Charge-transfer Absorption Band Research Articles

Synthesis and Properties of two CuI Complexes Involving Tetrathia-fulvalene-Fused Phenanthroline Ligand.

Two CuI complexes based on the π-conjugated tetrathiafulvalene-annulated phenanthroline ligands (TTF-Phen, L1 and L2), [CuI(Xantphos)(L1)]BF4 (1, Xantphos = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene) and [CuI(Binap)(L2)]BF4 (2, Binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), have been synthesized. They have been fully characterized, and their photophysical and electrochemical properties are reported together with those of L1 and L2 for comparison. Both CuI complexes show metal-to-ligand charge transfer (MLCT) absorption bands, whereas the 3MLCT luminescence is strongly quenched.

Pt(II) diimine complexes bearing difluoro-boron-dipyrromethene acetylide ligands: Synthesis, photophysics, aggregation included emission and optical power limiting properties

A series of Pt(II) complexes bearing difluoro-boron-dipyrromethene (Bodipy) acetylide ligands and different alkyl/aryl substituted 2,2′-bipyridyl ligands (Pt-1 – Pt-3) were synthesized and characterized. Their photophysics, aggregation included emission (AIE) and optical power limiting properties were systematically investigated via UV−Vis absorption, emission, transient absorption, and nonlinear transmission spectroscopy/technique. These complexes exhibit 1π-π* transitions/metal-to-ligand charge transfer (1MLCT) absorption bands in UV-Vis spectral region and green fluorescence assigned to 1π-π* state mixed with 1MLCT character. All complexes exhibit obvious AIE in CH3CN-water solution system. When water fraction increased, nanoparticles of Pt(II) complexes were formed owing to the aggregation, and the emission intensity and triplet excited state lifetime both increased. In addition, complexes Pt-1 – Pt-3 all exhibit triplet transient absorption (TA) in visible region, where reverse saturable absorption (RSA) could occur. The strength of RSA at 532 nm follows the trend: Pt-2 > Pt-3 > Pt-1. The significant AIE, TA and OPL properties of these complexes would be useful for rational design of transition-metal complexes with high emission quantum yield, broadband excited-state absorption, long excited-state lifetime, and strong nonlinear absorption for multi-functional applications.

Photosubstitution of Monodentate Ligands from RuII–Dicarboxybipyridine Complexes

We report the photophysical and photochemical properties of RuII–polypyridine complexes [Ru(bpy)(dcbpy)py2]2+ (1)2+ and [Ru(dcbpy)2py2]2+ (2)2+ (bpy = 2,2′‐bipyridine, dcbpy = 4,4′‐dicarboxy‐2,2′‐bipyridine, py = pyridine). These complexes combine a monodentate ligand with a chelate bipyridine substituted with carboxylate groups. At low pH, both complexes present metal‐to‐ligand charge transfer absorption bands in the visible region and room‐temperature photoluminescence with long excited‐state lifetimes (τ > 200 ns). At physiological pH, their absorption and emission maxima are displaced to higher energies, with a significant reduction of their emission lifetime. These species show photosubstitution of the monodentate pyridine upon irradiation at 450 nm. At low pH, the quantum yield for this process is very low, but at physiological pH, they are very active, with a φPS,450 value of 0.14 for (1)2+ and of 0.17 for (2)2+. The products of photosubstitution were identified as monoaquo complexes. Both the reactants and the products of the photosubstitution show photoluminescence, but with very different lifetimes, making it possible to monitor the reaction by the time constant of their decay. The ability of complexes (1)2+ and (2)2+ to photorelease monodentate ligands at physiological pH makes them attractive candidates for the delivery of biomolecules linked to more complex structures through the carboxylate functional group.

A Diarylethene Derived Chemosensor for Colorimetric Sensing of Cu2+

A diarylethene bearing a 8-hydroxyquinoline-linked salicylhydrazide Schift base unit has been synthesized. In CH3CN, the conpound displayed a highly selective and sensitive response to Cu2+ via perceptible color and UV–vis absorbance changes among the other tested metal ions. In the presence of Cu2+, the peak at 341 nm disappeared and concomitantly a new charge transfer absorption band emerged at 381 and 450 nm. Moreover, the compound formed host-guest complexe in 1:1 stoichiometry.

Observation of Supramolecular Chirality in a Hierarchically Self-Assembled Mixed-Stack Charge-Transfer Complex.

Supramolecular chirality was observed in a charge-transfer (CT) complex formed by self-assembly of β-cyclodextrin-linked pyrene and adamantane-linked pyromellitic diimide in water. In the complex, the pyrene-pyromellitic diimide CT interaction is reinforced several-fold by the strong β-cyclodextrin-adamantane inclusion binding interaction, as confirmed by 1 H NMR, absorption, fluorescence and isothermal titration calorimetry studies. The CT complex exhibited high stability, as inferred from the temperature-dependent absorption and NMR studies. The CT absorption band exhibited a positive Cotton effect indicative of supramolecular chirality for the complex. Twisted fibres were observed in TEM, AFM, and SEM experiments. A mechanism involving synergistic binding interactions leading to 2D sheets, followed by twisting of the sheet, is proposed for the formation of twisted fibres. This system offers a chance to investigate a combination of supramolecular CT and supramolecular chirality and explore their synergistic effects.

Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads.

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)3Mo2(μ-O2CC6H4N(C6H4CH3)2] (DAniF = N,N'-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)3Mo2(μ-OSCC6H4N(C6H4CH3)2] ([OS-ph-N]), and [(DAniF)3Mo2(μ-S2CC6H4N(C6H4CH3)2] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ΔE1/2(E1/2(2)(N/N•+) - E1/2(1)(Mo2IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]+, [OS-ph-N]+, and [SS-ph-N]+, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo2) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (Hab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm-1 in CH2Cl2. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo2) charge transfer.

Visible to near-infrared MSL/Mastcam multispectral imaging: Initial results from select high-interest science targets within Gale Crater, Mars

The Mastcam CCD cameras on the Mars Science Laboratory Curiosity Rover each use an 8-position filter wheel in acquiring up to 1600 × 1200 pixel images. The filter set includes a broadband near-infrared cutoff filter for RGB Bayer imaging on each camera and 12 narrow-band geology filters distributed between the two cameras, spanning the wavelength range 445–1013 nm. This wavelength region includes the relatively broad charge-transfer and crystal-field absorption bands that are most commonly due to the presence of iron-bearing minerals. To identify such spectral features, sequences of images taken with identical pointings through different filters have been calibrated to relative reflectance using pre-flight calibration coefficients and in-flight measurements of an onboard calibration target. Within the first 1000 sols of the mission, Mastcam observed a spectrally diverse set of materials displaying absorption features consistent with the presence of iron-bearing silicate, iron oxide, and iron sulfate minerals. Dust-coated surfaces as well as soils possess a strong positive reflectance slope in the visible, consistent with the presence of nanophase iron oxides, which have long been considered the dominant visible-wavelength pigmenting agent in weathered martian surface materials. Fresh surfaces, such as tailings produced by the drill tool and the interiors of rocks broken by the rover wheels, are grayer in visible wavelengths than their reddish, dust-coated surfaces but possess reflectance spectra that vary considerably between sites. To understand the mineralogical basis of observed Mastcam reflectance spectra, we focus on a subset of the multispectral data set for which additional constraints on the composition of surface materials are available from other rover instruments, with an emphasis on sample sites for which detailed mineralogy is provided by the results of CheMin X-ray diffraction analyses. We also discuss the results of coordinated observations with the ChemCam instrument, whose passive mode of operation is capable of acquiring reflectance spectra over wavelengths that considerably overlap the range spanned by the Mastcam filter set (Johnson et al. 2016). Materials that show a distinct 430 nm band in ChemCam data also are observed to have a strong near-infrared absorption band in Mastcam spectral data, consistent with the presence of a ferric sulfate mineral. Long-distance Mastcam observations targeted toward the flanks of the Gale crater central mound are in agreement with both ChemCam spectra and orbital results, and in particular exhibit the spectral features of a crystalline hematite layer identified in MRO/CRISM data. Variations observed in Mastcam multi-filter images acquired to date have shown that multispectral observations can discriminate between compositionally different materials within Gale Crater and are in qualitative agreement with mineralogies from measured samples and orbital data.

Theoretical study on the molecular structure, intermolecular interaction and spectral features of 2-aminopyridine/ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone complex

Density function theory (DFT) was performed to study the structures and binding energies of the 2-aminopyridine(2-AP)/2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) complex. Four distinct stable conformers (denoted as S1–S4) were located and the bonding characteristics of these conformers were investigated with NBO and AIM analysis. It was revealed that S1 is formed by the edge(2-AP)-to-face(DDQ) linkage, which is stabilized by a moderate $${\upsigma }$$ – $${\uppi }$$ interaction and hydrogen binding interaction. S2 and S3 are formed by the face(2-AP)-to-face(DDQ) linkage through $${\upsigma }$$ – $${\uppi }$$ and $${\uppi }$$ – $${\uppi }$$ interactions, and S4 is constructed by the edge(2-AP)-to-edge(DDQ) linkage through halogen bonding and hydrogen bonding interactions. The electronic excitation energies of the two stable conformers, S2 and S3, were calculated with time-dependent DFT (TDDFT), which revealed that the 2-AP $$({\uppi })$$ $$\rightarrow $$ DDQ( $${\uppi }$$ *) charge transfer transitions in conformers S2 and S3 contribute to the two new charge transfer absorption bands which were experimentally observed at 598 nm and 557 nm for the 2-AP/DDQ complex in chloroform. Four probable structures of the 2-AP/DDQ complex were located by DFT calculation. The bonding characteristics between the two molecules of the complex were investigated by NBO and AIM analysis. Two new absorption bands observed in the UV–Vis absorption spectrum of the complex were attributed according to the TDDFT results.

Synthesis and photophysical properties of cyclometalated 4′-phenyl-2,2′:6′,2″-terpyridyl Pt(II) chloride complexes with different aryl substituents

A series of 4′-phenyl-2,2′:6′,2″-terpyridyl Pt(II) chloride complexes bearing different aryl substituents (1a–1f) was designed and synthesized. The photophysical properties of these complexes have been systematically investigated. All complexes exhibit strong 1π,π* absorption bands in the UV region; and broad, structureless metal-to-ligand charge transfer (1MLCT) absorption bands in the visible region. When excited at the charge-transfer absorption band, the complexes exhibit room temperature luminescence (λem=555–614nm) in CH3CN. The emissions of complexes 1c–1f with electron-donating or extended π-conjugation substituents are assigned to the 3MLCT states. Meanwhile, the emitting state of 1a and 1b exhibits a significant intraligand 3π,π* character. Complexes 1a–1d exhibit moderate triplet transient absorptions from visible to NIR region, where reverse saturable absorption (RSA) occurs. Their photophysical properties have been investigated with the aim to provide a basis for elucidating the structure-property correlations and developing new nonlinear optical materials.

Four photoelectroactive composite films based on [BW11O39M(H2O)]7− (M = Zn, Cu) and bipolar hemicyanines

Four composite films consisting of Zn(II) and Cu(II) substituted tungstoborates K7[BW11O39M(H2O)] (BW11M, M=Zn, Cu) and two bipolar hemicyanines of (E)-1,1′-(propane-1,3-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H3Br2) and (E)-1,1′-(decane-1,10-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H10Br2), were prepared by electrostatic layer-by-layer self-assembly technique. These films of (BW11M/Ha)n (M=Zn, Cu; a=3, 10) exhibited ultraviolet-visible characteristic absorption bands of BW11M and HaBr2 with their absorption intensities increasing linearly with increasing the layer number n, indicating that the composite films were successfully and uniformly deposited. With respect to the H3Br2 and H10Br2 aqueous solutions, the intramolecular charge transfer absorption bands of H3Br2 and H10Br2 in the films exhibited bathochromic shifts to different extents depending on the hemicyanines and BW11M in the films. These films could generate stable, cathodic photocurrents with photocurrent action spectra being consistent with corresponding absorption spectra of the hemicyanines in the films, when illuminated with 100mW/cm2 polychromatic light (730nm>λ>325nm). The effects of the alkyl chain length, BW11M, layer number of the (BW11M/Ha)n films, and applied bias potential and electron acceptor in the electrolyte solution on the photocurrent densities are discussed.

Dinuclear PhotoCORMs: Dioxygen-Assisted Carbon Monoxide Uncaging from Long-Wavelength-Absorbing Metal–Metal-Bonded Carbonyl Complexes

We describe a new strategy for triggering the photochemical release of caged carbon monoxide (CO) in aerobic media using long-wavelength visible and near-infrared (NIR) light. The dinuclear rhenium-manganese carbonyl complexes (CO)5ReMn(CO)3(L), where L = phenanthroline (1), bipyridine (2), biquinoline (3), or phenanthrolinecarboxaldehyde (4), each show a strong metal-metal-bond-to-ligand (σMM → πL*) charge-transfer absorption band at longer wavelengths. Photolysis with deep-red (1 and 2) or NIR (3 and 4) light leads to homolytic cleavage of the Re-Mn bonds to give mononuclear metal radicals. In the absence of trapping agents, these radicals primarily recombine to reform dinuclear complexes. In oxygenated media, however, the radicals react with dioxygen to form species much more labile toward CO release via secondary thermal and/or photochemical reactions. Conjugation of 4, with an amine-terminated poly(ethylene glycol) oligomer, gives a water-soluble derivative with similar photochemistry. In this context, we discuss the potential applications of these dinuclear complexes as visible/NIR-light-photoactivated CO-releasing moieties (photoCORMs).

Toward Broadband Reverse Saturable Absorption: Investigating the Impact of Cyclometalating Ligand π-Conjugation on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium Complexes

The synthesis, photophysics and reverse saturable absorption of a series of bis-cyclometalated Ir(III) complexes Ir(C∧N)2L·PF6, where L = 3,8-bis[9,9-di(2-ethylhexyl)-9H-fluoren-2-yl]-1,10-phenanthroline and C∧N = 2-phenylpyridine (ppy, 1), 2-phenylquinoline (pqu, 2), 1-phenylisoquinoline (piq, 3), 2-phenylbenzo[g]quinoline (pbq, 4), and 2,3-diphenylbenzo[g]quinoxaline (dpbq, 5), are reported. By gradually increasing the π-conjugation along the pyridine or pyrazine ring of the C∧N ligands, the energies of the lowest singlet (S1) and triplet (T1) excited states are significantly reduced, as reflected by the pronouncedly red-shifted charge transfer absorption bands at >450 nm and the emission band(s) in their UV–vis absorption and emission spectra, respectively. Additionally, our density functional theory (DFT) calculations confirm that the natures of the S1 and T1 states vary with the increased π-conjugation of the C^N ligands, with the S1 state changing from the exclusive 1LLCT (ligand-to-ligand charge tr...

Absorption Tails of Donor:C60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation.

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D-A complexes occur at photon energies below the optical gaps of both the donors and the C60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

A study of asymmetrical mixed-valent Mo2-Mo2 complexes in the class III regime.

Three novel asymmetrical dimolybdenum dimers, [Mo2(DAniF)3]2(μ-OOCCOS) (DAniF = N,N'-di(p-anisyl)formamidinate) ([OO-OS]), [Mo2(DAniF)3]2(μ-S2CCO2) ([SS-OO]), and [Mo2(DAniF)3]2(μ-SSCCOS) ([SS-OS]), have been synthesized and characterized by either single-crystal X-ray crystallography or 1H NMR spectroscopy. The structural asymmetry for these compounds gives rise to a redox asymmetry, which enlarges the potential separation (ΔE1/2) between the two [Mo2] units. The mixed-valance (MV) species [OO-OS]+, [SS-OO]+ and [SS-OS]+, prepared by one-electron chemical oxidation of the neutral precursors, exhibit an intense and symmetrical intervalence charge transfer (IVCT) absorption band in the near-IR region, along with the high energy metal (δ) to ligand (π*) (ML) and ligand (π) to metal (δ) charge transfer (LMCT) absorptions. The LMCT band, which is absent in the neutral precursors, is reflective of the cationic [Mo2]+ unit in the MV species; therefore, it is evidenced that in the MV complexes optical electron transfer from the electron donor to acceptor occurs, while the thermal process is energetically unfavorable. The C(1)-C(2) bonds (1.44-1.48 Å) that connect the two [Mo2] units are significantly shorter than a C-C single bond, showing that the two Mo2 centers are strongly coupled. For the series, TD-DFT calculations show that the molecular orbitals have an unsymmetrical charge density distribution over the two dimolybdenum sites. For each of the complex systems, the calculated orbital energy gaps, SOMO(δ - δ)-LUMO(bridging ligand π*), HOMO-8(bridging ligand π)-SOMO(δ - δ) and SOMO(δ - δ)-HOMO-1(δ + δ), are in good agreement with the observed MLCT, LMCT and IVCT absorption band energies, respectively. The consistency in energy between the IVCT band and the SOMO(δ - δ)-HOMO-1(δ + δ) gap permits assignment of the MV complexes to Class III in the Robin-Day scheme.

Optically probing the localized to delocalized transition in Mo2-Mo2 mixed-valence systems.

Four thienylene (C4H2S) bridged Mo2 dimers, [Mo2(DAniF)3]2(μ-OOCC4H2SCOO) (DAniF = N,N'-di(p-anisyl)formamidinate), [Mo2(DAniF)3]2(μ-N(H)SCC4H2SCN(H)S), [Mo2(DAniF)3]2(μ-OSCC4H2SCSO) and [Mo2(DAniF)3]2(μ-SSCC4H2SCSS), have been synthesized and studied in terms of electronic coupling. The subtle structural differences between these compounds vary largely the extent of electron delocalization; consequently, a systematic transition from Class II to Class III via Class II-III is achieved, which is probed using spectral parameters of intervalence charge transfer (IVCT) absorption (band energy, intensity and shape) for the mixed-valence complexes. Significantly, the species in Class II-III displays a low energy, half cut-off and solvent-dependent IVCT band, while a high energy, less asymmetrical IVCT band is observed for the complex in Class III. These results give fresh and detailed understanding of the system transition.

Excited States of Triphenylamine-Substituted 2-Pyridyl-1,2,3-triazole Complexes.

A new 2-pyridyl-1,2,3-triazole (pytri) ligand, TPA-pytri, substituted with a triphenylamine (TPA) donor group on the 5 position of the pyridyl unit was synthesized and characterized. Dichloroplatinum(II), bis(phenylacetylide)platinum(II), bromotricarbonylrhenium(I), and bis(bipyridyl)ruthenium(II) complexes of this ligand were synthesized and compared to complexes of pytri ligands without the TPA substituent. The complexes of unsubstituted pytri ligands show metal-to-ligand charge-transfer (MLCT) absorption bands involving the pytri ligand in the near-UV region. These transitions are complemented by intraligand charge-transfer (ILCT) bands in the TPA-pytri complexes, resulting in greatly improved visible absorption (λmax = 421 nm and ϵ = 19800 M-1 cm-1 for [Pt(TPA-pytri)Cl2]). The resonance Raman enhancement patterns allow for assignment of these absorption bands. The [Re(TPA-pytri)(CO)3Br] and [Pt(TPA-pytri)(CCPh)2] complexes were examined with time-resolved infrared spectroscopy. Shifts in the C≡C and C≡O stretching bands revealed that the complexes form states with increased electron density about their metal centers. [Pt(TPA-pytri)Cl2] is unusual in that it is emissive despite the presence of deactivating d-d states, which prevents emission from the unsubstituted pytri complex.

The influence of Pb-doping on the crystal structure and luminescence properties of LiEuMo2O8 red-emitting phosphors

High efficiency red phosphors, showing spectral range of 610–620nm and a narrow-line emission, are helpful for achieving high color rendering index (CRI) and low correlated color temperature (CCT) phosphor-converted white-light diodes. Red phosphors activated by trivalent europium (Eu3+) thus have attracted a renewed interest for pc-WLEDs. In our case, we prepare a series of Pb2+-doped LiEuMoO8 red-emitting phosphors by solid-state reactions. The introduction of Pb2+ ions expands and distorts slightly the unit cell volume of LiEuMo2O8 matrix, and red-shifts the charge transfer absorption (CTA) band of LiEuMo2O8, which are demonstrated by X-ray powder diffraction, absorption spectra and micro-Raman spectra. Photoluminescence and photoluminescence excitation spectra results show that the excitation efficiency of Eu3+ is improved significantly in the near ultraviolet region when Pb2+ ions are doped into the host lattice. At x=0.015, the red-emitting intensity of LiEuMo2O8 is enhanced by 50%, and the internal quantum yields is 53% under excitation at 395nm. The enhanced red emission is discussed in term of the relaxation of parity-forbidden selection rules for trivalent europium luminescent center, and the sensitization effect of Pb2+ as a sensitizer for Eu3+ ions is negligible in LiEuMo2O8 under near ultraviolet excitation.

Star-shaped D-π-A compounds with a 1,3,5-triazine core and N-aryl chromophore substituted fluorene arms: Synthesis, aggregation induced emission and two-photon absorption

Three star-shaped D-π-A compounds with a 1,3,5-triazine core and N-aryl chromophore (3,6-di-tert-butylcarbazole, phenoxazine and phenothiazine) substituted fluorene arms were designed and synthesized. Their photophysical properties were investigated systematically via spectroscopic and theoretical methods. All compounds exhibit strong 1π–π* transitions in the UV region and intense 1π–π*/intramolecular charge transfer (1ICT) absorption bands in the UV–vis region. Bathochromic shifts of the 1ICT bands were caused by the electron-donating N-aryl chromophores. All compounds exhibit aggregation induced emission properties in a solvent, and relatively high quantum efficiency in solid-states (Φ = 10%–20%). Moreover, these compounds show two-photon absorption (TPA) properties due to their good planarity and large π-conjugation. The largest TPA cross section value reaches up to 5000 GM at 800 nm. The multifunctional properties make these compounds potential candidates for application in organic light-emitting materials and two-photon bioimaging.

Direct Evidence of Chelated Geometry of Catechol on TiO2 by a Combined Solid-State NMR and DFT Study

Catechol on TiO2 is a model system for a class of molecules that bind and interact very strongly with metal oxides. This interaction gives rise to a marked charge-transfer absorption band that can be used to sensitize the complex to visible light. In solar cells, these are called type II sensitizers in contrast with type I sensitizers where an excitation of the molecule with subsequent charge injection is the main mechanism for placing an electron in the conduction band of the semiconductor. The adsorption geometry of these molecules is critical in their functioning. Nuclear magnetic resonance (NMR) spectroscopic methods can be used to elucidate structural information about the local geometry at the substrate–molecule interface. NMR methods coupled with density functional theory (DFT) allow for the detailed characterization of molecular binding modes. In the present work, we report a solid-state NMR and DFT study of catechol on TiO2. DFT-GIPAW chemical shift predictions for the 13C CP-MAS experiments unam...

Luminescence and scintillation properties of Mg-codoped LuAG:Pr single crystals annealed in air

The influence of the Mg2+ codoping and annealing in the air on the scintillation and luminescence characteristics of Pr-doped lutetium-aluminum garnet crystals (LuAG) was studied to find a possible positive effect of Pr4+. The overall scintillation efficiency under X-ray excitation of the annealed Pr- doped samples decreased with increasing Mg concentration. This was explained by increased overlap of the Pr3+ 5d-4f emission with the charge-transfer (CT) absorption band of the Pr4+ ion stabilized by Mg2+. This absorption caused even greater decrease of the light yield, as the light is collected from the whole sample volume in the pulse-height spectrum measurement. Electron centers based on oxygen vacancies (F+ centers) which can have negative influence on scintillation mechanism were revealed by the electron paramagnetic resonance (EPR) experiments. The O- hole center was detected as well and the temperature dependences of the EPR spectra showed that its thermal stability was decreased by Mg codoping. Negligible changes in the decay time of Pr3+ 5d-4f photoluminescence exclude the non-radiative energy transfer from relaxed 5d1 excited state of Pr3+ towards CT absorption transition of Pr4+ and suggest that simple reabsorption of the emitted light takes place. Unlike for the Ce-doped analogs, promoting the Pr3+ luminescence center to the tetravalent state by divalent ion co-doping and air annealing did not improve scintillation properties of Pr-doped LuAG.