Unraveling the Electronic Structure of Azolehemiporphyrazines: Direct

Article pubs.acs.org/JPCA

Unraveling the Electronic Structure of Azolehemiporphyrazines: Direct Spectroscopic Observation of Magnetic Dipole Allowed Nature of the Lowest π−π* Transition of 20π-Electron Porphyrinoids Atsuya Muranaka,*,†,‡ Shino Ohira,† Naoyuki Toriumi,∥ Machiko Hirayama,† Fumiko Kyotani,† Yukie Mori,†,⊥ Daisuke Hashizume,§ and Masanobu Uchiyama*,†,∥ †

Advanced Elements Chemistry Research Team, RIKEN Center for Sustainable Resource Science (CSRS) and Elements Chemistry Laboratory, Wako-shi, Saitama 351-0198, Japan ‡ Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS), Wako-shi, Saitama 351-0198, Japan ∥ Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan S Supporting Information *

ABSTRACT: Hemiporphyrazines are a large family of phthalocyanine analogues in which two isoindoline units are replaced by other rings. Here we report unambiguous identification of 20π-electron structure of triazolehemiporphyrazines (1, 2) and thiazolehemiporphyrazine (3) by means of X-ray analysis, various spectroscopic methods, and density functional theory (DFT) calculations. The hemiporphyrazines were compared in detail with dibenzotetraazaporphyrin (4), a structurally related 18π-electron molecule. X-ray analysis revealed that tetrakis(2,6-dimethylphenyloxy)triazolehemiporphyrazine (1b) adopted planar geometry in the solid state. A weak absorption band with a pronounced vibronic progression, observed for all the hemiporphyrazines, was attributed to the lowest π−π* transition with the electricdipole-forbidden nature. In the case of intrinsically chiral vanadyl triazolehemiporphyrazine (2), a large dissymmetry (g) factor was detected for the CD signal corresponding to the lowest π−π* transition with the magnetic-dipole-allowed nature. Molecular orbital analysis and NICS calculations showed that the azolehemiporphyrazines have a 20π-electron system with a weak paratropic ring current.

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INTRODUCTION

of a detailed comparison with 18π-electron dibenzotetraazaporphyrin (4) (Chart 2). Pyridine-type hemiporphyrazine is the most widely studied compound, but it readily adopts nonplanar conformations.12 We chose to explore azolehemiporphyrazines in this study because their structural resemblance to tetrapyrrolic macrocycles was expected to facilitate theoretical analysis. We believe that an in-depth understanding of the electronic structures of cross-conjugated hemiporphyrazines is of great importance in connection with rational design of organic materials for optoelectronic applications. Long alkyl chains or bulky aryl groups were introduced into the azolehemiporphyrazines to increase the solubility and to allow easier chromatographic separation. An optically active oxovanadium(IV) complex (2) was designed to detect intrinsic circular dichroism (CD) of the triazolehemiporphyrazine.

Hemiporphyrazines are a class of phthalocyanine analogues in which the two opposing isoindole units are replaced by other moieties, such as pyridine, triazole, and benzene.1−3 The first hemiporphyrazine was described in 1952 by Elvidge and Linstead.4 Since then, hemiporphyrazines have attracted considerable interest, not only from the viewpoint of their fundamental properties but also in materials science.5−10 Hanack and co-workers reported that hemiporphyrazines have large two-photon absorption cross sections.11 A common feature of hemiporphyrazines, besides their thermal stability, is their lability in aqueous acidic media due to their Schiff base or iminic nature. These macrocyles have been considered to be formally a 20π-electron system,12 but many questions about the electronic structure of hemiporphyrazines remain unanswered. One of the most troublesome problems is that hemiporphryazines are cross-conjugated porphyrinoids.13,14 This is in contrast to [4n]π-electron porphyrins with a macrocyclic conjugated circuit (Chart 1).15−22 Here we describe unambiguous identification of the 20πelectron structure of azolehemiporphyrazines (1−3) by means © 2014 American Chemical Society

Received: January 7, 2014 Revised: May 8, 2014 Published: May 27, 2014 4415

dx.doi.org/10.1021/jp5001557 | J. Phys. Chem. A 2014, 118, 4415−4424

The Journal of Physical Chemistry A

Article

and 1a′) were obtained by heating equimolar amounts of isoindolinediimine and 1-dodecyl-1,2,4-triazole-3,5-diamine.32 The ratio of the two isomers was found to be ca. 50:50, based on the integral ratio of the 1H NMR spectra. The isomers were successfully separated by medium-pressure silica-gel column chromatography and fully characterized by 1H NMR, MS, IR, and UV−vis spectroscopy. Figure 1 shows the IR and 1H NMR

Chart 1. Structures of Hemiprophryazines and Porphyrins

Chart 2. Structures of Azolehemiporphyrazines and Dibenzotetraazaporphyrin Used in This Study

Figure 1. (a) Partial 1H NMR spectra (CDCl3) and (b) IR spectra of two regioisomers of triazolehemiporphyrazine (1a and 1a′).

spectra of the two isomers. As can be seen in Figure 1a, a single peak was observed for 1a, whereas 1a′ exhibited two distinct NH signals. These signals were observed in the downfield region, which is indicative of a paratropic ring current effect.26 The chemical shift difference between two NH signals of 1a′ (0.28 ppm) was similar to that in the syn isomer of dicyanotriazolehemiporphyrazine (0.3 ppm).33 In the IR spectrum of 1a in the solid state, a single NH stretching vibration was detected at 3298 cm−1, whereas two peaks (3363 and 3291 cm−1) were observed for 1a′ (Figure 1b). These spectral patterns were reproduced very well by DFT calculations, and therefore the higher and lower wavenumber peaks observed for 1a′ were assigned to the symmetric and antisymmetric NH stretching modes, respectively (Supporting Information). It is evident from these results that the inner NH protons of the N-alkylated triazolehemiporphyrazine are localized on the isoindole moiety in solution and in the solid state. Intrinsically chiral triazolehemiporphyrazine (2) was synthesized by means of a metal insertion reaction of 1a with vanadyl sulfate hydrate in DMF.34 Because of the positions of peripheral groups, 2 becomes optically active, even though it does not have optically active substituents.35 Resolution of 2 was achieved by high performance liquid chromatography using a CHIRALPAK IC column (Cellulose tris(3,5-dichlorophenyl-

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RESULTS AND DISCUSSION Synthesis. The first synthesis of triazolehemiporphyrazine was carried out by Campbell by reacting phthalonitrile and 1,2,4-triazole-3,5-diamine.23 Smirnov et al. described the compound by reaction of isoindolinediimine and 1,2,4triazole-3,5-diamine and prepared several metal complexes.24,25 The triazolehemiporphyrazine without any peripheral substituents is quite insoluble in almost all common organic solvents. Torres and co-workers synthesized substituted triazolehemiporphyrazines and their metal complexes to enhance the solubility.26−31 Following the method reported by Torres et al.,26 mixtures of two regioisomers of N-alkylated triazolehemiporphyrazine (1a 4416



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Figure 3. Selected bond distances (Å) from the X-ray structures of (a) 1b, (b) 4, and (c) 1,2,4-triazole-3,5-diamine.42

The reaction of the same diiminoisoindoline with 2,4diamino-5-phenylthiazole monohydrobromide gave a novel hemiporphyrazine, in which two thiazole rings were incorporated into the macrocyclic skeleton. The thiazolehemiporphyrazine (3) was obtained as a mixture of two regioisomers. The inner NH protons were observed in the downfield region (13.68, 13.89, and 14.15 ppm), and the isomer ratio was ca. 50:50. Dibenzotetraazaporphyrin (4) was synthesized via mixed condensation of 1,3-diiminoisoindoline and 4,5-dicyano-4octene.36,37 The 1H NMR spectrum of 4 was characteristic of aromatic phthalocyanine derivatives. Thus, the inner NH protons were observed in the upfield region (−1.93 ppm). X-ray Structure. X-ray investigation of triazolehemiporphyrazines has not been carried out so far. Here, we succeeded in determining the molecular structure of the triazolehemiporphyrazine with aryl ether substituents (1b). Single crystals of 1b suitable for X-ray diffraction were obtained from chloroform solution with vapor diffusion of pyridine. As can be seen in Figure 2a, the hemiporphyrazine adopts an essentially planar structure. The outer NH protons are located in the opposite direction, resulting in an anti-form. It was found that the outer NH groups form hydrogen bonds to pyridine molecules (Supporting Information). The X-ray geometry of the reference compound (4) was also determined (Figure 2b). Selected bond distances in the X-ray structures of these macrocycles are shown in Figure 3. The C(pyrrole)−N(meso) bond distances in 4 are 1.32−1.34 Å, indicating that the macrocycle is a highly conjugated system. In contrast, a distinct difference in the bond lengths was seen for 1b. The averaged C(pyrrole)−N(meso) bond in 1b (1.29 Å) was shorter than that in 4, whereas the averaged C(triazole)−N(meso) bond (1.38 Å) was longer. This type of localization of the CN

Figure 2. X-ray structures of 1b (a) and 4 (b). The thermal ellipsoids are scaled to the 50% probability level. Solvent molecules are omitted for clarity.

carbamate)-immobilized to silica gel, Daicel) eluting with CH2Cl2 at 1.0 mL/min flow speed at 40 °C. Triazolehemiporpyrazine with aryl ether substituents (1b) was synthesized by reacting 5,6-bis(2,6-dimethylphenyloxy)1,3-diiminoisoindoline with 1,2,4-triazole-3,5-diamine. A pure sample of 1b is insoluble in CHCl3, but the solubility increases when mixed solvents are used. We measured the 1H NMR spectra of 1b in CDCl3/(CD3)2CO (5:2 v/v). At 20 °C, the inner and outer NH protons were detected at 14.73 and 12.31 ppm, respectively. The inner NH proton peak was split into three peaks at low temperature, indicating that the exchange between the syn- and anti-isomers becomes sufficiently slow at low temperature (Supporting Information). Judging from the integral ratio, the ratio of the syn- and anti-isomers was ca. 1:2 at −60 °C. 4417



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seen in the expanded system.10,44 The MCD intensity corresponding to the visible band was almost zero. Complex MCD signals were observed for the relatively intense absorption band in the 350−400 nm region. Figure 5 shows the dependence of the electronic absorption spectra on the structure of azolehemiporphyrazines. All of the

double bonds is observed in the X-ray structures of common hemiporphyrazines.38−40 The difference in bond length between the C(pyrrole)−N(meso) and C(triazole)−N(meso) bonds was 0.09 Å, which was a little more than the difference between the C(pyrrole)−N(meso) bonds in germanium(IV) phthalocyanine containing an antiaromatic 20π-electron circuit (0.069 Å).41 As shown in Figure 3c, the bond lengths of the triazole unit in 1b were almost identical with those in the X-ray structure of 1,2,4-triazole-3,5-diamine.42 When methanol was used instead of pyridine, 1b crystallized in a different space group. The hemiporphyrazine skeleton was essentially planar, and the structural data, such as bond lengths, were almost the same as those for the crystal obtained from CHCl3/pyridine solution (Supporting Information). UV−Vis, MCD, and CD Spectra. Although the UV−vis spectra of several triazolehemiporphyrazines have been previously reported,5,28,33 detailed analysis has not yet been carried out. We measured the electronic absorption and magnetic circular dichroism (MCD) spectra of the macrocycles studied. MCD spectra often give valuable information for full assignment of bands observed in the optical spectra of porphyrinoids.43 As shown in Figure 4, the spectral pattern of

Figure 5. Electronic absorption spectra of various types of azolehemiporphyrazines measured in CHCl3. The spectrum of 1b was obtained in CHCl3/methanol (96:4 v/v).

triazolehemiporphyrazines exhibited a weak absorption band in the visible region and intense absorption bands in the UV region. The absorption spectrum of thiazolehemiporphyrazine (3) was significantly red-shifted relative to those of the triazolehemiporphyrazines and its absorption edge reached ca. 800 nm. According to Michl’s 4N-electron perimeter model, chiral perturbation on a [4n]π-electron system induces circular dichroism (CD) with a large dissymmetry factor (g-factor: Δε/ε) for the lowest π−π* transition, as is the case with the n−π* transition in carbonyl compounds.45 However, to our knowledge, no one has detected a large g-factor in [4n]πelectron macrocycles. We therefore designed an optically active triazolehemiporphyrazine (2) to test this prediction. Hemiporphyrazines are essentially achiral, but the anti-isomer of triazolehemiporphyrazines will become chiral when a metal and an axial ligand are introduced. This type of chiral molecule is suitable to detect CD signals arising from the [4n]π-electron structure. Figure 6 shows the electronic absorption and CD spectra of enantiopure 2. The feature in the absorption spectrum of the vanadyl complex was similar to that of metal-free hemiporphyrazine (1a), but the peak positions appeared to be slightly red-shifted. The first fraction obtained from chiral HPLC showed positive CD signals in the visible region, whereas the UV region was dominated by negative CD signals. Importantly, a large g-factor corresponding to the forbidden band in the visible region was observed. The g-factor corresponding to the lowest-energy band at 618 nm was more than 0.01. Geometry Optimizations and MO Analysis. Semiempirical calculations of triazolehemiporphyrazines have been reported by Torres and co-workers.26 We performed geometry optimization calculations of the azolehemiporphyrazines and dibenzotetraazporphyrin using more accurate DFT methods.

Figure 4. MCD and electronic absorption spectra of 1a (a) and 4 (b) in CHCl3 at room temperature.

the dibenzotetraazaporphyrin (4) was characteristic of a lowsymmetry tetraazaporphyrin with D2h symmetry. Thus, two intense Q-bands (674 and 644 nm) with coupled Faraday B terms were observed in the visible region.36 In the case of the N-alkylated triazolehemiporphyrazine (1a), a weak absorption band with a pronounced vibronic progression was seen in the visible region. This weak absorption band was also seen in other triazolehemiporphyrazines.5,28,33 The spectral feature in the short wavelength region of triazolehemiporphyrazines seems to be similar to that of 3 + 3 type expanded hemiporphyrazines, but such a weak absorption band is not 4418



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Figure 6. Anisotropy (g) factor profile, CD, and electronic absorption spectra of two enantiomers of 2 in CHCl3 at room temperature (first fraction, blue line; second fraction, red line). Figure 7. Frontier molecular orbitals, energy levels, symmetry labels, and perimeter labels of the optimized structures of tetraazaporphyrin (left), anti-triazolehemiporphyrazine (middle), and anti-thiazolehemiporphyrazine (right) calculated at the level of B3LYP/6-31G(d,p). Isosurface plots of molecular orbitals of the 18π-electron perimeter model are also shown.

The peripheral groups were omitted and replaced with hydrogen for the calculations. The geometrical parameters for the optimized structures were in good agreement with the Xray data, which justifies the suitability of the calculation methodology. The planar structures were found to be the most stable for all the azolehemiporphyrazines, even when a highly nonplanar geometry was used as an initial structure (Supporting Information). This is in contrast to a nonplanar optimized geometry of pyridine-type hemiporphyrazine.12 Bond length differences between C(pyrrole)−N(meso) and C(azole)−N(meso) were predicted for the azolehemiporphyrazines, whereas such a difference was not seen in dibenzotetraazaporphyrin (Supporting Information). Frontier molecular orbitals for the anti-forms of the azolehemiporphyrazines are shown in Figure 7. Clearly, the nodal patterns of the HOMO−1, HOMO, and LUMO of the azolehemiporphyrazines are associated with the HOMO, LUMO, and LUMO+1 of the 18π-electron dibenzotetraazaporphyrin, respectively. The HOMO and LUMO of the hemiporphyrazines are derived from a pair of degenerate LUMOs of the 18π-electron perimeter model, whereas the HOMO−1 is derived from one of the degenerate HOMOs.45−48 These results make it clear that azolehemiporphyrazines have a 20π-electron system and that the electronic structure can be analyzed by application of the perimeter model. The HOMO−LUMO energy gap of triazolehemiporphyrazine (ca. 3.0 eV) was larger than that of dibenzotetraazaporphyrin (ca. 2.2 eV), which is in a sharp contrast with the case of typical [4n]/[4n+2]π porphyrinoids.49 As compared to triazolehemiporphyrazine, thiazolehemiporphyrazine has a smaller HOMO−LUMO gap (ca. 2.7 eV), which may be responsible for the observed red-shift of the absorption spectrum of thiazolehemiporphyrazine. The origin of the decrease in the HOMO−LUMO gap of thiazolehemiporphyrazine can be explained as follows. Because carbon and sulfur atoms are less electronegative than a nitrogen atom, the replacement of nitrogen atoms by carbon and sulfur atoms destabilizes the energy of the HOMO.50,51 The LUMO energy does not change significantly because the magnitude of the coefficient on these atoms is very small.

TDDFT. To rationalize the observed spectral properties, we calculated the electronic absorption spectra using the DFToptimized structures. Table 1 summarizes the results for the lowest-energy π−π* states of azolehemiporphyrazines. As shown in Figure 8, the calculated spectral features of antitriazolehemiporphyrazine and dibenzotetraazaporphyrin appear to be in reasonable agreement with the observed spectral patterns. Two electric-dipole-allowed transitions (Q1 and Q2) were predicted in the visible region for the 18π-electron tetraazaporphyrin. As for the hemiporphyrazine, the transition to the lowest-energy π−π* state was electric-dipole-forbidden. This transition consists of the HOMO−LUMO transition and is attributable to the S band according to the perimeter model.45−48 Strictly forbidden electric dipole transitions gain intensity by vibronic coupling, so the vibronic progression observed in the visible region is associated with the S band. A large transition magnetic dipole moment was calculated for the S transition, and its direction was along the z-axis. These properties arise from the symmetry of the molecules (Because both the HOMO and LUMO of anti-triazolehemiporphyrazine with C2h symmetry have irreducible representation of bg, the direct product of the molecular orbitals transforms as ag). As illustrated in Figure 8b, the HOMO−LUMO transition involves a rotation of charge about the z-axis. Two electricdipole-allowed transitions at 380 nm (2.63 × 104 cm−1) and 345 nm (2.90 × 104 cm−1) were assigned to the N1 and N2 bands. The N1 transition was polarized along the y-axis, whereas the polarization of the N2 transition was along the xaxis. The observed 380 and 360 nm absorption bands of 1a are attributable to these transitions. To predict the MCD signs for these bands, relative orbital energy differences, ΔHL, ΔHSL, and ΣHL, were calculated (Supporting Information).45 We found that anti-triazolehemiporphyrazine can be classified as an orbital-splitting-dominated system of S-perturbed perimeters (ΣHL > ΔHSL). Because ΔHL is negative, the expected MCD 4419



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Table 1. TDDFT Results for the Lowest-Energy π−π* States of Azolehemiporphyrazines (B3LYP/6-31G(d,p))

a

compounda

λ (nm)

E (eV)

f

μb (au)

mc (au)

composition (weight, %)

triazoleHp (anti-form) triazoleHp (syn-form) thiazoleHp (anti-form) thiazoleHp (syn-form) VO N-Me triazoleHp (anti-form)

528 572 621 676 561

2.35 2.17 2.00 1.83 2.21

0.000 0.012 0.000 0.024 0.000

0.000 0.473 0.000 0.735 0.006

3.758 3.347 3.710 3.300 3.688

dibenzoTAP

583

2.13

0.177

1.843

0.000

108 → 109 (94.9%) 108 → 109 (90.1%) 116 → 117 (99.6%) 116 → 117 (94.5%) 131α → 132α (46.3%), 130β → 131β (48.6%) 107 → 108 (68.9%), 104 → 109 (12.2%)

Hp = hemiporphyrazine, TAP = tetraazaporphyrin. bTransition electric dipole moment. cTransition magnetic moment.

Figure 9. Calculated CD and absorption spectra of the oxovanadium N-methylated triazolehemiporphyrazine (UB3LYP/6-31G(d,p)). The broken lines indicate the experimental spectra of one enantiomer of 2. The inset shows the optimized geometry corresponding to 2a.

from the theoretical CD sign corresponding to the S band, the first and second fractions of the vanadyl complex were assigned to 2a and 2b, respectively (Chart 2). NICS. To further understand the electronic structure of azolehemiporphyrazines, NICS(0) indices52 were calculated at the center of the inner 16-membered ring (point a) and the midpoint of two C−N bonds (point b). As shown in Figure 10, the values for the reference dibenzotetraazaporphyrin (4) were comparable with those reported for aromatic porphyrins.53 In marked contrast, the NICS(0) values for the anti-forms of triazolehemiporphyrazine and thiazolehemiporphyrazine were positive, but small in magnitude, suggesting the existence of weak paratropic ring currents. Although these values were considerably lower than those of antiaromatic porphyrins,16 the presence of paratropic ring current in the azolehemiporphyrazines was strongly supported by the fact that the NICS values of three-unit models were less positive than those of the hemiporphyrazines. This is in contrast to the 3 + 3 type expanded hemiporphyrazines whose NICS values are almost zero.10 The weak paratropic ring currents may arise from the relatively large HOMO−LUMO energy gap.54 The syn-forms of these hemiporphyrazines also showed positive NICS values (syn-triazolehemiporphyrazine +4.63 (a), +4.43 (b); synthiazolehemiporphyrazine +5.45 (a), +6.21 (b)). When we calculated 1H NMR chemical shifts for the optimized structures of the azolehemiporphyrazines and the reference tetraazaporphyrin, there was good agreement between the calculated and experimental shifts. The inner NH protons of the anti- and syn-isomers of triazolehemiporphyrazine were predicted to appear in the downfield region (anti-form 15.15 ppm; syn-form 15.10, 15.60 ppm). The calculated chemical shift for the NH proton of thiazolehemi-

Figure 8. (a) Absorption stick spectra of anti-triazolehemiporphyrazine (top) and dibenzotetraazaporphyrin (bottom) calculated at the level of B3LYP/6-31G(d,p). The broken lines indicate the experimental spectra of 1a and 4. (b) Illustration of the difference in the HOMO−LUMO transitions of the two systems.

sign pattern for the two N transitions is a −, + pattern in ascending energy. This prediction agrees with the observed MCD signs. We performed TDDFT calculations for other azolehemiporphyrazines. Similar results were obtained and S, N1, and N2 bands were assigned (Supporting Information). The energy splitting between N1 and N2 bands was found to be smaller for the syn-form than for the anti-form. The transition magnetic dipole moment of the syn-form was a little smaller than that of the anti-form, whereas the syn-form has a larger electric dipole moment than the anti-form (Table 1). Figure 9 shows the calculated absorption and CD spectra of oxovanadium N-methylated triazolehemiporphyrazine. The oxovanadium complex has a nonzero CD signal because the electric and magnetic transition dipole moments were nonzero (Table 1). Because the magnetic moment was much larger than the electric moment, a large g-factor was predicted. Judging 4420



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[4n]π-electron porphyrinoids. Application of hemiporphyrazines to optoelectronic materials is currently being examined.55

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EXPERIMENTAL SECTION General Information. 3,5-Diamino-1-dodecyl-1,2,4-triazole32 and 5,6-bis(2,6-dimethylphenyloxy)-1,3-diiminoisoindoline55 were synthesized according to literature procedures. NMR spectra were obtained on JEOL AL-300, AL-400 NMR, and BRUKER AVANCE III HD spectrometers. Chemical shifts are expressed in δ (ppm) values. 1H NMR spectra were referenced to a tetramethylsilane or CHCl3 as an internal standard. High-resolution mass spectra (HRMS) were recorded on a JEOL JMS-700 (FAB) or Waters Synapt G2 (ESI) spectrometer. Medium-pressure liquid chromatography was carried out with a Yamazen WPrep2XY system. IR spectra were recorded with a JASCO FT/IR-4100 using an ATR unit. UV spectra were recorded with a JASCO V-670 spectrometer. CD spectra were recorded with a JASCO J-820 spectrodichrometer using a 10 mm quartz cell. The magnitude of the CD signal is expressed in terms of molar circular dichroism Δε/M−1 cm−1. MCD measurements were made with the above spectrodichrometer equipped with a JASCO electromagnet which produced magnetic fields of up to 1.0 T with parallel and antiparallel fields. The magnitude of the MCD signal is expressed in terms of magneto-molar circular dichroic absorption ΔεM/M−1 cm−1 T−1. Synthesis of 1a and 1a′. Following the reported method,26 mixtures of two regioisomers of N-alkylated triazolehemiporphyrazine (1a and 1a′) were obtained by heating equimolar amounts of isoindolinediimine and 1-dodecyl-1,2,4-triazole-3,5diamine. The ratio of the two isomers was found to be ca. 1:1, based on the integral ratio of the 1H NMR spectra. These isomers were separated by medium-pressure silica-gel column chromatography using CHCl3/CH3OH 99:1 (v/v) as eluent. The first band was 1a and the second one 1a′. 1a: 1H NMR (400 MHz, CDCl3/TMS): δ (ppm) = 15.08 (2H, s), 7.94 (2H, d, J = 7 Hz), 7.82 (2H, d, J = 7 Hz), 7.58 (4H, m), 4.18 (4H, t, J = 7 Hz), 2.17 (4H, m), 1.9−1.2 (36H, m), 0.85 (6H, t, J = 7 Hz). UV/vis (CHCl3): λmax (ε × 10−4) = 533 (0.05), 494 (0.13), 462 (0.17), 439 (0.16), 380 (3.99), 368 (3.61), 359 (5.07), 350 (4.15), 340 nm (3.60). HRMS (ESI) m/z calcd for C44H61N12 (M + H+): 757.5142, found 757.5137. 1a′: 1H NMR (400 MHz, CDCl3/TMS) δ (ppm) = 15.23 (1H, s), 14.95 (1H, s), 7.89 (2H, m), 7.81 (2H, m), 7.58 (2H, m), 7.54 (2H, m), 4.16 (4H, t, J = 7 Hz), 2.17 (4H, m), 1.9−1.2 (36H, m), 0.86 (6H, t, J = 7 Hz). UV/vis (CHCl3): λmax (ε × 10−4) = 585 (0.02), 539 (0.09), 501 (0.16), 469 (0.18), 443 (0.16), 378 (4.76), 357 (5.80), 339 (3.85), 320 nm (2.51). HRMS (ESI) m/z calcd for C44H61N12 (M+H+): 757.5142, found 757.5142. Synthesis of 1b. A mixture of 5,6-bis(2,6-dimethylphenyloxy)-1,3-diiminoisoindoline (83 mg, 0.216 mmol), and 3,5diamino-1,2,4-triazole (26 mg, 0.216 mmol) was stirred in dry 2-ethoxyethanol (8 mL) at 135 °C under argon atmosphere for 24 h. After the mixture was cooled to room temperature, the solvent was removed in vacuo. Purification by silica chromatography using CHCl3 as eluent furnished the title compound as an orange solid (68 mg, 71%). 1H NMR (495 MHz, CDCl3/(CD3)2CO 5:2 (v/v) TMS) δ (ppm) = 14.73 (2H, s), 12.31 (2H, s), 7.17 (12H, s), 6.98 (2H, s), 6.72 (2H, s), 2.24 (24H, s). UV/vis (CHCl3/CH3OH 96:4 (v/v)): λmax (ε × 10−4) = 483 (0.14), 430 (0.21), 370 (4.70), 350 (5.39), 334 nm (3.41). HRMS (ESI) m/z calcd for C52H45N12O4 (M + H+): 901.3687, found 901.3680.

Figure 10. NICS(0) (ppm, B3LYP/6-31G(d,p)) calculated for the optimized structures of anti-triazolehemiporphryazine, anti-thiazolehemiporphyrazine, and dibenzotetraazaporphyrin. The NICS value at b represents the value at the midpoint of two C−N bonds in two fivemembered rings. NICS values of three-unit models with a planar geometry were also calculated for comparison.

porphyrazine (anti-form 14.59 ppm; syn-form 14.43, 15.10 ppm) also reproduced the experimental data. These results strongly support the idea that the inner NH protons in the present azolehemiporphyrazines are localized on the isoindole moiety.

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CONCLUSION In conclusion, a comprehensive study of triazolehemiporphyrazines (1, 2), thiazolehemiporphryazine (3) and the structurally related tetraazaporphyrin (4) has shown that azolehemiporphyrazines have a planar 20π-electron system despite their cross-conjugated structure. Although a cyclic, planar molecule with a [4n]π-electron structure generally has antiaromatic character, a weak paratropic ring current was estimated for the present system. The weak paratropic ring currents may arise from the relatively large HOMO−LUMO energy gap. All the hemiporphryazines exhibited a weak absorption band in the visible region, and the origin of the spectral features can be explained in terms of Michl’s 4N-electron perimeter model. A large anisotropy (g) factor was detected for the CD signal of the inherently chiral hemiporphyrazine (2). To our knowledge, the present report is the first to present direct spectral evidence that a cyclic [4n]π-electron molecule has a large transition magnetic dipole moment for the lowest π−π* transition. Considerable efforts have been devoted to exploring and understanding the peculiar electronic properties of [4n]πelectron porphyrinoids. This work provides new insights into the electronic structure of hemiporphyrazines and related 4421



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Synthesis of 2. A mixture of 1a (5.4 mg, 7 μmol) and vanadyl sulfate hydrate (35 mg, ca. 150 μmol) was heated at 140 °C in dehydrated DMF (5 mL) under argon for 6 days. The reaction mixture was cooled, diluted with CHCl3 and filtered. The filtrate was evaporated and the residue was chromatographed on silica-gel with CHCl3 as eluent. The first reddish violet band was collected and evaporated to give the desired compound (2 mg, 35%). IR 997 cm−1 (VO). UV/vis (CHCl3): λmax (ε × 10−4) = 572 (0.04), 527 (0.08), 492 (0.09), 463 (0.08), 391 (2.86), 370 (3.80), 356 nm (3.21). HRMS (FAB) m/z calcd for C44H58N12OV (M+): 821.4296, found 821.4297. Optical resolution was carried out by HPLC through Daicel Chiralpak IC with CH2Cl2 as eluent. Synthesis of 3. A mixture of 5,6-bis(2,6-dimethylphenyloxy)1,3-diiminoisoindoline (72 mg, 0.184 mmol), 2,4-diaminophenylthiazole monohydrobromide (52 mg, 0.184 mmol), and triethylamine (0.03 mL) was stirred in dry 2-ethoxyethanol (0.60 mL) at 135 °C under argon atmosphere for 24 h. After the mixture was cooled to room temperature, the reaction mixture was poured into methanol and filtered. After the precipitate was dissolved in CHCl3, purification by silica gel column chromatography using CHCl3/toluene 1:1 (v/v) furnished the desired compound as an orange solid (40 mg, 40%). 1H NMR (300 MHz, CDCl3/TMS) δ (ppm) = 14.15 (0.5H, s), 13.89 (1H, s), 13.68 (0.5H, s), 7.70−7.63 (4H, m), 7.23−7.12 (18H, m), 6.71−6.64 (4H, m), 2.24 (24H, s). UV/ vis (CHCl3): λmax (ε × 10−4) = 480 (4.77), 449 (5.39), 306 nm (3.54). HRMS (ESI) m/z calcd for C66H53N8O4S2 (M + H+): 1085.3632, found 1085.3612. Synthesis of 4. Magnesium turning (120 mg, 5 mmol) was dissolved in 1-butanol (12 mL), which was dried over molecular sieves 4A, by refluxing in the presence of a piece of iodine under argon for 24 h. Here were added 1,3diiminoisoindoline (220 mg, 1.5 mmol) and 4,5-dicyano-4octene36,37 (405 mg, 2.5 mmol) and the resultant mixture was refluxed under argon for 24h. The solvent was removed under reduced pressure, and the residue was stirred in a mixture of CH2Cl2 (50 mL) and CF3CO2H (2.5 mL) at room temperature for 3 h. The resultant solution was poured into aqueous NH3 and was extracted with CHCl3. The organic layer was dried over Na2SO4 and evaporated. The residue was chromatographed on silica gel with toluene as eluent repeatedly to give the desired compound (1.3 mg, 0.3%). 1H NMR (400 MHz, CDCl3/TMS) δ (ppm) = 9.25 (4H, m) 8.08 (4H, m), 4.05 (8H, t, J = 7.5 Hz), 2.41 (8H, quint J = 7.5 Hz), 1.33 (12H, t, J = 7.5 Hz), −1.93 (2H, s). UV/vis (CHCl3): λmax (ε × 10−4) = 674 (7.10), 644 (1.05), 614 (0.95), 580 (6.56), 537 (1.66), 345 nm (5.50). HRMS (FAB) m/z calcd for C36H38N8 (M+): 582.3220, found 582.3232. Computational Methods. All calculations were performed at the DFT level, by means of the hybrid Becke3LYP (B3LYP)56 functional as implemented in Gaussian 09,57 because this level of theory has most often been applied in other hemiporphyrazines. The 6-31G(d,p) basis set was used for all atoms.58 The unrestricted UB3LYP method was used for the oxovanadium(IV) complex (2) due to a doublet ground state. Vibrational frequency computations verified the nature of the stationary points. The excitation energies, oscillator strengths, and rotatory strengths were obtained at the density functional level using the time-dependent perturbation theory (TDDFT) approach.35,59 The anti- and syn- isomers of Nmethyltriazolehemiporphyrazine and the oxovanadium complex of the anti-isomer were calculated to obtain theoretical IR and

CD spectra. NMR properties and NICS(0) indices were calculated using the GIAO-B3LYP/6-31G(d,p) method. The calculated chemical shifts were analyzed by subtracting the isotropic shift for each hydrogen atom from the corresponding shift for TMS calculated using the same method (31.7551 ppm). NICS(0) indices were calculated at the center of the inner 16-membered ring (point a) and the midpoint of two C− N bonds in two 5-membered rings (point b). The geometries of three-unit models were obtained by removing one subunit from the optimized structures. Molecular orbitals were drawn using MolStudio R4.0 (Rev. 2.0).

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ASSOCIATED CONTENT

S Supporting Information *

Single crystal X-ray structure analysis (crystallographic data, crystal structures, molecular structures), 1H NMR spectral data, chiral HPLC profile, MCD and electronic absorption spectra, optimized structures, bond distances, IR spectra, molecular orbitals, energy levels, and perimeter labels, TDDFT results, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*A. Muranaka: e-mail, [email protected]. *M. Uchiyama: e-mail, [email protected]. Present Address ⊥

Department of Chemistry, Faculty of Science, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI Scientific Research (S) (No. 24229001 (to M.U.)), Young Scientists (A) (No. 21750052 (to A.M.)), Young Scientists (B) (No. 24685009 (to A.M.)), and JST PRESTO program (to A.M). This research was also supported by Research Foundation for Opt-Science and Technology, Mochida Memorial Foundation, Tokyo Biochemical Research Foundation, Foundation NAGASE Science Technology Development, and Sumitomo Foundation (to M.U.). RIKEN Integrated Cluster of Clusters (RICC) provided the computer resources for the DFT calculations. The authors thank Prof. Nagao Kobayashi (Tohoku University) for providing 4,5-dicyano-4-octene. We also thank Dr. Etsuko Tomiyama (RIKEN) for the X-ray analysis of dibenzotetraazaporphyrin (4).

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REFERENCES

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Unraveling the Electronic Structure of Azolehemiporphyrazines: Direct

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