The photoinduced electron transference of porphyrin-anthraquinone dyads bridged with different lengths of links

Article abstract of DOI:10.1016/j.saa.2010.11.006

The photoinduced electron transference (PET) interaction in porphyrin containing donor-acceptor (D-A) molecules is of great importance in nature and a significant part of the PET research has been devoted to the study of its mechanism ("through-space" or "through-bond") in these decades. Herein we synthesized a series of covalently linked porphyrin-anthraquinone dyads (Por-C_n-AQ) bridged with flexible alkoxy chains at different lengths (n = 1, 4, 10) and investigated their intramolecular PET using a combination of electronic absorption, steady-state fluorescence and decayed luminescence spectra. The experimental results show that the PET efficiency depends on the length of the flexible linkage between the porphyrin and anthraquinone moieties. Meanwhile, theoretical calculation applying the density functional theory (DFT) was also carried out to give the frontier orbital distribution and the optimized structures of these dyads. It is found that the orientation of the dyad with high PET efficiency is disadvantageous to π-π interaction. Thus, the PET of these dyads seemingly is best compatible with a "through-bond" (superexchange) mechanism.

Full text of DOI:10.1016/j.saa.2010.11.006

Spectrochimica Acta Part A 78 (2011) 437–442
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The photoinduced electron transference of porphyrin–anthraquinone dyads
bridged with different lengths of links
a,∗
b,∗∗
c
d
b
Ping Zhao , Jin-Wang Huang
, Lian-Cai Xu , Li Ma , Liang-Nian Ji
a
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, No. 280, Waihuandong Road, Education Mega Centre, Guangzhou 510006, PR China
MOE Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, No. 135, Xingangxi Road,
b
Guangzhou 510275, PR China
c
Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, PR China
Chemistry and Chemical Engineering School, Henan University of Technology, Zhengzhou 450001, PR China
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 April 2010
Received in revised form 18 October 2010
Accepted 15 November 2010
The photoinduced electron transference (PET) interaction in porphyrin containing donor–acceptor (D–A)
molecules is of great importance in nature and a significant part of the PET research has been devoted to
the study of its mechanism (“through-space” or “through-bond”) in these decades. Herein we synthesized
a series of covalently linked porphyrin–anthraquinone dyads (Por–Cn–AQ) bridged with flexible alkoxy
chains at different lengths (n = 1, 4, 10) and investigated their intramolecular PET using a combination
of electronic absorption, steady-state fluorescence and decayed luminescence spectra. The experimental
results show that the PET efficiency depends on the length of the flexible linkage between the porphyrin
and anthraquinone moieties. Meanwhile, theoretical calculation applying the density functional theory
Keywords:
Porphyrin–anthraquinone
Electron transfer
Fluorescence quantum yields
Fluorescence lifetime
(DFT) was also carried out to give the frontier orbital distribution and the optimized structures of these
“Through-bond” mechanism
dyads. It is found that the orientation of the dyad with high PET efficiency is disadvantageous to ␲–␲ inter-
action. Thus, the PET of these dyads seemingly is best compatible with a “through-bond” (superexchange)
mechanism.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
transfer (PET) [9–12]. Various efforts have been made to understand
the PET mechanism of the porphyrin–quinone dyads.
The importance and complexity of electron-transfer reactions
It is widely accepted that the intramolecular PET in porphyrin
containing donor–acceptor (D–A) molecules mainly employs
“through-space” and/or “through-bond” (superexchange) mech-
anisms [6–13]. A “through-space” interaction is defined as a
mechanism that electrons transfer from donor (D) to acceptor
(A) via the direct overlap of their respective electronic wave
functions, which could often be understood as through a ␲–␲ inter-
action; however, the indirect “through-bond” (superexchange)
interaction is defined as a mechanism which involves the par-
ticipation of not only D and A but also the spacer molecules
between them. The nature of the bridge linking the D and A
components is thus a significant subject of much interest since
PET in many D–A molecules is known to be dominated by
“through-bond” interactions [3,4,7,8,11,13]. Although there are a
few reports on the research of PET affected by the bridge between
D and A [7,8], so far no studies were found focused on the PET
of D–A molecules linked by saturated alkoxy chains at differ-
ent lengths. As part of our continuing interest in the study of
PET processes [14,15], we designed a series of covalently linked
porphyrin–anthraquinone dyads bridged with different lengths
of flexible alkoxy chains (Por–Cn–AQ, n = 1, 4, 10, see molecular
structure in Scheme 1) and investigated the intramolecular PET
in nature have lead many researchers to look for ways to study
the fundamental chemistry of these processes in simplified model
systems [1,2]. Recently, there has been a great deal of research
activity on the photochemical properties of porphyrin contain-
ing donor–acceptor (D–A) molecular assemblies in an attempt to
mimic the primary light-driven step in natural photosynthetic reac-
tion centers [3–5]. Since it is well known that the key step of the
photosynthesis in nature is the electron transfer from the excited
chlorophyll molecules to the quinone hybrids, covalently/non-
covalently linked porphyrin–quinone molecules have been studied
extensively as models for the light-initiated charge separation step
in the photosynthesis [6–8]. Many current investigations are con-
cerned with porphyrin–quinone molecules in order to understand
the role of factors like distance, orientation, energetics and medium
in determining the rate of intramolecular photoinduced electron
∗
Corresponding author. Tel.: +86 20 39352620; fax: +86 20 39352620.
Corresponding author. Tel.: +86 20 84113317; fax: +86 20 84112245.
E-mail addresses: zhaoping666@163.com (P. Zhao), ceshjw@163.com
J.-W. Huang).
∗
∗
(
1
386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.11.006

4
38
P. Zhao et al. / Spectrochimica Acta Part A 78 (2011) 437–442
Scheme 1. The synthetic scheme of Por–Cn–AQ dyads.
−
interaction mechanism of these dyads using spectral and theoretic
methods.
5% Na CO . Yield: 1.55 g, 90%. ESI-MS m/z 342.2 ([M−H] , calcd. for
2
3
C
1
H BrNO : 342.9).
10 3
6
BrPAAQ (bromopentanamino-anthraquinone) and BrUAAQ
bromoundecanamino-anthraquinone) were similarly prepared,
(
2
. Experimental
replacing bromoacetic acid by 5-bromopentanoic acid and 11-
bromoundecanoic acid, respectively.
2.1. Materials
−
BrPAAQ. Yield: 92%. ESI-MS m/z 427.2 ([M−H] , calcd. for
C19
C25
H BrNO : 385.2).
16 3
1
-Amino-9,10-anthraquinone (AQ), bromoacetic acid, 5-
−
BrUAAQ. Yield: 86%. ESI-MS m/z 469.4 ([M−H] , calcd. for
BrNO : 469.4).
bromopentanoic acid, 8-bromooctanoic acid, 11-bromoundecanoic
acid were commercially available from Sigma. Solvents utilized
for spectroscopic and electrochemical experiments were further
purified using standard procedures [16].
H
28
3
2
.2.2. Synthesis of Por–Cn–AQ dyads
,10,15,20-Tetramethoxyphenylporphyrin (TTP) and 5-(4-
hydroxyphenyl)-10,15,20-tris(4-methoxylphenyl)porphyrin
HTTP) were synthesized according to the traditional method
5
2
2
.2. Synthesis
(
.2.1. Synthesis of anthraquinone derivates
reported previously [17].
BrMAAQ (bromomethoxylamino-anthraquinone). Bromoacetic
The synthetic scheme of Por–Cn–AQ dyads 1, 2 and 3 is shown
in Scheme 1.
acid (10 mmol, 1.5 g) dissolved in thionyl chloride (35 mL) was
refluxed for 2 h. After cooling to room temperature, thionyl chlo-
ride was removed under reduced pressure. Dry benzene (30 mL)
was used to wash the residue and then removed. To this residue,
a solution of AQ (5 mmol, 1.1 g) dissolved in dry benzene was
added and then refluxed for 4 h. The solvent was removed under
reduced pressure. BrMAAQ was then obtained after washing with
1: a mixture of HTTP (0.15 mmol, 0.095 g), BrMAAQ (0.3 mmol,
0.102 g) and anhydrous K CO3 (0.7 g) in DMF (20 mL) was stirred
2
for 48 h at room temperature. Then the reaction mixture was
poured into water (50 mL) and filtered. The crude material was
extracted by a mixture of CHCl –EtOH (25/1, v/v) and purified
3
by chromatography. Evaporation of solvent afforded 200 mg of

P. Zhao et al. / Spectrochimica Acta Part A 78 (2011) 437–442
439
1
. Yield: 92% (Found: C, 79.08%; H, 5.20%; N, 7.29%. Calcd. for
C63H47N5O ·H O: C, 79.14%; H, 5.17%; N, 7.32%). ESI-MS m/z
4
2
+
1
9
1
9
6
2
37.2 (M , calcd for C H N5O : 938.1). H NMR (CDCl ): ı:
63 47 4 3
3.493 (s, 1H, NH–C O), 9.492(d, J = 5.92 Hz, 6H, 2,6-pyridinium),
.139(s, 4H, ␤-pyrrole), 9.043(d, J = 4.50 Hz, 4H, ␤-pyrrole), 8.950(s,
H, 3,5-pyridinium), 8.392(d, J = 5.92 Hz, 2H, 2,6-phenyl), 8.014(s,
H, 3,5-phenyl), 7.785(d, J = 6.53 Hz, 4H, AQ-phenyl), 7.089(s, 2H,
AQ-phenyl), 5.089(s, 1H, AQ-phenyl), 2.702(s, 9H, CH -phenyl),
3
−
2.781(s, 2H, NH pyrrole).
Compounds 2 and 3 were similarly prepared, replacing BrMAAQ
by BrPAAQ and BrUAAQ, respectively.
: yield: 87% (Found: C, 77.89%; H, 5.62%; N, 6.84%. Calcd. for
C66H53 N5O .2H O: C, 78.01%; H, 5.65%; N, 6.89%). ESI-MS m/z 979.2
2
4
2
+
1
(
M , calcd for C66H53N5O : 980.2.) H NMR (500 MHz, CDCl ): ı:
4
3
1
9
6
2.591 (s, 1H, NH–C O), 9.448(d, J = 5.94 Hz, 6H, 2,6-pyridinium),
.12(s, 4H, ␤-pyrrole), 9.05(s, 4H, ␤-pyrrole), 8.978(d, J = 5.51 Hz,
H, 3,5-pyridinium), 8.00(d, J = 8.13 Hz, 2H, 2,6-phenyl), 7.255(d,
J = 8.06 Hz, 2H, 3,5-phenyl), 4.290(s, 2H, O–CH ), 2.705(s, 9H, CH -
Fig. 1. The electronic absorption spectra of TTP, AQ and dyad 1 in CH2Cl2 (10 ␮M).
2
3
phenyl), 1.834(t, 4H, –CH –), −2.857 (s, 2H, NH pyrrole).
2
3
: yield: 87% (Found: C, 76.17%; H, 6.50%; N, 6.18%. Calcd. for
there was no appreciable interaction between the ground-state
porphyrin moiety and the ground-state anthraquinone moiety for
these dyads [8,11,15].
C72H65 N5O .4H O: C, 76.10%; H, 6.47%; N, 6.16%). ESI-MS m/z
4
2
+
1
1
063.2 (M , calcd for C72H65 N5O : 1064.3.) H NMR (500 MHz,
4
CDCl ): ı: 12.010 (s, 1H, NH–C O), 9.450(d, J = 5.94 Hz, 6H, 2,6-
3
pyridinium), 9.10(s, 4H, ␤-pyrrole), 9.00(s, 4H, ␤-pyrrole), 8.98(d,
J = 5.51 Hz, 6H, 3,5-pyridinium), 8.01(d, J = 8.13 Hz, 2H, 2,6-phenyl),
3.2. Steady-state fluorescence spectra
7
9
.262(d, J = 8.06 Hz, 2H, 3,5-phenyl), 4.310(s, 2H, O–CH ), 2.711(s,
2
Fig. 3 depicts the steady-state emission spectra of dyads 1, 2, 3
H, CH -phenyl), 1.834(t, 14H, –CH –), −2.957(s, 2H, NH pyrrole).
3
2
and the reference compound TTP as well as its 1:1 mixture with AQ,
which were excited at 420 nm. From Fig. 3, we can find that, com-
pared with TTP, the emission spectra of the 1:1 mixture of TTP and
AQ is only slightly quenched, which may result from the dynamic
dash between the TTP and AQ molecules [8,11,14,15]. However,
significant quenching was observed for the emission spectra of
Por–Cn–AQ dyads 1, 2 and 3 with respect to that of TTP and the
2.2.3. Theoretical details
The full geometry optimization computations of the Por–Cn–AQ
dyads were carried out with the DFT-B3LYP method and 6-31G*
basis set, and the frequency analysis confirmed the obtained
geometries to be genuine minimum. There is no symmetry appear-
ing in these compounds. In order to vividly depict the details of
the frontier molecular orbitals, the stereographs of some related
frontier molecular orbitals of the complexes were drawn with the
Chemcraft 1.6 program based on the computational results. All cal-
culations were performed with the G03 (d01) program-package
1
:1 mixture of TTP and AQ. This strong quenching can be attributed
to the occurrence of a PET reaction from the porphyrin singlet
state to the appended quinone, as is true for the covalently linked,
porphyrin–anthraquinone systems reported earlier [7,8,11,13,21].
The fluorescence quantum yields (˚) of these compounds are sum-
marized in Table 1, from which we can find that ˚1 (0.023) < ˚2
[
18]. The solvent effect was not considered in this research.
(
0.031) < ˚3 (0.049) (˚ , ˚2 and ˚3 are the fluorescence quantum
1
yields for dyads 1, 2 and 3, respectively). It seems that the fluo-
rescence quantum yields increase with the length of the flexible
carbon linkages. Since a lower ˚ value indicates a higher fluores-
cence quenching, the quenching efficiency decreases as the linkage
3
. Results and discussion
3.1. Electronic absorption spectra study
The ground-state electronic absorption spectra of dyad 1 along
with the reference compounds TTP and AQ in dichloromethane
are given in Fig. 1. It is found that AQ has a strong absorption
band around 243 nm whereas TTP has a strong absorption band at
around 419 nm (Soret band) and four weak absorption bands in the
range of 500–650 nm (Q band). Here the Soret band of porphyrin
is an a1u(␲)/e.g.(␲*) electron transition, assigned to the second
excited state S2 generated by ␲/␲* transition, while the Q band
corresponds to an a2u(␲)/e.g.(␲*) electron transition, belonging to
the first excited state S1 generated by ␲/␲* transition [8,19,20].
From Fig. 1, it can be clearly observed that dyad 1 exhibits the
characteristic absorption bands of both TTP and AQ. Meanwhile,
Fig. S1 in Supplemental materials shows similar absorption spectra
for dyads 2 and 3.
Fig. 2 gives the electronic absorption spectra of dyad 1 and 1:1
mixture of TTP and AQ at equal concentration. Compared with the
mixture, slight decrease in intensity and weak red shift of the Soret
band for porphyrin moiety were observed in the spectrum of 1,
which may result from the close coupling of the porphyrin and
AQ moieties [9]. Similar results were also found in dyads 2 and
Fig. 2. The electronic absorption spectra of 1 and the 1:1 mixture of TTP and AQ in
CH2Cl2 (10 ␮M). Inset: the electronic absorption spectra of TTP and AQ.
3
(shown in Fig. S2 in Supplemental materials), indicating that

4
40
P. Zhao et al. / Spectrochimica Acta Part A 78 (2011) 437–442
Fig. 4. The fluorescence-decay profile as measured at 653 nm for dyads 1, 2, 3 and
TTP in CH2Cl2. ꢀex = 420 nm.
Fig. 3. The emission spectra of TTP, the 1:1 mixture of TTP and AQ, dyads 1, 2 and 3
in CH2Cl2 (10 ␮M). ꢀex = 420 nm.
sponds to the linkage, with kET values following an order as
becomes longer. Because it is widely admitted that the spacer
between donor and acceptor will participate the PET interaction of
D–A molecules in a “through-bond” (superexchange) mechanism,
it is supposed that these Por–Cn–AQ dyads employ the “through-
bond” mechanism in its intramolecular PET interaction. This could
be further confirmed by the observation of the decayed lumines-
cence spectra of these dyads.
5
−1
5
−1
4
−1
)
kET,1 (3.32 × 10 S ) > kET,2 (1.76 × 10 S ) > kET,3 (9.49 × 10 S
(
kET,1, k , k for 1, 2 and 3, respectively). The Por–Cn–AQ dyad
ET,2 ET,3
with shorter linkage has faster the PET rate, indicating that the
spacer plays a significant role in the electron transfer process. This
result is in good agreement with the steady-state emission spectra
above.
3.4. Theoretical explanation
3.3. Decayed luminescence spectra
Theoretical mechanistic questions concerning intramolecular
The difference of electron transfer from porphyrin moiety to
electron transfer in covalently linked donor–acceptor complexes
have focused on the issue of “through-space” versus “through-
bond” electron transmission pathway. The former mechanism
should be strongly dependent on the donor–acceptor orientation,
since the electron will transfer from HOMO (the highest occupied
molecular orbital) or HOMO-x (the occupied molecular near the
HOMO) of the molecule to LUMO (the lowest unoccupied molecu-
lar orbital) or LUMO-x (the unoccupied molecular near the LUMO)
through ␲–␲ stacking interaction by this mechanism [12,13,22,23];
however, the latter mechanism would be less sensitive to geo-
metrical factors between the donor and acceptor moieties but be
strongly dependent on the number of atoms connecting the donor
and acceptor, since the reside between the donor and the accep-
tor will mix their electronic states with those of the donor and
acceptor at this case, and thus the “through-bond” mechanism is
synonymous with superexchange [13,24,25].
From the calculation results of DFT method, we get the
HOMO and LUMO distribution and geometrical optimized struc-
tures of the Por–Cn–AQ dyads, as shown in Fig. 5. From Fig. 5,
we can find that for all the dyads, the HOMO is mainly dis-
tributed at porphyrin moiety while LUMO is mainly distributed
at anthraquinone moiety. The energies of HOMO and LUMO for
dyads 1, 2 and 3 were calculated in Table 1. The PET occurs
from porphyrin moiety to anthraquinone moiety when excited at
anthraquinone moiety between 1, 2 and 3 can also be detected from
their rate constants of PET process. Decayed luminescence spec-
tra of these dyads as well as their reference compound TTP were
given in Fig. 4, and the obtained lifetimes were listed in Table 1.
Compared with the lifetime of TTP (ꢁ = 8.18 ns), the excited state
lifetimes of all the Por–Cn–AQ hybrids are strongly reduced. More-
over, it is found that the lifetime of the excited states increases with
the flexible linkages in the dyads becomes longer: ꢁ1 (2.47 ns) < ꢁ2
(
3.36 ns) < ꢁ3 (4.61 ns) (ꢁ , ꢁ2 and ꢁ3 are the lifetimes for dyads 1, 2
1
and 3, respectively). It seems that the length of the flexible linkage
between the two moieties significantly influences the lifetimes of
the excited state in the Por–Cn–AQ hybrids.
Table 1 also listed the PET rates of the dyads, kET, which were
achieved using following Eq. (1):
1
kET =
(1)
ꢁ
− 1/ꢁ0
where ꢁ and ꢁ0 are the lifetime of the excited state in the
presence and absence of quencher. The PET rate strictly corre-
Table 1
Some experimental and theoretical details of dyads 1, 2 and 3.
Compound
4
20 nm, which is the characteristic excitation wavelength of por-
phyrin.
From the geometrical optimized structures (shown in Fig. 5)
1
2
3
˚a
0.023
0.031
0.049
ꢁ
(ns)
2.47
3.36
4.61
and the dihedral angles between porphyrin and anthraquinone
moieties (given in Table 1) of the three dyads, we can find that
the two planes in dyads 1 and 3 are almost vertical to each other
kET(S−1)
3.32 × 10
5
1.76 × 10
5
9.49 × 10
4
LUMO (eV)
HOMO (eV)
Dihedral angleb
−2.9468
−4.8216
−3.0312
−4.7509
−3.0121
−4.6856
◦
◦
◦
◦
o
88.3
23.0
73.8
(the dihedral angles are 88.3 and 73.8 for 1 and 3, respectively),
indicating a small extent of ␲–␲ stacking in these two dyads.
Thus, if a “through space” mechanism is predominantly opera-
tive in the PET of this system, the electron transfer efficiency of
a
˚
TPP(CH2Cl2) = 0.11.
Refers to the dihedral angles between porphyrin and anthraquinone moieties
in the dyads.
b

P. Zhao et al. / Spectrochimica Acta Part A 78 (2011) 437–442
441
Fig. 5. The geometrical optimized structures and the distributions of HOMO and LUMO for dyads 1, 2 and 3. Shown as big atoms for the corresponding molecular orbitals.
dyads 1 and 3 will be much lower than dyad 2, in which the
dihedral angle between the two planes is 23.0o with a relatively
stronger ␲–␲ interaction. However, the experiments give a totally
different result. As noted above, the fluorescence quantum yields
electronic states with those of the donor and acceptor. To some
extent, it is also possible that the “through-space” mechanism may
have a role to play in the efficient PET observed in the present
set of Por–Cn–AQ dyads (especially in dyad 2 with a small dihe-
dral angle) but it is not apparent from the results obtained in this
study.
(
˚ < ˚ < ˚ ), the lifetimes (ꢁ < ꢁ2 < ꢁ ) and the photoinduced
1 2 3 1 3
electron transfer rate kET (kET, 1 < kET, 2 < kET, ) of these dyads are
3
in an order which has no close relationship with the geometri-
cal structures but closely related to the length of flexible linkage
between porphyrin and anthraquinone moieties. Seemingly, these
results are best compatible with “through-bond” (superexchange)
mechanism, since it is widely admitted that, in a “through-bond”
The above theoretical results can explain the electron trans-
ference properties of these porphyrin–anthraquinone dyads to a
certain extent. However, our theoretical calculation skill is not
so mature and needs further development. For example, model-
ing the porphyrin–anthraquinone system in solvent environment
instead of modeling isolated compounds would provide more valu-
able information for such kind of studies. Studies addressing this
issue are currently in progress.
(
superexchange) electron transmission pathway, the molecules
that reside between the donor and the acceptor may influence the
rate of donor–acceptor electron transfer by means of mixing their

4
42
P. Zhao et al. / Spectrochimica Acta Part A 78 (2011) 437–442
4
. Conclusion
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lows an order of 1 > 2 > 3, which strictly corresponds to the length
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Acknowledgements
This work was financially supported by Guangdong Pharmaceu-
tical University and the Medical Research Foundation of Guangdong
(
B2010158).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
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