Abstract
AbstractCarbene‐metal‐amides (CMAs) are a promising family of donor–bridge–acceptor molecular charge‐transfer (CT) emitters for organic light‐emitting diodes. A universal approach is demonstrated to tune the energy of their CT emission. A blueshift of up to 210 meV is achievable in solid state via dilution in a polar host matrix. The origin of this shift has two components: constraint of thermally‐activated triplet diffusion, and electrostatic interactions between guest and polar host. This allows the emission of mid‐green CMA archetypes to be tuned to sky blue without chemical modifications. Monte‐Carlo simulations based on a Marcus‐type transfer integral successfully reproduce the concentration‐ and temperature‐dependent triplet diffusion process, revealing a substantial shift in the ensemble density of states in polar hosts. In gold‐bridged CMAs, this shift does not lead to a significant change in luminescence lifetime, thermal activation energy, reorganization energy, or intersystem crossing rate. These discoveries offer new insight into coupling between the singlet and triplet manifolds in CMA materials, revealing a dominant interaction between states of CT character. The same approach is employed using materials which have been chemically modified to alter the energy of their CT state directly, shifting the emission of sky‐blue chromophores into the practical blue range.
Highlights
A decade later, exhibit high efficiency and acceptor molecular charge-transfer (CT) emitters for organic light-emitting diodes
Of order 3.0/11.4% of the electron density in the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) resides on the metal, with largest contributions from the 5dyz/5py atomic orbitals, respectively, where the carbazole is taken to lie in the x-z plane
By applying a MonteCarlo simulation of 3D triplet diffusion, we find that a Dexter-type dependency of hopping probability on intermolecular distance reproduces the observed concentration-dependent migration rate, with the long-time saturation of peak position occurring where triplets are able to relax to the tail of their density of states (DOS)
Summary
Excitation from S0 to S1 is dominated by a HOMO–LUMO transition (natural transition orbitals comprise 98% HOMO–LUMO), which spans the metal bridge and has significant charge-transfer (CT) character, shifting electron density back from the amide to the carbene group. This reduces the electrostatic dipole to ≈5 D and reverses its sign.[4,20]. In addition to direct absorption to the singlet CT state, optical absorption spectra of CMA1 show features related to ligandcentered excitations of the carbene and amide groups (Figure S1, Supporting Information). The CT absorption band is broader and peaks at 388 nm, between the values of toluene (407 nm) and dichloromethane (385 nm) solutions
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