14 July 2000

Ž .Chemical Physics Letters 324 2000 354–358www.elsevier.nlrlocatercplett

Ultrafast third-order non-linear optical response of Diels–Alderadduct of fullerene C with a metallophthalocyanine60

Wentao Huang a, Shufeng Wang a, Ruisheng Liang a, Qihuang Gong a,),Wenfeng Qiu b, Yunqi Liu b, Daoben Zhu b

a National Laboratory for Mesoscopic Physics and Department of Physics, Peking UniÕersity, Beijing 100871, Chinab Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

Received 29 February 2000

Abstract

The third-order optical non-linearity of a Diels–Alder adduct of fullerene C with a copper phthalocyanine was60

investigated using the femtosecond time-resolved optical Kerr gate method at 830 nm wavelength. Its non-resonantsecond-order hyperpolarizability was estimated to be 5.4=10y31 esu. The non-linearity enhancement from the formation ofthe intramolecular charge-transfer complex is discussed. q 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

Delocalized p-conjugated molecules and poly-mers have been considered as promising candidatesfor future practical applications in non-linear opticalŽ .NLO devices. To obtain the non-resonant largethird-order NLO response, various chemical struc-tures have been designed and different compoundssynthesized. As an effective rule, the formation of

Ž .charge-transfer CT complexes has been applied tothe enlargement of third-order optical non-linearityw x1,2 . By mixing several components in solutions orsolid films to form intermolecular CT complexes, orby combining selected radicals to synthesize intra-molecular ones, large enhancement of optical non-linearity in many CT systems has been experimen-

) Corresponding author. Fax: q86-10-6275-6567; e-mail:qhgong@ibm320h.phy.pku.edu.cn

w xtally and theoretically discussed 3–12 . As a uniquethree-dimensional cage molecule, C has a large60

p-conjugated electronic system and one of its ac-knowledged characters is its electron deficiency andability to undergo multistage reduction. With connec-tion to an electron sufficient moiety or suchmolecules, intramolecular or intermolecular CT is

w xeasily obtained 13–15 . Evidence for the enhance-ment of the optical non-linearity was exhibited. Li etal. have studied the third-order optical non-linearity

w xof C –poly-aminonitrile 16 . Kafafi et al. and Gong60

et al. have researched the NLO responses of films ofw Ž XC with meso-tetraphenyl, poly- 2-methoxy,5- 2 -60

. xethylhexyloxy –p-phenylenevinylene , and poly-te-w xtrahydrofurfuryl methacrylate, respectively 17,18 .

All results revealed great CT enhancement in theNLO property mainly due to promotion of the delo-calization of electrons inducing electric polarizationon a larger spatial scale and producing C nywhile a60

CT complex formed.

0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 00 00617-5

( )W. Huang et al.rChemical Physics Letters 324 2000 354–358 355

In this Letter, we investigate the third-order NLOproperties of a Diels–Alder adduct of C with a60

Ž .copper phthalocyannine CuPc . For their extremelylarge two-dimensional p-conjugated electronic sys-

Ž .tems, the metallophthalocyanines MPc have shownrelatively splendiferous NLO properties and have

w xgained great attention for years 2 . The adduct com-bines the unique structures of C and CuPc, which60

may lead to special characters. In fact, second-orderhyperpolarizability g of the adduct was measured tobe as large as 5.4=10y31 esu using the femtosecond

Ž .time-resolved optical Kerr effect OKE technique.

2. Experiments

2.1. Method

In order to obtain the NLO signal of a samplesolution, we performed the ultrafast optical Kerr gatemethod with the ultrashort laser pulses generated

Žfrom a Ti:Sapphire laser Mira 900F, Coherent Co..Ltd., USA . The laser output with wavelength cen-

tered at 830 nm was split into strong pump and weakprobe beams with an intensity ratio of 11:1. Thepolarization plane of the probe beam was rotated by458 with respect to the linear polarization of thepump beam. An optical delay line driven by a com-puter-controlled step motor was inserted into theoptical path of the pump beam to obtain a time-re-solved signal. Two beams were focused to overlapspatially in the sample solution filling a 1-mm-thick

Ž .glass cell. An analyzer polarizer with a transmis-sion axis perpendicular to the probe beam’s originalpolarization was inserted just before a photodiode soonly the generated orthogonal signal, the optical Kerrsignal, could be detected. In order to decrease theundesirable noise, the pump and probe beams werechopped at different frequencies, and the signal wasamplified at the sum frequency by a lock-in ampli-fier connected to the photodiode, then the ArDtransferred to a computer which was also used tomonitor and control the measurement. By replacingthe sample cell with a 0.3-mm-thick BBO crystal, wedetermined the zero delay point and obtained theautocorrelation signal of the laser beam. The pulseduration was then deduced to be 120 fs. A more

detailed apparatus configuration has been describedw x16,19 .

2.2. Material

Asymmetrical CuPc was synthesized by a mixedcondensation reaction of diiminoisoindoline productsof two corresponding phthalonitriles. Several drops

w x Ž .of 1,8-diazabicyclo 5,4,0 undec-7-ene DBU wasused as a catalyst. The target compound CuPc–C60

was obtained from a Diels–Alder adduct of unsym-metrical CuPc and 10-fold excess of C in toluene60

under argon for 2 days. After reaction, the mixturewas separated by chromatography though a silica gelcolumn using toluene as eluent to get pure CuPc–C .60

Its chemical structure was characterized by FD-MS,w xUV-VIS, IR, NMR and elemental analysis 20 . Re-

serving the unique structures of CuPc and C ,60

CuPc–C may have excellent properties such as60

superconductivity or ferromagnetism and potentialapplications as a NLO material. The structures ofmolecules C , CuPc and CuPc–C are illustrated60 60

in Fig. 1.Fig. 2 shows the UV-VIS absorption spectra of

C , CuPc and CuPc–C in toluene at the same60 60Ž y6 .concentration 1=10 M . For solution of CuPc–

C , two separate absorption bands were observed60

Fig. 1. Molecular structures of C , CuPc and CuPc–C .60 60

( )W. Huang et al.rChemical Physics Letters 324 2000 354–358356

Fig. 2. Absorption spectra of C , CuPc and CuPc–C in toluene60 60

at a concentration of 1=10y6 M.

and no absorption was found around 830 nm. Thesolutions of CuPc–C in toluene at concentrations60

of 13.3=10y5 M, 8.9=10y5 M and 5.9=10y5 Mwere prepared for the ultrafast OKE measurement.

Ž .Meanwhile, cyclic voltammetry CV measurementwas used to estimate the energies of charge-sep-arated states. The CVs of C , CuPc and CuPc–C60 60

Žin pyridine 0.1 M tetra-n-butylammonium tetrafluo-.roborate, Bu NBF at a scan rate of 50 mVrs are4 4

shown in Fig. 5.

3. Results and discussions

We followed the standard reference measurementto obtain the sample solution’s third-order NLO sus-

Ž3. w xceptibility x 21 . The following equation1r2 2I n a lS SŽ3. Ž3.x sx 1Ž .S R ya l ya l r2ž /ž /I n 1ye eŽ .R R

was used for calculation. The subscripts ‘S’ and ‘R’represent, respectively, the parameters for the sampleand reference, and I and n indicate the intensity ofthe OKE signal and refractive index, respectively.The last fraction comes from the sample absorptionand equals 1 while the sample has no absorptionaround the employed laser wavelength. The excellentand stable third-order optical response solvent, CS ,2

was selected as reference. The ns1.62 for CS and2

its x Ž3. is estimated to be 1=10y13 esu on thew xfemtosecond timescale 22 . Since the sample solu-

tion concentration is very low, we can use the n oftoluene at 208C as its refractive index.

Fig. 3. The OKE response of the solution of CuPc–C in toluene60

with concentrations of 13.3=10y5 M, 8.9=10y5 M and 5.9=

10y5 M.

The OKE signals of CuPc–C in toluene solu-60

tions at different concentrations were measured andare illustrated in Fig. 3. The relatively weak back-ground noise and contribution of the solvent has

Ž .been subtracted. According to Eq. 1 , with themeasured signal of CS under identical conditions,2

the x Ž3. of the solution at 8.9=10y5 M was de-duced to be 1.14=10y13 esu. From the figure, wenotice that the NLO response is on the same timescaleas the pulse duration and no delayed process isobserved. This instantaneous response proves thatthe NLO response of CuPc–C in toluene was60

mainly derived from the delocalized electron re-w xsponse 23 .

Meanwhile, a linear dependence of the measuredx Ž3. on the solution concentration was proved and isshown in Fig. 4. It was concluded that our experi-

Fig. 4. Linear fitting of the concentration dependence of third-orderNLO susceptibility x Ž3..

( )W. Huang et al.rChemical Physics Letters 324 2000 354–358 357

mental measurement was reliable and the followingequation

gsx Ž3.rNL 2Ž .can be introduced to estimate the second-order hy-perpolarizability g of the sample molecule. In Eq.Ž .2 , N is the number density of the molecule and L

wŽ 2is the local field correction factor given by n q. x4 Ž . Ž3.2 r3 . Using Eq. 2 , we calculated g from the x

of the sample solution at a certain concentration andaveraged the three values as the most acceptable g

of the sample molecule; a magnitude of 5.4=10y31

esu was deduced which is relatively large amongthose for different C derivatives compared with60

our previous reports, in which the g was found to beless than 9.0=10y35 esu for C and 3.2=10y33

60

esu for C -poly-amininonitrile under almost the60

same conditions. The value for CuPc–C is 600060

times of that for C and 170 times larger than that60w xfor C -poly-amininonitrile 16 . As for the NLO of60

CuPc, it is well known that metallophthalocyaninesŽ .MPc have larger third-order non-linearity amongorganic compounds and the magnitude for non-reso-

y32 y33 w xnant g is ca. 10 –10 esu 2,24 . As MPcobviously aggregate in solution, even at very low

y5 w xconcentration, for example 1=10 M 25 , it isdifficult for us to measure the magnitude of g forCuPc due to our experimental limits. However, themagnitude of g for CuPc–C , which is one order of60

magnitude larger than the values reported for MPcs,

obviously indicates the enhancement of third-ordernon-linearity of CuPc–C .60

We believe that such enhancement is attributed tothe formation of an intramolecular CT complex ofCuPc–C . In Fig. 2, the absorption spectra of C ,60 60

w xCuPc and CuPc–C give an intuitive illustration 7 .60

The two characteristic absorption peaks of CuPc–C60

at 676 and 282.5 nm are obviously derived from itsCuPc and C parts, respectively. The absorption60

decreases compared to CuPc at 676 nm where C60

has no absorption, and increases compared to thesum of the absorptions of C and CuPc at 282.5 nm.60

This shape shifting implies an electron density changeof the two component parts and suggests electronictransfer from CuPc to C . This is further proved60

from the cyclic voltammetric results in Fig. 5. TheCV of Pc–C shows four quasi-reversible reduction60

couples. The redox potentials of the couples arey0.49, y0.84, y1.17 and y1.46 V, respectively.The first reversible reduction of CuPc–C occurs at60

y0.49 V and corresponds only to the formation ofthe C mono-anion according to the feature of C60 60

and CuPc, as shown in Fig. 5. Comparison with thefirst reduction potential of C at y0.39 V clearly60

shows a decrease of 100 mV; thus it is more difficultto deoxidize the C moiety of CuPc–C than C60 60 60

itself, indicating that the C moiety contains more60

electrons than C molecule and thus the intramolec-60

ular CT is confirmed. Similar conclusions have beenw xdiscussed by Linssen et al. 26 . The formation of the

Ž . Ž . Ž .Fig. 5. Cyclic voltammetry of C insert small one , CuPc . . . and CuPc–C — .60 60

( )W. Huang et al.rChemical Physics Letters 324 2000 354–358358

CT complex can enhance the delocalized movementof the p electrons, which also enhances the optical

w xnon-linearity 7 .

4. Conclusion

In summary, in forming a CT complex of CuPc–C , the instantaneous second-order hyperpolariz-60

ability of the CuPc–C molecule was estimated to60

be as large as 5.4=10y31 esu. This extremely largemagnitude is orders of magnitude greater than for thecomponents CuPc or C . The results lead to the60

conclusion that NLO properties can be enhancedstrongly by forming an intramolecular CT complex.

Acknowledgements

The work was supported by the National Climb-Ž .ing Project 97.3 of China under Grant

G1999075207 and the National Natural ScienceFoundation of China under Grants 19884001,19974004, 69890228 and 19525412. This work wasalso supported by the Chinese Academy of Sciences.

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