Phosphorescence at Low Temperature by External Heavy‐Atom Effect in Zinc(II) Clusters

Abstract

Luminescent ZnII clusters [Zn4L4(μ3‐OMe)2X2] (X=SCN (1), Cl (2), Br (3)) and [Zn7L6(μ3‐OMe)2(μ3‐OH)4]Y2 (Y=I− (4), ClO4− (5)), HL=methyl‐3‐methoxysalicylate, exhibiting blue fluorescence at room temperature (λmax=416≈429 nm, Φem=0.09–0.36) have been synthesised and investigated in detail. In one case the external heavy‐atom effect (EHE) arising the presence of iodide counter anions yielded phosphorescence with a long emission lifetime (λmax=520 nm, τ=95.3 ms) at 77 K. Single‐crystal X‐ray structural analysis and time‐dependent density‐functional theory (TD‐DFT) calculations revealed that their emission origin was attributed to the fluorescence from the singlet ligand‐centred (1LC) excited state, and the phosphorescence observed in 4 was caused by the EHE of counter anions having strong CH−I interactions.

Coordinated Halide and Pseudo Halide‐Dependent Structures and Photoluminescence of Defective Double Cubane Zinc(II) Clusters

“…Their photoluminescent behaviors indicate the contributions of intraligand 1 π-π* transition to their luminescence, which was characterized from the DFT and TDDFT calculations using similar tetranuclear Zn(II) complex. [10] The emission maxima of the complexes shift to significantly lower energy than HL, which are consistent with the increase in HOMO energy in cases that reflect different electronegativities between protons and the Zn(II) ions. [13] The above observations indicate that the emission maxima of 1 to 5 are not necessarily affected by coordinated halides or pseudo halide ions.…”

“…Its emission spectrum displays a fluorescence band (λ max = 363 nm). [10] All complexes (1 to 5) exhibit photoluminescence properties in the solid state ( Figure 2b and Figure S7), displaying blue luminescence at room temperature (298 K). The emission spectra of each complex show unstructured broad emission bands with maxima (λ max ) at 415-428 nm, which are quite similar with slight differences.…”

“…[14] Also, the use of heavy atoms such as iodide and lanthanide ions increases both inter-system crossing (ISC) rate and phosphorescence decay time of the excited-state luminophore due to the orbital interaction between the heavy atoms and the luminophore. [10,15] The phosphorescence spectra of complexes 1 to 5 measured at 77 K after a 20 ms delay after excitation show broad bands with maximum values around 520 nm ( Figure S10). Phosphorescence peaks were observed and in particular, complex 3 with coordinated iodide was more prominent than others.…”

“…Each complex shows an average emission lifetime (ԏ av ) of 3.10 to 7.07 nanoseconds (ns) at 298 K, which are consistent with the expected values from intraligand 1 π-π* transitions in Zn(II) complexes. [10] Their QYs range from 0.22 to 0.53. At low temperature (77 K), meanwhile, ԏ av range from 4.77 to 8.45 ns, with QYs ranging 0.70~0.88 ns, which are higher than those at 298 K because, in general, the fluorescence life time and QYs of most fluorescent materials are decreased by thermal quenching via multiphoton relaxation pathways as the temperature increases from cryogenic to ambient temperatures.…”

“…To the best of our knowledge, QY of complex 5 at room temperature (Φ em = 0.53) is the second-highest among the reported multinuclear Zn(II) complexes, [9a] and it is higher than that of [Zn 4 (mmsa) 4 (μ 3 -OCH 3 ) 2 (NCS) 2 ] (Φ em = 0.36) which we have previously reported. [10] Unlike our previous tetranuclear Zn(II) cluster where methyl-esterified ligands were coordinated, benzyl substituents in the ligand of complex 5 seem to play an important role in improving QY, so their Hirshfeld surfaces and energy framework analyses [17] were applied to evaluate them. Hirshfeld surface appears to be particularly suitable for visualizing variations in the intermolecular interactions of compounds.…”