Synthesis and photophysical study of new push-pull 1,4-naphthoquinone dyes

Push-pull or acceptor-donor dyes are extensively studied for their versatile applications as dyes and sensors.(1) Naphthoquinones (NQ) can be found in many natural products and the rational design of new NQ with simple structural modifications can create colorful compounds. In this study, NQ was chosen as the acceptor and tetrahydroquinoline (THQ), diphenylamine (DPA), triphenylamine (TFA), carbazole (CBZ) and phenothiazine (PTZ) as the donor moieties. The synthesis of the molecules 1-5 occurs by an oxidative coupling(2) with triflic acid, as catalyst, in acetonitrile. The photophysical behavior of 1-5 was studied. The UV-visible absorption spectrum revealed an intramolecular charge transfer (ICT) band. In ACN, the λmax was 533, 494, 502, 458 and 531 nm for 1 to 5, respectively. However, only 1, 2 and 5 exhibited a significant solvatocromic effect. On changing the solvent from cyclohexane to t-butanol, a bathochromic shift was observed: 99, 70 and 80 nm, respectively for 1, 2 and 5. It was possible to distinguish up to 20% and 10% of ethanol in ethanol-hexane mixtures using 1 and 2, respectively, as colorimetric sensors. Compound 1 was observed to differ from the other derivatives in that its’ solution was discolored by the addition of trifluoracetic acid due to protonation of the basic NH site, thus suppressing the ICT process. The protonation was reversed when triethylamine was added. Compounds 1 to 5 were weakly fluorescent in apolar solvents (0.002 ≤ Φf CyH ≤ 0.16), max for emission in cyclohexane for 1 to 5 was: 598, 565, 596, 525 and 695 nm. Polar solvents quenched the fluorescence for all structures. Finally, the insertion of donor groups increased significantly NQ absorption in the visible region and 1 and 2 are possible candidates as colorimetric sensors for alcohols in apolar media.
1 Klymchenko, A. S. Acc. Chem. Res. 2017, 50, 366−375. 2 Jiang, J-H, Boominathan, S. S. K., Hu, W-P, Chen, C-Y, Vandavasi, J.K., Lin, Y-T, Wang, J-J. Eur. J. Org. Chem. 2016, 2284–2289