Fluorescence of anthracene

fluorescence

light that is emitted by molecules after they are excited to upper (excited) electronic states, as they relax down to the ground electronic state; keeps the same spin orientation

linear combination of atomic orbitals - molecular orbital (LCAO-MO)

one model that describes the allowed energy levels of molecule

ground state (S₀)

each molecular orbital contains 2 electrons, with their spins paired

first excited state (S₁)

obtained by promoting one electron from the highest occupied MO to the lowest unoccupied MO

Fluorescence steps

1. A molecule absorbs a photon: Ground state (S0) + photon → singlet excited state (S1)

2. The excited state decays back to the ground state by emitting a photon: S1 → S0 + photon

3. There are a number of other, non-radiative pathways of decay of S1 to S0. (S1 → S0 + heat (No Fluorescence!))

4. The non-radiative processes compete with fluorescence.

High fluorescence intensity (yield)

Rate constant of fluorescence >> non-radiative rate constants

Low fluorescence intensity

Non-radiative rate constants >> rate constant of fluorescence

Singlet (¹S) state

Only one spin state is possible with total spin angular momentum = 0

number of possible spin states (multiplicity)

2S + 1

triplet (³T) state

three distinguishable spin states, so that the possible values of the angular momenta are +1, 0, and -1

Radiative transitions

involve the absorption or emission of photons; energy of emission is always lower that that of absorption

Non-radiative transitions

excited state energy is dissipated in the form of heat; Excitation energy in excited states Sn is lost by collision with other molecules in their environment (e.g., solvent), or dissipation through internal vibrations that occur on a timescale of picoseconds to sub-nanoseconds thus generating heat; also called internal conversion; faster in liquids and solids than in gases

Jablonski diagram

describes and illustrates energy conversions in molecular energy levels; possible to excite the electron to higher molecular orbitals by shining light of higher and higher energy (shorter and shorter wavelength) on the molecule; Each state has a series of vibrational sublevels

Kasha’s rule

in almost all cases, emission is observed only from S1, the first excited state; If a molecule were excited to S3, for example, it would relax very quickly (within about 1 ps) to S1 before emitting: internal conversion; first order kinetics, like radioactive decay

phosphorescence

emission of light from T1 (first excited triplet state) to S0; spin flips

intersystem crossing (ISC)

process by which triplet states are usually populated from excited singlet states; enhanced by anything which favors spin flipping (spin-orbital coupling), such as the presence of “heavy atoms”; much longer lifetime and wavelengths than fluorescence

mirror symmetry

absorption: almost all of the molecules will be in the lowest vibrational state of S0 and we get a series of absorptions to different vibrational levels of S1

fluorescence: the molecule most always relaxes back to the lowest vibrational level of S1 before fluorescing. (Kasha’s rule.) However, once in that level, we can get emission to different vibrational levels of S0

highest energy emission band roughly corresponds to the lowest energy absorption band; this band is called (0,0).

symmetry will not be exact, because

(1) The vibrational spacings in the ground state and excited states will not be exactly the same

(2) The (0,0) in absorption will not correspond exactly to the (0,0) in emission

vibrational energy spacing

excited electronic state can be estimated from the absorption spectrum, and ground electronic state can be estimated from the fluorescence spectrum
∆E = (4 + 1/2)hν′ − (3 + 1/2)hν′ = hν′

λ04 (S0, v = 0 −→ S1, v′ = 4) = 354 nm

λ03 (S0, v = 0 −→ S1, v′ = 3) = 372 nm

Stokes shift

emission is red-shifted; solvent before absorption stabilises ground state’s dipole/dispersion THEN rearrange themselves to stabilise excited state → lower the energy of excited singlet before emission

all fluorescence is strongly solvent dependent, but polar fluorophores are more solvent dependent

experimental setup

2 monochromators; adjust both the wavelength of excitation and wavelength of emission

To measure a fluorescence spectrum, we would set the excitation monochromator at some wavelength where the sample absorbs strongly, and then scan the fluorescence monochromator

vice versa for absorption spectra

(1) The “shape” of the fluorescence spectrum, i.e. relative numbers of fluorescence quanta at different wavelengths, should be independent of the excitation wavelength.

(2) The shape of the fluorescence spectrum should qualitatively resemble the shape of the absorption spectrum (remember: fluorescence intensity will be proportional to the amount of absorbed light).

Quenching

reacts with the fluorophore molecule in the excited state, and stimulates its decay to the ground state by a radiationless transition mechanism: the excited state energy is converted to heat and the fluorescence is ‘quenched’

fluorophores

molecules that are capable of emitting fluorescence after photoexcitation; sensitive to solvent and quenchers

Dynamic quenching

involves diffusional collisions between fluorophore and Quencher

S 1 + Q → S0 + Q + heat

static quenching

quencher forms a complex with the fluorophore before photoexcitation. The fluorescence quantum yield (Φ) is also diminished.

S0 + Q → [S0 ..Q]

[S0 ..Q] + hν → [S1 ..Q] → [S 0 ..Q] + heat

rate constants of decay

1. Radiative decay (fluorescence), rate constant k_F

2. Non-radiative intersystem crossing, k_ISC

3. Non-radiative internal conversion to the ground state, k_IC

4. Radiative decay from phosphorescence, k_P

5. Non-radiative: Quenching, k_Q

overall rate constant, k_TOT , is the sum of all the individual rate constants

Flourescence quantum yield

phi₀ = k_F/k₀ = tau₀k_f

observed fluorescence liftime in absence of quencher

tau₀ = 1/k₀ ~ 1 - 500 ns

Stern-Volmer equation

F₀/F = 1 + tau₀ k_q[Q] = phi₀/phi_F
thus, plotting F₀/F vs. [Q] = linear relationship w/ intercept of 1 = [Q]=0 and slope tau₀ k_q