Structure–property relationships and third-order nonlinearities in diketopyrrolopyrrole based D–π–A–π–D molecules

Nine new quadrupolar chromophores based on diketopyrrolopyrrole were designed and prepared by cross-coupling reactions. The property tuning has been achieved by structural variation of the peripheral substituents (donor) and enlargement of the π-system. Fundamental properties of target molecules were studied by differential scanning calorimetry, electrochemistry, and absorption and emission spectra. Nonlinear optical properties were studied by measuring the third harmonic generation. The experimental data were completed by quantum-chemical calculations and structure–property relationships were elucidated.


Experimental procedures and characterization of compounds
Reagents and solvents were reagent-grade and were purchased from Penta, Aldrich, and Fluka and used as received. Dry THF was freshly distilled from Na/K alloy and diphenylmethanone under inert argon atmosphere. Used solvents were evaporated on a Heidolph Laborta 4001 rotary evaporator. Cross-coupling reactions were carried out at vacuum-inert line in Schlenk's flasks. Column chromatography was carried out with silica gel 60 (particle size 0.040-0.063 mm, 230-400 mesh; Merck) and commercially available solvents. Thin-layer chromatography (TLC) was conducted on aluminum sheets coated with silica gel 60 F 254 obtained from Merck, with visualization by UV lamp (254 or 360 nm). Melting points (mp) were measured on a Büchi B-540 melting-point apparatus in open capillaries and are uncorrected. 1 H and 13 C NMR spectra were recorded in CDCl 3 at 400/100 MHz with a Bruker AVANCE III or 500/125 MHz with Bruker Ascend TM at 25 °C. Chemical shifts are reported in ppm relative to the signal of Me 4 Si. The residual solvent signal in the 1 H and 13 C NMR spectra was used as an internal reference (CDCl 3 -7.25 and 77.23 ppm). Coupling constants (J) are given in Hz. The apparent resonance multiplicity is described as s (singlet), d (doublet), t (triplet), q (quartet) and m (multiplet). Signal overlap of 2-ethylhexyl chain CH group with signal of residual water can be observed in 1 H NMR spectra. High-resolution MALDI mass spectra were measured using method ''dried droplet'' on a MALDI mass spectrometer LTQ Orbitrap XL (Thermo Fisher Scientific, Germany) equipped with nitrogen UV laser (337 nm, 60 Hz). The LTQ Orbitrap instrument was operated in positive ion mode over a normal mass range (m/z 50-2000) with the resolution 100.000 at m/z 400. The used matrix was 2,5-dihydroxybenzoic acid (DHB). IR spectra were recorded as neat using HATR adapter on a Perkin Elmer FTI Spectrum BX spectrometer. UV-vis spectra were S4 recorded on a HP 8453 spectrophotometer in 1,4-dioxane (c 1 × 10 −5 mol/L). These solutions were diluted 10x for fluorescent measurement, absorbance A max (0.06-0.1).

Dibromo DPP derivative 8
Diketopyrrolopyrrole derivative 7 (1.12 g, 1.75 mmol) was dissolved in DCM (100 mL). N-Bromosuccinimide (622 mg, 3.50 mmol) and acetic acid (0.3 mL) were added. The reaction was stirred at 25 °C for 2 hours and quenched by water (100 mL). The product was extracted with DCM (3 × 100 mL), the combined organic extracts were dried with sodium sulfate, filtered and the solvent was evaporated. The

Chromophore 2b
The

Chromophore 3b
The

S14
THG measurement's method Figure S1: Principal set-up for the measurements of the THG in the reflected light geometry.
The 10 ns Nd:YAG laser was used as a source of the fundamental laser beam at wavelength 1064 nm. The pulse duration was about 10 ns, the pulse frequency repetition was equal to about 11 Hz. The output laser energy was tuned by rotating polarizer. The set-up allows to achieve the maximal energy density of about 199.05 J/m 2 . The beam dimer was varied within the 1.5 to 3.5 mm and its beam sequence was of a Gaussian-like form. The stability of the pulse was averaged over 200 pulses and did not exceed 0.4%. Its energy was controlled by Ge detector with 0.8 ns relaxation time. The fundamental beam has illuminated the sample's surface using a set of mirrors. The chromophores 1-5 were placed between two glass plates. The reflected light from the chromophore was analyzed using an interferometric filter at 355 nm and photomultiplier. The samples were put on a rotating table in order to find the maximal THG signal, which was analyzed with 2 GHz Tectronix oscilloscope. The measurements were operated by a PC. The control of the sample surfaces has shown that the local heating did not exceed 2 to 3 K. Particular attention was devoted S15 to elimination of the parasitic light scattering and this background did not exceed 0.6%.
The THG value was evaluated by the effective surfaces under the curves shown in the Figure 6 after the statistical averagment over 200 different surface points. The further increase of fundamental laser energy was limited by occurrence of specimen's photo-destruction. Higher THG responses was observed for 1a and 4b, however due to their high linear absorption at 355 nm, the renormalization for the absorption was done and the resulted THG magnitudes were different. The BiB 3 O 6 crystals were used as reference samples [2]. The absolute values are given in the Table 2, the reproducibility of the data was better than 2.3%.