On the thermodynamics of aggregation toward phosphorescent metallomesogens: From electronic tuning to supramolecular design

Two series with three Pt(II) complexes each (PtL_Ph-n, PtL_Fpy-n) bearing asymmetric tetradentate ligands as dianionic luminophores with variable alkyl chain lengths were synthesized. Hence, each ligand series is distinguished by one of its cyclometallating rings (phenyl vs. 2,6-difluoropyrid-3-yl). Steady-state and time-resolved photoluminescence spectroscopic studies in diluted solutions at room temperature and in glassy matrices at 77 K show that the emissive state is mainly centered on the invariantly electron-rich cyclometalated side while the second ring regulates the admixture of ligand-centered and metal-to-ligand charge-transfer character. Hence, the radiative rates can be controlled, as indicated by quantum-mechanical calculations, which also explain the temperature-dependent trend in the phosphorescence rate constants. Studies in condensed phases (single-crystal X-ray diffractometry, polarized optical microscopy, differential scanning calorimetry, steady-state and time-resolved photoluminescence micro(spectro)scopy) showed the development of a smectic A mesophase for the fluorinated species bearing the two longest alkyl chains. Nuclear magnetic resonance-based studies on the thermodynamics of aggregation in solution confirm the marked enthalpic stabilization of aggregates mediated by the polar 2,6-difluoropyrid-3-yl moiety (and to a lesser extent by dispersive forces between the alkyl chains). On the other hand, the negative entropy of aggregation is dominated by the restriction of degrees of freedom involving the peripheral alkyl moieties upon stacking, which becomes increasingly relevant for longer chains. All these factors control Pt···Pt coupling, a crucial interaction for the design of photofunctional mesogens based on Pt(II) complexes.