Accessing the triplet excited state in perylenediimides

Article abstract of DOI:10.1021/ja800333y

Here, we present a strategy designed to permit access to the PDI triplet manifold that preserves the desirable colorfastness and visible light-absorption properties associated with these dyes. To this end, three new Pt(II) complexes each bearing two PDI moieties tethered to the metal center via acetylide linkages emanating from one of the PDI bay positions have been synthesized, structurally characterized, and thoroughly examined by nanosecond laser flash photolysis. Upon ligation, the bright singlet-state fluorescence of the PDI chromophore is quantitatively quenched, and no long wavelength photoluminescence is observed from the Pt(II)-PDI complexes in deaerated solutions. In each of the Pt-PDI chromophores, quantitatively similar transient absorption difference spectra were obtained; the only distinguishing characteristic is in their single-exponential lifetimes (τ = 246 ns, 1.0 μs, and 710 ns). Triplet-state sensitization experiments of free PDI-CCH using thioxanthone confirmed the PDI triplet state assignments in each of the Pt-PDI structures. Copyright

Full text of DOI:10.1021/ja800333y

Published on Web 02/13/2008
Accessing the Triplet Excited State in Perylenediimides
Aaron A. Rachford, Se´bastien Goeb, and Felix N. Castellano*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
Received January 15, 2008; E-mail: castell@bgsu.edu
Perylenediimides (PDIs) continue to emerge as quintessential
chromophores in a variety of photofunctional materials.1,2 In
addition to their desirable visible light-harvesting and impressive
fluorescence properties, PDIs have established niche applications
in chemical sensing,3 organic field-effect transistors,4,5 light-emitting
diodes,6 and photovoltaics (PVs).5,7 Given recent reports highlight-
ing how polymeric PV performance is enhanced by utilizing long-
lived triplet excited states in the photoinduced charge separation
step,8,9 access to the PDI triplet state clearly represents a desirable
goal for solar energy conversion using these molecules. Despite
recent attempts,10 the PDI triplet state remains accessible only
through bimolecular sensitization11 or in sophisticated molecular
architectures involving a cascade of nonradiative steps.2,12-14 Here,
we designed molecules 1-3 in an effort to efficiently generate the
triplet state of PDI following photoexcitation in three distinct PtII
square planar structures. The platinum-acetylide motif was selected
as this combination is known to induce strong spin-orbit coupling
facilitating a rapid (kisc > 1011 s-1) singlet f triplet intersystem
crossing in the appended organic chromophore.9,15 Structures 2 and
3 examine the influence of cis- vs trans-PDI-acetylide ligation
whereas 1, which inherently possesses a more complex electronic
structure, assesses the consequence of positioning a triplet charge
transfer (3CT) excited state energetically proximate to the PDI
triplet.
Figure 1. (A) Absorption spectra of 1, 2, and 3. (B) Absorption and
fluorescence spectra of PDI-CCH. All spectra were collected in CH2Cl2.
CCH; however, integration of each absorption spectrum yielded
the expected 2:1 ratio (based on number of PDIs) for complexes 2
and 3 relative to PDI-CCH. A significant increase in extinction
coefficient is observed for 1 in the 375-500 nm region which
results from the overlapping CT transition, thereby rendering a
greater than 2:1 integrated absorption ratio relative to PDI-CCH.
PDI-CCH exhibits strong fluorescence (Φ ) 0.91) centered at 537
nm with a lifetime of 4.53 ns, Figure S1. Upon ligation via an
acetylide bridge in the bay region to PtII, the fluorescence is
quantitatively quenched in 1-3, and unfortunately, no correspond-
ing phosphorescence could be detected in the visible or near-IR
region under degassed conditions. However, all three complexes
1
sensitize O2 emission at 1270 nm in aerated CH2Cl2 with visible
light excitation, Figure S2. This result indirectly suggests that the
lowest energy excited state in these molecules can be assigned to
a triplet. It also stands to reason that the excited state in 1 is likely
similar to that in 2 and 3 due to a lack of any photoluminescence
emanating from these samples. Hence, nanosecond transient absorp-
tion spectroscopy was used to establish the nature of the lowest
triplet state in 1-3.
The transient absorption difference spectra of 1, 2, and 3 are
presented in Figure 2, panel A, B, and C, respectively, following
450 ( 2 nm pulsed laser excitation in CH2Cl2 at room temperature;
the single-exponential lifetimes measured in each case are indicted
on each panel. All major features of the three difference spectra
agree remarkably well with each other. Select single wavelength
kinetic traces and their respective fits with residuals are provided
as SI. The transient spectrum observed in each case appeared
promptly within the instrument response so no information could
be obtained on the spectral evolution of the transients. The major
visible absorption features measured in all three complexes are
qualitatively similar and red-shifted relative to bimolecular triplet-
state sensitization experiments performed on PDI-CCH using
thioxanthone as the sensitizer, Figure S3. Taken together, the
Complete synthetic and structural characterization details of 1-3
are provided as Supporting Information (SI). Briefly, treatment of
PDI-CC-TMS16 with NaOH in methanol produced the terminal
acetylene (PDI-CCH) which was reacted with the appropriate Pt-
(II) chloride precursor in CH2Cl2 in the presence of i-Pr2NH and
CuI, affording high yield (>75%) chloride-to-acetylide metathesis
in each case. All compounds were structurally characterized by
NMR, MALDI-MS, and elemental analysis.
Figure 1 presents the absorption spectra of 1, 2, and 3 as well as
the absorption and emission spectra of the PDI acetylide precursor,
PDI-CCH. All three PtII complexes exhibit absorptions dominated
by the π-π* transitions of the PDI moiety with maxima between
570 and 573 nm (ꢀ ≈ 65 500-68 000 M-1 cm-1). These absorptions
are significantly red-shifted relative to that of the PDI-CCH
chromophore (529 nm, ꢀ ) 80 000 M-1 cm-1), as is typically
observed when a conjugated terminal acetylene produces the
corresponding Pt-acetylide chromophore.15,17 Surprisingly, the
extinction coefficients of 1-3 are not double that of the PDI-
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J. AM. CHEM. SOC. 2008, 130, 2766-2767
10.1021/ja800333y CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
ingly, the excited state lifetime of 3 (τ ) 710 ns) lies between that
of 1 and 2 which implies that stereochemistry and the nature of
the coordinated ligands profoundly influence the excited state
dynamics in these PDI-acetylide chromophores.
We have directly observed the triplet state of the PDI chro-
mophore by establishing a PtII-acetylide linkage in its bay region.
The singlet fluorescence of the PDI moieties is quantitatively
quenched as a result of this linkage, producing characteristic
transient absorption difference spectra with concomitant long
lifetimes ranging from 246 ns to 1.0 µs. Each of these nonlumi-
1
nescent compounds readily sensitizes O2 emission in the near-IR
when excited with visible light in the presence of dissolved O2.
The current work represents an efficient starting point toward
intramolecularly accessing the long-lived PDI triplet state, proven
elusive in the past. Armed with the present strategy, polymer solar
cells harnessing the PDI triplet state now appear viable, but its broad
application in photonics and photofunctional materials also seems
imminent.
Acknowledgment. The authors thank Dr. Xianghuai Wang for
helpful discussions. This work was supported by the Air Force
Office of Scientific Research (FA9550-05-1-0276), the National
Science Foundation (CHE-0719050), the ACS-PRF (44138-AC3),
and the BGSU Research Enhancement Initiative.
Figure 2. Transient absorption difference spectra of 1 (A), 2 (B), and 3
(C). All transients were collected in deaerated CH2Cl2 at room temperature
with λex ) 450 nm (2.5-3 mJ/pulse). Single-exponential decay times are
indicated within each panel.
Supporting Information Available: Synthesis, experimental de-
tails, additional kinetic analyses and spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
experimental data support the conclusion that a long lifetime PDI-
localized triplet state is readily accessible and populated in 1-3.
The transient absorption spectrum of 1 (Figure 2A) does not
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