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 pi-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-car- borane, both linked to indolo[3,2,1-jk]carbazole. This compound exhibits a planar geometry and an electronabundant 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 LEbased emissive band only in the high-energy region (lambda em approximate to 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 pi-pi 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'e ' clairage 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 pi-conjugated carboranyl luminophores play an essential role in inducing intriguing photophysical characteristics.