Luminescence Spectroscopy (Chapter 15) fluorescence ...

What about Lifetimes?• Absorption:S 1 ←S 0 very fast 10-15-10-13 s• Relaxation:Resonant emission S 1 →S 0 fast 10-9-10-5 s (fluorescence)common in atomsstrong absorber ⇒ shorter lifetimeNon-resonant emission S 1 →S 0 fast 10-9-10-5 s (fluorescence)common in moleculesv. fast vibrational relaxationred shifted emission (Stokes shift)CEM 333 page 5.4Fig. 15-2

Non-resonant emission T 1 →S 0 slow 10-5-10 s (phosphorescence)Transitions between states of different multiplicities are improbable"forbidden" (e.g. T←S or T→S)Fig. 15-3phosphorescencefluorescenceexcitationCEM 333 page 5.5

Internal Conversion:radiationless transition to lower state whenvibrational energy levels "match"External Conversion: radiationless transition to lower state bycollisional deactivationIntersystem Crossing: transition with spin change (e.g. S to T)Fluorescence:emission not involving spin change (e.g.S→S, T→T), efficient, short-lived 10-5 sDissociation:excitation to vibrational state with enoughenergy to break bondPredissociation:relaxation to state with enough energy tobreak bondCEM 333 page 5.6

Energy Level Diagram: (Fig. 15-1)CEM 333 page 5.7

How likely is fluorescence?Fluorescence Quantum Yield - ratio of number of moleculesfluorescing to number excitedΦ fluor =# photons fluor# species excited(Φ fluor = 0.0 to 1.0)=k fluork fluor + k int con + k ext con + k ISC + k pre dis + k disWhat Factors Affectfluor?(1) Excitation λShort λ's break bonds increase k pre-dis and k disrarely observedmost commonσ* → σ π* → n π* → πemission usually from lowest lying excited state(2) Lifetime of stateTransition probability measured by εLarge ε implies short lifetimeLargest fluorescence from short lifetime/high ε stateπ*→π > π*→n (10-9-10-7 s > 10-7-10-5 s)CEM 333 page 5.8

(3) StructureFew conjugated aliphatics fluoresceMany aromatics fluoresceDesire short lifetime S 1 , no/slowly accessible T 1Fluorescence increased by # fused rings and substitution on/inringNHNPyridineNPyrroleNQuinolineIndole(4) RigidityRigid structures fluoresceIncrease in fluorescence with chelationFluoreneBiphenyl(5) Temperature, pH, solvent (p 363-364)CEM 333 page 5.9

Quantitative Luminescence Spectrophotometry:Fluorescence }FAbsorbed( )678= K' I 0 − I= K'⋅2.303εbc ⋅I 0= K⋅ cOnly works at low A (

Fluorometer - filters to isolate excitation and fluorescencewavelengths (but no scanning)Spectrofluorometer - two monochromators for excitation scanning orfluorescence scanningFig 15-7CEM 333 page 5.11

Total Luminescence Measurements:Fig 15-8CEM 333 page 5.12

Chemiluminescence: (see www article)Reaction produces molecule in electronically excited stateA + B → C * + DExample:C * → C + hνNO + O 3 → NO * 2 + O 2NO * 2 → NO 2 + hν (600 − 2800 nm)Used to detect NO from 1 ppb to 10 pptIntensity depends on rate of reaction of production of C*[ ]I CL = φ CLd C *dtFig. 15-11Advantages: (i) simple instrumentation (no excitation hν) (ii) highsensitivity (ppm to