environmental impact of light

Environmental Impact of Light

Fundamental processes in soil, atmospheric and aquatic systems

Chemical and biochemical changes2.iii.c (2 hrs)

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‘Dark’ Reactions

• Most organic compounds react very slowly, even with oxygen, at normal temperature.

• As a rule, they can be considered generally nonreactive.

• BUT !...

Environmental processes / Chemical and biochemical changes / Impact of light

3

Let Be Light !

• Visible and ultraviolet radiation can promote the reactivity of almost all compounds.

• Recall a well known fact that items exposed (northern hemisphere) on southern side of buildings decay more quickly than those on northern side.

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On the side facing south, the wooden (and metal) items decay faster

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Museum item

Side exposed to light Side not exposed to light

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Basics of Photochemistry

• Primary photophysical process.

• Subsequent chemical change(s)

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Photophysical process

• Actually, it means absorption of light quanta.• It promotes molecule from ground- to excited

state (energy rich one).

• But, to catch the light quanta, molecule has to have a CHROMOPHORE !

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Chromophores

• Chromophores are structural details in molecule(s) that enable high(er) probability of light absorption.

• Normally, it comprises double bonds in molecule, i.e., π-electrons:

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Chromophore

• Double bond

• Aromatic ring

• Far more efficient are polarized double bonds.

C=O:

C=N-

C=C

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Photophysical process• Once molecule acquires light quantum, it has

many ways to decay, involving the chemical change.

• Simplified approach tells that molecule can exist in ground state singlet or triplet– The same holds for excited state

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Ground-statesinglet

Excited-statesinglet

Excited-statetriplet

S1 T10S

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hν(d)(b)

(c)E

S1

S0

T1

hν hν(a)

ES1S0 T1

hν

hν

hν

(a)

(b)(c)

(d)

r

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• Singlets and triplets are known as multiplicities of state.

• Electronic transition among states of different multiplicities is not likely to happen (is ‘forbidden’),

• But, as many other things, something forbidden still could happen.

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• These ‘violations’ could be promoted by, so called, SENSITIZERS.

OHO OH

COO

+

S

N

NMe 2Me 2 N+

CN

CN

Fluoresceineee

Methylene blue Dicyanoanthracene

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• Sensitizers quickly, and in good yields populate excited triplet states.

• Triplets live long enough to encounter other molecules.

hν(d)(b)

(c)E

S1

S0

T1

hν hν(a)

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Sensitization

h ν

in te rs ys te m

C r o ss in g

S e n s (S 0) S e n s (S 1)

S e n s (S 1) S e n s (T 1)

S e n s (T 1) + V O C (S 0) S e n s (S 0) + V O C (T 1)))))

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Photophysics → Photochemistry

C=O:

CH3-CO-CH3 hν CH3CO + CH3 ))CH3COCOCH3 + CH3CH3

Norrish type I

+

R

O H

R

OH

hνR

R

R

OHR

Enol

Norrish type II

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• Primary chemical intermediates in photochemical metathesis are organic free radicals.

• Free radicals are highly reactive.• Radicals react with almost

everything!

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Additional sensitization(singlet oxygen)

h ν

in te rs y s te m

C r o ss in g

S e n s (S 0) S e n s (S 1)

S e n s (S 1) S e n s (T 1)

S e n s (T 1) + O 2 (T 0) S e n s (S 0) + O 2 (S 1)))))

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Coatings & Erosion

• VOC-s exposed to light can be promoted to more reactive compounds.

• Oxygen-containing derivatives can be aggressive to living organisms and to artifacts.

• Reactive intermediates may lead to polymers forming sticky coating.

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Everything matters

• Ozone is not emitted directly from industrial sources and vehicles.

• It is formed in troposphere as a result of reactions involving oxides of nitrogen and volatile organic compounds.

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What pollution we can control?

–Industry,–Traffic,–Agriculture– and (….. leisure)

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Internet Resources

• http://www.iaq.dk/index.html• http://www.airclim.org/airAndEnvironment/A

E_chp3.htm• http://www.epa.gov/region4/airqualitytoolkit/1

0_Glossaries/US NPS - Air Quality Glossary.pdf

• http://www.heritage.xtd.pl/pdf/full_czop.pdf• http://www.epa.gov/iaq/voc.html

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• http://www.environment.gov.au/soe/2006/publications/commentaries/atmosphere/glossary.html

• http://www.airimpacts.org/documents/local/aqbook.pdf