Electronic and Magnetic Interactions in 6-Oxoverdazyl Diradicals: Connection through N(1) vs C(3) Revisited.

Four isomeric di-6-oxoverdazyl diradicals connected at their N(1) or C(3) positions with either 1,3- or 1,4-phenylene linkers were obtained and characterized by spectroscopic, electrochemical, magnetic, and structural methods. These results were compared to those for the corresponding 6-oxoverdazyl monoradicals. UV-vis spectroscopy demonstrated that only the N(1)-connected para-through-benzene diradical has a distinct spectrum with significant bathochromic and hypsochromic shifts relative to the remaining species. Electrochemical analysis revealed two one-electron reduction processes in all diradiacals, while only the N(1)-connected para-through-benzene diradical exhibits two one-electron oxidation processes separated by 0.10 V. Variable temperature EPR measurements in polystyrene solid solutions gave negative mean exchange interaction energies J for all diradicals, suggesting the dominance of conformers with significant intramolecular antiferromagnetic interactions for the meta-through-benzene isomers. DFT calculations predict a small preference for the triplet state with the ΔES-T of about 0.25 kcal mol-1 for both meta-through-benzene connected diradicals.

[closo-B10H8-10-PhI-1-COOH]- Anion: An Intermediate for Functional Anionic Carboxylate Ligands.

A potentially general intermediate, [closo-B10H8-10-PhI-1-COOH]-, for a class of functional anionic carboxylic acids, [closo-B10H8-10-X-1-COOH]2-, was obtained in four steps and 26% overall yield from [closo-B10H10]2-. It was converted to the pyridinium derivative (X = C5H5N+) and subsequently to coordination complexes with (phen)2Cu2+ and (phen)2Zn2+ ions. Both the acid and Zn(II) complex exhibit a cage-to-pyridine charge-transfer band. The availability of such acids opens access to functional metal-ion complexes with compensated charges.

Blatter Diradicals with a Spin Coupler at the N(1) Position.

Reactions of a benzo[e][1,2,4]triazine with dilithiobenzenes lead to di-Blatter diradicals connected at the N(1) positions via a spin coupling unit, 1,4-phenylene or 1,3-phenylene. Electrochemical analysis in MeCN revealed four one-electron redox processes separated by 0.1-0.3 V in both diradicals. Variable temperature EPR measurements in polystyrene (PS) solid solutions gave the singlet-triplet energy gaps ΔES-T =2 J of -3.02(11) and -0.16(1) kcal mol-1 for 1,4-phenylene and 1,3-phenylene derivatives, respectively. The latter negative value was attributed to conformational properties of the diradical in the PS solid solution. Results suggest a simple and efficient access to a family of stable Blatter diradicals with a controllable S-T gap through a judicious choice of the arylene coupling unit. DFT calculations indicate that the triplet state is stabilized by (het)arylenes with low LUMO.

Paramagnetic Liquid Crystals With Close π-π Packing: The Effect of Blatter Radical Planarization on Behavior of Bent-Core Mesogens.

A new, 19 π-delocalized electrons planar Blatter radical building block was developed and used to obtain paramagnetic bent-core liquid crystals. The mesogens were investigated by optical, thermal, powder XRD and DFT methods in the pure form and as binary mixtures. Comparison of their properties with those of the classical Blatter radical analogues revealed that planarization of the central angular element results in a significantly higher stability of the mesophases and increased molecular organization suitable for the formation of ordered banana and columnar mesophases with tighter π-π interactions. These results indicate access to a new, potentially rich class of functional paramagnetic soft materials.

Photophysical Behavior of Self-Organizing Derivatives of 10- and 12-Vertex p-Carboranes, and their Bicyclo[2.2.2]octane and Benzene Analogues.

Four series of isostructural derivatives of 3-ring liquid crystalline derivatives containing p-carboranes (12-vertex A, and 10-vertex B), bicyclo[2.2.2]octane (C), or benzene (D) as the variable structural element were investigated for their mesogenic behavior and electronic interactions. Comparative studies demonstrated that the effectiveness of elements A-D in stabilization of the mesophase typically increases in the order: B

3-Substituted Blatter Radicals: Cyclization of N-Arylguanidines and N-Arylamidines to Benzo[e][1,2,4]triazines and PhLi Addition.

A series of 3-amino- and 3-alkyl-substituted 1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yls was prepared in four steps involving N-arylation, cyclization of N-arylguanidines and N-arylamidines, reduction of the resulting N-oxides to benzo[e][1,2,4]triazines, and subsequent addition of PhLi followed by aerial oxidation. The resulting seven C(3)-substituted benzo[e][1,2,4]triazin-4-yls were analyzed by spectroscopic and electrochemical methods augmented with density functional theory (DFT) methods. Electrochemical data were compared to DFT results and correlated with substituent parameters.

Toward N-peri-Annulated Planar Blatter Radical through aza-Pschorr and Photocyclization.

Preparation of the elusive N-peri-annulated planar Blatter radicals was attempted using aza-Pschorr and photocyclization methods. In both methods, substrates containing N-Me and N-Ac groups yielded a zwitterionic heterocycle lacking the N-substituent as the main product, while in one of them a carbazole derivative representing a new heterocyclic system was also obtained. The formation of the zwitterion and the carbazole suggests the formation of the desired planar Blatter radical, which undergoes facile fragmentation through homolysis of the N-R bond. This mechanism is supported by DFT computational results, which also suggest that N-Ar derivatives should be sufficiently stable for isolation. Electronic structures of three planar Blatter radicals annulated with the O, S, and N-Ph groups are compared.

Thermally induced dimensionality changes in derivatives of a "super stable" Blatter radical.

Two derivatives of a "super stable" Blatter radical (1,3-diphenyl-7-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) with N(1)-Ar = 2-CF3C6H4 and 2-MeOC6H4 were obtained and investigated using XRD and SQUID magnetometry methods. The investigation revealed strong antiferromagnetic interactions in both radicals, which are described using the Hatfield model. For the latter radical, an abrupt and reversible change in the χ(T) plot was observed at 29 K. It was ascribed to a structural transition, consistent with a two-dimensional to one-dimensional thermally activated crossover, as supported by specific heat measurements (CvHvs. T). It is suggested that the transition is related to an order-disorder transition of the CF3 group, which is corroborated using XRD structural analysis.

"Upper" Ring Expansion of the Planar Blatter Radical via Photocyclization: Limitations and Impact on the Electronic Structure.

Photocyclization of 8-aryloxybenzo[e][1,2,4]triazines leads to the formation of π-expanded flat Blatter radicals for three phenanthryloxy and pyren-1-yloxy derivatives, whereas no photoreaction is observed for the perylen-3-yloxy precursor. Two of the new radicals are nonplanar, out of which one is unstable to isolation. The radical with the fused pyrene ring constitutes the largest thus far paramagnetic polycyclic π-system containing seven fused rings with 27 sp2-hybridized atoms and 29 π-delocalized electrons. The investigation of the reaction conditions demonstrated the higher efficiency of photoformation of the parent radical in polar solvents, which suggests a polar transition state and the S1 photoreactive state. The effect of π expansion on the electronic structure was investigated with spectroscopic (UV-vis, electron paramagnetic resonance) and electrochemical methods augmented with density functional theory computational studies. The molecular structure of one of the radicals was determined with a single-crystal X-ray diffraction method.

Tethered Blatter Radical for Molecular Grafting: Synthesis of 6-Hydroxyhexyloxy, Hydroxymethyl, and Bis(hydroxymethyl) Derivatives and Their Functionalization.

Synthetic access to 7-CF3-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals containing 4-(6-hydroxyhexyloxy)phenyl, 4-hydroxymethylphenyl or 3,5-bis(hydroxymethyl)phenyl groups at the C(3) position and their conversion to tosylates and phosphates are described. The tosylates were used to obtain disulfides and an azide with good yields. The Blatter radical containing the azido group underwent a copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene under mild conditions, giving the [1,2,3]triazole product in 84% yield. This indicates the suitability of the azido derivative for grafting Blatter radical onto other molecular objects via the CuAAC "click" reaction. The presented derivatives are promising for accessing surfaces and macromolecules spin-labeled with the Blatter radical.

C(1)-Phenethyl Derivatives of [closo-1-CB11 H12 ]- and [closo-1-CB9 H10 ]- Anions: Difunctional Building Blocks for Molecular Materials.

C(1)-vinylation of [closo-1-CB9 H10 ]- (A) and [closo-1-CB11 H12 ]- (B) with 4-benzyloxystyryl iodide followed by hydrogenation of the double bond and reductive deprotection of the phenol functionality led to C(1)-(4-hydroxyphenethyl) derivatives. The phenol functionality was protected as the acetate. The esters were then treated with PhI(OAc)2 and the resulting isomers were separated kinetically (for derivatives of anion A) or by chromatography (for derivatives of anion B) giving the difunctionalized building blocks in overall yields of 9 % and 50 %, respectively. A similar series of reactions was performed starting with anions A and B and 4-methoxystyryl bromide and iodide. Significant differences in the reactivity of derivatives of the two carborane anions were rationalized with DFT computational results. Application of the difunctionalized carboranes as building blocks was demonstrated through preparation of two ionic liquid crystals. The extensive synthetic work is accompanied by single crystal XRD analysis of six derivatives.

UV-photoelectron spectroscopy of stable radicals: the electronic structure of planar Blatter radicals as materials for organic electronics.

The electronic structure of Blatter radicals and a series of C(10)-substituted derivatives of 2-phenyl-3H-[1,2,4]triazino[5,6,1-kl]phenoxazin-3-yl (planar Blatter radicals) containing H, F, Cl, Br, CN, CF3 and OMe substituents was investigated by gas phase UV-photoelectron spectroscopy. The energy of the SOMO of the radicals, determined to be about 6.5 eV, was correlated with their electrochemical oxidation potentials, E0/+11/2, relative to the Fc/Fc+ couple in CH2Cl2 giving the correction of 6.60(1) eV. The optical band gap Eoptg ∼ 1.7 eV of the radicals yielded the electronic transport gap, Eelg, of about 2.1 eV, which is similar to the electronic parameters of pentacene. The radicals were analyzed by EPR spectroscopy and single crystal XRD methods, and all experimental data were compared to DFT computational results obtained at the CAM-B3LYP/6-311G(d,p) level of theory.

Effect of π-System Extension on the Ionization Energy of the Planar Blatter Radical: Experimental and Theoretical Studies.

Fusion of benzene, naphthalene, and phenalene rings with the D ring of the planar Blatter radical leads to extension of the π-system and increased spin delocalization. The effect of this π-extension and the position of the ring fusion on the electronic structure of the radicals was investigated by UV-photoelectron spectroscopy and DFT CAM-B3LYP/6-311G(d,p) method. The experimental data obtained for 3 out of 8 derivatives were correlated with DFT-derived ionization energies.

Thermal and Photophysical Properties of Highly Quadrupolar Liquid-Crystalline Derivatives of the [closo-B12 H12 ]2- Anion.

Two series of 1,12-bis-zwitterionic derivatives of the [closo-B12 H12 ]2- anion (B), containing either two 4-alkoxypyridinium groups (1B[n]-p) or one 4-alkoxypyridinium and one 4-pentylthianium groups (2B[n]-p), were prepared and their structural (XRD, DFT), thermal, and photophysical properties were compared with those of the analogous derivatives of the [closo-B10 H10 ]2- anion (1A[n]-p and 2A[n]-p). Some 1,7-derivatives of B were isolated and investigated. Both series 1[n] and 2[n] exhibit nematic and crystalline polymorphism; the 12-vertex derivatives (B) have higher transition temperatures than those of the 10-vertex analogues (A). All compounds fluoresce with quantum yields higher for 1B (ΦF =0.37 for 1B[7]-p and ΦF =0.27 for 2B[7]-p) than those for the 10-vertex analogues (ΦF =0.04 for 2A[5]-p). DFT calculations demonstrate an order of magnitude lower first hyperpolarizability, ß(-ω,ω,0) , for 2B[7]-p than that for the 10-vertex analogue 2A[7]-p (1.7×10-30 vs. 18.9×10-30  esu at ω=0).

Tuning the Magnetic Properties of Columnar Benzo[e][1,2,4]triazin-4-yls with the Molecular Shape.

A homologous series of disc-like 1,3,6-trisubstituted benzo[e][1,2,4]triazin-4-yls 1[n] was synthesized and their structural, thermal, optical, magnetic, and electric properties were investigated. The results demonstrate that all members of the series display a Colh phase with clearing temperatures depending on the length of the alkoxy chains at the N(1) position, hence the shape of the disc. Powder XRD and magnetic data indicate a gradual change in the column diameter and magnetic behavior in the series in transition from half-disc in 1[0] (antiferromagnetic interactions) to full-disc geometry in the 1[12] homologue (ferromagnetic interactions with J/kB =+7.5 K). Studies of binary systems revealed that a 1 : 1 mixture of 1[0] and 1[12] exhibits modest stabilization of the Colh phase with an expanded range, and magnetic behavior typical for 1[0] in the rigid phase obtained from the melt. Electric measurements demonstrated hole mobility of ∼10-3  cm2  V-1 s-1 and dark conductivity of ∼10-11  Scm-1 in the mixture and individual compounds. The latter is enhanced up to 4 times by simultaneous illumination with UV light.

3-Substituted Benzo[ e][1,2,4]triazines: Synthesis and Electronic Effects of the C(3) Substituent.

A series of 19 structurally diverse C(3)-substituted derivatives of benzo[ e][1,2,4]triazine were synthesized from 3-chloro- (1c) and 3-iodobenzo[ e][1,2,4]triazine (1d) obtained in three steps from 2-nitroaniline in 37-55% yields. Nucleophilic aromatic substitution and metal-catalyzed (Pd, Cu) reactions led to functional derivatives that include alkyl (C5H11), (het)aryl (Ph, 2-thienyl, ferrocenyl), ArC≡C, amine (NHPh and morpholine), PO(OEt)2, sulfanyl (SBu- t), alkoxide (OEt, OMe), and CN. The synthesis of C(3)-CF3 derivative 1g via the Ruppert reaction with 1d and its 1-oxide analogue 2d led to the substitution followed by formal addition of HCF3 to the C═N bond. Pd-catalyzed carbonylation reactions of 1d and 2d did not give the corresponding C(3)-carboxylic acids. Therefore, acid 1f was obtained through hydrolysis of the CN. The substituent effect on the electronic structure of the benzo[ e][1,2,4]triazine ring was investigated by spectroscopic methods (UV-vis and NMR) augmented with density functional theory calculations. Results show significant effect of the C(3) substituent on the π-π*(1) transition energy and good correlation of the 1H NMR chemical shift with the substituent constant σp. Molecular and crystal structures of six derivatives were established with the single-crystal X-ray diffraction method, and the substituent impact on the molecular geometry was investigated.

Magnetostructural Investigation of Orthogonal 1-Aryl-3-Phenyl-1,4-Dihydrobenzo[e][1,2,4]triazin-4-yl Derivatives.

3-Phenyl-1,4-dihydrobenzo[e][1,2,4]triazinyl radicals with the N(1) position substituted with naphth-2-yl (1 b), naphth-1-yl (1 c), pyren-1-yl (1 d), anthracen-9-yl (1 e), 2-trifluoromethylphenyl (1 f), 3-trifluoromethylphenyl (1 g), and 2-iodophenyl (1 h) were characterized by using single-crystal X-ray diffraction, variable-temperature magnetic susceptibility, and DFT computational methods. The substituent at N(1) is essentially orthogonal to the heterocycle plane in 1 f and 1 h, and with a high torsion angle in 1 c and 1 d. Radicals 1 c and 1 h form unusual infinite chains with crisscrossing hetero-co-facial π-π interactions, whereas radical 1 d forms analogous homo-co-facial arrangements. Infinite chains of homo-co-facial π-π dimers are found in 1 b, 1 f and 1 g; in the latter the position of the CF3 group controls the slippage of the dimers in the chain. No π-π parallel arrangements were found in 1 e. Magnetic susceptibility measurements demonstrated strong antiferromagnetic interactions in 1 b (J=-264±4 cm-1 ) and 1 f (J=-134±1 cm-1 ), while weak intradimer ferromagnetic interactions were found in 1 g (J2 =+21±1 and J1 =-15±1 cm-1 ). Other derivatives exhibit typical weak antiferromagnetic exchange interactions in a range of -5 to -10 cm-1 .

Regioselective Functionalization of the [ closo-1-CB9H10]- Anion through Iodonium Zwitterions.

Reactions of [ closo-1-CB9H9-1-R]- (2, R = H, COOH, C5H11) with PhI(OAc)2 lead to mixtures of regioisomers [ closo-1-CB9H8-1-R-6-IPh] (5[6]) and [ closo-1-CB9H8-1-R-10-IPh] (5[10]) in ratios of ∼3:1 to 1:1, of which the former isomer undergoes selective reactions with nucleophiles (MeCN, pyridine, MeC(═NH)NH2, CN-). The products and the unreacted 10-isomers 5[10] are separated achieving kinetic resolution of the isomeric iodonium zwitterions. Pure 5[10] is reacted with nucleophiles (pyridine, 4-C7H15OPyridine, Me2NCHS, PhCO2-, CN-, N3-, I-, MeC(═NH)NH2, and MeCN), giving substitution products. The mechanism of the substitution is investigated with density functional theory (DFT) methods. Some of the nucleophilic substitution products are transformed further, expanding the scope of available functional groups for the [ closo-1-CB9H10]- anion. Four derivatives are characterized with single-crystal XRD methods: [ closo-1-CB9H9-10-N2] (4[10]a), [ closo-1-CB9H9-6-NC5H5] (9[6]a), [ closo-1-CB9H9-10-NC5H5] (9[10]a), and [ closo-1-CB9H9-10-NHC(NH2)Me] (10[10]a). Spectroscopic data for selected derivatives are interpreted in terms of transmission of electronic effects through the { closo-1-CB9} cluster (NMR) and interaction with substituents (IR, UV). The latter results are compared to those of TD-DFT computational methods.

Convenient Synthesis of [closo-B10H9-1-I]2- and [closo-B10H8-1,10-I2]2- Anions.

Two iodo derivatives, [closo-B10H9-1-I]2- (1) and [closo-B10H8-1,10-I2]2- (2), were obtained from the parent [closo-B10H10]2- anion in a two-step process and about 60% and 80% overall yields, respectively. Molecular and crystal structures of 2[Pr4N] were established with XRD methods: I422; a = b = 10.1531(1) Å, c = 18.0149(2) Å; α = ß = γ = 90°; Z = 2. Synthetic applications of 1[Bu4N] were demonstrated in a Pd(0)-catalyzed cross-coupling reaction with 4-MeOC6H4MgBr.

Substituent-Dependent Magnetic Behavior of Discotic Benzo[e][1,2,4]triazinyls.

Discotic mesogens containing the benzo[e][1,2,4]triazinyl radical as the central unit exhibit a Colh phase below 80 °C. Depending on the substituent at the N(1) position, they show different modes of thermal expansion and magnetic behavior, presumably due to differences in molecular organization. Thus, for 1-phenyl (1a) and 1-PhF-m (1b) derivatives, the Colh phase has positive thermal expansion coefficient κ and antiferromagnetic interactions, while for the 1-(3,4,5-(C12H25X)3C6H2) derivatives 1c (X = O) and 1d (X = S), κ is negative and weak ferromagnetic interactions in the crystalline phase are observed for 1c (J/kB = +4.76 K). Compounds 1a and 1c exhibit photoinduced hole transport (µ ≈ 1.3 × 10(-3) cm(2) V(-1) s(-1)) in the Colh phase.