In the solution state, there were no notable differences between the optical properties of a range of alkyl-substituted tetracenes. However, in the solid state, their photophysical properties changed with respect to the length, shape, number, and substitution pattern of the alkyl side chains, as well as the distribution of two regioisomers. Remarkably, in the solid state, 1,4,7,10-tetraisopropyltetracene exhibited the highest reported fluorescence quantum yield of any tetracene derivative (0.90). The changes in the optical characteristics of these tetracenes according to the arrangement of the tetracene rings and the color-change mechanism in the solid state are discussed. Moreover, the world record in solid-state fluorescence efficiency in acenes larger than anthracene is described. DOI 10.1002/tcr.201200003

We synthesized 1,4-dipropyltetracene on a 200-mg scale, the key step of which involved a Diels–Alder reaction between alkyl-substituted o-quinodimethane, generated in situ, and 1,4-naphthoquinone. The product was obtained as an orange solid, which was soluble in organic solvents including hexane. The optical properties of the product in solution showed no marked differences from those of other 1,4,7,10-tetraalkyltetracenes. Solid-state absorption and fluorescence spectra exhibited 20–30 nm blueshifts compared with those of 1,4,7,10-tetrapropyltetracene. X-ray analysis revealed that two propyl groups were coplanar with the tetracene ring, that there was no π overlap along the stacking direction, and that the molecules formed a herringbone structure. The peripheral alkyl chains were found to be important for controlling the molecular packing and optical properties in the solid state.

We have synthesized 1,4,7,10-tetraisoalkyltetracenes from a 2,6-naphthodiyne precursor and 2,5-diisoalkylfurans as the starting materials (isoalkyl: isopropyl, isobutyl, and isopentyl). The tetracene molecules exhibited crystallochromy: The solid-state colors of the isopropyl, isobutyl, and isopentyl derivatives were yellow, red, and orange-yellow, respectively. In contrast, there were no marked differences in the optical properties of these compounds in solution. The isopropyl derivative exhibited the highest fluorescence quantum yield in the solid state of 0.90 in the series of alkylated tetracenes. X-ray analysis revealed that there were significant structural differences in alkyl conformation and crystal packing. The crystallochromy effects are derived from the unique crystal-packing patterns. The fluorescence quantum yields in the solid state probably depend on the relative positional relationship of the nearest neighboring molecules in one column as well as the crystal rigidity.

The cover picture shows 1,4,7,10-tetraisoalkyltetracenes in the solid state under UV light irradiation and under natural light. The photophysical properties exhibit crystallochromy ranging from yellow to red and different fluorescence quantum yields depending on the length of the alkyl side chain. The isopropyl derivative achieved the highest quantum yield ever reported among tetracenes. Details are discussed in the article by C. Kitamura et al. on p. 3033 ff. The background of the cover picture is a night view of Himeji Castle, which is well known for its beauty and supposed resemblance to an egret (it was accorded UNESCO World Heritage Site status in 1993). The authors would like to thank Hiroko Kitamura for her help in designing the cover picture.

We synthesized a series of 1,4,7,10-tetraalkyltetracenes using a new 2,6-naphthodiyne precursor and 2,5-dialkylfurans as starting materials (alkyl=methyl to hexyl). Surprisingly, the solid-state color of the tetracenes ranges through yellow, orange, and red. Both yellow and red solids are obtained for the butyl derivative. Optical properties in solution show no marked differences; however, those in the solid state show characteristics that vary with alkyl side-chain length: methyl, propyl, and pentyl derivatives are orange; ethyl and butyl derivatives are yellow; and another butyl and hexyl derivative are red. X-ray analyses reveal that the molecular structures are planar, semi-chair, or chair forms; the chair form takes a herringbone-like arrangement and the other forms take slipped parallel arrangements. The mechanism of crystallochromy is discussed in terms of molecular structure, crystal packing, and calculations that take account of exciton coupling.