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Large conjugated substituents in the center of the cyanine (R’) introduce low-lying charge-transfer excited states. These excited states primarily involve a single excitation from the HOMO-1 localized primarily on the substituent to the LUMO on the cyanine. Conjugated Bridge Substituents ωB97xD/cc-pVDZ Effect of Bulky End Substituents • Force the carbazole to be more perpendicular to the cyanine • Reduce mixing of the cyanine and carbazole π systems • Decrease the TPA cross-section of the charge-transfer excited state Comparison of substituents • T-Butyl and Phenyl • Moderate reduction of δ TPA • Adamantyl • More steric bulk, so greater reduction of δ TPA • Reduced δ TPA may improve performance for all- optical switching Torsion of Bridge Substituents Anthracene • Energetic minimum at 90° Carbazole • Less steric hindrance to rotation • Energetic minimum at ~65 ° Excited States Anthracene • No mixing of cyanine and substituent π systems • CT excited state has a negligible TPA cross-section Carbazole • Cyanine and substituent π systems mix • CT excited state has a large TPA cross-section • May cause problems for all-optical switching Third-order Nonlinear Optical Properties of Cyanines Rebecca Gieseking , Sukrit Mukhopadhyay, Chad Risko, Jean-Luc Brédas School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, and Center for Organic Materials for All-Optical Switching Georgia Institute of Technology, Atlanta, Georgia 30332-0400 References 1 Stegeman, G. I.; Stolen, R. H. J. Opt. Soc. Am. B 6, 652 (1989). 2 Gieseking, R. L.; Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Brédas, J.-L. Adv. Mater. doi: 10.1002/adma.201302676 (2013). 3 Hales, J. M.; Matichak, J.; Barlow, S.; Ohira, S.; Yesudas, K.; Brédas, J.-L.; Perry, J. W.; Marder, S. R. Science 327, 1485 (2010). 4 Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Brédas, J.-L. Chem. Sci. 3, 3103 (2012). Funding and Support Understanding the magnitude of the transition dipole μ ee’ between the first two cyanine excited states is key to understanding the contribution of the T term to Re(γ). Transition Dipole Moment Between Excited States Second excited state e’ Major contributions from three excitations • Two single excitations • HOMO-1 → LUMO • HOMO → LUMO+1 • Contributions to μ ee’ have the same sign • Double HOMO → LUMO excitation • Contribution to μ ee’ of the opposite sign Conclusions Nonlinear Optical Properties of Cyanines Thiopyrylium cyanines meet the requirements of large |Re(γ)| and small Im(γ) in dilute solution. 3 However, aggregation leads to the loss of these properties in thin films. One strategy to reduce aggregation is to introduce bulky substituents on the ends (R) and bridge (R’, R”) of the cyanine to sterically hinder aggregation. E E ge E ge' 0 e OPA e' TPA g ħω ħω E E ge' E ge 0 e' TPA e OPA g ħω ħω Unlike most π-conjugated systems, cyanines have a large negative Re(γ). Because E ge is small and μ ge is large, the N term is very large and dominates the SOS expression. The large energetic window between the first two excited states allows the energy ħω of the incoming light to be close to the first excited state energy without significant TPA, giving significant pre- resonant enhancement of Re(γ). 2 9C Streptocyanine Transition Transition Density Transition Dipole [H → L] → [H-1 → L] +5.36 Debye [H → L] → [H → L+1] +5.50 Debye [H → L] → [H,H → L,L] -7.98 Debye + Smaller terms +1.70 Debye INDO/SDCI Aggregation of Thiopyrylium Cyanines 0 12 9 6 3 Offset (Å) Torsion () 0 4 8 0 30 60 90 12 0 2 1 0 12 9 6 3 Offset (Å) Torsion () 0 4 8 0 30 60 90 12 0 2 1 Torsion Radial Distance Offset Top view: Cyanine-Cyanine Geometries x y z Δx Δy Cyanine-Counterion Geometries Cyanine aggregation can cause substantial changes in their optical and NLO properties. 4 For all-optical switching applications, aggregation must be minimized. • Cyanine-counterion interactions • Symmetry breaking • Substantial decrease of |Re(γ)| • Cyanine-cyanine interactions • Additional two-photon absorbing excited states • Increase in Im(γ) To understand the effect of bulky substituents on aggregation, molecular dynamics simulations were performed using a modified OPLS-AA force field. Unsubstituted Thiopyrylium Cyanine • Cyanine-counterion interactions • Counterion sits very close to the cyanine backbone • Cyanine-cyanine interactions • H-aggregation is preferred • Many aggregate geometries are possible Thiopyrylium Cyanine with Bulky Substituents • Cyanine-counterion interactions • Counterion sterically cannot approach the cyanine backbone • Cyanine-cyanine interactions • Aggregation is sterically hindered in all geometries Substituent θ() δ TPA (GM) R = H 65 490 R = t-Butyl 75 380 R = Phenyl 75 300 R = Adamantyl 80 140 |g |e |e’ δ TPA = 490 GM OPA TPA OPA TPA δ TPA = 1 GM OPA TPA INDO/MRDCI Pure Component Transition Dipole Moments Second Excited State CI Coefficients Contributions to μ ee’ As the streptocyanine length increases, the magnitude of μ ee’ is determined by the changes in (1) the transition dipole moments of the pure component transitions and (2) the relative contributions of each excitation to the second excited state. • All of the pure transition dipole moments increase with increasing length • Contributes to an increase in μ ee’ with increasing length • The CI coefficient of the double [H,H → L,L] transition increases with increasing length • Contributes to a decrease in μ ee’ with increasing length • The two competing effects mostly cancel with increasing length • μ ee’ has a weak dependence on streptocyanine length In all-optical switching, one optical signal is modulated by a second optical signal without converting the light to an electrical signal. All-optical switching applications generally require materials with a large third-order polarizability γ. In particular, the real part of the third-order nonlinearity |Re(γ)| must be large and the imaginary part Im(γ), corresponding to two-photon absorption (TPA), must be small. 1 Of particular interest are materials with a large |Re(γ)| in the telecommunications window, 0.8- 0.95 eV (1300-1550 nm). All-Optical Switching Third-order NLO material ∝ + ′ ′ ′ − = Transition dipole between states g and e = Difference between state dipoles of e and g = Energy difference between states g and e The third-order polarizability γ is typically evaluated using a sum-over states (SOS) approach: D T N Key to designing cyanines for all-optical switching applications is achieving a clear understanding of how molecular structure and aggregation influence the electronic and optical properties. Our computational investigation of cyanines, using a combination of quantum-chemical and molecular dynamics approaches, have aided in achieving this understanding. In collaboration with the Marder and Perry research groups, these results have helped enable the design of cyanine-based materials with the necessary large |Re(γ)| and small Im(γ) necessary for device applications.
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