Boron Difluoride Curcuminoid Fluorophores with Enhanced Two‐Photon Excited Fluorescence Emission and Versatile Living‐Cell Imaging Properties

The synthesis of boron difluoride complexes of a series of curcuminoid derivatives containing various donor end groups is described. Time‐dependent (TD)‐DFT calculations confirm the charge‐transfer character of the second lowest‐energy transition band and ascribe the lowest energy band to a “cyanine‐like” transition. Photophysical studies reveal that tuning the donor strength of the end groups allows covering a broad spectral range, from the visible to the NIR region, of the UV–visible absorption and fluorescence spectra. Two‐photon‐excited fluorescence and Z‐scan techniques prove that an increase in the donor strength or in the rigidity of the backbone results in a considerable increase in the two‐photon cross section, reaching 5000 GM, with predominant two‐photon absorption from the S₀–S₂ charge‐transfer transition. Direct comparisons with the hemicurcuminoid derivatives show that the two‐photon active band for the curcuminoid derivatives has the same intramolecular charge‐transfer character and therefore arises from a dipolar structure. Overall, this structure–relationship study allows the optimization of the two‐photon brightness (i.e., 400–900 GM) with one dye that emits in the NIR region of the spectrum. In addition, these dyes demonstrate high intracellular uptake efficiency in Cos7 cells with emission in the visible region, which is further improved by using porous silica nanoparticles as dye vehicles for the imaging of two mammalian carcinoma cells type based on NIR fluorescence emission.     

Spectral and structural characterization of amidate-bridged platinum-thallium complexes with strong metal-metal bonds

The reactions of [Pt(NH3)2(NHCOtBu)2] and TlX3 (X = NO3-, Cl-, CF3CO2-) yielded dinuclear [{Pt(ONO2)(NH3)2(NHCOtBu)}Tl(ONO2)2(MeOH)] (2) and trinuclear complexes [{PtX(RNH2)2(NHCOtBu)2}2Tl]+ [X = NO3- (3), Cl- (5), CF3CO2- (6)], which were spectroscopically and structurally characterized. Strong Pt-Tl interaction in the complexes in solutions was indicated by both 195Pt and 205Tl NMR spectra, which exhibit very large one-bond spin-spin coupling constants between the heteronuclei (1J(PtTl)), 146.8 and 88.84 kHz for 2 and 3, respectively. Both the X-ray photoelectron spectra and the 195Pt chemical shifts reveal that the complexes have Pt centers whose oxidation states are close to that of Pt(III). Characterization of these complexes by X-ray diffraction analysis confirms that the Pt and Tl atoms are held together by very short Pt-Tl bonds and are supported by the bridging amidate ligands. The Pt-Tl bonds are shorter than 2.6 Angstrom, indicating a strong metal-metal attraction between these two metals. Compound 2 was found to activate the C-H bond of acetone to yield a platinum(IV) acetonate complex. This reactivity corresponds to the property of Pt(III) complexes. Density functional theory calculations were able to reproduce the large magnitude of the metal-metal spin-spin coupling constants. The couplings are sensitive to the computational model because of a delicate balance of metal 6s contributions in the frontier orbitals. The computational analysis reveals the role of the axial ligands in the magnitude of the coupling constants.     

Metal-dependent ferro- versus antiferromagnetic interactions in molecular crystals of square Planar (M(II) imino-nitroxide radical) complexes (M = Pt,Pd)

The synthesis and structural, spectral, and magnetic characterizations of two new complexes of formula [Pt(IM(2)Py)Cl(2)] (A) and [Pd(IM(2)Py)Cl(2)] (B) are reported. IM(2)Py stands for the imino-nitroxide radical ligand 2-(ortho-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl. Their crystal structures were solved at room temperature and at 120 K revealing structural phase transitions from pseudo-orthorhombic to monoclinic systems for the two compounds which remain isostructural in the whole temperature range explored. Structural parameters for A: T = 293 K [120 K], monoclinic (P2(1)/n) [P2(1)/c], a = 7.906(2) [7.989(3)] A, b = 17.872(9) [10.168(4)] A, c = 10.357(3) [17.623(6)] A, beta = 90.732(13) degrees [95.940(2)] degrees, Z = 4 [4]. Structural parameters for B: T = 293 K [120 K], monoclinic (P2(1)/n) [P2(1)/c], a = 7.900(3) [7.9730(2)] A, b = 17.907(9) [10.1806(3)] A, c = 10.299(3) [17.7171(4)] A, beta = 90.524(14) degrees [95.747(2)] degrees, Z = 4 [4]. In both complexes, the metal coordination is essentially planar. The average Pt-N, Pt-Cl and Pd-N, Pd-Cl bond lengths are 1.996(6) [1.88], 2.295(2) [2.248(8)] A and 2.015(7) [2.029(8)], 2.287(3) [2.294(3)] A, respectively. The solid state structure is characterized by a pairlike molecular packing stacked in columns parallel to the a axis; this dimer character is reinforced at low temperature. Despite their structural similarity, the investigation of the magnetic properties revealed that dominant ferromagnetic interactions govern the behavior of the Pt derivative A, whereas antiferromagnetic interactions take place for the Pd compound B. A rationalization for this rather intriguing difference is proposed in light of the spin population deduced from density functional theory calculations. The electronic absorption spectra of A and B present structured absorption bands in the visible which are attributed to MLCT transitions. Both compounds are nonluminescent at room temperature. However, a weak emission is detected for A in butyronitrile glasses at 77 K, indicating that the MLCT excited state is strongly quenched at low temperature.     

Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far‐Red (Polarized) Luminescence

The physicochemical properties of cationic dioxa ( 1 ), azaoxa ( 2 ), and diaza ( 3 ) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pK_R+=20.4, =?0.72 V) compared to its azaoxa 2 (pK_R+=15.2, =?0.45 V) and dioxa 1 (pK_R+=8.8, =?0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far‐red regions. From 1 to 3 , a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post‐functionalization reactions (12 examples, compounds 4 – 15 ). The electronic absorption is modulated from the orange to the far‐red spectral range (560–731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first‐principles calculations.     

Polarized Neutron Diffraction to Probe Local Magnetic Anisotropy of a Low‐Spin Fe(III) Complex

We have determined by polarized neutron diffraction (PND) the low‐temperature molecular magnetic susceptibility tensor of the anisotropic low‐spin complex PPh₄[Fe^III(Tp)(CN)₃]?H₂O. We found the existence of a pronounced molecular easy magnetization axis, almost parallel to the C₃ pseudo‐axis of the molecule, which also corresponds to a trigonal elongation direction of the octahedral coordination sphere of the Fe^III ion. The PND results are coherent with electron paramagnetic resonance (EPR) spectroscopy, magnetometry, and ab initio investigations. Through this particular example, we demonstrate the capabilities of PND to provide a unique, direct, and straightforward picture of the magnetic anisotropy and susceptibility tensors, offering a clear‐cut way to establish magneto‐structural correlations in paramagnetic molecular complexes.     

Solvent effects on 195Pt and 205Tl NMR chemical shifts of the complexes [(NC)5Pt--Tl(CN)n]n- (n=0-3), and [(NC)5Pt--Tl--Pt(CN)5]3- studied by relativistic density functional theory

The 295Pt and 205Tl NMR chemical shifts of the complexes [(NC)5Pt-Tl(CN)n]n- n=0-3, and of the related system [(NC)5Pt--Tl--Pt(CN)5]3- have been computationally investigated. It is demonstrated that based on relativistically optimized geometries, by applying an explicit first solvation shell, an additional implicit solvation model to represent the bulk solvent effects (COSMO model), and a DFT exchange-correlation potential that was specifically designed for the treatment of response properties, that the experimentally observed metal chemical shifts can be calculated with satisfactory accuracy. The metal chemical shifts have been computed by means of a two-component relativistic density functional approach. The effects of electronic spin-orbit coupling were included in all NMR computations. The impact of the choice of the reference, which ideally should not affect the accuracy of the computed chemical shifts, is also demonstrated. Together with recent calculations by us of the Pt and Tl spin-spin coupling constants, all measured metal NMR parameters of these complexes are now computationally determined with sufficient accuracy in order to allow a detailed analysis of the experimental results. In particular, we show that interaction of the complexes with the solvent (water) must be an integral part of such an analysis.     

A Terminal Fluoride Ligand Generates Axial Magnetic Anisotropy in Dysprosium Complexes

The first dysprosium complexes with a terminal fluoride ligand are obtained as air‐stable compounds. The strong, highly electrostatic dysprosium–fluoride bond generates a large axial crystal‐field splitting of the J=15/2 ground state, as evidenced by high‐resolution luminescence spectroscopy and correlated with the single‐molecule magnet behavior through experimental magnetic susceptibility data and ab initio calculations.     

Efficient sensitization of europium, ytterbium, and neodymium functionalized tris-dipicolinate lanthanide complexes through tunable charge-transfer excited states

A series of push-pull donor-pi-conjugated dipicolinic acid ligands and related tris-dipicolinate europium and lutetium complexes have been prepared. The ligands present broad absorption and emission transitions in the visible spectral range unambiguously assigned to charge-transfer transitions (CT) by means of time-dependent density functional theory calculations. The photophysical properties (absorption, emission, luminescence quantum yield, and lifetime) of the corresponding europium complexes were thoroughly investigated. Solvatochromism and temperature effects clearly confirm that Eu(III) sensitization directly occurs from the ligand CT state. In addition, modulation of the energy of the CT donating state by changing the nature of the donor fragment allows the optimal energy of the antennae for europium sensitization to be determined, and this optimal energy was found to be close to the (5)D 1 accepting state. Finally, this CT sensitization process has been successfully extended to near-infrared emitters (neodymium and ytterbium).