Structural Chemistry
https://doi.org/10.1007/s11224-021-01837-4
ORIGINAL RESEARCH
Optical and electronic properties of para‑functionalized
triphenylamine‑based dyes: a theoretical study
Stelyus L. Mkoma1,2 · Yohana Msambwa1 · Fortunatus R. Jacob3 · Lucy W. Kiruri4 · Grace A. Kinunda3 ·
Sixberth Mlowe1 · Geradius Deogratias3
Received: 9 July 2021 / Accepted: 7 September 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Molecular engineering of dyes has become a popular andmost successful approach towards improvement of photovoltaic
power conversion efficiency of dye-sensitized solar cells (DSSCs). We report the geometrical, optical, and electronic properties for para-substituted triphenylamine (TPA)-based dyes with D-π-π-A architecture. Results were realized through density
functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. We used B3LYP/6–31 + G(d,p)
and CAM-B3LYP/6–31 + G(d,p) level of theory for DFT and TD-DFT, respectively. Six electron-donating (ED) and electronwithdrawing (EW) groups were symmetrically grafted to the para-direction of the phenyl rings. Two anchoring groups
namely: cyanoacrylic acid (CA) and hydantoin (HY) were used. Excellent relationships between electronic energies and
the Hammett constants (σp) have been reported. The results show that variation of both anchoring groups and substituents
significantly affect the absorption of the dyes; maximum absorption for CA dyes was found ranging between 514–571 nm
and 470–503 nm for ED and EW groups, respectively, while for HY dyes demonstrated maximum absorption between
502–537 nm and 480–496 nm for ED and EW, respectively. A linear correlation between σp and λmax with R2 > 0.97 was
obtained. In addition, the mapping of the HOMO and LUMO energies suggests the intramolecular charge transfer and a
strong electronic coupling between dye and semiconductor. Our theoretical calculations show that electron-donating substituents enhance the optoelectronic properties of the dyes. Analysis of chemical descriptors suggests that dyes containing
alternative anchoring group HY substituted with –NH2 and –N(CH3)2 may demonstrate improved performance of DSSCs.
Keywords Triphenylamine · DFT · TD-DFT · Anchoring group · Donors · Acceptors
Introduction
Solar energy is considered the most abundant among renewable energy sources with high-potential to address the challenges arising from environmentally problematic energy
sources such as fossil fuels, coal, and oil [1, 2]. Dye-sensitized
* Geradius Deogratias
dgeradius@udsm.ac.tz
1
Department of Chemistry, Dar es Salaam University College
of Education, University of Dar es Salaam, P. O. Box 2329,
Dar es Salaam, Tanzania
2
Department of Chemistry and Environmental Sciences,
Marian University College, P. O. Box 47, Bagamoyo,
Tanzania
3
Chemistry Department, College of Natural and Applied
Sciences, University of Dar es Salaam, P. O. Box 35061,
Dar es Salaam, Tanzania
4
Department of Chemistry, Kenyatta University,
P. O. Box 43844‑00100, Nairobi, Kenya
solar cells (DSSCs) have received greater attention among the
industry and academia since the seminal work by O’Regan
and Grätzel in 1991 [3]. The DSSCs would potentially outperform the traditional c-Si owing to their competitive characteristics such as tunable aesthetics features, straightforward synthesis of materials on a wide range of substrates [4]. Unlike
c-Si solar cells, DSSCs demonstrate good performance even
under diffuse light conditions [4–7]. The subject of development and discovery of novel dyes is of interest towards the
improvement of DSSCs’ power conversion efficiency (PCE)
[4]. The PCE of ~ 17% has been achieved in laboratory settings [8], however, the reached PCE remains insufficient for
most of the application purposes. Thus, further investigation
is crucial to the improvement of DSSCs’ PCE.
Molecular engineering of dyes has become a popular and
most successful approach which has resulted in fast improvement
of DSSCs’ PCE [3, 9–11]. The D-π-A, D-π-π-A, and D-A-π-A
structural motifs are often preferred in order to improve the optical
and electronic properties such as the broader coverage of absorp-
13
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Structural Chemistry
tion spectrum in the UV–Visible and near infra-red regions.
The abbreviations D and A here stand for donor and anchoring
groups, respectively, while π represents the linker/bridge [12–14].
Donor groups such as triphenylamine (TPA), butyldimethylsilyl,
carbazole, and 2,6-diphenyl-4H-pyranylidene [15–17], π-bridges
such as thiophene, furan, oligo-phenylene-vinylene, and pyrrole
[15, 18, 19], and anchoring groups for example carboxylic acid,
rhodamine, and malononitrile groups have been used [9, 20–22].
Carboxylic acid based anchoring groups such as cyanoacrylic
acid (CA) appear frequently in recent studies, however, dyes
featuring these anchoring groups suffer from low stability due
to photo-degradation under long illumination times and dissociate from semiconductor’s surface [22, 23]. The heterocyclic
hydantoin (HY) anchoring group was reported to offer strong
adsorption stability and improved optoelectronic properties in
comparison to CA group [9, 20]. Furthermore, HY promotes
electron injection into the semiconductor and dye regeneration
[24].
Despite considerable efforts from both experimental
and theoretical investigations, few studies have reported on
modifications of the donor part and the influence of electronwithdrawing (EW) and electron-donating (ED) groups on
optical characteristics of the resulting materials [21, 25–27].
Studies have been conducted on the role of –OCH3 and
–CN in the donor, π-linker and acceptor part of the DSSCs
[27–29]. It is anticipated that other substituent groups may
have an influence on the dye’s properties.
In this work, we performed structural modification through
symmetrical substitution of ED and EW groups to the paraposition of the two terminal phenyl rings. The optoelectronic
and chemical reactivity parameters of novel dye molecules featuring CA and HY anchoring groups based on
D-π-π-A framework as displayed in Fig. 1 are discussed. The
relationships between the obtained optoelectronic characteristics and Hammett constants displayed in Table 1 are reported.
Computational details
The 3-D geometries were generated using Avogadro package [31] and brought to minimum conformational energies using Universal Force Fields (UFF)
Fig. 1 Molecular structures
of the designed dyes featuring
cyanoacrylic (CA) and hydantoin (HY) anchoring groups
for different R substituents on
para-position of two terminal
phenyl rings along with structural features (bond lengths and
dihedral angles)
13
Table 1 The substituents and the corresponding Hammett constants
at para position σp [30]
EW substituent
Hammett
constant (σp)
ED substituent
Hammett
constant
(σp)
CN
CF3
COCH3
Cl
Br
F
0.66
0.54
0.50
0.23
0.23
0.06
SH
CH3
OH
OCH3
NH2
N(CH3)2
0.15
−0.17
−0.37
−0.27
−0.66
−0.83
embedded in Avogadro. The equilibrium geometries
were obtained using density functional theory (DFT) at
B3LYP/6–31 + G(d,p) level of theory by applying the
conductor-like polarized continuum model (C-PCM) of
the chloroform solvent. The absence of imaginary frequencies confirmed that the obtained geometries correspond
to a stationary point on the potential energy surfaces. The
UV–Vis spectra were simulated under time-dependent
density functional theory (TD-DFT) method with CAMB3LYP/6–31 + G(d,p) level of theory within C-PCM of
the chloroform solvent. The level of theory adopted in
this work was validated by Deogratias et al. [32] which
adequately reproduced the electronic absorption spectra
reported by Guo et al. [9]. The chemical reactivity parameters were evaluated based on HOMO and LUMO energy
in accordance with Koopmans’ theory [33].
Results and discussion
Geometrical properties
The modification on the investigated dyes includes only
the para-substitution of the phenyl rings as the ortho- and
meta-substituents would introduce steric effects. The estimated ground-state geometrical parameters (bond distance
and dihedral angles) for the dyes containing methoxy
(R = –OCH3) group and their derivatives are presented in
Structural Chemistry
Table S1. As it would be expected, the bond distances
­d1 and ­d2 are equivalent due to symmetrical substituents.
For CA containing dyes the average carbon–nitrogen bond
distance is 1.421 Å and 1.430 Å for EW and ED, respectively. On average, the distances ­d1 and ­d2 are shortened
by ~0.01 Å for HY dyes when compared to CA dyes. The
bond distance d­ 3 exhibits greater variations among all the
bond lengths with the highest standard deviation of 0.08
and 0.07 for CA and HY dyes, respectively, among dyes
with EW groups while a standard deviation of 0.05 was
noted for the dyes containing ED groups. Furthermore,
a positive correlation between the Hammett constants
for the substituents and d­ 3 was observed with R2 values
between 93 − 94%. The substitution of ED groups resulted
in shorter d­ 3 distances for both CA and HY dyes. It is worthy to mention that HY(d3) was higher by 0.02 to 0.04 Å
when compared to CA(d3). The effect of the substituent
groups decreases gradually for the remote C–C bond distances ­di (i = 4, 5, and 6) to ­d7(C = C).
The dihedral angles (ϑi) of the optimized geometries featuring CA and HY dyes are displayed in Table S2. In both
CA and HY dyes, the dihedral angles ϑ1, ϑ2, and ϑ3 determine the non-planarity of the dye. Comparatively, the ED
substituents resulted in larger dihedral angles than EW; this
implies that the incorporation of electron-donating groups
will result into reduction of dye aggregation, –NH2 and
–N(CH3)2 resulted in the largest ϑ1, ϑ2, and ϑ3 values. The
ϑi (i = 4–7) are smaller when compared to ϑi (i = 1–3), the
fact that dihedral angles ϑi (i = 4–7) are small, they increase
the planarity which in turn facilitate intramolecular
charge transfer. For HY dyes, ϑi (i = 4–5) are larger than those
of CA dyes. This reflects larger π-orbital overlap between
neighbouring backbones facilitating intramolecular charge
transfer which can be reflected in the blue-shifted electronic
spectra for HY dyes relative to CA dyes.
Frontier molecular orbitals
Frontier molecular orbitals (FMOs) are among the important factors that affect the intramolecular charge transfer
(ICT) properties of the dyes. The surface mapping of the
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energies for the 24
studied dyes featuring CA and HY anchoring groups are
presented in Fig. 2. The HOMO of CA-based dyes with
both EW and ED groups are mainly spread over the TPA
(donor) and π-linkers (benzothiadiazole and thiophene)
moieties whereas the LUMO is mainly delocalized on
the π-linkers and anchoring group. The observed distribution suggests intermolecular charge transfer [17, 20].
This localization facilitates the electron transfer from the
TPA donor sub unit through the π-linkers to the anchoring
group direction. The LUMOs for the EW and ED groups in
HY dyes are mostly delocalized over the anchoring group
and π-linker but partially on its adjacent phenyl group. The
observed LUMO charge distribution may results in strong
electronic coupling between dye and semiconductor [34].
Electronic absorption properties
The performance of DSSCs depends on the spectral coverage within visible and near infra-red regions. The absorption spectra for CA and HY dyes are presented in Fig. 3.
The electronic transitions, excitation energies (ΔE, eV),
wavelengths (λ, nm), oscillator strengths (f), excited-state
lifetimes (τ, ns), light-harvesting efficiencies (LHE), and
transition contributions are displayed in Tables 2 and 3. The
maximum absorption wavelength (λmax) of CA and HY dyes
with methoxy group (­ OCH3) in chloroform were calculated
at TD-DFT/CAM-B3LYP/6–31 + G(d,p) level and found to
be 532 nm and 513 nm, respectively. The λmax found at this
level of theory were in agreement with the experimental
observations (Guo et al. [9]). Thus, the same level of theory
was applied to novel designed sensitizers in this study for
simulation of electronic properties. The UV–Vis absorption spectra for the dyes containing EW and ED substituents
groups featuring CA and HY anchoring groups are shown
in Fig. 3. The absorption spectral bands for CA and HY
dyes with EW groups lie within the visible region. Strong
electron-withdrawing groups –CN, –CF3, and –COCH3 led
to blue-shifted electronic spectra while the red-shifted
peaks were observed for –Cl, –Br, and –F groups when compared to parent materials. A difference of 101 nm between
the least and highest values of λmax for EW and ED was
observed for CA dyes and 58 nm for HY dyes. For CA dyes,
ED resulted into a red-shifted electronic spectra ranging
between 37 and 68 nm when compared to EW and 22–41 nm
for HY dyes.
The calculated ∆E for R = –Cl coincides with that of
R = –Br, this is consistent with the Hammett constant value
of –Cl and –Br. The calculated λmax for ED groups follow
the order –SH < –CH3 < –OH < –OCH3 < –NH2 < –N(CH3)2.
This implies that dyes containing –NH2 and –N(CH3)2 may
have better PCE than –OCH3 dyes. All designed dyes exhibited excellent LHE exceeding 95%.
Figure 4 shows a positive correlation (R2 > 0.97) for the
dependence of λmax on substituents. The positive slope indicates that the increase in the Hammett constant σp results in
the increase in λmax. The ED groups exhibit lower vertical
excitation energies when compared to EW substituents. A
very important characteristic of the regression lines is the
difference in the slopes of 0.3068 and 0.1876 for CA and HY
dyes, respectively. Beyond doubt, this is associated with the
anchoring groups used; the CA dyes have larger deviations
13
Structural Chemistry
Fig. 2 Frontier molecular orbitals of the studied dyes simulated
at CAM-B3LYP/6–31 + G(d,p)
with PCM, chloroform solvent.
Red represent the positive phase
and green the negative phases of
the wave function [24]
13
Structural Chemistry
Fig. 3 UV–Vis electronic
absorption spectra in chloroform for a CA-EW, b HY-EW,
c CA-ED, and d HY-ED-based
dyes calculated at CAMB3LYP/6–31 + G(d,p) with
PCM (chloroform)
in λmax than those of HY dyes and the y-intercept relate to
the R groups. This may be attributed to the presence of
lone pairs in oxygen and nitrogen atoms in the hydantoin
anchoring group.
From Tables 2 and 3, the ­S0 → ­S1 electronic transitions
for the investigated dyes are associated with λ max, with
major contribution from both HOMO → LUMO and
HOMO-1 → LUMO transitions. Nevertheless, slightly
higher f values are observed in the CA dyes than HY
dyes. Dyes with CA anchoring group had f in the range
1.631–1.548 for EW and 1.419–1.546 for ED groups
whereas dyes with HY group showed f in the range
1.454–1.499 for EW and 1.360–1.479 for ED groups. The
Table 2 Characteristics of
electronic absorption data
obtained for CA-based
dyes (electronic transitions,
excitation energies (ΔE, eV),
wavelengths (λ, nm), oscillator
strengths (f), excited state
lifetimes (τ, ns), light-harvesting
efficiencies (LHE, %), and main
transition configurations
reason for the differences is probably due to the small
transition dipole moment that leads to slight red-shift for
dyes substituted with ED groups.
The excited state lifetime (τ) relates to the decay process of the dye from excited to ground state. The lifetime
has an influence on the dye's stability and affects the charge
transfer ability of the dye; consequently, to the efficiency
of the solar cell. The longer the excited state lifetime the
stable the molecule in its excited state; this can facilitate
the charge transfer due to efficient exciton dissociation [35,
36]. The τ values are reported in Table 2 for CA-based
dyes and Table 3 for HY-based dyes. The results of excited
state lifetime are in the range of 2.03–3.45 ns for CA dyes
EW
Transitions
∆E
λ
f
τ
LHE
Contributions
CN
CF3
COCH3
Cl
Br
F
ED
SH
CH3
OH
OCH3
NH2
N(CH3)2
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
Transitions
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
2.64
2.61
2.60
2.50
2.50
2.47
∆E
2.41
2.38
2.35
2.33
2.24
2.17
470
475
477
495
496
503
λ
514
521
528
532
553
571
1.628
1.599
1.631
1.582
1.596
1.548
f
1.546
1.519
1.504
1.500
1.456
1.419
2.03
2.12
2.10
2.32
2.31
2.45
τ
2.56
2.68
2.78
2.83
3.15
3.45
97.6
97.5
97.7
97.4
97.5
97.2
LHE
97.2
97.0
96.9
96.8
96.5
96.2
H → L (58%); H-1 → L (36%)
H → L (59%); H-1 → L (35%)
H → L (56%); H-1 → L (39%)
H → L (59%); H-1 → L (34%)
H → L (59%); H-1 → L (34%)
H → L (60%); H-1 → L (32%)
Contributions
H → L (58%); H-1 → L (36%)
H → L (60%); H-1 → L (31%)
H → L (60%); H-1 → L (31%)
H → L (60%); H-1 → L (32%)
H → L (60%); H-1 → L (32%)
H → L (59%); H-2 → L (32%)
13
Structural Chemistry
Table 3 Characteristics of
electronic absorption data
obtained for HY-based dyes
(Electronic transitions,
excitation energies (ΔE, eV),
wavelengths (λ, nm), oscillator
strengths (f), excited state
lifetimes (τ, ns), light-harvesting
efficiencies (LHE, %), and
contributions
EW
Transitions
∆E
λ
f
τ
LHE
Contributions
CN
CF3
COCH3
Cl
Br
F
ED
SH
CH3
OH
OCH3
NH2
N(CH3)2
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
Transitions
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
S0 → ­S1
2.58
2.59
2.57
2.53
2.53
2.50
∆E
2.47
2.45
2.42
2.42
2.35
2.31
480
479
483
489
491
496
λ
502
505
512
513
527
537
1.476
1.462
1.499
1.479
1.489
1.454
f
1.479
1.451
1.429
1.440
1.393
1.360
2.34
2.36
2.33
2.43
2.42
2.53
τ
2.55
2.64
2.75
2.73
2.99
3.18
96.7
96.5
96.8
96.7
96.8
96.5
LHE
96.7
96.5
96.3
96.4
96.0
95.6
H → L (67%); H-1 → L (14%)
H → L (66%); H-1 → L (17%)
H → L (65%); H-1 → L (22%)
H → L (62%); H-1 → L (27%)
H → L (63%); H-1 → L (26%)
H → L (63%); H-1 → L (26%)
Contributions
H → L (58%); H-1 → L (35%)
H → L (61%); H-1 → L (31%)
H → L (61%); H-1 → L (31%)
H → L (60%); H-1 → L (32%)
H → L (59%); H-1 → L (34%)
H → L (57%); H-1 → L (32%)
and 2.33–3.18 ns for HY dyes. It is interesting to note that
ED groups may lead to efficient charge transfer than EW
groups. Contrary to the EW substituents, τ values for ED
groups are higher for CA than for HY dyes.
Energy level and charge transfer properties
In DSSC devices, electrons are either injected from a
dye into a semiconductor’s conduction band or from
electrolyte to the dye. Therefore, energy level alignment between the frontier molecular orbitals with both
conduction band (CB) of a semiconductor and redox
potential of electrolyte is an important factor. Charge
transfer characteristics of dyes depend on the HOMO and
LUMO energy levels. According to Koopmans’ theorem
[33, 37], the HOMO energy also known as ground state
oxidation potential (GSOP) of the dye can be abbreviated
as Eox . The excited state oxidation potential (ESOP) is
dye∗
abbreviated as Eox involves both GSOP and excitation
dye∗
dye
energies (∆E) as Eox = Eox + ∆E. For efficient charge
injection, the ESOP should be higher than conduction
band edge of the semiconductor (normally T
­ iO 2); also
GSOP should be more negative than the redox potential
of electrolyte. The GSOP, ESOP, free energy of charge
injection and dye regeneration abilities are reported in
Table 4.
All calculated GSOP are more negative than the potential
of redox couple of iodide/tri-iodide electrolyte of −4.80 eV
[38]. Thus ΔGreg are negative implying that the designed
dyes can be regenerated by receiving an electron from the
electrolyte. The estimated ESOP values lie above the conduction band of T
­ iO2 except for CA dyes containing stronger
electron-withdrawing groups R = CN, C
­ F3, and ­COCH3. The
conduction band of T
­ iO2 is set at −4.21 eV a theoretical
dye
Fig. 4 The dependence of maximum electronic absorption to Hammett constants for a cyanoacrylic acid and b hydantoin dyes
13
Structural Chemistry
Table 4 The ground and excited
state oxidation potentials, free
energies of charge injection
(ΔGinj) and dye regeneration
(ΔGreg) for EW and ED
substituted CA and HY-based
dyes
CA–dye
HY–dye
EW
–GSOP
–ESOP
ΔGinj
–ΔGreg
–GSOP
–ESOP
–ΔGinj
–ΔGreg
CN
CF3
COCH3
Cl
Br
F
ED
SH
CH3
OH
OCH3
NH2
N(CH3)2
7.03
6.95
6.88
6.70
6.70
6.65
–GSOP
6.46
6.44
6.38
6.34
6.14
5.96
4.39
4.35
4.29
4.20
4.20
4.18
–ESOP
4.04
4.06
4.04
4.01
3.90
3.79
0.18
0.14
0.08
−0.01
−0.01
−0.03
ΔGinj
−0.17
−0.15
−0.17
−0.20
−0.31
−0.42
2.23
2.15
2.08
1.90
1.90
1.85
–ΔGreg
1.66
1.64
1.58
1.54
1.34
1.16
6.78
6.73
6.69
6.56
6.56
6.50
–GSOP
6.35
6.33
6.27
6.24
6.06
5.89
4.19
4.15
4.12
4.03
4.03
4.00
–ESOP
3.88
3.88
3.85
3.82
3.70
3.59
0.02
0.06
0.09
0.18
0.18
0.21
–ΔGinj
0.33
0.33
0.36
0.39
0.51
0.62
1.98
1.93
1.89
1.76
1.76
1.70
–ΔGreg
1.55
1.53
1.47
1.44
1.26
1.09
value [39]. Spontaneous electron injection into the semiconductor CB increases with increasing electron-donating
strength and decreases with increasing electron-withdrawing
power. The energy levels alignment diagrams of the CA and
HY dyes with respect to CB of T
­ iO2 and redox potential of
I − ∕I3− for EW and ED groups are shown in Fig. 5.
Fig. 5 Energy levels of alignment a CA-EW, b HY-EW, c CA-ED, and d HY-ED with respect to CB of ­TiO2 and redox potential of I − ∕I3− for
EW and ED groups
13
Structural Chemistry
Fig. 6 Linear regression analysis on the dependence of IP and EA on Hammett constants; the plots a, b and c, d are for the dyes containing
cyanoacrylic acid and hydantoin anchoring groups, respectively
The values of ∆Ginj for HY dyes are larger than those of
corresponding CA dyes in the range of −0.02 to −0.21 eV for
EW groups and −0.33 to − 0.62 eV for ED groups. These values are higher in magnitude than the minimum ∆Ginj required
(−0.2 eV) [40, 41] which implies that the HY dyes favour
spontaneous injection of electron from dye’s excited state
into CB of the ­TiO 2 semiconductor, thus increasing
the dyes efficiency. Therefore, strong electron-donating groups
(–OCH3, –NH2, and –N(CH3)2) are expected to offer higher
PCE due to their relatively higher ∆Ginj values. The ∆Ginj values for CA dyes are characterized by positive values for strong
electron-withdrawing groups (–CN, –CF3 to –COCH3) and
slightly less negative values for Cl, Br, and F.
IP, EA, and chemical reactivity parameters
Quantification of reactivity parameters of the designed dyes
were performed through analysis of ionization potentials
13
(IPs), electron affinities [42] and their derivatives. The
IP values for CA and HY dyes are between 5.96–7.03 eV
and 5.89–6.78 eV, respectively. Also, the EA values
range between 2.35–2.48 eV and 2.01–2.17 eV for CA
and HY dyes, respectively. In most cases, the IP values
decreases with increasing electron-donating abilities of the
R groups. It can be noted that, the change of R groups significantly affects the IP values more than it is for EA, the standard deviation values are 0.325 and 0.273 among CA and HY
molecules, respectively. We observed small standard deviations in terms of EA of about 0.041 for CA and 0.048 for
HY dyes. The decrease in IP values with increasing electrondonating groups is desirable to facilitate the hole and electron injection. In contrast, larger IP values reflect an increasing injection barrier of the charge. Figure 6 illustrates good
correlation between quantum chemical descriptors IP/EA
and Hammett constants with R2 higher than 0.90. The functional groups with more negative Hammett constants exhibit
Structural Chemistry
Table 5 The chemical
reactivity quantities ionization
potential IP, electron affinity
EA, electronic chemical
potential µ, global hardness
ɳ, electrophilicity ω,
electroaccepting power ω+, and
electrodonating power ω− (all
in eV) for EW and ED groups
substituted in CA-based dyes
EW
IP
EA
–µ
ɳ
ω
ω+
ω–
CN
CF3
COCH3
Cl
Br
F
ED
SH
CH3
OH
OCH3
NH2
N(CH3)2
7.03
6.95
6.88
6.70
6.70
6.65
IP
6.46
6.44
6.38
6.34
6.14
5.96
2.48
2.46
2.46
2.43
2.42
2.41
EA
2.41
2.39
2.38
2.38
2.36
2.35
4.76
4.71
4.67
4.56
4.56
4.53
–µ
4.43
4.41
4.38
4.36
4.25
4.16
2.28
2.25
2.21
2.14
2.14
2.12
ɳ
2.03
2.03
2.00
1.98
1.89
1.80
2.48
2.46
2.47
2.44
2.43
2.42
ω
2.42
2.40
2.40
2.40
2.38
2.39
2.87
2.85
2.88
2.86
2.85
2.85
ω+
2.88
2.85
2.86
2.86
2.88
2.93
7.63
7.56
7.56
7.42
7.42
7.38
ω–
7.31
7.26
7.24
7.22
7.13
7.09
lower IP values. The negative σp values observed for dyes
substituted with ED groups indicate high charge donating
characteristics for the dyes containing ED groups.
Global reactivity descriptors are appropriate benchmarks
to study the electronic properties of dyes; among them is an
electrophilicity index (ω). The electrophilicity index values
for the designed dyes for cyanoacrylic acid and hydantoin
groups are presented in Tables 5 and 6, respectively. It can
be seen that dyes with EW substituents exhibit higher electrophilicity index, while sensitizers with ED substituents
demonstrate lower electrophilicity index. For instance, the
strongest withdrawing group (–CN) has an electrophilic
value of (ω = ~2.5 (2.2) eV) while the strongest electrondonating group has ω value of about 2.4 (2.0) eV, for
cyanoacrylic acid (hydantoin).
This observation shows that the activating/deactivating effects can be promoted by substituent EW/ED groups
in both cyanoacrylic acid and hydantoin dyes. Contrary
Table 6 The chemical
reactivity quantities ionization
potential IP, electron affinity
EA, electronic chemical
potential µ, global hardness
ɳ, electrophilicity ω,
electroaccepting power ω+, and
electrodonating power ω− (all
in eV) for EW and ED groups
substituted in HY-based dyes
to chemical hardness (ɳ), the absolute values of chemical potential (μ) increases in the order weak EW < strong
EW < weak ED < strong ED for both cyanoacrylic acid and
hydantoin containing dyes. The μ values lie between −5.03
and −4.16 eV for CA and −5.00 and −3.95 eV for HY-based
dyes. The high μ values observed for dyes substituted with
ED groups particularly –NH2 and –N(CH3)2 indicate that
they have higher reactivity than those with EW groups. The
resistance to charge transfer is lower for dyes containing
ED groups. Larger ω, ω+, and ω− values are found for CA
dyes than HY dyes, however, lower ω, ω+, and ω− values are
obtained for CA and HY dyes substituted with ED groups
indicating better electron-donation ability and high dye stabilization energy. From chemical descriptors, we can conclude
that dyes substituted with strong ED groups, i.e., –NH2 and
–N(CH3)2 could be potential and most appropriate materials
among the designed dyes for improving the performance of
DSSCs.
EW
IP
EA
–µ
ɳ
ω
ω+
ω–
CN
CF3
COCH3
Cl
Br
F
ED
SH
CH3
OH
OCH3
NH2
N(CH3)2
6.78
6.73
6.69
6.56
6.56
6.50
IP
6.35
6.33
6.27
6.24
6.06
5.89
2.17
2.13
2.12
2.09
2.09
2.08
EA
2.07
2.05
2.05
2.03
2.02
2.01
4.47
4.43
4.41
4.33
4.32
4.29
–µ
4.21
4.19
4.16
4.14
4.04
3.95
2.31
2.30
2.28
2.23
2.23
2.21
ɳ
2.14
2.14
2.11
2.10
2.02
1.94
2.17
2.13
2.13
2.10
2.09
2.09
ω
2.07
2.05
2.05
2.04
2.02
2.01
2.39
2.34
2.34
2.31
2.31
2.30
ω+
2.31
2.27
2.28
2.26
2.27
2.29
6.86
6.77
6.74
6.63
6.63
6.59
ω–
6.52
6.46
6.44
6.40
6.31
6.24
13
Structural Chemistry
Conclusions
References
Geometrical and electronic properties of twelve dyes are
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substitution of electron-withdrawing and electron-donating
groups. An alternative anchoring group to cyanoacrylic
acid (hydantoin) was investigated. DFT and TD-DFT
methods were employed to obtain the reported properties.
We observed large variations for the bond lengths closer
to the regions of modification. All dyes exhibit excellent
planarity essential for intramolecular charge transfer. The
ED containing dyes resulted in red-shifted electronic spectra
when compared to EW containing dyes. Lower energy λmax
values (in eV) are observed for strong ED substituents specifically
for –NH2 and –N(CH3)2 groups. Also, the studied chemical reactivity and the possible correlation analysis with
different electronic energy parameters were performed.
The results suggest that HY dyes substituted with –NH2
and –N(CH3)2 groups show enhanced optoelectronic and
chemical reactivity properties than other dyes. Excellent
relationship between the Hammett constants and electronic
energies were obtained. The results from this work are useful towards the improvement of PCE of DSSCs.
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Supplementary information The online version contains supplementary material available at https://d​ oi.o​ rg/1​ 0.1​ 007/s​ 11224-0​ 21-0​ 1837-4.
Acknowledgements SLM acknowledges the support received from
Rev. Fr. Valentine Bayo CSSp. The authors are thankfully to Mr. Rene
Costa and Mr. Peter I. Kirenga for their dedicated assistance during
development of this work. We would like to thank the South Africa
Centre for High Performance Computing (CHPC) for providing computational resources on their Lengau cluster for this research.
Author contribution All authors contributed to the study conception and design. SLM prepared the materials, performed analysis and
interpretation of data, drafted the work, and revised it critically for
important intellectual content. Authors YM, FRJ, GAK, LWK, SM, and
GD guided, edited, interpreted, revised the manuscript, and ensured
accuracy or integrity of this work, LWK performed all calculation for
the findings reported in this work. All authors read and approved the
final manuscript.
Funding Financial support was received from the University of Dar
es Salaam through its Directorate of Research Grant Number DUCE20151 supported FRJ, GAK, YM, SM, and GD, the Kenya Education
Network Trust (KENET) for mini-grant supported LWK, and Marian
University College supported SLM.
Availability of data and material The datasets generated during and/or
analyzed during the current study are available from the corresponding
author on reasonable request.
Declarations
Conflict of interest The authors declare no competing interests.
13
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