Millisecond long-lived charge separated state at room temperature in a flexibly linked diphenylaminopolyene-C60 dyad

Article abstract of DOI:10.1039/b410201k

Long-lived photoinduced charge separation involving one-step electron transfer is achieved in diphenylaminopolyene based C₆₀-donor dyads with a short, flexible linkage. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b410201k

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Millisecond long-lived charge separated state at room temperature in a
flexibly linked diphenylaminopolyene-C60 dyad
Yanong Han,{ Laura Dobeck, Aijun Gong, Fanqing Meng, Charles W. Spangler and Lee H. Spangler*
Received (in Columbia, MO, USA) 7th July 2004, Accepted 4th November 2004
First published as an Advance Article on the web 11th January 2005
DOI: 10.1039/b410201k
The successful synthesis of a new kind of Wittig salt (compound 1,
Scheme 1) provides an easy way to obtain DPAP-C60 dyads in
good yield. The dyads were characterized by 1H NMR, 13C NMR
and MALDI-TOF MS apparatus. The absorption bands of the
C60 and DPAP moieties are present in the spectra of the linked
compounds D1 and D2.
Long-lived photoinduced charge separation involving one-step
electron transfer is achieved in diphenylaminopolyene based
C60-donor dyads with a short, flexible linkage.
Extensive research has been directed towards the synthesis of
electron donor–acceptor linked compounds in the hope of finding
systems which will form long-lived charge separated (CS) states
with high quantum efficiency, as is achieved in the photosynthetic
reaction.1–5 The lifetimes of CS states are sensitive to the energy of
the CS state, the type of linkage between the donor and acceptor
molecules, and also the molecular topology and distance and
orientation of the donor and acceptor moieties.6–11 Large efforts
have been made to achieve a long-lived CS state by tuning these
factors with the longest currently reported CS state lifetime of a
dyad at room temperature solution being 260 ms for a zinc
imidazoporphyrin-C60 dyad.6 Relatively unexplored is CS lifetime
dependence on the flexibility of the linkage between the donor and
acceptor in the dyad.
The photoexcitation of both D1 and D2 in deaerated
benzonitrile (BN) solution with 360 nm laser light results in
fluorescence with the emission maximum at 540 nm. The emission
intensities of D1 and D2 are quenched 35- and 9-fold respectively,
compared to emission from the reference compound DPAP. This
indicates that the excited state of DPAP is quenched efficiently by
linked C60. The emission intensity of the single armed D1 is four
times smaller than that of D2 at the same DPAP concentration.
The fluorescence decay of the DPAP unit in BN solution was
monitored from 415 to 715 nm after excitation with 400 nm light
from a frequency doubled ultrafast Ti : sapphire laser (76 MHz,
150 fs pulse width). The fluorescence lifetime of the unlinked
DPAP (t0 5 1.8 ns) was significantly reduced in D1(tf 5 9 ps).
This is likely due to ET from the excited state of DPAP (DPAP*)
to C60. The rate constant of forward ET (ket) from DPAP* to C60
was estimated from ket 5 1/tf 21/t0, where t0 is the fluorescence
lifetime of the DPAP unit, and tf is that in the linked compound.
Among a wide variety of donor molecules available for linking
covalently to C60, the triphenylamine group is one with good
electron donating ability to excited state C60 molecules.12 We
report herein photoinduced intramolecular electron transfer (ET)
processes in diphenylaminopolyene (DPAP) based C60-donor
dyads (DPAP-C60) with a short, flexible linkage. The decay
mechanism of the CS state was elucidated with the use of nano-
second laser flash photolysis. Dual decay components of the CS
state were observed, with first order lifetimes of about 300 ns and
1 ms respectively. To our knowledge, this millisecond slow
component lifetime is the longest first order lifetime at room
temperature ever reported for intramolecular charge recombina-
tion (CR) in donor–acceptor dyad systems involving one-step
electron transfer. A new method was introduced to simultaneously
analyze the backward electron transfer (BET) process
which involves both intra- (first order) and intermolecular (second
order) CR.
The ket value of D1 was evaluated to be 1.1 6 1011 s21
.
The time profile of the fluorescence intensity of the DPAP* in
D2 shows biexponential decay, with fluorescence lifetimes of 20
and 1800 ps respectively. This may be due to the restriction that
only one DPAP* could be quenched by C60 if both arms are
photoexcited. The ket value from DPAP* to C60 for D2 was
estimated to be 4.9 6 1010 s21
.
Transient absorption spectra of DPAP-C60 were carried out
following nanosecond laser excitation at 532 nm. Only absorption
One and two armed DPAP-C60 dyads (D1 and D2) were
synthesized as shown in Scheme 1. Anhydrous tetrahydrofuran
(THF) was used freshly distilled from sodium and benzophenone-
dried tetrahydrofuran. Other solvents were used as received. Wittig
reaction methodologies were used to extend the conjugated lengths
of these molecules. The donor chromophores were synthesized
with a free OH functionality for attachment to malonyl dichloride.
The resulting diester was then coupled to C60 using Hirsh–Bingel
methodology, yielding the target dyads as dark solids in high yield.
{ Present address: Yatai Technologies Ltd., Tianzhu Road, Hefei, Anhui
230088, China.
*spangler@montana.edu
Scheme 1 Synthesis of donor chromophores and dyads.
Chem. Commun., 2005, 1067–1069 | 1067
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Fig. 2 Plot of d[CS]/dt vs. [CS] for 0.2 mM D2 charge separation state
decay in deaerated BN. Circles: experimental data resulting from different
pump powers (0.5–5 mJ); solid line: fit with second order polynomial.
to first order at low concentration (t . 500 ms), and at high
concentration it is significantly affected by intermolecular BET.
The decay behavior follows:
d½CSꢀ
~{k1st½CSꢀ{k2nd½CSꢀ½CSꢀ
dt
where k1st (51/tlifetime) and k2nd are intra- and intermolecular decay
rate constants respectively. The dependence of the charge
separation (CS) state decay rate on [CS] was examined by
employing different laser powers over a wide range and it exhibits
a good second order polynomial fit (see Fig. 2). This fitting process
gives us values for both first and second order rate constants, so we
can calculate the lifetimes expected for the first order process alone.
The first order lifetimes of D1 and D2 were determined to be 1.1
and 0.7 ms, respectively. These values are approximately two times
larger than what we get from single exponential fitting at low
concentrations and long times, since the intermolecular BET
contribution is still present at the lowest concentrations used.
To our knowledge, the millisecond lifetime is the longest CS
lifetime in solution ever reported for intramolecular charge
recombination at room temperature. Evolution of the 8800 cm21
CS band shape over a 300 ns timescale suggests conformational
changes resulting in some solvent separation of the linked DPAP+
and C602. This may lead to a new synthetic strategy to attain long
lifetime charge separation donor–acceptor dyads by using flexible
linkages and controlling the folding. Further research is under way
to probe this point.
Fig. 1 Top: photoinduced absorption (PIA) spectra obtained by using
532 nm laser excitation of DPAP-C60 in deaerated BN, compared with the
chemically induced absorption (CIA) spectrum of the cation generated by
doping the donor with SbCl5. Bottom: time profiles (ms range) of DPAP-
C60 charge separated state decay. Inset are the ns range time profiles.
3
*
of the C60 moiety was detected at 740 nm in deaerated toluene.
From the decay of the time profile, the lifetime of DPAP-3C60* was
evaluated to be 18 ms, which is close to that of pristine C60. It
*
indicates that the intersystem crossing process from DPAP-1C60
to DPAP-3C60* is the predominant one with no charge separation
in nonpolar solvents.
The CS state was detected through laser flash photolysis
(pumped by a 7 ns 532 nm laser) of DPAP-C60 (both D1 and D2)
in deaerated benzonitrile (BN) solution as shown in Fig. 1.
Following the excitation, an absorption band at 8800 cm21 with a
shoulder at 9600 cm21 appears immediately. This is attributable to
the DPAP cation and fullerene anion. The transient absorption in
the near visible region (15 000 cm21 band) is also due to DPAP+.
The photoinduced DPAP+ spectrum is confirmed by doping the
donor with a strong oxidizing agent (SbCl5) to chemically generate
the cation.
We would like to acknowledge DOE grant DE-FG02-
01ER45869 and ARO grant DAAD 19-00-1-0162 for financial
support and Reed Howald for helpful discussions.
Yanong Han,{ Laura Dobeck, Aijun Gong, Fanqing Meng,
Charles W. Spangler and Lee H. Spangler*
Department of Chemistry, Montana State University, Bozeman,
MT 59717, USA. E-mail: spangler@montana.edu; Fax: 01 406 994 5407;
Tel: 01 406 994 4399
The temporal decay profiles of both D1 and D2 show two
obvious components. The time constant of the fast component is
about 330 ns, while that of the slow component is about 1 ms. The
ratios of populations of the slow and fast components are about
0.15 and 0.2 for D1 and D2 respectively, as shown in Fig. 1.
The CS states decay through back electron transfer (BET) to the
ground state. The slow component decays for both D1 and D2 on
the ms timescale are neither strictly first order nor second order, as
shown in Fig. 1. The BET occurs through two processes: it is close
Notes and references
1 (a) M. R. Wasielewski, Chem. Rev., 1992, 92, 435; (b) F. D. Lewis,
X. Liu, J. Liu, S. E. Miller, R. T. Hayes and M. R. Wasielewski, Nature,
2000, 406, 51.
1068 | Chem. Commun., 2005, 1067–1069
This journal is ß The Royal Society of Chemistry 2005

View Article Online
2 (a) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 1993, 26,
198; (b) C. C. Page, C. C. Moser, X. Chen and P. L. Dutton, Nature,
1999, 402, 47.
3 (a) S. Fukuzumi and D. M. Guldi, in Electron Transfer in Chemistry, ed.
V. Balzani, Wiley-VCH, Weinheim, 2001, vol. 2, pp 270–337.
4 K. Ohkubo, H. Imahori, J. Shao, Z. Ou, K. M. Kadish, Y. Chen,
G. Zheng, R. K. Pandey, M. Fujitsuka, O. Ito and S. Fukuzumi,
J. Phys. Chem. A, 2002, 106, 10991.
9 N. Martin, L. Sanchez, B. Illescas and I. Perez, Chem. Rev., 1998, 98,
2527.
10 (a) H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito,
Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 2607; (b)
D. M. Guldi, C. Luo, T. Da Ros, M. Prato, E. Dietel and A. Hirsch,
Chem. Commun., 2000, 375; (c) S. Fukuzumi, H. Imahori, H. Yamada,
M. E. El-Khouly, M. Fujitsuka, O. Ito and D. M. J. Guldi, J. Am.
Chem. Soc., 2001, 123, 2571.
5 C. Luo, D. M. Guldi, H. Imahori, K. Tamaki and Y. Sakata, J. Am.
Chem. Soc., 2000, 122, 6535.
11 (a) F. D’Souza, G. R. Deviprasad, M. S. Rahman and J.-P. Choi,
Inorg. Chem., 1999, 38, 2157; (b) T. Da Ros, M. Prato, D. M. Guldi,
E. Alessio, M. Ruzzi and L. Pasimeni, Chem. Commun., 1999, 635; (c)
F. D’Souza, N. P. Rath, G. R. Deviprasad and M. E. Zandler,
Chem. Commun., 2001, 267; (d) F. D’Souza, G. R. Deviprasad,
M. E. El-Khouly, M. Fujitsuka and O. Ito, J. Am. Chem. Soc., 2001,
123, 5277; (e) F. D’Souza, G. R. Deviprasad, M. E. Zandler,
M. E. El-Khouly, M. Fujitsuka and O. Ito, J. Phys. Chem. B, 2002,
106, 4952.
6 H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. Luo, Y. Sakata
and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 6617.
7 Y. Kashiwagi, K. Ohkubo, J. A. McDonald, I. M. Blake, M. J. Crossley,
Y. Araki, O. Ito, H. Imahori and S. Fukuzumi, Org. Lett., 2003, 5,
2719.
8 (a) D. M. Guldi, M. Maggini, G. Scorrano and M. Prato, J. Am. Chem.
Soc., 1997, 119, 974; (b) P. A. Liddell, D. Kuciauska, J. P. Sumida,
B. Nash, D. Nguyen, A. L. Moore, T. A. Moore and D. Gust, J. Am.
Chem. Soc., 1997, 119, 1400.
12 Y. Han and L. H. Spangler, J. Phys. Chem. A, 2002, 106,
1701.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1067–1069 | 1069