Role of kinks in dyeing crystals: Confocal luminescence microscopy from single molecules to square centimeters

Large (1 cm³) potassium hydrogen phthalate (KAP) crystals grown in the presence of the fluorescent dye 2′,7′-dichlorofluorescein (DCF) show patterns of luminescence characteristic of selective inter- and intrasectoral zoning. Despite this selectivity, the polarization dependence of the luminescence of single DCF molecules inside the crystals indicated broad orientational distributions consistent with nonspecific mixed crystal growth mechanisms (Wustholz, K.; Kahr, B.; Reid, P. J. J. Phys. Chem. B, 2005, 109, 16357-16362). In an effort to reconcile this apparent discrepancy, KAP crystals were grown from 10^-9 to 10^-4 M DCF solutions and examined using confocal luminescence and polarized absorption microscopies, where possible. Single molecules and molecular ensembles were investigated. The fluorescence excitation dichroism was strongly dependent on the concentration of DCF, suggesting that different mixed crystal growth mechanisms were at work at different guest concentrations. Hottenhuis et al. (J. Cryst. Growth, 1986-1989) had earlier established that certain trivalent cations bind preferentially to distinct kink sites of KAP. By blocking particular kink sites with Fe ³⁺ or Ce³⁺ at high DCF-solution concentrations, the orientation of DCF in the crystals was modulated, thus establishing that the dye not only recognizes some propagating steps as opposed to others but also preferentially chooses between kinks propagating in opposing directions on the same step, evidence that kink selectivity plays a vital role in the dyeing of crystals.