Molecular Luminescence. Absorption Instrumentation. UV absorption spectrum. lg ε. A = εbc. monochromator. light source. Rotating mirror (beam chopper)

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1 Molecular Luminescence Absorption Instrumentation light source I 0 sample I detector light source Rotating mirror (beam chopper) motor b sample I detector reference I 0 UV absorption spectrum lg ε A = εbc ε is the molar absorptivity magnitude of molecular absorption per mole large ε means large probability of absorption

2 Chromophores Fluorescence Instrumentation light source I0 sample F detector Excitation/Fluorescence Excitation Fluorescence Wavelength

3 Christian 5th ed, Fig p444 Fluorescence resonance fluorescence (cf atomic fluo.) excitation - to any excited state fluorescence excited state to original state (no change in wavelength emitted) non-resonance fluorescence excitation between any states fluorescence to intermediate level (changed wavelength emitted) wavelength > excitation (Stokes shift) singlet/triplet excited states excited triplet less energetic than singlet singlet - diagmagnetic, triplet - paramagnetic triplet less probable - spin flip necessary - energy required to unpair the electrons direct excitation to triplet is highly improbable Skoog, Hollier & Nieman, 5th edn, 1998, p357

4 Pauli exclusion electron spin no 2 electrons w/ same 4 quantum numbers no more than 2 electrons in one orbital 2 paired electrons have opposite spin all electrons paired - singlet state excitation - electrons still have original orientation - singlet state excitation - electrons have same orientation, unpaired - triplet state Fluorescence Instrumentation light source I 0 sample F detector Excitation/Fluorescence Excitation Fluorescence Wavelength

5 Christian 5th ed, Fig p446 rates - absorption/fluorescence absorption to s fluorescence slower, depends on lifetime of excited state, and the molar absorptivity (ε) (could be 10 3 to 10 5 ) excited state lifetime is inversely proportional to (ε) (could be 10-7 to 10-9 s) phosphorescence is a forbidden transition, change in multiplicity or spin - slower to 10 s or more Which Compounds Fluoresce? Most molecules absorb, only a few fluoresce Fluorescence depends on structure For molecules that fluoresce, fluorescence is greater if absorption is greater many aromatic, hetrocyclic compounds fluoresce Compounds with multiple conjugated double bonds, i.e. extended π systems

6 Fluorescent compounds, cont. Compounds with one or more electron donating groups - OH, -NH 2,-OCH 3 polycyclic compounds e.g. vitamin K, purines, and nucleosides, conjugated polyenes such as Vitamin A -NO 2, -COOH, -CH 2 COOH, -Br, -I, and azo groups, tend to inhibit fluorescence Fluorescent Compounds, cont. Degree of fluorescence can be altered by substituents, e.g. ph dependence - only the ionized or unionized form fluorescent. e.g. C 6 H 5 OH fluorescent, C 6 H 5 O - is not. Compounds may be derivatized e.g. malic acid can be reacted with β-napthol in conc H 2 SO 4 to give a fluorescent derivative Rigidity promotes fluorescence fluorene Φ = 1.0 C H 2 biphenyl Φ = 0.2 chelation, or derivatization Al N O-H O N 1/3 Al H hydroxyquinoline (non-fluorescent) fluorescent

7 Relationship fluo vs concentration F = K Φ P 0 (2.3 εbc) F = fluorescence intensity K = constant (includes instrumental factors) Φ= Quantum efficiency - energy lost by collisional/vibrational or other deactivation (ratio of molecules that fluoresce to total molecules) εbc = absorption expression P 0 = power of incident light beam i.e. F α K C Calibration Curve fluorescence proportional to concentration and light source power not measuring ratio cf absorption fluorescence v. sensitive limited only by light source intensity fluorescence concentration high conc. self-absorption of fluorescence linear region Quenching high concentrations, other molecules can cause collisions - quench fluorescence - signal reduced reduces quantum efficiency of fluorescence k f Φ = k f + k i + k ec + k ic + k pd + k d k f is rate constant for fluorescence k i is for intersystem crossing k ec external conversion (eg collisions) k ic internal conversion k pd predissociation or k d dissociation

8 Lifetime of excited state # of atoms, or molecules, dn, spontaneously decaying from state 2 to state 1 during time dt is: dn = N 2 A 21 dt (A 21 is the Einstein coefficient) exponential decay rate is observed t τ N 2 (t) = N 2 (0) e (first order kinetics) where N 2 (0) is an initial population in an excited state where N 2 (t) is the population at time t and τ is a constant - radiative lifetime of excited state determined mainly by A 21 (τ and A 21 reciprocally related) values of A 21 about 10 8 s -1, so τ ~ 10-8 lifetime measurements real decay laser pulse measured decay usually use lasers ps or ns lasers energy transfer, lifetime of excited state species mathematically deconvolute multiple species 2 or more difficult to do Skoog, Hollier & Nieman, 5th edn, 1998, Fig , p373