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

Luminescent Platinum(II) Complexes with Stimuli-Responsive Flexible Lewis Pair Ligands: Spectroscopic and Computational Studies

A series of new luminescent bimetallic platinum(II) complexes with stimuli-responsive flexible Lewis pair (FlexLP) ligands are described. The FlexLP ligands consist of a dimesitylboron Lewis acid and diphenylphosphine oxide Lewis base which are in equilibrium between the unbound open form and the Lewis adduct, controlled by the hydrogen bond donating strength of the solvent. Spectroscopic techniques and density functional theory (DFT) calculations were used to interpret the photophysics of the platinum(II) complexes. All complexes exhibit tunable absorption in the region of 300-500 nm and green to orange photoluminescence, depending on the ratio of weak (THF) to strong (MeOH) hydrogen bond donating solvent employed. Spectroscopic and computational data shows that phosphine and peripheral acetylide ligands on the platinum(II) centers have limited influence on the emission energy, indicating the emission originates from the FlexLP-dominated fluorescence. Using time-resolved transient absorption spectroscopy it is shown that the complexes undergo intersystem crossing (ISC) to the triplet excited state upon photoexcitation, and the ISC efficiency is affected by the peripheral acetylide ligands. The triplet excited state lifetime can also be manipulated by the state of the FlexLP ligand, with the closed form complexes having longer lifetimes than the open form complexes.

Tetrazole-Functionalized Organoboranes Exhibiting Dynamic Intramolecular N→B-Coordination and Cyanide-Selective Anion Binding

Starting from two different cyano-functionalized organoboranes, we demonstrate that 1,3-dipolar [3+2] azide-nitrile cycloaddition can serve to generate libraries of alkyl-tetrazole-functionalized compounds capable of intramolecular N→B-Lewis adduct formation. Due to the relatively low basicity of tetrazoles, structures can be generated that exhibit weak and labile N→B-coordination. The reaction furnishes 1- and 2-alkylated regio-isomers that exhibit different effective Lewis-acidities at the boron centers, and vary in their optical absorption and fluorescence properties. Indeed, we identified derivatives capable of selectively binding cyanide over fluoride, as confirmed by 11B NMR. This finding demonstrates the potentialities of this synthetic strategy to systematically fine-tune the properties of lead structures that are of interest as chemical sensors.

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.

Triphenylamine/4,4'-Dimethoxytriphenylamine-Functionalized Thieno[3,2-b]thiophene Fluorophores with a High Quantum Efficiency: Synthesis and Photophysical Properties

A wide series of 10 new triphenylamine (TPA)/4,4'-dimethoxytriphenylamine (TPA(OMe)₂)-functionalized thieno[3,2-b]thiophene (TT) fluorophores, 4a-e and 5a-e, bearing different electron-donating and electron-withdrawing substituents (-PhCN, -PhF, -PhOMe, -Ph, and -C₆H₁₃) at the terminal thienothiophene units were designed and synthesized by the Suzuki coupling reaction. Their optical and electrochemical properties were investigated by experimental and computational studies. Solid-state fluorescent quantum yields were recorded to be from 20 to 69%, and the maximum solution-state quantum efficiency reached 97%. Moreover, the photophysical characterization of the novel chromophores demonstrated a significant Stokes shift, reaching 179 nm with a bathochromic shift. They exhibited tuning color emission from orange to dark blue in solution and showed fluorescence lifetime reaching 4.70 ns. The relationship between triphenylamine (TPA)/4,4'-dimethoxytriphenylamine (TPA(OMe)₂)-derived triarylamines and different functional groups on thieno[3,2-b] thiophene units was discussed.

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.

Turn-on fluorescence sensors based on dynamic intramolecular N→B-coordination

A series of ten aryl-triazole-functionalized boranes bearing BMes₂-groups and capable of forming intramolecular five-membered N→B-coordinated heterocycles, has been prepared by 1,3-dipolar cycloaddition.