Nonlinear Optical Pigments. Two-Photon Absorption in Crosslinked Conjugated Polymers and Prospects for Remote Nonlinear Optical Thermometry

Nonlinear optical (NLO) pigments are compounds insoluble in solvents that exhibit phenomena related to nonlinear optical susceptibilities (χ(n) where n = 2,3,...), e.g., two-photon absorption (2PA) which is related to the imaginary part of χ(3). Determination of spectrally-resolved 2PA properties for NLO pigments of macromolecular nature, such as coordination polymers or crosslinked polymers, has long been a challenging issue due to their particulate form, precluding characterizations with standard techniques such as Z-scan. In this contribution, we investigate thus far unknown spectrally-resolved 2PA properties of a new subclass of NLO pigments—crosslinked conjugated polymers. The studied compounds are built up from electron-donating (triphenylamine) and electron-withdrawing (2,2’-bipyridine) structural fragments joined by vinylene (Pol1) or vinyl(4-ethynylphenyl) (Pol2) aromatic bridges. 2PA properties of these polymers have been characterized in broad spectral range by specially modified two-photon excited fluorescence (TPEF) techniques: solid state TPEF (SSTPEF) and internal standard TPEF (ISTPEF). The impact of self-aggregation of aromatic backbones on the 2PA properties of the polymers has been evaluated through extended comparisons of NLO parameters, i.e., 2PA cross sections (σ2) and molar-mass normalized 2PA merit factors (σ2/M) with those of small-molecular model compounds: Mod1 and Mod2. By doing this, we found that the 2PA response of Pol1 and Pol2 is improved 2–3 times versus respective model compounds in the solid state form. Further comparisons with 2PA results collected for diluted solutions of Mod1 and Mod2 supports the notion that self-aggregated structure contributes to the observed enhancement of 2PA response. On the other hand, it is clear that Pol1 and Pol2 suffer from aggregation-caused quenching phenomenon, well reflected in time-resolved fluorescence properties as well as in relatively low values of quantum yield of fluorescence. Accordingly, despite improved intrinsic 2PA response, the effective intensity of two-photon excited emission for Pol1 and Pol2 is slightly lower relative to Mod1 and Mod2. Finally, we explore temperature-resolved luminescence properties under one- (377 nm), two- (820 nm), and three-photon excitation (1020 nm) conditions of postsynthetically Eu3+-functionalized material, Pol1-Eu, and discuss its suitability for temperature sensing applications.


Synthetic procedures Materials
Starting materials were of reagent grade purity and were obtained from commercial sources (Aldrich and Avantor Chemicals, Poland). THF, toluene, dioxane and methanol used for syntheses were kept over 3 Å molecular sieves.

Synthesis of 2,2'-bipyridine-4,4'-dicarboxylic acid, (1a)
In a two-neck flask was placed 4,4-dimethyl-2,2-bipyridine (4.000 g, 21.7 mmol) and 98% sulfuric acid (70 cm 3 ) was carefully added dropwise. After cooling to to room temperature, potassium dichromate (3.3g, 11.3mmol) was added in portions (the solution color turns dark green). Both processes are strongly exothermic; hence, in each case, the reaction flask is immersed in water bath for better dissipation of produced heat. Next, the reaction mixture was heated to 65 °C and was maintained at that temperature for 6 h under stirring. After that time, reaction mixture was cooled to room temperature, the reaction mixture poured into 500 cm 3 of water, forming greenish precipitate which was filtered out under reduced pressure, and washed with a small amount of water. Next, crude compound was dissolved in a minimal amount of ammonia affording dark-colored solution which was filtered through paper filter (chromium-containing precipitate was discarded), and the filtrate was acidified with 1M HCl to obtain suspension of 2,2'-bipyridine-4,4'-dicarboxylic acid. It was filtered out under reduced pressure, washed with small amount of water, and dried. Pure 2,2'-Bipyridine-4,4'-dicarboxylic acid was obtained (4.811g, 91%). 1 H NMR (400 MHz, D2O+NaOD) δ 8.64 (2H, s), 8.23 (2H, s), 7.75 (2H, s). Note that peaks appear as singlets due to proton-exchange broadening.
The mixture was stirred until the glassy solid was obtained. Next, a solution of triphenylamine (5.00 g, 20.4 mmol) in chloroform (50 cm 3 ) was added. Reaction mixture was refluxed for 6 hours and then cooled to room temperature. After cooling, the mixture was poured in portions into the 200 cm 3 of 20% aqueous solution of sodium acetate. Biphasic mixture was stirred until organic layer changed color to yellow. Organic layer was separated.

Synthesis of tris(4-formylphenyl)amine, (3)
A modified two-step Vilsmeier-Haack Formylation reaction was employed to obtain tris(4-formylphenyl)amine. This protocol rests on synthesis of bis(4-formylphenyl)aniline in the first synthetic step, followed by additional formylation reaction in the second step. In two-neck flask was placed DMF (14.5 cm 3 , 187.4 mmol) and POCl3 (20.00 cm 3 , 214.6 mmol) was added in small portions at 0 °C under the atmosphere of nitrogen. The mixture was stirred till the glassy solid was obtained. Next, triphenylamine (2.50 g, 10.2 mmol) was added.
Reaction mixture was kept at 95 deg C. for 4h under nitrogen and then cooled to room temperature. After cooling, glassy solid was dissolved in chloroform and the mixture was poured in portions into the 300 cm 3 of 20% aqueous solution of sodium acetate.
Biphasic mixture was stirred until organic layer changed color to orange. The organic layer was separated and was washed with water (50 cm 3 ), two times with NaCl saturated solution (2 x 50 cm 3 ) and finally dried with anhydrous Na2SO4. Crude bis(4formylphenyl)aniline was dissolved in a minimal amount of chloroform was passed through short layer of silica gel in order to remove dark-colored impurities. No additional purifications of crude intermediate product were performed, and in the second step the crude bis(4-formylphenyl)aniline was subjected to additional formylation. In a two-neck flask was placed DMF (14.51 cm 3 , 187.4 mmol) and POCl3 (20.05 cm 3 , 214.6 mmol) was added in small portions at 0 °C under the atmosphere of nitrogen. The mixture was stirred until the glassy solid was obtained. Next, a powdered crude bis(4-formylphenyl)aniline was added. Reaction mixture was kept at 95 deg C. for 2h under nitrogen and then cooled to room temperature. After cooling, glassy solid was dissolved in chloroform and the mixture was poured in portions into the 300 cm 3 of 20% aqueous solution of sodium acetate. Organic layer was separated and was washed with water (50 cm 3 ), two times with NaCl saturated solution (2 x 50 cm 3 ), and finally dried with anhydrous Na2SO4. After evaporation yellow solid was subjected to column chromatography (dichloromethane) to give pure tris(4formylphenyl)amine (1.609 g, 48%).

Synthesis of 4-N,N-diphenylamino-1-bromobenzene, (4c)
In a two-neck flask were placed triphenylamine (9.210 g, 37.50 mmol) and N-Bromosuccinimide (NBS, 6.680 g, 37.50 mmol) were dissolved in 120 mL carbon tetrachloride and obtained solution was refluxed for 5 h. After that time reaction mixture was cooled to room temperature. Precipitate (succinimide) was filtered out and discarded. Filtrate was evaporated, and obtained oil was crystallized from ethanol once. Pure 4-N,N-diphenylamino-1-bromobenzene was obtained as fine white needles