A Twist on Nonlinear Optics: Understanding the Unique Response of π-Twisted Chromophores.

Materials with large nonlinear optical (NLO) response have the ability to manipulate the frequency and phase of incident light and exhibit phenomena that form the basis of modern telecommunication systems. In molecule-based materials, the second- and third-order NLO performance is related to the hyperpolarizability (ß) and second hyperpolarizability (γ) of the constituent molecules. The search for higher ß materials is driven by the desire to keep pace with expanding demand for high speed data transmission, while discovery of high γ chromophores is crucial for the development of emergent photonic technologies reliant on manipulation of "light-with-light". For decades, it was believed that for highest performance, organic NLO materials must be composed of planar π-system chromophores, and much exploratory research focused on subtle molecular modifications, which generally yielded incremental increases in µß, where µ is the molecular dipole moment. The surprising recent discovery that twisted π-system chromophores can exhibit dramatically higher ß values than their planar analogues has revealed a new design paradigm and stimulated the development of high performance twisted intramolecular charge transfer (TICT) chromophores, which are composed of electron-donating and electron-accepting π-substituents joined by a sterically constrained twisted biaryl fragment. In such chromophores, the twisting of the π-system enforces charge separation in the electronic ground state, leading to large dipole moments and low-lying charge-transfer excitations. This unique electronic structure forms the basis for enhanced NLO response, with an archetypal TICT chromophore, TMC-2, exhibiting very large second- ( µß = 24 000 × 10-48 esu) and third-order (γ = 1.4 × 10-33 esu) metrics in dilute low-polarity solutions. This Account summarizes several approaches to enhance µß in various environments, including (1) manipulating the biaryl torsional angle, (2) modifying the electron accepting fragment, (3) extending conjugation, (4) adding multiple twisted fragments, (5) modifying chromophore side chains, and (6) tuning the chromophore environment. Another set of modifications is explored to enhance γ, including (1) coupling to a cyanine dye to hybridize the cyanine and TICT orbitals, (2) manipulating the donor and acceptor group identity. The extensive modifications described above yield a detailed understanding of TICT chromophore molecular NLO response and unambiguous evidence that such chromophores have the potential to revolutionize organic electro-optics.

Electronic Coupling in Metallophthalocyanine-Transition Metal Dichalcogenide Mixed-Dimensional Heterojunctions.

Mixed-dimensional heterojunctions, such as zero-dimensional (0D) organic molecules deposited on two-dimensional (2D) transition metal dichalcogenides (TMDCs), often exhibit interfacial effects that enhance the properties of the individual constituent layers. Here we report a systematic study of interfacial charge transfer in metallophthalocyanine (MPc) - MoS2 heterojunctions using optical absorption and Raman spectroscopy to elucidate M core (M = first row transition metal), MoS2 layer number, and excitation wavelength effects. Observed phenomena include the emergence of heterojunction-specific optical absorption transitions and strong Raman enhancement that depends on the M identity. In addition, the Raman enhancement is tunable by excitation laser wavelength and MoS2 layer number, ultimately reaching a maximum enhancement factor of 30x relative to SiO2 substrates. These experimental results, combined with density functional theory (DFT) calculations, indicate strong coupling between nonfrontier MPc orbitals and the MoS2 band structure as well as charge transfer across the heterojunction interface that varies as a function of the MPc electronic structure.

Organic Salts Suppress Aggregation and Enhance the Hyperpolarizability of a π-Twisted Chromophore.

Twisted intramolecular charge-transfer (TICT) chromophores exhibit extraordinary hyperpolarizabilities, ß, and are therefore promising for electro-optic technologies. Nevertheless, centrosymmetric aggregate formation severely diminishes ß in concentrated solutions or in polymer matrices. Herein, the remarkable effects of organic salts on the linear and nonlinear optical response of a high ß benzimidazolium-based TICT chromophore, B2TMC-2, are reported. Addition of Bu4 P+ Br- to B2TMC-2 solution in CHCl3 induces a halochromic blueshift, primarily reflecting interactions between Bu4 P+ Br- and the B2TMC-2 cationic portion. DC electric-field induced second-harmonic generation (EFISH) measurements on B2TMC-2+Bu4 P+ Br- solution reveal a large µß=-22,160×10-48  esu, unprecedented for any chromophore with such a broad optical transparency window. Moreover, Bu4 P+ Br- is shown to suppress B2TMC-2 aggregation, thereby preserving high µß in concentrated solutions. This phenomenon should be applicable to many other NLO chromophores.

Unprecedented Large Hyperpolarizability of Twisted Chromophores in Polar Media.

Twisted intramolecular charge transfer (TICT) chromophores exhibit uniquely large second-order optical nonlinearities ( µß). However, their promise as electro-optic (E-O) materials is yet untapped, reflecting a strong tendency to aggregate in low-polarity media, leading to a dramatic fall in µß. Until now, TICT chromophores in deaggregating polar solvents suffered decreased response due to polarity-driven changes in electronic structure. Here we report a new series of benzimidazolium-based TICT chromophores with interaryl torsional angles in the range of 64-77°. The most twisted, B2TMC-2, exhibits a large µßvec = -26,000 × 10-48 esu (at 1907 nm) in dilute nonpolar CH2Cl2 solution, which is maintained in polar DMF ( µßvec= -20,370 × 10-48 esu) as measured by DC electric field-induced second harmonic generation (EFISH). Sterically enforced interaryl torsional angles are confirmed by single-crystal X-ray diffraction and solution phase Nuclear Overhauser Effect (NOE) NMR, and spectroscopic characterization reveals a zwitterionic/aromatic ground state electronic structure associated with the high µß. We show that increasingly disrupted conjugation is correlated with increased µß even at intermediate twist angles. The excellent performance and reduced aggregation in polar solvents opens new avenues for bridging microscopic and macroscopic chromophore performance.

Cooperative coupling of cyanine and tictoid twisted π-systems to amplify organic chromophore cubic nonlinearities.

We report a new class of hybrid π-electron chromophores with a 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 reveals exceptional third-order response exceeding the sum of the individual chromophore contributions.