Chromophore triplet state

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

Figure 1.2. Jablonski energy level diagram showing the singlet state and the triplet state with its zero-field splittings for a planar aromatic chromophore.

For = Ph the 5-phenyloxadiazole chromophore of (52) is excited, the O—N bond breaks, and the intermeditate nitrene (55) is either intercepted by a nucleophilic solvent like methanol (R, R = Ph) or closes the ring (from a triplet state <91JCS(P2)187 to a quinazolinone (57) (Scheme 19) <68TL242i>. Further reaction paths of (55) are also shown in Scheme 19. 

Recently, however, examples of intramolecular triplet sensitization have been described. Alkyl aryl ketones with pendant azido groups in the alkyl moiety were irradiated into the benzoyl chromophore. The excited alkylphenyl ketones undergo intersystem crossing to their triplet states within picoseconds and they can then transfer their triplet energy to the nearby alkyl azido group. This process... 

Absorption by the chromophore ligand is commonly (6) followed by inter-system crossing (ISC) to the triplet state of the ligand. This state can decay either by emission (ligand phosphorescence) or by... 

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