Adsorption\nof Polycyclic Aromatics on Crystal Surface\nof Coordination Cages: Photophysical Properties and Forster\nResonance Energy Transfer

Self-assembly reactions of PdX<sub>2</sub> (X<sup>–</sup> = NO<sub>3</sub><sup>–</sup>, BF<sub>4</sub><sup>–</sup>, ClO<sub>4</sub><sup>–</sup>, ReO<sub>4</sub><sup>–</sup>, PF<sub>6</sub><sup>–</sup>, and CF<sub>3</sub>SO<sub>3</sub><sup>–</sup>) with 9,10-bis((isoquinolin-5-yloxy)methyl)anthracene\n(L) in Me<sub>2</sub>SO give rise to single crystals of coordination\ncages, [X@Pd<sub>2</sub>L<sub>4</sub>]X<sub>3</sub>, irrespective\nof X<sup>–</sup> anions, in high yields. The intracage Pd<b>···</b>Pd distance is significantly sensitive to\nthe nestled X<sup>–</sup> anion, which can serve as a gauge\nfor recognition of ubiquitous polyatomic anions. One interesting feature\nis that, via π–π interactions, various polycyclic\naromatics (PAs) are characteristically adsorbed on the crystal surface\nof designed coordination cages with a wall of anthranyl moiety. This\nunprecedented ensemble system shows an efficient Förster resonance\nenergy transfer (FRET) process from the anthracene (ANT) to pyrene\n(PYR) via a large degree of spectral overlap between the ANT emission\nand PYR absorption bands, in contrast to a simple mixture of ANT and\nPYR.