White emission from cascade energy-transfer via multiple luminescence in structural controlled single-molecules: ortho-Carboranyl-Substituted Indolo[3,2,1-jk]carbazole

In this study, we present a strategic methodology for molecular design aimed at harnessing intriguing photophysical characteristics from π‒aromatic group substituted o-carborane compounds. We investigate the optical properties of two compounds, one with H- (ICH) and the other with trimethylsilyl- (ICSi) substituted o-carborane, both linked to indolo[3,2,1-jk]carbazole. This compound exhibits a planar geometry and an electron-abundant nature. Interestingly, in their crystalized solid states, both compounds exhibit multiple luminescent properties that can be divided into three emissive parts: locally excited (LE), crystallization-induced excimer (EX), and intramolecular-charge-transfer (ICT) states. However, in solution, the PL spectra reveal only the LE-based emissive band only in the high-energy region (λem ≈ 380 nm). Analysis of the crystal structures of these compounds reveals notable geometric difference, providing crucial insights to elucidate the origin of each emissive pattern. The well-overlapped π‒π interaction between ICH dimer molecules and the perfect orthogonal geometry around the o-carborane of ICSi molecule explain why the EX- and ICT-based emissions dominate the PL spectra of ICH and ICSi crystals, respectively. In particular, ICH crystals exhibited an intriguing feature: white emission (Commission internationale de l'éclairage 1931 coordinates = 0.29, 0.30), resulting from the collaboration of a multi-luminescent pattern within a single-molecule packing system. Furthermore, the reduced rate constants (kIET) for intramolecular energy transfer (IET) from LE-to EX- and ICT-based emission strongly indicate the occurrence of a cascade IET phenomenon in the crystalline packing system. These findings validate that specific molecular design and functional substitutions around the o-carborane of π‒conjugated carboranyl luminophores play an essential role in inducing intriguing photophysical characteristics.