Photomodulation of Lewis basicity in a pyridine-functionalized 1,2-dithienylcyclopentene

Article abstract of DOI:10.1039/b501779c

The ability of a pyridine ligand on the photoresponsive 1,2-dithienylethene backbone to coordinate to a ruthenium porphyrin is modulated by interconverting the compound between its electronically insulated ring-open and electronically connected ring-closed form. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501779c

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COMMUNICATION
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Photomodulation of Lewis basicity in a pyridine-functionalized
1,2-dithienylcyclopentene{
Hema D. Samachetty and Neil R. Branda*
Received (in Cambridge, UK) 3rd February 2005, Accepted 6th April 2005
First published as an Advance Article on the web 21st April 2005
DOI: 10.1039/b501779c
refractive index and charge transfer, all of which are useful in
photo-optical and photo-electronic device applications.3
The ability of a pyridine ligand on the photoresponsive 1,2-
dithienylethene backbone to coordinate to ruthenium
a
The synthetic and supramolecular chemist would argue that
electronic differences are most dramatically manifested by how
molecules react or interact with others. Despite this fact, there are
few examples4–6 of molecular systems that take advantage of the
changes in electronic communication between the two heterocycles
on the DTE backbone to modulate chemical reactivity even
though the importance of this concept in regulating drug delivery,
sensing and catalysis is obvious. It is this concept that is the focus
of this study.
porphyrin is modulated by interconverting the compound
between its electronically insulated ring-open and electronically
connected ring-closed form.
The development of molecular systems that respond to light by
undergoing reversible changes in their structures and properties is
an important goal as it has the potential to significantly impact
numerous technologies related to materials science and structural
biology.1 This bold statement is based on the fact that some well-
chosen photoresponsive compounds can be toggled between two
thermally stable, geometrically distinct forms, each displaying
unique steric and electronic properties. Photoresponsive com-
pounds containing the dithienylethene (DTE) backbone are
particularly well suited to take on the role as switching elements
because they undergo thermally irreversible cyclization reactions
when stimulated with UV and visible light (isomers 1 and 2 in
Scheme 1, for example).2 Due to the significant differences in the
extent of linear p-conjugation between the two DTE isomers, these
photoresponsive systems offer many choices of variations in
physical properties such as absorbance, luminescence, magnetism,
The photoregulation of reactivity is illustrated by the ring-open
(1) and ring-closed (2) DTE isomers of the mono-alkylated
bis(pyridine) shown in Scheme 1. Due to the lack of coplanarity
and linear p-conjugation in the ring-open isomer 1, the two
pendant pyridine rings are electronically insulated from each other
and the nucleophilic pyridine will not sense the electronic pull of
the electron-deficient pyridinium cation. Photocyclization creates a
linear p-conjugated pathway running along the molecular back-
bone in 2. The two formerly independent pyridine rings are now
electronically coupled and it is reasonable to expect that the free
pyridine will sense the effects of the electron-deficient pyridinium
cation residing at the other end of the conjugated system. In this
communication, we show that the outcome is a lowering of the
nucleophilicity as is observed when electron-withdrawing groups
are appropriately placed on nitrogen ligands.
1
{ Electronic supplementary information (ESI) available: details of all H
NMR and IR spectroscopic studies. See http://www.rsc.org/suppdata/cc/
b5/b501779c/
*nbranda@sfu.ca
Scheme 1
2840 | Chem. Commun., 2005, 2840–2842
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We chose to demonstrate the difference in Lewis basicity of the
pyridine ligands in 1 and 2 by monitoring a supramolecular event.
The axial coordination of the pyridine to a ruthenium porphyrin as
shown in Scheme 1 is a convenient way to assess the differences in
coordination behaviour because the N–Ru binding is slow on the
NMR time scale,7 and sharp, unchanging peaks for the statistical
mixture of ‘free’ (1 and 2) and ‘bound’ (1(TTP) and 2(TTP))
pyridines are clearly visible when less than one molar equivalent of
Ru(TTP)(CO)(EtOH) is added to CD2Cl2 solutions of the
compounds. This provides a means to instantaneously measure
the relative ratios of all compounds in a single NMR experiment
and assess the binding behaviour of the ring-open and ring-closed
DTE isomers.
Fig. 2 Partial 1H NMR (600 MHz, CD2Cl2) spectra showing the peaks
corresponding to the CH3 protons in a 50 : 50 mixture of ring-open isomer
1 and ring-closed isomer 2 (top trace) and when slightly less than 1 molar
equivalent of Ru(TTP)(CO)(EtOH) is added (bottom trace). The 50 : 50
mixture was obtained by irradiating a solution of 1 with 365 nm light until
the appropriate ratio was achieved.
The ring-open isomer 1 is prepared in one step by mono-
alkylating the known bis(pyridine)8 with 4-bromobenzyl bromide.
The changes that occur in the UV-vis absorption spectra when a
CH2Cl2 solution of 1 (2.5 6 1025 M) is irradiated with 365 nm
light{ to convert it to the ring-closed isomer 2 are shown in Fig. 1.
The ring-open isomer is regenerated upon irradiation with visible
light and alternate irradiation with 365 nm light and wavelengths
greater than 490 nm show no apparent signs of photodegradation
after 10 cycles (Fig. 1). Similar cycling experiments can be analyzed
corresponding to the thiophene methyl protons are shown in
Fig. 2). This is deduced by comparing the relative ratios of the
peak integrals for the ‘bound’ to ‘free’ ligands, which is measured
to be 52 : 48 in the case of 1(TTP) : 1 and 42 : 58 in the case of
2(TTP) : 2. Although the changes are small, they are significant
and imply that the ring-open isomer is the more effective ligand.11
The magnitude of the differences may be ascribed to the fact that
the ‘free’ pyridine ring is twisted 20–30u out of coplanarity from
the p-conjugated backbone in the ring-closed isomer 2 as predicted
by molecular modeling."
1
using H NMR spectroscopy, which also shows no degradation
even after prolonged irradiation (365 nm for over 2 h) nor
formation of the photostable side-products that are occasionally
observed for DTE compounds lacking substituents at the C-4
position of the thiophene rings.9,10
A CD2Cl2 solution of 1 (1.1 6 1023 M) can be irradiated with
365 nm light until a 50 : 50 mixture of ring-open and ring-closed
isomers is generated as monitored by measuring the relative
integrals of the corresponding pairs of signals for the two isomers
(Fig. 2).{ Partial axial coordination is then accomplished by simply
treating this 50 : 50 mixture with slightly less than 1 molar
equivalent of Ru(TTP)(CO)(EtOH) (TTP).§ The 1H NMR spectra
of both complexes 1(TTP) and 2(TTP) show significant upfield
shifts for all hydrogen atoms of the DTE ligand as anticipated for
protons lying within the shielding cone of the porphyrin (Fig. 2).
As expected, the protons immediately adjacent to the pyridine
nitrogen of the core unit are the most dramatically affected and are
seen to move as much as 7.1 ppm upfield upon complexation.{
The 1H NMR spectrum reveals that there is slightly less than 1.5
times more of the ring-open isomer axially coordinated to the
metalloporphyrin than its ring-closed counterpart (only the signals
In a competition study, 0.79 molar equivalents of 3-bromo-2-
methyl-5-pyridylthiophene (compound 3 in Scheme 2) were added
to the above NMR sample. This pyridine provides a competing
Lewis basic nitrogen to axially coordinate to the metalloporphyrin
and monitoring its coordination behaviour is a convenient way to
provide additional support to the first findings. By measuring the
difference in the ratios of ‘bound’ to ‘free’ 3, the effect that the
monocation has on this coordination can be assessed when it is in
its all-ring-open form as opposed to its all-ring-closed form.
Irradiating the NMR sample using light greater than 490 nm
1
results in the complete ring-opening of 2 and 2(TTP). The H
NMR spectrum shows that half of the added pyridine is axially
coordinated and the ratio of the peak integrals for the signals
corresponding to 3(TTP) : 3 is measured to be 33 : 67. The amount
of axially coordinated additive increases (3(TTP) : 3 5 39 : 61)
when the same NMR sample is irradiated with 365 nm light to
Fig. 1 Changes in the UV-vis absorption spectra of a CH2Cl2 solution
(2.5 6 1025 M) of 1 when irradiated with 365 nm light for a total of 30 s
(left). Modulated absorptions at 370 nm (%) and 679 nm ($) during
alternating irradiation at 365 nm and .490 nm (right).
Scheme 2
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Hema D. Samachetty and Neil R. Branda*
Department of Chemistry, Simon Fraser University, 8888 University
Drive, Burnaby, BC, Canada V5A 1S6. E-mail: nbranda@sfu.ca
induce the ring-cyclization and quantitatively produce 2 and
2(TTP). This observation reveals that the ring-closed form of the
DTE is a weaker ligand than its ring-open counterpart and
supports the results obtained in the first set of NMR experiments.
The final tool we used to diagnose the ability of the DTE
compound to act as a Lewis base is infrared spectroscopy because
monitoring the changes in the CLO stretching frequency is another
convenient means to illustrate the electronic differences between
the two forms of the nitrogen base (1 and 2). These results are
shown in Scheme 2.
Notes and references
{ Standard lamps used for visualizing TLC plates (Spectroline E-series,
470 mW cm22) were used to carry out the ring-closing reaction of 1 to 2 and
1(TTP) to 2(TTP). The ring-opening reactions were carried out using the
light of a 150 W tungsten source that was passed through a 490 nm cutoff
filter to eliminate higher energy light.
§ The titration was carried out by slowly adding small portions (as a solid)
of an approximately weighed amount of the metalloporphyrin.
" Semi-empirical calculations (AM1 and PM3) were performed using
Spartan ’02 from Wavefuntion, Inc.
The metalloporphyrin reagent, Ru(TTP)(CO)(EtOH), has its
CLO stretch at 1946 cm21 in the IR spectrum (cast in KBr pellet).{
The coordination compound formed when this metalloporphyrin
is complexed to the pyridylthiophene additive (3(TTP)) has its
CLO stretch at 1951 cm21 due to the decrease in Ru A CLO
backbonding by the presence of the pyridyl p-acid. Compound 3 is
a good representative of the DTE ring-open isomer 1 and 1(TTP)
1 (a) Molecular Switches, ed. B. L. Feringa, Wiley-VCH, Weinheim,
Germany, 2001; (b) Organic Photochromic and Thermochromic
Compounds, ed. J. C. Crano and R. J. Gugliemetti, Plenum,
New York, 1999, vol. 1 and 2.
2 (a) M. Irie, in Organic Photochromic and Thermochromic Compounds,
ed. J. C. Crano and R. J. Gugliemetti, Plenum, New York, 1999, vol. 1,
p. 207; (b) M. Irie, Chem. Rev., 2000, 100, 1685; (c) M. Irie, in Molecular
Switches, ed. B. L. Feringa, Wiley-VCH, Weinheim, Germany, 2001,
p. 37.
3 H. Tian and S. Yang, Chem. Soc. Rev., 2004, 33, 85.
4 (a) For an example where the acidity of a phenol –OH increases due
to the creation of linear p-conjugation between the phenol and a
pyridinium across a DTE backbone, see: S. H. Kawai, S. L. Gilat and
J.-M. Lehn, Eur. J. Org. Chem., 1999, 2359; (b) For an example where
the coordination geometry around a copper(II) ion is photoregulated
using the bis(pyridine) version of the DTE, see: K. Matsuda,
K. Takayama and M. Irie, Inorg. Chem., 2004, 43, 482. Although this
study demonstrates that there is a difference in the p-acceptor character
of the pyridyl ligand in the two forms of the DTE, the authors only
suggest it is due to variations in electronics in the pyridine of the closed
form and do not attribute it to electronic communication between the
free pyridine and the one that is electron poor owing to the
coordination. In fact they later state that the distance between the two
metal centres is too far for them to be effectively coupled.
5 For examples where azobenzene has been used to regulate reactivity
based on the geometric changes, see: (a) A. Ueno, K. Takahashi and
T. Osa, J. Chem. Soc. Chem. Commun., 1981, 94; (b) R. Cacciapaglia,
S. Di Stefano and L. Mandolini, J. Am. Chem. Soc., 2003, 125, 2224; (c)
F. Wu¨rthner and J. Rebek, Jr., Angew. Chem., Int. Ed. Engl., 1995, 34,
446; (d) F. Wu¨rthner and J. Rebek, Jr., J. Chem. Soc., Perkin Trans. 2,
1995, 1727.
6 For an example where DTE has been used to regulate reactivity based
on geometric changes, see: D. Sud, T. B. Norsten and N. R. Branda,
Angew. Chem., Int. Ed., 2005, 44, 2019.
7 (a) S. S. Eaton and G. R. Eaton, Inorg. Chem., 1977, 16, 72; (b)
J. W. Faller, C. C. Chen and C. J. Malerich, J. Inorg. Biochem., 1979,
11, 151; (c) K. Chichak and N. R. Branda, Chem. Commun., 2000, 1211;
(d) K. Chichak, M. C. Walsh and N. R. Branda, Chem. Commun., 2000,
847.
8 S. L. Gilat, S. H. Kawai and J.-M. Lehn, J. Chem. Soc., Chem.
Commun., 1993, 1439.
9 M. Irie, T. Lifka, K. Uchida, S. Kobatake and Y. Shindo, Chem.
Commun., 1999, 747.
10 A. Peters, R. McDonald and N. R. Branda, Adv. Mater. Opt. Electron.,
2000, 10, 245.
11 For an example where photochemical fragmentation of a malachite
green dye leads to complete decomplexation of a cation/aza-crown ether,
see: K. Kimura, R. Mizutani, M. Yokoyama, M. Okamoto and H. Doe,
J. Am. Chem. Soc., 1997, 119, 2062.
12 A. Takata, M. Saito, A. Murakami, S. Nakamura, M. Irie and
K. Uchida, Adv. Funct. Mater., 2003, 13, 755.
13 T. J. Wigglesworth, A. J. Myles and N. R. Branda, Eur. J. Org. Chem.,
2005, 1233.
has its CLO stretch at
a .
similar frequency, 1949 cm21
Monomethylbipyridine 4 is an appropriate analogue for the
ring-closed isomer 2 and the coordination compound prepared
when the metalloporphyrin is treated with 4 has its CLO stretch at
1956 cm21 as expected for the stronger p-acid. Samples prepared
from all-ring-closed 2(TTP) have the CLO stretching frequency at
1954 cm21 as predicted from the model compound 4(TTP).
Irradiating the KBr pellet of 1(TTP) with 365 nm light results in
the shifting of the CLO stretch to higher energy (1952 cm21) albeit
very slowly (over 2 h) and not completely to the frequency
corresponding to the ring-closed isomer. The fact that the
photocyclization appears to be hindered in the pellet can be
ascribed to the environmental constraints imposed on the ring-
open form by the solid KBr matrix. It is well-documented that two
equal energy conformers of DTE derivatives coexist and that only
one of them is free to undergo the ring-closing reaction.2 The close
packing in the solid KBr pellet likely prevents the free rotation and
renders the mixture partially inactive. Similar constraints of the
medium on DTE derivatives have been described.12,13 This
limitation can be overcome in a similar manner as previously
described12 by commencing the IR studies with the pellet prepared
from the all-ring-closed isomer 2(TTP). Exposure of this pellet
(CLO stretch at 1954 cm21) to light of wavelengths greater than
490 nm (1 h) results in the shifting of the stretching frequency to
1949 cm21 corresponding to the CLO stretch of the ring-open
complex 1(TTP). Re-irradiation of the pellet sends the CLO stretch
back to the higher energy value (1954 cm21) corresponding to the
ring-closed isomer.
We have shown here that there is a difference in the reactivity
(via a supramolecular event) between the ring-open and ring-closed
isomers of a DTE switch due to the reversible interconversion of
an electronically insulating and conducting backbone. We are
currently exploring the use of this technology in catalysis and
reactivity.
This work was supported by the Natural Sciences and
Engineering Research Council of Canada, the Canada Research
Chair Program and Simon Fraser University. We thank Nippon
Zeon Corporation for supplying the octafluorocyclopentene
needed to prepare the photochromic compounds.
2842 | Chem. Commun., 2005, 2840–2842
This journal is ß The Royal Society of Chemistry 2005