J. Phys. Chem. B 1999, 103, 10741-10745
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Cooperative Enhancement of Two-Photon Absorption in Multi-branched Structures Sung-Jae Chung, Kyoung-Soo Kim, Tzu-Chau Lin, Guang S. He, Jacek Swiatkiewicz, and Paras N. Prasad* Departments of Chemistry and Physics, Photonics Research Laboratory, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 ReceiVed: August 11, 1999; In Final Form: October 1, 1999
Recent reports of molecular structures with considerably enhanced two-photon absorption cross-section have generated considerable interest in this phenomenon from both fundamental and applications perspectives. In this letter, we report cooperative enhancement of two-photon absorption in multi-branched structures which may lead to new design criteria for development of highly efficient two-photon materials. The multi-branched structures were synthesized using coupling of two and three two-photon active asymmetric donor-acceptor chromophores linked together by a common amine group. The two-photon cross-sections measured both with nanosecond and femtosecond pulses show that the value for the trimer is more than six times larger than that for the monomer, and not just three times larger as expected from the number density increase.
Introduction The two-photon process involving simultaneous absorption of two photons was theoretically predicted by M.Goppert-Mayer in 1931,1 and was experimentally observed in the 1960s.2-5 Pioneering work by Rentzepis in data storage6 and by Webb7 in microscopy demonstrated early potential applications of twophoton processes. However, until recently, most known molecular two-photon absorptivities were too small to find usage in many practical applications. Reports of new dyes with increased two-photon absorption (TPA) cross-sections and large upconverted fluorescence yields by our group8-11 and other groups12,13 have generated considerable interest in the development of highly efficient two-photon materials and a myriad of new applications has opened up.14 New applications include twophoton upconversion lasing,15,16 two-photon optical power limiting,17-19 three-dimensional optical data storage,20,21 and two-photon photodynamic therapy.22 Multiphoton microscopy appears to be of great value as an imaging technique for numerous biological systems23,24 as well as for nondestructive evaluation of organic paints and coatings.24 Molecular structures used so far to develop efficient twophoton chromophores utilize either a symmetrically (terminating in two electron-donor or two electron-acceptor groups) or asymmetrically (terminating in a donor and an acceptor) substituted π-conjugation unit.9-14 The effect of varying the electron-donating or electron-accepting strength of the end groups, introducing additional groups in the middle to vary the charge redistribution, and varying the effective conjugation length has been studied in order to achieve at design criteria to produce structures with enhanced two-photon activity.13 In this paper we report the discovery of cooperative enhancement of two-photon absorption in multi-branched structures, which may lead to new design criteria for development of multibranched polymers and dendrimers with very strong two-photon activities. We report on the synthesis of these new structures, which involve linkage of asymmetrically substituted two- or three-chromophore units through a common amino group. Both * Corresponding author.
the nonlinear transmission method utilizing nanosecond pulses and the Z-scan technique utilizing femtosecond pulses have been used to obtain effective two-photon absorption cross-sections and to examine the effect of pulse-width on the effective crosssection. Finally, we discuss some possible mechanisms of this cooperative enhancement. Experimental Section Synthesis. We have used triphenylamine as the π-electron donor25 together with 2-phenyl-5-(4-tert-butylphenyl)-1,3,4oxadiazole26,27 as the π-electron acceptor. This structural unit serves as a building block in the monomeric material. The respective monomeric, dimeric, and trimeric structures synthesized and studied are N-[4-{2-(4-{5-[4-(tert-butyl)phenyl]-1,3,4oxadiazol-2-yl}phenyl)-1-ethenyl}phenyl]-N,N-diphenylamine (abbreviated as PRL-101), N,N-bis[4-{2-(4-{5-[4-(tert-butyl)phenyl]-1,3,4-oxadiazol-2-yl}phenyl)-1-ethenyl}phenyl]-Nphenylamine (abbreviated as PRL-501), and N,N,N-tris[4-{2(4-{5-[4-(tert-butyl)phenyl]-1,3,4-oxadiazol-2-yl}phenyl)-1ethenyl}phenyl]amine (abbreviated as PRL-701). Their structures are shown in Figure 1. The synthetic routes to the chromophores are shown in Scheme 1. In the final steps, PRL-101 and PRL-501 were synthesized by Wittig reaction28 of phosphonium bromide of the oxadiazole compound (8) with the aldehyde-functionalized triphenylamine compounds (1 and 4) in 63% and 47% yields. PRL-701 was obtained by reaction of the vinyl oxadiazole compound (9) with tribromo triphenylamine (3) under palladium-catalyzed Heck conditions,29 32% yield. All compounds were thoroughly washed with methanol and then were purified by column chromatography. The pure chromophores are highly soluble in a variety of organic solvents such as methylene chloride, chloroform, THF, 1,1,2,2-tetrachloroethane, etc., and are fluorescent greenish yellow in the solid state. Characterization. All optical characterizations were conducted using solutions of compounds in 1,1,2,2-tetrachloroethane. UV-visible absorption and fluorescence studies were conducted using a Shimadzu UV-3101PC spectrophotometer and a Shimadzu RF5000U spectrofluorophotometer. TPA cross-
10.1021/jp992846z CCC: $18.00 © 1999 American Chemical Society Published on Web 11/18/1999
10742 J. Phys. Chem. B, Vol. 103, No. 49, 1999
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Figure 1. Chemical Structures of PRL-101, PRL-501, and PRL-701.
Figure 2. UV-visible absorption spectra of PRL-101, PRL-501, and PRL-701 solutions in 1,1,2,2-tetrachloroethane (1 × 10-5 mol/L).
section values of the three compounds in solution were measured with both nanosecond (810 nm) and femtosecond (796 nm) pulses. We have used direct nonlinear transmission measurements19 at ∼810 nm with ∼8 ns pulses from a Nd:YAG pumped dye laser at the intensity levels of several hundreds of megawatt per square centimeter. The experimental setup and data processing procedure are basically the same as those described in previous publications.19,30 Femtosecond Z-scan measurements were performed using the same solutions as for the nanosecond nonlinear transmission measurements. The ultrafast laser system and the Z-scan technique are described elsewhere.31 For these experiments, the average pulse length was 173 fs and irradiance at the focal point was 36 GW/cm2 at a wavelength of 796 nm. The repetition rate was kept low at 32 Hz, to avoid any cumulative effect from slow photophysical/photochemical processes. The irradiance was reduced to a level at which the
Figure 3. One-photon emission spectra of PRL-101, PRL-501, and PRL-701 solutions in 1,1,2,2-tetrachloroethane (1 × 10-5 mol/L) obtained at the excitation wavelength of 405 nm.
nonlinear absorption coefficient was found to be constant, to reduce contributions from other nonlinear effects. Results and Discussion Figure 2 compares the UV-visible absorption spectra of the chromophores in solution. It shows that there are two strong absorption bands attributed to the solute molecules. The absorption at about 300-320 nm is localized in the phenylene rings, whereas the longer wavelength region absorption (about 400 nm) has considerable charge-transfer character.32,33 PRL101 shows a peak of its one-photon absorption at 399 nm, whereas for PRL-501 and PRL-701 the peaks are shifted to 417 and to 426 nm, respectively. One-photon emission spectra of the chromophores, obtained at the excitation wavelength of 405 nm, are compared in Figure 3. As one can see, the λmax
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J. Phys. Chem. B, Vol. 103, No. 49, 1999 10743
SCHEME 1. Synthetic Routes for PRL-101, PRL-501, and PRL-701
values of emission also shift from 503 nm for PRL-101 to 510 nm for PRL-501 and 516 nm for PRL-701, respectively. That is, the same trend is observed in the UV-visible absorption and the emission spectra of chromophores as one increases the content of the chromophore moiety per molecule. This is a clear indication of some interactions between chromophore moieties in the molecule, resulting in charge redistribution and extended delocalization. No absorption in the wavelength range from 550 to 1000 nm is observed in the single-photon spectra. The twophoton energy of ∼810 nm radiation falls within the strong linear absorption band of all these materials in solutions. Very strong frequency-upconverted fluorescence emission is easily observed from solutions excited with a Q-switched Nd:YAG
pumped dye laser at 810 nm. This observation indicates that a two-photon absorption process is occurring within the samples. The values of the two-photon absorption cross-section calculated for the materials by fitting both the nonlinear transmission data for the nanosecond experiment and the Z-scan data for the femtosecond experiment are listed in Table 1. The values obtained in experiments with nanosecond pulses are more than two orders of magnitude larger than those evaluated from the corresponding experiments with femtosecond laser pulses. Such large differences have also been observed in other materials31 and can be attributed to the possible contribution of excited state absorption during excitation. Nevertheless, both experiments clearly indicate a relative increase of the effective nonlinear
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TABLE 1: Optical Properties of PRL-101, PRL-501, and PRL-701 in 1,1,2,2-Tetrachloroethanea compound solvent (mol/L) λmax linear abs. nm (upconv. em. nm) σ (×10-20 cm4/GW)
PRL-101
PRL-501
PRL-701
CH2Cl2CH2Cl2 CH2Cl2CH2Cl2 CH2Cl2CH2Cl2 (1 × 10-2) (1.4 × 10-2) (9 × 10-3) 399 (503) 60 (0.35)
417 (510) 208 (1.1)
426 (516) 587 (2.4)
a Experimental σ values determined with nanosecond pulses at 810 nm in ∼8 ns range and femtosecond pulses at 796 nm in ∼ 173 fs range (given in parentheses) are reported; the uncertainties in the experimental σ values are estimated to be (15.
absorption cross-section as the number of chromophore moieties increases, which is not linear (the relative values are 1:3.5:9.8 with nanosecond pulses and 1:3.1:6.8 with femtosecond pulses). The two-photon absorption cross-section σ2 is related to the imaginary part of the molecular third-order nonlinear polarizability (second hyperpolarizability, γ). In the formalism of the sum-over-states it can be written as34
[
Im γ(-ω;ω,-ω,ω) ) Im P M2ge ∆µ2ge
(Ege - pω - iΓge)(Ege - 2pω - iΓge)(Ege - pω - iΓge) +
∑(E e′
ge
2 M2ge Mee′
- pω - iΓge)(Ege - 2pω - iΓge′)(Ege - pω - iΓge) M4ge
(Ege - pω - iΓge)(Ege + pω + iΓge)(Ege - pω - iΓge)
D
T
N
]
In this expression P corresponds to a permutation operator over the optical frequencies; Mge is the transition dipole moment between the ground state, g, and the first-lying charge-transfer excited state; Ege denotes the transition energy; Γge is the associated damping factor, and ∆µeg is the difference in the dipole moments. For asymmetric dipolar structures, as used here, both the first (D) and second (T) terms contribute. The third (N) term is the negative term for which only one-photon resonances occur and which therefore makes no contribution to the TPA sum. If each arm of the multi-branched structure is taken as a linear conjugated structure, one can assume that the largest tensor component of γ will be γ1111 along the length. Using a simple tensor component addition,35 one would expect the orientationally averaged second hyperpolarizability to follow a 1:2:3 ratio, regardless of the value of the bond angle in the amino group as long as this angle does not change in going from the monomer to the trimer. Accordingly, one would expect the relative two-photon cross-section for the dimer and trimer to be σdimer/σmonomer ) 2 and σtrimer/σmonomer ) 3, provided that the assumption of the constant bond angle is valid and that the multi-branched molecule consists of noninteracting units. Clearly these relative values are much smaller than what we observe under femtosecond conditions (which provide a more accurate assessment of the true two-photon absorption cross-section). Therefore, the individual chromophore units are definitely interacting and delocalization is extending through the various arms. This delocalization effect is also evident from a reduction in the excitation energy as seen in the UV-visible absorption and emission spectra discussed above. Various power laws for the dependence of γ on the excitation energy, ∆E, have been predicted depending on the type of theoretical formulation used.35,36 A simple electron-free model
predicts a 1/∆E6 dependence of χ(3). Using the experimentally observed peaks in the UV-visible absorption spectra for the estimates of band gap, this model predicts relative values for PRL-101, PRL-501, and PRL-701 to be 1:2.6:4.5, taking into account also the number density increase. Even these values are smaller than what we observe. Therefore, delocalization effect itself can not explain the observed enhancement. We therefore suggest that because the same electron donor group is being shared by all of the electron acceptors, the charge redistribution also influences the dipole terms (∆µ for the first term and/or the transition dipole moment coupling the intermediate states for the second term). The molar extinction coefficients, , for the three compounds also do not follow the number of density ratios [PRL-701/PRL-101 ) 2.2, PRL-501/ PRL-101 ) 1.4], indicating a change in transition dipole. Higherorder multipolar terms may also make contribution. Further theoretical work needs to be done to establish the mechanism of cooperative enhancement in these multi-branched structures. Conclusion In conclusion, we have synthesized, using Wittig and Heck reactions, new multi-branched chromophores with different numbers of two-photon moieties per molecule. All of our chromophores exhibit very large two-photon absorption crosssection as determined by the use of nanosecond and femtosecond pulses. We have discovered cooperative enhancement of twophoton absorption in these multi-branched structures. Acknowledgment. This work was supported in part by the Air Force Office of Scientific Research Directoriate of Chemistry and Life Science through contract number F4962093C0017 and in part by the Army through a subcontract from Laser Photonics Technology Inc. References and Notes (1) Goppert-Mayer. M. Ann. Phys. 1931, 9, 273. (2) Rentzepis, P. M.; Pao, Y. H. Appl. Phys. Lett. 1964, 156, 964. (3) Peticolas, W. L. Annu. ReV. Phys. Chem. 1967, 18, 233. (4) McClain, W. M. Acc. Chem. Res. 1974, 7, 129. (5) Birge, R. R.; Pierce, B. M. J. Chem. Phys. 1979, 70, 165. (6) Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989, 245, 843. (7) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73. (8) He, G. S.; Gvishi, R.; Prasad, P. N.; Reinhardt, B. A. Opt. Commun. 1995, 117, 133. (9) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl. Phys. Lett. 1995, 67, 2433. (10) He, G. S.; Yuan, L.; Cheng, N.; Bhawalkar, J. D.; Prasad, P. N.; Brott, L. L.; Clarson, S. J.; Reinhardt, B. A. J. Opt. Soc. Am. B 1997, B14, 1079. (11) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt, J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Chem. Mater. 1998, 10, 1863. (12) Ehrlich, J. B.; Wu, X. L.; Lee, I.-Y. S.; Hu, Z.-Y.; Rockel, H.; Marder, S. R.; Perry, J. W. Opt. Lett. 1997, 22 (24), 1843. (13) Albota, M.; Beljonne, D.; Bredas, J.-L.; Ehrlich, J. E.; Fu, J.-Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCordMaughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramaniam, G.; Webb, W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653. (14) Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Rep. Prog. Phys. 1996, 59, 1041. (15) Bhawalkar, J. D.; He, G. S.; Park, C.-K.; Zhao, C. F.; Ruland, G.; Prasad, P. N. Opt. Commun. 1996, 124, 33. (16) Zhao, C. F.; Gvishi, R.; Narang, U.; Ruland, G.; Prasad, P. N. J. Phys. Chem. 1996, 100, 4526. (17) He, G. S.; Xu, G. C.; Prasad, P. N.; Reinhardt, B. A.; Bhatt, J. C.; Dillard, A. G.; McKellar, R. Opt. Lett. 1995, 20, 435. (18) He, G. S.; Weder, C.; Smith, P.; Prasad P. N. J. Quantum Electron. 1998, 34, 2279. (19) Tutt, L. W.; Boggess, T. F. J. Quantum Electron. 1993, 17, 299.
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J. Phys. Chem. B, Vol. 103, No. 49, 1999 10745 (29) Dieck, H. A.; Heck, R. F. J. Am. Chem. Soc. 1974, 96, 1133. (30) To compare the TPA cross-section of materials without interference from solvent polarity effect, the same solvent, 1,1,2,2-tetrachloroethane, was used. (31) Swiatkiewicz, J.; Prasad, P. N.; Reinhardt, B. A. Opt. Commun. 1998, 157, 135. (32) Gruen, H.; Gaeraner, H. J. Phys. Chem. 1989, 93, 7144. (33) Beljonne, D.; Bredas, J. L.; Cha, M.; Torruellas, W. E.; Stegeman, G. I.; Hofstraat, J. W.; Horsthuis, W. H. G.; Mohlmann, G. R. J. Chem. Phys. 1995, 103, 7834. (34) Kogej, T.; Beljonne, D.; Meyers, F.; Perry, J. W.; Marder, J. L.; Bredas. J. L. Chem. Phys. Lett. 1998, 298, 1. (35) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (36) Norman, P.; Luo, Y.; Agren, H. Chem. Phys. Lett. 1998, 296, 8.
