Molecular Rectangles Featuring Two Parallel NCN‐Coordinated Platinum Units: Enhancing Near‐Infrared Emission Through Excimer Formation

TL;DR

Scientists designed a new rectangular molecule with two platinum units that emit near-infrared light through a special excited state.

Contribution

The novel structure promotes intramolecular excimer formation for near-infrared emission through two Pt(NCN) units in a macrocyclic framework.

Findings

The complexes show strong near-infrared photoluminescence with λ_max up to 761 nm.

Intramolecular excimers form when Pt(NCN) units are brought into close proximity.

Molecular distortion is required for excimer formation, which is hindered in rigid media.

Abstract

New macrocyclic molecules are described that incorporate Pt(NCN) units on opposite edges of a rectangular structure, with xanthene units constituting the other two sides. Here, NCN represents a cyclometallating tridentate ligand based on 2,6‐di(2‐pyridyl)benzene or its pyrimidine analog. The complexes display strong photoluminescence peaking in the near‐infrared region of the spectrum in solution (λ
max up to 761 nm). Photophysical data and DFT calculations indicate that the emission arises from “intramolecular excimers”—triplet excited states that form when the two Pt(NCN) units within the molecule are brought into close proximity to interact interfacially. In doped polymers, the necessary molecular distortion is inhibited, but related excited states that emit in a similar region can still form through intermolecular interactions.

Macro‐cycling into the near‐infrared: By incorporating two cyclometallated Pt(II) units into opposite sides of a molecular rectangle, the formation of near‐infrared‐emitting excited states that span the two units is promoted in solution, compared to the corresponding, acyclic, tweezer‐like complexes. A degree of molecular reorganization is nevertheless required to form such states, which is inhibited in more rigid media.

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