Enantioselective Assembly of a Ruthenium(II) Polypyridyl Complex into a Double Helix

Evolution can increase the complexity of matter by self‐organization into helical architectures, the best example being the DNA double helix. One common aspect, apparently shared by most of these architectures, is the presence of covalent bonds within the helix backbone. Here, we report the unprecedented crystal structures of a metal complex that self‐organizes into a continuous double helical structure, assembled by non‐covalent building blocks. Built up solely by weak stacking interactions, this alternating tread stairs‐like double helical assembly mimics the DNA double helix structure. Starting from a racemic mixture in aqueous solution, the ruthenium(II) polypyridyl complex forms two polymorphic structures of a left‐handed double helical assembly of only the Λ‐enantiomer. The stacking of the helices is different in both polymorphs: a crossed woodpile structure versus a parallel columnar stacking.     

Chirality and magnetism in a novel series of isotypic borophosphates: M(II)[BPO4(OH)2] (M(II) = Mn, Fe, Co)

Three novel isotypic borophosphates M(II)[BPO4(OH)2] (M(II) = Mn, Fe, Co) have been hydrothermally synthesized and structurally characterized. The isotypic compounds crystallize in the chiral space group P3(1)21 or P3(2)21. Their crystal structures are characterized by edge-sharing helical M(II)O6-octahedral chains wound around 3(1) or 3(2) screw axes (along [001]) interconnected by borophosphate single chains of alternating BO2(OH)2 and PO4 tetrahedra perpendicular to [001], resulting in three-dimensional framework structures. The helical M(II)O6-octahedral chains lead to low-dimensional magnetic properties.     

Iron(II) and zinc(II) monohelical binaphthyl-salen complexes with overlapping benz[a]anthryl sidearms

The synthesis of the polyaromatic aldehyde 1-hydroxybenz[a]anthracene-2-carboxaldehyde is reported via a seven step protocol from 9,10-dihydroanthracene, with an overall yield of 30%. Two equivalents of the aldehyde are condensed with (R)-1,1'-binaphthyl-2,2'-diamine to produce a new binaphthyl-salen ligand, which is subsequently complexed to iron(II) and zinc(II) ions. The ligand and complexes are characterized by single-crystal X-ray crystallography. The complexes have distinct helical structures with overlapping benz[a]anthryl sidearms, and only M-helices are observed. The ligand and complexes are further characterized by solution (1)H and (13)C NMR spectroscopy as well as UV-visible and ECD spectroscopies. These studies indicate that there is a single component in solution, consistent with the solid state characterization.     

Spectroscopic evidence for diastereomeric helical segments of polysilane bearing enantiopure (S)- and (R)-3,7-dimethyloctyl groups

We synthesized two optically active helical polysilanes, poly[(S)-3,7-dimethyloctyl-3-phenylpropylsilane] (PS1) and poly[(R)-3,7-dimethyloctyl-3-phenylpropylsilane] (PS2), bearing a flexible and rodlike silicon main chain and enantiopure alkyl side chains with (S)- and (R)-chiral centers, respectively, at the γ-positions. PS1 and PS2 underwent a thermodriven helix–helix transition at 10 °C in isooctane. Circular dichroism (CD) and UV studies demonstrated the transition characteristics, such as the transition temperature, population of right- and left-handed helical motifs, global shape, and screw pitch. At −80 °C, the dissymmetry ratio suggested that a preferential right-handed or left-handed screw sense was present in the polymer chains of PS1 and PS2, respectively. However, above the transition temperature, the appearance of a bisignate cotton band in the CD spectra suggested that both right-handed screw-sense, tight helical segments and left-handed screw-sense, loose helical segments coexisted in the same chains of PS1 and PS2. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4518–4527, 2004     

Homochiral 1D helical chain based on an achiral Cu(II) complex

Self-assembly of an achiral [Cu(L)] complex produced a homochiral helical chain [Cu(L)](3)·2H(2)O (1) (L = 2-dimethylaminoethyl(oxamato)). Interestingly, complex 1 obtained in our laboratory exhibits only a left-handed helical chain without any chiral source. Single-crystal X-ray analysis revealed the absolute structure and homochirality of its helical chain structure in the space group of P3(2). Solid-state circular dichroism (CD) spectra confirmed the high enantio excess of the crystals obtained in different synthesis batches. Magnetic susceptibility measurements reveal a relatively strong intrachain antiferromagnetic interaction between Cu(II) centers via an oxamato bridge (J = -74.4 cm(-1)).     

The conformational effect of form I helical poly(L-proline) and form II helical poly(L-proline) to the complex formation with helical poly(L-glutamic acid) through hydrogen bonding

The complex formation between helical poly-L-glutamic acid (PLGA) and helical poly-L-proline (PLP) was studied in a methanol-water (2 : 1) cosolvent and a propanol-water cosolvent (9 : 1). Reduced viscosity, circular dichroism, pH, and molar absorptivity were measured. The experimental results exhibit that the interpolymer complex was formed between helical PLGA and helical PLP through hydrogen bonding. When the complex was formed the unit mole ratio of PLGA : PLP(II) is 2 : 1 and PLGA : PLP(I) is 1.5 : 1, the ability of complex formation of PLP (II) with PLGA is better than that of PLP(I). On complexation the conformations of PLGA and PLP change and this change is more enhanced in the PLGA-PLP(II) than the PLGA-PLP(I) complex; its cause is studied. © 1993 John Wiley & Sons, Inc.     

Triple-stranded helicates of zinc(II) and cadmium(II) involving a new redox-active multiring nitrogenous heterocyclic ligand: synthesis, structure, and electrochemical and photophysical properties

The protonated form [H(2)(L)](CF(3)SO(3))(2) (1) of a new redox-active bis-bidentate nitrogenous heterocyclic ligand, viz., 3,3'-dipyridin-2-yl[1,1']bi[imidazo[1,5-a]pyridinyl] (L), and its zinc(II) and cadmium(II) complexes (2 and 3) have been synthesized and characterized by single-crystal X-ray diffraction analysis. In the solid state, both 2 and 3 have triple-stranded helical structures involving ligands that experience twisting and bending to the extent needed by the stereoelectronic demand of the central metal ion. The metal centers in the zinc(II) complex [Zn(2)(L)(3)](ClO(4))(4) (2) are equivalent, each having a distorted octahedral geometry, flattened along the C(3) axis with a Zn1···Zn1# separation of 4.8655(13) ?. The cadmium complex [Cd(2)(L)(3)(H(2)O)](ClO(4))(4) (3), on the other hand, has a rare type of helical structure, showing coordination asymmetry around the metal centers with a drastically reduced Cd1···Cd2 separation of 4.070 ?. The coordination environment around Cd1 is a distorted pentagonal bipyramid involving a N(6)O donor set with the oxygen atom coming from a coordinated water, leaving the remaining metal center Cd2 with a distorted octahedral geometry. The structures of 2 and 3 also involve anion-π- and CH-π-type noncovalent interactions that play dominant roles in shaping the extended structures of these molecules in the solid state. In solution, these compounds exhibit strong fluxional behavior, making the individual ligand strands indistinguishable from one another, as revealed from their (1)H NMR spectra, which also provide indications about these molecules retaining their helical structures in solution. Electrochemically, these compounds are quite interesting, undergoing ligand-based oxidations in two successive one-electron steps at E(1/2) of ca. 0.65 and 0.90 V versus a Ag/AgCl (3 M NaCl) reference. These molecules are all efficient emitters in the red and blue regions because of ligand-based π*-π fluorescent emissions, tuned appropriately by the attached Lewis acid centers.     

Trinuclear Double Helicates of Iron(II) and Nickel(II): Self-assembly and resolution into helical enantiomers

The synthesis of the linear tris[terpyridine] 1 with three tridentate binding sites is described. The reaction with metal ions of octahedral coordination geometry, such as Fe^II or Ni^II, leads to the self-assembly of trinuclear complexes [M₃( 1 )₂]⁶⁺, which display properties in agreement with a double helical structure. The trinuclear iron (II) helicate has been resolved into its enantiomers.     

Homochiral cobalt(II) and strontium(II) amino-carboxylate–phosphonate hybrids

Two homochiral metal amino-carboxylate–phosphonate hybrids, namely, [Co₂Cl(S-HL)(H₂O)₅]Cl · H₂O 1 and Sr₂(S-HL)(NO₃)₂(H₂O) · H₂O 2 (S-H₃L = S-HO₂CC₄H₇NCH₂PO₃H₂) have been synthesized by the reaction of the enantiopure S-H₃L ligand with cobalt(II) chloride or strontium nitrate under acidic condition at room temperature. The structure of compound 1 features a novel 3D framework with helical chains and channels. Compound 2 has a layered structure in which the 1D chains of edge-sharing SrO₈ and SrO₉ polyhedra are interconnected by phosphonate ligands.     

A series of chiral coordination polymers containing helicals assembled from a new chiral (R)-2-(4'-(4'-carboxybenzyloxy)phenoxy)propanoic acid: syntheses, structures and photoluminescent properties

Ten new chiral coordination polymers, namely, [Ni(L)(H(2)O)(2)] (1), [Co(L)(H(2)O)(2)] (2), [Cd(L)(H(2)O)] (3), [Cd(L)(phen)] (4), [Mn(2)(L)(2) (phen)(2)]·H(2)O (5), [Cd(2)(L)(2)(biim-4)(2)] (6), [Zn(2)(L)(2)(biim-4)(2)] (7), [Cd(L)(pbib)] (8), [Cd(L)(bbtz)] (9) and [Cd(L)(biim-6)] (10), where phen = 1,10-phenathroline, biim-4 = 1,1'-(1,4-butanediyl)bis(imidazole), pbib = 1,4-bis(imidazole-1-ylmethyl)benzene, bbtz = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene, biim-6 = 1,1'-(1,6-hexanedidyl)bis(imidazole), and H(2)L = (R)-2-(4'-(4'-carboxybenzyloxy)phenoxy)propanoic acid, have been synthesized under hydrothermal conditions. Their structures have been determined by single-crystal X-ray diffraction analyses and further characterized by infrared spectra (IR), powder X-ray diffraction (PXRD), elemental analyses and thermogravimetric (TG) analyses. Compounds 1 and 2 exhibit similar 1D left-handed helical chains, which are further extended into 3D supramolecular structures through O-H···O hydrogen-bonding interactions, respectively. Compound 3 shows a 2D double-layer architecture containing helical chains. Compound 4 features two types of 2D undulated sheets with helical chains, which are stacked in an ABAB fashion along the c direction. Compound 5 possesses a 1D double chain ribbon structure containing unusual meso-helical chains, which is linked by π-π interactions into a 2D supramolecular layer. These layers are further extended by hydrogen-bonding interactions to form a 3D supramolecular assembly. Compounds 6 and 7 are isostructural and exhibit 2D (4(4))-sql networks with helical chains. Neighboring sheets are further linked by C-H···O hydrogen-bonding interactions to generate 3D supramolecular architectures. Compounds 8-10 are isostructural and display 3D 3-fold interpenetrating diamond frameworks with helical chains. The effects of coordination modes of L anions, metal ions and N-donor ligands on the structures of the coordination polymers have been discussed. The luminescent properties of 3, 4 and 6-10 have also been investigated in detail.     

Versatility of 2,6-diacetylpyridine (dap) hydrazones in generating varied molecular architectures: synthesis and structural characterization of a binuclear double helical Zn(II) complex and a Mn(II) coordination polymer

A binuclear complex of Zn(ii) with formula [Zn(dap(A)(2))](2).2.25DMF (.2.25DMF) and a Mn(ii) coordination polymer with formula [Mn(3)(dap(In)(2))(3)(H(2)O)(2).2DMSO](n) (.2DMSO)(n) have been prepared and structurally characterized [dap(A)(2) = dideprotonated form of 2,6-diacetylpyridine bis(anthraniloyl hydrazone); dap(In)(2) = doubly deprotonated form of 2,6-diacetylpyridine bis(isonicotinoyl hydrazone)]. In the Zn(ii) complex the molecular units are double helical, with the Zn(ii) ions in a square pyramidal environment. The Mn(ii) complex on the other hand is a coordination polymer containing two different types of hepta-coordinated Mn(ii) ions, which differ in their axial ligands. The magnetic properties of the Mn(ii) complex, along with those of a double helical pyridine bridged binuclear Ni(ii) complex, earlier synthesized by us, are also reported. The ability of the 2,6-diacetylpyridine bis(aroyl hydrazone) ligands to form double helical complexes is analyzed in terms of the conformational flexibility of the ligands. The differences in the magnetic properties of the micro-N bridged binuclear complexes formed by 1,1 azido N-bridging ligands, and pyridine N-bridging ligands, is analyzed with the help of EHMO calculations.     

Solvothermal Synthesis and Crystal Structure of a 1D Helical-chain Cobalt（II） Complex Containing （Quinolin-8-yloxy）acetate

The cobalt(II) complex with (quinolin-8-yloxy)acetate, [CoCl(C11H8NO3)]n (1), has been prepared via the solvothermal method and characterized by IR, elemental analysis, UV- Vis diffuse-reflection spectra and single-crystal X-ray diffraction analysis. Complex 1 is a novel carboxylate-bridged one-dimensional helical cobalt(II) polymer, and the Co(II) centre exhibits an approximately square pyramidal CoClNO3 coordination geometry. It crystallizes in monoclinic, space group P21/n, with a = 9.1594(10), b = 6.8864(7), c = 17.290(2) , β = 102.629(3)o, C11H8ClCoNO3, Mr = 296.56, V = 1064.2(2) 3, Z = 4, Dc = 1.851 g/cm3, F(000) = 596, μ = 1.856 mm-1, the final R = 0.0308 and wR = 0.0807. Interestingly, the chain complexes are assembled to form two-dimensional networks through intermolecular face-to-face π-π stacking interactions with the centroid-to-centroid distance of 3.559(1)  and the dihedral angle of 8.4(1)° between the aromatic rings.     

Characterization of collagen model peptides containing 4-fluoroproline; (4(S)-fluoroproline-pro-gly)10 forms a triple helix,but (4(R)-fluoroproline-pro-gly)10 does not

Collagen model peptide (Pro-Pro-Gly)10 has a triple helical structure and undergoes a thermal transition to a single random coil structure. The transition temperature of the analogous model peptides depends largely on amino acid substitution. Substitution of Pro by 4-hydroxyproline (Hyp) or 4-fluoroproline (fPro) has especially attracted attention because the position of substitution and chirality of the hydroxyl group or fluorine atom affect the transition temperatures. Here, we demonstrated that (4(S)-fPro-Pro-Gly)10 takes a triple helical structure, but (4(R)-fPro-Pro-Gly)10 exists in a single chain structure. This is not consistent with the case of Hyp substitution in our previous report where both (4(S)-Hyp-Pro-Gly)10 and (4(R)-Hyp-Pro-Gly)10 are in a single random coil state.     

Co‐Assembled Supramolecular Nanostructure of Platinum(II) Complex through Helical Ribbon to Helical Tubes with Helical Inversion

We demonstrated the morphology transformation of co‐assemblies based on terpyridine‐based ligands ( 1R and 1S ) possessing R‐ or S‐alanine analogues and their platinum(II) complex ( 2R‐Pt and 2S‐Pt ). The right‐handed helical ribbon of the co‐assembly formed with 0.5 equivalents of 2R‐Pt to 1R was converted into the left‐handed helical ribbon with 0.6 equivalents of 2R‐Pt . The left‐handed helical ribbon structure of the co‐assembly became a tubular structure in the presence of 0.8–1.0 equivalents of 2R‐Pt . The morphology transformation via helical inversion at the supramolecular level was due to an orientation change of the amide groups caused by non‐covalent Pt???Pt interactions between the terpyridine of 2R‐Pt and that of 2R‐Pt . This study provides insights into controlling the morphology of the transformation of helical ribbons into tubular structures through helicity inversion in co‐assembled supramolecular nanostructures based on platinum(II) complexes.     

Synthesis,characterization and X-ray crystallographic investigation of 2-D hybrid hydrogen bonded and rectangular grid networks in Cu(II) and Co(II) metal complexes

Two copper/cobalt metal complexes, [Cu(l-cys)(2,2′-bpy)(H₂O)] (1) and {[Co(l-cys)(4,4′-bpy)(H₂O)]·H₂O}_n (2), with the N-donor ligands 2,2′-bipyridyl/4,4′-bipyridyl and the l-cysteate dianion (l-cys) have been synthesized by different reaction methods and structurally characterized. Compound 1 exists as a discrete monomeric unit in which the metal ions possess a distorted square-pyramidal coordination environment provided by nitrogen atoms from the 2,2′-bpy ligand and the amino-carboxylate group of l-cys in a chelated coordination, constituting the square base, and with a water molecule occupying the axial coordination site to complete the penta coordination. Packing and hydrogen bonding interactions of 1 reveal that the screw related monomeric units are involved in intermolecular hydrogen bonding with the formation of helical bilayers via O–H?O and N–H?O interactions. These hydrogen bonded bilayered helical nets are involved in stacking and C–H?O interactions which generate a two dimensional hydrogen bonded network in the bc-plane. Complex 2 is a two dimensional coordination polymer which is insoluble in common polar and non-polar solvents. The coordination around the metal center possesses a distorted octahedral geometry. The adjacent metal centers are bridged via the carboxylate group of the l-cys moiety in a syn–anti fashion, generating a one dimensional helical network along the b-axis. Adjacent helical chains are further pillared by the 4,4′-bpy ligand through the terminal nitrogen atoms, generating a two dimensional square grid type coordination network. Both the complexes are characterized well by various physico-chemical techniques such as CHN analysis, IR spectroscopy, PXRD and CD analysis.     

Four zinc(II) helical coordination polymers and 78-membered six-node zinc metallacycle assembled from diastereopure N,N'-bis(acetylacetone)cyclohexanediimine

The interaction of the diastereopure N,N'-bis(acetylacetone)cyclohexanediimine ligands, L(1)(1R,2R), L(2)(1S,2S), and their 1:1 mixture, with Zn(II) chloride has been investigated. Four new coordination polymers, [ZnL(1)Cl2 x H2O]n (1), [ZnL(2)Cl2 x H2O]n (2), [ZnL(2)Cl2]n (3), and [Zn6(L(2))6Cl12 x 2 H2O]n (4), each consisting of an infinite single helical chain displaying different pitches and/or handedness have been isolated. The complexed Schiff base ligands are present in their deprotonated enol forms, and the nitrogen atoms, which do not coordinate, are protonated because of proton transfer from the adjacent enol oxygen (coupled with concomitant N-H...O bond formation); each bound ligand is thus pseudo-zwitterionic. The respective zinc centers are bound to two chloro ligands and two oxygen donors from acacH-imine units belonging to different N,N'-bis(acetylacetone)cyclohexanediimine ligands such that the coordination at each zinc is distorted tetrahedral. Compounds 1 and 2, prepared from enantiopure L(1) and L(2), respectively, are enantiomers with similar structures, with the helical pitch in each being 17.0 A. Overall, the structure of 3 may be described as a one-dimensional helical chain with a pitch of 17.3 A, with each period corresponding to two L(2) ligands and two metal centers. The structure of [(Zn6L(2)6Cl12) x 2 H2O]n (4) contains six Zn(II) centers connected via six L(2) ligands to form a "bowed" helical repeat unit, with the pitch of the helix corresponding to 43.5 A. Supramolecular (intra- and intermolecular) aspects of all these unusual polymeric structures are discussed. Finally, the synthesis and characterization of an unprecedented six zinc-node discrete supramolecular assembly, [Zn6(L(1))3(L(2))3Cl12] (5), incorporating a 78-membered metallacycle, is also reported.     

A pyrimidine-like nickel(II) DNA base pair

4-(2'-Pyridyl)-pyrimidinone deoxyriboside is synthesized and characterized as a DNA metallo base-pair; this novel nucleoside forms a self-pair in the presence of Ni(II) and stabilizes double helical DNA to the same extent as a G.C pair.     

Syntheses and structures of isoindoline complexes of Zn(II) and Cu(II): an unexpected trinuclear Zn(II) complex

Deprotonation of the tridentate isoindoline ligand 1,3-bis[2-(4-methylpyridyl)imino]-isoindoline, 4'-MeLH, and reaction with hydrated zinc(II) perchlorate produces an unexpected trinuclear Zn(II) complex, [Zn(3)(4'-MeL)(4)](ClO(4))(2).5H(2)O (1), whereas reaction with hydrated copper(II) perchlorate in methanol produces the expected mononuclear product, [Cu(4'-MeL)(H(2)O)(2)]ClO(4) (2). X-ray diffraction shows that the trinuclear Zn(II) complex (1) contains a linear zinc backbone, and the arrangement of ligands about the outer chiral zinc(II) atoms is helical. The two terminal zinc ions exhibit approximate C(2) site symmetry, with tetrahedral coordination by two pyrrole and two pyridyl nitrogen atoms of the potentially tridentate isoindoline ligands. The central zinc ion exhibits approximate tetrahedral symmetry, with coordination by four pyridyl nitrogen atoms of four different isoindoline ligands. Pyridyl-pyrrole intramolecular pi-stacking interactions contribute to the stability of the trinuclear cation. The structure of the mononuclear copper(II) complex cation in 2 is best described as a distorted trigonal bipyramid. The isoindoline anion binds Cu(II) in both axial positions and one of the equatorial positions; water molecules occupy the other two equatorial positions.     

Complexes of 2,6-bis[N-(2'-pyridylmethyl)carbamyl]pyridine: formation of mononuclear complexes, and self-assembly of double helical dinuclear and tetranuclear copper(II) and trinuclear nickel(II) complexes

The potentially pentadentate ligand 2,6-bis[N-(2'-pyridylmethyl)carbamyl]pyridine (H2L1), readily prepared from reaction of a diester of pyridine-2,6-dicarboxylic acid (H2dipic) and 2-aminomethylpyridine (ampy), shows limited tendency to form 1:1 M:L complexes with labile metal ions, although [CuL1] and [NiL1] were observed as minor species, the latter characterized by a crystal structure analysis. A mononuclear complex formed with inert Co(III) was characterized by a crystal structure as the neutral 1:2 complex [Co(L1)(HL1)] with two ligands acting as tridentate ligands, one coordinated by the central pyridine and its two flanking deprotonated amido groups, and the other by the central pyridine, one amido and one terminal pyridine group, with the remaining poorly coordinating protonated amide remaining unbound along with other terminal pyridine groups. Fe(III) is known to form a symmetrical 1:2 complex, but that complex is anionic due to binding of all four deprotonated amido groups; the unsymmetrical neutral Co(III) complex converts into a symmetrical anionic species only on heating for hours in aqueous base in the presence of activated carbon. The most remarkable tendency of H2L1, however, is towards the formation of robust double helical complexes: a dinuclear Cu(II) complex [Cu2L1(2)] forms, as well as a trinuclear Ni(II) complex [Ni(3)(L1)2(OAc)2(MeOH)2]. Moreover, in the presence of added H2dipic, the tetranuclear complex [Cu4(L1)2(dipic)2(OH2)2] is obtained. All helical complexes have been characterized by X-ray crystal structure analyses, and all crystals feature a racemic mixture of left- and right-handed double helices stabilized by inter-ligand pi-stacking (inter-ring distances of 3.2-3.8 A) of ligands which each span several metal ions. Using the chelating ligand pentane-2,4-dione (acac), each of the two pairs of adjacent monodentate ligands in [Ni3(L1)2(OAc)2(OH2)2] have been shown to be available for substitution without destroying the helical structure, to form [Ni3(L1)2(acac)2], also characterized by a crystal structure.     

Synthesis and photophysical studies of chiral helical macrocyclic scaffolds via coordination-driven self-assembly of 1,8,9,16-tetraethynyltetraphenylene. formation of monometallic platinum(II) and dimetallic platinum(II)-ruthenium(II) complexes

This paper is concerned with the synthesis and reactions of enantiopure 1,8,9,16-tetraethynyltetraphenylene (3). We obtained 3 in 34% yield through four steps starting from 1,8,9,16-tetrahydroxytetraphenylene (2a) via a functional group interconversion strategy. On the basis of this chiral "helical" building block, three rigid helical macrocycles 14, 15, and 22 were designed. Complexes 14 and 15 were constructed via coordination-driven self-assembly with platinum(II) complexes 8 and 9b, while 22 cannot be obtained successfully. Then macrocycle 28 was designed on the structural basis of 22 to which octyl chains were introduced, in the hope of improving the solubility of the complex. Macrocycle 28 was finally formed and was characterized by NMR spectroscopy, elemental analysis, and electrospray mass spectrometry. For the enantiopure 15 and 28, circular dichroism (CD) spectra also exhibited chiral properties. Complexes 27 and 28 both exhibited an intense emission band at 621 nm in acetonitrile at 298 K upon excitation at λ > 420 nm.