Chem G8316_10 Supramolecular Organic Chemistry

Transcription

1 Chem G8316_10 Supramolecular Organic Chemistry Lecture 5, Wednesday, February 3, 2010 Photophysics of aromatic hydrocarbons Supramolecular effects on the photophysics of aromatic hydrocarbons 1

2 Course grade: to be determined by (1) participation in class discussions, (2) presentation of a 15 minute powerpoint presentation to the class on a selected topic in supramolecular organic chemistry or organic supramolecular photochemistry (3) a written report on a selected topic in supramolecular organic chemistry or organic supramolecular photochemistry 2

3 Supramolecular topics for consideration Micelles Dendrimers DNA Proteins Zeolites Cyclodextrins Carcerands Molecular sensors Nanoparticles Endofullerenes Spin chemistry A topic from JACS in 2009 or 2010 Some other topic 3

4 Timeline for presentations and reports 1. Presentations will be given on three days (5 presentations per day); Monday, March 1, Wednesday, March 3 and Monday, March 8 2. Printed and electronic copies of the reports are due by Monday, March 15 at 5 PM 4

5 Controlling paths along a reaction pathways of radical pairs with supramolecular effects At any of the stages escape from the cage to form free radicals is possible 5

6 Zeolites as primitive enzyme models Active site of an enzyme Supercage of a zeozyme 6

7 Photochemistry of α,α Dimethyl dibenzyl ketone Any opportunity to investigate supramolecular effects on the stereochemistry of radical pair reconbintation Primary Geminate Pairs RP(1) Secondary Geminate Pairs RP(2) Free radicals FR O hν O - CO + meso-dpp P r (1) P g (2) P fr (2) P = P(1) + P(2) P(2) = P g (2) + P fr (2) O DPP: Primary Geminate Coupling (DPB) g : Secondary Geminate Coupling (DPB) fr : Free Radcial Coupling + Regioisomers 7

8 Controlling stereochemistry of radical-radical recombination with chiral co-guests Create a chiral supercage by adding a chiral molecule as a co-guest DiMe-DBK Chiral co-guest 8

9 Spin chemistry of radical pairs in supramolecular systems Magnetic isotope effect will increase cage reactions Magnetic field effect will decrease cage reactions T + Free Radicals Cage Reaction S T T 0 S Cage Reaction T - Free Radicals Zero Magnetic Field High Magnetic Field 9

10 10

11 Enhancements of enantioselectivity in cyclodextrins and in zeolites O hν O hν O O R 2 O R 2 O R 2 O Out 38 In O (13.4) O O R2 R 2 39A Enantiomer A 39B Enantiomer B O O CH 2 C 6 H 5 hν β-cd O O CH 2 C 6 H 5 + O O CH2 C 6 H 5 (13.5) 40 40A ee = 33% 40B O O O C 6 H 5 O O C 6 H 5 O C 6 H 5 hν + NaY/Ephedrine H H 42 43A 43B ee = 78% (13.6)

12 The state energy diagram. Photophysical processes: radiative and radiationless 12

13 Photophysical processes Supramolecular effects 13

14 From the state energy diagram to absorption and emission spectra Absorption Emission 14

15 Exemplars of elementary systems 15

16 Pyrene as an exemplar of the photophysics of aromatic hydrocarbons Pyrene hν * * + hν 16

17 UV/Vis Spectrum of Pyrene in Ethanol 17

18 Fluorescence Emission at Room Temperature Excitation 382 nm (0,3) (0,2) S 1 x x (0,1) (0,0) S 0 Emission 335 nm nm

19 Comparison of pyrene s absorption and emission spectra Mirror image relationship between S 0 S 1 and S 1 S 0 19

20 Phosphorescence Emission at 77 K 2.0 Phosphorescence Spectrum 1.5 (0,0) S 1 ISC x (0, 1) T (0,2) S o nm

21 Fluorescence Response to Solvent Polarities I 1 I 3 Ratio of 1st to 3rd vibrational band intensities is dependent on the polarity of the solvent. 15 I F x Less Polar More Polar 5 Lower I 1 /I 3 Ratio Higher I 1 /I 3 Ratio nm 21

22 Comparison of Pyrene Emission in Different Solvents x Cyclohexane I 1 /I 3 =.57 Lit.58 x Methanol I 1 /I 3 = 1.28 Lit % Difference nm x Toluene I 1 /I 3 = 1.04 Lit x Acetone I 1 /I 3 = 1.65 Lit x nm Ethanol I 1 /I 3 = 1.15 Lit % Difference x Acetonitrile I 1 /I 3 = 1.79 Lit nm nm Pyrene fluorescence provides a means of measuring the polarity of a host as the environment experienced by a guest 22

23 Pyrene as an exemplar of excimer formation hν * * + * Excimer * - hν + 23

24 Radicals form complexes with everything!

25 Electronically excites states make complexes with any other molecules. The issue is how strong. These electronically excited complexes are called excimers (R = N) or exciplexes (R N) Filled shell Half Filled shell 25

26 Formation of an exciplex between an electron donor (D) and an electron acceptor (A) 26

27 Excimer formation between an electronically excited pyrene (*Py) and ground state pyrene (Py) 27

28 Exciplex formation between an electronically excited pyrene (*Py) and ground state diethyl aniline 28

29 (Py) Enhanced excimer formation due to preorganization of two pyrenes in a cyclodextrin cavity 29

30 capsule: preorganization of interacting groups to avoid self quenching 30

31 The heavy atom effect on spin transitions The heavy atom effect is an atomic number effect that is related to the coupling of the electron spin and electron orbit motions (spin-orbit coupling, SOC) Most commonly, the HAE refers to the rate enhancement of a spin forbidden photophysical radiative or radiationless transition the is due to the presence of an atom of high atomic number, Z, in the system The heavy atom may be either internal to a molecule (molecular) or external (supramolecular) 31

32 Spin-orbit coupling energies for selected atoms

33 The heavy atom effect on the emission of aromatic compounds What is the heavy atom effect due to? 33

34 Using the heavy atom effect to develop a supramolecular strategy for observing phosphorescence of aromatic hydrocarbons at room temperature Make more triplets through the heavy atom effect Make triplets emit faster in competition with quenching processes 34

35 Micelles as hosts micelle: effect of heavy atom counterions Na: Z = 11 Tl: Z = 81 Heavy atom produces more triplets and the triplets produced phosphoresce at a faster rate 35

36 Cyclodextrins as hosts effect of CH 2 Br 2 as co-guest 36

37 OA capsule as hosts capsule: excimer formation enhanced [4 + 4]Photodimerization of anthracene inhibited (supramolecular steric effect) 37

38 Zeolites as hosts influence of heavy atom charge compensating cations 38

39 Zeolites as hosts deaggregration with water 39

40 Carcerands as hosts

41 Carcerands Cram s taming of cyclobutadiene For many years attempts to isolate cyclobutadiene in solution at room temperature failed because one diene undergoes a very rapid Diels-Alder reaction with a second diene molecule (a dimerization) + Cram s idea was to synthesize cyclobutadiene in a host system that would provide supramolecular steric hindrance to prevent dimerization 41

42 Cram s breakthrough publication: The Taming of Cyclobutadiene, Angew. Chem. 30, 102 (1991)

43 Structural and topological models of carcerands Structural model Topological model

44 Some photochemistry performed on guests in carcerands O 2 can get inside carcerand Benzyne can be trapped in cacerand

45 Summary of reaction intermediates stabilized

46 Electronic energy transfer

47 Energy transfer: Requirement of overlap of emission and absorption spectra

48 Two mechanisms of electronic energy transfer: dipoledipole and exchange

49 Schematic of electron transfer with excited state as electron acceptor

50 Photoinduced electron transfer Requirement: ΔG < *E (redox energy)

51 Exemplar of photoinduced electron transfer

52 Can undergo electron and energy transfer processes?

53 can transfer energy and electrons to molecules outside the walls of the carcerand, but at rates that are much slower than those for fluid solution