Towards a Charged Homo[2]catenane Employing Diazaperopyrenium

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Towards a Charged Homo[2]catenane Employing Diazaperopyrenium Homophilic Recognition Xirui Gong, Jiawang Zhou, Karel J. Hartlieb, Claire Miller, Peng Li, Omar K. Farha, Joseph T. Hupp, Ryan M. Young, Michael R. Wasielewski, and J. Fraser Stoddart J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03407 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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Towards a Charged Homo[2]catenane Employing Diazaperopyrenium Homophilic Recognition Xirui Gong,†# Jiawang Zhou,†# Karel J. Hartlieb,† Claire Miller,†‡ Peng Li,† Omar K. Farha,†║ Joseph T. Hupp,† Ryan M. Young,†‡ Michael R. Wasielewski†‡* and J. Fraser Stoddart†§* †Department of Chemistry and ‡Argonne-Northwestern Solar Energy Research (ANSER) center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113 §Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road, Tianjin, 300092, P. R. China ║Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia Supporting Information Placeholder ABSTRACT: An octacationic diazaperopyrenium (DAPP2+)-

based homo[2]catenane (DAPPHC ), wherein no less than eight positive charges are associated within a mechanically interlocked molecule, has been produced in 30% yield under ambient conditions as a result of favorable homopilic interactions, reflecting a delicate balance between strong π-π interactions and the destabilizing penalty arising from Coulombic repulsions between DAPP2+ units. This DAPPHC8+ catenane is composed of two identical mechanically interlocked tetracationic cyclophanes, namely DAPPBox4+, each of which contains one DAPP2+ unit and one extended viologen (ExBIPY2+) unit, linked together by two pxylylene bridges. The solid-state structure of the homo[2]catenane demonstrates how homophilic interactions play an important role in the formation of DAPPHC8+ in which the mean ring planes of the two DAPPBox4+ cyclophanes are oriented at about 60° to each other with a centroid-to-centroid separation of 3.7 Å between the mean planes of the outer ExBIPY2+ and inner DAPP2+ units, and 3.6 Å between the mean planes of the two inner DAPP2+ units. We show that irradiation of the DAPPHC8+ catenane at 330 nm in acetonitrile solution results in simultaneous energy and electron transfer. The lattoccurs from the inner DAPP2+ dimer to the outer ExBIPY2+ unit, leading to the generation of a temporary chargeseparated state within a rigid and robust homo[2]catenane. Compared to DAPPBox4+, both forward- and back-electron transfer in DAPPHC8+ occur with faster rates, owing to the closer proximity between the electron donor and acceptor in the homo[2]catenane than in the separated cyclophane. 8+

While wholly synthetic catenanes — molecules with non-trivial topologies — have attracted interest for more than half a century based on their aesthetic appeal, the rich physicochemical and materials properties of these exotic molecular compounds is only beginning to be recognized.1-6 Recent attention has been paid to highly charged catenanes, exhibiting not only multiple redox states7, 8 but also photophysical processes3, 9 for their potential usage in energy storage and optoelectronic applications.9 A series of octacationic and rigid catenanes has been prepared by radicalradical templation8, 11 which makes it possible for two tetracationic cyclophanes to mechanically interlock with each other. Radicalradical templation helps to reduce high Coulombic repulsion during catenane synthesis by taking advantage of the dimerization12,

13 of radical cations formed by heterogeneous zinc dust reduction of 4-4ꞌ-bipyridinium units. Although many chemical and physical properties of those catenanes have been observed, their yields of preparation (from 5 to 16%) can to be improved and their syntheses require considerable care and attention to detail. It would be valuable, therefore, to explore efficient templation methods for the construction of complicated MIMs systems with high charge density.

Recently, we have demonstrated the ability of dicationic diazaperopyrenium (DAPP2+) units (Scheme 1), which have an extended,

Scheme 1. Homophilic molecular recognition as well as heterophilic recognition of DAPP2+ units

electron-rich aromatic core region flanked on either side by electron-deficient pyridinium rings, to perform both homophilic recognition14, 15 — involving the interaction of structurally and electronically similar, if not identical, species (e.g. pimerization of viologens) — and heterophilic recognition15, 16 — consisting of constitutionally different species (e.g., donor-acceptor interactions) — in a single redox state. While the homophilic recognition17 of DAPP2+ units, compensating the Coulombic repulsion through strong π-π interactions between the large aromatic core at the center of DAPP2+, has been found in their solid-state superstructures, most of the current work focuses on the use of the heterophilic recognition of DAPP2+ units in mechanically interlocked molecules (MIMs),18 graphene exfoliation applications,19 hostguest inclusion complexes,20 and anticancer activity.21 In addition to recognition ability, some early literature also hinted that


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Journal of the American Chemical Society DAPP2+ units can exhibit photophysical properties22-24 similar to those of their PDI relatives.

Scheme 2. The final two key steps in the synthesis of octacationic homo[2]catenane DAPPHC•8PF6

Here we report the synthesis (Scheme 2) of an octacationic homo[2]catenane DAPPHC8+, obtained as a result of templation involving homophilic interactions between two DAPP2+ units in a bisbromomethylbenzyl precursor. This source of templation is a result of strong π-π stacking interactions outweighing the Coulombic repulsion between a pair of DAPP2+ units, which enables the production of DAPPHC8+ without having to resort to radical templation. The fully charged homo[2]catenane can be isolated directly from the crude reaction mixture by reverse-phase HPLC. The mechanically interlocked nature of the pure product was established beyond any doubt by mass spectrometry and by 1H NMR spectroscopy in CD3CN. The solid-state structure, obtained from single-crystal X-ray crystallography, revealed the relative geometrical disposition of the two cyclophanes. Concurrent ultrafast intramolecular energy and electron transfer have been demonstrated in this homo[2]catenane by using transient absorption (TA) spectroscopy. We reasoned that the homophilic interactions between two DAPP2+ units could be used as a source of templation in the synthesis of the homo[2]catenane DAPPHC8+ on the basis of a comparison of the strength of complex formed between MeDAPP2+ and the positively charged ExBox4+ as well as DAPPBox4+, respectively. The strength of the association (Ka) of the ExBox4+ and MeDAPP2+ was probed (see the SI) by an 1H NMR titration experiment at 298 K in CD3CN and found to be 6.22×102 ± 0.38×102 M-1. The corresponding Ka value for DAPPBox4+ ⊃ MeDAPP2+ was found to be 3.87×103 ± 0.63×103 M-1, i.e., almost six times larger, indicating that the homophilic interactions between the two DAPP2+ units are strong enough to template homo[2]catenane formation.

Figure 1. 1H NMR Spectra (500 MHz) of DAPPHC8+ and DAPPBox4+ recorded in CD3CN at 298 K. The chemical shift of Hδ in DAPPHC8+ compared with the chemical shift in DAPPBox4+ reveals the mechanically interlocked nature of the molecule. The highly symmetrical nature of DAPPHC8+ with the two DAPP2+ units residing inside and the two ExBIPY2+ units on the outside was supported by the 1H NMR spectrum (compared with that of DAPPBox•4PF6 in Figure 1) of DAPPHC•8PF6. The same co-conformation was shown to be present in the solid-state structure of DAPPHC•8TFA. Diffusion of i-Pr2O vapor into a MeCN solution of DAPPHC•8TFA afforded cubic orange single crystals after one week, which were then analyzed by X-ray crystallography. The solid-state (super)structure (Figure 2a) of the homo[2]catenane revealed that the dihedral angle between the mean planes of the mechanically interlocked cyclophanes is 57°. The geometrical arrangement of aromatic units in the two cyclophanes appears to maximize π-π interactions — with centroid -to-centroid separations (Figure 2b) of 3.7 and 3.6 Å — and simultaneously minimizes Coulombic repulsions. The relative rotations of the DAPP2+ units in the homo[2]catenane is similar to the packing of MeDAPP2+ units in the solid-state.25

The homo[2]catenane DAPPHC•8PF6 was prepared from the bisbromomethylbenzyl derivative of DAPP2+ in 30% yield by means of the four steps summarized in Scheme 2 and discussed in detail in the SI. Strong evidence for the formation of the homo[2]catenane was obtained by high resolution mass spectrometry (HRMS) performed on the TFA– salt prior to chromatography. A peak corresponding to [M – 2TFA]2+ was present in the gas phase at m/z 1107.7819 compared with the calculated value of 1107.7814.



Figure 2. Solid-state (super)structure of the DAPPHC8+ obtained from X-ray crystallography on single crystals of the TFA– salt. (a) Stick representation of DAPPHC8+ shows that of dihedral angle between the two mechanically interlocked cyclophanes is 57° in the homo[2]catenane. (b) Plan view of the stick representation of the DAPPHC8+ displaying the molecule’s geometry. The two absorption bands for DAPPHC8+ dissolved in MeCN, which were observed (Figure 3) around 300 and 450 nm, can be assigned to the π-π* transitions of ExBIPY2+ and DAPP2+ units, respectively. It is worth noting that the broadening of the absorption band centered on 450 nm contrasts with the dual absorption bands observed for MeDAPP2+ and DAPPBox4+, indicating strong π-π stacking interaction between the two DAPP2+ units located in the center of the homo[2]catenane. These observations encouraged us to investigate the photophysical properties of DAPPHC8+ using femtosecond and nanosecond transient absorption techniques, and to compare how the excited-state dynamics differ from those of DAPPBox4+.26 The overall features of the homo[2]catenane are similar to those observed in the DAPPBox4+ cyclophane itself: co-absorption of the 330 nm excitation (Figure 4) by each subunit leads to concurrent energy transfer from ExBIPY2+ to DAPP2+ and electron transfer from 1*DAPP2+ to 1* ExBIPY2+; selectively exciting DAPP2+ at lower energy (414 nm) results only in fluorescence and intersystem crossing to the lower triplet state. The rigid interlocked structure and the orthogonal orientation of the two DAPP2+ subunits, however, in the homo[2]catenane alter the rates of these process. Specifically, the energy transfer rate is found to be (0.3 ps)-1 compared to (0.5 ps)-1 in DAPPBox4+.

Figure 3. Absorption spectra of MeExBIPY2+, MeDAPP2+, DAPPBox4+, and DAPPHC8+ in MeCN at room temperature. Note that the DAPP2+ unit feature from 400 to 500 nm in the homo[2]catenane become structureless, indicating strong π–π interactions. This faster observed rate most likely arises from competitive energy transfer to two energy acceptors in DAPPHC8+, one of which is now in closer proximity to the donor but with less favorable electronic coupling and thus a slower energy transfer rate. The S1 lifetime of DAPPHC8+ is also found to be shorter (13.3 ns vs. 20 ns in DAPPBox4+), possibly owing to the orthogonal orientation of the two DAPP2+ subunits causing faster spin-orbit induced intersystem crossing (SO-ISC).

Figure 4. Visible and NIR fsTA spectra for DAPPHC8+ in MeCN at room temperature following 330 nm excitation. The rates of electron transfer27 induced by 330 nm excitation are likewise enhanced. Unlike in DAPPBox4+, the characteristic ExBIPY+• absorption band at 1160 nm is present immediately upon excitation in DAPPHC8+, so the forward electron transfer (FET) must occur within the 200 fs instrument response. The back electron transfer (BET) occurs in 1.6 ns in the homo[2]catenane rather than in the 3.2 ns observed in DAPPBox4+. Again, we attribute the increase in both the FET and BET rates to an increase the number of available electron acceptors, one of which is in closer proximity (3.7 vs. 7.1 Å) and subsequently is more strongly coupled to the donor. DFT calculations (vide infra) suggest that the electron from HOMO orbital on the π-stacked DAPP units may delocalize across both units; the orthogonal orientation between the π systems of the electron donor (inner DAPP2+) and acceptor (ExBIPY2+) is then favorable for spin-orbit charge transfer intersystem crossing28-30 (SOCT-ISC), which also can potentially accelerate the BET rate. Additionally, we observe two different relaxation components with time constants of 6.0 ps and 153 ps in the singlet and charge-separated states, respectively, which likely originate from inter-and intra-Box relaxation in DAPPHC8+. See the SI. The 153 ps relaxation is similar to the 106 ps relaxation observed previously in the case of DAPPBox4+, suggesting that this relaxation results from intra-Box motions. By switching the excitation to 414 nm, we no longer excite the ExBIPY2+ unit and do not observe any energy or electron transfer, confirming once again that the EnT is from the ExBIPY2+ unit to the DAPP2+ unit in the homo[2]catenane and that ET is occuring from the higher lying singlet excited state of ExBIPY2+ as well. According to DFT calculations of the frontier molecular orbitals, the electron is predominantly localized on the inner DAPP2+ dimer HOMO orbital as well as on the low-lying orbitals (LUMO, LUMO+1 and LUMO+2). The LUMO+3 orbital starts showing contributions from the outer extended viologen units, and in LUMO+4 the electron exclusively occupies the extended viologen. See the SI. Our calculations further corroborate the TA observations that excitation of the lower absorption band (414 nm) does not lead to intramolecular ET within DAPPHC8+. In summary, a feasible synthesis of an octacationic homo[2]catenane (DAPPHC8+), which is capable of accommodating eight positive charges in an interlocked nanocube, has been achieved with a 30% yield. 1H NMR Spectroscopy and X-ray single-crystal structure reveal the mechanically interlocked nature


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Journal of the American Chemical Society of DAPPHC8+. The solid-state structure of the homo[2]catenane is consistent with the proposition that the homophilic recognition between two DAPP2+ units works as driving force to form this charge-dense catenane, i.e., strong π–π interactions help to balance the energy penalty arising from Coulombic repulsions. In addition, the homophilic recognition of charged DAPP2+ units has been further illustrated by the association between tetracationic cyclophanes (ExBox4+ as well as between DAPPBox4+) and MeDAPP2+. Our results have demonstrated that the 1:1 complexes, ExBox4+ ⊃ MeDAPP2+ and DAPPBox4+ ⊃ MeDAPP2+, can be formed. Lastly, we have shown that kinetically competitive intramolecular energy and electron transfer processes occur within DAPPHC8+. Our results demonstrate that it is possible to take advantage of this favorable self-templated DAPP2+ dimerization in the future to construct more complicated MIMs architectures with high charge densities.

ASSOCIATED CONTENT Supporting Information Experimental details, including synthesis, NMR, UV-Vis-NIR, fluorescence emission and excitation, fsTA data, and electrochemical experiments, are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *[email protected]; *[email protected];

ORCID Jiawang Zhou: 0000-0002-2399-0030 Peng Li: 0000-0002-4273-4577 Ryan M. Young: 0000-0002-5108-0261 Omar K. Farha: 0000-0002-9904-9845 Joseph T. Hupp: 0000-0003-3982-9812 Michael R. Wasielewski: 0000-0003-0097-5716 J. Fraser Stoddart: 0000-0003-3161-3697

Author Contributions #

X.G. and J.Z. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was conducted as part of the Joint Center of Excellence in Integrated Nanosystems (JCIN) at King Abdulaziz City for Science and Technology (KACST) and Northwestern University (NU). The authors thank both KACST and NU for their continued support of this research. This work was also supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, US DOE under grant no. DEFG02-99ER14999 (M.R.W.). J.T.H. gratefully acknowledge financial support by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences (grant No. DE-FG02 87ER13808) and Northwestern University. O.K.F. gratefully acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences (grant No. DEFG02-17ER16362).

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Towards a Charged Homo[2]catenane Employing Diazaperopyrenium

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