A new class of deep-blue emitting Cu(i) compounds – effects of counter ions on the emission behavior

Abstract

Three deep blue emitting Cu(I) compounds, [Cu(PPh3)tpym]PF6, [Cu(PPh3)tpym]BF4, and [Cu(PPh3)tpym]BPh4 (tpym = tris(2-pyridyl)methane, PPh3 = triphenylphosphine) featuring the tripodally coordinating tpym and the monodentate PPh3 ligands were studied with regard to their structural and photophysical properties. The compounds only differ in their respective counter ions which have a strong impact on the emission properties of the powder samples. For example, the emission quantum yield can be significantly increased for the neat material from less than 10% to more than 40% by exchanging BPh4(-) with PF6(-). These effects can be linked to different molecular packings which depend on the counter ion. In agreement with these results, it was found that the emission properties also strongly depend on the surrounding matrix environment which was elucidated by investigating photophysical properties of the compounds as powders, doped into a polymer matrix, and dissolved in a fluid solution, respectively. The observed differences in the emission behavior can be explained by different and pronounced distortions that occur in the excited state. These distortions are also displayed by density functional theory (DFT) calculations.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

“…The individual value depends on molecular properties, for example, on the extent of SOC of higher lying singlets to the lowest triplet state, but also on the local environment, such as a fluid or a rigid matrix . In general, a significant prompt fluorescence (S 1 →S 0 ) is not detected, but a very bright long‐lived phosphorescence (T 1 →S 0 ) is frequently observed at low temperature . At higher temperature and in a situation of a fast thermal equilibration, population of the higher lying singlet state is governed by the Boltzmann distribution.…”

“…However, mixing‐in of different, higher lying singlets S n (with n >1) can be significant. If so, the triplet state of Cu I complexes, for example, exhibits a distinct ZFS of several cm −1 (a few 0.1 meV) and the T 1 →S 0 transition rate for the phosphorescence can become as large as 5×10 4 s −1 (20 μs) . Accordingly, a second, effective radiative decay channel is opened in addition to the TADF decay path.…”

“…Such a small ZFS value is a consequence of weak SOC of the T 1 substates with higher lying states. Furthermore, the emission decay time of τ (T 1 )=1200 μs (at 30 K) is extremely long if compared to Cu(dmp)(phanephos) + 21 (Section 3) and many other Cu I complexes . Again, this is a consequence of the weak SOC with respect to the lowest triplet state.…”

“…If so, the triplet state of Cu I complexes, for example, exhibits ad istinct ZFS of severalc m À1 (a few 0.1 meV) and the T 1 !S 0 transition rate for the phosphorescence can become as large as 5 10 4 s À1 (20 ms). [30,34,97,136,137] Accordingly,asecond, effective radiatived ecay channel is openedi na ddition to the TADF decay path. These combined radiatived ecay pathsc an distinctly shorten the ambient temperaturee mission decay time.…”

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

“…Our attempts to obtain the unsubstituted ligands E(2-py) 3 (E = As,S b, Bi)u sing the in situ reaction of 2-lithiopyridine with ECl 3 were only successful for E = As, [25] apparently as ar esult of reductive elimination of bipyridine from E(2-py) 3 and the generation of elemental Sb or Bi for the heavier Group 15 elements.T his is in line with ap revious report which showed that the ligand Bi(2-py) 3 could not be obtained even from the reaction of BiCl 3 with the more stable 2-(ZnBr)-pyridine reagent. Our attempts to obtain the unsubstituted ligands E(2-py) 3 (E = As,S b, Bi)u sing the in situ reaction of 2-lithiopyridine with ECl 3 were only successful for E = As, [25] apparently as ar esult of reductive elimination of bipyridine from E(2-py) 3 and the generation of elemental Sb or Bi for the heavier Group 15 elements.T his is in line with ap revious report which showed that the ligand Bi(2-py) 3 could not be obtained even from the reaction of BiCl 3 with the more stable 2-(ZnBr)-pyridine reagent.…”