Intraligand Charge Transfer Research Articles

Redox-active ferrocene-dithiolenes hybrid complexes showing switchable nonlinear optical properties

Density functional theory(DFT) calculations were carried out to investigate the geometry structures, redox properties and second-order nonlinear optical(NLO) properties of ferrocene(Fc)-dithiolenes hybrid complexes. The switchable second-order NLO properties of these complexes are induced by the redox process. The oxidized process significantly affects the geometrical structures of the dithiolene moieties, that is, the embowed dithiolene moieties change into planar structures. It supports that the dithiolene moieties are the oxidized center. The βtot values of the cationic species are at least 4 and 10 times those of their corresponding parent complexes, respectively. Further, the time-dependent DFT calculation illustrates that the low-energy absorption(which is helpful for the large NLO response) is mainly assigned to intra-ligand charge transfer [π(ex-dithiolene)→π*(ex-dithiolene)]. These results suggest the potential use of the novel Fc-based dithiolenes complexes as versatile and fascinating NLO switching materials.

Osmium Bisterpyridine Complexes with Redox-Active Amine Substituents: A Comparison Study with Ruthenium Analogues.

Five osmium complexes with redox-active amine substituents, [Os(ttpy)(Ntpy)](PF6)2 (1(PF6)2), [Os(Ntpy)2](PF6)2 (2(PF6)2), [Os(ttpy)(NPhtpy)](PF6)2 (3(PF6)2), [Os(Ntpy)(NPhtpy)](PF6)2 (4(PF6)2), and [Os(NPhtpy)2](PF6)2 (5(PF6)2), have been prepared, where ttpy is 4'-tolyl-2,2':6',2″-terpyridine, Ntpy is 4'-(di-p-anisylamino)-2,2':6',2″-terpyridine, and NPhtpy is 4'-(di-p-anisylaminophen-4-yl)-2,2':6',2″-terpyridine. X-ray crystallographic data of 2(PF6)2 and 4(PF6)2 are presented. These complexes show rich visible absorptions attributed to the singlet metal-to-ligand charge-transfer ((1)MLCT), triplet MLCT, and intraligand charge-transfer transitions. Complexes 3(PF6)2 and 5(PF6)2 show weak emissions around 720 nm at room temperature. All complexes show stepwise oxidations of the osmium ion and the amine segment. However, the redox potentials and the order of the Os(III/II) and N(•+/0) processes vary significantly, depending on the electronic nature of the amine substituents. In the singly oxidized state, either Os(II) → N(•+) MLCT or N → Os(III) ligand-to-metal charge-transfer transitions in the near-infrared region have been observed. For complexes 2(PF6)2, 4(PF6)2, and 5(PF6)2 with two amine substituents, no evidence has been observed for the presence of osmium-mediated amine-amine electronic coupling. Density functional theory (DFT) and time-dependent DFT calculations have been performed to complement these experimental results. The one-electron-oxidized forms 3(3+) and 5(3+) show distinct electron paramagnetic resonance (EPR) signals in CH3CN at room temperature. However, complexes 1(3+), 2(3+), and 4(3+) are EPR silent under similar conditions. In addition, a comparison study has been made between these osmium complexes and the previously reported ruthenium analogues.

Synthesis, crystal structures and theoretical studies of dinuclear Mn(II) and Ni(II) complexes of phenol-based “end-off” compartmental ligand

Two novel complexes [Mn2(phmp)2](ClO4)2 (1) and [Ni2(phmp)(μ-H2O)(H2O)4](NO3)3 (2) were synthesized using an “end-off” compartmental ligand [H-phmp = 4-Methyl-2,6-bis-(pyridin-2-yl-hydrazonomethyl)-phenol] with Mn(II)-perchlorate and Ni(II)-nitrate salts as metal precursors. Both these complexes were characterized by spectroscopic (IR, UV–Vis) and X-ray crystal structure analysis. The ligand, H-phmp acts as a pentadentate NNONN donor for both the metal complexes and the geometry of the complexes 1 and 2 is distorted octahedral. The DFT optimized bond distances and angles are well correlate to the X-ray structure bond parameters. For complex 1 the transitions at 435 nm and 428 nm have mixed MLCT (dπ(Mn) → π*(L)) and ILCT (π(L) → π*(L)) character whereas other transitions correspond to intra-ligand charge transfer transitions (ILCT). For complex 2, weak transitions at 464 and 405 nm correspond to ligand to metal charge transfer transitions (LMCT) (π(L) → dπ(Ni)) and most of the other transitions have ILCT character. As anticipated, various weak forces, i.e. anion–π/π–anion/anion–π/π–NH interactions as well as C–H/π interaction, play a key role in stabilizing the self-assembly process observed for both compounds.

Cyclometalated Platinum(II) Terpyridylacetylide with a Bis(arylamine) Donor as a Proton-Triggered Luminescence Chemosensor for Zn(2+).

A cyclometalated Pt(II) acetylide derivative bearing a bis(arylamine) donor (D) and a terpyridine (TPY) receptor (A) was successfully synthesized and characterized. This three-component D-π-A system displays an intense low-energy absorption band at λmax = 470 nm, resulting from overlapping the dπ(Pt) → π*(C^N^N) metal-to-ligand charge transfer and π(C≡C-Ar) → π*(C^N^N) ligand-to-ligand charge transfer transitions with the π(bis(arylamine)) → π*(C^N^N) intraligand charge transfer transition. Upon protonation of the bis(arylamine) donor, a strong emission from the phosphorescence Pt(II) complex unit is recovered at λmax = 580 nm. With introduction of the TPY receptor, this complex possesses quite high affinity for Zn(2+) (Ka = 6.86 × 10(9) mol(-2)·dm(6)) to form the heterotrinuclear Pt-Zn-Pt complex in CH2Cl2 solution. Though this coordination effect is seriously inhibited by protons in acidic medium, the 2.4-fold luminescence intensity enhancement is obtained yet, strongly suggesting the presence of the intramolecular energy transfer process from the Zn(II)-TPY complex core to the phosphorescence Pt(II) complex units at two ends. The H(+)-triggered and Zn(2+)-enhanced luminescence can be reversibly switched on and off upon successive additions of H(+) and OH(-). Furthermore, this complex displays an unexpected Zn(2+)-selective luminescence enhancement effect in acidic solution.

Comparison of Redox Activity between 2-Aminothioether and 2-Aminothiophenol: Redox-Induced Dimerization of 2-Aminothioether via C-C Coupling.

Three chemical reactions of two 2-aminothioethers and 2-aminothiophenol with CpRu(II)Cl(PPh3)2 (Cp(-) = cyclopentadienyl anion), under identical reaction conditions, are reported. While 2-(methylthio)aniline, H2L(1) and an analogous substrate, 2-(phenylthio)aniline yielded dicationic dinuclear complexes [(PPh3)CpRu(II)(L(3/)L(4))Ru(II)Cp(PPh3)]Cl2 (where L(3) = (4E)-4-(4-imino-3-(methylthio)cyclohexa-2,5-dienylidene)-2-(methylthio)cyclohexa-2,5-dienimine ([1a]Cl2) and L(4) = (4E)-4-(4-imino-3-(phenylthio)cyclohexa-2,5-dienylidene)-2-(phenylthio)cyclohexa-2,5-dienimine ([1b]Cl2)), the reaction with 2-aminothiophenol (H2L(2)) produced a mononuclear complex [(PPh3)CpRu(II)(L(2))]Cl (where L(2) = 6-iminocyclohexa-2,4-dienethione) ([2]Cl). All these complexes are obtained in high yields (65%-75%). Formations of the products from the above reactions involve a similar level of oxidation of the respective substrate, although their courses are completely different. A comparison between the above two chemical transformations are scrutinized thoroughly. Characterizations of these complexes were made using a host of physical methods: X-ray crystallography, nuclear magnetic resonance (NMR), cyclic voltammetry, ultraviolet-visible (UV-vis), electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT). The complexes [1a]Cl2 and [1b]Cl2 showed intense metal-to-ligand charge transfer transition in the long wavelength region of the spectrum, at 860 and 895 nm, respectively, and displayed two reversible electron transfer (ET) processes at [1a](2+): -0.28 and -0.52 V; [1b](2+): -0.13 and -0.47 V, along with an irreversible ET process at 0.76 and 0.54 V, respectively. The ET processes at negative potentials are due to successive reductions of the bridging ligand, which are characterized by EPR and UV-vis spectroscopy. The one-electron reduced compound, [1a](+), showed a intraligand charge transfer transition (ILCT) at 1530 nm. The complex [2](+) showed a reversible ET process at -0.36 V and two irreversible ET processes at -1.04 and 1.18 V, respectively. DFT calculations were used to support the spectral and redox properties of the complexes and also to throw light on the difference of redox behavior between thioether and thiophenol substrates.

Nonlinear Optical Chromophores with Two Ferrocenyl, Octamethylferrocenyl, or 4-(Diphenylamino)phenyl Groups Attached to Rhenium(I) or Zinc(II) Centers

The compounds 4,4′-bis[(E)-2-R-vinyl]-2,2′-bipyridyl {R = ferrocenyl [(Fcv)_2bpy], octamethylferrocenyl [(Me_8Fcv)_2bpy] or 4-(diphenylamino)phenyl [(Dapv)_2bpy]} are used to prepare eight new complexes with Zn^(II)Cl_2, Zn^(II)(OAc)_2, or fac-Re^ICl(CO)_3 centers. The recently reported complex fac-Re^ICl(CO)_3[(Dapv)_2bpy] (Horvath, R. et al. Inorg. Chem. 2013, 52, 1304) is also studied. Electronic absorption spectra show intense d → π^* metal-to-ligand charge-transfer (MLCT) and π → π^* intraligand charge-transfer (ILCT) absorption bands, the relative energies of which correlate logically with the molecular structure. Cyclic voltammetry reveals a reversible oxidation wave for the Fc/Me_8Fc complexes, accompanied by quasireversible or irreversible ligand-based reductions. The Re complexes also show irreversible Re^(II/I) waves. Single-crystal X-ray structures are reported for (Me_8Fcv)_2bpy, Zn^(II)Cl_2[(Me_8Fcv)_2bpy], Zn^(II)(OAc)_2[(Fcv)_2bpy]·CHCl_3, and fac-Re^ICl(CO)_3[(Me_8Fcv)_2bpy]·0.5CHCl_3. Molecular first hyperpolarizabilities β are measured in DCM solutions via the hyper-Rayleigh scattering (HRS) technique at 1300 nm. Stark (electroabsorption) spectroscopic studies on only the MLCT bands in frozen butyronitrile allow the indirect estimation of lower limits for the overall static first hyperpolarizabilities β_0. Time-dependent density functional theory (TD-DFT) calculations on selected complexes confirm the expected assignments of their low energy absorption bands, with the best results obtained by using the M06 functional and Def2-TZVP/SVP/TZVPP mixed basis set. DFT predicts that the total static first hyperpolarizability β_(tot) increases in the Zn^(II)Cl_2 series in the order R = Fc < Me_8Fc < Dap, consistent with the HRS and Stark data. The computed β values increase substantially on moving from the gas phase to a DCM or MeCN solvent medium, and the essentially 2D nature of the chromophores leads to dominant β_(xxy) tensor components.

Pt(II) Metal Complexes Tailored with a Newly Designed Spiro-Arranged Tetradentate Ligand; Harnessing of Charge-Transfer Phosphorescence and Fabrication of Sky Blue and White OLEDs

Tetradentate bis(pyridyl azolate) chelates are assembled by connecting two bidentate 3-trifluoromethyl-5-(2-pyridyl)azoles at the six position of pyridyl fragment with the tailored spiro-arranged fluorene and/or acridine functionalities. These new chelates were then utilized in synthesizing a series of Pt(II) metal complexes [Pt(Ln)], n = 1-5, from respective chelates L1-L5 and [PtCl2(DMSO)2] in 1,2-dimethoxyethane. The single-crystal X-ray structural analyses were executed on 1, 3, and 5 to reveal the generalized structures and packing arrangement in crystal lattices. Their photophysical properties were measured in both solution and solid state and are discussed in the context of computational analysis. These L1-L5 coordinated Pt(II) species exhibit intense emission, among which complex 5 shows remarkable solvatochromic phosphorescence due to the dominant intraligand charge transfer transition induced by the new bis(pyridyl azolate) chelates. Moreover, because of the higher-lying highest occupied molecular orbital of acridine, complex 5 can be considered as a novel bipolar phosphor. Successful fabrication of blue and white organic light-emitting diodes (OLEDs) using Pt(II) complexes 3 and 5 as the phosphorescent dopants are reported. In particular, blue OLEDs with 5 demonstrated peak efficiencies of 15.3% (36.3 cd/A, 38.0 lm/W), and CIE values of (0.190, 0.342) in a double-emitting layer structure. Furthermore, a red-emitting Os(II) complex and 5 were used to fabricate warm-white OLEDs to achieve peak external quantum efficiency, luminance efficiency, and power efficiency values as high as 12.7%, 22.5 cd/A, and 22.1 lm/W, respectively.

Highly luminescent Sm(III) complexes with intraligand charge-transfer sensitization and the effect of solvent polarity on their luminescent properties.

Samarium complexes with the highest quantum yields to date have been synthesized, and their luminescence properties were studied in 12 solvents. Sensitization via a nontriplet intraligand charge-transfer pathway was also successfully demonstrated in solution states with good quantum yields.

Homoleptic and heteroleptic complexes of chromium(III) containing 4′-diphenylamino-2,2′:6′,2″-terpyridine ligands

Two heteroleptic bis(2,2′:6′,2″-terpyridine)chromium(III) complexes [Cr(1)(4′-(4-tolyl)tpy)][CF3SO3]3 and [Cr(2)(4′-(4-tolyl)tpy)][CF3SO3]3 (1=4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-diphenylaniline, 2=4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-bis(4-methoxyphenyl)aniline, 4′-(4-tolyl)tpy=4′-(4-tolyl)-2,2′:6′,2″-terpyridine) have been prepared and their spectroscopic and electrochemical properties compared with those of [Cr(4′-(4-tolyl)tpy)2][CF3SO3]3 and [Cr(1)2][CF3SO3]3. The single crystal structure of [Cr(4′-(4-tolyl)tpy)2][CF3SO3]3·2MeCN is presented, and the effects of accommodating three triflate anions and two MeCN molecules per cation are discussed in terms of related structures. The coordination of 1 or 2 to chromium(III) red-shifts the intra-ligand charge transfer (ILCT) band and this band exhibits a negative solvatochromic effect in some solvents. However, in H2O, MeOH, DMSO and DMF, the tpy ligands are labile; changes in the absorption spectra of solutions of [Cr(2)(4′-(4-tolyl)tpy)][CF3SO3]3 are consistent with the formation of [Cr(4′-Xtpy)(Solv)3]3+ (Solv=solvent) rather than complete ligand displacement or a ligand redistribution.

Effects of protonation on the optical and photophysical properties of ReCl(CO)3(dppz–TAA) and [Ru(bpy)2(dppz–TAA)]2+

The effect of protonation on the spectroscopic properties of Re(I) and Ru(II) complexes with the ligand 11-triphenylamine-dipyrido[3,2-a:2′,3′-c]phenazine (dppz-TAA) are investigated. For [Re(CO)3Cl(dppz-TAA)] protonation with triflic acid in CH2Cl2 results in a depletion of the intraligand charge-transfer transition at 500nm (∊=17700M−1cm−1) and the concomitant grow-in of a band at 780nm (∊=18200Lmol−1cm−1). Resonance Raman spectra of the protonated species show a strong band at 1628cm−1 attributed, through DFT modeling, to a TAA mode with bending of the phenazine NH linkage. The low energy band is also predicted by TD DFT. The behavior of [Ru(bpy)2(dppz-TAA)]Cl2 was studied in aqueous solution as a function of pH. The electronic absorption spectrum shows modest changes as a function of pH. The excited state properties show a short-lived emissive state (λmax=500nm) that is diminished in intensity at lower pH. The compound also has a long-lived dark state with absorption features at 420 and 700nm; the lifetime of this state is also pH sensitive varying from 2500 to 500ns. The dark long-lived state is probed with transient resonance Raman spectroscopy and is assigned as a 3π,π∗ ligand state. This is consistent with DFT studies. From these data the pKa for [Ru(bpy)2(dppz-TAA)]Cl2 is estimated as 4.5.

DFT and TD-DFT Study of Bis[2-(5-Amino-[1,3,4]-Oxadiazol-2-yl) Phenol](Diaqua)M(II) Complexes [M = Cu, Ni and Zn]: Electronic Structures, Properties and Analyses

Ground state geometries, spectral (IR and UV-Vis) properties, analysis of frontier molecular orbitals (FMOs), natural bond orbital (NBO) analysis and molecular electrostatic potential (MEP) surfaces of three transition metal complexes [Cu(AOYP)2(OH2)2] (A), [Ni(AOYP)2(OH2)2] (B) and [Zn-(AOYP)2(OH2)2] (C), have been studied theoretically by the Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods. AOYP is the oxadiazole ligand 2-(5-amino-[1,3,4]-oxadiazol-2-yl)phenol. The geometries of these complexes were initially optimized using two basis sets: LAN2DZ and a generic basis set, the latter of which was selected for subsequent analysis. The stability of the complexes arising from intramolecular interactions and electron delocalization was estimated by natural bond orbital (NBO) analysis. The NBO results showed significant charge transfer from lone pair orbitals on the AOYP donor atoms O19, O21, N15 and N36 to central metal ions in the complexes, as well as to the benzene and oxadiazole rings. The electronic spectrum of (A) showed bands at 752 and 550 nm mainly attributable to ligand-to-metal charge transfer (LMCT) transitions, and a band at 446 nm assigned to a d-d transition. The electronic spectrum of (B) consisted of bands at 540, 463 and 395 nm mainly due to d-d transitions. Calculated electronic bands for (C) occurred at 243, 238 and 235 nm, arising from intraligand charge transfer (ILCT) transitions within AOYP. A good agreement in terms of band positions was found between experimental and calculated absorption spectra of the complexes.

Zinc complexes exhibiting highly efficient thermally activated delayed fluorescence and their application to organic light-emitting diodes.

Metal complexes emitting thermally activated delayed fluorescence based on intra-ligand charge transfer and enhanced by metallization were synthesized. Organic light-emitting diodes using a thermally stable zinc complex processed by vacuum vapor deposition achieved an external quantum efficiency of nearly 20%.

In situ spectroelectrochemical and theoretical study on the oxidation of a 4H-imidazole-ruthenium dye adsorbed on nanocrystalline TiO2 thin film electrodes.

Terpyridine 4H-imidazole-ruthenium(II) complexes are considered promising candidates for use as sensitizers in dye sensitized solar cells (DSSCs) by displaying broad absorption in the visible range, where the dominant absorption features are due to metal-to-ligand charge transfer (MLCT) transitions. The ruthenium(III) intermediates resulting from photoinduced MLCT transitions are essential intermediates in the photoredox-cycle of the DSSC. However, their photophysics is much less studied compared to the ruthenium(II) parent systems. To this end, the structural alterations accompanying one-electron oxidation of the RuIm dye series (including a non-carboxylic RuIm precursor, and, carboxylic RuImCOO in solution and anchored to a nanocrystalline TiO2 film) are investigated via in situ experimental and theoretical UV-Vis absorption and resonance Raman (RR) spectroelectrochemistry. The excellent agreement between the experimental and the TDDFT spectra derived in this work allows for an in-depth assignment of UV-Vis and RR spectral features of the dyes. A concordant pronounced wavelength dependence with respect to the charge transfer character has been observed for the model system RuIm, and both RuImCOO in solution and attached on the TiO2 surface. Excitation at long wavelengths leads to the population of ligand-to-metal charge transfer states, i.e. photoreduction of the central ruthenium(III) ion, while high-energy excitation features an intra-ligand charge transfer state localized on the 4H-imidazole moiety. Therefore, these 4H-imidazole ruthenium complexes investigated here are potential multi-photoelectron donors. One electron is donated from MLCT states, and additionally, the 4H-imidazole ligand reveals electron-donating character with a significant contribution to the excited states of the ruthenium(III) complexes upon blue-light irradiation.

Synthesis, Structural Characterisation, and Spectroscopic Properties of Copper(I) Complexes and their Applications to Dye-Sensitised Solar Cells

Based on the ligand 1,4-bis(2-(diphenylphophino)benzylideneamino)benzene (pbb), two complexes, [Cu2(pbb)(2,2′-bipyridine)2](BF4)2 (1) and [Cu2(pbb)(phen)2](BF4)2 (2), have been prepared and characterised by IR, 1H NMR, 31P NMR, 19F NMR, and 11B NMR spectroscopy and X-ray crystal structure analysis. Structural analysis reveals that complexes 1 and 2 contain 1D infinite chains and 2D supramolecular networks constructed by C–H···π and π···π interactions, and an ordered-layer-lattice of BF4– is located between these 2D networks. The results show that C–H···π and π···π interactions play an important role in the formation of 2D supramolecular networks. The UV-vis absorption peaks of complexes 1 and 2 display intraligand charge transfer and metal to ligand charge transfer (MLCT) absorption. Complexes 1 and 2 display efficient luminescent emission assigned to MLCT excited states, and the maximum emissions of these complexes in acetonitrile solution are different from those of the solid-state samples. In addition, complexes 1 and 2 have been exploited as sensitisers in dye-sensitised solar cells, and efficiencies are also observed.

Ruthenium-bis-terpyridine Complex with Two Redox-Asymmetric Amine Substituents: Potential-Controlled Reversal of the Direction of Charge-Transfer

A ruthenium-bis-terpyridine complex [Ru(NPhtpy)(Ntpy)]2+ (22+) with two redox-asymmetric amine units has been prepared, where NPhtpy is 4′-(di-p-anisylaminophen-4-yl)-2,2′:6′,2″-terpyridine and Ntpy is 4′-(di-p-anisylamino)-2,2′:6′,2″-terpyridine. This complex displays two consecutive redox couples at +0.82 and +1.02 V vs Ag/AgCl, which are assigned to the N•+/0 processes of the amine components of the NPhtpy and Ntpy ligands, respectively. The mono-oxidized complex 23+ obtained by oxidative electrolysis shows the presence of the charge transfer from ruthenium(II) to the oxidized aminium radical cation of the NPhtpy ligand (MNNPhtpyCT) around 1000 nm. In the dioxidized form (24+), the MNNPhtpyCT transition decreased distinctly and an opposite charge transfer from ruthenium(II) to the oxidized aminium radical cation of the Ntpy ligand (MNNtpyCT) appeared at 1380 nm. Complexes [Ru(NPhtpy)(tpy)]2+ (tpy is 2,2′:6′,2″-terpyridine), [Ru(Ntpy)(tpy)]2+, and [Ru(NPhtpy)2]2+ have been prepared and studied for the purpose of comparison. TDDFT calculations show that the involvement of the intraligand charge transfer from both NPhtpy and Ntpy ligands is responsible for the enhancement of the visible absorptions of these complexes with respect to [Ru(tpy)2]2+. DFT and TDDFT calculations have been performed on 23+ and 24+ to provide information on the spin distributions and the nature of the near-infrared absorptions. Complex 23+ shows an isotropic EPR signal at room temperature, consistent with an unpaired electron localized on the nitrogen atom.

Synthesis, characterization, spectroscopic and electrochemical studies of donor–acceptor ruthenium(II) polypyridine ligand derivatives with potential NLO applications

In this work we report the preparation of six new donor–metal–acceptor (D–M–A) type complexes of ruthenium(II) with the highly absorbing “chromophoric” ligand 4,4′-bis(2-(4-methoxyphenyl)styryl)-2,2′-bipyridine, (L-OCH3, donor moiety) and substituted polypyridinic ligands with electron acceptor character (NN-A). The NN-A studied ligands were pyrazino[2,3-f][1,10]phenanthroline (ppl), 11-R-dipyrido[2,3-a:2′,3′-c]phenazine (dppz-R; R is H, NO2, or CN) and 10,11-[1,4-naphtalenedione]dipyrido[3,2-a:2′,3′-c]phenazine (Aqphen). The complexes were characterized by IR, NMR, UV–Vis spectroscopy and cyclic voltammetry. The potential NLO response of the complexes was evaluated by solvatochromic studies. Although the communication between D and A exists, the effect of the change of the acceptor moiety on the properties of the complexes is small and the behavior of the complexes is governed mainly by the donor ligand. The Metal to Ligand Charge Transfer bands (MLCT) exhibited by all complexes in the visible region have dominant electronic density transfer character from the metal to the chromophoric L-OCH3 ligand. The hypsochromic shift of this low energy absorption band on going from a less polar (benzene) to a more polar solvent (acetonitrile) indicated that a redistribution of the electronic density among the metal and the donor ligand is observed. This behavior permits to predict a NLO response for these types of complexes. The combination of high molar absorptivity with intraligand charge transfer (ILCT) mixing into the MLCT bands are encouraging for the generation of new materials with interesting NLO properties.

Pt(II) bipyridyl complexes bearing substituted fluorenyl motif on the bipyridyl and acetylide ligands: synthesis, photophysics, and reverse saturable absorption.

A series of Pt(II) diimine complexes bearing benzothiazolylfluorenyl (BTZ-F8), diphenylaminofluorenyl (NPh2-F8), or naphthalimidylfluorenyl (NI-F8) motifs on the bipyridyl or acetylide ligands (Pt-4-Pt-8), (i.e., {4,4'-bis[7-R1-F8-(≡)n-]bpy}Pt(7-R2-F8- ≡ -)2, where F8 = 9,9'-di(2-ethylhexyl)fluorene, bpy = 2,2'-bipyridine, Pt-4: R1 = R2 = BTZ, n = 0; Pt-5: R1 = BTZ, R2 = NI, n = 0; Pt-6: R1 = R2 = BTZ, n = 1; Pt-7: R1 = BTZ, R2 = NPh2, n = 1; Pt-8: R1 = NPh2, R2 = BTZ, n = 1) were synthesized. Their ground-state and excited-state properties and reverse saturable absorption performances were systematically investigated. The influence of these motifs on the photophysics of the complexes was investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The intense absorption bands below 410 nm for these complexes is assigned to predominantly (1)π,π* transitions localized on either the bipyridine or the acetylide ligands; while the broad low-energy absorption bands between 420 and 575 nm are attributed to essentially (1)MLCT (metal-to-ligand charge transfer)/(1)LLCT (ligand-to-ligand charge transfer) transitions, likely mixed with some (1)ILCT (intraligand charge transfer) transition for Pt-4-Pt-7, and predominantly (1)ILCT transition admixing with minor (1)MLCT/(1)LLCT characters for Pt-8. The different substituents on the acetylide and bipyridyl ligands, and the degrees of π-conjugation in the bipyridyl ligand influence both the (1)π,π* and charge transfer transitions pronouncedly. All complexes are emissive at room temperature. Upon excitation at their respective absorption band maxima, Pt-4, Pt-6, and Pt-8 exhibit acetylide ligand localized (1)π,π* fluorescence and (3)MLCT/(3)LLCT phosphorescence in CH2Cl2, while Pt-5 manifests (1)ILCT fluorescence and (3)ILCT phosphorescence. However, only (1)LLCT fluorescence was observed for Pt-7 at room temperature. The nanosecond transient absorption study was carried out for Pt-4-Pt-8 in CH3CN. Except for Pt-7 that contains NPh2 at the acetylide ligands, Pt-4-Pt-6 and Pt-8 all exhibit weak to moderate excited-state absorption in the visible spectral region. Reverse saturable absorption (RSA) of these complexes was demonstrated at 532 nm using 4.1 ns laser pulses in a 2 mm cuvette. The strength of RSA follows this trend: Pt-4 > Pt-5 > Pt-7 > Pt-6 > Pt-8. Incorporation of electron-donating substituent NPh2 on the bipyridyl ligand significantly decreases the RSA, while shorter π-conjugation in the bipyridyl ligand increases the RSA. Therefore, the substituent at either the acetylide ligands or the bipyridyl ligand could affect the singlet and triplet excited-state characteristics significantly, which strongly influences the RSA efficiency.

Selective Sensitization of Eu(III) and Tb(III) Emission with Triarylboron-Functionalized Dipicolinic Acids

Three triarylboron- (Mes2BAr-) functionalized dipicolinic acids, namely, 4-(4-(dimesitylboranyl)-2,3,5,6-tetramethylphenyl)pyridine-2,6-dicarboxylic acid (H2L1), 4-(4-(4-dimesitylboranyl)-2,3,5,6-tetramethylphenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2L2), and 4-(4-(4-dimesitylboranyl) phenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2L3), have been designed and synthesized. Lanthanide(III) complexes with the general formula of [NBu4]3[LnL3] (Ln = Eu or Tb; L = L1, L2, or L3) were obtained. The new triarylboron-functionalized ligands were found to be effective in the selective sensitization of the emissions of Eu(III) and Tb(III) ions with a high quantum efficiency (e.g., 0.54 for [NBu4]3[TbL13] in the solid state) upon excitation at ∼330 nm. An intraligand charge-transfer (ILCT) transition from the mesityl or aryl group to the boron or boron-aryl unit was found to play a key role in the activation of the Eu(III) and Tb(III) emissions, based on TD-DFT computational data and luminescence titration experiments performed using fluoride and cyanide ions.

Host-guest interaction dictated selective adsorption and fluorescence quenching of a luminescent lightweight metal-organic framework toward liquid explosives.

In this article, we report the successful preparation of a Mg-based luminescent MIL-53 metal-organic framework (MOF), namely [Mg2(BDC)2(BPNO)]·2DMF (1) (BDC = 1,4-benzene dicarboxylate, BPNO = 4,4'-dipyridyl-N,N'-dioxide, DMF = N,N-dimethylformamide) in a mixed solvent containing a 2 : 3 volume ratio of DMF and ethanol (EtOH) under solvothermal conditions. Desolvated compound 1a can be used as an absorbent for selective adsorption and separation of liquid explosives, including nitroaromatic (nitrobenzene (NB)) and nitroaliphatic (nitromethane (NM) and nitroethane (NE)) compounds, through single crystal-to-single crystal (SC-SC) transformations. As one of the weakly luminescent MOFs, the luminescence of compound 1a could be quenched by the incorporation of the three liquid nitro explosives. On the basis of single crystal analysis, we provide direct evidence that both the selective adsorption and fluorescence quenching of the desolvated compound 1a are dictated by host-guest interactions between guest liquid explosives and the host framework. Such findings differ from those reported in previous works, which were dominated by surficial close contact interactions. Moreover, based on the experimentally obtained single-crystal structures, we explain that the luminescence of 1a follows the intraligand π*→π emission states or weak ligand to ligand charge transfer (LLCT), with little incorporation of intraligand charge transfer (ILCT).

Bis-Tridentate Ruthenium Complexes with a Redox-Active Amine Substituent: Electrochemical, Spectroscopic, and DFT/TDDFT Studies

Seven bis-tridentate ruthenium complexes with a general formula of [Ru(N-L1)(L2)]2+ (the anions are PF6–) have been prepared, where N-L1 is a 2,2′:6′,2″-terpyridine (tpy) derivative with a redox-active di-p-anisylamino (MeO-L1), di-p-tolylamino (Me-L1), or bis(p-chlorophenyl)amino (Cl-L1) substituent and L2 is tpy, bis(N-methylbenzimidazolyl)pyridine (Mebip), or trimethyl 2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylate (Me3tctpy). The electrochemical results suggest that the amine segment of [Ru(MeO-L1)(tpy)]2+, [Ru(MeO-L1)(Mebip)]2+, [Ru(MeO-L1)(Me3tctpy)]2+, and [Ru(Cl-L1)(Me3tctpy)]2+ is oxidized prior to the RuIII/II process, and the one-electron-oxidized forms of these complexes show intense RuII → N•+ metal-to-ligand charge transfer (CT) transitions around 1200–1500 nm. In contrast, the ruthenium ion is more easily oxidized than the amine segment in [Ru(Cl-L1)(Mebip)]2+ and only weak ligand-to-metal CT transitions are observed after one-electron oxidation. The one-electron-oxidation processes of [Ru(Me-L1)(tpy)]2+ and [Ru(Cl-L1)(tpy)]2+ are possibly associated with both the amine segment and ruthenium component. The substitution of the amine group expands the visible absorptions of these complexes, as a result of the presence of additional intraligand CT transitions. The complex [Ru(Cl-L1)(Me3tctpy)]2+ with multiple electron-withdrawing substituents emits at 732 nm with a quantum yield of 1.3%. DFT calculations have been performed to provide information on the frontier orbital energy alignment and spin distributions of one-electron-oxidized forms. The absorption spectra have been rationalized with the aid of TDDFT calculations.