Chem G8316_10 Supramolecular Organic Chemistry

Introduction of some general features of supramolecular chemistry

Lecture 2, Monday, January 25, 2010 10:35 AM-11:50 AM

1 

Brief introduction to modern molecular organic photochemistry

Examples of molecular organic supramolecular photochemistry

The beginnins of supramolecular organic chemistry: Cram, Lehn, Petersen

3 

Some exemplar host systems, H

Molecular photochemistry of guests, G

Supramolecular photochemistry of guest@host, G@H

The enzyme guest@host paradigm

G G+

Host Guest Guest@Host

4 

We’ll be using this paradigm to discuss supramolecular systems

5 

I B O

GI B O

GI B O

G

Inside

(b)

(a)

Isopentane Neopentane

Boundary Outside

G

G G

(c)

SUPRAMOLECULAR ISOMERS

CONSTITUTIONAL ISOMERS

TOPOLOGICAL ISOMERS

GUEST

n-Pentane

HOST

Topological representation of supramolecular complexes

6 

G2I B O

G2I B O

G1

G2I B O

G1

G2

I B O

G1 G1

G1

I B O

G2 G2

I B O

G1

G1 G2

G1 G1

G1

G1

G1

G2 G2

G2

G2

G2

a b

d

c

e f

Topological Representations of Simple Two Guest-Single Host Supramolecular Systems(Radical Pairs in Micelles, Zeolites, Cyclodextrins, etc.)

Molecular and Supramolecular Organic Photochemistry

7 

R represents a guest molecule. The circle represents a host molecule.

Top: paradigm for molecular organic photochemistry Bottom: paradigm for supramolecular organic photochemistry

8 

I

hν

P R

*R

hν = supramolecular

host

Paradigm for molecular and supramolecular photochemistry

Overall photochemical transformation

Secondary thermal processes

Geminate radical pair intermediate

Electronically excited state

INSIDE

BOUNDARY

OUTSIDE

TOPOLOGICAL

INSIDE INSIDE INSIDE INSIDE

Zeolite ZSM-5 Faujasite Zeolites Cyclodextrins Micelles

From the circle to real structures: from topology to 3D geometry

The “circle” (host) can accelerate (b) or inhibit (c) the rate of a reaction

10 

Energy diagram representa7on of supramolecular control of a reac7on 

The top reac7on (a) is indiscriminate since the ac7va7on energies for R going to P1 or P2 are iden7cal 

Exemplars of two unimolecular photochemical reactions

11 

12 

An exemplar of supramolecular control of a photoreac7on with two compe7ng paths 

13 

Acceleration and inhibition of the Type II photoreaction

14 

Exemplars of elementary supramolecular hosts 

15 

Exemplars of elementary guest@host systems 

Cartoons of micelle structure

C12 C16SDS CTABCore (2-3 nm)

Stern Layer (up to a few A)

Gouy-Chapman Layer(up to several hundred A)

Water molecule

SO3-

Na+

N+Br-

Surfactant

Cartoon of the hydrophobic effect: (1) water is more ordered about the surfactant monomer (left) than ordinary water causing an relative increase in water organization and a decrease;

(2) Water is less ordered about the hydrophobic skin of the micelle causing a relative increase in entropy.

Less water entropy

More water entropy

Why do micelles form at all?

Surfactants gather at interfaces: the air/water interface and the water/solid interface

Structures formed from surfactants in aqueous solution

The critical micelle concentration phenomenon: Sudden break in properties near a certain concentration of surfactant.

Monomers only Monomers plus micelles

Use of the cmc to study a photochemical reaction

NH

H

2 SBD

6 SBD

70 Å29 Å

8 Å

N-CH2-CH2-CO-NH-CH2-CH2-N

Dendrimers: Macromolecules as micelles

generation surface groups diameter (Å)

0 .5

1 .5

2 .5

3 .5

4 .5

5 .5

6 .5

7 .5

6

12

24

48

96

192

384

768

27.9

36.2

48.3

66.1

87.9

103.9

126.8

147.3

separation of the surface groups (Å)

12.4

12.8

12.7

12.6

11.5

10.3

9.8

7.7

16

32

64

128

256

512

1024

surface groups

8

Polyvalency in dendrimers: surface groups

24 

Exemplars of cavitands: Octa acid (OA), Cucubituril (CB) and Cyclodextrin (CD)

25 

Cavitand dimensions:

~ 5 Å

~ 9 Å

~ 12 Å

26 

Octa acid - A water soluble host

O OH

OO O OO O

H H HH

O OH HH

O OOO

O O

H

O O

OH HOOH HO

HO OHHO

O OO O

O OO

guest

gues

t 2 nm

1 nm

2:1 complex (capsule)

guest

1:1 complex (cavitand) 2:2 complex (capsule)

Octa acid (OA) - A water soluble cavitand or capsule host

cavitand guest

13.73 A0

5.46 A0

11.36 A0

O OH

OO O OO O

H H HH

O OH HH

O OOO

O O

H

O O

OH HOOH HO

HO OHHO

O OO O

O OO

Gibb, C. L. D. Gibb, B. C. J. Am. Chem. Soc., 2004, 126(37), 11408-11409.

Characteristics

 Water soluble at pH ~ 9.0

  Ability to form 2:2, 2:1 complex with a guest

6.1 Å 11.7 Å9.2 Å

OA complexes with aromatic hydrocarbons

‘Dry’ and ‘Non-polar’

Cartoon of pyrene forming a supramolecular complex with OA

Summary: Encapsulation of aromatics within octa acid

O OH

OO O OO O

H H HH

O OH HH

O OOO

O O

H

O O

OH HOOH HO

HO OHHO

O OO O

O OO

9.2 Å

6.8 Å

6.1 Å 11.7 Å 9.2 Å

~ 5 Å ~ 9 Åˇ~ 12 Åˇ

Cyclodextrins as supramolecular hosts

Cyclodextrins

D-glucopyranoside units connected into a cycle via

1,4-glycosidic linkages

5 Å

9 Å

7 Å

33 

Porous Solids: Zeolites

Discovered “boiling stones” which he called “zeolites” from the Greek: zeo (boil) and lithos (stone).

Baron Cronstedt 1722-1765

Cronstedt’s boiling stones, zeolites: Today’s Philosopher’s Stone?

A zeolite, as found in Nature

Discovery of zeolites

Zeolites: Sythetic

Center for Microcrystal Assembly

200 nm 1 µm 2.5 µm

2 µm

Zeolite-A

ZSM-5 Zeolite-X or Y

Zeolite-L

0.4 nm 0.5 nm

0.7 nm 0.7 nm

More than 65% of the earth’s crust consists of 3D crystalline polyaluminosilicates (3D-CPAS): feldspar, zeolite, and ultramarine. Zeolite is a class of 3D-CPAS having nanochannels and nanocavities.

HYDROPHILICHYDROPHOBICHYDROPHILIC

FRAMEWORK CONSTITUTION

Mx+(AlO2)x(SiO2)y(H2O)mG

ZEOLITE COMPOSITION (MONOVALENT CATIONS)

O O O O O O

MM

O

-

OO O

-+ +

A. Exchangeable monovalent cations. Mobile speciesLocated at exchangeable sites. Steric and electrostatic effects.

B. Tetrahedral atoms (Al and Si) with bridging oxygen atoms. Compose the zeolitic framework or "walls" of the internal surface.

C. Intracrystalline water. Usually bound to cations and can be reversibly removed.

D. Adsorbed guest molecules. May be on the external or on the internal surface.

A B C D

OOSiAl

OSi

OSi

OAl

O

FAU External Surface FAU Supercage

8 Å

Hole on External Surface

8 Å Window Leading to Supercage

Internaldiameter of supercage:13 Å

A supercage for controlling the stereoselectivity, regioselectivity, chemioselectivity and magnetoselectivity

of radical pair reactions

The FAU family of zeolites (MX and MY) M variable, Si/Al ratio variable (cation density variable)

Photoreactions within zeolites

•  Key is the cation binding to the included organic molecule. Confined space also imposes restrictions.

•  Cations and coadsorbed guest moleculs Cations: Li +, Na+, K+, Rb+, Cs+

Active site of an enzyme Supercage of

a zeozyme

Zeolites as primitive enzyme models

C6H5CH2COCH2C6H5 (dibenzyl ketone, (DBK) adsorbed in the supercage of a FAU zeolite

Silicalite Channel Orientation

[010][100]

The MFI family of zeolites (ZSM-5 and Silicalite)

43 

Structure of MFI zeolites

5.3 Å x 13.3 Å 6.6 Å x 11.5 Å

Molecular diameterallows diffusion into the internal surface

Molecular diameterdoes not allow diffusion into the internal surface

ca 5.5 Å

9Å

4.5

Å5.

4 Å

20 Å

6.7 Å

I

Elliptical 10-membered ring

Circular 10-membered ring

20 Ä

ca 5.4 Å

20 Å

ca 5.5 Å

O CH3O

CH3

44 

R + hν *R

Photochemical

Photophysical Thermal

I P

General photochemical paradigm without spin

Creating a robust paradigm for molecular organic photochemistry of guest molecules

45 

LU

HO

*R PR Ih!

(NB1)1(NB2)

1(HO)1(LU)1(HO)2 (HO)2

NB1 NB2

Rh!

P

Add HO and LU orbitals!

46 

Rh!

R(S0)

P

Rh!

*R I P

Rh! 1*R

ISC 3*RISC3 I 1I P

HO

LU

1*R(S1)3*R(T1) P(S0)

3 I(D) 1I(D)

(a)

(b)

(c)

(d)(NB1)

1 (NB2)1

Add spin!

47 

The state energy diagram and competing photophysical and photochemical processes

Photophysical radiationless processes

Photophysical radiative processes

48 

Exemplars of two unimolecular photochemical reactions

49 

50 

Some examples of photochemical systems that are subject to significant supramolecular control in guest@host systems.

A number of examples will be discussed during the course

51

Chemical sociology

Atomic: electronic sociology enforced by attraction to nucleus and electron-electron repulsions. Atom = superelectron

Molecular: atomic sociology enforced by covalent bonds. Molecule = superatom

Supramolecular: molecular sociology enforced by non-covalent bonds. Supramolecular structure = Supermolecule