Interchange of L-, Z-, and Bound-Ion-Pair X-Type Ligation on Cadmium Selenide Quantum Belts

Wurtzite CdSe quantum belts (QBs) having Z-type ligation, such as {CdSe[Cd(oleate)₂]_0.19} QBs, undergo a facile ligation exchange with AX salts (A = R₄N, Na; X = OH, Cl, Br, NO₃, OBz, OAc) to afford QBs having bound-ion-pair X-type ligation and empirical formulas CdSe[X]_x[A]_x. The exchange to AX ligation is accompanied by shifts of the quantum-belt absorption spectra by as large as 340 meV (for X = OH) relative to the spectrum of L-type {CdSe[n-octylamine]_0.53} QBs. AX ligation is also investigated using the Na⁺ salts of d- and l-phenylalanine. These chiral X ligands induce inverse chiroptical effects in the circular dichroism spectra corresponding to the electronic transitions of the CdSe QBs, providing a strong evidence of the direct ligation of the X groups on the QB surfaces. AX ligation appears to consist of two populations on the QBs, one for which the AX ligands are persistently bound and another for which the AX ligands are readily removed by washing. These generate two ligation stoichiometries referred to as depleted and saturated ligation. The empirical formulas for the depleted ligation are in the approximate range of CdSe[X]_0.1-0.3[A]_0.1-0.3, whereas those for saturated ligation are in the approximate range of CdSe[X]_0.4-0.8[A]_0.4-0.8. These ranges are consistent with approximately one and two X^- ligands per three-coordinate surface Cd atom. AX ligation is readily exchanged to either L-type primary amine or Z-type Cd(oleate)₂ ligation. However, AX salts do not displace L-type primary amine ligation under the conditions studied. All ligation exchanges are rapid and complete at room temperature, and with the exception of L-type to bound-ion-pair X-type ligation, fully reversible.