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May 5, 2017 - electronic structures and spectroscopic signatures of these systems. ..... Michl's notation) orbital should be more stable in N-confused...
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Magnetic Circular Dichroism of Transition-Metal Complexes of Perfluorophenyl-N-Confused Porphyrins: Inverting Electronic Structure through a Proton Samantha Doble,† Allen J. Osinski,‡ Shelby M. Holland,§ Julia M. Fisher,§ G. Richard Geier, III,*,§ Rodion V. Belosludov,*,∥ Christopher J. Ziegler,*,‡ and Victor N. Nemykin*,†,⊥ †

Department of Chemistry and Biochemistry, University of MinnesotaDuluth, Duluth, Minnesota 55812, United States Department of Chemistry, University of Akron, Akron, Ohio 44325-3601, United States § Department of Chemistry, Colgate University, Hamilton, New York 13346, United States ∥ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ⊥ Department of Chemistry, University of Manitoba, Winnipeg, MB R3T 2N2, Canada ‡

S Supporting Information *

ABSTRACT: Neutral and deprotonated anionic Ni(II), Pd(II), Cu(II), and Cu(III) complexes of tetrakis(perfluorophenyl)-N-confused porphyrin (PF-NCP) were prepared and investigated by UV−visible and magnetic circular dichroism (MCD) spectroscopies. As in the previously reported Ni(II) adduct of tetraphenyl N-confused porphyrin, we observe sign reverse (positive to negative intensities with increasing energy) features in the MCD spectra of the neutral Ni(II), Pd(II), and Cu(II) complexes of PF-NCP, which is indicative of rare ΔHOMO < ΔLUMO relationships. Upon deprotonation of Ni(II), Pd(II), and Cu(II) complexes, these features revert to those of more typical porphyrin MCD spectra consistent with a ΔHOMO > ΔLUMO condition. The Cu(III) PF-NCP complex shows features similar to those of the deprotonated divalent metal systems. Spectroscopic features in all target complexes as well as previously published metal-free and Ni(II) NCP systems were correlated with the density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Calculation data are consistent with the tautomeric rearrangement of the electronic structures of NCP cores playing dominant roles, with smaller contribution from the central metal ions in the observed optical and magneto-optical properties. This is true for all described NCP systems to date, as they affect the stabilization/destabilization of the N-confused porphyrin-centered Gouterman orbitals.

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HOMO−1, while ΔLUMO is the energy difference between the macrocycle-centered LUMO and LUMO+1). In normal metal-containing porphyrins, the ΔLUMO is zero (the orbitals are degenerate as the effective symmetry of such porphyrins is D4h or C4v) and the ΔHOMO is nonzero, which results in the typical ΔLUMO < ΔHOMO relationship. In the low-symmetry porphyrinoids such as chlorins and bacteriochlorins, the LUMO and LUMO+1 are nondegenerate and in many cases the ΔLUMO > ΔHOMO energy relationship is observed, which results in readily observable spectroscopic changes in the magnetic circular dichroism (MCD) spectra.23−29 N-confused porphyrin (NCP, Figure 1) or 2-aza-21carbaporphyrin is one of the most thoroughly studied isomers of porphyrin.30,31 In NCP, one of the pyrrole rings is inverted

INTRODUCTION The electronic structure of normal porphyrins has been extensively studied,1−3 but much work remains in the investigation of the electronic structures of isomers and analogues of porphyrins. Isomers of porphyrins include ring rearranged macrocycles, such as porphycene, while analogues of porphyrins are skeletal modified macrocycles such as expanded, contracted or ring modified systems.4−17 Both types of macrocycles share many of the characteristics of normal porphyrin. However, modification of the ring alters the electronic structure of the isomers and analogues of porphyrin, and we have been investigating these systems for the past several years.18−22 In particular, when asymmetry is incorporated into the macrocycle skeleton, an inversion of the magnitude of the ΔHOMO versus the ΔLUMO is observed (within the borders of Gouterman’s four-orbital model23−25 and Michl’s perimeter model,26−29 ΔHOMO is the energy difference between the macrocycle-centered HOMO and © XXXX American Chemical Society

Received: March 27, 2017 Revised: April 26, 2017

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Figure 1. Structures of porphyrin, N-confused porphyrin (showing both inner and outer tautomers), and complexes 1−4.

form.21 Thus, MCD spectra available on NCPs studied so far are indicative of a complex interplay between type of the NCP core (external versus internal tautomer) as well as type of the central ion (proton versus nickel). The small set of NCPs studied by MCD spectroscopy to date, however, precludes observation of general trends in this class of porphyrinoids. In this report, we expand the range of studied NCPs, with an MCD study of the nickel, palladium and copper complexes of tetra(perfluorophenyl)-N-confused porphyrin (PF-NCP, Figure 1).43 By introducing strongly electron-withdrawing mesoperfluorophenyl groups, we sought to determine whether the sign-reversed MCD spectrum of NiNCP was maintained or whether the spectrum was restored to the typical ΔHOMO > ΔLUMO relationship. Thus, we investigated the Ni(II) (1) and Pd(II) (2) complexes of PF-NCP in both their protonated and deprotonated states. Additionally, we investigated the Cu(II) complex (3) of PF-NCP in both forms, and were able to probe the effect of metal oxidation state on the MCD spectra via the Cu(III) complex (4, Figure 1). DFT calculations on the new MPF-NCP systems as well as previously reported NiNCP, [NiNCP]−, and H2NCP were conducted at the same level of theory and allowed us to generalize to some extent trends in electronic structures and spectroscopic signatures of these systems.

compared to normal porphyrin; a carbon atom occupies the internal binding position while a nitrogen atom is located on the periphery of the pyrrole ring. Although modified from normal porphyrin, NCP retains an 18-electron annulene ring structure. Even though the overall structures of normal porphyrin and NCP are similar, the inversion of one of the pyrrole rings alters the electronic structure of NCP, which results in different chemical and physical properties.32−37 In particular, NCP exhibits two isolable tautomers: an internal tautomer with three hydrogen atoms in the core of the ring and an external tautomer with a hydrogen atom on the external nitrogen atom and only two hydrogen atoms in the core.38,39 On the basis of the MCD spectra and theoretical calculations on cis- and trans-isomers of Cu(III) complexes of doubly Nconfused porphyrins, in 2008, Kobayashi and co-workers suggested that the formation of N-confused macrocycle should result in A ΔLUMO > ΔHOMO energy relationship.40 This would lead to a positive-to-negative in ascending energy sequence of the MCD signals in Q-band region.40 Later, we showed that the MCD spectra of the metal-free meso-phenyl20 and meso-(4-methoxycarbonyl)phenyl41 NCPs (H2NCP and H2NCP(CO2Me), respectively) have a typical for porphyrins ΔLUMO < ΔHOMO relationship in the Q-band region, which is not altered by protonation or deprotonation of the nitrogen centers or by the polarity of the solvent. In contrast, we showed that the nickel meso-phenyl NCP (NiNCP) and its externally methylated variant (NiNCPMe) exhibit sign-reversed MCD spectra in the Q-band region, indicating that the ΔHOMO < ΔLUMO condition is observed.21 Although the sign-reversed MCD spectrum is seen with other porphyrinoids including corroles and reduced porphyrins,18,19,42 the MCD spectrum of NiNCP can be reverted to the typical for porphyrins ΔHOMO > ΔLUMO relationship by simple deprotonation of the external NH proton; this cannot occur in the methylated

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EXPERIMENTAL SECTION Materials and Instrumentation. All solvents were purchased from commercial sources and dried using standard approaches prior to experiments. DDQ and a methanol solution of (NBu4)OH were purchased from Aldrich used without further purification. Metal-free perfluorophenyl-Nconfused porphyrin was synthesized using a two-step, oneflask method recently described by Geier and co-workers.44 Metalation of perfluorophenyl-N-confused porphyrin by copper B

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Figure 2. UV−vis and MCD spectra of 1 (top left), 2 (top right), 3 (bottom left), and 4 (bottom right) in dichloromethane.

porphyrins with transition metals that have six or more dorbital electrons, the absorption bands are also blue-shifted relative to the freebase; in NiTPP at the B-band appears at 415 nm with one Q-band absorption at 524 nm.55 For the perfluoronated variant NiPF-TPP, the bands are further blueshifted, with a B-band at 407 nm with two Q-band absorptions at 526 and 559 nm.56 The confusion of the pyrrole ring in H2NCTPP significantly impacts the spectra versus normal H2TPP as well as induces tautomer dependent spectroscopic properties.38 We have discussed the absorption spectroscopy of both tautomeric forms previously.39 Upon metal insertion, the absorption profile depends greatly on whether the macrocycle is acting as a trianion (internal tautomer) or a dianion (external tautomer). NCP compounds have red-shifted B-bands relative to normal porphyrins.20,57,58 For instance, NiNCP, where the ring is acting as a dianion, has a B-band at 425 nm with a shoulder at 458 nm, and multiple Q-band absorptions at 555, 594, 655, 716, and 789 nm.21 When deprotonated, the bands shift as the macrocycle becomes trianionic type, and the B-band appears at 424 nm along generally red-shifted Q bands at 537, 590, 667, and 719 nm.21 Figure 2 shows the UV−visible spectra of PF-NCP complexes 1 - 4 along with their MCD spectra. Compared to NiNCP, in Ni(II) complex 1, the B-band shifts to 421 nm, as do the higher energy transitions in Q-band region (564 nm with a 592 nm shoulder compared to 594 nm with a 665 nm in NiNCP).21 The lower energy transitions in Q-band region appear shifted to the red: 735 and 808 nm compared to 716 and 789 nm for NiNCP.21 The other metal complexes 2 - 4 exhibit quite different absorption spectra in spite of the fact that complexes 2 and 3 have divalent d8 and d9 transition metal ions and complex 4 has a trivalent d8 central ion. In Pd(II) complex 2, we observe a split B-band (422 and 442 nm) and five transitions in Q-band region at 537, 578, 638, 695, and 761 nm. Unlike in Ni(II) complex 1, all transitions in Q-band region are of comparable intensity. In Cu(II) complex 3, the B-band region is dominated by broad red-shifted (440 nm) band, while four clear transitions can be observed in the Q-band region at 543, 587, 676, and 735 nm. We also sought to investigate an Nconfused porphyrin system where the macrocycle acts as a

and palladium was performed using a modified method developed by Furuta.43 Nickel complex 1 was prepared using a modification of a method initially reported by Chen, Tung and co-workers for a different NCP45 (see Supporting Information). UV−vis−NIR data were obtained on a JACSO V-670 spectrometer with dichloromethane as the solvent. MCD data were recorded using an OLIS DCM 17 CD spectropolarimeter using a permanent 1.4 T DeSa magnet. The spectra were recorded twice for each sample, once with a parallel field and again with an antiparallel field, and their intensities were expressed by molar ellipticity per tesla. Computational Aspects. All computations were performed using the Gaussian 09 software package running under Windows or UNIX OS.46 Molecular orbital contributions were compiled from single point calculations using the QMForge program.47 All geometries were optimized without any symmetry restrictions. Frequencies were calculated for all optimized geometries in order to ensure that final geometries represent minima on the potential energy surface. In all calculations, the hybrid B3LYP exchange-correlation functional48 and 6-31G(d) basis set49 were used for all atoms. In addition, DFT and TDDFT calculations of all MPF-NCP and [MPF-NCP]− complexes were conducted using hybrid TPSSh exchange-correlation functional to study an exchange-correlation functional dependence (Supporting Information). TDDFT calculations were conducted for the first 50 excited states in order to ensure that all π−π* transitions of interest were accounted for. Solvent effects were modeled by PCM approach using DCM as a solvent.50

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RESULTS AND DISCUSSION Comparing the absorption spectra of both free base and the metal complexes of 5,10,15,20-tetra(pentafluorophenyl)-porphyrin (PF-TPP) to normal 5,10,15,20-tetraphenylporphyrin (TPP) and its metal adducts, the most significant changes are blue shifts of the absorption bands in the former compound.51,52 H2TPP exhibits an intense B-band at ∼420 nm and four weak Q-band absorptions at 515, 550, 591, and 647, while in H2PF-TPP these same bands shift to ∼410 nm for the B-band with the Q-band absorptions at 504, 539, 582, and 638 nm.53,54 For hypso-type porphyrins, observed in metalloC

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The Journal of Physical Chemistry A trianion without deprotonation of the external nitrogen position. This can occur when N-confused porphyrin binds trivalent metal ions in its internally protonated form. The earliest form of this type of binding was seen in the Ag(III) adduct, where the metal ion resides in the core of ring.59 An analogous structure can be achieved for copper upon oxidation of the Cu(II) complex 3. Furuta reported that the Cu(III) complex is produced upon oxidation of the Cu(II) compound with DDQ.60 As shown in Figure 3, titration of DDQ into a

Figure 4. Transformation of 1 (top left) into 1−, 2 (top right) into 2−, and 3 (bottom) into 3− during titration with tetrabutylammonium hydroxide in dichloromethane.

Figure 3. Transformation of 3 into 4 with DDQ in dichloromethane.

formation of anionic species have clear central-metal dependence. In case of the Ni(II) complex 1, the most intense peak in the B-band region increases in shifts to 419 nm and a prominent shoulder at ∼480 nm appears in the spectrum. The Q-band region also alters significantly with all observable peaks shifting to higher energies at 537, 587, 681, and 741 nm. Similar changes were observed for Cu(II) complex 3 upon its deprotonation. In particular, the most intense peak in the Bband region increases in intensity and shifts slightly to 452 nm with a formation of clear shoulder at ∼475 nm. In the Q-band region the most intense bands shift to higher energies (540, 582, 649, and 683 nm) although a broad low intensity shoulder appears at ∼760 nm in this case. Upon deprotonation of Pd(II) complex 2, the initial split B-band converts to three peaks (399, 440, and 462 nm) and the five initial peaks in the Q-band region undergo red shifts and transform into four bands at 528, 567, 653, and 701 nm. This behavior is significantly different from that of the nickel and copper analogues. If there is a common trend to the changes of these spectra upon deprotonation, it is a general hypsochromic shift of the Qband transitions. MCD spectra of anionic complexes 1−−3− are shown in Figure 5 and are indicative of the normal for porphyrins ΔHOMO > ΔLUMO energy relationship, which was also observed in [NiNCP]−.21 Indeed, in all cases examined in this study, anionic transition-metal PF-NCPs have negative MCD signals associated with the lowest energy bands in corresponding UV−vis spectra. Thus, all divalent transition-metal NCPs studied so far exhibit the same behavior, i.e. they possess the rarely observed inverse (ΔHOMO < ΔLUMO) relationship in neutral state and normal for porphyrins (ΔHOMO > ΔLUMO) relationship in anionic state.

solution of Cu(II) complex 3 affords the oxidized complex 4. The B-band region in 4 is dominated by three overlapping transitions at 446, 409, and 378 nm with band at 446 nm bathochromically shifted compared to complex 3. The Q-band region in 4 is dominated by transitions observed at 521, 556, 633, and 724 nm. Overall, UV−vis spectra of the neutral complexes 1−4 are clearly indicative of their central-metal dependency. The MCD spectra of the neutral complexes 1−4 can be seen in Figure 2. For Ni(II) complex 1, MCD spectrum in the Qband region can be described by four Faraday B-terms observed at 560, 594, 738, and 811 nm. The two lowest energy signals are very weak and have positive amplitudes, while bands at 594 and 560 nm are much stronger and have positive and negative amplitudes, respectively. The MCD spectrum of 1 in the Bband region is dominated by an intense positive signal at 430 nm. Similarly, for the Pd(II) complex 2, low-energy Q-band region of MCD spectrum can be described as a superposition of three positive signals observed at 643, 697, and 761 nm along with two negative signals at 543 and 578 nm (Figure 2). Again, the MCD spectrum of 2 in B-band region is dominated by a positive signal at 447 nm. Cu(II) complex 3 has a MCD spectrum close to that in compound 2 with positive bands observed at 676 and 738 nm and negative signals detected at 541 and 588 nm. Because of the extremely low intensity for the Cu(III) complex 4, we were not able to clearly define a sign of the MCD transitions associated with the broad, low-energy band at 724 nm observed in the UV−vis spectrum. In contrast, a clear positive signal at 631 nm and negative bands at 583 and 552 nm were observed in the MCD spectrum in the Q-band region of 4, which resembles the spectroscopic signature of complex 1 with a similar d8 configuration. Overall, MCD spectra of the complexes 1−4 are indicative of the unusual ΔHOMO < ΔLUMO energy relationship as evidenced by reverse sign sequence (positive to negative in ascending energy) features. Titration of the neutral complexes 1−3 with a strong organic base removes the external proton from the inverted pyrrole ring and leads to formation of anionic 1−, 2−, and 3− complexes. These anionic complexes have significantly different UV−vis spectra (Figure 4). Spectroscopic changes associated with the

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DFT AND TDDFT CALCULATIONS To further elucidate the electronic structures in 1−4 and anionic 1 − −3 − complexes, we carried out theoretical investigations on these systems using hybrid B3LYP and TPSSh exchange-correlation functionals, which have proven to be reliable for determining the vertical excitation energies of porphyrins, phthalocyanines, and their analogues.18−22,61 The B3LYP-predicted energy diagram for all target compounds is D

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Figure 5. UV−vis and MCD spectra of 1− (top), 2− (bottom left), and 3− (bottom right) in dichloromethane.

shown in Figure 6, while representative frontier orbital images are pictured in Figures 7 and 8. Figure S1, in the Supporting Information, depicts the energy diagram for TPSSh calculations on the same systems. Both exchange-correlation functionals give very similar results for NCPs 1−4 and anions 1−−3−. In particular, both functionals are indicative of the presence of Gouterman’s type frontier orbitals in diamagnetic Ni(II) and Pd(II) compounds 1, 2, 1−, and 2−. Indeed, DFT-predicted HOMO resembles Gouterman’s “a2u” orbital (“s” orbital in Michl’s perimeter model for porphyrins), HOMO−1 resembles Gouterman’s “a1u” orbital (“a” orbital in Michl’s notation), while LUMO and LUMO+1 resemble Gouterman’s “eg” orbitals (pair of “-a” and “-s” orbitals in Michl’s notation) with the LUMO+1 orbital having a node at the N-confused

Figure 6. DFT-predicted (B3LYP) partial energy diagram NCPs 1−4 and 1−−3−.

Figure 7. DFT-predicted (B3LYP) Gouterman’s type frontier orbitals for 1, 2, 1−, and 2−. E

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Figure 8. DFT-predicted (B3LYP) Gouterman’s type and copper-centered “dx2−y2” frontier orbitals for 3, 3−, and 4.

nitrogen atom (Figure 7). In the case of the copper-containing complexes 3, 3−, and 4, the expected frontier in-plane orbital with significant contribution from the copper ion’s “dx2‑y2” orbital was predicted by DFT calculations. For the diamagnetic Cu(III) complex 4 such orbital was predicted to be LUMO (Figure 8), which correlates well with the metal-centered oxidation of the Cu(II) complex 3. In case of the paramagnetic complexes 3 and 3−, the copper-centered orbital was predicted to be LUMO+2 (β-set) and HOMO−1 (α-set) or HOMO (TPSSh for 3 only), which reflects a large spin-polarization in 3 and 3− (Figure 8 and Supporting Information, Figure S1). Independently of the exchange-correlation functional that was used, DFT predicts major spin density localization at the Cu(II) center, which agrees well with the experimental EPR

Figure 9. Comparison of DFT-predicted (B3LYP) Gouterman’s type orbital energy levels and orbital diagrams for previously reported NCP and TPP compounds and compounds 1 and 4.

Figure 10. Experimental and TDDFT-predicted UV−vis spectra of N-confused porphyrins 1−4 in dichloromethane. F

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Figure 11. Experimental and TDDFT-predicted UV−vis spectra of N-confused porphyrins 1−−3− in dichloromethane.

calculations at both B3LYP and TPSSh levels suggest that the unoccupied Gouterman’s “eg” (−a and −s in Michl’s notation) orbitals in H2NCP-i and Cu(III) NCP 4 are stabilized compared to the parent H2TPP and NiPF-TPP. Simple perturbation theory predicts that Gouterman’s “a1u” (a in Michl’s notation) orbital should be more stable in N-confused systems compared to the parent porphyrins, but DFT predicts that such stabilization in H2TPP/H2NCP-e, NiTPP/NiNCP, and NiPF-TPP/1 is negligibly small (0.02−0.05 eV). Thus, to explain observed trends in electronic structure and spectroscopy of N-confused porphyrins, we need consider additional factors such as influence of the central ion and the nature of Nconfused nitrogen atom (pyrrolic N−H type versus aromatic N: type of the nitrogen atom). Indeed, when H2TPP/H2NCPe, NiTPP/NiNCP, and NiPF-TPP/1 pairs with external N−H fragments in N-confused ring are compared, the destabilization of Gouterman’s “a2u” (s in Michl’s notation) orbital and stabilization of one of “eg” (−a in Michl’s notation) orbital is significantly larger in the case of nickel complexes, which can be traced back to the central ion contribution to these orbitals. As a result, ΔHOMO < ΔLUMO in all nickel systems, while the ΔHOMO > ΔLUMO relationship holds for all metal-free NCPs studied so far. Transformation of the external pyrrolic type N−H fragment into a aromatic-type N: nitrogen atom (H2TPP/H2NCP-i and NiTPP/4 pairs) dramatically reduces ΔLUMO value compared to all other NCPs studied so far. The ΔHOMO gap in NCP 4 with trivalent central ion is very small compared to the same gap in H2NCP-i tautomer. Finally, destabilization of the −a orbital (one with nonzero contribution from the external N-confused nitrogen), upon deprotonation of NiNCP and 1 is about twice as large compared to the destabilization of the −s orbital (one with node at the external N-confused nitrogen), which reduces ΔLUMO gap in these complexes and inverts the MCD signals to the normal porphyrin trend.62−64 We also used TDDFT methods to predict the UV−vis spectra of the compounds investigated in this study with the results shown in Figures 10 and 11 (B3LYP calculations) and Supporting Information Figures S4 and S5 (TPSSh calculations). For both the B3LYP and TPSSh calculations, we observe good agreement between theory and experiment. The

spectra available for CuNCPs. In case of all divalent complexes, DFT predicts rather minor influence of the central metal on the energies of Gouterman’s type frontier, NCP-centered πorbitals. DFT calculations also predict correctly the ΔHOMO < ΔLUMO relationship for the neutral complexes 1−4 and the ΔHOMO > ΔLUMO relationship for anionic complexes 1−− 3−. To gain insight into the general trends of MCD spectroscopy of NCPs, we have conducted DFT calculations on NiNCP, NiTPP, NiPF-TPP, H2TPP, H2NCP-i, and H2NCP-e compounds (NCP-i = “internal” tautomer of N-confused porphyrin, and NCP-e = “external” tautomer of N-confused porphyrin) at the same level of theory (B3LYP and TPSSh exchangecorrelation functionals). The generalized energy diagrams for such comparative calculations are shown in the Supporting Information, Figures S2 and S3. First, it should be noted that independent of the exchange-correlation functional used, all DFT calculations were able to correctly predict ΔHOMO/ ΔLUMO relationship and sign of the MCD transitions in the Q-band region, which proves the validity of the calculations. Second, it seems that introduction of the perfluorophenyl groups instead of phenyl substituents into porphyrin and Nconfused core has a simple electron-withdrawing effect. Indeed, the energy differences between respective s (“a2u”), a (“a1u”), −a (“eg”), and −s (“eg”) orbitals in NiNCP and 1 (0.49, 0.35, 0.43, and 0.42 eV) as well as NiTPP and NiPF-TPP (0.53, 0.36, 0.43, and 0.43 eV) are very close to each other. The only difference between frontier orbitals in NiTPP and NiPF-TPP is the order of the occupied s (“a2u”), a (“a1u”) orbitals as the former orbital is the HOMO in NiTPP and HOMO−1 in NiPF-TPP. Next, according to the perturbation theory approach, introduced in application to MCD spectroscopy of doubly N-confused porphyrins by Kobayashi and co-workers,40 inversion of the pyrrole ring in the porphyrin core should result in destabilization of Gouterman’s “a2u“ and one of “eg“ orbitals (s and −s orbitals in Michl’s notation) and stabilization of Gouterman’s “a1u” and another “eg“ orbitals (a and −a orbitals in Michl’s notation, Figure 9). Our DFT calculations and measurements of spectroscopic properties of N-confused porphyrins indicate that such a simplified approach can be misleading. For instance, DFT G

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The Journal of Physical Chemistry A TDDFT-predicted B-band regions in complexes 1−3 and 1‑−3‑ should be primarily dominated by the HOMO−1 → LUMO, HOMO−1 → LUMO+1, and HOMO → LUMO+1 singleelectron excitations which fit well within the borders of Gouterman’s four-orbital model. For Cu(III) compound 4, the B-band region is primarily dominated by single-electron excitations from the HOMO−1 → LUMO+2, HOMO−1 → LUMO+1, HOMO → LUMO+2, and HOMO → LUMO+1 as the LUMO in this compound is copper-centered “dx2‑y2” orbital. All transitions listed above can be characterized as π−π* transitions and adhere to the Gouterman four orbital paradigm. For both functionals, the TDDFT-predicted energies in the Qband region for 1−4 and 1‑−3‑ are slightly overestimated. However, as expected from the Gouterman’s four-orbital model, TDDFT predicts that the Q-band region in respective complexes should be dominated by the HOMO → LUMO (complexes 1−3 and 1‑−3‑) or HOMO → LUMO+1 (compound 4) single-electron excitations.

provided by the Miller-Cochran Fund, administered by Colgate University. R.V.B. is grateful for the support of the HITACHI SR16000-M1 supercomputing facility by the Computer Science Group and E-IMR center at the Institute for Materials Research, Tohoku University, Sendai.

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(1) Ngaojampa, C.; Namuangruk, S.; Surakhot, Y.; Promarak, V.; Jungsuttiwong, S.; Kungwan, N. Influence of Phenyl-Attached Substituents on the Vibrational and Electronic Spectra of MesoTetraphenylporphyrin: A DFT Study. Comput. Theor. Chem. 2015, 1062, 1−10. (2) Kay, C. W. M. The Electronic Structure of the Photoexcited Triplet State of Free-Base (Tetraphenyl)porphyrin by Time-Resolved Electron−Nuclear Double Resonance and Density Functional Theory. J. Am. Chem. Soc. 2003, 125, 13861−13867. (3) Nguyen, K. A.; Day, P. N.; Pachter, R.; Tretiak, S.; Chernyak, V.; Mukamel, S. Analysis of Absorption Spectra of Zinc Porphyrin, Zinc Meso-Tetraphenylporphyrin, and Halogenated Derivatives. J. Phys. Chem. A 2002, 106, 10285−10293. (4) Vogel, E.; Köcher, M.; Schmickler, H.; Lex, J. Porphycene-A Novel Porphin Isomer. Angew. Chem., Int. Ed. Engl. 1986, 25, 257− 259. (5) Dolphin, D.; Johnson, A. W.; Leng, J.; van den Broek, P. The Base-Catalysed Cyclisations of 1,19-Dideoxybiladienes-Ac. J. Chem. Soc. C 1966, 880−884. (6) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoo, M.-C.; Kim, D.; Osuka, A. Tribenzosubporphines: Synthesis and Characterization. Angew. Chem., Int. Ed. 2006, 45, 961−964. (7) Xue, Z.; Shen, Z.; Mack, J.; Kuzuhara, D.; Yamada, H.; Okujima, T.; Ono, N.; You, X.-Z.; Kobayashi, N. A Facile One-Pot Synthesis of Meso-Aryl-Substituted [14]Triphyrin(2.1.1). J. Am. Chem. Soc. 2008, 130, 16478−16479. (8) Sessler, J. L.; Davis, J. M. Sapphyrins: Versatile Anion Binding Agents. Acc. Chem. Res. 2001, 34, 989−997. (9) Panda, P. K.; Kang, Y.-J.; Lee, C.-H. A Benzodipyrrole-Derived Sapphyrin. Angew. Chem., Int. Ed. 2005, 44, 4053−4055. (10) Inokuma, Y.; Matsunari, T.; Ono, N.; Uno, H.; Osuka, A. A Doubly N-Fused Benzohexaphyrin and Its Rearrangement to a Fluorescent Macrocycle upon DDQ Oxidation. Angew. Chem., Int. Ed. 2005, 44, 1856−1860. (11) Wu, D.; Descalzo, A. B.; Weik, F.; Emmerling, F.; Shen, Z.; You, X.-Z.; Rurack, K. A Core-Modified Rubyrin with Meso-Aryl Substituents and Phenanthrene-Fused Pyrrole Rings: A Highly Conjugated near-Infrared Dye and Hg2+ Probe. Angew. Chem., Int. Ed. 2008, 47, 193−197. (12) Ishida, M.; Kim, S.-J.; Preihs, C.; Ohkubo, K.; Lim, J. M.; Lee, B. S.; Park, J. S.; Lynch, V. M.; Roznyatovskiy, V. V.; Sarma, T.; et al. Protonation-Coupled Redox Reactions in Planar Antiaromatic MesoPentafluorophenyl-Substituted O-Phenylene-Bridged Annulated Rosarins. Nat. Chem. 2013, 5, 15−20. (13) Chandrashekar, T. K.; Prabhuraja, V.; Gokulnath, S.; Sabarinathan, R.; Srinivasan, A. Fused Core-Modified Meso-Aryl Expanded Porphyrins. Chem. Commun. 2010, 46, 5915−5917. (14) Karthik, G.; Lim, J. M.; Srinivasan, A.; Suresh, C. H.; Kim, D.; Chandrashekar, T. K. Conformational Change from a Twisted FigureEight to an Open-Extended Structure in Doubly Fused 36π CoreModified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity. Chem. - Eur. J. 2013, 19, 17011− 17020. (15) Ghosh, A.; Srinivasan, A.; Suresh, C. H.; Chandrashekar, T. K. [40]π Fused and Nonfused Core-Modified Nonaphyrins: Syntheses and Structural Diversity. Chem. - Eur. J. 2016, 22, 11152−11155. (16) Okujima, T.; Ando, C.; Agrawal, S.; Matsumoto, H.; Mori, S.; Ohara, K.; Hisaki, I.; Nakae, T.; Takase, M.; Uno, H.; et al. Template Synthesis of Decaphyrin without Meso-Bridges: Cyclo[10]pyrrole. J. Am. Chem. Soc. 2016, 138, 7540−7543.

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CONCLUSIONS In conclusion, the transition-metal perfluorophenyl-N-confused porphyrin complexes 1−4 exhibit the unusual reversed sign sequence MCD spectra as compared to other porphyrinoid variants. The deprotonation of the external pyrrolic hydrogen in complexes 1- 3, changes the electronic structure from rare ΔLUMO > ΔHOMO to the usual ΔLUMO < ΔHOMO relationship which we have previously shown to occur with the deprotonation of NiNCP.21 DFT calculations on all NCPs studied by MCD spectroscopy so far are in agreement with the experimental data and are indicative of the important role of not only the central metal ion but also tautomeric type of the N-confused π-system (i.e., external versus internal tautomer).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b02908. Synthetic details on preparation of the target NCPs. DFT-predicted molecular orbital diagrams for 1−4. TDDFT (TPSSh) predicted UV−vis spectra of 1−4 and 1−−3− (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(G.R.G.) E-mail: [email protected]. *(R.V.B.) E-mail: [email protected]. *(C.J.Z.) E-mail: [email protected]. *(V.N.N.) E-mail: [email protected]. ORCID

G. Richard Geier III: 0000-0001-9751-6190 Christopher J. Ziegler: 0000-0002-0142-5161 Victor N. Nemykin: 0000-0003-4345-0848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.J.Z. acknowledges the University of Akron for support of this research. Generous support from the NSF CHE-1464711, Minnesota Supercomputing Institute, NSERC, CFI, University of Manitoba, and WestGrid Canada to V.N.N. is greatly appreciated. A summer research fellowship for SMH was H

DOI: 10.1021/acs.jpca.7b02908 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A (17) Kuzuhara, D.; Mack, J.; Yamada, H.; Okujima, T.; Ono, N.; Kobayashi, N. Synthesis, Structures, and Optical and Electrochemical Properties of Benzoporphycenes. Chem. - Eur. J. 2009, 15, 10060− 10069. (18) Ziegler, C. J.; Sabin, J. R.; Geier, G. R., III; Nemykin, V. N. The First TDDFT and MCD Studies of Free Base Triarylcorroles: A Closer Look into Solvent-Dependent UV-Visible Absorption. Chem. Commun. 2012, 48, 4743−4745. (19) Rhoda, H. M.; Crandall, L. A.; Geier, G. R., III; Ziegler, C. J.; Nemykin, V. N. Combined MCD/DFT/TDDFT Study of the Electronic Structure of Axially Pyridine Coordinated Metallocorroles. Inorg. Chem. 2015, 54, 4652−4662. (20) Ziegler, C. J.; Erickson, N. R.; Dahlby, M. R.; Nemykin, V. N. Magnetic Circular Dichroism Spectroscopy of N- Confused Porphyrin and Its Ionized Forms. J. Phys. Chem. A 2013, 117, 11499−11508. (21) Sripothongnak, S.; Ziegler, C. J.; Dahlby, M. R.; Nemykin, V. N. Controllable and Reversible Inversion of the Electronic Structure in Nickel N-Confused Porphyrin: A Case When MCD Matters. Inorg. Chem. 2011, 50, 6902−6909. (22) Rhoda, H. M.; Akhigbe, J.; Ogikubo, J.; Sabin, J. R.; Ziegler, C. J.; Brückner, C.; Nemykin, V. N. Magnetic Circular Dichroism Spectroscopy of Meso-Tetraphenylporphyrin-Derived Hydroporphyrins and Pyrrole-Modified Porphyrins. J. Phys. Chem. A 2016, 120, 5805−5815. (23) Gouterman, M. J. The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. III. (24) Gouterman, M. Spectra of Porphyrins. J. Mol. Spectrosc. 1961, 6, 138−163. (25) Seybold, P. G.; Gouterman, M. Porphyrins. J. Mol. Spectrosc. 1969, 31, 1−13. (26) Waluk, J.; Michl, J. Perimeter Model and Magnetic Circular Dichroism of Porphyrin Analogs. J. Org. Chem. 1991, 56, 2729−2735. (27) Michl, J. Magnetic Circular Dichroism of Cyclic π-Electron Systems. 1. Algebraic Solution of the Perimeter Model for the A and B Terms of High-Symmetry Systems with a (4N + 2)-Electron [N]annulene Perimeter. J. Am. Chem. Soc. 1978, 100, 6801−6811. (28) Michl, J. Magnetic Circular Dichroism of Cyclic π-Electron Systems. 2. Algebraic Solution of the Perimeter Model for the B Terms of Systems with a (4N + 2)-Electron [N]annulene Perimeter. J. Am. Chem. Soc. 1978, 100, 6812−6818. (29) Michl, J. Magnetic Circular Dichroism of Cyclic π-Electron Systems. 3. Classification of Cyclic.pi. Chromophores with a (4N + 2)Electron [N]annulene Perimeter and General Rules for Substituent Effects on the MCD Spectra of Soft Chromophores. J. Am. Chem. Soc. 1978, 100, 6819−6824. (30) Chmielewski, P. J.; Latos-Grażyński, L.; Rachlewicz, K.; Glowiak, T. Tetra-P-Tolylporphyrin with an Inverted Pyrrole Ring: A Novel Isomer of Porphyrin. Angew. Chem., Int. Ed. Engl. 1994, 33, 779−781. (31) Furuta, H.; Asano, T.; Ogawa, T. N-Confused Porphyrin”: A New Isomer of Tetraphenylporphyrin. J. Am. Chem. Soc. 1994, 116, 767−768. (32) Furuta, H.; Maeda, H.; Osuka, A. Confusion, Inversion, and Creationa New Spring from Porphyrin Chemistry. Chem. Commun. 2002, 25, 1795−1804. (33) Chmielewski, P. J.; Latos-Grażyński, L. Core Modified Porphyrins − a Macrocyclic Platform for Organometallic Chemistry. Coord. Chem. Rev. 2005, 249, 2510−2533. (34) Harvey, J. D.; Ziegler, C. J. Developments in the Metal Chemistry of N-Confused Porphyrin. Coord. Chem. Rev. 2003, 247, 1− 19. (35) Srinivasan, A.; Furuta, H. Confusion Approach to Porphyrinoid Chemistry. Acc. Chem. Res. 2005, 38, 10−20. (36) Maeda, H.; Furuta, H. A Dozen Years of N-Confusion: From Synthesis to Supramolecular Chemistry. Pure Appl. Chem. 2006, 78, 29−44. (37) Harvey, J. D.; Ziegler, C. J. The Metal Complexes of NConfused Porphyrin as Heme Model Compounds. J. Inorg. Biochem. 2006, 100, 869−880.

(38) Furuta, H.; Ishizuka, T.; Osuka, A.; Dejima, H.; Nakagawa, H.; Ishikawa, Y. NH Tautomerism of N-Confused Porphyrin. J. Am. Chem. Soc. 2001, 123, 6207−6208. (39) Belair, J. P.; Ziegler, C. J.; Rajesh, C. S.; Modarelli, D. A. Photophysical Characterization of Free-Base N-Confused Tetraphenylporphyrins. J. Phys. Chem. A 2002, 106, 6445−6451. (40) Muranaka, A.; Homma, S.; Maeda, H.; Furuta, H.; Kobayashi, N. Detection of Unusual ΔHOMO