Communication
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three samples are weakly emissive, with fluorescence quantum
yields of 0.16 for p-PDI, <0.01 for MeO-PDI, and <0.01 for
MeS-PDI, which is in sharp contrast to the parent PDI.[11] We
also note that the Stokes shift between the absorption and
fluorescence peaks is largely reduced in MeO-PDI and MeS-PDI.
Time-resolved fluorescence studies clarify that fluorescence
time profiles of MeO-PDI and MeS-PDI can be well-fitted with
a biexponential function, indicative of the significant charge-
transfer character of the excited states.[12] The emission of
p-PDI decays in a simple monoexponential manner, resulting in
a lifetime of 0.96Æ0.01 ns (Table 1 and Figure S1, Supporting
Information). The short-lived components of MeO-PDI and
MeS-PDI cannot be well resolved due to the limitation of our
instrumental response function (IRF), which is in agreement
with the observed FF values. The enormous fluorescence
quenching as well as the accelerated emission decay suggests
the efficient nonradiative decay channel is active in the studied
model system.
61.4 ms in deaerated solution. Apparently, the Ar-PDI* is effi-
ciently quenched by the dissolved molecular oxygen due to
3
energy transfer from Ar-PDIs’ T1 to ground-state O2. Consider-
ing the highly electron-deficient structure of Ar-PDIs, the possi-
bility of the electron-transfer reaction from 3Ar-PDI to 3O2 is
ruled out and the quantum efficiency of energy transfer FEnT=
(1ÀtEnT/tT) is estimated to be 98, 99, and 99% for p-PDI, MeO-
PDI, and MeS-PDI, respectively.
We then use bimolecular triplet sensitization techniques to
obtain triplet absorption cross-sections and to determine trip-
let quantum yields of the studied systems (for details, see Ex-
perimental Section in the Supporting Information). These trip-
lets are optically probed to generate the triplet-induced ab-
sorption spectrum (Figure 4a) with long-lasting triplet lifetime
and high extinction coefficient (18700, 16800, and
13400mÀ1 cmÀ1 for p-PDI, MeO-PDI, and MeS-PDI, respectively).
The triplet sensitization experimental results agree well with
the proposed T1-induced absorption from femtosecond transi-
ent absorption spectroscopy, which confirms the triplet being
formed on an ultrafast timescale following optical excitation of
the studied Ar-PDI compounds. Note that all Ar-PDIs have
a similar T1 spectral pattern, although the overlapping ground-
state bleaching makes different contributions to net transient
absorption. It is straightforward to independently determine
that the triplet yield for p-PDI is 8% in use of the single triplet
absorption cross-section (for details see the Supporting Infor-
mation). The values for MeO-PDI and MeS-PDI are 54 and 86%,
respectively.
To unravel the origin of the unusual emission behavior and
investigate the underlying photophysical processes of Ar-PDIs,
femtosecond transient absorption spectra were recorded in
CH2Cl2 after selective photoexcitation of Ar-PDI. Upon 480 nm
pulsed photoexcitation of MeO-PDI, ground-state bleaching at
~530 and 600–730 nm positive absorption features emerge
within IRF and are attributed to the generation of lowest sin-
glet-excited PDI13. The 1PDI* decays monoexponentially with
a lifetime of 28Æ3 ps, in line with the time-resolved fluores-
cence data. Meanwhile, an additional absorption band peaked
at 555 nm increases in concert with the decay of the 720 nm
feature, with a time constant of 27Æ5 ps. The 555 nm absorp-
tion persists across the entire time window of the transient ab-
sorption measurement. We assign this long-lived feature to the
triplet excited state of PDI[10] (vide infra, see also in triplet sen-
sitization measurements). Similar spectral dynamics are also
shown in MeS-PDI. The transient absorption at 555 nm rises up
with a time constant that corresponds to the decay at 720 nm.
Global fitting of 720 and 555 nm traces (squares in Figure 3)
show one decay component of 84Æ5 ps and one rise compo-
nent of 91Æ8 ps. A pseudoisosbestic point at ~590 nm is ob-
served (Figure 3c), which is direct evidence for a singlet-to-trip-
let intersystem crossing process. As for p-PDI, a triplet-oriented
spectral pattern is also fairly observed (Figure 3a).
The above triplet generation upon direct photoexcitation
and kinetics analysis prove clearly a highly efficient ISC process.
We propose an energetic diagram for related photophysical
processes. Theoretical computational studies were performed
by using DFT methods at the B3LYP/6-31G(d) level. It can be
found that the appended aromatic groups of Ar-PDIs exhibit
ring twists relative to the planar PDI skeleton. In the optimized
structure, the LUMOs of the PDIs are all localized on the poly-
aromatic core. However, the HOMOs exhibit distinct characters,
in which the electronic density for p-PDI is located at the PDI
moiety, whereas those for MeO-PDI and MeS-PDI are rather
spread over the aryl groups. Strong charge-transfer interac-
tions are introduced in the system. It has been reported that
spatial separation of the HOMO and LUMO will result in the
decreased singlet triplet energy offset (DEST).[14] The calculated
excitation energy difference of the low-lying excited singlet
(Sn) and triplet states (Tn) for Ar-PDIs are shown to largely
reduce when nꢀ2. This would enhance the spin-orbital cou-
pled intersystem crossing from singlet to triplet manifold, as is
evidenced by the measured triplet yield sequence of MeS-
PDI>MeO-PDI>p-PDI. Moreover, the replacement of the
oxygen atom with a sulfur atom may also lead to an additional
increase in the spin-orbital interactions. Furthermore, the
Low-temperature phosphorescence spectroscopy and nano-
second flashlamp photolysis were conducted to estimate the
energy level of the lowest triplet excited state and to obtain
the related characters of the triplet state. In N2-saturated solu-
tion, phosphorescence bands can be observed with 897 nm
for p-PDI, 927 nm for MeO-PDI, and 938 nm for MeS-PDI, re-
spectively. Figure 4a reveals the time profile of p-PDI in a nitro-
gen-saturated CH2Cl2 solution on the microsecond timescale.
The Tn–T1 absorption at 555 nm decays single-exponentially
with a time constant of 30.6 ms. A much faster lifetime of
0.69 ms is obtained for p-PDI in an air-saturated solution. For
MeO-PDI, the Tn-T1 transient absorption was also monitored
and resulted in tEnT =0.52 ms for the air-saturated solution and
t=59.7 ms for the nitrogen-saturated solution (Figure 4c). Simi-
larly, the tEnT of Ms-PDI is 0.71 ms in aerated solution and
1
higher energy of the Ar-PDI’s T1 state (~1.3 eV) than that of O2
as well as its large electron affinity makes the energy-transfer
reaction between them highly efficient. Comparison between
p-PDI, MeO-PDI, and MeS-PDI suggests that the electron-do-
nating ability of substitutions on the headland positions of
PDIs influence the photophysics significantly.
Chem. Eur. J. 2016, 22, 4717 – 4722
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Yu, Zhenyi
