Planejamento, síntese e estudo de novos cristais líquidos luminescentes derivados da triazina

Abstract

In this work, two series of discotic molecules were designed, synthesized and characterized, one symmetrical and the other non-symmetrical, both derived from the s-triazine heterocycle. The methodology used initially used the trimerization of benzonitriles in an acid medium and later the triphenyltriazine was functionalized with one, two or three arms of the phenanthryl or [4]helicenyl type from Perkin-Mallory reactions. The final molecules were designed with short-chain alkyl ester-type groups to induce glass transitions and also columnar mesomorphism. All compounds were characterized using spectroscopic and spectrometric techniques. The thermal properties of the final molecules were investigated by DSC, TGA, POM and XRD techniques.

The phenanthryl- and helicenyl-substituted symmetrical triazine esters showed glass transitions conveniently above room temperature and within the hexagonal columnar liquid crystal phase, resulting in a solid columnar order at room temperature. Clearing points > 375°C were recorded for phenanthryl derivatives. For the ideal alignment of a liquid crystal, it is important that the melting and clearing temperatures are attainable, which was achieved only for the moderately non-planar symmetrical (> 205°C) helicenyl derivatives.

To overcome this challenge, a second series of non-symmetrical molecules was prepared from the functionalization of triphenyltriazine with one or two phenanthryl or helicenyl arms but keeping the methyl benzoate group in the structure. Non-symmetrical triazines also showed hexagonal columnar liquid-crystalline behavior, but with lower melting points (< 290°C). Glass transitions were also observed at approximately 70, 50, 40 and 0°C for the non-symmetrical compounds, depending on the substitution pattern.

The photophysical properties were evaluated for the symmetrical triazines in solution and solid phase. The materials showed absorbance between 272-342 nm and emission between 409-458 nm. The average quantum yield was 22% for phenanthryl derivatives and 12% for helicenyl. From the photophysical data it was possible to infer the energies of the first excited singlet state S1 of 3.31 eV for phenanthrenes and 2.95 eV for helicenes. DFT calculations showed good correlation between experimental and calculated energy values for S1 and allowed a good estimate for T1. The calculated values for the T1 orbital of the symmetric triazines are close to 2.5 eV, therefore, these materials can be considered as matrices for TADF emitters that emit light in the yellow-green range of the visible spectrum.