Blue emitting organic semiconductors under high pressure: status and outlook

Abstract

This review describes essential optical and emerging structural experiments that use high GPa range hydrostatic pressure to probe physical phenomena in blue-emitting organic semiconductors including π-conjugated polyfluorene and related compounds. The work emphasizes molecular structure and intermolecular self-organization that typically determine transport and optical emission in π-conjugated oligomers and polymers. In this context, hydrostatic pressure through diamond anvil cells has proven to be an elegant tool to control structure and interactions without chemical intervention. This has been highlighted by high pressure optical spectroscopy whilst analogous x-ray diffraction experiments remain less frequent. By focusing on a class of blue-emitting π-conjugated polymers, polyfluorenes, this article reviews optical spectroscopic studies under hydrostatic pressure, addressing the impact of molecular and intermolecular interactions on optical excitations, electron-phonon interaction, and changes in backbone conformations. This picture is connected to the optical high pressure studies of other π-conjugated systems and emerging x-ray scattering experiments from polyfluorenes which provides a structure-property map of pressure-driven intra- and interchain interactions. Key obstacles to obtain further advances are identified and experimental methods to resolve them are suggested.

High‐Pressure Chemistry and the Mechanochemical Polymerization of [5]‐Cyclo‐p‐phenylene

“…Figure shows the computed variation of the intensity ratio of two important Raman bands, that is, I / I as a function of the pressure. The increase of the ratio with an increasing pressure is in good correlation with the literature where we also see an increase of the ratio when the interring torsion increases.…”

“…Although the slopes, which are the MGPs, vary, their general behavior is in fact similar to those seen for linear para ‐hexaphenyl . The actual MGP values computed based on Figure S16 in the Supporting Information are around 1–3, which are in the generally observed range …”

“…An interesting connection to the behavior of our system with previously studied conjugated molecular crystals under high pressure can be seen by comparing their mode Grüneisen parameters (MGPs) . A selection of the correlation between the log [ V (0)/ V ( P )] and log [ i ( P )/ i (0)] where V ( P ) is the unit cell volume at the pressure P , and i ( P ) is the frequency of the i ‐th Raman band, are shown in Figure S16 in the Supporting Information.…”

“…The literature contains descriptions and experiments on conjugated systems, which show structural changes and phase transformations under hydrostatic pressure in a pressure range of approximately 1–20 GPa . For example, the intermolecular packing in fluorene (a molecule that can be viewed as planarized biphenyl) changes from a herringbone pattern towards π stacking at 3.6 GPa .…”

“…Particularly relevant for [ n ]CPPs are high pressure studies of linear oligophenyls that correlated the intensity ratio of two important Raman bands, that is I / I to interring torsion . Another important result in the same work was to correlate the mode Grüneisen parameters (MGPs) of the various vibrational Raman modes with local volume changes and a vibrational anharmonicity …”