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) − MoS 2 heterojunctions using optical absorption and Raman spectroscopy to elucidate M core (M = first row transition metal), MoS 2 layer number, and excitation wavelength effects. Observed phenomena include the emergence of heterojunctionspecific 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 MoS 2 layer number, ultimately reaching a maximum enhancement factor of 30x relative to SiO 2 substrates. These experimental results, combined with density functional theory (DFT) calculations, indicate strong coupling between nonfrontier MPc orbitals and the MoS 2 band structure as well as charge transfer across the heterojunction interface that varies as a function of the MPc electronic structure.

Twisted intramolecular charge transfer (TICT) chromophores exhibit uniquely large second-order optical nonlinearities ( μβ). 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 μβ. 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 μβ = -26,000 × 10 esu (at 1907 nm) in dilute nonpolar CHCl solution, which is maintained in polar DMF ( μβ= -20,370 × 10 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 μβ. We show that increasingly disrupted conjugation is correlated with increased μβ even at intermediate twist angles. The excellent performance and reduced aggregation in polar solvents opens new avenues for bridging microscopic and macroscopic chromophore performance.

CONSPECTUS: 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 (β) and second hyperpolarizability (γ) of the constituent molecules. The search for higher β 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 μβ, where μ is the molecular dipole moment. The surprising recent discovery that twisted π-system chromophores can exhibit dramatically higher β 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-(μβ = 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 μβ 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.

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.

Twisted intramolecular charge transfer (TICT) chromophores exhibit a promising third-order nonlinear optical (NLO) response, γ, which is potentially useful for all-optical switching. Here we explore the third-and fifth-order NLO response of a newly synthesized chromophore, TMC-3′, which incorporates both stilbene and TICT motifs with optical function. Solution Z-scan measurements in CH 2 Cl 2 reveal modest two-photon absorptions (2PA) at 1100 and 800 nm (σ 2PA > 80 GM), and two contributions to the nonlinear refraction. The first component is attributed to the third-order bound-electronic response, while the second is assigned to a fifth-order process involving 2PA induced excited state refraction (2PA-ESR). Quantum computation suggests that the differences between the present work and previous studies lie primarily in the addition of the stilbene related states, as well as the increase in ground state dipole moment. This work yields insight into the thirdand fifth-order NLO response of TICT chromophores and demonstrates that subtle structural modifications significantly impact their functionality.