Adsorption of Polycyclic Aromatics on Crystal Surface of Coordination Cages: Photophysical Properties and Förster Resonance Energy Transfer

Self-assembly reactions of PdX2 (X– = NO3–, BF4–, ClO4–, ReO4–, PF6–, and CF3SO3–) with 9,10-bis((isoquinolin-5-yloxy)methyl)anthracene (L) in Me2SO give rise to single crystals of coordination cages, [X@Pd2L4]X3, irrespective of X– anions, in high yields. The intracage Pd···Pd distance is significantly sensitive to the nestled X– anion, which can serve as a gauge for recognition of ubiquitous polyatomic anions. One interesting feature is that, via π–π interactions, various polycyclic aromatics (PAs) are characteristically adsorbed on the crystal surface of designed coordination cages with a wall of anthranyl moiety. This unprecedented ensemble system shows an efficient Förster resonance energy transfer (FRET) process from the anthracene (ANT) to pyrene (PYR) via a large degree of spectral overlap between the ANT emission and PYR absorption bands, in contrast to a simple mixture of ANT and PYR.