Abstract: NIR dyes have become popular for many applications, including biosensing and imaging. For this reason, the mol. switch mechanism of the xanthene dyes makes them useful for in vivo detection and imaging of bioanalytes. Our group has been designing NIR xanthene-based dyes by the donor-acceptor-donor approach; however, the equilibrium between their opened and closed forms varies depending on the donors and spacer. We synthesized donor-acceptor-donor NIR xanthene-based dyes with an alkyne spacer via the Sonogashira coupling reaction to investigate the effects of the alkyne spacer and the donors on the maximum absorption wavelength and the mol. switching (ring opening) process of the dyes. We evaluated the strength and nature of the donors and the presence and absence of the alkyne spacer on the properties of the dyes. It was shown that the alkyne spacer extended the conjugation of the dyes, leading to absorption wavelengths of longer values compared with the dyes without the alkyne group. In addition, strong charge transfer donors shifted the absorption wavelength towards the NIR region, while donors with strong π-donation resulted in xanthene dyes with a smaller equilibrium constant DFT/TDDFT calculations corroborated the exptl. data in most of the cases. Dye 2 containing the N,N-dimethylaniline group gave contrary results and is being further investigated.
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Preparation of compound 2-2 [95] The compound 2-1(18 g, 55.86 mmol) was dissolved in THF (200 mL) and n-buLi (24.58 mL, 61.45 mmol, 2.5 M in Hexane) was slowly added thereto at -78C. 1 hours later, DMF (5.6 mL, 72.65 mmol) was added and the mixture was stirred for 12 hours at room temperature. Distilled water was added thereto and the product was extracted with EA. After drying with MgSO4 and distilling under reduced pressure, a compound 2-2 (11 g, 40.54 mmol, 72.58 %) was given by column separation.
51%
To a solution of 0.90 g 9-(4-bromo-phenyl)-9H-carbazole (1) (2.8 mmol) in 30 ml anhydrous Et2O was added 1.34 ml of 2.5 M n-BuLi in hexane (3.4 mmol) at -78 C under nitrogen atmosphere. After the reaction mixture was warmed to 0 C, 0.26 ml DMF(3.4 mmol) was added at -78 C. The resulting mixture was allowed to warm to room temperature and stirred at room temperature overnight. Then the mixture was poured into ice water and 5% HCl (aq.) (5 ml) was added and the two layers were separated. The aqueous layer was extracted with Et2O (3 × 20 ml) and the combined organic layer was washed with brine (15 ml), dried with MgSO4 and evaporated under reduced pressure, a yellow power was obtained by crystallized from ethanol (0.39 g, yield:51%). m.p. 148-150 C refPreviewPlaceHolder[31]. 1H NMR (300 MHz, CDCl3) δppm 10.12-10.20 (s, 1H), 8.12-8.16 (m, 4H), 7.78-7.86 (d, 2H), 7.49-7.63 (d, 2H), 7.32-7.44 (t, 2H), 7.19-7.30 (t, 2H). MS(M+) 271.0997. Anal. Calc. for C19H13ON: C, 84.13%; H, 4.80%; N, 5.17%. Found: C, 84.40%; H, 4.77%; N, 5.12%.
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