Lowest Energy Absorption Band Research Articles

Effect of One- and Two-Electron Reduction of Terbium(III) Double-Decker Phthalocyanine on Single-Ion Magnet Behavior and NIR Absorption.

Reduction of terbium(III) double-decker phthalocyanine, TbIIIPc2 (1), by sodium fluorenone ketyl in the presence of bis(triphenylphosphoranylidene)ammonium cations yields one-electron-reduced (PPN+){TbIII(Pc2-)2}-·2.5C6H4Cl2 (2) containing the dianionic Pc2- macrocycles, whereas a stronger NaCpCo(CO)2 reductant in the presence of an excess of cryptand yields two-electron-reduced {Cryptand(Na+)}2{(Pc2-)TbIII(Pc•3-)}2- (3) containing the Pc•3- radical trianionic macrocycle. Isolated pairs of the {TbIII(Pc2-)2}- anions are formed in 2, whereas compound 3 has unique 3D packing of the macrocycles with weak π-orbitals overlapping in all three directions. This is the first example of the two-electron-reduced lanthanide double-decker containing Pc•3- radical trianion studied in solid state. Compound 2 manifests single-ion magnet (SIM) behavior with a large effective spin-reversal energy barrier of Ueff = 538 cm-1 in comparison with previously studied (Bu4N+){TbIII(Pc2-)2}- ( Ueff = 230 cm-1). Thus, changes in cation size and shape affect the molecular packing of {TbIII(Pc2-)2}- and increase the spin-reversal energy barrier. At the same time, two-electron-reduced species 3 containing TbIII and Pc•3- show no signs of SIM such as hysteresis loop at 1.9 K, and no peaks are observed on the temperature dependencies of in-phase (χ') and out-of-phase (χ″) signals. In contrast to EPR silent 2, both compounds 1 and 3 manifest broad signals from paramagnetic TbIII ions. Narrower signals attributed to Pc•3- are of high intensity only in 3. In addition to the absorption bands of Pc in the UV and visible spectral range, compound 3 manifests the lowest energy absorption band in solid-state spectra even in the near IR range at 4700 cm-1 (2130 nm), whereas such bands are not observed in the spectrum of 2. These data show that the reduction of the Pc macrocycles in the lanthanide double-deckers leads to the appearance of new very low-energy new transitions associated with Pc•3- whose energy is the lowest among known reduced metal phthalocyanines.

Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs.

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near-infrared regions of the spectrum due to the transfer of electronic excited-state energy into vibrations. We describe how this undesirable "energy gap law" can be sidestepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centers. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λmax = 610 nm, Φ = 0.85 in deoxygenated CH2Cl2 at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53 800 M-1 cm-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T1 → S0 phosphorescence process, allowing it to compete effectively with nonradiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.

Circular Dichroism and Isotropy - Polarity Reversal of Ellipticity in Molecular Films of 1,1'-Bi-2-Naphtol.

We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1'-Bi-2-naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.

Synthesis, characterization, thermal and luminescent properties of thiophenol-functionalized platinum(II) bis(acetylide) complexes

Two thiophenol-functionalized trans-platinum(II) bis(acetylide) complexes, having one thiophenol moiety in each alkenyl backbone with general formula trans-[(PPh3)2Pt{C≡C–Ar–CH=CH(SC6H5)}2], (2a–2b), (Ar = phenylene or 2,5-dimethylphenylene), were synthesized in good yields and good regioselectivities. Compared to the absorption bands of the trans-platinum(II) bis(acetylide) complexes 1, the positions of the lowest energy absorption bands were redshifted by 17–18 nm, after functionalization with thiophenol. For both of the platinum(II) complexes 2, the lowest energy absorption bands in chloroform solution at room temperature were observed in the range 376–379 nm, and under excitation at the absorption maximum both complexes showed emission maxima in the range of 408–419 nm. All the new Pt complexes have been characterized by physico-chemical and spectroscopic methods.

Benzoylguanidines as Anion-Responsive Systems.

A series of benzoylguanidinium salts was prepared and the changes in UV/Vis spectra, triggered by the presence of anions, were investigated. All compounds undergo deprotonation with basic anions like dihydrogenphosphate and acetate in acetonitrile. The most pronounced spectral changes were obtained by deprotonation of N1 -benzoyl-N3 -(p-nitrophenyl) guanidinium chloride which shows the naked-eye visible color change from colorless to yellow. Measured pKa (BH+ ) in acetonitrile ranges from 12-16, which is comparable to the pyridinium cations. The proton transfer equilibria were also tested in acetonitrile/water mixture where all but the most acidic derivatives showed pKa (BH+ ) of 4-6 units which corresponds to apparent association constants of 104 -106 dm3 mol-1 . UV/Vis spectra of neutral and protonated forms were modelled by the TD-DFT approach using CAM-B3LYP and PBE0 functionals and compared to CC2 results. In the case of CAM-B3LYP, a parameter ω, defining amount of long-range exchange correction, was varied to achieve the best agreement with the experimental spectra. The optimized ω parameters are 0.10 a0 -1 for neutral benzoylguanidines and 0.20 a0 -1 for neutral nitrobenzoyl and protonated systems. The larger ω parameter in the latter is ascribed to more pronounced charge transfer character of the HOMO-LUMO transition - the one responsible for the lowest energy absorption band.

Visible Light-Activated CO Release and 1O2 Photosensitizer Formation with Ru(II),Mn(I) Complexes.

Two diimine-bridged Ru(II),Mn(I) complexes with a [(bpy)2Ru(BL)Mn(CO)3Br]2+ architecture, where bpy = 2,2'-bipyridine and BL = 2,3-bis(2-pyridyl)pyrazine (dpp; Ru(dpp)Mn) or 2,2'-bipyrimidine (bpm; Ru(bpm)Mn), were designed to both dissociate multiple equivalents of CO and produce 1O2 when irradiated with visible light. Analysis of the complexes by Fourier transform infrared (FTIR) spectroscopy and cyclic voltammetry suggest a stronger π-accepting ability for bpm compared to that of dpp. Both complexes absorb light throughout the UV and visible regions with lowest energy absorption bands comprising overlapping Ru(dπ)→BL(π*) and Mn(dπ)→BL(π*) singlet metal-to-ligand charge transfer (1MLCT) and Br(p)→dpp(π*) singlet halide-to-ligand charge transfer (1XLCT) transitions. This lowest energy band is centered at 510 nm (ε = 12 000 M-1cm-1) for Ru(dpp)Mn and 553 nm (ε = 3240 M-1cm-1) for Ru(bpm)Mn, and the absorption band extends to nearly 700 nm in each case. Irradiation with visible light (both 470 and 627 nm) releases all three CO ligands, as observed by a combination of UV-vis, FTIR, and gas chromatography. The exchange of the first CO ligand with a solvent molecule occurs more efficiently for Ru(dpp)Mn (Φ470 = 0.22 ± 0.03 in H2O; 0.37 ± 0.06 in CH3CN) than for Ru(bpm)Mn (Φ470 = 0.049 ± 0.008 in H2O and 0.16 ± 0.03 in CH3CN), and the CO dissociation efficiency is unaffected by irradiation wavelength. The differences between Ru(dpp)Mn and Ru(bpm)Mn are proposed to result from the variation in electron density distribution across each formally reduced BL in the Mn(dπ)→BL(π*) 1MLCT excited state based on the nature of BL. Exhaustive photolysis causes the decomplexation of oxidized Mn(II), and the resulting [(bpy)2Ru(BL)]2+ complexes produce 1O2 with quantum yields (ΦΔ) of 0.37 ± 0.03 and 0.16 ± 0.01 for Ru(dpp) and Ru(bpm), respectively, with 460 nm irradiation. This bimetallic architecture presents the opportunity to use visible light to codeliver both CO and 1O2, both of which have biological relevance in photoactivated therapeutics, with spatiotemporal control.

A series of oxyfluoride chains containing asymmetric basic building units of both early- and late-transition metal cations

Four new transition metal oxyfluorides revealing chain structures composed of asymmetric basic building units of [MO2F4]2− (M = Mo and W) and [M’(pz/mpz)4]2+ (M′ = Cu and Ni) polyhedra, i.e., [Cu(mpz)4][MoO2F4] (1), [Cu(pz)4]2[WO2F4]2 (2), [Ni(pz)4]2[MoO2F4]2 (3), and [Ni(pz)4][WO2F4] (4) (pz = pyrazole; mpz = 3-methyl pyrazole) have been synthesized via hydrothermal reactions. The linear chain structures found from the title compounds are attributable to the trans-directing [MO2F4]2- octahedral moiety and hydrogen bonding interactions. Strong Jahn–Teller distortions are observed for Cu2+ cations with d9 configuration in compounds 1 and 2. All the compounds show both lower-energy and higher-energy absorption band gaps, which originate from the d–d transitions and the distortion of octahedral geometry of M2+ cations, respectively. The magnetism of [Cu(mpz)4][MoO2F4] turns out to be described by noninteracting Cu2+ spins rather than a spin chain due to long exchange path mediated via the nonmagnetic Mo6+ ions. The title compounds have been also thoroughly characterized using spectroscopic and thermal analyses along with calculations of local dipole moments and the extent of out-of-center distortions.

Spectroscopic and Electrochemical Properties of Electronically Modified Cycloplatinated Formazanate Complexes.

Formazanates are a class of monoanionic, chromophoric, redox-active chelating ligands, and until recently, the coordination chemistry of formazanates with third-row transition metals was unexplored. Described here is a series of heteroleptic cyclometalated platinum formazanate complexes of the type Pt(C^N)(Fz), where "C^N" is the cyclometalating ligand and "Fz" is the formazanate. This work includes a set of 14 complexes derived from four different cyclometalating ligands and five different formazans, greatly expanding the structural and electronic diversity of this class of complexes. The formazans fall into two major categories: triarylformazans with either electron-donating (Me, OMe) or electron-withdrawing (CN) para substituents, and even more electron-deficient diarylformazans where the center position of the backbone is a cyano group. Seven of the new compounds are characterized by single-crystal X-ray diffraction, and all are characterized by cyclic voltammetry and UV-vis absorption spectroscopy to provide insight into the frontier orbitals. The complexes can be electrochemically reduced by three electrons, with the first two electrons populating formazanate-centered frontier orbitals and the third being added to the Pt-C^N fragment. The formazanate-based reduction potentials are highly sensitive to the substitution pattern of the formazanate and can be tuned over a wide range of ca. 0.8 V by altering substituents. The formazanates also impart these complexes with very strong, low-energy visible absorption bands assigned to HOMO → LUMO transitions, which are greatly perturbed from the free ligand due to interactions of the formazanate frontier orbitals with platinum-centered orbitals. Unlike the redox properties, the formazanate-derived visible absorption band is relatively insensitive to substituent effects. Additional visible absorption bands, attributed to Pt(d) → C^N(π*) MLCT transitions, can be tuned over a wide range by changing the structure of the C^N ligand, giving rise to strong panchromatic absorption for some members of the series.

Excitation energy transfer processes in BODIPY based donor-acceptor system - Synthesis, photophysics, NLO and DFT study

Excited state intramolecular proton transfer (ESIPT) meso flanked BODIPY dyad are synthesized and investigated by using spectroscopic, fluorescence lifetime decay, electrochemical and DFT method. Excitation (λexc = 290 nm) of the Bn-OH-BDY dyad leads to emission (λemi = 545 nm) totally directed by the BODIPY subunits pointing to excitation energy transfer (EET) from the ESIPT to BODIPY core. A large shift of 255 nm has been observed between excitation and emitted light. Attachment of ESIPT unit does not cause any perturbation in the low-energy absorption band of the Bn-OH-BDY dyad. The electrochemical study shows an irreversible peak of single-electron oxidation of ESIPT core at 0.38 eV and reduction of BODIPY core at 1.023 eV. DFT calculation studies show the existence of charge transfer (CT) state from ESIPT to BODIPY core. The results obtained from photophysics, electrochemical study and DFT support EET from excited ESIPT core to BODIPY subunits. The DFT results show dominating electronic contribution to the linear polarizability (α) and first hyperpolarizability (β). NLO response was measured using Z scan technique. The values obtained for nonlinear absorption coefficient (β) in DMSO is −5.95 × 10−12 (m/W) and for third-order susceptibility (χ)3 it is 14.19 × 10−13 (e.s.u). The geometries of the dyad were optimized by using B3LYP/6–311++ G (d,p) level of theory.

Molecular Structure, Optical, and Magnetic Properties of Free‐Base Naphthalocyanine Dianions

The reduction of free‐base naphthalocyanine (H2Nc) with sodium fluorenone ketyl and an excess of bis(triphenylphosphoranylidene)ammonium chloride (PPN+)(Cl–) unexpectedly results in the synthesis of the (PPN+)2(H2Nc2–) salt (1). As sodium fluorenone ketyl can generate only H2Nc·– radical anions, the formation of H2Nc2– is most probably realized through the disproportionation of H2Nc·– to H2Nc0 and H2Nc2–. The H2Nc2– dianions in 1 form layers in which π–π stacks of H2Nc2– are formed along the a direction with a short interplanar distance of 3.30 Å and a HOMO–HOMO (HOMO = highest occupied molecular orbiatl) overlap integral of 3.1 × 10–3. The H2Nc2– macrocycle is distorted owing to the formation of alternate short and long C–N(imine) bonds. The distortion of the macrocycle increases with the negative charge on H2Nc owing to the occupation of the lowest unoccupied molecular orbital (LUMO). Calculations indicate that the distortion disappears in the triplet state of H2Nc2– as the occupation of the LUMO and LUMO+1 orbitals averages their effects on the geometry of the macrocycle. The optical spectrum of 1 shows a lowest energy absorption band at λ = 1230 nm, which is attributed to the HOMO → LUMO+1 excitation and a blueshift of the Q‐band of H2Nc. The elecron paramagnetic resonance (EPR) silence of 1 indicates the diamagnetic singlet state of the H2Nc2– dianions. The DFT calculations indicate that the open‐shell broken‐symmetry singlet state is lower in energy than the corresponding closed‐shell singlet and triplet states. The large singlet–triplet energy gap indicates that the corresponding triplet state is thermally inaccessible.

Versatile Control of Directed Supramolecular Assembly via Subtle Changes of the Rhodium(I) Pincer Building Blocks.

Various rhodium(I) pincer complexes with different structural features have been prepared and found to display interesting self-assembly properties due to the extensive Rh(I)···Rh(I) interactions. The incorporation of electron-withdrawing -CF3 substituent has been found to improve the stability of the complexes and also facilitate the directed assembly of complex molecules, providing an opportunity for the systematic investigation of the various noncovalent interactions in their versatile self-assembly behaviors and insights into the structure-property relationship in governing the intermolecular interactions. An isodesmic growth mechanism is identified for the solvent-induced aggregation process. The complex molecules exhibit intense low-energy absorption bands corresponding to the absorptions of the dimers, trimers, and higher order oligomers upon aggregation, with energies related to the electronic properties of the tridentate N-donor ligand. Chiral auxiliaries have also been introduced into the rhodium(I) complexes to build up helical supramolecular assemblies and soft materials.

Luminescence of defect clusters in the disordered anion sublattice of CaF2 crystals

The fast quenching of CaF2 crystals preheated up to 1000 °C allows to stabilize the high-temperature disordering of anion sublattice typical for the superionic state of the crystal. Such procedure results in the formation of structured ultraviolet to vacuum ultraviolet (UV–VUV) absorption with a lengthy long-wavelength tail stretching into the visible spectral range. Such absorption in the quenched crystals cannot be related to any local center and testifies to the presence of nano-sized complexes of displaced anions, the electron structure of which is drastically changed as compared with a regular lattice. Possibly, these clusters are frozen dynamic clusters that were discovered earlier in the superionic state with quasi-elastic diffuse neutron-scattering technique. Four luminescence bands in near UV and visible spectral range correspond to four low-energy absorption bands. Unlike absorption bands that are of zone nature, the luminescence occurs from the localized state that arises due to strong electron-vibration coupling in the clusters. Absorption and luminescence states are divided by a potential barrier that is responsible for the spacing between corresponding excitation and emission bands as well as the temperature quenching of luminescence. The luminescence study reveals the features of the electronic structure of clusters within preheated/quenched CaF2 crystals. The quenching method allows to study the high-temperature superionic state of crystals with fluorite structure using a wide set of experimental techniques.

Construction of Polycyclic π-Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8-Diphenyl-9,10-bis(phenylethynyl)phenanthrene.

Polycyclic aromatic hydrocarbons (PAHs) that incorporate either heptagons or pentagons consist of non-planar molecular structures with unusual optoelectronic properties, and the design of a relatively simple and efficient method to construct these highly fused π-conjugated systems with odd-membered rings is in high demand. This work describes the use of silver(I) cations to promote the efficient synthesis of azulene-embedded PAH 2, which is a structural isomer of tribenzo[fg,ij,rst]pentaphene 3, via tandem oxidative transannulation between the phenyl and arylethynyl moieties. This method involves a carbophilic interaction of the silver(I) cation with the acetylene units, which facilitates an electron transfer in the initial step. The synthesized PAH 2 and the protonated cation 2 H⋅BF4 were fully characterized by X-ray crystallographic analysis, electronic absorption, electrochemical measurement, and quantum chemical calculation. The azulene-embedded PAH 2 exhibited a low-energy absorption band and amphoteric redox events, which were characterized as non-alternant characteristics originating from the azulene unit.

Metal-Stabilized Quinoidal Dibenzo[ g, p]chrysene-Fused Bis-dicarbacorrole System.

We report here a metal complexation-based strategy that permits access to a highly stable expanded porphyrin-type quinoidal polycyclic aromatic hydrocarbons (PAH). Specifically, double insertion of Pd(II) ions into a dibenzo[ g, p]chrysene-fused bis-dicarbacorrole (bis-H3) gives rise to a bis-metalated species (bis-Pd) that undergoes a facile benzenoid-quinonoid transformation. In contrast to what is true for the corresponding mono-Pd(II) complex, which has organic radical character, well resolved 1H NMR and 19F NMR spectra are seen for bis-Pd. This complex is also electron paramagnetic resonance (EPR) silent over a range of temperatures. On the basis of crystallographic analyses, Raman spectroscopic studies, harmonic oscillator model of aromaticity (HOMA), and nucleus-independent chemical shift (NICS) calculations, we suggest that the dibenzo[ g, p]chrysene bridge in bis-Pd has quinoidal character and that the system as a whole is a closed shell species. As expected for a quinoidal system, bis-Pd is characterized by a lowest energy absorption band that is shifted into the NIR (λmax = ca. 1420 nm (ε > 1.5 × 105 M-1 cm-1) for bis-Pd vs 780 nm (ε < 5.0 × 103 M-1 cm-1) for bis-H3). On the other hand, bis-Pd displays solvent dependent ground state and transient absorption spectral features. Such findings provide support for a zwitterionic resonance contribution to what is a predominantly a quinonoid-type ground state. The use of specific metalation to fine-tune the electronic features of polytopic ligands, as reported here, opens the door to what might be a potentially generalizable approach to the design of quinoidal PAH structures with long wavelength solvatochromic absorption features.

Fast switching soluble electrochromic polymers obtained from a 4,9-Dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-embedded system

A series of conjugated polymers containing 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene (IDT) and 3,4-ethylenedioxythiophene (EDOT) or benzo[c][1,2,5]thiadiazole (Tz) are successfully synthesized via Stille coupling polymerization with reasonable yields as well as high molecular weights. The synthesized polymers can dissolve in chloroform, toluene, THF, etc. Additionally, the physical, optical and electrochemical properties of these synthesized polymers are fully characterized. After a film is spin-coated on the ITO electrode, the prepared polymer films present remarkable electrochromic behaviors. Increasing the composition of electron-rich EDOT along with the conjugated backbone of the synthesized D-A polymer is shown to induce red-shift of the short-wavelength absorption band and blue-shift of the long-wavelength counterpart. On the other hand, as the strong electron-deficient benzo[c][1,2,5]thiadiazole (Tz) incorporates into all-donor polymer mainchain, a new low-energy absorption band due to the intramolecular charge transfer of D-A interaction emerges and the corresponding intensity intensifies with the composition of electron-deficient moieties increasing. In addition, the prepared IDT-based polymers also exhibit fast switching response and good stability.

Coupling d6 Ir(III) and d8 Pt(II) Chromophores.

Two classes of widely studied luminescent metal complexes are octahedral d6 (i.e., Ir3+) and square planar d8 (i.e., Pt2+) polypyridyl complexes, which have distinctly different photophysics and photoreactivity. In this study we report a series of d6-d8 IrIII-PtII hybrid complexes arising from coordination of metalloligands IrL2(benzene-1-thioether-2-thiolate) or Ir(L)2(benzene-1,2-dithiolate) anion [L = 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), or 1-phenylisoquinoline (piq)] to Pt(terpy)2+ (terpy = 2,2':6',2″-terpyridine). X-ray crystal structures of the Ir-Pt complexes show the IrL2 and Pt(terpy) chromophores are cofacially oriented with interplanar distances of 3.268-3.442 Å. Density functional theory (DFT) calculations show that the highest occupied molecular orbital and the lowest unoccupied molecular orbital are localized in the IrL2 and the Pt(terpy), respectively. All the complexes display a low-energy absorption band (λmax = 460-534 nm, εmax = (0.75-2.13) × 103 M-1 cm-1), which is attributed to interchromophore-charge-transfer (ICCT) transition, according to time-dependent DFT calculations. The 3ICCT excited state is emissive, giving long-lived phosphorescence that reaches as low as near-infrared (λmax = 668-710 nm, τ = 0.17-0.79 μs).

Theoretical investigation of the photophysical properties of donor-acceptor dyes containing coumarin and naphthoquinone moieties linked by an aminomethylene bridge

Mannich bases HL1 and HL2 derived from lawsone, heptylamine and 3-formylcoumarins have been synthesized for the investigation of their photophysical properties. The cyclic voltammetry data showed a charge transfer (CT) process from the coumaryl to the naphthoquinonoid group, in spite of the absence of conjugation between these two fragments, with the nitrogen atom playing an important role. Photophysical studies by UV–Visible and fluorescence spectroscopy in acetonitrile revealed low emission quantum yield (Φ), thus confirming the quenching of the coumarin fluorophore by the naphthoquinone moiety. The role of the nitrogen atom in this spacer was investigated by perturbing the electronic density at this site by protonation. The emission spectra of the hydrochlorides HL1·HCl and HL2·HCl revealed increase in the quantum yields in both cases. The geometries of all compounds were optimized at B3LYP/6-31 + G (d,p) level. TD-DFT calculations were carried out with different functionals. The hybrids B3LYP and PBE0 provided the best results with respect to the lowest energy transition and M06-HF failed in this prediction, whereas the long range corrected functionals - CAM-B3LYP and ωB97XD - did not improve the description of this transition. For HL1 and HL2, the lowest energy absorption band around 440 nm (λ3) is assigned to an aminomethyl coumarin (HOMO-1) to naphthoquinone (LUMO) CT. However, for HL1·HCl and HL2·HCl, the contribution of the nitrogen atom was suppressed by protonation and the lowest energy transition is assigned only to the coumarin (HOMO) to naphthoquinone (LUMO) CT.

Synthesis, characterization and photophysical properties of homoleptic platinum(II) complexes with 2,2′-biimidazole-based ligands

Eight pairs of cis–trans isomeric homoleptic platinum(II) complexes based on N-alkyl- or aryl-substituted 2,2′-biimidazole ligands were synthesized, and their photophysical properties were investigated. The cis and trans isomers readily interconvert at slightly elevated temperature, implying that the activation barrier for this process is low. Single crystal X-ray diffraction analysis revealed that the complexes have an ideal square-planar geometry. Their UV–Vis spectra showed lower energy absorption bands in the range of 345–378 nm, which are assigned to the typical MLCT mixed with LC transitions. In frozen glass solution at 77 K and also in the powder state, these complexes exhibit green emission ranging from 440 to 540 nm with photoluminescence quantum yields of 3.3–24.4%. The emitting excited state is dominated by 3ππ* character with some contributions from 3MLCT according to the excitation spectra.

Emissive mononuclear Cu(I) triphenylphosphine complexes with functionalized 6-tert-butoxycarbonyl-2,2′-bipyridine

Two new emissive mononuclear copper(I) triphenylphosphine complexes with functionalized 6-tert-butoxycarbonyl-2,2′-bipyridine chelating ligands were prepared and characterized. Each Cu(I) cation has a markedly distorted P2N2 tetrahedral array with two different Cu—N bond distances, due to the addition of the bulky tert-butoxycarbonyl group at the 6-position of the 2,2′-bipyridyl ring. A weak low-energy broad absorption band appears at 330—500 nm in their CH2Cl2 solutions, which is influenced by the methylation of the 2,2′-bipyridyl ring. At room temperature, the photoluminescence can be clearly observed both in degassed CH2Cl2 solution and in the solid state, because of the introduction of the bulky tert-butoxycarbonyl substituent, and the emission properties are remarkably affected by both the methylation and the alkoxycarbonyl variation of the 2,2′-bipyridyl ring. It is shown that the introduction of the bulky tert-butoxycarbonyl group is favorable for improving the emission properties of the 6-alkoxycarbonyl-2,2′-bipyridine-based cuprous complexes.

Synthesis, characterization, X-ray crystal structure and DFT calculations of 4-([2,2':6',2''-terpyridin]- 4'-yl)phenol

La síntesis de derivados terpiridinicos (Tpy) se ha investigado ampliamente debido a su potencial para la conversión de energía solar. En este artículo se sintetizó y caracterizó el 4-([2,2':6',2'' terpiridin]-4'-il)fenol (TpyOH), a través de varias metodologías como la cristalografía de rayos X y herramientas computacionales. El análisis de rayos X de monocristal mostró que el TpyOH es plano, con ángulos diedros de 5,03° entre el piridinilo central y el anillo fenólico, con presencia de ángulos de 6,05 y 12,2º en la porción terpiridínica. En el cristal, las moléculas están unidas por enlaces de hidrógeno intermoleculares y mediante interacciones de apilamiento π-π. Utilizando cálculos DFT dependientes del tiempo (TD-DFT) y teniendo en cuenta el efecto de los disolventes, se investigaron y compararon los espectros de absorción y fluorescencia de TpyOH. Las energías de transición TD-DFT de S0→Sn y S1→S0 concuerdan con los resultados experimentales. El análisis de orbitales moleculares de frontera mostró que la banda de absorción de baja energía corresponde a transferencia de carga intraligando (ICT); mientras que la banda de alta energía es común en las transiciones π-π* del resto Tpy. La emisión debido a la transición S1→S0 corresponde a ICT, con una contribución del 90% proveniente de transiciones HOMO→LUMO.