External Heavy-Atom Effect via Orbital Interactions Revealed by Single-Crystal X‑ray Diffraction

Enhanced spin–orbit coupling
through external heavy-atom
effect (EHE) has been routinely used to induce room-temperature phosphorescence
(RTP) for purely organic molecular materials. Therefore, understanding
the nature of EHE, i.e., the specific orbital interactions between
the external heavy atom and the luminophore, is of essential importance
in molecular design. For organic systems, halogens (e.g., Cl, Br,
and I) are the most commonly seen heavy atoms serving to realize the
EHE-related RTP. In this report, we conduct an investigation on how
heavy-atom perturbers and aromatic luminophores interact on the basis
of data obtained from crystallography. We synthesized two classes
of molecular systems including N-haloalkyl-substituted
carbazoles and quinolinium halides, where the luminescent molecules
are considered as “base” or “acid” relative
to the heavy-atom perturbers, respectively. We propose that electron
donation from a π molecular orbital (MO) of the carbazole to
the σ* MO of the C–X bond (π/σ*) and n electron
donation to a π* MO of the quinolinium moiety (n/π*) are
responsible for the EHE (RTP) in the solid state, respectively.