A new tetrathiafulvalene-phenoxynaphthacenequinone dyad: Switching on the intramolecular electron-transfer with UV light irradiation and metal ion coordination

Article abstract of DOI:10.1039/c0cc02110e

The metal ion-promoted intramolecular electron transfer within TTF-based dyad 1 containing a photochromic PNQ unit can be switched on with UV light irradiation by taking advantage of the photochromic feature of the PNQ unit and the fact that the trans and

Full text of DOI:10.1039/c0cc02110e

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A new tetrathiafulvalene–phenoxynaphthacenequinone dyad: switching
on the intramolecular electron-transfer with UV light irradiation and
metal ion coordinationwz
Lina Jia, Guanxin Zhang,* Deqing Zhang,* Junfeng Xiang, Wei Xu and Daoben Zhu
Received 26th June 2010, Accepted 4th August 2010
DOI: 10.1039/c0cc02110e
The metal ion-promoted intramolecular electron transfer within
TTF-based dyad 1 containing a photochromic PNQ unit can be
switched on with UV light irradiation by taking advantage of the
photochromic feature of the PNQ unit and the fact that the trans
and ana forms of PNQ show different electron accepting
capacity.
Accordingly, it is interesting to examine the TTF–PNQ dyad
and it is anticipated that the intramolecular electron transfer
within the TTF–PNQ dyad in the presence of metal ions can
be tuned by light irradiation. With these considerations in
mind the TTF–PNQ dyad 16 (Scheme 1) in which the TTF and
PNQ units are linked by a glycol chain was designed and
investigated. The absorption and ESR spectral investigations
clearly manifest that intramolecular electron transfer takes
place in dyad 1 after cascade input of UV light (310 nm) and
metal ions (Sc3+, Pb2+ and Zn2+).
As strong electron donors TTF (tetrathiafulvalene) and its
derivatives have been widely employed as very important
building blocks for organic conducting materials, molecular
machines, fluorescence switches and chemical sensors.1–4
Among them, TTF-based D–A systems have been extensively
studied for the intramolecular charge transfer/photoinduced
electron transfer processes.2,3 We have recently reported the
metal ion-promoted electron transfer within TTF–quinone
dyads in which the TTF and quinone units are covalently
linked by an oligoethylene glycol chain.4 The investigations
manifest that the coordination of the oligoethylene glycol
chain and the radical anion of quinone with metal ions is
crucial for facilitating the electron transfer. Further studies
indicate that such metal ion-promoted electron transfer is
dependent on the electron accepting capacities of the quinone
units of the TTF–quinone dyads;4b the stronger an acceptor
the quinone unit is, the more efficient the intramolecular
electron transfer becomes for the TTF–quinone dyad in the
presence of certain metal ions. Accordingly, it would be
possible to tune the intramolecular electron transfer within
TTF–quinone dyads by varying the electron accepting
capacities of the quinone units.
The syntheses of dyad 1 and relevant compounds are
deposited in ESI. Fig. 1 shows the absorption spectrum of
dyad 1 and those after UV light irradiation for different times.
Dyad 1 exhibits weak absorptions above 400 nm. After UV
light irradiation new absorption bands around 450 nm and
479 nm emerged gradually as depicted in Fig. 1. These new
absorptions are due to the ‘ana’ form of the PNQ unit in dyad
1, which was generated from the ‘trans’ form of the PNQ unit
upon UV light irradiation according to previous reports.5 A
control experiment with compound 2 under UV light irradiation
also confirms this assumption. The photostationary state was
achieved after UV light irradiation for 110 s. Further visible
light (500 nm) induced a decrease for the absorption bands
around 450 nm and 479 nm and the absorption spectrum of
dyad 1 can be restored after further visible light irradiation for
100 s as shown in Fig. S1. The inset of Fig. 1 demonstrates the
reversible absorbance variation at 479 nm for dyad 1 after
alternating UV and visible light irradiation for several cycles.
Thus, the ‘trans’ form and the ‘ana’ form of the PNQ unit in
dyad 1 can be reversibly transformed by UV and visible light
irradiation.7
Phenoxynaphthacenequinone (PNQ), as a typical photo-
chromic compound, undergoes photoisomerization from
‘trans’ form to ‘ana’ form upon UV light irradiation and the
reverse conversion occurs upon further visible light irradiation
as illustrated in Scheme 1.5 More importantly, the ‘ana’ form
of PNQ is a stronger electron acceptor compared to the
corresponding ‘trans’ form.5c,e This is also confirmed by
theoretical calculations (see ESI); the LUMO energy of the
‘ana’ form (ꢀ3.02 eV) is lower than that of the corresponding
‘trans’ form (ꢀ2.58 eV). Thus, the electron accepting capacity
of PNQ can be tuned by UV/visible light irradiation.
The photoisomerization of the PNQ unit in dyad 1 was
investigated with 1H-NMR spectroscopy. As shown in Fig. S3,
Beijing National Laboratory for Molecular Sciences, Organic Solids
Laboratory, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: dqzhang@iccas.ac.cn
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Synthesis and
characterization data for 1, 2 and 4; other related data. See DOI:
10.1039/c0cc02110e
Scheme 1 Photoisomerization of phenoxynaphthacenequinone and
chemical structures of compounds 1–4.
c
322 Chem. Commun., 2011, 47, 322–324
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Fig. 2 (A) Absorption spectra of dyad 1 recorded in degassed CH2Cl2
(1.0 ꢂ 10ꢀ5 M) after UV (310 nm) irradiation for 110 s and further
addition of different amounts of Pb2+ [Pb(ClO4)2]. (B)The ESR
spectrum of dyad 1 (1.0 ꢂ 10ꢀ3 M) in CH2Cl2 after UV light
(310 nm) irradiation for 25 min in the presence of 5.0 eq. of Pb2+
[Pb(ClO4)2] recorded at room temperature; the solution was degassed
before measurement.
Fig. 1 Absorption spectra of dyad 1 in degassed CH2Cl2 (1.0 ꢂ 10ꢀ5 M)
after UV light (310 nm) irradiation for different times; inset shows the
variation of absorbance at 479 nm for the solution of dyad 1 after
alternating UV and visible light irradiation.
the signals due to the aromatic protons of dyad 1 were
changed clearly after UV light irradiation. For example, the
signals of H10 (see Scheme 1, 6.83 ppm) and H11 (7.20 ppm)
became weak gradually and two new signals at 7.00 ppm and
7.23 ppm emerged upon UV light irradiation. The molar ratio
of the ‘ana’ form and the ‘trans’ form of the PNQ unit of dyad
1 in the photostationary state was ca. 7 : 3 based on the
1H-NMR data. Similarly, the molar ratio of the ‘ana’ form
and the ‘trans’ form of compound 2 in the photostationary
state was estimated to be ca. 6 : 1 under the same conditions.
Further examination of the absorption spectrum of dyad 1
and that after UV light irradiation indicates that the inter-
action between the TTF and PNQ (both ‘trans’ form and ‘ana’
form) units within the dyad can be neglected. The absorption
spectrum of dyad 1 is almost the sum of those of compounds 2
and 3 (see Fig. S4). Similarly, the absorption spectrum of dyad
1 after UV light irradiation contains mainly the absorptions of
the TTF unit and the ‘ana’ form of the PNQ unit (see Fig. 1).
Moreover, the cyclic voltammetric investigations also manifest
that the interaction between the TTF and PNQ (both ‘trans’
form and ‘ana’ form) units within the dyad can be negligible.
Dyad 1 exhibits two quasi-reversible oxidation waves with
According to previous studies,4,9 the absorption bands around
450 and 790 nm can be ascribed to the formation of the radical
cation of the TTF unit (TTFꢁ+) in dyad 1. In fact, this
conclusion was supported by the observation of new absorptions
around 450 nm and 790 nm for reference compound 3 after the
direct oxidation with 1.0 eq. of Fe3+ (see Fig. S7). It should be
noted that a similar absorption spectral change was detected
for dyad 1 after addition of Pb2+ and further UV light
irradiation (Fig. S10). These absorption spectral investigations
clearly indicate that the TTF unit within dyad 1 in which the
PNQ unit exists in the ‘ana’ form was oxidized, i.e. intra-
molecular electron transfer can take place within dyad 1 after
the addition of Pb2+
.
Furthermore, strong ESR signals were detected for dyad 1
after UV light irradiation and addition of Pb2+ as depicted in
Fig. 2B. The solution of dyad 1 in the presence of 5.0 eq. of
Pb2+ after UV light irradiation exhibited strong ESR signals
(doublet, g = 2.01001). The ESR signals are likely due to the
radical cation of the TTF unit in dyad 1 according to previous
studies.4 In comparison, no ESR signals were observed for
dyad 1 in the presence of Pb2+ without UV light irradiation.
These ESR data also indicate that intramolecular electron
transfer occurs within dyad 1 after sequential UV light
E1/2
= 0.55 V and E1/2
= 0.93 V, as shown in Fig. S5,
ox2
ox1
which are rather close to those of compounds 3 and 4.8 After
UV light irradiation the two oxidation potentials due to the
TTF unit (E1/2ox1 = 0.55 V and E1/2ox2 = 0.93 V) keep almost
unaltered. However, a new reduction wave at ꢀ0.74 V appears
(see Fig. S5), which should be due to the ‘ana’ form of the
PNQ unit in dyad 1 according to previous reports.5
irradiation and the addition of Pb2+
.
Besides Pb2+, other metal ions were also examined with
regard to the intramolecular electron transfer within dyad 1.
Both absorption and ESR studies (see Fig. S8–S9) show that
Sc3+ and Zn2+ can also induce the intramolecular electron
transfer between the TTF and PNQ (as ana form) units of
dyad 1. New absorptions around 450 nm and 790 nm were
observed for dyad 1 in the presence of either Zn2+ or Sc3+
after UV light irradiation (see Fig. S8). Similarly, ESR signals
were detected for dyad 1 after UV light irradiation and
addition of either Zn2+ or Sc3+ (see Fig. S9). The above
results clearly indicate that metal ion-promoted electron
transfer cannot take place within dyad 1 before UV light
irradiation, but it occurs efficiently after UV light irradiation
as schematically illustrated in Scheme 2. Therefore, the intra-
molecular electron transfer within dyad 1 in the presence of
metal ions can be tuned with UV light irradiation.
In the following we demonstrate that intramolecular
electron transfer occurs within dyad 1 in the presence of metal
ions after UV light irradiation. Addition of metal ions such as
Pb2+ and Sc3+ to the solution of dyad 1 led to almost no
absorption spectral change (see Fig. S6). However, when the
solution of dyad 1 was exposed to UV light for a certain time,
+
new absorptions due to the TTFꢁ were detected for dyad 1
after the addition of metal ions. For instance, new absorption
around 790 nm emerged gradually after the addition of Pb2+
to the solution of dyad 1 which was irradiated with UV light
(310 nm) for 110 s as depicted in Fig. 2A; moreover, the
intensity of this new absorption was increased by increasing
the amounts of Pb2+ added to the solution and maximum
absorption was reached after the addition of 5.0 eq. of Pb2+
.
In order to understand the metal ion-promoted electron
transfer within dyad 1 after UV light irradiation, the cyclic
voltammograms of reference compound 2 in the presence of
Additionally, the absorption around 450 nm was also slightly
enhanced after introducing Pb2+ to the solution of dyad 1.
c
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Notes and references
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Scheme 2 Illustration of the mechanism for the intramolecular
electron transfer after UV light irradiation and introducing metal ions.
metal ions (Pb2+ and Sc3+) after UV light irradiation were
recorded as depicted in Fig. S11. The ‘ana’ form of 2 generated
after UV light irradiation showed a reduction peak at ꢀ0.61 V.
Interestingly, the reduction peak was positively shifted after
introducing either Pb2+ or Sc3+. For instance, the reduction
peak potential of the ‘ana’ form of 2 (generated after UV light
irradiation for 25 min) was shifted from ꢀ0.61 V to ꢀ0.52 V in
the presence of 5.0 eq. of Sc3+. In the presence of Sc3+, the
cyclic voltammograms of 2 after UV light irradiation for
25 min were recorded at different scanning rates. By following
Fukuzumi’s method,10 the reduction potential of the ana form
of 2 was estimated to be ꢀ0.47 V in the presence of 5.0 eq. of
Sc3+ (Fig. S11–S12). Therefore, it can be concluded that the
electron accepting capacity of the ‘ana’ form of PNQ can be
enhanced in the presence of metal ions. Accordingly, it is
anticipated that the electron transfer between the TTF and
PNQ (‘ana’ form) units in dyad 1 would become more feasible
in the presence of metal ions. But, the electron transfer is still
not thermodynamically favorable by considering the oxidation
potential of the TTF unit (E1/2ox1 = 0.55 V). As for the metal
ion-promoted electron transfer within TTF–quinone dyads,
we propose that oxygen atoms of the oligoethylene glycol
chain, oxygen atoms of the radical anion of the PNQ
+
(‘ana’ form) unit and the sulfur atom from TTFꢁ in dyad 1
may coordinate with Pb2+/Sc3+/Zn2+ to stabilize the electron-
transfer state by increasing the interaction between the corres-
ponding cation and anion (see Scheme 2).11 It should be
mentioned that the electron accepting capacity of PNQ (trans
form) before UV light irradiation is rather weak,5c,e and an
obvious reduction peak cannot be detected even in the
presence of metal ions. Therefore, it is understandable that
intramolecular electron transfer within dyad 1 cannot take
place in the presence of metal ions before UV light irradiation.
In summary, a new TTF-based dyad 1 with a photochromic
PNQ unit was synthesized and studied. Both absorption and
ESR spectral investigations manifest that intramolecular elec-
6 The detailed synthetic procedures and characterization data for 1,
2 and 4 are provided in ESI.
7 It should be noted that the photoisomerization process of the PNQ
unit in dyad 1 is slow compared to that of compound 2 without the
TTF unit as shown in Fig. S2, where the variation of the absorbance
at 450 nm vs. the UV light irradiation time for dyad 1 and compound
2 is displayed. This is probably due to the photoinduced electron
transfer between the TTF and PNQ units in dyad 1, and as a result
the excited state of PNQ will be quenched to some extent.
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tron transfer within dyad 1 in the presence of Pb2+/Sc3+
/
Zn2+ cannot occur, but it can take place after UV light
irradiation. Therefore, the metal ion-promoted electron transfer
within dyad 1 is switched on with UV light irradiation by
taking advantage of the photochromic feature of the PNQ unit
and the fact that the ‘trans’ and ‘ana’ forms of PNQ show
different electron accepting capacity.
11 The other oxygen atoms may also bind metal ion to further
stabilize the charge-transfer state.
c
324 Chem. Commun., 2011, 47, 322–324
This journal is The Royal Society of Chemistry 2011