Successful bifunctional photoswitching and electronic communication of two platinum(II) acetylide bridged dithienylethenes

Article abstract of DOI:10.1021/ja907434x

(Chemical Equation Presented) Coordinating two dithienylethenes to a platinum center results in the reversible ring closure of both photochromic units in a model for a photoresponsive =-conjugated polymer. This system demonstrates how metal-sensitized pho

Full text of DOI:10.1021/ja907434x

Published on Web 10/30/2009
Successful Bifunctional Photoswitching and Electronic Communication of
Two Platinum(II) Acetylide Bridged Dithienylethenes
Matthew N. Roberts,† Carl-Johan Carling,‡ Jeffrey K. Nagle,§ Neil R. Branda,*,‡ and
Michael O. Wolf*,†
Department of Chemistry, UniVersity of British Columbia, VancouVer, British Columbia V6T 1Z1, Canada,
4D LABS, Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia V5A 1S6, Canada, and
Department of Chemistry, Bowdoin College, Brunswick, Maine 04011
Received September 2, 2009; E-mail: mwolf@chem.ubc.ca; nbranda@sfu.ca
Scheme 1
The addition of molecular switching elements into conjugated
polymers would provide a means to modulate the optoelectronic
properties these versatile materials offer to molecular electronic,1
sensing,2 and logic3 technologies. Photoresponsive molecular
systems based on the dithienylethene (DTE) architecture are
particularly suitable choices as “on/off” modulators of conjugation
due to their undergoing reversible ring-closing reactions between
two isomers, each having markedly different optical and electronic
properties.4
Despite efforts directed toward developing conjugated materials
from DTEs,5 systems that incorporate multiple chromophores into a
single conjugated backbone while retaining the photoswitching be-
havior of all components have been elusive. The major problem is
rapid energy transfer (ET) from the singlet excited state of a ring-
open isomer to an adjacent ring-closed DTE that takes precedence
over photocyclization. The few examples that have multiple, adjacent
DTEs undergoing photoinduced ring-closing tend to have more
localized excited states and, therefore, properties similar to those of a
single DTE unit.6 Therefore, little benefit has been gained from
integrating multiple DTEs into a single polymer, and nontraditional
approaches to photochromic switching must be exercised to eliminate
this paradox.
Here, we demonstrate how metal-sensitized population of the triplet
manifold can be used to ring-close more than one DTE via the organic
chromophores’ triplet states.7 This is one of the first systems to show
how the interesting optoelectronic properties typical for conjugated
oligomers can be reversibly modulated by photoswitching multiple
DTEs. Compound 1oo (Scheme 1) is used to illustrate this unprec-
edented approach and is based on the fact that triplet states in Pt-
acetylide-based conjugated oligomers are localized on only one
conjugated ligand rather than delocalized over the whole oligomer as
in the singlet state.8 Pt-bis(acetylide) makes an excellent sensitizer in
1oo since ligand-localized triplet states can be populated by excitation
with visible light to trigger the cyclization of both DTE photoswitches
without the ring-open DTE transferring its excited state energy to the
adjacent ring-closed DTE in 1oc. Cyclization of the two ring-open
isomers produces a fully π-conjugated pathway that extends through
the Pt and over the full length of both DTEs.9
In both cases, lower energy absorptions (590-630 nm) corresponding
to the ring-closed isomers appear (Figure 1a). In the case of 1oo, both
the long-axis transition at ∼350 nm and the ring-closed DTEs’ πfπ*
bands gradually red-shift by 16-20 nm (Figure 1b), resulting from
the conversion of the ring-open isomer to a changing mixture of two
ring-closed isomers (1oc and 1cc). The ring-closing of only one DTE
unit (1oo f 1oc) can be observed in the early stages of the irradiation,
where isosbestic points at 307, 327, and 364 nm exist.10 These
isosbestic points disappear with further irradiation once the more
complex equilibrium of isomers is established (1oo / 1oc / 1cc).
Ring-opening of all compounds is achieved by irradiation with visible
light (typically at wavelengths greater than 470 nm).
The sequential ring-closing (1oo f 1oc f 1cc) is supported by 1H
and 31P NMR spectroscopy, which both show the formation of
significant amounts of 1oc prior to 1cc, ultimately producing a
photostationary state (PSS) consisting of 80% 1cc and 20% 1oc. A
plot of the relative concentrations of each component (31P NMR)
against time is consistent, qualitatively, with consecutive pseudo-first-
order kinetics (Figure 1c).10 The NMR studies also show that the
chemical shifts for the phosphorus and the thienyl proton closest to
the metal move downfield as 1oo is converted to 1oc and then to 1cc10
signifying increased donation of electron density from the phosphine
to Pt in the case of the 31P signals, and decreased electronic shielding
from the alkyne due to a withdrawing of electron density by the
adjacent ring-closed DTE for the 1H signals. These effects will become
important when explaining the electrochemical properties later in this
report.
Compound 1oo is prepared by coupling two acetylene-terminated
DTE photoswitches to trans-Pt(PBu3)2Cl2.10 The absorption spec-
trum of 1oo in CH2Cl2 (Figure 1a) contains a band at 280 nm
corresponding to the πfπ* thienyl transitions found in DTE 2o
and another at 350 nm assigned to a long-axis πfπ* transition
involving both the metal and alkynyl ligands.9
Whereas photocyclization of DTE 2o is accomplished by irradiation
with light of wavelengths shorter than 340 nm, complex 1oo also
photocyclizes when it is irradiated at wavelengths as long as 415 nm.10
The 20 nm red shift that appears in the absorption spectra when
1oo is converted to the PSS is attributed to increased delocalization
of the singlet state over both DTE units in 1cc relative to 1oc. This is
in agreement with other reports that show singlet state delocalization
† University of British Columbia.
‡ Simon Fraser University.
§ Bowdoin College.
9
16644 J. AM. CHEM. SOC. 2009, 131, 16644–16645
10.1021/ja907434x CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. (a) UV-vis absorption spectra of CH2Cl2 solutions of 1oo and 2o, and the photostationary states generated when solutions of 1oo and 2o are
irradiated with 365 and 254 nm light, respectively. (b) Changes in the UV-vis absorption spectra of a CH2Cl2 solution of 1oo (2.3 × 10-5 M) when it is
irradiated with 365 nm light. (c) Relative percentages of 1oo (9), 1oc (O), and 1cc ([) as determined by 31P NMR spectroscopy when a CD2Cl2 solution
of 1oo is irradiated with 365 nm light. (d) Changes in the differential pulse voltammograms of a CH2Cl2 solution of 1oo as it is irradiated with 365 nm light.
(e) vis-NIR absorption spectra of a CH2Cl2 solution of 1oo/1oc (black) and 1oc/1cc (blue) before oxidation and after 1 equiv of oxidant is added to generate
1oo+/1oc+ (dash) and 1oc+/1cc+ (red).
in Pt-bis(acetylide)s spanning the entire system, including several repeat
units in oligomeric and polymeric analogues.11 DFT-calculated orbital
plots of 1oo, 1oc, and 1cc suggest that electron density in the HOMO
is limited to the metal and its two proximate alkynyl thiophene rings
in 1oo but extends over both DTE thiophenes when ring-closed. In
1cc, the HOMO is delocalized over the entire molecule with significant
orbital density on both DTEs and at the metal.10
the Pt linkage slows the rate of ET to the lowest energy excited state.
Ring-closing must proceed at a faster rate than ET to the remaining
closed DTE. Without the metal, the triplet state is inaccessible and
delocalization in the singlet state prevents ring-closing of adjacent
DTEs.5b
Our system represents a unique approach to photoswitching
conjugated oligomers by taking advantage of heavy-metal-induced
population of the triplet manifold, from which both DTEs are
independently photoactive. The result is a fully conjugated system with
electronic communication in the ground state. Analogous DTE-Pt-
bis(acetylide) conjugated polymers should enable photomodulation of
conductivity.
Acknowledgment. This research was supported by the Natural
Sciences and Engineering Research Council (NSERC) of Canada,
the Canada Research Chairs Program, the University of British
Columbia, and Simon Fraser University. The authors thank Dr.
Hema D. Samachetty for help with the kinetic analysis.
Differential pulse voltammetry of 1oo shows a single oxidation wave
at 1.00 V (vs SCE) corresponding to oxidation of the ring-open DTE
(Figure 1d), less positive than that of 2o due to Pt coordination. When
1oo is converted to 1oc, two new waves (0.65, 0.90 V) assigned only
to the ring-closed DTE appear, while the remaining ring-open isomer
of 1oc is likely oxidized at potentials more positive than the solvent’s
limit.10 The fully ring-closed isomer (1cc) exhibits a single oxidation
wave at 0.76 V. Because the ring-closed DTE isomer is a stronger
π-acid than its ring-open counterpart, it accepts more electron density
via backbonding from the Pt in 1oc explaining the cathodic shift of
the first oxidation potential of 1oc relative to 1cc. This is supported
by DFT calculations, which estimate PtsC and CtC bond lengths
that indicate greater backbonding to the ring-closed isomer, and by
IR data, which show a red shift of the acetylide stretch upon ring-
closing.10 The first reaction (1oo f 1oc) produces a large red shift
(∼24 cm-1) due to π-backbonding and the π-acidity of the adjacent
ring-closed DTE in 1oc. In 1cc, both ring-closed DTEs compete for
electron density through a conjugated system that includes the metal
center resulting in a smaller red shift (∼4 cm-1) for the second reaction
(1ocf1cc).
Ground state electronic communication in 1cc is illustrated by
comparing the vis-near-IR spectra of each system after it is chemically
oxidized with 1 equiv of [(4-Br-C6H5)3N][SbCl6].10 Oxidation of a
solution containing only 1oo and 1oc results in the disappearance of
the πfπ* band of 1oc (Figure 1e). The oxidation of a solution of the
PSS generates several new bands, notably one in the near-IR region
(λmax ) 1301 nm) assigned as an intervalence charge-transfer (IVCT)
transition, the result of electronic coupling between the DTEs in 1cc+
to give optically induced exchange of the electron-hole pair.12 The
lack of an analogous low energy band in the spectrum of 1oc+ indicates
that the cation is more localized in this species than in 1cc+. No near-
IR absorption bands are observed for any of the species when 2 equiv
of oxidant are added.
Supporting Information Available: Syntheses and characterizations
of all chromophores. This material is available free of charge via the
Internet at http://pubs.acs.org.
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