Conjugated polyenes excited triplet states

Collisional energy transfer is frequently called sensitization. It is an important tool in photochemistry because it provides access to excited triplet states of compounds whose singlets do not intersystem cross efficiently (such as alkenes and conjugated polyenes). It also provides access directly to the triplet states of compounds for determination of the multiplicity of the state responsible for a photochemical reaction. 

Figure 3.8 shows the carbon-carbon stretching force constants thus determined for the S0 and Ta states. Changes in those force constants upon triplet excitation (from open circles to filled circles) can be characterized as follows Concerning the conjugated polyene chain, the decrease in the C=C stretching force constant is in the order, C11=C12 > C9=C10 > C7=C8 > C13=C14, whereas the increase in the C-C stretching force constant is in the order, C8-C9 > C10-C11 > C12-C13. 

Energies of the lowest singlet and triplet excited states of linear polyenes containing 2 to 20 conjugated double bonds were recently calculated (94). These values are considerably lower than the known experimental values, given in this table—e.g., calc, of 3-carotene 11,5 kcal/mole (48 kj/mole). 

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