"T-Shaped" Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane-Containing Organoboron Complexes

Crystals of separate enantiomers of the [2.2]paracyclophane (pCP)-containing organoboron complexes (S_p)-1BF₂-H, (S_p)-1BF₂-iPr, and (S_p)-1BF₂-tBu were prepared. These crystals have the same space group (P2₁2₁2₁) but molecules in them participate in different intermolecular edge-to-face π-π interactions. These crystals all have fluorescence (FL) profiles consisting of two components with shorter (<10 ns) and longer (>10 ns) FL lifetimes. The former is associated with an excited monomer FL, while the latter is attributed to a "T-shaped" excimer FL, which derives from an excited dimer with a perpendicular orientation of π-planes. The "T-shaped" excimer FL was found to be promoted by intermolecular edge-to-face π-π interaction between each pCP moiety in the (S_p)-1BF₂-H crystal, which we termed pCP-to-pCP π-π interaction. Theoretical calculations suggest that a remarkable intermolecular orbital interaction occurs between HOMOs involved in the pCP-to-pCP π-π interaction. This electronic effect causes the S₀-S₁ transition dipole moment in the dimer to be larger than that in the monomer, which is suggested to be a major factor responsible for enhancing the "T-shaped" excimer FL.