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

Abstract

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.