Chemistry Reference
In-Depth Information
k
vib
>
10
12
s
-1
ν
4
ν
5
ν
6
ν
7
1
ES
ν
3
ν
2
ν
1
ν
0
k
r
=
10
3
-10
10
s
-1
E
h
ν
h
ν'
1
GS
ν
4
ν
5
ν
6
ν
7
ν
3
ν
2
ν
1
ν
0
r
e
ES
r
e
GS
r
Figure 8.4
Morse potential energy surface diagram for electronic (
1
GS,
1
ES) and vibrational
(
n
) states of a chromophore. Included are typical fi rst-order rate constants.
k
vib
= vibronic
relaxation rate constant,
k
r
= rate constant of emission of light with average energy
h
ν
,
r
=
ν
′
internuclear distance,
r
GS
S
equilibrium internuclear distance,
r
e
ES
ES
equilibrium internu-
=
=
e
clear distance.
k
x
ΦΦ
x
=
(8.2)
ES
∑
k
where F
ES
is the quantum effi ciency of population of the reactive ES. An important
descriptor of the electronic excited state is its inherent lifetime, t
0
, simply defi ned
as (
k
)
− 1
.
Conversion between electronic states of systems is typically represented with
Jablonski or state diagrams. These diagrams are simplifi ed versions of the Morse
potential energy surfaces, as shown in Figure 8.4, where each electronic state is
represented as a line instead of a surface. Decay processes of electronic excited
states, ES, may occur by multiple pathways (Figure 8.5). Radiative processes are
typically represented as straight arrows ( ) and nonradiative processes are pre-
sented as wavy arrows ( ). Internal conversion,
k
ic
, is nonradiative relaxation
without change in electron spin, while intersystem crossing,
k
isc
, is accompanied by
a change in spin. A molecular orbital depiction of the processes is shown in Figure
8.6 . Fluorescence,
k
f
, is defi ned as emission of a photon and decay to the ground
state without change in electronic spin. Radiative decay with a change in spin state
is defi ned as phosphorescence,
k
p
. It should be noted that several examples in the
literature incorrectly defi ne fl uorescence or phosphorescence by the lifetime of the
Σ
