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<b>···</b>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.