“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 (Sp)-1BF2-H, (Sp)-1BF2-iPr, and (Sp)-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 two component fluorescence (FL) profiles 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 -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 (Sp)-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.