Halogen Impact on the Supramolecular Organization of Chiral Phthalimide Emitters Displaying Room Temperature Phosphorescence

Abstract

Room temperature phosphorescence from organic materials has attracted an increasing attention in the recent years due to their potential application in various advancing technologies, notably in bioimaging and displays. In this context, heavy atoms such as halogen ones revealed useful tools to enhance the spin-orbit coupling (SOC) of molecular organic phosphors. However, the effect of halogen at the supramolecular level remains less understood, especially in the field of molecular crystals where additional factors can impact the phosphorescence emission. Here, we investigate external effect of halogens on the phosphorescence of chiral phthalimides molecular crystals. The results show that changing the nature of the halogen atom onto the phthalimide core leads to an evolution of the photophysical properties of the materials which does not necessarily follow the classical trend imposed by the expected internal heavy atom effect. Beyond this aspect, we showed that the halogen atom has a profound impact on the packing between the chromophores at the supramolecular level which is of paramount importance towards the optical properties (PLQY and lifetimes) of the different phosphors examined.

Keywords: Chiral phthalimides; Circularly polarized luminescence; Organic molecular crystals; Room temperature phosphorescence.

Scheme 1

Chemical structures of the chiral phthalimides previously reported (Ref. , top) and the compounds investigated in this study, with their main photophysical characteristics related to the structural and electronical features bring the halogen atoms, and pictures of the luminescence obtained in solid state at room temperature.

Figure 1

Left: a) PL spectra of the different phthalimide derivatives at 77 K in 2‐MeTHF, under 310 nm excitation. Steady state (black) and delayed (red) emission spectra of the solids at RT (in aerated conditions) of b) F₈‐Pht (λ_exc=310 nm), c) Br₈‐Pht (λ_exc=360 nm), d) H₈‐Pht (λ_exc=310 nm), e) H₄F₄‐Pht; and d) Lifetime decay profiles of phosphorescence with the corresponding values of lifetimes (τ) for Br₈‐Pht at 550 nm under 312 nm excitation at RT. Right: Photophysical data of the compounds recorded in chloroform at 298 K, in 2‐methyltetrahydrofuran at 77 K, and in solid‐state (crystals) at 298 K under inert conditions.

Figure 2

Main features extracted from X‐Ray single crystal data obtained for H₈‐Pht, H₄F₄‐Pht, F₈‐Pht and Br₈‐Pht with intermolecular centroid‐centroid distances (bottom) between the phthalimides dimers.

Figure 3

a) and b) Hirshfeld surfaces of H₄F₄‐Pht and Br₈‐Pht, respectively, (mapped with d_norm ) showing the neighbouring molecules within their packing mode; c), d) and e) Breakdown of the HS surface into specific intermolecular C⋅⋅C and C⋅⋅Br interactions.