Theoretical Study on the Relationship between Diradical Character and Second Hyperpolarizabilities of Four‐Membered‐Ring Diradicals Involving Heavy Main‐Group Elements

By using spin‐unrestricted density functional theory methods, the relationship between the diradical character y and the second hyperpolarizability γ (the third‐order nonlinear optical (NLO) properties at the molecular scale) for four‐membered‐ring diradical compounds, that is, cyclobutane‐1,3‐diyl, Niecke‐type diradicals, and Bertrand‐type diradicals, were investigated by focusing on the substitution effects of heavy main‐group elements as well as of donor/acceptor groups on the y and γ values. It has been found that i) γ is enhanced in the intermediate y region for these four‐membered‐ring diradicals, ii) Niecke‐type diradicals with intermediate y values, which are realized by tuning the combination of the main‐group elements involved, exhibit larger γ values than Bertrand‐type diradicals, and iii) the y value and thus γ value can be controlled by modifying the both‐end donor/acceptor substituents attached to carbon atoms in Nicke‐type C₂P₂ diradicals. These results demonstrate that four‐membered‐ring diradicals involving heavy main‐group elements exhibit high controllability of the y and γ, which indicates the potential applications of four‐membered‐ring diradicals as a building block of highly efficient open‐shell NLO materials.     

Synthesis,solid‐state,and charge‐transport properties of conjugated polythiophene‐S,S‐dioxides

An alkylated semiconducting polymer comprising alternating bithiophene‐[all]‐S,S‐dioxide and aromatic monothiophene units in the polymer backbone was synthesized with the intent of modifying the energy gap and lowest unoccupied molecular orbital for use as a stable n‐type semiconductor. Films spun from this semiconducting polymer were characterized utilizing X‐ray scattering, near edge X‐ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, and thin‐film field effect transistors to determine how oxidation of the thiophene ring systems impacts the structural and electronic properties of the polymer. The thiophene‐S,S‐dioxide polymers have lower optical and electrical band gaps than corresponding thiophene polymers. X‐ray scattering results indicate that the polymers are well ordered with the π–π stacking distances increased by 0.4 Å relative to analogous thiophene polymers. The electrical stability of these polymers is poor in transistors with a drop in the field effect mobility by approximately one order of magnitude upon addition of just 5% of the thiophene‐S,S‐dioxide unit in a copolymer with thiophene. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Interplay between the Diradical Character and Third‐Order Nonlinear Optical Properties in Fullerene Systems

To reveal new structure–property relationships in the nonlinear optical (NLO) properties of fullerenes that are associated with their open‐shell character, we investigated the interplay between the diradical character (y_i) and second hyperpolarizability (longitudinal component, γ_zzzz) in several fullerenes, including C₂₀ , C₂₆ , C₃₀ , C₃₆ , C₄₀ , C₄₂ , C₄₈ , C₆₀ , and C₇₀ , by using the broken‐symmetry density functional theory (DFT; LC‐UBLYP (μ=0.33)/6‐31G*//UB3LYP/6‐31G*). We found that the large differences between the geometry and topology of fullerenes have a significant effect on the diradical character of each fullerene. On the basis of their different diradical character, these fullerenes were categorized into three groups, that is, closed‐shell (y_i=0), intermediate open‐shell (0<y_i<1), and almost pure open‐shell compounds (y_i?1), which originated from their diverse topological features, as explained by odd‐electron‐density and spin‐density diagrams. For example, we found that closed‐shell fullerenes include C₂₀ , C₆₀ , and C₇₀ , whereas fullerenes C₂₆ and C₃₆ and C₃₀ , C₄₀ , C₄₂ , and C₄₈ are pure and intermediate open‐shell compounds, respectively. Interestingly, the γ_zzzz enhancement ratios between C₃₀ / C₃₆ and C₄₀ / C₆₀ are 4.42 and 11.75, respectively, regardless of the smaller π‐conjugation size in C₃₀ and C₄₀ than in C₃₆ and C₆₀ . Larger γ_zzzz values were obtained for other fullerenes that had intermediate diradical character, in accordance with our previous valence configuration interaction (VCI) results for the two‐site diradical model. The γ_zzzz density analysis shows that the large positive contributions originate from the large γ_zzzz density distributions on the right‐ and left‐extended edges of the fullerenes, between which significant spin polarizations (related to their intermediate diradical character) appear within the spin‐unrestricted DFT level of theory.     

Stable Cross‐Conjugated Tetrathiophene Diradical

A tetracyano quinoidal tetrathiophene, having a central bi(thieno[3,4‐c]pyrrole‐4,6‐dione) acceptor, has been studied. The recovered aromaticity of the thiophenes produces a diradical species with cross‐conjugation between the inter‐dicyano and inter‐dione acceptor paths. A diradical character of y₀=0.61 and a singlet–triplet gap of ?2.76 kcal mol^?1 were determined. Competition between the two cross‐conjugated paths enhances the disjointed character of the SOMOs and results in the confinement of the diradical to the molecular center, enabling a thermodynamic diradical stabilization featuring a half‐life of 262 hours. Cross‐conjugation effects have been also addressed in the anionic species (up to a radical trianion).     

A Stable All‐Thiophene‐Based Core‐Modified [38]Octaphyrin Diradicaloid: Conformation and Aromaticity Switch at Different Oxidation States

A soluble and stable core‐modified [38]octaphyrin, MC‐T8 , containing eight thiophene rings was synthesized by Yamamoto coupling followed by oxidative dehydrogenation. X‐ray crystallographic analysis revealed a nearly planar backbone, and the molecule is globally aromatic with a dominant 38π conjugation pathway. The dication MC‐T8²⁺ is antiaromatic, and the backbone is distorted, with a different orientation of the thiophene rings. The tetracation MC‐T8⁴⁺ becomes aromatic again, with a shallow‐bowl‐shaped geometry. Both the neutral compound and the dication demonstrated open‐shell diradical character with a small singlet–triplet energy gap (?2.70 kcal mol^?1 for MC‐T8 and ?3.78 kcal mol^?1 for MC‐T8²⁺ ), and they are stable owing to effective spin delocalization.     

Synthesis and characterization of fluorene‐based oligomers and polymers incorporating N‐arylphenothiazine‐S,S‐dioxide units

A series of 3,7‐bis(9,9‐di‐n‐hexylfluoren‐2‐yl)‐N‐arylphenothiazine‐S,S‐dioxide trimers and (9,9‐di‐n‐octylfluorene‐2,7‐diyl‐co‐N‐arylphenothiazine‐S,S‐dioxide) co‐polymers, with varying ratios of phenothiazine‐S,S‐dioxide units, have been prepared in good yields by palladium‐catalyzed cross‐coupling reactions. The materials are deep blue emitters and show no solvatochromism or evidence for an intramolecular charge‐transfer state. The photoluminescence quantum yields of the trimers are ?_PL 15–30% in solution and 14–25% in films. The polymers demonstrated very high values in solution (?_PL 74–84%) and ?_PL values in films of 28–47%. The estimated HOMO energy levels are between ?5.64 and ?5.62 eV for the polymers with 15% incorporation of the phenothiazine‐S,S‐dioxide units. An analogous N‐arylphenothiazine co‐polymer shows significantly red shifted absorption and emission. Solution electrochemical data and density functional theory calculations are also presented. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Stable Expanded Porphycene‐Based Diradicaloid and Tetraradicaloid

The synthesis of a bithiophene‐bridged 34π conjugated aromatic expanded porphycene 1 and a cyclopentabithiophene bridged 32π conjugated anti‐aromatic expanded porphycene 2 by a McMurry coupling strategy is presented. Magnetic measurements and theoretical calculations reveal that both 1 and 2 exhibit an open‐shell singlet ground state with significant radical character (y₀=0.63 for 1 ; y₀=0.68, y₁=0.18 for 2 ; y₀: diradical character, y₁: tetraradical character) and a small singlet–triplet energy gap (ΔE_S‐T=?3.25 kcal mol^?1 for 1 and ΔE_S‐T=?0.92 kcal mol^?1 for 2 ). Despite the open‐shell radical character, both compounds display exceptional stability under ambient air and light conditions owing to effective delocalization of unpaired electrons in the extended cyclic π‐conjugation pathway.     

1‐[(E)‐2‐Arylethenyl]‐2,2‐diphenylcyclopropanes: Kinetics and Mechanism of Rearrangement to Cyclopentenes

Kinetic measurements for the thermal rearrangement of 2,2‐diphenyl‐1‐[(E)‐styryl]cyclopropane ( 22a ) to 3,4,4‐triphenylcyclopent‐1‐ene ( 23a ) in decalin furnished ΔH =31.0±1.2 kcal mol^?1 and ΔS =?6.0±2.6 e.u. The lowering of ΔH^≠ by 20 kcal mol^?1, compared with the rearrangement of the vinylcyclopropane parent, is ascribed to the stabilization of a transition structure (TS) with allylic diradical character. The racemization of (+)‐(S)‐ 22a proceeds with ΔH =28.2±0.8 kcal mol^?1 and ΔS =?5±2 e.u., and is at 150° 106 times faster than the rearrangement. Seven further 1‐(2‐arylethenyl)‐2,2‐diphenylcyclopropanes 22 , (E)‐ and (Z)‐isomers, were synthesized and characterized. The (E)‐compounds showed only modest substituent influence in their k_rac (at 119.4°) and k_isom (at 159.3°) values. The lack of solvent dependence of rate opposes charge separation in the TS, but a linear relation of log k_rac with log p.r.f., i.e., partial rate factors of radical phenylations of ArH, agrees with a diradical TS. The ring‐opening of the preponderant s‐trans‐conformation of 22 gives rise to the 1‐exo‐phenylallyl radical 26 that bears the diphenylethyl radical in 3‐exo‐position, and is responsible for racemization. The 1‐exo‐3‐endo‐substituted allylic diradical 27 arises from the minor s‐gauche‐conformation of 22 and is capable of closing the three‐ or the five‐membered ring, 22 or 23 , respectively. The discussion centers on the question whether the allylic diradical is an intermediate or merely a TS. Quantum‐chemical calculations by Houk et al. (1997) for the parent vinylcyclopropane reveal the lack of an intermediate. Can the conjugation of the allylic diradical with three Ph groups carve the well of an intermediate?     

2‐Ethyl‐1‐[5‐(4‐methylphenyl)pyrazol‐3‐yl]‐3‐(thiophene‐2‐carbonyl)isothiourea: molecular ribbons containing three types of hydrogen‐bonded ring

The title compound, C₁₈H₁₈N₄OS₂, was prepared by reaction of S,S‐diethyl 2‐thenoylimidodithiocarbonate with 5‐amino‐3‐(4‐methylphenyl)‐1H‐pyrazole using microwave irradiation under solvent‐free conditions. In the molecule, the thiophene unit is disordered over two sets of atomic sites, with occupancies of 0.814 (4) and 0.186 (4), and the bonded distances provide evidence for polarization in the acylthiourea fragment and for aromatic type delocalization in the pyrazole ring. An intramolecular N—H...O hydrogen bond is present, forming an S(6) motif, and molecules are linked by N—H...O and N—H...N hydrogen bonds to form a ribbon in which centrosymmetric R₂²(4) rings, built from N—H...O hydrogen bonds and flanked by inversion‐related pairs of S(6) rings, alternate with centrosymmetric R₂²(6) rings built from N—H...N hydrogen bonds.     

A Peri‐tetracene Diradicaloid: Synthesis and Properties

Peri‐acenes are good model compounds for zigzag graphene nanoribbons, but their synthesis is extremely challenging owing to their intrinsic open‐shell diradical character. Now, the successful synthesis and isolation of a stable peri‐tetracene derivative PT‐2ClPh is reported; four 2,6‐dichlorophenyl groups are attached onto the most reactive sites along the zigzag edges. The structure was confirmed by X‐ray crystallographic analysis and its electronic properties were systematically investigated by both experiments and theoretical calculations. It exhibits an open‐shell singlet ground state with a moderate diradical character (y₀=51.5 % by calculation) and a small singlet–triplet gap (ΔE_S‐T=?2.5 kcal mol^?1 by SQUID measurement). It displays global aromatic character, which is different from the smaller‐size bisanthene analogue BA‐CF3 .     

meta‐ and para‐Phenylenediamine‐Fused Porphyrin Dimers: Synthesis and Magnetic Interactions of Their Dication Diradicals

meta‐ and para‐Phenylenediamine‐fused nickel(II) porphyrin dimers were synthesized by S_NAr reaction of meso,β,β‐trichloro nickel(II) porphyrin with meta‐ and para‐phenylenediamines and subsequent Pd‐catalyzed intramolecular C?H arylation. Their tetrachlorinated dication diradicals are very stable, allowing SQUID magnetometry and revealing clear open‐shell characters for both meta and para isomers with ferro‐ and anti‐ferromagnetic interactions, respectively. The nitrogen analogue of Thiele's hydrocarbon usually displays predominant closed‐shell nature but its hidden diradical characters increase either in a twisted conformation or upon insertion of an additional phenylene spacer. The observed distinct diradical nature of the para‐congener indicates that diradical properties can be enhanced also by efficient spin delocalization.     

Tuning the Electronic Properties and Acid‐Response Behavior of N‐Heteroacene‐Based π‐Conjugated Liquids by Changing the Number of π‐Conjugated Substituents

We have designed and synthesized two room‐temperature‐fluorescent π‐conjugated liquids based on the N‐heteroacene framework ( 1 and 2 ). These two π‐conjugated liquids, which contained one and two thiophene rings, respectively, exhibited different electronic properties and rheology behaviors. Single‐crystal X‐ray analysis of dithiophene‐appended compound 4 revealed that two thiophene rings hindered the interactions of the imino N atoms with acids through the formation of interactions between the S atoms of the thiophene rings and the imino N atoms of the pyrazine group. On the other hand, monothiophene‐appended molecules 1 and 3 each contained an unhindered imino N atom on the opposite site to the thiophene ring. Upon dissolving various acids with different pK_a values in compounds 1 and 2 , these slight structural differences gave rise to marked differences in their acid‐response behaviors, thereby resulting in the emission of variously colored fluorescence in the liquid state. Furthermore, when acids with lower pK_a values was dissolved in compounds 1 and 2 , phase transition occurred from an isotropic liquid state to a self‐organized liquid‐crystalline phase.     

Derivatizing Tribenzothiophene‐Fused Hexa‐peri‐hexabenzocoronenes with Tunable Optoelectronic Properties

A series of trisbenzothieno[1,2:7,8:13,14]hexa‐peri‐hexabenzocoronenes were synthesized and characterized by a combination of NMR, 2D NMR, MALDI‐TOF MS, UV/Vis absorption spectroscopy, and 2D‐WAXS measurement. By structural modulation like decoration of electro‐donating alkoxyl chain, and conversion from an electron‐rich thiophene ring into an electron‐poor thiophene‐S,S‐dioxide moiety, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the hexabenzocoronenes derivatives can be effectively tuned which is further verified by the DFT calculations and cyclic voltammetry.     

Stacking patterns of thieno[3,2‐b]thiophenes functionalized by sequential palladium‐catalyzed Suzuki and Heck cross‐coupling reactions

The crystal structures of three 5‐alkenyl‐2‐arylthieno[3,2‐b]thiophenes, namely 3,6‐dibromo‐5‐(4‐tert‐butylstyryl)‐2‐(naphthalen‐1‐yl)thieno[3,2‐b]thiophene, C₂₈H₂₂Br₂S₂, (I), 3,6‐dibromo‐5‐(4‐methylstyryl)‐2‐(naphthalen‐1‐yl)thieno[3,2‐b]thiophene, C₂₅H₁₆Br₂S₂, (II), and 3,6‐dibromo‐2‐(4‐tert‐butylphenyl)‐5‐(4‐methylstyryl)thieno[3,2‐b]thiophene, C₂₅H₂₂Br₂S₂, (III), have been determined in order to evaluate the geometry of the molecules. The π‐conjugated system containing the thieno[3,2‐b]thiophene skeleton, the ethylene bridge and the phenyl rings is almost planar. The aromatic ring directly attached to the thieno[3,2‐b]thiophene moiety is not coplanar with the thieno[3,2‐b]thiophene moiety itself due to steric hindrance of the bromo substituent. The crystal packings are characterized by π–π stacking [only for (II)] and C—Br...π interactions. The long axes of the molecules in (I) are oriented in two directions; for the two other structures the long axis is oriented in one direction only.     

Stable Red‐Emissive Cationic Dithienotropylium Dyes

A series of thiophene‐fused tropylium ions, containing various electron‐donating amino groups at the terminal positions, was synthesized. The fusion of the thiophene rings, as well as the presence of the terminal amino groups endows the cationic tropylium ion with excellent stability and high pK_R⁺ values. X‐ray crystallographic analysis of these compounds revealed a pronounced quinoidal character for the amino‐substituted dithienotropylium skeletons. These compounds exhibit attractive photophysical properties such as strong absorption in the visible region combined with red fluorescence. Theoretical calculations suggested that the 3,3′‐bithiophene substructure should be crucial for attaining these photophysical properties.     

Stable Oxindolyl‐Based Analogues of Chichibabin's and Müller's Hydrocarbons

Chichibabin's and Müller's hydrocarbons are classical open‐shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR‐2 and OxR‐3 , based on the newly developed oxindolyl radical. X‐ray crystallographic analysis on OxR‐2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y₀=11.1 %) and large singlet–triplet gap (ΔE_S‐T=−10.8 kcal mol⁻¹). Variable‐temperature NMR studies on OxR‐2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG^≠_298K=15–16 kcal mol⁻¹). OxR‐3 exhibits a much larger diradical character (y₀=80.6 %) and a smaller singlet–triplet gap (ΔE_S‐T=−3.5 kcal mol⁻¹), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon‐centered monoradical and diradicaloid.     

Porphyrin/Quinoidal‐Bithiophene‐Based Macrocycles and Their Dications: Template‐Free Synthesis and Global Aromaticity

We report the template‐free synthesis and characterization of a new type of porphyrin/quinoidal‐bithiophene‐based conjugated macrocycle. X‐ray crystallographic analysis of the dimer ( 2MC ) revealed a cyclophane‐like geometry with large dihedral angles between the porphyrin and the neighboring thiophene rings, and NMR measurements and theoretical calculations confirmed a localized aromatic character of the porphyrin/thiophene rings and quinoidal character of the bithiophene linkers. Restricted rotation of the thiophene rings linked to the porphyrin unit was observed by variable‐temperature NMR measurements. The dication ( 2MC²⁺ ) adopts a chair‐shaped conformation to facilitate π‐electron delocalization around the whole macrocycle. As a result, the molecule is globally aromatic, with a dominant 54 π conjugation pathway. The trimer ( 3MC ) also shows localized aromatic character of porphyrin rings and conformational flexibility, but its dication ( 3MC²⁺ ) is rigid and globally aromatic with a dominant 82 π conjugation pathway.     

Advantage of the N‐Alkylation Strategy for Retaining the Molecular Planarity for Oligothiophene/Imidazole/1,10‐Phenanthroline‐Based Heterocyclic Semiconducting and Fluorescent Compounds

A family of planar oligothiophene/imidazole/1,10‐phenanthroline (OTIP)‐based heterocyclic, aromatic, semiconducting, and fluorescent compounds with N‐substituted alkyl chains (allyl, n‐butyl, n‐octyl, n‐dodecyl, and n‐cetyl) have been designed and synthesized. They all have specific N‐coordination sites, various donor–acceptor spacers, good molecular planarity, suitable solubility, and high thermal stability. In comparison with conventional double β‐alkylation of the thiophene ring, our results reveal that the single imidazole N‐alkylation strategy for OTIPs has the advantage of maintaining the planarity of the whole molecule, in addition to improving the solubility, which can be clearly verified by the small dihedral angles between adjacent thiophene/imidazole/1,10‐phenanthroline (TIP) rings in eight X‐ray single‐crystal structures. In particular, n‐dodecyl‐ and n‐cetyl‐substituted OTIPs ( 7 and 8 ) with the same molecular length of 2.37 nm (M_W=939 and 1052), show good molecular planarity with the aforementioned dihedral angles of 8.9(5) and 10.4(5)°. Furthermore, special attention has been paid to the physicochemical properties of seven symmetrical OTIPs ( 6 – 8 , 13 – 15 , and 19 ), including two to six thiophene rings in the middle of their molecular structures. To the best of our knowledge, this is the first synthetic, structural, and spectral investigation into the N‐alkylation of OTIP‐based compounds.     

Toward Benzobis(thiadiazole)‐based Diradicaloids

We theoretically predicted that acetylene‐bridged benzo[1,2‐c ;4,5‐c ′]bis[1,2,5]thiadiazole (BBT) oligomers would show a quick increase of diradical character with the extension of chain length. To validate the hypothesis, six stable BBT‐based diradicaloids were synthesized and fully characterized by X‐ray crystallographic analysis and various spectroscopic measurements. Three of them showed prominent paramagnetic activity at elevated temperatures due to thermal population from the open‐shell singlet ground state to triplet excited state. It was also found that substitution by electron‐donating triphenylamine groups at the termini promoted the diradical character and reduced the singlet–triplet energy gap, and at the same time, resulted in intense near‐infrared absorption.     

An Unusually Small Singlet–Triplet Gap in a Quinoidal 1,6‐Methano[10]annulene Resulting from Baird’s 4n π‐Electron Triplet Stabilization

Within the continuum of π‐extended quinoidal electronic structures exist molecules that by design can support open‐shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open‐shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open‐shell structure is accompanied by structural reorganization from a contorted Möbius aromatic‐like shape in S₀ to a more planar shape in the Hückel aromatic‐like T₁. This stability was attributed to Baird’s Rule which dictates the aromaticity of 4n π‐electron triplet excited states.