70-TCTA (fs) C60-MQEtCz (fs) C70-MQEtCz (fs) C60-BCzMB (fs) C70-BCzMB (fs)
800 800 800 780 780 780 775 775 780 780 780 780 780 780
0.93 0.93 0.93 0.89 0.89 0.89 unavailable 0.88 0.8 0.8 0.8 0.8 0.8 0.8
>1000 ∼825 ∼850 ∼90 ∼120 ∼170 ∼400 ∼140 ∼125 ∼120 ∼140 ∼130 ∼150 ∼135
ref 31 31 31 30 30 30 32 33 this this this this this this
work work work work work work
SCHEME 1: Synthetic Route of C70 and C60 Derivatives
from the side group moiety to the fullerene and excited state takes above 5 ps since it involves charge separation and recombination process.28 Furthermore, it would be impossible for triplet excited-state absorption to occur since typical intersystem crossing times between singlet and triplet excitedstates are a few nanoseconds. All these experiments assured that the measured σ2 values result mainly from TPA processes. Among the C70 derivatives reported here, functionalized groups play a crucial role in enhancing TPA cross sections. For the concentrations in the range of (0.98-1.34) × 10-4 M, the TPA cross sections were found to be 347, 110, and 78.2 × 10-48 cm4 s photon-1 molecule-1, respectively, for the samples C70TCTA, C70-MQEtCz, and C70-BCzMB. This clearly indicates that the electron-donating efficiency of the functionalized groups in these molecules increases in the order of BCzMB group < MQEtCz group < TCTA group. By comparing these molecular structures, in the molecule C70-TCTA, the side functionalized group is a typical multibranched structure with large π-conjugated system, and TCTA is also considered as the best electron donor, which has been extensively used as hole-transporting layer in the organic light-emitting diodes.29 Therefore, the TPA cross-section of C70-TCTA is enhanced mainly due to the strong electron-donating ability and degree of π-conjugates in TCTAgroup. However, the TPA cross-section of C70-BCzMB is about five times lower than that of C70-TCTA, which is attributed to the single bond (methylene) located between the carbazole unit and the benzene unit. This single bond greatly reduces the degree of conjugated-electron delocalization in BCzMB-group. Mean-
while, given the flexible methylene group between the carbazole unit and the benzene unit, the electronic density is no longer restricted in the ground and excited states, and hence, the ICT is weakened in this molecule, resulting in a sizable influence on the TPA cross-section of C70-BCzMB. Our analysis reveals that the order of ICT abilities between the functionalized groups and fullerene is C70-TCTA > C70-MQEtCz > C70-BCzMB. The electron acceptors also make an important contribution to the enhancement in the TPA. There is the same donor but different acceptor in molecules C70-TCTA and C60-TCTA. By examining their TPA cross sections measured with the concentration of 10-4 M, we find that the values of σ2 are 347 and 164 × 10-48 cm4 s photon-1 molecule-1 for C60-TCTA and C70TCTA, respectively. It is clear that the σ2 of C70-TCTA is two times higher than that of C60-TCTA, implying that the strong electron-accepting group of C7016 can significantly enhance the TPA cross-section in the molecules of the D-π-A system. Likewise, the functionalized fullerenes exhibit larger TPA cross sections compared to the pristine C70 (σ2 ) 0.81 × 10-48 cm4 s photon-1 molecule-1) and C60 (σ2 ) 0.56 × 10-48 cm4 s photon-1 molecule-1). This may be attributed to the charge transfer exists between the electron donor and acceptor. Figure 3a clearly shows that there is a drop in the light transmittance at the focus, and the degree of the drop is in the order of C70-TCTA > C60-TCTA > C70-MQEtCz > C70-BCzMB in the concentration range of ∼10-4 M. This trend is also in agreement with the optical limiting measurements described in the following. With adjusting the concentration, all the solutions
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showed a linear transmission (T) of ∼80% recorded at input laser irradiances less than 30 GW/cm2 (or 4.5 mJ/cm2 in fluence). As displayed in Figure 6, the transmittance starts to deviate from the linear transmission and decreases if the incident irradiance is increased beyond 30 GW/cm2, a typical optical limiting effect manifests itself with the light transmittance reduced down to 33, 41, 36, 43, 39, and 50% at ∼150 GW/cm2, respectively, for the dyads C70-TCTA, C60-TCTA, C70-MQEtCz, C60-MQEtCz, C70-BCzMB, and C60-BCzMB. This limiting performance correlates to the TPA properties of these fullerene derivatives determined by the Z-scans in Figure 3. The limiting threshold is the quantity that is often used to describe the limiting performance and it is defined as the incident irradiance at which the transmittance falls to 50% of the linear transmittance. The measured limiting thresholds are summarized in Table 3. From Table 3, the present work has limiting thresholds for the compounds of C70 less than those of compounds of C60, which should be attributed to the charge transfer and surface area of C70. And C70-TCTA also shows the best optical limiting response to femtosecond laser pulses (see Table 3, which summarizes all the TPA materials reported up to now). Such enhanced nonlinear attenuation in the light transmittance may be attributed mainly to TPA in TCTA moiety as indicated by its linear absorption spectrum. As shown in Figure 2 because the minimum gap is 700 nm (or 1.77 eV) between the ground and lowest-lying excited singlet state, it needs to absorb two photons in order to excite the ground-state electrons to the excited state. It is anticipated that if the lowest-lying excited singlet state is populated to a large degree with higher laser excitation, singlet excited-state absorption (ESA) should occur between the first and second (or higher-order) excited singlet states, and ESA intensifies the optical limiting effect at higher input energy (>240 GW/cm2) of femtosecond laser pulses. Conclusions We have demonstrated an approach toward the design and synthesis of novel fullerene derivatives with high TPA cross sections. We have observed greatly enhanced TPA cross sections of 34700, 16400, 11000, and 7820 GM, respectively for C70TCTA, C60-TCTA, C70-BCzMB, and C70-MQEtCz in toluene with the concentration of ∼10-4 M. More importantly, the TPA cross-section is increased to 1.04 × 105 GM for C70-TCTA at a lower concentration of 10-5 M. The concentration dependence is attributed to the formation of molecular aggregates at higher concentrations.Theirradiance-dependentZ-scansandpump-probe experiments show that the measured nonlinear absorption in the irradiances up to 240 GW/cm2 be purely due to the TPA processes. As far as the structure-TPA relationship concerns, our findings show that the TPA cross sections increase parabolically with the degree of conjugation and the number of branches. It is shown that strong D and A with large conjugation and multibranched structures is an effective way to enhance TPA cross sections. On the basis of the TPA in the fullerene derivatives, superior performance is realized for the optical limiting of femtosecond laser pulses at 780 nm. Our findings open a new avenue for designing highly efficient TPA molecules for nonlinear optical applications, in particular, for optical limiting. Experimental Section Materials. 1,3,5-Tribromobenzene, tris(4-bromophenyl)amine, 4,4′-dibromobiphenyl, 4,4′-bis (bromomethyl)biphenyl, carbazole, N-ethyl-carbazole-3-carbaldehyde, 2-methylquinolin-8-ol, and C60, C70 were purchased from TCI. CuI, K2CO3, and 1,10-
Ouyang et al. phenathroline were obtained from Guangzhou chemical reagent company. The solvents were dried using standard procedures. The compounds TCTA, MQEtCz, and BCzMB were prepared according to the literature.34 All other reagents were used as received from commercial sources, unless otherwise stated. Characterization. Melting points were determined using an Electro-thermal IA 900 apparatus and the thermometer was uncorrected. FT-IR spectra were recorded on a Perkin-Elmer Fourier transform infrared spectrometer and measured as KBr pellets. 1H NMR spectra were determined in DMSO, CDCl3 or (CD3)2CO with a Bruker DRX 400 MHz spectrometer. Chemical shifts (δ) were given relative to tetramethylsilane (TMS). The coupling constants (J) were reported in Hz. Elemental analyses were recorded with a Perkin-Elmer 2400 analyzer. ESI-Ms, APCI-Ms, and EI-Ms spectra were performed with a FINNIGAN Trace DSQ mass spectrometer at 70 eV using a direct inlet system. MALDI-TOF-MS spectra were acquired using a Voyager-DE STR MALDI-TOF mass spectrometer (PerSeptive Biosystems) in the linear positive mode with delayed extraction. Samples were analyzed using a dichloromethane solution of the sample (1 L) mixed with 10 L of dithranol matrix (10 mg mL-1 in 70:30 CH2Cl2/EtOH) before loading onto a metal sample plate. The experiment course was monitored by TLC. Column chromatography was carried out on silica gel (100-200 mesh). Elemental analyses were performed by the FINNIGAN C, H, N, S analyzer. Synthesis of Compound BCzMB-CHO. Anhydrous DMF (27 mL, 12 m mol) was cooled in an ice-water bath, and then phosphorus oxychloride (2.4 mL, 1.2 m mol) was added dropwise, with stirring, from a syringe placed through the rubber septum. A bright yellow Vilsmeier-Haack reagent was formed. BCzMB (0.523 g, 1.2 m mol) was added quickly to the Vilsmeier-Haack reagent. After stirring for 30 min, the mixture was heated at 90 °C for one night. After cooling to room temperature, 1 M sodium acetate was added to neutrality and the mixture was stirred vigorously for 1 h. the solution was extracted with dichloromethane, the organic phase was dried in vacuum. Then mixture, purified by flash column chromatography with CH2Cl2 and petroleum ether as eluent, afforded a yellow powder. Yield: 0.249 g, 44.8%; mp 243-245 °C. FTIR (KBr) ν (cm-1): 3065.74, 3011.34, 2948.31, 1682.76, 1597.68, 1496.67, 1454.47, 1326.78, 1216.53, 1121.83, 1012.59, 917.59, 832.44. 742.16, 727.34, 627.58. APCI-MS (M++H+): 464.9. 1H NMR (CDCl3) δ: 10.08 (s, 1H), 8.62 (s, 1H), 8.15 (d, 1H, J ) 8.8 Hz), 8.12-8.09 (m, 2H), 7.94 (d, 1H, J ) 9.2 Hz), 7.47-7.44 (m, 1H), 7.42-7.40 (m, 2H), 7.39-7.37 (m, 2H), 7.36 (d, 2H, J ) 8.4 Hz), 7.34-7.32 (m, 1H), 7.30-7.20 (m, 2H), 7.01 (d, 4H, J ) 8.2 Hz); 5.46 (s, 4H). Anal. Found: C, 85.54; H, 5.17; N, 6.24. Calcd for C33H24N2O: C, 85.32; H, 5.21; N, 6.03. Synthesis of Compound C60-BCzMB. A mixture of C60 (150 mg, 0.23 m mol), BCzMB-CHO (54 mg, 0.115 m mol), and N-methylglycine (82 mg, 0.92 m mol) was refluxed for 10 h in degassed anhydrous chlorobenzene under nitrogen atmosphere. After solvent removal, the residue was purified by chromatography with toluene and cyclohexane as eluent to give a black solid. Yield: 56 mg, 40.2%. FT-IR (KBr) ν (cm-1): 3009.72, 2995.51, 1596.14, 1501.42, 1408.47, 1319.55, 1215.64, 1127.78, 1017.16, 833.52. 739.36, 719.12, 523.46. MALD-TOF-Ms (M+): 1210.7. 1H NMR (C6D6) δ: 8.16 (d, 2H, J ) 7.6 Hz), 8.08 (d, 1H, J ) 7.6 Hz), 7.41 (t, 2H, J ) 7.6 Hz), 7.34-7.30 (m, 3H), 7.18 (d, 2H, J ) 7.6 Hz), 7.12-7.08 (m, 4H), 7.05 (d, 2H, J ) 8 Hz), 6.67 (s, 4H), 5.07 (s, 1H), 4.83 (s, 2H), 4.70 (s, 2H),
Properties of C70/C60 Derivatives with Carbazole 4.02 (d, 1H, J ) 9.2 Hz), 2.82 (s, 3H). Anal. Found: C 94.17, H 2.37, N 3.83. Calcd for C95H29N3: C 94.14, H 2.39, N 3.47. Synthesis of compound C70-BCzMB. A mixture of C70 (201.1 mg, 0.24 m mol), N-methylglycine (76.1 mg, 0.86 m mol) and BCzMB-CHO (110 mg, 0.24 m mol) in dry chlorobenzene (230 mL) was heated at reflux under nitrogen for 8 h. After cooling the solution to room temperature, the solvent was removed, and the residue was purified by chromatography with toluene and cyclohexane as eluent to give a black solid. Yield: 32 mg, 21.7%. FT-IR (KBr) ν (cm-1): 3037.56, 2997.31, 2921.73, 1599.17, 1502.42, 1417.57, 1316.29, 1121.97, 1011.23, 831.22, 794.36, 748.93, 673.32, 641.88, 475.33. MALD-TOFMS (M++1): 1331.5. 1H NMR (CDCl3+CS2) δ: 8.18 (d, 2H, J ) 7.6 Hz), 8.07 (d, 1H, J ) 7.6 Hz), 7.43-7.28 (m, 6H), 7.16-7.03 (m, 7H), 6.63 (s, 4H), 5.27 (s, 1H), 4.78 (s, 4H), 4.52 (s, 1H), 4.18 (s, 1H), 2.48, 2.54 (major isomer), 2.58 (s, 3H). Anal. Found: C, 94.79; H, 2.11; N, 3.29. Calcd for C105H29N3: C, 94.66; H, 2.18; N, 3.16. Synthesis of Compound TCTA-CHO. The synthesis process is similar to that of BCzMB-CHO, DMF (27 mL, 12 m mol), phosphorus oxychloride (2.4 mL, 1.2 m mol), and TCTA (0.888 g, 1.2 m mol). Yellow powder was obtained. Yield: 0.417 g, 45.3%; mp 297-299 °C. FT-IR (KBr) ν (cm-1): 3082.54, 2955.27, 2877.44 1681.82, 1598.41, 1508.63, 1417.47, 1305.88, 1215.39, 1152.94, 1025.75, 922.16, 811.23. 746.53, 722.35, 424.33. APCI-MS (M++H+): 769.1. 1H NMR (CDCl3) δ: 10.12 (s, 1H), 8.67 (s, 1H), 8.21 (d, 1H, J ) 7.6 Hz), 8.15 (d, 4H, J ) 8 Hz), 7.98 (d, 1H, J ) 8.2 Hz), 7.58 (d, 4H, J)8.8 Hz), 7.49-7.55 (m, 15H), 7.41-7.45 (m, 5H), 7.29 (t, 4H, J ) 8.4 Hz). Anal. Found: C, 85.81; H, 4.54; N, 7.13. Calcd for C55H36N4O: C, 85.91; H, 4.72; N, 7.29. Synthesis of Compound C60-TCTA. The synthesis process is similar to that of C60-BCzMB, C60 (150 mg, 0.23 m mol), TCTA-CHO (54 mg, 0.115 m mol), and N-methylglycine (82 mg, 0.92 m mol), black solid was obtained. Yield: 74 mg, 42.7%. FT-IR (KBr) ν (cm-1): 3051.32, 2975.59, 1594.41, 1498.57, 1406.57, 1332.72, 1216.73, 1121.19, 1021.48, 839.25. 798.43, 754.51, 676.13, 644.62, 521.37, 481.22. MALD-TOFMs (M++1): 1516.3. 1H NMR (CDCl3+CS2) δ: 8.10 (d, 5H, J ) 7.8 Hz), 7.57 (d, 6H, J ) 8.8 Hz), 7.51 (d, 7H, J ) 8.4 Hz), 7.47-7.43 (m, 6H), 7.41 (t, 6H, J ) 7.8 Hz), 7.29-7.25 (m, 5H), 5.27-5.17 (m, 3H), 4.37 (s, 1H), 2.91 (s, 3H). Anal. Found: C, 92.81; H, 2.54; N, 4.75. Calcd for C117H41N5: C, 92.67; H, 2.71; N, 4.62. Synthesis of Compound C70-TCTA. The synthesis process is similar to that of C70-BCzMB, C70 (201.1 mg, 0.24 mmol), N-methylglycine (76.1 mg, 0.86 mmol), and TCTA-CHO (184 mg, 0.24 mmol), black solid. Yield: 80 mg, 20.5%. FT-IR (KBr) ν (cm-1): 3071.46, 2988.54, 1598.48, 1501.46, 1413.09, 1322.76, 1215.83, 1118.17, 1010.18, 834.52. 794.81, 752.43, 671.52, 640.78, 480.23. MALD-TOF-Ms (M++1): 1636.0. 1H NMR (CDCl3+CS2) δ: 8.27 (d, 1H, J ) 7.8 Hz), 8.08-8.11 (m, 5H), 7.69 (t, 1H, J ) 7.8 Hz), 7.39-7.58 (m, 23H), 7.23-7.26 (m, 5H), 5.27 (s, 1H), 4.74 (s, 1H), 4.39 (s, 1H), 2.45, 2.53 (major isomer), 2.6 (s, 3H). Anal. Found: C, 93.34; H, 2.25;N, 4.34. Calcd for C127H41N5: C, 93.21; H, 2.51; N, 4.28. Synthesis of Compound MQEtCz-CHO. The synthesis process is similar to that of BCzMB-CHO, DMF (27 mL, 1.2 m mol), phosphorus oxychloride (2.4 mL, 1.2 m mol), (E)-9-ethyl3-(2-(8-methoxyquinolin -2-yl)vinyl)-9H-carbazole (0.458 g, 1.2 m mol). Yellow powder was obtained; yield ) 0.221 g, 45.7%; mp ) 205-207 °C. FT-IR (KBr) ν (cm-1): 2925, 1733, 1675, 1324, 1589, 1558, 1483, 1376, 1330, 1233, 1192, 1131, 1102, 965, 816, 756, 713, 583 APCI-MS (M++H+): 406.8. 1H
J. Phys. Chem. B, Vol. 113, No. 44, 2009 14571 NMR (CDCl3) δ: 10.12 (s, 1H), 8.64 (s, 1H), 8.40 (s, 1H), 8.14 (d, 1H, J ) 8.7 Hz), 8.04 (d, 1H, J ) 8.4 Hz), 7.93-7.82 (m, 3H), 7.48-7.37 (m, 2H), 7.29-7.26 (m, 2H), 7.09 (d, 1H, J).7.6 Hz), 4.40 (q, 2H, J ) 7.20 Hz), 4.11 (s, 1H), 1.33 (t, 3H, J ) 7.5 Hz). Anal. Found: C, 79.71; H, 5.37; N, 6.93. Calcd for C27H22N2O2: C, 79.79; H, 5.46; N, 6.89. Synthesis of Compound C60-MQEtCz. The synthesis process is similar to that of the C60-BCzMB, C60 (150 mg, 0.23 m mol), MQEtCz-CHO (46 mg, 0.115 m mol), and N-methylglycine (82 mg, 0.92 m mol), black solid; 41.5% yield. FT-IR (KBr) ν (cm-1): 3015.33, 2925.43, 1594.25, 1556.44, 1488.36, 1461.23, 1322.71, 1259.44, 1231.91, 1106.68, 978.86, 856.57, 527.42, 477.43. MALDI-TOF-MS (M+): 1152.4. 1H NMR (C6D6) δ: 8.39 (d, 1H, J ) 16 Hz), 8.17 (s, 1H), 7.78 (d, 2H, J ) 8.8 Hz), 7.63 (d, 1H, J ) 16 Hz), 7.47 (s, 1H), 7.40 (d, 1H, J ) 8.4 Hz), 7.33-7.29 (m, 3H), 7.08 (s, 1H), 7.01 (d, 1H, J ) 8.4 Hz), 6.81 (t, 1H, J ) 8 Hz), 5.13 (s, 1H), 5.05 (d, 1H, J ) 8.8 Hz), 4.09 (d, 1H, J ) 8.8 Hz), 3.77 (s, 3H), 3.67 (q, 2H, J ) 7.6 Hz), 2.82 (s, 3H), 0.94 (t, 3H, J ) 7.6 Hz). Anal. Found: C, 92.51; H, 2.47; N, 3.77. Calcd for C89H27N3O: C, 92.63; H, 2.34; N, 3.64. Synthesis of Compound C70-MQEtCz. The synthesis process is similar to that of C70-BCzMB, C70 (201.1 mg, 0.24 m mol), N-methylglycine (76.1 mg, 0.86 m mol), and MQEtCzCHO (92 mg, 0.24 m mol), black solid. Yield: 68 mg, 22.3%. FT-IR (KBr) ν (cm-1): 3015.48, 2989.72, 2891.42 1601.17, 1498.72, 1409.05, 1312.59, 1124.36, 1012.09, 831.71. 791.57, 751.07, 676.25, 642.54, 484.67. MALD-TOF-Ms (M+): 1272.8. 1 H NMR (CDCl3+CS2) δ: 8.34 (d, 1H, J ) 16 Hz), 8.14 (s, 1H), 7.72 (d, 2H, J ) 8.8 Hz), 7.58-7.42 (m, 3H), 7.31- 7.26 (m, 3H), 7.05-7.02 (m, 2H), 6.78 (m, 1H), 5.13(s, 1H), 4.69 (s, 1H), 4.32 (s, 1H) 3.75 (s, 3H), 3.62 (q, 2H, J ) 7.6 Hz), 2.47, 2.55 (major isomer), 2.61 (s, 3H), 0.89 (t, 3H, J ) 7.6 Hz). Anal. Found: C, 93.37; H, 2.08; N, 3.39. Calcd for C99H27N3O: C, 93.32; H, 2.12; N, 3.30. Z-Scans and Optical Limiting Measurements. The molecular two-photon absorption (TPA) cross sections were measured by a Z-scan technique. The laser pulses were produced by a mode-locked Ti:sapphire laser (Quantronix, IMRA), which seeded a Ti:sapphire regenerative amplifier, and focused onto a 1-mm-thick (or 1-cm-thick) quartz cuvette containing the solutions of these molecules with the minimum beam waist of 25 ( 5 µm. The incident and transmitted laser pulse energy were monitored by moving the cuvette along the propagation direction of the laser pulses. The maximum on-axis irradiance of the laser pulses was ∼240 GW/cm2 at the focus. The Z-scan data (or the transmittance as a function of z position) were normalized to the linear (small-signal) transmittance. The nonlinear-optical signal from the solvent was negligible, compared to the signals from the fullerene derivatives. The Z-scans confirmed that the samples had surprisingly good photostability, which was verified by indifference between the linear absorption spectra measured before and after intense laser irradiation in the Z-scans. In addition, the TPA coefficients, β, could be extracted from the best fitting between the Z-scan theory and the data,35 and the TPA cross sections were then calculated from the definition σ2 ) βpω/N, where pω is the photon energy and N is the molecular concentration. The Z-scan experimental system was calibrated using a piece of cadmium sulfide (CdS) bulk crystal as a reference because it possesses large TPA at the wavelength of 780 nm and has been well investigated in our laboratory. The TPA coefficient of CdS was determined to be 6.4 ( 0.6 cm GW-1, which was in good agreement with theoretical values within the experiment uncertainty. The optical
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limiting measurements were carried out in the same experiment setup as the Z-scans, except that the incident pulse energy and lens were varied and the toluene solutions of C70-TCTA, C60TCTA, C70-MQEtCz, C60-MQEtCz, C70-BCzMB, and C60BCzMB were contained in 1-cm-path quartz cuvette and fixed at the focus. Z-Scan Theories on the Two-Photon Absorbers. Owing to the condition that the photon energy of laser pulses is less than the band gap energy (Ephoton < Eg), the attenuation of a light beam passing through an optical medium can be generally expressed by the following phenomenological expression:
dI ) -(R0 + dz'
∑ RmI
m-1
+ σaNe-h)I
(1)
m)2
where σa is the singlet excited-state absorption cross-section of photoexcited charge carriers; Ne-h is the number density of the photoexcited charge carriers; R0 is the linear absorption coefficient, Rm is the multiphoton absorptive coefficient (m ) 2 for TPA); and I is the irradiance within the sample. If we keep the TPA term and ignore all other terms on the right side of eq 1, we can analytically solve eq 1 for open-aperture Z-scans on two-photon absorbers. By assuming a spatially and temporally Gaussian profile for incoming laser pulses, the normalized energy transmittance can be derived as,
TOA(z) )
1 1/2
π q0
∫-∞∞ [1 + q0 exp(-x2)] dx
q0 ) β × I0 × Leff I0 )
I00 z2 1+ 2 z0
(2) (3) (4)
Leff ) [1 + exp(-R0L)]/R0
(5)
z0 ) πω0 /λ
(6)
2
where z is the z-position, and z0 is the Rayleith range; ω0 is the minimum beam waist at the focal point (z ) 0); λ is the laser free-space wavelength; I0 is the on-axial peak intensity at different z-position; q0 is the on-axis peak phase shift caused by TPA processes; I00 is the on-axial peak intensity at the focal position and Leff denotes the effective sample length; L is the sample length. If the excited-state absorption plays an important role, the above measured TPA cross-section should depend on the laser irradiance. However, the independence of the TPA cross-section eliminates the role of the excited-state absorption. Pump-probe Measurement. The standard time-resolved transient absorption measurement (or degenerate pump-probe experiment) was carried out using 450-fs laser pulses at 1 kHz repetition rate with lower average power to minimize accumulative thermal effects. The laser pulses were produced by the same laser used in the Z-scans described above. To eliminate any coherent effects, the polarizations of the pump and the probe pulses were perpendicular to each other. The pump and probe beams were focused onto the solutions of these samples with a minimum beam waist of 30 ( 5 µm. The probe pulse energy was a small fraction (
