Ligand design and nuclearity variation towards dual emissive Pt() complexes for singlet oxygen generation, dual channel bioimaging, and theranostics

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Vol. 1; no. 14; pp. 5636 - 5647
• Main Authors: Shafikov, Marsel Z, Suleymanova, Alfiya F, Kutta, Roger J, Gorski, Alexander, Kowalczyk, Aleksandra, Gapi ska, Magdalena, Kowalski, Konrad, Czerwieniec, Rafa
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 07.04.2022
• Subjects: Atomic energy levels; Chromophores; Coordination compounds; Design modifications; Dichloromethane; Electronic structure; Emission; Energy gap; Fluorescence; Iridium; Ligands; Medical imaging; Phosphorescence; Photoluminescence; Singlet oxygen; Time constant; Transition metals
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/d2tc00257d

Organic ligands comprising thiophene ring(s) afford complexes of transition metals, such as Pt( ii ) and Ir( iii ), with photoluminescence readily tunable via ligand modifications. In this work we demonstrate the targeted design of a N^C-C^N type ditopic ligand with a central thiophene ring as the cyclometalating core and variation of the nuclearity of the complex to fine-tune the photophysical properties of the material. The mononuclear complex Pt-1 shows red T 1 → S 0 phosphorescence with the emission maximum at λ = 660 nm. The dinuclear complex Pt-2 shows near infrared (NIR) T 1 → S 0 phosphorescence peaking at λ = 710 nm. In both cases the phosphorescence is quenched by molecular oxygen generating singlet oxygen molecules with high efficiencies of Δ 83% ( Pt-1 ) and Δ 70% ( Pt-2 , respectively) in air-equilibrated CH 2 Cl 2 solutions under ambient conditions. The red phosphorescence of Pt-1 is accompanied by green S 1 → S 0 fluorescence with the maximum at λ = 495 nm. This makes Pt-1 a dual emissive material with two emissions stemming from a single chromophore moiety. Transient absorption studies revealed a relatively low rate of ISC from the S 1 state to the triplet manifold with a time constant τ (ISC) of about 4 ps. The slow ISC in Pt-1 is rationalized by a specific electronic structure with a relatively large energy gap Δ E (S 1 → T 1 ) 0.63 eV and the higher triplet state T 2 being higher in energy than the singlet state S 1 . In dinuclear Pt-2 , state T 2 lies below S 1 opening fast T 2 → S 1 ISC paths with a time constant τ of only 0.13 ps. The unique dual emission of Pt-1 was beneficial for its imaging in HeLa cells as it enabled switching between green fluorescence and red phosphorescence channels of detection in the time-span of the single confocal luminescence microscopy experiment. Pt-1 represents a prototype of new theranostic agents combining cytotoxic activity with a unique dual wavelength mode of detection. Mono- and dinuclear Pt( ii ) complexes of a new ditopic ligand with a thiophene cyclometalating core are reported. The mononuclear complex shows dual emission of fluorescence and phosphorescence and is demonstrated as a dual channel bioimaging probe.