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The interactions of light with matterIgnoring fluorescence, the interactions of light with matter can be expressed thus:

Io = Ireflected + Iscattered + Iabsorbed + Itransmitted

transparentmaterial

translucentmaterial

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The Beer-Bouger-Lambert Law and εmax

The Beer’s law: At a given λ the proportion of light absorbed by a transparent medium is independent of the intensity of the incident light and is proportional to the number of absorbing molecules through which the light passes.

I0 is the intensity of incident lightI is the intensity of transmitted lightk is the absorption coefficientl is the path length through the sampleε is the molar extinction coefficient or molar absorptivityc is the concentration in mol/L=> k ≈ 2.303 ε c

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Electronic, Vibrational, and Rotational States of Molecules

S = 0

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The Franck-Condon Principle

∆E

shapes of absorption bands

vertical transition

S = 0

S = 1

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Classification of Electronic Transitions in Molecules

∆E = hν

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ε < 10 forbiddenε < 1000 weakly allowedε < 106 allowedε > 106 strongly allowed

Classification of Electronic Transitions in Molecules

∆E = hν

A small ε is equivalent with a low probability of this transition (here, it has to do with orbital symmetry not with different populations of the states)

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Chromophores and Auxochromic Groups

• The position and intensity of an absorption band depends on the nature of the electronic transition involved;

• Electrons in σ-orbitals usually have lower energy than electrons in π-orbitals and electrons in n-orbitals have the highest energy; The excitation energy of these electrons decreases form σ to n since they are excited into the same unoccupied orbital (LUMO) and ∆E therefore decreases;

• These isolated groups are called chromophores and the absorption spectrum of a molecule containing several isolated chromophores is usually given by the sum of the individual absorptions;

• Functional groups that contain n-orbitals and are bond to a chromophore are called auxochromic groups and they usually move the absorption to lower energy (higher wavelength); An increase in intensity might also be observed;

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Absorption of Isolated Chromophores

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Absorption of Alkenes

π→π∗

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Absorption of benzene

derivatives

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Absorption of Carbonyl Groups

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Absorption of Carbonyl Groups

Conjugated chromophore (C=C) with auxochromic group (C=O)

LCAO (Linear Combination of Atomic Orbitals)

Conjugation of the carbonyl group with a C=C group decreases the energy of the p* and increases the energy of π. Thus π→π∗ and n →π∗ move to lower energy.π Donors and σ acceptors such as OR and NR increase π* and decrease n.

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Absorption of α,β-unsaturated Carbonyl Groups

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Solvent Effects in UV-VIS Spectra of Ketones

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Intramolecularcharge transfer (push-pull effect) and solvent effects

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Twisted Intramolecular Charge Transfer (TICT)

ground state

excited state

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300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

H-aggregate

4b

5b6b

Abs

orba

nce

λ in nm

NN

N

N N N

N

N

S RS

S

S

SS

S

S RR

R

R R

R

R

Cu

Absorption of a Copper Phthalocyanine Dye and Aggregation

Aggregation and UV-VIS Spectra

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A biphenyl molecule is non-planar in its ground state (S=0) but becomes planar in the S=1 state.Does this conformational change take place before the excitation, during the excitation, or after the excitation process?

Conformation and UV-VIS Spectra

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