Fluorescence Fluorescent corals

Jablonski Diagram

Fluorescence From v = 0 down Absorption From v = 0 up So expect the emission and absorption spectra to overlap here Mostly don’t – because of changes of energy due to solvent interactions

Fluorescence is always at a longer wavelength than absorption, because of the loss of vibrational energy This is known as Stoke’s shift

Mirror image rule (Kasha’s Rule) The shape of an emission spectrum of a simple molecule is the mirror image of the absorption spectrum. The vibrational levels have approximately the same space in the ground state and the first excited level because the shape of the molecule does not really change.

Excited Singlet state – paired electrons but one is excited No splitting of electronic energy levels occurs when the molecule is in a magnetic field Triplet state – unpaired electrons –the spins of the two electrons are parallel Paramagnetic Triplet because of spectroscopic multiplicity

Intersystem Crossing When the lowest vibrational state of S1 has the same energy as an upper vibrational level of the triplet state. Lifetime of a triplet state is long – there are lots of chances for loss of energy in collisions We do not see phosphorescence in liquids at room temperature

What molecules fluoresce? Rigid, coplanar (reduces collisional dectivation) Conjugated Have to freeze to get phosphorescence or put on filter paper Use micelles, cyclodextrin All reduce collisional deactivation

Note the difference in rigidity

Applications

Advantages of Fluorescence over Absorption Greater selectivity and freedom from spectral interferences Fewer species which luminesce Can vary the absorption (excitation) and emission wavelengths Lower LOD than Absorption for same compound F is linear with conc over 3-4 orders of magnitude

Lower LOD than Absorption for same compound Fluorescence is read directly by detector Absorption is a ratio F is linear with conc over 3-4 orders of magnitude (extending to lower conc range)

Lysergic acid diethylamide 50 µg is active Plasma or urine Make basic Extract with 98:2 n-heptane:isoamyl alcohol Excitation: 335 nm Emission: 435 nm

Phosphorescence Radiative relaxation from T1 to G Is forbidden – so has long lifetime 10-6 – 10 sec To make a glow-in-the-dark toy, what you want is a phosphor that is energized by normal light and that has a very long persistence. Two phosphors that have these properties are Zinc Sulfide and Strontium Aluminate. Strontium Aluminate is newer -- it's what you see in the "super" glow-in-the-dark toys. It has a much longer persistence than Zinc Sulfide does. The phosphor is mixed into a plastic and molded to make most glow-in-the-dark stuff.

Phosphorescence Occurs in solids Which may be frozen solvents Reduces the number of collisions Paramagnetic species increase the likelihood of intersystem crossing So reduce fluorescence and phosphorescence

Shape of Emission spectrum Does not change with excitation wavelength BUT the intensity changes The most fluorescence will occur when a lot of light is absorbed Can find an excitation λ by running an absorption spectrum Use this to find λemmax and then λexmax

Quantum Yield = Φ ΦF = number of fluorescence quanta emitted divided by number of quanta absorbed to a singlet excited state Φ F = ratio of photons emitted to photons absorbed

Quenching

Xenon arc lamp Laser High power is more important than stability A reference system is added to measure the stability

Xenon Arc Lamp Unstable Some portion of initial light goes to reference detector to ratio with F signal to compensate for changes in lamp intensity Sometimes a fluorescent standard of rhodamine is included May have to restrict intensity of light to minimize sample decomposition (photobleaching)

Sources of UV produce ozone. Fan disperses this and cools lamp. Ozone is toxic but also absorbs certain wavelengths Detector at right angles to lamp Two wavelength selectors Slits: narrow for high resolution Wider(5-10 nm) to give greater sensitivity

L = k[P0 – P] P = P0 10-abc (Beer’s law) = k[P0 – P0 10-abc] = kP0[1 – 10-abc] Note: L is proportional to P0

10-abc = e-(2.3abc) Expand in a series:

Calibration using Raman peak for water Raman peak maxima of water at various Exc λ’s Excitation wavelength/nm Raman emission/nm 200 212 250 272 300 337 350 397 400 463 450 530 500 602

Excitation wavelength/nm Positions of the Raman bands of various solvents when excited at selected wavelengths Solvents Excitation wavelength/nm 313 366 405 436 Water 350 418 469 511 Acetonitrile 340 406 457 504 Cyclohexane 344 409 458 499 Chloroform 346 411 461 502

Excitation wavelengths for quinine 450 450 Excitation wavelengths for quinine Excitation (Em at 450 nm) Absorbance

0.05 M H2SO4 Ex at 250 nm

Excitation spectrum with emission at 450 nm