Abdou Boucekkine

Syntheses, structures, two-photon absorption cross-sections and computed second hyperpolarisabilities of quadrupolar A–π–A systems containing E-dimesitylborylethenyl acceptors

A series of bis(E-dimesitylborylethenyl)-substituted arenes, namely arene = 1,4-benzene, 1,4-tetrafluorobenzene, 2,5-thiophene, 1,4-naphthalene, 9,10-anthracene, 4,4′-biphenyl, 2,7-fluorene, 4,4′-E-stilbene, 4,4′-tolan, 5,5′-(2,2′-bithiophene), 1,4-bis(4-phenylethynyl)benzene, 1,4-bis(4-phenylethynyl)tetrafluorobenzene and 5,5″-(2,2′:5′,2″-terthiophene), have been synthesised viahydroboration of the corresponding diethynylarenes with dimesitylborane. Their absorption and emission maxima, fluorescence lifetimes and quantum yields are reported along with the two-photon absorption (TPA) spectra and TPA cross-sections for the 5,5′-bis(E-dimesitylborylethenyl)-2,2′-bithiophene and 5,5′-bis(E-dimesitylborylethenyl)-2,2′:5′,2″-terthiophene derivatives. The TPA cross-section of the latter compound of ca. 1800 GM is the largest yet reported for a 3-coordinate boron compound and is in the range of the largest values measured for quadrupolar compounds with similar conjugation lengths. The X-ray crystal structures of 1,4-benzene, 2,5-thiophene, 4,4′-biphenyl and 5,5″-(2,2′:5′,2″-terthiophene) derivatives indicate π-conjugation along the BCC–arene–CCB chain. Theoretical studies show that the second molecular hyperpolarisabilities, γ, in each series of compounds are generally related to the HOMO energy, which itself increases with increasing donor strength of the spacer. A strong enhancement of γ is predicted as the number of thiophene rings in the spacer increases.

Photoswitching of the second-order nonlinearity of a tetrahedral octupolar multi DTE-based copper(I) complex

The modulation of the quadratic NLO response of an octupolar metal-based chromophore featuring four photochromic dithienylethene units is reported. Quantum mechanical simulations are consistent with a full switching of the DTE units and reproduce the strong enhancement of the NLO response.

Lanthanide(III) and Actinide(III) Complexes [M(BH4)2(THF)5][BPh4] and [M(BH4)2(18-crown-6)][BPh4] (M = Nd, Ce, U): Synthesis, Crystal Structure, and Density Functional Theory Investigation of the Covalent Contribution to Metal-Borohydride Bonding

Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3.C4H8S, 4.THF, 5, and 6.THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7.C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U...B with respect to the Ce...B distance, which is accompanied by a lengthening of the U-Hb bonds and an opening of the Hb-B-Hb angle (Hb = bridging hydrogen atom of the eta3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 -->M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M-Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the pi bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.

Linear and nonlinear optical properties of tris-cyclometalated phenylpyridine Ir(III) complexes incorporating π-conjugated substituents.

The synthesis, luminescence, and nonlinear optical properties of a new series of Ir(ppy)3 (ppy = 2-phenylpyridine) complexes incorporating π-extended vinyl-aryl substituents at the para positions of their pyridine rings are reported. Appropriate substitution of the pyridyl rings allows the tuning of the luminescence properties and the second-order nonlinear optical response of this unusual family of three-dimensional chromophores. Theoretical calculations were performed to evaluate the dipole moments, to gain insight into the electronic structure, and to rationalize the observed optical properties of the investigated complexes.

Experimental and Theoretical Studies of Quadrupolar Oligothiophene-Cored Chromophores Containing Dimesitylboryl Moieties as π-Accepting End-Groups: Syntheses, Structures, Fluorescence, and One- and Two-Photon Absorption

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)-dimesitylborylvinyl groups (4 V-5 V), and five thiophene rings with two terminal aryldimesitylboryl groups (5 B), as well as an analogue of 5 V with a central EDOT ring (5 VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66-89%). Crystal structures of 4 V, 5 B, bithiophene 2 V, and five thiophene-derived intermediates are reported. Chromophores 4 V, 5 V, 5 B and 5 VE have photoluminescence quantum yields of 0.26-0.29, which are higher than those of the shorter analogues 1 V-3 V (0.01-0.20), and short fluorescence lifetimes (0.50-1.05 ns). Two-photon absorption (TPA) spectra have been measured for 2 V-5 V, 5 B and 5 VE in the range 750-920 nm. The measured TPA cross-sections for the series 2 V-5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V-5 V with the related compounds 5 B and 5 VE, giving insight into the structure-property relationship for this class of chromophore. DFT and TD-DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two-photon-excited fluorescence applications.

Synthesis, photophysics and molecular structures of luminescent 2,5-bis(phenylethynyl)thiophenes (BPETs)

The Sonogashira cross-coupling of two equivalents of para-substituted ethynylbenzenes with 2,5-diiodothiophene provides a simple synthetic route for the preparation of 2,5-bis(para-R-phenylethynyl)thiophenes (R = H, Me, OMe, CF3, NMe2, NO2, CN and CO2Me) (1a–h). Likewise, 2,5-bis(pentafluorophenylethynyl)thiophene (2) was prepared by the coupling of 2,5-diiodothiophene with pentafluorophenylacetylene. All compounds were characterised by NMR, IR, Raman and mass spectroscopy, elemental analysis, and their absorption and emission spectra, quantum yields and lifetimes were also measured. The spectroscopic studies of 1a–h and 2 show that both electron donating and electron withdrawing para-subsituents on the phenyl rings shift the absorption and emission maxima to lower energies, but that acceptors are more efficient in this regard. The short singlet lifetimes and modest fluorescence quantum yields (ca. 0.2–0.3) observed are characteristic of rapid intersystem crossing. The single-crystal structures of 2,5-bis(phenylethynyl)thiophene, 2,5-bis(para-carbomethoxyphenylethynyl)thiophene, 2,5-bis(para-methylphenylethynyl)thiophene and 2,5-bis(pentafluorophenylethynyl)thiophene were determined by X-ray diffraction at 120 K. DFT calculations show that the all-planar form of the compounds is the lowest in energy, although rotation of the phenyl groups about the CC bond is facile and TD-DFT calculations suggest that, similar to 1,4-bis(phenylethynyl)benzene analogues, the absorption spectra in solution arise from a variety of rotational conformations. Frequency calculations confirm the assignments of the compounds’ IR and Raman spectra.

Density functional theory investigations of the homoleptic tris(dithiolene) complexes [M(dddt)(3)](-q) (q = 3, 2 ; M = Nd(3+) and U(3+/4+)) related to lanthanide(III)/actinide(III) differentiation.

The structures of the homoleptic lanthanide and actinide tris(dithiolene) complexes [M(dddt)(3)](q-) (q = 3, M = Nd(3+) and q = 3 or 2, M = U(3+/4+)) have been investigated using relativistic Density Functional Theory (DFT) computations including spin-orbit corrections coupled with the COnductor-like Screening Model (COSMO) for a realistic solvation approach. The dithiolene ligands are known to be very efficient at stabilizing metal high oxidation states. The aim of the work is to explain the peculiar symmetric folding of the three Mdddt metallacycles in these complexes, some of them existing under a polymeric form, in relation with the Ln(III)/An(III) differentiation. In the [M(dddt)(3)(py)](q-) species, where an additional pyridine ligand is linked to the metal center, the Mdddt moieties appear to be almost planar. The study brings to light the occurrence of a M...C=C interaction explaining the Mdddt folding of the [U(dddt)(3)](q-) uranium species, the metal 5f electrons playing a driving role. No such interaction appears in the case of the Nd(III) complex, and the folding of the rather flexible dddt ligands in the polymeric structure of this species should be mainly due to steric effects. Moreover, the analysis of the normal modes of vibration shows that the U(III) complex [U(dddt)(3)](3-), which has not yet been isolated, is thermodynamically stable. It appears that the X-ray characterized U(IV) complex [U(dddt)(3)](2-) should be less stable than the calculated U(III) complex in a polar solvent.

The first hexadithienylethene-substituted tris(bipyridine)metal complexes as quadratic NLO photoswitches: combined experimental and DFT studies.

Flip the NLO switch! A straightforward access to hexadithienylethene-based photochromes by using a coordination-chemistry approach through the combination of bis(dithienylethene)bipyridyl ligands and metal ions (M = Zn, Fe) is reported. The photostability of the isomeric forms of the iron(II) complexes allows the photoswitching of second-order nonlinear optical (NLO) responses (see figure), and the results are rationalized with the help of time-dependent density functional theory.

Density functional theory investigation of the redox properties of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes.

The redox behavior of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes L(3)UCl with L = C(5)H(5) (Cp), C(5)H(4)Me (MeCp), C(5)H(4)SiMe(3) (TMSCp), C(5)H(4)(t)Bu ((t)BuCp), C(5)Me(5) (Cp*), and C(4)Me(4)P (tmp), has been investigated using relativistic density functional theory calculations, with the solvent being taken into account using the conductor-like screening model. A very good linear correlation (r(2) = 0.99) has been obtained between the computed electron affinities of the L(3)UCl complexes and the experimental half-wave reduction potentials E(1/2) related to the U(IV)/U(III) redox systems. From a computational point of view, our study confirms the crucial importance of spin-orbit coupling and solvent corrections and the use of an extended basis set in order to achieve the best experiment-theory agreement. Considering oxidation of the uranium(IV) complexes, the instability of the uranium(V) derivatives [L(3)UCl](+) is revealed, in agreement with experimental electrochemical findings. The driving roles of both the electron-donating ability of the L ligand and the U 5f orbitals on the redox properties of the complexes are brought to light. Interestingly, we found and explained the excellent correlation between variations of the uranium Hirschfeld charges following U(IV)/U(III) electron capture and E(1/2). In addition, this work allowed one to estimate theoretically the half-wave reduction potential of [Cp*(3)UCl].

DFT study of conjugated biheterocyclic oligomers exhibiting a very low HOMO–LUMO energy gap

Abstract Density functional theory (DFT) is applied to study the structure and electronic properties of oligomers based on bithiophene bridged by a sp 2 carbon substituted by a chalcogen atom (O, S, Se and Te), and their polybifurane and polybipyrrole analogues. The important reduction of the energy gap which is observed for the whole series of biheterocyclic compounds, when going down the chalcogen group, is explained on the basis of an orbital interaction analysis. Bithiophene polymers bridged by a selenium or a tellurium substituted carbon atom are expected to exhibit very low energy band gaps.