Stimuli-Triggered reversible switching mechanism between H-and J-Type supramolecular assemblies of cationic porphyrins adsorbed on tungsten(VI) oxide surface

Journal of Porphyrins and Phthalocyanines

J. Porphyrins Phthalocyanines 2018; 22: 1–12

DOI: 10.1142/S1088424618500372

Published at http://www.worldscinet.com/jpp/

Stimuli-triggered reversible switching mechanism between

H- and J-type supramolecular assemblies of cationic

porphyrins adsorbed on tungsten(VI) oxide surface

a,b

a

c

Kenta Adachi *, Yukimasa Ura and Naoya Kanetada

a

Department of Chemistry, Graduate School of Sciences & Technology for Innovation, Yamaguchi University,

Yamaguchi, 753-8512, Japan

Opto-Energy Research Center, Yamaguchi University, Yamaguchi, 753-8511, Japan

b

c

Department of Environmental Science & Engineering, Graduate School of Science & Engineering,

Yamaguchi University, Yamaguchi, Japan

Received 9 January 2018

Accepted 28 February 2018

ABSTRACT: Supramolecular organic dye–inorganic semiconductor nanocrystal assemblies are

potentially useful in a broad range of technologies and applications, including photovoltaic systems, but

the molecular basis of the adsorption of dye molecules onto the semiconductor surfaces remains poorly

understood. Herein, we investigated the pH-dependent adsorption and conformational change of two

cationic porphyrin stereoisomers [5,10-diphenyl-15,20-di(N-methyl-4-pyridyl)porphyrin (cis-DMPyP)

and 5,15-diphenyl-10,20-di(N-methyl-4-pyridyl)porphyrin (trans-DMPyP)] on the tungsten(VI) oxide

(

WO ) colloid nanoparticle in aqueous media by means of UV-vis absorption spectroscopy. In accordance

3

with the combination of a modiﬁed Langmuir adsorption model and Kasha’s exciton coupling model,

the molecular orientation and stacking arrangement of DMPyP derivatives on the WO colloid surface

3

are discussed in detail. In the trans-DMPyP/WO aqueous system, trans-DMPyP molecules adopted

3

ﬂat-on orientation with respect to the WO colloid surface and eventually formed head-to-tail J-dimers

3

regardless of pH conditions. cis-DMPyP molecules in the acidic system also lay ﬂat-on and mainly

formed J-dimers on the WO colloid surface, whereas ones in the neutral system exhibited a dominant

3

edge-on orientation and had a higher tendency to form face-to-face H-dimers. Additionally, we have also

convincingly demonstrated the pH-triggered switchable p-stacking geometry of cis-DMPyP molecules

from H- to J-dimer and vice versa on the WO colloid surface. Such ﬁndings will undoubtedly provide a

3

pertinent guideline for the rational design of stimuli-responsive organic-inorganic materials.

KEYWORDS: porphyrin, tungsten oxide, adsorption, aggregation, organic/inorganic hybrid material,

stimuli-responsive.

organic materials conceived by dynamic supramolecular

INTRODUCTION

approaches [4, 5]. For instance, chlorophyll molecules,

which are structurally similar to porphyrin, aggregate with

the assistance of robust scaffold proteins into ring-shaped

and rod-like dye oligomers in purple and green bacteria,

respectively [6]. In these supramolecular organizations,

the porphyrin skeletons are assembled in a suitable

spatial arrangement and orientation, which enables

efﬁcient energy transfer between the donor and acceptor

chromophores upon photoexcitation [7]. This kind of

porphyrinoid aggregate is the so-called “light-harvesting

Nowadays, “aggregates,” composed of a large number

of small molecules, have garnered considerable attention.

That is because special traits which are not available in an

individualmoleculecanbebroughtoutonlyifmoleculesare

assembled with each other [1–3]. Organic dye aggregates

are ubiquitous in nature, and are of signiﬁcant interest for

*

correspondence to: Kenta Adachi, email k-adachi@yama-

guchi-u.ac.jp; tel. & fax: +81 83 933 5731.

2

K. ADACHI ET AL.

system” in photosynthetic organisms. For artiﬁcial photo-

synthetic systems, it is also an issue of paramount

importance to design the aggregate structure of porphyrins

in the dye molecule [27–29]. As a further step toward

the tailored functional design of the organic-inorganic

assemblies, it is worthwhile to investigate the adsorption/

aggregation process of porphyrin derivatives on the

inorganic semiconductor surface in response to external

stimuli.

[8]. The arrangement and orientation of organic dyes’

aggregates could be affected by many factors such as the

temperature, pH, ionic strength, concentration of the dyes,

and additives in the solution [9–12]. Recently, several

porphyrin-based supramolecular nanostructures have been

successfully assembled in a solution by using various

surfactants as a structure-directing template [13, 14].

Stimuli-responsive materials are gaining attention across

many different ﬁelds because of their potential applications

in the environmental, biomedical, chemical, industrial, and

pharmaceutical ﬁelds [15–17]. Among these materials, a

large number of organic dyes’ supramolecular assemblies

in response to external stimuli have been also achieved

and reported [18]. However, it remains an unbeatable

challenge to ﬁnd more and more effective ways to achieve

a fully external stimuli-controlled reversible aggregation of

organic dye molecules. Generally, dye aggregates through

the cooperative effect of weak intermolecular non-covalent

interactions such as hydrogen bond, p–p stacking, van der

Waals, and electrostatic attractive/repulsive interaction

exhibit ﬂexibility, which may be signiﬁcant for a

microscopic transformation of the molecular stacking [19,

In the present study, we have chosen the water-soluble

porphyrin derivative and the colloidal tungsten(VI)

oxide (WO ) aqueous solution systems. WO is an

3

intensively studied representative of a group of metal-

oxide semiconductors with a wide band gap (<3.0 eV),

and it is well-known for its unique characteristics

such as photocatalytic activity, photo- and electro-

chromic coatings, and microelectronic applications

[30, 31]. More recently, it has been demonstrated that

photochromic WO and related nanocolloid particles

3

exhibit colorimetric sensing properties for a-amino

acid compounds in aqueous solutions, which will have

numerous applications in continuous in vivo monitoring

[32, 33]. Herein, we will present a comparative study

of two cationic porphyrin derivatives [5,10-diphenyl-

15,20-di(N-methyl-4-pyridyl)porphyrin (cis-DMPyP)

and 5,15-diphenyl-10,20-di(N-methyl-4-pyridyl)por-

phyrin (trans-DMPyP), see Scheme 1] in the WO₃

colloid aqueous solution depending upon the pH

value by means of UV-vis absorption spectroscopy.

The adsorption and aggregation characteristics of

20]. If one can modulate the arrangement and orientation

of chromophores in the supramolecular nanostructures by

external stimuli, such dye-aggregates will show changes

in optical and electronic properties in response to the

corresponding stimuli.

Since the integration of organic and inorganic matter

is a promising way to exploit favorable properties of

both material classes and to achieve novel light-emitting,

sensing, photovoltaic, and lasing properties, active

research is now underway to better understand organic/

inorganic hybrid structures consisting of an organic dye

and an inorganic semiconducting or metallic nanocrystal

DMPyP derivatives on the WO colloid surface were

3

quantitatively investigated by detailed analysis based on

Langmuir adsorption and exciton coupling theory. The

differences in the arrangement of DMPyP derivatives

adsorbed on the surface depending upon the substituents

were also evaluated. The pH-responsive switchable

aggregation was achieved in the cis-DMPyP/WO₃

aqueous system.

[

21–23]. Organic dyes adsorbed on the

inorganic material surface can be easily

aggregated and have different orientations

of their chromophores with respect to the

surface (i.e. ﬂat-on or edge-on) [24–26]. Of

particular interest is the use of highly-ordered

dye aggregates as organic components of

hybrid conjugates, because dye aggregate

formation on the inorganic semiconductor

surfaces has been demonstrated to affect

strongly the electronic and optical properties

of organic chromophores in hybrid organic/

inorganic photovoltaic devices such as a dye-

sensitized solar cells (DSSC). In our previous

studies, the adsorption/aggregation behaviors

of cationic phenothiazine and xanthene dyes

on the inorganic semiconductor surface were

favored by controlling the pH and ionic

Scheme 1. Structure of meso-diphenyl-di(N-methyl-4-pyridyl) porphyrin

strength in the aqueous solution and by (DMPyP) derivatives used in this study, showing the protonation equilibria

substituent(s) introduction and modiﬁcation of the porphyrin skeleton forming monoprotonated and diprotonated forms

J. Porphyrins Phthalocyanines 2018; 22: 2–12

STIMULI-TRIGGERED REVERSIBLE SWITCHING MECHANISM BETWEEN H- AND J-TYPE SUPRAMOLECULAR ASSEMBLIES

3

iodide and the corresponding di-4-pyridyl-substituted

porphyrin derivatives (cis-DPyP and trans-DPyP,

EXPERIMENTAL

respectively) according to the reported method [28].

Brieﬂy, DPyP derivative (0.32 mmol) and excess methyl

Materials

iodide (0.05 mol) were successively added to dried

dimethylformamide (DMF, 100 mL) at room temperature.

After being reﬂuxed for ca. 1 h, the crude product was

recrystallized from a hot methanol/diethyl ether mixture

to give the required derivative as a purple powder.

Sodium tungstate(VI) dihydrate (Na WO ·2H O),

2

4

2

concentrated hydrochloric acid (conc. HCl) (Analytical

Reagent, Wako Pure Chemical, Osaka, Japan), and tubular

membranes with 3500 dialytic modulus (Cellu-Sep T1,

Membrane Filtration Products, Inc., TX, USA) were

cis-DPyP: Yield: 3.4%; R = 0.40 (4th fraction);

used to prepare tungsten(VI) oxide (WO ) colloids. The

f

3

1

H NMR (CDCl , 400 MHz), d, ppm: 9.04 (m, 4H,

water used for all sample preparation was ﬁrst distilled

and then passed through a Milli-Q system (Millipore,

USA), resulting in the speciﬁc resistivity of 18.2 MWcm.

All other chemicals were of reagent grade and purchased

from Nacalai Tesque Inc. (Kyoto, Japan) or Wako Pure

Chemical (Osaka, Japan), and were used as received

without further puriﬁcation.

3

2

,6-position pyridyl), 8.90 (d, 2H, b-pyrrol), 8.87 (s, 2H,

b-pyrrol), 8.84 (s, 2H, b-pyrrol), 8.80 (s, 2H, b-pyrrol),

.21 (m, 2H, 4-position phenyl), 8.16 (m, 4H, 3,5-position

pyridyl), 7.78 (m, 8H, 2,3,5,6-position phenyl), -2.84

8

(s, 2H, NH).

trans-DPyP: Yield: 1.7%; R = 0.68 (3rd fraction);

f

1

H NMR (CDCl , 400 MHz), d, ppm: 9.06 (m, 4H,

3

2

4

,6-position pyridyl), 8.90 (d, 4H, b-pyrrol), 8.80 (d,

H, b-pyrrol), 8.20 (m, 2H, 4-position phenyl), 8.16 (m,

H, 3,5-position pyridyl), 7.78 (m, 8H, 2,3,5,6-position

Preparation of size-controlled WO colloid solution

3

Colloidal WO is easily prepared by the acid-catalyzed

3

phenyl), -2.84 (s, 2H, NH).

cis-DMPyP: Yield: 83.4%; H NMR (400 MHz,

DMSO-d , d): 9.47 (m, 4H, 2,6-position pyridyl), 9.13

sol-gel synthesis method in aqueous media [34].

Na WO ·2H O (100 g, 0.3 mol) was dissolved in 100 mL

1

2

4

2

of water. Adding conc. HCl (7 mL, 0.7 M) dropwise into

the Na WO ·2H O aqueous solution, a cream white

6

(

s, 2H, b-pyrrol), 9.02 (m, 2H, b-pyrrol), 8.99 (m, 2H,

2

4

2

b-pyrrol), 8.91 (s, 2H, b-pyrrol), 8.90 (m, 4H, 3,5-position

pyridyl), 8.25 (m, 2H, 4-position phenyl), 7.91 (m, 8H,

deposit was formed. After efﬁcient stirring for ca. 1 h, a

colorless transparent aqueous WO colloid solution (pH

3

2

,3,5,6-position phenyl), 4.72 (s, 6H, -CH ), -2.84 (s, 2H,

3

.3, 0.023 M WO ) was eventually obtained, which was

3

-

+

NH); MALDI-TOF-MS m/z: 901 [cis-DMPyP + 2I ] + H ;

then closed in a semipermeable tubular membrane and

dialyzed in a 1000-mL beaker containing Milli-Q water

for a period of 8 h. The Milli-Q water was periodically

replaced until chloride ions could not be detected by

ion chromatography. The concentration of chloride ion

Anal. calcd. for C H N I : C, 58.68; H, 3.81; N, 9.33.

44 34 6 2

Found : C, 58.58; H, 3.84; N, 9.27.

1

trans-DMPyP: Yield: 77.8%; H NMR (400 MHz,

DMSO-d , d): 9.46 (m, 4H, 2,6-position pyridyl), 9.04

6

(

8

7

m, 4H, b-pyrrol), 9.03 (m, 4H, 3,5-position pyridyl),

in the WO colloid solution was determined by using a

3

.98 (d, 4H, b-pyrrol), 8.24 (m, 2H,4-position phenyl),

portable-type IC analyzer (PIA-1000, Shimadzu, Japan)

equipped with an anion-exchange Shim-pack IC-A3

column (Shimadzu, Japan) at 30°C. The eluent used in

.91 (m, 8H, 2,3,5,6-position phenyl), 4.71 (s, 6H,

-

[

CH ), -2.84 (s, 2H, NH); MALDI-TOF-MS m/z: 901

3

-

+

-3

trans-DMPyP + 2I ] + H ; Anal. calcd. for C H N I :

this study was 4-hydroxybenzoic acid (8.0 × 10 M)/

44 34

6 2

C, 58.68; H, 3.81; N, 9.33. Found: C, 58.45; H, 3.89;

N, 9.21.

bis(2-hydroxyethyl)iminotris-(hydroxymethyl)methane

-

3

(

bis-tris; 3.2 × 10 M), and the ﬂow rate was 300 mL/min.

Synthesis of cationic diphenyl-di(N-methyl-

-pyridyl)-substituted porphyrin (DMPyP)

Adsorption-aggregation behaviors of DMPyP

4

derivatives on the WO colloid surface

3

derivatives

A typical experiment would consist of ﬁrst mixing

the DMPyP derivative aqueous solution, water, sodium

sulfate (ionic strength adjustment), and hydrochloric

According to the Rothemund–Lindsey procedure

[

35, 36], diphenyl-di(4-pyridinyl)porphyrin derivatives

5,10-diphenyl-15,20-di(4-pyridyl)porphyrin (cis-DPyP)

acid or sodium hydrate. Then the WO colloid aqueous

3

and 5,15-diphenyl-10,20-di(4-pyridyl)porphyrin (trans-

DPyP)] were synthesized from pyrrole condensed with

solution would be added and stirred for 10 min at room

temperature. It was preliminarily established that this

time is sufﬁcient for the establishment of adsorption-

aggregation equilibrium. The pH values of the DMPyP/

4

-pyridinecarboxaldehyde and benzaldehyde in reﬂuxing

propionic acid, and the crude products were puriﬁed by

silica-gel column chromatography using chloroform/

methanol (30:1) as eluent. 5,10-Diphenyl-15,20-di(4-

pyridyl-N-methyl)porphyrin diiodide (cis-DMPyP) and

WO aqueous solution were periodically checked with an

3

F-14 pH meter (Horiba, Japan) equipped with a 6366-

10D glass electrode (Horiba, Japan). The DMPyP/WO₃

aqueous solution was injected into a single compartment

cell (0.1 or 1.0 cm path length) with two quartz windows.

5

,15-diphenyl-10,20-di(4-pyridyl-N-methyl)porphyrin

diiodide (trans-DMPyP) were prepared with methyl

J. Porphyrins Phthalocyanines 2018; 22: 3–12

4

K. ADACHI ET AL.

Afterward, static UV-vis absorption spectra of the

weak Q bands in the visible region at about 500–700 nm,

DMPyP/WO aqueous solution were obtained on a V-730

spectrometer (Jasco, Japan).

arising from the a (p)−e (p) and a (p)−e (p) transition

3

1u

g

2u

g

[39], respectively. Under acidic conditions (pH = 1.5),

the Lambert–Beer law was obeyed for both DMPyP

-

6

Other equipment

derivatives in the concentration range from 5.0 × 10

-4

to 1.0 × 10 M at the maximum absorption wavelength

not shown). However, the absorption spectra of DMPyP

Elemental analyses were determined using a Perkin-

Elmer 2400 series II CHNS/O elemental analyzer.

Nitrogen adsorption-desorption isotherm was measured

at -195.8°C with a TriStar II 3020 analyzer (Micro-

(

derivatives showed a nonlinear concentration dependence

at neutral pH condition (pH = 6.0), e.g. the intensity of

Soret band gradually decreased with the increase of the

DMPyP concentration, and the absorption spectra showed

a discernible shoulder 390 nm in higher concentration

meritics, USA). Dried WO colloid samples (ca. 0.5 g)

3

were prepared by evaporating water from the WO₃

colloid aqueous solution, and then outgassed with helium

for 2 h at 200°C prior to the adsorption measurement.

The Brunauer-Emmett-Teller (BET) method [37] was

applied to calculate the speciﬁc surface area. By using

the Barrett-Joyner-Halenda (BJH) model [38], the pore-

size distributions were calculated from the adsorption

branch isotherms. As previously reported [29], the

-6

than 1.0 × 10 M. Some isosbestic points were observed

in the visible region of their spectra, indicating that the

initial and ﬁnal species were in equilibrium [40]. It is

well-known that porphyrin compounds tend to interact

with each other by attractive p–p stacking owing to their

extended ﬂat aromatic surface [41]. Actually, the blue-

shift, broadening and lowering in the intensity of Soret

band clearly indicate the dimerization with a “face-

to-face” arrangement. To determine the dimerization

2

-1

.

speciﬁc surface area is 11.8 m g and the pore size is

1

about 50 nm. Hydrogen nuclear magnetic resonance ( H

NMR) spectra of the porphyrin derivatives were recorded

on an AVANCE DPX-400 (400 MHz) spectrophotometer

constant (K ) of DMPyP derivatives in neutral water,

D

the molar absorbance coefﬁcient at maximum peak was

plotted as a function of the total DMPyP concentration,

assuming the following monomer and dimer equilibrium

(Bruker, USA) with tetramethylsilane (TMS) as the

internal standard. MALDI-TOF mass spectra were

acquired on a Voyager DE PRO (Applied Biosystems,

USA) using a-cyano-4-hydroxycinnamic acid as a

(Equations 1 and 2):

matrix. The TEM image of WO colloids was measured

KD

3

ꢀꢀꢀꢁ

DMPyP ꢂꢀ ꢀꢀ (DMPyP)

2

(1)

(2)

2

with a JEM-2000EX II instrument (JEOL, Japan). The

TEM samples were prepared by dropping the WO colloid

3

[

(DMPyP)₂]

sample solution onto a copper grid covered with a carbon

K =

D

2

[

DMPyP]

ﬁlm. The size distribution histogram of WO colloids was

3

assessed by averaging the sizes of 100 particles directly

from the TEM images. The resulting samples were

composed of almost spherical particles with diameters

over the range 6–22 nm (see Fig. S1).

Using the monomer molar absorbance coefﬁcient

(

e ) obtained from the absorbance in the dilute solution

m

-6

<1.0 × 10 M), the values of K_Dand dimer molar

absorbance coefﬁcient (e ) for DMPyP derivatives were

d

calculated from the curve-ﬁtting in the inset of Fig. S2.

These obtained values are listed in Table S1 (K : 1.25 ×

D

4

-1

4

-1

RESULTS AND DISCUSSION

10 M for cis-DMPyP and 0.89 × 10 M for trans-

DMPyP). Interestingly, in spite of the structural similarity

of the two molecules, cis-DMPyP molecules form more

stable aggregates than trans-DMPyP ones.

Molecular forms of DMPyP derivatives

in aqueous media

Free-base (non-metallated) water-soluble porphyrin

compounds can bind to one or two protons on pyrrolinic

nitrogens, resulting in the mono- and di-protonated

species [42]. Generally, the porphyrin species exhibit

signiﬁcant spectral changes during protonation. As

shown in Fig. S3, UV-vis absorption spectra of dilute

To theorize the nature of the interaction between

DMPyP derivatives and WO₃colloids, a detailed

understanding of the behavior and habits of both cis-

and trans-DMPyP in aqueous solutions is required. Two

DMPyP derivatives (cis- and trans-DMPyP) were well-

synthesized via the Rothemund–Lindsey procedure. Both

-6

cis- and trans-DMPyP aqueous solutions (1.0 × 10 M:

the formation of DMPyP dimers in the solution can

be neglected) greatly changed with the pH in a range

between 1 and 6. From these spectral changes, the two

-4

DMPyP derivatives are quite soluble (>1.0 × 10 M) in

water. At ﬁrst, we studied the concentration dependence

on the absorption spectra for cis- and trans-DMPyP in

acidic (pH = 1.5) and neutral (pH = 6.0) aqueous solution,

as shown in Fig. S2. At low concentrations of DMPyP

acid dissociation constants (pK_a₁and pK ) of cis- and

a2

trans-DMPyP were pK = 3.03 and 3.08, pK = 2.88

a1

a2

-

6

derivatives (<1.0 × 10 M), the UV-vis absorption spectra

in aqueous solution are dominated typically by two

intense bands: the Soret band around 400–450 nm and the

and 2.90, respectively, in agreement with values reported

in the literature (pK = ca. 3.0 and pK = ca. 2.9) [43,

a1

a2

44]. Herein, it is noted that free-base DMPyP in neutral

J. Porphyrins Phthalocyanines 2018; 22: 4–12

STIMULI-TRIGGERED REVERSIBLE SWITCHING MECHANISM BETWEEN H- AND J-TYPE SUPRAMOLECULAR ASSEMBLIES

5

water (pH 6.0) tends to form dimers even though no

signiﬁcant association of di-protonated DMPyP is seen

in acidic aqueous media (pH 1.5). Spectral studies

have demonstrated that the self-association behavior

of DMPyP derivatives in water is obviously governed

by the arrangement of two peripheral cationic groups

on the meso-positions of the porphyrin ring and/or the

protonation of the porphyrin core.

the other hand, the absorption spectral changes in the

cis-DMPyP/WO aqueous system at acidic and neutral

3

pH were strikingly different. As the concentration of

the WO colloid increases, the monomer Soret band for

3

cis-DMPyP decreased sharply and shifted to the longer

wavelength at pH 1.5 (Fig. 1(a)), but shifted to the shorter

wavelength at pH 6.0 (Fig. 1(b)). Oddly enough, both

acidic and neutral cis-DMPyP/WO aqueous systems

3

exhibit weak red-shifted Q bands.

When small-molecule chromophores assemble in

the aqueous solution, they often form H-type or J-type

aggregates, depending on the relative alignment of the

transition dipole moments on adjacent molecules [4, 5].

Speciﬁcally taking into account the angle between the

direction of the transition dipole moments and the line

linking the molecular centers (θ) and the angle between

the transition dipole moments of the monomers in the

aggregate (a), several speciﬁc cases can be considered

(Scheme 2). In the context of Kasha’s molecular exciton

coupling theory [45], face-to-face stacked chromophores

(54.7° < θ ≤ 90° and a = 0; H-type aggregate) are

expected to exhibit a blue-shifted band (H-band) because

only the higher energy exciton transition is allowed. On

the other hand, head-to-tail arranged chromophores (0°

≤ θ < 54.7° and a = 0; J-type aggregate) exhibit a red-

shifted band (J-band) because only the lower energy

excitation is allowed. However, for a twisted face-to-

face (54.7° < θ ≤ 90° and 0° < a ≤ 90°) or an oblique

head-to-tail aggregate (0° ≤ θ < 54.7° and 0° < a ≤ 90°),

both H- and J-bands will be observed in the absorption

spectra, because both the higher and lower energy exciton

transitions are allowed. In these cases, the H-band will

Adsorption and aggregation behaviors

of DMPyP derivatives on the WO colloid surface

3

The WO colloid particles had a unique adsorption

3

property for various organic dyes in an aqueous solution.

Furthermore, a self-aggregation phenomenon of organic

dyes adsorbed on theWO colloid surface occurred, as had

3

been reported in our previous study [10–12]. The UV-vis

absorption spectra in the DMPyP/WO aqueous system

3

were measured under acidic (pH = 1.5) and neutral (pH =

6

.0) conditions. Both acidic and neutral DMPyP/WO₃

aqueous solutions were essentially transparent in the

visible region under the present experimental conditions,

and stable for several weeks without precipitate at room

temperature. As shown in Fig. 1, the addition of the WO₃

colloid aqueous solution resulted in signiﬁcant spectral

changes of both DMPyP derivatives in the visible region.

In particular, a gradual decrease in the monomer Soret

band intensity of trans-DMPyP was observed with a

titration of the WO colloid aqueous solution, along with

3

a red-shift in absorption edge to a longer wavelength and

an appearance of a new weak band at around 370 nm

regardless of pH conditions (Figs 1(c) and (1d)). On

Fig. 1. Change of UV-vis absorption spectra of (a) cis-DMPyP in acidic water, (b) cis-DMPyP in neutral water, (c) trans-DMPyP in

acidic water, and (d) trans-DMPyP in neutral water as a function of added WO colloid at 25°C. Concentration conditions: [DMPyP

3

-

6

-2

derivative] = 1.0 × 10 M; [Na SO ] = 3.3 × 10 M; pH 1.5 (acidic condition), pH 6.0 (neutral condition); molar ratios ([WO ]/

DMPyP derivative]) as follows: (A) 0, (B) 4, (C) 8, (D) 10, (E) 25, (F) 40, (G) 50, and (H) 80. The arrows indicate the direction of

2

4

3

[

the absorbance changes, as the WO colloid concentration is increased

3

J. Porphyrins Phthalocyanines 2018; 22: 5–12

6

K. ADACHI ET AL.

Scheme 2. Orientation dependence in exciton coupling between two chromophores (ellipses) and their electric dipoles (arrow in

ellipse). The solid and dashed arrows from ground states to excited states represent allowed and forbidden transitions, respectively

be more intense for the twisted face-to-face aggregate,

whereas the oblique head-to-tail aggregate is achieved

if the J-band is more intense. In the trans-DMPyP/WO₃

aqueous system, oblique J-aggregates were present

regardless of pH conditions. Moreover, the contrasting

anchoring groups and/or the di-protonated porphyrin core

of DMPyP derivatives, and the p–p stacking interaction

between the DMPyP derivatives adsorbed on the surface)

can be considered as the most probable adsorption and

aggregation mechanism, respectively. The adsorption

and aggregation processes of DMPyP derivatives in

spectral shifts in the cis-DMPyP/WO aqueous system

3

indicate that cis-DMPyP chromophores in the acidic

the DMPyP/WO aqueous system need to be identiﬁed

3

WO colloid aqueous solution adopt the J-aggregation

separately to quantitatively evaluate each other. Actually,

stable isosbestic points were observed in the UV-vis

absorption spectra during the titration experiment in

which the DMPyP derivative concentration was held

constant and combined with aqueous solutions of the

3

mode, whereas ones in the neutral WO colloid aqueous

3

solution exhibit H-aggregation tendency. It is interesting

to note that the neutral cis-DMPyP/WO aqueous system

3

exhibits weak red-shifted Q bands, not blue-shifted ones

(

see Fig. 1(b)), though the reason is not clear at the present

different concentration of WO colloid, implying that

3

stage. In tetrapyrrolic chromophores, the derivation and

interpretation of the spectra of their aggregates are still

complicated by the presence of two perpendicular dipole

moments [4, 5].

a sequence of adsorption and aggregation of DMPyP

derivatives in the WO colloid aqueous solution was

3

proceeding apace without forming an intermediate or

multiple products. Considering the dynamic equilibrium

between the monomer and aggregate of the DMPyP

Generally, metal oxide particles in an aqueous system

are hydrated and M–OH groups cover their surface. The

M–OH sites on the surface of particles can react with

derivative on the WO colloid surface, the adsorption

3

constant onto the WO colloid surface (K′) and the

3

+

-

H or OH from dissolved acids or bases, and positive

aggregation constant on the WO colloid surface (K′ )

3

agg

+

-

(

M–OH ) or negative (M–O ) charges develop on the

can be deﬁned as follows (Equations 3 and 4):

2

surface. Thus, metal oxides have a characteristic pH,

where the positive and negative sites are in equal amount

K′

ꢀꢀꢀꢁ

DMPyP ꢂ ꢀ ꢀꢀ (DMPyP)

ad

(

isoelectric point, IEP) [46]. At pH values lower than

(

3)

[

(DMPyP)_a_d]

the IEP the particle surface charge is positive, and at pH

values higher than the IEP, the particle surface charge

K′ =

[

DMPyP]

is negative. Since the IEP of WO is 0.2−0.5 [47], the

3

K′

WO colloid surface is negatively charged in this aqueous

agg

3

ꢀꢀꢀꢀꢁ

n(DMPyP)_a_dꢂꢀꢀ ꢀꢀ ((DMPyP) )

ad

n

solution (pH = 1.5 and 6.0). As can be seen from the

above section, the aggregation (including dimerization)

of DMPyP derivatives in acidic and neutral water is

almost negligible under this dye concentration (1.0 ×

4)

[

((DMPyP) ) ]

ad

n

ad

n

K′ =

agg

[DMPyP]

-

6

1

0

M). On the basis of the well-known peculiarities

where [DMPyP] and [(DMPyP) ] are the concentration

ad

of the physicochemical properties of metal oxide

nanoparticles and cationic porphyrin derivatives in the

aqueous system, we suggest that two different driving

forces (the electrostatic interaction between the negatively

of the DMPyP derivatives in the aqueous solution and

on the WO colloid surface, respectively. [((DMPyP) ) ]

representstheconcentrationoftheaggregateoftheDMPyP

3

ad n

derivatives on the WO colloid surface. The subscript or

3

charged WO colloid surface and the peripheral cationic

superscript “n” is the apparent number of monomers in

3

J. Porphyrins Phthalocyanines 2018; 22: 6–12

STIMULI-TRIGGERED REVERSIBLE SWITCHING MECHANISM BETWEEN H- AND J-TYPE SUPRAMOLECULAR ASSEMBLIES

7

the unit aggregate addressed in an absorption spectral

change. Assuming that the concentration of monomer

DMPyP derivatives on the WO₃colloid surface is

negligibly smaller than that of aggregate on the surface,

the equilibrium concentration of the aggregate on the

stoichiometric analysis of the experimental data using

Equation 6 suggested the aggregation of the dimer

(n = 2) of both cis- and trans-DMPyP on the WO colloid

3

surface, meaning that the aggregate formed on the WO₃

colloid surface must include two DMPyP molecules

as a minimum number. These DMPyPs’ aggregation

behaviors agree with the results previously reported on the

adsorption/aggregation of tetrakis(4-pyridyl)porphyrin

WO colloid surface is spectrometrically evaluated as the

3

following (Equation 5):

.

(

[DMPyP]_i_n_i−[DMPyP]) V

[

((DMPyP) ) ] =

(5)

(TPyP) derivatives on the WO

Table 1, we summarize the adsorption and aggregation

dimerization) parameters of DMPyP derivatives at two

3

colloid surface [28]. In

ad

n

n.S

(

where the subscript ‘ini’ denotes initial concentration.

signiﬁcant pH experimental conditions. Our data clearly

S and V are the total surface area of the WO particles

3

showed that the pH in the DMPyP/WO aqueous system

3

(calculated using BET speciﬁc area value) and the volume

affected both adsorption and dimerization behaviors on

of the aqueous phase, respectively. [((DMPyP) ) ] can be

ad n

the WO colloid surface. In the cis-DMPyP/WO aqueous

3

calculated from the UV-vis absorption spectra at different

concentrations using the approximation method of West

and Pearce [48]. Figure 2 shows apparent adsorption

system, the K′ value determined in acidic media is larger

than the one in neutral media, whereas the K′ in acidic

agg

media is smaller than the one in neutral media. A similar

isotherms for DMPyP derivatives in the WO colloid

3

tendency is also seen in the trans-DMPyP/WO aqueous

3

aqueous solution system at pH 1.5 and 6.0. These

isotherm shapes were very akin to ones for the Langmuir

case. In the Langmuir adsorption theory, the interaction

between the species adsorbed on the surface, however, is

not considered at all. Since the basic Langmuir isotherm

system. As results, we ﬁnd that the porphyrin core

protonation is facilitated upon the adsorption of DMPyP

derivatives, but prevents their dimerization reaction on

the WO colloid surface, which seems to be reasonable

3

given that it would be more favorable for the stable

adsorption-aggregation equilibrium in the DMPyP/WO₃

aqueous system due to the balance of the electrostatic

model is not applicable in the DMPyP/WO aqueous

3

systems, it is modiﬁed with the introduction of the

aggregation constant term as the following (Equation 6):

(attractive/repulsive) interaction between cationic

DMPyP molecules and negatively charged WO surface.

3

n

Note that speciﬁc large saturated concentration was

observed for cis-DMPyP at pH = 6.0. Since the absorption

spectra of cis-DMPyP only showed hypsochromic shift

K′ (aK′[DMPyP])

agg

[

((DMPyP) ) ] =

(6)

ad

n

(

a + K′[DMPyP])

on the addition of WO colloid aqueous solution at

3

Herein, a refers to the saturated concentration

neutral pH, unlike trans-DMPyP, the arrangement of cis-

on the WO colloid surface. As shown in Fig. 2, the

DMPyP molecules adsorbed on the WO surface must

3

Table 1. Spectroscopic parameters of aggregate, saturated concentrations (a), aggregation constants (K_a′_g_g₍_d_i_m_e_r₎) of DMPyP derivatives

on the WO colloid surface, adsorption constants (K′) onto the WO colloid surface at 25°C in two ion strength conditions, and the

3

tilting angles (f) of the porphyrin plane from the tangent line to the WO particle surface.

3

-2

-1

.

2

DMPyP derivative

pH

Ion strength/

a/mol dm

K′_a_g_g₍_d_i_m_e_r₎/mol dm

K′/dm

f/degree

.

-3

mol dm

-

8

-1

0

.01

(0.97 ± 0.10) × 10

(2.07 ± 0.21) × 10

(0.98 ± 0.12) × 10

(0.96 ± 0.17) × 10

(1.27 ± 0.14) × 10

(1.97 ± 0.20) × 10

(1.18 ± 0.15) × 10

(1.40 ± 0.17) × 10

(3.80 ± 1.13) × 10

(2.41 ± 0.71) × 10

(2.59 ± 0.94) × 10

(1.67 ± 0.34) × 10

(3.11 ± 1.02) × 10

(1.91 ± 0.58) × 10

(2.03 ± 0.79) × 10

(1.25 ± 0.39) × 10

20.3

″

1

6

1

6

.5

.0

.5

.0

8

0

.1

cis-DMPyP

8

.01

87.2

″

0

.1

.01

20.9

″

-

8

-1

0

.1

trans-DMPyP

.01

22.6

″

-

8

-1

0

.1

J. Porphyrins Phthalocyanines 2018; 22: 7–12

8

K. ADACHI ET AL.

Fig. 2. Adsorption isotherms of (a) cis-DMPyP in acidic water, (b) cis-DMPyP in neutral water, (c) trans-DMPyP in acidic water,

and (d) trans-DMPyP in neutral water onto the WO colloid surface. Concentration conditions: [DMPyP derivative] = 1.0 ×

3

-

6

-6

-3

-2

1

(

0 M; [WO ] = 1.0 × 10 −1.0 × 10 M; [Na SO ] = 3.3 × 10 M (circle), 3.3 × 10 M (triangle); pH 1.5 (acidic condition), pH 6.0

neutral condition). The solid lines are ﬁtted by Equation 6. (e) Box approximating DMPyP derivative molecule. Areas for each face

3

2

4

A (ﬂat-on) and B (edge-on) were determined from the CPK model structure

be correlated to their preferred aggregate geometries on

a nanoscale. The detailed information of geometrical

structure in the DMPyP derivatives’aggregates formed on

pH values in the cis-DMPyP/WO aqueous system. Upon

increasing the pH value from 1 to 6, the cis-DMPyPs’

3

J-dimer band at 463 nm was observed to disappear,

and then a new H-dimer band at 372 nm appeared in

the region higher than pH 3.5. Moreover, an isosbestic

point was obtained at ca. 440 nm in the absorption

spectra, implying stoichiometric conversion of J-dimers

to H-dimers. In order to evaluate the reversibility of

the WO colloid surface will be argued in a later section.

3

Moreover, in an effort to further delineate the

adsorption characteristics of the DMPyP derivatives

onto the WO colloid surface, we set out to quantify

3

the inﬂuence of ionic strength on the isotherm under a

constant concentration of DMPyP derivative. As shown

in Fig. 2 and Table 1, the determined adsorption constants

of DMPyP derivatives decreased with increasing sodium

sulfate concentration. It is evident that electrostatic

interactions between cationic DMPyP derivative

molecules and the negatively charged WO₃colloid

surface dominate the adsorption behavior of the DMPyP

this color changing in the cis-DMPyP/WO aqueous

3

system, this pH titration was carried out over several

cycles using the same solution, and the absorption

spectrum was recorded after each step (pH 1.5 and 6.0).

Figure 3(b) shows the variation of the maximum molar

absorbance coefﬁcients of cis-DMPyP in the acidic

and neutral WO aqueous solution, indicating the high

3

derivatives on the WO colloid surface.

degree of reversibility of pH-responsive color switching

3

between cis-DMPyPs’ J- and H-dimers formed on

the WO colloid surface. The J- to H-type aggregate

3

pH-Induced switch between H- and

J-aggregation mode of cis-DMPyP stacks

transformation observed in this study originates from a

change in the packing of the porphyrin cores on the WO₃

colloid surface. Hence, we surmise that this cis-DMPyPs’

transformation involves the different location of the

N-methyl-4-pyridyl substituent(s) and/or the different

intermolecular interactions, since no pH-stimulus

induces switching back and forth from J- to H-dimer in

on the WO colloid surface

3

Spectroscopic discrimination between the H- and J-

type dimerization modes of cis-DMPyP molecules in the

WO colloid aqueous solution was observed by UV-vis

3

absorption spectral measurements. For novel approaches

in controlling morphology and functioning of smart and

promising stimuli-responsive supramolecular materials,

it is worthwhile to realize that the porphyrin aggregates

formed on the metal oxide would be transformed

depending upon pH. Accordingly, we investigated the

pH-responsive color characteristics in the cis-DMPyP/

the trans-DMPyP/WO aqueous system (not shown). The

3

similar pH-responsive switching was also observed in the

cationic/zwitterionic xanthene dye/WO aqueous system

3

as reported previously [27], indicating that protonation/

dissociation of the xanthene derivatives adsorbed on

the WO surface induces reversible H- to J-aggregate

3

WO aqueous system. Figure 3(a) depicts the typical

transformation. Herein, we demonstrated that the drastic

3

progressive absorption spectral changes in the Soret

aggregation mode switching of cis-DMPyP dimers

band region of cis-DMPyP in the WO colloid aqueous

formed on the WO colloid surface can be successfully

3

solution under various pH conditions. The inset shows

changes of the maximum molar absorbance coefﬁcients

at 372 (H-band) and 463 nm (J-band) depending upon the

achieved via controlling the external stimuli conditions,

such as the pH change in the surrounding water medium.

Recently, Eguchi et al. [49] investigated the orientation

J. Porphyrins Phthalocyanines 2018; 22: 8–12

STIMULI-TRIGGERED REVERSIBLE SWITCHING MECHANISM BETWEEN H- AND J-TYPE SUPRAMOLECULAR ASSEMBLIES

9

of the presence and orientation of the dye monolayer

adsorbed on the metal oxide surface [27–29]. Therefore,

the adsorption parameters of the DMPyP derivatives

calculated for the Langmuir model evidence their

energetically preferred adsorption geometries. Using

the observed saturated concentration (a) values of

DMPyP derivatives, the occupied area per molecule

on the WO colloid surface was calculated for every

3

DMPyP/WO aqueous system. For acidic and neutral

3

trans-DMPyP/WO aqueous systems, similar molecular

3

area values were obtained (169 and 173 Å/molecule,

respectively). Because both experimental values are

quite close to the projection area ones (estimated from

the CPK model optimized at PM3 level with Gaussian

0

9 program [50]) of the trans-DMPyP derivatives lying

ﬂat-on (see Fig. 2(e)), a nearly ﬂat-lying structure would

be assumed for the monolayer adsorption/aggregation

morphology of trans-DMPyP derivatives on the WO₃

colloid surface. The area per molecule in the acidic cis-

DMPyP/WO aqueous system was estimated to be 171

3

2

Å /molecule, indicating that di-protonated cis-DMPyP

molecules are also lying ﬂat-on the WO surface in the

3

same manner as in the trans-DMPyP/WO aqueous

3

system. However, the area per molecule in the neutral

2

cis-DMPyP/WO aqueous system was 80 Å /molecule,

3

much smaller than that in the acidic same system,

suggesting that the free-base cis-DMPyP molecules

are standing perpendicular to the WO colloid surface

3

and adopt an edge-on arrangement for the situation of

strong molecule–molecule (probably p–p) interaction.

Consequently, this minimizes the interaction area with

Fig. 3. (a) Change of absorption spectra in the cis-DMPyP/WO₃

aqueous system at 25°C as a function of pH. Concentration

-

6

conditions: [cis-DMPyP] = 1.0 × 10 M; [WO ] = 1.0 ×

3

the underlying WO substrate.

3

-

3

-2

1

0

M; [Na SO ] = 3.3 × 10 M. The arrows indicate the

2

4

The primary mechanism through which molecular

aggregate structures in the nature system are formed

is generally self-assembly as a result of intrinsic

intermolecular interactions [51]. Herein, depending on

the DMPyP derivatives’ geometric (cis/trans) isomerism

and pH condition, we keenly observed various types of

alignments that adopt either a “ﬂat–on” or “edge–on”

orientation of DMPyP derivatives with respect to the

direction of the absorbance changes with increasing pH

value. The insets show the molar absorbance coefﬁcients at

maximum wavelengths under various pH conditions. The

lines in the insets are added as a visual guide. (b) Reversible

switching between H- and J-dimerization in the cis-DMPyP/

WO aqueous system at 25°C. Intensity changes in the molar

3

absorbance coefﬁcients at the H-band (ﬁlled circles) and

J-band (open circles) maxima under 10 cycles of the acidic

(

pH 1.5) and neutral (pH 6.0) treatments

WO colloid surface. Two different driving forces (not

3

only the p–p intermolecular interaction between aromatic

DMPyP moieties but also the electrostatic interaction

between cationic DMPyP and negatively surface-

of cis-DMPyP molecules adsorbed on the synthetic

clay (Sumecton SA) surface in various solvents, and

likewise concluded that the tilting angle of cis-DMPyP

with respect to surface could be reversibly controlled by

changes in surrounding solvent environment. Although

difference does exist in adsorbates and/or solvents, it is

interesting that there is a similar trend for cis-DMPyP

molecules adsorbed on the surface toward stimuli-

induced conformational transformations.

charged WO colloid particles) were used to assess the

3

adsorption–aggregation behavior in the DMPyP/WO₃

aqueous system. As the WO colloid particles used

3

in this study are almost an ideal spherical shape with

radius R, the adsorption onto curved surfaces should be

considered. According to previous calculations [28], we

can hypothesize and evaluate the probable arrangement

for the DMPyP derivatives adsorbed on the WO colloid

3

surface, as shown in Scheme 3. Considering that the

Favorable geometry of DMPyP aggregates

curvature radius R of WO colloid particles nearly equals

3

formed on the WO surface

3

to 8.8 nm (from TEM image, Fig. S1), we predicted

the molecular area (per DMPyP derivative) as follows

(Equation 7):

The UV-vis absorption spectroscopy has proven to be

crucial and suitable for in situ quantitative information

J. Porphyrins Phthalocyanines 2018; 22: 9–12

10

K. ADACHI ET AL.

occupied molecular area

CONCLUSION

(7)



l 

2R





lsinφ

 (lsinφ + R)tanφ





In summary, a detailed qualitative and quantitative

study on the adsorption–aggregation of cationic porphyrin

stereoisomers [5,10-diphenyl-15,20-di(N-methyl-4-pyri-

dyl)porphyrin (cis-DMPyP) and 5,15-diphenyl-10,20-

di(N-methyl-4-pyridyl)porphyrin (trans-DMPyP)] on

2

−1

tan⁻¹

≈

2R tan







where l and f are the length of the box approximating

DMPyP derivative molecule calculated from CPK

molecular model (see Fig. 2(e)) and the tilting angle

of the porphyrin plane from the tangent line to a WO₃

particle surface, respectively. The f values of cis- and

trans-DMPyP derivatives, respectively, were estimated

to be 20.3° and 20.9° in the acidic system (pH 1.5) and

to be 87.2° and 22.6° in the neutral system (pH 6.0).

For trans-DMPyP derivative, the tilting angles did not

vary appreciably with a variation in ambient pH. In

contrast, it is interesting to notice that the f value in the

neutral system was much larger than one in the acidic

system for cis-DMPyP derivative. The present results,

together with previously published studies [26–29, 52],

clearly indicate that the adsorption arrangement of a

porphyrin derivative adlayer on the metal-oxide surface

is variable depending on the position of the electrically

charged sites on each porphyrin derivative monomer.

We already observed the paradigm for switchable

red- and blue-shifted Soret band absorption in the cis-

DMPyP/WO₃aqueous system depending upon pH.

Given these facts and trends, it comes as no surprise

that DMPyP derivative molecules adsorb onto the WO₃

colloid surface via the electrostatic interaction and

then form ordered H-aggregates (face-to-face fashion)

or J-aggregates (head-to-tail fashion) that reﬂect the

intermolecular interactions.

the tungsten(VI) oxide (WO ) colloid nanoparticle

3

surface were investigated by spectroscopic method in

combination with theoretical calculations (Langmuir

adsorption isotherm and exciton coupling theory).

Modiﬁed Langumuir isotherms in the DMPyP/WO₃

aqueous system suggested that the DMPyP derivatives are

adsorbed onto the negatively chargedWO colloid surface

3

by the electrostatic interaction, and eventually form well-

ordered dimers via diverse intermolecular interaction.

Exciton-theoretical interpretation of spectral shifts has

permitted us to discern that cis-DMPyP molecules in the

acidic system (pH 1.5) formed the head-to-tail J-dimers

on the WO colloid surface (“ﬂat-on” orientation with

3

respect to the WO colloid surface), whereas ones in the

3

neutral system (pH 6.0) had a higher tendency to form the

face-to-face H-dimers (“edge-on” orientation) unlike in

the trans-DMPyP/WO aqueous system where J-dimers

3

were always present regardless of pH conditions. More

interestingly, we have experimentally demonstrated that

the reversible switching between the H- and J-aggregation

modes depending upon the pH value in the cis-DMPyP/

WO aqueous systems. The ﬁndings obtained in this

3

study may provide a good starting point for further

development of more accurate models to control stimuli-

responsive molecular alignment and orientation of

various functional dyes on the metal oxide surface.

Acknowledgments

This study was partially supported by the “Regional

Enterprise Support Foundation Project” sponsored by

Yamaguchi bank and by the Scientiﬁc Research of the

Grant-in-Aid for Young Scientists (B) (No. 24750069)

and Scientiﬁc Research (C) (No. 16K05817) from Japan

Society for the Promotion of Science (JSPS). ICP-AES

and TEM measurements documented in this study were

performed at the Center for InstrumentalAnalysis and the

Innovation Center, respectively, Yamaguchi University.

The authors declare no competing ﬁnancial interest.

Supporting information

Additional information, including Figs S1−S3,

Table S1 (TEM image of the WO colloid particles;

3

concentration- and pH-dependence of UV-vis absorption

spectra of DMPyP derivatives in water) and additional

parameters of DMPyP derivatives’ aggregates in aqueous

media are given in the supplementary material. This

material is available free of charge via the Internet at

http://www.worldscinet.com/jpp/jpp.shtml.

Scheme 3. Schematic illustration of proposed model for

adsorption of the DMPyP derivatives onto theWO particle with

3

radius R. Herein, the rectangles represent DMPyP derivatives

J. Porphyrins Phthalocyanines 2018; 22: 10–12

STIMULI-TRIGGERED REVERSIBLE SWITCHING MECHANISM BETWEEN H- AND J-TYPE SUPRAMOLECULAR ASSEMBLIES

11

2

5. Florio GD, Bründermann E,Yadavalli NS, Santer S,

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Article Doi

Stimuli-Triggered reversible switching mechanism between H-and J-Type supramolecular assemblies of cationic porphyrins adsorbed on tungsten(VI) oxide surface

DOI: 10.1142/S1088424618500372

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Article abstract of DOI:10.1142/S1088424618500372

Full text of DOI:10.1142/S1088424618500372

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