Chemistry 2 Lecture 13 Everything

Energy is the most important thing

Extraterrestrial solar spectrum 6000 K thermal spectrum

Atmospheric absorption Absorption by water, Carbon dioxide and ozone. Scattering too! O3 H2O H2O & CO2 overtones!

Blue ice is due to overtone absorption overtones!

Absorption of light by the earth Chlorophyll fluorescence in satellite image ISC IC Absorption S2 T2 While the earth fluoresces a little, the majority of incoming energy is internally converted into heat, and re-radiated in vibrational infrared transitions of water, rocks, asphalt… ISC IC S1 Fluorescence ISC T1 Phosphorescence S0

Greenhouse (average)

Absorption and re-emission of infrared radiation by atmospheric molecules ATMOSPHERE E/2 E E/4 E E/2 2E EARTH

The greenhouse effect is due to IR absorption With no atmosphere, average temperature on earth is T0. If earth was blackbody with albedo of 0.40, then E E=168 But, with single layer blackbody atmosphere absorbing outgoing radiation, Earth heats to irradiate twice the incoming energy. E 2E EARTH But the atmosphere does not absorb all outgoing radiation…. And is best represented as a multlayer. Clouds and weather complicate matters.

Greenhouse gases If atmosphere was purely N2 and O2, all surface-emitted radiation would escape into space. Gases which have oscillators that overlap the emission spectrum of earth’s 300K blackbody convey blackbody behaviour to the atmosphere (statistical emission/absorption). Peak emission of 300K object is 590cm-1.

Green plants Green plants absorb CO2 and synthesize sugars using light energy from the sun. S2-S0 S1-S0 S2 S1 S0 IC T1 T2 Absorption Fluorescence ISC Phosphorescence Can be modelled as a particle on a ring system.

Light harvesting The energy in photosynthesis is transferred from chlorophyll to chlorophyll and funnelled into the reaction centre. chlorophylls S2-S0 S1-S0

Energy transfer is internal conversion in bichromophoric molecule * S2-S0 IC Absorption S2 T2 IC S1 Fluorescence hn T1 S0 hn

Photodynamic cancer therapy Cytotoxic singlet oxygen is produced from energy transfer after intersystem crossing. Specific tissues can be targeted by laser irradiation of triplet sensitizer. ISC S1 T1 hn S1 energy transfer T0 S0 O2 triplet sensitizer Skin transmits red and near infrared light effectively (800nm is great), but absorbs most visible and UV. Much effort goes into finding near IR absorbing triplet sensitizers for cancer therapy.

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triplet-triplet annihilation (TTA) sensitizer sensitizer S1 S1 S1 S1 ISC ISC T1 TTA T1 T1 T1 S0 S0 S0 S0 emitter emitter

spin states of two triplets Q 1/9 collisions statistically gives singlet which can decay into excited S1 of one chromophore, and S0 opf the other.

requirements for TTA upconversion sensitizer S1 Step down by >>kBT S1 ISC More than half S1 T1 TTA T1 S0 emitters

single threshold solar cells unoccupied energy levels IC! ~32% max V electrons

Up-conversion cell Up-conversion unit Limiting efficiency of an Upconversion cell is about 50% cell Up-conversion unit

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Good Luck! Week 13 homework Electronic spectroscopy worksheet in the tutorials Complete the practice problems at the end of the lectures Note: ALL of the relevant past exam problems have been used as practice problems. Other questions on past papers include parts which are no longer part of the course.