Lewis Pair-Functionalized Pt(II) Complexes with Tunable Emission Color and Triplet-State Properties.

Two emissive Pt(II) complexes containing dynamic "flexible" Lewis pair (FlexLP) ligands are reported. The FlexLP ligand encompasses a diphenylphosphine oxide Lewis base and a dimesitylborane Lewis acid attached to a bithiophene scaffold, which can switch between an open unbound Lewis pair and a bound P-O-B Lewis adduct depending on the hydrogen bond-donating (HBD) strength of the solvent. [Pt(FlexLP)2] contains two FlexLP ligands, and [Pt(FlexLP)(Py)] contains one FlexLP ligand and one pyrene ligand. UV-vis absorption and fluorescence studies demonstrate that the FlexLP ligands switch between the open Lewis pair and the closed Lewis adduct in MeOH, a strong HBD solvent, and acetone, a weak HBD solvent, respectively, and exhibit tunable emission color depending on the acetone/MeOH solvent ratio. Transient absorption spectroscopy reveals a large difference in the triplet-state lifetime depending on the conformation of the FlexLP ligands for both complexes. In the closed form, the triplet-state lifetimes of the two complexes are over an order of magnitude longer compared to that of the complexes in the open conformation. Calculations of optimized geometries suggest that this difference in triplet-state lifetime is due to a difference in the thiophene-thiophene torsion angle between the two conformations.

Stimuli-responsive flexible Lewis pair-modified nanoparticles for fluorescence imaging.

A stimuli-responsive fluorophore, encompassing a Lewis acid-base pair, binds to primary amines on mesoporous silica nanoparticles, which may serve as environment-sensitive drug carriers. The fluorophore switches conformation, exhibiting different emission color and lifetimes, allowing for the detection of the water content of the nanoparticles' surroundings through fluorescence spectroscopy and microscopy.

Fluorescence Switching of Intramolecular Lewis Acid-Base Pairs on a Flexible Backbone.

A class of intramolecular Lewis acid-base pairs containing Lewis acidic dimesitylboranes paired with phosphine oxide, sulfoxide, and sulfone Lewis basic groups are explored. The absorption and emission properties of the compounds are investigated in different solvent environments, and switching of the photophysical behavior between the Lewis acid-base adducts and free acid-base pairs is examined. We find that phosphine oxide Lewis base groups are effective partners in flexible Lewis pairs; however, the analogous sulfoxide and sulfone groups are not. Additionally, the absorption and emission wavelengths can be systematically tuned by varying the conjugation length and electron-donating and -accepting substituents on the backbone.

Solvent- and Temperature-Responsive Platinum(II)-Functionalized Flexible Lewis Pairs.

Metal-functionalized flexible Lewis pair (flexLP) complexes are described. Platinum(II) terpyridine (Pt-tpy) units added to a flexLP backbone through copper(I)-catalyzed coupling reactions yield monometallic (1) and bimetallic (2) Pt-tpy flexLP complexes. NMR and UV-vis absorption studies demonstrate that the complexes switch between the open unbound Lewis pair and closed Lewis adduct with solvents of varying hydrogen-bond-donating (HBD) strength as well as varying temperature. The charge-transfer (CT) energies of the complexes can be tuned by controlling the equilibrium between open and closed forms, demonstrating the ability to alter the photophysical behavior of the metal complexes by stimuli-responsive flexLP ligands.