Supramolecular organic photochemistry: Control of covalent bond formation through noncovalent supramolecular interactions and magnetic effects
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Perspective
Supramolecular organic photochemistry: Control of
covalent bond formation through noncovalent
supramolecular interactions and magnetic effects
Nicholas J. Turro*
Department of Chemistry, Columbia University, 3000 Broadway, MC 3119, New York, NY 10027
Supramolecular organic photochemistry, a field concerned with the interaction of light with supramolecular assemblies of organic
molecules, has been inspired by the remarkable structural and dynamic features of guest@host chemistry, particularly as exemplified by
enzymes. Exemplars of supramolecular organic photochemistry from soft-matter hosts (micelles) and hard-matter hosts (porous solids)
are discussed with an emphasis on how noncovalent interactions, which are at the heart of supramolecular chemistry, can be
systematically exploited to control the catalytic and magnetic effects on the formation of covalent bonds from photochemically produced
pairs of radicals.
From Molecular to Supramolecular Enzymes are remarkably specific both released and the active site becomes ca-
Chemistry in the selection of the guest molecules they pable again of binding another molecular
bind and in the reactions they catalyze (3). guest.
The high degree of catalytic selectivity of
T he concept of the organic molecule as
an assembly of atoms held together by
covalent bonds is a key intellectual unit in
enzymes results from the very specific
structural demands of binding of a guest to
Host@Guest Supermolecules As
Nanoscopic Reaction Vessels
chemistry. Chemists have mastered con- an ‘‘active site’’ of the protein framework The active sites of enzymes, whose spatial
cepts of the covalent bond and demon- of the enzyme. The structure and action of dimensions are of the order of a nanome-
ter (1 nm ⫽ 10 Å), are truly nanoscopic
PERSPECTIVE
strated their mastery through the use of enzymes has provided chemists with both
the concepts to design the synthesis of a stimulus and an inspiration to design reaction vessels for conducting catalytic
remarkably complex organic molecules ‘‘synthetic enzymes’’ to provide exemplars reactions, involving the making and break-
and to correlate molecular structure and for the understanding of supramolecular ing of covalent bonds of bound guest
dynamics with the physical and chemical structure and dynamics that display excel- molecules. A challenge in supramolecular
properties of organic compounds. In the lent catalytic and selectivity efficiencies catalysis of synthetic enzymes is to design
same manner that molecular chemistry for practical applications. A key supramo- systems that selectively and efficiently cat-
alyze covalent bond formation from in-
SPECIAL FEATURE
may be considered to be the chemistry of lecular feature of enzymes is their capacity
atomic assemblies, which are held to- for ‘‘molecular recognition’’ by which ad- herently reactive species such as pair of
gether by covalent interatomic bonds, su- mission to, and binding with, the active radicals through the noncovalent, inter-
pramolecular organic chemistry may be site by a particular molecular guest is molecular interactions associated with
considered to be the chemistry of molec- extremely selective and is based on the guest@host complexes. A range of nano-
ular assemblies that are held together by enzyme’s ability to recognize the guest’s scopic reactors have been used (4) to
noncovalent intermolecular bonds (1, 2). size, shape, and chemical characteristics. conduct organic photochemical reactions
The process of molecular recognition of in the fluid or ‘‘soft-matter’’ phases (mi-
Enzymes As Exemplars of Guest@Host the guest from all other molecules in a celles, microemulsions, liquid crystals, and
Supermolecules surrounding aqueous environment and of polymer films) and in the solid or ‘‘hard-
Perhaps the highest levels of sophistica- transport of the guest to the active site matter’’ phases (zeolites, silica gel, crys-
tion of applications of supramolecular involves a sequence of diffusional steps tals, and semiconductors). We shall con-
chemistry are found in living systems, starting with the recognition of the sub- sider one exemplar from soft matter
consisting of elegant supramolecular as- strate by the exterior of the global enzyme (micelles) and one from hard matter
semblies of organic molecules that make supramolecular assembly, followed by (zeolites).
up the machinery whose structures and binding of the guest to the external surface In ordinary molecular solvents, a dis-
of the enzyme. The guest is then vectori- solved molecule may be considered as a
dynamics enable and support life func-
ally shuttled from the external surface guest that is surrounded by a wall or as
tions. An important class of life’s super-
through the enzyme’s internal structure solvent molecules that form a host ‘‘cage’’
molecules are guest@host complexes (the
until it reaches and is bound to the active about the guest (5). Micelles and zeolites
@ symbol indicates noncovalent bonding
site. The active site possesses a size兾shape may be viewed as ‘‘supercages’’ that sur-
between molecular species) for which a
round the guest. A supercage influences
‘‘guest’’ molecule, whose chemistry is of geometry and chemical functionality that
the chemical and兾or physical properties of
direct interest, is modified catalytically as are complementary to the size兾shape of
the guest in a qualitative and兾or quanti-
the result of noncovalent binding to a the guest and which allow specific chem-
tative manner compared with the molec-
macromolecular ‘‘host.’’ For example, en- istry of the guest to be catalyzed by the
ular solvent cage as a reference. The active
zymes are macromolecular protein hosts host. Once the chemistry is achieved, be-
site of an enzyme represents the cage par
that catalyze the important reactions cause the product’s size兾shape兾chemical
of noncovalently bound guest organic characteristics are not complementary to
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molecules. those of the active site, the product is *E-mail: turro@chem.columbia.edu.
www.pnas.org兾cgi兾doi兾10.1073兾pnas.032657999 PNAS 兩 April 16, 2002 兩 vol. 99 兩 no. 8 兩 4805– 4809porous and cannot effectively constrain
the separation of the geminate pair.
The cage effect for the probability of
recombination of geminate radical pairs
produced by the photolysis of ketones in
micelles has been used as an exemplar to
demonstrate the supramolecular effects
on covalent bond formation (10). For the
same ketones as guests in micelle hosts,
the probability of geminate pair recombi-
nation can be systematically controlled
from a few percent up to about 100% by
controlling the complementary hydropho-
bicity of the guest geminate pair and the
host micelle core (Table 1).
Magnetic and Supramolecular Effects on
Fig. 1. Schematic representation of a micelle. The black circle represents the ionic portion of the head Covalent Bond Formation
group (e.g., SO3⫺), whereas the long tail depicts the hydrophobic alkyl chain. SDS (C12) and cetyltrimeth-
ylammonium bromide (C16) are typical anionic and cationic micelle-forming surfactants, respectively. Magnetic field and magnetic isotope ef-
fects are commonly observed for photo-
chemical reactions that produce triplet
excellence for controlling the chemistry of form a stable molecule. In solution (mo- geminate radical pairs in supercages (11,
incarcerated guest molecule. lecular chemistry), the geminate radical 12). The intersystem crossing (ISC) step
pair is a highly reactive species and does converting 3I (triplet geminate pair) to 1I
Micelles As Soft-Matter Enzyme Mimics not require energetic activation for cova- (singlet geminate pair) is responsible for
Micelles are supramolecular assemblies lent bond formation, which occurs very the common occurrence of spin effects in
consisting of bipolar molecules called sur- rapidly and efficiently. Enzymes can acti- photoreactions in supramolecular sys-
factants. Surfactants, named because of vate bond breaking of key covalent bonds tems. The key concept in spin chemistry is
their surface active properties, typically of the guest through the geometric dispo- that the ISC step 3I to 1I is a magnetic
consist of a relatively long hydrophobic sition of chemical functionality that geo- reactivity switch! The switch is based on
organic chain that serves as a ‘‘tail’’ to a metrically complements the bound guest. spin selection rules, which forbid 3I from
polar or ionic head group (Fig. 1). In In organic supramolecular photochemis- directly forming singlet products, 1P,
dilute aqueous solution, surfactants exist try (8), photons absorbed by the guest through radical–radical combination (or
as monomer (i.e., a conventional solubi- provide the activation required to break disproportionation) until ISC to a singlet
lized molecule), but above a certain con- covalent bonds of the guest. state,1 I, occurs.
centration, the surfactants spontaneously A qualitative model (11) that integrates
and cooperatively organize to form a su- The ‘‘Cage Effect’’ As a Signature of supramolecular chemistry and spin chem-
pramolecular assembly (6) termed a ‘‘mi- Supramolecular Systems istry is shown in Fig. 2. When the time
celle.’’ The micelle possesses an internal When photolysis causes the cleavage of a scale of the ISC step is of the order of
hydrophobic core consisting of the or- bond in a guest adsorbed in a micelle to nanoseconds to microseconds, large spin
ganic chains and an external surface con- produce a pair of radicals (a geminate effects caused by applied laboratory mag-
sisting of the hydrophilic polar or ionic radical pair), there is a certain probability, netic fields or by magnetic isotopes are
head groups (Fig. 1 Right). On a time P, that the geminate pair will recombine in expected theoretically and found experi-
average globular micelles are roughly the micelle and a probability, (1 ⫺ P), that mentally on the value of P (11, 12). Be-
spherical in shape and of the order of 1–3 the partners of the pair will diffuse out of cause the size of the supercages of micelles
nm in diameter (Fig. 1 Left), which qual- the micelle. The probability, P, that the are of the order of 1 nm, spin effects are
ifies micelles as nanoscopic reactors for geminate pair will recombine in the mi- commonly observed on the values of P for
bound guest molecules. The micelle may celle is termed the geminate ‘‘cage effect.’’ covalent bond formation between gemi-
be viewed as a primitive model of an In nonviscous solvents, the cage effect of nate triplet radical pairs in nanoscopic
enzyme with the hydrophobic core as the recombination of the geminate pair pro- supercages. The model of Fig. 2 provides
analogue of the active site. The micelle duced from photolysis of ketones is a few an intellectual and scientific basis for ex-
core is a ‘‘soft,’’ liquid-like structure and percent or less (9), because the walls of the perimental variation of supramolecular
differs from that of an ordinary organic molecular solvent cage are very soft and guest@host complexes in the search of
solvent in that the guest is constrained to
remain in the hydrophobic region of the Table 1. Cage effects on secondary geminate pair recombination
micelle, which provides a ‘‘supercage’’ or
nanoscopic reactor space for the guest. Ketone Earth’s field 13,000 gauss Surfactant
1 30% 16% C16
Molecular and Supramolecular Organic 2 59% 31% C16
Photochemistry 3 95% 76% C16
Let us consider a strategy in which con- 2 30% 15% C12
ventional molecular photochemistry (7) is
used to cleave a covalent bond in a bound
guest molecule to produce a geminate
radical pair that is bound in an ‘‘active
site,’’ which will control the subsequent
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covalent bond formation of the pair to See Fig. 1 for the structures of C12 and C16.
4806 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.032657999 TurroFig. 2. Model of a nanoscopic supercage and the origin of magnetic effects on covalent bond formation. The large circle represents a supramolecular
host that serves as a nanoscopic supercage for a geminate pair of radicals (the small circles represent a dynamic triplet radical pair, 3I). A triplet geminate
radical pair is produced by photolysis. The pair undergoes a random walk in the supercage under the constraints of the ‘‘viscosity’’ and spatial dimen-
sions of the supercage. Under the influence of magnetic interactions the pair undergoes ISC to a singlet during the random walk. After reencounter,
the singlet pair forms a covalent bond with a probability, P. The latter undergo relative diffusion (D ⫽ diffusion constant) from an initial site about a
supercage of radius L with an average reencounter period of . For a random walk, is proportional to L2兾D. For a supercage with a diameter of the order
of about 1 nm (10 Å), the value of is of the order of nanoseconds to microseconds for values of D corresponding to nonviscous to moderately viscous
solvents.
PERSPECTIVE
reaction control by spin effects: (i) control Magnetic Effects on Covalent Bond walk separation and reencounter of the
the relative diffusional motion (D) of the Formation in Supramolecular Systems geminate pair) in relationships between
pair, which will depend on the size, shape, the magnitude of the cage effect and the
and chemical structure of the partners of Fig. 3 provides a schematic integration of
the photochemistry (formation of the size and ‘‘viscosity’’ of the supercage
the guest pair; (ii) control the size (L),
geminate triplet pair), the spin dynamics microreactor.
shape, and chemical structure of the host
SPECIAL FEATURE
supercage; and (iii) control the magnetic (triplet to singlet intersystem crossing of The application of an external magnetic
parameters (hyperfine coupling, applied the geminate pair), chemical dynamics field strongly (10) reduces the probability
fields, g-factors, etc.) influencing the rate (covalent bond formation of the geminate of cage recombination of geminate pairs
of ISC of the pair. Generally, other pro- pair), and diffusional dynamics (random in micellar supercages (Table 1). This
cesses (e.g., escape from the supercage,
spin-independent chemical reactions, etc.)
compete with the ISC step. Because the
ISC step is controlled through magnetic
effects and product formation selectivity
depends on the competition between ISC
and spin independent reactions, magnetic
effects can operate on the 3I to 1I process
to exert a control on eventual covalent
bond formation selectivity from 3I.
A second important supramolecular
aspect of the model is the intermolecular
radical–radical interactions of electron
exchange that plays an important role in
determining the magnitude of the ob-
served spin effects (11). Thus, the com-
bination of weak noncovalent supramo-
lecular interactions between the radicals
of a pair and weak exchange and mag-
netic interactions between the electron
Fig. 3. Representation of the hyperdynamics involved in the recombination reaction of a triplet
spins of the pair can control the reaction geminate radical pair. The arrows moving along the singlet and triplet energy surface indicate the motion
pathways of radical–radical reactions to of the representative point of the nuclei of the radical pair. The spin dynamics is represented by the vector
selectively make strong covalent bonds in notation 1. The diffusional dynamics is represented below the surfaces as in Fig. 2. The boundary for a
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supercages. hypothetical supercage is shown. The shaded circles represent geminate radicals.
Turro PNAS 兩 April 16, 2002 兩 vol. 99 兩 no. 8 兩 4807effect results from the strong coupling of
an external magnetic field with the elec-
tron spins that inhibits the electron spins
from undergoing ISC. This action slows
down the rate of singlet formation and
allows the radicals to escape more effi-
ciently out of the supercage, reducing the
probability of the cage effect (Figs. 2 and
3). The presence of a 13C at the carbonyl
position of the acyl radical of the primary
pair leads to significant magnetic isotope
effects on the cage reactions of the pri-
mary geminate radical pair (12). This ef-
fect occurs because the presence of 13C (a
magnetic isotope) at the carbonyl carbon
of the radical pair accelerates the rate of Fig. 4. Schematic of the external surface (Left) and the supercage (Right) of an FAU zeolite.
ISC of the primary pair relative to 12C (a
nonmagnetic isotope that occurs in 99%
natural abundance), thereby speeding up zeolite. In these cases, the enzyme may be accomplish the required substrate trans-
the rate of ISC to the singlet pair and viewed as suppressing a high inherent formation selectively, efficiently, and cat-
causing more efficient covalent bond for- molecular reactivity of a species by direct- alytically. Zeolites mimic these properties
mation and a higher cage effect. ing alternative reactions of a guest toward of enzymes to a certain measure.
a specific pathway, which is not possible in
Zeolites As Hosts for Supramolecular homogeneous solution conditions because Negative Catalysis As a Mechanism for
Organic Photochemistry the alternative reaction is too slow to Selective Covalent Bond Formation in
Zeolites are crystalline porous solids pos- compete with other available, very fast, Supramolecular Systems
sessing an ‘‘open’’ or porous internal sur- reactions. In some cases, enzymes achieve selectivity
face whose framework is constructed from Although zeolites are robust porous sol- by transforming substrates into unstable
SiO4 and AlO4 tetrahedra connected ids and a form of hard matter, they possess and reactive intermediates that would nor-
through oxygen bridges (13). The open a number of structural similarities to mally undergo rapid and nonselective re-
framework structure starts with pores on ‘‘soft-matter’’ enzymes (16). To the extent actions in homogeneous media. Enzy-
the external surface that determine the that these similarities are valid, the chem- matic selectivity in these cases is provided
guest molecules which can diffuse into the ist is inspired to combine the attractive by a sort of negative catalysis (17), if we
interconnected channels of the internal features of the robust, chemically inert define catalysis in terms of the action of a
surface. The dimensions of the pores and framework of an inorganic structure, a host in promoting a chemical transforma-
channels are of the order of 3–10 Å, the zeolite crystal, with the spectacular selec- tion without being consumed and do not
size of small organic molecules such as tivity and activity of enzymes. Nature de- demand that a catalyzed reaction be char-
benzene (molecular cross section about 5 mands high selectivity and catalytic con- acterized by a faster rate than some ref-
Å). In some cases, the channels of the ditions to carry out life’s chemistry and erence uncatalyzed reaction. Negative ca-
internal surface form intersections that does so with enzymes through the use of a talysis slows down some reactions of a
are roughly spherical and are considerably very special supramolecular host struc- reactive intermediate that would occur in
larger than the channels. For example (14, ture, the protein ‘‘framework’’ of an en- homogeneous solution and thereby allows
15), the diameter of the roughly cylindrical zyme. The protein framework provides (i) the reactive intermediate to undergo re-
pores and channels of zeolites possessing the ‘‘walls’’ of the active site and protects actions that are too slow to be observed
the MFI topology (silicalite and ZSM-5) it from undesirable side reactions; (ii) the under ‘‘molecular’’ conditions. Reaction
are about 5 Å, but the diameter of the binding sites on the external surface, selectivity is achieved by using supramo-
roughly spherical intersections is about 9 which are ‘‘portals’’ for selective binding, lecular effects to hinder certain rapid,
Å. The intersections are the likely ‘‘active and the ‘‘channels’’ on the internal surface indiscriminate molecular reactions and
sites’’ for reactions in zeolites, because through which the substrate molecules are leaving other target reactions to occur ‘‘by
they offer the greatest degree of void ‘‘sieved’’ on their vectorial excursion to default.’’ A number of enzymatic reac-
space for reacting molecules to interact. the active site; (iii) the sterically demand- tions involve carbon-centered free radi-
Thus, although zeolites as solids are for-
ing ‘‘void’’ space at the active site for cals, which are reactive intermediates that
mally ‘‘hard matter,’’ their internal void
adsorption of the substrate; (iv) and the tend to react by radical–radical reactions
space is ‘‘soft.’’ As a result, zeolites can
chemical ‘‘tools’’ within the active site to nonselectively and at diffusion-controlled
serve as hosts for guest molecules. In the
case of the FAU family of zeolites (Fig. 4),
the pores on the external surface are about
8 Å in diameter and the internal super-
cages are about 13 Å in diameter.
Zeolites As Hard-Matter Enzyme Mimics
Many of the selective reactions catalyzed
by zeolites can be viewed as reactions that
are nonselective under ‘‘molecular’’ con-
ditions (homogeneous solvents) but which
have become selective as the result of the
size兾shape兾chemical effects imposed on
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the guest@host complex by the protein Scheme 1.
4808 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.032657999 Turroisotope effects. For example (21), photol-
ysis of 13C carbonyl carbon-enriched
dibenzylketone (DBK) yields 4 as the ma-
jor product, whereas photolysis of 12C
Scheme 2. carbonyl carbon (natural abundance)
DBK yields 5 as the major product (Eqs.
4 and 5, Schemes 2 and 3). These magnetic
rates in ordinary solvents (17). An impli- framework. Examples of the effect of cat- isotope effects are explained, as in the
cation of these examples of negative ca- ion on the photochemistry of dibenzyl- case for the cage effect discussed above, as
talysis is that the highly reactive radicals ketone (DBK) (1) are given in equations being the result of faster ISC of the triplet
are ‘‘stabilized’’ and protected from reac- 1–3 of ref. 20 for the DBK@zeolite com- to the singlet for the 13C-enriched gemi-
tion as the result of their guest relationship plexes, where the zeolite is a faujasite nate pair, which allows covalent bond for-
in the active site. Negative catalysis is also possessing internal supercages whose di- mation to occur faster in competition with
a property displayed by zeolite hosts for ameters are about 13 Å (Fig. 4). This relative rotational motion of the pair.
guests that undergo covalent bond forma- family of zeolites possesses cations in the
tion by geminate radical combination. supercage to balance the negative charges Conclusion
in the framework. For an appreciation of The enzymatic catalysis of covalent bonds
Supramolecular and Magnetic Control of the size of the void space of the supercage, between carbon atoms is a critical step in
Covalent Bond Formation Between Guest up to 6 benzene molecules can fit snugly in many essential life processes catalyzed by
Geminate Radical Pairs in Zeolite Hosts the supercage. enzymes. The principles of supramolecu-
The supercages of the internal surface of With Li⫹ as the cation, the major lar chemistry allow the design of simple
zeolites provide nanoscopic reactors that product, 1, results from decarbonylation models of enzymes that can mimic their
control covalent bond formation of gem- followed by coupling of benzyl radicals catalytic action. Two examples, micelles
inate radical pairs produced by photolysis (Eq. 1, Scheme 1); with Na⫹ as the and zeolites, can serve as enzyme mimics
of guest molecules adsorbed in the zeolite cation, the major product is the isomeric for controlling, in catalytic fashion, the
host (18, 19). In contrast to the soft and rearranged ketone, 2 (Eq. 2); with K⫹ as selectivity of formation of covalent bonds
flexible walls of micellar supercages, the the cation, the major product is the iso- between geminate radical pairs generated
walls of zeolite supercages are hard and meric ketone, 3 (Eq. 3). The structures of by photolysis of ketone as ketone@micelle
inflexible. Thus, the steric effects resulting the products are controlled by the free or ketone@zeolite supermolecules. A
from the interactions of a geminate radical volume available in the supercage con- model of supercages whose dimensions
pair adsorbed in a zeolite supercage can be taining the guest ketone and the photo- are of the order of 1 nm reveals the
possibility of significant magnetic effects
PERSPECTIVE
severe. These steric interactions are re- chemically generated geminate radical
sponsible for the size兾shape selectivity of pair and the competing rates of covalent on the formation of covalent bonds be-
reactions that occur in zeolites. The steric bond formation and rotation of the gem- tween geminate pairs in supercages. Such
interactions experienced by a guest in a inate radical pair. effects are readily observed and very sig-
zeolite supercage can be modified by the As in the case of micelles, product for- nificant in determining the probability
adsorption of a co-guest or by variation of mation from photolysis of ketones bound and selectivity of covalent bond formation
the cations that are associated with the as guests in zeolite hosts is strongly influ- between geminate radical pairs bound to
supercages.
SPECIAL FEATURE
negative aluminate anions of the zeolite enced by magnetic field and magnetic
I thank the members of my research group, past
and present, and many collaborators for partici-
pating in the development of the ideas and ex-
periments that form the basis of this Perspective.
I also thank the National Science Foundation and
Department of Energy for financial support
Scheme 3. through the Environmental Molecular Science
Institute program at Columbia University.
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