1-alkyl-1H-imidazole-based dipolar organic compounds for dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.200900244

A series of donor-π-acceptor-type organic dyes based on 1-alkyllH-imidazole spacers 1-5 have been developed and characterized. The two electron donors are at positions 4 and 5 of the imidazole, while the electron-accepting cyanoacrylic acid is incorporated at position 2 by a spacer-containing heteroaromatic rings, such as thiophene and thiazole. Detailed investigation on the relationship between the structure, spectral and electrochemical properties, and performance of DSSC is described here. Dye-sensitized solar cells (DSSCs) using dyes as the sensitizers exhibit good efficiencies, ranging from 3.06 to 6.35%, which reached 42-87% with respect to that of N719-based device (7.33%) fabricated and measured under similar conditions. Timedependent density functional theory (TDDFT) calculations have been performed on the dyes, and the results show that both electron donors can contribute to electron injection upon photo-excitation, either directly or indirectly by internal conversion to the lowest excited state.

Full text of DOI:10.1002/asia.200900244

DOI: 10.1002/asia.200900244
1-Alkyl-1H-imidazole-Based Dipolar Organic Compounds for Dye-
Sensitized Solar Cells
Marappan Velusamy, Ying-Chan Hsu, Jiann T. Lin,* Che-Wei Chang, and
Chao-Ping Hsu*[a]
Abstract: A series of donor–p–accept-
spectral and electrochemical properties,
and performance of DSSC is described
with respect to that of N719-based
or-type organic dyes based on 1-alkyl-
1H-imidazole spacers 1–5 have been
developed and characterized. The two
electron donors are at positions 4 and 5
of the imidazole, while the electron-ac-
cepting cyanoacrylic acid is incorporat-
ed at position 2 by a spacer-containing
heteroaromatic rings, such as thiophene
and thiazole. Detailed investigation on
the relationship between the structure,
device (7.33%) fabricated and mea-
sured under similar conditions. Time-
dependent density functional theory
(TDDFT) calculations have been per-
formed on the dyes, and the results
show that both electron donors can
contribute to electron injection upon
photo-excitation, either directly or indi-
rectly by internal conversion to the
lowest excited state.
here.
Dye-sensitized
solar
cells
(DSSCs) using dyes as the sensitizers
exhibit good efficiencies, ranging from
3.06 to 6.35%, which reached 42–87%
Keywords: charge transfer · donor–
acceptor systems · dyes · photophy-
sics · sensitizers
Introduction
tizers are relatively cheaper, and molecules with high molar
extinction coefficients are easily attained. Therefore, there is
also tremendous progress in metal-free sensitizer-based
DSSCs in recent years, and efficiencies exceeding 9% have
been reached,[5] using sensitizers which have very high molar
extinction coefficients in the visible region.
Inorganic semiconductor photovoltaic cells have been devel-
oped for over two decades; nevertheless, research in this
area has never ceased because solar energy is readily avail-
able and environmentally friendly. To counteract the deficit
of energy and the increasing decline of natural resources, or-
ganic compounds have also attracted considerable interest
after Grꢀtzelꢁs[1] and Heegerꢁs[2] seminal works on ruthenium
complex-based dye-sensitized solar cells (DSSCs) and poly-
mer-based photovoltaic cells. DSSCs with high conversion
efficiency (~11%) approaching that of amorphous-silica-
based photovoltaic cells have been achieved by using cis-di(-
thiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylate)ruthen-
The metal-to-ligand charge-transfer (MLCT) transition
plays an important role in light-harvesting processes for
ruthenium-based sensitizers. In contrast, typical metal-free
organic dyes consist of the electron donor, the electron ac-
ceptor with an anchoring group (such as carboxylic acid),
and a conjugated spacer to facilitate charge transfer from
the donor to the acceptor. Various metal-free organic sensi-
tizers including coumarin-,[6] indoline-,[7] cyanine-,[8] hemi-
cyanine-,[9] merocyanine-,[10] perylene,[11] xanthene-,[12] triar-
ylamine-,[13] oligoene-,[14] and thiophene-based[6b,15] dyes
have been reported for DSSCs. Choice of the donor and the
acceptor for the sensitizer is somewhat limited, and aryl- (or
alkyl-) amine and 2-cyano-acrylic acid appear to be the most
popular donor and acceptor, respectively. Tuning of the
charge-transfer absorption for better light harvesting can be
practiced by variation of the spacer.
ACHTUNGTRENNUNG
ium(II) (N3) and analogues as the sensitizer.[3] Though there
is still room for improvement of the efficiency of ruthenium-
based DSSCs,[4] prototype ruthenium sensitizers are never-
theless costly and normally have absorption coefficients at
~20000mꢀ1 cmꢀ1 or below. In comparison, metal-free sensi-
[a] Dr. M. Velusamy, Dr. Y.-C. Hsu, Prof. Dr. J. T. Lin, C.-W. Chang,
Prof. Dr. C.-P. Hsu
Institute of Chemistry
We have been interested in metal-free sensitizers for
DSSCs application. Previously, we reported several series of
arylamine-based sensitizers with various heteroaromatic
rings in the spacers, and achieved high-performance DSSCs
using these sensitizers.[15a,b,16] Among these, 1H-phenanthro-
Academia Sinica, 115 Nankang, Taipei (Taiwan)
Fax : (+886)2-27831237
E-mail: jtlin@chem.sinica.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900244.
Chem. Asian J. 2010, 5, 87 – 96
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87

FULL PAPERS
A
imidazolyl intermediate was performed with nBuLi and N-
formylpiperidine in THF. The final target compounds were
synthesized by treatment with cyanoacetic acid in refluxing
acetic acid in the presence of a catalytic amount of ammoni-
um acetate. The structures of all synthesized compounds
were characterized by NMR, MS, and elemental analysis.
based[16b] dyes with two terminal arylamines appear to be in-
teresting. Recombination of the injected electrons with the
oxidized dyes was retarded through a cascade of charge sep-
aration in the former, and recombination of the injected
electrons with the oxidized electrolytes was suppressed in
both cases because of the presence of an extra hydrophobic
arylamine-containing segment. In continuation of our stud-
ies on organic sensitizers for DSSCs, 1-alkyl-1H-imidazolyl
unit was used in the spacer based on the following reasons:
1) it is relatively easy to introduce electron donors at posi-
tions 2 and 4, and an electron acceptor at position 5 of 1-
alkyl-1H-imidazoles to form conjugated dipolar com-
pounds;[17] 2) through appropriate modification of the conju-
gated chain, it is possible to tune the charge-transfer transi-
tion of dipolar molecules; 3) charge recombination after
electron injection may be retarded because of decreasing
positive charge density at the donor by electronic delocaliza-
tion of the two substituents at positions 4 and 5 of the imida-
zolyl ring; 4) cascade energy/electron transfer or panchro-
matic absorption may be possible with two different sub-
stituents at positions 4 and 5. In this paper, we report the
synthesis and characterization of new dipolar dyes contain-
ing a 1-alkyl-1H-imidazole moiety. DSSCs using these dyes
as the sensitizers are also discussed.
Photophysical Properties
The optical absorption spectra of the dyes in THF are dis-
played in Figure 2 and the data are collected in Table 1. The
absorption band at <400 nm is attributed to the p–p* transi-
tion. The broad band at >400 nm stems from the charge-
transfer transition from the donors at positions 4 and 5 of
the imidazole to the acceptor, 2-cyanoacrylic acid (see be-
low).[17b,c] The charge-transfer characteristic of the low
energy band is supported by the following observations:
1) this band disappears in compounds na (n=1–5); 2) re-
placement of the 2-cyanoacrylic acid in n (n=1–5) by a
weaker acceptor, aldehyde nb (n=1–5) leads to a blue-shift
of lmax (434 vs 405 nm for 2 and 458 vs 444 nm for 5); 3) the
wavelength of the charge-transfer band increases in the
order of 1<2<3, which is consistent with the order of in-
creasing electron-donating strength of the capping segment,
OMe<Ph2N<Me2N. Although the diarylamino moiety is
considered to be a much stronger electron-donating group
than the methoxy moiety,[18] the small difference in the lmax
value (16 nm) between 1 and 2 indicates that the charge
transfer is only of moderate strength. Similarly, replacement
of the 2-cyanoacrylic acid in 5 by a weaker acceptor, an al-
dehyde, leads to blue-shift of lmax by only 14 nm (458 nm vs
444 nm). Similar to our previous observations,[16c,d] replace-
ment of the p-phenylene unit in 2 by a fluorene unit only
leads to a small change in the absorption wavelength of the
charge-transfer band. Interestingly, incorporation of an elec-
tron-deficient thiazole ring in the spacer, results in both a
bathochromic shift and hyperchromic effect of the charge-
transfer band, that is, 2 versus 5. The extinction coefficients
of the charge-transfer bands in these compounds are only
moderate, ranging from ~10000 to ~27000mꢀ1 cmꢀ1. This
may be attributed to the twist of the donor at position 5 of
the imidazole from co-planarity with the imidazole ring,
which hampers the effective conjugation between the two.
In this series of compounds, the two donor groups (aryl
amine or methoxyphenyl) are attached at two different posi-
tions: position 5, which allows a resonance with electrons to
flow from the donor through the imidazolyl group, whereas
position 4 does not allow such a resonance. For the excita-
tions involving position 5, we expect that the resonance
allows a stronger interaction between the donor and the rest
of the molecule, leading to a more delocalized transition,
and therefore larger oscillator strength. On the contrary, ex-
citations involving position 4 are expected to be more local-
ized and thus have smaller oscillator strength, as the donor
is more isolated from the rest of the molecule. However,
there exists steric hindrance that increases the dihedral
angles between the phenyl group in the donor and the imi-
The synthetic procedures of the compounds 1–5 are shown
in Scheme 1. The structures of the new dyes are illustrated
in Figure 1. The first step of the synthesis of compounds 1–5
was condensation of the corresponding diketone derivatives
with aldehydes in the presence of ammonium acetate in
acetic acid, followed by N-alkylation to form 1,2,4,5-tetra-
substituted imidazoles in good yields.[17] Formylation of the
Scheme 1. Outline of the synthetic scheme for dyes.
88
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Chem. Asian J. 2010, 5, 87 – 96

1-Alkyl-1H-imidazole-Based Dipolar Organic Compounds
dazole moiety, and therefore
decreases electronic interaction
between them. From our calcu-
lated results detailed below, the
dihedral angle (angle A) be-
tween the phenyl group at posi-
tion 4 and the imidazole ring,
ranges from ~2.5 to 288 (see
computation section). In com-
parison, the dihedral angle
(angle B) between the phenyl
group at position 5 and the imi-
dazole ring is significantly
larger, ranging from ~56 to 888.
Apparently, the donor at posi-
tion 5 of the imidazole is more
sterically crowded than that at
position 4 because of the pres-
ence of the N-alkyl group in ad-
dition to the steric interaction
between the two donors. Simi-
lar phenomenon was observed
in other 1,2,4,5-tetra-substituted
imidazolyl congeners.[17a,c]
A blue-shift of the charge-
transfer band in more polar sol-
vent, that is, negative solvato-
chromism, was recognized in
these compounds. For example,
the absorption wavelength in-
creases in the order of DMF (2,
422 nm; 4, 418 nm)<THF (2,
438 nm; 4, 428 nm)<toluene
(2, 447 nm; 4, 447 nm). This
may be attributed to the strong
interaction of polar solvent
molecules with the sensitizer,
ꢀ
which weakens the O H bond
of the carboxylic acid and con-
sequently decreases the elec-
Figure 1. The structure of the dyes.
tron-withdrawing nature of the
COOH group. Additional support for this argument comes
from the blue-shift of lmax upon NEt3 treatment of the THF
solution of the dye, in which complete deprotonation of the
COOH group is expected to occur. The MLCT band of the
black dye[19] and the charge-transfer band of some metal-
free organic dye[15c,16b–d] were reported to behave similarly.
No noticeable changes of lmax values or band broadening
were observed in these dyes upon adsorption on the TiO2
nanoparticles (Figure 3) except for 4 and 5. The red tailing
of the absorption bands for 4 and 5 adsorbed on TiO2 is
mostly likely caused by the J-aggregation of the dyes.[15d,20]
Additional absorption in the shorter wavelength region for
4 on the TiO2 nanoparticles compared to that in the solution
may stem from H-aggregation of the dye molecules.[15d,21]
Though the absorption band of 3 has much weaker intensity
than that of 2 in the solution, the two dyes adsorbed on the
Figure 2. Absorption spectra of the dyes recorded in THF.
Chem. Asian J. 2010, 5, 87 – 96
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89

J. T. Lin, C.-P. Hsu et al.
FULL PAPERS
Table 1. Electrooptical parameters of the dyes.[a]
tial (E0ꢀ0*) of the sensitizer was
estimated from the first oxida-
tion potential at the ground
state and the zero–zero excita-
tion energy E0ꢀ0 according to
Equation (1), in which, the
zero–zero excitation energy was
estimated from the absorption
onset. The electron injection
from the excited sensitizers to
the conduction band of TiO2
should be energetically favora-
ble because of the more nega-
Dye
labs [nm]
E
1/2(ox) [mV]
HOMO/LUMO [eV]
E0ꢀ0 [eV]
E0ꢀ0* [V]
[eꢃ10ꢀ4 mꢀ1 cmꢀ1
]
1
2
418 (2.00), 276 (1.56)
434 (1.94), 339 (3.04),
307 (3.99)
577
370, 512
5.38/2.88
5.17/2.74
2.50
2.43
ꢀ1.22
ꢀ1.36
3
4
5
446 (1.08), 300 (1.58),
268 (1.94)
430 (2.30), 370 (5.81),
312 (3.59)
458 (2.70), 339 (3.14),
306 (4.14)
120, 259
330, 448
344, 502
4.92/2.61
5.13/2.60
5.14/2.85
2.31
2.53
2.29
ꢀ1.49
ꢀ1.50
ꢀ1.25
[a] Absorption data and electrochemical data were collected in DCM solutions. Scan rate, 100 mVsꢀ1; electro-
lyte, (n-C4H9)4NPF6; potentials are quoted in reference to the internal ferrocene standard (E1/2 =+212 mV vs
Ag/AgNO3).
tive E0ꢀ0
*
values (ꢀ1.22~
Figure 3. Absorption spectra of the dyes adsorbed on TiO2 films.
Figure 4. Cyclic voltammograms of compounds 1 and 5 in DCM.
TiO2 nanoparticles have comparable absorption intensity be-
cause of the higher adsorbed dye density of 3 than 2 (see
below). The compound are emissive in THF, and the emis-
sion wavelength is in the order of 4 (535 nm)<1 (558 nm)<
2 (590 nm)<3 (595 nm)<5 (602 nm), which is roughly par-
allel to the trend of the absorption wavelength.
ꢀ1.50 V vs NHE, see Table 1) compared to the conduction-
band-edge energy level of the TiO2 electrode (ꢀ0.5 V vs
NHE).[21] The first oxidation potentials of the dyes (0.82~
1.28 V vs NHE) are more positive than the Iꢀ/I3 redox
ꢀ
couple (0.4 V vs NHE),[15e,22] therefore, dye regeneration
should also be energetically favorable.
Electrochemical Properties
The electrochemical properties of the compounds were stud-
ied by cyclic voltammetry, and all data are listed in Table 1.
Representative cyclic voltammograms are shown in
Figure 4. A quasireversible one-electron oxidation wave ob-
served for 1 is attributed to the oxidation of the imidazole-
based conjugated spacer. For compounds 2–5, the one-elec-
tron redox wave observed at lower potential is attributed to
the oxidization of one of the amines, and the two-electron
redox wave occurring at a higher potential is caused by the
unresolvable waves from the oxidation of the second amine
and the imidazole-based conjugated spacer. As expected,
the N,N-dimethylamine in 3 is oxidized at a significantly
lower potential[16d] compared to the triarylamine in 2–5. The
energy levels of the HOMOs of these materials were calcu-
lated with reference to ferrocene (4.8 eV) and ranged from
4.92 to 5.38 eV. The optical edge was utilized to derive the
band gap and the LUMO energies. The excited-state poten-
*
E0ꢀ0 ¼ EoxꢀE0ꢀ0
ð1Þ
Photocurrent–Voltage Characteristics
Compounds 1–5 were used as the sensitizers for dye-sensi-
tized nanocrystalline anatase TiO2 (20 nm particles, film~
18 mm thick) solar cells (DSSCs). The cells with an effective
area of 0.25 cm2 were fabricated with an electrolyte com-
posed of 0.05m I2/0.5m LiI/0.5m tert-butyl pyridine in aceto-
nitrile solution. The device performance of solar cells under
AM 1.5 illumination are summarized in Table 2. Figure 5
shows the photocurrent–voltage (J–V) curves of the cells.
The action spectra of incident photon-to-current conversion
efficiency (IPCE) for the cells are plotted in Figure 6. The
efficiencies of the DSSCs are in the order of 2 (6.35%)>1
(5.41%)>4 (4.87%)ꢁ5 (4.76%)>3 (3.06%). The efficien-
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1-Alkyl-1H-imidazole-Based Dipolar Organic Compounds
Table 2. Performance parameters of DSSCs constructed using the dyes.[a]
standard ruthenium dye N719-based cell), and the lowest
short-circuit current (Jsc) and open-circuit voltage (Voc). In
view of the high dye density (1, 6.02ꢃ10ꢀ7; 2, 4.50ꢃ10ꢀ7; 3,
7.48ꢃ10ꢀ7; 4, 3.74ꢃ10ꢀ7; 5, 3.84ꢃ10ꢀ7 molcmꢀ2) of 3 on the
TiO2 surface, such an outcome can be rationalized by the
following reasons: 1) 3 has a relatively small molar extinc-
tion coefficient of the charge-transfer band; 2) the high
HOMO energy level of 3 results in slower regeneration of
the oxidized sensitizer after electron injection. Consequent-
ly, more facile charge recombination between the conduc-
tion-band electrons and dye cations leads to lower Jsc and
Voc values. The lower efficiencies of the devices based on 4
and 5 compared to 1 can be attributed to the lower dye den-
sities of 4 and 5 on TiO2 arising from the larger sizes of the
molecules. The device based on 5 has a performance inferior
to those of 1 and 2 despite the broader and intense absorp-
tion band of 5 compared to 1 and 2. Apparently, the charge-
transfer band overlaps with the delocalized p–p* transition,
which does not contribute to the electron injection (see
computation section). The recombination lifetime (tR) of
the photoinjected electron with the oxidized dye was mea-
sured by transient photovoltage[24] at open circuit. To avoid
dark current, I2 was omitted and the electrolyte used was
LiI in CH3CN. The tR values are 3.9, 2.6, 1.0, 5.2, 1.6, and
8.9 ms for the cells based on 1–5 and N719, respectively. The
rather short tR value (1.0 ms) of the 3-based cell, attributed
to the relatively slower regeneration of the oxidized sensitiz-
er (see above), is consistent with its low efficiency. The
longer lifetime for 4 may be caused by the longer distance
between the donor and the anchoring group. Though the
distance between the donor and the anchoring group in 5 is
also large among the five compounds, surprisingly, the
device based on 5 has a much smaller tR value when com-
pared to that of 4.
Cell
Jsc [mAcmꢀ2
]
Voc [V]
FF
h [%]
1
2
3
4
12.76
15.00
7.24
10.01
11.55
16.15
0.66
0.67
0.61
0.69
0.63
0.72
0.64
0.64
0.69
0.71
0.65
0.63
5.41
6.35
3.06
4.87
4.76
7.33
5
N719
[a] Experiments were conducted using TiO2 photoelectrodes with approx-
imately 18 mm thickness and 0.25 cm2 working area on the FTO (15 W/
sq.) substrates.
Figure 5. J–V curves of DSSCs based on the dyes.
The cell stability for 2 was checked briefly by measuring
the efficiency after a period of 24 h. The cell was exposed to
the light source only during the measurements for the first
96 h. After 96 h, the cell was irradiated for 1 h before mea-
surement. A 4% drop in efficiency was observed after 96 h,
and a ~15% drop was noticed after 192 h. The decrease in
the efficiency was mainly caused by the decrease in Jsc. Both
leakage of the solvent and degradation of the sensitizer may
contribute to the decline in the efficiency.
Figure 6. IPCE plots for the DSSCs.
Theoretical Approach
cy of 2-based device reaches 87% of the standard ruthenium
dye N719-based cell (conversion efficiency=7.33%) fabri-
cated and measured under similar conditions. Interestingly,
1-based device also has an efficiency reaching >70% of the
N719-based cell[23] even though 1 has a much weaker elec-
tron donor (methoxophenyl) than 2 (diphenylamino). The
higher efficiency (and therefore, higher short-circuit current)
of the 2-based device than the 1-based device can be ration-
alized by the red-shift of the absorption spectra of 2 com-
pared to 1, that is, better light-harvesting efficiency of 2.
The device from 3 has the lowest efficiency (<50% of the
To gain further insight into the correlation between struc-
ture and the physical properties as well as the device perfor-
mance, quantum chemistry computation was conducted.
Model compounds in which the substituent at position 4
(compounds, n’, n=1–5) or 5 (compounds n’’, n=1–5) of the
imidazole was replaced by a hydrogen atom, were also cal-
culated for comparison. Representative structures for 1’ and
1’’ are illustrated in Figure 1. The molecules were divided
into five to six segments: the arylamine group in the same
side with the N-methyl (or N-ethyl) group (DR), another ar-
ylamine group (DNR), the imidazole ring next to DR and
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91

J. T. Lin, C.-P. Hsu et al.
FULL PAPERS
DNR (B1), the thiophene ring next to B1 (B2), the thiazole
ring next to B2 (B3), and 2-cyanoacrylic acid (Acid). The re-
sults for the theoretical approach are included in Table 3
and Figure S1 of the Supporting Information, respectively.
resonance allows a stronger interaction between the donor
and the rest of the molecule, leading to higher oscillator
strength and smaller charge separation, whereas for the
latter, lower oscillator strength and larger charge separation
are expected. The calculated results for model compounds
n’ and n’’ are consistent with this expectation. The lowest
energy transition (S0!S1; >98% HOMO!LUMO) in both
n’ and n’’ has significant charge transfer from arylamine (or
methoxyphenyl). This transition has very different character-
istics between n’ and n’’: 1) the energy of the transition is
lower by ~0.3–0.5 eV for compound n’’ (donor attached at
position 4); 2) the oscillator strength (f) of the transition in
n’ (donor attached at position 5) is two-fold larger than that
in n’’; 3) compound n’’ has better charge separation between
the donor and Acid than compound n’, and the Mulliken
charge difference ranges from 1.02 to 1.42 jej for n’’, which
reduces to 0.69–1.37 jej for n’. Charge transfer is more
facile from the donor at position 4 of the imidazole to the
acceptor because of the smaller twist angle between the
donor and the imidazole ring (Figure S2 of the Supporting
Information).
The S1 and S2 states of our experimentally studied mole-
cules n are mainly composed of excitations originating from
electrons in one of the donors at positions 4 and 5, respec-
tively (except for 1). The lowest energy transition (S0!S1;
>99% HOMO!LUMO) is mainly from DNR (position 4)
to Acid, and significant charge transfer from DR (position 5)
to Acid is observed at higher energy transition, S0!S2 (>
97% HOMO-1!LUMO) for 2–5 and S0!S3 (>92%
HOMO-2!LUMO) for 1 and 2. For 1, excitation from the
same DNR contributes to the first two excitations, with a
small mix from the excitation originated from DR (Table S3
and Figure S1 in the Supporting Information). Therefore, it
is believed that S0!S2 and S0!S3 transitions can also inject
electrons directly to the conduction band of TiO2, or indi-
rectly through internal conversion to S0!S1. The contribu-
tions of the S0!S2 and S0!S3 transitions support the high
conversion efficiency of 2-based
Table 3. Calculated low-lying transition for the dyes.[a]
Dye
State
Excitation[a]
lcal [eV]
f[b]
1
S1
S2
S3
S1
S2
S3
S1
S2
S3
S1
S2
S3
S1
S2
S3
H!L (99%)
H1!L (94%)
H2!L (92%)
H!L (100%)
H1!L (99%)
H2!L (97%)
H!L (99%)
H1!L (97%)
H2!L (95%)
H!L (100%)
H1!L (100%)
H2!L (98%)
H!L (100%)
H1!L (100%)
H2!L (99%)
2.34
3.18
3.57
2.01
2.42
2.90
1.99
2.72
3.04
1.91
2.28
2.58
1.75
2.19
2.54
0.505
0.410
0.208
0.201
0.127
0.761
0.268
0.086
0.726
0.115
0.000
0.577
0.226
0.177
0.794
2
3
4
5
[a] H=HOMO, L=LUMO, H1=The next highest occupied molecular
orbital, or HOMO-1, H2=HOMO-2. In the parentheses is the popula-
tion of a pair of MO excitations. [b] Oscillator strength.
Mulliken charges for the S1, S2, and S3 states were also cal-
culated as a projection from the time-dependent DFT
(TDDFT) results. Differences in the Mulliken charges in the
excited state and ground state were calculated and grouped
into the same segments in the molecules to estimate the
extent of charge separation upon excitation (Figure 7).
As discussed above, in this series of compounds, the two
donor groups (aryl amine or methoxyphenyl) play different
roles in the photophysics: the one attached at position 5
(DR), which allows a resonance with electrons, flows from
the donor through the imidazolyl group, while position 4
(DNR) does not allow such a resonance. For the former, the
DSSC. Though S0!S2 and S0!
S3 transitions in 1 have higher
energies are not likely to con-
tribute to electron injection, the
rather high f value of S0!S1
may be responsible for its high
conversion
though the methoxy unit is only
weak electron donor. The
efficiency
even
a
high dye density of 1 on the
TiO2 surface may also contrib-
ute to good conversion efficien-
cy.
The combination of several
excited states is favorable in the
design of sensitizing dyes for
DSSCs. While the lower excited
state offers a good charge sepa-
ration, higher excited state can
Figure 7. Plot of difference in Mulliken charge between ground state and excited state for 1–5.
92
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 87 – 96

1-Alkyl-1H-imidazole-Based Dipolar Organic Compounds
be optically active and helps in harvesting photons. The two
distinct characters of donor positions appear to follow such
an advantageous design for molecules studied in the present
work: the donor attached at the 4-position offers charge sep-
aration, whereas the one at the 5-position, in which reso-
nance is allowed, can help increase absorption, as indicated
by the computational results for the model compounds n’
and n’’.
DSSC (7.33%) fabricated and measured under similar con-
ditions. Theoretical investigations indicate that both donors
at positions 4 and 5 can contribute to electron injection into
TiO2 upon photo-excitation, either directly or indirectly by
internal conversion to the lowest excited state.
Experimental Section
However, the oscillator strengths and charge separation
for the molecules n did not quite follow those for n’ and n’’
(Tables S2–S3 and Figure S3 in the Supporting Information).
The oscillator strength values for S1 are larger than those for
S2 in all compounds in the n series. In 3 and 4, charge sepa-
rations for the S2 states are larger than those of the S1 state.
We believe, it is mainly occurring from the large dihedral
angles between the donor and the imidazolyl moieties
caused by the steric hindrance. As shown in Figure S1 in the
Supporting Information, the effect of the N-alkyl group is
evident by comparison of the dihedral angles when the sub-
stituent at position 4 (compounds 1’–5’) or 5 (compounds
1’’–5’’) is replaced by a hydrogen atom. The dihedral angles
between the donor and the imidazole ring are significantly
larger in n’ than in n’’ in all isomeric pairs: 1’/1’’, 44 to 98; 2’/
2’’, 54 to 48; 3’/3’’, 54 to 28; 4’/4’’, 45 to 28; 5’/5’’, 56 to 18.
The dihedral angles for n are even larger in the DR branch.
The large dihedral angles for DR pose an adverse effect to
the electron resonance, reducing the interaction between the
donor and the attached imidazole. This result indicates that
even though in theory molecules n have a good potential for
offering a good charge separation while absorbing light effi-
ciently using its dual donor design, the desirable effect may
be compromised by the final molecular structure. Unlike the
previously reported cases[16a,c,d] in this series of compounds,
we see less correspondence between the extent of charge
separation and the final DSSC performance. It is possible
that the theoretical predictions for some of the important
parameters, such as the dihedral angles between the donors
and the imidazolyl group, do not reflect the actual situation.
The twisting forces are expected to be weak, and the mole-
cules could have a typical environment on the TiO2 particle
surface that may affect the structure of the dye molecules.
Nevertheless, the computational results indicate the impor-
tance of controlling the electronic interaction between
neighboring groups, which should be useful in the future
design of organic DSSC dyes.
General Information
Unless otherwise specified, all the reactions and solvent distillations were
performed under nitrogen atmosphere using standard Schlenk tech-
niques. THF was distilled from sodium and benzophenone. DMF was dis-
tilled from CaH2. Dichloromethane for spectroscopic measurements were
distilled from calcium hydride. 1H NMR spectra were recorded on a
Bruker 400 MHz spectrometer operating at 400.135 MHz. Absorption
spectra were recorded on a Cary 50 probe UV/Vis spectrophotometer.
All chromatographic separations were carried out on silica gel (60M,
230–400 mesh). Mass spectra (FAB) were recorded on a VG70–250S
mass spectrometer. Elemental analyses were performed on a Perkin–
Elmer 2400 CHN analyzer.
Syntheses
The starting materials (1a–5a) were prepared by adopting literature pro-
cedures.[17] 4,5-Bis(4-methoxyphenyl)-1-methyl-2-(thiophen-2-yl)-1H-imi-
dazole (1a) and 5-(4,5-bis(4-methoxyphenyl)-1-methyl-1H-imidazol-2-yl)
thiophene-2-carbaldehyde (1b) were synthesized according to the litera-
ture procedures.[17b,25] Compounds 2b–5b were synthesized by similar
procedures, and only the preparation of 3b is described in detail. The
target compounds (1–5) were obtained by essentially following a similar
procedure, and an illustrative example is given below for 1.
4,4’-(1-Ethyl-2-(thiophen-2-yl)-1H-imidazole-4,5-diyl)bis(N,N-diphenyl-
benzenamine) (2a): Colorless solid (90%); H NMR (CDCl3): d=1.21 (t,
1
J=7.2 Hz, 3H), 4.03 (q, J=7.2 Hz, 2H), 6.93–6.97 (m, 4H), 7.04–7.08 (m,
7H), 7.09–7.11 (m, 2H), 7.12–7.15 ( m, 4H), 7.18–7.21 (m, 6H), 7.25–7.29
(m, 4H), 7.37 (d, J=4.8 Hz, 2H), 7.46 ppm (d, J=8.7 Hz, 2H); HRMS
(APCI): m/z (%) calcd for C45H37N4S1: 665.2739 [M++H]; found:
665.2735.
4,4’-(1-Ethyl-2-(thiophen-2-yl)-1H-imidazole-4,5-diyl)bis(N,N-dimethyl-
benzenamine) (3a): Yellow solid (70%); 1H NMR (CDCl3): d=1.16 (t,
J=7.2 Hz, 3H), 2.86 (s, 6H), 3.01 (s, 6H), 3.96 (q, J=7.2 Hz, 2H), 6.58
(d, J=9.0 Hz, 2H), 6.76 (d, J=8.8 Hz, 2H), 7.09 (dd, J=3.6 Hz, 1H),
7.22 (d, J=9.5 Hz, 2H), 7.34–7.36 (m, 2H), 7.43 ppm (d, J=9.0 Hz, 2H);
HRMS (APCI): m/z (%) calcd for C25H29N4S: 417.2113 [M++H]; found:
417.2114.
7,7’-(1-Ethyl-2-(thiophen-2-yl)-1H-imidazole-4,5-diyl)bis(9,9-diethyl-N,N-
diphenyl-9H-fluoren-2-amine) (4a): Pale yellow solid (70%); 1H NMR
(CDCl3): d=0.23 (t, J=7.3 Hz, 6H), 0.39 (t, J=7.3 Hz, 6H), 1.19 (t, J=
7.2 Hz, 3H), 1.72–1.74 (m, 4H), 1.88–1.92 (m, 4H), 4.05 (q, J=7.2 Hz,
2H), 6.93–7.0 (m, 5H), 7.02–7.04 (m, 1H), 7.05–7.14 (m, 10H), 7.17–7.27
(m, 8H), 7.27–7.36 (m, 1H), 7.36–7.52 (m, 7H), 7.58 (d, J=7.7 Hz, 1H),
7.52–7.69 (m, 1H), 7.68 ppm (d, J=7.7 Hz, 1H); MS (FAB): m/z (%):
953.46 [M++H].
4,4’-(1-Ethyl-2-(5-(thiazol-2-yl)thiophen-2-yl)-1H-imidazole-4,5-diyl)-
bis(N,N-diphenylbenzenamine) (5a): Yellow solid (68%); 1H NMR
(CDCl3): d=1.26 (t, J=7.2 Hz, 3H), 4.1 (q, J=7.2 Hz, 2H), 6.93–6.98
(m, 4H), 7.04–7.08 (m, 8H), 7.10–7.15 (m, 4H), 7.18–7.22 (m, 6H), 7.25–
7.29 (m, 5H), 7.44–7.47 (m, 3H), 7.52 (d, J=4.0 Hz, 1H), 7.77 ppm (d,
J=3.2 Hz, 1H); HRMS (APCI): m/z (%) calcd for C48H38N5S2: 748.2569
[M++H]; found: 748.2582.
Conclusions
In summary, we have synthesized dipolar 1,2,4,5-tetra-substi-
tuted imidazole derivatives, in which, two electron-donating
units are incorporated at positions 4 and 5 of 1-alkyl-1H-imi-
dazole, and electron-accepting cyanoacetic acid is incorpo-
rated at position 2 of the imidazole ring by a conjugated
spacer. DSSCs using these compounds exhibit efficiencies
ranging from 5.41–6.35% under AM 1.5 illumination. The
best performance of the device reaches 87% of N719-based
5-(4,5-Bis(4-(diphenylamino)phenyl)-1-ethyl-1H-imidazol-2-yl)thio-
phene-2-carbaldehyde (2b): Orange solid (78%); 1H NMR (CDCl3): d=
1.25 (t, J=7.2 Hz, 3H), 4.11 (q, J=7.2 Hz, 2H), 6.94–6.96 (m, 4H), 7.05–
7.13 (m, 8H), 7.15–7.17 (m, 4H), 7.19–7.21 (m, 6H), 7.25–7.29 (m, 4H),
7.44 (d, J=8.7 Hz, 2H), 7.60 (d, J=4.0 Hz, 1H), 7.75 (d, J=4.0 Hz, 1H),
Chem. Asian J. 2010, 5, 87 – 96
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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93

J. T. Lin, C.-P. Hsu et al.
FULL PAPERS
9.91 ppm (s, 1H); HRMS (APCI): m/z (%) calcd for C46H37N4O1S1:
693.2688 [M++H]; found: 693.2692.
(d, J=4.0 Hz, 1H), 8.13 ppm (s, 1H); 13C NMR ([D6]DMSO): d=15.69,
26.97, 40.08, 108.54, 112.03, 112.24, 116.94, 118.44, 119.00, 122.54, 124.43,
126.70, 126.85, 130.28, 131.66, 136.26, 136.45, 138.09, 138.42, 138.80,
141.24, 149.03, 150.25, 164.25, 165.59 ppm; HRMS (APCI): m/z (%)
calcd for C29H30N5O2S1: 512.2120 [M++H]; found: 512.2114; elemental
analysis: calcd (%) for C29H29N5O2S: C 68.08, H 5.71, N 13.69; found:
C 67.95, H 5.64, N 13.48.
5-(4,5-Bis(4-(dimethylamino)phenyl)-1-ethyl-1H-imidazol-2-yl)thio-
phene-2-carbaldehyde (3b): Compound 3a (1.66 g, 4 mmol) dissolved in
THF (50 mL) was cooled to ꢀ788C under nitrogen atmosphere. n-Butyl-
lithium (2.8 mL, 1.6m in hexane, 4.4 mmol) was added dropwise with vig-
orous stirring. It was brought to 08C during 1 h, and kept at this tempera-
ture for additional 30 min. Again, the mixture was cooled to ꢀ788C and
dry dimethylformamide (2 mL) was added at once. The solution was
brought to room temperature and stirred overnight. The reaction was
quenched by the addition of dilute HCl (2 mL) in water (40 mL) and ex-
tracted with diethyl ether. The organic extract was dried over anhydrous
MgSO4 and filtered. The filtrate was evaporated to yield the crude prod-
uct as dark yellow solid. It was purified by column chromatography on
silica gel using hexane/dichloromethane mixture (1:1) as eluent, to afford
orange solid (80%). 1H NMR (CDCl3): d=1.23 (t, J=7.2 Hz, 3H), 2.87
(s, 6H), 3.02 (s, 6H), 4.05 (q, J=7.2 Hz, 2H), 6.59 (d, J=6.9 Hz, 2H),
6.76 (d, J=6.8 Hz, 2H), 7.21 (d, J=6.8 Hz, 2H), 7.41 (d, J=6.9 Hz, 2H),
7.57 (d, J=4.0 Hz, 1H), 7.73 (d, J=4.0 Hz, 1H), 9.90 ppm (s, 1H);
HRMS (APCI): m/z (%) calcd for C26H29N4O1S1: 445.2062 [M++H];
found: 445.2055.
(E)-3-(5-(4,5-Bis(7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-
1H-imidazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (4): Red solid
(65%); 1H NMR ([D6]DMSO): d=0.14 (t, J=7.3 Hz, 6H), 0.31 (t, J=
7.3 Hz, 6H), 1.20 (t, J=7.2 Hz, 3H), 1.51–1.69 (m, 4H), 1.84–2.30 (m,
4H), 4.06 (q, J=7.2 Hz, 2H), 6.88–6.90 (m, 1H), 6.97–7.10 (m, 15H),
7.24–7.35 (m, 10H), 7.55 (s, 1H), 7.57–7.64 (m, 4H), 7.29–7.85 (m, 3H),
8.19 ppm (s, 1H); 13C NMR ([D6]DMSO): d=8.26, 8.32, 15.42, 31.65,
40.08, 55.15, 55.83, 118.58, 118.67, 119.02, 119.83, 120.32, 120.60, 121.22,
122.54, 122.80, 123.17, 123.45, 125.27, 125.35, 128.11, 129.33, 129.43,
131.84, 132.29, 135.45, 136.11, 136.67, 138.06, 138.32, 139.02, 139.27,
141.55, 146.34, 147.19, 147.33, 147.35, 148.93, 150.19, 150.83, 151.22,
163.26 ppm; HRMS (APCI): m/z (%) calcd for C71H62N5O2S: 1048.4624
[M++H]; found: 1048.4629; elemental analysis: calcd (%) for
C71H61N5O2S: C 81.34, H 5.86, N 6.68; found: C 81.20, H 5.72, N 6.63.
(E)-3-(2-(5-(4,5-bis(4-(diphenylamino)phenyl)-1-ethyl-1H-imidazol-2-
yl)thiophen-2-yl)thiazol-5-yl)-2-cyanoacrylic acid (5): Red solid (70%);
1H NMR ([D6]DMSO): d=1.18 (t, J=7.2 Hz, 3H), 4.06 (q, J=7.2 Hz,
2H), 6.86 (d, J=8.7 Hz, 2H), 6.96–7.00 (m, 4H), 7.02–7.03 (m, 4H),
7.08–7.11 (m, 6H), 7.25–7.30 (m, 7H), 7.32–7.37 (m, 3H), 7.38 (d, J=
6.8 Hz, 2H), 7.52 (d, J=4.1 Hz, 1H), 7.82 (d, J=4.0 Hz, 1H), 8.22 (s,
1H), 8.36 ppm (s, 1H); 13C NMR ([D6]DMSO): d=15.62, 40.08, 111.09,
118.45, 121.50, 122.62, 122.86, 123.24, 123.74, 123.91, 124.97, 125.62,
126.87, 128.64, 129.46, 129.70, 130.37, 131.90, 132.02, 135.34, 136.94,
137.05, 138.95, 145.51, 146.58, 147.10, 147.95, 151.40, 163.06, 163.79 ppm;
HRMS (APCI): m/z (%) calcd for C52H39N6O2S2: 843.2576 [M++H];
found: 843.2579; elemental analysis: calcd (%) for C52H38N6O2S2:
C 74.08, H 4.54, N 9.97; found: C 73.72, H 4.49, N 9.85.
5-(4,5-Bis(7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-1H-
imidazol-2-yl)thiophene-2-carbaldehyde (4b): Yellow solid (75%);
1H NMR (CDCl3): d=0.24 (t, J=7.3 Hz, 6H), 0.39 (t, J=7.2 Hz, 6H),
1.25 (t, J=7.20 Hz, 3H), 1.71–1.76 (m, 4H), 1.89–1.92 (m, 4H), 4.12 (q,
J=7.2 Hz, 2H), 6.93–7.01 (m, 5H), 7.03–7.18 (m, 11H), 7.18–7.31 (m,
9H), 7.34–7.52 (m, 5H), 7.58–7.70 (m, 3H), 7.78 (d, J=3.4 Hz, 1H)
9.94 ppm (s, 1H); MS (FAB): m/z (%): 981.45 [M++H].
2-(5-(4,5-Bis(4-(diphenylamino)phenyl)-1-ethyl-1H-imidazol-2-yl)thio-
phen-2-yl)thiazole-5-carbaldehyde (5b): Red solid (75%); 1H NMR
(CDCl3): d=1.29 (t, J=7.2 Hz, 3H), 4.10 (q, J=7.0 Hz, 2H), 6.94–6.99
(m, 4H), 7.05–7.09 (m, 8H), 7.10–7.15 (m, 4H), 7.18–7.23 (m, 6H), 7.25–
7.30 (m, 4H), 7.44 (d, J=8.7 Hz, 2H), 7.48 (d, J=4.0 Hz, 1H), 7.67 (d,
J=4.0 Hz, 1H), 8.33 (s, 1H), 10.0 ppm (s, 1H); HRMS (APCI): m/z (%)
calcd for C49H38N5O1S2: 776.2518 [M++H]; found: 776.2523.
Assembly and Characterization of DSSCs
(E)-3-(5-(4,5-Bis(4-methoxyphenyl)-1-methyl-1H-imidazol-2-yl)thiophen-
2-yl)-2-cyanoacrylic acid (1): A mixture of 1b (0.404 g, 1 mmol) and cya-
noacetic acid (0.102 g, 1.2 mmol), ammonium acetate (16 mg, 0.2 mmol),
and acetic acid (20 m) were heated to reflux for 8 h. After cooling, the
precipitate formed was filtered and thoroughly washed with water and
cold methanol. After drying in air, the crude product was purified by
column chromatography on silica gel with dichloromethane/methanol
mixture as eluent to afford a red solid (70%). 1H NMR ([D6]DMSO):
d=3.57 (s, 3H), 3.69 (s, 3H), 3.82 (s, 3H), 6.80 (d, J=8.5 Hz, 2H), 7.07
(d, J=8.4 Hz, 2H), 7.29–7.33 (m, 4H), 7.58 (d, J=3.7 Hz, 1H), 7.75 (d,
J=3.7 Hz, 1H), 8.22 ppm (s, 1H); 13C NMR ([D6]DMSO): d=32.90,
55.04, 55.25, 108.44, 113.69, 114.64, 118.96, 121.92, 125.48, 126.76, 127.53,
130.82, 132.21, 136.48, 136.55, 137.30, 138.59, 140.08, 141.60, 158.05,
159.62, 164.52 ppm; HRMS (ESI): m/z (%) calcd for C26H22N3O4S1:
472.1331 [M++H]; found: 472.1320; elemental analysis: calcd (%) for
C26H21N3O4S: C 66.23, H 4.49, N 8.91; found: C 66.10, H 4.47, N 8.87.
The photoanode used was the TiO2 thin film coated on FTO glass sub-
strate[26] with a dimension of 0.5ꢃ0.5 cm2, and a platinized FTO was used
as a counter electrode. The active area was controlled at a dimension of
0.6ꢃ0.6 cm2 by adhered polyester tape (3m) with a thickness of 60 mm on
the Pt electrode. After heating the TiO2 film to 808C, the film was taken
out from the oven and dipped into the THF solution containing dye sen-
sitizers (3ꢃ10ꢀ4 m) for at least 12 h. After rinsing with THF, the photoa-
node was placed on top of the counter electrode and tightly clipped to-
gether to form a cell. A 0.6ꢃ0.6 cm2 cardboard mask was clipped onto
the device to constrain the illumination area. Electrolyte was then inject-
ed into the space, and the cell was sealed with Torr Seal cement (Varian,
MA, USA). The electrolyte was composed of lithium iodide (LiI, 0.5m),
iodine (I2, 0.05m), and 4-tert-butylpyridine (TBP, 0.5m) dissolved in ace-
tonitrile. The photoelectrochemical characterizations on the solar cells
were carried out using a modified light source, 300 W Xe lamp (Oriel
6258) equipped with a water-based IR filter and AM 1.5 filter (Oriel
81088). Photocurrent–voltage characteristics of the DSSCs were recorded
with a potentiostat/galvanostat (CHI650B, CH Instruments, Inc., USA)
at a light intensity of 100 mWcmꢀ2 measured by a thermopile probe
(Oriel 71964). The light intensity was further calibrated by an Oriel refer-
ence solar cell (Oriel 91150) and adjusted to be 1.0 sun. The monochro-
matic quantum efficiency was recorded through a monochromator (Oriel
74100) at short circuit condition.
(E)-3-(5-(4,5-Bis(4-(diphenylamino)phenyl)-1-ethyl-1H-imidazol-2-yl)th-
iophen-2-yl)-2-cyanoacrylic acid (2): Red solid (65%); 1H NMR
([D6]DMSO): d=1.19 (t, J=7.2 Hz, 3H), 4.05 (q, J=6.8 Hz, 2H), 6.88
(d, J=8.7 Hz, 2H), 6.98 (d, J=7.6 Hz, 2H), 7.02–7.04 (m, 6H), 7.09–7.13
(m, 6H), 7.26–7.33 (m, 9H), 7.35–7.39 (m, 3H), 7.55 (d, J=4.0 Hz, 1H),
7.77 (d, J=4.0 Hz, 1H), 8.14 ppm (s, 1H); 13C NMR ([D6]DMSO): d=
15.58, 40.08, 108.73, 118.80, 121.45, 122.58, 122.84, 123.20, 123.71, 123.90,
124.95, 126.94, 128.61, 129.43, 129.68, 130.53, 131.88, 136.26, 136.61,
137.18, 138.11, 139.05, 141.05, 145.52, 146.56, 147.06, 147.93, 163.79 ppm;
HRMS (APCI): m/z (%) calcd for C49H38N5O2S1: 760.2746 [M++H];
found: 760.2728; elemental analysis: calcd (%) for C49H37N5O2S: C 77.45,
H 4.91, N 9.22; found: C 77.32, H 4.84, N 9.10.
Measurement of Recombination Time Constant by Transient Photovoltage
The photovoltage transients of assembled devices were recorded with a
digital oscilloscope (LeCroy, WaveSurfer 24Xs). Pulsed laser excitation
was applied by a Q-switched Nd:YAG laser (Continuum, model Minilite
II) with 1 Hz repetition rate at 532 nm and a 5 ns pulse width at half-
height. The beam size was slightly larger than 0.5ꢃ0.5 cm2 to cover the
area of the device with an incident energy of 1 mJcmꢀ2. The recombina-
tion lifetime of photoinjected electrons with oxidized dyes was measured
(E)-3-(5-(4,5-Bis(4-(dimethylamino)phenyl)-1-ethyl-1H-imidazol-2-yl)th-
iophen-2-yl)-2-cyanoacrylic acid (3): Red solid (70%); 1H NMR
([D6]DMSO): d=1.13, (t, J=7.2 Hz, 3H), 2.83 (s, 6H), 2.98 (s, 6H), 4.0
(q, J=6.8 Hz, 2H), 6.57 (d, J=9.0 Hz, 2H), 6.82 (d, J=8.7 Hz, 2H), 7.19
(d, J=8.7 Hz, 2H), 7.26 (d, J=8.9 Hz, 2H), 7.50 (d, J=4.0 Hz, 1H), 7.73
94
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1-Alkyl-1H-imidazole-Based Dipolar Organic Compounds
by transient photovoltage at open circuit with the presence of LiI electro-
lyte (0.5m). The average electron lifetime can be estimated approximate-
ly by fitting a decay of the open circuit voltage transient with expACTHNUTRGEN(UNG -t/tR),
in which, t is time and tR is an average time constant before recombina-
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The predicted structures of the molecules were optimized by using
B3LYP hybrid functional and 6-31G* basis sets. For each of the mole-
cules, a number of conformational isomers were examined and the one
with the lowest energy was used. For the excited states, we have em-
ployed the time-dependent density functional theory (TDDFT) with the
B3LYP functional. All of the analyses were performed using Q-Chem 3.0
software.[27] There exist a number of previous works that employed
TDDFT to characterize excited states with charge-transfer character.[28]
In some cases, underestimation of the excitation energies was seen.[29]
Therefore, in the present work, we use TDDFT to characterize the
extent of charge-shift, and avoid drawing conclusions from the excitation
energy.
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nancial support.
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