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This experiment revealed the spectral signature corresponding
to TNTCꢀ at (lmax =490 nm). The transient spectroscopy experi-
tration of TNT from t =290–340 ps (47 mm) to t =420–450 ps
(260 mm). Figure 8G–I indicate the spectral signatures with an
4
4
ments using TNT alone (Figure 8B) reveal an excited state ab-
sorption centered at lmax =470 nm. This is found to be in
a good agreement with the previously reported literature on
increasing resemblance to the spectroelectrochemistry of
ꢀ
TNTC . The values of t , however, are significantly different
4
than that observed in the pure TNT sample. This is because t4
is likely associated with the timescale of the electron transfer
from the Ir complex 3MLCT state to form the TNTCꢀ/
[34]
nitroaromatics (Figure S33 in the Supporting Information).
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This feature decays with a time constant of 1.2–1.4 ps and
IV
2+
comprises internal conversion to the S state and ISC to the p–
[Ir (ppy) (dmbpy)] charge-separated pair. This key evidence
1
2
p* T state. The T state has a lifetime of 294–339 ps. An addi-
strongly suggests the excited state electron transfer between
2
1
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tional decay feature with a lifetime 28–80 ps is observed but
its assignment is ambiguous and may be associated with intra-
molecular vibrational redistribution (IVR) followed by geometri-
cal relaxation within the triplet state (Figure S34 in the Sup-
the Ir complex and the TNT acceptor. The last time constant is
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associated with the Ir complex and is consistent with the life-
time titrations reported in Figure S24. A Stern–Volmer plot (Fig-
ure S39 in the Supporting Information) demonstrates that the
quenching constant calculated from the ultrafast experiments
closely match the one obtained from TCSPC fluorescence tech-
niques (ꢁ10%).
[
34]
porting
Information).
Further,
experiments
using
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[
Ir (ppy) (dmbpy)]BF alone in air-equilibrated acetonitrile (Fig-
2
4
ure 8C) reveal the evolution of several spectral features emerg-
ing from different processes (see Figures S31 and S32 in the
Supporting Information) in agreement with previously pub-
The thermodynamic confirmation of the results obtained
from the static and dynamic quenching experiments and ultra-
fast transient absorption is provided by cyclic voltammetry
(CV) (Figures S25–S28 and Table S5 in the Supporting Informa-
tion). CV data revealed the reduction and oxidation potentials
used for the calculation of HOMO (from the first oxidation po-
tential) and LUMO energies (from the first reduction potential)
(Table S6, and Figures S29 and S30 in the Supporting Informa-
tion) measured with respect to the vacuum level, Figure 9.
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[35]
lished data on Ir cyclometalated complexes. Ultrafast transi-
ent absorption experiments were then performed on solutions
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of [Ir (ppy) (dmbpy)]BF containing increasing amounts of TNT
2
4
(47, 90, and 260 mm). Different concentrations of TNT were
chosen to ascertain the influence of the amount of quencher
on the evolution of the relevant spectral signatures. At a first
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glance, all the features recorded for the Ir complex in pres-
ence of TNT seem to be matched with the features observed
in the control experiments (Figure S36 in the Supporting Infor-
mation). However, the maximized signal for DA occurs earlier
in presence of the quencher (10, 3, 1 ps for [TNT]=47, 90, and
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2
60 mm, respectively), while that for the Ir complex achieves
a maximum signal in approximately 100 ps. Normalization of
the ultrafast data illustrated that differences in the spectral sig-
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natures of the Ir /TNT mixtures and the control experiments
are more evident in the 100–1500 and 1500–6000 ps time
ranges (Figure 8D–I). Together with the changes in the
growth/decay time constants moving to higher [TNT], the ad-
ditional spectral signatures increasingly resemble the spectro-
electrochemical features of one-electron reduced TNT, that is,
ꢀ
TNTC . The first time constant comprises both growth and
decay signatures, however, it is considerably fast with respect
to the time resolution of the experiment and only an approxi-
mate treatment could be made here. The t decreases as the
1
concentration of TNT increases; a plausible explanation is
linked to the superposition of the prompt growth and decay
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features of both the Ir complex and TNT. The effects of TNT
concentration starts to be more evident on the longer time-
frames of the ultrafast experiments (see Figure S38 in the Sup-
porting Information). The second time constant is related
Figure 9. HOMO–LUMO diagram showing the energy levels determined ex-
perimentally for the organometallic complexes and TNT and calculated
[
36]
values for TNT, 2,4-DNT, DMDNB.
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solely to the IVR and vibrational cooling of the Ir complex
3
within the MLCT state and to the S !T ISC for TNT. The third
From the bar graph in Figure 9 it is possible to see why
DMDNB has no effect on the luminescence of the metal com-
plexes utilized; its LUMO is positioned at a too high energy to
act as an electron acceptor from the MLCT excited states. On
the other hand, both 2,4-DNT and TNT have LUMO energies fa-
vorable to engage in an excited state electron transfer process.
However, the LUMO of 2,4-DNT is located at an energy compa-
rable to that of the LUMO of the complexes, which provides
1
2
time constant, t , is related to the spectral signatures of TNT
3
itself. Because this time constant is not present in the lower
concentration mixture, it appears to be associated with the
TNT excited states produced in the pump laser pulse and re-
sults from non-selective light excitation. The fourth time con-
stant is not present in the control experiments performed with
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the Ir complex and its magnitude increases with the concen-
Chem. Eur. J. 2015, 21, 4056 – 4064
4062
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