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

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