Design and synthesis of a new class of photochromic diarylethenecontaining dithieno[3,2-b:2',3'-d]pyrroles and their switchable luminescence properties

Article abstract of DOI:10.1002/chem.200900581

A series of dithienylethene-containing dithieno[3,2-b:2',3'-d]pyrroles were synthesized and their photophysical study was done. Tetrabromination of dithieno[3,2-b:2',3'-d]pyrrole followed by subsequent Suzuki cross-coupling reaction in the presence of an

Full text of DOI:10.1002/chem.200900581

COMMUNICATION
DOI: 10.1002/chem.200900581
Design and Synthesis of a New Class of Photochromic Diarylethene-
Containing Dithieno[3,2-b:2’,3’-d]pyrroles and Their Switchable
Luminescence Properties
Hok-Lai Wong,[a] Chi-Chiu Ko,[a, b] Wai Han Lam,[a] Nianyong Zhu,[a] and
Vivian Wing-Wah Yam*[a]
Conjugated acenes and heteroacenes have received enor-
mous attention in the past decade or so owing to their prob-
able applications in the fabrication of high-performance
semiconductor devices.[1] Amongst them, dithieno[3,2-b:2’,3’-
d]pyrroles represent one class of interesting compounds that
can function as efficient fluorescent materials,[2] low-band-
gap field-effect transistors, and organic semiconductors.[3] In
addition, the recent interest in the development of nondop-
ed organic light-emitting diodes (OLEDs), which often re-
quires bipolar design to facilitate exciton formation,[4a–e] has
led to the exploration of common electron donors such as
triarylamine and carbazole.[4f–m] Being an analogue of carba-
zole, dithienopyrrole derivatives have also been explored for
use as a new class of bipolar materials in high efficiency
OLEDs.[4n]
tended our interest towards the functionalization of dithie-
no[3,2-b:2’,3’-d]pyrroles owing to their desirable semicon-
ducting properties and their ease of functionalization at the
pyrrole ring. Introduction of an optically addressable switch-
ing unit into dithieno[3,2-b:2’,3’-d]pyrrole system may give
rise to new classes of interesting materials with photoswitch-
able luminescence and electronic properties. Herein are de-
scribed the synthesis and the photophysical study of a new
class of photochromic dithieno[3,2-b:2’,3’-d]pyrrole deriva-
tives 1–5.
On the other hand, photochromic diarylethenes also pos-
sess excellent and promising switching properties,[5] with
high fatigue resistance, high stability, and fast response time.
Our group has recently directed our interests in functionaliz-
ing materials with diarylethenes and studying their photo-
chromic behavior.[6] With our recent experience in working
on photochromic fused-thiophene systems,[6f] we have ex-
Of the different methods employed in synthesizing the di-
thieno[3,2-b:2’,3’-d]pyrrole core,[7] the synthetic methodolo-
gy developed by Rasmussen and co-workers[7a,e] was adopt-
ed. Tetrabromination of dithieno[3,2-b:2’,3’-d]pyrrole fol-
lowed by subsequent Suzuki cross-coupling reaction in the
presence of an excess of 2,5-dimethylthien-3-yl boronic acid,
[a] H.-L. Wong, Dr. C.-C. Ko, Dr. W. H. Lam, Dr. N. Zhu,
Prof. Dr. V. W.-W. Yam
Centre for Carbon-Rich Molecular and
Nano-Scale Metal-Based Materials Research
Department of Chemistry and
HKU-CAS Joint Laboratory of New Materials
The University of Hong Kong, Pokfulam Road
Hong Kong (P.R. China)
aqueous Na2CO3 and a catalytic amount of [PdACTHUNGETRNNU(G PPh3)4] in
THF under reflux conditions (Scheme 1) afforded the de-
sired compounds in moderate yield.
Fax : (+852)2857-1586
E-mail: wwyam@hku.hk
[b] Dr. C.-C. Ko
Present address: Department of Biology and Chemistry
City University of Hong Kong
Tat Chee Avenue, Kowloon, Hong Kong (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900581.
Scheme 1. Synthetic route for 1–5.
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Compounds 1–5 have been characterized by 1H NMR
spectroscopy, EI-MS, and give satisfactory elemental analy-
ses, while the structures of compound 2 and 3 have been de-
termined by X-ray crystallography. A representative per-
spective drawing of 2 is depicted in Figure 1. The N-aryl
370 nm and a broad absorption at about 558 nm in the UV/
Vis spectra (Figure 2), typical of the absorptions of the
closed form.[5,6] Two well-defined isosbestic points at about
Figure 1. Perspective drawing of 2. Hydrogen atoms have been omitted
for clarity and thermal ellipsoids are shown at the 30% probability level.
plane is twisted from the dithienopyrrole core by about 748
for 2 and 668 for 3. The diarylethene groups in 2 are both in
an anti-parallel configuration, while only one of them is
found to be in an anti-parallel configuration in 3, as shown
in Figure S1 in the Supporting Information.
Figure 2. UV/Vis absorption spectral changes of 5 (1.92ꢁ10ꢁ5 m) upon ir-
radiation at 360 nm (top) and its fluorescence spectral changes (lexc
=
352 nm) in degassed benzene (bottom).
Compounds 1–5 dissolve in benzene and they showed sim-
ilar UV/Vis absorption spectra with an intense band at
about 350 nm. An additional shoulder at approximately
325 nm was observed for 1–4. The intense band at 350 nm is
ascribed to the p!p* transitions of the dithienopyrrole core
mixed with the dimethylthiophene moieties, while the
shoulder at 325 nm that could only be found in 1–4 may be
attributed to the p!p* transition resulting from the pres-
ence of the N-aryl rings. The relative insensitivity of the ab-
sorption energies of the 350 nm band to the nature of the N-
substituent is suggestive of the non-coplanarity of the N-aryl
ring to the plane of the dithienopyrrole core, as similarly ob-
served in the X-ray crystal structures of 2 and 3. Upon exci-
tation at the transition band at 360 nm in degassed benzene,
the solutions of compound 1–5 showed photochromic behav-
ior, turning from colorless to reddish purple, with the
growth of an intense absorption band at approximately
290 and 352 nm were observed, suggesting the photocycliza-
tion of only a pair of diarylethene units, probably due to the
effective energy transfer from the dithienylethene moiety to
the 8a,8b-dimethyl-1,8-dithia-as-indacene moiety (closed
form) after the first cyclization,[6f,8] as illustrated in
Scheme 2. Further prolongation of irradiation did not pro-
duce the double photocyclized product. Upon excitation at
the absorption band of the closed form at 470–600 nm, the
UV/Vis absorption spectral changes were reversed, resulting
from the regeneration of the open form. The cyclo-reversi-
bility has been demonstrated in 4 with at least three repeat-
ing cycles (Figure S2 in the Supporting Information).
On excitation into the p!p* absorption band (lꢀ
400 nm) of 1–5, intense blue fluorescence with lifetimes of
less than one nanosecond was observed. The photophysical
Scheme 2. Photochromic reactions for 1–5.
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Luminescence Switching
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Table 1. Photophysical data of 1–5 in benzene at 298 K.
(2b) and tetrabrominated di-
thienopyrroles (2c) as well as
the relative insensitivity of
these oxidation potentials to
the change of the N-substituent,
the irreversible oxidation waves
at higher oxidation potentials
are likely attributed to the oxi-
dations of the peripheral dime-
thylthiophene units.
To gain a deeper insight into
the nature of the electronic ab-
sorption and emission proper-
ties of this class of compounds,
Absorption
Emission
lem [nm] (Flum)
labs [nm] (e [dm3 molꢁ1 cmꢁ1])
1
2
3
4
5
open
closed
open
closed
open
closed
open
closed
open
closed
326 sh (18360), 348 (23300)
370 sh (31830), 380 (32860), 532 sh (7330), 558 (8070)
325 sh (20240), 349 (26720)
368 sh (33700), 380 (35010), 532 sh (7210), 558 (7930)
326 sh (19940), 349 (25250)
370 sh (33870), 380 (34870), 530 sh (7510), 558 (8420)
324 sh (15230), 349 (19840)
368 sh (31730), 382 (37120), 528 sh (7930), 558 (8940)
350 (35590)
298 (8160), 372 (49450), 528 sh (8310), 556 (9300)
417 (0.14)
–
410 (0.16)
–
418 (0.13)
–
422 (0.14)
–
417 (0.18)
–
data are summarized in Table 1. Photocyclization of all the
compounds resulted in a reduced fluorescence intensity
(Figure 2). This is probably due to the existence of lower
lying excited states of the closed form, which would quench
the emission and inactivate the second photocyclization
step. The photochromic quantum yields have also been mea-
sured (Table 2). The photocyclization quantum yield[9] is
found to be much higher than that for photocycloreversion,
which is typical of the diarylethene systems.[5,6] Half-lives of
the closed form of compound 4 have also been investigated
at 298 and 328 K (Table S4 in the Supporting Information).
All of the compounds exhibit a quasi-reversible oxidation
couple and two irreversible oxidative waves at higher poten-
tials (Table 3). No reduction wave was observed upon scan-
ning up to ꢁ2.5 V. The first oxidation couple is probably
due to the formation of the cation radical in the dithienopyr-
role core. This assignment is further supported by the sensi-
tivity of the oxidation potential to the change of N-substitu-
ents on the dithienopyrrole. In light of the absence of the
second and the third oxidation wave in the unsubstituted
calculations using DFT, TDDFT, HF, and CIS methods were
performed for the open and closed forms of 1–5.[10]
The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) for the PBE0
optimized structure of the open forms of 1–5 are mainly
contributed from the respective p and p* orbitals of the di-
thienopyrrole core, mixed slightly with the p and p* orbitals
of the peripheral thiophene rings. However, the HOMO and
LUMO for the PBE0 optimized structure of closed forms of
1–5 are the p and p* orbitals localized on the condensed
thiophene moiety, respectively. Figure 3 shows the spatial
plots of selected frontier molecular orbitals of the two forms
of 1. A HOMO–LUMO energy gap of the open forms is in
the range of 4.20–4.24 eV (Table S6 in the Supporting Infor-
mation), while a significant decrease in energy gap is found
in the closed forms (2.63–2.71 eV) as a more extended p
conjugation in the condensed thiophene moiety leads to the
increase in energy of p and decrease in energy of p* orbi-
tals.
Table 2. Photochemical quantum yields of 1–5 in degassed benzene solu-
tion at 293 K.
Photocyclization, F350
Photocycloreversion, F500
1
2
3
4
5
0.37
0.38
0.37
0.35
0.38
0.11
0.10
0.08
0.11
0.09
Table 3. Electrochemical data of 1–5 in THF.[a]
E1/2 [V]
DEp [mV]
Epa [V]
1
2
3
4
5
2b
2c
+0.78
+0.77
+0.75
+0.80
+0.74
–
89
95
91
101
95
–
+1.03, +1.42
+1.03, +1.41
+1.03 +1.50
+1.03, +1.44
+1.03, +1.41
+1.10
–
–
+1.27
[a] All potentials are versus SCE. Data were recorded under argon with
0.10m nBu4NPF6 as the supporting electrolyte. E1/2 is (Epa +Epc)/2; Epa
and Epc are peak anodic and peak cathodic potentials, respectively. Ep is
the peak-to-peak separation.
Figure 3. Spatial plots (isovalue=0.04) of selected TDDFT/CPCM fron-
tier molecular orbitals of the open form (a) and closed form (b) in 1.
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V. Wing-Wah Yam et al.
Selected singlet–singlet transitions of the open and closed
forms in 1–5 are listed in Table S7 and Table S8, respective-
ly, in the Supporting Information. The spectral assignment is
based on the correlation of the experimental band maxima
with the calculated excitation wavelengths of the transitions
with significant oscillator strengths. The absorption bands of
the open form are computed at 343–347 nm for 1–5. This
transition mainly corresponds to an excitation from HOMO
to LUMO, which can be assigned as the p!p* transition of
the dithienopyrrole core, slightly mixed with the p!p* tran-
sition of the peripheral thiophene rings. These calculated
wavelengths are in excellent agreement with the lmax in the
major electronic absorption bands of the open forms.
For the closed forms in 1–5, the low-energy absorption
band corresponds to the transition to the first singlet excited
state (S1) computed at 578–595 nm, which is mainly consist-
ed of an excitation from HOMO to LUMO, and can be as-
signed as the p!p* transition of the condensed thiophene
moiety. The most intense transition with the calculated exci-
tation wavelengths (377–389 nm) compares well with the
lmax in the high-energy absorption band observed in the
electronic absorption spectra. For compounds 1–3 and 5, the
transition is mainly composed of a combination of two exci-
tations HOMOꢁ1!LUMO and HOMO!LUMO+1. As
shown in Figure 3, the HOMOꢁ1 and LUMO+1 are p and
p* orbitals concentrated on the dithienopyrrole core and
the peripheral thiophenes, respectively. In view of the topol-
ogies of the MOs involved in the transition, the high-energy
absorption band can be assigned as the p!p* transition
with charge transfer character. In addition to the
HOMOꢁ1!LUMO excitation, the most intense transition
at 381 nm for 4 is composed of HOMO!LUMO+2 and
HOMO!LUMO+3 excitations, in which the LUMO+2
and LUMO+3 can be considered as the p* orbitals of the
dithienopyrrole core and the peripheral thiophenes, but with
a significant contribution from the N-aryl ring.
Experimental Section
Details of the syntheses and characterization of compounds 1–5 can be
found in the Supporting Information. CCDC-722353 (2) and 722352 (3)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
V.W.-W.Y. acknowledges support from The University of Hong Kong
under the Distinguished Research Achievement Award Scheme, the Uni-
versity Development Fund, the Faculty Development Fund, and the
URC Strategic Research Theme on Molecular Materials. This work has
been supported by a GRF Grant (Project No. HKU7057/07P) and a
CAV Grant (Project No. HKU2/05C) from the Research Grants Council
of Hong Kong Special Administrative Region (P.R. China). H.-L.W. ac-
knowledges the receipt of a postgraduate studentship and W.H.L. the re-
ceipt of a University Postdoctoral Fellowship, both from The University
of Hong Kong. We also thank the Computer Center at The University of
Hong Kong for providing the computational resources.
Keywords: density functional calculations · luminescence ·
materials · nitrogen heterocycles · photochromism · sulfur
heterocycles
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The first singlet excited state of the open forms in 1–5
was optimized using the CIS method. On the basis of these
S1 optimized geometries, the lowest energy transition is
computed at 393–394 nm using the TDDFT/CPCM method
at the PBE0 level and consisted of p!p* transition of the
dithienopyrrole core (Table S9 in the Supporting Informa-
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relative to the corresponding ground states occur mainly in
the dithienopyrrole core, in which the C=C bonds are
ꢁ
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(Table S10 in the Supporting Information). The feature is
understandable, as the excited state involves population of
the LUMO which contains quinoid character.
In conclusion, a series of dithienylethene-containing di-
thieno[3,2-b:2’,3’-d]pyrroles has been synthesized. Photo-
physical, photochromic, electrochemical, and computational
studies have been carried out. The crystal structures of 2
and 3 have been determined. Further investigations on their
possible applications are in progress.
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Luminescence Switching
COMMUNICATION
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Received: March 4, 2009
Published online: August 19, 2009
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