instructor: dr. marinella sandros
DESCRIPTION
Nanochemistry NAN 601. Instructor: Dr. Marinella Sandros. Lecture 7: Quantum Chemistry_Fluorescence. Let’s start with photon energy. Light is quantized into packets called photons Photons have associated: frequency, (nu) wavelength, ( = c ) speed, c (always) - PowerPoint PPT PresentationTRANSCRIPT
Instructor: Instructor:
Dr. Marinella SandrosDr. Marinella Sandros
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NanochemistrNanochemistryy
NAN 601NAN 601
Lecture 7: Quantum Chemistry_Fluorescence
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Light is quantized into packets called photons Photons have associated:
◦ frequency, (nu)◦ wavelength, ( = c)◦ speed, c (always)◦ energy: E = h
higher frequency photons higher energy more damaging
◦ momentum: p = h/c The constant, h, is Planck’s constant
◦ has tiny value of: h = 6.6310-34 J·s
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Sunny day (outdoors):◦ 1015 photons per second enter eye (2 mm pupil)
Moonlit night (outdoors):◦ 51010 photons/sec (6 mm pupil)
Moonless night (clear, starry sky)◦ 108 photons/sec (6 mm pupil)
Light from dimmest naked eye star (mag 6.5):◦ 1000 photons/sec entering eye◦ integration time of eye is about 1/8 sec 100
photon threshold signal level
http://www.ars-chemia.net/Classes/101/PPT/08_Quantum_Chemistry.pdf
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Every particle or system of particles can be defined in quantum mechanical terms◦ and therefore have wave-like properties
The quantum wavelength of an object is: = h/p (p is momentum)
◦ called the de Broglie wavelength typical macroscopic objects
◦ masses ~ kg; velocities ~ m/s p 1 kg·m/s◦ 10-34 meters (too small to matter in macro
environment!!) typical “quantum” objects:
◦ electron (10-30 kg) at thermal velocity (105 m/s) 10-8 m
◦ so is 100 times larger than an atom: very relevant to an electron!
All matter (particles) has wave-like properties◦so-called particle-wave duality
Particle-waves are described in a probabilistic manner◦electron doesn’t whiz around the nucleus, it has a
probability distribution describing where it might be found◦allows for seemingly impossible “quantum tunneling”
Spring 2008 7
Why was red light incapable of knocking electrons out of certain materials, no matter how bright◦ yet blue light could readily do so even at modest intensities◦ called the photoelectric effect◦ Einstein explained in terms of photons, and won Nobel Prize
Spring 2008 8
What caused spectra of atoms to contain discrete “lines”◦ it was apparent that only a
small set of optical frequencies (wavelengths) could be emitted or absorbed by atoms
Each atom has a distinct “fingerprint”
Light only comes off at very specific wavelengths◦ or frequencies◦ or energies
Note that hydrogen (bottom), with only one electron and one proton, emits several wavelengths
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Squint and things get fuzzy◦ opposite behavior from particle-based pinhole
camera Eye floaters
◦ look at bright, uniform source through tiniest pinhole you can make—you’ll see slowly moving specks with rings around them—diffraction rings
Shadow between thumb and forefinger◦ appears to connect before actual touch
Streaked street-lights through windshield◦ point toward center of wiper arc: diffraction
grating formed by micro-grooves in windshield from wipers
◦ same as color/streaks off CD
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particle? wave?
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The pattern on the screen is an interference pattern characteristic of waves
So light is a wave, not particulate
Lets watch this movie!!!
http://www.youtube.com/watch?v=DfPeprQ7oGc
http://www.ars-chemia.net/Classes/101/PPT/08_Quantum_Chemistry.pdf
http://www.ars-chemia.net/Classes/101/PPT/08_Quantum_Chemistry.pdf
http://www.ars-chemia.net/Classes/101/PPT/08_Quantum_Chemistry.pdf
http://www.ars-chemia.net/Classes/101/PPT/08_Quantum_Chemistry.pdf
Luminescence• Emission of photons from electronically excited
states
• Two types of luminescence:Relaxation from singlet excited state
Relaxation from triplet excited state
Singlet and triplet states• Ground state – two electrons per orbital; electrons
have opposite spin and are paired
• Singlet excited state Electron in higher energy orbital has the opposite spin orientation relative to electron in the lower orbital
• Triplet excited state The excited valence electron may spontaneously reverse its spin (spin flip). This process is called intersystem crossing. Electrons in both orbitals now have same spin orientation
Types of emission• Fluorescence – return from excited singlet state to
ground state; does not require change in spin orientation (more common of relaxation)
• Phosphoresence – return from a triplet excited state to a ground state; electron requires change in spin orientation
• Emissive rates of fluorescence are several orders of magnitude faster than that of phosphorescence
Energy level diagram (Jablonski diagram)
Fluorescence process: Population of energy levels• At room temperature (300 K), and for typical
electronic and vibration energy levels, can calculate the ratio of molecules in upper and lower states
kTE
n
n
lower
upper exp
k=1.38*10-23 JK-1 (Boltzmann’s constant)E = separation in energy level
Fluorescence process: Excitation• At room temperature, everything starts out at
the lowest vibrational energy level of the ground state
• Suppose a molecule is illuminated with light at a resonance frequency
• Light is absorbed; for dilute sample, Beer-Lambert law applieswhere is molar absorption (extinction) coefficient (M-1 cm-1); its magnitude reflects probability of absorption and its wavelength dependence corresponds to absorption spectrum
• Excitation - following light absorption, a chromophore is excited to some higher vibrational energy level of S1 or S2
• The absorption process takes place on a time scale (10-15 s) much faster than that of molecular vibration → “vertical” transition (Franck-Condon principle).
clA
Fluorescence process: Non-radiative relaxation
• In the excited state, the electron is promoted to an anti-bonding orbital→ atoms in the bond are less tightly held → shift to the right for S1 potential energy curve →electron is promoted to higher vibrational level in S1 state than the vibrational level it was in at the ground state
• Vibrational deactivation takes place through intermolecular collisions at a time scale of 10-12 s (faster than that of fluorescence process)
So
S1
Fluorescence process: Emission
• The molecule relaxes from the lowest vibrational energy level of the excited state to a vibrationalenergy level of the ground state(10-9 s)
• Relaxation to ground state occurs faster than time scale of molecular vibration → “vertical” transition
• The energy of the emitted photonis lower than that of the incidentphotons
So
S1
Stokes shift The fluorescence light is red-shifted (longer
wavelength than the excitation light) relative to the absorbed light ("Stokes shift”).
Internal conversion (transition occurring between states of the same multiplicity) can affect Stokes shift
Solvent effects and excited state reactions can also affect the magnitude of the Stoke’s shift
Invariance of emission wavelength with excitation wavelength
• Emission wavelength only depends on relaxation back to lowest vibrational level of S1
• For a molecule, the same fluorescence emission wavelength is observed irrespective of the excitation wavelength
So
S1
I. Principles of Fluorescence
Mirror image rule• Vibrational levels in the excited
states and ground states are similar
• An absorption spectrum reflects the vibrational levels of the electronically excited state
• An emission spectrum reflects the vibrational levels of the electronic
ground state
• Fluorescence emission spectrum is mirror image of absorption
spectrum
S0
S1
v=0
v=1
v=2v=3v=4v=5
v’=0v’=1v’=2v’=3v’=4v’=5
Internal conversion vs. fluorescence emission
As electronic energy increases, the energy levels grow more closely spaced
It is more likely that there will be overlap between the high vibrational energy levels of Sn-1 and low vibrational energy levels of Sn
This overlap makes transition between
states highly probable
Internal conversion is a transition occurring between states of the same multiplicity and it takes place at a time scale of 10-12 s (faster than that of fluorescence process)
The energy gap between S1 and S0 is significantly larger than that between other adjacent states → S1 lifetime is longer → radiative emission can compete effectively with non-radiative emission
Mirror-image rule typically applies
when only S0 → S1 excitation takes
place
Deviations from the mirror-image rule
are observed when S0 → S2 or transitions
to even higher excited states also
take place
Intersystem crossing refers to non-radiative transition between states of different multiplicity
It occurs via inversion of the spin of the excited electron resulting in two unpaired electrons with the same spin orientation, resulting in a state with Spin=1 and multiplicity of 3 (triplet state)
Transitions between states of different multiplicity are formally forbidden
Spin-orbit and vibronic coupling mechanisms decrease the “pure” character of the initial and final states, making intersystem crossing probable
T1 → S0 transition is also forbidden → T1 lifetime significantly larger than S1 lifetime (10-3-102 s)
S0
S1T1absorption fluorescence
phosphorescence
Intersystemcrossing
Inte
nsi
ty
Wavelength
Absorbance
DONOR
Absorbance
Fluorescence Fluorescence
ACCEPTOR
Molecule 1 Molecule 2
Fluorescence energy transfer (FRET)In
tensi
ty
Wavelength
Absorbance
DONOR
Absorbance
Fluorescence Fluorescence
ACCEPTOR
Molecule 1 Molecule 2
Non-radiative energy transfer – a quantum mechanical process of resonance between transition dipoles
Effective between 10-100 Å onlyEmission and excitation spectrum must significantly
overlapDonor transfers non-radiatively to the acceptor
Quantum yield of fluorescence, f, is defined as:
In practice, is measured by comparative measurements with reference compound for which has been determined with high degree of accuracy.
Ideally, reference compound should have◦ the same absorbance as the compound of interest at given
excitation wavelength◦ similar excitation-emission characteristics to compound of
interest (otherwise, instrument wavelength response should be taken into account)
◦ Same solvent, because intensity of emitted light is dependent on refractive index (otherwise, apply correction
◦ Yields similar fluorescence intensity to ensure measurements are taken within the range of linear instrument response
absorbed photons ofnumber
emitted photons ofnumber f
Quantum yield of fluorescence
Another definition for f is
where kr is the radiative rate constant and k is the sum of the rate constants for all processes that depopulate the S1 state.
The observed fluorescence lifetime, is the average time the molecule spends in the excited state, and it is
k
krf
kf
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Fluorescence lifetime
Fluorescence emission distribution•For a given excitation wavelength,
the emission transition is distributed among different vibrational energy levels
•For a single excitation wavelength, can measure a fluorescence emission spectrum
Inte
nsi
ty
Emission Wavelength (nm)
ExcEmm
Effect on fluorescence emission• Suppose an excited molecule emits fluorescence
in relaxing back to the ground state
• If the excited state lifetime, is long, then emission will be monochromatic (single line)
• If the excited state lifetime, is short, then emission will have a wider range of frequencies (multiple lines)
1) Which more closely resembles an absorption spectrum an emission or an excitation spectrum?
1) What is the difference between fluorescence and phosphorescence?
2) Define quantum yield?
1) An excitation spectrum is essentially identical to an absorption spectrum.
2) Fluorescence – return from excited singlet state to ground state; does not require change in spin orientation (more common of relaxation)
Phosphoresence – return from a triplet excited state to a ground state; electron requires change in spin orientation
3) Quantum yield is absorbed photons ofnumber
emitted photons ofnumber f