Push-pull hyperbranched molecules. A theoretical study

Article abstract of DOI:10.1002/poc.1417

The electronic properties of the ground state, unrelaxed and relaxed first excited states of push-pull hyperbranched molecules bearing amino and nitro terminal groups have been studied at BB1K/cc-pvdz//HF/6-31g(d), TD-BB1K/cc-pvdz//HF/6-31g(d) and TD-BB1K

Full text of DOI:10.1002/poc.1417

Research Article
Received: 15 April 2008,
Revised: 2 June 2008,
Accepted: 3 June 2008,
Published online in Wiley InterScience: 9 July 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1417
Push–pull hyperbranched molecules.
A theoretical study
a
a
a
´
Estrella Ramos , Patricia Guadarrama , Gerardo Teran
and Serguei Fominea
*
The electronic properties of the ground state, unrelaxed and relaxed first excited states of push–pull hyperbranched
molecules bearing amino and nitro terminal groups have been studied at BB1K/cc-pvdz//HF/6-31g(d), TD-BB1K/
cc-pvdz//HF/6-31g(d) and TD-BB1K/cc-pvdz//CIS/6-31g(d) levels of theory, respectively. It was demonstrated that
dendritic architecture of push–pull molecules favours the charge transfer in the excited state compared to linear
molecules. The possibility of adopting a plane conformation is an important condition for the charge transfer in an
excited state. According to the calculations 1:1 ratio of donor and acceptor groups is another important precondition
for the manifestation of strong charge separation in the excited state. In case of excess of nitro groups over the amino,
some of the excitations participating in the S0 ! S1 transition favour the charge transfer in the excited state in the
opposite directions, thus decreasing the charge separation. Copyright ß 2008 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: dendrimers; push–pull molecules; charge transfer TD-DFT; excited state; conjugation
To gain better understanding of this mechanism of charge
INTRODUCTION
separation, we used the quantum chemistry tools to study the
electronic properties of the ground and excited states of
donor–acceptor hyperbranched molecules and the correspond-
ing linear models.
Donor–acceptor systems[1–4] are very interesting and attractive
candidates for applications in molecular electronics, light
harvesting and photocatalysis and also find multiple applications
in organic photovoltaic cells[5] to convert sunlight into electrical
energy.[6] Recently, there has been a great interest in the
preparation of photoactive macromolecules on the nanometre
scale, and among these, branched molecules have drawn great
deal of interest due to their architecture.[7–12] A theoretical study
on conjugated phenyl-cored thiophene dendrimers has recently
been published describing the optical response and the
excited-state properties.[13]
We have developed an efficient synthesis of conjugated
dendrons[14] using the Wittig reaction of b,b-dibromostyrens
and substituted phenyl acetylenes to afford fully conjugated
branched molecules bearing donor and acceptor groups. All
oligomers were found to be blue emitters with their emission
maxima correlating with the number of the repeating units. It is
known that efficient conversion of light into other energy sources
requires the formation of charge-separation states that exhibit
long lifetimes.[1] Several approaches have been explored in the
last few years for this purpose. The most common of them is
probably the design of multicomponent systems that separate
the photogenerated charges over long distances, either by a
cascade of sequential charge migration steps[15] or by the
dissociation of the charged components in supramolecular
ensembles.[16,17] However, recent studies have demonstrated
that there are still many optimizable parameters in simple
donor–acceptor models that play a decisive role in the kinetics of
charge separation and recombination. Taking the latter into
account, another possibility of exploration is the use of branched
molecules containing multiple donor and acceptor groups to
enhance the charge separation in the excited state.
COMPUTATIONAL DETAILS
All calculations were carried out using Gaussian 03 suite of
programs.[18] The theoretical model selection was based on its
ability to reproduce the experimental absorption and emission
spectra of synthesized molecules. Table 1 shows the experimental
and theoretical long wave absorption and emission maxima for
various branched conjugated molecules. As seen, the selected
model is able to reproduce reasonably well both absorption and
emission spectra of selected molecules. Their molecular struc-
tures are shown in Scheme 1. Letter c in Scheme 1 denotes the
linear analogue of the corresponding dendritic molecules
denoted as Na, where N ¼ 5 or 7. When N ¼ 4 or 6 the letter a
corresponds to the molecules with excess of amino groups while
b means excess of nitro groups. Molecules 1a, 2a and 3a are the
reference molecules used for the calibration of the theoretical
model.
*
Correspondence to: S. Fomine, Instituto de Investigaciones en Materiales,
´
Universidad Nacional Autonoma de Mexico, Apartado Postal 70-360, CU,
Coyoacan, Mexico DF 04510, Mexico.
´
´
´
E-mail: fomine@servidor.unam.mx
´
E. Ramos, P. Guadarrama, G. Teran, S. Fomine
Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma
a
´
´
de Mexico, Apartado Postal 70-360, CU, Coyoacan, Mexico DF 04510, Mexico
´
´
J. Phys. Org. Chem. 2009, 22 9–16
Copyright ß 2008 John Wiley & Sons, Ltd.

E. RAMOS ET AL.
included 10 000 iterations. Lowest energy structures obtained
were then used as inputs for ab initio optimizations without any
symmetry restrictions.
The ground state geometry was optimized at HF/6-31G (d)
level of theory. Electronic properties of the ground state were
calculated using BB1K functional[20] in combination with cc-pvdz
basis set. The electronic properties of the unrelaxed S1 state were
calculated using the TD-BB1K/cc-pvdz model at HF/6-31G(d)
optimized geometry. BB1K functional is available in Gaussian 03
Table 1. Experimental (lexp) and theoretical (lt) long wave
absorption and the corresponding emission (le-expand le-t
)
maxima in chloroform, respectively (nm)
Molecule
Lexp
lt
le-exp
le-t
1a
2a
3a
4a
377 (Reference 14)
365 (Reference 21)
368
347
375
394
443
—
—
532
454
—
—
510
372
380
using
a
combination of keywords BB95 and IOp(3/76 ¼
0580004200). BB1K-TDDFT excitation energies were shown to
be in excellent agreement with experiment for the excitations
involving electron transfer.[21]
First, the extensive conformational searches have been carried
out using a mixed torsional/large scale low mode sampling
algorithm incorporated in Macromodel 9.0 suite of program
using OPLS-AA force field.[19] Each conformational search
The geometries of relaxed S1 states were obtained at CIS/
6-31G(d) level. The electronic properties of the relaxed S1 state
were calculated at TD-BB1K/cc-pvdz level using CIS/6-31G(d)
optimized geometry. All calculations were carried out in the gas
Scheme 1. Structures of studied molecules
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2009, 22 9–16

PUSH–PULL HYPERBRANCHED MOLECULES
phase unless otherwise mentioned. The solvation in chloroform
was taken into account using the PCM model incorporated in
Gaussian 03 suite of programs.[18] The molecular cavity was
specified using the United Atom Topological Model applied on
radii optimized for the PBE0/6-31G(d) level of theory (uaks
keyword).
EXPERIMENTAL
All reagents were used from Aldrich without any purification.
Synthesis of b,b-dibromo-4-nitrostyrene is described in Refer-
ence [22]. The visible absorption and emission (excitation at
350 nm) spectra were measured in chloroform using Cary–Eclipse
Varian fluorescence spectrometer.
Synthesis of b,b-di(4(-aminophenylethynyl)-4-
nitrostyrene(4a)
A mixture of b,b-dibromo-4-nitrostyrene (1.3 mmol), dichlorobis-
(triphenylphosphine)palladium (5 mg) and triethylamine (0.2 ml)
was stirred in 20 ml of polyethyleneglycol-200 under N2
atmosphere. 4-ethynylaniline (2.6 mmol) was added dropwise
to the solution. The mixture was heated to 80 8C for 8 h.
The reaction mixture was extracted with ether and the raw
product was purified by column chromatography on SiO2-
ammonia (eluent: hexane:ethylacetate, 1:3) to obtain an orange
solid in 60% yield.
1H NMR (CDCl3): 8.2 (d, 2H, ortho to NO2), 7.6 (d, 2H, metha
—
to NO ), 7.5 (s, 1H, HC C), 7.3 (d, 2H, metha to NH ), 6.6 (d, 2H,
—
2
2
ortho to NH2).IR (cmꢀ1): 2864 (CH), 3430 (NH), 2357–1946 (C C),
—
—
1600 (Carom),1454 (N—O).
Figure 1. Ground state geometries of studied molecules. The numbers
are the differences of the corresponding bond lengths in the ground and
˚
excited states in A
RESULTS AND DISCUSSION
Ground state geometry
Figure 1 shows HF/6-31G(d) optimized ground state structures.
Nitro and amino groups were selected as strong and typical
acceptor and donor groups, respectively to study the properties
of branched push–pull molecules. Molecules 5c and 7c are the
linear analogues of branched molecules of different generations.
As seen, the lowest energy conformers of 5c and 7c are quite
different. While the global minimum of 7c is linear, 5c molecule is
bent due to energetically favourable interactions between nitro
and amino groups. For 7c, the bent conformation similar to 5c is
not favoured for steric reasons. To estimate the effect of dendritic
architecture on the molecular planarity which is an important
characteristic of conjugated systems, the RMS deviation from
plane for 4a and 4b dendrons. The situation is different for 6a and
6b, where 6a is more planar compared to 6b due to hydrogen
bonding between adjacent amino groups as follows from the
inspection of their geometry (shortest distance between
˚
adjacent NH2 groups is of 2.42 A). In 4a this interaction is not
possible due to restrictions imposed by the molecular geometry.
Therefore, the interactions between terminal groups favour the
dendron planarity. This conclusion is also supported comparing
the deviations from plane for 6b and 7a. Even though 7a is more
branched compared to 6b, the former molecule is more planar.
This is due to interactions between lone pairs of oxygens of nitro
group with hydrogen atoms of amino group (Fig. 1).
˚
plane per atom in A was calculated for all studied molecules and
the results are shown in Table 2.
As seen, there are various factors affecting the planarity of
conjugated molecules in the ground state. All other things being
equal, the planarity of the molecules decreases with branching
degree. Thus, as expected, linear molecule 7c shows RMS
Excited state geometry
There are two major differences between the geometries of S0
and S1 states of studied molecules (as shown in Supporting
Information). The first one is that the RMS deviation from plane is
much less for the excited state than for the ground state (Table 2).
The second one is the shortening of C—N distances in S1 state
(Fig. 1). These changes are the indications of the charge transfer
from amino to nitro group occurring in the excited state.
According to these results, the electron transfer in the excited
state can be represented by resonant structures with formal
˚
deviation from plane 0.00 A. On the other hand, when comparing
4a, 4b, 6a and 6b molecules one can notice that while for the
molecules 4a,b nitro groups favour the planarity of dendrons, this
is not the case for the next generation 6a,b molecules. The
explanation of this phenomenon is that the electron withdrawing
groups favour planarity reducing electron repulsion between
aromatics as seen from the comparison of RMS deviation from
J. Phys. Org. Chem. 2009, 22 9–16
Copyright ß 2008 John Wiley & Sons, Ltd.
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E. RAMOS ET AL.
˚
Table 2. Dipole moment (m), (D), topological diameter (Dt) and deviation from plane (Dev) (A)
m
Devb
Molecule
S0rel
S1
S1rel
Dt
Dm/Dta
S0
S1
g/ec
4a
4b
5a
5c
6a
6b
7a
7c
8.10
9.24
8.37
3.07
12.41
7.53
11.24
9.14
21.38
19.81
27.64
19.06
32.73
19.50
25.46
22.12
15.24
13.44
20.22
11.12
28.22
19.22
19.08
20.92
14
16
22
22
30
32
38
38
0.51
0.26
0.54
0.37
0.53
0.37
0.21
0.31
32.7(0.71)
14.2(0.31)
83.6(1.1)
20.8(0.43)
161.5(1.52)
428.0(4.04)
585.0 (2.98)
0.190 (0.00)
10.2 (0.22)
12.0 (0.26)
11.6(0.15)
9.92 (0.21)
71.8(0.68)
162 (1.53)
468 (2.39)
0.19 (0.00)
3.23
1.19
7.33
2.05
2.23
2.64
1.25
1
´
a Dm ¼ S1rel ꢀ S0rel, Dm/Dt (D/A).
˚
b RMS deviation from the least-squares plane, with deviation per atom in parenthesis.
c Ratio S0/S1 of RMS deviation from plane per atom (deviation per atom).
double bonds between the aromatic rings and amino and nitro
groups, respectively, as shown in Fig. 2. The planarization and
C—N distances shortening in the excited state although
confirming the electron transfer from amino to nitro groups in
excited state cannot be taken as a measure of this transfer
because both these parameters depend on steric factors and the
electron count in the molecules. However, several conclusions
can be made comparing similar molecules. Thus, as seen from
Fig. 1 differences in C—NO2 and C—NH2 distancesbetween
ground and excited states are significantly larger for branched
molecules 4a and 4b, respectively, compared to linear analogue
5c demonstrating dendritic effect on the electron transfer in the
excited state. Similar observation can be made for the molecules
6a, 6b and 7c. As seen from Fig. 1 differences in C—NO2 and
C—NH2 distance between S0 and S1 states for 6a and 6b are of
emission energies where no apparent correlation between the
emission energy and the effective conjugation length (the
number of aromatic rings) is observed as seen from Table 3.
The lowest emission energy is found for 5a followed by 7a.
These molecules have multiple donor and acceptor groups,
resulting in better stabilization of CT excited state. 5a has two
nitro and two amino groups, while 7a has four nitro and four
amino groups. As seen from Table 3 these molecules show the
largest Stock shifts, supporting this hypothesis. Table 4 shows the
total energies for excited states, as well as the relaxation energies.
The direct comparison of the energy difference between relaxed
and unrelaxed S1 states (Table 4) shows that the largest
relaxation energy is for 5a closely followed by 7a. Thus, the best
stabilization of CT excited state occurs for the dendritic structures
with equal number of donor and acceptor groups. Steric factors
play an important role since the excessively branched molecule
7a is unable to adopt the geometry plane enough to stabilize
the S1 state to the same extent as 5a does, as seen from
the comparison of RMS deviation from plane data for S0 and S1
states (Table 2). The ratio g/e is 7.33, the largest for 5a dendrimer
while it is 1.25, for 7a dendrimer reflecting steric hindrances in
this molecule.
The charge transfer degree on photoexcitation can be
estimated by the comparison of the dipole moments in S0
and S1 states (as shown in Table 2). However, the direct
comparison of the dipole moments is not totally correct due to
the size differences between the molecules. To compare the
charge transfer in the excited states we used Dm/Dt index where
Dm is the difference of the dipole moments between the ground
and excited states of the same molecule and Dt is the topological
diameter of the system i.e. the maximum topological distance in
the molecule. Table 2 shows the dipole moments in the ground
and excited states as well as Dm/Dt indexes. As seen the dipole
moments in the ground state depend mostly on the geometry of
global minimum conformation. Thus, the small dipole moment of
5c is due to its bent conformation and 4b has larger dipole
moment than 4a in the ground state. The situation changes for
the next generation dendrons (6a and 6b) where 6a has larger
dipole moment due to conformational differences (as shown in
Supporting Information). As seen from Table 2 the excitation from
˚
0.006 and 0.014 A, while for linear 7a molecule these differences
˚
are of 0.001 and 0.002 A, respectively.
Excitation, emission energies and dipole moments
Table 3 shows the calculated S0 ! S1 transition energies. As seen
from Table 3 the vertical excitation energies of the dendrimer and
the corresponding linear analogues decrease with the number of
aromatic rings in the molecule and do not depend on the
molecular architecture. Thus, NO2 group causes larger red shift
compared to NH2 (4a, 4b and 6a, 6b) since NO2 introduces larger
conjugation compared to NH2. The situation changes for
Figure 2. Resonant structures representing electron transfer in excited
state
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Copyright ß 2008 John Wiley & Sons, Ltd.
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PUSH–PULL HYPERBRANCHED MOLECULES
Table 3. Calculated orbitals involved in S0!S1 excitation and the corresponding expansion coefficients (Ci), Stock shifts (nm), long
wave (nm) absorption and emission (in brackets) maxima of studied molecules
Molecule
S0 ! S1
Ci
Stocks shift
l
4a
4b
5a
HOMO ! LUMO
0.65
0.65
99.4
93.0
162.6
371.25(470.69)
371.72(464.75)
412.52(575.09)
HOMO ! LUMO
HOMO ꢀ 2 ! LUMO
HOMO ! LUMO
ꢀ0.22
0.60
HOMO ! LUMO þ 2
HOMO ! LUMO
ꢀ0.18
0.68
0.14
5c
6a
118.0
89.9
390.75(508.74)
405.33(495.22)
HOMO ꢀ 4 ! LUMO
HOMO ! LUMO
0.61
ꢀ0.18
HOMO ! LUMO þ 1
HOMO ! LUMO þ 2
HOMO ꢀ 1 ! LUMO
HOMO ꢀ 1 ! LUMO þ 1
HOMO ! LUMO
0.16
6b
7a
ꢀ0.11
0.13
0.61
107.1
153.2
409.14(516.19)
434.09(587.30)
HOMO ! LUMO þ 4
HOMO ꢀ 3 ! LUMO
HOMO ꢀ 3 ! LUMO þ 3
HOMO ꢀ 2 ! LUMO
HOMO ꢀ 1 ! LUMO
HOMO ! LUMO
0.19
ꢀ0.29
0.12
ꢀ0.19
0.12
0.37
HOMO ! LUMO þ 1
HOMO ! LUMO þ 3
HOMO ! LUMO þ 2
HOMO ! LUMO þ 4
HOMO ! LUMO þ 6
HOMO ꢀ 2 ! LUMO
HOMO ꢀ 2 ! LUMO þ 1
HOMO ꢀ 1 ! LUMO
HOMO ꢀ 1 ! LUMO þ 1
HOMO ꢀ 1 ! LUMO þ 2
HOMO ! LUMO
0.10
ꢀ0.24
ꢀ0.11
0.14
0.13
7c
ꢀ0.11
ꢀ0.10
0.36
73.6
402.06(475.68)
ꢀ0.16
0.11
ꢀ0.34
0.35
0.12
HOMO ! LUMO þ 1
HOMO ! LUMO þ 2
S0 to S1 state results in significant increase of dipole moment
indicating the charge transfer along the molecule. As follows
from TD calculations in all cases the most important contribution
into S0 ! S1 transition represents HOMO–LUMO excitation.
Figure 3 shows the molecular orbitals contributing to the
excitation with Ci expansion coefficients of at least 0.1.
The corresponding coefficients are shown in Table 3. As seen,
for the molecules 4a, 4b and 5c the only important excitation is
HOMO–LUMO one.
The number of excitations contributing to S0 ! S1 transition
increases with molecular size, however, in all cases except for 7c
HOMO–LUMO excitation prevails. As seen from MO plots
(Fig. 3a and b) the interpretation of S0 ! S1 transition for 4a,
4b and 5c is rather straightforward meaning the electron transfer
from the ‘donor’ moiety to the ‘acceptor’ moiety. In the case of 4a
and 4b, the ‘donor’ and the ‘acceptor’ moiety include two amino
and two nitro groups, respectively, while in the case of 5c the
‘donor’ and ‘acceptor’ moiety involve one amino and one nitro
group, respectively. As a consequence 4a and 4b excited state
geometries show greater contribution from quinoid structure
(Fig. 2) to the geometry of their excited states compared to linear
5c (Fig. 3b). The introduction of two amino and two nitro groups
into molecular structure (5a) further improves the charge transfer
in the excited state as seen from Fig. 3b. HOMO includes atomic
orbitals of two NH2 groups and LUMO includes the corresponding
Table 4. Total energies of unrelaxed S1 states and the
corresponding relaxation energies in a.u. and eV, respectively
Molecule
S1
Era
4a
4b
5a
5c
6a
6b
7a
7c
ꢀ1238.676305
ꢀ1387.780282
ꢀ2134.667903
ꢀ1260.714612
ꢀ2732.439740
ꢀ3179.745945
ꢀ5420.385729
ꢀ2029.627397
0.484
0.411
0.835
0.676
0.527
0.619
0.838
0.509
a Relaxation energy defined as (S1 ꢀ S1rel).
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E. RAMOS ET AL.
Figure 3. (a) Molecular orbitals involved in S0 ! S1 transition for 4a and 4b molecules; (b) molecular orbitals involved in S0 ! S1 transition for 5a and 5c
molecules; (c) molecular orbitals involved in S0 ! S1 transition for 6a and 6b molecules; (d) molecular orbitals involved in S0 ! S1 transition for 7a and 7c
molecules
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PUSH–PULL HYPERBRANCHED MOLECULES
orbitals of two nitro groups. In fact, 5a has the highest Dm/Dt
index for the relaxed S1 state of all molecules (0.54), indicative of
the best charge transfer on excitation. Unlike 4a, in the case of
6a the most important HOMO–LUMO excitation for S0 ! S1
transition involves only half of all amino groups available in
molecule. Another excitation contributing to the charge transfer
is HOMO ꢀ 4 ! LUMO (Fig. 3c) with smaller coefficient (as shown
in Table 3). However, as seen from Fig. 3c one of the four NH2
groups in 6a does not participate in the charge transfer. Similar
situation holds for 6b where only HOMO ! LUMO excitation
contributes to the charge transfer. Three other excitations do not
contribute significantly to the electron transfer from amino to
nitro groups as seen from Fig. 3c. Dm/Dt index is in line with the
orbital analysis of S0 ! S1 excitation; for 6a Dm/Dt is less than for
5a and for 6b Dm/Dt is less than for 6b (as shown in Table 2). As
can be seen from Table 3 and Fig. 3d all the excitations involved in
S0 ! S1 transition in 7a contribute to the charge transfer from
the ‘donor’ to ‘acceptor’ moiety. However, the Dm/Dt index
(Table 2) calculated for the relaxed S1 state is the lowest of all
studied molecules. This observation agrees with the fact that 7a
shows the largest deviation from plane in both ground and
excited states and the lowest ratio out of all molecules except for
7c since 7c is essentially plane even in the ground state. In fact,
the Dm/Dt index is higher for 7c compared to 7a due to the plane
geometry of the former. As seen from Fig. 3c and Table 3 the only
excitation contributing to the charge transfer in the excited state
in 7c is HOMO–LUMO transition.
As can be seen from Table 2 the dipole moments of dendrons
4b and 6b in the ground state are larger than those of 4a and 4b,
however, in the excited state the situation reverses; 4a and 6a
have larger dipole moments than 4b and 6b. Therefore, Dm/Dt
index is higher for 4a and 6a compared to 4b and 6b (Table 2)
evidencing better charge transfer in the excited state for
dendrimers with multiple amino groups compared to multiple
nitro groups. This difference between nitro and amino groups can
be understood analysing excitations contributing to S0 ! S1
transition in different molecules. All contributions with expansion
coefficients greater than 0.01 were analysed for molecules 4a, 4b,
6a and 6b. While for dendrons 4a and 6a all excitations
involved in S0 ! S1 transition implied the charge transfer from
‘donor’ to ‘acceptor’ moieties in case of 4b and 6b excitations
involving MOs shown in Fig. 4 favours the charge transfer in the
opposite direction, thus reducing the dipole moment in excited
state compared to 4a and 6b dendrons.
CONCLUSIONS
Calculations showed that dendritic structure of push–pull
molecules favours the charge transfer in the excited state. Thus,
the contribution to the excited state geometry from zwitter ionic
quinoid structure is larger for dendritic 4a and 4b molecules
(Fig. 2) compared to linear 5c. An important condition for
the manifestation of dendritic effect is the possibility for the
molecule to adopt planar conformation in excited state. Thus, the
charge transfer in the excited state is higher for 4a, 4b and 5a
compared to 6a, 6b and 7a due to steric hindrances to adopt
plane conformation for the latter. According to the analysis of the
dipole moments in the ground and excited states, the most
efficient charge transfer occurs in 5a dendron evidencing that 1:1
ratio of donor to acceptor group is another important
precondition for the molecule to show strong charge transfer
in the excited state. All other things being equal, the excess of
amino groups is preferable to the excess of nitro groups for
studied dendrimers to manifest strong charge transfer in the
excited state due to the fact that in molecules 4b and 6b some of
the excitations contributing to S0 ! S1 transition favour the
charge transfer in the opposite direction.
SUPPORTING INFORMATION
Cartesian coordinates of optimized geometries of studied
molecules are available in the supplementary material.
Acknowledgements
This research was carried out with the support of grant 49290
from CONACyT.
REFERENCES
[1] Eds.: V., Balzani, P. Piotrowiak, , Electron Transfer in Chemistry, Vols. 1-5,
Wiley-VCH, New York, NY, 2001.
[2] M. R. Wasielewski, Chem. Rev. 1992, 92, 435.
[3] M. N. Paddon-Row, Electron and Energy Transfer, In Stimulating
Figure 4. Some of the molecular orbitals involved in S0 ! S1 transition
for molecules 4b and 6b
¨
Concepts in Chemistry (Eds.: F., Vogtle, J., Stoddart, M. Shibasaki),
Wiley-VCH, New York, NY, 2000, pp 267–291.
J. Phys. Org. Chem. 2009, 22 9–16
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E. RAMOS ET AL.
´
[4] J. Cremer, E. Mena-Osteritz, Neil, G. Pschierer, K. Mu¨llen, P. Ba¨uerle,
Org. Biomol. Chem. 2005, 3, 985.
[17] L. Sanchez, N. Mart´ın, D. M. Guldi, Angew. Chem. Int. Ed. 2005, 44,
5374.
[5] B. O’Regan, M. Gra¨tzel, Nature 1991, 353, 737.
[6] A. M. Ramos, S. C. J. Meskers, P. A. van Hal, J. Knol, J. C. Hummelen,
R. A. J. Janssen, J. Phys. Chem. A 2003, 107, 9269.
[7] T. H. Kwon, M. K. Kim, J. Kwon, D. Y. Shin, S. J. Park, C. L. Lee, J. J. Kim,
J. I. Hong, Chem. Mater. 2007, 19, 3673.
[18] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, V. G. Zakrzewski, A. Montgomery, R. E. Stratmann, Jr,
J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi,
B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M.
Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc; Wallingford CT,
2004.
[8] A. Petrella, J. Cremer, L. De Cola, P. Ba¨uerle, R. M. Williams, J. Phys.
Chem. A 2005, 109, 11687.
˜
´
[9] V. Parra, T. Del Cano, M. J. Gomez-Escalonilla, F. Langa, M. L. Rodrı-
´
guez-Mendez, J. A. De Saja, Synth. Met. 2005, 148, 47.
[10] S.-C. Lo, P. L. Burn, Chem. Rev. 2007, 107, 1097.
[11] J. Qiao, C. Yang, Q. He, F. Bai, Y. Li, J. Appl. Polym. Sci. 2004, 92, 1459.
[12] C.-Q. Ma, E. Mena-Osteritz, T. Debaerdemaeker, M. M. Wienk, R. A.
Janssen, P. Ba¨uerle, Angew. Chem. Int. Ed. 2007, 46, 1679.
¨
¨
[13] M. E. Kose, W. J. Mitchell, N. Kopidakis, C. H. Chang, S. E. Shaheen, K.
[19] MacroModel 9.0, supplied by Schrodinger, Portland, OR.
Kim, G. Rumbles, J. Am. Chem. Soc. 2007, 129, 14257.
[14] L. Fomina, P. Guadarrama, S. Fomine, R. Salcedo, T. Ogawa, Polymer
1998, 39, 2629.
[20] Y. Zhao, B. J. Lynch, D. G. Truhlar, J. Phys. Chem. A. 2004, 108, 2715.
[21] J. To, A. Sokol, S. French, N. Kaltsoyannis, R. Catlow, J. Chem. Phys.
2005, 122, 144704.
´
[22] S. Fomine, L. Fomina, R. Jimenez, E. Popova, T. Ogawa, Mendeleev
Commun. 1998, 8, 117.
[15] D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34, 40.
[16] D. M. Guldi, N. Mart´ın, J. Mater. Chem. 2002, 12, 1978.
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