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...

Theoretical study of static (Hyper)polarizabilities of twisted intramolecular charge transfer chromophores

“…The two rings in the chromophores with four ortho-methyls are twisted with dihedral angles of [54][55][56][57][58][59][60][61][62] . The dihedral angles are less than the experimental value measured by Kang et al [39] For example, the measured dihedral angle of T8 is 85. 6 , compared with the computed value of 61.23…”

“…In the planar structure, the donor/acceptor driven charge flow is significantly favored through the conjugation paths, but is seriously blocked by the inter-ring bond in the twisted structure. An alternative electron-transfer mechanism has been proposed by several authors [39,71,72] for the twisted structures in which the overlap between the orbitals of the two fragments are reduced to facilitate electron transfer from the more electronegative part to the less electronegative part upon optical excitation, leading to large hyperpolarizabilities. Analysis based on two-state model reveals that the twisted structures possess relatively low excitation energies and large dipole moment changes from the ground to first excited states.…”

“…A number of theoretical studies [41][42][43][44] have been conducted to rationalize the origin of the enhanced responses of this new class of promising NLO chromophores. In fact, the D-p-A molecules with twisted p-electron systems have been predicted [39] to have unprecedentedly large firstorder hyperpolarizabilities, b, using the finite-field (FF) or sum-over-states approach at semi-empirical level of theory before the synthesis of tictoid molecules. Two major electronic structures, quinoid and charge-separated forms, were assigned to the tictoids.…”

Electronic responses of donor acceptor substituted twisted biphenyls

“…However, for both compounds, the large pyridinium-phenyl twist angle enhances their charge-separated character and the resulting large ground state dipole moment leads them to form aggregates. Also, upon changing the solvent used in EFISH experiments from dichloromethane to dimethylformamide, the response drops by a factor of five [6] indicating that the solvation cage may play a determinant role in the electronic properties of these chromophores.…”

New building blocks and growth processes for high-performance self-assembled electro-optic materials

“…For molecules of type II we will show that disruption of the π-conjugation via a sterically induced molecular twisting results in unusual molecular characteristics and unprecedented hyperpolarizabilities (μβ values as high as -466,000 × 10 -48 esu at 1907 nm) [4]. Guest-host poled polymers containing these chromophores reveal very large electro-optic coefficients (r 33 ) up to 320 pm/V at 1310 nm.…”