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 (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.