Metal-centered Excited States Research Articles

Probing the photophysical capability of mono and bis(cyclometallated) Fe(ii) polypyridine complexes using inexpensive ground state DFT.

The abundance and low toxicity of iron with respect to ruthenium would certainly make it valuable for photophysical applications if one could circumvent its tendency to make high-spin compounds and the kinetic lability of its polypyridine complexes, both related to the presence of low-lying quintet metal-centered excited states. The aim of this study was to probe the photophysical potential of six cyclometallated Fe(ii) polypyridine complexes by means of ground state DFT and TDDFT calculations. Quantitative and qualitative indicators were extracted from such calculations and bring us to the conclusion that two complexes should display promising photophysical properties: Fe(NCN)(NNC) and Fe(NNC)2.

Influence of alkyl chain lengths on the properties of iridium(III)-based piezochromic luminescent dyes with triazole-pyridine type ancillary ligands

To investigate the relationship between structures and piezochromic luminescence behavior and to develop excellent materials for efficient light-emitting electrochemical cells, a series of iridium(III) complexes with different length of alkyl chains have been designed and synthesized. The results show that all complexes exhibit not only naked-eye visible and reversible piezochromic behavior, but chain length-dependent emission properties: the shorter the alkyl chain is, the more remarkable mechanochromic behavior and higher recrystallization temperature will be. In light of powder X-ray diffractometry and differential scanning calorimetric data, the interconversion between crystalline and amorphous states upon external stimuli is response for the present piezochromism. Additionally, these complexes show high quantum yields of 55–65% in neat films as well as excellent redox reversibility. Despite changed alkyl chains are introduced into these complexes, the negligible effect on emitting colors and excited-state characters of them in both solutions and neat films have been observed. Such photophysical properties are also interpreted with the help of theoretical calculations. Moreover, the theoretical results suggest that the intrinsic intramolecular π–π stacking confirmed by the crystal structures can effectively restrict the opening of the structures in metal-centered excited-state, demonstrating their stability. The structure–property relationships and the results demonstrated here will help researchers develop and design more promising iridium(III)-based piezochromic materials and attractive phosphors for optical devices.

Cloning of the R1 and R2 subunits of ribonucleotide reductase from Helicobacter pylori

Nonradiative deactivation of metal-centered excited state by multiphonon relaxation (MR) is one of the most important processes in the photophysics of trivalent lanthanoid ions. In molecular complexes, the dominant drivers for this phenomenon are strongly anharmonic, high-energy local-mode vibrations such as O–H, N–H, or C–H stretching modes in surrounding ligands and solvent molecules. This chapter discusses MR of lanthanoid luminescence with a special focus on the unique situation for molecular luminophores, in particular highlighting the differences with solid-state lanthanoid materials. Various aspects relevant to this topic are covered, from the properties of vibrational local-mode overtones, the comparison of different models for the quantitative description of nonradiative deactivation, to MR isotope effects.

Photo- and electro-chromic organometallics with dithienylethene (DTE) linker, L2CpM-DTE-MCpL2: Dually stimuli-responsive molecular switch

Photochromic dithienylethene (DTE) derivatives, M-DTE-M (M: M(η(5)-C(5)R(5))L(2); M = Fe, Ru; R = H, Me; L = CO, phosphine), with direct σ-bonded, redox-active organometallic attachments are prepared and their response to photo- and electro-chemical stimuli as well as wire-like and switching performance has been investigated. These properties turn out to be dependent on the metal and the auxiliary ligands. The DTE complexes with the MCp(CO)(2) and RuCp(CO)(PPh(3)) fragments undergo reversible photochemical ring-closing and -opening of the DTE moiety upon UV and visible-light irradiation, respectively, whereas the other FeCp(CO)(PPh(3)) and Fe(η(5)-C(5)R(5))(dppe) derivatives are virtually inert with respect to the photochemical ring closing process. Electrochemical analysis of the DTE complexes reveals that 2e-oxidation of the open isomer O also brings about the ring closure of the DTE moiety to afford the Fischer-carbene-type, dicationic closed derivatives C(2+) with the π-conjugated system different from that in the neutral ones C obtained photochemically. Subsequent reduction of C(2+) furnishes the neutral closed species C. Thus the ring closure is mediated not only by the conventional photochemical process but also by the sequential oxidation-reduction process, i.e. the organometallic DTE complexes are found to be dually photo- and electro-chromic. It is notable that the oxidative procedures are viable for the photochemically inert derivatives. Wire-like and switching performance has been evaluated on the basis of the comproportionation constant K(C) for the 1e-oxidized mixed valence monocationic species obtained by the electrochemical analysis and the switching factor SF (K(C)(C)/K(C)(O)), respectively. The K(C)(C) (7.5 × 10(4)) and SF values (5.4 × 10(3)) for phosphine-substituted derivatives are significantly large, as a result of the distinct π-conjugated systems of the DTE moieties involved in the O- (with cross-conjugation) and C-forms (fully conjugated). Compared to the previously reported acetylide-type complexes bridged by a DTE linker, (dppe)Cp*M-C≡C-DTE-C≡C-MCp*(dppe), both parameters have been significantly improved by factors of ~150. Time-dependent DFT analysis for the photochemical processes has revealed that the ring-closing process occurs not only via the ligand centered singlet excited state but also via the ligand centered triplet state resulting from energy transfer processes between the ligand- and metal-centered excited states and that this proposed mechanism can account for the photochemical reactivity of ruthenium complexes superior to that of the corresponding iron derivatives.

Dual Emission from Highly Conjugated 2,2′:6′:2″‐Terpyridine Complexes—A Potential Route to White Emitters

Here, we present a new class of terpyridine complexes of the transition-metal ions, iron(II), ruthenium(II), and osmium(II), overcoming the poor emission properties typical for this class of polypyridyl complexes. These complexes show, besides an increased room-temperature emission quantum yield and a prolonged lifetime of the metal-to-ligand charge-transfer (MLCT) states, dual emission from two well-separated excited states of the same molecule. These experimental findings are attributed to a highly stabilized ligand chromophore, where photoinduced excited-state planarization causes an enhancement of electron delocalization. This planarization, in turn, reduces the potential energy of the S(1) state and minimizes electronic coupling to the MLCT state, which is prone to non-radiative deactivation via metal-centered excited states. Due to their dual emission the complexes presented here show emission covering the entire Vis spectral range upon excitation of the ππ* states in the near UV. Thus, by structurally tuning the electronic coupling of the ππ* and the MLCT states a new synthetic route toward white emitters, which can subsequently be incorporated into polymers, is opened.

Remarkable Tuning of the Photophysical Properties of Bifunctional Lanthanide tris(Dipicolinates) and its Consequence on the Design of Bioprobes

Derivatives of dipicolinic acid with a polyoxyethylene pendant arm at the pyridine 4-position have been functionalized for potential grafting with biological material. Four ligands with different terminal functions (alcohol, methoxy, phtalimide and amine) have been synthesized, which react with trivalent lanthanide ions Ln (III) to yield triple helical [Ln(L) 3] (3-) complexes, as shown by NMR and UV-vis titrations. The tris chelates display large thermodynamic stability with log beta 13 approximately 19-20 for all Eu (III) complexes for instance. Photophysical measurements reveal adequate sensitization of the metal-centered luminescence in the europium (eta sens = 33-72%) and terbium complexes, which is modulated by the nature of the terminal function. The lifetimes of the metal-centered excited states are long, up to 1.4 ms for [Eu(L) 3] (3-) and 1.6 ms for [Tb(L) 3] (3-) at room temperature, in line with hydration numbers essentially equal to zero. Quantum yields are as high as 29% for the [Eu( L ( NH2 )) 3] (3-) and 18% for the [Tb( L ( OH )) 3] (3-) tris chelates in water at physiological pH. These series of complexes demonstrate the extent of fine-tuning achievable for lanthanide luminescent probes and are simple models for investigating the effect of binding to biological molecules on the metal-centered luminescent properties.

Excitation Energy Dependence in Near-infrared Photoluminescence Spectra and Dynamics of PuO22+ in Cs2U(Pu)O2Cl4

AbstractRecently we reported the observation of near-infrared photoluminescence from metal-centered 5f electronic excited states of PuO22+ doped into polycrystalline Cs2U(Pu)O2Cl4.[1] Photoluminescence dynamics following pulsed excitation show complicated decay patterns suggesting that multiple luminescent states are involved. Here we report the results of two recent sets of experiments showing that photoluminescence processes depend significantly on the energy of photoexcitation. In the first case, decay kinetics following excitation at a lower energy are missing an in-growth term that is present when exciting at higher energy. In the second case, we have observed that lower excitation energy produces significantly reduced number of emission transitions than higher excitation energy. Both observations suggest that higher energy excitation populates feeder states that decay to emitting states, causing signal from the latter to have an in-growth followed by a decay characteristic of their intrinsic lifetimes, whereas lower energy excitation leads to more direct population of luminescent states.

Cyclopalladated complexes based on Coumarin Dye

Luminescing complexes of platinum metals are promising candidates for the design of photoactive molecular organized metal complex systems and devices based on them [1, 2]. Complexes of Pd(II) in contrast to Pt(II) rarely luminesce in solutions at room temperature because of nonradiative deactivation of low-energy metal-centered excited states [3 5]. At the same time, Pd(II) readily enters into cyclopalladation reactions with various heterocyclic compounds H(C^N) to form {Pd(C^N)} chromophoric groups [6]. For example, the cyclopalladation of Nile red yields an organometallic luminophore luminescing in the red spectral region [5].

The photophysics of Ni(II) meso-tetraphenylbenzoporphyrin: A combined theoretical and experimental investigation

A combined theoretical and kinetic spectrometric investigation of Ni(II)-tetraphenyl tetrabenzoporphyrin (NiTPTBP) has demonstrated that photo-excitation into the B band (400 nm) of the ground state absorption spectrum rapidly generates the compound in its Q state. The intriguing time dependence of stimulated emission generated from the Q state implies that this state is populated via two (or more) radiationless channels, one is very rapid (<1 ps) and another less rapid (∼6 ps). The theoretical results show that two excited states with significant oscillator strengths lie close in energy within the B band envelope. These are the 5 1E (2.72 eV; f = 0.95) and the 4 1E (2.63 eV; f = 0.19) and both these could be directly excited by the 400 nm excitation pulse, in which case these two upper states could be the origins of the two kinetic populations of the Q state. Less than 2 ps post excitation onset the spectral characteristics of a hot metal-centered excited state appear, identified as 2 1E. The excess vibrational energy of this state is dissipated by IVR and intermolecular modes. Eventually the cooled (d,d) state repopulates the initial ground state with a rate that is extremely solvent dependent. The 2 1E state is computed to possess some 20% of MLCT character, which is the possible source of the solvent dependent deactivation rate.

Synthesis and optical properties of binuclear gold(I) complexes with bridging phosphine ligands: luminescence from intraligand and metal-centered excited states

The complexes Au2Cl2(P-P) with P-P=biphep (2,2′-bis(diphenylphosphino)-1,1′-biphenyl), binap (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) and xantphos (9,9′-dimethyl-4,5-bis(diphenyl-phosphino)xanthene), were prepared and characterized by elemental analysis and ESI-MS. The solid compounds show a r.t. phosphorescence. While the binap complex emits from an intraligand (IL) triplet, the luminescence of the biphep complex originates from a metal-centered (MC) triplet which is presumably lowered by gold–gold interaction. The xantphos complex displays a dual phosphorescence. In this case, the emitting triplets are of the IL and MC type.

A strategy for improving the room-temperature luminescence properties of Ru(II) complexes with tridentate ligands.

Several ruthenium(II) complexes with new tridentate polypyridine ligands have been prepared, and their photophysical properties have been studied. The new tridentate ligands are tpy-modified systems (tpy = 2,2':6',2' '-terpyridine) in which aromatic substituents designed to be coplanar with the tpy moiety are introduced, with the aim of enhancing delocalization in the acceptor ligand of the potentially luminescent metal-to-ligand charge-transfer (MLCT) state and increasing the MLCT-MC energy gap (MC = metal-centered excited state). Indeed, the Ru(II) complexes obtained with this new family of tridentate ligands exhibit long-lived luminescence at room temperature (up to 200 ns). The enhanced luminescence properties of these complexes support this design strategy and are superior to those of the model Ru(tpy)22+ compound and compare favorably with those of the best Ru(II) complexes with tridentate ligands reported so far.

High-Resolution Optical Spectroscopy of Na3[Ln(dpa)3]·13H2O with Ln = Er3+, Tm3+, Yb3+

The title compounds were synthesized and studied by solution and single-crystal absorption, luminescence, and excitation spectroscopy. The f-f luminescence is induced in the Tm(3+) and Yb(3+) complexes in solution by exciting into the (1)Pi-(1)Pi absorptions of the ligand in the UV. A single-configurational coordinate model is proposed to rationalize the nonradiative relaxation step from ligand-centered to metal-centered excited states in [Yb(dpa)(3)](3-) (dpa = 2,6-pyridinedicarboxylate). Direct f-f excitation is used in crystals of Na(3)[Tm(dpa)(3)].13H(2)O and Na(3)[Yb(dpa)(3)].13H(2)O to induce f-f luminescence. From low-temperature, high-resolution absorption, luminescence, and excitation spectra, the ligand-field splittings in the relevant states can be determined. It was impossible to induce NIR to VIS upconversion in any of the complexes. This is mainly due to the fact that nonradiative relaxation among the f-f excited states is highly competitive, even in [Yb(dpa)(3)](3-) with an energy gap between (2)F(5/2) and (2)F(7/2) of about 10000 cm(-1). It can be rationalized on the basis of an adapted energy gap law. No luminescence at all could be detected in Na(3)[Er(dpa)(3)].13H(2)O.

Mono- and dinuclear ruthenium(II) and osmium(II) polypyridine complexes built around spiro-bridged bis(phenanthroline) ligands: synthesis, electrochemistry, and photophysics.

Two new dyads have been synthesized in which terminal Ru(II) and Os(II) polypyridine complexes are separated by sterically constrained spiro bridges. The photophysical properties of the corresponding mononuclear complexes indicate the importance of the decay of the lowest-energy triplet states localized on the metallo fragments through the higher-energy metal-centered excited states. This effect is minimized at 77 K, where triplet lifetimes are relatively long, and for the Os(II)-based systems relative to their Ru(II)-based counterparts. Intramolecular triplet energy transfer takes place from the Ru(II)-based fragment to the appended Os(II)-based unit, the rate constant being dependent on the molecular structure and on temperature. In all cases, the experimental rate constant matches surprisingly well with the rate constant calculated for Förster-type dipole-dipole energy transfer. As such, the disparate rates shown by the two compounds can be attributed to stereochemical factors. It is further concluded that the spiro bridging unit does not favor through-bond electron exchange interactions, a situation confirmed by cyclic voltammetry.

Ultrafast Photophysics and Photochemistry of [Ni]-Bacteriochlorophyll a

The ultrafast photodynamics of bacteriochlorophyll with the central Mg2+-ion replaced by Ni2+ ([Ni]-BChl) in toluene and pyridine have been studied by femtosecond time-resolved fluorescence and absorption spectroscopy with a time resolution of 100 fs in the spectral range of 470−900 nm. Excitation of tetracoordinated [Ni]-BChl in toluene in its red-most absorption band leads to four transient states which decay single-exponentially with lifetimes of 100 fs, 450 fs, 4 ps, 25 ps. Except for the 4 ps component these kinetics arise from an internal conversion cascade conserving the overall multiplicity of the singlet configuration. The spectral characteristics of the intermediates indicate a deactivation process via several π- and metal-centered excited states. The 4 ps kinetic results from a relaxation within the metal d-states caused by a change of size of the central nickel ion upon electronic excitation. A different deactivation pattern is observed for [Ni]-BChl in pyridine, where the central metal is add...

Interactions between the Excited States of Ruthenium(II)−Diimine Complexes and Phenols in Aqueous Solution

Interactions between the MLCT excited states of Ru(bpy)32+ (bpy = 2,2‘-bipyridine) or Ru(bpz)32+ (bpz = 2,2‘-bipyrazine) and phenol or monochlorophenols have been investigated in aqueous solution using steady-state and time-resolved spectrofluorimetry. The presence of phenol (PhOH), which does not quench *Ru(bpy)32+ in mildly acidic solution, causes the emission to undergo a blue shift with increasing [PhOH]; the emission quantum yields and the excited-state lifetimes decrease with increasing [PhOH] at low temperatures (5−35 °C) but increase with increasing [PhOH] at higher temperatures (35−70 °C). This behavior is understood in terms of the variations in the rate constants of the temperature-independent radiative and nonradiative decays of *Ru(bpy)32+ and the activation-controlled population of the metal-centered excited state as [PhOH] is changed. The presence of chlorophenols has no effect on the photophysics of *Ru(bpy)32+; PhOH reductively quenches *Ru(bpz)32+.

A Study on Delocalization of MLCT Excited States by Rigid Bridging Ligands in Homometallic Dinuclear Complexes of Ruthenium(II)

For the structurally rigid homometallic dinuclear complexes (ttp)Ru(tpy-tpy)Ru(ttp)4+ and (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, we have obtained ground-state absorption spectra and transient-absorption difference spectra at room temperature and luminescence spectra and lifetimes in the temperature interval from room temperature to the rigid matrix (90 K); the solvent was acetonitrile or butyronitrile (tpy is 2,2‘:6‘,2‘‘-terpyridine, ttp is 4‘-p-tolyl-2,2‘:6‘,2‘‘-tpy, and ph is 1,4-phenylene). The gathered spectroscopic data indicate that after absorption of visible light, formation of the luminescent metal-to-ligand charge transfer (MLCT) excited states takes place, which involves the bridging ligand (BL). Since we found that (ttp)Ru(tpy-tpy)Ru(ttp)4+ is a good luminophore (λmax = 720 nm, Φ = 4.7 × 10-3, and τ = 570 ns) while both (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+ (λmax = 656 nm, Φ = 1.1 × 10-4, and τ = 4 ns) and the reference mononuclear complex Ru(ttp)22+ (λmax = 640 nm, Φ = 3.2 × 10-5, and τ = 0.9 ns) are not, we have explored the effects brought about by the delocalization and energy content of the luminescent state. The study of the temperature dependence of the luminescence lifetimes indicates that two main nonradiative paths, i and ii, are responsible for deactivation of the luminescent state. Path i directly connects the luminescent and ground states; within the frame of the “energy-gap law”, vibronic analysis of low-temperature luminescence profiles enables one to correlate the delocalization of the M → BL CT state and the extent of structural distortions occurring at the accepting ligand. Thermally activated decay via a metal-centered (MC, of dd orbital origin) excited state characterizes path ii, with a MLCT−MC energy separation ΔE = 3800, 2300, and 1600 cm-1 for (ttp)Ru(tpy-tpy)Ru(ttp)4+, (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, and Ru(ttp)22+, respectively. At room temperature, for this limited series of complexes it is found that nonradiative processes governed by the “energy-gap law” play a minor role as compared to thermally activated processes, the ratios of the rate constants being knract/knrdir ≈ 16, 1900, and 7000 for (ttp)Ru(tpy-tpy)Ru(ttp)4+, (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, and Ru(ttp)22+, respectively.

Electron Delocalization in Ruthenium(II) and Osmium(II) 2,2‘-Bipyridyl Complexes Formed from Ethynyl-Bridged Ditopic Ligands

Photophysical and electrochemical properties have been recorded for a series of mono- and binuclear ruthenium(II) and osmium(II) 2,2‘-bipyridyl complexes that contain an ethynyl-bridged ditopic ligand. In particular, the electrochemical properties are indicative of electron delocalization over an extended π*-orbital in the π-radical anions. The site of attachment of the ethynyl substituent to the 2,2‘-bipyridyl ring affects the various properties, especially absorption and emission spectral maxima. In most cases, the rates of nonradiative deactivation of the lowest-energy triplet excited states are slower than expected for a corresponding complex not possessing a conjugated substituent. This effect is rationalized in terms of electron delocalization over part of the ditopic ligand within the triplet state and its significance depends markedly on the triplet energy of the complex in question. The lowest-energy triplet mixes to some extent with an upper-lying triplet that is more strongly coupled to the ground state. According to the nature of the metal complex, this higher-energy triplet might originate from (i) charge transfer from metal center to parent ligand, (ii) a π,π* state localized on the ditopic ligand, or (iii) a metal-centered excited state. For the OsII complexes at 77 K electron delocalization over an extended π*-orbital is accompanied by a reduction in the amount of nuclear displacement between triplet and ground states and by a smaller vibronic coupling matrix element relative to the parent complex. These two factors combine, within the framework of the energy-gap law, to decrease the rate at which electronic energy can be dissipated among medium-frequency vibrational (i.e., −CC− and −CN−) modes. This realization permits a quantitative explanation of the measured rate constants for nonradiative decay of the triplet excited states of these ethynyl-substituted metal complexes.

Synthesis and characterizations of cyclometalated iridium(III) solvento complexes

Complexes of the type [Ir(L)[sub 2](S)[sub 2]][OTf] have been prepared by reactions of [Ir(L)[sub 2]Cl][sub 2] with AgOTf in an appropriate solvent medium (L = 2-phenylpyridine (ppy) or 2-(p-tolyl)pyridine (ptpy); S = H[sub 2]O or CH[sub 3]CN; OTf = CF[sub 3]SO[sub 3][sup [minus]]). These solvento complexes have been characterized by [sup 1]H and [sup 13]C NMR spectroscopies, UV-visible absorption and emission spectroscopies, and cyclic voltammetry. Estimates of radiative lifetimes based upon weak integrated absorption bands in the 460-490 nm region are in agreement with emission lifetimes monitored in glasses at 77 K. Low-lying excited states responsible for these absorption and emission bands are assigned to an admixture of ligand-localized and metal-to-ligand charge-transfer character. Quenching of the emissions in ambient solutions is discussed in terms of ligand labilization due to either thermal population of metal-centered excited states or direct labilization in the MLCT excited state due to enhanced trans effects of the Ir-C bonds on the Ir-S bonding.

PH Control of photoreactivity of ruthenium(II) pyridyltriazole complexes: photoinduced linkage isomerism and photoanation

The photophysical and photochemical behavior of mixed ligand complexes of Ru(II) with 3-(pyridin-2-yl)-1,2,4-triazole (HPTN) have been examined in solutions of varying acidity. Protonation of both the N-2 bound (HPTN-2) and the N-4 bound (HPTN-4) isomers of [(bpy){sub 2}Ru(HPTN)]{sup +} results in a decrease in the luminescence lifetime in solution and an increase in reactivity upon photolysis in CH{sub 2}Cl{sub 2}. The excited state of the deprotonated form of both complexes decays by a single exponential at pHs above 3; the data were fit to a general expression for excited-state decay involving protonation of the excited complex. Results of temperature-dependent luminescence decays suggest the protonated complexes decay via population of a metal-centered excited state, while the deprotonated complexes do not efficiently populate this state at temperatures at or below room temperature. Photolysis of either protonated isomer in CH{sub 2}Cl{sub 2} results in linkage isomerism; equilibrium is established for photolysis at 4:1 [(bpy){sub 2}Ru(HPTN-4)]{sup 2+} to [(bpy){sub 2}Ru(HPTN-2)]{sup 2+}. In the presence of coordinating counterions photoanation occurs to yield the cis-anion complex with loss of pyridyltriazole. The deprotonated form of both isomers is inert to photosubstitution in CH{sub 2}Cl{sub 2} at room temperature. Thus, the substitutional photolability can be controlled by controllingmore » protonation of the ground state of the complex. 41 refs., 9 figs., 3 tabs.« less

Independent control of charge-transfer and metal-centered excited states in mixed-ligand polypyridine ruthenium(II) complexes via specific ligand design

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIndependent control of charge-transfer and metal-centered excited states in mixed-ligand polypyridine ruthenium(II) complexes via specific ligand designWilliam F. Wacholtz, Roy A. Auerbach, and Russell H. SchmehlCite this: Inorg. Chem. 1986, 25, 2, 227–234Publication Date (Print):January 1, 1986Publication History Published online1 May 2002Published inissue 1 January 1986https://doi.org/10.1021/ic00222a027RIGHTS & PERMISSIONSArticle Views669Altmetric-Citations105LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts