Vaulted trans-bis(salicylaldiminato)platinum(II) crystals: heat-resistant, chromatically sensitive platforms for solid-state phosphorescence at ambient temperature

Abstract

The synthesis, structure, and solid-state emission of vaulted trans-bis(salicylaldiminato)platinum(II) complexes are described. A series of polymethylene (1: n=8; 2: n=9; 3: n=10; 4: n=11; 5: n=12; 6: n=13) and polyoxyethylene (7: m=2; 8: m=3; 9: m=4) vaulted complexes (R=H (a), 3-MeO (b), 4-MeO (c), 5-MeO (d), 6-MeO (e), 4-CF3O (f), 5-CF3O (g)) was prepared by treating [PtCl2(CH3CN)2] with the corresponding N,N'-bis(salicylidene)-1,ω-alkanediamines. The trans coordination, vaulted structures, and the crystal packing of 1-9 have been unequivocally established from X-ray diffraction studies. Unpredictable, structure-dependent phosphorescent emission has been observed for crystals of the complexes under UV excitation at ambient temperature, whereas these complexes are entirely nonemissive in the solution state under the same conditions. The long-linked complex crystals 4-6, 8, and 9 exhibit intense emission (Φ77K =0.22-0.88) at 77 K, whereas short-linked complexes 1-3 and 7 are non- or slightly emissive at the same temperature (Φ77K <0.01-0.18). At 298 K, some of the long-linked crystals, 4a, 4b, 5c, 5e, 6c, 6e, and 9b, completely lose their high-emission properties with elevation of the temperature (Φ298K <0.01-0.02), whereas the other long-linked crystals, 5a, 6a, 9a, and 9d, exhibit high heat resistance towards emission decay with increasing temperature (Φ298K =0.21-0.38). Chromogenic control of solid-state emission over the range of 98 nm can be performed simply by introducing MeO groups at different positions on the aromatic rings. Orange, yellow-green, red, and yellow emissions are observed in the glass and crystalline state upon 3-, 4-, 5-, and 6-MeO substitution, respectively, whereas those with CF3 O substituents have orange emission, irrespective of the substitution position. DFT calculations (B3LYP/6-31G*, LanL2DZ) showed that such chromatic variation is ascribed to the position-specific influence of the substituents on the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) levels of the trans-bis(salicylaldiminato)platinum(II) platform. The solid-state emission and its heat resistance have been discussed on the basis of X-ray diffraction studies. The planarity of the trans-coordination sites is strongly correlated to the solid-state emission intensities of crystals 1-9 at lower temperatures. The specific heat-resistance properties shown exclusively by the 5a, 6a, 9a, and 9d crystals are due to their strong three-dimensional hydrogen-bonding interactions and/or Pt···Pt contacts, whereas heat-quenchable crystals 4a, 4b, 5c, 5e, 6c, 6e, and 9b are poorly bound with limited interactions, such as non-, one-, or two-dimensional hydrogen-bonding networks. These results lead to the conclusion that Pt···Pt contacts are an important factor in the heat resistance of solid-state phosphorescence at ambient temperature, although the role of Pt···Pt contacts can be substituted by only higher-ordered hydrogen-bonding fixation.