Thermally Activated Delayed Fluorescence and Phosphorescence Quenching in Iminophosphonamide Copper and Zinc Complexes

Abstract

The synthesis of copper and zinc complexes of four variably substituted iminophosphonamide ligands is presented. While the copper complexes form ligand-bridged dimers, the zinc compounds are monomeric. Due to different steric demand of the ligand the arrangement of the ligands within the dimeric complexes varies. Similar to the structurally related iminophosphonamide complexes of alkali metals and calcium, the steady-state and time-resolved photoluminescence (PL) of four of the seven compounds studied here as solids in a temperature range of 5-295 K can be described within the scheme of thermally activated delayed fluorescence (TADF). Accordingly, they exhibit bright bluegreen phosphorescence at low temperatures (< 100 K), which turns into delayed fluorescence by increasing the temper-ature. However, unusually, the fluorescence is practically absent in two copper complexes which otherwise still conform to the TADF scheme. In these cases, the excited singlet states decay essentially non-radiatively and their thermal population from the corresponding low-lying triplet states efficiently quenches PL (phosphorescence). Three other copper and zinc complexes only exhibit prompt fluorescence, evidencing a wide variation of photophysical properties in this class of compounds. The excited states of the copper complex with especially pronounced phosphorescence quenching were also investigated by low-temperature timeresolved infrared spectroscopy and quantum chemical calculations.

Ultra‐Long Lived Luminescent Triplet Excited States in Cyclic (Alkyl)(amino)carbene Complexes of Zn(II) Halides

“…Roesky et al. recently reported Zn II iminophosphonamide (NPN) complexes (Scheme 1B) displaying delayed fluorescence in the solid state [28] . In this case, twisting of the NPN units in the T 1 state is the mechanism that supports the thermally driven spin flip to the singlet excited state as the parent compound with planar ligand arrangement showed prompt fluorescence [28] .…”

“…recently reported Zn II iminophosphonamide (NPN) complexes (Scheme 1B) displaying delayed fluorescence in the solid state [28] . In this case, twisting of the NPN units in the T 1 state is the mechanism that supports the thermally driven spin flip to the singlet excited state as the parent compound with planar ligand arrangement showed prompt fluorescence [28] . However, the quantum yields are relatively low ( ϕ <0.05) at 297 K, and thus this study nicely pinpoints the challenges and obstacles to overcome for the design of efficient Zn II ‐based triplet emitters.…”

“…Interestingly, despite the d 10 electron configuration of Zn II and its high abundance (~75 ppm in the Earth's crust), only few triplet emitters have been developed so far, but a large number of purely fluorescent Zn II complexes is known. [18][19][20][21][22][23][24][25][26][27][28] This may be due to the fact that MLCT states cannot be formed due to the much higher ionization energy of Zn II compared to Cu I , which precludes the metal d-orbitals to be involved in the excited states and mediate SOC. Thus, predominantly intra-ligand CT (ILCT) or LC states have been studied for phosphorescence or TADF.…”

Cooperative Effects in Heterometallic Complexes

“…The current special issue of Chemistry -a European Journal issue comprises 23 contributions in total, 17 of which stem from work that was conducted within the framework of the 3MET.de center, [1][2][3][4]6,[8][9][10][11]14,[16][17][18][19][20][21]23] and six from outside. [5,7,12,13,15,22] There are two reviews: one is focused on elucidating the magnetic benefits and challenges of cooperativity within 3d-4f based coordination clusters of archetypical butterfly type, [21] and the other one highlights more generally the advancement of inorganic coordination chemistry by spectroscopy of isolated molecules in view of the applied methods and the achieved applications.…”

“…[5,7,12,13,15,22] There are two reviews: one is focused on elucidating the magnetic benefits and challenges of cooperativity within 3d-4f based coordination clusters of archetypical butterfly type, [21] and the other one highlights more generally the advancement of inorganic coordination chemistry by spectroscopy of isolated molecules in view of the applied methods and the achieved applications. [18] Three communications, [3,12,22] one concept article [8] and 17 full papers [1,2,[4][5][6][7][9][10][11][13][14][15][16][17]19,20,23] document and discuss particular 3MET species and their relevance for our understanding of the TM interactions within. A contribution on the development and application of a complete active space spin-orbit configuration interaction program designed for molecule magnets deserves special mention: this manuscript appears in a more specialized sister journal.…”