3,14-Bis(4-formylphenyl)-17,17-di(n-pentyl)tetrabenzo[a,c,g,i]fluorene showing solvatochromism and crystallochromism in fluorescence

Abstract

The substituent effect on optical properties of 3,14-diphenyl-17,17-di(n-pentyl)tetrabenzofluorene derivatives bearing electron-withdrawing groups (NC, CH3OCO, CH3SO2, and OHC) at 4-position of the phenyl groups were investigated. These compounds were readily prepared by using Suzuki–Miyaura cross-coupling as a key step. The 4-cyano, 4-methoxycarbonyl, and 4-methylsulfonyl derivatives display intense blue to blue-green fluorescence in CH2Cl2 and in the solid states. On the other hand, the title compound forms two polymorphs, and thus, exhibit crystallochromism in fluorescence; the prismatic crystal emits intense yellow fluorescence, and the other crystal form does blue-green fluorescence. Moreover, the compound shows large solvatofluorochromism (ΔλEM = 67 nm). The crystallographic analysis of the prismatic crystal reveals that the tetrabenzofluorene moiety of the compound possesses a helical structure with a C2 symmetry and the molecules form a herringbone-like structure. The density function theory calculations predict the remarkable intramolecular charge-transfer character of the compound. The prediction is in good agreement with the observed fluorescence properties.