Ground state hn Ground state reactants products The overall reaction 1.1 Ground state Excited state Reaction Ground state reactants reactants Intermediates products 1.2 Ground state hn Ground state reactants products Ground state Excited state Reaction Ground state reactants reactants Intermediates products 1.3 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Assumed fully characterized by standard analytical techniques 1.4 EXAMPLE Ground state Excited state Reaction Ground state reactants reactants Intermediates products CH3 O Butyrophenone 1.5 hn Ground state Excited state Reaction Ground state reactants reactants Intermediates products Light absorption LUMO HOMO 1.6 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Singlet Triplet States States 1.7 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Singlet Triplet States States LUMO Lowest excited states HOMO TRIPLETS SINGLETS 1.8 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Singlet Triplet States States LUMO Upper excited states HOMO TRIPLETS SINGLETS 1.9 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Singlet Triplet States States S n Tn .... .... S 1 T 1 S 0 1.10 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Singlet Triplet States States S n T n .... S Kasha's 1 T 1 Rule S 0 1.11 Ground state Excited state Reaction Ground state reactants reactants Intermediates products n,p* p,p* s,p * . 1.12 Ground state Excited state Reaction Ground state reactants reactants Intermediates products small S big S 1 1 T 1 T 1 S S 0 0 n,p* p,p* 1.13 Ground state Excited state Reaction Ground state reactants reactants Intermediates products k Singlet isc Triplet States States Intersystem crossing from the first excited singlet state to the lowest triplet state The importance of spin 1.14 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Singlet Triplet States States Internal conversion Fluorescence Intersystem crossing from the triplet to the ground state Phosphorescence Ground Singlet State 1.15 EXAMPLE Ground state Excited state Reaction Ground state reactants reactants Intermediates products CH3 CH 3 Triplet butyrophenone has n,p* O O* character and is formed following very rapid intersystem crossing after excitation to the singlet state Butyrophenone 1.16 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Modified excited state reactants In some cases excited states may undergo significant modifications before they participate in the reaction of interest, e.g.: • Associative processes (excimer, exciplex) • Structural (adiabatic) modifications 1.17 Ground state Excited state krxn Reaction Ground state reactants reactants Intermediates products The reactive excited state transforms into ground but short lived species which eventually yield the final products of reaction 1.18 Ground state Excited state krxn Reaction Ground state reactants reactants Intermediates products The transformation may involve atom or electron transfer or bond breaking or formation. It is usually an adiabatic process. Spin is conserved A small number of elementary transformations are normally sufficient to explain most photo- reactions, which may reflect combinations of this basic reaction set 1.19 EXAMPLE Ground state Excited state Reaction Ground state reactants reactants Intermediates products H-atom transfered CH CH2• CH3 3 OH O hn O* k rxn • Biradical generated in the triplet state 1.20 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Ground state species, in principle stable but whose lifetime is limited by intramolecular or intermolecular reactivity The nascent reaction intermediates usually remember the spin state of the precursor excited state 1.21 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Transition state ‡ Intermediate Reactive species excited state Products 1.22 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Typical examples Free radicals biradicals radical ions carbocations ylides ortho-xylylenes carbenes 1.23 EXAMPLE Ground state Excited state Reaction Ground state reactants reactants Intermediates products CH2• • OH 1.24 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Intermediate Intermediate Intermediate # 1 # 2 # n Several reaction intermediates may be involved between the precursor excited state and the final products 1.25 Ground state Excited state Ground state reactants reactants products The involvement of reaction intermediates on the way between excited states and products is very common, but not essential 1.26 k Ground state Excited state Reaction p Ground state reactants reactants Intermediates products The conversion of reaction intermediates into final products of reaction may be a single-step or multistep process and may be determined by many factors, e.g.: • Chemical reaction • Conformational changes • Spin evolution 1.27 EXAMPLE Ground state Excited state Reaction Ground state reactants reactants Intermediates products CH CH • CH • CH 2 CH2 3 CH3 2 2 O OH • OH OH hn O* k rxn • Triplet Singlet biradical biradical 1.28 Ground state Excited state Reaction Ground state reactants reactants Intermediates products Ground state products are most frequently closed shell molecules and can be fully characterized using normal analytical techniques 1.29 EXAMPLE Ground state Excited state Reaction Ground state reactants reactants Intermediates products OH O CH 2 CH 2 CH3 CH3 CH2• CH 3 O hn O* k rxn • OH O Product forming reactions OH Ph 1.30 Ground state hn Ground state reactants products The overall reaction 1.31 EXAMPLE The overall reaction Ground state hn Ground state reactants products O CH 2 CH2 CH3 O hn OH Ph 1.32.