View source for Host–guest chemistry

{{Short description|Supramolecular structures held together other than by covalent bonds}}
In [[supramolecular chemistry]],<ref>{{cite book|last1=Steed|first1=Jonathan W.|last2=Atwood|first2=Jerry L.|title=Supramolecular Chemistry|date=2009|publisher=Wiley|isbn=978-0-470-51234-0|pages=1002|edition=2nd.}}</ref> '''host–guest chemistry''' describes [[inclusion compound|complexes]] that are composed of two or more [[molecule]]s or [[ion]]s that are held together in unique structural relationships by forces other than those of full [[covalent bond]]s. Host–guest chemistry encompasses the idea of molecular recognition and interactions through [[non-covalent bonding]]. Non-covalent bonding is critical in maintaining the 3D structure of large molecules, such as proteins and is involved in many biological processes in which large molecules bind specifically but transiently to one another.

Although non-covalent interactions could be roughly divided into those with more electrostatic or dispersive contributions, there are few commonly mentioned types of non-covalent interactions: [[ionic bonding]], [[hydrogen bond]]ing, [[van der Waals forces]] and [[hydrophobic effect|hydrophobic interactions]].<ref name="Molecularcellbiology">{{cite book | title = Molecular Cell Biology | year = 2008 | isbn = 978-0-7167-7601-7 | author1 = Lodish, H.|author2 = Berk, A.| author3 = Kaiser, C.| publisher = Macmillan }}</ref>

Host-guest interaction has raised dramatical attention since it was discovered. It is an important field, because many biological processes require the host-guest interaction, and it can be useful in some material designs. There are several typical host molecules, such as, cyclodextrin, crown ether, ''et al''.

[[File:Cucurbit-6-uril ActaCrystallB-Stru 1984 382.jpg|thumbnail|200px|Crystal structure of a host–guest complex with a p-xylylenediammonium bound within a [[cucurbituril]]
<ref>{{cite journal |last= Freeman |first= Wade A. 
|journal= [[Acta Crystallographica B]] 
|year= 1984 
|pages= 382–387 
|title= Structures of the ''p''-xylylenediammonium chloride and calcium hydrogensulfate adducts of the cavitand 'cucurbituril', C<sub>36</sub>H<sub>36</sub>N<sub>24</sub>O<sub>12</sub> 
|doi= 10.1107/S0108768184002354 
|volume=40|issue= 4 
|bibcode= 1984AcCrB..40..382F 
}}</ref>]]

[[File:Encapsulating Assembly ChemEurJ 1996 v2 p989.jpg|thumbnail|200px|A guest [[nitrogen|N<sub>2</sub>]] is bound within a host hydrogen-bonded capsule
<ref>{{cite journal| last1=Valdés| first1=Carlos| last2=Toledo| first2=Leticia M.| last3=Spitz|first3=Urs|last4=Rebek|first4=Julius
|title=Structure and Selectivity of a Small Dimeric Encapsulating Assembly
|journal=Chem. Eur. J.
|year=1996
|volume=2
| issue=8|pages=989–991
|doi=10.1002/chem.19960020814}}</ref>]]

"Host molecules" usually have "pore-like" structure that is able to capture a "guest molecules". Although called molecules, hosts and guests are often ions. The driving forces of the interaction might vary, such as hydrophobic effect and van der Waals forces<ref name=":05">{{Cite book |url=https://www.worldcat.org/oclc/992802408 |title=Comprehensive supramolecular chemistry II |date=2017 |others=J. L. Atwood, George W. Gokel, Leonard J. Barbour |isbn=978-0-12-803199-5 |location=Amsterdam, Netherlands |oclc=992802408}}</ref><ref>{{Cite journal |last=Stoddart |first=J. F. |date=1988 |title=Chapter 12. Host–guest chemistry |url=http://xlink.rsc.org/?DOI=OC9888500353 |journal=Annu. Rep. Prog. Chem., Sect. B: Org. Chem. |language=en |volume=85 |pages=353–386 |doi=10.1039/OC9888500353 |issn=0069-3030}}</ref><ref>{{Citation |last=Harada |first=Akira |title=Supramolecular Polymers (Host-Guest Interactions) |date=2013 |url=http://link.springer.com/10.1007/978-3-642-36199-9_54-1 |encyclopedia=Encyclopedia of Polymeric Nanomaterials |pages=1–5 |editor-last=Kobayashi |editor-first=Shiro |access-date=2023-02-15 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-36199-9_54-1 |isbn=978-3-642-36199-9 |editor2-last=Müllen |editor2-first=Klaus}}</ref><ref name=":12">{{Cite journal |last1=Seale |first1=James S. W. |last2=Feng |first2=Yuanning |last3=Feng |first3=Liang |last4=Astumian |first4=R. Dean |last5=Stoddart |first5=J. Fraser |date=2022 |title=Polyrotaxanes and the pump paradigm |url=http://xlink.rsc.org/?DOI=D2CS00194B |journal=Chemical Society Reviews |language=en |volume=51 |issue=20 |pages=8450–8475 |doi=10.1039/D2CS00194B |pmid=36189715 |s2cid=252682455 |issn=0306-0012}}</ref>

Binding between host and guest can be highly selective, in which case the interaction is called [[molecular recognition]]. Often, a [[dynamic equilibrium]] exist between the unbound and the bound states:

:<math>H + G \rightleftharpoons\ HG</math>
:H ="host", G ="guest", HG ="host–guest complex"

The "host" component is often the larger molecule, and it encloses the smaller, "guest", molecule. In biological systems, the analogous terms of host and guest are commonly referred to as [[enzyme]] and [[Substrate (biochemistry)|substrate]] respectively.<ref name="textbook">{{cite book |last1=Anslyn |first1=Eric V. |title=Modern Physical Organic Chemistry |last2=Dougherty |first2=Dennis A. |publisher=MacMillan |year=2005 |isbn=978-1-891389-31-3}}</ref>
==Inclusion and clathrate compounds==
[[File:Cd(CN)2CCl4.jpg|thumb|Cd(CN)<sub>2</sub>·CCl<sub>4</sub>: [[Cadmium cyanide]] clathrate framework (in blue) containing [[carbon tetrachloride]] (C atoms in gray and disordered Cl positions in green).]]
Closely related to host–guest chemistry, are '''inclusion compounds''' (also known as an '''inclusion complexes''').  Here, a  [[complex (chemistry)|chemical complex]] in which one [[chemical compound]] (the "host") has a cavity into which a "guest" compound can be accommodated.  The interaction between the host and guest involves purely [[van der Waals bonding]]. The definition of inclusion compounds is very broad, extending to channels formed between molecules in a crystal lattice in which guest molecules can fit. 
{{Quote box
| title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
| quote = '''Inclusion Compound''': A complex in which one component (the host) forms a cavity or, in the case of a crystal, a crystal lattice containing spaces in the shape of long tunnels or channels in which molecular entities of a second chemical species (the guest) are located. There is no covalent bonding between guest and host, the attraction being generally due to van der Waals forces.<ref>{{cite web|title=inclusion compound (inclusion complex)|doi=10.1351/goldbook.I02998 |url=https://goldbook.iupac.org/html/I/I02998.html}}</ref>
| align = left
| width = 30%
}}

Yet another related class of compounds are [[clathrate]]s, which often consisting of a [[lattice (group)|lattice]] that traps or contains molecules.<ref name=Ullmann>Atwood, J. L. (2012) "Inclusion Compounds" in ''Ullmann's Encyclopedia of Industrial Chemistry''. Wiley-VCH, Weinheim. {{doi| 10.1002/14356007.a14_119}}</ref> The word ''clathrate'' is derived from the [[Latin language|Latin]] {{wikt-lang|la|clathratus}} ({{lang|la|clatratus}}), meaning 'with bars, [[Crystal structure|lattice]]d'.<ref>[http://lysy2.archives.nd.edu/cgi-bin/WORDS.EXE?clathrate Latin dictionary] {{webarchive|url=https://web.archive.org/web/20120414202654/http://lysy2.archives.nd.edu/cgi-bin/WORDS.EXE?clathrate |date=2012-04-14 }}</ref>

==Molecular encapsulation==
'''Molecular encapsulation''' concerns the confinement of a guest within a larger host.  In some cases, true host-guest reversibility is observed, in other cases, the encapsulated guest cannot escape.<ref>{{cite book|title=The Encapsulation Phenomenon Synthesis, Reactivity and Applications of Caged Ions and Molecules|author= Yan Voloshin, Irina Belaya, Roland Krämer|year=2016|publisher=Springer|isbn=9783319277387}}</ref>
[[File:Nitrobenzene bound within hemicarcerand from Chemical Communications (1997).jpg|thumbnail|200px|Molecular encapsulation of a nitrobenzene bound within a [[hemicarcerand]].<ref>{{cite journal| author=Donald J. Cram and coworkers|journal=Chem. Commun.|year=1997|page=1303-1304}}</ref>]]

An important implication of encapsulation (and host-guest chemistry in general) is that the guest behaves differently from the way it would when in solution. Guest molecules that would react by bimolecular pathways are often stabilized because they cannot combine with other reactants.  The spectroscopic signatures of trapped guests are of fundamental interest. Compounds normally highly unstable in solution have been isolated at room temperature when molecularly encapsulated. Examples include [[cyclobutadiene]],<ref>Cram, D. J.; Tanner, M. E.; Thomas, R., ''The Taming of Cyclobutadiene'' [[Angewandte Chemie International Edition]] Volume 30, Issue 8, Pages 1024 - 1027 '''1991'''</ref> [[aryne]]s or cycloheptatetraene.<ref>{{cite journal|title=Stabilization of Reactive Organometallic Intermediates Inside a Self-Assembled Nanoscale Host|author=Dorothea Fiedler, Robert G. Bergman, Kenneth N. Raymond|journal=Angewandte Chemie International Edition|volume=45|year=2006|issue=5 |pages=745–748 |doi=10.1002/anie.200501938|pmid=16370008 }}</ref><ref>{{cite journal |author1=Fraser Hof |author2=Stephen L. Craig |author3=Colin Nuckolls |author4=Julius Rebek Jr. |date=May 3, 2002 |title=Molecular Encapsulation |journal=Angewandte Chemie International Edition  |volume= 41 |issue=9 |pages=1488–1508 |doi=10.1002/1521-3773(20020503)41:9<1488::AID-ANIE1488>3.0.CO;2-G |pmid= 19750648}}</ref> Large metalla-assemblies, known as [[metallaprism]]s, contain a conformationally flexible cavity that allows them to host a variety of guest molecules.  These assemblies have shown promise as agents of drug delivery to cancer cells.

Encapsulation can control reactivity. For instance, excited state reactivity of free 1-phenyl-3-tolyl-2-proponanone (abbreviated A-CO-B) yields products A-A, B-B, and AB, which result from decarbonylation followed by random recombination of radicals A• and B•. Whereas, the same substrate upon encapsulation reacts to yield the controlled recombination product A-B, and rearranged products (isomers of A-CO-B).<ref>{{Cite journal|last=Kaanumalle|first=Lakshmi S|date=Oct 20, 2004|title=Controlling Photochemistry with Distinct Hydrophobic Nanoenvironments.|url=https://pubs.acs.org/doi/10.1021/ja0450197|journal=J. Am. Chem. Soc. |volume=126|issue=44|pages=14366–14367|doi=10.1021/la203419y|pmid=15521751}}</ref>

== Macrocyclic hosts ==
Organic hosts are occasionally called '''cavitands'''. The original definition proposed by [[Donald J. Cram|Cram]] includes many classes of molecules: [[cyclodextrins]], [[calixarene]]s, [[pillararene]]s and [[cucurbituril]]s.<ref>{{cite book |last1=Cai |first1=X. |last2=Gibb |first2=B. C. |editor1-last=Atwood |editor1-first=Jerry |title=Comprehensive Supramolecular Chemistry II |date=2017 |publisher=Elsevier |isbn=9780128031995 |pages=75–82 |chapter-url=https://www.sciencedirect.com/science/article/pii/B978012409547212582X |chapter=6.04 - Deep-Cavity Cavitands in Self-Assembly}}</ref>
===Calixarenes===
[[Calixarene]]s and related formaldehyde-arene condensates ([[resorcinarene]]s and [[pyrogallolarene]]s) form a class of hosts that form inclusion compounds.<ref name=":05"/><ref>{{cite book |last1=Wishard |first1=A. |last2=Gibb |first2=B.C. |title=Calixarenes and beyond |publisher=Springer |isbn=978-3-319-31867-7 |pages=195–234 |chapter=A chronology of cavitands|doi=10.1007/978-3-319-31867-7_9 |year=2016 }}</ref>  A related family of formaldehyde-derived oligomeric rings are [[pillararene]]s (pillered arenes). One famous illustration of the stabilizing effect of host-guest complexation is the stabilization of [[cyclobutadiene]] by such an organic host.<ref name="Cram2003">{{cite journal|title=''The Taming of Cyclobutadiene'' Donald J. Cram, Martin E. Tanner, Robert Thomas|journal=Angewandte Chemie International Edition in English|volume=30|issue=8|pages=1024–1027|year=1991|doi=10.1002/anie.199110241|last1=Cram|first1=Donald J.|last2=Tanner|first2=Martin E.|last3=Thomas|first3=Robert}}</ref> 

===Cyclodextrins and cucurbiturils===
[[File:Pillar5arene Feb2013.png|thumb|Chemical structure of pillar[5]arene]]
[[Cyclodextrin]] (CD) are tubular molecules composed of several glucose units connected by ether bonds. The three kinds of CDs, α-CD (6 units), β-CD (7 units), and γ-CD (8 units) differ in their cavity sizes: 5, 6, and 8 Å, respectively.  α-CD can thread onto one PEG chain, while γ-CD can thread onto 2 PEG chains. β-CD can bind with thiophene-based molecule.<ref name=":05"/> Cyclodextrins are well established hosts for the formation of inclusion compounds.{{ref|1}}{{ref|2}}{{ref|3}} Illustrative is the case of [[ferrocene]] which is inserted into the cyclodextrin at 100&nbsp;°C under hydrothermal conditions.<ref>{{cite journal|title=A unique tetramer of 4:5 -cyclodextrin–ferrocene in the solid state|author1=Yu Liu |author2=Rui-Qin Zhong |author3=Heng-Yi Zhang |author4=Hai-Bin Song |journal=Chemical Communications|issue=17|year=2010|pages=2211–2213|doi=10.1039/B418220K|pmid=15856099}}</ref>

[[Cucurbituril]]s are [[macrocycle|macrocyclic molecules]] made of [[glycoluril]] ({{chem2|\dC4H2N4O2\d}}) [[monomer]]s linked by [[methylene bridge]]s ({{chem2|\sCH2\s}}). The oxygen atoms are located along the edges of the band and are tilted inwards, forming a partly enclosed cavity ([[cavitand]]). . Cucurbit[n]urils have similar size of γ-CD, which also behave similarly (''e.g.'', 1 cucurbit[n]uril can thread onto 2 PEG chains).<ref name=":05" />

=== Cryptophanes ===
[[File:Fig_2_Sample_of_some_macrocyclic_rings.png|thumb|359x359px|a) Structure of Cryptophanes. b) Structure of Resorcinarenes and Pyrogallolarenes. c) Structure of cucurbit[n]urils. Redrawn from.<ref name=":05"/>]]
The structure of [[cryptophane]]s contain 6 phenyl rings, mainly connected in 4 ways . Due to the phenyl groups and aliphatic chains, the cages inside cryptophanes are highly hydrophobic, suggesting the capability of capturing non-polar molecules. Based on this, cryptophanes can be employed to capture xenon in aqueous solution, which could be helpful in biological studies.<ref name=":05"/>

===Crown ethers and cryptands===
[[File:Fig_1_threading_of_macrocyclic_ring.png|thumb|386x386px|a) Structure of 18-crown-6. b) Threading of crown ether and 1,2,3-triazole (rotaxane). Redrawn from [3]. c) Inclusion of a-CD and polyethylene glycol (PEG) d) Threading of b-cyclodextrin and thiophene-based molecule. Redrawn from.<ref name=":05"/>]]
[[Crown ether]]s bind cations.  Small crown ethers, e.g. [[12-Crown-4|12-crown-4]] bind well to small ions such as Li+ and large crowns, such as 24-crown-8 bind better to larger ions.<ref name=":05"/> Beyond binding ionic guests, crown ethers also bind to some neutral molecules, ''e.g.'', 1, 2, 3- triazole. Crown ethers can also be threaded with slender linear molecules and/or polymers, giving rise to supramolecular structures called [[rotaxane]]s. Given that the crown ethers are not bound to the chains, they can move up and down the threading molecule.<ref name=":12"/>  Crown ether complexes of metal cations (and the corresponding complexes of [[Cryptand]]s) are not considered to be inclusion complexes since the guest is bound by forces stronger than van der Waals bonding.

==Polymeric hosts==
[[Zeolite]]s have open framework structures with cavities in which guest species can reside.  [[Aluminosilicate]]s being their composition, zeolites are rigid.  Many structures are known, some of which are considerably useful as catalysts and for separations.<ref name=Ullmann/>

Silica clathrasil are compounds structurally similar to clathrate hydrates with a SiO<sub>2</sub> framework and can be found in a range of marine sediment.<ref>{{Cite journal |last1=Momma |first1=Koichi |last2=Ikeda |first2=Takuji |last3=Nishikubo |first3=Katsumi |last4=Takahashi |first4=Naoki |last5=Honma |first5=Chibune |last6=Takada |first6=Masayuki |last7=Furukawa |first7=Yoshihiro |last8=Nagase |first8=Toshiro |last9=Kudoh |first9=Yasuhiro |date=September 2011 |title=New silica clathrate minerals that are isostructural with natural gas hydrates |journal=Nature Communications |language=en |volume=2 |issue=1 |pages=196 |doi=10.1038/ncomms1196 |pmid=21326228 |bibcode=2011NatCo...2..196M |issn=2041-1723|doi-access=free }}</ref>

Clathrate compounds with formula ''A''<sub>8</sub>''B''<sub>16</sub>''X''<sub>30</sub>, where ''A'' is an [[alkaline earth metal]], ''B'' is a [[Boron group|group III]] element, and ''X'' is an element from [[Carbon group|group IV]] have been explored for thermoelectric devices. Thermoelectric materials follow a design strategy called the ''phonon glass electron crystal'' concept.<ref>{{Cite journal |last1=Nolas |first1=G. S. |last2=Cohn |first2=J. L. |last3=Slack |first3=G. A. |last4=Schujman |first4=S. B. |date=1998-07-13 |title=Semiconducting Ge clathrates: Promising candidates for thermoelectric applications |journal=Applied Physics Letters |language=en |volume=73 |issue=2 |pages=178–180 |doi=10.1063/1.121747 |bibcode=1998ApPhL..73..178N |issn=0003-6951|doi-access=free }}</ref><ref>{{cite journal | vauthors=((Beekman, M.)), ((Morelli, D. T.)), ((Nolas, G. S.)) | journal=Nature Materials | title=Better thermoelectrics through glass-like crystals | volume=14 | issue=12 | pages=1182–1185 | date=2015 | issn=1476-4660 | doi=10.1038/nmat4461| pmid=26585077 | bibcode=2015NatMa..14.1182B }}</ref> Low [[thermal conductivity]] and high electrical conductivity is desired to produce the [[Thermoelectric effect|Seebeck Effect]]. When the guest and host framework are appropriately tuned, clathrates can exhibit low thermal conductivity, i.e., ''phonon glass'' behavior, while electrical conductivity through the host framework is undisturbed allowing clathrates to exhibit ''electron crystal''. 

[[Hofmann clathrates]] are [[coordination polymer]]s with the formula Ni(CN)<sub>4</sub>·Ni(NH<sub>3</sub>)<sub>2</sub>(arene).  These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been exploited commercially for the separation of these hydrocarbons.<ref name=Ullmann/> [[Metal organic framework]]s (MOFs) form clathrates.
[[File:IRMOF-1 wiki.png|thumb|[[MOF-5]], an example of a [[metal organic framework]]: the yellow sphere represents the guest cavity.]]

[[Urea]], a small molecule with the formula {{chem2|O\dC(NH2)2}}, has the peculiar property of crystallizing in open but rigid networks. The cost of efficient molecular packing is compensated by hydroge-bonding. Ribbons of hydrogen-bonded urea molecules form tunnel-like host into which many organic guests bind. Urea-clathrates have been well investigated for separations.<ref name="Worsch 2002">{{cite book | date=2002 | last1=Worsch | first1=Detlev | last2=Vögtle | first2=Fritz | title=Topics in Current Chemistry | chapter=Separation of enantiomers by clathrate formation | publisher=Springer-Verlag | isbn=3-540-17307-2 | doi=10.1007/bfb0003835 | pages=21–41}}</ref>  Beyond urea, several other organic molecules form clathrates: [[thiourea]],  [[hydroquinone]], and [[Dianin's compound]].<ref name=Ullmann/>

==Thermodynamics of host-guest interactions==
{{main| Determination of equilibrium constants}}

When the host and guest molecules combine to form a single complex, the equilibrium is represented as
:<math>H + G \leftrightharpoons HG</math>
and the equilibrium constant, K, is defined as
:<math>K=\frac{[HG]}{[H] [G]}</math>
where [X] denotes the concentration of a chemical species X (all activity coefficients are assumed to have a numerical values of 1).
The mass-balance equations, at any data point, 
:<math>T_H = [H] + K [H] [G]</math> 
:<math>T_G = [G] + K [H] [G]</math> 
where <math>T_G</math> and <math>T_H</math> represent the total concentrations, of host and guest, can be reduced to a single quadratic equation in, say, [G] and so can be solved analytically for any given value of K. The concentrations [H] and [HG] can then derived. 
:<math>[H] = T_H - T_G + [G]</math>
:<math>[HG] = K [H] [G]</math>
The next step in the calculation is to calculate the value, <math>X^{calc}_i</math>, of a quantity corresponding to the quantity observed <math>X^{obs}_i</math>. Then, a sum of squares, U, over all data points, np, can be defined as 
:<math>U=\sum_{i=1,np} (X^{obs}_i -X^{calc}_i)^2</math>
and this can be minimized with respect to the stability constant value, K, and a parameter such the chemical shift of the species HG (nmr data) or its molar absorbency (uv/vis data). This procedure is applicable to 1:1 adducts.

===Experimental techniques===
[[File:Nmr titration.gif|thumb|right|300px|Set of NMR spectra from a host–guest titration]]
[[File:UV-vis spectrum of host-guest binding.jpg|thumb|240px|right|Typical ultraviolet–visible spectra for a host–guest system]]
With [[nuclear magnetic resonance]] (NMR) spectra the observed [[chemical shift]] value, {{mvar|{{overbar|δ}}}}, arising from a given atom contained in a reagent molecule and one or more complexes of that reagent, will be the concentration-weighted average of all shifts of those chemical species. Chemical exchange is assumed to be rapid on the NMR time-scale. 

Using UV-vis spectroscopy, the [[absorbance]] of each species is proportional to the concentration of that species, according to the [[Beer–Lambert law]].

:<math>A_\lambda = \ell\sum_{i = 1}^N \varepsilon_{i,\lambda} c_i</math>

where λ is a wavelength, <math>\ell</math> is the optical path length of the cuvette which contains the solution of the ''N'' compounds ([[chromophore]]s),
<math>\varepsilon_{i,\lambda} </math> is the molar absorbance (also known as the extinction coefficient) of the ''i''th chemical species at the wavelength λ, ''c''<sub>i</sub> is its concentration. When the concentrations have been calculated as above and absorbance has been measured for samples with various concentrations of host and guest, the Beer–Lambert law provides a set of equations, at a given wavelength, that which can be solved by a linear least-squares process for the unknown extinction coefficient values at that wavelength.

Host-guest structures can be probed by their luminescence. A rigid matrix protects emitters from being quenched, extending the lifetime of  phosphoresce.<ref>{{Cite journal |last1=Dai |first1=Wenbo |last2=Niu |first2=Xiaowei |last3=Wu |first3=Xinghui |last4=Ren |first4=Yue |last5=Zhang |first5=Yongfeng |last6=Li |first6=Gengchen |last7=Su |first7=Han |last8=Lei |first8=Yunxiang |last9=Xiao |first9=Jiawen |last10=Shi |first10=Jianbing |last11=Tong |first11=Bin |last12=Cai |first12=Zhengxu |last13=Dong |first13=Yuping |date=2022-03-21 |title=Halogen Bonding: A New Platform for Achieving Multi-Stimuli-Responsive Persistent Phosphorescence |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.202200236 |journal=Angewandte Chemie International Edition |language=en |volume=61 |issue=13 |pages=e202200236 |doi=10.1002/anie.202200236 |pmid=35102661 |s2cid=246443916 |issn=1433-7851}}</ref> In this circumstance, α-CD and CB could be used,<ref>{{Cite journal |last1=Yan |first1=Xi |last2=Peng |first2=Hao |last3=Xiang |first3=Yuan |last4=Wang |first4=Juan |last5=Yu |first5=Lan |last6=Tao |first6=Ye |last7=Li |first7=Huanhuan |last8=Huang |first8=Wei |last9=Chen |first9=Runfeng |date=January 2022 |title=Recent Advances on Host–Guest Material Systems toward Organic Room Temperature Phosphorescence |url=https://onlinelibrary.wiley.com/doi/10.1002/smll.202104073 |journal=Small |language=en |volume=18 |issue=1 |pages=2104073 |doi=10.1002/smll.202104073 |pmid=34725921 |s2cid=240421091 |issn=1613-6810}}</ref><ref>{{Cite journal |last1=Xu |first1=Wen-Wen |last2=Chen |first2=Yong |last3=Lu |first3=Yi-Lin |last4=Qin |first4=Yue-Xiu |last5=Zhang |first5=Hui |last6=Xu |first6=Xiufang |last7=Liu |first7=Yu |date=February 2022 |title=Tunable Second-Level Room-Temperature Phosphorescence of Solid Supramolecules between Acrylamide–Phenylpyridium Copolymers and Cucurbit[7]uril |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.202115265 |journal=Angewandte Chemie International Edition |language=en |volume=61 |issue=6 |pages=e202115265 |doi=10.1002/anie.202115265 |pmid=34874598 |s2cid=244922727 |issn=1433-7851}}</ref> in which the phosphor is served as a guest to interact with the host. For example, 4-phenylpyridium derivatives interacted with CB, and copolymerize with [[acrylamide]]. The resulting polymer yielded ~2 s of phosphorescence lifetime. Additionally, Zhu et al used crown ether and potassium ion to modify the polymer, and enhance the emission of phosphorescence.<ref>{{Cite journal |last1=Zhu |first1=Weijie |last2=Xing |first2=Hao |last3=Li |first3=Errui |last4=Zhu |first4=Huangtianzhi |last5=Huang |first5=Feihe |date=2022-11-08 |title=Room-Temperature Phosphorescence in the Amorphous State Enhanced by Copolymerization and Host–Guest Complexation |url=https://pubs.acs.org/doi/10.1021/acs.macromol.2c00680 |journal=Macromolecules |language=en |volume=55 |issue=21 |pages=9802–9809 |doi=10.1021/acs.macromol.2c00680 |bibcode=2022MaMol..55.9802Z |s2cid=253051272 |issn=0024-9297}}</ref>

Another technique for evaluating host-guest interactions is [[calorimetry]]. 

==Aspiration applications==
Host guest complexation is pervasive in biochemistry.  Many protein hosts recognize and hence selectively bind other biomolecules.  When the protein host is an enzyme,  the guests are called substrates.  While these concepts are well established in biological systems, the applications of synthetic host-guest chemistry remains mostly in the realm of aspiration.  One major exception, being zeolites where host-guest chemistry is their ''raison d'etre.''

=== Self-healing ===
[[File:Fig_3_self-healing_mechanism.png|thumb|308x308px|Self-healing mechanism of host-guest interaction by a) using host-small guest molecule and b) host-polymer. Redrawn from <ref name=":3">{{Cite journal |last1=Ikura |first1=Ryohei |last2=Park |first2=Junsu |last3=Osaki |first3=Motofumi |last4=Yamaguchi |first4=Hiroyasu |last5=Harada |first5=Akira |last6=Takashima |first6=Yoshinori |date=December 2022 |title=Design of self-healing and self-restoring materials utilizing reversible and movable crosslinks |journal=NPG Asia Materials |language=en |volume=14 |issue=1 |pages=10 |doi=10.1038/s41427-021-00349-1 |bibcode=2022npjAM..14...10I |issn=1884-4049|doi-access=free }}</ref><ref name=":4">{{Cite journal |last1=Xie |first1=Jing |last2=Yu |first2=Peng |last3=Wang |first3=Zhanhua |last4=Li |first4=Jianshu |date=2022-03-14 |title=Recent Advances of Self-Healing Polymer Materials via Supramolecular Forces for Biomedical Applications |url=https://pubs.acs.org/doi/10.1021/acs.biomac.1c01647 |journal=Biomacromolecules |language=en |volume=23 |issue=3 |pages=641–660 |doi=10.1021/acs.biomac.1c01647 |pmid=35199999 |s2cid=247082155 |issn=1525-7797}}</ref>]]
A [[self-healing material|self-healing]] hydrogel constructed from modified cyclodextrin and adamantane .<ref name=":3" /><ref name="Park 2002008">{{Cite journal |last1=Park |first1=Junsu |last2=Murayama |first2=Shunsuke |last3=Osaki |first3=Motofumi |last4=Yamaguchi |first4=Hiroyasu |last5=Harada |first5=Akira |last6=Matsuba |first6=Go |last7=Takashima |first7=Yoshinori |date=October 2020 |title=Extremely Rapid Self-Healable and Recyclable Supramolecular Materials through Planetary Ball Milling and Host–Guest Interactions |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.202002008 |journal=Advanced Materials |language=en |volume=32 |issue=39 |pages=2002008 |doi=10.1002/adma.202002008 |pmid=32844527 |s2cid=221326154 |issn=0935-9648}}</ref> Another strategy is to use the interaction between the polymer backbone and host molecule (host molecule threading onto the polymer). If the threading process is fast enough, self-healing can also be achieved.<ref name=":4" />

===Encapsulation and release: fragrances and drugs===
Cyclodextrin forms inclusion compounds with [[fragrance]]s which are more stable towards exposure to light and air. When incorporated into textiles the fragrance lasts much longer due to the [[slow-release]] action.<ref>{{cite journal|title=Fragrance-release Property of β-Cyclodextrin Inclusion Compounds and their Application in Aromatherapy |first1=C. X. |last1=Wang |first2=Sh. L. |last2=Chen |journal=Journal of Industrial Textiles|volume=34|pages=157–166|year=2005|issue=3 |doi=10.1177/1528083705049050|s2cid=95538902 }}</ref>

Photolytically-sensitive caged compounds have been examined as containers for [[drug delivery|releasing a drug or reagent]].<ref>{{cite journal|last=Ellis-Davies|first=Graham C. R.|title=Caged compounds: Photorelease technology for control of cellular chemistry and physiology|journal=[[Nature Methods]]|year=2007|doi=10.1038/nmeth1072|pmid=17664946|volume=4|issue=8|pages=619–628|pmc=4207253}}</ref><ref>{{Cite journal |last1=Blanco-Gómez |first1=Arturo |last2=Cortón |first2=Pablo |last3=Barravecchia |first3=Liliana |last4=Neira |first4=Iago |last5=Pazos |first5=Elena |last6=Peinador |first6=Carlos |last7=García |first7=Marcos D. |date=2020 |title=Controlled binding of organic guests by stimuli-responsive macrocycles |url=http://xlink.rsc.org/?DOI=D0CS00109K |journal=Chemical Society Reviews |language=en |volume=49 |issue=12 |pages=3834–3862 |doi=10.1039/D0CS00109K |pmid=32395726 |issn=0306-0012|hdl=2183/31671 |s2cid=218599759 |hdl-access=free }}</ref>

=== Encryption ===
An [[encryption]] system constructed by pillar[5]arene, spiropyran and pentanenitrile (free state and grafted to polymer) was constructed by Wang ''et al''. After UV irradiation, spiropyran would transform into merocyanine. When the visible light was shined on the material, the merocyanine close to the pillar[5]arene-free pentanenitrile complex had faster transformation to spiropyran; on the contrary, the one close to pillar[5]arene-grafted pentanenitrile complex has much slower transformation rate. This spiropyran-merocyanine transformation can be used for message encryption.<ref>{{Cite journal |last1=Ju |first1=Huaqiang |last2=Zhu |first2=Chao Nan |last3=Wang |first3=Hu |last4=Page |first4=Zachariah A. |last5=Wu |first5=Zi Liang |last6=Sessler |first6=Jonathan L. |last7=Huang |first7=Feihe |date=February 2022 |title=Paper without a Trail: Time-Dependent Encryption using Pillar[5]arene-Based Host–Guest Invisible Ink |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.202108163 |journal=Advanced Materials |language=en |volume=34 |issue=6 |pages=2108163 |doi=10.1002/adma.202108163 |pmid=34802162 |bibcode=2022AdM....3408163J |s2cid=244482426 |issn=0935-9648}}</ref> Another strategy is based on the metallacages and polycyclic aromatic hydrocarbons.<ref>{{Cite journal |last1=Hou |first1=Yali |last2=Zhang |first2=Zeyuan |last3=Lu |first3=Shuai |last4=Yuan |first4=Jun |last5=Zhu |first5=Qiangyu |last6=Chen |first6=Wei-Peng |last7=Ling |first7=Sanliang |last8=Li |first8=Xiaopeng |last9=Zheng |first9=Yan-Zhen |last10=Zhu |first10=Kelong |last11=Zhang |first11=Mingming |date=2020-11-04 |title=Highly Emissive Perylene Diimide-Based Metallacages and Their Host–Guest Chemistry for Information Encryption |url=https://pubs.acs.org/doi/10.1021/jacs.0c09904 |journal=Journal of the American Chemical Society |language=en |volume=142 |issue=44 |pages=18763–18768 |doi=10.1021/jacs.0c09904 |pmid=33085462 |s2cid=224824066 |issn=0002-7863}}</ref> Because of the fluorescnece emission differences between the complex and the cages, the information could be encrypted.

=== Mechanical property ===
Although some host-guest interactions are not strong, increasing the amount of the host-guest interaction can improve the mechanical properties of the materials. As an example, threading the host molecules onto the polymer is one of the commonly used strategies for increasing the mechanical properties of the polymer. It takes time for the host molecules to de-thread from the polymer, which can be a way of energy dissipation.<ref name="Park 2002008"/><ref>{{Cite journal |last1=Jin |first1=Jia-Ni |last2=Yang |first2=Xi-Ran |last3=Wang |first3=Yan-Fang |last4=Zhao |first4=Lei-Min |last5=Yang |first5=Liu-Pan |last6=Huang |first6=Liping |last7=Jiang |first7=Wei |date=2023-01-18 |title=Mechanical Training Enabled Reinforcement of Polyrotaxane-Containing Hydrogel |url=http://dx.doi.org/10.1002/ange.202218313 |journal=Angewandte Chemie |volume=135 |issue=8 |doi=10.1002/ange.202218313 |issn=0044-8249}}</ref><ref>{{Cite journal |last1=Wang |first1=Shuaipeng |last2=Chen |first2=Yong |last3=Sun |first3=Yonghui |last4=Qin |first4=Yuexiu |last5=Zhang |first5=Hui |last6=Yu |first6=Xiaoyong |last7=Liu |first7=Yu |date=2022-01-20 |title=Stretchable slide-ring supramolecular hydrogel for flexible electronic devices |journal=Communications Materials |language=en |volume=3 |issue=1 |pages=2 |doi=10.1038/s43246-022-00225-7 |bibcode=2022CoMat...3....2W |issn=2662-4443|doi-access=free }}</ref> Another method is to use the slow exchange host-guest interaction. Though the slow exchange improves the mechanical properties, simultaneously, self-healing properties will be sacrificed.<ref>{{Cite journal |last1=Huang |first1=Zehuan |last2=Chen |first2=Xiaoyi |last3=O'Neill |first3=Stephen J. K. |last4=Wu |first4=Guanglu |last5=Whitaker |first5=Daniel J. |last6=Li |first6=Jiaxuan |last7=McCune |first7=Jade A. |last8=Scherman |first8=Oren A. |date=January 2022 |title=Highly compressible glass-like supramolecular polymer networks |url=https://www.nature.com/articles/s41563-021-01124-x |journal=Nature Materials |language=en |volume=21 |issue=1 |pages=103–109 |doi=10.1038/s41563-021-01124-x |pmid=34819661 |bibcode=2022NatMa..21..103H |s2cid=244532641 |issn=1476-1122}}</ref>

===Sensing===
Silicon surfaces [[Functional group|functionalized]] with tetraphosphonate cavitands have been used to singularly [[Molecular recognition|detect]] [[sarcosine]] in water and [[urine]] solutions.<ref>{{Cite journal|last=Biavardi|first=Elisa|date=February 14, 2011|title=Exclusive recognition of sarcosine in water and urine by a cavitand-functionalized silicon surface|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|volume=109|issue=7|doi=10.1073/pnas.1112264109|pmid=22308349|pages=2263–2268|pmc=3289311|bibcode=2012PNAS..109.2263B|doi-access=free}}</ref>

Traditionally, chemical sensing has been approached with a system that contains a covalently bound indicator to a receptor though a linker. Once the analyte binds, the indicator changes color or fluoresces. This technique is called the indicator-spacer-receptor approach (ISR).<ref>{{cite journal | title = Newer optical-based molecular devices from older coordination chemistry | journal = [[Dalton Transactions]] | year = 2003| volume = 10 | pages = 1902–1913 | doi = 10.1039/b212447p | author1 = de Silva, A.P. | author2 = McCaughan, B | author3 = McKinney, B.O. F. | author4 = Querol, M. | issue = 10}}</ref> In contrast to ISR, indicator-displacement assay (IDA) utilizes a non-covalent interaction between a receptor (the host), indicator, and an analyte (the guest). Similar to ISR, IDA also utilizes colorimetric (C-IDA) and fluorescence (F-IDA) indicators. In an IDA assay, a receptor is incubated with the indicator. When the analyte is added to the mixture, the indicator is released to the environment. Once the indicator is released it either changes color (C-IDA) or fluoresces (F-IDA).<ref>{{cite journal | title = Supramolecular Analytical Chemistry | journal = [[Journal of Organic Chemistry]] | year = 2007| volume = 72 | pages = 687–699 | doi = 10.1021/jo0617971 | author1 = Anslyn, E. | pmid=17253783 | issue = 3}}</ref>
[[File:Sensing.png|thumb|300px|right|Types of Chemosensors. <span style="color:black">(1.)</span> Indicator-spacer-receptor (ISR) <span style="color:black">(2.)</span> Indicator-Displacement Assay (IDA)]]

IDA offers several advantages versus the traditional ISR chemical sensing approach. First, it does not require the indicator to be covalently bound to the receptor. Secondly, since there is no covalent bond, various indicators can be used with the same receptor. Lastly, the media in which the assay may be used is diverse.<ref>{{cite journal | title = Indicator-displacement assays | journal = [[Coord. Chem. Rev.]] | year = 2006 | volume = 250 | pages = 3118–3127 | doi = 10.1016/j.ccr.2006.04.009 | author1 = Nguyen, B. | author2 = Anslyn, E. | issue = 23–24}}</ref>

[[File:IDA Dyes.png|thumb|175px|left|Indicator-Displacement Assay Indicators. <span style="color:black">(1.)</span> Azure A <span style="color:black">(2.)</span> thiazole orange]]
Chemical sensing techniques such as C-IDA have biological implications. For example, [[protamine]] is a coagulant that is routinely administered after cardiopulmonary surgery that counter acts the anti-coagulant activity of herapin. In order to quantify the protamine in plasma samples, a colorimetric displacement assay is used. Azure A dye is blue when it is unbound, but when it is bound to herapin, it shows a purple color. The binding between Azure A and heparin is weak and reversible. This allows protamine to displace Azure A. Once the dye is liberated it displays a purple color. The degree to which the dye is displaced is proportional to the amount of protamine in the plasma.<ref>{{cite journal | doi = 10.1016/0049-3848(94)90158-9 | title = A method for the quantitation of protamine in plasma | journal = [[Thrombosis Research]] | year = 1994 | volume = 74 | issue = 4 | pages = 427–434 | pmid = 7521974 | author1 = Yang, V. | author2 = Fu, Y. | author3 = Teng, C. | author4 = Ma, S. | author5 = Shanberge, J.| hdl = 2027.42/31577 | url = https://deepblue.lib.umich.edu/bitstream/2027.42/31577/1/0000505.pdf| hdl-access = free }}</ref>

F-IDA has been used by Kwalczykowski and co-workers to monitor the activities of [[helicase]] in ''E.coli''. In this study they used thiazole orange as the indicator. The helicase unwinds the dsDNA to make ssDNA. The fluorescence intensity of thiazole orange has a greater affinity for dsDNA than ssDNA and its fluorescence intensity increases when it is bound to dsDNA than when it is unbound.<ref>{{cite journal | title = A method for the quantitation of protamine in plasma | journal = [[Nucleic Acids Research]] | year = 1996 | volume = 24 | pages = 1179–1186 | doi = 10.1093/nar/24.7.1179 | author1 = Eggleston, A. | author2 = Rahim, N. | author3 = Kowalczykowski, S | author4 = Ma, S. | author5 = Shanberge, J. | issue = 7| pmc = 145774 | pmid=8614617}}</ref><ref>{{cite journal |last1=Biancardi|first1=Alessandro |last2=Tarita|first2=Biver |last3=Alberto |first3=Marini |last4=Benedetta |first4=Mennucci |last5=Fernando |first5=Secco |title=Thiazole orange (TO) as a light-switch probe: a combined quantum-mechanical and spectroscopic study |journal=Physical Chemistry Chemical Physics |date=2011 |volume=13 |issue=27 |pages=12595–12602 |doi=10.1039/C1CP20812H |pmid=21660321 |bibcode=2011PCCP...1312595B }}</ref>

===Conformational switching===
A [[crystalline solid]] has been traditionally viewed as a static entity where the movements of its atomic components are limited to its vibrational equilibrium. As seen by the transformation of graphite to diamond, solid to solid transformation can occur under physical or chemical pressure. It has been proposed that the transformation from one crystal arrangement to another occurs in a cooperative manner.<ref>{{cite journal | title = Guest Transport in a nonporous Organic Solid via Dynamic van der Waals Cooperativity | journal = [[Science (journal)|Science]] | year = 2002 | volume = 298 | pages = 1000–1002 | doi = 10.1126/science.1077591 | author1 = Atwood, J | author2 = Barbour, L | author3 = Jerga, A | author4 = Schottel, L | pmid=12411698 | issue = 5595|bibcode = 2002Sci...298.1000A| s2cid = 17584598 }}</ref><ref>{{cite journal | title = Dynamic porous properties of coordination polymers inspired by hydrogen bonds| journal = [[Chemical Society Reviews]] | year = 2005 | volume = 34 | pages = 109–119 | doi = 10.1039/b313997m | author1 = Kitagawa, S | author2 = Uemura, K | issue = 2 | pmid = 15672175}}</ref> Most of these studies have been focused in studying an organic or metal-organic framework.<ref>{{cite journal | title = Methane and Carbon Dioxide Storage in a Porous van der Waals Crystal | journal = [[Angewandte Chemie]] | year = 2005 | volume = 44 | pages = 1816–1820 | doi = 10.1002/anie.200461704 | author1 = Sozzani, P | author2 = Bracco, S | author3 = Commoti, A | author4 = Ferretti, R | author5 = Simonutti, R | pmid=15662674 | issue = 12}}</ref><ref>{{cite journal | title = A Contrivance for a Dynamic Porous Framework: Cooperative Guest Adsorption Based on Square Grids Connected by Amide−Amide Hydrogen Bonds | journal = [[J. Am. Chem. Soc.]] | year = 2004 | volume = 126 | pages = 3817–3828 | doi = 10.1021/ja039914m | author1 = Uemura, K | author2 = Kitagawa, S | author3 = Fukui, K | author4 = Saito, K | pmid=15038736 | issue = 12}}</ref> In addition to studies of macromolecular crystalline transformation, there are also studies of single-crystal molecules that can change their conformation in the presence of organic solvents. An organometallic complex has been shown to morph into various orientations depending on whether it is exposed to solvent vapors or not.<ref>{{cite journal | title = Guest-Induced Conformational Switching in a Single Crystal | journal = [[Angewandte Chemie]] | year = 2006 | volume = 45 | pages = 5856–5859 | doi = 10.1002/anie.200602057 | author1 = Dobrzanska, L | author2 = Lloyd, G | author3 = Esterhuysen, C | author4 = Barbour, L | pmid=16871642 | issue = 35}}</ref>

===Environmental applications===
Host guest systems have been proposed to remove hazardous materials. Certain calix[4]arenes bind cesium-137 ions, which could in principle be applied to clean up radioactive wastes. Some receptors bind carcinogens.<ref>{{cite journal | title = Structural Characterization of the [Cs(p-tert-butylcalix[4]arene -H) (MeCN)] Guest–Host System by 13C-133Cs REDOR NMR | journal = [[Journal of Physical Chemistry B]] | year = 2001 | volume = 105 | pages = 5887–5891| doi = 10.1021/jp004559x | author1 = Eric Hughes | author2 = Jason Jordan | author3 = Terry Gullion | issue = 25}}</ref><ref>{{cite journal | title = Extraction of Carcinogenic Aromatic Amines from Aqueous Solution Using Calix[n]arene Derivatives as Carriers | journal = [[Journal of Hazardous Materials]] | year = 2009 | volume = 168 | pages = 1170–1176 | doi = 10.1016/j.jhazmat.2009.02.150 | author1 = Serkan Erdemir | author2 = Mufit Bahadir | author3 = Mustafa Yilmaz | pmid=19345489 | issue = 2–3}}</ref>

===Alcohol===
According to [[food chemistry|food chemist]] Udo Pollmer of the European Institute of Food and Nutrition Sciences in [[Munich]], alcohol can be molecularly encapsulated in [[cyclodextrin]]es, a sugar derivate. In this way, encapsuled in small capsules, the fluid can be handled as a powder. The cyclodextrines can absorb an estimated 60 percent of their own weight in alcohol.<ref>[http://www.wz-newsline.de/sro.php?redid=67450 Alcohol powder: Alcopops from a bag] {{webarchive|url=https://web.archive.org/web/20070927000016/http://www.wz-newsline.de/sro.php?redid=67450 |date=2007-09-27 }}, Westdeutsche Zeitung, 28 October 2004 (German)</ref> A US [[patent]] has been registered for the process as early as 1974.<ref>[https://patents.google.com/patent/US3795747 Preparation of an Alcohol Containing Powder], General Foods Corporation March 31, 1972</ref>

== See also ==
*[[Cryptophane]]

==Further reading==
*{{cite journal |doi=10.1021/cr200213s |title=Fluorescent Dyes and Their Supramolecular Host/Guest Complexes with Macrocycles in Aqueous Solution |date=2011 |last1=Dsouza |first1=Roy N. |last2=Pischel |first2=Uwe |last3=Nau |first3=Werner M. |journal=Chemical Reviews |volume=111 |issue=12 |pages=7941–7980 |pmid=21981343 }}
*{{cite journal |doi=10.1021/cr5005315 |title=Supramolecular Amphiphiles Based on Host–Guest Molecular Recognition Motifs |date=2015 |last1=Yu |first1=Guocan |last2=Jie |first2=Kecheng |last3=Huang |first3=Feihe |journal=Chemical Reviews |volume=115 |issue=15 |pages=7240–7303 |pmid=25716119 }}
*{{cite journal |doi=10.1021/cr200333h |title=NMR Insights into Dendrimer-Based Host–Guest Systems |date=2012 |last1=Hu |first1=Jingjing |last2=Xu |first2=Tongwen |last3=Cheng |first3=Yiyun |journal=Chemical Reviews |volume=112 |issue=7 |pages=3856–3891 |pmid=22486250 }}
*{{cite journal |doi=10.1021/acs.chemrev.9b00839 |title=Functional Supramolecular Polymeric Networks: The Marriage of Covalent Polymers and Macrocycle-Based Host–Guest Interactions |date=2020 |last1=Xia |first1=Danyu |last2=Wang |first2=Pi |last3=Ji |first3=Xiaofan |last4=Khashab |first4=Niveen M. |last5=Sessler |first5=Jonathan L. |last6=Huang |first6=Feihe |journal=Chemical Reviews |volume=120 |issue=13 |pages=6070–6123 |pmid=32426970 }}
*{{cite journal |doi=10.1021/cr5006342 |title=Photoresponsive Host–Guest Functional Systems |date=2015 |last1=Qu |first1=Da-Hui |last2=Wang |first2=Qiao-Chun |last3=Zhang |first3=Qi-Wei |last4=Ma |first4=Xiang |last5=Tian |first5=He |journal=Chemical Reviews |volume=115 |issue=15 |pages=7543–7588 |pmid=25697681 }}

==References==
{{reflist}}

{{Branches of chemistry}}

{{DEFAULTSORT:Host-guest chemistry}}
[[Category:Supramolecular chemistry]]
[[Category:Equilibrium chemistry]]