Enhanced Donor–π–Acceptor Character of a Porphyrin Dye Incorporating Naphthobisthiadiazole for Efficient Near-Infrared Light Absorption

Article abstract of DOI:10.1002/ejoc.201701736

Donor–π–acceptor (D–π–A) molecules play an important role in various fields including polymer solar cells (PSCs) and dye-sensitized solar cells (DSSCs). In PSCs, naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) has emerged as a promising acceptor owing to its higher electron-accepting ability compared with 2,1,3-benzothiadiazole (BT), which is a widely used acceptor unit in DSSCs as well as PSCs. On the other hand, various D–π–A-type porphyrin dyes have been developed for use in DSSCs and have achieved high power conversion efficiencies (η). Given that enhanced D–π–A character improves light-harvesting ability in the near-infrared (NIR) region, we have designed and synthesized for the first time a novel D–π–A-type porphyrin sensitizer, ZnPNTz, incorporating the NTz unit. The porphyrin ZnPNTz shows excellent absorption in the NIR region together with a large Stokes shift as a result of the NTz moiety. A DSSC fabricated with ZnPNTz exhibits photocurrent generation at wavelengths up to 860 nm. Therefore, we have demonstrated that the NTz unit is a promising electron-acceptor unit for novel D–π–A small molecules exhibiting unique optical properties in the NIR region.

Full text of DOI:10.1002/ejoc.201701736

DOI: 10.1002/ejoc.201701736
Full Paper
Porphyrins
Enhanced Donor–π–Acceptor Character of a Porphyrin Dye
Incorporating Naphthobisthiadiazole for Efficient Near-Infrared
Light Absorption
Tomohiro Higashino,[a] Yuma Kurumisawa,[a] Shimpei Nimura,[a] Hitomi Iiyama,[a] and
Hiroshi Imahori*[a,b]
Abstract: Donor–π–acceptor (D–π–A) molecules play an im- improves light-harvesting ability in the near-infrared (NIR) re-
portant role in various fields including polymer solar cells gion, we have designed and synthesized for the first time a
(PSCs) and dye-sensitized solar cells (DSSCs). In PSCs, novel D–π–A-type porphyrin sensitizer, ZnPNTz, incorporating
naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) has emerged as the NTz unit. The porphyrin ZnPNTz shows excellent absorption
a promising acceptor owing to its higher electron-accepting in the NIR region together with a large Stokes shift as a result
ability compared with 2,1,3-benzothiadiazole (BT), which is a of the NTz moiety. A DSSC fabricated with ZnPNTz exhibits
widely used acceptor unit in DSSCs as well as PSCs. On the photocurrent generation at wavelengths up to 860 nm. There-
other hand, various D–π–A-type porphyrin dyes have been de- fore, we have demonstrated that the NTz unit is a promising
veloped for use in DSSCs and have achieved high power con- electron-acceptor unit for novel D–π–A small molecules exhibit-
version efficiencies (η). Given that enhanced D–π–A character ing unique optical properties in the NIR region.
Introduction
and nonlinear optical (NLO) devices.[3] In recent years, PSCs and
DSSCs have attracted considerable attention because of their
Donor–π–acceptor (D–π–A) molecules have been actively de- potential to solve environmental and energy issues. Although
veloped as promising functional dyes in various fields, including PSCs and DSSCs possess advantages such as lightweight, flexi-
polymer solar cells (PSCs),[1] dye-sensitized solar cells (DSSCs),[2] ble structures, and low-cost production, their power conversion
Figure 1. Representative examples of D–A polymers for use in PSCs.
[a] Department of Molecular Engineering, Graduate School of Engineering,
efficiencies (η) are still insufficient. To realize practical applica-
tions, further improvement of the light-harvesting ability in the
visible and near-infrared (NIR) regions is prerequisite.
Kyoto University,
Nishikyo-ku, Kyoto, 615-8510, Japan
E-mail: imahori@scl.kyoto-u.ac.jp
http://www.moleng.kyoto-u.ac.jp/~moleng_05/index_e.html
[b] Institute for Integrated Cell-Material Sciences, Kyoto University,
Sakyo-ku, Kyoto, 606-8501, Japan
In PSCs, one of the most important approaches towards the
improvement of the value of η is the development of new semi-
conducting donor–acceptor (D–A) polymers with alternating
donor and acceptor units.[1] D–A polymers can afford absorp-
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201701736.
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tion bands in the NIR region that produce large short-circuit
On the other hand, the photovoltaic performance of DSSCs
current densities (JSC) and high η values. Thus, various kinds strongly depends on sensitizers. Various ruthenium complexes
of D–A polymers have been synthesized.[4] Among them, were developed as highly efficient sensitizers in the early stages
2,1,3-benzothiadiazole (BT) has been widely used as an ac- of the development of DSSCs.[7] In recent years, organic D–π–A
ceptor unit and achieved high η values.[5] Recently, naphtho- molecules have been actively explored, aiming to reduce their
[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) has emerged as a promis- production cost.[8,9] Among them, D–π–A-type porphyrin sensi-
ing acceptor unit for high-performance D–A polymers (Fig- tizers have attracted considerable attention (Figure 2).[10,11] In
ure 1).[6] The high electron-accepting ability of the NTz unit 2011, the benchmark porphyrin sensitizer YD2-o-C8 was re-
improved the light-harvesting ability of D–A polymers owing to ported.[10b] After that, various D–π–A-type porphyrin sensitizers
its enhanced intramolecular charge-transfer (ICT) character. For were prepared that exceeded η values of 10 %. By introducing
instance, the PSC based on PBDT-DTNT shows a higher η value the electron-accepting BT unit, GY50 exhibited an improved
than that based on PBDT-DTBT owing to its broad absorption η value due to enhanced NIR absorption reaching up to
covering the range 300–800 nm.[6a]
750 nm.[10f] Additionally, a DSSC based on XW17 showed pho-
Figure 2. D–π–A-type porphyrin sensitizers for use in DSSCs.
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tocurrent generation at wavelengths above 800 nm, because ance of the DSSC prepared from this substrate using XW17 as
the replacement of the diarylamino moiety by a triarylamine- a reference.
based electron-donating group led to further redshift of the
absorption in the NIR region.[11d]
Results and Discussion
To further explore the potential of D–π–A-type porphyrin
dyes, the rational integration of suitable electron-accepting
units into a porphyrin π core would be highly desirable. To date,
Synthesis
the BT unit has been one of the most widely used electron Although the NTz unit has been introduced into various D–A
acceptors in DSSCs.[8e–g,10e,10f,10i,11c–11e] However, to the best of polymers for use in PSCs because of its strong electron-accept-
our knowledge, no application of the NTz unit in D–π–A mol- ing character, examples of NTz-based small molecules are still
ecules has been reported, despite the rapid development of limited due to difficulties in their synthesis.[12] Therefore, we
D–A polymers incorporating the NTz unit in PSCs. Thus, we en- aimed to develop a synthetic approach to NTz-based molecules
visioned that the use of the NTz unit in D–π–A-type porphyrin (Scheme 1). We first attempted the monobromination of NTz
sensitizers would enhance the NIR light absorption, and thereby (1) by reaction with 1 equivalent of NBS, but obtained an insep-
improve their power conversion efficiency. Herein we designed arable mixture of mono- and dibrominated compounds owing
and synthesized a novel D–π–A-type porphyrin sensitizer, to their poor solubility in common organic solvents. A decrease
ZnPNTz (Figure 2). It is noteworthy that ZnPNTz is the first in the amount of NBS or the reaction time could not prevent
example of an organic molecule with an unsymmetrically sub- the formation of dibrominated NTz. On the other hand, the re-
stituted NTz unit. We examined the optical and electrochemical action with 2.2 equivalents of NBS provided dibrominated com-
properties of ZnPNTz and evaluated the photovoltaic perform- pound 2 in high yield (ca. 80 %).[6a] Then we performed the
Scheme 1. Attempted synthesis of the unsymmetrically functionalized naphthobisthiadiazole.
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Stille coupling reaction of 2 with arylstannane 3. The reaction debrominated, and butylated compounds. After optimization of
of 2 with 1 equivalent of 3 at 120 °C resulted in a complicated the conditions, we found that the reaction with 0.5 equivalents
mixture of undesired products, such as diarylated, proto- of 3 at 45 °C afforded the desired product 4. Although the yield
Scheme 2. Synthesis of the D–π–A porphyrin with the NTz unit.
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was low (9.6 %), an increase in temperature or reaction time concentration of ZnPNTz provided a larger amplitude of the
was not effective on account of the formation of byproducts.
In the next step we synthesized the porphyrin core (5)
according to a typical procedure by acid-catalyzed condensa-
tion of dipyrromethane with the appropriate aldehyde
(Scheme 2).[10b] The bromination of 5 with NBS and zinc inser-
tion into the resulting 6 provided the porphyrin intermediate
7. Sonogashira coupling of 7 with the triarylamine-based elec-
tron-donating group 8, which was prepared according to a liter-
ature procedure,[11d] afforded 9 in 25 % yield. A further ethynyl
group was introduced into the porphyrin by Sonogashira cou-
pling reaction of 9 with triisopropylsilylacetylene. After the des-
ilylation of 10 with tetrabutylammonium fluoride (TBAF), the
coupling reaction of 11 with the unsymmetrically substituted
NTz 4 gave the porphyrin 12. Finally, the reaction of 12 with an
excess amount of NaOH provided the D–π–A-type porphyrin
ZnPNTz in satisfactory yield (55 %). We also synthesized
benzothiadiazole derivative XW17 as a reference according to
a literature procedure.[11d]
fast component. Therefore, the fast component can be attrib-
uted to the aggregated state of ZnPNTz, which exists even in
solution owing to the large planar structure of the NTz unit.
This result implies that the broadened fluorescence relates to
the aggregated state as well as to the enhanced ICT character.
By averaging the peak energies of the absorption at the longest
wavelength and of the emission at the shortest wavelength for
ZnPNTz and XW17, the optical HOMO–LUMO gaps were esti-
mated to be 1.63 and 1.75 eV, respectively.
Optical and Electrochemical Properties
The UV/Vis/NIR absorption and steady-state fluorescence spec-
tra of ZnPNTz and XW17 were recorded in THF (Figure 3). Al-
though the Soret band of ZnPNTz is almost the same as that
of XW17, ZnPNTz shows a redshifted (14 nm) and broadened
Q-band, which indicates enhanced D–π–A character as a conse-
quence of the high electron-accepting ability of the NTz unit.
Notably, the absorption edge of ZnPNTz is shifted by around
50 nm, exceeding 800 nm. The fluorescence spectrum of
ZnPNTz displays a broad peak at 823 nm, which is remarkably
shifted compared with that of XW17 at 725 nm. The Stokes
shift of ZnPNTz (2014 cm–1) is three times as large as that of
XW17 (658 cm–1). This large Stokes shift suggests significant
conformational change in the singlet excited state of ZnPNTz,
arising from the NTz unit. We attempted to record the absorp-
tion and fluorescence spectra in various solvents. Although
ZnPNTz and XW17 are soluble in toluene, they possess insuffi-
cient solubility for measurement in polar solvents (e.g., meth-
Figure 3. (a) UV/Vis/NIR absorption and (b) normalized fluorescence spectra
of ZnPNTz (red) and XW17 (blue) in THF. For the fluorescence measurements,
samples were excited at 468 nm. To obtain the whole fluorescence spectra,
we used two detectors, one for wavelengths less than 825 nm, and the other
for wavelengths more than 825 nm.
The electrochemical properties of ZnPNTz and XW17 were
anol, acetonitrile, DMF, and DMSO). The absorption and fluores- studied by cyclic voltammetry (CV) and differential pulse vol-
cence spectra recorded in THF reveal redshifted Q-bands and tammetry (DPV) in THF versus NHE with tetrabutylammonium
emission compared to those in toluene, respectively (see Fig- hexafluorophosphate as the electrolyte (see Figure S9 in the
ures S6 and S7 in the Supporting Information). More impor- Supporting Information). The two porphyrins exhibit two re-
tantly, the redshift and broadening are more marked for versible reduction and oxidation peaks. The oxidation potentials
ZnPNTz than for XW17. This indicates that the ICT character is of ZnPNTz (+0.80 and +0.96 V) are largely similar to those of
more evident in ZnPNTz, as a result of the introduction of the XW17 (+0.75 and +0.92 V). On the other hand, the reduction
strong electron-withdrawing NTz into the porphyrin core potentials of ZnPNTz (–0.83 and –1.10 V) are shifted in the
through the triple bond. The fluorescence lifetimes (τ) were positive direction by around 0.2 V relative to those of XW17
measured by the time-correlated single-photon counting (–1.07 and –1.26 V). These large shifts have been attributed to
(TCSPC) technique (see Figure S8). The decay curve of the fluo- the high electron-accepting ability of the NTz unit. The electro-
rescence intensity of XW17 in THF was fitted as a single expo- chemical HOMO–LUMO gaps of ZnPNTz (1.63 eV) and XW17
nential to give τ1 = 0.94 ns [amplitude (A1) = 100 %]. In contrast, (1.82 eV) are consistent with the corresponding optical HOMO–
the decay curve of ZnPNTz was fitted by a fast component LUMO gaps. From the oxidation potential and optical HOMO–
(τ1 = 0.15 ns, A1 = 53 %) and a slow component (τ2 = 0.89 ns, LUMO gap, the oxidation potential of the porphyrin singlet ex-
A2 = 47 %). The lifetime of the slow component is similar to cited state of ZnPNTz was determined to be –0.83 V. We deter-
that of XW17, which suggests that the slow component is de- mined the driving forces for electron injection (ΔGinj) from the
rived from the monomeric state. It is noteworthy that a higher porphyrin singlet excited state to the conduction band (CB) of
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TiO2 (–0.5 V vs. NHE)[13] and for the regeneration (ΔGreg) of the
–
porphyrin radical cation by the I–/I3 redox shuttle (+0.4 V vs.
NHE;[13] Table 1). All of the electron-transfer (ET) processes
should occur efficiently because these driving forces are more
negative than –0.3 eV.
Table 1. Electrochemical properties of the D–π–A-type porphyrins.
[a]
[a]
[b]
[c]
[d]
Dye
Eox1
Ered1
E0–0
E*ox
ΔGinj
ΔGreg
[V]
[V]
[V]
[V]
[eV]
[eV]
ZnPNTz
XW17
+0.80
+0.75
–0.83
–1.07
1.63
1.75
–0.83
–1.00
–0.33
–0.50
–0.40
–0.35
[a] Determined by DPV (vs. NHE). [b] Determined by adding E0–0 to Eox1
.
[c] Driving force for electron injection from the porphyrin singlet excited state
to the CB of TiO2 (0.5 V vs. NHE). [d] Driving force for the regeneration of the
Figure 4. Photocurrent–voltage characteristics of the DSSCs based on ZnPNTz
(red) and XW17 (blue) under the best conditions for achieving the highest η
values.
porphyrin radical cation by the I–/I3 redox shuttle (+0.4 V vs. NHE).
–
Table 2. Photovoltaic performances of the DSSCs based on ZnPNTz and
XW17.[a]
Photovoltaic Properties
Dye
JSC [mA cm–2
]
VOC [V]
ff
η [%]
The photovoltaic properties of the DSSCs were evaluated under
standard AM 1.5 conditions using an electrolyte solution con-
ZnPNTz
7.33
(7.04 0.05)
15.96
0.648
0.765
3.63
(3.47 0.03)
8.14
(0.654 0.002) (0.754 0.003)
0.700 0.729
–
taining the I–/I3 redox shuttle. We first examined the effect of
XW17
immersion solvent to optimize the cell performance. After the
optimization with a fixed immersion time of 3 h, the DSSCs
based on ZnPTNz and XW17 displayed η values of 2.3 and
7.0 % using ethanol/toluene (1:1 and 4:1 v/v, respectively) as
the best immersion solvent (see Table S1 in the Supporting
Information). Then we investigated the effect of immersion time
and found that an immersion time of 3 h was the best (see
Figure S10). By fixing the immersion time to 3 h, we also as-
sessed the effect of chenodeoxycholic acid (CDCA), a co-adsorb-
ent for the suppression of porphyrin aggregation on TiO2. The
co-adsorption of CDCA improved the cell performance. Maximal
η values of 3.6 and 8.1 % were attained with 20 equivalent of
CDCA for ZnPNTz and 15 equivalent of CDCA for XW17, respec-
tively (see Figure S11). The large dependence on the amount
of CDCA implies strong aggregation behavior of the porphyrins
on TiO2. The larger amount of CDCA required for ZnPNTz than
for XW17 also indicates that the NTz unit has a greater tend-
ency towards aggregation than the BT unit owing to its larger
planar structure. Overall, the highest value of η achieved for the
DSSC based on ZnPNTz under our optimized conditions was
3.63 % [JSC = 7.33 mA cm–2, VOC (open-circuit voltage) = 0.648 V,
ff (fill factor) = 0.765]. In contrast, the DSSC based on XW17
(15.67 0.07) (0.710 0.003) (0.714 0.005)
(7.94 0.06)
[a] Photovoltaic parameters deriving from the highest η value. The values in
parentheses denote average values from eight independent experiments. Er-
ror bars represent a standard error of the mean.
under our conditions exhibits an η value of 8.14 % (JSC
=
15.96 mA cm–2, VOC = 0.700 V, ff = 0.729). The photocurrent–
voltage characteristics of the DSSCs under the finally optimized
conditions are depicted in Figure 4 and the photovoltaic param-
eters are summarized in Table 2. The lower η value of the DSSC
based on ZnPNTz is mainly ascribed to the low JSC value.
The absorption of the ZnPNTz-sensitized TiO2 film displays
improved light-harvesting ability at around 700–800 nm (Fig-
ure 5a) and the photocurrent spectral response of the DSSC
based on ZnPNTz reaches up to 860 nm (Figure 5b). Consider-
ing the light-scattering layers in the real DSSCs, the light-
harvesting efficiencies of ZnPNTz and XW17 are almost unity.
Accordingly, the electron injection efficiency (φinj) and/or
charge collection efficiency (ηcol) should provide the difference
Figure 5. (a) UV/Vis/NIR absorption spectra of the porphyrin-sensitized TiO2
electrodes with ZnPNTz (red) and XW17 (blue). Light-scattering TiO2 layers
were not used to obtain an accurate absorption profile. TiO2 electrodes were
obtained by immersing the electrodes in porphyrin (0.2 mM) solutions of
ethanol/toluene (1:1 v/v, for ZnPNTz, 4:1, v/v for XW17) at 25 °C for 3 h in
the absence of CDCA co-adsorbent. (b) Photocurrent action spectra of the
DSSCs based on ZnPNTz (red) and XW17 (blue) under the best conditions
for achieving the highest η values.
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in the incident photon-to-current efficiency (IPCE) values. The is slightly larger than that based on WX17 (30.1 Ω), which con-
strong tendency of ZnPNTz and XW17 towards aggregation tradicts the difference in the VOC values. This suggests that CR
may reduce the φinj values because of self-quenching of the between the oxidized porphyrin and TiO2 rather than between
excited states on TiO2. Nevertheless, dye aggregation should be the oxidized redox shuttle and TiO2 governs the VOC values.[15]
suppressed by the addition of CDCA. As described above, the We have already reported that the geometry of the porphyrin
large Stokes shift of ZnPNTz suggests a large conformational on TiO2 controls the CR between the oxidized porphyrin and
change in the singlet excited state. Therefore, the re-organiza- TiO2.[16] Porphyrins with a longer bridge between the porphyrin
tion energy (λ) of ZnPNTz should be larger than that of XW17. core and the anchoring group tend to be tilted onto TiO2. Tak-
Given the larger λ value and smaller driving force for ZnPNTz, ing into account the fact that ZnPNTz possesses a slightly
the electron injection process should be slow.[14] Consequently, longer bridge than WX17, ZnPNTz on TiO2 is likely to be tilted
the lower value of φinj for ZnPNTz than for XW17 would be more heavily than WX17. In such a case, the CR in ZnPNTz
responsible for the low IPCE values. Note that ZnPNTz consists would be faster than that in WX17, thereby lowering the ηcol
of several parts that independently can cause aggregation, such value of ZnPNTz. Therefore, the inferior photovoltaic perform-
as the NTz unit, porphyrin core, and triarylamine group. It is ance of the DSSC based on ZnPNTz results from the low φinj
unlikely that whole molecules will aggregate closely at the and ηcol values.
same time; even if the NTz units aggregate tightly, the other
parts, that is, the porphyrin cores and triarylamine groups,
would not aggregate strongly, making it possible to induce
large conformational change.
DFT Calculations
To obtain an insight into the differences in the photovoltaic We carried out DFT calculations on the model porphyrins at the
performances, we employed electrical impedance spectroscopy B3LYP/6-31G(d) level of theory (Figure 6). The 2,6-bis(alk-
(EIS). EIS Nyquist plots were obtained under standard AM 1.5 oxy)phenyl groups at the meso positions are almost perpendic-
illumination and open-circuit conditions (see Figure S12 in the ular to the porphyrin plane. On the other hand, the triarylamine
Supporting Information). The ET process at the TiO2/dye/elec- and NTz moieties adopt a coplanar conformation. Although the
trolyte interface is represented by Rp, which corresponds to a alkoxy chains on the meso-aryl groups are known to wrap the
charge recombination (CR) resistance between the TiO2 and porphyrin core,[11c,17] the highly coplanar structure could be re-
electrolyte. The Rp value of the DSSC based on ZnPNTz (31.8 Ω) sponsible for the strong aggregation tendency. The calculated
Figure 6. Energy diagrams and selected Kohn–Sham orbitals of model D–π–A porphyrins on the optimized structures. To reduce the calculation time and
cost, alkoxy groups were replaced by methoxy groups.
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HOMO–LUMO gap of ZnPNTz (1.54 eV) is less than that of imply that a more rigid dye structure, electron-rich anchoring
XW17 (1.77 eV), which agrees with the trend of the optical and groups, and a suitable bridge between the dye and the anchor-
electrochemical HOMO–LUMO gaps. The orbital distributions of ing group are essential for achieving highly efficient DSSCs.
the HOMOs are delocalized on the electron-donating triaryl- Nevertheless, the NTz unit as a strong electron-withdrawing
amine moiety and porphyrin core due to effective π expansion group is still promising for exploring novel D–π–A small mol-
through the ethynyl linkage. In contrast, the orbital distribu- ecules that exhibit unique optical properties in the NIR region.
tions of the LUMOs are considerably localized on the electron-
accepting NTz of ZnPNTz and BT of XW17. Importantly, the
lower orbital distribution of the LUMO on the carboxyphenyl
moiety of ZnPNTz compared with XW17 rationalizes its low φinj
value, because the orbital distributions of LUMOs on anchoring
and reagents were used without further purification unless other-
groups are known to affect the electron coupling between por-
phyrin dyes and TiO2.[18]
We also performed time-dependent DFT (TD-DFT) calcula-
tions to evaluate the UV/Vis/NIR absorption properties (see
Table S2 in the Supporting Information). The TD-DFT calcula-
tions allow the excitation states of both porphyrins to be as-
signed to the respective Soret and Q-bands. The lowest-energy
excitation consists mainly of a HOMO→LUMO transition, which
corresponds to the Q-band. The excitation energy of ZnPNTz
(1.36 eV) is less than that of XW17 (1.50 eV), which agrees with
the redshift of the Q-band. It is noteworthy that the HOMO–
Experimental Section
Instrumentation and Materials: Commercially available solvents
wise mentioned. Silica gel column chromatography was performed
with UltraPure Silica Gel (230–400 mesh, SiliCycle) unless otherwise
noted. TLC was performed with Silica gel 60 F254 (Merck). UV/Vis/
NIR absorption spectra of solutions and films were recorded with
a Perkin–Elmer Lambda 900 UV/Vis/NIR spectrometer. Steady-state
fluorescence spectra were recorded with a HORIBA Nanolog spec-
trometer. 1H and 13C NMR spectra were recorded with a JEOL EX-
400 spectrometer (operating at 400 MHz for 1H and 100 MHz for
13C) using the residual solvent as internal reference for 1H (CDCl3:
δ = 7.26 ppm) and 13C (CDCl3: δ = 77.16 ppm). HRMS were recorded
with a Thermo Fischer Scientific EXACTIVE spectrometer for ESI and
APCI measurements. Electrical impedance spectra were recorded
1→LUMO transition is allowed for ZnPNTz, whereas it is forbid- with an SP-150 (Bio-Logic) spectrometer.
den for XW17. This HOMO–1→LUMO transition leads to
Synthesis: 5,10-Dibromonaphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole
(2),[6a] methyl 4-tributylstannylbenzoate (3),[19] 4-ethynyl-2,5-
bis(hexyloxy)-N,N-bis[4-(hexyloxy)phenyl]aniline (8),[11d] porphyrin
9,[11d] and XW17[11d] were prepared according to literature proce-
dures.
broader Q-bands for ZnPNTz than for XW17. In addition, the
HOMO→LUMO+1 transition can be characterized as a Soret
band. Because the HOMO and LUMO+1 are mainly localized on
the porphyrin core and triarylamine moiety, the NTz unit has
little impact on the Soret band, which results in rather similar
Soret bands for ZnPNTz and XW17. Therefore, these results
support the efficient NIR light absorption of ZnPNTz arising
from the enhanced D–π–A character of the NTz unit.
5-Bromo-10-(4-methoxycarbonylphenyl)naphtho[1,2-c:5,6-c′]-
bis[1,2,5]thiadiazole (4): Naphthobisthiadiazole 2 (50 mg,
0.12 mmol) and benzoate 3 (26 mg, 0.062 mmol, 0.5 equiv.) were
added to dry DMF (10 mL) in a flask and the reaction mixture was
degassed with argon for 15 min. Then, PdCl2 (2.2 mg, 0.012 mmol,
10 mol-%), tBu3P·HBF4 (3.6 mg, 0.012 mmol, 10 mol-%), CuI (1.2 mg,
0.0062 mmol, 5 mol-%), and CsF (47 mg, 0.31 mmol, 2.6 equiv.)
were added to the mixture. The reaction mixture was heated at
45 °C with stirring for 17 h. Then, the reaction mixture was washed
with water, extracted with hexane/AcOEt (4:1), dried with anhy-
drous Na2SO4, and the solvent was removed. The residue was puri-
fied by column chromatography on silica gel using CHCl3 and hex-
ane as the eluent to give 4 as a yellow solid (5.4 mg, 0.012 mmol,
9.8 %). M.p. 257–258 °C. 1H NMR (CDCl3, 400 MHz): δ = 9.22 (s, 1 H),
9.02 (s, 1 H), 8.26 (d, J = 8.4 Hz, 2 H), 8.20 (d, J = 8.0 Hz, 2 H), 3.99
Conclusions
We have synthesized for the first time the D–π–A-type por-
phyrin sensitizer ZnPNTz bearing the NTz unit as a new type
of NTz-based D–π–A molecule. The porphyrin ZnPNTz shows
superior absorption properties in the NIR region in comparison
with the reference porphyrin XW17 containing the BT unit due
to its enhanced D–π–A character, which can be rationalized by
the stronger electron-accepting nature of NTz compared with
the BT unit. The extremely large Stokes shift for ZnPNTz sug-
gests a large conformational change in the singlet excited state
of ZnPNTz arising from the NTz unit. Notably, the DSSC based
on ZnPNTz exhibits photocurrent generation at wavelengths
(s, 3 H) ppm. FTIR (ATR): ν = 3852, 3751, 3651, 2540, 2184, 2059,
˜
1967, 1723, 1280, 953, 764 cm–1. MS (ESI): calcd. for C18H10BrN4O2S2
[M + H]+ 456.9423; found 456.9414. Owing to low solubility, we
could not obtain the 13C NMR spectrum with a sufficient signal-to-
noise (S/N) ratio.
up to 860 nm, although it shows a lower η value (3.6 %) than Porphyrin 10: Porphyrin 9 (56 mg, 0.028 mmol) was added to dry
THF (1.9 mL) and NEt3 (0.38 mL) in a Schlenk tube and the solution
was degassed with argon for 30 min. Then, triisopropylsilylacetylene
(0.018 mL, 0.083 mmol, 3 equiv.), [Pd(PPh3)2Cl2] (3.9 mg,
0.0055 mmol, 20 mol-%), and CuI (1.8 mg, 0.0092 mmol, 33 mol-%)
were added to the solution and the reaction mixture was heated at
70 °C for 18 h under argon. The mixture was then cooled to ambient
temperature, washed with water, extracted with CH2Cl2, and dried
with anhydrous Na2SO4. The solvent was removed and the crude
product was purified by column chromatography on silica gel using
CH2Cl2/hexane as eluent to give porphyrin 10 as a green solid
the DSSC based on XW17 (8.1 %). The inferior photovoltaic per-
formance is mainly attributed to its low φinj and ηcol values
for the following reasons. 1) The enhanced D–π–A character
bestowed by the NTz moiety induces a large re-organization
energy and resulting slow ET process. 2) The lower orbital distri-
bution on the carboxy moiety of the LUMO of ZnPNTz com-
pared with XW17 provides insufficient electron coupling be-
tween the porphyrin dye and TiO2 for efficient electron injec-
tion. 3) The more tilted geometry of ZnPNTz on TiO2 acceler-
ates CR between the oxidized porphyrin and TiO2. These results (53 mg, 0.025 mmol, 90 %). M.p. >300 °C. 1H NMR (CDCl3, 400 MHz):
Eur. J. Org. Chem. 0000, 0–0
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8
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δ = 9.81 (d, J = 4.8 Hz, 2 H), 9.63 (d, J = 4.4 Hz, 2 H), 8.84 (d, J =
4.4 Hz, 2 H), 8.82 (d, J = 4.8 Hz, 2 H), 7.67 (t, J = 7.6 Hz, 2 H), 7.46
(s, 1 H), 7.00 (m, 8 H), 6.81 (d, J = 7.2 Hz, 4 H), 6.76 (s, 1 H), 4.07 (t,
J = 6.4 Hz, 2 H), 3.95 (t, J = 6.6 Hz, 4 H), 3.90 (t, J = 6.2 Hz, 2 H),
3.82 (t, J = 6.2 Hz, 8 H), 2.11–2.07 (m, 2 H), 1.82–1.64 (m, 8 H), 1.50–
477 cm–1. UV/Vis (THF): λ (ε) = 343 (4300), 456 (150000), 706
(81000
M =
–1 cm–1) nm. Fluorescence (THF, λex = 468 nm): λmax
823 nm. MS (APCI): calcd. for C143H184N9O10S2Zn [M + H]+
2315.2957; found 2315.2894. M.p. >300 °C. Owing to low solubility,
we could not obtain the 13C NMR spectrum with a sufficient S/N
1.31 (m, 39 H), 1.26–0.99 (m, 34 H), 0.97–0.74 (m, 46 H), 0.67–0.38 ratio.
(m, 28 H) ppm. 13C NMR (CHCl3, 100 MHz): δ = 160.1, 155.5, 154.7,
Electrochemical Measurements: Electrochemical measurements
were made by using an ALS 630a electrochemical analyzer. Redox
potentials were determined by differential pulse voltammetry (DPV)
152.4, 151.7, 150.6, 148.0, 132.1, 131.6, 131.2, 130.8, 129.9, 124.0,
121.2, 119.3, 115.3, 115.0, 111.9, 110.1, 109.4, 105.5, 102.1, 100.1,
97.1, 96.6, 93.4, 69.8, 69.6, 68.9, 68.5, 32.1, 32.0, 31.9, 31.8, 29.90,
29.86, 29.6, 29.5, 29.40, 29.36, 29.29, 29.1, 28.80, 28.75, 26.3, 26.0,
in THF containing 0.1
M tetrabutylammonium hexafluorophosphate
(Bu4NPF6). A glassy carbon (3 mm diameter) working electrode,
Ag/AgNO3 reference electrode, and Pt wire counter electrode were
employed. Ferrocene (+0.642 V vs. NHE) was used as the internal
standard for the DPV measurements.
25.7, 25.4, 22.8, 22.7, 19.3, 14.2, 12.1 ppm. FTIR (ATR): ν = 3854,
˜
3710, 3651, 2922, 2854, 2554, 2335, 2208, 2162, 1968, 1591, 1505,
1456, 1379, 1238, 1211, 1101, 1000, 713 cm–1. MS (APCI): calcd. for
C135H198N5O8SiZn [M + H]+ 2109.4296; found 2109.4310.
DFT Calculations: All DFT calculations were carried out by using
the Gaussian 09 program with the restricted B3LYP functional
(Becke's three-parameter hybrid exchange functionals and the Lee–
Yang–Parr correlation functional) employing the 6-31G(d) basis set
for C, H, N, O, S, and Zn.[20] All the porphyrin structures were fully
optimized without any symmetry restriction.
Porphyrin 12: A 1
M solution of TBAF in THF (0.040 mL, 0.040 mmol,
5 equiv.) was added to a solution of porphyrin 10 (17 mg,
0.0081 mmol) in dry THF (1.8 mL). The solution was stirred at ambi-
ent temperature for 30 min under argon. The mixture was
quenched with water and then extracted with CH2Cl2. The organic
layer was dried with anhydrous Na2SO4 and the solvent was re-
moved. The residue and 4 were dissolved in a mixture of dry THF
(3.5 mL) and NEt3 (0.70 mL) and the solution was degassed with
argon for 30 min. Then, [Pd2(dba)3] (1.8 mg, 0.0020 mmol, 25 mol-
%) and AsPh3 (4.9 mg, 0.016 mmol, 2 equiv.) were added to the
mixture. The solution was heated at reflux for 7 h under argon. The
mixture was cooled to ambient temperature, washed with water,
extracted with CH2Cl2, and dried with anhydrous Na2SO4. The sol-
vent was removed under reduced pressure and the crude product
was purified by column chromatography on silica gel using CH2Cl2/
hexane as eluent to give 12 as a brown solid (9.0 mg, 0.0039 mmol,
48 %). M.p. >300 °C. 1H NMR (CDCl3, 400 MHz): δ = 10.05 (d, J =
4.8 Hz, 2 H), 9.81 (d, J = 4.8 Hz, 2 H), 9.49 (s, 1 H), 9.07 (s, 1 H), 8.96
(d, J = 4.4 Hz, 2 H), 8.83 (d, J = 4.4 Hz, 2 H), 8.12 (m, 4 H), 7.72 (t,
J = 8.8 Hz, 2 H), 7.46 (s, 1 H), 7.02 (m, 8 H), 6.81 (d, J = 9.2 Hz, 4 H),
6.77 (s, 1 H), 4.07 (t, J = 6.4 Hz, 2 H), 3.97 (s, 3 H), 3.91–3.96 (m, 6
H), 3.88 (t, J = 7.0 Hz, 8 H), 2.15–2.08 (m, 2 H), 1.83–1.66 (m, 8 H),
Preparation of the DSSCs: The TiO2 electrodes and sealed cells for
photovoltaic measurements were prepared according to the litera-
ture.[13,21] A nanocrystalline TiO2 layer of 12 μm (d = 30 nm,
PST30NR-D, Dyesol) was coated on a FTO glass plate by screen
printing. Then, a sub-microcrystalline TiO2 layer of 4 μm (d =
400 nm, CCIC:PST400C, JGC-C&C) was formed on the top of the FTO.
The TiO2 electrode was immersed in a 0.2 m
M porphyrin solution
in ethanol/toluene (1:1 v/v for ZnPNTz, 4:1 v/v for XW17) at 25 °C
with CDCA. A sandwich cell was prepared by using the dye-an-
chored TiO2 electrode as the working electrode and a Pt counter
electrode, which were assembled with a hotmelt-ionomer electrode
Surlyn polymer gasket (DuPont, 50 μm). The electrolyte solution
was composed of 1.0
0.05 LiI, 0.01 guanidinium thiocyanate, and 0.5
pyridine in acetonitrile/valeronitrile (85:15).
M
1,3-dimethylimidazolium iodide, 0.03
M I2,
M
M
M
4-tert-butyl-
Photovoltaic Measurements: Incident photon-to-current efficien-
cies (IPCE) and photocurrent–voltage (I–V) performance were meas-
ured by using an action spectrum measurement setup (CEP-2000RR,
BUNKOUKEIKI) and a solar simulator (PEC-L10, Peccell Technologies)
with simulated sunlight of AM 1.5 (100 mW cm–2), respectively: IPCE
[%] = 100 × 1240i/(Winλ), in which i [A cm–2] is the photocurrent
density, Win [W cm–2] is the incident light intensity, and λ [nm] is
the excitation wavelength. During the photovoltaic measurements,
black plastic tape was attached to the back of the TiO2 electrode
except for the TiO2 film region to avoid light scattering. The convo-
lution of the spectral response in the photocurrent action spectrum
with the photon flux of the AM 1.5G spectrum provided the esti-
mated JSC value, which is in good agreement with the JSC value
obtained from the I–V performance.
1.50–0.74 (m, 98 H), 0.67–0.43 (m, 28 H) ppm. FTIR (ATR): ν = 3724,
˜
3616, 2927, 2852, 2357, 2180, 2150, 2046, 2014, 1587, 1504, 1465,
1238, 1103, 999, 767, 630, 552, 527, 429 cm–1. MS (APCI): calcd. for
C
144H186N9O10S2Zn [M + H]+ 2329.3050; found 2329.3125. Owing to
low solubility, we could not obtain the 13C NMR spectrum with a
sufficient S/N ratio.
D–π–A-Type Porphyrin ZnPNTz: Porphyrin 12 (7.5 mg,
0.0032 mmol) was dissolved in dry THF (1.9 mL) and MeOH (1.9 mL)
and then a solution of NaOH (20 % w/w in water, 0.38 mL,
1.9 mmol) was added. The solution was heated at 50 °C for 30 min
upon which TLC (silica, CH2Cl2) indicated complete hydrolysis of the
ester. A saturated NH4Cl aqueous solution was added to the reac-
tion mixture and the mixture was diluted with CH2Cl2, washed with
brine, dried with anhydrous Na2SO4, and the solvent was removed.
The crude product was purified by column chromatography on sil-
ica gel using CH2Cl2/MeOH as eluent. Recrystallization from CH2Cl2/
acetonitrile gave ZnPNTz as a brown solid (7.0 mg, 0.0030 mmol,
Acknowledgments
This work was supported by the JSPS [KAKENHI Grant Number
25220801 (H. I.)].
1
94 %). H NMR (CDCl3, 400 MHz): δ = 9.99 (d, J = 4.4 Hz, 2 H), 9.81
(d, J = 4.8 Hz, 2 H), 9.39 (s, 1 H), 8.96 (d, J = 4.4 Hz, 2 H), 8.94 (s, 1
H), 8.84 (d, J = 4.4 Hz, 2 H), 8.12 (m, 4 H), 8.03 (t, J = 8.0 Hz, 2 H),
7.80 (m, 2 H), 7.73 (t, J = 8.0 Hz, 2 H), 7.47 (s, 1 H), 7.07–7.00 (m, 8
H), 6.82 (d, J = 9.2 Hz, 4 H), 6.79 (s, 1 H), 4.11 (t, J = 6.4 Hz, 2 H),
3.98–3.89 (m, 14 H), 2.12–2.04 (m, 2 H), 1.83–1.76 (m, 4 H), 1.67–
1.24 (m, 18 H), 1.13–0.72 (m, 80 H), 0.68–0.52 (m, 28 H) ppm. FTIR
Keywords: Functional organic materials · Donor–acceptor
systems · Dye sensitized solar cells · Porphyrinoids · Fused-
ring systems · Light harvesting · Heterocycles
(ATR): ν = 3825, 3713, 3675, 2922, 2855, 2162, 2014, 1980, 1968,
1686, 1587, 1504, 1455, 1238, 1210, 1101, 999, 783, 726, 651,
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Received: December 13, 2017
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Full Paper
Porphyrins
We have designed and synthesized
for the first time a novel D–π–A-type
porphyrin sensitizer incorporating a
naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiaz-
ole (NTz) unit. Owing to the strong
electron-accepting ability of the NTz
unit, the porphyrin shows excellent ab-
sorption in the NIR region and the
DSSC fabricated from this sensitizer
exhibits photocurrent generation at
wavelengths up to 860 nm.
T. Higashino, Y. Kurumisawa, S. Nimura,
H. Iiyama, H. Imahori* ................... 1–12
Enhanced Donor–π–Acceptor Char-
acter of a Porphyrin Dye Incorporat-
ing Naphthobisthiadiazole for Effi-
cient Near-Infrared Light Absorption
Porphyrin Dyes Functionalized for Efficient NIR Absorption by Hiroshi Imahori @KyotoU_News #OutstandingOrganics
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DOI: 10.1002/ejoc.201701736
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