Sulfur-substituted perylene diimides: Efficient tuning of LUMO levels and visible-light absorption: Via sulfur redox

Article abstract of DOI:10.1039/c9cc07040k

A series of sulfide and sulfone substituted perylene diimides (PDIs) with different LUMO levels covering a range of 0.72 eV were synthesized through simple sulfur redox chemistry. The LUMO level of phenylsulfone substituted PDI reached a record-breaking-4

Full text of DOI:10.1039/c9cc07040k

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Sulfur-substituted perylene diimides: efficient
tuning of LUMO levels and visible-light absorption
via sulfur redox†
Cite this: Chem. Commun., 2019,
55, 13570
Received 10th September 2019,
Accepted 11th October 2019
b
Yongxin Zhou,a Bo Xue,a Chenyu Wu,
Siqi Chen,a Hui Liu,a Tonggang Jiu,c
a
a
Zhibo Li
and Yingjie Zhao
*
DOI: 10.1039/c9cc07040k
rsc.li/chemcomm
A series of sulfide and sulfone substituted perylene diimides (PDIs) aromatic planes can be influenced. As a result, when the
with different LUMO levels covering a range of 0.72 eV were synthe- reactions undergo an anionic transition state, the anionic
sized through simple sulfur redox chemistry. The LUMO level of intermediates can be stabilized by the anion–p interactions
phenylsulfone substituted PDI reached a record-breaking À4.75 eV. between the electron-deficient surface and the intermediates to
In addition, the maximum emission ranged from 540 to 692 nm.
finally achieve the catalysis purpose13,25 Some applications
have been realized including anion binding, transport across
Perylene diimides (PDIs) have been recognized as promising lipid bilayer membranes, and catalysis of reactions with anionic
electron acceptor materials for n-type semiconductors owing to transition states.10–13 Although PDIs have promising charac-
their low-lying lowest unoccupied molecular orbital (LUMO) teristics, a detailed investigation into the control of the energy
levels and highly reversible redox waves in cyclic voltammetry levels has been less reported.10,26 Therefore, we are particularly
(CV).1–9 Recently, the electron deficient p surfaces of PDIs have interested in making p-expanded PDIs featuring low-lying LUMO
been drawing attention because they can attract anions or levels. PDI molecules can be easily functionalized at the core by
anionic intermediates which make them suitable candidates introducing functional groups to the ‘‘bay’’ positions to tune
for sensing, anion binding, transport, and anion–p catalysis.10–13 the reduction potential and the LUMO levels. Some efforts have
In addition, PDIs exhibit other interesting properties—such as been devoted to functionalize the p-surface of the PDIs to lower
high fluorescence quantum yields, high photochemical stability the LUMO energy level.10,27,28–CN, –NO2, and –F, which are all
and low cost, which allow PDIs to be used in many other classical electron-drawing groups, have been introduced into
newly developed applications.14–20 Among all these PDIs and the p-surface to improve the electron-transport and ambient
applications, the low-lying LUMO level was considered as a key stability.27–31 Herein, we studied sulfur-substituted perylene
point to result in excellent n-channel performances and make diimides at the bay positions of PDIs. Sulfur bridges were
them attractive to assemble into complex architectures. The particularly attractive for varying the LUMO energy level
low-lying LUMO level could reduce the energy barrier between through simple and robust sulfide oxidation chemistry.13,32,33
the Fermi level of the source and drain contacts of the semi- The electron-donating sulfide can be converted in situ into
conductor and therefore facilitate electron injection and strong electron-withdrawing sulfone acceptors and avoid reduc-
transportation.3,21–24 In addition, the optimized LUMO level tive radical formation. The LUMO levels of naphthalenedii-
could enhance the ambient stability of PDIs in the presence of mides (NDIs) have been well studied by Matile and co-workers
impurities such as water and oxygen. On the other hand, through the sulfide oxidation chemistry.10,13,32–35 Regular
related to these low LUMOs, the intrinsic positive quadrupole different p-acidity surface and LUMO levels of NDIs could be
moment of these PDIs which results in electron-deficient obtained for attracting the anionic intermediate. In this work,
LUMO levels of the PDIs ranging from À4.03 to À4.75 eV were
achieved. LUMO levels as low as À4.75 eV were realized
a Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education
(compound 8) among the obtained PDIs. These innovative PDIs
with low LUMO levels are of interest as electron transporters in
n-type semiconductor materials and also have some significance
for the study of anion–p interactions and anion–p catalysis.
The structures and synthesis of the sulfide and sulfone PDIs
(2–9) are shown in Fig. S1 (ESI†). The details of all the synthesis
procedures are listed in the ESI.† For the ultimate goal
Department, College of Polymer Science and Engineering, Qingdao University of
Science and Technology, Qingdao 266042, P. R. China. E-mail: yz@qust.edu.cn
b University of New South Wales, Sydney, New South Wales 2018, Australia
c Qingdao Institute of Bioenergy and Bioprocess Technology,
Chinese Academy of Sciences, Qingdao 266101, P. R. China
† Electronic supplementary information (ESI) available: Synthesis procedures,
1H-NMR spectra, 13C-NMR spectra, MS, absorption spectra, fluorescence spectra,
cyclic voltammetry measurements, etc. See DOI: 10.1039/c9cc07040k
13570 | Chem. Commun., 2019, 55, 13570--13573
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compound 9 from 6, even under extremely harsh conditions, no
target product was obtained. We did observe the existence of 9
in the reaction mixture by MALDI-TOF mass spectrometry.
However, the separation of 9 has so far not been successful.
The aromatic core of 9 is probably too electron deficient to
exist. All of the products show good solubility in common
organic solvents at room temperature.
The different substituted PDIs were characterized by absorp-
tion and fluorescence spectroscopy first (Fig. 1 and 2). The
unsubstituted PDI (1) was used as a reference compound here.
The incorporation of the substituents with different electron
donating and accepting effects on the bay region significantly
affects the absorption and fluorescence properties in solution.
Compared to 1 (labs = 523 nm, lem = 540 nm), the absorption
and fluorescence maximum of 2 with electron-donating alkyl-
sulfide substituent groups shifted to 560 nm and 692 nm.
The Stokes shift changed from 17 nm to 132 nm. However,
the fluorescence quantum yield decreased from 95% to 3.6%. The
Fig. 2 Absorption (a) and fluorescence (b) spectra of the different sub-
stituted PDIs in CH2Cl2, normalized to maxima; pictures of PDIs under
sunlight (c) and 365 nm UV light (d).
The LUMO energy levels of compounds 1–8 were determined
oxidation of the arylsulfoxide to arylsulfone reverted the electron- by CV in CH2Cl2 against the ferrocenium/ferrocene (Fc+/Fc)
donating nature into strong electron-withdrawing acceptors. couple as the internal standard (Fig. S19, ESI†). All the com-
The absorption and fluorescence maximum of 7 blue shifted pounds exhibit two distinct, reversible reduction steps.
back to labs = 527 nm and lem = 565 nm. The fluorescence
The energy level of the LUMO was calculated by assuming
quantum yield increased to 98%. For the phenyl series, similar À5.1 eV for Fc+/Fc. The LUMO levels are summarized in Fig. 1.
tendency was observed. The absorption and fluorescence The LUMO level of the alkylsulfide substituted 2 is À4.06 eV
maximum of 3 are labs = 552 nm and lem = 678 nm which are which is similar to that of the unsubstituted 1 (À4.03 eV). This
slightly blue shifted compared to 2 which may be due to the indicated that the alkylsulfide substituent group had very little
conjugation effect of the aromatic benzene group. Oxidation of influence on the p-surface of PDI. However, the phenylsulfide
3 gave 8 with labs = 542 nm and lem = 582 nm. The fluorescence substituent group shows obvious electron-withdrawing charac-
quantum yield changed from 0.9% for 3 to 95% for 8. Interest- ter which lowers down the LUMO level to À4.15 eV (3). The
ingly, for pentafluorophenylsulfide substituted 6, no obvious LUMO level of 6 with two electron-withdrawing pentafluoro-
red shift was observed compared to 1. The absorption phenylsulfide groups was found to be À4.37 eV. Full oxidation
maximum is labs = 531 nm which is only 8 nm red shift. of the four sulfide groups in 2, 3 led to sulfone groups and
However, the fluorescence maximum of 6 red shifted to lem
=
lowered the LUMO levels by 0.65 eV and 0.60 eV to À4.71 eV for
637 nm (97 nm red shift compared to 1). In addition, the 7 and À4.75 eV for 8 respectively. Partial oxidation of the sulfide
fluorescence quantum yield remains at 34%. This result is will lead to sulfoxide groups which is also an electron with-
interesting because this may provide a way to construct near- drawing group. The electron withdrawing ability of the sulfoxide
infrared fluorescent dye with high fluorescence quantum yield is between the sulfide and sulfone. We did observe the sulfoxide
by the dual effect of the pentafluorophenyl and the sulfide.
intermediates by MALDI-TOF mass spectrometry and Thin-Layer
Fig. 1 Summary of electrochemical and photophysical properties of compounds 1–9. [a] LUMO energies were obtained by CV and are reported in
electron volts relative to À5.1 eV for Fc+/Fc; [b] the LUMO energy level of 9 was calculated at the B3LYP/6-31+G** level of theory; [c] the emission
spectra were excited at absmax; [d] the fluorescence quantum yield of the PDIs was determined by using 1 as the standard. All the measurements were
done in CH2Cl2 at room temperature (1 Â 10À5 M).
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Chromatography. However, no separation and purification were energy levels by 0.1–0.5 eV compared with the experimentally
done because the chirality of the sulfoxide bridges makes the measured values (Fig. 1), the general tendency is mainly the
substituted PDIs complicated on the one hand and the purpose of same, where more electron-withdrawing substituents tend
this work is pursuing extremely low LUMO level of the PDIs on the to lower the LUMO energy level. More electron-withdrawing
other hand. The LUMO value of À4.75 eV obtained for 8 is already substituents are known to yield lower LUMO energy levels given
remarkable. To the best of our knowledge, this is the lowest the same chromophore.36,37 This trend generally complies with
LUMO level for the reported PDI derivatives. The unsuccessful our experimental observation for 1–9 that more electron-
synthesis and separation of 9 is possibly attributed to the too low withdrawing substituents indeed lead to lower LUMO energy
LUMO which makes it unstable in the air. The calculated LUMO levels, despite some slight deviation within experimental
of 9 by DFT at the B3LYP/6-31G** level is À4.80 eV which is errors. In particular, 9 was calculated to exhibit a LUMO energy
reasonable and consistent with the changing tendency. The level of À4.80 eV, lower than any of the dyes 1–8, despite the
pentafluorophenyl together with the sulfone led to an extremely overestimation by DFT computation. Indeed, the highly electro-
low LUMO level of 9. Besides, we also measured the LUMO level withdrawing pentafluorophenyl groups and sulfoxide groups
of the well-known compounds 4 and 5 which shows a value of add up to the most electro-withdrawing substituents among the
À4.24 eV and À4.28 eV as control experiments. These values are substituents of all the dyes. As TD-DFT calculations reveal that
in accordance with the previously reported results.
all of the dyes 1–9 have a S1 excitation with 98% contribution
To elucidate the structure–property relationships behind from HOMO - LUMO transitions, it is rational to use HOMO
these observed property changes by structure variation of the and LUMO energy levels to discuss their labs. Indeed, a narrower
PDI derivatives, we subsequently performed density functional HOMO–LUMO energy gap should lead to more red-shifted
theory (DFT) calculations at the B3LYP/6-31+G** level of absorption/fluorescence by requiring less excitation energy.38
theory using the Gaussian 09 D01 software package to obtain Particular attention was drawn to 2 and 3 which exhibit
geometry-optimized structures and frontier molecular orbital significantly red-shifted visible-light absorption compared to
information of dyes 1–9 (Fig. 3). All the alkyl chains were other dyes, while the absorption peak of 6 also displays some
simplified to the methyl substituent to reduce the computation red shift (Fig. 2a). By examining the visualized frontier orbitals
time as a common practice. Time-dependent (TD) DFT calcula- of 2, 3 and 6, we clearly observed a significant portion of HOMO
tions were performed to reveal the contribution of the transi- localized on the S atoms, each of which has two pairs of lone
tion between the highest occupied molecular orbital (HOMO) electrons. Because there is no LUMO on the S atom for each of
and the LUMO to the lowest singlet excited state (S1) excitation the three dyes, apparently the substituents attached to S have a
(Fig. 3, below the dye labels). Although DFT calculations for significant effect on the HOMOs, but should have much less
these PDI derivatives tend to overestimate the exact LUMO impact on the LUMOs. Indeed, the highly electro-donating
Fig. 3 Molecular geometries (top) and visualized frontier orbitals (isovalue = 0.03, bottom) of PDI derivatives 1–9, calculated at the B3LYP/6-31+G**
level of theory. HOMO and LUMO energy levels and their gaps were denoted in eV. Contribution of HOMO - LUMO transition to S1 excitation was
denoted below the dye label.
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Conflicts of interest
There are no conflicts to declare.
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