Exceptional Molecular Hyperpolarizabilities in Twisted π‐Electron System Chromophores

Abstract

Molecule-based electrooptic (EO) materials are of intense research interest for understanding how light interacts with matter and for applications in photonic technologies such as high-speed optical communications, integrated optics, and optical data processing and storage. [1] In such materials, the second-order susceptibility tensor governing EO response (r 33 ), is governed both by the net polar microstructural order and the microscopic molecular first hyperpolarizability tensor (b). Large b values are essential for large EO response, and the quest for higher performance EO chromophores presents a daunting challenge.[1] To date, effective chromophores have been designed according to similar principles embodied in the classical "two-level" model: conjugated p systems endcapped with donor (D) and acceptor (A) moieties.[2] Elegant efforts have sought maximum b by optimizing D and A strengths and conjugation pathways, [3] directed by "bond length alternation" [4] and "auxiliary donor and acceptor" models.[5] Such strategies utilize extended planar p-conjugation, resulting in chromophores that are inherently elaborate structurally, complicating synthesis and introducing potential chemical, thermal, and photochemical instabilities.[6] Alternative routes to very large-b chromophores would clearly be desirable, and there is growing evidence that simple two-level systems may not provide access. [7] Recent theoretical work suggests that unconventional chromophores with twisted p-electron systems bridging D and A substituents (TICTOID = twisted intramolecular chargetransfer; Scheme 1) may exhibit unprecedented hyperpolarizabilities through non-classical mechanisms.[8] These would have relatively simple biaryl structures in which b is sterically tunable through R 1 , R 2 modification of the interplanar dihedral angle (q). Large b magnitudes are predicted at q % 70-858, [8a] with twist-induced reduction in D-p-A conjugation leading to charge-separated zwitterionic ground states. The intriguing question is whether such molecules, with small numbers of p-electrons, could thereby exhibit far larger b values than conventional planar chromophores.We report here the first realization of such TICTOID chromophores, that they have unprecedented hyperpolarizabilities on the order of 10-20 larger than previously observed, [1][2][3][4][5] and that these are not simple two-level systems. The new chromophores (Scheme 1) were designed according to the following criteria: 1) Synthetically challenging tetraortho-alkylbiaryl cores should enforce large interplanar angles, judging from bimesityl (q % 908).[9] 2) Dicyanomethanide and pyridinium groups should be effective D and A substituents, with dicyanomethanide more stable than phenoxide.[10] 3) Pyridine alkylations should enhance solubility and processability (TMC-1 and TMC-2), with styrenic substitution further enhancing b (TMC-2).TMC syntheses (Scheme 2) begin with precursor 1, synthesized as reported elsewhere. [10] Pd-catalyzed NaCH(CN) 2 coupling affords 2, which is then regioselect...

Heterophenes revisited

“…Because of strain, the π-systems in teraryl system and in the bridged double bond are twisted. Such a twisted system may thus induce significant enhancement in hyperpolarizability [36,37]. The five-membered heteroaromatic rings in 34 may not only serve as electron donors, but also accommodate the appropriate geometry to enable the interactions between the oligoaryl systems and the double bond, leading to unusual photophysical and NLO properties.…”

“…In this regard, one might consider that the bridging alkene group might be an electron acceptor. Recently, it has been shown that twisted π-electron system chromophores in a biaryl system having charged donor and acceptor moieties exhibit ultralarge molecular hyperpolarizability with exceptionally high µβ values [36,37]. It is envisaged that 34 and related compounds may be second-order NLO active.…”

Acentric Nanostructured Assembly as a Strategy for the Design of Organic Electrooptic Materials

“…In octopolar NLO compounds, non-centrosymmetric molecules with no dipole moment, however, a significant increase in second order NLO performance, when compared to chromophores designed after the "traditional" push-pull approach, has not been achieved to date. A new paradigm for the optimization of molecular hyperpolarizabilities has been recently proposed by the use of "twisted" D--A chromophores [14]. These molecular modules (compound 4 in Fig.…”

Applying universal scaling laws to identify the best molecular design paradigms for second-order nonlinear optics

“…[5][6][7][8] The performance of a molecule is evaluated by comparing its response with that of the optimal structure that uses the same number of electrons * jperezmo@skidmore.edu (the quantum limit). An analysis based on the quantum limit theory applied to experimental data allows one to determine the mechanisms that dominate the nonlinear optical response at the molecular level, [9][10][11][12][13][14][15][16][17][18] introduce new paradigms for optimization, [14,[19][20][21][22][23][24] and establish fundamental scaling laws. [25,26] In this paper we introduce a new analysis that we apply to experimental data in the literature to identify the best molecular candidates for the largest second-order nonlinear optical response.…”