Intraligand Charge Transfer State Research Articles

The interplay between structural features and the efficiency of the energy transfer process in terbium complexes with triarylcyclopentadienyl ligands

Terbium and gadolinium complexes with triarylcyclopentadienyl ligands having electron-donating methoxy substituents in the ortho- (CpPh2Ar−o) and para- (CpPh2Ar−p) positions of one of the phenyl rings have been synthesized. Different structural types, including tetranuclear [{CpPh2Ar−oTb (THF)}2 (μ2-Cl)2(μ3-Cl)3K (THF)]2 (Tb1), [{CpPh2Ar−oTb}2 (μ2-Cl)2 (μ3-Cl)3K]2 (Tb3), [{(CpPh2Ar−oTb)2 (μ2-Cl)3}2 (μ2-Cl)2] (Tb5), [{CpPh2Ar−pTb (THF)}2 (μ2-Cl)2 (μ3-Cl)3K (THF)]2 (Tb6), binuclear [{CpPh2Ar−oTb (THF)Cl}2 (μ2-Cl)2] (Tb4), [{CpPh2Ar−pTb (THF)}2 (μ2-Cl)2 (μ3-Cl)3K (THF)]2 (Tb7), [CpPh2Ar−p2Tb (μ2-Cl) (μ3-Cl)K]2 (Tb8) and mononuclear [CpPh2Ar−o2TbCl] (Tb2) complexes have been obtained and confirmed by single crystal X-ray diffraction analysis. All designed terbium complexes exhibit high quantum yields of the photoluminescence (up to 65 %). The introduction of a methoxy group into one of the phenyl rings of the triarylcyclopentadienyl ligand leads to a more than twofold increase in the quantum yield of the terbium ion luminescence. The estimated values of radiative rate krad of the terbium ion vary in the range of 0.23–0.69 ms. The presence of the methoxy group as well as the mutual disposition of the phenyl rings promotes the appearance of an intraligand charge transfer states involved in the energy transfer process. Due to a lowering of the energy of the interconfigurational 4f −5d transitions caused by the relatively strong crystal field of the Cp ligand in bis(cyclopentadienyl) terbium complexes, the quantum yield of the photoluminescence is high at the short lifetimes of the 5D4 level of the terbium ion.

Intramolecular charge transfer enables highly-efficient X-ray luminescence in cluster scintillators

Luminescence clusters composed of organic ligands and metals have gained significant interests as scintillators owing to their great potential in high X-ray absorption, customizable radioluminescence, and solution processability at low temperatures. However, X-ray luminescence efficiency in clusters is primarily governed by the competition between radiative states from organic ligands and nonradiative cluster-centered charge transfer. Here we report that a class of Cu4I4 cubes exhibit highly emissive radioluminescence in response to X-ray irradiation through functionalizing biphosphine ligands with acridine. Mechanistic studies show that these clusters can efficiently absorb radiation ionization to generate electron-hole pairs and transfer them to ligands during thermalization for efficient radioluminescence through precise control over intramolecular charge transfer. Our experimental results indicate that copper/iodine-to-ligand and intraligand charge transfer states are predominant in radiative processes. We demonstrate that photoluminescence and electroluminescence quantum efficiencies of the clusters reach 95% and 25.6%, with the assistance of external triplet-to-singlet conversion by a thermally activated delayed fluorescence matrix. We further show the utility of the Cu4I4 scintillators in achieving a lowest X-ray detection limit of 77 nGy s−1 and a high X-ray imaging resolution of 12 line pairs per millimeter. Our study offers insights into universal luminescent mechanism and ligand engineering of cluster scintillators.

Tuning the photophysical properties of lanthanide(iii)/zinc(ii) 'encapsulated sandwich' metallacrowns emitting in the near-infrared range.

A family of Zn16Ln(HA)16 metallacrowns (MCs; Ln = YbIII, ErIII, and NdIII; HA = picoline- (picHA2−), pyrazine- (pyzHA2−), and quinaldine- (quinHA2−) hydroximates) with an ‘encapsulated sandwich’ structure possesses outstanding luminescence properties in the near-infrared (NIR) and suitability for cell imaging. Here, to decipher which parameters affect their functional and photophysical properties and how the nature of the hydroximate ligands can allow their fine tuning, we have completed this Zn16Ln(HA)16 family by synthesizing MCs with two new ligands, naphthyridine- (napHA2−) and quinoxaline- (quinoHA2−) hydroximates. Zn16Ln(napHA)16 and Zn16Ln(quinoHA)16 exhibit absorption bands extended into the visible range and efficiently sensitize the NIR emissions of YbIII, ErIII, and NdIII upon excitation up to 630 nm. The energies of the lowest singlet (S1), triplet (T1) and intra-ligand charge transfer (ILCT) states have been determined. LnIII-centered total (QLLn) and intrinsic (QLnLn) quantum yields, sensitization efficiencies (ηsens), observed (τobs) and radiative (τrad) luminescence lifetimes have been recorded and analyzed in the solid state and in CH3OH and CD3OD solutions for all Zn16Ln(HA)16. We found that, within the Zn16Ln(HA)16 family, τrad values are not constant for a particular LnIII. The close in energy positions of T1 and ILCT states in Zn16Ln(picHA)16 and Zn16Ln(quinHA)16 are preferred for the sensitization of LnIII NIR emission and ηsens values reach 100% for NdIII. Finally, the highest values of QLLn are observed for Zn16Ln(quinHA)16 in the solid state or in CD3OD solutions. With these data at hand, we are now capable of creating MCs with desired properties suitable for NIR optical imaging.

Optically Controlled Electron Transfer in a ReI Complex.

Ultrafast optical control of intramolecular charge flow was demonstrated, which paves the way for photocurrent modulation and switching with a highly wavelength-selective ON/OFF ratio. The system that was explored is a fac-[Re(CO)3 (TTF-DPPZ)Cl] complex, where TTF-DPPZ=4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine. DFT calculations and AC-Stark spectroscopy confirmed the presence of two distinct optically active charge-transfer processes, namely a metal-to-ligand charge transfer (MLCT) and an intra-ligand charge transfer (ILCT). Ultrafast transient absorption measurements showed that the ILCT state decays in the ps regime. Upon excitation to the MLCT state, only a long-lived 3 MLCT state was observed after 80 ps. Remarkably, however, the bleaching of the ILCT absorption band remained as a result of the effective inhibition of the HOMO-LUMO transition.

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.

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.

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.

Energy transfer in ternary TbEDTA chelates with a series of dipicolinic acid derivatives

The energy transfer in lanthanoid chelates was studied using the ternary TbEDTA (EDTA = ethylenediaminetetraacetic acid) chelates with several dipicolinic acid derivatives (pyridine-2,6-dicarboxylic acid (L1, dpa), 4-(9H-fluoren-3-yl)pyridine-2,6-dicarboxylic acid (L2), 4-(dibenzo [b,d]furan-2-yl)pyridine-2,6-dicarboxylic acid (L3), 4-(dibenzo [b,d]thiophen-2-yl)pyridine-2,6-dicarboxylic acid (L4) and 4-(9H-carbazol-3-yl)pyridine-2,6-dicarboxylic acid (L5)) and spectroscopic methods (absorption and luminescence spectroscopy and the effect of argon treatment and temperature on luminescence lifetime(s)). The results revealed that the ILCT (intra-ligand charge transfer) state is inefficient in exciting the Tb(III) ion and the emissive states of the ligands, as well as the triplet states, act as quenching states by receiving energy back transfer from the Tb(III) ion. The stability constants for these ternary complexes were also determined.

Polyphenylcyclopentadienyl Ligands as an Effective Light-Harvesting π-Bonded Antenna for Lanthanide +3 Ions.

A new approach to design "antenna-ligands" to enhance the photoluminescence of lanthanide coordination compounds has been developed based on a π-type ligand-the polyphenyl-substituted cyclopentadienyl. The complexes of di-, tri-, and tetraphenyl cyclopentadienyl ligands with Tb and Gd have been synthesized and all the possible structural types from mononuclear to di- and tetranuclear complexes, as well as a coordination polymer were obtained. All types of the complexes have been studied by single-crystal X-ray diffraction and optical spectroscopy. All terbium complexes are luminescent at ambient temperature and two of them have relatively high quantum yields (50 and 60%). Analysis of energy transfer process has been performed and supported by quantum chemical calculations. The role of a low-lying intraligand charge transfer state formed by extra coordination with K+ in the Tb3+ ion luminescence sensitization is discussed. New aspects for design of lanthanide complexes containing π-type ligands with desired luminescence properties have been proposed.

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

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

Intraligand Charge-Transfer Excited States in Re(I) Complexes with Donor-Substituted Dipyridophenazine Ligands

The donor-acceptor ligands 11-(4-diphenylaminophenyl)dipyrido[3,2-a:2',3'-c]phenazine (dppz-PhNPh2) and 11-(4-dimethylaminophenyl)dipyrido[3,2-a:2',3'-c]phenazine (dppz-PhNMe2), and their rhenium complexes, [Re(CO)3X] (X = Cl(-), py, 4-dimethylaminopyridine (dmap)), are reported. Crystal structures of the two ligands were obtained. The optical properties of the ligands and complexes are dominated by intraligand charge transfer (ILCT) transitions from the amine to the dppz moieties with λabs = 463 nm (ε = 13 100 M(-1) cm(-1)) for dppz-PhNMe2 and with λabs = 457 nm (ε = 16 900 M(-1) cm(-1)) for dppz-PhNPh2. This assignment is supported by CAM-B3LYP TD-DFT calculations. These ligands are strongly emissive in organic solvents and, consistent with the ILCT character, show strong solvatochromic behavior. Lippert-Mataga plots of the data are linear and yield Δμ values of 22 D for dppz-PhNPh2 and 20 D for dppz-PhNMe2. The rhenium(I) complexes are less emissive, and it is possible to measure resonance Raman spectra. These data show relative band intensities that are virtually unchanged from λexc = 351 to 532 nm, consistent with a single dominant transition in the visible region. Resonance Raman excitation profiles are solvent sensitive; these data are modeled using wavepacket theory yielding reorganization energies ranging from 1800 cm(-1) in toluene to 6900 cm(-1) in CH3CN. The excited state electronic absorption and infrared spectroscopy reveal the presence of dark excited states with nanosecond to microsecond lifetimes that are sensitive to the ancillary ligand on the rhenium. These dark states were assigned as phenazine-based (3)ILCT states by time-resolved infrared spectroscopy. Time-resolved infrared spectroscopy shows transient features in which Δν(CO) is approximately -7 cm(-1), consistent with a ligand-centered excited state. Evidence for two such states is seen in mid-infrared transient spectra.

Complexes with Redox-Active Ligands: Synthesis, Structure, and Electrochemical and Photophysical Behavior of the Ru(II) Complex with TTF-Annulated Phenanthroline

Ru(II) complexes with chelating ligands, 4',5'-ethylenedithiotetrathiafulvenyl[4,5-f][1,10]phenanthroline (L1), 1,3-dithiole-2-thiono[4,5-f][1,10]phenanthroline (L2), and 1,3-dithiole-2-ono[4,5-f][1,10]phenanthroline (L3), have been prepared and their structural, electrochemical, and photophysical properties investigated. Density functional theory (DFT) calculations indicate that the highest occupied molecular orbital of [Ru(bpy)2(L1)](PF6)2 (1) is located on the tetrathiafulvalene (TTF) subunit and appears ~0.6 eV above the three Ru-centered d orbitals. In agreement with this finding, 1 exhibits three reversible oxidations: the two at lower potentials take place on the TTF subunit, and the one at higher potential is due to the Ru(3+)/Ru(2+) redox couple. Complexes [Ru(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)](PF6)2 (3) exhibit only the Ru(3+)/Ru(2+)-related oxidation. The optical absorption spectra of all complexes reveal a characteristic metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of 1 the MLCT band is augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition of L1, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of 1 compared to those of 2 and 3 is attributed to the reductive quenching of the emissive state via electron transfer from the TTF subunit to the Ru(3+) center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the presence of O2, complex 1 undergoes a photoinduced oxidative cleavage of the central C═C bond of the TTF fragment, resulting in complete transformation to 3. This photodegradation process was studied with (13)C NMR and optical absorption spectroscopy.

Quadratic susceptibility and first hyperpolarizability of the complex of Cr(CO)3 with [2.2]paracyclophane

The peculiarities of chemical bonding and nonlinear optical properties of the complex of Cr(CO)3 with [2.2]paracyclophane (Cr–pCp) have been studied by a combined experimental–theoretical approach. The Cr–pCp complex exhibits second harmonic generation, the efficiency of which was measured by the powder technique at 1064.2nm fundamental wavelength and estimated theoretically. The comparison of electronic structure of the Cr–pCp with benzene tricarbonyl chromium analogue has revealed an important role of intraligand charge transfer state in pCp moiety conditioned by a strong transannular interaction between aromatic rings which is responsible for the enhanced molecular hyperpolarizability of Cr–pCp.

A Donor–Acceptor Tetrathiafulvalene Ligand Complexed to Iron(II): Synthesis, Electrochemistry, and Spectroscopy of [Fe(phen)2(TTF-dppz)](PF6)2

The synthesis and photophysical properties of the complex [Fe(phen)(2)(TTF-dppz)](2+) (TTF-dppz = 4',5'-bis-(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine, phen = 1,10-phenanthroline) are described. In this complex, excitation into the metal-ligand charge transfer bands results in the population of a high-spin state of iron(II), with a decay lifetime of approximately 1.5 ns, in dichloromethane, at room temperature. An intraligand charge transfer state can also be obtained and has a lifetime of 38 ps. A mechanism for the different states reached is proposed based on transient absorption spectroscopy.

Photophysical properties of a new series of water soluble iridium bisterpyridine complexes functionalised at the 4′ position

Four new hetero- and homo-leptic iridium(III) bisterpyridine complexes have been prepared which incorporate aniline (tpy-φ-NH(2)), benzoic acid (tpy-φ-COOH), and benzyl alcohol (tpy-φ-CH(2)OH) substituents at the 4' positions of the tpy ligands (tpy = 2,2':6',2''-terpyridine, φ = phenylene). The electrochemical behaviour and ground and excited state spectroscopic properties of the complexes are reported, and the X-ray crystal structures of a homoleptic benzyl alcohol [Ir(tpy-φ-CH(2)OH)(2)](PF(6))(3), homoleptic aniline [Ir(tpy-φ-NH(2))(2)](PF(6))(3), and heteroleptic benzyl alcohol/aniline substituted complex [Ir(tpy-φ-CH(2)OH)(tpy-φ-NH(2))](PF(6))(3) have been solved. Complexes with aniline substituents were found to display absorption bands at around 430 nm corresponding to intraligand charge transfer (ILCT) that are sensitive to changes in solvent and pH. Strong emission in the visible region involving the ILCT state is observed in two of the complexes (Φ(e) = 0.7% and 2.6%) in acetonitrile. In the heteroleptic aniline/benzyl alcohol complex the Stokes shift is shown to be linearly related to solvent polarisability according to the Lippert equation, but only for solvents with weak hydrogen bonding interactions. Additionally, in water, emission from the ILCT state is quenched and only weak ligand centred (LC) emission is observed. The long lifetimes and quantum yields of these complexes make them interesting candidates for probes in sensing applications, especially [Ir(tpy-φ-CH(2)OH)(tpy-φ-NH(2))(2)](PF(6))(3) due to its unusual sensitivity to the solvent environment.

Back-to-Back Dinuclear Platinum Terpyridyl Complexes: Synthesis and Photophysical Studies

Two back-to-back terpyridine ligands using fluorenyl as bridging group (1-L and 2-L) and their corresponding dinuclear platinum(II) complexes (1 and 2) were synthesized and characterized. Their electronic absorption, photoluminescence, and the triplet transient difference absorption were systematically investigated. Both ligands possess intense (1)pi,pi* absorption in the UV region, and they exhibit structured (1)pi,pi* fluorescence around 400 nm. With addition of p-toluenesulfonic acid to the ligands, both the absorption band and the emission band are red-shifted because of the increased electron-withdrawing ability of the protonated terpyridines and possible mixture of some intraligand charge transfer (ILCT) character. For complexes 1 and 2, they both exhibit broad and strong absorption between 400 and 500 nm, which is assigned as the (1)pi,pi*/(1)ILCT/(1)MLCT (metal-to-ligand charge transfer) transition. The involvement of (1)ILCT in the lowest excited state is evident by the acid titration experiment of the ligands. At room temperature, the complexes exhibit dual emission that admixes fluorescence and phosphorescence from the (1,3)pi,pi*/(1,3)ILCT/(1,3)MLCT states. The assignment of the emitting states is based on the distinct emission lifetimes, different sensitivity to oxygen quenching, and different temperature dependency. Both complexes exhibit emission at 77 K, which is assigned as the mixture of (3)pi,pi*/(3)MLCT. 1 and 2 also exhibit two triplet excited-state absorption bands in the visible to the NIR region, which are tentatively attributed to the (3)pi,pi* and (3)MLCT/(3)ILCT state. In addition, the connection pattern between the fluorenyl component and the terpyridyl components influences the excited-state characteristics of both the ligands and the complexes. Ligand 1-L and its corresponding platinum complex 1 that have the triplet bond connection between the fluorenyl and terpyridyl components exhibit a red-shifted low-energy absorption band, an emission band, and a transient absorption band compared to ligand 2-L and complex 2 that have the fluorenyl directly attached to terpyridyl components. These differences could be rationalized by the enhanced conjugation between the fluorenyl and terpyridyl components in 1-L and 1 because of the better coplanarity induced by the triple bond.

Molecular logic circuit based on a multi-state mononuclear platinum(ii) terpyridyl complex

A new type of multi-state platinum(ii) terpyridyl acetylide complex: platinum(ii) 4'-(4-dimethylaminophenyl)-2,2' : 6',2''-terpyridyl ferrocenyl acetylide (1), together with its reference complexes: platinum(ii) 4'-(4-methylphenyl)-2,2' : 6',2''-terpyridyl ferrocenyl acetylide (2), platinum(ii) 4'-(4-dimethylaminophenyl)-2,2' : 6',2''-terpyridyl phenyl acetylide (3) and platinum(ii) 4'-(4-methylphenyl)-2,2' : 6',2''-terpyridyl phenyl acetylide (4), are designed and synthesized. UV-vis-NIR absorption and electrochemical studies demonstrate that complex 1 possesses three ordered intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) states, and the successive modulation of the intramolecular ILCT, LLCT and MLCT states in a single mononuclear platinum(ii) complex 1 is well evidenced by their reference complexes 2-4. More interestingly, a near-infrared absorption band ranging from 720-1000 nm was clearly observed for complexes 1 and 2 upon oxidation of the σ-bonded ferrocene-ethynyl group by Fe(ClO(4))(3), which was tentatively assigned to platinum(ii)-disturbed ligand-to-metal charge transfer (LM'CT) of the ferrocenium group. These promising features allow us to integrate the multiple states to reproduce basic logic operations at the molecular level. A four-state (LLCT, ILCT, MLCT and LM'CT) molecular switch of complex 1 that responds to the stimulation of H(+), Fe(ClO(4))(3) and Zn producing four outputs has been encoded in binary digits. The logic function executed by complex 1 is equivalent to that of a combinational logic circuit integrating seven or eight logic functions as AND, OR, NOT. A three-input-four-output molecular logic circuit has been established.

A sensitive phosphorescent thiol chemosensor based on an iridium(iii) complex with α,β-unsaturated ketone functionalized 2,2′-bipyridyl ligand

An iridium(III)-containing phosphorescent chemosensor Ir(ppy)(2)(L)(PF(6)) (1, ppy = 2-phenylpyridine) containing a 2,2'-bipyridyl ligand (L) functionalized with an alpha,beta-unsaturated ketone for selective detection of thiol was synthesized and characterized by spectroscopic and photophysical measurements. The structure of complex 1 was determined by X-ray crystallography. In order to get an insight into 1,4-addition reactions of thiol to complex 1, the adduct 2 from reaction of 1 with benzenethiol was successfully prepared and characterized. Complex 1 shows a lowest energy absorption at ca. 450 nm, primarily ascribable to an intraligand charge transfer (ILCT) transition from the HOMO (pi) resident on the fragment -C(O)C(6)H(4)N(C(2)H(5))(2) to the LUMO (pi*) localized on the 2,2'-bipyridyl moiety in the functionalized 2,2'-bipyridyl ligand as suggested from DFT computational studies. Complex 1 is weakly emissive at ca. 587 nm at ambient temperature, arising likely from the (3)ILCT excited state. Upon addition of thiol to a semi-aqueous solution of complex 1, the lowest energy absorption is obviously blue-shifted and the emission is remarkably enhanced due probably to a conversion from the primary ILCT state to the predominant [pi(ppy)-->pi*(L)] LLCT and the [5d(Ir)-->pi*(L)] MLCT state caused by the formation of the 1-thiol adduct. The sensing properties of 1 to thiol were also investigated by ESI-MS spectrometry and (1)H NMR spectroscopy.

Photophysics of an Intramolecular Hydrogen‐Evolving Ru–Pd Photocatalyst

Photoinduced electron-transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)(2)Ru(tpphz)PdCl(2)](2+) (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3'',2'',-h:2''',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)(2)Ru(tpphz)](2+) subunit Ru with that of the supramolecular catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the molecular ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramolecular complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-determining steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-separated states.