Visible absorbing croconium dyes with intramolecular hydrogen bonding: A combined experimental and computational study

Abstract Croconium molecules CR1-CR4 with break-in conjugation (C N Bonding) was synthesized by condensation of croconic acid and arylamines. By using combined experimental and theoretical methods like UV–visible spectra, DFT and TDDFT studies, we have characterized electronic absorption properties. The reported molecules are having absorption in visible region ranging from 450 to 550 nm with large extinction coefficient (2.5–5.0 × 10 4  M −1  cm −1 ). We find that CR2 and CR4 are showing 50 to 100 nm red shifted absorption than CR1 and CR3 . This red shift is possibly due to presence of intramolecular hydrogen bonding in CR2 and CR4 . Further this is supported by DFT studies, in case of CR2 and CR4 shows strong intramolecular hydrogen bonding between oxygen of carboxylate group (at ortho position of phenyl ring) and hydrogen of nitrogen attached to the central croconate ring. It is also observed that, there is small diradicaloid character in these molecules. This study is helpful in design and synthesis of new croconium dyes which are useful in materials applications.