Chiral Aggregates of Triphenylamine-Based Dyes for Depleting the Production of Hydrogen Peroxide in the Photochemical Water-Splitting Process

DOI: 10.1002/hlca.201900065

FULL PAPER

Chiral Aggregates of Triphenylamine-Based Dyes for Depleting the

Production of Hydrogen Peroxide in the Photochemical

Water-Splitting Process

Beatrice Adelizzi,^{a, b}Andreas T. Rösch,^{a, b}Daan J. van Rijen,^{a, b}R. Simone Martire,^{a, b}

Serkan Esiner,^{a, b}Martin Lutz,^cAnja R. A. Palmans,^{a, b}and E. W. Meijer*^{a, b}

a

Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513,

NL-5600 MB Eindhoven, The Netherlands, e-mail: e.w.meijer@tue.nl

Institute for Complex Molecular Systems, Eindhoven University of Technology, The Netherlands

b

c

Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8,

NL-3584 CH Utrecht, The Netherlands

Dedicated to Prof. François Diederich for his outstanding contributions to science and for a lifelong friendship

Recent studies on water-splitting photoelectrochemical cells (PECs) have demonstrated the intriguing

possibility of controlling the spin state in this chemical reaction to form H₂and O₂by exploiting the chirality of

organic π-conjugated supramolecular polymers. Although this fascinating phenomenon has been disclosed, the

chiral supramolecular materials reported thus far are not optimized for acting as efficient photosensitizer for dye-

sensitized PECs. In this work we report on the design, synthesis, and characterization of chiral supramolecular

aggregates based on C₃-symmetric triphenylamine-based dyes that are able to both absorb visible light and

control the spin state of the process. Variable temperature-dependent spectroscopic measurements reveal the

assembly process of the dyes and confirm the formation of chiral aggregates, both in solution as well as on solid

supports. Photoelectrochemical measurements on TiO₂-based anodes validate the advantage of using chiral

supramolecular aggregates as photosensitizer displaying higher photocurrent compared to achiral analogues.

Moreover, fluorimetric tests for the quantification of the hydrogen peroxide produced, confirm the possibility of

controlling the spin of the reaction exerting spin-selection with chiral supramolecular polymers. These results

represent a further step towards the next-generation of organic-based water-splitting solar cells.

Keywords: dye-sensitized photoelectrochemical cells, water splitting, supramolecular chemistry, chirality, chiral

induced spin selectivity.

and technical issues.^[6]These devices closely resemble

the structures used in n- and p-type dye-sensitized

Introduction

The production of H₂and O₂through photo- or solar cells (DSSCs).^[7]The DS-PECs are based on a high

electro-chemical processes has attracted many re- band-gap n- and/or p-type semiconductor functional-

searches for the last thirty years.^[1–5]Among the ized with molecular sensitizers and are able to

several devices studied for this aim, dye-sensitized efficiently harvest the visible portion of the solar

photoelectrochemical cells (DS-PEC) offer an optimal spectrum. However, the systems used are often

compromise between efficiency, production-costs, affected by a significant overpotential and by the

formation of peroxides and superoxide radicals as

byproducts

(especially

for

TiO₂

based

photoanodes).^[8–11]These byproducts, along with

reducing the efficiency of the water-splitting process,

Supporting information for this article is available on the

WWW under https://doi.org/10.1002/hlca.201900065

Helv. Chim. Acta 2019, 102, e1900065

Figure 1. Molecular structure of triphenylamine-based D-π-A dyes 1 and 2. The reference cyano-acetic 1 is used to synthesize the

analogues bearing three cyano-acetamides with chiral (S)-3,7-dimethyloctyl chains, S-2, or three achiral n-octyl chains, a-2. An

analogue with a short chiral chain S₄-2 is synthesized for obtaining the crystal structure.

are known to poison and damage the surface, there-

by reducing the device’s lifetime.^[8]Recently a asymmetrically substituted conjugated molecules with

possible solution to this problem was proposed by

donor-π-bridge-acceptor structure (D-π-A).^[17–19]

Organic dyes for the visible spectrum are often

a

Ron Naaman and was demonstrated in collaboration However, bulkiness and asymmetry are qualities that

with our group.^[12,13]The functionalization of TiO₂may hamper the helical supramolecular organization.

anodes with chiral supramolecular polymers that are Therefore, coplanar aromatic systems combined with

able to act as spin filters – through an effect called C₃-symmetrical amide functionalities bearing chiral

chiral induced spin selectivity (CISS)^[14]– demon- chains are proposed to be ideal in achieving helical

strated the possibility of simultaneously reducing the aggregation of organic dyes. This design for chirally

production of H₂O₂and increasing the current.

ordered supramolecular polymers has been studied in

This proof-of-concept highlighted the intriguing detail and proved its importance in the formation of

potential of controlling the spin of a reaction by controlled 1D aggregates.^[20,21]

exploiting the chirality of an organic film and created

Here, we report on the design, the synthesis, and

a promising route to next-generation water-splitting the supramolecular characterization of new chiral

photoelectrochemical cells. The prototype cells used, supramolecular dyes and their application in DS-PECs.

however, were not optimized, especially with respect To merge the two prerequisites discussed above, we

to the TiO₂that was functionalized with chiral designed a C₃-symmetrical D-π-A dye which bears

supramolecular polymers based on porphyrins or three carboxamides functionalized with chiral chains

tripyridylamines. These anodes were neither designed (Figure 1).

to efficiently absorb visible light nor to adequately

We selected a branched triphenylamine dye, similar

inject electrons into the TiO₂.^[15,16]These issues to many dyes reported,^[4][19][22]and directly related to

evidenced the necessity of combining design rules the cyanoacetic acid analogue, 1, which has been

required

for

obtaining

helically

aggregated previously reported by Shang et al.^[23]The small library

supramolecular polymers with those for obtaining of the cyanoacetamide derivatives, 2, displays a

organic dyes that absorb visible light efficiently.

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triphenylamine core as donor unit, thiophene as π-

Helv. Chim. Acta 2019, 102, e1900065

°

Scheme 1. Synthetic route from 1 to 2. i) pentafluorophenyl trifluoroacetate, Et₃N, DMF, 0 C to 20 C, 4 h; ii) related primary amine,

°

Et₃N, DMF, 20 C, 18 h.

bridge, and a cyanoacetamide group which acts both through activation of 1 with pentafluorophenyl tri-

as acceptor and as supramolecular unit. S-2 comprises fluoroacetate and consecutive substitution with the

chiral (S)-3,7-dimethyloctyl side chains for relaying a appropriate amine. SiO₂-gel chromatography and

bias in the helical sense of the assembly, while the consecutive preparative thin layer chromatography

achiral analogue, a-2, has n-octyl chains. In addition, (TLC) yielded the desired product in high purity. In

an analogue molecule with short chiral chains, S₄-2, is contrast to 1, which is a dark-red solid, S-2, a-2, and

designed to obtain a crystal structure.

S₄-2 are bright red-orange powders.

The current study is ranging from molecular syn-

The molecules synthesized were characterized both

thesis to device operation. It is inspired by the in solution (¹H- and ¹³C-NMR spectroscopy, mass

beautiful work of François Diederich, where he excels spectrometry) and in bulk (IR spectroscopy, DSC, and

in combining original organic synthesis and compre- polarized optical microscopy) to confirm the structures

hensive physical-organic characterization with fascinat- assigned (Supporting Information, Figures S1 and S2).

ing applications. This study gives insight into how Subsequently, UV/Vis spectroscopic measurements in

complex supramolecular monomers assemble and the molecularly dissolved state (DMSO, c=20 μM;

confirms the advantage of using a chiral light-harvest- Figure 2,a) were performed to evaluate the absorption

ing layer in water-splitting devices.

range of S-2 and a-2.

Both the molecules display analogous spectra and

molar extinction coefficients (ɛ) in line with ester

analogues published previously.^[24–28]S-2 and a-2

display a broad absorption between 300 and 600 nm

with a maximum absorption at λ=474 nm. This, in

Results and Discussion

Synthesis and Molecular Characterization

The target molecules S-2, a-2, and S₄-2 were synthe- combination with ɛ of 6.9 10⁴cm^{À 1}^{À 1}, demonstrates

M

sized through amide coupling from 1 (Scheme 1). The their suitability for harvesting visible light.

synthesis of 1 was performed similarly to the proce-

dure reported in literature (Experimental Section,

Scheme 2).^[23]The amide coupling was then performed

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Scheme 2. Synthetic scheme followed for the synthesis of 2 derivatives.

Supramolecular Polymerization

We then evaluated the aggregation of S-2 and a-2 in

dilute solutions. In apolar methylcyclohexane (MCH),

the supramolecular monomers tend to aggregate

upon cooling. The aggregation processes can be

followed by evaluating the variation of the optical

features as a function of temperature. First, the

aggregation was evaluated through UV/Vis spectro-

°

Figure 2. Molar extinction coefficient, ɛ, at 20 C, for S-2 (red

lines) and a-2 (black lines) in a) molecularly dissolved state in

DMSO and b) in aggregated state in methylcyclohexane.

°

scopy on S-2 and a-2 in MCH at 20 C. Both S-2 and a-

2 display a reduced ɛ and a broader absorption

spectrum, including a red-shifted shoulder at λ=

515 nm, compared to measurements taken in DMSO

(Figure 2,b). The difference in ɛ for a-2 and S-2 is

probably due to an increased degree of aggregation

of a-2 due to its lower solubility in MCH. To further

investigate the assembly and evaluate the ability of S-

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2 to form chiral aggregates, variable temperature UV/ which does not display preferred helical organization,

Vis and circular dichroism (CD) measurements were while the second shows preferred handedness. In

°

performed by cooling S-2 from 80 C to 0 C in MCH addition, the shapes of both the cooling curves display

°

and acquiring spectra every 5 C. Upon cooling, UV/Vis a gradual increment (in absolute values) of the UV and

measurements display a decrease in intensity of the CD intensities. Although the cooling curves in CD

main absorption band at λ=448 nm and the rise of a spectroscopy indicate a slightly cooperative nature of

shoulder at λ=515 nm (Figure 3,a). CD Spectroscopy the polymerization, the curves can be fitted with an

reveals a bisignate Cotton effect with a negative isodesmic model. This isodesmic character is probably

extremum at λ=517 nm (Figure 3,b), verifying that the due to a π-π driven aggregation, with a donor–

chirality of periphery of S-2 biases the helicity of the acceptor contribution, rather than a cooperative

supramolecular polymers, resulting in the formation of supramolecular

growth

by

triple

hydrogen

chiral aggregates at room temperature. Analogous bonding.^[30–32]This hypothesis, is also supported by IR

measurements on a-2 display similar aggregation spectroscopy in bulk (Supporting Information, Figur-

processes but the absence of CD signals, which as es S1 and S2). All results are consistent with the fact

expected, indicates the formation of aggregates with that, under analogous conditions, S-2 assembles at a

no biased helicity (Supporting Information, Figure S3).

lower temperature than the well-known amide-based

By following the evolution of UV and CD intensities C₃-symmetric molecules that polymerize cooperatively

as a function of temperature, it is possible to obtain through hydrogen bonding.^[16,21]

insights into the mechanism of aggregation of S-2.^[29]

Variable-temperature fluorescence measurements

Three cooling curves varying the concentration of S-2 (S-2, MCH, c=60 μM) confirmed the formation of

°

were recorded (λ=522, 80 C to 0 C, cooling rate: supramolecular polymers (Figure 3,c). At high temper-

À 1

°

15 Ch ; Figure 3,d and 3,e). Interestingly, the cooling ature, molecularly dissolved S-2 displays an emission

curves display slightly different starting temperatures band at λ=505 nm with vibrational fine structures.

in the UV and in the CD trace, which may be related to This band first increases in intensity while cooling

°

the formation of two different aggregates, the first of from 80 C to 40 C. Consecutively, the monomer

°

Figure 3. Variable temperature measurements of S-2 in MCH measured from 80 C (red lines) to 0 C (blue line), intermediate

spectra every 5 C (grey lines), cooling rate: 15 Ch^{À 1}. a) Absorbance spectra for c=25 μM; b) CD spectra for c=60 μM and_Àc₁)

°

emission spectra for c=60 μM. b) Related cooling curves for d) absorbance, e) CD cooling curves (λ=522 nm, cooling rate: 15 Ch

c=25 μM, 60 μM, and 80 μM), and f) emission cooling curves (c=60 μM).

,

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to the aggregate state which is dominated by an

intermolecular charge-transfer between donor and

acceptor moieties. The cooling curves obtained by

plotting the fluorescence maxima for the monomer

(Figure 3,f, red open dots) and for the supramolecular

aggregate (Figure 3,f, blue closed dots) display a

starting temperature for aggregation coinciding with

the one observed for the CD cooling curves (Figure 3,f

vs. Figure 3,d). This observation indicates that the

process which gives chirality to the supramolecular

aggregates is the same that modifies the emission

properties. The lack of an isosbestic point in the

emission spectra is in agreement with the UV- and CD-

data, indicating a complex aggregation behavior,

instead of a simple nucleation-elongation mechanism.

To verify our hypothesis that donor-acceptor inter-

actions and π-stacking are the driving interactions for

the aggregation of S-2, we grew crystals of S₄-2

suitable for single crystal X-ray diffraction. Needle-like

crystals were afforded by vapor diffusion of diethyl

ether into a concentrated CHCl₃solution of S₄-2. The

X-ray analyses confirmed the molecular structure of

the monomer (Figure 4,a) which crystallizes in the non-

centrosymmetric space group C2 with two independ-

ent molecules in the asymmetric unit (Figure 4,b) and

approximately 8% of disordered solvent molecules in

the unit cell volume (CCDC-1897910 contains the

supplementary crystallographic data for this manu-

script). The molecules lack molecular symmetry in the

crystal structure (Figure 4,a and 4,b) and due to the

different conformations of the molecules, the non-

crystallographic twofold axis between the independ-

ent molecules is only approximate (Figure 4,b and 4,c).

Interestingly, five of the six independent amide groups

are connected by NÀ H···NC hydrogen bonds to form

an infinite, two-dimensional network in the b, c-plane

and no hydrogen bonds between amides are detected.

Packing plots (Figure 4,c and 4,d) display the 3D

arrangements of the molecules. The high conjugation

between the donating core and the cyanoacetamides

renders the three branches of the molecule particularly

planar, while the molecule itself shows a propeller

geometry; well-known for triphenylamines.^[33–35]This,

in combination with the competing À CN groups,

hampers the classic hydrogen bonding network of C₃

supramolecular polymers in the crystalline state.

Figure 5. a) Absorbance and b) CD spectra of S-2 (red lines)

and a-2 (black lines) on FTO/TiO₂substrates. TiO₂thickness

5 μm, S-2 and a-2 spincoated from CHCl₃, c=10 mM, 1000 rpm,

°

annealing 100 C 30 min.

Figure 4. X-ray crystal structure of S₄-2. a) Molecular structure

of one of the independent molecules in b) the asymmetric unit

formed by two monomers. c) Crystalline packing in a unit cell

projected along the b axis. Color coded by symmetry operation

(blue, C₂axis, light blue screw axis). d) Observed 1D arrange-

ment in the crystal structure, seen along axis b. a), b), and d)

Crystal structures color coded based on the atoms, C are grey,

N blue, O red, S yellow. H not displayed.

We then tested the formation of chiral assemblies

upon deposition of S-2 both on glass (Supporting

Information, Figure S4,a) and on TiO₂surfaces (Figure 5

emission band rapidly decreases in intensity while a and Supporting Information, Figure S4,b). TiO₂Sub-

broad band at λ=607 nm grows reaching its max- strates functionalized with S-2 or a-2 (spin-coated

°

imum at 0 C. The red-shifted band can be attributed from CHCl₃, c=10 mM and annealing 100 C, 30 min)

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exhibit absorption spectra attributed to the aggre-

gated state (Figure 5,a and Supporting Information,

Figure S4, top). CD Spectroscopy on these surfaces

confirms the presence of chiral aggregates for S-2,

while a-2 again does not form aggregates with any

preferred chirality (Figure 5,b and Supporting Informa-

tion, Figure S4, bottom). Remarkably, the structure of

the chiral assemblies on TiO₂matches well with the

aggregates in solution as evidenced by CD spectro-

scopy (Figure 5,b and Supporting Information, Fig-

ure S4,b). These films were used in our devices.

Device Testing

Anatase TiO₂photoanodes were fabricated following

Grätzel’s procedure for DSSCs.^[36]The electrodes were

prepared on glass/FTO (fluorine doped tin oxide)

substrates following the procedure reported for Solar-

onix T/SP paste® (commercially available TiO₂paste,

anatase 20 nm nanoparticles).^[36]The as-prepared

anodes were then characterized through X-Ray pow-

Figure 6. a) Schematic representation of the set up used (left)

and of the TiO₂anode functionalized with chiral supramolecular

der diffraction (XRD), ellipsometry and scanning elec- polymers (right). b) Current vs. time measurement of the

devices under illumination and V=0 vs. Ag/AgCl. Measure-

tron microscopy (SEM; Supporting Information, Fig-

ments reported as mean � sd (lines � solid background) (n=

ure S5). To ensure the quality of the individual

substrates, I–V curves were recorded for each photo-

anode, and a set of anodes with analogous electro-

chemical behavior was chosen for further experiments

4).

(Supporting Information, Figure S6). TiO₂Substrates rent density, J, can be correlated with the number of

were used bare or functionalized with S-2 or a-2, water-splitting events and hence with the production

giving the respective electrodes S-2/TiO₂and a-2/ of H₂at the cathode and O₂(and hydrogen peroxides)

TiO₂, (molecules deposited through spin-coating from at the anode.^[12]In line with studies reported

CHCl₃c=10 mM and consequent annealing for 30 min previously,^[12,13]these measurements confirm higher

°

at 100 C).

current for the chiral device compared to achiral

To evaluate the H₂O₂production, after 20 minutes

Photoelectrochemical measurements were per- analogues and bare TiO₂.

formed in a three-electrode cell, with Ag/AgCl (3 M

KCl) as reference electrode and a Pt-plate as counter of constant irradiation and a potential kept at 0 V vs.

electrode (Figure 6,a, left, Supporting Information, Fig- Ag/AgCl, an aliquot was taken from the electrolyte

ure S7). A 0.1 ^MNa₂SO₄(pH=6.56) aqueous solution solution and analyzed. A fluorimetric test based on

was used as the electrolyte (63 mL). Bare TiO₂devices horseradish peroxidase^[37,38]was used to quantify the

were tested both under dark conditions and under concentration of produced H₂O₂, based on a calibra-

illumination (halogen lamp, 12 V, 50 W) to corroborate tion curve (Supporting Information, Figure S9). The

that in the dark no current is measured at 0 V vs. Ag/ results show that S-2/TiO₂devices exhibit an average

AgCl (Supporting Information, Figure S8). Next, func- production of H₂O₂that is 37% lower than that of a-2/

tionalized and bare devices (S-2/TiO₂, a-2/TiO₂and TiO₂(Table 1). Remarkably, this happens although S-2/

bare TiO₂) were tested by chronoamperometry meas- TiO₂displays higher current than a-2/TiO₂suggesting

uring the magnitude of the current as a function of that the depletion of H₂O₂byproducts correlates with

time (potential kept at 0 V vs. Ag/AgCl; Figure 6,a). For a higher device efficiency. Lower [H₂O₂] were also

all samples, the experiments were performed four detected for bare TiO₂. However, this is due to both

times to evaluate the reproducibility of the system. the lower light absorption (due to the lack of photo-

The related results are reported as means � the sensitizer) and the lower photocurrent. In order to

associated standard deviation (Figure 6,b, lines and appropriately compare the results of H₂O₂production,

corresponding areas respectively). The measured cur- we normalized the detected [H₂O₂], with the current

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flowing through the device. To do so, we assumed the increased photocurrent for S-2/TiO₂over a-2/TiO₂

current measured at the 1000^thsec (0 V vs. Ag/AgCl) as demonstrates the benefit of exploiting chiral dyes for

representative of the current in the steady state. We photocatalytic applications and stresses the impor-

divided the [H₂O₂] by this value (Supporting Informa- tance of controlling electrons’ spins in chemical

tion, Tables S1 and S2). This normalization further reactions involving multiple electrons. The encourag-

highlights the effect of the homo-chirality. The ratio of ing results obtained in this study are, nevertheless, still

[H₂O₂] to current density, [H₂O₂]·J^{À 1}, for chiral devices far from a real exploitation in photoelectrochemical

is (9.1�3.7)μM H₂O₂cm²mA^{À 1}which is nearly half of cells. However, we are confident that further optimiza-

the value obtained for achiral devices. This result tion of the system will result in an effective use of the

quantifies the effective gain of using chiral aggregates CISS effect in this important application.

as photosensitizers and highlights the impact of the

CISS effect on photoelectrochemical reactions.

Experimental Section

Materials and Methods

Conclusions

All reagents were purchased from Aldrich and used as

After the first reports on using the CISS effect in the received, unless otherwise specified. Copper powder

process of water-splitting, we add a new set of was acquired from Aldrich (<425 μm, 99.5% trace

experiments to further strengthen the proposal made metals basis). All solvents were purchased from

by Ron Naaman. Here, we report on the synthesis of Biosolve and dry solvents were obtained using MBraun

chiral and achiral supramolecular dyes designed to solvent purification system (MB SPS-800). Deuterated

both supramolecularly polymerize in a helical fashion compounds were obtained from Cambridge Isotopes

and to harvest visible light. Absorption studies demon- Laboratories. Reactions were followed by thin-layer

strate the broad absorption range and suitability as chromatography (TLC) using 60-F254 silica gel plates

visible sensitizers for both chiral and achiral analogues. from Merck.

The temperature-dependent spectroscopic measure-

ments in apolar solutions verify the formation of chiral

Automated column chromatography was per-

supramolecular aggregates through a proposed iso- formed on a Grace Reveleris X2 using Reveleris Silica

1

desmic mechanism. Fluorescence measurement and X- Flash Cartridges. H-NMR and ¹³C-NMR measurements

ray analysis indicate that the aggregation occurs were conducted on a Varian Mercury 200 MHz and/or a

mainly through donor-acceptor interactions.

Varian Gemini 400 MHz (100 MHz for ¹³C). Proton

Chronoamperometry measurements under light chemical shifts are reported in ppm downfield from

irradiation corroborate the higher photocurrent for tetramethylsilane (TMS). Carbon chemical shifts are

devices with chiral dyes compared to the achiral reported using the resonance of CDCl₃as internal

equivalent. These results highlight the validity of the standard. MALDI-TOF-MS were acquired using a

CISS effect in improving the cell’s performances. The PerSerptive Biosystem Voyager-DE PRO spectrometer

combination of the reduced H₂O₂production with the using α-cyano-4-hydroxycinnamic acid (CHCA) and 2-

[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]

malononitrile (DCTB) as matrices. IR Spectra were

recorded on a Perkin–Elmer spectrum two FT-IR

spectrometer. Variable temperature IR spectra were

recorded on a Bruker Tensor 27 GladiATR with temper-

ature controller. Polarization optical microscopy (POM)

measurements were done using a Jenaval polarization

microscope equipped with a Linkam THMS 600 heating

device, with crossed polarizers. The thermal transitions

were determined with DSC by using a DSC Q2000 TA

under a nitrogen atmosphere with heating and cool-

ing rates of 10 Kmin^{À 1}.

Table 1. Hydrogen peroxide values for tested devices after

1200 sec of visible irradiation and V=0 V vs. Ag/AgCl.

Device

Fluorescence [H₂O₂]^[b]

J^[c]

[H₂O₂]·J^{À 1[d]}

intensity^[a]

[μM]

[μAcm^{À 2}] [μM cm²mA^{À 1}

]

[À ]

S-2/TiO₂1191�336

a-2/TiO₂1897�871

Bare TiO₂440�150

0.39�0.12 45�7

0.62�0.29 35�5

0.14�0.05 6�2

9.1�3.7

17.5�6.7

24.2�10.4

[a]

Fluorescence intensity detected with the fluorimetric test

based on horseradish-peroxidase. ^[b]Concentration H₂O₂based

on calibration curve. ^[c]Current measured after 1000 sec, 0 V vs.

[d]

Ag/AgCl. Determined ratio of produced peroxide as a

For all spectroscopic measurements, cells with an

optical path length of 1 cm were employed, and

function of the photocurrent measured.

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Synthetic Procedures

spectroscopic-grade solvents were employed. Solu-

tions were prepared by weighing the necessary

amount of compound for the given concentration and The cyanoacetic acid derivative, 1, was synthesized

dissolved with a weighted amount of solvent based on adjusting a reported procedure^[23](Scheme 2), while

its density. The stock solutions were heated up, the cyanoacetamide derivatives (a-2, S-2, S₄-2) were

sonicated till complete dissolution and slowly cooled synthesized from 1 through activation with pentafluor-

down to room temperature every time before use ophenyl trifluoroacetate.

unless otherwise specified. All the spectroscopic

measurements were performed with freshly prepared

Tris(4-bromophenyl)amine (=4-Bromo-N,N-bis-

solutions (max. one week after the preparation of the (4-bromophenyl)aniline; 3). In a two-neck round

stock solution).

bottom flask, triphenylamine (2.5 g, 10.0 mmol) was

dissolved in anhydrous DMF (25 mL) under argon

°

UV/Vis and circular dichroism (CD) measurements atmosphere. The solution was cooled to 0 C by ice-

were performed on a Jasco J-815 spectropolarimeter, water bath and NBS (5.2 g, 32.0 mmol) in DMF (10 mL)

for which the sensitivity, time constants, and scan was added drop-wise under stirring. After the addition,

rates were chosen appropriately. Corresponding tem- the mixture was stirred for 4 h at room temperature.

perature-dependent measurements were performed The mixture was then poured into water (400 mL),

with a Jasco PFD-425S/15 Peltier-type temperature leading to the formation of a white precipitate. The

controller with a temperature range of 263–393 K and precipitate was filtered and dried under reduced

adjustable temperature slope. In all experiments the pressure. The crude product was recrystallized from

linear dichroism was also measured and in all cases no heptane to afford pure 3 as a white solid (3.8 g, 77%).

linear dichroism was observed. Separate UV/Vis spec- ¹H-NMR (400 MHz, CDCl₃): 7.36–7.34 (m, 6 H); 6.93–

tra were obtained from a Perkin–Elmer UV/Vis spec- 6.91 (m, 6H). APCI-MS: 482.9 (C₁₈H₁₃Br₃N⁺, [M+H]⁺;

trometer Lambda 40. Fluorescence spectra were meas- calc. 481.9).

ured with Jasco FMO-427S/15 fluorimeter implemented

in the CD spectrometer.

Tris[4-(2-thienyl)phenyl]amine (=4-(Thiophen-2-

yl)-N,N-bis[4-(thiophen-2-yl)phenyl]aniline; 4). In a

The layer thicknesses were determined on a Veeco three-neck round bottom flask, tris(4-bromophenyl)

Dektak150 profilometer after manually creating amine (3; 3 g, 6.2 mmol), thiophene-2-boronic acid

scratches with a sharp knife. Doctor blade coating of pinacol ester (4.7 g, 22.4 mmol) and tetrakis(triphenyl-

the TiO₂layers was performed using an Erichsen Model phosphine)palladium(0) (0.36 g, 0.31 mmol) were dis-

360 13 mm quadruple film applicator with gap heights solved in dioxane (100 mL) under argon atmosphere.

of 30, 60, 90, and 120 μm. SEM and EDX was A 1 ^MNa₂CO₃aqueous solution (30 mL) was intro-

performed on a FEI Nova600i SEM with an EDX detector duced under stirring. The mixture was stirred and

°

(136 eV resolution). Ellipsometry was performed using heated at 100 C for 18 h, then cooled to room

a Sentech 805 SE spectroscopic ellipsometer with a temperature and diluted with CHCl₃(100 mL). The

wavelength range 800–1700 nm. X-Ray Diffraction organic phase was then washed with water (150 mL),

(XRD) measurements were performed on a Rigaku 1 ^MHCl (150 mL), and brine (150 mL), dried over

Geigerex powder diffractometer with Bragg-Brentano anhydrous MgSO₄, filtered, and evaporated under

geometry, using copper radiation, wavelength vacuum. The crude product was purified through SiO₂

0.154 nm, at 40 kV and 30 mA. The samples were flash column chromatography (CH₂Cl₂/heptane 7:3 v/v)

1

°

prepared on a glass substrate and measured from 5

to afford 4 as a yellow solid (1.8 g, 57%). H-NMR

°

till 80 2θ with a step size of 0.02 and a dwell time of (400 MHz, CDCl₃): 7.52 (d, J=8.7, 2 H); 7.26–7.22 (m,

15 seconds. The photoelectrochemical cell and the 2 H); 7.14 (d, J=8.6, 2 H); 7.07 (dd, J=5.1, 3.6, 1 H).

related measurements are described in a following MALDI-MS: 491.13 (C₃₀H₂₂NS₃⁺, [M+H]⁺; calc. 491.08).

section.

Tris[4-(5-formyl-2-thienyl)phenyl]amine

Single crystal X-ray diffraction analysis described in (=5,5’,5’’-[Nitrilotri(4,1-phenylene)]tri(thiophene-2-

Supporting Information.

carbaldehyde); 5). Tris[4-(2-thienyl)phenyl]amine (4)

(800 mg, 1.63 mmol) was dissolved in 1,2-dichloro-

ethane (50 mL) in a two-neck round bottom flask

under argon. DMF (595 mg, 8.14 mmol) and POCl₃

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(1.25 g, 8.14 mmol) were added dropwise and the Et₃N (586 mg, 5.79 mmol, 1 ml) were dissolved in dry

solution was stirred and refluxed for 17 h. The mixture DMF (10 mL) under argon. The solution was cooled to

°

was then cooled to room temperature; CH₂Cl₂(80 mL) 0 C using an ice bath. In a second round bottom flask,

and a saturated aqueous solution of sodium acetate pentafluorophenyl

trifluoroacetate

(487 mg,

(160 mL) were added. The mixture was then stirred at 1.74 mmol) was dissolved in dry DMF (1.5 mL) under

room temperature for 2 h. The organic phase was argon atmosphere. This solution was added dropwise

°

washed with water (3×150 mL), dried over anhydrous to the mixture containing 1, stirred at 0 C for 1 h and

MgSO₄, filtered, and evaporated under vacuum, afford- then at room temperature for 4 h. Next, a solution of

ing compound 5 as a yellow solid (1.04 g, 99%). IR: the appropriate alkylamine (1.74 mmol) was diluted in

2793, 1651, 1589, 1528, 1504, 1436, 1385, 1320, 1293, dry DMF (1.5 mL) under argon and then added to the

1269, 1222, 1183, 1113, 1014, 960, 830, 800, 756, 736, mixture dropwise. The mixture was stirred for 40 h at

1

728, 697, 672, 648, 606, 594, 548, 499, 480, 547. H- room temperature. After completion, the mixture was

NMR (400 MHz, CDCl₃): 9.89 (s, 1 H); 7.74 (d, J=4.0, diluted with CHCl₃(150 mL), and the organic phase

1 H); 7.62 (d, J=8.8, 2 H); 7.36 (d, J=4.0, 1 H); 7.19 (d, was washed with 1 ^NHCl (2×200 mL) and water (3×

J=8.7, 2 H). MALDI-MS: 575.12 (C₃₃H₂₁NO₃S₃⁺, M⁺; 200 mL). The organic phase was dried over anhydrous

calc. 575.07).

MgSO₄, filtered, and evaporated under vacuum, afford-

ing a red oil. The crude product was precipitated from

the red oil by pouring it in diethyl ether (120 mL) and

3,3’,3’’-[Nitrilotris(4,1-phenylene-5,2-thiophene-

diyl)]tris[2-cyano-2-propenoic acid] (=3,3’,3’’-{Nitri- filtered, affording a red solid. The crude product was

lotris[(4,1-phenylene)thiene-5,2-diyl]}tris(2-cyano- first purified through SiO₂flash column chromatogra-

prop-2-enoic acid); 1). In a two-neck round bottom phy (CHCl₃/AcOEt 10:0.3 v/v) and then through

flask, tris[4-(5-formyl-2-thienyl)phenyl]amine (5; preparative TLC (CHCl₃/AcOEt 10:0.3 v/v). After recrys-

450 mg, 0.78 mmol), cyanoacetic acid (300 mg, tallization from acetonitrile, the product was obtained

3.51 mmol), and ammonium acetate (42 mg, pure as a red solid. The yield was estimated to be

0.55 mmol) were dissolved in glacial acetic acid about 15%.

(16 mL). The mixture was stirred and refluxed for 40 h

under argon atmosphere. After completion, the mix-

Data of a-2: IR: 3357, 3027, 2924, 2854, 2205, 1663,

ture was cooled to room temperature, and the crude 1578, 1521, 1496, 1435, 1354, 1324, 1276, 1225, 1184,

product was filtered and washed with CHCl₃(about 1113, 1065, 940, 831, 802, 734, 734, 689, 609, 545, 504.

50 mL). The crude product was purified by washing in ¹H-NMR (400 MHz, CDCl₃): 8.37 (s, 3 H); 7.68 (d, J=4.1,

CHCl₃under reflux conditions for 15 h, to afford 1 as a 3 H); 7.62 (d, J=8.7, 6 H); 7.35 (d, J=4.0, 3 H); 7.18 (d,

dark red solid (406 mg, 67%). IR: 3175, 2824, 2218, J=8.7, 6 H); 6.24 (t, J=5.8, 3 H); 3.41 (td, J=7.2, 5.8,

1738, 1690, 1565, 1493, 1416, 1321, 1258, 1185, 1063, 6 H); 1.64–1.57 (m, 6 H); 1.43–1.21 (m, 30 H); 0.94–

1

936, 799, 734, 683, 608, 582, 503. H-NMR (400 MHz, 0.84 (m, 9 H). ¹³C-NMR (100 MHz, CDCl₃): 160.5926;

(D₆)DMSO): 12.13 (s, 3 H); 8.49 (s, 3 H); 8.02 (d, J=4.1, 154.42; 152.54; 147.48; 147.45; 144.41; 138.42; 135.02;

3 H); 7.79 (d, J=8.5, 6 H); 7.73 (d, J=4.0, 3 H); 7.21 (d, 128.32; 127.68; 124.65; 123.74; 99.42; 99.15; 40.63;

J=8.6, 6 H). ¹³C-NMR (100 MHz, (D₆)DMSO): 164.13; 31.78; 29.71; 29.48; 29.23; 29.17; 26.87; 22.64; 14.09.

152.93; 147.58; 147.05; 142.11; 134.61; 128.24; 128.10; MALDI-MS: 1109.53 (C₆₆H₇₅N₇O₃S₃⁺, M⁺; calc. 1109.51).

À 1

°

125.18; 125.10; 117.03; 98.39. MALDI-MS: 776.09 DSC (ramp: 10 Kmin ) Tg: 61.18 C.

(C₄₂H₂₄N₄O₆S₃⁺, M⁺; calc. 776.09).

Data of S-2: IR: 3355, 3027, 2953, 2924, 2867, 2855,

General Synthesis for (2E,2’E,2’’E)-3,3’,3’’-(5,5’,5’’-(Ni- 2205, 1731, 1661, 1579, 1521, 1496, 1464, 1435, 1379,

trilotris(benzene-4,1-diyl))tris(thiophene-5,2-diyl))tris(2-

cyano-N-alkylamides), i.e., (2E,2’E,2’’E)-3,3’,3’’-{Nitrilo- 1065, 1017, 959, 939, 830, 802, 753, 734, 689, 666, 627,

tris[(4,1-phenylene)thiene-5,2-diyl]}tris(2-cyano-N-

609, 599, 544, 505, 465. ¹H-NMR (400 MHz, CDCl₃): 8.36

octylprop-2-enamide) (a-2), (2E,2’E,2’’E)-3,3’,3’’-{Ni- (s, 3 H); 7.68 (d, J=4.0, 3 H); 7.62 (d, J=8.7, 6 H); 7.35

1365, 1355, 1324, 1275, 1263, 1226, 1184, 1109, 1097,

trilotris[(4,1-phenylene)thiene-5,2-diyl]}tris{2-

(d, J=4.0, 3 H); 7.18 (d, J=8.7, 6 H); 6.19 (t, J=5.7, 3 H);

3.44 (dq, J=8.2, 6.1, 6 H); 1.76–1.07 (m, 30 H); 0.94 (d,

cyano-N-[(3S)-3,7-dimethyloctyl]prop-2-enamide}

(S-2),

and

(2E,2’E,2’’E)-3,3’,3’’-{Nitrilotris[(4,1- J=6.5, 9 H); 0.88 (d, J=0.7, 18 H). ¹³C-NMR (100 MHz,

phenylene)thiene-5,2-diyl]}tris{N-[(2S)-butan-2-yl]-

CDCl₃): 171.15; 160.53; 152.52; 147.43; 144.36; 138.40;

2-cyanoprop-2-enamide} (S₄-2). In a two-neck round 135.00; 128.30; 127.66; 124.63; 123.72; 117.50; 99.15;

bottom flask, compound 1 (300 mg, 0.39 mmol) and 60.40; 39.21; 38.76; 37.09; 36.52; 30.70; 27.96; 24.62;

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Helv. Chim. Acta 2019, 102, e1900065

[4] B. Cecconi, N. Manfredi, T. Montini, P. Fornasiero, A.

22.70; 22.60; 21.06; 19.50; 14.21. MALDI-MS: 1193.63

(C₇₂H₈₇N₇O₃S₃⁺, M]⁺; calc. 1193.60).

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Data of S₄-2: IR: 3348, 2963, 2929, 2873, 2207, 1663,

1572, 1518, 1493, 1435, 1360, 1323, 1282, 1266, 1242,

1227, 1219, 1184, 1157, 1127, 1115, 1090, 1060, 1014,

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621, 602, 544, 510, 498, 452, 407. H-NMR (400 MHz,

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The authors acknowledge Dr. Chidambar Kulkarni for

the help with the TiO₂substrates, Ingeborg Schreur for

SEM. Would like to thank Dr. Stefan Meskers, Prof. Dr.

René Janssen, and Prof. Dr. Ron Naaman and his group

for fruitful discussions. We acknowledge funding from

the Dutch Ministry of Education, Culture and Science

(Gravity program 024.001.035).

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Author Contribution

B. A. conceived the project and designed the experi-

ments. B. A., A. T. R., A. J. R., and R. S. M. performed the

experiments. B. A., A. T. R., and S. E. analyzed the data.

M. L. determined the crystal structure of S₄-2. B. A.,

A. T. R., A. R. A. P., and E. W. M. wrote the manuscript.

A. R. A. P. and E. W. M. supervised the research.

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Received February 24, 2019

Accepted March 22, 2019

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Article Doi

Chiral Aggregates of Triphenylamine-Based Dyes for Depleting the Production of Hydrogen Peroxide in the Photochemical Water-Splitting Process

DOI: 10.1002/hlca.201900065

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