Presentation on theme: "Chemistry 2 Lecture 13 Everything. Learning outcomes from lecture 12 Be able to explain Kasha’s law by describing internal conversion Be able to define."— Presentation transcript:
Chemistry 2 Lecture 13 Everything
Learning outcomes from lecture 12 Be able to explain Kasha’s law by describing internal conversion Be able to define fluorescence quantum yield Be able to describe intersystem crossing and how it leads to phosphoresence Be able to explain why the phosphorescence occurs at lower energy (“red-shifted”) and is slower than fluorescence The sequence of events that can occur after absorption, including emission, fluorescence, phosphorescence, non-radiative decay, internal conversion and intersystem crossing. The use of Jablonski diagrams to describe these processes. Assumed knowledge
Energy is the most important thing
Extraterrestrial solar spectrum 6000 K thermal spectrum
Atmospheric absorption Absorption by water, Carbon dioxide and ozone. Scattering too! H 2 O & CO 2 H2OH2O O3O3 overtones!
Blue ice is due to overtone absorption overtones!
Absorption of light by the earth Chlorophyll fluorescence in satellite image S2S2 S1S1 S0S0 IC T1T1 T2T2 Absorption Fluorescence ISC Phosphorescence 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…
Absorption and re-emission of infrared radiation by atmospheric molecules EARTH E ATMOSPHERE E E/2 E/4 2E2E E E
The greenhouse effect is due to IR absorption EARTH E=168 2E2E E E But, with single layer blackbody atmosphere absorbing outgoing radiation, Earth heats to irradiate twice the incoming energy. With no atmosphere, average temperature on earth is T 0. If earth was blackbody with albedo of 0.40, then 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 N 2 and O 2, 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 CO 2 and synthesize sugars using light energy from the sun. Can be modelled as a particle on a ring system. S 1 -S 0 S 2 -S 0 S2S2 S1S1 S0S0 IC T1T1 T2T2 Absorption Fluorescence ISC Phosphorescence
Light harvesting The energy in photosynthesis is transferred from chlorophyll to chlorophyll and funnelled into the reaction centre. S 1 -S 0 S 2 -S 0 chlorophylls
Energy transfer is internal conversion in bichromophoric molecule S 2 -S 0 h * S2S2 S1S1 S0S0 IC T1T1 T2T2 Absorption Fluorescence h
Photodynamic cancer therapy S0S0 S1S1 T1T1 ISC h triplet sensitizer energy transfer O2O2 T0T0 S1S1 Cytotoxic singlet oxygen is produced from energy transfer after intersystem crossing. Specific tissues can be targeted by laser irradiation of 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.
spin states of two triplets STQ TTQ QQQ 1/9 collisions statistically gives singlet which can decay into excited S 1 of one chromophore, and S 0 opf the other.
requirements for TTA upconversion S0S0 S1S1 T1T1 sensitizer T1T1 TTA S1S1 ISC emitters More than half S 1 Step down by >>k B T
single threshold solar cells ~32% max electrons unoccupied energy levels V IC!
Up-conversion cell Up-conversion unit Limiting efficiency of an Upconversion cell is about 50%
Nothing wrong with this picture!
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 (either on the worksheets or as ‘end of lecture problems’. Other questions on past papers include parts which are no longer part of the course.