set 05
TRANSCRIPT
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UV-vis and Fluorescence
Spectroscopy
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Literature (Selection)
General Texts:
M. Klessinfer, J. Michl, Excited States and Photochemistry of Organic Molecules, VCH Publishers, 1995.
J.M. Hollas, Modern Spectroscopy, Wiley&Sons, 1996.
D.C. Harris, M.D. Bertolucci, Symmetry and Spectroscopy: An Introduction to Vibrational and ElectronicSpectroscopy, Dover Publications, 1990. D.C.
H.-H. Perkampus, H.C. Grinter, T.L. Threfall, UV-vis Spectroscopy and Its Applications, Springer, 1992.
B.J. Clark, T. Frost, M.A. Russell, UV Spectroscopy - Techniques, instrumentation and data handling,
Chapman&Hall, 1993.J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Publishers, 1999.
M.G. Gore, Spectrophotometry and Spectrofluorimetry: A Practical Approach, Oxford Univ. Press, 2000.
L. Brand Fluorescence Spectroscopy, Academic Press, 1997.
Specialized Topics:
J.D. Coyle, Introduction to Organic Photochemistry, Wiley&Sons, 1988.
N.J. Turro, Modern Molecular Photochemistry, University Science Books, 1991.V. Balzani, F. Scandola, Supramolecular Photochemistry, Ellis Horwood, 1991.
J.R. Lakowicz, Topics in Fluorescence Spectroscopy (Vol 1-6), Plenum Publishers, 1997.
Data Collections:
H.H. Perkampus, UV-vis Atlas of Organic Compounds (Part 1 and 2 ), VCH Publishers, 1992.
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UV-vis Range of the Electromagnetic Spectrum
[cm1]
[nm]
x-rays
visible region
far UV near UV infrared
420 470 530 580 620 700 75040020010
blue
green
yellow
orange
red
purple
1.31042.51045104106
c =
(cm1
) =1
=c
1 eV = 8066 cm
1
= 23 kcalmol
1
96.5 kJmol
1
E= h =h c
= h c
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Selection Rule
Not every possible transition is seen in a spectrum
> the statements of required characteristics are called selection rules:
Allowed transitions = ground and excited state for a possible transition possess
the required characteristics
Forbidden transitions = ground and excited state do not meet the characteristics
LaPortes Rule:In centrosymmetric environments transitions can only occur between states
of opposite parityu > g or g > u (d > p, s > p are allowed, but not d > d, s > d etc.)
Spin Selection Rule:Transitions may occur only between energy states with the same spin
multiplicity
=> most possible transitions are actually forbidden!Nevertheless, in certain cases also forbidden transitions can be observed,
but their intensity is much weaker.
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Jablonski Diagrams
energy
A
F
Ph
IC
ISC
Absorption
Fluorescence
Phosphorescence
Internal Conversion
Intersystem Crossing
Radiative process
Non-radiative process
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Vibrational and Rotational Energy Levels
In contrast to single, isolated atoms electronic transitions in molecules
appear as broad absorption bands due to the presence of vibrational androtational energy levels:
Sharp-line absorption
typical for isolatedatoms in the gas-phase
Absorption band with
vibrational structuretypical for small or rigid
molecules
Structureless broad
absorption typical forlarge molecules insolution
Potential energy curves and vibrational
levels of a molecule (rotational sublevelsare not shown)E
total= E
el+ E
vib+ E
rot
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Vibrational and Rotational Energy Levels
The structure of absorption bands depends strongly on the acquisition
conditions:
Vibrational structure of the n,* absorption bandof 1,2,4,5-tetrazine:I: Vapor phase spectrum at room temperatureII: Spectrum at 77K in isopentane/methycyclohexane matrixIII: Spectrum in cyclohexane at room temperatureIV: Spectrum in water at room temperature
Electronic, vibrational and rotational energylevel diagram
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Franck-Codon Principle
Absorption of light occurs in the range of 1015 s, which is much shorter than
the time needed for vibrational changes
According to the Franck-Codon principle, the absorption corresponds to avertical transition of the ground state to the excited state energy hypersurface
> all bond lengths, angles, conformations and solvation cages areconserved during the transition
Unsymmetric band resulting fromapproximately equal equilibriuminternuclear distances in the groundand excited state> intense 0 > 0 transition
Symmetric band internuclear distancesin the excited state larger than in theground state> intense 0 > 2 transition
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Classification of Electronic Transitions
In organic molecules electronic transitions can be classified by indicating the
molecular orbitals involved:
Absorption range for various electronictransitions:
The corresponding transitions can beabbreviated as:
> *
> *
n > *n > *
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Labeling of Electronic Transitions
Beside the Kashanomenclature based on simplified molecular orbitals, there
are various other commonly used descriptions for electronic transitions:
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Spectrum Acquisition
monochromatorlight
source
sample PMT
A = log I0I
= c l
Lambert-Beer (1760/1852):
A: absorbance
: molar extinction coefficientl: path length of sample cuvette
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Spectrum Acquisition
UV spectra of azulene 2 in cyclohexaneA) log () =f()B) log () =f( )C =f()D) =f( )
Most instruments record the absorbanceA as a function of the wavelength
Wavenumbers are proportional to the energy of the absorption, and usuallythe better choice for quantitative interpretations:
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Ab
sorptionofIsola
tedChromopho
ricGroups
(Lowestenergytransition
s)
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Interactions Between Chromophores
A molecule which contains several isolated chromophores will give a UV-vis
spectrum which essentially consists of the additive absorptions of theindividual groups
This is not the case for conjugated chromophores: With increasingconjugation the absorption energy and intensity decreases steadily ( > p*)
C
C
H
O
H
O
H
C
H
O
max = 303 nm
= 18
max = 450 nm
= 5
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Olefines and Polyenes
The > * transition in ethylene occurs at 165 nm (max = 16000).
Substitution with an atom containing non-bonding electrons (=auxochromic
groups, OH, OR, NH2, NHR, SH, SR, Hal) results in a bathochromic shift> the non-binding electrons interact with the -orbitals of the double bond
> the energy difference between the highest unoccupied molecular orbital(=HOMO) and the lowest unoccupied molecular orbital (=LUMO) decreases
energy
LUMO
HOMO
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Conjugated Olefins
Conjugation of two or more double bonds results in decreasing energydifference between the HOMO and LUMO:
Table:longest wavelength absorptions inconjugated all-trans polyenes
Note: the molar extinction coefficientis increases with increasing size of
the polyene
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Cis-/trans Isomers
Cis/trans isomers show different absorption spectra due to change of the
symmetry properties of the molecule and therefore also of the ground andexcited state wavefunctions:
The first overtone of -carotin is found at 340 nm. In the all- trans configuration this transition issymmetry forbidden, whereas in the depicted cis-configuration the transition is allowed.> this leads to the so called cis-peak of the carotines
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Empirical Rules
Woodward (1942) as well as Fieser and Scott derived a set of empirical rules
for the estimation of the long wavelength maxima in diens:
acyclic, transoid217 nm
cisoid (homoannular)253 nm
transoid (heteroannular)214 nm
Increments:
For each additional conjugated double bondFor each exocyclic position of a double bond
For each alkyl groupFor each of the following auxochromic groups:
+30 nm+ 5 nm
+ 5 nm
+ 6 nm+ 0 nm+30 nm+60 nm+ 5 nm+ 5 nm
O-alkylO-acylS-alkylN(alkyl)2ClBr
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Benzene and Aromatic Compounds
A) Energy diagram of the benzene -orbitals
B) Energy term diagram:
I: max 256 nm,1A1g >
1B2u (-band)II: max 203 nm,
1A1g >1B1u (p-band)
III: max 184 nm,1A1g >
1E1u (-band)
Absorption spectrum of benzene
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Substitution Effects
Long wavelength absorptions of somepara-disubstituted benzenes:
The strong bathochromic shift for 4-nitrophenol is due to a charge transfer absorption:
OHN
O
O
OHN
O
O
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UV/vis spectra of o-, m-, and p-nitrophenol:
A) in 10 mM HCl
B) in 5 mM NaOH
Note: the low energy charge transfer band isnot only observed for the orthoand para
isomer, but also for the metaisomer
A
B
Charge Transfer
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Condensed Aromatic Systems
As compared to benzene the HOMO and LUMO orbitals in condensed
aromatic compounds are not degenerate> there are four different electronic transitions possible:
A) Orbital diagram B) Term diagram C) Electronic transitions
(under consideration ofconfiguration interaction)
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Condensed Aromatic Systems
With increasing size the -,p-, and -band of benzene are shifting to longer
wavelengths
The bathochromic shift leads to absorption in the visible range:
BenzeneTetracene
Pentacene
Hexacene
colorlessoange-yellow
blue-purple
dark green
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Carbonyl Compounds
Excitation can occur to the antibonding *or * orbitals
With saturated aldehydes and ketones theallowed n > * and > * transitions are
observed in the vacuum-UV region> only the forbidden n(p) > * transitions
can be observed (275- 300 nm, e = 15-30)
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Influence of Auxochromes
Auxochromic substituents (OH, OR, NH2, NHR, SH, SR, Hal) at thecarbonyl group increase the energy of the * orbital (-donor), and decreasethe n orbital energy (-acceptor)
> the n,* transitions are shifted to higher energy (shorter wavelength)
Conjugation with a double bond has essentially no effect on the n-orbitalenergy, but increases the HOMO energy
> the ,* transition of enones with increasing chain length is shifted into the
visible range
en
ergy