Tunable luminescence of a novel organic co-crystal based on intermolecular charge transfer under pressure

Continuous tunable photoemission of a novel co-crystal (9-acetylanthracene-tetrafluoroterephthalonitrile, 9ACA-TFP) depending on intermolecular charge transfer (CT) is achieved in situ under high pressure via a diamond anvil cell. Compared with other methods for tuning the emission of organic co-crystals based on organic synthesis or doped organic co-crystals, this pressure-based tuning method is simple and effective. The high-pressure data show that this organic co-crystal (9ACA-TFP) exhibits continuous redshifts in its emission bands (from 490 to 620 nm) with the pressure increasing, and its colour and emission bands can recover to their initial state once the pressure is completely released. Raman and ultraviolet-visible absorption spectra with pressure were also measured. All Raman vibrational bands move to higher wavenumbers and exhibit decreased intensities with pressure, proving that the shortened bond lengths and donor-acceptor-donor (D-A-D) interplanar distance result in stronger intermolecular interactions. The in situ absorption spectra of the 9ACA-TFP co-crystal show a 134 nm redshift from 0 to 7.22 GPa, indicating that high pressure enhances the intermolecular CT transition between D and A. The co-crystal materials show on/off switching of photoemission upon cycling compression and decompression. This study is of sigificance in determining the intermolecular CT interaction -photoemission property relation of organic co-crystals.