Highly efficient photoproduction of charge-separated states in donor-acceptor-linked bis(acetylide) platinum complexes

Article abstract of DOI:10.1021/ja904241r

(Chemical Equation Presented) We report a highly efficient charge separation system, D-Pt-A, where D (triphenylamine) and A (naphthalenediimide) are bonded to the Pt moiety through highly twisted phenylene ethynylene linkages. The quantum yields for the f

Full text of DOI:10.1021/ja904241r

Published on Web 07/09/2009
Highly Efficient Photoproduction of Charge-Separated States in
Donor-Acceptor-Linked Bis(acetylide) Platinum Complexes
Shuichi Suzuki,† Ryoji Sugimura,† Masatoshi Kozaki,† Kazutoshi Keyaki,‡ Koichi Nozaki,*,‡
Noriaki Ikeda,§ Kimio Akiyama,*,| and Keiji Okada*,†
Department of Chemistry, Graduate School of Science, Osaka City UniVersity, Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan, Department of Chemistry, Graduate School of Science and Engineering, UniVersity of
Toyama, 3190 Gofuku, Toyama 930-8555, Japan, Department of Chemistry, Graduate School of Science, Osaka
UniVersity, Machikaneyama, Toyonaka, Osaka 560-0043, Japan, and Institute of Multidisciplinary Research for
AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan
Received May 25, 2009; E-mail: nozaki@sci.u-toyama.ac.jp; akiyama@tagen.tohoku.ac.jp; okadak@sci.osaka-cu.ac.jp
In the 21st century, research in pure and applied science has
mainly been focused on discovering artificial systems that mimic
natural photosynthetic systems.1 Recently, considerable advance-
ments in multichromophoric systems, including multistep electron
transfer, have been made.2 However, these systems do not always
exhibit efficient, long-lived charge-separated (CS) states. The most
promising strategy is a spin-control approach using a chromophore
undergoing rapid intersystem crossing.3 Metal (Ru, Ir, Pt) complexes
that exhibit intense phosphorescence at room temperature via strong
spin-orbit coupling (SOC) are desirable for this purpose.4 Such
complexes provide a new route for generating triplet dyes, as has
been recently demonstrated.5 This approach can be applied to CS
systems.6 Eisenberg and co-workers showed that some D-Pt-A
complexes, in which Pt is linked with an electron donor (D) and
acceptor (A), undergo photoinduced charge separation to produce
the CS state D+-Pt-A-.7 The CS-state formation efficiency was
estimated as 10-30% in some cases,7b with a lifetime of 75-230
ns.7 This low efficiency was attributed to the occurrence of rapid
electron back-transfers in partial CS states (D+-Pt--A or
D-Pt+-A-).7b Further, the identification of the spin state of
D+-Pt-A- has not been reported. Here we report a highly efficient
system (ΦCS ) 0.96-0.97) with clear identification of the long-
lived CS state.
energy transfer from the locally excited triplet state of the Pt
chromophore (3LEPt) occurs.
Figure 1. Molecular structures of 1, 2, and 3 and UV-vis and (inset)
phosphorescence spectra of 1 (red line) and model complex 3 (black dashed
line) in toluene at room temperature.
1 showed the following redox potentials (in V vs Fc/Fc+) in
CH2Cl2: E1/2(MTA/MTA+) ) +0.16; Ep(Pt/Pt+) ) +0.92; E1/2
(MNDI/MNDI-) ) -1.06; E1/2(MNDI-/MNDI2-) ) -1.51; Ep(Pt/
Pt-) ) -1.89. The CS state energies (ECS) in solvents of different
polarities were evaluated as follows using the dielectric continuum
model:9 1.87 eV in toluene; 1.18 eV in THF; 0.95 eV in
butyronitrile (Tables S1 and S2 and Figure S1 in the Supporting
Information). The energy of 3LEPt estimated from the 0-0 transition
in the phosphorescence spectrum of 3 in glassy MTHF (2.48 eV,
Figure S2) is higher than that of 3LEMNDI (∼2.0 eV)6c,10 but lower
The new triads MTA-Pt-MNDI (1) and MTA-Pt-NDI (2)
contain dimethoxydimethyltriphenylamine (MTA) as the donor and
a naphthalene diimide [(M)NDI] as the acceptor. These chro-
mophores were bonded to the Pt moiety through highly twisted
phenylene ethynylene linkages. MTA+ and (M)NDI- exhibit strong,
well-characterized absorptions in different wavelength regions (see
below); this enables the detection and quantification of the CS states.
than that of LEMTA (2.88 eV).11
3
Figure 2a shows transient absorption spectra of 1 in toluene at
room temperature under 400 nm laser-pulse excitation (400 fs
fwhm). Immediately after excitation (1 ps), a broad peak with λmax
) 650 nm was observed. A similar 650 nm absorption was observed
in the transient spectra of 3 (Figure S3); this indicates that the 650
nm absorption corresponds to the T-T absorption of 3LEPt induced
by very fast intersystem crossing in 1LEPt. Within 1 ns, the initially
observed 650 nm absorption disappeared, and new absorptions with
maxima at 470, 610, and 750 nm appeared. The 470 and 610 nm
absorptions are assigned to MNDI-, while the 750 nm absorption
is assigned to MTA+; these radical ions were independently
generated by electrochemical reduction and oxidation of 1 (Figure
S4). Thus, the transient spectrum after 1 ns is clearly assigned to
the CS state.
1 and 2 exhibit comparable spectral and redox properties. The
UV-vis spectrum of 1 in toluene shows the longest absorption
band (MMLL′CT,8 λmax ) 430 nm) attributed to the Pt chromophore
(Figure 1). The shorter absorption bands are attributed to MTA
(λmax ) 300 nm) and MNDI (λmax ) 380 nm). These absorptions
were almost superimposed with those of their components (D, A,
and Pt), suggesting negligible electronic interactions between them
in the ground state. The model compound 3 shows strong
phosphorescence (λem ) 560 nm) at room temperature in solution
under deaerated conditions, while 1 shows considerably weaker
phosphorescence (Figure 1, inset), suggesting that fast electron or
† Osaka City University.
‡ University of Toyama.
§ Osaka University.
A priori, two routes of for CS-state formation are possible: (A)
electron transfer via a triplet state of the acceptor (3LEPt f 3LEMNDI
| Tohoku University.
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10.1021/ja904241r CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
442 nm laser-pulse excitation in MTHF at room temperature, 1
showed a polarized spectrum characteristic of the spin-correlated
radical pair (SCRP) state14 with an AEAEAE pattern (Figure 2c;
Figure S6 shows the spectra in other solvents). The observed phase
pattern was reproduced by assuming a triplet precursor (3LEPt) of
the SCRP state, a positive exchange interaction (J ) +0.05 mT)15
in the CS state, and the following radical ion parameters: g ) 2.0035
16
for MNDI- and 2.0032 for MTA+; aN ) 0.9 mT.17
In conclusion, we have developed a highly efficient CS system
with a long lifetime for the CS state (SCRP state) using D-Pt-A
complexes with highly twisted π linkers; this approach allows
efficient formation of D+-Pt-A-, in which the SOC effect is
minimized by the elimination of the unpaired electron from the Pt
moiety, giving rise to a long-lived CS state. Further studies,
including temperature and medium effects on the lifetime of the
CS states and molecular design to control the SOC effect, are in
progress.
Figure 2. (a) Picosecond and (b) nanosecond transient spectra of 1 in
toluene. (c) Time-resolved EPR spectrum recorded in direct detection mode
(A, absorption; E, emission) after laser-pulse irradiation (442 nm) of 1 in
MTHF at room temperature; the red dashed line is the simulated spectrum
for the SCRP state.
Acknowledgment. This work was partially supported by a
Grant-in-Aid (20750034) from MEXT, Japan.
Supporting Information Available: Detailed synthetic methods for
1 and 2, Tables S1-S3, and Figures S1-S6. This material is available
free of charge via the Internet at http://pubs.acs.org.
f CS) and (B) sequential electron transfer via a partial CS state
(3LEPt f [D-Pt+-A- or D+-Pt--A] f CS).
Interestingly, the formation rates of MNDI- and MTA+ are
different and solvent-dependent. In toluene, τ470 ) 220 ps and τ750
) 250 ps (Figure 2a). In butyronitrile, τ470 ) 126 ps and τ750 ) 21
ps. In THF, there are two-component rises: τ470(1) ) 146 ps (77%),
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