Quinoxalino[2,3-b']porphyrins Behave as π-Expanded

Jul 4, 2007 - Zheng-Li Cai,‡ Jeffrey R. Reimers,*,‡ and Maxwell J. Crossley*,‡. Department of Chemistry, UniVersity of Houston, Houston, Texas ...
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8762

J. Phys. Chem. B 2007, 111, 8762-8774

Quinoxalino[2,3-b′]porphyrins Behave as π-Expanded Porphyrins upon One-Electron Reduction: Broad Control of the Degree of Delocalization through Substitution at the Macrocycle Periphery Karl M. Kadish,*,† Wenbo E,† Paul J. Sintic,‡ Zhongping Ou,† Jianguo Shao,† Kei Ohkubo,§ Shunichi Fukuzumi,*,§ Linda J. Govenlock,‡ James A. McDonald,‡ Andrew C. Try,‡ Zheng-Li Cai,‡ Jeffrey R. Reimers,*,‡ and Maxwell J. Crossley*,‡ Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, School of Chemistry, The UniVersity of Sydney NSW 2006, Australia, and Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ReceiVed: April 5, 2007

The synthesis and redox properties of a series of free-base and metal(II) quinoxalino[2,3-b′]porphyrins and their use in an investigation of the substituent effects on the degree of communication between the porphyrin and its β,β′-fused quinoxalino component are reported. ESR, thin-layer spectroelectrochemistry, and quantum chemical calculations of the resultant radical anions from one-electron reduction indicate that localization of the unpaired electron across both the porphyrin and the fused quinoxalino group can be controlled, the system as a whole behaving as a highly polarizable π-expanded porphyrin radical anion. ESR studies on the radical anions of zinc(II) quinoxalino[2,3-b′]porphyrin derivatives indicate that nitrogen-atom spin distribution changes as a function of chemical substitution: 27% quinoxaline character when the porphyrin ring bears a 7-nitro substituent, 34% quinoxaline character in the unsubstituted parent, and 51-61% nitroquinoxaline character when the quinoxalino unit has one or more nitro groups. Close analogies are found between the calculated and observed nitrogen-atom spin distributions, indicating that the calculations embody the key chemical effects. The calculations also indicate that the nitrogen-atom spin distributions closely parallel the important total porphyrin, quinoxaline, and nitro spin distributions, indicating that the observed quantities realistically depict the change in the nature of the delocalization of the radical anion as a function of chemical substitution. The profound effects observed indicate long-range communication of the type that is essential in molecular electronics applications.

Introduction Many meso- and β-pyrrolic linked porphyrin arrays have been prepared in recent years with applications aimed toward electronic conduction.1-11 Quinoxalino[2,3-b′]porphyrins are molecular components of larger multiporphyrin arrays synthesized by Crossley and co-workers over the past decades and proposed as “molecular wires”.12-14 One example is the pseudoone-dimensional linear tetrakis-porphyrin (1) shown in Chart 1. This linked tetrakis-porphyrin has the individual macrocycles laterally bridged by coplanar tetraaza-anthracene units fused at the β,β′-pyrrolic positions of the porphyrin and has several features useful for applications in molecular electronics. It has a defined length and rigidity, permitting the molecule to span supporting elements such as monolayers and bilayers. It also has a sheath of bulky tert-butyl groups that insulate the conjugated core, and extended π-conjugated delocalization which should facilitate the rapid transfer of information from one end of the molecule to the other.15-17 Quinoxalinoporphyrins are also molecular components in the three-dimensional tetrakis* Authors to whom correspondence may be sent. E-mail: kkadish@ uh.edu; [email protected]; [email protected]; [email protected]. † University of Houston. ‡ The University of Sydney. § Osaka University.

porphyrin (2) that was synthesized as a model of the arrangement of pigments in the photosynthetic reaction center. The β,β′fused porphyrins in (2) were very effective in mediating photoinduced electron transfer from the zinc(II) porphyrin to the gold(III) porphyrin, leading to a long-lived charge-shift state.18 Quinoxalinoporphyrins lend themselves to facile synthetic elaboration by incorporation of different central metal ions or selective functionalization at β-pyrrolic positions.12,19 This should enable “fine-tuning” of electronic properties of these compounds by altering relative energies of the highest-occupied molecular orbital (HOMO) and lowest-occupied molecular orbital (LUMO), one end goal being the creation of logically constructed arrays that can meet specific electronic requirements.13,15,16 The incorporation of electron-withdrawing or electron-donating substituents into porphyrins would allow controloftheredoxpropertiesforcreatingelectronicgradients,13,20-25 whereas in the field of molecular electronics, functionality of this type may be used to store charge and thereby act as a molecular-information storage device.26 In order to develop new materials based on these compounds for “molecular electronics” applications there is a need to understand and establish the degree of communication (couplingdelocalization) between the porphyrin ring and the β,β′-fused quinoxalino unit. For applications there is a need to finely

10.1021/jp0726743 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007

Redox Properties of Quinoxalino[2,3-b′]porphyrins

J. Phys. Chem. B, Vol. 111, No. 30, 2007 8763

CHART 1

CHART 2

control the redox properties of such extended systems that have not previously been examined in a systematic manner. Herein we report the syntheses and redox properties of an extensive series of free-base and metal(II) quinoxalino[2,3-b′]porphyrins with electron-withdrawing (NO2, Cl, Br) or electrondonating substituents (OCH3, NH2). The redox properties are investigated by electrochemistry, ESR, thin-layer spectroelectrochemistry, and quantum-chemical analysis. The investigated compounds are shown in Chart 2 together with reference quinoxaline compounds. The shorthand notation for the compounds utilized in the text is given by a number to indicate the macrocycle and a letter to indicate the central metal ion or freebase derivative, examples being 6a to 6e to represent (PQ)M where PQ ) 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b′]porphyrin and M ) 2H, CuII, ZnII, NiII, or PdII and 12a and 12c to represent the free-base macrocycle

[PQ(NO2)2]2H and its zinc(II) derivative [PQ(NO2)2)]Zn, respectively. Results and Discussion Synthesis. Syntheses of (P)M (3), (PQ)M (6), [PQ(6NO2)]2H (11a), and [(NO2)PQ]M (13) were achieved following published procedures.18,27-29 The free-base quinoxalinoporphyrins 5a and 7a-12a were synthesized in good yield by condensation of porphyrin-2,3-dione30 (20) with R-substituted-1,2-diaminobenzenes (21-26) (Scheme 1).31-34 In each case IR spectra of the product showed no carbonyl stretches at ca. 1733 cm-1, indicating reaction of the R-dione of 20; 1H NMR showed the expected splitting pattern for the protons of the fused quinoxalino unit, and elemental analysis confirmed the molecular formulas.

8764 J. Phys. Chem. B, Vol. 111, No. 30, 2007 SCHEME 1

a

Kadish et al. SCHEME 2

a

a Reagents and conditions: i, SnCl ‚H O, CH Cl -MeOH, stir; ii, 2 2 2 2 standard conditions for metal(II) chelation.27

a

Reagents and conditions: i, CH2Cl2 or pyridine, stir; ii, standard conditions for metal(II) chelation.27

The 5′-nitro functionality on PQ(5NO2)2H (9a) was reduced with tin(II) chloride dihydrate and concentrated hydrochloric acid to give the corresponding 5′-amino analogue [PQ(5NH2)]2H (4a) (Scheme 2). Preparation of the copper(II), zinc(II), nickel(II), and palladium(II) chelates was achieved by refluxing a solution of the above free-base porphyrins in the presence of a salt containing the appropriate metal ion (Schemes 1 and 2).27 Synthesis and characterization of the copper(II), zinc(II), nickel(II), and palladium(II) chelates of compounds (4, 5, 7-12) are given in Supporting Information, pages S2-S13). Once again yields were high with the exception of the palladium(II) derivatives where reduced solubility of the free-base quinoxalinoporphyrins in the chloroform-acetic acid reaction solution lead to lower overall yield. Redox Chemistry and Substituent Effects. The substituted quinoxalinoporphyrins were examined as to their electrochemistry in CH2Cl2 containing 0.1 M TBAP. A summary of the half-wave and peak potentials for each redox process of all examined compounds in CH2Cl2 is given in Table 1. Key points to consider in analyzing the electrochemical data in this table are the half-wave potentials for oxidation and reduction, the reversibility of the processes, the electrochemical HOMOLUMO gap, and the differences in potentials between the first and second reductions or the first and second oxidations when both processes are reversible. Cyclic voltammograms of the substituted quinoxalinoporphyrins (a) [PQ(OCH3)2]2H (5a), (b) (PQ)2H (6a), (c)

(PQBr)2H (7a), and (d) (PQCl2)2H (8a) in CH2Cl2 containing 0.1 M TBAP are shown in Figure 1. There is only a small effect of the electron-donating (OCH3) or -withdrawing substituents (Br, Cl) on the oxidation and reduction potentials, although the reduction potentials of [PQ(OCH3)2]2H (5a) are slightly shifted in a positive direction due to the electron-donating substituent (OCH3). These results are in accord with previous calculations27 that show little spin distribution residing on the 6′- and 7′-quinoxalino positions in the porphyrin LUMO, leading to the expectation that substitution at these sites should have a minimal effect. In contrast, reduction potentials of the NO2-substituted quinoxalinoporphyrins that are substituted at either the β-pyrrolic periphery of the macrocycle or on the fused quinoxalino unit exhibit significant positive shifts as compared with (PQ)2H (6b) or (PQ)M (6b-e). An example of this is shown in Figure 2 for (PQ)Cu (6b), (PQCl2)Cu (8b), and [PQ(NO2)2]Cu (12b) in CH2Cl2. This implies that nitro substitution changes the fundamental nature of the orbital structure of the molecule. The first reduction of [PQ(NO2)2]Cu (12b) (E1/2 ) -0.64 V vs SCE in Figure 2c) is significantly more positive than E1/2 for reduction of (PQ)Cu (6b) (-1.19 V vs SCE in Figure 2a) or (PQCl2)Cu (8b) (-1.11 V vs SCE in Figure 2b), whereas the first oxidation potentials of the three compounds remain rather constant. Thus, the electrochemical HOMO-LUMO gap of 12b (1.68 V) is much smaller than that of 6b (2.16 V) or 8b (2.13 V). The second reduction and second oxidation of (PQ)Cu (6b) are both irreversible in CH2Cl2 due to the presence of a previously described coupled chemical reaction,14 and this is also observed in the case of the ZnII, NiII, and PdII derivatives in this solvent. In the case where the NO2 group is attached at the 7-position on the β-pyrrolic periphery, [(NO2)PQ]M (13), the positive shift

Redox Properties of Quinoxalino[2,3-b′]porphyrins

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TABLE 1: Half-Wave Potentials (V vs SCE) in CH2Cl2, 0.1 M TBAP macrocycle (P)M (3)a

[PQ(5NH2)]M (4)

[PQ(OCH3)2]M (5)

(PQ)M (6)a

(PQBr)M (7)

(PQCl2)M (8)

[PQ(5NO2)]M (9)

[PQ(NO2)(NH2)]M (10)

[PQ(6NO2)]M (11)

[PQ(NO2)2]M (12)

[(NO2)PQ]M (13)

PhNO2 (14) Ph(NO2)2 (15) Q (16) Q(CH3)2(5NO2) (17) Q(6NO2) (18) Q(NO2)2 (19)

M 2H CuII ZnII NiII PdII 2H CuII ZnII 2H CuII ZnII 2H CuII ZnII NiII PdII 2H CuII ZnII 2H CuII ZnII NiII 2H CuII ZnII 2H CuII ZnII PdII 2H CuII ZnII 2H CuII ZnII NiII PdII 2H CuII ZnII PdII

oxidation second first 1.28b 1.06 0.93c 1.52b 1.23b 1.14 1.02b 1.36b 1.14 0.98 0.99c 1.30b 1.10b 1.34 1.46b 1.38b 1.15 0.99 1.40b 1.15 1.13b 1.35 1.09 1.16 1.01 1.50b 1.08 0.95 1.14 1.21 1.03 1.39b 1.14 1.17b 1.08c 1.48b 1.39 1.20 1.04 1.56b

0.95e 0.92 0.72 0.93c 1.05 1.03b 0.92b 0.78 1.02b 0.94 0.75 0.99c 0.97 0.72 0.95 1.06 1.05b 1.00 0.76 1.07b 1.02 0.77 0.97 1.02 1.02 0.79 1.00b 0.93 0.74 1.04 1.06b 1.04 0.80 1.09b 1.04 0.86 1.08c 1.20 1.08b 1.03 0.84 1.22

first

second

-1.22 -1.36 -1.42 -1.31 -1.32 -1.12 -1.17 -1.31 -1.18 -1.22 -1.34 -1.13 -1.19 -1.31 -1.12 -1.16 -1.12 -1.14 -1.23 -1.10 -1.11 -1.20 -1.06 -1.06 -1.03 -1.08 -0.82 -0.78 -0.81 -0.78 -0.89 -0.84 -0.88 -0.68 -0.64 -0.70 -0.65 -0.66 -0.84 -0.92 -0.98 -0.93 -1.08 -0.77 -1.63b -1.00 -0.84 -0.60

-1.55 -1.92b -1.95b -1.32 -1.56 -1.61 -1.42 -1.62 -1.70 -1.33 -1.72b -1.75d -1.59b -1.62b -1.30 -1.52 -1.66 -1.25 -1.49 -1.59 -1.43 -1.19 -1.38b -1.32b -1.06 -1.17 -1.20b -1.18 -1.10 -1.18 -1.20b -1.03 -1.05 -1.13 -1.07 -1.08 -0.94 -1.19 -1.16 -1.22

reduction third

fourth

fifth

HOMO-LUMO gap (V) 2.17 2.28 2.14 2.24 2.37

-1.84b 2.09 2.16 2.09 2.12 2.16 2.03 2.07 2.22 -1.84b

-1.66b -1.74b -1.62b -1.59b -1.59b -1.41b -1.47b -1.50 -1.72b -1.40b -1.36 -1.44 -1.66b,c -1.46 -1.40 -1.88b -1.92b -1.90b

2.14 1.99 2.13 1.97 2.03 2.08 2.05 1.87 -1.90b -1.67b -1.71b -1.67b -1.94b

1.71 1.55 1.82 1.88 1.78

-1.48b -1.58b -1.66b,c -1.68b -1.70b

-1.92b -1.78b -1.84b -1.82b -1.84b

1.68 1.56 1.73 1.86 1.95 1.82 2.15

-1.03 -1.55b -1.56b -1.06

-1.86b

a Taken from ref 14. b Peak potential at a scan rate of 0.1 V/s for irreversible reaction. c Two overlapping one-electron redox process. d Chemical reactions distort observed value.14 e Distorted waveform most likely indicative of an interfering chemical reaction.

in the first reduction potential is less dramatic. For example, [(NO2)PQ]Cu (13b) has a first reduction potential of -0.92 V and a resultant HOMO-LUMO gap of 1.95 V. This indicates that porphyrin β-pyrrolic substitution by a strong electronwithdrawing group also affects the LUMO, but its impact is less than that of substitution at the 6′- and 7′-positions on the fused quinoxalino unit in 10-12, suggesting again that the electronic structures of the nitro derivatives are highly perturbed. Substitution at the 6′- and 7′-positions (R3 and R4) of the quinoxalino unit in (PQ)M by electron-withdrawing Br and Cl groups leads to a small positive shift in E1/2 for the first two reductions, but the largest potential shift is observed for the five [PQ(NO2)2]M (12) derivatives, all of which are shifted positively by close to 500 mV with respect to potential for reduction of the (PQ)M (6) complexes with the same metal ion and reduced in the narrow potential range of E1/2 ) -0.64 to

-0.70 V in CH2Cl2. When the fused quinoxalino unit bears a substituent, most second reductions are reversible, unlike the case of the unsubstituted analogues (compare, for example, (PQCl2)Cu (8b) and (PQ)Cu (6b) in Figure 2), indicating a stabilization of the electrogenerated monoanions and dianions on the electrochemical time scale. The measured electrochemical HOMO-LUMO gaps for all of the quinoxalinoporphyrins with reversible first oxidation and first reduction potentials in CH2Cl2 are listed in the last column of Table 1 and range from a high of 2.22 V for (PQ)Pd (6e) to a low of 1.56 V for [PQ(NO2)2]Zn (12c). Despite the longtime use of reporting “diagnostic” HOMO-LUMO gaps for oxidation or reduction at a conjugated metalloporphyrin macrocycle (2.25 ( 0.15 V) for TPP complexes,35 the actual value depends very much on the type of central metal ion, and this is seen for quinoxalinoporphyrins in the currently investigated

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Figure 1. Cyclic voltammograms of substituted quinoxalinoporphyrins: (a) [PQ(OCH3)2]2H 5a, (b) (PQ)2H 6a, (c) (PQBr)2H 7a, and (d) (PQCl2)2H 8a in CH2Cl2 containing 0.1 M TBAP.

Figure 2. Cyclic voltammograms of copper complexes (PQ)Cu (6b), (PQCl2)Cu (8b) and [PQ(NO2)2]Cu (12b) in CH2Cl2 containing 0.1 M TBAP.

series of macrocycles where the trends fit almost exactly what was reported earlier for a number of substituted TPP complexes,35 i.e., the smallest gap for the ZnII derivatives and largest

for the PdII compounds. As metalation is a convenient process typically applied after synthesis of the free-base analogue, this property of the nitroquinoxalinoporphyrins offers a ready

Redox Properties of Quinoxalino[2,3-b′]porphyrins

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Figure 3. Cyclic voltammograms of nitroquinoxaline (18) and nitroquinoxalinoporphyrins (11a) and (11b) in pyridine containing 0.1 M TBAP.

SCHEME 3

synthetic route to band gap control in molecular devices constructed from this class of molecule. It is also clear from the data in Table 1 that a significant band gap lowering occurs for the nitroquinoxalinoporphyrins (9-12) as compared to compounds 3-8, and this can be accounted for by the fact that the electron-withdrawing nitro substituents stabilize the LUMO orbital more than the HOMO of these compounds. In order to better understand the more complex redox reactivity of compounds 9-12, an examination was made of related molecules that constitute the structure of these nitroquinoxalinoporphyrins. An examination of the reversible redox potentials in Table 1 shows clearly that the first reduction

potentials for compounds in the series 9-12 occur at potentials very similar or even identical to the E1/2 values for the first reduction of the nitroquinoxaline model compounds 17-19. An example is shown in Figure 3 which illustrates cyclic voltammograms for the reduction of [PQ(6NO2)]2H (11a), its copper derivative [PQ(6NO2)]Cu (11b), and its quinoxaline component Q(6NO2) (18). It is immediately evident that the first reduction of all three compounds occurs at virtually the same potential. This suggests that the first reduction is porphyrin-localized when the quinoxalino unit is not nitro substituted, and nitroquinoxalino-localized otherwise. This crude picture is subsequently

8768 J. Phys. Chem. B, Vol. 111, No. 30, 2007 refined through spectroelectrochemical, quantum-chemical, and ESR studies. In contrast to the first reduction, the second reduction of [PQ(6NO2)]Cu (11b) is shifted negatively by 120 mV as compared to that of [PQ(6NO2)]2H (11a) (Figure 3, panels b and c) as is expected for electron additions involving the conjugated π-ring system. Copper porphyrins reduced at the conjugated macrocycle to give a porphyrin anion or dianion should always be more difficult to reduce than free-base porphyrins with the same macrocycle,35,36 and this is clearly seen in the present study for each comparison of E1/2 values for the second reductions of free-base and CuII compounds in the series 3-8 where the initial electron-transfer site is unambiguously at the quinoxalinoporphyrin π-ring system. Spin Delocalization As Seen through Spectroelectrochemical Studies. Systematic changes in spectra of both the neutral and one-electron-reduced species of 5-8 with changes in the electron-donating or -withdrawing groups were examined by thin-layer spectroelectrochemistry as shown in Figure 4. A decrease in the Soret band and appearance of the broad absorption band at 785-788 nm with isosbestic points upon the first one-electron reduction are characteristic of porphyrin π-radical anions,14 and all of the spectra shown in this figure have this character. Shown in Figure 5 are spectra observed for radical anions of the nitro-containing compounds, with the spectrum for [(NO2)PQ]2H(13a) showing clear evidence of a porphyrin macrocycle reduction. However, only slight changes in the spectra are observed upon reduction of [PQ(6NO2)]2H (11a) and [PQ(NO2)2]2H(12a), indicating minor participation of the macrocycle in the process. Interestingly, [PQ(5NO2)]2H (9a) has more macrocycle-like reduction character, and so it is clear that chemical substitution can significantly modify the nature of the radical anion of quinoxalinoporphyrins. This result is in qualitative agreement with the previous interpretations of the electrochemical data. Computation of the Reduction Potentials and the Degree of Spin Delocalization. Theoretical calculations involving molecules 6, 9, 10, 11, 12, 14, 15, 16, 17, 18, and 19 were performed, and Figure 6 plots the calculated changes in the first reduction potentials as compared to (PQ)2H (6a) (obtained as the difference in free energies between the neutral and ionic species) versus the experimentally measured difference in E1/2 values in CH2Cl2. Over the 1 V range investigated, good qualitative agreement was generally found between the calculated and observed data, the most significant differences being for [PQ(5NO2)]Zn (9c) and the model compound 1,2-dinitrobenzene Ph(NO2)2 (15). While porphyrin metalation with ZnII is known to destabilize porphyrin anions, leading to a harder reduction,35,36 calculations predict an opposite effect. This may be associated with the fact that specific solvent-metal interactions are neglected in the calculations as these treat the solvent only as a dielectric continuum. Also, the differences found for Ph(NO2)2 (15) may possibly be attributed to the small size of the molecule increasing the atomic charge densities of the anion and hence enhancing specific interactions with the solvent. Most importantly, however, the calculations reproduce the major qualitative features found in reduction potentials of the free-base nitroquinoxalinoporphyrins 9a-12a and the nitroquinoxalines 17-19. As the calculations semiquantitatively reproduce the observed first reduction potentials, they should provide insight into the nature of the processes involved. From the calculated atomic spin distributions, the fraction of the radical anion localized on the porphyrin macrocycle, the quinoxaline

Kadish et al.

Figure 4. Thin-layer UV-visible spectral changes during the first reduction of non-nitro substituted free-base quinoxalinoporphyrins (5a8a) in CH2Cl2 containing 0.2 M TBAP.

unit, and the substituents may be deduced, and the results are given in Table 2. These predict that the charge is not simply localized fully on either the porphyrin macrocycle or the quinoxaline, as qualitative interpretation of the electrochemical data suggested, but vary from 8% quinoxaline character in [(NO2)PQ]Zn to 24% character in (PQ)Zn to 61-65% total nitroquinoxaline character in the nitroquinoxalines. Indeed, the spectroelectrochemical results shown in Figures 4 and 5 indicate

Redox Properties of Quinoxalino[2,3-b′]porphyrins

J. Phys. Chem. B, Vol. 111, No. 30, 2007 8769

Figure 5. Thin-layer UV-visible spectral changes obtained during the first reduction of (a) [(NO2)PQ]2H (13a), (b) [PQ(5NO2)]2H (9a), (c) [PQ(6NO2)]2H (11a), and (d) [PQ(NO2)2]2H (12a) in CH2Cl2, 0.2 M TBAP.

considerable variation of the nature of the electronic transitions with substitution, consistent with a gradual impact of substitution on electronic structure. Spin Delocalization in Radical Anions of Quinoxalinoporphyrins. The degree of spin delocalization upon one-electron addition to quinoxalinoporphyrins was examined by ESR measurements. The radical anions of quinoxalinoporphyrins

were produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA)2]37,38 to the quinoxalinoporphyrins in MeCN as shown in Scheme 3. The (BNA)2 is known to act as a unique electron donor to produce the radical anions of electron acceptors.38 The ESR spectrum of the radical anion of (PQ)Zn (6c) obtained under photoirradiation are shown in Figure 7a. The computer simula-

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Figure 6. Calculated versus observed differences in the first reduction potential E1/2 from that for (PQ)2H (6a), E1/2(6a), in CH2Cl2 solution: (b) [PQ(R1R2R3)]2H (9a-12a), (O) [PQ(5NO2)]Zn(9c), (4) nitrobenzenes and nitroquinoxalines (14-19).

TABLE 2: DFT-Calculated Total and Nitrogen-Atom Spin Density Ratios P:Q:NO2, Compared to ESR-Observed Nitrogen-Atom Hyperfine Coupling Constant Ratios for Atoms in the Porphyrin Macrocycle (P), in the Quinoxaline Group (Q), and on Attached Nitro Groups (NO2) nitrogen-atom contributions

total

molecule

ESR observed

DFT calculated

DFT calculated

[(NO2)PQ]Zn (13c) (PQ)Zn (6c) [PQ(5NO2)]Zn (9c) [PQ(6NO2)]Zn (11c) [PQ(NO2)2]Zn (12c)

57:27:16 66:34:0 49:36:15 48:39:13 39:51:10

68:19:13 62:38:0 43:43:14 43:42:15 42:48:10

77:8:15 76:24:0 37:43:20 44:35:20 40:42:19

tion spectrum (Figure 7b) agrees with the observed ESR spectrum (Figure 7b), affording the hyperfine coupling (hfc) constants in Figure 7c, where the calculated spin densities by the DFT method are also listed. The aN values of one nitrogen (5.44 and 4.11 G) correspond to the hyperfine coupling on N(21) and N(23), which have relatively large spin densities. The aN value of two equivalent nitrogens (2.48 G) corresponds to the hyperfine coupling on N(9′ and 10′). No hyperfine coupling is observed on the other two equivalent nitrogens, N(22 and 24), where very small spin densities are seen. Of the six nitrogens in this molecule, 66% of the total aN values thus arise from the four nitrogens on the porphyrin macrocycle, whereas 34% arises from the two quinoxalino nitrogens. For all molecules studied using ESR, the observed ratios, along with the corresponding DFT-calculated ones, are given in Table 2. For (PQ)Zn, the calculated ratio is 62:38, in good agreement with the experimental ESR data. Very similar observed values of 49:36:15 and 48:39:13 are found for the mono-nitro compounds [PQ(5NO2)]Zn(9c) and [PQ(6NO2)]Zn(11c), respectively, compounds whose ESR spectra are shown in Supporting Information S16 and S17. For these compounds, the calculated spin-density ratios are in quantitative agreement with the experimental measurements. For [PQ(NO2)]Zn (12c), the calculated spin delocalization ratios on nitrogens for the entire porphyrin, quinoxaline, and nitro groups are 42:48:10, also close to the observed ratio of 39:51:10 (see Supporting Information S18).

Figure 7. (a) ESR spectrum of the radical anion of (PQ)Zn (6c: 5.0 × 10-4 M) obtained under photoirradiation of the porphyrin compound with (BNA)2 (3.0 × 10-3 M) in deaerated PhCN at 298 K. (b) The computer simulation spectrum of the radical anion of (PQ)Zn with (c) its deduced hyperfine coupling constants (a, G) and the DFT-calculated distribution of spin densities.

In contrast to the case of the NO2 substitution on the quinoxalino unit, NO2 substitution on the β-pyrrolic 7-position of the porphyrin ring [(NO2)PQ]Zn (13c) results in a change of the hyperfine pattern as shown in Figure 8a. The simulated spectrum (Figure 8b) agrees with the observed spectrum, indicating that the spin distribution is significantly affected by the NO2 substitution on the porphyrin β-pyrrolic position. The observed nitrogen-atom hyperfine coupling ratios are 57:27: 16, in good agreement with the calculated nitrogen spin-density ratios of 68:19:13 (Table 2). The ESR results thus indicate a steadily changing nitrogenatom spin distribution as a function of chemical substitution: 27% quinoxaline character in [(NO2)PQ])Zn (13c), 34% qui-

Redox Properties of Quinoxalino[2,3-b′]porphyrins

J. Phys. Chem. B, Vol. 111, No. 30, 2007 8771 interpret the observed reduction potentials, and to explain the significant qualitative changes of band shape observed by spectroelectrochemistry. Conclusions The redox potentials of substituted quinoxalinoporphyrins are shown to be dependent on the position and nature of the substituents, as well as the HOMO-LUMO gap in these molecules. In particular, nitro substitution is found to have a large effect in reducing the HOMO-LUMO gap. In particular, the spectroelectrochemistry suggests a change in the orbital being reduced, with the ESR and computational approaches providing quantitative measures of the mixing of the implied porphyrin and quinoxaline orbitals. These features are significant as it is relatively easy to chemically vary the nitroquinoxalinoporphyrins, by both metalation and reaction of the nitro groups. Indeed, the nitroquinoxalinoporphyrins [PQ(NO2)(NH2)]M (10) and [PQ(NO2)2]M (12) are themselves especially valuable building blocks toward the synthesis of oligoporphyrins.39 Nitroquinoxalinoporphyrin chemistry is thus both accessible and controllable for possible applications in molecular-electronic devices. The profound effects observed indicate long-range communication of the type that is essential in molecular electronics applications. There are many ways in which nitroquinoxalinoporphyrins and quinoxalinoporphyrins in general, could be employed in molecular electronic devices. For example, functionalizing the tail end of a porphyrin array with a β,β′-fused nitroquinoxalino group, will allow charge to be localized on the periphery of the terminal component of the wire where it is accessible for further applications.40,41 This further highlights the ability of functionalized porphyrins to act in a similar fashion to other chemical models40,41 that have proven useful in the developing field of molecular electronics. Experimental Section

Figure 8. ESR spectrum of (a) the radical anion of [(NO2)PQ]Zn (13c: 5.0 × 10-4 M) obtained under photoirradiation of the porphyrin compound with (BNA)2 (3.0 × 10-3 M) in deaerated PhCN at 298 K. (b) Computer simulation spectrum of the radical anion of [(NO2)PQ]Zn with (c) the deduced hyperfine coupling constants (a, G) and the DFT-calculated distribution of spin densities.

noxaline character in (PQ)Zn (6c), and 51-61% total nitroquinoxaline character in the ZnII nitroquinoxalinoporphyrins (9c, 11c, 12c). Close analogies are found between the calculated and observed nitrogen-atom spin distributions, indicating that the calculations embody the key chemical effects. Calculations also indicate that the nitrogen-atom spin distributions closely parallel the important total porphyrin, quinoxaline, and nitro spin distributions, indicating that the observed quantities realistically depict the change in the nature of the delocalization of the radical anion as a function of chemical substitution. As the calculations also reproduce the key electrochemical data, it is clear that the observed ESR spin distributions can be used to

General Procedures. Melting points were recorded on a Reichert melting point stage and are uncorrected. Microanalyses were performed at the Microanalytical Unit, The University of New South Wales. Infrared spectra were recorded on a PerkinElmer model 1600 FT-IR spectrophotometer, as solutions in the stated solvents. UV-visible spectra were routinely recorded on a Hitachi 150-20 spectrophotometer in chloroform that was de-acidified by filtration through an alumina column. 1H NMR spectra were recorded on either a Bruker AC-200 (200 MHz), a Bruker DPX-400 (400 MHz), or a Bruker AMX600 (600 MHz) spectrometer as stated. Deuterochloroform was used as the solvent with tetramethylsilane (TMS) as an internal standard. Signals are recorded in terms of chemical shift (in ppm), multiplicity, coupling constants (in Hz), and assignment, in that order. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were recorded on a VG TofSpec spectrometer. Mass spectra were obtained as an envelope of the isotope peaks of the molecular ion. The mass corresponding to the envelope’s maxima is reported and was compared with the maxima of a simulated spectrum. High-resolution electrospray ionization Fourier transform ion cyclotron resonance spectrometry (HR-ESI-FT/ICR) was recorded on a Bruker Daltonincs BioAPEX II FT/ICR mass spectrometer equipped with a 4.7 T MAGNEX superconducting magnet and an Analytica external ESI source. Column chromatography was routinely carried out using the gravity feed column technique on Merck silica gel type 9385 (230-400 mesh). All solvents used were routinely distilled prior to use, unless otherwise stated.

8772 J. Phys. Chem. B, Vol. 111, No. 30, 2007 Light petroleum refers to the fraction of bp 60-80 °C. Ether refers to diethyl ether and was distilled over crushed calcium chloride and stored over sodium wire. Ethanol-free chloroform was obtained by distillation from calcium chloride and passage through alumina. Merck AR grade methanol was used. Where solvent mixtures were used, the proportions are given by volume. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-6′,7′-dimethoxyquinoxalino[2,3-b′]porphyrin (5a). A solution of 2,3-dioxo5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin30 (20) (599 mg, 0.547 mmol) and 1,2-diamino-4,5-dimethoxybenzene33 (21) (279 mg, 1.66 mmol) in dichloromethane (35 mL) was stirred for 10 min. The mixture was then filtered through a plug of silica (type 9385, dichloromethane). The filtrate was evaporated to dryness and purified by column chromatography over silica (type 7736, dichloromethane-light petroleum, 2:5). The major red-brown band yielded 5a (646 mg, 96%) as a purple, microcrystalline solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 3348w (NH), 2964s, 2904w, 2869w, 1731w, 1592m, 1494m, 1476w, 1427m, 1393w, 1363m, 1287w, 1236s, 1180w, 1160w, 1141w, 1013w cm-1. UV-vis (CHCl3): 305 (log  4.27), 383sh (4.65), 438 (5.49), 527 (4.32), 562 (3.81), 597 (4.00), 649 (2.91) nm. 1H NMR (400 MHz, CDCl3): δ -2.52 (2 H, br s, inner NH), 1.49 (36 H, s, tertbutyl H), 1.53 (36 H, s, tert-butyl H), 4.04 (6 H, s, methoxy H), 7.06 (2 H, s, quinoxaline H), 7.80 (2 H, t, J ) 1.8 Hz, aryl H), 7.93 (2 H, t, J ) 1.8 Hz, aryl H), 7.98 (4 H, d, J ) 1.8 Hz, aryl H), 8.11 (4 H, d, J ) 1.8 Hz, aryl H), 8.80 (2 H, s, β-pyrrolic H), 8.80 and 9.10 (4 H, ABq, J ) 4.9 Hz, β-pyrrolic H). MS (MALDI-TOF): 1226 (M+ requires 1226). Anal. Calcd for C84H100N6O2: C, 82.31; H, 8.22; N, 6.86. Found: C, 82.40; H, 8.18; N, 6.73. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-6′-bromoquinoxalino[2,3-b′]porphyrin (7a). A degassed solution of 20 (135 mg, 0.123 mmol) and 1,2-diamino-4-bromobenzene34 (22) (34.60 mg, 0.185 mmol) in dry dichloromethane (7 mL) was stirred for 1 h. The solvent was removed under vacuum and the residue purified by chromatography over silica (type 7734, dichloromethane-light petroleum, 2:3). The major brown band yielded 7a (146 mg, 95%) as a purple, microcrystalline solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 3348w (NH), 2964s, 2905w, 2868w, 1593m, 1477m, 1394w, 1364m, 1292w, 1248w, 1148w, 1121w cm-1. UV-vis (CHCl3): 360 (log  4.42), 436 (5.21), 531 (4.24), 568 (3.77), 600 (4.04), 650 (2.85) nm. 1H NMR (400 MHz, CDCl3): δ -2.51 (2 H, br s, inner NH), 1.48 (18 H, s, tert-butyl H), 1.50 (18 H, s, tert-butyl H), 1.54 (36 H, s, tert-butyl H), 7.68 (1 H, d, J ) 8.9 Hz, quinoxaline H), 7.80-7.83 (3 H, m, 2 × aryl H and 1 quinoxaline H), 7.92-7.99 (7 H, m, 6 × aryl H and 1 quinoxaline H), 8.10 (4 H, d, J ) 1.8 Hz, aryl H), 8.79 (2 H, s, β-pyrrolic H), 8.98-9.00 (2 H, m, β-pyrrolic H), 9.06-9.08 (2 H, m, β-pyrrolic H). MS (MALDI-TOF): 1246 (M+ requires 1245). HR-ESI-FT/ICR Found: 1245.691727. C82H95N6 + H requires 1245.68948. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-6′,7′-dichloroquinoxalino[2,3-b′]porphyrin (8a). A solution of 20 (1.34 g, 1.23 mmol) and 1,2-diamino-4,5-dichlorobenzene (23) (450 mg, 2.54 mmol) in dichloromethane (400 mL) was heated at reflux for 24 h. The solvent was removed under vacuum and the crude residue purified by passing through a plug of silica (type 9385, dichloromethane-light petroleum, 1:2). The filtrate was evaporated to dryness to give 8a (1.38 g, 91%) as a dark, red-brown

Kadish et al. microcrystalline solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 3426s (NH), 2961s, 1590m, 1472m, 1458m, 1422w, 1358m, 1262s, 1203s, 1139w cm-1. UV-vis (CHCl3): 363 (log  4.61), 421 (5.34), 436sh (5.22), 533 (4.37), 568 (3.84), 654 (3.69) nm. 1H NMR (400 MHz, CDCl3): δ -2.52 (2 H, br s, inner NH), 1.49 (36 H, s, tert-butyl H), 1.53 (36 H, s, tert-butyl H), 7.81 (2 H, t, J ) 1.8 Hz, aryl H), 7.91 (2 H, s, quinoxaline H), 7.94 (2 H, t, J ) 1.7 Hz, aryl H), 7.95 (4 H, d, J ) 1.7 Hz, aryl H), 8.09 (4 H, d, J ) 1.85 Hz, aryl H), 8.80 (2 H, s, β-pyrrolic H), 9.00 and 9.07 (4 H, ABq, J ) 4.9 Hz, β-pyrrolic H). MS (MALDI-TOF): 1235 (M+ requires 1235). Anal. Calcd for C82H94N6Cl2: C, 79.64; H, 7.82; N, 6.80. Found: C, 80.00; H, 7.85; N, 6.70. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-5′-nitroquinoxalino[2,3-b′]porphyrin (9a). A solution of 20 (1.50 g, 1.37 mmol) and 1,2-diamino-3-nitrobenzene (24) (300 mg, 1.96 mmol) in dichloromethane (85 mL) and pyridine (15 mL) was stirred under nitrogen in the dark for 5 days. Workup and chromatography over silica (type 9385, dichloromethane-light petroleum, 1:1) gave 9a (1.52 g, 90%) as a purple, microcrystalline solid. An analytically pure sample was obtained by recrystallization from a chloroform-methanol mixture, mp >300 °C. IR (CHCl3): 3364w, 2964s, 2903m, 2861m, 1593m, 1536m (NO2), 1476m, 1364m, 1163m, 1130m cm-1. UV-vis (CHCl3): 338 (log  4.23), 352 (4.42), 420 (5.41), 455sh (4.84), 537 (4.19), 568sh (3.83), 605 (4.08), 656 (2.94) nm. 1H NMR (400 MHz, CDCl3): δ -2.51 (1 H, br s, inner NH), -2.49 (1 H, br s, inner NH), 1.48 (18 H, s, tert-butyl H), 1.50 (18 H, s, tert-butyl H), 1.53 (18 H, s, tert-butyl H), 1.54 (18 H, s, tertbutyl H), 7.77 (1 H, dd, J ) 8.4 and 7.4 Hz, quinoxaline H 7′), 7.80-7.82 (2 H, m, aryl H), 7.91 (1 H, t, J ) 1.8 Hz, aryl H), 7.93 (2 H, d, J ) 1.8 Hz, aryl H), 7.94 (1 H, t, J ) 1.8 Hz, aryl H), 7.97 (2 H, d, J ) 1.8 Hz, aryl H), 8.00 (1 H, dd, J ) 8.4 and 1.4 Hz, quinoxaline H 8′), 8.04 (1 H, dd, J ) 7.4 and 1.4 Hz, quinoxaline H 6′), 8.08 (2 H, d, J ) 1.8 Hz, aryl H), 8.09 (2 H, d, J ) 1.8 Hz, aryl H), 8.79 (2 H, s, β-pyrrolic H), 8.91 and 8.97 (2 H, ABq, J ) 5.0 Hz, β-pyrrolic H), 9.00 and 9.09 (2 H, ABq, J ) 5.0 Hz, β-pyrrolic H). MS (MALDI-TOF): 1211 (M+ requires 1211). Anal. Calcd for C82H95N7O2: C, 81.30; H, 7.90; N, 8.10. Found: C, 81.50; H, 8.00; N, 8.00. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-5′-aminoquinoxalino[2,3-b′]porphyrin (4a). A solution of 5,10,15,20tetrakis(3,5-di-tert-butylphenyl)-5′-nitroquinoxalino[2,3-b′]porphyrin (9a) (1.42 g, 1.17 mmol) and tin(II) chloride (2.4 g, 13 mmol) in dichloromethane (150 mL) and hydrochloric acid (10 M, 3 mL) was stirred in the dark for 7 days. Workup and chromatography over silica (type 7734, dichloromethane-light petroleum, 1:2) gave 4a (1.05 g, 76%) as a brown, microcrystalline solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 3495w (NH2), 3365w (NH2), 3350w (NH), 2965s, 2904m, 2868m, 1592s, 1477m, 1364m, 1163m, 923 m cm-1. UV-vis (CHCl3): 326 (log  4.43), 398sh (4.96), 432 (5.15), 452sh (5.01), 534 (4.36), 566 (3.81), 601 (3.99), 655 (3.29), 663sh (3.25) nm. 1H NMR (400 MHz, CDCl3): δ -2.47 (2 H, br s, inner NH), 1.48 (36 H, s, tert-butyl H), 1.53 (36 H, s, tert-butyl H), 4.66 (2 H, bs, NH2), 6.85 (1 H, dd, J6′,7′ ) 8.2, J6′,8′ ) 1.4 Hz, quinoxaline H 6′), 7.15 (1 H, dd, J8′,7′ ) 8.2, J8′,6′ ) 1.4 Hz, quinoxaline H 8′), 7.51 (1 H, dd, J7′,8′ ) 8.2, J7′,6′ ) 1.4 Hz, quinoxaline H 7′), 7.79-7.81 (2 H, m, aryl H), 7.87 (1 H, t, J ) 1.7 Hz, aryl H), 7.91 (1 H, t, J ) 1.7 Hz, aryl H), 7.97 (2 H, d, J ) 1.7 Hz, aryl H), 8.02 (2 H, d, J ) 1.7 Hz, aryl H), 8.08-8.11 (4 H, m, aryl H), 8.78 (2 H, s, β-pyrrolic

Redox Properties of Quinoxalino[2,3-b′]porphyrins H), 8.91 and 9.04 (2 H, ABq, J ) 4.7 Hz, β-pyrrolic H), 9.04 and 8.99 (2 H, ABq, J ) 4.7 Hz, β-pyrrolic H). MS (MALDITOF): 1181 (M+ requires 1180). Anal. Calcd for C82H97N7: C, 83.4; H, 8.3; N, 8.3. Found: C, 83.2; H, 8.4; N, 8.3. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-6′-amino-7′-nitroquinoxalino[2,3-b′]porphyrin (10a). A solution of 20 (400 mg, 0.366 mmol) and 1,2,4-triamino-5-nitrobenzene31 (25) (96.0 mg, 0.571 mmol) in chloroform (20 mL) was heated at reflux for 17 h. The solvent was removed under vacuum and the residue purified by chromatography over silica (type 9385, dichloromethane-light petroleum, 1:1). The major brown band gave 10a (115 mg, 76%) as a purple, microcrystalline solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 3517w (NH), 3407w (NH), 3351w (NH), 3024s, 2964s, 2904w, 2867w, 1632s (NH), 1593s, 1567m, 1550w, 1528m (NO2), 1476m, 1427w, 1394m, 1320s (NO2), 1294m, 1265w, 1226w, 1137w, 1058w cm-1. UV-vis (CHCl3): 316 (log  4.34), 429 (5.38), 532 (4.12), 610 (4.00), 660 (3.32) nm. 1H NMR (400 MHz, CDCl ): δ -2.42 (2 H, br s, inner NH), 3 1.51 (36 H, s, tert-butyl H), 1.56 (36 H, s, tert-butyl H), 5.87 (2 H, br s, NH2), 7.05 (1 H, s, quinoxaline H), 7.82 (2 H, t, J ) 1.8 Hz, aryl H), 7.92 (1 H, d, J ) 1.8 Hz, aryl H), 7.957.97 (4 H, m, aryl H), 7.98 (2 H, t, J ) 1.8 Hz, aryl H), 8.11 (4 H, d, J ) 1.8 Hz, aryl H), 8.69 (1 H, s, quinoxaline H), 8.78 (2 H, s, β-pyrrolic H), 8.97-9.09 (4 H, m, β-pyrrolic H). MS (MALDI-TOF): 1226 (M+ requires 1226). Anal. Calcd for C82H96N8O2: C, 80.35; H, 7.90; N, 9.14. Found: C, 80.50; H, 7.90; N, 8.85. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-6′,7′-dinitroquinoxalino[2,3-b′]porphyrin (12a). 1,2-Diamino-4,5-dinitrobenzene32 (26) (17 mg, 0.09 mmol) was added to a solution of 20 (133 mg, 0.12 mmol) in pyridine (10 mL). The mixture was heated at reflux in the dark for 3 h. Workup and chromatography over silica (type 9385, dichloromethane-light petroleum, 1:3) gave 12a (115 mg, 76%) as a brown, amorphous solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 3351w (NH), 2965s, 2869m, 1593m, 1546m (NO2), 1477m, 1363m (NO2), 1292m, 1248w, 1155w, 1107w cm-1. UV-vis (CHCl3): 425sh (log  5.17), 431 (5.17), 616 (3.95), 667 (3.39) nm. 1H NMR (400 MHz, CDCl3): δ -2.48 (2 H, br s, inner NH), 1.49 (36 H, s, tert-butyl H), 1.53 (36 H, s, tert-butyl H), 7.83 (2 H, t, J 1.5 Hz, aryl H), 7.96 (4 H, d, J 1.5 Hz, aryl H), 7.98 (2 H, m, aryl H), 8.10 (4 H, d, J 1.5 Hz, aryl H), 8.35 (2 H, s, quinoxaline H), 8.81 (2 H, s, β-pyrrolic H), 9.02 (2 H, d, J 5.0 Hz, β-pyrrolic H), 9.11 (2 H, d, J ) 5.0 Hz, β-pyrrolic H). MS (MALDI-TOF): 1255 (M+ requires 1256). Anal. Calcd for C82H94N8O2: C, 78.50; H, 7.50; N, 8.90. Found: C, 78.80; H, 7.60; N, 9.00. {5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-6′,7′-dinitroquinoxalino[2,3-b′]porphyrinato}zinc(II) (12c). A solution of (12a) (99 mg, 0.080 mmol) in dichloromethane (45 mL) and methanol (5 mL) was heated at reflux with zinc(II) acetate dihydrate (100 mg, 0.46 mmol) for 3 h. Workup and chromatography over silica (type 7734; dichloromethane-light petroleum, 2:3) gave 12c (98 mg, 94%) as a dark-brown, microcrystalline solid. An analytically pure sample was obtained by recrystallization from a dichloromethane-methanol mixture, mp >300 °C. IR (CHCl3): 2964s, 2866w, 1594s, 1541m (NO2), 1443s, 1364m, 1342m (NO2), 1296w, 1228w, 1164w, 1107w, 1002w cm-1. UV-vis (CHCl3): 340 (log  4.42), 416 (5.03), 429sh (5.02), 505 (4.46), 541sh (4.23), 672sh (3.75) nm. 1H NMR (400 MHz, CDCl3): δH 1.48 (36 H, s, tert-butyl H), 1.53

J. Phys. Chem. B, Vol. 111, No. 30, 2007 8773 (36 H, s, tert-butyl H), 7.81 (2 H, bs, aryl H), 7.95 (4 H, bs, aryl H), 7.97 (2 H, bs, aryl H), 8.08 (4 H, bs, aryl H), 8.44 (2 H, s, quinoxaline H), 8.92 (2 H, s, β-pyrrolic H), 9.02 and 9.06 (4 H, ABq, J ) 5.0 Hz, β-pyrrolic H). MS (MALDI-TOF): 1319 (M+ requires 1319). Anal. Calcd for C82H92N8O4Zn + CH2Cl2: C, 71.10; H, 6.70; N, 7.90. Found: C, 70.60; H, 6.60; N, 7.80. Syntheses of other metalloquinoxalinoporphyrins and their spectroscopic characterizations (1H NMR, UV-visible, IR, MS, HR-ESI-FT/ICR) are described in Supporting Information, pages S2-S13. Quinoxaline and its analogues were purchased from commercial sources. Electrochemical Measurements. Cyclic voltammetry was carried out by using an EG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat. A homemade threeelectrode cell was used for cyclic voltammetric measurement and consisted of a platinum button or glassy carbon working electrode, a platinum counter electrode, and a homemade saturated calomel reference electrode (SCE). The SCE was separated from the bulk of the solution by a fritted glass bridge of low porosity, which contained the solvents/supporting electrolyte mixture. UV-visible spectroelectrochemical experiments were performed with a home-built thin-layer cell that had a light-transparent platinum networking electrode.42 Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. Time-resolved UV-visible spectra were recorded with a Hewlett-Packard model 8453 diode array spectrophotometer. Absolute dichloromethane (CH2Cl2) and pyridine were received from Aldrich Co. and used as received without further purification. Tetra-n-butylammonium perchlorate (TBAP) was purchased from Sigma Chemical or Fluka Chemika Co., recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for at least one week prior to use. ESR Measurements. Radical anions of the quinoxalinoporphyrins were prepared by the photoreduction of the compounds with dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA)2]37,38 in deaerated PhCN. Typically, a quinoxalinoporphyrin was dissolved in PhCN (5.0 × 10-4 M in 300 µL) containing (BNA)2 (2.5 × 10-4 M) and purged with argon for 10 min. The solution was bubbled with Ar gas through a syringe that has a long needle. ESR spectra were recorded on a JEOL JES-RE1XE spectrometer under irradiation of a high-pressure mercury lamp (USH-1005D) by focusing at the sample cell in the ESR cavity at 298 K. The magnitude of modulation was chosen to optimize the resolution and signal-to-noise (S/N) ratio of the observed spectra under nonsaturating microwave power conditions. The g values were calibrated using a Mn2+ marker. Calculations. Calculations of the properties of molecules were performed using density functional theory (DFT) with the B3LYP density functional43 and the 6-31G(d) basis set. All calculations were performed using GAUSSIAN-03.44 These calculations involved independent optimization of the neutral molecules and their anions or dianions in the presence of a selfconsistent reaction field45 modeling solvation in CH2Cl2. Graphical outputs of the computational results were generated with the Gauss View software program (ver. 3.09) developed by Semichem, Inc.46 Acknowledgment. The support of the Robert A. Welch Foundation (K.M.K., Grant E-680) and the Texas Advanced Research program to K.M.K. under Grant No. 003652-00182001 is gratefully acknowledged. This work was also supported by a Discovery Research Grant (DP0208776) to M.J.C. and J.R.R. from the Australian Research Council and a Grant-in-

8774 J. Phys. Chem. B, Vol. 111, No. 30, 2007 Aid (No. 17750039) to K.O. from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank the Australian Partnership for Advanced Computing (APAC) for provision of the computational resources. Supporting Information Available: Complete citation for ref 44, synthesis and characterization of the metalated derivatives of compounds 4-12 (S2-S13), analytical data for compounds 13a,b,c,e (S14-S15), and ESR spectra of [PQ(5NO2)]Zn (9c), [PQ(6NO2)]Zn (11c), and [PQ(NO2)2]Zn (12c) together with their simulation spectra, hfc values, and spin densities (S16S18). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051. (2) Anderson, H. L. Chem. Commun. 1999, 2323. (3) Nakamura, Y.; Aratani, N.; Shinokubo, H.; Takagi, A.; Kawai, T.; Matsumoto, T.; Yoon, Z. S.; Kim, D. Y.; Ahn, T. K.; Kim, D.; Muranaka, A.; Kobayashi, N.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 4119. (4) (a) Screen, T. E. O.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.; Anderson, H. L. J. Mater. Chem. 2003, 13, 2796. (b) McEwan, K. J.; Fleitz, P. A.; Rogers, J. E.; Slagle, J. E.; McLean, D. G.; Akdas, H.; Katterle, M.; Blake, I. M.; Anderson, H. L. AdV. Mater. 2004, 16, 1933. (5) Ikeda, T.; Lintuluoto, J. M.; Aratani, N.; Yoon, Z. S.; Kim, D.; Osuka, A. Eur. J. Org. Chem. 2006, 3193. (6) Duncan, T. V.; Susumu, K.; Sinks, L. E.; Therien, M. J. J. Am. Chem. Soc. 2006, 128, 9000. (7) Aratani, N.; Takagi, A.; Yanagawa, Y.; Matsumoto, T.; Kawai, T.; Yoon, Z. S.; Kim, D.; Osuka, A. Chem. Eur. J. 2005, 11, 3389. (8) Yoon, D. H.; Lee, S. B.; Yoo, K.-H.; Kim, J.; Lim, J. K.; Aratani, N.; Tsuda, A.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2003, 125, 11062. (9) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735. (10) Kang, B. K.; Aratani, N.; Lim, J. K.; Kim, D.; Osuka, A.; Yoo, K.-H. Chem. Phys. Lett. 2005, 412, 303. (11) Oike, T.; Kurata, T.; Takimiya, K.; Otsubo, T.; Aso, Y.; Zhang, H.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2005, 127, 15372. (12) Crossley, M. J.; Burn, P. L. J. Chem. Soc., Chem. Commun. 1991, 1569. (13) Sendt, K.; Johnston, L. A.; Hough, W. A.; Crossley, M. J.; Hush, N. S.; Reimers, J. R. J. Am. Chem. Soc. 2002, 124, 9299. (14) Ou, Z.; E, W.; Shao, J.; Burn, P. L.; Sheehan, C. S.; Walton, R.; Kadish, K. M.; Crossley, M. J. J. Porphyrins Phthalocyanines 2005, 9, 142. (15) Hush, N. S.; Reimers, J. R.; Hall, L. E.; Johnston, L. A.; Crossley, M. J. Ann. N. Y. Acad. Sci. 1998, 852, 1. (16) Lu¨, T. X.; Reimers, J. R.; Crossley, M. J.; Hush, N. S. J. Phys. Chem. 1994, 98, 11878. (17) Reimers, J. R.; Bilic, A.; Cai, Z.-L.; Dahlbom, M.; Lambropoulos, N. A.; Solomon, G. C.; Crossley, M. J.; Hush, N. S. Aust. J. Chem. 2004, 57, 1133.

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