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Cooperative Coupling of Cyanine and Tictoid Twisted #Systems to Amplify Organic Chromophore Cubic Nonlinearities Yanrong Shi, Alexander J.-T. Lou, Guang S. He, Alexander Baev, Mark T. Swihart, Paras N Prasad, and Tobin J. Marks J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b01042 • Publication Date (Web): 26 Mar 2015 Downloaded from http://pubs.acs.org on March 31, 2015
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Cooperative Coupling of Cyanine and Tictoid Twisted π-Systems to Amplify Organic Chromophore Cubic Nonlinearities Yanrong Shi,1 Alexander J.-T. Lou,1 Guang S. He,2 Alexander Baev,2 Mark T. Swihart,3 Paras N. Prasad,2,4,* Tobin J. Marks1,* 1
Department of Chemistry and the Materials Research Center, Northwestern University, Evanston IL 60208, USA The Institute for Lasers, Photonics and Biophotonics, State University of New York at Buffalo, Buffalo, New York 3 14260, USA Department of Chemical and Biological Engineering, State University of New York at Buffalo, Buffalo, New York 14260, USA 4Department of Chemistry, Korea University, Seoul 136-701, Korea
2
Supporting Information Placeholder ABSTRACT: We report a new class of hybrid π-electron chromophores with large, sign-tunable third-order nonlinear optical (NLO) response, achieved via cooperative coupling of cyanine dye bond-length alternation effects with the rich density of states in zwitterionic twisted π-system chromophores. A combined synthetic, linear/nonlinear spectroscopic, and quantum chemical study revealsexceptional thirdorder response exceeding the sum of the individual chromophore contributions.
Interest in organic third-order nonlinear optical (NLO) materials has grown dramatically in recent years, due to their promise in optical telecommunications, ultra-high speed signal processing, real time target recognition, and aberration-corrected imaging, where linear and nonlinear absorption must be minimized, and in optical power limiting, 1 where nonlinear absorption must be maximized. Target figures of merit for all-optical signal processing are: W≡n2I/(α1λ)>>1 and T≡α2λ/n210 M cm ) in both solvents, typical of cyanines, the Cy-TICT absorption is significantly weaker and broader in both solvents, and is in no way explicable as a simple superposition of the TMC-2' and IR-1061-BArF spectra. Furthermore, the Cy-TICT spectrum in CH2Cl2 is independent of concentration over a 23-fold range (Figure S5). The ground state to first excited electronic state transi-
Figure 2. Solution optical absorption spectra of: (a)IR-1061BArF, (b) Cy-TICT in the indicated solvents. Large third-order response is invariably accompanied by 1c resonance enhancement, usually one- or two-photon, with response dependent on input wavelength. Z-scan results for Cy-TICT/CH2Cl2 at 1305 nm and 1410 nm are in the SI (Figures S6 and S7). Note that both the Cy-TICT/CH2Cl2 induced refractive index change and nonlinear absorption fall with increasing input wavelength, indicating resonance enhancement. In contrast, quartz exhibits a constant non-resonant -16 2 nonlinear refractive index n2 value = 3·10 cm /W from 600 13 nm - 1600 nm. The Cy-TICT/CH3CNZ-scan shows a significantly lower nonlinearity, indicating solvent-dependent response (Figure S8). Furthermore, comparing these results to IR-1061-BArF/CH2Cl2 Z-scan data (Figure S9) indicates that for the same solvent and wavelength, the Cy-TICT nonlinear refractive index and nonlinear absorption are significantly greater than those of IR-1061-BArF.
Figure 3. (a) Open-aperture Z-scan (nonlinear transmittance, f=5 cm) using ~160-fs 2.9 µJ laser pulses at 1305 nm for CyTICT/CH2Cl2; (b) Nonlinear transmittance as a function of input pulse intensity, showing optical power limiting. Solid curve is the best fit to two-photon absorption (2PA) theory. Figure 3a shows open-aperture Z-scan results for 0.01 M Cy-TICT/CH2Cl2 at 1305 nm; the dashed line is the best fit to 14 a saturable two-photon absorption (2PA) model. Figure 3b shows nonlinear transmittance as a function of input pulse intensity, and the best-fit curve, clearly indicates optical
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power limiting behavior. Table I summarizes the third-order parameters of the present chromophores. At ~1300 nm, CyTICT/CH2Cl2 exhibits both a larger refractive index change and greater nonlinear absorptivity than does IR-1061-BArF; the former property enables optical switching and the latter, optical power limiting. Comparing the Cy-TICT third-order nonlinearity parameters with those of TMC-2 reported earli9 er and IR-1061-BArF (Table II) clearly shows the marked synergistic effects of combining the twist and cyanine fragments.
Table I. Third-Order NLO parameters for chromophores from Z-scan measurements in solution* Chromophore/Solvent Wavelength (nm)
Cy-TICT
Cy-TICT
IR1061-BArF
(CH2Cl2)
(CH3CN)
(CH2Cl2)
1305
1410
1305
1305
-2.04
-0.72
-1.11
-1.20
Contribution to Re. χ(3) at -3.88 0.01 M (10-14 esu)
-1.37
-2.11
-2.28
Re. γ (10-33 esu)
-6.43
-2.27
-3.50
-3.78
0.018
0.0045
0.0033
0.0053
2PA cross-section (GM) 45.5
10.6
8.34
13.4
Contribution to Im. χ(3) 3.54 at 0.01 M (10-15 esu)
0.953
0.649
1.04
Im. γ (10-35 esu)
15.8
10.8
17.3
Linear transmittance (for ~0.70 1-mm solution)
~0.95
~0.75
~0.90
T-parameter
0.8813
0.388
0.5764
Contribution to n2 at 0.01 M (10-6 cm2/GW)
2PA coefficient β (cm/GW)
58.8
1.1515
*Experimental uncertainty: ±10%.
Table II. Comparison of Cy-TICT, IR-1061-BArF, and TMC-2 NLO parameters Parameter
Cy-TICT (1305 nm)
IR1061BArF (1305 nm)
[9]
TMC-2
(1172 nm)
Contribution to n2 at 0.01 M
-2.04
-1.20
0.15
-6.43
-3.78
0.50
45.50
13.40
1.50
58.80
17.30
