Designing Efficient Azobenzene and Azothiophene Nonlinear Optical Photochromes

Abstract

International audienceThe present contribution constitutes an extensive density functional theory (DFT) investigation of the nonlinear optical (NLO) properties of a large number of molecules belonging to the azobenzene and azothiophene families of photochromic compounds which can act as NLO switches. Toward the design of systems simultaneously presenting both large total nonlinear response values, beta(tot), and large contrast, beta(ratio), between the cis and trans isomers, we have focused not only on the monomers but also on azobenzene dimers, the latter containing two N=N bonds along the molecular backbone. After it was established that the inclusion of implicit solvation is not important in drawing qualitative conclusions on the NLO switching ability for the investigated systems, gas-phase calculations have shown that for the asymmetric pushpull azobenzene and azothiophene candidates, the combination of strong donating groups such as NPh2, N(Ph-OMe)(2), and N(Ph-NMe2)(2) with the dicyanoethene group CH=C(C=N)(2) as an acceptor delivers large ?trans (150-217 x 10(-30) esu) and non-negligible beta(cis) (18-55 x 10(-30) esu) values as well as substantial contrast, beta(ratio) (3.9-8.7). For the investigated double azobenzenes, it is found that, with a careful choice of donor and acceptor groups, the contrast, beta(ratio), can be significantly increased compared to that of the monomers while maintaining large beta values that facilitate their detection with standard experimental techniques (e.g., electric-field-induced second-harmonic generation). Our results set the stage on which further theoretical and experimental studies can be based in the search for efficient and versatile NLO switches

The choice of appropriate density functional for the calculation of static first hyperpolarizability of azochromophores and stacking dimers

“…The search for a DF, describing adequately both energy of excitation and first hyperpolarizability, led to a conclusion that none of the DFs with the account of various degrees of HF exchange was ideal in this sense, although M06‐2X and CAM‐B3LYP, in the author's opinion, come closest to the experimental values. However, range‐separated ωB97X‐D seems to be preferable when just first hyperpolarizabilities are calculated; the same conclusion is made in other papers . In the article by Garza et al by the example of PNA it is shown that the ωB97X‐D functional gives hyperpolarizability values in good agreement with experiment, the error depending upon the choice of the basis set and being less than 10%.…”

“…At present the number of works dealing with the calculations of molecular polarizabilities of stacked clusters is essentially fewer than those where the energy of binding π–π interactions is studied. As a rule the choice of an adequate computational level was done by the example of isolated chromophores . To select the calculation method to be used for reliable NLO characteristics estimation for both isolated chromophores and stacked clusters, it is necessary to come to an agreement on the reference data.…”

“…As a rule the choice of an adequate computational level was done by the example of isolated chromophores. [11,43,[60][61][62][63][64][65] To select the calculation method to be used for reliable NLO characteristics estimation for both isolated chromophores and stacked clusters, it is necessary to come to an agreement on the reference data. When studying the chromophores with long conjugated bridge one often uses the data obtained at MP2 level as reference ones, [11,38,43,63] although there exists a common opinion that MP2 is more appropriate for small systems, as it provides too slow increase of characteristics with the chain length growth.…”

4,4′-Bis[4-(4′-hydroxyphenyl)phenylazo]diphenyl Ether

“…Aromatic azo compounds are widely used in the chemical industry as dyes, pigments [6][7][8], food additives [9], indicators [10], radical reaction initiators [11] and therapeutic agents [12,13]. In addition, azobenzenes have shown promising applications in photo-optical media [14], photo-switches [15], photo-mechanical systems [16], micro patterning [17], nonlinear optical media [18], molecular shuttles, [19] nanotubes, [20] and in the manufacture of protective eye glasses and filters [21]. Moreover, the light induced reversible isomerization of azobenzenes between cis and trans forms makes them excellent candidates to modulate the relative movement of different moieties; thus leads to the development of light driven motors [22].…”

Relation between Nonlinear Optical Properties of Push–Pull Molecules and Metric of Charge Transfer Excitations

“…[1][2][3][4][5][6][7][8][9][10][11][12] In particular, much effort has recently been invested in designing molecules with a large two-photon absorption strength (δ TPA ) -a quantity related to the imaginary part of the second hyperpolarizability. [13][14][15][16][17][18][19] The quest after simple relationships between nonlinear optical properties of molecules and electronic structure parameters has a long history.…”