Light-controlled reversible modulation of frontier molecular orbital energy levels in trifluoromethylated diarylethenes

Article abstract of DOI:10.1002/chem.201605511

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their pelectronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/

Full text of DOI:10.1002/chem.201605511

A Journal of
Accepted Article
Title: Light-Controlled Reversible Modulation of Frontier Molecular
Orbital Energy Levels in Trifluoromethylated Diarylethenes
Authors: Martin Herder, Fabian Eisenreich, Aurelio Bonasera, Anna
Grafl, Lutz Grubert, Michael Pätzel, Jutta Schwarz, and
Stefan Hecht
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To be cited as: Chem. Eur. J. 10.1002/chem.201605511
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Light-Controlled Reversible Modulation of Frontier Molecular
Orbital Energy Levels in Trifluoromethylated Diarylethenes
Martin Herder, Fabian Eisenreich, Aurelio Bonasera, Anna Grafl, Lutz Grubert, Michael Pätzel, Jutta
Schwarz, and Stefan Hecht*[a]
Abstract: Among bistable photochromic molecules diarylethenes
(DAEs) possess the distinct feature that upon photoisomerization
they undergo a large modulation of their π-electronic system,
accompanied by a marked shift of the HOMO/LUMO energies and
hence oxidation/reduction potentials. The electronic modulation can
be utilized to remote-control charge as well as energy transfer
processes and it can be transduced to functional entities adjacent to
the DAE core, thereby regulating their properties. In order to exploit
such photoswitchable systems it is important to precisely adjust the
absolute position of their HOMO and LUMO levels and to maximize
the extent of the photoinduced shifts of these energy levels. Here,
we present a comprehensive study detailing how variation of the
substitution pattern of DAE compounds, in particular using strongly
electron-accepting and chemically stable trifluoromethyl groups
either in the periphery or at the reactive carbon atoms, allows for the
precise tuning of frontier molecular orbital levels over a broad energy
range and the generation of photoinduced shifts of more than 1 eV.
Furthermore, the effect of different DAE architectures on the
transduction of these shifts to an adjacent functional group is
discussed. Whereas substitution in the periphery of the DAE motif
as molecular geometry, dipole moment, and electronic structure,
and hence many other functions can be switched on and off by
light. Consequently the interest in applying photoswitchable
molecules has recently grown in diverse fields ranging from
pharmacology[2] over molecular machines[3] to organic
electronics.[4]
For the realization of such “smart” systems diarylethenes
(DAEs)[5] are
a
highly attractive class of photochromic
compounds with a number of properties that render them
superior to others: in general, the photochemical interconversion
between their open and closed form is very efficient, both
isomers are thermally stable, and they can be designed to
possess outstanding fatigue resistance. Looking into the
literature two principal strategies can be identified how the
photochromic reaction of DAEs can be translated into a remote-
controllable function:[6] Some examples in the areas of
photoactuation,[7] responsive supramolecular assemblies[8] or
binding to biological targets[9] rely on the modulation of
molecular geometry and flexibility of the DAE core upon
isomerization, though it is generally small compared to other
photoswitchable molecules, such as azobenzenes. The vast
majority of applications of DAEs, however, are based on the
marked electronic changes occurring upon isomerization with a
reorganization of their π-electronic structure accompanied by a
significant shift of the energies of their frontier molecular orbitals
(Figure 1a).
has
only
minor
implications
on
the
photochemistry,
trifluoromethylation at the reactive carbon atoms strongly disturbs
the isomerization efficiency. However, this can be overcome by
using a nonsymmetrical substitution pattern or by combination with
donor groups, rendering the resulting photoswitches attractive
candidates for the construction of remote-controlled functional
systems.
One way to utilize this feature in order to gain remote-control
over a specific function is to implement inter- or intramolecular
energy or charge transfer processes between the DAE and a
functional moiety, whose efficiency will be different in the two
isomeric forms. Exemplarily, the emission of fluorophores
attached to DAEs can be quenched by energy or electron
transfer typically to the ring-closed isomer with its reduced
energy gap.[10] Likewise other excited state processes such as
singlet oxygen sensitization[11] or triplet triplet annihilation[12] can
be reversibly quenched by DAEs. Another obvious application is
the photomodulation of conductance through single DAE
molecules[13] as well as the charge transport through molecular
ensembles within organic electronic devices.[14] In recent work
we demonstrated that by combining DAEs and high-performing
organic semiconductors by a simple blending approach effective
photocontrol over charge carrier mobilities in transistors can be
achieved and flexible, non-volatile optical memories be
realized.[15] In these devices, the precise alignment of the HOMO
and LUMO levels of the two DAE isomers relative to the ones of
the semiconducting host material is the key to induce light-
induced reversible charge trapping by the photochromic
molecules. Consequently, to further improve these systems we
are highly interested in the underlying electronic structure-
property relationships of DAEs and want to understand: i) how to
Introduction
Photoswitchable compounds, which can be reversibly
interconverted between two (meta)stable isomers by irradiation
with light,[1] are powerful tools for the design of molecules and
materials thereof that exert their function on demand and in a
highly controlled fashion. The simplest remote-controllable
function of a photochromic molecule is the absorption of light of
distinct wavelengths and thus the emergence of color. However,
there are more physical and chemical properties of the
photochromic moiety that are altered upon isomerization, such
[a]
Dr. M. Herder, F. Eisenreich, Dr. A. Bonasera, A. Grafl, Dr. L.
Grubert, Dr. M. Pätzel, J. Schwarz, Prof. Dr. S. Hecht
Department of Chemistry & IRIS Adlershof
Humboldt-Universität zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
Supporting information for this article is given via a link at the end of
the document
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precisely engineer the absolute energetic positioning of the
HOMO/LUMO levels of the isomers and ii) how to maximize the
relative shifts in HOMO/LUMO energies upon isomerization.
In our previous work we found that introduction of chemically
inert trifluoromethyl (CF3) groups is particularly attractive: Either
in the periphery of the DAE structure at the terminal phenyl
groups, where they significantly improve the fatigue resistance
of the switching process,[16] or as a replacement of the methyl
groups in reactive α-position of the hetaryl rings, where a huge
impact on the oxidation and reduction potentials was
observed.[17] To obtain a more systematic insight on the impact
of trifluoromethyl substitution on the photochemistry and in
particular on the electronic properties of DAEs, we now prepared
a set of model compounds possessing CF3 groups either in the
periphery or in the α-position within thiophene and thiazole
series and compared these with standard DAE derivatives
possessing electron neutral or donor groups (Figure 1b).
Besides relying on charge (or energy) transfer processes
another strategy to remote-control functions is to transduce
electronic changes occurring within the DAE core to an adjacent
functional group, such as a reactive group, a catalyst or a
supramolecular binding motif. Thereby, the reshuffling of double
bonds upon the isomerization reaction allows for the reversible
electronic coupling of the functional group with donor or acceptor
groups, thus modulating its electron density and hence
acidity/basicity and/or electrophilicity/nucleophilicity. In principle,
several strategies involving three different architectures with
distinct π-conjugation pathways are conceivable (Figure 2). In
many cases the functional unit serves as the bridge of the DAE
scaffold, thus its properties are modulated upon ring-closure by
turning the central double bond into a single bond and by
bringing donor or acceptor groups attached to the hetaryl rings
into conjugation to the bridge (Figure 2a). Alternatively, the
functional unit can be placed in the periphery of the DAE, for
example as substituent of one hetaryl ring (Figure 2b). The
isomerization reaction effects a coupling/decoupling with donor
or acceptor groups, placed either on the opposite hetaryl ring or
in the opposite α-position of the same hetaryl ring. For all three
architectures examples of DAEs modulating the properties of a
functional unit can be found in the literature.[18] However, a direct
comparison to evaluate which structure provides the largest
transduction of electronic effects to a given functional unit upon
switching is difficult.
Figure 2. Structural approaches for transducing the photoinduced electronic
changes of DAEs to an attached functional group by coupling/decoupling it
with electron-donating/-withdrawing groups (EDG/EWG): a) Incorporation of
the functional unit as the bridge. b) Attachment of the functional unit in the
periphery using donor/acceptor substituents either on the opposite (left) or the
same (right) thiophene ring. Below DAEs with redox active N,N-dimethylaniline
and triphenyltriazine moieties and differently placed trifluoromethyl groups to
evaluate their effect are shown.
Figure 1. a) Modulation of DAE core HOMO/LUMO levels upon light (or redox)
induced isomerization. b) Trifluoromethylated DAE derivatives for investigating
the extent of their HOMO/LUMO level modulation.
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To address this question, model compounds 6-8 were
designed bearing redox active N,N-dimethylaminophenyl or
triphenyltriazine moieties in the periphery and the changes in
their oxidation or reduction potential, respectively, occurring
upon photoisomerization were monitored. Trifluoromethyl groups
were chosen as acceptor moieties, either in the form of 3,5-
bis(trifluoromethyl)phenyl substituents (6 and 7) or directly
attached to the α-position of the thiophene ring (8). Compound 6
can be regarded as a "mismatched" case since two effects are
opposing each other: The π-system of the DAE becomes
extended during the course of cyclization, leading to a decrease
of the amine’s oxidation potential in the closed isomer, whereas
the established electronic coupling between the amine and the
acceptor group on the other thiophene ring should increase the
amine’s oxidation potential. In contrast, compounds 7 and 8
resemble "matched" cases, for which the change in conjugation
and coupling/decoupling of the acceptor units cooperate, i.e.
lowering the oxidation or reduction potentials of the amine or
triazine, respectively, upon ring-closure. Thus, for model DAEs 7
and 8 more pronounced potential differences between the
isomers are expected. Note that the bridge-functionalized
architecture has already been investigated by us in a previous
iodo-5-phenylthiophene
10
or
2-iodo-5-(4-N,N-dimethyl-
aminophenyl)thiophene 25 with TMSCF3 in presence of
stoichiometric amounts of CuI and KF. Due to the electron
withdrawing nature of the CF3 group the resulting α-
trifluoromethylated thiophenes 11 and 26 could easily be
metalated in the adjacent β-position using LDA and transformed
into their pinacolboronic esters, which finally were cross-coupled
with
1,2-dibromocyclopentene
13
or
the
already
monofunctionalized analogue 28 to afford DAEs 3-CF3, 5-CF3,
and 6, respectively.
In the thiazole series, 5-iodo-2-phenylthiazole 15 could be
trifluoromethylated in yields superior to that of Urata and
Fuchikami’s method using the in-situ generation of Hartwig’s
“trifluoromethylator” [(phen)CuCF3] (phen = phenanthroline).[23]
However, attempts to increase the scale of the reaction from the
suggested 0.5 mmol scale drastically decreased the yield in our
hands. The resulting α-CF3 thiazole 16 could be metalated using
LDA and transformed into its organostannane 20 in only
moderate yield due to partial decomposition. To improve the
metalation step a bromine substituent was introduced in the
4-position of the thiazole via 5-bromo-2-phenylthiazole 17, which
reacted in a halogen-dance-reaction[24] to 4-bromo-5-iodo-2-
phenylthiazole 18 and was then trifluoromethylated in the 5-
position using the [(phen)CuCF3] complex with excellent
study.[19] The reduction potential of
a bridging maleimide
"matched" with acceptor substituents in the periphery was
reduced by 240 mV upon ring-closure, while the "mismatched"
selectivity.
The
resulting
4-bromo-2-phenyl-5-
derivative bearing donor groups showed a change of only 70 mV. trifluoromethylthiazole 19 could subsequently be lithiated at very
This electronic difference was directly related to a difference in
the function of the maleimide, i.e. the strength of the binding to a
receptor via hydrogen-bonding interactions.
low temperatures (-100 °C) and transformed into the
organostannane 20, which however had again to be purified by
column chromatography lowering the yield. Finally, cross-
Here, we report on the synthesis and the photochemistry as
well as the redox properties of various types of
trifluoromethylated DAEs. We discuss the implications of the
different substitution patterns on the absolute position of the
frontier molecular orbital energy levels and their relative
photoinduced shifts. Furthermore, we analyze the interplay of
substituent and conjugation effects in “matched” and
“mismatched” DAE architectures to uncover the optimal strategy
for achieving maximum electronic changes upon photoswitching.
coupling
of
the
organostannane
20
with
1,2-
dibromocyclopentene 13 gave DAE 4-CF3, while the reaction of
an already monofunctionalized bromocyclopentene 22 with 4-
bromo-2-phenyl-5-trifluoromethylthiazole
19
gave
the
nonsymmetrically substituted analogue 4-mixed.
Results and Discussion
Synthesis. The syntheses of DAEs 1-CyH6, 1-CyF6, 2-CyH6, 2-
CyF6, 3-CH3, 4-CH3, 5-CH3, and 6 were reported by us
previously[16] and followed the established strategies of cross-
coupling of pre-functionalized thiophene or thiazole building
blocks with the respective dihalogenated bridging units[20] or the
(stepwise)
functionalization
of
the
1,2-bis(2-chloro-5-
methylthien-4-yl)cyclopentene building block.[21] Details for the
analogous synthesis of DAEs 2-MI and 7 can be found in the
Supporting Information.
For the synthesis of α-CF3 substituted DAEs the
heterocycles were pre-assembled with the key step being the
trifluoromethylation of the thiophene or thiazole rings
(Scheme 1). In case of the thiophene derivatives,
trifluoromethylation was achieved in moderate yields following a
procedure by Urata and Fuchikami[22] consisting of reacting 2-
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Scheme 1. Synthesis of α-trifluoromethylated diarylethenes: (a) PhB(OH)2, Pd(PPh3)4, Na2CO3, toluene, 100 °C, 16 h, 99%; (b) NaI, NCS, AcOH, rt, 3 d, 81%; (c)
TMSCF3, CuI, KF, DMF/NMP 1:1, 50 °C, 24 h, 49%; (d) 1) LDA, THF, -78 °C, 10 min, 2) (i-PrO)Bpin, -78 °C to rt, 2 h, quant.; (e) 1,2-dibromocyclopentene 13,
Pd(OAc)2, SPhos, K3PO4, toluene, 100 °C, 24 h, 46%; (f) 2-bromoacetaldehyde diethylacetal, p-TsOH, EtOH/H2O, 100 °C, 16 h, 97%; (g) 1) n-BuLi, THF, -80 °C
to -10 °C, 30 min, 2) I2, -10 °C, 30 min, 82%; (h) TMSCF3, CuCl, KOtBu, phenanthroline, DMF, 50 °C, 18 h, 77%; (i) 1) LDA, THF, -78 °C, 45 min, 2) Bu3SnCl, -
78 °C, 15 min, 68%; (j) Br2, AcOH, rt, 16 h, 64%; (k) 1) LDA, THF, -78 °C, 30 min, 2) I2, -78 °C, 30 min, 92%; (l) TMSCF3, CuCl, KOtBu, phenanthroline, DMF,
50 °C, 18 h, 80%; (m) 1) n-BuLi, THF, -100 °C, 45 min, 2) Bu3SnCl, -85 °C, 30 min; (n) 1,2-dibromocyclopentene 13, PdCl2(PPh3)2, DMF, 100 °C, 18 h, 18%; (o)
Pd(OAc)2, SPhos, K3PO4, toluene, 100 °C, 16 h, 61%; (p) 1) n-BuLi, THF, -78 °C, 45 min, 2) (i-PrO)Bpin, -78 °C to rt, 16 h, quant.; (q) 19, Pd(OAc)2, Cs2CO3,
DavePhos, EtOH/THF/DMF 4:4:1, 70 °C, 16 h, 43%; (r) 1) n-BuLi, THF, -78 °C, 1 h, 2) B(OBu)3, -78 °C to rt, 1 h. 3) 4-bromo-N,N-dimethylaniline, PdCl2(dppf),
Na2CO3, THF/EtOH, 75 °C, 4 h, 79%; (s) 1) n-BuLi, THF, -60 °C, 45 min, 2) I2, -60 °C to rt, 20 min, 78%; (t) TMSCF3, CuI, KF, DMF/NMP 1:1, 55 °C, 16 h, 69%;
(u) 1) LDA, THF, -78 °C, 30 min, 2) (i-PrO)Bpin, -78 °C to rt, 30 min, quant. (v) 1,2-dibromocyclopentene 13, PdCl2(dppf), K3PO4, dioxane, 100 °C, 18 h, 27%; (w)
28, Pd(OAc)2, SPhos, toluene, 100 °C, 20 h, 56%.
Photochemistry. The photochemical properties of α-methyl
functionalized DAEs 1-CyH6, 1-CyF6, 2-CyH6, 2-CyF6, 3-CH3, 4-
CH3, 5-CH3, and 6 were previously described in detail[16] and are
summarized in Table 1. They generally show an efficient ring-
closure upon UV irradiation with quantum yields ΦAB in the range
0.4 – 0.7, whereas quantum yields for ring-opening upon visible
light irradiation ΦBA are more than one order of magnitude
smaller. This ensures high conversions to the ring-closed isomer
in the photostationary state (PSS) reached upon UV irradiation,
generally exceeding 90%, though both isomers absorb UV light.
Only derivative 6 possessing a strong donor and acceptor
substituent on either side of the DAE structure represents a
marked exception with its quantum yields in both isomerization
directions being smaller by one order of magnitude when
compared to the symmetrically donor or acceptor substituted
analogues 5-CH3 and 1-CyH6, respectively.
Supporting Information) the ring-open isomer of 2-MI shows a
weak, broad charge transfer (CT) band extending to the visible
range besides the strong π-π* transition, which is a typical
property of DAEs substituted with the strongly electron-
withdrawing maleimide bridge.[17,19,25] Potentially due to the
competing CT process the quantum yield for ring-closure is
somewhat
(perfluoro)cyclopentene-bridged DAEs. However, in contrast to
analogous dithiazolylmaleimides bearing strong donor
substituents on the thiazole rings,[17] cyclization of 2-MI is still
possible with reasonable efficiency. For DAE 7,
smaller
than
for
the
corresponding
a
a
bathochromically shifted absorbance is observed of both the
open and closed isomers as compared to that of prototype DAE
1-CyH6, reflecting the extension of the π-system by the
triphenyltriazine moiety. While ring-closure of 7 is associated
with a high quantum yield ΦAB = 0.62, the reverse ring-opening
process is much less efficient when compared to 1-CyH6,
presumably due to stabilization of the closed isomer by the
elongated π-system.[26]
Compounds 2-MI and 7 also show the expected behavior for
α-methyl functionalized DAEs with small deviations resulting
from the substituents. In UV/vis spectra (see section 1.2 of the
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Table 1. Photophysical properties of investigated diarylethenes in acetonitrile (25 °C).
λmax / nm (ε / 104 M-1 cm-1)
open isomer
284 (3.16)
comp.
PSS (λirr)[a]
ΦAB (λirr)[b]
ΦBA (λirr)[c]
closed isomer
548 (1.96)
590 (1.54)
522 (1.34)
538 (1.13)
[16]
1-CyH6
97% (313 nm)
96% (313 nm)
95% (313 nm)
89% (313 nm)
0.44 (313 nm)
0.58 (313 nm)
0.56 (313 nm)
0.45 (313 nm)
0.0046 (546 nm)
0.017 (546 nm)
0.015 (546 nm)
0.039 (546 nm)
[16]
1-CyF6
300 (3.45)
[16]
2-CyH6
332 (1.80)
[16]
2-CyF6
2-MI
307 (3.11)
310 (2.60),
376 (0.44, shoulder)
532 (1.10)
78% (313 nm)
0.14 (313 nm)
0.031 (546 nm)
[16]
3-CH3
3-CF3
4-CH3
278 (3.25)
284 (3.31)
316 (2.13)
520 (1.78)
493 (2.01)
500 (1.55)
98% (313 nm)
63% (300 nm)
94% (313 nm)
0.47 (313 nm)
0.061 (300 nm)
0.53 (313 nm)
0.0093 (546 nm)
0.076 (546 nm)
0.021 (546 nm)
[16]
38% (280 nm)
39% (300 nm)
4-CF3
283 (2.53)
469 (1.72)
0.067 (300 nm)
0.11 (436 nm)
4-mixed
292 (2.33)
331 (4.33)
482 (1.67)
537 (2.66)
84% (300 nm)
99% (313 nm)
0.17 (300 nm)
0.74 (313 nm)
0.013 (436 nm)
0.0029 (546 nm)
[16]
5-CH3
0.51 (313 nm)
0.58 (365 nm)
5-CF3
6[16]
7
345 (4.02)
326 (3.37)
528 (4.05)
570 (2.54)
568 (2.63)
98% (350 nm)
97% (313 nm)
97% (313 nm)
0.011 (546 nm)
0.0022 (546 nm)
0.0023 (546 nm)
0.051 (313 nm)
0.62 (313 nm)
269 (6.27),
358 (3.28)
311 (2.42),
346 (2.10)
8
522 (2.80)
98% (313 nm)
0.77 (313 nm)
0.011.(546 nm)
[a] Amount of closed isomer in the PSS reached after UV irradiation, determined by UPLC. [b] Quantum yields for ring-closure, obtained by
evaluation of kinetic traces using ferrioxalate actinometry. [c] Quantum yields for ring-opening, obtained by the initial slope method using
Aberchrome 670 actinometry (λirr = 546 nm) or ferrioxalate actinometry (λirr = 436 nm).
While the presence of CF3 groups in the periphery of the
DAE structure does not alter the photochemical isomerization
between open and closed isomers, their introduction in the α-
position of the heterocycles has dramatic effects. Compared to
the parent dithiazolylcyclopentene 4-CH3, the doubly α-
trifluoromethylated analogue 4-CF3 shows marked hypsochromic
shifts of the lowest energy absorption bands in both the open
and closed isomer (Figure 1a, see section 1.2 of the Supporting
Information). Remarkably, the quantum yield for ring-closure is
decreased by almost one order of magnitude to ΦAB = 0.067
rendering the doubly CF3 substituted compound
inefficient DAE photoswitch.
a rather
In order to investigate whether the usual DAE
photochemistry can be restored upon replacing only one of the
α-CH3 groups by an α-CF3 group, the nonsymmetrically
substituted derivative 4-mixed was synthesized. In fact, the
photochemical characteristics of 4-mixed are somewhere in
between those of the two symmetrical compounds 4-CH3 and 4-
CF3: The hypsochromic shift of the absorbance is smaller than
for 4-CF3 (Figure 3a), the decrease of ΦAB to a value of 0.17 is
less pronounced, ring-opening is as efficient as for 4-CH3, and
the amount of the ring-closed isomer in the PSS of 84% is
reasonable. Importantly, these results show that an α-CF3 group
and its strong influence on the electronic properties of the DAE
scaffold (vide infra) may be utilized at least on one of the
heterocyclic building blocks without disturbing the efficiency of
photoisomerization too much.
while at the same time the quantum yield for ring-opening ΦBA
=
0.11 is significantly higher when compared to 4-CH3. The rate-
increasing effect of α-CF3 groups on ring-opening was already
observed for maleimide-bridged dithiazolylethenes[17] and may
be due to a general destabilization of the closed isomer by
strongly electron-withdrawing groups attached to the reactive
carbon atoms.[27] Since for 4-CF3 the ratio of quantum yields for
ring-opening and ring-closure is less favorable the population of
the closed isomer in the PSS upon UV illumination is only 39%,
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Figure 4. Comparison of UV/vis spectra of 3-CH3, 6, and 8 showing the
differing conjugation patterns between donor and acceptor groups. Solid lines
represent spectra of pure open isomers in acetonitrile (25 °C), dashed lines
represent calculated spectra of the pure closed isomers as calculated from a
series of measured spectra at different stages of the irradiation for which the
photoconversion was determined by UPLC.
The reason for the severe loss of photocyclization efficiency
in case of α-CF3 substitution on the hetaryl rings and its recovery
upon addition of strongly donating 4-N,N-dimethylamino groups
to the peripheral phenyl rings remains unclear. It may be due to
a change in the relative stabilities of photoreactive and inactive
ground state conformations of the ring-open isomer[28] or due to
an alternating electronic structure leading to the population of
excited states with unfavorable orbital topologies for ring-
closure.[29] Notably, despite the pronounced donor-acceptor
substitution of the thiophene rings in DAEs 5-CF3 and 8
photocyclization in polar solvents is not prevented by the
formation of a twisted intramolecular charge transfer (TICT)
state, as it is the case for DAEs bearing electron deficient
bridges in combination with electron rich hetaryl rings.[17,30]
Comparison of the UV/vis spectra of the prototype DAE 3-
CH3 with that of the model compounds 6 and 8 (Figure 4)
reveals the different conjugation paths between the aniline as a
"functional unit" and the acceptor moieties, as claimed in
Figure 2. The open form of DAE 8, with the donor and the
acceptor being electronically coupled, shows the most
bathochromically shifted absorbance of the investigated open
isomers. However, in the closed form the position of the
absorbance band in the visible range is almost identical to that
of unsubstituted 3-CH3. This shows that the CF3 group is
efficiently decoupled from the π-system. In contrast, the
absorbance of the closed isomer of 6 is shifted bathochromically
by 50 nm, revealing the electronic coupling of the donor and the
acceptor after ring-closure.
Figure 3. Impact of CF3 substitution on UV/vis spectra of a) dithiazolylethenes
4-CH3, 4-CF3, and 4-mixed and b) dithienylethenes 3-CH3, 3-CF3, and 5-CF3.
Solid lines represent spectra of pure open isomers in acetonitrile (25 °C),
dashed lines represent spectra of the pure closed isomers as calculated from
a series of measured spectra at different stages of the irradiation for which the
photoconversion was determined by UPLC.
For the thiophene containing DAEs we observed similar
trends for α-CF3 substitution by comparing compounds 3-CH3
and 3-CF3 (Figure 3b). Again, replacing both methyl groups by
CF3 imparts a hypsochromic shift of the absorption bands, a
marked decrease of the quantum yield for ring closure, an
increased quantum yield for ring-opening, and consequently a
reduced amount of the ring-closed isomer in the PSS.
Surprisingly, when strongly donating N,N-dimethylamino groups
are attached to the phenyl rings in combination with α-CF3
groups in the symmetrical DAE 5-CF3 as well as the
nonsymmetrically substituted DAE 8 the typical photochemical
behavior is retained. In fact, both show highly efficient ring-
closure with quantum yields ΦAB = 0.58 and ΦAB = 0.77,
respectively, and almost quantitative conversion to the closed
isomer in the PSS.
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10.1002/chem.201605511
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crystallography (Figure 5b).[32] Note that the other isomer
detected by UPLC/MS possesses an almost identical
absorbance spectrum and the same m/z ratio (Figure S11b).
The fact that there is no strong preference for one of these
annulated isomers may be of importance for the theoretical
elucidation of the mechanism of by-product formation. To the
present either the cleavage of one C-S bond or the
transformation of the central cyclohexadiene core into
a
methylcyclopentene diradical like structure are discussed as the
initial fatigue reaction coordinate in the excited state (Scheme
S1).[33] However, when molecular symmetry is broken as in 4-
mixed, for both mechanisms a preference for one of the two
possible annulated isomers due to the destabilization of
intermediate radical species by the CF3 group could be expected.
Thus our experimental finding hints to
a more concerted
mechanism of the formal type I dyotropic rearrangement as it is
considered for other thermally activated reactions of this kind.[34]
Remarkably, if the DAE is substituted with a strong donor in
the periphery, which generally leads to
a poor fatigue
resistance,[16] attachment of α-CF3 groups may lead to a
significant improvement. While 5-CH3 is one of the DAEs with
the worst fatigue resistance in our hands its analogue 5-CF3
shows only little decomposition after 11 switching cycles (see
Figure S10 in the Supporting Information). Together with the
excellent efficiency of the photochromic isomerization and the
bathochromically shifted absorbance of the open isomer (vide
supra) this renders the combination of N,N-dimethylaminophenyl
and α-CF3 groups as hetaryl substituents an interesting motif for
the construction of DAEs.
Figure 5. a) Formation of two annulated isomers differing in the relative
positioning of the CH3 and CF3 groups as by-products of the photochemical
isomerization of 4-mixed. b) ORTEP-drawing (50
% probability thermal
ellipsoids) of the molecular structure of the isolated annulated isomer of
4-mixed in the single crystal as determined by X-ray diffraction. Hydrogen
atoms and co-crystallized solvent (benzene) are omitted for clarity.
Modulation of HOMO and LUMO energies. In this work HOMO
and LUMO energy levels of DAE derivatives are calculated from
Besides having a strong impact on absorbance spectra and
isomerization quantum yields trifluoromethyl substitution of
DAEs also alters their photochemical fatigue behavior. As
a1
c1
the first oxidation and reduction potentials Ep
and Ep ,
determined by cyclic voltammetry in acetonitrile, according to the
following equation (e = elementary charge):
reported
earlier,[16]
substitution
with
3,5-bis(trifluoro-
methyl)phenyl groups in the periphery strongly suppresses the
formation of an annulated isomer[31] as a by-product of the UV
induced cyclization reaction. In contrast, DAEs bearing α-CF3
groups exhibit significant fatigue upon repeated switching
between the open and closed isomers (see section 1.3 of the
Supporting Information). Qualitatively, the switching cycles of α-
trifluoromethylated DAEs 3-CF3, 4-CF3, and 4-mixed as
compared to their methylated analogues 3-CH3 and 4-CH3
reveal a rather similar overall performance independent of the
nature of the group in the α-position. However, by inspecting
UV/Vis spectra and following the switching experiments with
UPLC/MS we noted that for DAEs bearing α-CF3 groups on both
hetaryl rings (3-CF3, 4-CF3, 5-CF3) the fatigue is not due to the
typical selective formation of the annulated isomer but due to the
slow decomposition of the DAEs to several yet unidentified
photoproducts (see Section 1.3 of the Supporting Information for
details). Interestingly, if only one α-CF3 group is present (4-
mixed) two annulated regioisomers, which differ in the
positioning of the anti-configured methyl and trifluoromethyl
groups, are formed in a 1:1 ratio as the only by-products upon
prolonged UV irradiation (Figure 5a, Figure S11a). We could
isolate one of these annulated isomers and determine its
structure by NMR spectroscopy (Figures S12-S13) and X-ray
EHOMO/LUMO = -e · Epa1/c1 – 4.8 eV
(1)
The offset of 4.8 eV represents the ionization potential of
ferrocene on the vacuum scale as postulated by Pommerehne
and coworkers.[35] It is important to note that the utilization of
redox potentials as a measure for absolute and relative frontier
orbital energies is a rough estimate due to a number of
reasons:[36]
1) Formally, electrochemistry is not able to directly measure
MO levels of a neutral molecule as in the moment an electron is
removed or introduced a new species evolves with an altered
electronic structure. Furthermore, while HOMO and LUMO
energies are scaled in vacuum, redox potentials are generally
obtained in solution in the presence of large amounts of an
electrolyte. Thus, by using equation (1) electronic reorganization
and the differing solvation energies of oxidized and reduced
species are disregarded. However, if structurally similar
compounds are compared, it can be assumed that solvation and
interaction with the electrolyte are comparable and that relative
energies are quite accurate within a margin of ca. 0.1 eV.[36]
Comparing values obtained in different solvents or between
solution and the solid state is problematic and unreliable.
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Table 2. Anodic and cathodic peak potentials[a] determined by cyclic voltammetry in acetonitrile and derived HOMO and LUMO energies.[b]
open isomer
Epa1 / V
1.08
closed isomer
Epa1 / V
0.24
comp.
Epc1 / V
< -2.80
-2.40
EHOMO / eV ELUMO / eV
Epa2 / V
0.53
-
Epc1 / V
-1.84
-1.27
-1.61
-1.08
-1.21
-2.17
-1.89
-1.99
-1.70
-1.87
-2.40
-2.22
-2.02
-1.91
-2.23
EHOMO / eV ELUMO / eV
[16]
1-CyH6
-5.88
-6.59
-5.86
-6.80[c]
-6.43
-5.59
-6.29
-5.64
-6.32
-5.88
-4.93
-5.22
-5.02
-5.73
-5.20
-
-5.04
-5.56
-5.41
-6.15
-5.90
-4.83
-5.25
-5.20
-5.68
-5.48
-4.43
-4.67
-4.67
-4.91
-4.81
-2.96
-3.53
-3.19
-3.72
-3.59
-2.63
-2.91
-2.81
-3.10
-2.93
-2.40
-2.58
-2.78
-2.89
-2.57
[16]
1-CyF6
1.79
-2.40
-2.42
-2.69
-3.23
-
0.76
[16]
2-CyH6
1.06
-2.38
0.61
0.88
-
[16]
2-CyF6
2-MI
2.00[c]
1.63
-2.11
1.35
-1.57
1.10
-
[16]
3-CH3
3-CF3
4-CH3
4-CF3
0.79
< -2.80
-2.70
0.03
0.27
0.67
0.61
1.03
0.90
-
1.49
-2.10
-1.95
-2.41
-2.30
-
0.45
[16]
0.84
-2.85
0.40
1.52
-2.39
0.88
4-mixed
1.08
-2.50
0.68
[16]
5-CH3
0.13
< -2.80
-2.78
-0.37
-0.13
-0.13
0.11
5-CF3
6[16]
7[e]
0.42
-2.02
-2.09
-2.59
-
-
0.22[d]
0.93
-2.71
0.09
0.46
-
-2.21
8
0.40[f]
< -2.80
0.01
[a] All values are reported against the ferrocene/ferrocenium redox couple used as external standard. [b] EHOMO/LUMO = -e · Epa1/c1 – 4.8 eV. [c]
Approximate value due to overlapping onset of solvent oxidation. [d] Epa2 = 0.64 V. [e] In methylene chloride. [f] Epa2 = 0.95 V.
2) Generally, the half-wave potential of a fully reversible
peak is taken as a measure for the formal potential of the redox
process. However, in case of DAEs also irreversible processes
are observed, in particular for oxidation of the open isomers.
Therefore, we report peak potentials for all observed redox
waves. In case of fully reversible processes the difference
between the half-wave potential and the peak potential amounts
to 29 mV and can be neglected. For irreversible processes,
which are due to a chemical reaction following the electron
transfer, e.g. ring-closure for some ring-open DAEs, the
observed peak potential is shifted to lower values compared to
the formal potential of the electron transfer. This shift can
amount up to approximately 350 mV for very fast chemical
reactions. However, for slower reactions the shift is smaller and
in case of quasireversible processes it can be neglected. Thus,
HOMO and LUMO levels of open isomers showing fully
irreversible redox processes have to be treated with care. In
contrast, redox processes of most closed isomers are reversible
or quasireversible and hence HOMO and LUMO levels can be
regarded as more reliable.
exist in the literature.[36] Therefore, care has to be taken which
conversion scale is applied when comparing to literature data.
Though these arguments make the utilization of cyclic
voltammetry for the estimation of frontier orbital energies appear
rather unreliable, the procedure often is successfully applied for
the prediction of specific device properties in the field of organic
electronics.[37] The method is an easy, cheap, and fast way to
select derivatives that fulfill specific energetic requirements out
of a collection of compounds. Thereby, for the reasons stated
above, comparing relative energies of structurally similar
derivatives is much more precise than their positioning on an
absolute scale. Note that in the literature[38] correlations between
HOMO energies found by cyclic voltammetry with values
determined by ultraviolet photoelectron spectroscopy (UPS)
generally show a linear dependence even for structurally very
different compounds, though the slope may slightly deviate from
unity.[39]
3) The offset of 4.8 eV relating the ferrocene/ferrocenium
redox couple to the vacuum scale is under debate and a number
of different conversion scales with values between 4.5 - 5.4 eV
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Figure 6. Tuning of HOMO and LUMO energies, as obtained from oxidation and reduction potentials by cyclovoltammetry, via donor and acceptor substitution of
symmetrical DAEs in their open (blue) and closed (red) form. The bridging unit as well as the heterocyclic building blocks for each compound are depicted below.
For compound 2-CyF6 the oxidation potential of the open isomer is in the range of the onset of solvent oxidation, thus only an approximate value can be given,
indicated by a dashed line.
Electrochemical data of all investigated compounds can be
found in Table 2 and in Section 2 of the Supporting Information.
Derived HOMO and LUMO levels of symmetrically substituted
DAEs are collected in Figure 6, giving an overview over the
covered range of absolute frontier orbital energies and the
extent of their modulation by the photoisomerization process. A
similar representation, also including some other DAE
derivatives reported earlier by us,[15b,16,17,19] is shown in Figure
S21 of the Supporting Information. From these data the following
general trends can be derived:
donor/acceptor character a precise fine-tuning of the
energetic levels can be achieved.
The nature of the bridging unit has an effect of similar
•
magnitude:
Diarylperfluorocyclopentene
derivatives
possess HOMO and LUMO energies about 0.5 - 1 eV
lower than their perhydrocyclopentene analogues (1-
CyH6/2-CyH6
vs.
1-CyF6/2-CyF6).
The
electron-
withdrawing capability of the N-tert-butylmaleimide bridge
in the closed isomer is slightly smaller than that of
perfluorocyclopentene (2-MI vs. 2-CyF6), however, the
electron affinity of the open isomer of 2-MI is strongly
increased (and higher than that of 2-CyF6) due to the
redox active carbonyl groups present in the bridge.
Substitution with α-CF3 groups has a strong effect on
HOMO and LUMO levels of open isomers, as can be seen
by comparing DAEs 3-CH3 and 3-CF3 as well as 4-CH3
and 4-CF3, which show shifts between 0.5 – 0.7 eV. In
contrast, the effect on closed isomers is smaller with shifts
between 0.3 – 0.4 eV. The latter can readily be explained
by the quaternary carbon atoms, which decouple the CF3
groups from the π-system in the closed isomers. Thus, α-
•
Absolute energies of the frontier molecular orbitals can be
systematically tuned over a broad range covering more
than 2.4 eV for HOMO levels (5-CH3 vs. 2-CyF6) and at
least 1.8 eV for LUMO levels (4-CH3 vs. 2-CyF6). Note that
LUMO levels of most open dithienylethenes are expected
to be positioned above -1.9 eV, the limit of accessible
potentials in the CV experiments.
•
•
The variation of the substituent in the periphery from
strongly donating (4-Me2N in 5-CH3) to strongly accepting
(3,5-(CF3)2 in 1-CyH6) shifts HOMO levels of open and
closed isomers over a total range of 0.95 eV and 0.61 eV,
respectively. By using substituents with an intermediate
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trifluoromethyl substitution significantly increases the
relative shifts of energy levels upon ring-closure.
•
•
Further fine-tuning of absolute energy levels can be
achieved by exchanging thiophene with thiazole
heterocycles in otherwise identical structures. For
analogous substitution patterns HOMO and LUMO levels
of thiazole derivatives are positioned between 0.1 – 0.5 eV
lower in energy than their thiophene counterparts.
Large relative shifts in HOMO and LUMO levels comparing
open and closed isomers are obtained upon increasing
acceptor substitution of the DAE scaffold. Thereby,
dithienylethenes generally show a larger isomerization
induced variation than their thiazole analogues. Maximum
relative shifts of more than 1 eV can be achieved using the
strongly acceptor substituted DAE 1-CyF6 as well as the
α-CF3 substituted derivative 3-CF3.
Figure 7. Correlation between experimental and theoretical (B3LYP/
PCM(acetonitrile)/6-311+G(d,p)) HOMO (filled symbols) and LUMO (empty
symbols) energies of open (blue) and closed (red) isomers.
All together the variation of substituents, in particular using
chemically robust fluorinated groups, either in the periphery, in
the bridge, or at the ring-closing carbon atoms represents a
versatile toolbox to specifically tailor the electronic properties of
DAE derivatives according to the energetic requirements within
the desired application. We have successfully applied this
toolbox for the construction of light-controllable organic thin film
transistors by incorporating specifically selected DAE switches
into various small molecule as well as polymeric semiconducting
matrices.15 Thereby, the alignment of the HOMO or LUMO
energies of the two isomers of a DAE with respect to that of a p-
type or n-type semiconductor allowed for the reversible trapping
of holes or electrons within the matrix dependent on the
switching state of the DAE. From our work it has become
evident that small variations in the relative energy levels
between the DAE and the semiconductor directly affect the
photoswitching performance of the overall device, illustrating the
importance of being able to precisely fine-tune the electronic
properties of the photoswitchable charge trap.
In order to rationalize and possibly predict the influence of
distinct substitution patterns on the electronic properties of the
DAE scaffold we performed DFT calculations and theoretically
assessed HOMO and LUMO energies. Thereby we note that
theoretically predicted MO energies typically are sensitive to the
choice of the functional and size of the basis set and that an
empirical correlation to experimental values is only valid for a
homologous series of chemical compounds. Comparing the
HSEh1PBE[40] and B3LYP[41] functionals using the 6-311+G(d,p)
basis set we found that B3LYP provides a slightly better
agreement between experimental and theoretical data. Notably,
after implementing the solvent acetonitrile using the polarizable
continuum model (PCM) the dispersion of theoretical HOMO and
LUMO levels around the experimental values was significantly
reduced such that we obtained a surprisingly good linear
correlation (Figure 7). The slope of the correlation is nearly unity
for both HOMO and LUMO energies, whereas there is a slightly
different offset for the two data sets. Based on this correlation it
is possible to theoretically predict frontier orbital levels of de
novo designed DAE derivatives with specific electronic
properties.
Transduction of electronic changes to redox active groups.
In order to have a model on how much of the electronic changes
within the DAE core can be transduced to an adjacent functional
group nonsymmetrically substituted DAEs 6-8 were designed
bearing redox markers and acceptor groups in different
configurations (see Figure 2). In terms of modulating the
electron density of an N,N-dimethylaniline group by the
switching process compounds
6
and
8
represent the
“mismatched” and “matched” case, respectively, for the coupling
with the acceptor group. For both compounds the first oxidation
processes are fully reversible in the ring-open and ring-closed
forms and can be assumed to be located at the N,N-
dimethylaniline unit (Figure 8). Note that, similar to symmetrically
N,N-dimethylaniline substituted compounds 5-CH3 and 5-CF3,
for the closed isomer of 8 a two-electron oxidation wave is
observed. From the difference of the anodic and cathodic peak
currents it can be derived that two one-electron oxidation
processes with a potential splitting of less than 100 mV are
occuring.
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the opposite terminus of the DAE should not interfere with the
aniline.
In contrast, for the ring-closed isomer of 6, with the N,N-
dimethylaniline moiety being coupled to the electron-withdrawing
3,5-bis(trifluoromethyl)phenyl group, it was expected that its first
oxidation would appear at higher potentials than the first
oxidation of the closed isomer of 8, for which the π-electronic
system is decoupled from the α-CF3 group. Instead, it is
experimentally found that the oxidation of the closed isomer of 6
at Epa1 = -0.13 V occurs at a slightly lower potential than that of 8
at Epa1 = 0.01 V. This shows that in the closed form the effect of
the acceptor group at the opposite terminus through the
conjugated DAE core is rather limited while the CF3 group in α
position of the hetaryl ring still has a significant impact on the π-
electronic system, though it is separated by an additional sp3-
carbon. As a result, the difference between the two DAE
derivatives 6 and 8 in their relative potential shift upon ring-
closure is small. Though the shift is slightly larger in the
"matched" compound 8 with a value of 390 mV, the shift of
350 mV for the "mismatched" compound 6 is comparable.
A similar potential shift of 300 mV between the open and
closed isomers is observed for the reversible reduction of DAE 7,
which is located at the triphenyltriazine moiety. As this
compound already represents the "matched" case, the synthesis
of the "mismatched" analogue was not attempted.
Conclusions
In this work we demonstrate that the HOMO and LUMO
energies of DAE photoswitches can be precisely tuned over a
broad range by donor/acceptor substitution in the periphery,
variation of the bridging moiety, substitution with CF3 groups in
the α-positions of the hetaryl rings, and exchange of thiophene
with thiazole heterocycles. Some of these derivatives exhibit
remarkable photoisomerization induced shifts of the energies of
their frontier molecular orbital energy levels of more than 1 eV.
By creating a large collection of electronically modulated DAEs,
by estimating the effects of structural modifications, and by
Figure 8. Cyclic voltammograms (dE/dt = 1 V s-1) of DAEs a)
6 in
acetonitrile/0.1 M Bu4NPF6, b) 8 in acetonitrile/0.1 M Bu4NPF6, and c) 7 in
methylene chloride/0.1 M Bu4NPF6. All concentrations were 1⋅10-3 M before
irradiation. For compound 6 only low conversion to the closed isomer could be
achieved after extended irradiation of the electrochemical cell for 2.5 h. Note
that for a) and c) solutions became more concentrated during the irradiation
procedure due to long irradiation times needed in case of a) and high volatility
of the solvent in case of c).
establishing
a
theoretical model based on correlating
The potential of the first oxidation process of the open
isomer of 8 is higher by 180 mV when compared to 6 and by
270 mV when compared to 5-CH3, which is in accordance with
the expectation that the CF3 group in the α-position of the
thiophene ring would decrease the electron density of the aniline
moiety (Figure 2). A very similar potential is observed for the
symmetrically substituted analogue 5-CF3 showing that the two
termini of the DAE are electronically isolated in the ring-open
form. Given the much larger electronic effect of α-CF3 groups on
the electron density within the DAE core itself, observed by
comparing compounds 3-CH3/4-CH3 and 3-CF3/4-CF3 (vide
supra), it can already be inferred that transduction of the
electronic changes to an adjacent functional unit such as the
N,N-dimethylaniline moiety is significantly less pronounced. Note
that compared to the open isomer of 5-CH3 the first oxidation of
6 is shifted by 90 mV to higher potentials, although both
compounds are expected to possess the same redox behavior in
the first step as in the ring-open form of 6 the acceptor moiety at
experimental and computational data it is now possible to
custom-design specific DAE derivatives that fulfill energetic
requirements for potential applications, in particular in
combination with other (opto)electronically active materials to
reversibly enable and disable charge transport (or energy
transfer) between the building blocks. In the future we will be
able to use this insight to screen the electronic properties of
hitherto unknown DAE derivatives in silico and focus the
synthetic efforts only on the most promising candidates.
In contrast to the large electronic modulation induced within
the DAE core upon photoisomerization, transduction of these
intrinsic changes to an adjacent, π-conjugated functional unit is
less pronounced. For model compounds possessing aniline or
triazine redox moieties potential shifts between 300 – 400 mV
were observed upon ring-closure. Thereby, different patterns of
substitution, involving acceptor groups with either "matched" or
"mismatched" effects on the π-electronic system of the DAE,
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turned out to have only a small influence. In light of the desired
property changes of functional units attached to DAEs, such as
[4]
[5]
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a
larger
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In addition to the remarkable electronic properties of α-
trifluoromethylated DAEs, interesting insights into their
photochemistry were obtained. DAEs symmetrically substituted
with α-CF3 groups on both hetaryl rings show a strong reduction
of the cyclization quantum yield, whereas the ring-opening
quantum yield was significantly increased when compared to the
methyl substituted derivatives. Importantly, installing only one
CF3 group on one hetaryl ring restores the usual photochemical
behavior of DAEs. The combination of a 4-N,N-dimethylaniline
donor and a CF3 acceptor placed in the two α-positions of one
thiophene moiety provides attractive new DAE derivatives, which
exhibit a marked bathochromic shift of the absorbance of the
open isomer, an increased quantum yield for ring-closure, and
good fatigue resistance.
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Acknowledgements
The authors thank Dr. Beatrice Braun-Cula for obtaining single
crystal X-ray structural data and Jana Hildebrandt for synthetic
support. F.E. thanks the Fonds der Chemischen Industrie for
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providing
a doctoral fellowship. Generous support by the
European Research Council (ERC via ERC-2012-STG_308117
“Light4Function”), the European Commission (via MSCA-ITN
“iSwitch” GA No. 642196), and the German Research
Foundation (DFG via SFB 658, project B8) is gratefully
acknowledged.
Keywords: photochromism • diarylethenes • frontier molecular
orbital energies • quantum yields • cyclic voltammetry
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Entry for the Table of Contents
FULL PAPER
Photocontrol over energy levels: By
decorating photochromic diarylethenes
with fluorinated substituents HOMO
and LUMO energy levels of both
isomers are precisely tuned over a
broad range and the effect of the
switching event is maximized. The
light-induced electronic shifts can be
transduced to adjacent functional
groups in order to modulate their
properties.
Martin Herder, Fabian Eisenreich, Aurelio
Bonasera, Anna Grafl, Lutz Grubert,
Michael Pätzel, Jutta Schwarz, and
Stefan Hecht*
Page No. – Page No.
Light-Controlled Reversible
Modulation of Frontier Molecular
Orbital Energy Levels in
Trifluoromethylated Diarylethenes
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