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.

No abstract

No abstract

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.

An organoboron complex containing a [2.2]paracyclophanyl group (2 BF2) exhibits remarkable solvatofluorochromism associated with a large Stokes shift. With increased solvent polarity, the fluorescence (FL) color of 2 BF2 changes from sky blue (λFL=485 nm in cyclohexane (c‐C6H12)) to orange (λFL=576 nm in CH3CN). This behavior derives from the intramolecular charge transfer (ICT) character of the excited state 2 BF2* in which the [2.2]paracyclophanyl group serves as the electron donor while the remaining 2 BF2 structure (apart from the sub‐benzene ring) functions as the electron acceptor. Wave deconvolution of FL spectra in a solvent mixture of CH2Cl2/c‐C6H12, and a plot of the FL emission energy versus the CH2Cl2/c‐C6H12 ratio, suggest that FL of 2 BF2 occurs from two types of 2 BF2* excited states. The results of DFT calculations support this explanation and further emphasize that the ICT character of 2 BF2* is derived from a large conformational change associated with the electron‐donating character of the [2.2]paracyclophanyl group. Moreover, the fact that 2 BF2 is more sensitive than Reichardt's dye to polarity changes in a low polarity CH2Cl2/c‐C6H12 solvent mixture is also of importance.