Ligand-ligand Charge-transfer Research Articles

Dramatic enhancement in lithium-ion battery capacity through synergistic effects of electronic transitions in light-assisted organic coordination cathode material Co(bpy)(dhbq)2

Integration of photoactive and lithium storing units into a single cathode endows it with notable capacity in the presence of suitable light. However, the interfacial effect between the two materials causes significant loss of photogenerated electrons during their transfer which is one of the biggest obstacles in the development of current photo-assisted rechargeable batteries. In this paper, a bifunctional cobalt-coordinated organic cathode is fabricated by combining 2,2′-bpy and DHBQ via Co by adopting the spin evaporation technique. It optimizes the pristine interfaces of photoactive and lithium storage units into a photoactive unit-metal interface and a metal-lithium storage unit interface through application of Co. In the presence of light, Co causes a strong metal-ligand charge transfer. Meanwhile, ligand-ligand charge transfer also takes place between the multi-ligands. The synergistic effect of these two phenomena offers a discharge capacity of 387 mAh g -1 which is significantly higher than that of 316 mAh g -1 recorded in the absence of light. The demonstrated design of bifunctional metal-ligand cathode by incorporation of photoactive ligands into lithium storage ligands through applications of metal centers can open the pathways for establishing a new type of photo-assisted lithium-ion batteries with higher efficiency and lower cost.

Heterobimetallic Cu(I)/Fe(II) azomethine bridged coordination-organometallic hybrid complexes: Synthesis, luminescence, electrochemical and catalytic properties

A new series of heterobimetallic complexes of the type [Cu(L)(PPh3)2]X (1–4) [where L = trans-4-(ferrocenylideneamino)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one; PPh3 = triphenylphosphine, X = NO3−(1), ClO4− (2), BF4− (3) , PF6− (4)] were prepared by reaction of L with [Cu(MeCN)2(PPh3)2]X and characterized by elemental analysis, FTIR, 1H NMR, 31P NMR and UV–visible spectral studies. The thermal stability of (1–4) has been investigated by using thermogravimetric analysis (TGA). X-ray powder diffraction of the representative complex 1 used to clarify the crystal structure of the complex. The electrochemical behaviour of complexes (1–4) displayed quasireversible redox behaviour corresponding to Cu(I)/Cu(II) and Fe(II)/Fe(III) couples. All complexes show red emission as a result of ligand-ligand charge transfer (LLCT) and metal-ligand charge transfer (MLCT) or admixture of them. The catalytic efficiency of the complexes (1–4) was tested and it was found that the complex 4 worked as an effective catalyst for the C-N coupling reaction of aromatic amines and aryl halides.

Intra- and Intermolecular Charge-Transfer Dynamics of Carbene-Metal-Amide Photosensitizers.

A series of steady-state and time-resolved spectroscopies were performed on a set of eight carbene-metal-amide (cMa) complexes, where M = Cu and Au, that have been used as photosensitizers for photosensitized electrocatalytic reactions. Using ps-to-ns and ns-to-ms transient absorption spectroscopies (psTA and nsTA, respectively), the excited-state kinetics from light absorption, intersystem crossing (ISC), and eventually intermolecular charge transfer were thoroughly characterized. Using time-correlated single photon counting (TCSPC) and psTA with a thermally activated delayed fluorescence (TADF) model, the variation in intersystem crossing (ISC), (S1 → T1) rates (∼3-120 × 109 s-1), and ΔEST values (73-115 meV) for these compounds were fully characterized, reflecting systematic changes to the carbene, carbazole, and metal. The psTA additionally revealed an early time relaxation (rate ∼0.2-0.8 × 1012 s-1) attributed to solvent relaxation and vibrational cooling. The nsTA experiments for a gold-based cMa complex demonstrated efficient intermolecular charge transfer from the excited cMa to an electron acceptor. Pulse radiolysis and bulk electrolysis experiments allowed us to identify the character of the transient excited states as ligand-ligand charge transfer as well as the spectroscopic signature of oxidized and reduced forms of the cMa photosensitizer.

Synthesis, structure and optoelectronic properties of iridium(III) complexes bearing a four-membered Ir-N-C[sbnd]N chelate structure

Herein four novel iridium(III) complexes (tfpmd)2Ir(dpppy) (Ir1a, tfpmd = 2-(4-(trifluoromethyl) phenyl) pyrimidine, dpppy = P,P-diphenyl-N-(pyridin-2-yl)phosphinic amide), (tfpmd)2Ir(dppq) (Ir1b, dppq = P,P-diphenyl-N-(quinolin-2-yl)phosphinic amide), (tfpqz)2Ir(dpppy) (Ir2a, tfpzq = 4-(4-(trifluoromethyl) phenyl) quinazoline) and (tfpqz)2Ir(dppq) (Ir2b) were synthetized, and their structures, opto-physical, and electro-chemical properties were investigated in detail. The crystal structure shows that these complexes exhibit pseudo-octahedral geometry around iridium atom coordinated by C, N atoms from cyclometalated and ancillary ligands. Interestingly, two coordinated nitrogen atoms from the ancillary ligand form an unusual quaternary ring with the central iridium ion. Ir1a and Ir1b emit pure-green light with Commission Internationale de L'Eclairage (CIE) color coordinates of (0.25, 0.67) and (0.22, 0.67), respectively, in CH2Cl2 solution. With the increase of the degree of conjugation of the cyclometalated ligand from tfpmd to tfpqz, the luminescence of complexes Ir2a and Ir2b is greatly red-shifted, and pure-red emission with CIE coordinates of (0.66, 0.34) and (0.66, 0.34), respectively, is obtained. The photophysical behaviors were reasonably explained by quantum chemical calculations. The calculation results shows that their emission is ascribed to the mixed 3MLCT (metal-ligand charge transfer, dπ(Ir)→π*(cyclometalated ligand)) and 3LLCT (ligand-ligand charge transfer, π(ancillary ligand)→π*(cyclometalated ligand)) states. Owing to their good thermal and electrochemical stabilities, organic light emitting diode (OLED) devices with dual-emitting layer structure and a space interlayer were fabricated and exhibited high electroluminescent properties and mild efficiency roll-off (14%-21%).

A Theoretical Investigation into the Homo- and Hetero-leptic Cu(I) Phosphorescent Complexes Bearing 2,9-dimethyl-1,10-phenanthroline and bis [2-(diphenylphosphino)phenyl]ether Ligand.

Cu(I) complexes have received widespread attention as a promising alternative to traditional noble-metal complexes. Herein, we systematically study the properties of Cu(I) complexes from homo- to hetero-ligands, and found the following: (1) hetero-ligands are beneficial to regulate phosphorescent efficiency; (2) when the hetero-ligands in a tetracoordinated Cu(I) complex are 1:1, the ligands coordinate along the dx2-y2 direction of Cu(I) ion, which can observably suppress structural deformation; (3) unlike the P^P ligand, the N^N ligand can enhance the participation of Cu(I) during the transition process; (4) the addition of an appropriate amount of P^P ligand can effectively raise the energy level of HOMO (highest occupied molecular orbital), enhance the proportion of LLCT (ligand–ligand charge transfer), and thereby increase the available singlet emission transition moments which can be borrowed, thus promoting the radiative decay process. As a result, this work provides a detailed understanding of the effects of different ligands in Cu(I) complexes, and provides a valuable reference and theoretical basis for regulating and designing the phosphorescent properties of Cu(I) complexes in the future.

Low-cost photo-switches based on stilbene-appended Zn(II)-terpyridine complexes.

We report herein the synthesis, characterization, photophysics, and photo-isomerization behaviors of three Zn(II)-terpyridine complexes of the type [Zn(tpy-pvp-X)2]2+ (X = H, Me, and NO2) covalently tethered with stilbene moiety. The complexes exhibit absorption bands stretching up to the edge of the visible domain due to ligand → ligand charge transfer (LLCT) transitions and strong emission at room temperature in the visible due to radiative deactivation of 3LLCT state having lifetime within 1.0-3.0ns. The stilbene motifs in the complexes undergo trans to cis isomerization upon irradiating with UV and visible light accompanied by significant alteration of their absorption, emission, and 1H NMR spectral profiles. Apart from the variation of electron donating and electron withdrawing substituent (X), the isomerization studies were also carried out in three different solvents (DCM, MeCN, and DMSO) to further tune their kinetic and thermodynamic parameters. The rate, rate constant and quantum yield of isomerization were estimated in all the solvents. The reverse process (cis to trans) also occurs very slowly on keeping but could be accelerated upon heating. Trans to cis photoisomerization leads to quenching of emission in case of 1 and 2, whereas backward thermal cis to trans conversion leads to restoration of emission. By contrast, for the nitro-derivative (3) forward process induces emission enhancement, while backward process gives rise to emission quenching. In essence, "on-off" and "off-on" emission switching is feasible for 1 and 2, whereas "off-on" and "on-off" emission switching occurs in case of 3. Emission spectral responses upon successive action of photonic and thermal input lead to the fabrication of INHIBIT and IMPLICATION logic gates. DFT and TD-DFT computational investigations were also undertaken to visualize their electronic structures, correct assignment of the spectral bands, and mode of isomerization process.

UiO-66-based metal organic frameworks for the photodegradation of acetaminophen under simulated solar irradiation

This work reports the solvothermal synthesis of UiO-66-based MOFs with three different ligands and its application to the breakdown of aqueous acetaminophen under simulated sunlight. This pharmaceutical is a representative contaminant of emerging concern that enters water bodies mainly through wastewater treatment plant discharges. The synthesis approach of the MOFs was fitted using a zirconium alkoxide as metal cluster precursor and 2-aminoterephthalic and 2,5-dihydroxyterephthalic acids as ligands to prepare UiO-66-NH2 and UiO-66-(OH)2 MOFs, respectively. These new MOFs have enhanced visible light harvesting and narrower band gap than the UiO-66. Among all, UiO-66-NH2 yielded the highest removal of acetaminophen under simulated solar irradiation in batch test. The activity and stability of UiO-66-NH2 were demonstrated for the first time in a continuous flow test, where stable performance was observed upon 30 h on stream. The degradation pathway of acetaminophen was elucidated based on coupling, ring-opening, and oxidation reactions. DFT calculation confirmed that the indirect semiconductor behavior of UiO-66-NH2 upon light excitation occurred through ligand-ligand charge transfer. Overall, promising UiO-66-based MOFs photocatalysts were obtained for effective degradation of acetaminophen with the assistance of solar light.

Synthesis, structure and properties of a new Sm(III) rare-earth metal coordination complex with 2,5-dihydroxy-terephthalic acid ligand

A new Sm(III) coordination complex, [Sm(DHTA)4(H2O)3]n, was synthesized with 2,5-dihydroxyterephthalic acid (H2DHTA) as ligand. The crystal structure and properties were characterized by single crystal X-ray diffraction, IR, elemental analysis and TGA analysis. The single-crystal X-ray diffraction analysis showed that Sm(III) ion is nine-coordinate with six oxygen atoms of four DHTA ligands and three oxygen atoms of three H2O molecules, forming a tri-capped trigonal-prism configuration. The adjacent units of the complex are extended into a 3 D supramolecular network. In the IR spectrum of the complex, there are asymmetric and symmetric stretching vibrations of C = O of the carboxyl group. The complex has two weight loss steps in the TG plot. The complex’s UV-Vis absorptions at 400 and 261 nm are attributed to the DHTA. Furthermore, there is a fluorescent maximum emission at 476 nm, which is derived from the ligand. The fluorescence is attributed to the ligand-ligand charge transfer of the complex.

Computational Approach to Understanding the Electrocatalytic Reaction Mechanism for the Process of Electrochemical Oxidation of Nitrite at a Ni-Co-Based Heterometallo-Supramolecular Polymer.

Here, we report a semiempirical quantum chemistry computational approach to understanding the electrocatalytic reaction mechanism (ERM) of a metallic supramolecular polymer (SMP) with nitrite through UV/vis spectral simulations of SMP with different metal oxidation states before and after interactions with nitrite. In one of our recent works, by analyzing the electrochemical experimental data, we showed that computational cyclic voltammetry simulation (CCVS) can be used to predict the possible ERM of heterometallo-SMP (HMSMP) during electrochemical oxidation of nitrite (IslamT.ACS Appl. Polym. Mater.2020, 2( (2), ), 273−284). However, CCVS cannot predict how the ERM happens at the molecular level. Thus, in this work, we simulated the interactions between the repeating unit (RU) of the HMSMP polyNiCo and nitrite to understand how the oxidation process took place at the molecular level. The RU for studying the ERM was confirmed through comparing the simulated UV/vis and IR spectra with the experimental spectra. Then, the simulations between the RU of the polyNiCo and various species of nitrite were done for gaining insights into the ERM. The simulations revealed that the first electron transfer (ET) occurred through coordination of NO2– with either of the metal centers during the two-electron-transfer oxidation of nitrite, while the second ET followed a ligand–ligand charge transfer (LLCT) and metal–ligand charge transfer (MLCT) pathway between the NO2 species and the RU. This ET pathway has been proposed by analyzing the transition states (TSs), simulated UV/vis spectra, energy of the optimized systems, and highest occupied molecular orbital–lowest occupied molecular orbital (HOMO–LUMO) interactions from the simulations between the RU and nitrite species.

Theoretical assessment of TD-DFT applied to a ferrocene-based complex

We present a theoretical study of the electronic absorption spectrum of the 2-ferrocenyl-1,8-naphthyridine (FcNP) complex using the time-dependent density functional theory (TD-DFT) approach. The calculations were carried out at the DFT/LANL2DZ(Fe)/6-31++G(d) level of theory with nine DFT functionals of the type GGA (PBE, B97D), HGGA (B3LYP, PBE0), HMGGA (M05, M06) and range-separated (wB97XD, CAM-B3LYP, LC-wPBE) in the condensed phase (CH2Cl2). The experimental spectrum for FcNP presents three electronic absorption bands (A, B, C) in the UV region and one band (D) in the visible region. We found that there is no unique functional that reproduces the entire electronic spectrum. GGA and LC-wPBE functionals predict large errors (up to 0.57eV) for the UV bands. On the contrary, HGGA (PBE0), HMGGA (M05, M06) and range-separated (wB97XD, CAM-B3LYP) functionals give small errors for these bands, 0.18, 0.12 and 0.25eV, respectively. Band D is well described by all the functionals with errors up to 0.26eV, except by the M05 and M06 functionals, which fail in this region with significant errors (0.52 and 0.56eV). The analysis of the excitation energies shows that the PBE0, wB97XD and CAM-B3LYP functionals predict the best agreement with the experimental data. The two stronger bands (A, B) in the UV region are assigned to a ligand–ligand charge transfer (L-LCT) that involves the cyclopentadienyl rings (Cp) and the naphthyridine fragment (NP), and then associated with π→π∗ electronic transitions. The two weaker bands (C, D) are assigned to a charge transfer from ligand (Cp) to metal (Fe)-ligand (NP) (L-MLCT) which is mainly addressed through the dxy and dz2 atomic orbitals, respectively. No charge transfer is observed between metal and NP ligand. It was shown that the theoretical methods used for understanding the electronic absorption properties of FcNP were adequate because the results showed an excellent agreement with experimental ones.

Singlet Oxygen Production of Photocatalytic Ir(III) Complex Impregnated unto Zeolite NaY Supercages

Trivalent iridium complexes have been recently studied due to the extent of its overlap with the solar spectrum as well as the accessibility of its ligand-ligand charge transfer (LLCT) transition in the UV region. Entrapment of the complex inside faujasitic zeolite NaY converts the dye molecule into a heterogeneous catalyst that has greater stability and usability than the free complex. Ship-in-a-bottle assemblies of trisphenanthroline iridium (III) complex unto zeolite NaY were found to produce singlet oxygen upon illumination. The possible use of these solid-state catalyst for waste water treatment was examined by using 9,10-dimethylanthracene (DMA) as a chemical trap for singlet oxygen. The degradation spectra was observed to be pseudo-first order with respect to DMA. Varying the time of illumination showed that the amount of DMA degraded was directly dependent on the length of UV irradiation.

Theoretical studies of ground and excited states in a series of Zn(II) complexes, derived from thiourea and thiosemicarbazide

Theoretical studies for a series of mono- and binuclear zinc (II) complexes Zn(CH3COO)(2)(H2L)(2) [H2L = N-2-propenyl-N'-2-pyridinylthiourea] (A), Zn-2(CH3COO)(2)(H3L-a)(2) [H3L-a=2-[(2-hydroxy-phenyl)methylene]hydrazine-N-phenylcarbothioamide] (B), and Zn(H3L-b)(2) [H3L-b = 2-[(2-hydroxy-phenyl) methylene]hydrazine-N-(2-propenyl)carbothioamide] (C) have been performed on their structures and excited-state absorption spectra. The singlet ground-state geometries are fully optimised at three DFT levels, i.e., B3LYP, B3PW91, and M06. Different geometries, i.e., strongly distorted tetrahedral coordination environment in complex A, distorted square-pyramidal environment in complex B, and irregular octahedral mode in complex C are identified. Consequently, the spectroscopic properties are calculated by means of time-dependent density functional theory (TDDFT) with the Polarisable Continuum Model (PCM) based on the optimised gas-phase geometries. Three absorption peaks are identified for every complex, which are in good agreement with the experimental ones. For complex A, all three absorption peaks centered at 280.33 nm, 268.09 nm, and 250.87 nm, respectively, are ascribed to the (p, pi) -> pi* transition with a mixed intraligand charge-transfer (ILCT)/ligand-ligand charge-transfer (LLCT) character. The composition of frontier orbitals involved in major absorption bands for the three complexes shows similarities, which results in the almost homologous transition attributions and characteristics. A remarkable bathochromic shift in the lowest-lying absorption band is observed for complexes B and C as compared with complex A, which is attributed to the decreased H (HOMO)-L (LUMO) energy gap (Delta E vertical bar HOMO-LUMO vertical bar) by the formation of conjugate metallocycles in complexes B and C.

π Donation and Its Effects on the Excited-State Lifetimes of Luminescent Platinum(II) Terpyridine Complexes in Solution

Introducing electron-donating groups extends the excited-state lifetimes of platinum(II)-terpyridine complexes in fluid solution. Such systems are of interest for a variety of applications, viz., as DNA-binding agents or as components in luminescence-based devices, especially sensors. The complexes investigated here are of the form [Pt(4'-X-T)Y](+), where 4'-X-T denotes a 4'-substituted 2,2':6',2″-terpyridine ligand and Y denotes the coligand. The π-donating abilities of -X and -Y increase systematically in the orders -NHMe < -NMe2 < -(pyrrolidin-1-yl) and -CN < -Cl < -CCPh, respectively. The results presented include crystal structures of two new 4'-NHMe-T complexes of platinum, as well as absorption, emission, and excited-state lifetime data for nine complexes. Excited-state lifetimes obtained in deoxygenated dichloromethane vary by a factor of 100, ranging from 24 μs for [Pt(4'-pyrr-T)CN](+) to 0.24 μs for [Pt(4'-ma-T)Cl](+), where ma-T denotes 4'-(methylamino)-2,2':6',2″-terpyridine and pyrr-T denotes 4'-(pyrrolidin-1-yl)-2,2':6',2″-terpyridine. Analysis of experimental and computational results shows that introducing a simple amine group on the terpyridine and/or a π-donating coligand engenders the emitting state with intraligand charge-transfer (ILCT) and/or ligand-ligand charge-transfer (LLCT) character. The excited-state lifetime increases when the change in orbital parentage lowers the emission energy, suppresses quenching via d-d states, and encourages delocalization of the excitation onto the ligand(s). At some point, however, the energy is low enough that direct vibronic coupling to the ground-state surface becomes important, and the lifetime begins to decrease again.

Theoretical studies on the spectroscopic properties of a series of palladium (II) complexes with 1-allyl-3-(2-pyridyl)thiourea

To explore the spectroscopic properties, a series of palladium complexes are investigated theoretically through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Full geometry optimizations of 1-allyl-3-(2-pyridyl)thiourea (APTU) and its corresponding Pd (II) complexes [Pd(APTU)Cl2] (A), trans-[Pd(APTU)2]2+ (B), cis-[Pd(APTU)2]2+ (C), together with two thiolic tautomeric APTU-based Pd (II) complex cis-[Pd(APTU)2] (D) and trans-[Pd(APTU)2] (E) are carried out at the B3LYP, B3PW91, and M06 levels. All of them present slightly distorted square-planar geometries. On the basis of the optimized geometries, the absorption spectra in ethanol are evaluated by using three TD-DFT methods with the PCM model. Experimental absorption spectra are well reproduced by our time-dependent density functional theory calculations. Both UV–vis absorption peaks at 219.64 and 254.80nm for complex A are mainly attributed to (p,π,d)→π* with a mixed character of intraligand charge-transfer (ILCT)/ligand–ligand charge-transfer (LLCT)/metal-ligand charge-transfer (MLCT) transitions. Another weak absorption band of complex A is located at 313.17nm largely originated from HOMO−6 to LUMO transition. There are two isomers with small energy difference for [Pd(APTU)2]2+, i.e., trans-[Pd(APTU)2]2+ (B) and cis-[Pd(APTU)2]2+ (C). Only when both isomers are considered, four absorption peaks can be obtained, which is in line with the experimental results. The absorption peaks of complexes D and E present obvious red shift that is associated with their lower HOMO–LUMO gap (ΔE|HOMO–LUMO|) among five complexes. Differences and similarities in molecular orbitals and electronic transition characters for five complexes are discussed in detail.

Theoretical studies on the spectroscopic properties of a series of halide Zinc (II) complexes with pyridinylimine and pyridinylmethylamine derivatives

The ground structures of pyridinylimine derivatives (L1-a=N-(pyridin-2-ylmethylene)aniline and L1-b=N1,N1-(dimethyl-N4-pyridin-2-ylmethylene)benzene-1,4-diamine) and pyridinylmethylamine derivatives (L2-a=N-(pyridin-2-ylmethyl)aniline and L2-b=N1,N1-(dimethyl-N4-pyridin-2-ylmethy)benzene-1,4-diamine) and their corresponding Zn (II) complexes ([Zn(L1-a)Cl2] (A), [Zn(L1-b)Cl2] (B), [Zn(L2-a)Cl2] (C), and [Zn(L2-b)Cl2] (D)) are optimized at three DFT levels, i.e., B3LYP, B3PW91, and M06. The spectroscopic properties are calculated using time-dependent density functional theory (TD-DFT) in gas and in solution. The occupied orbitals involved in the transitions have mixed character of the Cl atom p orbital and ligand-based π orbital, while the lowest unoccupied molecular orbital (LUMO) presents π* orbital character. Two UV–Vis absorption peaks located at 252 and 353nm are assigned to (p, π)→π* transition with mixed intraligand charge-transfer (ILCT)/ligand–ligand charge-transfer (LLCT) character and π→π* transition with ILCT character for complex A, respectively. With respect to complex B, the absorption bands show red shift with two peaks at 281 and 470nm with the appearance of an electron-releasing group (–N(CH3)2) because the energy gap (ΔE|HOMO–LUMO|) is decreased. Red shift phenomenon is also observed between other two complexes C and D. Comparison between theoretical and experimental results for the structures and spectra is discussed in detail.

A novel carbazole-based dye outperformed the benchmark dye N719 for high efficiency dye-sensitized solar cells (DSSCs)

Two novel high molar extinction coefficient heteroleptic Ru(II) isomers, NCSU-10 and NCSU-10′, based on carbazole antenna were synthesized with the aid of Knoevenagel reaction, to study the influence of the carbazole antenna and anchoring group (COOH) isomerization on the light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), short-circuit photocurrent density (JSC), and total solar-to-electric conversion efficiency (η) for DSSCs, and their device performances were compared to the benchmark dye N719. The photophysical and photoelectrochemical properties discussed herein addressed the significant impact of the carbazole antenna and the position of the anchoring group on JSC and η in DSSCs. Tetrabutylammonium (TBA) substituted NCSU-10 achieved efficient sensitization of nanocrystalline TiO2 over the whole visible range, extending into the near IR region (ca. 870 nm) with an excellent power conversion efficiency (η) of 9.37% under an irradiation of full sunlight (100 mW cm−2) with mask compared to 8.17% of N719 under optimized conditions. NCSU-10 outperformed N719 by 45% in molar absorptivity, 18.8% in JSC, and 14.6% in the total conversion efficiency. Molecular modeling studies (DFT/TD-DFT) of NCSU-10 and NCSU-10′ showed that the HOMO is delocalized not only on Ru and NCS but also on the carbazole with a large coefficient, indicating that the second charge generation transfer in the visible region at ∼400 nm is a mixture of metal-to-ligand charge transfer (MLCT) and strong ligand–ligand charge transfer (LLCT) with a significant HOMO coefficient originating from the carbazole antenna (π) to the bipyridyl electron acceptor (π*). Moreover, DFT calculations showed that the 4,4′-isomer (NCSU-10) is a significantly stronger electron acceptor than the 5,5′-isomer (NCSU-10′), which explained the inferior electron injection and significantly lower JSC of the 5,5′-isomer.

Photophysical properties of ligand localized excited state in ruthenium(ii) polypyridyl complexes: a combined effect of electron donor–acceptor ligand

We have synthesized ruthenium(II) polypyridyl complexes (1) Ru(II)(bpy)(2)(L(1)), (2) Ru(II)(bpy)(2)(L(2)) and (3) Ru(II)(bpy)(L(1))(L(2)), where bpy = 2,2'-bipyridyl, L(1) = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L(2) = 4-(N,N-dimethylamino-phenyl)-(2,2'-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the presence of electron donating -NMe(2) functionality in L(2) and electron withdrawing catechol fragment in L(1) ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is attributed to quenching of the (3)MLCT state by the low-emissive (3)LLCT state. The emission due to ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a new luminescence band which appeared in the 670-760 nm region. The LLCT excited state of complex 3 is populated either via direct photoexcitation in the LLCT absorption band (350-700 nm) or through internal conversion from the photoexcited (3)MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the (3)MLCT state in a time resolved emission study. The internal conversion to LLCT and ILCT excited states are observed to be as fast as ∼200 ps and ∼700 ps for complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand localized excited state of newly synthesized heteroleptic ruthenium(II) polypyridyl complexes.

Tailoring the Photoluminescence Properties of Ionic Iridium Complexes

Density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations were performed to investigate the structural, electronic, and optical properties of ionic Ir complexes with several different substituents on the cyclometalated ligand. Geometric parameters, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap, and Mulliken charge on different parts of the molecule were obtained and correlated to the calculated emission and absorption energies. We also discuss the influence of the position of fluoro-substituent on the spectroscopic properties of Ir complexes. As a major trend, the investigated complexes exhibit band shifts that correlate with the electron-withdrawing nature of the ligand substituent. Our results also show that the lowest emission wavelength is observed at ortho position with respect to the coordinating carbon. The different variations observed are discussed in terms of emissive states and, more especially, in terms of the mixture of ligand-ligand charge-transfer (LLCT) and metal-ligand charge-transfer (MLCT) states.

Enhanced Fluorescence of Ni(II) Complex Compounds in the Presence of DNA Components

DNA-mediated charge transfer currently attracts considerable interest because of its relevance for molecular electronics. In this work the fluorescence behavior of the ligand-ligand charge transfer complexes Ni(diimine)(thiol)2, where diimine = 2,2′-bipiridine and its 4,4′ derivatives and thiol = thioaminoacid cisteine and its methyl esther were studied in the presence of DNA components. Substitution of an electron withdrawing group in 4,4′ position of 2′2′-bipiridine causes red shift in the absorption spectrum. For all synthesized complex compounds it has been observed a significant increase of fluorescence emission intensity in the presence of DNA nucleobases.

Synthesis, spectral characterization and redox studies of [1-alkyl-2-(naphthyl- β -azo)imidazole] palladium(II) catecholates

1-Alkyl-2-(naphthyl-β-azo)imidazoles [β-NaiR; R = Me (a), Et (b), CH2Ph (c)] react with Pd(MeCN)2Cl2 to yield Pd(β-NaiR)Cl2(2), the i.r. spectra of which support the presence of a cis-PdCl2 configuration. The complexes react with catechols in the presence of Et3N to yield ternary complexes [Pd(β-NaiR)(O,O)] [O,O = pyrocatecholato (cat) (3), 4-t-butylcatecholato (tbcat) (4), 3,5-di-t-butylcatecholato (dtbcat) (5), and tetrachlorocatecholato (tccat) (6)], which were characterized by elemental analysis, i.r. and 1H-n.m.r. spectral data. Redox studies by cyclic voltammetry suggest the existence of four successive redox couples wherein two responses, positive to s.c.e. are due to catechol to semiquinone and semiquinone to quinone oxidation, respectively; the couples at negative to s.c.e. are referred to azo reductions. The complexes exhibit ligand-ligand charge-transfer transitions in the near-i.r. region. The band position is largely dependent upon the substitutent on the catechol frame and exhibits negative solvatochromic effects. The transition is qualitatively assigned as the HOMO (cat) → LUMO (β-NaiR) transition. This fact is also supported by theoretical calculations using the PM3 method.