Crystals of separate enantiomers of the 2.2paracyclophane (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 fluorescence (FL) profiles consisting of two components with shorter (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 (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.
Irii et al. (Sun,) studied this question.