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
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
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
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
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
Zeolites as primitive enzyme models Active site of an enzyme Supercage of a zeozyme 6
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
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
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
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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)
The state energy diagram. Photophysical processes: radiative and radiationless 12
Photophysical processes Supramolecular effects 13
From the state energy diagram to absorption and emission spectra Absorption Emission 14
Exemplars of elementary guest@host systems 15
Pyrene as an exemplar of the photophysics of aromatic hydrocarbons Pyrene hν * * + hν 16
UV/Vis Spectrum of Pyrene in Ethanol 17
Fluorescence Emission at Room Temperature 350 1.0300 Excitation 382 nm (0,3) (0,2) S 1 x10 6 0.8250 200 0.6 x10 6 150 0.4 100 0.2 50 (0,1) (0,0) S 0 Emission 335 nm 2 1 0 0.0 250260 280 300 320 340 350 360 nm 400 450 500 18
Comparison of pyrene s absorption and emission spectra Mirror image relationship between S 0 S 1 and S 1 S 0 19
Phosphorescence Emission at 77 K 2.0 Phosphorescence Spectrum 1.5 (0,0) S 1 ISC x10 6 1.0 (0, 1) T 1 0.5 (0,2) S o 0.0 600 650 nm 700 750 800 20
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 x10 3 10 Less Polar More Polar 5 Lower I 1 /I 3 Ratio Higher I 1 /I 3 Ratio 360 380 400 420 440 460 480 500 nm 21
Comparison of Pyrene Emission in Different Solvents x10 6 1.0 0.8 0.6 0.4 0.2 Cyclohexane I 1 /I 3 =.57 Lit.58 x10 3 15 10 5 Methanol I 1 /I 3 = 1.28 Lit. 1.35 5.2% Difference 0.0 360 380 400 420 440 460 480 500 0 360 380 400 420 nm 440 460 480 500 x10 6 1.0 0.8 0.6 0.4 0.2 Toluene I 1 /I 3 = 1.04 Lit. 1.04 x10 3 800 600 400 200 Acetone I 1 /I 3 = 1.65 Lit. 1.64 x10 3 15 10 5 360 380 400 420 nm 440 460 480 500 Ethanol I 1 /I 3 = 1.15 Lit. 1.18 2.5% Difference x10 3 15 10 5 360 380 400 420 440 460 480 500 Acetonitrile I 1 /I 3 = 1.79 Lit. 1.79 0 360 380 400 420 nm 0 440 460 480 500 360 380 400 420 440 460 480 500 nm Pyrene fluorescence provides a means of measuring the polarity of a host as the environment experienced by a guest 22
Pyrene as an exemplar of excimer formation hν * * + * Excimer * - hν + 23
Radicals form complexes with everything!
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
Formation of an exciplex between an electron donor (D) and an electron acceptor (A) 26
Excimer formation between an electronically excited pyrene (*Py) and ground state pyrene (Py) 27
Exciplex formation between an electronically excited pyrene (*Py) and ground state diethyl aniline 28
(Py) 2 @Cyclodextrin: Enhanced excimer formation due to preorganization of two pyrenes in a cyclodextrin cavity 29
Thioketone@OA capsule: preorganization of interacting groups to avoid self quenching 30
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
Spin-orbit coupling energies for selected atoms
The heavy atom effect on the emission of aromatic compounds What is the heavy atom effect due to? 33
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
Micelles as hosts Naphthalene@SDS 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
Cyclodextrins as hosts Phenanthrene@Cyclodextrin: effect of CH 2 Br 2 as co-guest 36
OA capsule as hosts Anthracene@OA capsule: excimer formation enhanced [4 + 4]Photodimerization of anthracene inhibited (supramolecular steric effect) 37
Zeolites as hosts Naphthalene@MX: influence of heavy atom charge compensating cations 38
Zeolites as hosts Anthracene@NaX: deaggregration with water 39
Carcerands as hosts Guest@carcerand
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
Cram s breakthrough publication: The Taming of Cyclobutadiene, Angew. Chem. 30, 102 (1991)
Structural and topological models of carcerands Structural model Topological model
Some photochemistry performed on guests in carcerands O 2 can get inside carcerand Benzyne can be trapped in cacerand
Summary of reaction intermediates stabilized
Electronic energy transfer
Energy transfer: Requirement of overlap of emission and absorption spectra
Two mechanisms of electronic energy transfer: dipoledipole and exchange
Schematic of electron transfer with excited state as electron acceptor
Photoinduced electron transfer Requirement: ΔG < *E (redox energy)
Exemplar of photoinduced electron transfer
Biacetyl@carcerand Can guest@carcerand undergo electron and energy transfer processes?
Biacetyl@carcerand 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 Bi@carcerand Bi@carcerand