Synthesis, characterization and density functional theory investigations of the electronic, photophysical and charge transfer properties of donor-bridge-acceptor triaminopyrazolo[1,5-a]pyrimidine dyes

Article abstract of DOI:10.1016/j.saa.2013.04.010

We have synthesized multifunctional dyes 3-(4-methyl-phenylazo)-6-(4-nitro- phenylazo)-2,5,7-triaminopyrazolo[1,5-a]pyrimidine (4a) and 3-(4-methyl- phenylazo)-6-(4-acetyl-phenylazo)-2,5,7-triaminopyrazolo[1,5-a]pyrimidine (4b), then characterized by IR, ¹H NMR and ¹³C NMR techniques. The ground state geometries have been computed by using density functional theory at B3LYP/6-31G^a?- level of theory. The absorption spectra have been calculated by using time dependent density functional theory with and without solvent. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are delocalized and localized on throughout the backbone, respectively. Solvent also play important role towards elevating the dipole moment. Significant red shift in absorption wavelengths have been observed in methanol compared to without solvent. We discussed the electron injection, electronic coupling constant and light harvesting efficiency.

Full text of DOI:10.1016/j.saa.2013.04.010

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 223–229
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and density functional theory investigations
of the electronic, photophysical and charge transfer properties of donor–bridge–
acceptor triaminopyrazolo[1,5-a]pyrimidine dyes
Abdullah G. Al-Sehemi a,b, , Ahmad Irfan a, , Ahmed M. Fouda a
⇑
⇑
a
Department of Chemistry, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
Unit of Science and Technology, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
Pyrazole-carboxamides have been
synthesized and characterized.
DFT and TDDFT were applied to study
the optical and charge transfer
properties.
ꢀ
ꢀ
ꢀ
The p-conjugation lowers the energy
level of the frontier molecular
orbitals.
The lowered LUMO energy level
would be beneficent for the flow of
electrons.
The electron injection, LHE and
electronic coupling constant have
been computed.
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 10 April 2013
We have synthesized multifunctional dyes 3-(4-methyl-phenylazo)-6-(4-nitro-phenylazo)-2,5,7-triam-
inopyrazolo[1,5-a]pyrimidine (4a) and 3-(4-methyl-phenylazo)-6-(4-acetyl-phenylazo)-2,5,7-triamino-
pyrazolo[1,5-a]pyrimidine (4b), then characterized by IR, 1H NMR and C NMR techniques. The
13
⁄
Keywords:
ground state geometries have been computed by using density functional theory at B3LYP/6-31G level
Electronic materials
Chemical synthesis
DFT calculations
Electronic structure
Transport properties
of theory. The absorption spectra have been calculated by using time dependent density functional theory
with and without solvent. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied
molecular orbitals (LUMOs) are delocalized and localized on throughout the backbone, respectively. Sol-
vent also play important role towards elevating the dipole moment. Significant red shift in absorption
wavelengths have been observed in methanol compared to without solvent. We discussed the electron
injection, electronic coupling constant and light harvesting efficiency.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
cost and structural flexibility [1–3]. Dye-sensitized solar cells
(
DSSCs) have attracted a lot of interest. Organic dyes are the poten-
Recently, organic charge transfer materials have attracted con-
siderable significance due to their easy chemical modification, low
tial contenders which have been intensively used in DSSCs [4–8].
Recently a great attention has been focused to the azo dyes bearing
one or more AN@NA groups, this is because of their biochemical
and physicochemical properties as well as their wide applications
in many other fields like semiconductors, organic light emitting
diodes (OLEDs), organic field effect transistors (OFETs), photovolta-
ics etc. [9,10]. Pyrazolo [1,5-a]pyrimidine derivatives used as inter-
mediates in the synthesis of dye industry. The donor–pi–acceptor
⇑
Corresponding authors. Address: Department of Chemistry, Faculty of Science,
King Khalid University, Abha 61413, P.O. Box 9004, Saudi Arabia (A.G. Al-Sehemi).
Tel.: +966 72418632; fax: +966 72418426 (A. Irfan).
E-mail addresses: agmasq@gmail.com (A.G. Al-Sehemi), irfaahmad@gmail.com
(
A. Irfan).
1
386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.010

2
24
A.G. Al-Sehemi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 223–229
(
D–pi–A) materials are being used in different fields e.g. the next-
using a Bruker AV 500 MHz spectrometer. Absorption spectra were
recorded on Schimadzu 1601 PC UV–Vis in DMSO as solvent.
generation full-color flat-panel displays as well as dye sensitized
solar cells. These have advantage of tremendous chemical, thermal,
and photochemical stabilities. Moreover alleviate structural tuning
to alter the electronic, photophysical, charge transfer and morpho-
logical properties [11–17].
1
Synthesis of 2-arylazo-malononitrile (2a-c)
1
4 ml of hydrochloric acid solution was added to (0.01 mol) of
aromatic amine (1a-c) and the mixture was cooled to 0 °C, then a
solution of sodium nitrite (0.7 g, in 3 ml H O) was added portion
To improve the electron transport properties, some different pi-
conjugated bridges are introduced into the designed molecules
2
[
11]. We have synthesized multifunctional dyes by N@NA groups
wise with stirring. The clear diazonium salt was added to a stirred
solution of malononitrile (0.66 g, 0.01 mol) in sodium acetate
and pyrazolo[1,5-a]pyrimidine moiety (see Scheme 1) which
would be better candidates in the field of dye-sensitized solar cells
(
4.6 g) in 31 aqueous ethanol 50% with vigorous stirring at 0 °C.
(
DSSCs). We have investigated the electronic, charge transfer, and
After the complete addition of diazonium salt stirring was contin-
ued for 5 h at 0–5 °C. The separated product was filtered off,
washed several times with water, dried and finally recrystallized
photophysical properties. Moreover, we have shed light on the
electronic coupling constant and electron injection to show their
ability to be used as DSSC materials. During the electron injection
investigations, we observed that strength of the electron with-
drawing group (anchoring group in the present study as well) play
vital role. Then we designed another dye by substituting the
2
from ethanol/H O to afford (2a-c).
2-(4-methyl-phenylazo)-malononitrile (2a)
Yellow crystals, (Yield 90%), m.p. 160–162 °C, FT-IR (KBr, cmꢁ1),
(NH, 3167), (CN, 2216), (C@N, 1586).
(
COOH) group in place of nitro (Supporting information) and then
investigated the electronic coupling constant and electron injec-
tion by computational means.
As a whole we apply the density functional theory (DFT), and
time-dependent density functional theory (TD-DFT) methods to
investigate the electronic, charge transfer and photophysical
properties such as the frontier molecular orbitals (HOMOs and
LUMOs), the HOMO and LUMO energies difference (Egap), the
absorption spectra are obtained and discussed in detail. The aim
of the present work is to provide an in-depth understanding of
electronic, charge transfer, photophysical properties and electron
injection of these compounds, and to offer excellent candidates
for DSSC materials. This study would have impact and useful
information can shed some light to design new DSSC functional
materials.
2
-(4-acetyl-phenylazo)-malononitrile (2b)
Yellow crystals, (Yield 95%), m.p. 171–173 °C, FT-IR (KBr, cm ),
NH, 3218), (CN, 2222), (C@N, 1599).
ꢁ1
(
2-(4-nitro-phenylazo)-malononitrile (2c)
Yellow crystals, (Yield 94%), m.p. 212–214 °C, FT-IR (KBr, cmꢁ1),
(
NH, 3220), (CN, 2225), (C@N, 11,593).
Synthesis of 3,5-diamino-4-(4-methyl-phenylazo)-1Hpyrazole (3a)
.1 ml pyridine was added to a mixture of 2a (0.55 g, .003 mol)
0
and hydrazine hydrate (0.177 g) in 30 ml ethanol and the reaction
mixture was then refluxed for 3 h. The formed product during re-
flux was filtered off, washed with hot ethanol to afford 3a. Yellow
Experimental details
ꢁ1
Melting points were determined with a Stuart Scientific Co., Ltd.
apparatus. IR spectra were determined as KBr pellets on a Jasco FT/
IR 460 plus spectrophotometer. 1H NMR spectra was recorded
crystals, (Yield 60%), m.p. 174–176 °C, FT-IR (KBr, cm ) (3396,
1
6
3290, 3181), (C@N, 1613). H NMR (DMSO-d ) d (ppm), 7.79 (2H,
3
d, 2,6-ArAH), 7.23 (2H, d, 3,5ArAH), 2.34 (s, 3H, CH ).
+
-
Cl
NH 2
N
N
CN
NaNO2/HCl
NCCH2CN
H
X
N
N
C
CH3COONa
- 5 oC
CN
0
2a-c
X
X
a, X= -CH3
b, X= -NO2
1
a-c
c, X= -COCH3
NH 2
NH
CN
N2H4
H
N
H3C
N
C
H3C
N
N
CN
2a
N
H2N
2
b,c
3a
H3C
N
N
N
NH 2
N
H2N
N
N
N
X
NH 2
4
4
a, X = NO2
b, X = COCH3
4
a,b
Scheme 1. Schematic diagram and synthesis route of the investigated dyes.

A.G. Al-Sehemi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 223–229
225
Synthesis of 3-(4-methyl-phenylazo)-6-(4-nitro-phenylazo)-2,5,7-
triaminopyrazolo[1,5-a]pyrimidine (4a) and 3-(4-methyl-phenylazo)-
are delocalized on throughout the backbone and electron with-
drawing groups. The donor group methyl has no any participation
in the formation of HOMOs. Similarly, LUMO and LUMO+1 are
localized on the entire molecules. But in 4a, most of the LUMO
and LUMO+1 are localized towards the acceptor group and its
neighbor rings. The nitrobenzene has no any contribution in the
formation of HOMO-1. In 5, the carboxylic group is participating
in the formation of LUMO and LUMO+1. The charge distribution
on acceptor group in 5 is not so significant. The distribution of
charge in unoccupied molecular orbitals is revealing intra-molecu-
lar charge transfer in 5. The HOMO energies (EHOMO), LUMO ener-
6-(4-acetyl-phenylazo)-2,5,7-triaminopyrazolo[1,5-a]pyrimidine (4b)
Under reflux condition
To a solution of 3a (0.65 g, 0.003 mol) in ethanol 30 ml was added
2
b or 2c (0.003 mol) followed by 0.1 ml of pyridine and the reaction
mixture was heated under reflux for 4 h. The separated product dur-
ing reflux was filtered off while hot and washed with ethanol and
then boiled with ethanol and filtered while hot, dried and finally
recrystalized from DMF to give 4a and 4b respectively.
ꢁ
1
4
a, Yield 65%, m.p. >300 °C, FT-IR (KBr, cm ) (NH
247), (C@N, 1610), (NO
, 1511, 1319). 1H NMR (DMSO-d
ppm), 8.15 (bs, 2H, NH -5), 7.15 (bs, 2H, NH , 2,7), 8.33 (2H, d,
), 7.94 (2H, d, 2,6-ArAHANO ), 7.50 (2H, d, 2,6-
), 7.36 (2H, d, 3,5-ArAHACH ), 2.52 (s, 3H, CH ).
b, Yield 71%, m.p. >300 °C, FT-IR (KBr, cm ) (NH , 3392, 3279,
181), (C@N, 1613), (COCH ) d (ppm),
, 1669). 1H NMR (DMSO-d
.6 (bs, 2H, NH -5), 6.94 (bs, 2H, NH , 2,7), 8.07 (2H, d, 3,5-ArAH,
), 7.96 (2H, d, 2,6-ArAHACOCH ), 7.61 (2H, d, 2,6-
ArAHACH ), 7.30 (2H, d, 3,5-ArAHACH ), 3.33 (s, 3H, COCH ),
.52 (s, 3H, CH ).
2
, 3462, 3386,
gies (ELUMO), HOMO–LUMO energy gaps (Egap) and dipole
3
(
3
2
6
) d
moments have been tabulated in Table 1. By substituting the acetyl
methyl in place of nitro boost up the HOMO and LUMO energies.
The HOMO and LUMO energies reduced from gas phase to solvent
(methanol). The HOMO–LUMO energy gap increases for 4b while
decreases for 4a at ground state. The introduction of carboxylic
group in place of nitro (5), augment the HOMO and LUMO energies.
From Table 1, we can see that energy gap of the 4b is larger than
the 4b. We observed decrease in energy gap by using solvent for
4a while solvent has no significant effect for 4b. It revealed that
4a would be red shifted in solvent compared to gas phase spec-
trum. This can be verified from Section 4.2. Previously [24], we ex-
plained that ‘‘CH’’/N substitution affect the dipole moment
dependently on the substituted group. In the present study, we
also found that substituent play vital role towards increasing or
decreasing the dipole moment. Secondly, solvent also play impor-
tant role towards elevating the dipole moment.
2
2
,5-ArAH, NO
ArAHACH
2
2
3
3
3
ꢁ
1
4
2
3
8
3
6
2
2
COCH
3
3
3
3
3
2
3
Under microwave irradiation
A mixture of 3a (0.65 g, 0.003 mol) and 2b or 2c (0.003 mol) were
placed in a mortar and mixed using pestle for ca. 3 min. The homog-
enized mixture was then transferred into a Pyrex test tube followed
by 6 ml ethanoland 0.1 ml pyridine. The test tube was then loaded to
household LG microwave oven. The reaction, monitored by TLC
[
1
Diethyl ether/Ethylacetate; 2:1], was found to be completed after
min. The reaction mixture was cooled to room temperature, then
boiled with hot ethanol and filtered while hot to afford 4a (Yield
5%, m.p. >300 °C) and 4b (Yield 88%, m.p. >300 °C) respectively.
Photophysical properties
In Table 2, we presented the experimental and calculated absorp-
tion wavelengths (k). The oscillator strengths and major transitions
have also been tabulated in Table 2. The computed absorption wave-
length of 4a is in good agreement with the experimental evidence.
The maximum absorption wavelength of 4a is 7 nm red shifted com-
pared to 4b. This is due to the more electron withdrawing effect and
smaller HOMO–LUMO energy gap for 4a. The transitions have been
observed from HOMO to LUMO in both the dyes. We observed red
shift in the absorption wavelength in solvent (methanol). Significant
effect has been observed for 4a, i.e., 33 nm red shift in methanol
compared to without solvent. The red shift in the absorption spec-
trum is directly linked with the smaller energy gap. The computed
absorption spectrum of 4a in methanol is in good agreement with
the experimental evidence (see Supporting information). Generally,
the absorption wavelengths are solvent dependent which is in good
agreement with Tsai and Wang experimental studied that azo dyes
are strongly solvent dependent [9]g. It is stated that computed spec-
trum in solvent is reproduced as experimental one. The maximum
absorption wavelengths of 4b and 5 are being blue shifted compared
to 4a. This blue shift is directly linked with the strength of the deac-
tivating group.
8
Computational details
All the calculations have been performed by using Gaussian09
package [18]. The ground state geometries have been optimized
using density functional theory (DFT) [19–26]. The Becke’s three
parameter gradient-corrected exchange potential and the Lee–
Yang–Parr gradient-corrected correlation potential (B3LYP) [27–
⁄
2
9], and 6-31G basis set [30] has been used for all the calculations.
The ground state geometries have also been optimized in solvent
methanol) at the same level of theory. The frequencies have also
(
been computed at the same level of theory and no imaginary fre-
quency has been observed. The absorption spectra have been com-
puted by using time dependent density functional theory (TDDFT)
which has been proved an efficient approach [31,32] at TD-B3LYP/
⁄
6
-31G level of theory. Moreover, absorption wavelengths have also
been computed by using solvent (methanol). The polarizable contin-
uum model (PCM) [33–36] is used for evaluating bulk solvent effects
at all stages. Guillaumont and Nakamura calculated the maximum
absorption wavelength of several organic dyes (indigo, azobenzene,
phenylamine, hydrazone, and anthraquinone) with an average devi-
ation close to 0.20 eV [37]. Recently, we have shed light on the elec-
Charge transfer properties
⁄
⁄
tron injection behavior using B3LYP/6-31G and TD-B3LYP/6-31G
level of theories and concluded that this level is good and reliable
38]. Thus in the present study same level of theory has been applied
The description of the electron transfer from a dye to a semicon-
ductor, the rate of the charge transfer process can be derived from
the general classical Marcus theory [39–43],
[
to investigate the electron injection [38].
1
=2
inject:
2
k
inject: ¼ jVRPjð2=hð
p
=kkBT
Þ
exp½ꢁð
D
G
þ kÞ =4kkBT
ꢂ
ð1Þ
Results and discussions
In Eq. (1), kinject. is the rate constant (in Sꢁ1) of the electron injection
Electronic structure
from dye to TiO , kBT is the Boltzmann thermal energy, h the Planck
2
inject.
constant, ꢁ
D
G
is the free energy of injection and k is the reor-
Fig. 1 illustrates the frontier molecular orbitals (HOMO, HOMO-
ganization energy of the system, |VRP| (is the coupling constant be-
1
, LUMO and LUMO+1). The HOMOs of all the studied compounds
tween the reagent and the product potential curves. Eq (1) revealed

2
26
A.G. Al-Sehemi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 223–229
LUMO
HOMO
HOMO-1
4a
LUMO+1
LUMO
HOMO
4
b
LUMO+1
HOMO-1
LUMO
HOMO
LUMO+1
HOMO-1
5
Fig. 1. The distribution patterns of the frontier molecular orbitals (HOMOs and LUMOs) of 4a and 4b.
Table 2
Table 1
a
⁄
Experimental and calculated absorption wavelengths (k) in nm of TD-B3LYP/6-31G
level of theory.
The HOMO energy (EHOMO), LUMO energy (ELUMO) HOMO–LUMO energy gap (Egap
and dipole moment (
)
⁄
l
) in eV for ground at B3LYP/6-31G level of theory.
f
k
Transition
Dyes
E
HOMO
E
LUMO
E
gap
l
4
ab
1.1183
0
0.8718
0.4224
0.8970
471
401
464
383
504
396
466
370
463
368
H ? L
H ? L + 1
H ? L
H ? L + 1
H ? L
H-1 ? L + 1
H ? L
H-1 ? L + 1
H ? L
H-1 ? L + 1
4
4
4
4
5
a
ꢁ5.51
ꢁ5.29
ꢁ5.60
ꢁ5.47
ꢁ5.30
ꢁ2.62
ꢁ2.34
ꢁ2.83
ꢁ2.47
ꢁ2.34
2.89
2.94
2.77
3.00
2.96
14.78
7.89
19.60
10.65
7.70
.1549
b
a
4bb
a
a
b
4
ac
0
.2090
a
c
The geometries have been optimized in solvent (methanol).
4b
1.6978
0.4428
1
c
5
.6651
.4526
0
that larger |VRP| leads to higher rate constant which would result
better sensitizer. The use of the Generalized Mulliken–Hush formal-
ism (GMH) allows evaluating |VRP| for a photoinduced charge
a
b
c
Experimental absorption wavelengths of 4a 479 nm while 4b 465 nm.
In gas phase.
In solvent (methanol).

A.G. Al-Sehemi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 223–229
227
transfer [40,41]. The Hsu et al. explained that |VRP| can be evaluated
as [41]
Table 3
The
D
Ginject., oxidation potential, |VRP|, LHE of investigated dyes at TD-B3LYP/6-31G⁄
⁄
and PCM-B3LYP/6-31G level of theory.
jVRPj ¼
D
E
RP=2
ð2Þ
Ginject.
ꢃ
System
D
dye
OX
dye
OX
k
ICT
max
LHE
|VRP|
E
E
From Eq. (2), it can found that if
D
E
RP would enhance then the cou-
4
4b
5
a
ꢁ0.86
ꢁ1.19
ꢁ1.18
5.60
5.47
5.50
3.13
2.80
2.82
2.46
2.66
2.68
0.8732
0.9799
0.9783
0.43
0.53
0.59
pling constant would also improve resulting develop the electron
injection. The injection driving force can be formally expressed
within Koopmans approximation as
h
i
h
i
dye
LUMO
dye
HOMO
dye
LUMO
dye
HOMO
TiO2
D
E
RP ¼ E
þ 2E
ꢁ E
þ E
þ ECB
ð3Þ
ꢃ
ꢃ
dye
D
Ginject. and E dO yX e
and EOX mean average error (MAE) [51]. Here,
have been evaluated using Eqs. (6) and (7).
TiO2
Here ECB is the conduction band edge. Though it is often difficult to
TiO2
accurately determine ECB , because it is highly sensitive to the con-
The light harvesting efficiency (LHE) of the dye has to be as high
as possible to maximize the photocurrent response. Here, LHE is
expressed as [52]:
TiO2
ditions, e.g. the pH of the solution thus we used ECB ¼ ꢁ4:0eV [44],
an experimental value corresponding to conditions where the semi-
conductor is in contact with aqueous redox electrolytes of fixed pH
LHE ¼ 1 ꢁ 10ꢁ ¼ 1 ꢁ 10ꢁf
where A (f) is the absorption (oscillator strength) of the dye associ-
ated to the k . The oscillator strength is directly derived from the
TDDFT calculations and writes:
A
ð9Þ
7
.0 [45,46].
dye
LUMO
More quantitatively for a closed-shell system E
to the reduction potential of the dye (E ), whereas the HOMO en-
ergy is related to the potential of first oxidation (i.e.,
corresponds
dye
RED
ICT
max
dye
HOMO
dye
ꢁ
E
¼ EOX ). As a result, Eq. (3) becomes,
h
i
h
i
2
ICT
max
2
f ¼ 3
k
jl
~
0
ꢁ ICTj
ð10Þ
dye
HOMO
dye
dye
OX
TiO2
D
E
RP ¼ E
ꢁ EOX ¼ ꢁ E þ ECB
ð4Þ
where
l
0
ꢁ ICT is the dipolar transition moment associated to the
The Eq. (4) can be rewritten as
ICT
electronic excitation. In order to maximize f, both k
must be large [53,54].
and
l
0
ꢁ ICT
max
h
i
dye
ꢁ0
dye
OX
dye
RED
TiO2
þ ECB
D
E
RP ¼ E0 ꢁ 2E þ E
ð5Þ
Ginject., oxidation potential,
D
In Table 3, we have presented the
and |VRP| of the studied dyes. The electron injection and electronic
coupling constant of for 4a are ꢁ0.86 and 0.43, respectively while
We propose to establish a reliable theoretical scheme to evalu-
ate the dye’s excited state oxidation potential, and quantify the
electron injection onto a titanium dioxide (TiO ) surface. The free
4
b are ꢁ1.16 and 0.58, respectively. Recently we have investi-
2
gated the electronic coupling constant and electron injection of
different hydrazone sensitizers, i.e., 0.195 and ꢁ0.390, respec-
tively for the parent system. We found that by substituting the
anthracene as donor group improved the electronic coupling con-
stant and electron injection to 0.540 and ꢁ1.08, respectively [38].
In the present study our new synthesized sensitizer 4b would be
better than the hydrazone sensitizers. Moreover, new designed
sensitizer 5 would be good contender of DSSC materials. Previ-
ously, it was studied that by elongating the bridge efficient mate-
rials can be designed [55]. This study revealed that if we attach
anchoring group having strong ability to anchor along with elon-
gating the bridge can lead to further improvement towards effi-
cient sensitizers. The light harvesting efficiency of 4a has been
observed 0.8732. By introducing acetyl group in place of nitro im-
prove the LHE to 0.9799. The substitution of carboxylic group
leads the LHE to 0.9783. Both the new designed derivatives have
LHE superior to 4a, see Table 3.
energy change (in electron volts, eV) for the electron injection
can be expressed as [45].
ꢃ
inject
dye
OX
TiO2
D
G
¼ E
ꢁ ECB
ð6Þ
ꢃ
dye
where E
is the oxidation potential of the dye in the excited state,
and ECB is the reduction potential of the semiconductor conduction
band. Two models can be used for the evaluation of EOX [39,47,48].
The first implies that the electron injection occurs from the unre-
laxed excited state. For this reaction path, the excited state oxida-
OX
TiO2
ꢃ
dye
tion potential can be extracted from the redox potential of the
dye
OX
ground state, E
3
which has been calculated the PCM-B3LYP-6-
⁄
⁄
1G approach using the restricted and unrestricted formalisms
and the vertical transition energy corresponding to the photoin-
duced intramolecular charge transfer (ICT),
ꢃ
dye
OX
dye
ICT
E
¼ EOX ꢁ k
ð7Þ
max
ICT
max
where k
is the energy of the ICT. Note that this relation is only
valid if the entropy change during the light absorption process
can be neglected. For the second model, one assumes that electron
injection occurs after relaxation. Given this condition, EOX is ex-
Conclusions
ꢃ
dye
In the framework of our present investigations, we can draw the
following conclusions:
pressed as [48]:
ꢃ
dye
OX
dye
OX
dye
0ꢁ0
E
¼ E ꢁ E
ð8Þ
(
a) The highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMO) are delocal-
ized and localized on throughout the backbone in 4b,
respectively.
dye
ꢁ0
where E0 is the 0–0 transition energy between the ground state
and the excited state. To estimate the 0–0 ‘‘absorption’’ line, we
0 1
need both the S (singlet ground state) and the S (first singlet ex-
cited state) equilibrium geometries, QS0 and QS1, respectively.
Though electron injection from unrelaxed excited states has been
2 2
observed in TiO [49] and SnO [50], the relative contribution of
an ultrafast injection path is not clear, and the most experimental
groups assume that the electron injection dominantly occurs after
(
(
b) The HOMO and LUMO energies reduced from gas phase to
solvent.
(
c) Solvent play important role towards elevating the dipole
moment.
d) The maximum absorption wavelengths are red shifted in
solvent.
relaxation [48]. Preat et al. concluded that the absolute difference
inject.
(e) The computed spectra in solvent are in good agreement with
experimental evidence.
between the relaxed and unrelaxed
D
G
. is constant for TPA
dye
derivatives, and is of the same order of magnitude than the EOX

2
28
A.G. Al-Sehemi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 223–229
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f) This study revealed that if we attach anchoring groups hav-
0
2
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[
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