Chemistry Reference
In-Depth Information
FIGURE 12.16
(a) Fluorescence intensity (count rate
n
c
(kHz) vs. time
t
(s)) and decay time
(fluorescence decay time,
.
(b) Second-order intensity correlation
C
(
t
) of the different intensity levels clearly demonstrat-
ing different off-times (triplet related) for different intensity levels. (c) Expanded view of the
first intensity level of a single molecule of compound
11
, demonstrating short off-times (triplet
related) and one long off event (related to formation of a radical anion). (d) Kinetic scheme of
ground- and excited-state pathways of multichromophoric systems like compound
t
(ns) vs. time,
t
(s)) trajectory of a single molecule of compound
11
11
.
Copyright from Ref. [82].
level [114-116]. Although this mechanism is already possible in systems containing a
single PMI chromophore (like compound
8
) due to the T
1
!
T
n
absorbance at 488 nm
(see also Figure 12.15a) [84], the decrease of
t
off
in compound
11
is promoted by
efficient singlet-triplet annihilation (S
1
þ
T
1
!
S
0
þ
T
n
) followed by reverse in-
tersystem crossing (T
n
!
S
n
). For the assignment of the additional triplet-formation
mechanism it is necessary to estimate its probability and rate constants based on a five-
level model as shown in Figure 12.16d. The model includes the singlet levels S
0
,S
1
,
S
n
, and the triplet levels T
1
and T
n
, and the following pathways are present: (1)
excitation S
0
!
S
1
(
k
S
0
S
1
), (2) fluorescence S
1
!
S
0
(
k
fl
), (3) singlet-singlet anni-
hilation S
1
þ
S
1
!
S
0
þ
S
n
(
g
SS
), (4) intersystem crossing S
1
!
T
1
(
k
S
1
T
1
), (5)
triplet formation from higher singlet states S
n
!
T
n
(
k
tf
), (6) singlet-triplet annihi-
lation S
1
þ
T
1
!
S
0
þ
T
n
(
g
ST
), (7) reverse intersystem crossing T
1
!
S
0
(
k
T
1
S
0
),
(8) higher excited-state reverse intersystem crossing T
n
!
S
n
(
k
T
n
S
n
), and (9) internal
conversion from higher excited states (
k
IC
).
Besides the above-mentioned on-off blinking process, there is an additional rare
event leading to longer fluorescence intermittence than the triplet off-times. The cause
of this is probably the formation of a radical anion on one of the PMIs [82,84]. This
radical anion can quench the fluorescence of the other PMIs by singlet-radical anion
energy transfer. Figure 12.15b shows the spectral overlap of this process and an
