Supramolecular Photophysics • Manipulating photophysics of organic molecules through weak interactions and confinement • Use of organic photophysics in understanding supramolecular structures • Supramolecular organic photohysics: Sensors, molecular motors, etc. Octa acid(OA)Cucurbiturils Chemistry in bowls, baskets, cages and cavitiesChemistry in bowls, baskets, cages and cavities Cyclodextrins Calixarenes Dendrimers Zeolites CrystalsMicelles A brief review of photophysical processes kisc krt kqtkqskrs - h! T1 S1 S0 - h! ' Fluorescence: • High radiative rate constant, 10-10 to 10-8 s-1 • Precursor state (S1) has short lifetime • Not susceptible to quenching Phosphorescence: • Low radiative rate constant, 10-6 to 10 s-1 • Precursor state (T1) has long lifetime • Very much susceptible to quenching • Emission quantum yield depends on S1 to T1 crossing 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 that is due to the presence of an atom of high atomic number, Z. The heavy atom may be either internal to a molecule (molecular) or external (supramolecular). Spin-orbit coupling energies for selected atoms Correlation of Experimental Parameters with Theory Heavy Atom Effect - Theory PS-T = !Si!Tj 2 " S HSO" T 2 ! i,j #ES-T HS-O ! "(L#S) PS-T ! (HSO)2 PS-T ! "2 PS-T = Transition Probability HSO = Spin-Orbit Hamiltonian ! = Spin-Orbit Coupling Constant !i,j = Vibrational Wavefunctions ! = Electronic Wavefunctions L = Orbital Angular Momentum S = Spin Angular Momentum Examples of internal heavy atom effect Room temperature phosphorescence in solution Internal heavy atom effect Strategy to record phosphorescence at room temperature through supramolecular approach Make more triplets through the heavy atom effect Make triplets emit faster in competition with quenching processes Stage 1 Stage 2 Phenanthrene@Cyclodextrin: effect of CH2Br2 as co-guest Cyclodextrins as hosts Induced Intersystem Crossing Depends on the SOC: Cations as the heavy atom pertutber Atom Ionic Spin-Orbit Radius of Coupling ! cm-1 the Cation (Å) Li 0.86 (+) 0.23 Na 1.12 11.5 K 1.44 38 Rb 1.58 160 Cs 1.84 370 Tl 1.40 3410 Pb 1.33 (2+) 5089 Crown ethers, Micelles and Zeolites as hosts Room temperature phosphorescence in solution External heavy atom effect: Crown ether apparoach Tl: Z = 81Na: Z = 11 Naphthalene@SDS micelle: effect of heavy atom counterions Micelles as hosts Heavy atom produces more triplets and the triplets produced phosphoresce at a faster rate Wavelength (nm) 300 400 500 600 700 LiX CsX RbX Intensity (Arb.) 0.0 1.0 Emission Spectra of Naphthalene Included in MY Zeolites Phosphorescence from Diphenyl Polyenes k0 (s -1 ) ! 2 (cm -2 ) 104 106 107 108 105 103 102 101 100 10-1 M Exchanged Zeolite + O O O O O O M+ M External Heavy Atom Effect on Triplet Decay Rates of Naphthalene S0 S0 T1 Self-quenching S0 S0 kdiffhν T1 1O2 S0 + kdiff 3O2 Diffusion controlled self-quenching and oxygen-quenching in solution Room temperature phosphorescence S0 hν T1 S0 T1 S0 1O2+ 3O2 Slow Prevention of self quenching and oxygen quenching with the help of containers Room temperature phosphorescence from thioketones in solution Camphorthione Fenchthione Adamantanethione SS Me Me MeS Me Me Me SS Me Me Me Pyrene as an exemplar of excimer formation hν * * * - hν + * + 23 Excimer Pyrene Excimer 1.5 1.0 0.5 0.0 650600550500450400350 wavelength (nm) in methylcyclohexane solution 10 mM 1 mM 0.1 mM 0.01 mM 7.5 mM 5 mM 2.5 mM normalizedfluorescenceintensity (Py)2@Cyclodextrin: Enhanced excimer formation due to preorganization of two pyrenes in a cyclodextrin cavity Anthracene@NaX: Cation controlled aggregation Zeolites as hosts 70x10 3 60 50 40 30 20 10 0 FluorescenceInt. 600550500450400 Wavelength (nm) Wavelength (nm) FluorescenceInt. ---- Anthracene in water Photophysics of OA-Anthracene Complex Sandwich pair emission- slow addition of host to the guest in borate buffer ---- Anthracene in octa acid τ = 263 ns 100 10 1 10 2 10 3 10 4 2000150010005000 Counts Time in ns Sandwich excimer – τ 210 - 225 ns Photodimerization Excimer emission Isotropic solution OA complex Photodimerization Excimer emission Product too large to fit in Stabilizing Unstable Molecules 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 “taming” of cyclobutadiene Cram’s idea was to synthesize cyclobutadiene in a host system that would provide supramolecular steric hindrance to prevent dimerization + Cram’s breakthrough publication: “The Taming of Cyclobutadiene”, Angew. Chem. 30, 1024 (1991) Stabilizing Unstable Molecules Stabilizing Reactive Intermediates Energy, electron and spin transfer through the walls of a carcerand Energy and spin transfer Electron transfer How good is the wall of a carcerand at protecting the guest? Communication Electron Energy Spin Biacetyl@carcerand Can a guest@carcerand undergo electron transfer, spin transfer and energy transfer processes? Biacetyl@carcerand can transfer energy and electrons to molecules outside the walls of the carcerand. Bi@carcerand Bi@carcerand ET and et are slowed down by several orders of magnitude (~ 10-3-10-4) N N * et * et Electron transfer between caged and free molecules ONH H N O O Cl N O CH3 O * 420 480 540 600 660 0 2x105 4x105 6x105 8x105 FluorescenceIntensity Wavelength (nm) Energy transfer between caged and free molecules Fluorescence Response to Solvent Polarities nm 15 10 5 x10 3 500480460440420400380360 Ratio of 1st to 3rd vibrational band intensities is dependent on the polarity of the solvent. I1 I3 Less Polar More Polar Lower I1/I3 Ratio IF Higher I1/I3 Ratio Comparison of pyrene emission in different solvents Pyrene fluorescence provides a means of measuring the polarity of a host as the environment experienced by a guest 800 600 400 200 x10 3 500480460440420400380360 15 10 5 0 x10 3 500480460440420400380360 nm 1.0 0.8 0.6 0.4 0.2 0.0 x10 6 500480460440420400380360 15 10 5 0 x10 3 500480460440420400380360 nm 15 10 5 0 x10 3 500480460440420400380360 nm 1.0 0.8 0.6 0.4 0.2 x10 6 500480460440420400380360 nm Cyclohexane I1/I3 = .57 Lit .58 Toluene I1/I3 = 1.04 Lit. 1.04 Ethanol I1/I3 = 1.15 Lit. 1.18 2.5% Difference Methanol I1/I3 = 1.28 Lit. 1.35 5.2% Difference Acetone I1/I3 = 1.65 Lit. 1.64 Acetonitrile I1/I3 = 1.79 Lit. 1.79 Pyrene as a polarity probe Octa acid’s interior micropolarity probed All above probes form 2:1 host-guest complexes. CH3 O O OH3CO O ON CHO Interior of octa acid is benzene-like ‘Dry’ and ‘Non-polar’ Supramolecular Sensors Chemistry in Confined Spaces