Excited State Chemistry - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Excited State Chemistry - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Excited State Chemistry – it should be recognized that an electronic excited state is a distinct chemical entity with its own chemical properties Sometimes fluorescence & phosphorescence emission are complicated by things that the excited states do, e.g.: 1) Chemical Reactions – the excited state can undergo chemical reactions that the ground state can’t, e.g. dyes fade in sunlight
this is photochemistry
docsity.com In chemical analysis, photodecomposition is a serious problem – rate of photo- decomposition is proportional to number of excited states, i.e. source intensity 2) Acid-Base Chemistry – Ka for excited singlet state can differ by as much as 106 from Ka for ground state e.g. Phenol ground state pKa = 10.0 excited singlet pKa = 4.00 At pH’s between 4 & 10 you see fluorescence characteristics of phenolate anion = good way to study excited state chemistry docsity.com Fluorescence Quenching of Fulvic Acid, Q = Cu2+ Hays, 1996 Emission scans of 15 mg/L Fulvic Acid with varying [Cu2+] and excitation at 340 nm docsity.com Stern Volmer Equation I, - I an = K [M] I The Ryan Equation IML} I-I C, Io - Tres docsity.com 1:1 Complex Formation M + L = ML [ML] K = ------------- [M] [L] Where M = metal ion; L = ligand; ML = complex docsity.com Fluorescence binding curves for Cu, Co, Mn & Al at pH values of 5-8 docsity.com Fluorescence Enhancement 15 mg/L Fulvic Acid at pH 4 (a)112 µM Al3+ (b) 0 µM Al3+ Emission Wavelength in nm with 360 nm Excitation Fl uo re sc en ce In te ns ity A rb itr ar y U ni ts docsity.com Fluorescence Enhancement 15 mg/L Fulvic Acid at pH 4 (a)112 µM Al3+ (b) 0 µM Al3+ Emission Wavelength in nm with 340 nm Excitation Fl uo re sc en ce In te ns ity A rb itr ar y U ni ts docsity.com Fluorescence Enhancement Curve with One-Site Model 15 mg/L of Fulvic Acid with increasing Al(III) concentrations at pH 4 (o) docsity.com Fluorescence Enhancement Curve with Two-Site Model 15 mg/L of Fulvic Acid with increasing Al(III) concentrations at pH 4 (o) docsity.com Individual Fluorescence Intensities Making Up the Overall “I” I = IL1 + IML1 + IL2 + IML2 docsity.com 2) Oxygen quenching is also a problem (sometimes degas a solution to see if F goes up) Another excited state process is eximer formation
an excited state dimer 1A* + A
AA*
AA + hν Heavy Atom Effect – atoms with high atomic number cause spin selection rules to be less rigidly obeyed
promotes IS at the expense of fluorescence Observed if heavy atom is internal (in molecule) or external (in solvent) emission shifted to longer λ docsity.com Solvent Effects – essentially the same as for absorption
for π-π* transitions
increase solvent polarity
excited state stabilized relative to ground state
smaller ∆E
λmax shifts to longer λ Relationship between structure & fluorescence Remembering that KF ФF = ---------------------KF + KIC + KIS Also that larger ε
larger KF (rate of fluorescence) docsity.com 1) Nature of Transition - π-π* vs. n-π* Previously stated π-π*
large ε (i.e. probable) Therefore large KF
large ΦF generally e.g. PAHs are efficient fluorophores n-π*
small ε
small KF,small ΦF (longer τF) e.g. carbonyls rarely fluoresce efficiently Fluorescence only observed for compounds with an extended π-electron system. Saturated aliphatics don’t fluoresce Mainly aromatics docsity.com 5) Effect of Substitution Effect of Substitution on the Fluorescence of Benzene Wavelength of Relative Intensity Compound Formula Fluorescence, nm of Fluorescence Benzene GH 270-310 10 | Toluene CoHacH, 270-320 lL? | Propylbenzene C.H;C;H,; 270-320 17 Fluorobenzene C,H5F 270-320 10 Chlorobenzene GHC! 275-345 a Bromobenzene C.H;Br 290-380 5 lodobenzene C,H,I os 0 Phenol C,H;OH 285-365 18 Phenolate ion C.H.0- 310400 10 Anisole C,H;,OCH, 285-345 20 Aniline C.H,NH. 310-405 20 Anilinium ion C.H,NH; — 0 Benzoic acid C,H,COOH 310-390 3 | Benzonitrile C,H,CN 280-360 20 | Nitrobenzene C.H,NO, — 0 docsity.com 5) Effect of Substitution (conclusions) - no effect of alkyl groups - heavy atom effect - electron donating substituents - increase - electron withdrawing – kills fluorescence - n-π* generally no good docsity.com Applications of Fluorescence (general) 1) Quantitative analysis – usually trace 2) Derivatization – to convert non-fluorescent compounds to fluorescent 3) HPLC detection – with & without derivatization (pre or post column) 4) Study chemistry of process involving a change in fluorescence (I or λ) 5) Study excited state chemistry 6) Qualitative identification – limited use docsity.com Spectrofluorometer Emission monochromator Excitation monochromator eis * i - : = = Grating == =—— Grating =|— Reference photomultiplier tube ro , a: photomultiplier tube White splitta reflector Absorbance compensating HY am \ 4) Xenon a lamp Sample compartment ® docsity.com Phosphoroscope — Sample cell Dewar flask with liquid nitrogen Emission shutter aperture Excitation shutter aperture From excitation monochromator To emission monochromator FIGURE 9-7 Schematic of a device for alternately exciting and observing phosphorescence. (Reprinted with permission from: T. C. O'Haver and J. D. Winefordner, Anal. Chem., 1966, 38, 603. Copyright 1966 American Chemical Society.) docsity.com