Steric Control in the π‐Dimerization of Oligothiophene Radical Cations Annelated with Bicyclo[2.2.2]octene Units

A Two series of oligothiophenes 2 (nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3 a – c , annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of π‐dimerization and the structure–property relationships of the π‐dimers of oligothiophene radical cations. Their radical‐cation salts were prepared through chemical one‐electron oxidation by using nitrosonium hexafluoroantimonate. From variable‐temperature electron spin resonance and electronic absorption measurements, the π‐dimerization capability was found to vary among the members of the 2 (nT)^{+ .} SbF₆^? series and 3 ^{+ .} SbF₆^? series of compounds. To examine these results, density functional theory (DFT) calculations at the M06‐2X/6‐31G(d) level were conducted for the π‐dimers. This level of theory was found to successfully reproduce the previously reported X‐ray structure of ( 2 (3T))₂²⁺ having a bent π‐dimer structure with cis–cis conformations. The absorption bands obtained by time‐dependent DFT calculations for the π‐dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the π‐dimerization were divided into four factors: 1) SOMO–SOMO interactions, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion, and the effects of each factor on the structural differences and chain‐length dependence are discussed in detail.     

Multistep π Dimerization of Tetrakis(n‐decyl)heptathienoacene Radical Cations: A Combined Experimental and Theoretical Study

Radical cations of a heptathienoacene α,β‐substituted with four n‐decyl side groups (D4T7 ^{. +}) form exceptionally stable π‐dimer dications already at ambient temperature (Chem. Comm. 2011 , 47, 12622). This extraordinary π‐dimerization process is investigated here with a focus on the ultimate [D4T7 ^{. +}]₂ π‐dimer dication and yet‐unreported transitory species formed during and after the oxidation. To this end, we use a joint experimental and theoretical approach that combines cyclic voltammetry, in situ spectrochemistry and spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The impact of temperature, thienoacene concentration, and the nature and concentration of counteranions on the π‐dimerization process is also investigated in detail. Two different transitory species were detected in the course of the one‐electron oxidation: 1) a different transient conformation of the ultimate [D4T7 ^{. +}]₂ π‐dimer dications, the stability of which is strongly affected by the applied experimental conditions, and 2) intermediate [D4T7]₂ ^{. +} π‐dimer radical cations formed prior to the fully oxidized [D4T7]₂ ^{. +} π‐dimer dications. Thus, this comprehensive work demonstrates the formation of peculiar supramolecular species of heptathienoacene radical cations, the stability, nature, and structure of which have been successfully analyzed. We therefore believe that this study leads to a deeper fundamental understanding of the mechanism of dimer formation between conjugated aromatic systems.     

A New Conformation With an Extraordinarily Long, 3.04 Å Two‐Electron,Six‐Center Bond Observed for the π‐[TCNE]22− Dimer in [NMe4]2[TCNE]2 (TCNE=Tetracyanoethylene)

[NMe₄]₂[TCNE]₂ (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe₄)CN in MeCN has ν_CN IR absorptions at 2195, 2191, 2172, and 2156 cm^?1 and a ν_CC absorption at 1383 cm^?1 that are characteristic of reduced TCNE. The TCNEs have an average central C?C distance of 1.423 Å that is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non‐eclipsed, centrosymmetric π‐[TCNE]₂^2? dimer with nominal C₂ symmetry, 12 sub van der Waals interatomic contacts <3.3 Å, a central intradimer separation of 3.039(3) Å, and comparable intradimer C???N distances of 3.050(3) and 2.984(3) Å. The two pairs of central C???C atoms form a ?C?C???C?C of 112.6° that is substantially greater than the 0° observed for the eclipsed D_2h π‐[TCNE]₂^2? dimer possessing a two‐electron, four‐center (2e^?/4c) bond with two C???C components from a molecular orbital (MO) analysis. A MO study combining CAS(2,2)/MRMP2/cc‐pVTZ and atoms‐in‐molecules (AIM) calculations indicates that the non‐eclipsed, C₂ π‐[TCNE]₂^2? dimer exhibits a new type of a long, intradimer bond involving one strong C???C and two weak C???N components, that is, a 2e^?/6c bond. The C₂ π‐[TCNE]₂^2? conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J/k_B=1000 K (700 cm^?1; 86.8 meV; 2.00 kcal mol^?1; H=?2 JS_a?S_b); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcal mol^?1 higher in energy than the closed‐shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc‐pVTZ calculations indicate that the C₂ and D_2h conformers have two different local metastable minima with the C₂ conformer being 1.3 kcal mol^?1 less stable. The different natures of the C₂ and D_2h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e^?/6c bond with one C???C and two C???N bonding components for the C₂ conformer as compared to the 6e^?/4c bond for the D_2h conformer with two C???C bonding components.     

The Divergent Dimerization Behavior of N‐Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals

Herein, we demonstrate that the dimerization behavior of amine‐substituted dicyanomethyl radicals can be switched from σ‐ to π‐dimerization simply by varying the electron‐donating substituents. For dicyanomethyl radicals with a 4,4′‐ditolylamine ( DT^. ) or a phenothiazine ( PT^. ) substituent, the monomeric radical form and the corresponding dimer connected by a reversible C−C bond (σ‐dimer) are in equilibrium in solution. On the other hand, the radical with the julolidine skeleton ( JD^. ) does not undergo σ‐dimerization and was isolated as a stable radical in spite of the absence of bulky protecting groups. X‐ray single‐crystal analysis revealed that JD^. forms the π‐dimer in the crystalline state, and variable‐temperature spectroscopy showed that JD^. is in equilibrium with the π‐dimer in toluene solution. DFT calculations point to the importance of electrostatic interactions as a driving force for the π‐dimerization of JD^. because of its polarized structure.     

The Origin of the Room‐Temperature Stability of [TTF].+⋅⋅⋅[TTF].+ Long,Multicenter Bonds Found in Functionalized π‐[R‐TTF]22+ Dimers Included in the Cucurbit[8]uril Cavity

A computational study is performed to identify the origin of the room‐temperature stability, in aqueous solution, of functionalized π‐[R‐TTF]₂²⁺ dimers (TTF=tetrathiafulvalene; R=(CH₂OCH₂)₅CH₂OH) included in the cavity of a cucurbit[8]uril (CB[8]) molecule. π‐[R‐TTF]₂²⁺ dimers in pure water are weakly stable, and are mostly dissociated at room temperature. Upon addition of CB[8] to an aqueous π‐[R‐TTF]₂²⁺ solution, a (π‐[R‐TTF]₂?CB[8])²⁺ inclusion complex is formed. The same complex is obtained after the sequential inclusion of two [R‐TTF]^.+ monomers in the CB[8] molecule. Both processes are thermodynamically and kinetically allowed. π‐[R‐TTF]₂²⁺ dimers dissolved in pure water present a [TTF]^.+???[TTF]^.+ long, multicenter bond, similar to that already identified in π‐[TTF]₂²⁺ dimers dissolved in organic solvents. Upon their inclusion in CB[8], the strength and other features of the [TTF]^.+???[TTF]^.+ long, multicenter bond are preserved. The room temperature stability of the π‐[R‐TTF]₂²⁺ dimers included in CB[8] is shown to originate in the π‐[R‐TTF]₂²⁺???CB[8] interaction, the strength of which comes from a strongly attractive electrostatic component and a dispersion component. Such a dominant electrostatic term is caused by the strongly polarized charge distribution in CB[8], the geometrical complementarity of the π‐[R‐TTF]₂²⁺ and CB[8] geometries, and the amplifying effect of the 2+ charge in π‐[R‐TTF]₂²⁺.     

Orientational Preference of Long,Multicenter Bonds in Radical Anion Dimers: A Case Study of π‐[TCNB]22− and π‐[TCNP]22−

The similar shape and electronic structure of the radical anions of 1,2,4,5‐tetracyanopyrazine (TCNP) and 1,2,4,5‐tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon?carbon bond in π‐[TCNP]₂^2? and in π‐[TCNB]₂^2?, in good accord with the Maximin Principle predictions. Instead, the two known structures of π‐[TCNP]₂^2? have a D_2h(θ=0°) and a C₂(θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six‐membered rings). The only known π‐[TCNB]₂^2? structure has a C₂(θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (E_int) and the variation of the E_int components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the E_int(θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of E_int(θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the E_int(θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)^.? ??? cation⁺ interactions are also included in the model aggregate are the experimental data reproduced computationally.     

2,3,4,5‐Tetrakis­[(2‐hydroxy­ethyl)­thio]­tetra­thia­fulvalene tetra­fluoro­borate

The title complex, C₁₄H₂₀O₄S₈^+.BF₄^?, is a charge‐transfer complex with typical charges for the donor and anion of +1 and ?1, respectively. Two centrosymmetrically related donors form a face‐to‐face π‐dimer with a strong intermolecular S?S interaction. These π‐dimers stack along the a axis to form a donor column. The structure is extensively hydrogen bonded.     

A third polymorph of 4‐(2,6‐difluorophenyl)‐1,2,3,5‐dithiadiazolyl

The crystal structure of a third polymorphic form of the known 4‐(2,6‐difluorophenyl)‐1,2,3,5‐dithiadiazolyl radical, C₇H₃F₂N₂S₂, is reported. This new polymorph represents a unique crystal‐packing motif never before observed for 1,2,3,5‐dithiadiazolyl (DTDA) radicals. In the two known polymorphic forms of the title compound, all of the molecules form cis‐cofacial dimers, such that two molecules are π‐stacked with like atoms one on top of the other, a common arrangement for DTDA species. By contrast, the third polymorph, reported herein, contains two crystallographically unique molecules organized such that only 50% are dimerized, while the other 50% remain monomeric radicals. The dimerized molecules are arranged in the trans‐antarafacial mode. This less common dimer motif for DTDA species is characterized by π–π interactions between the S atoms [S...S = 3.208 (1) Å at 110 K], such that the two molecules of the dimer are related by a centre of inversion. The most remarkable aspect of this third polymorph is that the DTDA dimers are co‐packed with monomers. The monomeric radicals are arranged in one‐dimensional chains directed by close lateral intermolecular contacts between the two S atoms of one DTDA heterocycle and an N atom of a neighbouring coplanar DTDA heterocycle [S...N = 2.857 (2) and 3.147 (2) Å at 110 K].     

Theoretical Studies on the Hydrogen‐bonding and π‐Stacking Interactions in the m‐Nisoldipine Polymorphism Dimers

The intermolecular interactions in the dimers of m‐nisoldipine polymorphism were studied by B3LYP calculations and quantum theory of "atoms in molecules" (QTAIM) studies. Four geometries of dimers were obtained: dimer I (a‐dimer, O···H? N), dimer II (b‐dimer, O···H? N), dimer III (b‐dimer, π‐stacking‐c), and dimer IV (b‐dimer, π‐stacking‐p). The interaction energies of the four dimers are along the sequence of II>I>III>IV. The intermolecular distance of the interactions follows the order: I (O···H? N)II>III>IV, and the electrostatic character decreases along the sequence: I>II>III>IV.     

Unusually long, multicenter, cation(δ+)···anion(δ-) bonding observed for several polymorphs of [TTF][TCNE

The α, β, and δ polymorphs of [TTF][TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) exhibit a new type of long, multicenter bonding between the [TTF]^δ+ and [TCNE]^δ? moieties, demonstrating the existence of long, hetero‐multicenter bonding with a cationic^δ+???anionic^δ? zwitterionic‐like structure. These diamagnetic π‐[TTF]^δ+[TCNE]^δ? heterodimers exhibit a transfer of about 0.5 e^? from the TTF to the TCNE fragments, as observed from experimental studies, in accord with theoretical predictions, that is, [TTF^δ+???TCNE^δ?] (δ?0.5). They have several interfragment distances <3.4 Å, and a computed interaction energy of ?21.2 kcal mol^?1, which is typical of long, multicenter bonds. The lower stability of [TTF]^δ+[TCNE]^δ? with respect to typical ionic bonds is due, in part, to the partial electron transfer that reduces the electrostatic bonding component. This reduced electrostatic interaction, and the large interfragment dispersion stabilize the long, heterocationic/anionic multicenter interaction, which in [TTF^δ+???TCNE^δ?] always involves two electrons, but have ten, eight, and eight bond critical points (bcps) involving C? C, N? S, and sometimes C? S and C? N components for the α, β, and δ polymorphs, respectively. In contrast, γ‐[TTF][TCNE] possesses [TTF]₂²⁺ and [TCNE]₂^2? dimers, each with long, homo‐multicenter 2e^?/12c (c=center, 2 C+4 S) [TTF]₂²⁺ cationic⁺???cationic⁺ bonds, as well as long, homo‐multicenter 2e^?/4c [TCNE]₂^2? anionic^????anionic^? bonding. The MO diagrams for the α, β, and δ polymorphs have all of the features found for conventional covalent C? C bonds, and for all of the previously studied multicenter long bonds, for example, π‐[TTF]₂²⁺ and π‐[TCNE]₂^2?. The HOMOs for α‐, β‐, and δ‐[TTF][TCNE] have 2c C? S and 3c C? C? C orbital‐overlap contributions between the [TTF]^δ+? and [TCNE]^δ? moieties; these are the shortest intra [TTF???TCNE] separations. Thus, from an orbital‐overlap perspective, the bonding has 2c and 3c components residing over one S and four C atoms.     

Phenylene‐bridged Porphyrin meso‐Oxy Radical Dimers

Stable meta‐ and para‐phenylene bridged porphyrin meso‐oxy radical dimers and their Ni^II and Zn^II complexes were synthesized. All the dimers exhibited optical and electrochemical properties similar to the corresponding porphyrin meso‐oxy radical monomers, indicating small electronic interaction between the two spins. Intramolecular spin‐spin interaction through the π‐spacer was determined to be J/k_B=?15.9 K for m‐phenylene bridged Zn^II porphyrin dimer. The observed weak antiferromagnetic interaction has been attributed to less effective conjugation between the porphyrin radical and linking π‐spacer due to large dihedral angle. In the case of Zn^II complexes, both para‐ and meta‐phenylene bridged dimers formed 1D‐chain in solutions and in the solid states through Zn‐O coordination.     

Site‐Directed Dimerization of Bowl‐Shaped Radical Anions to Form a σ‐Bonded Dibenzocorannulene Dimer

Designed site‐directed dimerization of the monoanion radicals of a π‐bowl in the solid state is reported. Dibenzo[a,g]corannulene (C₂₈H₁₄) was selected based on the asymmetry of the charge/spin localization in the C₂₈H₁₄^.? anion. Controlled one‐electron reduction of C₂₈H₁₄ with Cs metal in diglyme resulted in crystallization of a new dimer, [{Cs⁺(diglyme)}₂(C₂₈H₁₄?C₂₈H₁₄)^2?] ( 1 ), as revealed by single crystal X‐ray diffraction study performed in a broad range of temperatures. The C?C bond length between two C₂₈H₁₄^.? bowls (1.560(8) Å) measured at ?143 °C does not significantly change upon heating of the crystal to +67 °C. The single σ‐bond character of the C?C linker is confirmed by calculations. The trans‐disposition of two bowls in 1 is observed with the torsion angles around the central C?C bond of 172.3(5)° and 173.5(5)°. A systematic theoretical evaluation of dimerization pathways of C₂₈H₁₄^.? radicals confirmed that the trans‐isomer found in 1 is energetically favored.     

trans‐4‐Acetylcyclo­hexane­carboxyl­ic acid and ()‐trans‐2‐acetyl­cyclo­hexane­carboxyl­ic acid: hydrogen‐bonding patterns in two isomeric keto acids

Both title compounds, C₉H₁₄O₃, display carboxyl‐dimer hydrogen‐bonding patterns. The 4‐acetyl isomer adopts a chiral conformation with negligible disordering of the methyl and carboxyl groups and forms centrosymmetric dimers across the b and c edges of the chosen cell [O?O = 2.667 (3) Å and O—H?O = 175°]. Intermolecular C—H?O close contacts were found for both carbonyl groups. In the 2‐acetyl isomer, there is no intramolecular interaction between the carboxyl and acetyl groups and the hydrogen bonding involves centrosymmetric carboxyl dimerization across the ab and ac faces of the chosen cell [O?O = 2.668 (2) Å and O—H?O = 173°]. The carboxyl group is negligibly disordered, but significant rotational disordering was found for the acetyl methyl group. An intermolecular C—H?O close contact was found involving the ketone group.     

The Tetracyanopyridinide Dimer Dianion, σ‐[TCNPy]22−

The reaction of 2,3,5,6‐tetracyanopyridine (TCNPy) and Cr(C₆H₆)₂ forms diamagnetic σ‐[TCNPy]₂^2? possessing a 1.572(3) Å intrafragment sp³–sp³ bond. This is in contrast to the structurally related 1,2,4,5‐tetracyanobenzene and 1,2,4,5‐tetracyanopyrazine that form π‐dimer dianions possessing long, multicenter bonds.     

Ferromagnetic Interactions in a 1D Alternating Linear Chain of π‐Stacked 1,3‐Diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl Radicals

X‐ray studies show that 1,3‐diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 6 ) adopts a distorted, slipped π‐stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7‐(thien‐2‐yl)benzotriazin‐4‐yl 6 show two fully reversible waves that correspond to the ?1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie–Weiss behavior in the 5–300 K region with C=0.378 emu K mol^?1 and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J₁=+7.12 cm^?1, J₂=+1.28 cm^?1).     

Cyclophane‐Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18‐Porphyrinyl Dicyanomethyl Diradicals

2,18‐Bis(dicyanomethyl)‐substituted Ni^II porphyrin 8 and Zn^II porphyrin 11 were prepared and subjected to oxidation with PbO₂ in CH₂Cl₂ at 298 K to give cyclophane‐type chlorin dimers ( 9 )₂ and ( 12 )₂ as a consequence of double recombination of biradicals 9 and 12 , respectively. Dimer ( 9 )₂ takes a syn‐conformation of two distorted Ni^II chlorins but ( 12 )₂ takes an anti‐conformation of relatively planar Zn^II chlorins. At 298 K, dimer ( 9 )₂ is stable and its ¹H NMR spectrum is sharp but becomes broad at high temperature, while the ¹H NMR spectrum of ( 12 )₂ is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of ( 12 )₂ is much less than that of ( 9 )₂. The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer ( 16 )₂ to give homo dimers ( 9 )₂ and ( 15 )₂.     

Facile Synthesis and Properties of 2‐λ5‐Phosphaquinolines and 2‐λ5‐Phosphaquinolin‐2‐ones

Treatment of 2‐ethynylanilines with P(OPh)₃ gives either 2,2‐diphenoxy‐2‐λ⁵‐phosphaquinolines or 2‐phenoxy‐2‐λ⁵‐phosphaquinolin‐2‐ones under transition‐metal‐free conditions. This reaction offers access to an underexplored heterocycle, which opens up the study of the fundamental nature of the N?P^V double bond and its potential for delocalization within a cyclic π‐electron system. This heterocycle can serve as a carbostyril mimic, with application as a bioisostere for pharmaceuticals based on the 2‐quinolinone scaffold. It also holds promise as a new fluorophore, since initial screening reveals quantum yields upwards of 40 %, Stokes shifts of 50–150 nm, and emission wavelengths of 380–540 nm. The phosphaquinolin‐2‐ones possess one of the strongest solution‐state dimerization constants for a D–A system (130 M ^?1) owing to the close proximity of a strong acceptor (P?O) and a strong donor (phosphonamidate N? H), which suggests that they might hold promise as new hydrogen‐bonding hosts for optoelectronic sensing.     

Specificity of the redox photoreaction of dimers of thiadicarbocyanine dyes

The primary steps of the redox reaction of dimers of the thiadicarbocyanine dye and its 5,5′-dichloro derivative in aqueous solutions were studied in the presence of 4-nitroacetophenone, ascorbic acid, or hydroquinone. In water the dye molecules (anion, M^?) mainly exist as dimers M₂ ^2?. The laser pulse irradiation (10 ns, 532 nm) results in the population of the lowest triplet level M₂ ^2?, whose depletion occurs due to both intersystem crossing to the ground state and photoinduced transition to the highest triplet state of the dimer followed by photoionization. Photoionization at low intensities of a laser pulse proceeds via the one-quantum mechanism going to the two-quantum mechanism with an increase in the laser pulse intensity. The photooxidation of the dimer in the lowest triplet state with 4-nitroacetophenone results in the formation of unstable radical anion M₂ ^?· that spontaneously dissociates to monomer M^? and radical M^· of the dye. In the presence of electron donors (ascorbic acid, hydroquinone), the dimers in the triplet state are not photoreduced, but the electron donors reduce M₂ ^?· and M^· to the dye dimer and monomer, respectively.     

Side‐chain conformations in the isomorphous polyfluorinated {4,4′‐bis[(2,2‐difluoroethoxy)methyl]‐2,2′‐bipyridine‐κ2N,N′}dichloridopalladium and ‐platinum complexes

The polyfluorinated title compounds, [M Cl₂(C₁₆H₁₆F₄N₂O₂)] or [4,4′‐(HCF₂CH₂OCH₂)₂‐2,2′‐bpy]M Cl₂ [M = Pd, ( 1 ), and M = Pt, ( 2 )], have –C(H_α)₂OC(H_β)₂CF₂H side chains with H‐atom donors at the α and β sites. The structures of ( 1 ) and ( 2 ) are isomorphous, with the nearly planar (bpy)M Cl₂ molecules stacked in columns. Within one column, π‐dimer pairs alternate between a π‐dimer pair reinforced with C—H…Cl hydrogen bonds (α,α) and a π‐dimer pair reinforced with C—H_β…F(—C) interactions (abbreviated as C—H_β…F—C,C—H_β…F—C). The compounds [4,4′‐(CF₃CH₂OCH₂)₂‐2,2′‐bpy]M Cl₂ [M = Pd, ( 3 ), and M = Pt, ( 4 )] have been reported to be isomorphous [Lu et al. (2012). J. Fluorine Chem. 137 , 54–56], yet with disorder in the fluorous regions. The molecules of ( 3 ) [or ( 4 )] also form similar stacks, but with alternating π‐dimer pairs between the (α,β; α,β) and (β,β) forms. Through (C—)H…Cl hydrogen‐bond interactions, one molecule of ( 1 ) [or ( 2 )] is expanded into an aggregate of two inversion‐related π‐dimer pairs, one pair in the (α,α) form and the other pair in the (C—H_β…F—C,C—H_β…F—C) form, with the plane normals making an interplanar angle of 58.24 (3)°. Due to the demands of maintaining a high coordination number around the metal‐bound Cl atoms in molecule ( 1 ) [or ( 2 )], the ponytails of molecule ( 1 ) [or ( 2 )] bend outward; in contrast, the ponytails of molecule ( 3 ) [or ( 4 )] bend inward.     

Theoretical Study of the Dimerization of Chlorophyll (a) and Its Hydrates: Implication for Chlorophyll (a) Aggregation

Dimeric structures chlorophyll (a) (Chla) and their mono‐ and dihydrated have been suggested to play an important role in the mechanism of photoreaction center chlorophyll special pairs PSI and PSII. Despite their functional importance, the molecular basis structures for interacting two Chla molecules and the structural stabilization role of H₂O in the formation of hydrated Chla dimer complexes is poorly understood. In this article, the different coordination modes between two interacting Chla molecules and the configurational (orientation and distance) features between the dimer and bound H₂O molecules are characterized by means of super molecule approach the density functional theory DFT. An estimation of the thermodynamic quantities is made for Chla dimerization and hydration processes. The results indicate that structure including ester linkages via H₂O hydrogen bonding is the most favorable conformation for the dihydrated chlorophyll (a) dimer at B3LYP/6‐31G*‐DCP level of calculation. The dispersion interaction is shown to be of great significance for the Chla dimer stabilization. In aqueous nonpolar solvent, the thermodynamics show that Chla has a slightly stronger driving force for full hydration than for dimerization and that hydration of the dimers is rather weakly exergonic. The tetrahydrated dimers having a similar arrangement to that in crystals of ethyl chlorophyllide (a) dihydrate are found to be more stable than the Chla dihydrated dimer. The data underscore the key role of H‐bonding in the stability of Chla‐H₂O adducts and, in particular, the great importance of the Chla monomeric dihydrated species in the hydration and dimerization of Chla in aqueous media. Clearly, the Chla dihydrates (Chla‐2 H₂O) are found more stable than the monohydrates (Chla‐H₂O) and the Chla dimers (Chla₂), owing to a particular structure in which cooperative interactions occur between the H₂O molecules and Chla. Calculations also indicate that the most thermodynamically preferred pathway for the formation of Chla dimer hydrates can be represented by two steps: the first corresponds to the formation of Chla monomeric dihydrates and the second is the dimerization of the dihydrates on to tetrahydrated Chla dimers. These results allow to obtain a new possible pathway for Chla dimer formation processes and could provide new insights to the aggregation of chlorophyll (a) in solution.