Effects of peripheral ring fluorination on the photophysical properties of hexaarylbenzenes

Hexaarylbenzenes (HABs) exhibit interesting photophysical properties that originate from the through-space charge-transfer interactions between their six twisted peripheral aromatic rings. Although the introduction of fluorine atoms into π-conjugated structures allows one to effectively control their electron density distribution and aggregation behavior, the effects of peripheral ring fluorination on HAB properties remain underexplored. This paper describes the syntheses and photophysical properties of three unsymmetrical HABs and their symmetrical regioisomers, namely compounds belonging to 0F (no fluorine atoms on the short molecular axes of the peripheral aromatic rings), 4F (four fluorine atoms as substituents on the short molecular axes of the aromatic rings with CF3 end groups), and 8F series (four fluorine atoms on the short molecular axes of each of the peripheral aromatic rings). All the HABs exhibited blue photoluminescence both in solution and in crystalline form. Molecules in the 4F series exhibited an increase in their Stokes shift with increasing solvent polarity and featured superior intramolecular charge-transfer properties, which produced a significant redshift of the PL band of the unsymmetrical molecule compared to that of its symmetrical counterpart. It is noteworthy that the 4F series exhibited photochromic behavior, that is, an additional absorption band in the longer-wavelength region was observed upon irradiating it with 280-nm UV light, and consequently the colorless solution acquired a yellow color . Thus, the partial fluorination of the HAB scaffold (4F series) produced photochromic behavior, while its other photophysical properties were preserved.