Linear and Nonlinear Optical Properties of Azobenzene Derivatives Modified with an (Amino)naphthalene Moiety

The design of two-photon absorbing azobenzene (AB) derivatives has received much attention; however, the two-photon absorption (2PA) properties of bis-conjugated azobenzene systems are relatively less explored. Here, we present the synthesis of six azobenzene derivatives and three bis-azobenzenes substituted (or not) at para position(s) with one or two amino group(s). Their linear and nonlinear absorption properties are studied experimentally and theoretically. The switching behavior and thermal stability of the Z-isomer are studied for unsubstituted mono- (1a, 2a) and bis-azobenzene (3a) compounds, showing that when the length of the π system increases, the half-life of the Z-isomer decreases. Moreover, along with the increase of π-conjugation, the photochromic characteristics are impaired and the photostationary state (PSS) related to E–Z photoisomerization is composed of 89% of the Z-isomer for 2a and 26% of the Z-isomer for 3a. Importantly, the 2PA cross-section increases almost five-fold on extending the π-conjugation (2a vs 3a) and by about one order of magnitude when comparing two systems: the unsubstituted π-electron one (2a, 3a) with D-π-D (2c, 3c). This work clarifies the contribution of π-conjugation and substituent effects to the linear and nonlinear optical properties of mono- and bis-azobenzene compounds based on the experimental and theoretical approaches.


SYNTHETIC AND ANALYTICAL GENERAL METHODS
Solvents and starting materials were purchased from commercial suppliers and were used as received. Reactions were monitored by thin layer chromatography (TLC) carried out on silica gel plates (Merck 60F-254). Column chromatography was performed using silica gel (Merck 60, particle size 0.040-0.063 mm). NMR-spectra were recorded on a Bruker AvanceTM 600 MHz spectrometer or on a JEOL 400 MHz spectrometer at 25°C using residual protonated solvent signals as internal standards for 1 H-and 13 C-spectra. Multiplicities are abbreviated as follows: singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). High resolution mass spectra (HRMS) were conducted with a WATERS LCT Premier XE mass spectrometer (ESI). High Pressure Liquid Chromatography (HPLC) was done using an Agilent 1260 Infinity II equipped with a chiralpak IB column in hexane/isopropanol.

SYNTHESIS AND CHARACTERIZATION
General procedure for azo coupling reaction 1 Aniline derivative (1equiv) was added into the mixture of H 2 O/EtOH (2/1) and HCl (2 equiv) at 0˚C soon afterward dropwise addition of cold water solution of NaNO 2 (1 equiv) over a period of 10 minutes, then the mixture was stirred for 1 h at 0˚C. After that time, the reaction mixture was neutralized to pH = 6 using potassium acetate. Next, the diazonium salt solution was added dropwise into the solution of 1-naphthylamine (1equiv) and HCl (1equiv) at H 2 O/EtOH (2:1). The suspension was left stirring for next 1 h at 0˚C and left overnight at room temperature. The solution was then neutralized with addition of ammonium solution (25%).
Crude product was purified by column chromatography to give the corresponding azobenzene derivative.
General procedure for oxidation of aniline to nitrosoarene derivatives 2 To a solution of the aniline derivative (1 equiv) in DCM was added the solution of Oxone (2 equiv) in water. The mixture was stirred at room temperature until the reaction was completed (progress of the reaction was monitored by TLC). Then, two phases were separated and the organic phase was washed with water, dried over MgSO 4 , filtered, and the solvent was removed under reduced pressure. The resulting nitroso derivatives were used without further purification in the following Mills reaction.

S3
General procedure for Mills reaction 3 To a solution of the aniline derivative (1equiv) in acetic acid was added the nitrosoarene (1equiv) and the resulting mixture was stirred overnight at room temperature. Then the obtained precipitate was filtered and washed thoroughly with water. The resulting solid was purified by column chromatography to give the corresponding azobenzene derivative.
General procedure for reduction of diazonium salts 4 A solution of amine derivative (1 equiv) and HCl (1 equiv) in H 2 O/EtOH (2:1) was chilled in an ice-salt bath and treated with cold water solution of sodium nitrate (1 equiv). The resulting solution was stirred for 30 min, and the hypophosphorous acid (50%, 20 equiv) was added dropwise over 10 min. The resulting mixture was left stirring at 0-5˚C for 2 h and then left overnight at room temperature. The resulting mixture was neutralized with ammonium solution (25%) and extracted with EtOAc (2x50 mL), washed with water and brine, dried over MgSO 4 , filtered and concentrated under reduced pressure. The resulting solid was purified by column chromatography to give the corresponding azobenzene derivative (2a, 3a).

Synthesis of 1a-1c
Compound 1b was purchased from Sigma-Aldrich and compound 1c was synthesized according to previously published procedure. 5 Scheme S1. Synthetic route for 1a.
Nitrosobenzene: Standard oxidation of aniline to nitrosoarene derivatives procedure was used with aniline (1.20 g, 12.9 mmol) in DCM (40 mL) and Oxone (7.92 g, 25.8 mmol) in H 2 O (120 mL) to give nitrosobenzene as brown crystals that was used as such in the next step.
1a: Standard Mills reaction procedure was used with nitrosobenzene (1.38 g, 12.9 mmol) and aniline (1.20 g, 12.9 mmol) in AcOH (70 mL). The residue was purified by column S4 chromatography using as eluent pure hexanes to DCM/hexanes 30/70 to yield 1 (1.34 g, 57%) as an orange solid. 1     2c: To a solution of 2 (0.16 g, 0.55 mmol, 1 equiv) in H 2 O/THF 1:3 v/v (40 mL), Na 2 S (0.396 g, 1.65 mmol, 3 equiv) was added and the mixture was refluxed for 4 h. The THF was evaporated and residues were diluted with 1 M NaOH and extracted with EtOAc (3 x 50 mL). Layers were separated and organic layers were combined, washed with brine and dried over MgSO 4 . After evaporation of the solvent, crude product was purified by column chromatography on SiO 2 gradient from CHCl 3 to 99/2 CHCl 3 /MeOH to give product as deep red powder (0.12 g, 82%).

3:
To a solution of 6 (0.83 g, 2.9 mmol, 1 equiv) in H 2 O/THF 1:3 v/v (90 mL), Na 2 S (2.1 g, 8.7 mmol, 3 equiv) was added and the mixture was refluxed for7 h. The THF was evaporated and residues were diluted with 1 M NaOH and extracted with EtOAc (3x50 mL). Layers were separated and organic layers were combined, washed with brine and dried over MgSO 4 . After evaporation of the solvent, crude product was purified by column chromatography on SiO 2 gradient from DCM to 98/2 DCM/MeOH to give product as an orange powder (0.51 g, 69%).            Figure S25. Electrostatic potential for 1a-3c for optimized structures.

S25
The pyramidalization of the amino group is a natural feature of the amino-compounds, explained by their tendency to adopting of the sp 3 hybridization at nitrogen. The pyramidalization angle for the amino groups in the analyzed systems is presented in the table below with the dihedral angles H-N-C-H as its measure (-180 degrees would correspond to the planar -NH 2 group): It can be clearly noticed that the amino-group is not planar, the nitrogen is slightly (about 3 degrees) pushed below the surface, but the hydrogens significantly deviate from planarity (C-C-N-H dihedrals all about 20 degrees). Thus, it will break the planar symmetry of the C 2h point group observed for the ideal unsubstituted azobenzene 1a, and in the case of 1c lead to C i -like symmetry, with the pyramidalization of both groups in opposite directions. The growing size of the analyzed azobenzene molecules additionally results in the asymmetric pyramidalization of both amino groups present in the system. Namely, for 1c both amino groups are characterized with the same dihedral angle (Table S2) equal to -138.6 degrees. For 2c, the small difference of these two dihedrals appears and it gets more pronounced for 3c (-144 .30 versus -142.48 degrees). This introduces further deviation from the C 2h -like symmetry point group.