Regulating emissions of a o-carborane-based molecule through molecular vibrations coupled with molecular orientations in solid states

Abstract

Utilizing molecular vibrations to regulate the singlet/triplet-involved emissions in multiple states is a formidable challenge. Here, we have explored o-carborane-based molecules as model systems to gain insight into how molecular vibrations induce the coupling of singlet locally excited (¹LE) and charge-transfer (¹CT) states, as well as triplet locally excited (³LE) and charge-transfer (³CT) states. Their three-dimensional conformations allowed the molecules to generate appropriate C₁–C₂ bond stretching vibrations and modulated the orientations between the C-substituted unit and C₁–C₂ bond in the o-carborane moiety in the crystalline states, facilitating vibronic coupling between the ¹LE (³LE) and ¹CT (³CT) states to generate effective multiple emissions. More importantly, the energy barriers between the ¹LE–¹CT/³LE–³CT states were remarkably small to make these reverse states accessible, which is indicative of the potential occurrence of back charge transfer from electron-withdrawing to electron-donating units. Lastly, the environmental pressure inhibited molecular vibration to regulate the emission. This study provides a valuable model for delving into the fundamental nature of multiple emissions arising from molecular vibrations in o-carborane clusters.