Remote sensitized photoisomerization via through-bond triplet-triplet energy transfer mediated by a salt bridge in a supramolecular dyad

Article abstract of DOI:10.1002/cphc.200900556

A supramolecular dyad, BP-(amidinium-carboxylate)-NBD is constructed, in which benzophenone (BP) and norbornadiene (NBD) are connected via an amidinium-carboxylate salt bridge. The photophysical and photochemical properties of the assembled BP-(amidinium-

Full text of DOI:10.1002/cphc.200900556

DOI: 10.1002/cphc.200900556
Remote Sensitized Photoisomerization via Through-Bond
Triplet–Triplet Energy Transfer Mediated by a Salt Bridge
in a Supramolecular Dyad
[
a]
[a]
[b]
[a]
[a]
[a]
Lei Han, Hengxing Wei, Shayu Li, Jinping Chen,* Yi Zeng, Ying-Ying Li,
[a]
[a]
[b]
[b]
Yongbin Han, Yi Li,* Shuangqing Wang, and Guoqiang Yang*
A supramolecular dyad, BP-(amidinium-carboxylate)-NBD is
constructed, in which benzophenone (BP) and norbornadiene
clane (QC). All of these observations suggest that the triplet–
triplet energy transfer occurs efficiently in the BP-(amidinium-
carboxylate)-NBD salt bridge system. The triplet–triplet energy
transfer process proceeds with efficiencies of approximately
(NBD) are connected via an amidinium-carboxylate salt bridge.
The photophysical and photochemical properties of the assem-
bled BP-(amidinium-carboxylate)-NBD dyad are examined. The
phosphorescence of the BP chromophore is efficiently
quenched by the NBD group in BP-(amidinium-carboxylate)-
NBD via the salt bridge. Time-resolved spectroscopy measure-
ments indicate that the lifetime of the BP triplet state in BP-
3
ꢀ1
7
ꢀ1
0.87, 0.98 and the rate constants 1.8ꢀ10 s , and 1.3ꢀ10 s
at 77 K and room temperature, respectively. The mechanism
for the triplet–triplet energy transfer is proposed to proceed
via a “through-bond” electron exchange process, and the non-
covalent bonds amidinium-carboxylate salt bridge can mediate
the triplet–triplet energy transfer process effectively for photo-
chemical conversion.
(amidinium-carboxylate)-NBD is shortened due to the quench-
ing by the NBD group. Selective excitation of the BP chromo-
phore results in isomerization of the NBD group to quadricy-
Introduction
In photosynthesis, sunlight energy is converted into a chemical
potential with unit efficiency. In the process of photochemical
conversion, the photoinduced electron/energy transfer process
plays an important role and has been inspiring intense stud-
troscopic determination for proton position in the proton-cou-
pled electron transfer pathways between purpurin/chlorine
[7]
and naphthalenediimide. Recently, the singlet energy transfer
between Zn-porphyrin and free base porphyrin dyads mediat-
ed by the amidinium–carboxylate salt bridge has been report-
ed by Otsuki, suggesting a through-bond mechanism via the
[
1]
ies. It has been demonstrated that non-covalent interactions,
such as hydrogen bonds and salt bridges exist extensively in
biological structures and play remarkable roles in many natural
processes including energy transfer and electron transfer. To
mimic natural processes, various supramolecular systems as-
[8]
salt bridge interface.
Compared with the research of electron transfer and singlet
energy transfer in salt bridge systems, the role of the amidini-
um–carboxylate salt bridge in triplet–triplet energy transfer
processes has never been investigated although it is very im-
portant. As we know, the triplet–triplet energy transfer is the
most common and important type of energy transfer involved
[
2]
[3]
sembled via hydrogen bonds or salt bridges have been de-
veloped. As the simple model, the energy/electron donor-ac-
ceptor pairs are constructed using the non-covalent interac-
tions to confirm whether or not these non-covalent bonds can
[
4]
[1a]
mediate energy/electron transfer. Among these, salt bridges
is particularly attractive given their remarkable role in stabiliz-
ing of the biologic structures including RNA stem loops, zinc
finger/DNA complexes and the active site of dihydrofolate re-
in chemical and biochemical processes. The mechanism for
[
a] Dr. L. Han, H. Wei, Dr. J. Chen, Dr. Y. Zeng, Dr. Y.-Y. Li, Dr. Y. Han,
Prof. Dr. Y. Li
[
5]
ductase. In particular, the salt bridge formed between carbox-
ylate and amidinium/guanidinium via both hydrogen bonds
and electrostatic interaction bearing strong associating force is
stable enough for investigation even in polar or protonic sol-
vents. Unlike the guanidinium-carboxylate salt bridge afforded
by Arg–Asp, the amidinium–carboxylate salt bridge shows only
one specific binding mode and can be used as a simple model
to investigate the energy/electron transfer processes. The rep-
resentational study of Nocera and his co-workers shows that
the proton-coupled electron transfer process can be signifi-
Key Laboratory of Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences, Beijing 100190 (P. R. China)
Fax: (+86)10-82543518
E-mail: chenjp@mail.ipc.ac.cn
yili@mail.ipc.ac.cn
[
b] Dr. S. Li, Dr. S. Wang, Prof. Dr. G. Yang
Beijing National Laboratory for Molecular Sciences (BNLMS)
Key Laboratory of Photochemistry, Institute of Chemistry
Chinese Academy of Sciences, Beijing 100190 (P. R. China)
Fax: (+86)10-82617315
E-mail: gqyang@iccas.ac.cn
[6]
cantly influenced by the amidinium–carboxylate salt bridge.
As a continuing research, they also supplied a method of spec-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.200900556.
ChemPhysChem 2010, 11, 229 – 235
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
229

J. Chen, Y. Li et al.
triplet–triplet energy transfer is usually described by Dexter
[
9]
electron-exchange interaction and may be visualized in terms
of two electron-transfer processes or one electron-transfer and
one hole-transfer processes. Our research group has examined
the triplet–triplet energy transfer in dendritic system extensive-
[10]
ly by the sensitized photoisomerization of norbornadiene. In
the present work, we create a system in which the energy
donor and acceptor are connected by an amidinium–carboxy-
late salt bridge. Benzophenone (BP) and norbornadiene (NBD)
are adapted as the energy donor and acceptor, which have
been used extensively in previous studies and are suitable for
[
1a,11]
Dexter triplet–triplet energy transfer.
The photoisomeriza-
tion of norbornadiene to quadricyclane (QC) is used as a probe
to detect the energy transfer occurrence. The structures of
model and target salt bridge systems are shown in Figure 1.
The BP chromophore can be selectively excited. After intersys-
Figure 2. Absorption spectra for BP–Am, NBD–COOH and the mixture of BP–
Am/NBD–COOH=1/1 in THF.
sum of the spectra of BP–Am and NBD–COOH almost coincides
with that of the mixture, suggesting no measurable ground-
state electronic interaction between BP–Am and NBD–COOH.
Significantly, the absorption of the BP group extends to a
longer wavelength than that of the NBD group, which sug-
gests that the singlet–singlet energy transfer from the excited
BP chromophore to the NBD group is endothermic and, conse-
quently, unlikely. Furthermore, this fact permits selective excita-
tion of the BP moiety above 350 nm in the salt bridge system.
No fluorescence was observed for both the target system
BP-(amidinium-carboxylate)-NBD and the donor model system
BP–(amidinium-carboxylate)–Ph with the excitation at 362 nm.
The phosphorescence emission spectra of BP–(amidinium-car-
boxylate)–NBD and BP–(amidinium-carboxylate)–Ph measured
in glassy 2-methyltetrahydrofuran (MTHF) at 77 K are shown in
Figure 3. All of them exhibit the phosphorescence characteris-
tic of benzophenone with maxima at 422, 453, 488 and
526 nm, while the overall intensities of the BP phosphores-
cence for BP–(amidinium-carboxylate)–NBD are significantly
lower than the corresponding model system of BP–(amidi-
nium-carboxylate)–Ph. As the ratio of NBD–COOH to BP–Am
changes from 1:1 to 2:1, the intensity of phosphorescence for
BP–(amidinium-carboxylate)–NBD decreases steeply, with the
quenching efficiency increasing from 14% to 40%. In the
Figure 1. Structures of the model system BP–(amidinium-carboxylate)–Ph
and the target system BP–(amidinium-carboxylate)–NBD.
tem crossing with 100% efficiency, the triplet energy of BP is
transferred to NBD, resulting in the isomerization of the latter
to the QC group. The efficiency and the absolute rate constant
of triplet–triplet energy transfer in salt bridge system are ex-
amined by steady-state and time-resolved spectroscopy.
Results and Discussion
Synthesis and Characterization of the Compounds
4
-Amidinium-benzophenone (BP-Am) was synthesized follow-
[
12]
ing the reported procedure by Garigipati.
Bicyclo-
[
2.2.1]hepta-2,5-diene-2,3-dicarboxylic acid [NBD–(COOH) ] was
2
13]
[
synthesized via the method reported by Alder.
2-
(
Methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-carboxylic
acid (NBD–COOH) and dimethyl bicyclo[2.2.1]hepta-2,5-diene-
,3-dicarboxylate (NBD–COOMe) were synthesized from NBD-
COOH) . The details of all synthesis and characterization are
2
(
2
described in the Experimental Section. All new compounds
1
13
were characterized by H NMR, C NMR, IR, and mass spec-
trometry (MALDI–TOF or EI).
Absorption and Phosphorescence Studies on the Triplet–
Triplet Energy Transfer
Figure 2 illustrates the absorption spectrum of the mixture of
BP–Am and NBD–COOH with the ratio of 1:1 in tetrahydrofuran
Figure 3. Phosphorescence spectra of the target and model systems in
ꢀ4
MTHF at 77 K (lex =362 nm, [PhCOOH]=[BP–Am]=1.0ꢀ10 m, [NBD–
ꢀ
4
ꢀ4
(THF), together with those of BP–Am and NBD–COOH. The
COOH]=1.0ꢀ10 and 2.0ꢀ10 m).
2
30
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ChemPhysChem 2010, 11, 229 – 235

Photoisomerizaiton of Norbornadiene
glassy solvent of MTHF at 77 K, the diffusion of molecules is re-
stricted, and the intermolecular phosphorescence quenching
can be excluded. A control experiment on the phosphores-
cence measurement of a mixture of BP–Am and NBD–COOMe
without the salt bridge also supports this contention (see Sup-
porting Information). All the results suggest that the phosphor-
escence of BP in the mixture of BP–Am and NBD–COOH can
only be quenched by the NBD group through the amidinium–
carboxylate salt bridge formation.
escence lifetime measurements on the different ratio mixtures
of NBD–COOH and BP–Am validate our presumption. The por-
tion of the short lifetime species increases, and the proportion
of the longer lifetime component is getting less with increas-
ing the percentage of NBD–COOH in the mixture. The ampli-
tudes of the shorter lifetime species (A ) and the longer life-
1
time component (A =1ꢀA ) represent the relative amounts of
2
1
the associated BP–(amidinium-carboxylate)–NBD and dissocia-
tive BP–Am, respectively. The association constant can be ex-
pressed as K =A C /[(C ꢀA C )(CꢀA C )], where C and C are
To clarify the reason for the quenching of BP phosphores-
cence by the NBD group in the BP–(amidinium-carboxylate)–
NBD system, we estimated the free energy change involved in
an electron-transfer process from the NBD group to the BP
a
1
0
0
1
0
1
0
0
the concentrations of adding amount of NBD–COOH and BP–
Am, respectively; A C , (C ꢀA C ) and (CꢀA C ) represent the
1
0
0
1
0
1 0
concentrations of associated species BP–(amidinium-carboxy-
late)–NBD, free BP–Am and NBD–COOH, respectively. By using
the amplitudes A and A obtained from the phosphorescence
[14]
chromophore following the Rehm–Weller Equation [Eq. (1)]:
1
2
þ
ꢀ
2
lifetime measurements of the mixture of BP–Am and NBD–
DG ¼ 23:06½EðDC =DÞꢀEðA=AC ÞꢁꢀE ꢀe =re
00
ð1Þ
COOH with different ratios, the K value of BP–(amidinium-car-
a
2
0
ꢀ1
ꢀ
e =2 ð1=r þ 1=r Þð1=e ꢀ1=eÞ ðkcal mol
Þ
3
ꢀ1
þ
ꢀ
boxylate)–NBD can be calculated to be 3.2ꢀ10 m , and the
plot of (A ) vs (1ꢀA ) (CꢀA C ) with a linear fit is shown in
1
1
1 0
E00 is the triplet excited-state energy of the 4-substituent ben-
Figure 4.
ꢀ
1 [15]
zophenone (69 kcalmol ). The oxidation potential of nor-
bornadiene E(NBDC /NBD) and the reduction potential of ben-
+
ꢀ
[11a]
zophenone E(BP/BPC ) are +1.45 V and ꢀ1.73 V
in acetoni-
2
trile with respect to SCE, respectively. e /re is the Coulombic
energy depending on the distance (r) between the separated
radical ions (r=12.3 ꢂ, estimated from the molecular simula-
[16]
tion) and the dielectric constant (e) of MTHF (e=6.97). The
last term in Equation (1) is the Born correction to the solvation
energy which depends on the radii of the donor cation (r+)
and the acceptor anion (r ). To estimate the Born correction to
ꢀ
the solvation energy, r+ and r are set equal to 2.5 ꢂ and
ꢀ
3.8 ꢂ, respectively, by assuming that both donor and acceptor
are spherical. Estimation from Equation (1) gives the free-
ꢀ1
energy change DG to be about 12.5 kcalmol , suggesting
that the electron transfer from the norbornadiene group to
the triplet state benzophenone chromophore would be very
inefficient if any did occur. On the other hand, benzophenone
Figure 4. Plot of the portion of shorter lifetime species (A ) vs (1ꢀA )
1
1
ꢀ
4
(CꢀC
0 1 0
A ) with linear fit. [BP–Am] (C )=1.0ꢀ10 m, [NBD–COOH] (C)=0,
ꢀ
5
ꢀ5
ꢀ5
ꢀ5
ꢀ4
1
.0ꢀ10 , 2.0ꢀ10 , 5.0ꢀ10 , 7.0ꢀ10 , 1.0ꢀ10 m.
ꢀ1
ꢀ1 [17]
(
E =69 kcalmol )–norbornadiene (E =53 kcalmol )
is a
T
T
typical triplet–triplet energy transfer pair with proper energy
levels. Therefore, the triplet–triplet energy transfer from the
triplet excited BP chromophore to the NBD group should be
responsible for the quenching of the BP phosphorescence in
the BP–(amidinium-carboxylate)–NBD system.
The efficiency (FET) and rate constant (kET) for the triplet–
triplet energy transfer from BP to NBD in the BP–(amidinium-
carboxylate)–NBD (1:1) salt bridge system can be calculated to
3
ꢀ1
be 0.87 and 1.8ꢀ10 s by using Equations (2) and (3), respec-
tively. Considering there is only 20% component forming the
salt bridge system in a mixture of BP–Am and NBD–COOH
with the ratio of 1:1, the observed energy transfer efficiency is
estimated to be 0.17 (0.87ꢀ0.20), which is compatible with the
steady-state phosphorescence quenching efficiency (14%). The
result of the time-resolved phosphorescence experiments vali-
dates the occurrence of the triplet–triplet energy transfer from
BP to NBD in BP–(amidinium-carboxylate)–NBD.
The time-resolved phosphorescence spectra are examined in
glassy 2-methyltetrahydrofuran at 77 K with the excitation at
362 nm. The phosphorescence decays of the donor compound
BP–Am and the donor model system BP–(amidinium-carboxy-
late)–Ph monitored at 453 nm are fitted to a mono-exponential
function, giving a lifetime of 3.9 ms. On the other hand, the
decay of 1:1 mixture of BP–Am and NBD–COOH at the same
condition can only be fitted double-exponentially with life-
times of 0.49 ms (A =20%) and 3.9 ms (A =80%). The longer
1
2
lifetime component with the same lifetime of BP–Am is as-
signed to the dissociative energy donor compound BP–Am,
and the shorter one is assigned to BP–Am associating with
NBD–COOH in the BP–(amidinium-carboxylate)–NBD system in
which the triplet–triplet energy transfer occurs. The phosphor-
FET ¼ 1ꢀt=t0
ð2Þ
ð3Þ
kET ¼ 1=tꢀ1=t0
ChemPhysChem 2010, 11, 229 – 235
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www.chemphyschem.org
231

J. Chen, Y. Li et al.
Laser Flash Photolysis Evidence for the Triplet–Triplet
Energy Transfer
The evidence for the triplet–triplet energy transfer in the BP–
(amidinium-carboxylate)–NBD system is further strengthened
by laser flash photolysis studies. To make sure most of BP–Am
combined with NBD–COOH, 10.0 equiv NBD–COOH was added
ꢀ
4
ꢀ3
to the solution ([BP–Am]=5ꢀ10 , [NBD–COOH]=5ꢀ10 m).
According to the association constant of the BP–(amidnium-
3
ꢀ1
carboxylat)–NBD system (K =3.2ꢀ10 m ) estimated from the
a
time-resolved phosphorescence measurement, the dissociative
BP–Am is less than 1% in this case. Pulsed-laser photolysis of
the model system BP–(amidinium-carboxylate)–Ph (BP–Am/Ph–
[
18]
COOH=1/10), was performed in deaerated acetonitrile:me-
[
19]
thanol (9:1 v/v) by using 355 nm selective excitation light,
giving rise to a strong transient absorption spectrum with
maxima at 550 nm and 700 nm, respectively. The flash photoly-
sis study of the target system BP–(amidinium-carboxylate)–
NBD (BP–Am/NBD–COOH=1/10) under identical conditions
leads to a similar transient absorption. The transient absorption
spectra for BP–(amidinium-carboxylate)–Ph and BP–(amidi-
nium-carboxylate)–NBD are depicted in Figure 5.
Figure 6. Decay trace of BP–(amidinium-carboxylate)–Ph at 550 nm in deaer-
ꢀ4
ated acetonitrile/methanol (9/1 v/v). [BP–Am]=5.0ꢀ10 m, lex =355 nm.
Inset: Decay trace of BP–(amidinium-carboxylate)–NBD in a shorter time-
scale.
the transient absorption of benzophenone at the same condi-
tions indicates that non-monoexponential behavior in the
model systems can be ascribed to the hydrogen abstraction by
triplet state of benzophenone from the solvent of methano-
[1a,21]
l.
The shorter lifetime component is assigned to the triplet
state of energy donor compound BP–Am, which is compatible
[10]
with our previous results. The longer one is assigned to the
BP ketyl redical that is formed by abstracting a proton from
the solvent, and the value is almost the same as reported in
[21a]
the literature (t>35 ms).
The target system (BP–Am/NBD–
COOH, 1/10) shows a much faster decay of the triplet state of
BP than that in the model system indicative of the occurrence
of the triplet-triplet energy transfer from BP to NBD in BP–
(
amidinium-carboxylate)–NBD. The decay curve of BP for the
target system (BP–Am/NBD–COOH, 1/10) was measured at a
short timescale, which can be fitted monoexponentially, giving
a lifetime of 75 ns. One may argue that the electron transfer
might occur in high polar solvent of acetonitrile–methanol at
room temperature. Calculation according to the Rehm–Weller
equation shows that the free energy change (DG) in methanol
Figure 5. Transient absorption spectra of the triplet states formed upon
laser photolysis of BP–(amidinium-carboxylate)–Ph and BP–(amidinium-car-
boxylate)–NBD (ratio=1/10) in deaerated acetonitrile-methanol (9/1 v/v)
(lex =355 nm) at the peak of the laser pulse. Concentrations of BP–Am in
ꢀ
4
the model and target systems are both 5.0ꢀ10 m.
ꢀ1
is still positive (>4 kcalmol ), suggesting that the electron
transfer from NBD to the triplet state BP chromophore is an
endothermal process. Furthermore, the intensity of the BP
anion radical absorption band relative to the triplet absorption
band for target system BP–(amidinium-carboxylate)–NBD is
much weaker than that for the model system BP–(amidinium-
carboxylate)–Ph (Figure 5), which means that the BP anion rad-
ical produced by electron transfer can be neglected in the
target system of BP–(amidinium-carboxylate)–NBD. The hydro-
gen abstraction reaction of the BP triplet state from the alco-
hol solvent is strongly suppressed by the efficient triplet
energy transfer from the BP triplet state to NBD. The efficiency
(F ) and the rate constant (k ) for the triplet–triplet energy
The transient absorption at 550 nm is assigned to the lowest
triplet state of the BP chromophore on the basis of the follow-
ing observations. First, this absorption is essentially identical to
that of the alkyl benzophenone-4-carboxylate triplet state in-
[
10a,b,11a]
dependently generated.
Second, the 550 nm species is
rapidly quenched by O . The absorption band at 700 nm is as-
2
signed to the BP anion radical, which was induced by reactions
[
20]
of the triplet state of BP in alcohol solvent. Taking the transi-
ent spectra of BP–(amidinium-carboxylate)–Ph and BP–(amidi-
nium-carboxylate)–NBD at 550 nm as a function of time gives
the decay curves. Figure 6 shows the decay traces of BP–(ami-
dinium-carboxylate)–Ph and BP–(amidinium-carboxylate)–NBD.
The decay curve for the model system can be fitted double-
ET
ET
transfer in BP–(amidnium-carboxylate)–NBD are estimated to
7
ꢀ1
be 0.98 and 1.3ꢀ10 s by using the lifetimes of the BP triplet
state according to Equations (2) and (3), respectively.
exponentially with the lifetimes of 46.6 ms (A =65%) and
The rate constant at room temperature is ~7000 times larger
than that at 77 K. The T–T energy transfer within the salt
1
2
4
.3 ms (A =35%) with acceptable c value. Measurement of
2
2
32
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ChemPhysChem 2010, 11, 229 – 235

Photoisomerizaiton of Norbornadiene
1
bridge system can be treated as intramolecular T–T energy
transfer, and the rate constants is related to the separation of
the donor and acceptor, the specific orbital interactions, and a
spectral overlap integral normalized for the extinction coeffi-
cient of acceptor. The spectral overlap is not affected much by
clane derivative relies mainly on its H NMR spectrum (see Sup-
porting Information), which is in close agreement with that re-
[10a,17a]
ported in the literature.
On the basis of the experimental
results mentioned above, the primary photophysical and pho-
tochemical processes in BP–(amidinium-carboxylate)–NBD can
be expressed by Scheme 2.
[
22]
the temperature. According to the literature,
the amidini-
um–carboxylate salt bridge is not a completely rigid structure
as a covalent connection. The vibration of the salt bridge
within the lifetime of benzophenone triplet excited state in sol-
ution makes the orbital interactions and the distance between
donor and acceptor favor to the occurrence of T–T energy
transfer. Because the salt bridge system is frozen at 77 K, the
vibration of salt bridge is suppressed, and consequently the T–
T energy transfer process is slow.
Scheme 2. The primary photophysical and photochemical processes of BP–
Photosensitized Isomerization of the Norbornadiene to the
Quadricyclane Group
(amidinium-carboxylate)–NBD
The photosensitized valence isomerization of NBD to QC has
been the intense subject of experimental and theoretical inves-
The quantum yield of the photosensitized isomerization of
NBD in the salt bridge system, Fiso(BP–salt bridge–NBD), can
be calculated according to Equation (4):
[
23]
[24]
tigations in view of its significance in solar energy storage
[
25]
and mechanism interests. Benzophenone and some other ar-
omatic ketones are known to sensitize the isomerization of
[
26]
F ðBP ꢀ salt bridge ꢀ NBDÞ ¼ F F F ðNBDÞ
ð4Þ
iso
isc ET iso
NBD to QC through triplet–triplet energy transfer.
Thus,
study of the photosensitized isomerization of the NBD group
in BP–(amidinium-carboxylate)–NBD may provide evidence of
triplet–triplet energy transfer in the salt bridge system.
where Fisc represents the quantum yield of the intersystem
crossing from the singlet to the triplet excited state of the BP
[1a]
group and is assumed to be unity. FET was obtained by
Irradiation with l=355 nm laser of a BP–(amidinium-carbox-
ꢀ
3
phosphorescence experiment mentioned above as 0.87.Fiso
-
ylate)–NBD solution ([BP–Am]=[NBD–COOH]=1.0ꢀ10 m) in
deaerated deuterium methanol/ethanol (9/1 v/v) at 77 K until
no more obvious increase in the intensity of phosphorescence
results in valence isomerization of a portion of norbornadiene
group to the quadricyclane one, as shown in Scheme 1. Under
(
NBD) represents the quantum yield of the isomerization reac-
[11a]
tion of the NBD triplet state and is 0.185.
These in turn give
F (BP–salt bridge–NBD) as 0.16 for BP–(amidinium-carboxy-
iso
late)–NBD.
Mechanism of the Triplet–Triplet Energy Transfer
The photophysical and photochemical studies reveal that the
triplet–triplet energy transfer from BP to NBD can occur effi-
ciently and subsequently leads to the isomerization of the NBD
group to QC in the formed salt bridge system BP–(amidinium-
carboxylate)–NBD. It has been well established that the triplet–
triplet energy transfer proceeds via the Dexter electron-ex-
change mechanism which requires orbital overlap of donor
and acceptor, and its rate constant decreases exponentially
with an increasing of the distance between donor and accept-
Scheme 1. Photoisomerization of norbornadiene to the quadricyclane
group.
this condition, only the BP chromophore absorbs the light and
the diffusion of molecules is entirely suppressed in solid solu-
tion. Thus, the isomerization of NBD to QC must be attributed
to the triplet–triplet energy transfer within the formed salt
bridge system BP–(amidinium-carboxylate)–NBD, and the trip-
let–triplet energy transfer via intermolecular collision can be
excluded completely. Irradiation of the mixture of BP–Am and
NBD–COOMe without the salt bridge under the same condi-
tions gives no quadricyclane formation. The yield of the iso-
merization product is almost 100% on the basis of the con-
sumption of the starting material, and ~33% of the NBD
[1a]
or. Usually, the triplet–triplet energy transfer efficiency is ex-
pected to become inefficient as the distance between donor
and acceptor beyond the sum of their Van de Waals radius (5–
[1a]
10 ꢂ).
The separation between the BP and NBD chromo-
phores in BP–(amidinium-carboxylate)–NBD, which is defined
as the distance between the C of the carbonyl in BP–Am and
the C of the bridgehead in NBD–COOH, is estimated by using
the HyperChem 6.0 program. Computation from the lowest
energy conformation shows that the distance between the BP
chromophore and the NBD group in BP–(amidinium-carboxy-
late)–NBD is about 12.3 ꢂ. At such separation, the triplet–trip-
let energy transfer from BP to NBD via a through-space mecha-
1
group is isomerized to QC judged from the H NMR data in our
experiment. The assignment of the product as the quadricy-
ChemPhysChem 2010, 11, 229 – 235
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
233

J. Chen, Y. Li et al.
nism would be very inefficient and this is contrary to the ex-
periment results. Therefore, we propose that the triplet–triplet
energy transfer in BP–(amidinium-carboxylate)–NBD proceeds
via a “through-bond” electron-exchange mechanism. The non-
covalent bonds (amidinium-carboxylate salt bridge) mediate
the triplet–triplet energy transfer process.
slowly added a 1m solution (6 mL) of trimethy aluminum in hep-
tane. After the addition was completed, the reaction mixture was
allowed to warm to room temperature and stirred for 3.5 h until
gas evolution had ceased. b) Preparation of amidine: A solution of
4
-benzoylbenzonitrile (1.2 g, 5.8 mmol) in 10 mL of dry toluene
(distilled from Na) was added to a 6m solution of the aluminum
amide reagent in toluene at room temperature. The solution was
heated under argon at 808C until the nitrile was consumed
(
36.5 h). The reaction mixture was cooled and the aluminum com-
Conclusions
plex was decomposed by carefully pouring the solution into a
slurry of silica gel (20 g) in dichloromethane. The mixture was stir-
red for 5 min and filtered. The filter pad was further washed with
methanol (100 mL). After removal of the solvent, the residue was
recrystallized from the mixture of the toluene and methanol to
The efficient triplet–triplet energy transfer from the triplet ex-
cited benzophenone chromophore to the norbornadiene
group through the amidinium-carboxylate salt bridge was con-
firmed by the steady-state and the time-resolved phosphores-
cence spectra, the flash photolysis and the photosensitized iso-
merization. The energy transfer efficiencies at 77 K and room
temperature are 0.87 and 0.98 with rate constants of 1.8ꢀ
afford a pale yellow solid (0.30 g, 23.1%): mp.=194–1968C;
1
H NMR ([D ]acetone): d=8.11 (d, J=8 Hz, 2H), 7.96 (d, J=8 Hz,
6
2H,), 7.80 (d, J=8 Hz, 2H), 7.71 (t, J=8 Hz, 1H), 7.59 ppm (q, J=
8
134.5, 131.9, 130.8, 130.7, 129.2, 128.4 ppm; MS (EI): 224(M ), 208,
1
1
13
Hz, 2H); C NMR ([D ]acetone): d=198.7, 166.3, 141.9, 136.2,
6
+
3
ꢀ1
7
ꢀ1
1
0 s and 1.3ꢀ10 s , respectively. A “through-bond” elec-
+
80, 120, 105. MALDI–TOF: 224 (M ); IR: n˜ =3270, 3033, 1667,
tron-exchange mechanism is proposed for the triplet–triplet
energy transfer in the supramolecular assembly BP–(amidi-
nium-carboxylate)–NBD. It suggests that not only the covalent
bonds, but also the non-covalent bonds (amidinium-carboxy-
late salt bridge) can mediate the triplet–triplet energy transfer
process. This finding may provide a new strategy for develop-
ing supramolecular systems for photochemical conversion and
storage of solar energy.
ꢀ1
275, 925, 861, 798, 708, 630 cm
.
Acknowledgements
Financial support for this work by National Natural Science Foun-
dation of China (Grant Numbers 20603042, 20733007 and
2
2
0772134), the National Basic Research Program (Grant Numbers
007CB808004), and the Chinese Academy of Sciences is grateful-
Experimental Section
ly acknowledged.
Materials: Reagents were purchased from Aldrich or Acros and
were used without further purification, unless otherwise noted. Tol-
uene was distilled over Na/Benzophenone under argon atmos-
phere. DMF was dried with anhydrous K CO . Solvents used for
Keywords: energy
photoisomerization · salt bridges · supramolecular chemistry
transfer
·
phosphorescence
·
2
3
spectroscopic measurements were all spectra grade.
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34
www.chemphyschem.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Published online on November 9, 2009
ChemPhysChem 2010, 11, 229 – 235
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
235