The crystal-packing structures of seven derivatives of diaroylmethanatoboron difluoride (1 a–gBF₂) are characterized by no overlap of the π-conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π-orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π-orbitals of adjacent molecules (type III). The crystal-packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer-like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π-orbitals generates new FL domains, referred to as “excited multimers”, which possess allowed S₀–S₁ electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π-orbitals in the type III crystals leads to “excited multimer” domains with forbidden S₀–S₁ electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.

A series of 17,17-dialkyl-3,14-diaryltetrabenzofluorenes were efficiently prepared by using Suzuki–Miyaura cross-coupling reactions of the corresponding 3,14-dibromo derivatives. Studies of the unique fluorescence properties of these compounds showed that they display intense blue to yellow fluorescence with high quantum yields in the solution state and blue to orange fluorescence with moderate quantum yields in the solid state. In addition, the fluorescence wavelength of the bis(p-nitrophenyl) derivative is remarkably solvent-dependent in a manner that correlates with the solvent polarity parameter E_T(30). The results of density function theory calculations suggest that the intramolecular charge-transfer character of the HOMO–LUMO transition is responsible for the large solvent effect. Moreover, addition of water to a tetrahydrofuran (THF) solution of this compound leads to quenching of the yellow fluorescence owing to an increase in the solvent polarity. However, when the amount of water fraction exceeds 70 %, a new fluorescence band appears at the same orange-red emission wavelength as that of the solid-state fluorescence. This observation suggests the occurrence of a crystallization-induced emission (CIE) phenomenon in highly aqueous THF.

The geometries, energy gap between singlet and triplet states (S–T gap), electronic transitions, and molecular orbitals of the diphenyl derivative of the silole-fused tetramethyleneethane (TME) (3,4-dimethylenesilole) biradical 7^•• were determined by using the density functional theory method at the unrestricted Becke, three-parameter, Lee–Yang–Parr/cc-pVDZ level. In a manner similar to that of the parent TME biradical, the S–T gap of 7^•• is small. The wavelengths of electronic transitions of ³7^•• (λ_ET = 450 nm) are calculated to be different from that of ¹7^•• (λ_ET = 673 nm). These differences are attributed to interactions between orbitals of the two allyl radical moieties, especially, the existence of double σ*–π* conjugation, that is, π*–σ*–π* conjugation, that takes place in the silole subunit. Copyright © 2011 John Wiley & Sons, Ltd.