Novel sterically demanding Schiff base dyes: An insight from experimental and theoretical calculations

Abstract

We report extensive experimental and theoretical studies on structural and photophysical properties of two new bulky Schiff base dyes 1 and 2, obtained from the reaction of cyclohexylamine with the 5-bromosalicylaldehyde or 2-hydroxy-1-naphthaldehyde. The structure of 1 exhibits the enol-imine form in the solid state and is characterized by a typical intramolecular O–H⋯N hydrogen bond. Contrarily, the structure of 2 was found to be in the cis-keto-enamine tautomer, stabilized by an intramolecular N–H⋯O hydrogen bond. Favoured intermolecular H⋯H, H⋯C, H⋯O and H⋯Br contacts in the structure of 1 and H⋯H, H⋯C and H⋯O in the structure of 2 are responsible for the overall crystal packing of the dyes. The UV–vis spectra of 1 in cyclohexane, THF, CH2Cl2 and CH3CN exhibit bands in the UV region, indicating the enol-imine form, while in MeOH the UV–vis spectrum of 1 exhibits an additional band in the visible region at 410 nm, which corresponds to the cis-keto-enamine tautomer. The absorption spectra of 2 in the same solvents, except cyclohexane, also contain a band in the visible region up to about 450 nm. This band exhibits two maxima, which can be assigned to an equilibrium between the cis-keto-enamine and zwitterion forms, of which the latter one is structurally the same as the former isomer. The UV–vis spectrum of 2 in cyclohexane reveals only traces of the cis-keto-enamine. The dye 1 was found to be emissive exclusively in MeOH, while 2 is emissive in all the studied solvents, of which the most pronounced and the most negligible emission was found for MeOH and cyclohexane, respectively. An intense emission band in the spectrum of 1 at 468 nm arises from the emission of two conformers of the cis-keto-enamine* form. The solution of 2 in MeOH showed dual emission with the bands centered at 346 and about 450 nm, of which the latter one is a superposition of two single bands. The two low-energy emission bands arise from the emission of the cis-keto-enamine* and zwitterion* forms, while the high-energy emission band most likely corresponds to the naphthalene fragment. Finally, π-resonanse stabilization (RAHB) effect is found to be crucial for tuning the tautomerization mechanism through the alteration of intramolecular O–H⋯N hydrogen bond.