Intraligand Charge Transfer Research Articles

Synthesis, characterization, and photophysical properties of a new 2,5-di(aryl)phosphole derivative and their trigonal copper–phosphole complexes

A new phosphole derivative, 2,5-di(2-quinolyl)-1-phenylphosphole (1), was synthesized by using the Fagan–Nugent method. Phosphole 1 was obtained as an air-stable solid in high yield (73%). Additionally, two new copper–phosphole complexes, [CuX(Phosphole)2] (X = Cl (2a), I (2b), Phosphole=1), have been synthesized by reaction of CuX (X = Cl, I) and 1. All compounds were characterized by NMR, ESI-MS, UV–Vis, and fluorescence spectroscopy. The photophysical properties of all compounds were analyzed. UV–Vis spectra of 2a-b show π–π* transitions with shifts very similar to the free phosphole due to that their symmetrical structures inhibiting an efficient intraligand charge transfer, ILCT. Compounds 1, 2a-b exhibit fluorescence between 460 and 583 nm with quantum yields of Φf =0.04 − 0.11. The emission energy of 2b is higher than 2a, suggesting that λ max is affected by the ligand-field strength of the halide in the complexes (I¯ < Cl¯). The suggested structures of 2a-b were analyzed computationally at the PBEPBE/def2svp level of theory. The rotation barrier of the quinolyl group was analyzed by a scan analysis. Then, the 3 D structures of 2a-b were obtained in good agreement with the experimental results.

Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands.

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (1MLCT) and thiazole (tz)-induced intraligand charge-transfer (1ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered 3MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived 3ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τEm,1 ≈ 40-300 ns and τEm,2 ≈ 100-200 ns, which are attributed to emission from the 3ILCT and 3MLCT manifolds, respectively.

Rigidity controlled structures of Zn(II)-based coordination complexes: Synthesis and photophysical property study

Crystal engineering has been a hot research area for over two decades, due to the potential applications for gas storage and energy conversion et al. In this work, we have demonstrated that the rigidity of the building ligand has played a decisive influence on the final structure of coordination complex. XXQ-1 ({[Zn3(μ2OH)2(TIMB)(BDC)2]·0.25H2O}n) and XXQ-2 {[Zn2(IBIMB)(BDC)2(H2O)]·5H2O}n (TIMB = 1,3,5-tris(imidazol-1-ylmethyl)benzene, IBIMB = 1-(1-imidazolyl)-3,5-bis(imidazole-1-ylmethyl)benzene, BDC = 1,4-benzenedicarboxylate) were successfully synthesized from the hydrothermal reaction. XXQ-1 owned 3D framework with a (3,7)-connected network, and XXQ-2 was a 4-fold interpenetrated 3D framework. The structural differences for XXQ-1 and XXQ-2 were ascribed to the distinct rigidity of imidazole containing ligands, where the TIMB and IBIMB owned three and two flexible imidazole arms, respectively. Photophysical property for XXQ-1 and XXQ-2 has been investigated, where the strong photoluminescence was originated from the intraligand charge transfer (ILCT). Both complexes owned semiconductor property with the band gap energy of 3.77 eV and 2.83 eV for XXQ-1 and XXQ-2, respectively. This work has provided a great example to adjust the structures of coordination complexes through the ligand modification.

Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes.

Six rhenium(I) κ3N-dicarbonyl complexes with 4'-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe2-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σp substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ2N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Unusual Reactivity and Photophysical Properties of Platinum(II) Pincer Complexes Containing 6,6'‐Diphenyl‐2,2'‐bipyridine Ligands

A series of novel 6,6'‐diphenyl‐2,2'‐bipyridine ligands modified with the substituents at phenyl and bipyridine moieties have been prepared and used in the synthesis of pincer platinum chloride complexes 1–3, [Pt(NNC–R)Cl], where NNC–R – dimethyl 6,6'‐diphenyl‐[2,2'‐bipyridine]‐5,5'‐dicarboxylate (R = H, (1)), dimethyl 6,6'‐bis(4‐fluorophenyl)‐[2,2'‐bipyridine]‐5,5'‐dicarboxylate (R = F (2)), dimethyl 6,6'‐bis(4‐methoxyphenyl)‐[2,2'‐bipyridine]‐5,5'‐dicarboxylate (R = OMe (3)). Reactions of the obtained compounds with triphenylphosphine proceed through unusual pathway, which gives the [Pt(NNC–R)(PPh3)2Cl] products (1a–3a) containing two phosphine ligands, with the chloride retained in coordination sphere and diphenyl‐bipyridine fragment coordinated in η1‐mode through metalation of a pyridine ring. All complexes have been characterized with mass‐spectrometry and NMR spectroscopy, solid‐state structure of 1–3 and 1a, 3a was revealed by using XRD crystallography. The complexes 1–3 are phosphorescent in dichloromethane solution and in solid state. Their photophysical characteristics were determined and analyzed by DFT calculations, which gave assignment of emissive excited state character with the major contribution from the intraligand charge transfer (3ILCT, Ph→bipyridine) and ligand centered (3LC, bipyridine fragment) transitions.

Multinuclear 2-(Quinolin-2-yl)quinoxaline-Coordinated Iridium(III) Complexes Tethered by Carbazole Derivatives: Synthesis and Photophysics.

Five mono/di/trinuclear iridium(III) complexes (1-5) bearing the carbazole-derivative-tethered 2-(quinolin-2-yl)quinoxaline (quqo) diimine (N^N) ligand were synthesized and characterized. The photophysical properties of these complexes and their corresponding diimine ligands were systematically studied via UV-vis absorption, emission, and transient absorption (TA) spectroscopy and simulated by time-dependent density functional theory. All complexes possessed strong well-resolved absorption bands at <400 nm that have predominant ligand-based 1π,π* transitions and broad structureless charge-transfer (1CT) absorption bands at 400-700 nm. The energies or intensities of these 1CT bands varied pronouncedly when the number of tethered Ir(quqo)(piq)2+ (piq refers to 1-phenylisoquinoline) units, π conjugation of the carbazole derivative linker, or attachment positions on the carbazole linker were altered. All complexes were emissive at room temperature, with 1-3 showing near-IR (NIR) 3MLCT (metal-to-ligand charge-transfer)/3LLCT (ligand-to-ligand charge-transfer) emission at ∼710 nm and 4 and 5 exhibiting red or NIR 3ILCT (intraligand charge-transfer)/3LMCT (ligand-to-metal charge-transfer) emission in CH2Cl2. In CH3CN, 1-3 displayed an additional emission band at ca. 590 nm (3ILCT/3LMCT/3MLCT/3π,π* in nature) in addition to the 710 nm band. The different natures of the emitting states of 1-3 versus those of 4 and 5 also gave rise to different spectral features in their triplet TA spectra. It appears that the parentage and characteristics of the lowest triplet excited states in these complexes are mainly impacted by the π systems of the bridging carbazole derivatives and essentially no interactions among the Ir(quqo)(piq)2+ units. In addition, all of the diimine ligands tethered by the carbazole derivatives displayed a dramatic solvatochromic effect in their emission due to the predominant intramolecular charge-transfer nature of their emitting states. Aggregation-enhanced emission was also observed from the mixed CH2Cl2/ethyl acetate or CH2Cl2/hexane solutions of these ligands.

Os(II) Oligothienyl Complexes as a Hypoxia-Active Photosensitizer Class for Photodynamic Therapy.

Hypoxia presents a challenge to anticancer therapy, reducing the efficacy of many available treatments. Photodynamic therapy is particularly susceptible to hypoxia, given that its mechanism relies on oxygen. Herein, we introduce two new osmium-based polypyridyl photosensitizers that are active in hypoxia. The lead compounds emerged from a systematic study of two Os(II) polypyridyl families derived from 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmb) as coligands combined with imidazo[4,5-f][1,10]phenanthroline ligands tethered to n = 0-4 thiophenes (IP-nT). The compounds were characterized and investigated for their spectroscopic and (photo)biological activities. The two hypoxia-active Os(II) photosensitizers had n = 4 thiophenes, with the bpy analogue 1-4T being the most potent. In normoxia, 1-4T had low nanomolar activity (half-maximal effective concentration (EC50) = 1-13 nM) with phototherapeutic indices (PI) ranging from 5500 to 55 000 with red and visible light, respectively. A sub-micromolar potency was maintained even in hypoxia (1% O2), with light EC50 and PI values of 732-812 nM and 68-76, respectively -currently among the largest PIs for hypoxic photoactivity. This high degree of activity coincided with a low-energy, long-lived (0.98-3.6 μs) mixed-character intraligand charge-transfer (3ILCT)/ligand-to-ligand charge-transfer (3LLCT) state only accessible in quaterthiophene complexes 1-4T and 2-4T. The coligand identity strongly influenced the photophysical and photobiological results in this study, whereby the bpy coligand led to longer lifetimes (3.6 μs) and more potent photo-cytotoxicity relative to those of dmb. The unactivated compounds were relatively nontoxic both in vitro and in vivo. The maximum tolerated dose for 1-4T and 2-4T in mice was greater than or equal to 200 mg kg-1, an excellent starting point for future in vivo validation.

NIR-Absorbing RuII Complexes Containing α-Oligothiophenes for Applications in Photodynamic Therapy.

The design of near-infrared (NIR)-active photosensitizers (PSs) for light-based cancer treatments such as photodynamic therapy (PDT) has been a challenge. While several NIR-RuII scaffolds have been reported, this approach has not been proven in cells. This is the first report of NIR-RuII PSs that are phototoxic to cancer cells, including highly pigmented B16F10 melanoma cells. The PS family incorporated a bis(1,8-naphthyridine)-based ligand (tpbn), a bidentate thiophene-based ligand (nT; n=0-4), and a monodentate 4-picoline ligand (4-pic). All compounds absorbed light >800 nm with maxima near 730 nm. Transient absorption (TA) measurements indicated that n=4 thiophene rings (4T) positioned the PDT-active triplet intraligand charge transfer (3 ILCT) excited state in energetic proximity to the lowest-lying triplet metal-to-ligand charge transfer (3 MLCT). 4T had low-micromolar phototoxicity with PIvis and PI733nm values as large as 90 and 12, respectively. Spectroscopic studies suggested that the longer-lived (τTA =3-6 μs) 3 ILCT state was accessible from the 3 MLCT state, but energetically uphill in the overall photophysics. The study highlights that phototoxic effects can be achieved with NIR-absorbing RuII PSs as long as the reactive 3 ILCT states are energetically accessible from the low-energy 3 MLCT states. It also demonstrates that tissue-penetrating NIR light can be used to activate the PSs in highly pigmented cells where melanin attenuates shorter wavelengths of light.

Living Long and Prosperous: Productive Intraligand Charge-Transfer States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced Light Absorption.

The ground- and excited-state properties of six rhenium(I) κ2N-tricarbonyl complexes with 4'-(4-substituted-phenyl)terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly 4-fold variation of the triplet metal-to-ligand charge-transfer (3MLCT) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe2 group, we observed in the κ2N complex the appearance of a very strong absorption band, red-shifted by ca. 100 nm with respect to the other complexes. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 vs 1.5 ns), and a character change from 3MLCT to intraligand charge transfer (3ILCT), despite the remote location of the substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as photosensitizers for hydrogen production, with the κ2N-NMe2 complex resulting in a stable and efficient photocatalytic system reaching TONRe values of over 2100, representing the first application of the 3ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.

Tuning the Excited State of Tetradentate Pd(II) Complexes for Highly Efficient Deep-Blue Phosphorescent Materials.

Deep-blue-light-emitting materials are urgently desired in high-performance organic light-emitting diodes (OLEDs) for full-color display and solid-state lighting applications. However, the development of stable and efficient deep-blue emitters remains a great challenge. Herein, a series of stable and efficient tetradentate Pd(II)-complex-based deep-blue emitters with rigid 5/6/6 metallocycles and no F atom were designed and synthesized. These deep-blue emitters employ various isoelectronic five-membered heteroaryl-ring-containing ligands to exhibit extremely narrow emission spectra peaking at 439-443 nm with a full width at half-maximum (fwhm) of only 22-38 nm in 2-methyltetrahydrofuran at room temperature. In particular, the design of an intramolecular hydrogen bond enabled the 1-phenyl-1,2,3-trazole-based Pd(II) complexes to achieve CIEy < 0.1 (0.069-0.078; CIE is Commission Internationale de L'Eclairage). Theoretical calculation and natural transition orbital analysis reveal that these deep-blue materials emit light exclusively from their ligand (carbazole)-centered (3LC) states. Moreover, the triplet excited-state property can be efficiently regulated through ligand modification with isoelectronic oxazole and thiazole rings or pyridine rings, resulting in sky-blue-to-yellow materials, which emit light originating from an admixture of metal-to-ligand charge-transfer (3MLCT) and intraligand charge-transfer states. The newly developed Pd(II) complexes are strongly emissive in various matrixes with a quantum efficiency of up to 51% and also highly thermally stable with a 5% weight-reduction temperature (ΔT5%) of up to 400 °C. Deep-blue OLEDs with CIEy < 0.1 employing Pd(II) complexes as emitters were successfully fabricated for the first time. This study demonstrates that the Pd(II) complexes can act as excellent phosphorescent light-emitting materials through rational molecular design and also provide a valuable method for the development of Pd(II)-complex-based efficient and stable deep-blue emitters.

In Vitro Photodynamic Therapy of Mononuclear and Dinuclear Iridium(III) Bis(terpyridine) Complexes

Three mononuclear or dinuclear bis(terpyridine) (tpy) iridium(III) complexes bearing pyren-1-yl (pyr) group(s) were synthesized. Their photophysical properties in water and in vitro photodynamic therapy (PDT) effects toward the human lung epithelial cancer cell line A549 and the human epidermal skin cancer cell line A431 were investigated to evaluate the effects of dinuclear versus mononuclear complexes and the impact of the oligoether substituent at the ligand. All complexes possessed pyr-tpy ligand-associated charge transfer (1CT)/1π,π* absorption bands at 350-550 nm, with the dinuclear complex Ir3 showing the much enhanced absorptivity of this band. These complexes exhibited dual emission upon excitation at >430 nm in most cases, with the emitting states being ascribed to 1ILCT (intraligand charge transfer) and 3π,π*/3CT states, respectively. All complexes exhibited relatively weak to moderate cytotoxicity in the dark but high photocytotoxicity upon broadband visible light irradiation. Among them, the dinuclear complex Ir3 showed the highest intracellular reactive oxygen species (ROS) generation and PDT efficiency compared to its mononuclear counterpart Ir1. Introducing an oligoether substituent on one of the tpy ligands in Ir2 also improved its intracellular ROS generation and PDT efficacy compared to those induced by Ir1. Ir3 induced both mitochondrial dysfunction and lysosomal damage upon light activation toward both cell lines, whereas Ir1 and Ir2 caused both mitochondrial dysfunction and lysosomal damage in A431 cells but only lysosomal damage in A549 cells. The dominant cell death pathway induced by Ir1-Ir3 PDT is apoptosis.

Unsymmetric Heteroleptic Ir(III) Complexes with 2-Phenylquinoline and Coumarin-Based Ligand Isomers for Tuning Character of Triplet Excited States and Achieving High Electroluminescent Efficiencies.

2-Phenylquinoline (PQ) and four coumarin-based ligand isomers with ease of synthesis have been selected to construct the unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)]-type complex phosphors for organic light-emitting diodes (OLEDs). Six unsymmetric heteroleptic Ir(III) complexes have been obtained by employing four coumarin-based ligand isomers (L-C5/L-C6/L-C7/L-C8) in the [Ir(PQ)(C∧N)(acac)] structure due to two different coordinating carbon atoms in ligands L-C6 and L-C7 to form C-Ir bond. Through adopting unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)] structure, these Ir(III) complexes can not only achieve impressive absolute quantum yield Φp (ca. 0.5-1.0), higher than that of complex [Ir(PQ)2(acac)] (ca. 0.4), but also realize a dual modulation of both emission color from orange (AIrC6out, λ = 578 nm) to red (AIrC5, λ = 622 nm) and the character of the lowest triplet excited states (T1), showing both 3MLCT character and 3ILCT (intraligand charge transfer) character in their T1 states. AIrC5, AIrC7out, and AIrC7in show MLCT character from Ir(III) center to ligand L-C5 or L-C7 and ILCT character in ligand L-C5 or L-C7 in their T1 states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character from Ir(III) center to ligand PQ and ILCT character in ligand PQ in their T1 states. Moreover, the color-tuning mechanism and the lowest triplet state characters are investigated in detail. AIrC6in and AIrC8 were selected as emitters to evaluate the electroluminescent (EL) performance due to their high ΦP of nearly up to unity. Optimal orange-emitting device B2 based on AIrC8 can give a maximum external quantum efficiency (ηext) of 23.9%, a maximum current efficiency (ηL) of 70.9 cd A-1, and a maximum power efficiency (ηP) of 60.7 lm W-1. All these impressive results can definitely demonstrate the effectiveness of our simple approach for tuning character of the triplet excited states and achieving high-performance Ir-based phosphors in OLEDs.

Improving triplet excited-state absorption and lifetime of cationic iridium(III) complexes by extending π-conjugation of the 2-(2-quinolinyl)quinoxaline ligand

The synthesis and photophysical properties (UV−vis absorption, emission, and transient absorption) of four cationic Ir(III) complexes (C^N)2Ir(R-quqo)+ (HC^N = 1-phenylisoquinoline (piq) and 1,2-diphenylpyreno[4,5-d]imidazole (dppi), quqo = 2-(2-quinolinyl)quinoxaline, R = H or fluorenyl) are reported. The UV–vis absorption and emission were simulated by time-dependent density functional theory (TDDFT). Influences of extending π-conjugation of the C^N ligand and the diimine ligand on the singlet and triplet excited-state absorption and lifetimes of these complexes were explored. All complexes exhibited intense ligand-localized 1π,π transitions, broad and structureless metal-to-ligand charge transfer (1MLCT) / ligand-to-ligand charge transfer (1LLCT) transitions, and very weak spin-forbidden 3MLCT/3LLCT/3π,π transitions in their UV–vis absorption spectra. The two complexes that bear fluorenyl-substituted quqo ligands (Ir-3 and Ir-4) also possessed a broad intraligand charge transfer (1ILCT) / 1π,π band at 430–550 nm. The predominant 3ILCT/3π,π characters of the triplet excited states of Ir-3 and Ir-4 improved their phosphorescent emission quantum yields and prolonged their triplet lifetimes compared to the weaker and short-lived emission of Ir-1 and Ir-2. In contrast to the very weak nanosecond transient absorption (TA) of Ir-1 and Ir-2, Ir-3 and Ir-4 possessed much stronger TA signals at 520−800 nm upon nanosecond laser excitation. These complexes exhibited moderate to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses, with the RSA trend following Ir-1 >Ir-2 ≈ Ir-3 >Ir-4. Considering the long triplet excited-state lifetimes and broadband TA, complexes Ir-3 and Ir-4 could be potential broadband RSA materials.

Intraligand charge transfer boosts visible-light-driven generation of singlet oxygen by metal-organic frameworks

Generation of singlet oxygen (1O2) with metal-organic frameworks (MOFs) is limited by either poor light harvesting ability of type I MOFs or low availability of type II MOFs. Herein, we report a facile and widely applicable strategy, post-synthetic modification of type I MOFs with photo-redox active acetylacetone (AA) via a diazo coupling reaction, to enhance the visible-light-driven generation of 1O2 by MOFs. The introduction of AA to MOFs with variant metal nodes, such as Zr, Ti, Al, and Cr clusters, leads to intraligand charge transfer (ILCT), which not only strengthens the ligand to metal cluster charge transfer (LCCT) but also expands the photo-responsive edge of MOFs. Via the coupled ILCT and LCCT processes, UiO-66-AA functions as a hybrid type I-type II MOFs, which can efficiently generate 1O2 even at indoor lighting conditions. This work would open an avenue for the synthesis of MOFs towards diverse visible-light photocatalysis.

Two temperature-dependent Cd(II)-based coordination polymers with mixed adenine nucleobase and benzene-1,4-dicarboxylic acid: synthesis, structures and fluorescence properties

Two Hade-based Cd(II) polymers, [Cd(Hade)(tp)]n (1) and {[Cd6(Hade)6(tp)6]·9H2O}n (2) (Hade = adenine, H2tp = benzene-1,4-dicarboxylic acid), were obtained from the same reaction system at different temperatures which have been characterized by single-crystal X-ray diffraction, elemental analysis, FT-IR spectroscopy, thermogravimetric analysis, and luminescence spectroscopy. Significantly resulting from the synergistic coordination of nucleobase and bicarboxylate groups, they are a 3-D pillared-layer structure for 1 and a 2-D grid structure where the dicadmium paddle-wheel motifs are double bridged by benzene-1,4-dicarboxylate anions for 2. The supramolecular architecture of 2 is essentially knitted by hydrogen bonding interactions produced by the exocyclic amino/endocyclic imino groups of the nucleobase and the carboxylate groups of the co-ligand to favorably stabilize the high-dimensional supramolecular architectures. At room temperature, 1 and 2 exhibit intense luminescent emissions originating from a Hade-based intraligand and/or photoinduced charge transfer upon adenine binding mode.

Green-Light Activation of Push-Pull Ruthenium(II) Complexes.

Synthesis, characterization, electrochemistry, and photophysics of homo‐ and heteroleptic ruthenium(II) complexes [Ru(cpmp)2]2+ (22+) and [Ru(cpmp)(ddpd)]2+ (32+) bearing the tridentate ligands 6,2’’‐carboxypyridyl‐2,2’‐methylamine‐pyridyl‐pyridine (cpmp) and N,N’‐dimethyl‐N,N’‐dipyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) are reported. The complexes possess one (32+) or two (22+) electron‐deficient dipyridyl ketone fragments as electron‐accepting sites enabling intraligand charge transfer (ILCT), ligand‐to‐ligand charge transfer (LL'CT) and low‐energy metal‐to‐ligand charge transfer (MLCT) absorptions. The latter peak around 544 nm (green light). Complex 22+ shows 3MLCT phosphorescence in the red to near‐infrared spectral region at room temperature in deaerated acetonitrile solution with an emission quantum yield of 1.3 % and a 3MLCT lifetime of 477 ns, whereas 32+ is much less luminescent. This different behavior is ascribed to the energy gap law and the shape of the parasitic excited 3MC state potential energy surface. This study highlights the importance of the excited‐state energies and geometries for the actual excited‐state dynamics. Aromatic and aliphatic amines reductively quench the excited state of 22+ paving the way to photocatalytic applications using low‐energy green light as exemplified with the green‐light‐sensitized thiol–ene click reaction.

Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes.

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 μs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing 3MLCT and 3ILCT contributions across the series.

Synthesis, characterization and molecular docking studies of Co(II) metal complex of sulfathiazole

Sulfathiazole (SFTZ) is a sulfonamide used for the treatment of bacterial infection. The cobalt complex of sulfathiazole was synthesized by reaction of sulfathiazole with CoCl2.6H2O. The metal complex was characterized based on AAS, UV, IR, 1H NMR spectroscopy and X-ray powder diffraction. The electronic spectrum of the ligand showed intra ligand charge transfer (ILCT) which were assigned to the chromophores present in the ligand, while that of the complex suggested intra ligand charge transfer (ILCT) and ligand to metal charge transfer (LMCT). The IR spectra of the complexes showed the involvement of amine, sulfonyl and cyano group in coordination to the metal ion. This showed that sulfathiazole acted as a tridentate ligand. 1H NMR spectrum of [Co(SFTZ)] complex further showed the involvement of the amine and sulfonyl group in coordination to the metal ions. The structure of [Co(SFTZ)] complex was assigned as trigonal. The crystal structure of [Co(SFTZ)] complex belongs to cubic system, space group P1, with cell parameters of a = 4.007 Å, b = 5.0078 Å, c = 5.9844 Å, 𝑉 = 30.61 Å3, α = 90o, β 90o, γ = 90o. Molecular docking suggested that the ligand/complex binded effectively with the E. Coli and S. aureus because their global binding energies were negative. The binding interactions of ligand/complex with E. Coli and S. Aureus were predicted. Molecular docking predicted the feasibility of the biochemical reactions before experimental investigation.&#x0D; &#x0D; Bull. Chem. Soc. Ethiop. 2020, 34(1), 83-92.&#x0D; DOI: https://dx.doi.org/10.4314/bcse.v34i1.8

A New Facial Homoleptic Tris‐cyclometalated Iridium(III) Complex with Oxygen‐bridged Triarylamine Units

AbstractA new facial homoleptic tris‐cyclometalated phosphorescent iridium(III) complex Ir(ppyNO)3 featuring three oxygen‐bridged triarylamine units has been synthesized via an efficient Suzuki–Miyaura cross‐coupling reaction of oxygen‐bridged triarylamine unit based boron reagent 1‐b with corresponding tribrominated iridium(III) complex Ir‐Br. It can not only show an effective green light emission, but also display a narrower full width at half maximum (FWHM=57 nm) and better solubility than that of the classical complex fac‐Ir(ppy)3 (FWHM=82 nm, ppy=2‐phenylpyridine). Furthermore, a longer emission lifetime (τ=1201 ns) of Ir(ppyNO)3 in polystyrene (PS) film can be found than that (τ=45 ns) in CH2Cl2 solution. In addition, the lowest triplet state (T1) of Ir(ppyNO)3 can exhibit both metal‐to‐ligand charge transfer (3MLCT) and intraligand charge transfer (3ILCT) excited state character according to DFT calculations.

Ru(II)-polypyridine complexes with alkynyl Schiff base ligand: influence of π-conjugation, donor/acceptor substituents, and counter anions on electrochemical, luminescence, and catalytic properties

Ru(II)-polypyridine complexes of the general formula [Ru(L1/L2)(phen)2]X2 (1a–6a) and [Ru(L1/L2)(bipy)2]X2 (1b–6b) (where X = ClO4, BF4, PF6; phen = 1,10-phenanthroline, bipy = 2,2′-bipyridine) were prepared by the reaction of [Ru(phen)2Cl2]·2H2O and [Ru(bipy)2Cl2]·2H2O with (E)-5-((4-methoxyphenyl)ethynyl)-N-(pyridin-2-ylmethylene)pyridin-2-amine (L1) and (E)-5-((4-nitrophenyl)ethynyl)-N-(pyridin-2-ylmethylene)pyridine-2-amine (L2) in the presence of NaBF4, NaClO4, and NaPF6. The electrochemical properties of all the complexes indicate reversible redox behavior corresponding to Ru(II)–Ru(III) couple and are susceptible to variation of electron-donating/accepting properties of substituent group on L1 and L2. All complexes showed room temperature luminescence corresponding to π→π* intra-ligand charge-transfer (ILCT) transition with chelation enhanced fluorescence and is finely tuned by increasing π-conjugation, size of counter anions, and variation of substituent group with different electronic effects in the complexes. All the complexes worked as an effective catalyst for the oxidation of benzyl alcohol to corresponding benzaldehyde in good yield at room temperature.