Synthesis, Crystal Structures, and Solid-State Optical Properties of Substituted Tetracenes

Abstract

Various tetracenes possessing substituents, such as alkyl and ester groups, were prepared. In solution, their optical properties exhibited no marked differences. However, in the solid state, their photophysics differed with respect to the length, shape, number, substitution pattern, and kind of substituent groups, which is called crystallochromy. 1,4,7,10-Tetra(n-alkyl)tetracenes exhibited the solid-state colors of yellow, orange, and red. Both their molecular structures and molecular arrangements were classified into three groups. The molecular packing correlated well with the solid-state color. In 1,4,7,10-tetra(isoalkyl)tetracenes, the increase of fluorescence quantum yield in the solid state was remarkable. The isopropyl derivative showed the highest fluorescence quantum yield of 0.90 among any tetracene derivatives. In the case of anti-/syn-regioisomeric tetracene mixtures, the change in the solid-state color before and after recrystallization was observed. The X-ray analysis revealed that more crystalline molecules were anti-regioisomers. Molecular packing of 1,4-di(n-propyl)tetracene was a herringbone fashion which was different from that of 1,4,7,10-tetra(n-propyl)tetracene. Both 2,3-di(n-alkyl)- and 2,3,8,9-tetra(n-alkyl)tetracenes had low crystallinity and exhibited slight crystallochromy. In the crystal of 2,3-tetra(n-butyl)tetracene, there were not only a 1:1 disorder of two components of the molecule but also the repeating bilayer structure of the tetracene rings and alkyl groups. In tetracene octaester derivatives, the molecule with shorter alkyl chain length in the ester moiety was more redshifted. Relationship between the solid-state color and the interplanar distance between two neighboring tetracene rings in the crystal was observed. In conclusion, the alkyl side chains could act as not only spacer but also molecular orientation adjusters, to control the photophysical properties in the solid state.