Fluorescence (Notes 16)

Transcription

1 Fluorescence (Notes 16) XV 74 Jablonski diagram Where does the energy go? Can be viewed like multistep kinetic pathway 1) Excite system through A Absorbance S 0 S n Excite from ground excited singlet S = 0 could be any of them (FC overlap P ij ) must change dipole (very fast process, fsec) 2) If S n > S 1 often rapid decay/relax down (use VR, NR) S n S 1 called: IC internal conversion ( E ~ 0 fastest steps, goes to excited vibration level) VR within state fast in condensed phase heat, vibra. media

2 XV 75 3) Options to release energy from S 1, = 0 next step (big gap): a) NR - non-radiative decay to S 0 less probable -implies 1. - IC to another singlet, S 0, but big separation, slow 2. - VR - Vibrational relaxation through collision vibrational energy taken to solvent (surround) relax to lowest vibrational state S 0, = 0 (relatively fast process < ns) b) Fluorescence emit photon to S 0 most probable for S 1 trap there (unlikely from S 2 ) Fluorescence lifetime / quantum yield reflects probability of this process - ns s typical competitive process with IC/VR to S 0 c) ISC cross to triplet -- intersystem crossing slow S 0 In triplet VR relax to = 0 again trap energy, long time Phosphorescence much slower ( S 0) & weaker aided by heavy atoms (spin-orbit coupling) Fluorescence intensity depends on how molecules are excited and on the probablity of transition to ground state. kinetic process compete between pathways in Jablonski diagram, typically excite by absorbance Multistep kinetic process: Competition between fluorescence, non-radiative decay, and intersystem crossing.

3 First order kinetics: -d[m*]/dt = k d [M*] (linear in conc.) Lifetime decay: = 1/k d - if only fluorescence: = 1/k f Other processes take away excitation, --lead to shorter observed lifetimes -d[m*]/dt = k f [M*] + k nr [M*] + k Q [M*][Q] = k d [M*] where k nr [non-radiative decay], k Q [quenching] observed lifetime: = 1/(k f + k nr + k Q [Q]) = 1/k d XV 76 Quantum yield ratio: photons fluoresce / photon absorb A! f = # photon (F)/# photon (A) = k f [M*]/ k d [M*] = Quantum Mechanics role 1) Quantum mechanics used to describe excited states much less accurate than for vibrations (excited state) requires a surface not just single geometry calculations need configuration interaction states become mix of configurations: ( ) n ( ) m idea change occupied orbitals of electron to change states states mix different orbital configurations (e.g. *) impact calculations large and less accurate 2) Quantum mechanics and symmetry used to describe which vibronic excitation are allowed with electronic state change S = 0 Electric field cannot change spin Except (Phosphoresce mix spins - spin-orbit coupling) Dipole must change: A ~ ex * el g d 2 integral zero if ex and g same dipole = 0, 1, 2 no restriction on - sym. modes = 1, for asymmetric modes (distort molec. to get dipole) A: Absorb: most transition start g = 0 (most populated) F: Fluorescence is same but ex = 0 by relaxation (VR)

4 Absorption and Fluorescence sample the same states, but processes give fluorescence added dimension XV 77 Intensity dipole strength D ij = ij 2 = 0.92x10-38 ( d (esu-cm) 2 [or x10-2 in units of Debye 2, 1 D = esu cm = 3.34x10-30 C m] Absorption detect same photons as excite, A = -log I/I 0 To go beyond spatial average, need to orient molecule use polarization (next section), lifetime no meaning Sensitivity limited difference of big #s: [log I log I 0 ] Fluorescence excite different photon (abs.) than detect (emit) Transfer of energy between states kinetic process Polarization can detect change in orientation while excited Can go outside of chromophore intramolecular processes Excitation fluorescence intensity vs. excitation wavelength -detect states that absorb - transfer energy to emitting state. Sensitive - selective absorbance method FRET fluorescence resonant energy transfer (Engel 19.13) Efficiency of transfer from Donor (D) and Acceptor (A) T = k T /(k T + k f ) compare rates: fluoresce. (k f ) & transfer (k T ) In terms of quantum efficiency: T = 1- ( f / f 0 ) Rate/efficiency depend on distance and spectral overlap T = R 0 6 /(R 0 6 +r 6 ) - transfer rate: k FRET = (1/ D )(R 0 /r) 6 where: D = Donor lifetime, r = distance D A R 0 = experimental param., rate transfer rate decay Result: FRET can provide a spectroscopic ruler For molecules in nm(å?) range note 6 th power!

5 XV 78 Resonance- overlap donor fluorescence - acceptor absorbance Pro n end-to-end vary efficiency vs. length (Engel book)

6 Bio-applications see Engel Ch (range overestimated?) tag part of protein or DNA with fluorophores (D and A) observe relative intensity of fluorescence from A or D or better change in its lifetime, = 1/k FRET Calibrate with known lengths eg dsdna, poly-pro Can use dyes, or proteins Tryptophan to dye XV 79

7 XV 80 BPTI was labeled with just D, see 6.8 ns decay, but with D and A get much faster decay (reduced, unfolded) Folded and unfolded change lengths, more D and less A emission for unfolded. Length distribution changes

8 Quenching process deactivates excited state/reduce F Collision can take away energy or cause molecule to change states to non-fluorescent one e.g. O 2 : M*(sing) + O 2 (trip) M*(trip) + O 2 (sing) XV 81 Process, multistage kinetic path M + h M* (excitation) M* M + h (fluorescence) M* + Q M + Q (quenching) Rates: no quenching: 0 f = k f /(k f +k nr +k ISC ) With quenching: f = k f /(k f +k nr +k ISC +k Q [Q]) Stern-Volmer relation: 0 f / f = 1 + k Q [Q]/ (k f +k nr +k ISC ) = 1 + K[Q] lifetime as well: 0 f / f - 1 = k Q [Q] with = 1/(k f +k nr +k ISC ) plot: F 0 /F vs. [Q] and slope is k Q Can use this to sense exposure of fluorphores to surface Reflects change in environment e.g. unfold tertiary Polarization-Useful if chromophore fluorescing species has different absorbance with one polarization called dichroism (linear) can use for analysis of orientation in fluorescence excite with one polarization (1 st photon) observe emission in and orientation (2 nd photon) fluorescence anisotropy degree of motion / flexibility ideal - measure change in polarization with time r = I ) - I + I )/( 2I )

9 XV 82

10 Fluorescence anisotropy Changes in fluorescence anisotropy 0.08 A ph6.8 DPH TMA-DPH DMPG B ph4.6 DOPG DSPG DMPG DOPG DSPG XV 83 (left) DPH A and F polarized long axis, used to sense lipid organization. DPH deep, TMA-DPH near surface. Measure change in polarization on lipid vesicle binding protein, see BLG cause more disorder in DMPG vesicle ph 6.8, more in all at 4.6. Aside: 3) Transitions seen are determined by symmetry group theory tool for organizing symmetry useful in small molecules (Chem 444) Biomolecules less use no symmetry use correlation to small molecule components

11 Phosphorescence XV 84