Tomas Kindahl

Luminescence, Singlet Oxygen Production, and Optical Power Limiting of Some Diacetylide Platinum(II) Diphosphine Complexes

A series of four new trans-diphosphine Pt(II) diacetylide complexes, with a thiophene and two benzenoid rings in each acetylide ligand, have been synthesized and characterized with respect to optical absorption, spectrally and time-resolved luminescence, and optically nonlinear properties such as two-photon absorption cross section and optical power limiting. Density functional theory (DFT) calculations of a few ground state conformations of three Pt(II) diacetylide structures showed similar total energy for each geometry-optimized rotamer but some differences in the vertical excitation energies and in the ligand-to-metal charge-transfer character. The wavelengths of the calculated excitations were found to be red-shifted compared with peaks in the optical absorption spectra, but the general trends and shifts of wavelengths between the different structures are well reproduced. Static emission spectra for degassed samples in THF solution of the larger compounds showed small Stokes shifts and low fluorescence quantum yields, indicating fast intersystem crossing to the triplet manifold. More pronounced differences between the compounds were displayed in the phosphorescence data, in terms of spectral emission wavelengths and decay times. For instance, the phosphorescence decay of the compound with the thiophene ring close to the Pt center was found to be significantly faster than for the other compounds. A possible relationship between triplet lifetime and conformation of the compounds is discussed. It was also demonstrated that the quenching of the excited triplet states in air-saturated samples involves energy transfer to the oxygen triplet state, and subsequent generation of singlet oxygen showing the typical emission at approximately 1275 nm. The amount of produced singlet oxygen followed the phosphorescence yields of the solute molecules. Two-photon absorption cross sections (sigma(2)) were measured and showed values on the order of 10 GM at 780 nm for all compounds. Optical power limiting measurements of the new complexes in THF using 5 ns pulses, showed only slightly better performance at the wavelength of 532 nm compared to that of similar platinum compounds with only two aryl rings in each ligand. At 600 nm the complexes with three aryl rings were significantly better optical limiters than the smaller compounds with two aryl rings in the ligands.

Silica Hybrid Sol−Gel Materials with Unusually High Concentration of Pt−Organic Molecular Guests: Studies of Luminescence and Nonlinear Absorption of Light

The development of new photonic materials is a key step toward improvement of existing optical devices and for the preparation of a new generation of systems. Therefore synthesis of photonic hybrid materials with a thorough understanding and control of the microstructure-to-properties relationships is crucial. In this perspective, a new preparation method based on fast gelation reactions using simple dispersion of dyes without strong covalent bonding between dye and matrix has been developed. This new sol-gel method is demonstrated through synthesis of monolithic siloxane-based hybrid materials highly doped by various platinum(II) acetylide derivatives. Concentrations of the chromophores as high as 400 mM were obtained and resulted in unprecedented optical power limiting (OPL) performance at 532 nm of the surface-polished solids. Static and time-resolved photoluminescence of the prepared hybrid materials were consistent with both OPL data and previous studies of similar Pt(II) compounds in solution. The impacts of the microstructure and the chemical composition of the matrix on the spectroscopic properties, are discussed.

Photophysical and DFT Characterization of Novel Pt(II)-Coupled 2,5-Diaryloxazoles for Nonlinear Optical Absorption

Several new bis-phosphine platinum(II) complexes with 2,5-diaryl-substituted oxazole-containing alkyne ligands have been synthesized and optically characterized in solution. Measurements of nonlinear absorption showed strong attenuation of laser light at 532 and 600 nm. The light absorption of the Pt complexes was shifted from the near-UV region for the ground state to the red region for the excited triplet state, and was associated with large extinction coefficients. The optical limiting effect can be explained by triplet-triplet excited state absorption in conjunction with fast excited singlet-to-triplet intersystem crossing and slow triplet-to-ground-state decay, in comparison with the pulse length of the laser. DFT calculations show good predictability of the S(0)-S(1) and S(0)-T(1) energy gaps and offer insight into the interaction strength between Pt and the alkyne ligands. The use of this type of ligand, with weak absorption for the Pt(II) complexes in the visual wavelength range as a key feature, enables the possibility to further improve these molecular systems for nonlinear absorption applications.

Efficient one-step synthesis of 4-amino substituted phthalimides and evaluation of their potential as fluorescent probes

The phthalimide scaffold is recognized from bioactive compounds and marketed drugs, but can also be used as fluorescent probes by introducing a 4-amino substituent. Unfortunately, a general and convenient method to synthesize various 4-amino substituted phthalimides has been lacking. To overcome this, an atom efficient one-step synthesis of 4-amino substituted phthalimides in good to excellent yields that tolerate a wide range of substituents has been developed. Several of the generated compounds display interesting solvatochromic properties with high quantum yield of fluorescence in non-polar solvents that are significantly reduced in polar protic solvents. Many of these compounds displayed non-toxic properties and non-detectable unspecific binding and can thus potentially be linked to a substrate and used as fluorescent probes. Furthermore, bioactive and fluorescent 4-amino substituted phthalimides with IC50-values in the low micromolar range in cell-based assays have been identified and could be used to study uptake and distribution. The developed convenient synthetic method is thus valuable not only to construct fluorescent probes and fluorescent bioactive compounds to gain information about target binding, but also from a structure activity point of view in the various areas where the phthalimides have displayed activity.

Structure-activity relationships for lipoprotein lipase agonists that lower plasma triglycerides in vivo.

The risk of cardiovascular events increases in individuals with elevated plasma triglyceride (TG) levels, therefore advocating the need for efficient TG-lowering drugs. In the blood circulation, TG levels are regulated by lipoprotein lipase (LPL), an unstable enzyme that is only active as a non-covalently associated homodimer. We recently reported on a N-phenylphthalimide derivative (1) that stabilizes LPL in vitro, and moderately lowers triglycerides in vivo (Biochem. Biophys. Res. Commun.2014, 450, 1063). Herein, we establish structure-activity relationships of 51 N-phenylphthalimide analogs of the screening hit 1. In vitro evaluation highlighted that modifications on the phthalimide moiety were not tolerated and that lipophilic substituents on the central phenyl ring were functionally essential. The substitution pattern on the central phenyl ring also proved important to stabilize LPL. However, in vitro testing demonstrated rapid degradation of the phthalimide fragment in plasma which was addressed by replacing the phthalimide scaffold with other heterocyclic fragments. The in vitro potency was retained or improved and substance 80 proved stable in plasma and efficiently lowered plasma TGs in vivo.

Palladium-Mediated Approach to Coumarin-Functionalized Amino Acids

Incorporation of the fluorogenic l-(7-hydroxycoumarin-4-yl)ethylglycine into proteins is a valuable biological tool. Coumarins are typically accessed via the Pechmann reaction, which requires acidic conditions and lacks substrate flexibility. A Pd-mediated coupling is described between o-methoxyboronic acids and a glutamic acid derived (Z)-vinyl triflate, forming latent coumarins. Global deprotection with BBr3 forms the coumarin scaffold in a single step. This mild and scalable route yielded five analogues, including a probe suitable for use at lower pH.

Synthesis of Multiring Fused 2-Pyridones via a Nitrene Insertion Reaction: Fluorescent Modulators of α-Synuclein Amyloid Formation.

An efficient, straightforward method for the synthesis of thiazolo-2-pyridone embedded peptidomimetic polyheterocycles via a catalyst-free, microwave-assisted, intramolecular C-H amination reaction is reported. All the synthesized polyheterocycles were evaluated for their fluorescent properties and effect on α-synuclein amyloid formation.

Synthesis and Spectroscopic Properties of Fluorinated Coumarin Lysine Derivatives

The site-selective incorporation of fluorescent amino acids into proteins has emerged as a valuable alternative to expressible protein reporters. For successful application, a robust and scalable, yet flexible, route to non-natural amino acids is required. This work describes an improved synthesis of coumarin-conjugated lysine derivatives where fluorinated variants are accessed. These analogues can be utilized at low pH and should find application probing biological processes that operate under acidic conditions.

Non-covalent Inhibitors of Mosquito Acetylcholinesterase 1 with Resistance-Breaking Potency

Resistance development in insects significantly threatens the important benefits obtained by insecticide usage in vector control of disease-transmitting insects. Discovery of new chemical entities with insecticidal activity is highly desired in order to develop new insecticide candidates. Here, we present the design, synthesis, and biological evaluation of phenoxyacetamide-based inhibitors of the essential enzyme acetylcholinesterase 1 (AChE1). AChE1 is a validated insecticide target to control mosquito vectors of, e.g., malaria, dengue, and Zika virus infections. The inhibitors combine a mosquito versus human AChE selectivity with a high potency also for the resistance-conferring mutation G122S; two properties that have proven challenging to combine in a single compound. Structure-activity relationship analyses and molecular dynamics simulations of inhibitor-protein complexes have provided insights that elucidate the molecular basis for these properties. We also show that the inhibitors demonstrate in vivo insecticidal activity on disease-transmitting mosquitoes. Our findings support the concept of noncovalent, selective, and resistance-breaking inhibitors of AChE1 as a promising approach for future insecticide development.