N-Donor/Fluorenyl o-Carborane Fluorophores with Strong Crystallization-Induced Emission

N-Donor/fluorenyl (D/Fl) substituted o-carborane triads (2 a–2 e) exhibit strong crystallization-induced emission. Crystallographic studies reveal H⋅⋅⋅π interactions in 2 a–2 e and π⋅⋅⋅π interactions in 2 c/2 d/2 e contributing to the close packing to varying degrees. In solution, the D-Cb-Fl triads exhibit strong quenching of charge-transfer (CT) emission with quantum efficiencies (Φ) less than 0.05, whereas in solvated aggregates, they display blue-shifted CT emission and enhanced Φ values (up to 0.15). Further enhancement in emission is seen in the crystalline state, with 2 b/2 c/2 d exhibiting moderate Φ values (34 %–43 %) and 2 a/2 e showing high Φ (85 %–95 %). The π⋅⋅⋅π stacked 2 c/2 e possess short lifetimes (ns), whereas the H⋅⋅⋅π stacked 2 a/2 b have much longer lifetimes (0.83 μs/0.16 ms). Theoretical calculations show the key role that multiple H⋅⋅⋅π interactions play in stabilizing the crystalline aggregates. An “ONIOM” theoretical model has mimicked the PL behavior of 2 a in the crystalline state.