Effect of dimer formation via hydrogen bonding on static and dynamic nonlinear optical characteristics of chromophores

Abstract

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...

Morphology and Intramolecular Interactions in P(VDF-TrFE) Electrospun Nanofibers Doped with Disperse Orange 3 Dye: A Joint Infrared Spectroscopy and Electron Microscopy Study

“…A comparable stabilization energy of DH comes by the stronger, directional bond of H* with O* and a less effective electrostatic interaction with the second, more distant oxygen atom, with r (H*...O) = 2.506 Å. Interestingly, the two equilibrium geometries of the DO3 dimers correspond to the only two minima of the potential energy surface of (DO3) 2 HTT structures, while—at the level of theory adopted—a more symmetric “chelated” geometry, where each H atom of the NH 2 group forms non-directional H bonds with a different atom of NO 2 , results to be not stable. This finding is in partial disagreement with the results reported in ref ( 10 ). In that work, besides a H-bonded dimer showing a structure very similar to DH , a second, quasi-chelated DO3 dimer is identified as further equilibrium structures of (DO3) 2 , showing an HTT configuration.…”

“…In other words, our modeling suggests that DO3 dimers (or possibly oligomers) can exhibit such a low ν sym, NH 2 II wavenumber, only if they adopt a local conformation as that of the DH model. Accordingly, a chelated dimer (not found among the stable structures at the level of theory here adopted but proposed in ref ( 10 ).) as the stable HTT structure is expected not to show a significant red shift of the symmetric NH stretching band because directional H bonds are incompatible with a cyclic NH 2 ....NO 2 arrangement.…”

“… 7 , 11 As guest molecules, azobenzenes with a push–pull character (i.e., π-conjugated azobenzene units bearing electron-donor and electron-acceptor substituents) are attractive candidates. 7 , 8 , 10 , 12 − 15 …”

“…The DO3 dye exhibits a strong charge separation induced by its electron-withdrawing nitro group and electron-donor amino group, and it has been recently proposed for NLO applications in blends with polymers. 7 , 10 In these works, infrared (IR) and Raman vibrational spectroscopies have allowed for a deep study of the relevant role of the intermolecular interactions between the DO3 dye and the host polymer matrix.…”

“…Therefore, non-covalent supramolecular interactions play a key role in the design of host–guest polymeric materials with NLO activity. In fact, they influence the efficiency and the retaining of the long-term stability of the NLO response of the material by keeping the orientational order of the non-centrosymmetric organization of the dye. − ,− Both interactions among polymer chains and those involving host–guest pairs can contribute to the stabilization of the final material architecture. , As guest molecules, azobenzenes with a push–pull character (i.e., π-conjugated azobenzene units bearing electron-donor and electron-acceptor substituents) are attractive candidates. ,,,− …”