Aggregation-induced emission with large redshift in 2,7-diphenyl-fluorenone: Reality or artifact?

Abstract

The luminescence property of 2,7-diphenyl-fluorenone (DPFO) was previously reported to be very unusual with a large aggregation-induced effect associated with a fluorescence redshift of 150 nm. The phenomenon is reexamined in this work. It is found that the abnormal observations are caused by the presence of a trace amount of impurity 2,7-diphenyl-fluorene (DPF) in the as-synthesized DPFO. The pure DPFO molecule does have an intense fluorescence (FL) in solid (528 nm), about 4–5 times larger than in its dilute dichloromethane solutions (542 nm), but with a blueshifted rather than redshifted FL wavelength in solid. The enormous FL enhancement and redshifted FL wavelength of the as-synthesized DPFO solid are due to the presence of impurity DPF. The FL of DPF is much stronger than that of DPFO in dilute solutions and it also has shorter FL wave-lengths. In a dilute solution of DPFO with a trace amount of DPF (~1%), the dominant FL peaks are from DPF. Because the electronic absorption peaks of DPF overlaps with DPFO, the electronic energy of DPF can transfer to DPFO. The energy transfer is faster with the increase of concentration because DPF and surrounding DPFO molecules become closer, which quenches the FL of DPF (356 and 372 nm) and enhances the FL of DPFO (542 nm in solution and 528 nm in solid). Therefore, at high concentrations or in solids, only peak at about 542 or 528 nm shows up, and peaks at 356 and 372 nm disappear.