4. Molecular many electron systems: electronic & nuclear movement Molecular orbital Electronic configuration Electronic states UV/Vis-absorption spectrum * : antibinding IPC Friedrich-Schiller Universität Jena 1

4. Molecular many electron systems: electronic & nuclear movement Jablonski-Scheme v = 1 J = 4

Microwave spectroscopy

J = 3 J = 2 J = 1 J = 0 v = 0

IR- & NIR spectroscopy

S 4 S 3 S 2

UV-VIS-spectroscopy

Internal conversion [10 -14 s] Intersystem crossing T n S 1 v = 4 v = 3 v = 1 v = 0 S 0 Fluorescence [10 -9 s] T 1 Phosphorescence [10 -3 s] IPC Friedrich-Schiller Universität Jena

5. UV-Vis-Absorption

5.1 Franck-Condon principle

 Interpret electronic absorption spectra based on |  | 2 of the vibrational levels  electronic transitions (~10 -16 s) are much faster than the vibrational period (~10 -13 s) of a given molecule thus nuclear coordinates do not change during transition 3 IPC Friedrich-Schiller Universität Jena

5. UV-Vis-Absorption

5.2 Franck-Condon principle

 = degree of redistribution of electron density during transition  = degree of similarity of nuclear configuration between vibrational wavefunctions of initial and final states.

  Transition probability is proportional to the square modulus of the overlap integral between vibrational wavefunctions of the two electronic states =

Franck-Condon-Factor

: IPC Friedrich-Schiller Universität Jena 5

5. UV-Vis-Absorption

5.1 Franck-Condon principle

|f  |i  6 |f  |i  IPC Friedrich-Schiller Universität Jena

5. UV-Vis-Absorption

5.2 Molecular electronic transitions

 Molecular electronic transitions: valence electrons are excited from one energy level to a higher energy level.  Electrons residing in the HOMO of a sigma bond can get excited to the LUMO of that bond:

σ → σ*

transition.  Promotion of an electron from a

π

bonding orbital to an antibonding π

*

π → π*

transition. orbital:  Auxochromes with free electron pairs denoted as

n

have their own transitions, as do aromatic pi bond transitions.  The following molecular electronic transitions exist: σ → σ* π → π* n → σ* n → π* aromatic π → aromatic π* p,p

* (C=C, C=O) n

p

* (C=O, C=N, C=S) n

s

* ( –Hal, -S-, -Se- etc.)

IPC Friedrich-Schiller Universität Jena 7

5. UV-Vis-Absorption

5.2 Transition metal complexes

 A biologically very important group of metal complex bonds are the porphyrin pigments such as: 

Hemoglobin (

pigment of the blood, central ion Fe 2+ ) Heme-group    octahedron structure motive The four ligand positions of the base of the pyramid are occupied by the

lone electron pairs

of nitrogen atoms of the plane porphyrin ring system The two corners of the pyramid are occupied by specific amino acids (histidine) and/or by an oxygen molecule (hemoglobin)  

Cytochromes

of respiratory chain

Chlorophyll

(green molecules in leaves, central atom Mg) IPC Friedrich-Schiller Universität Jena 8

5. UV-Vis-Absorption

5.2 Transition metal complexes

 Cytochrome C:  Pyramid corners of heme unit are occupied by N-atom of a

histidine

residue and S-atom of a mezhionine residue  Redox change of cytochromes predominatly occurs at the central iron atom [(Fe 2+ ) ↔ (Fe 3+ )] a -Peaks = sensitive for redox change (

analysis of mitochondria

) IPC Friedrich-Schiller Universität Jena 9

5. UV-Vis-Absorption

5.2 Transition metal complexes

 Hemoglobin (iron is always found as Fe 2+ ) Arterial oxygen-loaded blood Blood in veines free of oxygen = = light red deep red

End-on

coordination of O 2 (Fe 2+ / 75 pm /

low spin

) B Desoxy Hemoglobin (Fe 2+ / 92 pm /

high spin

) IPC Friedrich-Schiller Universität Jena 10

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism

Fundamental terms: 

Polarimetry, optical rotation, circular birefringence:

turning of the plane of linearly polarized light  

Optical rotatory dispersion (ORD):

Wavelength dependency of rotation 

Optically active

molecules exhibit different refractive indices for right

n R

and left

n L

polarized light 

n R

≠

n L

Allows determination of absolute configuration of chiral molecules 

Circular dichroism:

linearly polarized light is transformed into elliptically polarized light upon traveling through matter  Different absorption coefficients for left and right circular polarized light ( e

R

≠ e

L

).

IPC Friedrich-Schiller Universität Jena 11

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Polarimetry

 What happens if light interacts with chiral molecules?

   Enantiomeric molecules interact differently with circular polarized light.

Polarizability

Optically active substances

left n L a depends on direction of rotation of incoming circular polarized light polarized light  n R ≠ n L exhibit different refractive indices for right n R and IPC Friedrich-Schiller Universität Jena 12

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Polarimetry

:   Linearly polarized light Different refractive index for its left and right circular constituents   Relative phase shift between left and right Vector addition yields again

linear polarized light

with rotated polarization plane

E

y

phase shift E

x IPC Friedrich-Schiller Universität Jena 13

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Polarimetry

: Due to the different refractive indices a phase difference d = j L builds up in the active medium which is proportional to the path – j R length l.

When exiting the medium linear polarized light where the oscillation plane is rotated by d /2 arises l It follows: For a follows: Na-D line l a  (

n L

= 589 nm 

n R

) p l

l

2-Butanol a = 11.2

° (Messwert) T = 20 °C  (

n L



n

l = 1dm

R

)  11 .

2 589 deg 180  10 deg  9 0 .

1

m m

 Difference is rather small!

 3 .

66  10  7 IPC Friedrich-Schiller Universität Jena 14

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Polarimetry

 The measured angle-of-rotation results in: a in angular degree, length in decimeter(!) and c in g ml -1 .

 Specific rotation is a substance specific constant (dependent on temperature and wavelength) and is a measure for the optical activity of this particular substance.

 Molar rotation is defined as follows: IPC Friedrich-Schiller Universität Jena 15

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Optical rotatory dispersion(ORD)

 ORD measures molar rotation [ F ] as function of the wavelength!

 If the substance to be investigated has

ORD spectra of 17ß- and 17

a

hydroxy-5

a

-androstan no electronic absorption

within the investigated spectral region the following ORD spectra are obtained  Reason: refractive indices for left and right polarized light change differently with wavelength (rotatory dispersion is proportional to refractive index difference).

IPC Friedrich-Schiller Universität Jena 16

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Optical rotatory dispersion(ORD)

 Refractive indices for left and right polarized light exhibit anomalous dispersion in the

range of an absorption

band 

Cotton effect

Positiv negativ Cotton effect IPC Friedrich-Schiller Universität Jena 17

5. UV-Vis-Absorption

5.3 Polarimety & Optical rotatory dispersion & Circular dichroism Optical rotatory dispersion(ORD)

 Quantitative theoretical correlations between

molecular structure and ORD

(Cotton effect) are difficult to derive;

ORD-Spektren von 5

a

-Spirostan und 5

a

Spirostan-3-on

 Empirical investigation are important: ORD has been successfully applied for constitution elucidation e.g. to position carbonyl groups in complex optically active molecules.

 By comparing ORD curves for structurally isomeric ketons (reference material needed!) the keto group can be localized.

ORD curve of molecule (2) is a superposition of a negative curve i.e. molecular skeleton without a chromophore (background curve) and a positive Cotton curve (C=O chromophore).

IPC Friedrich-Schiller Universität Jena 18

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Circular Dichroism (CD)

 Enantiomeric molecules exhibit besides different refractive indices for left and right circular polarized light also

different absorption coefficients

: Circular Dichroism

E

y  It follows:  left and right circular components ORD : different retardation CD also different absorption

E

x  Transmitted light is elliptically polarized. IPC Friedrich-Schiller Universität Jena 19

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Circular Dichroism (CD)

 The ratio between short and the long elliptical axis is defined as tangent of an angle  , the so called

ellipticity

(tan  = b/a):  a = E R + E L  b = E R - E L  The specific ellipticity is defined as:  [10 -1 × deg × cm 2 × g -1 ] where  0bs ellipticity. is the experimentally determined  The molar ellipticity is defined as: [10 × deg × cm 2 × mol -1 ] IPC Friedrich-Schiller Universität Jena 20

5. UV-Vis-Absorption

5.3 Polarimety & Optical rotatory dispersion & Circular dichroism Circular Dichroism (CD)

 Signal heights are displayed either as absorption difference De or as ellipticity [ q ].  Molar ellipticity and circular dichroism can be interconverted: [q]  [ grad cm 2 dmol -1 ]  Correlation between ORD and CD:  ORD is based on the different refractive indices of left and right circular polarized light (n R ≠ n L )  CD results from the different absorption behavior for left and right circular polarized light ( e

R

≠ e

L

)  Connection of both phenomena via Kronig-Kramer relationship:  This relation allows the calculation of an ORD value for a particular wavelength l from the corresponding CD spectrum IPC Friedrich-Schiller Universität Jena 21

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Circular Dichroismus (CD)

 Simple model:  For an electronic transition to be CD active the following must be true: µ µ e m is the electronic transition dipole moment (= linear displacement of electrons upon transition into an excited state) is the magnetic transition moment (= radial displacement of electrons upon excited state transition)  Scalar product is characterized by a helical electron displacement.

 Depending on the chirality of the helix preferably more right or left circular polarized light will be absorbed, respectively.

22 Electronic transition Magnetic transition Optical activity IPC Friedrich-Schiller Universität Jena

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Circular Dichroism (CD)

 Application field: b

-sheet

a

-helix

23

random coil

IPC Friedrich-Schiller Universität Jena

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Circular Dichroism (CD)

 Application field: Typical reference CD spectra: Poly-L-Lysine in different conformations: a -Helix, b -sheet and random coil.

24 Temperature dependent CD spectra of insuline: For increasing temperature the molecule changes form a -helix into the denaturated random coil form with ß-sheet contributions. IPC Friedrich-Schiller Universität Jena

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Vibrational-Circular-Dichroism (VCD)

 Vibrational transitions in the IR and NIR v=1 v=0  VCD monitors difference in absorption between left and right circular polarized light IPC Friedrich-Schiller Universität Jena 25

5. UV-Vis-Absorption

5.3 Polarimetry & Optical rotatory dispersion & Circular dichroism Vibrational-Circular-Dichroism (VCD) (



)-Mirtazapine

 Determination of the absolute configuration

Advantages VCD vs. CD

 Electronic chromophore is not necessary  VCD exhibits more characterisitic bands IPC Friedrich-Schiller Universität Jena 26

27

6. Fluorescence Spectroscopy

IPC Friedrich-Schiller Universität Jena

6. Basic concepts in fluorescence spectroscopy v = 1 J = 4

Microwave spectroscopy

J = 3 J = 2 J = 1 J = 0 v = 0

IR- & NIR spectroscopy

S 4 S 3 S 2 S 1

UV-VIS-spectroscopy

Internal conversion [10 -14 s] Intersystem crossing T n 28 v = 4 v = 3 v = 1 v = 0 S 0 Fluorescence [10 -9 s] T 1 Phosphorescence [10 -3 s] IPC Friedrich-Schiller Universität Jena

6. Basic concepts in fluorescence spectroscopy

6.1 Stokes-Shift

= Stokes-Shift due to vibrational energy relaxation within electronic excited state  Energy differences between vibrational states which determine vibronic band intensities are very often the same for ground and electronic excited state  Emission spectrum = mirror image of absorption spectrum  Emission bands are shifted bathochromically i.e. to higher wavelengths IPC Friedrich-Schiller Universität Jena 29