Metal-Stabilized Quinoidal Dibenzo[g, p]chrysene-Fused Bis

Subscriber access provided by READING UNIV

Article

A Metal-Stabilized Quinoidal Dibenzo[g,p]chrysenefused Bis-dicarbacorrole System Xian-Sheng Ke, Yongseok Hong, Vincent M Lynch, Dongho Kim, and Jonathan L. Sessler J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02718 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

A Metal-Stabilized Quinoidal Dibenzo[g,p]chrysenefused Bis-dicarbacorrole System Xian-Sheng Ke†,§, Yongseok Hong‡,§, Vincent M. Lynch†, Dongho Kim*‡, and Jonathan L. Sessler*† † ‡

Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712-1224, United States. Department of Chemistry, Yonsei University, Seoul 03722, Korea

KEYWORDS. Polycyclic aromatic hydrocarbons, carbaporphyrins, corroles, multi-coordination complexes, near infrared. ABSTRACT: We report here a metal complexation-based strategy that permits access to a highly stable expanded porphyrin-type quinoidal PAH. Specifically, double insertion of Pd(II) ions into a dibenzo[g,p]chrysene-fused bis-dicarbacorrole (bis-H3) gives rise to a bis-metalated species (bis-Pd) that undergoes a facile benzenoid-quinonoid transformation. In contrast to what is true for the corresponding mono-Pd(II) complex, which has organic radical character, well resolved 1H- and 19F NMR spectra are seen for bisPd. This complex is also electron paramagnetic resonance (EPR) silent over a range of temperatures. On the basis of crystallographic analyses, Raman spectroscopic studies, harmonic oscillator model of aromaticity (HOMA) and nucleus-independent chemical shift (NICS) calculations, we suggest that the dibenzo[g, p]chrysene bridge in bis-Pd has quinoidal character and that the system as a whole is a closed shell species. As expected for a quinoidal system, bis-Pd is characterized by a lowest energy absorption band that is shifted into the NIR (lmax = ca. 1420 nm (ε > 1.5 × 105 M-1cm-1) for bis-Pd vs. 780 nm (ε < 5.0 × 103 M-1cm-1) for bis-H3). On the other hand, bis-Pd displays solvent dependent ground state and transient absorption spectral features. Such findings provide support for a zwitterionic resonance contribution to what is a predominantly a quinonoid-type ground state. The use of specific metalation to fine-tune the electronic features of polytopic ligands, as reported here, opens the door to what might be a potentially generalizable approach to the design of quinoidal PAH structures with long wavelength solvatochromic absorption features. Introduction Polycyclic aromatic hydrocarbons (PAHs) are very important, both in terms of fundamental research and material applications, such as organic electronics and liquid crystal display development.1 PAHs with quinoidal character have attracted particular interest in recent years due to their rather unique photophysical and magnetic properties.2 However, many quinoidal PAHs are highly reactive and inherently unstable under normal laboratory conditions; this is particularly true for large π-extended quinodal PAHs.2c To date, several approaches have pursued in an effort to increase the stability of quinoidal PAHs, either as closed shell species or in the form of what are formally diradical resonance contributors; these include attaching phenyl groups to take advantage of resonance effects3 or strong electron-withdrawing cyano groups as exemplified in compounds such as 7,7,8,8-tetracyanoquinodimethane (TCNQ)4, and using bulky triisopropylsilylacetylene (TIPS) groups5 or benzannulation6 to provide kinetic stability. In spite of these efforts, many large quinoidal PAHs are unstable and decompose in the air within a few hours or days.5b,7 While not yet applied to large PAH systems, metalation has been used to improve the stability of several quinoidal systems. In early work, Harman and coworkers found that reacting phenol with an osmium salt can lead to a metal-stablized quinone structure.8 Milstein and coworkers found that metal coordination can stabilize highly reactive quinone methides.9 This team even demonstrated the synthesis of the first stable metallaquinone.10 These successes led us to consider whether metal coordination could be used to generate and stabilize larger quinoidal PAHs. Here, we show that the two-fold insertion of Pd(II) into

a hybrid dibenzo[g, p]chrysene expanded carbaporphyrin (bisH3; Scheme 1) produces a predominantly quinoidal PAH system

with highly red-shifted and solvatochromic optical absorption features.

Carbaporphyrins, porphyrin analogues with carbon donors incorporated into the central core, have attracted considerable attention in recent decades.11 Many are now known, including benziporphyrins,12 azuliporphyrins,13 naphthiporphyrin,14 as well as carbaporphyrins with larger PAHs in the core, such as anthracene,15 phenanthrene,16 triphenylene17 and pyrene.18 Despite tremendous effort devoted to the development of quinoidal tetrapyrrolic porphyrin systems,19 as a general rule only small quinoidal structures, such as quinodimethane,20 quinone methides21 and benzoquinone methides,14a,22 have been stabilized to date using carbaporphyrin-type backbones. Stabilizing large quinoidal PAH structures within larger carbaporphyrin frameworks remains an unmet challenge. Based on Clar’s aromatic sextet rule (a classic rule used to evaluate the stability of closedshell PAHs and their reactivity), benzenoid PAHs contain a greater number of aromatic 4n + 2 π-electron rings relative to their quinoidal analogues. They also display higher stability and lower reactivity.7a,23 In other words, it is necessary to pay an energetic penalty, as it were, in order to convert a benzenoid system into the corresponding quinoidal form. To address this energetic challenge via a change in the underlying thermodynamics, we sought a PAH framework capable of coordinating more than one metal cation. As detailed below bis-H3 proved effective in this regard.

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1 Synthesis and limiting resonance forms of bis-Pd.

Recently, we reported a bis-dicarbacorrole (bis-H3) that contains a dibenzo[g, p]chrysene PAH core.24 The hetero Cu(III)-Pd(II) and mono-Pd(II) complexes of bis-H3 both proved to exist in the form of stable organic mono-radicals. The formation of these organic radicals is ascribed to one-electron transfer from the ligand backbone to the Pd center. We have now prepared the corresponding bis-Pd(II) complex (bis-Pd). In marked contrast to the corresponding mono-Pd(II) complex, bis-Pd gives rise to well resolved 1H and 19F NMR spectra and is EPR silent over a wide range of temperatures, as befits a closed shell system. DFT calculations revealed that a closedshell ground state is more favorable that an alternative openshell diradical state. Based on a combination of crystallographic analyses, Raman spectral studies, and HOMA and NICS calculations, we conclude that the dibenzo[g, p]chrysene bridge contains a quinoidal substructure. As a result, each erstwhile trianonic cavity become a dianonic cavity. This subtle change allows for the accommodation of two Pd(II) ions within an overall neutral structure. However, in spite of its net neutrality, bis-Pd shows solvent dependent ground and excited state properties. This solvent dependence is consistent with contributions from a zwitterionic resonance form, which is exceedingly rare in the context of PAH chemistry.6d A full discussion of these points now follows. Results and Discussion Synthesis and Characterizations The synthesis of bis-Pd is shown in Scheme 1. Reaction of bis-H3 with excess bis(benzonitrile)palladium chloride (Pd(PhCN)2Cl2) in PhCN at 180 oC gives bis-Pd in 85% yield. Bis-Pd was characterized by 1H and 19F NMR spectroscopy, 2D-correlation spectroscopy (COSY) (Figure S1-S3), UV−vis absorption and high-resolution ESI mass spectrometry (Figure S4), as well as by means of a single-crystal X-ray diffraction analysis. The proton NMR spectra of bis-Pd recorded in CD2Cl2 is shown in Figure 1. There are two doublets located at 6.89 and 6.98 ppm, which are assigned to the β-pyrrolic C-H proton resonances. Signals are also seen at 7.50 and 8.20 ppm that are assigned to the outer C-H protons in the dibenzo[g, p]chrysene moiety. Relative to the proton NMR spectrum of bis-H3,24 the chemical shifts of the pyrrolic β-H and outer C-H protons of bis-Pd are shifted to lower field. The chemical shift differences (Δδs) between bis-H3 and bis-Pd are 1.19-1.41 and 0.78-1.48 ppm for the pyrrolic β-H and outer C-H protons, respectively. In contrast, the outer C-H protons of the recently reported antiaromatic bis-Cu complex are slightly upfield shifted relative to bis-H3.24 These differences are consistent with a decrease in the

antiaromatic character of bis-Pd, as compared to bis-H3 and bis-Cu.

Figure 1. Partial 1H NMR spectra of bis-H3 with the methyl CH, inner N-H and C-H signals are not shown for the sake of clarity and bis-Pd with the methyl C-H signals not shown. The spectra were recorded in CD2Cl2 at room temperature. The labeling refers to the features for a single corrole-like subunit. Asterisks indicate residual peaks arising from the solvent. The UV-vis -NIR absorption spectra of bis-Pd was recorded in toluene. Upon the insertion of two Pd(II) cations into bis-H3, an intense NIR absorption band at ca. 1420 nm (ε = 1.66 × 105 M-1cm-1) is seen to grow in (Figure 2). The intensity of this relatively red-shifted band for bis-Pd bears analogy to what is seen in a number of bis-porphyrinoid systems,19i,25 as well as larger porphyrin arrays26 and tapes.27 In contrast, bis-H3 and bis-Cu are characterized by very weak or negligible NIR absorptions over 1000 nm;24 presumably, this reflects the very different electronic nature of bis-Pd (vide infra). In addition, bisPd gives rise to weak NIR emission centered at ca. 1530 nm in CH2Cl2 solution at room temperature when excited at 532 nm with a continuous wave (CW) laser. (Figure S12) The redox properties of bis-Pd, in CH2Cl2 were examined by cyclic voltammetry (CV) using tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) as the supporting electrolyte (Figure S13). A CV analysis of bis-Pd reveals two ligand-centered reversible reduction waves at ∼ −0.27 and −0.45 V (vs Fc/Fc+) and two ligand-centered reversible oxidation waves at ∼ +0.35 and +0.77 V (vs Fc/Fc+), as well as one ligand-centered irreversible oxidation wave at ∼ +0.99 V (vs Fc/Fc+). The difference (ΔE) between the first oxidation and reduction waves, as inferred from the CV data, are 1.63,24 1.5124 and 0.62 V for bis-H3, bis-Cu and bis-Pd, respectively, under the same conditions of analysis. The reduced ΔE value is consistent with the red-shifted absorption seen for bis-Pd relative to bis-H3 and bis-Cu,24 as well as with the HOMO-LUMO gap order calculated by DFT (i. e.,v2.049, 1.981 and 0.985 eV for bis-H3, bisCu and bis-Pd, respectively; Figures S31-S32)


Page 2 of 9

Page 3 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. UV-vis-NIR absorption spectra of bis-Pd recorded in toluene (compared with bis-H324). The insert shows a picture of bis-H3 and bis-Pd in toluene. The structure of the bis-Pd was further confirmed via a single crystal X-ray diffraction analysis. As shown in Figure 3, bisPd is characterized by a twisted conformation in the solid state. The distortion is greater than that seen in either bis-H3 and bisCu24 as reflected in the dihedral angle of 33.20 (Figure 3b) between the two corrole subunits. In bis-Pd, the two Pd ions are located within the adj-CCNN cores and are characterized by near square-planar coordination geometries. The Pd-C bond lengths (1.962 Å) are shorter than the Pd-N (2.012-2.015 Å) distance. This finding is consistent with previously reported Pd(II) complexes of dicarbaporphyrinoids with adj-CCNN cores,28 and presumably reflects the relatively greater basicity of the carbanion donor site present in bis-Pd and these latter systems. The absence of counter ions within the crystal lattice provides support for the palladium centers in bis-Pd being in the +2 oxidation state. This is in accord with what was seen in the recently reported hetero Cu(III) - Pd(II) congener.24 More insights into the bonding features were gained from a detailed bond length analysis of the core dibenzo[g, p]chrysene moiety. The C(5)-C(6) bond length in bis-H3 (1.397 Å) and bis-Cu (1.407 Å) are closer to those of a sp2-sp2 hybridized carbon-carbon double bond, while those for the surrounding bonds fall closed to those expected for a sp2-sp2 hybridized carboncarbon single bond (1.453 to 1.466 Å). These metric parameters are quite similar to those found in dibenzo[g, p]chrysene itself (Figure 3c). The converse is true in the case of bis-Pd; now, the C(5)-C(6) bond length (1.450 Å) is closer to that of a single bond, while the surrounding bond lengths are relatively short (1.435-1.440 Å). In addition, long-short alternating bond lengths are seen within all six benzene rings present in bis-H3 and bis-Cu. It was thus inferred that these subunits have benzenoid character (referred to herein as Clar), which again mimics what is seen in dibenzo[g,p]chrysene itself30 (Figure S33). This long-short bond alternation is substantially less obvious in the case bis-Pd (Figure S33). This was taken as initial evidence that the chrysene portion of bis-Pd may have quinoidal character. However, zwitterionic contribution to the overall structure (cf. Scheme 1) could not be entirely ruled out.

Figure 3. Single crystal X-ray diffraction structure of bis-Pd; a) top view with selected Pd-C and Pd-N bond lengths (Å), b) side view, and c) bond lengths (Å) of the central C(5)-C(6) bond and surrounding bonds for dibenzo[g,p]chrysene,29 bis-H3,24 bisCu,24 and bis-Pd, respectively. Thermal ellipsoids are scaled to the 50% probability level. Solvent molecules and meso-aryl substituents are omitted for clarity. To gain further insights into the bonding features of bisPd, the Raman spectra of bis-H3, bis-Cu, and bis-Pd were compared (Figure S14). A readily apparent band at ~1570 cm-1 is seen in the spectra of both bis-H3 and bis-Cu. This feature, which reflects predominantly a C=C stretching mode, appears near 1530 cm-1 in the case of bis-Pd. This supports the inference that in both bis-H3 and bis-Cu the dibenzo[g, p]chrysene portion has Clar character. In contrast, the bond order lowering seen for bis-Pd is typical of what is seen upon the transition from a benzenoid to quinoidal system.31 The stability of bis-Pd was determined by means of UVvis-NIR and 1H NMR spectroscopy. The change in the absorption spectrum of bis-Pd in toluene was monitored as a function of time with measurements being made every 5 days. Over a period of 25 days, the absorbance of the peak at 1420 nm decreased in intensity by ≤4% relative to the original sample (Figure S10). The 1H NMR spectrum in CD2Cl2 was likewise monitored for roughly 1 month. Again, no appreciable changes were observed (cf. Figure S11). On this basis, bis-Pd was considered to be quite stable. This stands in contrast to what is seen in the case of many reported qunoidal PAHs.5b,7 Experimental evidence for a closed shell structure The EPR spectrum of bis-Pd was recorded at various temperatures. No evidence of an EPR signal was seen, either in solution or in the solid state. This proved true both at 360K and at 90K (Figure S5). Follow up superconducting quantum interference device (SQUID) measurements revealed the presence of only diamagnetic signals. Such findings are consistent with the closed-shell structure proposed for bis-Pd.



Bis-Pd shows well-resolved 1H NMR spectral feartures at room temperature (Figure 1). Moreover, heating a sample of bis-Pd in 1,1,2,2-tetrachloroethane-d2 from 27 to 100 °C caused only a very small broadening and little change in the intensity of the aromatic proton signals (Figure S6). This observation stands in contrast to what is typically seen for singlet biradical species, where the proton NMR signals undergo notable broadening and can even disappear at higher temperatures.5c No appreciable sharpening of the signals relative to the room temperature 1H NMR spectrum was seen for a sample of bis-Pd studied at low temperature (-80 °C in CD2Cl2). However, the chemical shifts of the pyrrolic β-Hs signals were seen to change as a function of temperature (Figures S6 and S8). This could reflect thermally induced conformational motion.32 Previously we had found that the corresponding mono-Pd complex has radical character and that this is reflected in changes in the 19F NMR signals ascribed to the meso-pentaflurophenyl groups as a function of temperature.24 Therefore, variable-temperature (VT) 19F NMR spectroscopic studies of bisPd were also carried out (Figures S7 and S9). However, in contrast to the mono-Pd(II)complex, no obvious changes in the 19F NMR spectrum were observed as the temperature was raised or lowered. This was taken as further support for the closed-shell nature of bis-Pd. However, the relative contribution of quinoidal vs. zwitterionic contributors to the overall structure (cf. Scheme 1) could not be ascertained from these studies. To address this issue, a number of theoretical analyses were carried out. Theoretical analyses The molecular geometries of dibenzo[g, p]chrysene,29 bisH3, bis-Cu, and bis-Pd were optimized at the M06/LANL2DZp level using the X-ray structural data as the point of departure. The bond length patterns of the dibenzo[g, p]chrysene portions of both bis-H3 and bis-Cu were calculated to be similar to those in dibenzo[g, p]chrysene, a system well known for having a benzenoid (Clar) structure.30 In contrast, bis-Pd was characterized by relatively unique features as illustrated in Figures 4 and S23. This was true for several indicators of local aromaticity, such as bond length alternation (BLA) and harmonic oscillator model of aromaticity (HOMA). Since the dibenzo[g, p]chrysene parts of bis-H3 and bis-Cu were found to be similar, we carried out a comparative analysis of bis-Pd vs. bis-H3 (only). The results are summarized in Figures S23-S24 and discussed in detail below. In an idealized resonance system, the bond order is 1.5 and the BLA value (Å) is 0. In bis-H3, the calculated BLA values for a and c sites (0.019 for both) are smaller than that at b site (0.047) (Figure 4b). This is in accord with what was found for dibenzo[g, p]chrysene, which serves as a known Clar structure. In contrast, the BLA values for bis-Pd are reversed. The BLA values at sites a and c were calculated to be 0.030, while those at site b were calculated to be 0.019. This reversal can be taken as further support for the suggestion that bis-Pd is not a fully benzenoid system and may have substantial quinoidal character. Further support for the above notion also came from HOMA value calculations (Figure S23). Fully conjugated aromatic systems, such as benzene, have HOMA values of 1.0, while idealized non-conjugated systems display HOMA values of 0. The HOMA values of bis-H3 at sites a and c (0.84 and 0.83, respectively) are much larger than those at site b (0.21). This is consistent with the presence of local aromaticity at sites a and c and an overall benzenoid-type structure. In contrast, bis-Pd

shows reduced HOMA values at sites a and c (0.63 for both) and increased HOMA values at site b (0.31).

Figure 4 Structural analysis of dibenzo[g, p]chrysene subunit. a) Chemical structure of dibenzo[g, p]chrysene subunit in benzenoid and quinonoid form respectively. b) The BLA values for the benzene rings (hatched blue/hatched purple and pink/hatched purple bars for bis-H3 and bis-Pd, respectively). The arrow indicates the changes in the BLA value that occur upon formal conversion of bis-H3 into bis-Pd. c) Representative bonds (squares and circles for l and d, respectively) and mean plane deviation (MPD) (triangles) plots of bis-H3, bisCu, bis-Pd (S1 state), and bis-Pd (S0 state). We also compared the theoretical C(5)-C(6) and C(6)-C(7) bond lengths, denoted d and l in Figure 4a, for bis-H3 and bisPd (Figure 4c).33 The accepted Clar structure of dibenzo[g, p]chrysene requires that the d and l bond lengths should be close to those for pure double and single bonds, respectively. The d and l values for bis-H3 are consistent with a Clar structure (1.404 and 1.464 Åfor d and l, respectively). In bis-Pd the pattern is reversed, with the d and l values lying closer to those for single (1.462 Å) and double bonds (1.445 Å), respectively. An additional structural parameter, the mean plane deviation (MPD), was also analyzed. The MPD is defined by the standard deviation of the distance from the mean plane for the


Page 4 of 9

Solvent-dependent photophysical properties and analysis To determine whether small contributions from benzenoid zwitterionic forms, as illustrated in Scheme 1, were contributing to the electronic character of bis-Pd within what was predicted to be an overall quinoidal systems dependent studies were carried out. The absorption spectrum of bis-Pd recorded in toluene differs from those of bis-H3 and bis-Cu. In particular, absorption bands that extend into the NIR region (up to ca. 1650 nm) are seen, whose origin is ascribed to a reduced HOMO-LUMO energy gap. (Figure S32) This reduced gap can be appreciated qualitatively from the extensive bond delocalization present in the limiting resonance structures shown in Scheme 1. The strong intensity of the NIR absorption feature is consistent with contributions from the delocalized resonance structures for bisPd shown in Scheme 1. These contributions serve to enhance in relative terms the transition dipole moments that underlie the observed electronic transitions. That one of the resonance structures in Scheme 1 is zwitterionic led to the consideration that bis-Pd might display solvatochromism. This was tested by recording the UV-vis-NIR spectra in n-hexane, dichloromethane, and benzonitrile, in addition to toluene (Figure 5). As the polarity increases from nhexane (ε = 1.88) to benzonitrile (ε = 26.0), the NIR bands broaden and shift slightly to the red. These changes mirror those seen for species with intramolecular charge transfer (ICT) character. However, bis-Pd is symmetric and does not contain donor-acceptor (DA) chromophores and the HOMO and LUMO are delocalized throughout the molecule. Thus, typical DAbased ICT determinants could be ruled out (Figure S25). Rather, we suggest that increasing the solvent polarity enhances the relative contribution of zwitterionic resonance structures, such as that shown in Scheme 1.

a) Hexane Toluene DCM BCN

1.5 1.0

6000

6500

7000

7500

8000

8500

9000

9500

-1

Wavenumber / cm

0.5 0.0

b)

Hexane Toluene DCM BCN

Absorbance / a.u.

2.0 e / 105 cm-1M-1

carbon atoms comprising the dibenzo[g, p]chrysene part. It provides a measure of the degree of molecular planarity for the central core of the overall ligand system. The calculated MPD values were 0.269, 0.425, and 0.460 Å in the cases of bis-H3, bisCu, and bis-Pd, respectively. The larger MPD value of bis-Pd compared to bis-Cu is considered to reflect conversion from a more planar benzenoid structure to a distorted quinoidal form, rather than the effects of metalation per se. We performed quantum calculations to determine whether bis-Pd might possess a degree of radical character. This was done using the occupation numbers of the spin-unrestricted Hartree-Fock natural orbitals (UNOs) and the LANL2DZp basis set to calculate (yi) (Table S1).34 The radical character parameter yi is a value between 0 and 1, limits that correspond to pure closed-shell and pure radical species, respectively. A value of y1 of 0 was calculated for bis-Pd. We thus conclude that this complex is a closed-shell species, as would be inferred from the experimental findings (e.g., sharp 1H NMR signal; absence of EPR spectral features). Finally, the extent of local aromaticity was determined by means of nucleus-independent chemical shift (NICS) calculations (Figure S30). At site a, bis-H3 and bis-Cu show similar NICS(0) values (-4.7 and -5.9, respectively), consistent with the aromatic nature of these rings. The corresponding value in the case of bis-Pd is -1.8. This change is consistent with a reduced level of aromaticity and hence a substantial degree of quinoidal character.

1250 1200 1150 1100 1050 1000 950 900 850 800

400 600 800 1000 1200 1400 1600 1800 Wavelength / nm

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6

Excited state lifetime Relative energy

0

5

10 15 20 Dielectric constant / e

Energy /eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


τ / ps

Page 5 of 9

25

Figure 5. Solvent-dependent properties of bis-Pd. a) Steady state absorption spectra of bis-Pd recorded in n-hexane (ε = 1.88), toluene (ε = 2.38), dichloromethane (ε = 8.93), and benzonitrile (ε = 26.0). b) Relative energies calculated at the M06(SCRF)/LANL2DZp level (circles) and excited state lifetimes (squares) of bis-Pd plotted vs the dielectric constant of the solvent. The effect of solvents on the structure bis-Pd was further probed by means of self-consistent reaction field (SCRF) calculations. The relative energies and structural analyses of the optimized structures in the four test solvents n-hexane, dichloromethane, toluene, and benzonitrile were determined at the M06(SCRF)/LANL2DZp level. 35 In benzonitrile the relative energy is reduced by as much as 1.5 eV (Figure 5b). Such a decrease is consistent with this relatively polar solvent interacting with and stabilizing the zwitterionic form. The calculated energy gap between the HOMO and LUMO also decreases with increasing solvent polarity (Figure S26-29). The BLA and MPD values were also calculated using the optimized structures (Table S2). As the solvent polarity increases, the BLA values at sites a and c decrease from 0.2917 to 0.0286, while those at site b increase from 0.01977 to 0.02006 upon passing from n-hexane to ACN. Such changes are consistent with the partial recovery of benzenoid structural features as would be expected as contributions from zwitterionic forms such as shown in Scheme 1 increase. The calculated MPD value of 0.45977 in n-hexane decreases to 0.45954 in benzonitrile, indicating the enhanced structural planarity expected for a more benzenoid structure, such as seen in bis-Cu. These results support the conclusion that zwitterionic contributions to the overall electronic structure of bis-Pd become more important as the polarity of the medium increases. The solvent-induced changes in electronic and structural features were expected to be reflected in the excited state dynamics. Thus, we carried out femtosecond transient absorption measurements to monitor the excited dynamics and spectral


Journal of the American Chemical Society changes of bis-Pd (Figures 6 and S19-S21). Following photoexcitation at 530 nm the resulting evolution associated spectra (EAS) were fit using the glotaran program.36 In toluene, three decay components, with time constants of 0.6, 10, and 900 ps, were obtained in this way. The early spectra with a time constant of 0.6 ps are characterized by a broad and featureless excited state absorption (ESA) band, which could be assigned to internal conversion processes from initially formed states to the lowest excited state. The ensuing EAS evolved to reveal a narrow ESA band with a time constant of 10 ps. This spectral evolution was assigned to a structural relaxation from a lowest excited state (Franck-Condon state and quinoidal structure) to a structurally relaxed state (benzenoid-like structure), which underwent subsequent electronic relaxation with a time constant of 900 ps.

a) 20 10

DmA

0 Time / ps -5 0.3 0.5 1 3 20.1 50

-10 -20 -30 450

b)

500

550

600

650

700

150 300 500 1000 1480 2000 3000

750

Wavelength / nm 20 10

S1 state, the values of d, l, and MPD (1.438, 1.458, and 0.438, respectively) lie between those for bis-H3 and bis-Pd in the ground (S0) state (Figure 4c). Such a finding provides support for the notion that the dibenzo[g, p]chrysene portion of bis-Pd in the S1 state has a structure intermediate between the limiting quinoidal and benzenoid forms. Such a conclusion is consistent with our interpretation of the TA dynamics in that the benzenoid-like structure in the relaxed S1 state is different from the primarily quinoidal structure that dominates in the S0 and Franck-Condon states. Conclusion The bis-Pd(II) complex of a dibenzo[g,p]chrysene-fused bis-dicarbacorrole reported here differs from both the previously reported free base and bis-Cu(III) complex in that it is a closed-shell system with substantial quinoidal character. Upon formal conversion of bis-H3 to bis-Pd through Pd(II) complexation, the local aromaticity of the dibenzo[g, p]chrysene core decreases, which is rationalized in terms of a benzenoid to quinonoid transformation. The closed-shell nature of bis-Pd was inferred from variable-temperature 1H NMR, 19F NMR, and EPR spectroscopies, as well as DFT calculations. The quinoidal character of bis-Pd was further supported by crystallographic analyses, Raman spectral studies, as well as HOMA and NICS calculations. The solvent dependent ground and excited state absorption features provide support for a zwitterionic contribution to the overall electronic description of bis-Pd, which remains primarily a quinoidal system. This work serves to highlight how relatively simple changes in the coordination chemistry of a ligand system can affect substantially its electronic and structural features. It also underscores metal complexation as a new tool that can allow access to stable quinoidal PAH structures. In the present case, this metalation strategy provides an entry into a stable, well characterized bimetallic complex with a strong NIR absorption feature that is unusual for such a relatively low molecular size system.

0 DmA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ASSOCIATED CONTENT -10 -20

Supporting Information EAS in toluene Internal conversion (600 fs) Structural relaxation (10 ps) S1 lifetime (900 ps)

-30 450

500

550

600

650

700

750

Wavelength / nm

Synthesis and characterization details, EPR spectra, VT-NMR spectra, CV, supporting photophysical measurements, X-ray experimental, cif files for bis-Pd, and DFT calculation details. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Femtosecond-TA spectra of bis-Pd recorded in toluene. a) Representative TA spectra of bis-Pd monitored over the 450 to 770 nm spectral region (λpump = 530 nm). b) EAS spectra of bis-Pd in toluene.

AUTHOR INFORMATION

The same basic TA spectral features were seen irrespective of which of the four test solvents was used. However, a modest effect on the decay profiles and the lifetime of the electronic relaxations was seen. For instance, these two time constants were found to increase from 8.3 and 840 ps in n-hexane to 13 and 1100 ps in benzonitrile, respectively (Figures 6b and S1518). These findings are consistent with a modest contribution from zwitterionic forms, such as shown in Scheme 1, to the overall electronic structure of bis-Pd. To rationalize the TA dynamics of bis-Pd, we optimized the S1 excited state structure and used it to calculate the bond length patterns and the degree of planarization. In the relaxed

Author Contributions

Corresponding Author [email protected]; [email protected]

§

X.-S. Ke and Y. Hong contributed equally

Notes The authors declare no competing financial interest. ORCID Xian-Sheng Ke: 0000-0002-0562-1039 Dongho Kim: 0000-0001-8668-2644 Jonathan L. Sessler: 0000-0002-9576-1325 ACKNOWLEDGMENT


Page 6 of 9

Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society We thank the National Science Foundation (grant CHE - 1402004 to J.L.S.) and the Robert A. Welch Foundation for support (F0018). The work at Yonsei University was supported by the Strategic Research program (NRF-2016R1E1A1A01943379) administered through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, Information and Communication Technologies and Future Planning (to D.K.). We would like to thank Mr. Zhu-Lin Xie for help with the EPR measurements and Dr. Tianhan Kai for helpful discussions. REFERENCES (1) (a) Watson, M. D.; Fechtenkötter, A.; Müllen, K. Chem. Rev. 2001, 101, 1267. (b) Anthony, J. E. Chem. Rev. 2006, 106, 5028. (c) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902. (d) Wu, J.; Pisula, W.; Müllen, K. Chem. Rev. 2007, 107, 718. (e) Jung, B. J.; Tremblay, N. J.; Yeh, M.-L.; Katz, H. E. Chem. Mater. 2011, 23, 568. (f) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012, 112, 2208. (g) Sun, Z.; Ye, Q.; Chi, C.; Wu, J. Chem. Soc. Rev. 2012, 41, 7857. (2) (a) Casado, J.; Ponce Ortiz, R.; Lopez Navarrete, J. T. Chem. Soc. Rev. 2012, 41, 5672. (b) Sun, Z.; Zeng, Z.; Wu, J. Acc. Chem. Res. 2014, 47, 2582. (c) Zeng, Z. B.; Shi, X. L.; Chi, C. Y.; Navarrete, J. T. L.; Casado, J.; Wu, J. S. Chem. Soc. Rev. 2015, 44, 6578. (d) Frederickson, C. K.; Rose, B. D.; Haley, M. M. Acc. Chem. Res. 2017, 50, 977. (3) (a) Thiele, J.; Balhorn, H. Ber. Dtsch. Chem. Ges. 1904, 37, 1463. (b) Tschitschibabin, A. E. Ber. Dtsch. Chem. Ges. 1907, 40, 1810. (c) Montgomery, L. K.; Huffman, J. C.; Jurczak, E. A.; Grendze, M. P. J. Am. Chem. Soc. 1986, 108, 6004. (4) (a) Acker, D. S.; Hertler, W. R. J. Am. Chem. Soc. 1962, 84, 3370. (b) Garito, A. F.; Heeger, A. J. Acc. Chem. Res. 1974, 7, 232. (c) Laquindanum, J. G.; Katz, H. E.; Dodabalapur, A.; Lovinger, A. J. J. Am. Chem. Soc. 1996, 118, 11331. (d) Yanagimoto, T.; Takimiya, K.; Otsubo, T.; Ogura, F. J. Chem. Soc., Chem. Commun. 1993, 519. (e) Zhu, X.; Tsuji, H.; Nakabayashi, K.; Ohkoshi, S.-i.; Nakamura, E. J. Am. Chem. Soc. 2011, 133, 16342. (5) (a) Chase, D. T.; Rose, B. D.; McClintock, S. P.; Zakharov, L. N.; Haley, M. M. Angew. Chem., Int. Ed. 2011, 50, 1127. (b) Li, Y.; Heng, W.-K.; Lee, B. S.; Aratani, N.; Zafra, J. L.; Bao, N.; Lee, R.; Sung, Y. M.; Sun, Z.; Huang, K.-W.; Webster, R. D.; López Navarrete, J. T.; Kim, D.; Osuka, A.; Casado, J.; Ding, J.; Wu, J. J. Am. Chem. Soc. 2012, 134, 14913. (c) Rudebusch, G. E.; Zafra, J. L.; Jorner, K.; Fukuda, K.; Marshall, J. L.; Arrechea-Marcos, I.; Espejo, G. L.; Ponce Ortiz, R.; Gómez-García, C. J.; Zakharov, L. N.; Nakano, M.; Ottosson, H.; Casado, J.; Haley, M. M. Nat. Chem. 2016, 8, 753. (6) (a) Zeng, Z.; Sung, Y. M.; Bao, N.; Tan, D.; Lee, R.; Zafra, J. L.; Lee, B. S.; Ishida, M.; Ding, J.; López Navarrete, J. T.; Li, Y.; Zeng, W.; Kim, D.; Huang, K.-W.; Webster, R. D.; Casado, J.; Wu, J. J. Am. Chem. Soc. 2012, 134, 14513. (b) Zeng, Z.; Ishida, M.; Zafra, J. L.; Zhu, X.; Sung, Y. M.; Bao, N.; Webster, R. D.; Lee, B. S.; Li, R.-W.; Zeng, W.; Li, Y.; Chi, C.; Navarrete, J. T. L.; Ding, J.; Casado, J.; Kim, D.; Wu, J. J. Am. Chem. Soc. 2013, 135, 6363. (c) Zeng, Z.; Lee, S.; Zafra, J. L.; Ishida, M.; Zhu, X.; Sun, Z.; Ni, Y.; Webster, R. D.; Li, R.-W.; López Navarrete, J. T.; Chi, C.; Ding, J.; Casado, J.; Kim, D.; Wu, J. Angew. Chem., Int. Ed. 2013, 52, 8561. (d) Zeng, Z.; Lee, S.; Son, M.; Fukuda, K.; Burrezo, P. M.; Zhu, X.; Qi, Q.; Li, R.-W.; Navarrete, J. T. L.; Ding, J.; Casado, J.; Nakano, M.; Kim, D.; Wu, J. J. Am. Chem. Soc. 2015, 137, 8572. (7) (a) Sun, Z.; Lee, S.; Park, K. H.; Zhu, X.; Zhang, W.; Zheng, B.; Hu, P.; Zeng, Z.; Das, S.; Li, Y.; Chi, C.; Li, R.-W.; Huang, K.-W.; Ding, J.; Kim, D.; Wu, J. J. Am. Chem. Soc. 2013, 135, 18229. (b) Sun, Z.; Huang, K.-W.; Wu, J. J. Am. Chem. Soc. 2011, 133, 11896. (8) (a) Kopach, M. E.; Hipple, W. G.; Harman, W. D. J. Am. Chem. Soc. 1992, 114, 1736. (b) Kopach, M. E.; Harman, W. D. J. Am. Chem. Soc. 1994, 116, 6581. (9) (a) Vigalok, A.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 7873. (b) Vigalok, A.; Milstein, D. Acc. Chem. Res. 2001, 34, 798. (10) Ashkenazi, N.; Vigalok, A.; Parthiban, S.; Ben-David, Y.; Shimon, L. J. W.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2000, 122, 8797. (11) Lash, T. D. Chem. Rev. 2017, 117, 2313. (12) (a) Stȩpień, M.; Latos-Grażyński, L. Acc. Chem. Res. 2005, 38, 88. (b) Lash, T. D. Org. Biomol. Chem. 2015, 13, 7846. (13) (a) Lash, T. D.; Chaney, S. T. Angew. Chem. Int. Ed. Engl. 1997, 36, 839. (b) Lash, T. D. Acc. Chem. Res. 2016, 49, 471.

(14) (a) Lash, T. D.; Young, A. M.; Rasmussen, J. M.; Ferrence, G. M. J. Org. Chem. 2011, 76, 5636. (b) Szyszko, B.; Latos-Grażyński, L. Organometallics 2011, 30, 4354. (c) Szyszko, B.; Pacholska-Dudziak, E.; Latos-Grażyński, L. J. Org. Chem. 2013, 78, 5090. (d) Hong, J.-H.; Aslam, A. S.; Ishida, M.; Mori, S.; Furuta, H.; Cho, D.-G. J. Am. Chem. Soc. 2016, 138, 4992. (15) (a) Szyszko, B.; Latos-Grażyński, L.; Szterenberg, L. Chem. Commun. 2012, 48, 5004. (b) Sung, Y. M.; Szyszko, B.; Mysliborski, R.; Stepien, M.; Oh, J.; Son, M.; Latos-Grażyński, L.; Kim, D. Chem. Commun. 2014, 50, 8367. (c) Aslam, A. S.; Hong, J.-H.; Shin, J.-H.; Cho, D.-G. Angew. Chem., Int. Ed. 2017, 56, 16247. (16) (a) Szyszko, B.; Bialonska, A.; Szterenberg, L.; Latos-Grażyński, L. Angew. Chem., Int. Ed. 2015, 54, 4932. (b) Szyszko, B.; Małecki, M.; Berlicka, A.; Białek, M. J.; Białońska, A.; Kupietz, K.; Pacholska-Dudziak, E.; Latos-Grażyński, L. Chem. Eur. J. 2016, 22, 7602. (17) Gopee, H.; Kong, X.; He, Z.; Chambrier, I.; Hughes, D. L.; Tizzard, G. J.; Coles, S. J.; Cammidge, A. N. J. Org. Chem. 2013, 78, 9505. (18) Gao, R.; AbuSalim, D. I.; Lash, T. D. J. Org. Chem. 2017, 82, 6680. (19) (a) Smith, K. M. J. Chem. Soc. D 1971, 540. (b) Fuhrhop, J. H.; Baumgartner, E.; Bauer, H. J. Am. Chem. Soc. 1981, 103, 5854. (c) Traylor, T. G.; Nolan, K. B.; Hildreth, R. J. Am. Chem. Soc. 1983, 105, 6149. (d) Y. Nelson, N.; G. Khoury, R.; J. Nurco, D.; M. Smith, K. Chem. Commun. 1998, 1687. (e) Blake, I. M.; Anderson, H. L.; Beljonne, D.; Brédas, J.-L.; Clegg, W. J. Am. Chem. Soc. 1998, 120, 10764. (f) Blake, I. M.; Rees, L. H.; Claridge, T. D. W.; Anderson, H. L. Angew. Chem., Int. Ed. 2000, 39, 1818. (g) Blake, I. M.; Krivokapic, A.; Katterle, M.; Anderson, H. L. Chem. Commun. 2002, 1662. (h) Smith, M. J.; Clegg, W.; Nguyen, K. A.; Rogers, J. E.; Pachter, R.; Fleitz, P. A.; Anderson, H. L. Chem. Commun. 2005, 2433. (i) Hiroto, S.; Furukawa, K.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 12380. (j) Bill, N. L.; Ishida, M.; Bähring, S.; Lim, J. M.; Lee, S.; Davis, C. M.; Lynch, V. M.; Nielsen, K. A.; Jeppesen, J. O.; Ohkubo, K.; Fukuzumi, S.; Kim, D.; Sessler, J. L. J. Am. Chem. Soc. 2013, 135, 10852. (k) Koide, T.; Furukawa, K.; Shinokubo, H.; Shin, J.-Y.; Kim, K. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2010, 132, 7246. (l) Esdaile, L. J.; Rintoul, L.; Goh, M. S.; Merahi, K.; Parizel, N.; Wellard, R. M.; Choua, S.; Arnold, D. P. Chem. Eur. J. 2016, 22, 3430. (m) Zhang, H.; Phan, H.; Herng, T. S.; Gopalakrishna, T. Y.; Zeng, W.; Ding, J.; Wu, J. Angew. Chem., Int. Ed. 2017, 56, 13484. (n) Jun-i, Y.; Fukui, N.; Furukawa, K.; Osuka, A. Chem. Eur. J. 2018, 24, 1528. (20) Anand, V. G.; Saito, S.; Shimizu, S.; Osuka, A. Angew. Chem., Int. Ed. 2005, 44, 7244. (21) (a) Lash, T. D. Angew. Chem., Int. Ed. Engl. 1995, 34, 2533. (b) Venkatraman, S.; Anand, V. G.; Pushpan, S. K.; Sankar, J.; Chandrashekar, T. K. Chem. Commun. 2002, 462. (c) El-Beck, J. A.; Lash, T. D. Org. Lett. 2006, 8, 5263. (22) (a) Lash, T. D.; Rasmussen, J. M.; Bergman, K. M.; Colby, D. A. Org. Lett. 2004, 6, 549. (b) Bialek, M. J.; Latos-Grażyński, L. Chem. Commun. 2018, 54, 1837. (23) Clar, E. The Aromatic Sextet; Wiley: London, 1972. (24) Ke, X.-S.; Hong, Y.; Tu, P.; He, Q.; Lynch, V. M.; Kim, D.; Sessler, J. L. J. Am. Chem. Soc. 2017, 139, 15232. (25) (a) Tsuda, A.; Furuta, H.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 10304. (b) Ooi, S.; Tanaka, T.; Park, K. H.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2016, 55, 6535. (c) Bhowmik, S.; Kosa, M.; Mizrahi, A.; Fridman, N.; Saphier, M.; Stanger, A.; Gross, Z. Inorg. Chem. 2017, 56, 2287. (26) Yoon, M.-C.; Noh, S. B.; Tsuda, A.; Nakamura, Y.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2007, 129, 10080. (27) Fukui, N.; Kim, T.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2017, 139, 9075. (28) AbuSalim, D. I.; Ferrence, G. M.; Lash, T. D. J. Am. Chem. Soc. 2014, 136, 6763. (29) Hatakeyama, T.; Hashimoto, S.; Seki, S.; Nakamura, M. J. Am. Chem. Soc. 2011, 133, 18614. (30) Randić, M. Chem. Rev. 2003, 103, 3449. (31) (a) Trznadel, M.; Zagorska, M.; Lapkowski, M.; Louarn, G.; Lefrant, S.; Pron, A. J. Chem. Soc., Faraday Trans. 1996, 92, 1387. (b) Quillard, S.; Louam, G.; Buisson, J. P.; Boyer, M.; Lapkowski, M.; Pron, A.; Lefrant, S. Synth. Met. 1997, 84, 805. (c) Alvarez, M. P.; Ruiz Delgado, M. C.; Taravillo, M.; Baonza, V. G.; Lopez Navarrete, J. T.; Evans, P.; Jasti, R.; Yamago, S.; Kertesz, M.; Casado, J. Chem.Sci. 2016, 7, 3494. (d) Rudebusch, G. E.; Espejo, G. L.; Zafra, J. L.; Peña-Alvarez, M.;



Spisak, S. N.; Fukuda, K.; Wei, Z.; Nakano, M.; Petrukhina, M. A.; Casado, J.; Haley, M. M. J. Am. Chem. Soc. 2016, 138, 12648. (32) (a) Casarini, D.; Lunazzi, L.; Mazzanti, A. Eur. J. Org. Chem. 2010, 2010, 2035. (b) Desroches, M.; Mayorga Burrezo, P.; BoismenuLavoie, J.; Peña Álvarez, M.; Gómez-García, C. J.; Matxain, J. M.; Casanova, D.; Morin, J.-F.; Casado, J. Angew. Chem., Int. Ed. 2017, 56, 16212. (c) He, X.; Xue, Y.; Li, C.-C.; Wang, Y.; Jiang, H.; Zhao, L. Chem.Sci. 2018, 9, 1481. (33) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6931.

(34) Kamada, K.; Ohta, K.; Shimizu, A.; Kubo, T.; Kishi, R.; Takahashi, H.; Botek, E.; Champagne, B.; Nakano, M. J. Phys. Chem. Lett. 2010, 1, 937. (35) (a) Cances, E.; Mennucci, B. J. Chem. Phys. 2001, 114, 4744. (b) Chipman, D. M. J. Chem. Phys. 2000, 112, 5558. (c) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (36) Snellenburg, J. J.; Laptenok, S.; Seger, R.; Mullen, K. M.; van Stokkum, I. H. M. J. Stat. Softw. 2012, 49, 1.


Page 8 of 9

Page 9 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic

9 ACS Paragon Plus Environment

Metal-Stabilized Quinoidal Dibenzo[g, p]chrysene-Fused Bis

Recommend Documents