Yoshio Teki

Preparation and ESR detection of a ground-state nonet hydrocarbon as a model for one-dimensional organic ferromagnets

The spin state and molecular conformation of a novel alternant hydrocarbon, m-phenylenebis[(diphenylmethylen-3-yl)methylene] (I) , have been studied by electron-spin resonance. The tetracarbene 1 was generated by the photolysis of the corresponding tetradiazo compound, m-phenylenebis[m-(a-diazobenzyl)phenyldiazomethane] (2), which was synthesized as follows: The reaction of m-tolylmagnesium bromide with isophthalonitrile produced 1,3-di-(m-toIuoyl)benzene, which was oxidized in two steps to isophthalophenone-3,3’-dicarboxylic acid. The Friedel-Crafts reaction of the bis(acid chloride) in benzene gave 3,3’-dibenzoylisophthalophenone. The corresponding tetrahydrazone was oxidized with active MnO, to give 2. The eight-line ESR spectrum due to 1 was obtained when benzophenone single crystals doped with 2 were irradiated with the 405-nm mercury line at 4.2 K. The relative separation and integrated intensities of the lines are in accord with the AM, = f l allowed transitions between the fine-structure sublevels of the nonet spin manifold in the high-field limit. The resonance fields and signal intensities observed at the K band (25 GHz) were well-reproduced by a third-order perturbation calculation based on the spin Hamiltonian 7f = g@SH + D[Sz2 -S(S + 1)/3] + E(Sx2 S

Electron spin resonance line shapes of randomly oriented molecules in septet and nonet states by a perturbation approach

) with g = 2.002, D = +0.031 61 cm-I, E = 0.003 94 cm. I , and .S = 4, proving 1 to be in the nonet state. Only the nonet spectrum was observed after photolysis. The temperature dependence of its total signal intensity in the range 1.8-56 K showed the observed nonet state to be the ground state, while the other triplet, quintet, and septet states also expected from the eight parallel spins are located at least 300 cm-’ above the ground state. When the sample was warmed, the spectrum showed irreversible spectral transitions at 64 K to two sets of new nonet signals and again at 92 K to a fourth set of nonet signals, which finally decayed out at 160 K. The transitions are ascribable to molecular conformation changes, which led to four nonet isomers. A semiempirical calculation of their fine-structure tensors has been carried out assuming the dipole-dipole interaction between the electron spins and the one-center n-T interactions on the divalent carbon atoms to be predominant. By fitting them to the observed fine-structure tensors, we obtained the most probable conformations for each of the four nonet isomers. Evidence for a one-photon process in the photodissociation of 2 into 1 was also obtained. The novel hydrocarbon described in this paper has the highest spin multiplicity so far reported among organic as well as inorganic molecules. This unusually high-spin multiplicity results from the topological symmetry. Such a high-spin state is relevant to the design of organic ferromagnets. Most known organic molecules have singlet ground states and are therefore usually diamagnetic. Paramagnetic organic compounds, less frequently, are usually free radicals. Their doublet ground state stems from an odd number of electrons. These magnetic properties contrast with those of inorganic compounds for which high-spin multiplicity in the ground state is not unusual. Until the early 1960s, this difference was held to reflect the low symmetry of organic molecules: from the group theoretical point of view, at most triply degenerate molecular orbitals could be found. Therefore, Hund’s rule predicts a maximum spin of 3 / 2 . In fact, organic molecules with the maximum spin consistent with their symmetry have not been detected yet. The pentachlorocyclopentadienyl cation’ and the cyclopentadienyl cation2 have such degenerate orbitals. They have been synthesized and they have been proven to have a triplet ground state. Due to their C, symmetry, their highest occupied molecular orbitals are doubly degenerate, hence the parallel spins. It should be noted that such degeneracy can be lifted by Jahn-Teller distortion as observed with the pentaphenylcyclopentadienyl cation, 4 ringlet in the ground ~ t a t e . ~ . ~ For organic molecules, therefore, high-spin multiplicity may not be expected from the degeneracy due to geometrical symmetry. Higuchi did the early theoretical work on organic high-spin molecules in 1963. He calculated the fine-structure parameters due to electron spin-spin interactions for several aromatic hydrocarbons being hypothetical a t that time.5*6 The first high-spin molecule was reported by one of us (K. I.)’ and subsequently by Wasserman et aI.* in 1967. This aromatic hydrocarbon, mphenylenebis(phenylmethylene), is a quintet in the electronic ground state. Its fine-structure parameters obtained by electron-spin resonance (ESR) were in reasonable agreement with ‘Osaka City University. tlnstitute for Molt.cular Science. 0002-7863/86/ 1508-2147

Excited high spin states of novel π conjugated verdazyl radicals: photoinduced spin alignment utilizing the excited molecular field

0l.50/0 the values predicted by H i g ~ c h i . ~ , ~ This hydrocarbon was a prototype for the series of high-spin hydrocarbons detected thereafter: m-phenylenebis(methy1ene) (S = 2),* benzene1,3,5-tris(phenyImethylene) ( S = 3),1° biphenyl-3,3’-bis(phenylmethylene) (S = 0, 1, 2),” 1,3,5-benzenetriyltris[bis(biphenyl-4-yl)methyl] ( S = 3 / 2 ) , 1 2 and 3,3’-diphenylmethylenebis(phenylmethy1ene) (S = 3).13 In addition, quintet and septet nitrenes isoelectronic with the above-mentioned quintet and septet hydrocarbons have been detected since.*J4 Recently, we published a preliminary report of the detection by single-crystal ESR of an aromatic hydrocarbon, mphenylenebis[(diphenylmethylen-3-yl)methylene] (1) with nonet spin multiplicity (S = 4) in the electronic ground state.I5 Static ( I ) Breslow, R.; Hill, R.; Wasserman, E. J . Am. Chem. SOC. 1364, 86, 5349-5350. (2) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.; Breslow, R.; Hoffman, J. M., Jr.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck, V. J . J . A m . Chem. SOC. 1973, 95, 3017-3018. (3) Breslow, R.; Chang, H. W.; Yager, W. A. J . Am. Chem. SOC. 1963, 85, 2033-2034. (4) Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. J. Am. Chem. SOC. 1967,89, 1 1 12-1 119. (5) Higuchi, J. J . Chem. Phys. 1963, 38, 1237-1245. (6) Higuchi, J. J . Chem. Phys. 1963, 39, 1847-1852. (7) Itoh, K. Chem. Phys. Left . 1967, 1, 235-238. (8) Wasserman, E.; Murray, R. W.; Yager, W. A,; Trozzolo, A. M.; Smolinsky, G . J . Am. Chem. SOC. 1967, 89, 5076-5078. (9) Higuchi, J. Bull. Chem. SOC. Jpn. 1970, 43, 3773-3779. (IO) Takui, T.; Itoh, K. Chem. Phys. Lef f . 1973, 19, 120-124. (11) Itoh, K. Pure Appl. Chem. 1978, 50, 1251-1259. (12) Reibisch, K.; Kothe, H.; Brickmann, J. Chem. Phys. Letf . 1972, 17, 86-89. Brickmann. J.: Kothe. G. J . Chem. Phw. 1973. 59. 2807-2814. (13) Teki, Y.; Ta’kui; T.; Yagi, H.; Itoh, K.; Iwimura, ti. J.’Chem. Phys. (14) Wasserman, E.; Scheller, K.; Yager, W. A. Chem. Phys. Lef t . 1968, 1985, 83, 539-547.

Using Stable Radicals To Protect Pentacene Derivatives from Photodegradation

The electron spin resonance line shapes of randomly oriented molecules in septet and nonet states are analyzed in terms of the formulas derived from a perturbation treatment to third‐order in the fine‐structure energy. The method is applied to a ground‐state septet hydrocarbon, 3, 3’‐diphenylmethylenebis (phenylmethylene), and a ground‐state nonet hydrocarbon, m‐phenylenebis[(diphenylmethylen‐3‐yl)methylene], randomly oriented in mixed polycrystalline powders of benzophenone. It is shown that the g factor and the fine‐structure parameters are determined from the line shapes of a K‐band spectrum with nearly the same accuracy as in a single‐crystal experiment. Extra lines have been observed in addition to the canonical lines corresponding to the external magnetic field along the principal axes of the fine‐structure tensor. The appearance of extra lines in the spectra of septet and nonet molecules is discussed relative to the third‐order perturbation theory.

Spin alignment in organic high-spin molecules. A Heisenberg Hamiltonian approach

This paper reports the excited quartet (S = 3/2) and quintet (S = 2) states arising from the intramolecular radical-triplet pair in the purely organic π conjugated spin systems. A previous paper reported the excited quartet and quintet states of 9-anthracene-(4-phenyliminonitroxide) and 9,10-anthracene-bis(4-phenyliminonitroxide), respectively, in which iminonitroxide radicals are linked to the phenyl- or diphenylanthracene moiety (a spin-coupler) through the π conjugation. The similar excited quartet and quintet states were observed for the 9-anthra-cene-(4-phenylverdazyl) radical (1) and 9,10-anthracene-bis(4-phenylverdazyl) diradical (2) by time resolved electron spin resonance (TRESR). The TRESR spectrum was analysed by the ordinary spin Hamiltonian with the Zeeman and fine structure terms. For the quartet state of 1, the g value, fine structure splitting, and relative population of the Ms sublevels have been determined to be g = 2.0035, D = 0.0230 cm−1, E = 0.0, P 1/2′ = P −1/2′ = 0.5 and P 3/2′ = P −3/2′ = 0.0, respectively, by spectral simulation. The spin Hamiltonian parameters of the quintet state of 2 were determined to be g = 2.0035, D = 0.0128 cm−1, E = 0.0, P 2′ = P −2′ = 0.0, P 1′ = P −1′ = 0.37 and P 0′ = 0.26, respectively. Direct observation of the excited high spin state showed that photoinduced intramolecular spin alignment is realized between the excited triplet state (S = 1) of the phenyl- or diphenylanthracene moiety and the doublet spin (S = 1/2) of the dangling verdazyl radicals. Ab initio MO calculations (DFT) were carried out in order to clarify the mechanism of the photoinduced spin alignment.

Design, synthesis, and uniquely electron-spin-polarized quartet photo-excited state of a π-conjugated spin system generated via the ion-pair state

Organic transistors and organic devices have a variety of applications in molecular electronics and in the up-andcoming area of organic molecular spintronics. The use of inexpensive and facile ink-jet methods is possible for the fabrication of organic semiconductors. Pentacene and its derivatives 7] have attracted increasing interest as promising electronic materials owing to their high hole mobility. Pentacene is the most promising candidate for organic fieldeffect transistor (OFET) applications. However, its chemical instability in the presence of light and air prevents practical applications. Efforts have been made to improve the stability by the addition of substituents. 7] The most notable example is 6,13-bis(triisopropylsilylethynyl)pentacene, 10] in which the 6and 13-positions of pentacene are protected by triisopropylsilylethynyl groups. For this pentacene derivative, extremely high hole mobility of 1.8 cm V 1 s 1 was reported for the thin film. However, the addition of substituents to photoactive carbon sites prevents further functional modifications because both the 6and 13-positions are blocked by the substituents and also the characteristic nature of the pentacene moiety is changed. Herein, we report a new method that utilizes a stable radical to protect pentacene derivatives from photodegradation. During the course of our systematic studies of pconjugated spin systems with high-spin photo-excited states for functional materials, we have discovered that a combination of two unstable species (photoreactive pentacene and a radical) leads to remarkable protection from photodegradation and an enhancement in solubility in common organic solvents. These effects are advantageous for practical applications of acene derivatives in molecular electronic devices. Radicals are well-known energy scavengers of the photoexcited state. We have utilized this characteristic of radicals to scavenge the energy of the photoexcited state of pentacene. Two novel radical pentacene hybrids, Pen–Ph–OV (1a) and Pen–Ph–NN (2a) and their precursors (1 b and 2b) were synthesized (Scheme 1; Pen, Ph, OV, and NN denote pentacene, phenyl, oxo-verdazyl radical, and nitronyl nitroxide radical moieties, respectively). We demonstrate the remarkable photochemical stability induced by the attachment of

Reaching Optimal Light-Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes

Abstract Spin alignment in organic high-spin molecules with both unpaired π and non-bonding electrons of carbene has been studied with the help of exact numerical diagonalization of the Heisenberg Hamiltonian. This approach predicts correct spin states and their relative energy gaps of high-spin molecules not only for ground states but also for low-lying excited states. The significance of topological symmetry of π-electron networks in spin alignment is also shown.

Photo-induced spin alignment utilizing the excited molecular field between the excited triplet state of phenyl- or diphenylanthracene and the dangling nitroxide radicals: theoretical investigation of the mechanism for the intramolecular spin alignment

A functionalized verdazyl radical, 1, was synthesized, which consists of a bodipy acceptor (A), a phenyl-anthracene donor (D), and a stable verdazyl radical (R). Optical measurements, electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The efficient intra-molecular energy transfer (EnT) from the anthracene moiety to the bodipy functional component was observed by time-resolved fluorescence spectroscopy and TRESR measurements. A unique dynamic electron-spin polarization (DEP) was detected for the quartet photo-excited state of 1 by TRESR and pulsed ESR. Such a unique DEP was not detected for either the parent verdazyl radical2 without the bodipy acceptor or new compound 3 with an anthraquinone moiety instead of the bodipy component. The spectral simulation of the quartet spectrum of 1 revealed that the unique DEP was generated by competition between a mechanism involving the intra-molecular ion pair *A−–D+–R and spin–orbit intersystem crossing. The dynamic electron-spin polarization via the ion-pair state and the mechanism were discussed based on the experimental data and theoretical calculations. The electronic structure and the spin delocalization in the photo-excited quartet state of 1 were discussed by comparison with the results of 3.

Excited-State Dynamics of Pentacene Derivatives with Stable Radical Substituents†

Ground-state (GS) and excited-state (ES) properties of novel photomagnetic molecular devices (PMMDs) are investigated by means of density functional theory. These organic PMMDs undergo a ferromagnetic alignment of their intramolecular spins in the lowest ES. They are comprised of: 1) an anthracene unit (An) as both the photosensitizer (P) and a transient spin carrier (SC) in the triplet ES ((3)An*); 2) imino-nitroxyl (IN) or oxoverdazyl (OV) stable radical(s) as the dangling SC(s); and 3) bridge(s) (B) connecting peripheral SC(s) to the An core at positions 9 and 10. Improving the efficiency of the PMMDs involves strengthening the ES intramolecular exchange coupling (J(ES)) between transient and persistent SCs, hence the choice of 2-pyrimidinyl (pm) as B elements to replace the original p-phenylene (ph). Dissymmetry of the pm connectors leads to [SC-B-P-B-SC] regio-isomers int. and ext., depending on whether the pyrimidinic nitrogen atoms point towards the An core or the peripheral SCs, respectively. For the int. regio-isomers we show that the photoinduced spin alignment is significantly improved because the J(ES)/k(B) value is increased by a factor of more than two compared with the ph-based analogue (J(ES)/k(B)>+400 K). Most importantly, we show that the optimal J(ES)/k(B) value ( approximately +600 K) could be reached in the event of an unexpected saddle-shaped structural distortion of the lowest ES. Accounting for this intriguing behavior requires dissection of the combined effects of 1) borderline intramolecular steric hindrance about key An-pm linkages, which translates into the flatness of the potential energy surface; 2) spin density disruption due to the presence of radicals; and 3) possibly intervening photochemistry, with An acting as a light-triggered electron donor while pm, IN, and OV behave as electron acceptors. Finally, potentialities attached to the [(SC)-pm-An-pm](int) pattern are disclosed.

General conditions for the occurrence of off‐axis extra lines in powder‐pattern ESR fine structure

Abstract In the previous paper [J. Am. Chem. Soc., 122 (2000) 984], we reported the first observation of the photo-excited quartet (S=3/2) and quintet (S=2) states arising from the radical-triplet pairs on the purely organic π-conjugated spin systems. 9-Anthracen(4-phenyliminonitroxide) (1) and 9,10-anthracen-bis(4-phenyliminonitroxide) (2) were designed and synthesized. The time-resolved ESR (TRESR) experiments were carried out in order to study the photo-induced spin alignments on the excited states. In this paper, we report the ab initio molecular orbital calculations based on the density functional theory for the photo-excited quartet (S=3/2) state of 1 and the quintet (S=2) state of 2 as well as the brief summary of our TRESR experiments. The mechanism of the photo-induced spin alignment has been clarified based on the spin distribution obtained by the MO calculations.