Effects of conjugation on the properties of alkynylcarbazole compounds: experimental and theoretical study

Four novel dyes containing alkynylcarbazole, namely, 2-((9H-carbazol-3-yl)ethynyl)-9-ethyl-9H-carbazole, 3,6-bis((9-ethyl-9H-carbazol-2-yl)ethynyl)-9H-carbazole, 3-(phenylethynyl)-9H-carbazole and 3,6-bis(phenylethynyl)-9H-carbazole, were synthesized on the basis of single and double substitutes by following the Sonogashira coupling method. The synthesized dyes were then assessed as novel photosensitizers in visible-light photopolymerization to evaluate the effects of conjugation on the properties of aromatic compounds. A comparison between UV–vis and TD/DFT electron transition spectra shows that $$\lambda _{\mathrm{max}}$$ in theoretical ultraviolet spectra matched well with the experimental spectra; every conjugated alkynylcarbazole dye exhibits a wide absorption band in the range of 300–400 mm. Moreover, conjugation enhancement by switching carbazoleacetylene moieties caused a red shift in the absorption bands. The theoretical study showed that the maxima $$\lambda $$ of these molecules ranged from $$\sim $$ 330 to 370 mm, corresponding to $$\pi {\rightarrow }\pi ^*$$ and $${n}{\rightarrow }\pi ^*$$ electron transitions. Fluorescence spectroscopic data show that the strongest emission peaks exhibit a red shift because of the addition of conjugated acetylene groups. A combination of alkynyl dyes and iodonium under a halogen lamp atmosphere by visible-light photopolymerization displayed a positive response to the cationic polymerization of bisphenol-A epoxy resin A and free-radical polymerization of tripropylene glycol diacrylate.