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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