Photophysical Properties of {[Au(CN)₂]⁻}₂ Dimers Trapped in a Supramolecular Electron-Acceptor Organic Framework

A.S. Abouelwafa, C.E. Anson, A. Hauser, H.H. Patterson, F. Baril-Robert, X. Li and A.K. Powell

Inorganic Chemistry, , 2012 , p2147483647

Abstract:
Dicyanoaurate reacts with the organic acceptor molecule, 1,1′-bis-(2,4-dinitrophenyl)-4,4′-bipyridinium, DNP, to form a supramolecular complex with the general formula {[Au(CN)₂]₂DNP}·4H₂O. The complex was characterized by X-ray crystallography, and its photophysical properties were investigated in the solid-state. Although the initial (DNP)Cl₂ compound does not show photoluminescence behavior and the dicyanoaurate shows photoluminescence only in the UV range, the resulting supramolecular complex displays two simultaneous, essentially independent, photoluminescence bands in the visible range originating from individual contributions of the DNP unit and the dicyanoaurate dimers. This unusual simultaneous photoluminescence behavior displayed by both the dicyanoaurate donor units and the redox-active 4,4′-bipyridinium acceptor have lifetimes of 0.5 μs and several hundred μs, respectively.

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Photophysics of Galvinoxyl Free Radical Revisited

J. Grilj, C. Zonca, L.M. Lawson Daku and E. Vauthey

Physical Chemistry Chemical Physics, in press , 2012

Abstract:
The photophysical properties of the free neutral radical galvinoxyl were studied by a combination of femtosecond time-resolved spectroscopy and quantum chemical calculations. The electronic absorption spectrum is dominated by an intense band at 430 nm that is ascribed to the D9,10←D0 transitions. Upon photoexcitation at 400 nm, the population of the D9,10 states decays within less than 200 fs to the electronic ground state. This ultrafast internal conversion does not involve intramolecular modes with large amplitude motion as the measured dynamics does not show any significant dependence on the environment, but is most probably facilitated by a high density of electronic states of different character. Depending on the solvent, a weak transient band due to the galvinoxylate anion is also observed. This closed-shell species, which is fluorescent although its deactivation is also dominated by non-radiative decay, is generated upon biphotonic ionization of the solvent and electron capture. The ultrashort excited-state lifetime of the galvinoxyl radical precludes photoinduced disproportionation previously claimed to be at the origin of the formation of both anion and cation.

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Chromium(III)-trisoxalate, a versatile building block for luminescent materials

M. Milos and A. Hauser

Journal of Luminescence, in press , 2012

Abstract:
Chromium(III)-trisoxalate,[Cr(ox)₃]^3- (ox = C₂O₄^2-), incorporated into polymeric networks of composition [NaCr(ox)₃][M^II(bpy)₃] and [NaCr(ox)₃][M^III(bpy)₃]ClO₄ (bpy= 2,2'-bipyridine, M^{II =} Zn, Fe, Ru; M^{III =} Rh, Cr), results in interesting features ranging from phonon-assisted and resonant energy migration within the R1 line the ²E state to persistent spectral side-hole burning via the latter, and manifestations of specific nearest-neighbour π–π interactionsbetween bipyridine and oxalate.

Keywords: chromium(III)-tris-oxalate; energy migration; fluorescence line narrowing; spectral diffusion; persistent spectral hole burning

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Targeting π-Conjugated Multiple Donor–Acceptor Motifs Exemplified by Tetrathiafulvalene-Linked Quinoxalines and Tetrabenz[bc,ef,hi,uv]ovalenes: Synthesis, Spectroscopic, Electrochemical, and Theoretical Characterization

H.-P. Jia, J. Ding, Y.-F. Ran, S.-X. Liu, C. Blum, I. Petkova, A. Hauser and S. Decurtins

Chemistry - An Asian Journal, 6 (12) , 2011 , p3312 -3321

Abstract:
An efficient synthetic approach to a symmetrically functionalized tetrathiafulvalene (TTF) derivative with two diamine moieties, 2-[5,6-diamino-4,7-bis(4-pentylphenoxy)-1,3-benzodithiol-2-ylidene]-4,7-bis(4-pentylphenoxy)-1,3-benzodithiole-5,6-diamine (2), is reported. The subsequent Schiff-base reactions of 2 afford large π-conjugated multiple donor–acceptor (D–A) arrays, for example, the triad 2-[4,9-bis(4-pentylphenoxy)-1,3-dithiolo[4,5-g]quinoxalin-2-ylidene]-4,9-bis(4-pentylphenoxy)-1,3-dithiolo[4,5-g]quinoxaline (8) and the corresponding tetrabenz[bc,ef,hi,uv]ovalene-fused pentad 1, in good yields and high purity. The novel redox-active nanographene 1is so far the largest known TTF-functionalized polycyclic aromatic hydrocarbon (PAH) with a well-resolved ¹H NMR spectrum. The electrochemically highly amphoteric pentad 1 and triad 8 exhibit various electronically excited charge-transfer states in different oxidation states, thus leading to intense optical intramolecular charge-transfer (ICT) absorbances over a wide spectral range. The chemical and electrochemical oxidations of 1 result in an unprecedented TTF^⋅+ radical cation dimerization, thereby leading to the formation of [1^⋅+]₂ at room temperature in solution due to the stabilizing effect, which arises from strong π–π interactions. Moreover, ICT fluorescence is observed with large solvent-dependent Stokes shifts and quantum efficiencies of 0.05 for 1 and 0.035 for 8 in dichloromethane.

Keywords: charge transfer;donor|acceptor systems;nanographene;redox chemistry;tetrathiafulvalene

Links: unige:18010 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Study of the relaxation in diluted spin crossover molecular magnets in the framework of the mechano-elastic model

C. Enachescu, L. Stoleriu, A. Stancu and A. Hauser

Journal of Applied Physics, 109 (7) , 2011 , p711

Abstract:
We model here the behavior of spin transition compounds, considering molecules arranged in a 2D hexagonal lattice and interacting via springs. The role of impurities in the clustering and nucleation phenomena is analyzed, as well as the manner in which the impurities affect the relaxation curves. The switching of the individual molecules is checked using a Monte Carlo procedure. When a molecule changes its state, it also modifies its volume, and the new equilibrium positions of all the molecules are calculated. As in previously reported experiments, it is found here that bigger impurities slow down the relaxation from the metastable high-spin state to the low-spin state, while smaller impurities act in an opposite way. It is shown that if the concentration of the impurities is higher than a certain threshold, then they act as a barrier, trammeling the fast evolution of domains developing from the edges.

Keywords: magnetic impurities, metastable states, molecular magnetism, Monte Carlo methods, spin dynamics

Links: unige:17481 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Thermal hysteresis in spin-crossover compounds studied within the mechanoelastic model and its potential application to nanoparticles

C. Enachescu, P. Chakraborty, L. Stoleriu, A. Stancu and A. Hauser

Physical Review B, 84 (13) , 2011 , p134102

Abstract:
The recently developedmechanoelastic model is applied to characterize the thermal transition in spin-crossover complexes, with special attention given to the case of spin-crossover nanoparticles. In a two-dimensional system, hexagonal-shaped samples with open boundary conditions are composed of individual molecules that are linked by springs and can switch between two states, namely, the high-spin (HS) and the low-spin (LS) states. The switching of an individual molecule during the spin transition is decided by way of a Monte Carlo standard procedure, using transition probabilities depending on the temperature, the energy gap between the two states, the enthalpy difference, the degeneracy ratio, and the local pressure determined by the elongation or compression of its closest springs. The influence of external parameters, such as temperature sweeping rate and pressure, or intrinsic features of the system, such as the value of its spring constant, on the width of the thermal hysteresis, its shape, and its position are discussed. The particular case of spin-crossover nanoparticles is treated by considering them embedded into a polymer environment, which essentially affects the molecules situated at the edges and faces by decreasing their transition probabilities from HS to LS. Finally, the pressure hysteresis, obtained by varying the external pressure at constant temperature is discussed.

Links: unige:17482 Abstract Article PDF DOI (Digital Object Identifier)

Lanthanide-Mediated Supramolecular Cages and Host–Guest Interactions

B. El Aroussi, L. Guénée, P. Pal and J. Hamacek

Inorganic Chemistry, 50 (17) , 2011 , p8588 -8597

Abstract:
The structure and thermodynamic properties of lanthanide complexes with a new tripodal ligand L2 have been elucidated using different physicochemical methods. At stoichiometric ratios, the tetrahedral three-dimensional complexes with lanthanide cations are formed in acetonitrile with good stabilities. Despite minor structural changes comparing to previously investigated tripodal ligands, the resulting assembly exhibits different features revealed with the crystal structure of [Eu₄L2₄](OH)(ClO₄)₁₁ (orthorhombic, Pbcn). Interestingly, the highly charged edifice contains an inner cage encapsulating a perchlorate anion. Such lanthanide mediated cage-like assemblies are rare, and may be of interest for different sensing applications. Indeed, the anionic guest can be exchanged with different anions. The related host–guest equilibria were investigated with NMR techniques. Various aspects of these reactions are qualitatively discussed.

Links: unige:17235 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

A novel ruthenium(ii) complex for two-photon absorption-based optical power limiting in the near-IR range

M. Four, D. Riehl, O. Mongin, M. Blanchard-Desce, L.M. Lawson Daku, J. Moreau, J. Chauvin, J.A. Delaire and G. Lemercier

Physical Chemistry Chemical Physics, 13 (38) , 2011 , p17304 -17312

Abstract:
In this article, the synthesis of a novel high-conjugated ligand and its corresponding Ru(II) complex PTFTF:Ru is reported, along with the linear and nonlinear optical characterizations. Two-photon absorption based optical power limiting properties (OPL), especially in the near infrared, are described and compared to those of the analogous complexes previously published. Combined with a preliminary theoretical approach, this allows us to highlight several key parameters for OPL optimization in such molecular systems and more particularly the spectral overlap between TPA and excited-state absorption.

Links: unige:17238 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Thermodynamics, Structure and Properties of Polynuclear Lanthanide Complexes with a Tripodal Ligand: Insight into their Self-Assembly

J. Hamacek, C. Besnard, T. Penhouet and P.-Y. Morgantini

Chemistry - A European Journal, 17 (24) , 2011 , p6753 -6764

Abstract:
Self-assembly processes between a tripodal ligand and Ln^III cations have been investigated by means of supramolecular analytical methods. At an equimolar ratio of components, tetranuclear tetrahedral complexes are readily formed in acetonitrile. The structural analysis of the crystallographic data shows a helical wrapping of binding strands around metallic cations. The properties of this series of highly charged 3D compounds were examined by using NMR spectroscopy and optical methods in solution and in the solid state. In the presence of excess metal, a new trinuclear complex was identified. The X-ray crystal structure elucidated the coordination of metallic cations with two ligands of different conformations. By varying the metal/ligand ratio, a global speciation of this supramolecular system has been evidenced with different spectroscopic methods. In addition, these rather complicated equilibria were successfully characterised with the thermodynamic stability constants. A rational analysis of the self-assembly processes was attempted by using the thermodynamic free energy model and the impact of the ligand structure on the effective concentration is discussed.

Keywords: helical structures;lanthanides;polynuclear complexes;thermodynamics;tripodal ligands

Links: unige:17236 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

The kinetics features of a thermal spin transition characterized by optical microscopy on the example of [Fe(bbtr)₃](ClO₄)₂ single crystals: Size effect and mechanical instability

C. Chong, A. Slimani, F. Varret, K. Boukheddaden, E. Collet, J.-C. Ameline, R. Bronisz and A. Hauser

Chemical Physics Letters, 504 (1-3) , 2011 , p29 -33

Abstract:
We investigated by optical microscopy the thermal spin transition in single crystals of [Fe(bbtr)₃](ClO₄)₂ (bbtr = 1,4-di(1,2,3-triazol-1-yl) butane). The growth of the low-spin phase was observed for different crystal orientations and sizes. The process always started from a corner of the crystal but its further development depended on the size, shape and thermal history of the crystal. In crystals of smaller size, under isothermal conditions, the low-spin phase developed in a continuous way, through the propagation of a rather well defined transformation front, with a higher propagation velocity inside the planes perpendicular to the c axis. In larger crystals the spontaneous occurrence of inhomogeneous stresses led to a stepwise propagation process.

Links: unige:16831 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Near-Infrared to Visible Light Upconversion in a Trinuclear d-f-d Complex

L. Aboshyan-Sorgho, C. Besnard, P. Pattison, K.R. Kittilstved, A. Aebischer, J.-C.G. Bünzli, A. Hauser and C. Piguet

Angewandte Chemie International Edition, 50 , 2011 , p4108 -4112

Abstract:
The connection of two Cr^III sensitizers around a central Er^III acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of Cr^III-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green Er^III-centered emission.

Keywords: helical structures;lanthanides;photochemistry;self-assembly;upconversion

Links: unige:15714 Abstract Article PDF DOI (Digital Object Identifier)

Effect of the Addition of a Fused Donor−Acceptor Ligand on a Ru(II) Complex: Synthesis, Characterization, and Photoinduced Electron Transfer Reactions of [Ru(TTF-dppz)₂(Aqphen)]²⁺

N. Dupont, Y.-F. Ran, H.-P. Jia, J. Grilj, J. Ding, S.-X. Liu, S. Decurtins and A. Hauser

Inorganic Chemistry, 50 (8) , 2011 , p3295 -3303

Abstract:
The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)₂(Aqphen)]²⁺(TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2′,3′-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal−ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy.

Links: unige:15004 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Structural and vibrational properties of Ca₂FeH₆ and Sr₂RuH₆

H. Hagemann, V. D'Anna, L.M. Lawson Daku, S. Gomes, G. Renaudin and K. Yvon

Journal of Physics and Chemistry of Solids, 72 , 2011 , p286 -289

Abstract:
The structural and vibrational properties of the isostructural compounds Ca₂FeH₆ and Sr₂RuH₆ are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A₂TH₆ (A = Mg,Ca,Sr; T = Fe, Ru) in order to identify correlations between selected frequencies and the T-H bond length. The bulk moduli of Ca₂FeH₆ and Sr₂RuH₆ have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg₂FeH₆ and several other compounds of this structure.

Keywords: hydrogen storage materials; DFT calculations; lattice vibrations

Links: unige:14988 Abstract Article PDF DOI (Digital Object Identifier)

Low-Spin→High-Spin Relaxation Dynamics in the Highly Diluted Spin-Crossover System [Fe_xZn_1−x(bbtr)₃](ClO₄)₂

I. Krivokapic, P. Chakraborty, C. Enachescu, R. Bronisz and A. Hauser

Inorganic Chemistry, 50 (5) , 2011 , p1856 -1861

Abstract:
Whereas the neat polymeric iron(II) compound [Fe(bbtr)₃](ClO₄)₂, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [Fe_xZn_1−x(bbtr)₃](ClO₄)₂, x = 0.02 and 0.1, stay predominantly in the ⁵T₂ high-spin state down to cryogenic temperatures. However, the ¹A₁ low-spin state can be populated as metastable state via irradiation into the spin-allowed ⁵T₂→⁵E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spin→high-spin relaxation is 1840(20) cm⁻¹.

Links: unige:14980 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Unidirectional Photoisomerization of Styrylpyridine for Switching the Magnetic Behavior of an Iron(II) Complex: A MLCT Pathway in Crystalline Solids

A. Tissot, M.-L. Boillot, S. Pillet, E. Codjovi, K. Boukheddaden and L.M. Lawson Daku

Journal of Physical Chemistry C, 114 (49) , 2010 , p21715 -21722

Abstract:
The photoreactivity of two iron(II)−styrylpyridine frameworks Fe(stpy)₄(NCSe)₂ (stpy = 4-styrylpyridine) has been investigated for the very first time in a crystalline solid. A quantitative cis-to-trans isomerization of stilbenoids is shown to occur in the confined environment of the inorganic solid. The photochromic reaction was driven by a visible excitation into the metal-to-ligand charge transfer absorption of the high-spin all-cis complex. The solid-state transformation is accompanied by a unit-cell volume increase and an amorphization. Interestingly, the photoproduct formed by irradiating the high-spin all-cis reactant undergoes a spin conversion when the temperature is decreased. This observation is related to the “ligand-driven light-induced spin change” effect in a constrained environment.

Links: unige:14853 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Competition Between Photo-Excitation and Relaxation in Spin Crossover Complexes in The Frame of a Mechano-Elastic Model

C. Enachescu, L. Stoleriu, A. Stancu and A. Hauser

Physical Review B, 82 , 2010 , p104114

Abstract:
In this paper we use a recently proposed elastic model in order to study the competition between linear photoexcitation and cooperative relaxation in spin-crossover molecular magnets. The difference in molecular size between the two possible spin states, that is, the high-spin and the low-spin states, respectively, induces distortions of the crystal lattice. These determine the elastic interactions between molecules, treated here as connecting springs that are either compressed or extended from their equilibrium length, thus modulating the local probability for the high-spin→low-spin relaxation. The crossover of individual molecules within the lattice is checked by a standard Monte Carlo procedure. Using very simple assumptions and a minimum number of parameters, photoexcitation curves and hysteresis loops under continuous irradiation below the thermal transition temperature can thus be simulated. The formation of clusters is analyzed and the presence of inhomogeneities in the system is investigated.

Links: unige:14682 Abstract Article PDF DOI (Digital Object Identifier)

Significant variation of the singlet-quintet intersystem crossing rate constant in an iron(II) high-spin complex as a function of temperature

I. Krivokapic, P. Chakraborty, R. Bronisz, C. Enachescu and A. Hauser

Angewandte Chemie, 49 (45) , 2010 , p8509 -8512

Abstract:
In the dilute mixed-crystal system [Zn_1−xFe_x(bbtr)₃](ClO₄)₂, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K

Links: unige:14713 DOI (Digital Object Identifier)

Effect of External Pressure on the Excitation Energy Transfer from [Cr(ox)3]^3- to [Cr(bpy)3]³⁺ in [Rh_1-xCr_x(bpy)₃][NaM_1-yCr_y(ox)₃]ClO₄

M. Milos, P. Pal and A. Hauser

ChemPhysChem, 11 (14) , 2010 , p3161 -3166

Abstract:
Resonant excitation energy transfer from [Cr(ox)3]^3- to [Cr(bpy)3]³⁺ in the doped 3D oxalate networks [Rh_1-xCr_x(bpy)₃][NaM^III_1-yCr_y(ox)₃]ClO₄ (ox=C₂O₄^-, bpy=2,2’-bipyridine, M=Al,Rh) is due to two types of interaction, namely super exchange coupling and electric dipole–dipole interaction. The energy transfer probability for both mechanisms is proportional to the spectral overlap of the ²E→⁴A₂ emission of the [Cr(ox)₃]^3- donor and the ⁴A₂→²T₁ absorption of the [Cr(bpy)₃]³⁺ acceptor.The spin-flip transitions of (pseudo-)octahedral Cr³⁺ are known to shift to lower energy with increasing pressure. Because the shift rates of the two transitions in question differ, the spectral overlap between the donor emission and the acceptor absorption is a function of applied pressure. For [Rh_1-xCr_x(bpy)₃][Na-M_1-yCr_y(ox)₃]ClO₄ the spectral overlap is thus substantially reduced on increasing pressure from 0 to 2.5 GPa. As a result, the energy transfer probability decreases with increasing pressure as evidenced by a decrease in the relative emission intensity from the [Cr(bpy)₃]³⁺ acceptor.

Keywords: [Cr(bpy)3]3+; [Cr(ox)3]3−; 3D oxalate networks; excitation energy transfer; high pressures

Links: unige:14715 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Cluster evolution in spin crossover systems observed in the frame of a mechano-elastic model

C. Enachescu, M. Nishino, S. Miyashita, L. Stoleriu, A. Stancu and A. Hauser

Europhysics Letters, 91 (2) , 2010 , p27003

Abstract:
In this paper we study the cluster formation and evolution in spin crossover systems during the thermal transition in the frame of a mechano-elastic model applied to open boundary hexagonal lattices. The switching processes between the high-spin (HS) and low-spin (LS) state are studied by a method combining a Monte Carlo standard procedure on the spin state and the lattice relaxation. In the present study, we adopt the transition probabilities of the spin state taking into account the energy gap between the two states, the degeneracy ratio and the local pressure determined by the elongations of the closest springs. It is found that clusters of molecules in the same state tend to grow starting from corners, as in available experimental data. Some qualitative differences between the processes of cluster formation for the two hysteresis branches, i.e., HS to LS and LS to HS are pointed out. Moreover, we have studied the dependence of cluster formation on the strength of the elastic interactions, and also on the system size. The size dependence of the ratio between the system size and the maximum cluster length is very weak, which indicates the appearance of macroscopic domains.

Keywords: Spin crossover, general studies of phase transitions, domain effects, magnetization curves, hysteresis

Links: unige:14746 DOI (Digital Object Identifier)

Ab Initio Molecular Dynamics Study of an Aqueous Solution of [Fe(bpy)₃](Cl)₂ in the Low-Spin and in the High-Spin States

L.M. Lawson Daku and A. Hauser

Journal of Physical Chemistry Letters, 1 , 2010 , p1830 -1835

Abstract:
The mechanism of the photoinduced low-spin → high-spin spin crossover is actively being investigated in Fe(II) complexes in solution using ultrafast spectroscopies. These studies accurately inform on the reaction coordinate of the Fe(II) chromophore upon photoexcitation. However, they leave open questions regarding the role of the solvent. Here, we report the description from a fully ab initio molecular dynamics study of the structure of [Fe(bpy)₃]²⁺ in water and of the organization of its solvation shell in the low-spin and the high-spin states. In particular, the low-spin → high-spin change of states is shown to be accompanied (i) by a 0.191 Å lengthening of the Fe−N bond, in agreement with experiment, and (ii) by an increased thermal fluctuation of the molecular edifice, which both result from the weakening of the Fe−N bond. Furthermore, our results suggest that about two water molecules are expelled from the first solvation shell of [Fe(bpy)₃]²⁺, which consists of water molecules intercalated between the bpy ligands.

Links: unige:14751 Abstract Article HTML Article PDF DOI (Digital Object Identifier)