The effect of head-to-tail azochromophore dimer formation on the values of static and dynamic first hyperpolarizability is studied on the basis of calculations performed at M06-2X/aug-cc-pVDZ and ωB97X-D/aug-cc-pVDZ computational levels; the results are compared with those obtained at second-order Moller-Plesset pertubation theory (MP2)/aug-cc-pVDZ. Azochromophores DO3 and AAB-DCV, participating in the dimer formation, contain nitro-or dicyanovinylene acceptor moieties. The structure of the studied dimers is obtained at the B3LYP-D3/6-31++G (d,p) level with basis set superposition error (BSSE) correction. Dynamic first hyperpolarizabilities are calculated at radiation frequencies of 0.65 eV, 0.918 eV and 1.165 eV. The essential effect of dimer formation is demonstrated: it results in almost a 3.5 times increase of the first hyperpolarizability. In the series D1-D2-D3, β(2ω) values at 0.65 eV increase in a way similar to the static case: β(2ω) for D2 and D3 are 1.5 and 1.8 times higher than that for D1. The notable resonance enhancement of β(2ω) for the studied hydrogen-bonded dimers is demonstrated at radiation frequency of 1.165 eV. K E Y W O R D S azochromophore, chromophore dimer, hydrogen bonding, static and dynamic first hyperpolarizability 1 | INTRODUCTION The formation of supramolecular aggregates of organic push-pull chromophores and its effect on the nonlinear optical (NLO) response presents a relatively new direction in the field of new polymer NLO materials development. The establishment of supramolecular structure-property relation-ships can provide new insight into the design of such materials. [1,2] Various types of supramolecular aggregates were indicated in both experimental and theoretical research. [3,4] The first one is the chromophore clusters of the "head-to-tail" type, in which electron donor groups of one chromophore interact with the electron acceptor groups of the other one via hydrogen bonds (H-bonds); such an arrangement results in the essential enhancement of the NLO response with cooperative effect. [3][4][5][6][7][8][9] Another type of chromophores self-organization is represented by the π-stacked structures with either codirected or antiparallel dipole moments of chromophores; [3,4,[10][11][12][13] the latter case is characterized by vanishing NLO activity due to chromophores' centrosymmetric arrangement. In the codirected structures, the chromophores are shown to be mutually shifted, such that the shift degree determines the NLO characteristics of such structures. For the clusters with strongly shifted chromophores, a moderate increase of the molecular NLO response is predicted. [11,12] Atomistic modeling of chromophore-containing polymer materials allowed us to show the possibility of the formation of chromophore clusters of a different structure: both head-to-tail and stacked dimers. [14][15][16] The application of density functional theory (DFT) calculations made it possible to study the effect of stacked dimer formation on the quadratic NLO response of azochromophores. [12] The...

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The first example of polymer materials containing in the side chain original D-π-A chromophore with quinoxaline core in the π-electron bridge is presented and their quadratic nonlinear optical (NLO) activity is studied. Two synthetic procedures: esterification and radical copolymerization, are used to obtain methacrylic copolymers P1Ch and P2Ch, respectively, which contain chromophores with quinoxaline moiety in various concentrations (4, 6, 7, and 9 mol%). Atomistic modeling has shown that introduction of chromophores in polymer side chains results in less pronounced chromophore aggregation in the material compared with the case of relative composites. NLO coefficients of the thin films obtained on the basis of copolymers P1Ch and P2Ch and composite material PMMA/AEEA-VQV-TCF with relative chromophoreguest are up to 40, 21, and 43 pm/V, respectively, at chromophore content 25 wt%. Covalent attachment of chromophore moiety to the polymer side chain is shown to improve the temporal and thermal stability of the material NLO coefficient compared with that of the composite material.

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