Reversibly modulating a conformation-adaptive fluorophore in [2]catenane

•A conformation-adaptive fluorophore is reversibly modulated in [2]catenanes•The mechanically interlocked structure enables a robust fluorescent molecular switch•Two modulation modes, including wavelength and intensity, can be programmed Developing molecular systems with fluorescent tunability plays an important role in many practical applications such as sensors, photo-electric devices, imaging, and display devices. Learning from naturally occurring green fluorescent proteins that embed fluorophores in the noncovalent environments of peptide assemblies, here, we report that the fluorescent properties of a conformation-adaptive fluorophore can be reversibly modulated in the supramolecular environment of mechanically interlocked [2]catenanes. By comparing the switching ability of molecular systems with different supramolecular environments, mechanically interlocked systems are found to enable a unique fluorescent switching capability with the most remarkable signal change and excellent reversibility. This work provides a distinctive strategy for designing fluorescent switches by combining conformation-adaptive fluorophores with molecular switches and machines. Tuning molecular emission by chemical means has long been a fundamental topic, because the emerging methodologies and mechanisms of this topic usually bring a lot of opportunities in many multi-disciplinary applications. Here, we demonstrate the reversible switching of a conformation-adaptive fluorophore, 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC), by incorporating this fluorescent unit into a mechanically interlocked [2]catenane. Taking advantage of the mechanical bond of [2]catenane, the conformational freedom of the DPAC-macrocycle can be modulated by the co-conformational state of the [2]catenane, thus enabling the reversible switching of the fluorescent properties of DPAC. Owing to the mechanically interlocked structure, this fluorescent molecular system can be switched in a dual-mode (wavelength or intensity), visually recognizable, and highly reversible manner. This work provides a distinct mechanism of switching molecular emission by modulating conformation-adaptive fluorescent systems in mechanically interlocked structures. Tuning molecular emission by chemical means has long been a fundamental topic, because the emerging methodologies and mechanisms of this topic usually bring a lot of opportunities in many multi-disciplinary applications. Here, we demonstrate the reversible switching of a conformation-adaptive fluorophore, 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC), by incorporating this fluorescent unit into a mechanically interlocked [2]catenane. Taking advantage of the mechanical bond of [2]catenane, the conformational freedom of the DPAC-macrocycle can be modulated by the co-conformational state of the [2]catenane, thus enabling the reversible switching of the fluorescent properties of DPAC. Owing to the mechanically interlocked structure, this fluorescent molecular system can be switched in a dual-mode (wavelength or intensity), visually recognizable, and highly reversible manner. This work provides a distinct mechanism of switching molecular emission by modulating conformation-adaptive fluorescent systems in mechanically interlocked structures. Developing fluorescent molecular systems with chemical tunability and switchability has aroused a large amount of research interest,1Callan J.F. de Silva A.P. Magri D.C. Luminescent sensors and switches in the early 21st century.Tetrahedron. 2005; 61: 8551-8588Crossref Scopus (1053) Google Scholar, 2Van de Linde S. Sauer M. How to switch a fluorophore: from undesired blinking to controlled photoswitching.Chem. Soc. Rev. 2014; 43: 1076-1087Crossref PubMed Google Scholar, 3Sedgwick A.C. Wu L. Han H.H. Bull S.D. He X.P. James T.D. Sessler J.L. Tang B.Z. Tian H. Yoon J. Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents.Chem. Soc. Rev. 2018; 47: 8842-8880Crossref PubMed Google Scholar exhibiting many potential applications such as sensors,4Cao D. Liu Z. Verwilst P. Koo S. Jangjili P. Kim J.S. Lin W. Coumarin-based small-molecule fluorescent chemosensors.Chem. Rev. 2019; 119: 10403-10519Crossref PubMed Scopus (477) Google Scholar imaging,5Wu L. Huang C. Emery B.P. Sedgwick A.C. Bull S.D. He X.P. Tian H. Yoon J. Sessler J.L. James T.D. Förster resonance energy transfer (FRET)-based small-molecule sensors and imaging agents.Chem. Soc. Rev. 2020; 49: 5110-5139Crossref PubMed Google Scholar and the optical display.6Liu Y. Li C. Ren Z. Yan S. Bryce M.R. All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes.Nat. Rev. Mater. 2018; 3: 18020Crossref Scopus (675) Google Scholar Stimuli-responsive fluorescent switches are a species of molecular switches whose fluorescence can be reversibly switched between two or more steady states under external stimuli.7Zhang J. Fu Y. Han H.H. Zang Y. Li J. He X.P. Feringa B.L. Tian H. Remote light-controlled intracellular target recognition by photochromic fluorescent glycoprobes.Nat. Commun. 2017; 8: 987Crossref PubMed Scopus (103) Google Scholar The widely used stimulus modes include light,7Zhang J. Fu Y. Han H.H. Zang Y. Li J. He X.P. Feringa B.L. Tian H. Remote light-controlled intracellular target recognition by photochromic fluorescent glycoprobes.Nat. Commun. 2017; 8: 987Crossref PubMed Scopus (103) Google Scholar pH,8Cao Z.-Q. Miao Q. Zhang Q. Li H. Qu D.-H. Tian H. A fluorescent bistable [2] rotaxane molecular switch on SiO2 nanoparticles.Chem. Commun. 2015; 51: 4973-4976Crossref PubMed Google Scholar redox,9Lou Z. Li P. Han K. Redox-responsive fluorescent probes with different design strategies.Acc. Chem. Res. 2015; 48: 1358-1368Crossref PubMed Scopus (379) Google Scholar ions,10Langton M.J. Beer P.D. Rotaxane and catenane host structures for sensing charged guest species.Acc. Chem. Res. 2014; 47: 1935-1949Crossref PubMed Scopus (205) Google Scholar temperature,11Nakatani Y. Furusho Y. Yashima E. Amidinium carboxylate salt bridges as a recognition motif for mechanically interlocked molecules: synthesis of an optically active [2] catenane and control of its structure.Angew. Chem. Int. Ed. Engl. 2010; 49: 5463-5467Crossref PubMed Scopus (61) Google Scholar solvents,12Yang Z. Cao J. He Y. Yang J.H. Kim T. Peng X. Kim J.S. Macro-/micro-environment-sensitive chemosensing and biological imaging.Chem. Soc. Rev. 2014; 43: 4563-4601Crossref PubMed Google Scholar etc. Tremendous efforts have succeeded in modulating molecular fluorescent properties by dynamic tuning diversified electron/energy transfer mechanisms at the level of individual fluorophores3Sedgwick A.C. Wu L. Han H.H. Bull S.D. He X.P. James T.D. Sessler J.L. Tang B.Z. Tian H. Yoon J. Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents.Chem. Soc. Rev. 2018; 47: 8842-8880Crossref PubMed Google Scholar, 4Cao D. Liu Z. Verwilst P. Koo S. Jangjili P. Kim J.S. Lin W. Coumarin-based small-molecule fluorescent chemosensors.Chem. Rev. 2019; 119: 10403-10519Crossref PubMed Scopus (477) Google Scholar, 5Wu L. Huang C. Emery B.P. Sedgwick A.C. Bull S.D. He X.P. Tian H. Yoon J. Sessler J.L. James T.D. Förster resonance energy transfer (FRET)-based small-molecule sensors and imaging agents.Chem. Soc. Rev. 2020; 49: 5110-5139Crossref PubMed Google Scholar,7Zhang J. Fu Y. Han H.H. Zang Y. Li J. He X.P. Feringa B.L. Tian H. Remote light-controlled intracellular target recognition by photochromic fluorescent glycoprobes.Nat. Commun. 2017; 8: 987Crossref PubMed Scopus (103) Google Scholar, 8Cao Z.-Q. Miao Q. Zhang Q. Li H. Qu D.-H. Tian H. A fluorescent bistable [2] rotaxane molecular switch on SiO2 nanoparticles.Chem. Commun. 2015; 51: 4973-4976Crossref PubMed Google Scholar, 9Lou Z. Li P. Han K. Redox-responsive fluorescent probes with different design strategies.Acc. Chem. Res. 2015; 48: 1358-1368Crossref PubMed Scopus (379) Google Scholar, 10Langton M.J. Beer P.D. Rotaxane and catenane host structures for sensing charged guest species.Acc. Chem. Res. 2014; 47: 1935-1949Crossref PubMed Scopus (205) Google Scholar or a collection of fluorophore assemblies.13Mei J. Leung N.L. Kwok R.T. Lam J.W. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (4712) Google Scholar, 14Qian H. Cousins M.E. Horak E.H. Wakefield A. Liptak M.D. Aprahamian I. Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission.Nat. Chem. 2017; 9: 83-87Crossref PubMed Scopus (242) Google Scholar, 15Shao B. Baroncini M. Qian H. Bussotti L. Di Donato M. Credi A. Aprahamian I. Solution and solid-state emission toggling of a photochromic hydrazone.J. Am. Chem. Soc. 2018; 140: 12323-12327Crossref PubMed Scopus (49) Google Scholar These well-established fluorescent molecular switches exhibited their versatility and reliability as responsive emissive materials with great applicable prospects.16Kwok R.T.K. Leung C.W.T. Lam J.W.Y. Tang B.Z. Biosensing by luminogens with aggregation-induced emission characteristics.Chem. Soc. Rev. 2015; 44: 4228-4238Crossref PubMed Google Scholar, 17Wang Y. Yang J. Fang M. Yu Y. Zou B. Wang L. Tian Y. Cheng J. Tang B.Z. Li Z. Förster resonance energy transfer: an efficient way to develop stimulus-responsive room-temperature phosphorescence materials and their applications.Matter. 2020; 3: 449-463Abstract Full Text Full Text PDF Scopus (101) Google Scholar, 18Wang Q. Zhang Q. Zhang Q.W. Li X. Zhao C.X. Xu T.Y. Qu D.H. Tian H. Color-tunable single-fluorophore supramolecular system with assembly-encoded emission.Nat. Commun. 2020; 11: 158Crossref PubMed Scopus (94) Google Scholar, 19Zhang X. Rehm S. Safont-Sempere M.M. Würthner F. Vesicular perylene dye nanocapsules as supramolecular fluorescent pH sensor systems.Nat. Chem. 2009; 1: 623-629Crossref PubMed Scopus (492) Google Scholar, 20Ai Y. Chan M.H.-Y. Chan A.K.-W. Ng M. Li Y. Yam V.W.-W. A platinum(II) molecular hinge with motions visualized by phosphorescence changes.Proc. Natl. Acad. Sci. USA. 2019; 116: 13856-13861Crossref PubMed Scopus (24) Google Scholar Recently, our group exploited a distinct family of conformation-adaptive fluorophores based on 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC),21Zhang Z. Wu Y.S. Tang K.C. Chen C.L. Ho J.W. Su J. Tian H. Chou P.T. Excited-state conformational/electronic responses of saddle-shaped N,N ′-disubstituted-dihydrodibenzo[a,c]phenazines: wide-tuning emission from red to deep blue and white light combination.J. Am. Chem. Soc. 2015; 137: 8509-8520Crossref PubMed Scopus (190) Google Scholar, 22Zhang Z. Song W. Su J. Tian H. Vibration-induced emission (VIE) of N,N′-disubstituted-dihydribenzo[a,c]phenazines: fundamental understanding and emerging applications.Adv. Funct. Mater. 2020; 30: 1902803Crossref Scopus (30) Google Scholar, 23Zhang Z. Chen C.L. Chen Y.A. Wei Y.C. Su J. Tian H. Chou P.T. Tuning the conformation and color of conjugated polyheterocyclic skeletons by installing ortho-methyl groups.Angew. Chem. Int. Ed. Engl. 2018; 57: 9880-9884Crossref PubMed Scopus (56) Google Scholar, 24Humeniuk H.V. Rosspeintner A. Licari G. Kilin V. Bonacina L. Vauthey E. Sakai N. Matile S. White-fluorescent dual-emission mechanosensitive membrane probes that function by bending rather than twisting.Angew. Chem. Int. Ed. Engl. 2018; 57: 10559-10563Crossref PubMed Scopus (44) Google Scholar, 25Chen W. Chen C.L. Zhang Z. Chen Y.A. Chao W.C. Su J. Tian H. Chou P.T. Snapshotting the excited-state planarization of chemically locked N,N′-disubstituted dihydrodibenzo[a,c]phenazines.J. Am. Chem. Soc. 2017; 139: 1636-1644Crossref PubMed Scopus (87) Google Scholar, 26Zhou Z. Chen D.G. Saha M.L. Wang H. Li X. Chou P.T. Stang P.J. Designed conformation and fluorescence properties of self-assembled phenazine-cored platinum(II) metallacycles.J. Am. Chem. Soc. 2019; 141: 5535-5543Crossref PubMed Scopus (54) Google Scholar, 27Chen W. Guo C. He Q. Chi X. Lynch V.M. Zhang Z. Su J. Tian H. Sessler J.L. Molecular cursor caliper: a fluorescent sensor for dicarboxylate dianions.J. Am. Chem. Soc. 2019; 141: 14798-14806Crossref PubMed Scopus (52) Google Scholar, 28Zhang Z. Sun G. Chen W. Su J. Tian H. The endeavor of vibration-induced emission (VIE) for dynamic emissions.Chem. Sci. 2020; 11: 7525-7537Crossref PubMed Google Scholar, 29Sun G. Wei Y.-C. Zhang Z. Lin J.-A. Liu Z.-Y. Chen W. Su J. Chou P.-T. Tian H. Diversified excited-state relaxation pathways of donor-linker-acceptor dyads controlled by the bent-to-planar motion of donor.Angew. Chem. Int. Ed. Engl. 2020; 59: 18611-18618Crossref PubMed Scopus (11) Google Scholar a fluorophore that allows the precise modulation of its multi-color emission by sensing the conformational freedom of the photo-excited vibrational motion of the aromatic backbone.22Zhang Z. Song W. Su J. Tian H. Vibration-induced emission (VIE) of N,N′-disubstituted-dihydribenzo[a,c]phenazines: fundamental understanding and emerging applications.Adv. Funct. Mater. 2020; 30: 1902803Crossref Scopus (30) Google Scholar,28Zhang Z. Sun G. Chen W. Su J. Tian H. The endeavor of vibration-induced emission (VIE) for dynamic emissions.Chem. Sci. 2020; 11: 7525-7537Crossref PubMed Google Scholar Intramolecular cyclization has been proven as a reliable strategy to control the conformational freedom by molecularly engineering the ring size,25Chen W. Chen C.L. Zhang Z. Chen Y.A. Chao W.C. Su J. Tian H. Chou P.T. Snapshotting the excited-state planarization of chemically locked N,N′-disubstituted dihydrodibenzo[a,c]phenazines.J. Am. Chem. Soc. 2017; 139: 1636-1644Crossref PubMed Scopus (87) Google Scholar thus producing multi-color emission by a series of DPAC-containing macrocycles. Meanwhile, further investigations also have demonstrated the possibilities to form multi-color DPAC-containing macrocycles by versatile noncovalent molecular recognitions.26Zhou Z. Chen D.G. Saha M.L. Wang H. Li X. Chou P.T. Stang P.J. Designed conformation and fluorescence properties of self-assembled phenazine-cored platinum(II) metallacycles.J. Am. Chem. Soc. 2019; 141: 5535-5543Crossref PubMed Scopus (54) Google Scholar,27Chen W. Guo C. He Q. Chi X. Lynch V.M. Zhang Z. Su J. Tian H. Sessler J.L. Molecular cursor caliper: a fluorescent sensor for dicarboxylate dianions.J. Am. Chem. Soc. 2019; 141: 14798-14806Crossref PubMed Scopus (52) Google Scholar These preliminary findings further motivated us to explore the possibility to reversibly modulate the conformation-adaptive emission of individual DPAC fluorophores, whose realization might evolve the development of fluorescent switches based on this wavelength-tunable single-fluorophore system with a conformation-adaptive mechanism. Inspired by many biological fluorescent proteins that usually exhibit dynamic emission by modulating their fluorophores in the noncovalent environment of cavities,30Patterson G.H. Lippincott-Schwartz J. A photoactivatable GFP for selective photolabeling of proteins and cells.Science. 2002; 297: 1873-1877Crossref PubMed Scopus (1272) Google Scholar,31Zimmer M. Green fluorescent protein (GFP): applications, structure, and related photophysical behavior.Chem. Rev. 2002; 102: 759-781Crossref PubMed Scopus (936) Google Scholar here, we propose a biomimetic modulation strategy for designing DPAC-based fluorescent switches, that is, to modulate the conformation-adaptive emission of DPAC in the noncovalent environment of mechanically interlocked molecules (MIMs).32Bruns C.J. Stoddart J.F. The Nature of the Mechanical Bond: from Molecules to Machines. John Wiley & Sons, 2016Crossref Scopus (395) Google Scholar, 33Stoddart J.F. Mechanically interlocked molecules (MIMs)-molecular shuttles, switches, and machines (Nobel lecture).Angew. Chem. Int. Ed. Engl. 2017; 56: 11094-11125Crossref PubMed Scopus (481) Google Scholar, 34Sauvage J.P. From chemical topology to molecular machines (Nobel lecture).Angew. Chem. Int. Ed. Engl. 2017; 56: 11080-11093Crossref PubMed Scopus (426) Google Scholar, 35Goujon A. Moulin E. Fuks G. Giuseppone N. [c2]Daisy chain rotaxanes as molecular muscles.CCS Chem. 2019; 1: 83-96Google Scholar, 36Erbas-Cakmak S. Leigh D.A. McTernan C.T. Nussbaumer A.L. Artificial molecular machines.Chem. Rev. 2015; 115: 10081-10206Crossref PubMed Scopus (1153) Google Scholar, 37Mena-Hernando S. Pérez E.M. Mechanically interlocked materials. Rotaxanes and catenanes beyond the small molecule.Chem. Soc. Rev. 2019; 48: 5016-5032Crossref PubMed Google Scholar, 38David A.H.G. Casares R. Cuerva J.M. Campaña A.G. Blanco V. A [2]rotaxane-based circularly polarized luminescence switch.J. Am. Chem. Soc. 2019; 141: 18064-18074Crossref PubMed Scopus (56) Google Scholar, 39Chang J.-C. Tseng S.-H. Lai C.-C. Liu Y.-H. Peng S.-M. Chiu S.-H. Mechanically interlocked daisy-chain-like structures as multidimensional molecular muscles.Nat. Chem. 2017; 9: 128-134Crossref Google Scholar, 40Ceroni P. Credi A. Venturi M. Light to investigate (read) and operate (write) molecular devices and machines.Chem. Soc. Rev. 2014; 43: 4068-4083Crossref PubMed Google Scholar The unique mechanical bond in MIMs, especially catenanes, supports supramolecular confinement for an intramolecularly cyclized DPAC-ring, enabling the effective modulation of conformational freedom of DPAC-ring by the co-conformational state of the catenanes. As a result, the DPAC-containing catenanes can act as a fluorescent switch with excellent reversibility, pH-responsive ability, and wavelength tunability. We expect this work as a starting point of a series of DPAC-based fluorescent switches, as well as an important step toward conformation-adaptive molecular emissive materials. To introduce DPAC fluorophore into the mechanically interlocked structure of [2]catenane, it is necessary to engineer the DPAC unit into a macrocycle and meanwhile endow the macrocycle with molecular recognition ability. In view of the reliable host-guest chemistry of crown ethers,41Zhang Q. Rao S.-J. Xie T. Li X. Xu T.-Y. Li D.-W. Qu D.-H. Long Y.-T. Tian H. Muscle-like artificial molecular actuators for nanoparticles.Chem. 2018; 4: 2670-2684Abstract Full Text Full Text PDF Scopus (64) Google Scholar, 42Rao S.J. Zhang Q. Mei J. Ye X.H. Gao C. Wang Q.C. Qu D.H. Tian H. One-pot synthesis of hetero[6]rotaxane bearing three different kinds of macrocycle through a self-sorting process.Chem. Sci. 2017; 8: 6777-6783Crossref PubMed Google Scholar, 43Coutrot F. Romuald C. Busseron E. A new pH-switchable dimannosyl [c2]daisy chain molecular machine.Org. Lett. 2008; 10: 3741-3744Crossref PubMed Scopus (175) Google Scholar we embedded the DPAC unit into a crown-ether-like macrocycle by intramolecular cyclization with tetramethylene glycol chains (Figure 1A). Owing to the template of sodium ions of NaH, DPAC-ring was synthesized in a high yield of 64% by Williamson etherification (Scheme 1A). The chemical structure has been confirmed by 1H NMR, 13C NMR, and high-resolution mass spectrum (HR-MS) (detailed information can be found in supplemental information). X-ray single-crystal structure of DPAC-ring was also obtained (Figure 2A; Table S1). The distance between two oxygen atoms of the ring was 7.464 Å (O1···O5, Figure S1), supporting the adequate space for benzene (≈ 5.0 Å) to pass through the cavity. The optical properties of the host macrocycle were investigated (Figure S2). In acetonitrile, the maximum absorption/emission wavelength of the DPAC-ring was 350/490 nm, respectively (Figure 2B). In dichloromethane, the absorption and emission spectra showed a slight red-shift (353/491 nm), which was consistent with the results reported in previous literature.25Chen W. Chen C.L. Zhang Z. Chen Y.A. Chao W.C. Su J. Tian H. Chou P.T. Snapshotting the excited-state planarization of chemically locked N,N′-disubstituted dihydrodibenzo[a,c]phenazines.J. Am. Chem. Soc. 2017; 139: 1636-1644Crossref PubMed Scopus (87) Google ScholarScheme 1Synthesis routes of the DPAC-ring and two [2]catenanesShow full caption(A) Synthesis of DPAC-ring. The dihedral angle describing the bending between planes 1 (C1, N1, and N2) and 2 (N1, N2, and C2) is denoted as Θd for all titled compounds, and a larger dihedral angle indicates more planarization. The characteristic angles (ΘS1: ∠C3-N2-N1; ΘS2: ∠C4-N1-N2) are marked as well.(B) Synthesis of [2]catenane 6-H and [2]catenane 7-H. The characteristic hydrogens in catenanes are labeled.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 2Host-guest combination and photophysical properties of pseudorotaxane DPAC-ring·1-HShow full caption(A) Design strategy and single-crystal structures of DPAC-ring and pseudorotaxane DPAC-ring·1-H (Plane 1: C1-N1-N2; Plane 2: C2-N2-N1).(B) Normalized absorption and fluorescence spectra of DPAC-ring in CH3CN and CH2Cl2.(C) Job plot curve of DPAC-ring and 1-H complex in CH3CN.(D) Normalized fluorescence spectra of DPAC-ring upon titration with 1-H in CH3CN.(E) Fluorescence spectra of DPAC-ring: 1-H (1:12) complex upon titration with DBU in CH3CN (λex = 350 nm).View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A) Synthesis of DPAC-ring. The dihedral angle describing the bending between planes 1 (C1, N1, and N2) and 2 (N1, N2, and C2) is denoted as Θd for all titled compounds, and a larger dihedral angle indicates more planarization. The characteristic angles (ΘS1: ∠C3-N2-N1; ΘS2: ∠C4-N1-N2) are marked as well. (B) Synthesis of [2]catenane 6-H and [2]catenane 7-H. The characteristic hydrogens in catenanes are labeled. (A) Design strategy and single-crystal structures of DPAC-ring and pseudorotaxane DPAC-ring·1-H (Plane 1: C1-N1-N2; Plane 2: C2-N2-N1). (B) Normalized absorption and fluorescence spectra of DPAC-ring in CH3CN and CH2Cl2. (C) Job plot curve of DPAC-ring and 1-H complex in CH3CN. (D) Normalized fluorescence spectra of DPAC-ring upon titration with 1-H in CH3CN. (E) Fluorescence spectra of DPAC-ring: 1-H (1:12) complex upon titration with DBU in CH3CN (λex = 350 nm). Then the host-guest recognition ability of DPAC-ring was studied. Dibenzylammonium (DBA) (1-H) was selected as the guest due to its high binding affinity with crown-ether-based hosts.41Zhang Q. Rao S.-J. Xie T. Li X. Xu T.-Y. Li D.-W. Qu D.-H. Long Y.-T. Tian H. Muscle-like artificial molecular actuators for nanoparticles.Chem. 2018; 4: 2670-2684Abstract Full Text Full Text PDF Scopus (64) Google Scholar, 42Rao S.J. Zhang Q. Mei J. Ye X.H. Gao C. Wang Q.C. Qu D.H. Tian H. One-pot synthesis of hetero[6]rotaxane bearing three different kinds of macrocycle through a self-sorting process.Chem. Sci. 2017; 8: 6777-6783Crossref PubMed Google Scholar, 43Coutrot F. Romuald C. Busseron E. A new pH-switchable dimannosyl [c2]daisy chain molecular machine.Org. Lett. 2008; 10: 3741-3744Crossref PubMed Scopus (175) Google Scholar 1H NMR spectra of the host DPAC-ring and the guest dibenzylammonium (1-H) were compared with that of the stoichiometric mixture of 1-H and DPAC-ring in CD3CN (Figure S3), confirming the 1:1 pseudorotaxane structure because of the distinctive proton shift. A Job plot further confirmed the 1:1 host-guest complex of DPAC-ring and 1-H and the association constants (Ka) were calculated to be 52.49 M−1 in CH3CN at 25°C via 1H NMR titration44Hibbert D.B. Thordarson P. The death of the Job plot, transparency, open science and online tools, uncertainty estimation methods and other developments in supramolecular chemistry data analysis.Chem. Comm. 2016; 52: 12792-12805Crossref PubMed Google Scholar (Figures 2C and S4). This binding affinity was approximately in accordance with the corresponding Ka value of 360 M−1 for DB24C8·1-H complex in CH3CN.45Ashton P.R. Campbell P.J. Glink P.T. Philp D. Spencer N. Stoddart J.F. Chrystal E.J.T. Menzer S. Williams D.J. Tasker P.A. Dialkylammonium ion/crown ether complexes: the forerunners of a new family of interlocked molecules.Angew. Chem. Int. Ed. Engl. 1995; 34: 1865-1869Crossref Scopus (386) Google Scholar Moreover, DPAC-ring·1-H displayed a main peak at m/z 850.4218 in ESI-MS, which was assigned to the [DPAC-ring·1-H]+ (Figure S5). Moreover, X-ray single-crystal structure of pseudo[2]rotaxane DPAC-ring·1-H was obtained (Figure 2A; Table S2 and S3), showing the 1:1 host-guest combination mediated by hydrogen bonds. By comparing the dihedral angle (∠C1-N1-N2-C2, Θd) of pseudorotaxane DPAC-ring·1-H with that of DPAC-ring (136.98°/140.16°) and the distances of two oxygen atoms on benzyl (7.464 Å and 6.938 Å, Figures S1 and S6), it was found that the ground-state conformation of the ring was more twisted in the presence of guest 1-H. These observations suggested the enhanced ring tension after binding guest 1-H by host-guest combination, also leading to the further deplanarization of the aromatic backbone of DPAC unit. The recognition-induced deplanarization was further revealed by both blue-shifted absorption peak (350→347 nm, Figure S7) and a fluorescent peak of DPAC-ring after binding 1-H (490→477 nm, Table S4; Figure 2D). Owing to the noncovalent nature, the supramolecular recognition DPAC-ring·1-H could be dissociated by adding an excess of the base (1,8-diazabicyclo[5,4,0]-undec-7-ene, DBU), recovering the blue-shifted fluorescent peak back to the original state of DPAC-ring (Figure 2E). A blank experiment in the absence of 1-H confirmed the non-responsive property of DPAC-ring under acid/base stimuli (Figure S8; Table S4). Density functional theory (DFT) was used to further understand this host-guest system in solution. The geometries at the ground state of DPAC-ring and DPAC-ring·1-H were optimized to obtain a structure corresponding to the global minimum (Figure S9; Data S1), after preliminary conformer sampling at the GFN-FF level46Spicher S. Grimme S. Robust atomistic modeling of materials, organometallic, and biochemical systems.Angew. Chem. Int. Ed. Engl. 2020; 59: 15665-15673Crossref PubMed Scopus (98) Google Scholar (see supplemental information). Conformer sampling followed by DFT optimization allowed us to obtain the geometry of the most stable structure at the ground state. The absorption and emission of all the structures were computed both at the TD(10 states)-PBE0/def2-SVP (see Table 1) and the TD(5 states)-ωB97X-D/def2-SVP levels (see supplemental information), both showing qualitatively comparable results after inspection of the transitions involved in the photophysical processes. This strategy was further extended to the mechanically interlocked structures (vide infra). In all cases examined, the species subject to the TD-DFT computational analysis populate an excited state with a partial charge-transfer state, where the aniline nitrogen N1 and N2 (and their aryls) donate to the phenanthrene moiety (see supplemental information). The crown ether part of DPAC-ring exhibited a folded conformation (Figure S9A), indicating the higher mobility of the free DPAC-ring. The flexible glycol chain was unfolded after recognition with guest 1-H (Figure S9B). This structural observation suggested the substantial difference in the conformational freedom of DPAC-ring before and after the recognition of the guest. The calculated dihedral angles (141.16°) of DPAC-ring was larger than that of DPAC-ring·1-H (137.28°), a structural feature comparable to the one found in the single crystals (Table 1; Figure 2A). The S0→S1 absorption was calculated from the S0 optimized geometry, following the Franck-Condon excitation (Table S5). The vertical S1→S0 transition was obtained from the vertical emission from the optimized S1 state (Table S6).21Zhang Z. Wu Y.S. Tang K.C. Chen C.L. Ho J.W. Su J. Tian H. Chou P.T. Excited-state conformational/electronic responses of saddle-shaped N,N ′-disubstituted-dihydrodibenzo[a,c]phenazines: wide-tuning emission from red to deep blue and white light combination.J. Am. Chem. Soc. 2015; 137: 8509-8520Crossref PubMed Scopus (190) Google Scholar,25Chen W. Chen C.L. Zhang Z. Chen Y.A. Chao W.C. Su J. Tian H. Chou P.T. Snapshotting the excited-state planarization of chemically locked N,N′-disubstituted dihydrodibenzo[a,c]phenazines.J. Am. Chem. Soc. 2017; 139: 1636-1644Crossref PubMed Scopus (87) Google Scholar The absorption maxima of DPAC-ring and DPAC-ring·1-H are comparable, as for the experimental data. Analogously, the calculated emission peaks of DPAC-ring·1-H showed a blue-shift in respect of the free host (446→441 nm, Table 1), which was consistent with the experimentally observed trend (490→477 nm, Table 1; Figure 2D).Table 1Experimental and calculated absorption wavelengths in acetonitrile (S0→S1 transitions for all compounds), emission wavelengths (S1→S0 transitions for all compounds), and relevant oscillator strengthsAbsorptionEmissionCompoundλexp (nm)λcalcbValues obtained at the TD(10 states)-PBE0/def2-SVP level on geometries optimized either at the ωB97X-D/def2-SVP or at the TD(5 states)-ωB97X-D/def2-SVP level using SMD(CH3CN) as implicit solvent method. (nm)fλexp (nm)Φem (%)λcalc[b] (nm)fDPAC-ring3503630.12749019.64460.177DPAC-ring·1-HaThe solution of DPAC-ring·1-H (1:12) is a mixture of DPAC-ring, 1-H, and pseudorotaxane DPAC-ring·1-H.3473690.111477_4410.232[2]Catenane 6-H3473620.14044724.34340.226[2]Catenane 63623760.14846722.44370.214[2]Catenane 7-H3483740.15044721.24360.226[2]Catenane 73553590.1524456.94240.238a The solution of DPAC-ring·1-H (1:12) is a mixture of DPAC-ring, 1-H, and pseudorotaxane DPAC-ring·1-H.b Values