Engineering Chemistry S Datta

552 ENGINEERING CHEMISTRY
Types of Photophysical Pathways
A molecule excited to a higher energy state must return to the ground state. Promotions
are from S 0
to S 1
states, as promotion from S to triplet states is ‘forbidden’. Promotion to other
higher singlet states like S 2
takes place, but in liquids and solids these higher energy states
are short lived (10 –13 to 10 –11 sec.) and usually drop back to S 1
state. The energy lost by these
drops from S 2
, S 3
to S 1
states is given up to the environment by collision with other neighbouring
molcules and the process is called energy cascade. The decay from the higher energy states
initially populates many vibrational levels of S 1
, but finally comes down to the lowest vibrational
levels of S 1
, which is therefore the only important excited singlet state. This state can undergo
different physical and chemical processes. The pathways are shown and listed in modified
Jablonski diagram (Fig. 25.4) and in Table 25.1.
vc
S 2
IC
vc
IC
ISC
S 1
vc
h
ISC
T 1
h
h
f
vc
h
p
S 0
Fig. 25.4 Modified Jablonski diagram.