“Two-in-one” organic–inorganic hybrid Mn^II complexes exhibiting dual-emissive phosphorescence

Abstract

Unprecedented organic-inorganic hybrid complexes, [Mn(L)3]MnHal4, containing both four- and hexacoordinated Mn2+ ions were synthesized by reacting MnCl2 or MnBr2 with bis(phosphine oxide) ligands (L) such as dppmO2, dppeO2, and 2,3-bis(diphenylphosphinyl)-1,3-butadiene (dppbO2). In the [Mn(L)3]2+ cation of the complexes, the Mn2+ ion features a [MnO6] octahedral coordination environment (Oh), and the [MnHal4]2- anion adopts a tetrahedral geometry (Td). These "two-in-one" complexes exhibit strong long-lived luminescence (τav = 12-15 ms at 300 K) having interesting thermochromic behavior attributed to the thermal equilibrium between two emission bands. So, in an emission spectrum of the typical complex [Mn(dppbO2)3]MnBr4, the intense "red" (ca. 620 nm) and weak "green" (ca. 520 nm) bands, originating from Mn2+ ions in Oh and Td environments, respectively, are observed. Cooling from 300 to 77 K simultaneously leads to (i) redshift of both bands by ca. 20 nm, (ii) increasing their intensities, and (iii) causing a substantial change of their integral intensity ratio from about 4 : 1 to 2 : 1. As a result, the colour of the emission changes from orange (CIE 0.56, 0.45) at 300 K to deep red (CIE 0.62, 0.39) at 77 K. This behavior was rationalized using steady-state and time-resolved fluorescent spectroscopy at various temperatures. The high photoluminescence quantum yields (up to 61% at 300 K) and fascinating dual-emissive phosphorescence coupled with high thermal stability and solubility suggest a high potential of this novel class of emissive Mn2+ complexes as promising emitters for OLED devices and potential stimuli-responsive materials.

A Rare Type of Rhenium(I) Diimine Complexes with Unsupported Coordinated Phosphine Oxide Ligands: Synthesis, Structural Characterization, Photophysical and Theoretical Study

“…Phosphine oxide ligands and their metal complexes are an important area of coordination chemistry both from the viewpoint of academic research and their practical applications in a wide range of technologies such as OLED devices, extraction of rare metals, and many others. Coordination properties of ligands and relevant characteristics of the corresponding complexes depend strongly on the electronic characteristics of the ligand atoms taking part in bonding with metal ions.…”

Manganese(II) Thiocyanate Complexes with Bis(phosphine Oxide) Ligands: Synthesis and Excitation Wavelength‐Dependent Multicolor Luminescence

“…Indeed, the nanosecond order of the lifetimes determined for this band ( τ 1 = 6.3 ns, τ 2 = 176 ns) corroborates its fluorescent origin. The decay kinetics of the low‐energy emission band shows rather long lifetime of 0.92 ms that is typical for spin‐forbidden 4 T 1 (G)→ 6 A 1 transitions in Mn 2+ ions located in octahedral ligand field . The excitation spectrum of 4 (Figure a), monitored for the phosphorescence band, features broad unresolved bands of Mn 2+ ion terms.…”

“…In [Mn( L5 ) 3 ] 2+ , metal atom lies in the distorted octahedral surroundings with the Mn–O distances varying from 2.131 to 2.168 Å. On the whole, geometry of the cation is close to that of the related [Mn( L5 ) 3 ]MnBr 4 complex . The [Mn(NCS) 4 ] 2– anion of 5 features tetrahedral geometry of Mn atom ( τ 4 = 0.96).…”

“…In crystal packing of 1 , 3 and 4 , the closest Mn ··· Mn distances (11.423, 10.725 and 10.692 Å, respectively) are quite long in terms of possible Förster resonance energy transfer (FRET) from an photoexcited Mn 2+ ion to neighboring nonexcited Mn 2+ ion. Meanwhile, for compounds 2 , 5 and 6 , where these distances are shorter than 10 Å (8.683, 9.234 and 8.968 Å), the FRET channel may contribute to the luminescence properties …”

“…The compounds containing both tetrahedral and octahedral Mn II centers, e.g. [Mn(dppeO 2 ) 3 ]MnHal 4 , manifest a dual emission . Moreover, unique thermochromic luminescence associated with conversion of a tetrahedronal to trigonal bipyramidal geometry of Mn 2+ has been reported …”