Synthesis of zinc chlorin-spiropyran dyads and their self-aggregation properties

Article abstract of DOI:10.1016/j.jphotochem.2015.06.017

A photochromic spiropyran moiety has been covalently linked to a self-aggregative zinc analog of chlorophyllous pigments. Semi-synthetic zinc chlorins possessing a 3¹-hydroxy- or 3¹-methoxy group were used as the chlorophyll model, and the spiropyran part was introduced at the 17-position of the chlorin macrocycle. The spiropyran moiety reversibly isomerized to its opened merocyanine form in the dyads. The photoisomerization induced drastic change in electric dipole moment of the side chain, and photoswitchable chlorophyll aggregation was found in the pigment conjugates.

Full text of DOI:10.1016/j.jphotochem.2015.06.017

Accepted Manuscript
Title: Synthesis of zinc chlorin–spiropyran dyads and their
self-aggregation properties
Author: Tomohiro Miyatake Yuki Takamori Kanae
Yamaguchi
PII:
S1010-6030(15)00229-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2015.06.017
JPC 9941
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
16-3-2015
12-6-2015
16-6-2015
Please cite this article as: Tomohiro Miyatake, Yuki Takamori, Kanae
Yamaguchi, Synthesis of zinc chlorinndashspiropyran dyads and their self-
aggregation properties, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2015.06.017
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Synthesis of Zinc Chlorin–Spiropyran Dyads and Their
Self-Aggregation Properties
Tomohiro Miyatakea,b, *, Yuki Takamoria, and Kanae Yamaguchia
aDepartment of Materials Chemistry, Ryukoku University, Otsu, Shiga 520-2194, Japan
bInnovative Materials and Processing Research Center, Ryukoku University, Otsu, Shiga
520-2194, Japan
* Corresponding author. Tel.: +81 77 543 7467; Fax.: +81 77 543 7483. E-mail address:
miyatake@rins.ryukoku.ac.jp (T. Miyatake)
Graphical abstract
Highlights



Photochromic spiropyran was appended to self-aggregative chlorophyll derivatives.
Spiropyran moiety in the chlorophyll derivatives reversibly isomerized by photoirradiation.
Open form of spiropyran, merocyanine form, quenched fluorescence emission of the
chlorophyll moiety.


Self-aggregation properties of zinc 31-hydroxy-chlorins were affected by isomerization of
spiropyran moiety.
Photo-triggered reversible self-aggregation of zinc 31-methoxy-chlorin was observed.

Abstract
A photochromic spiropyran moiety has been covalently linked to a self-aggregative zinc
analogue of chlorophyllous pigments. Semi-synthetic zinc chlorins possessing a 31-hydroxy- or
31-methoxy group were used as the chlorophyll model, and the spiropyran part was introduced
at the 17-position of the chlorin macrocycle. The spiropyran moiety reversibly isomerized to its
opened merocyanine form in the dyads. The photoisomerization induced drastic change in
electric dipole moment of the side chain, and photoswitchable chlorophyll aggregation was
found in the pigment conjugates.
Keywords: Chlorophyll, Light-harvesting antenna, Photochromism, Photosynthesis,
Self-aggregation, Spiropyran
1. Introduction
Chlorophylls are metallo-tetrapyrrolic compounds that work in light-harvesting and energy
conversion in early events of photosynthesis [1]. In light-harvesting antennas and reaction
centers chlorophyll molecules are adequately arranged for efficient intermolecular energy or
electron transfers [2]. Generally, chlorophyll molecules formed pigment–protein complexes in
the photosynthetic apparatuses, and the proteins contribute to the organization of the
photosynthetic pigments. Apart from such pigment–protein complexes, self-aggregates of
chlorophyllous pigments were found in a light-harvesting antenna system of green
photosynthetic bacteria [3–5]. The supramolecular antenna system is called a chlorosome, in
which a number of bacteriochlorophyll(BChl)s-c, d, e and f molecules self-aggregate without
any assistance of proteins (left drawing of Fig. 1). The chlorosomal chlorophylls possess a

hydroxy group at the 31-position, and the functional group makes an intermolecular hydrogen
bond with the 131-oxo group of the neighboring chlorophyll and a coordination bond with the
central magnesium of the third molecule [3,4,6]. The specific intermolecular interactions as well
as the -stacking result in the formation of J-aggregate with showing largely red-shifted Qy
absorption band at around 720–740 nm compared to their original Qy band at around 650–660
nm. Interestingly, the chlorosomal aggregate can be reproduced in non-polar organic solvent
such as hexane [7,8]. The in vitro model studies of BChl aggregate encouraged us to make
artificial chlorophyllous aggregate by using synthetic model pigments possessing a hydroxy
group. Semi-synthetic model pigments, zinc chlorins with 31-hydroxy group (right drawing of
Fig. 1), successfully gave a chlorosomal model with the red-shifted Qy absorption band as well
as exciton coupled circular dichroism (CD) signals [8–17]. In addition, the artificial aggregates
actually acted as an effective light-harvesting system showing fast energy transfer among the
assembled pigments [12,13]. The in vitro studies of chlorosomal chlorophyll models [14–17] as
well as pigment modification studies on in vivo aggregates [18] indicated that the side chain at
the 17-propionate group affects the supramolecular structure of the chlorosomal aggregate. The
molecular modeling [19–21] and solid state NMR [22] studies on the BChl aggregates suggest
that chlorosomal BChls form inverse micelle like rod structure in which the side chain at the
17-position oriented toward the outside of the assembly. The modified chlorophyll molecules
with a hydrophilic oligooxyethylene group were prepared, and the amphiphilic zinc chlorins
formed stable micelle like aggregate in aqueous media [9,23]. The hydrophilic oxyethylene
group might be oriented toward the outside of the aggregate, and the length of the oxyethylene
chain affected the self-aggregation of the chlorophyllous pigments. Therefore, the chemical
structure of the 17-side chain is one of the important factors of the chlorophyll self-assembly.
Here, we prepared novel chlorophyll derivatives possessing a photochromic spiropyran moiety

at the 17-position of zinc chlorin (Scheme 1). Photochromic compounds are photofunctional
materials whose colors are reversibly changeable by light irradiation. Azobenzenes [24],
diarylethenes [25] and spiropyrans [26] are representative photochromic compounds, and many
other kinds of photochromic compounds have been studied [27]. Photoirradiation to these
photochromic compounds induces some changes in not only their optical properties but also
their molecular shapes, their electronic dipole moments, etc. Therefore, photochromic
compounds are promising functional building blocks providing photoresponsible
supramolecular self-assemblies [28–32]. Combinations of the photochromic compounds with
other kinds of pigments such as naphthalene [33], fluorescein [34], Bodipy [35], and porphyrins
[36,37] were performed. These photochromic dyads showed photoswitchable physical property
changes such as photo-triggered energy transfer, etc. The spiropyran photochromism (Fig. 2)
was introduced in our chlorophyll conjugates. The closed form consists of indoline and
benzopyran parts that are bound via spiro junction. UV irradiation of the spiropyran induces a
ring opening reaction with heterolytic C–O bond cleavage to afford merocyanine structure,
which has the zwitterionic or the quinoidal resonance forms. The resulting opened isomer has
extended -conjugation with an absorption band at around 500–600 nm, and photochemical
isomerization by VIS light irradiation as well as thermal reaction gave the original closed
spiropyran form. It should be pointed out that the photochromic reaction of spiropyran
accompanies significant change in its electric dipole moment. The opened merocyanine form
has larger dipole moment than the parent spiropyran form. The unique physicochemical
character in spiropyran photochromism has been applied to functional materials such as
light-triggered aggregation of gold nanoparticle [38], photoswitchable wettability changes on
material surfaces [39], light-gated nanochannels [40], optical ion sensors [41], light-induced
proton dissociation [42], photoswitched DNA-binding [43], etc. In our chlorophyll–spiropyran
conjugate, the alcoholic spiropyran chromophore 1 was esterified at the 17-propionic side chain

of the chlorin macrocycle. The polarity change at the 17-position of the chlorin is exploited for
the photonic control of the self-aggregation of chlorophyllous pigments. In the present study, the
photoactive (zinc) chlorin–spiropyran dyads were synthesized and the self-aggregation
properties were examined.
2. Experimental section
2.1. Apparatuses and materials
Visible absorption and fluorescence spectra were recorded by Shimadzu UV-3100
1
spectrophotometer and Hitachi F-4500 fluorescence spectrophotometer, respectively. H NMR
spectra were measured with a Bruker DPX-400 spectrophotometer with tetramethylsilane as an
internal standard. Fast atomic bombardment (FAB)-mass spectra (MS) were recorded with a
JEOL GCmateII spectrometer. MS samples were dissolved in dichloromethane (DCM), and
3-nitrobenzyl alcohol was used as a matrix. High performance liquid chromatography (HPLC)
was performed with a Shimadzu LC-10AS pump and an SPD-M10AV diode array detector.
Melting points were recorded with a Yanagimoto micro melting apparatus and were
uncorrected.
Methyl pyropheophorbide-a (3-vinyl-chlorin) was prepared according to reported
procedures [8,44]. 1-(2-Hydroxyethyl)-3,3-dimethylindolino-6'-nitrobenzopyrylospiran 1 and
1-ethyl-3-(1-aminopropyl)carbodiimide hydrochloride (EDC·HCl) were purchased from Tokyo
Kasei Kogyo Co., Ltd., and commercially available acetone, DCM, methanol, toluene were
purchased from Nacalai Tesque. Synthetic solvents, DCM and toluene, were distilled before use.
Other reaction reagents were purchased from Sigma-Aldrich. Flash column chromatography
(FCC) was performed with silica gel (Merck, Kieselgel 60, 40–63 m, 230–400 mesh). HPLC

was done with a packed ODS column (Cosmosil 5C18-ARII, Nacalai Tesque, 10 250 mm).
Spectroscopic-grade solvents for optical measurements were purchased from Nacalai Tesque
and used without further purification.
2.2. General procedures
All synthetic procedures were done in the dark under nitrogen. Oxidation of the 3-vinyl
group to the 3-formyl group [8], reduction of the 3-formyl group to the 3-hydroxymethyl group
[8], hydration of the 3-vinyl group to the 3-(1-hydroxyethyl) group [45] and zinc metalation [8]
were performed according to reported procedures.
2.3. Condensation of alcohols to form ether
Chlorin with the 3-hydroxymethyl group (160 mol) was dissolved with methanol (25 mL),
and concentrated sulfuric acid (5 mL) was added to the solution at 0°C. The reaction mixture
was stirred at 50°C for 2 days and poured into ice-chilled water. The product was extracted with
DCM and washed with saturated aqueous NaHCO3, water and dried with anhydrous Na2SO4.
The product was purified with FCC (10% Et2O / DCM) and recrystallization (DCM and hexane)
to give 3-methoxymethyl-chlorin.
2.4. Esterification of 17-propionic acid group of chlorin with alcoholic spiropyran 1
Chlorin with the 17-propionic acid chain (100 mol) was dissolved with dried DCM (20
mL). EDC·HCl (1.50 mmol, 288 mg) and N,N-dimethyl-4-aminopyridine (DMAP: 1.50 mmol,
183 mg) were added to the solution with stirring at 0°C. The excess amount (1.5 eq.) of 1 (146

mol, 51.2 mg) was added to the solution and stirred for 12 h. The reaction mixture was poured
into 2% HCl and DCM. The organic layer was separated and washed with 4% NaHCO3, water
and dried with Na2SO4. The product was purified with FCC (12–16% Et2O / DCM) and
recrystallization (DCM and hexane) to give the corresponding spiropyran-appended chlorin.
2.5. Transesterification of methyl ester group of chlorin with alcoholic spiropyran 1
Chlorins with a methyl ester group at the 17-propionic side chain (42.0 mol) were
dissolved in distilled toluene (15 mL), and 1 (63.0 mol, 22.1 mg) and bis(di-n-butylchlorotin)
oxide (126 mmol, 69.7 mg) were added. The mixture was refluxed for 18 h after which the
solvent was evaporated. The crude reaction mixture was purified with FCC (16% Et2O / DCM)
and recrystallization (DCM and hexane) to give the corresponding spiropyran-appended chlorin.
2.6. Spectral Data
2.6.1. Spiropyran-appended 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (2)
Esterification and reduction of the 3-formyl group gave black solids of 2 in 49% yield
(FCC 16% Et2O/CH2Cl2). Mp 159–163°C; vis (CH2Cl2) max 662 (relative intensity, 0.47), 605
1
(0.11), 537 (0.12), 505 (0.13), 410 nm (1.00); H NMR (CDCl3) (R/S) 9.48/9.44, 9.46/9.43,
8.55/8.54 (each 1H, s, chlorin (Chl)-5-,10-,20-H), 7.91/7.90 (1H, dd, J = 3, 9 Hz, benzopyran
(Bp)-7-H), 7.82/7.68 (1H, d, J = 3 Hz, Bp-5-H), 7.06/6.91 (1H, dt, J = 1, 8 Hz, indoline
(In)-6-H), 6.98/6.97 (1H, dt, J = 7, 1 Hz, In-4’-H), 6.80/6.69 (1H, dt, J = 1, 7 Hz, In-5’-H),
6.61/6.60 (1H, d, J = 9 Hz, In-7-H), 6.57/6.30 (1H, d, J = 10 Hz, Bp-4-H), 6.45/6.40 (1H, d, J =
8 Hz, Bp-8-H), 5.90/5.88 (2H, s, Chl-3-CH2), 5.58/5.37 (1H, d, J = 10 Hz, Bp-3-H), 5.18/5.13,
5.05/5.04 (each 1H, d, J = 20 Hz, Chl-132-H2), 4.40/4.38 (1H, dt, J = 1, 7 Hz, Chl-18-H),

4.28–4.21 (1H, m, Chl-17-H), 4.12-3.90 (2H, m, -COOCH2), 3.68/3.66 (2H, q, J = 8 Hz,
Chl-8-CH2), 3.64/3.62, 3.42/3.41, 3.25/3.24 (each 3H, s, Chl-2-, 7-, 12-CH3), 3.16–2.97 (2H, m,
-NCH2), 2.62–2.49, 2.44–1.97 (1H+3H, m, Chl-17-CH2CH2), 1.77/1.75 (3H, d, J = 7 Hz,
Chl-18-CH3), 1.68/1.67 (3H, t, J = 8 Hz, Chl-81-CH3), 1.15/1.12, 1.00./0.94 (each 3H, s,
In-3-CH3), 0.30 (1H, br, Chl-NH), −1.80/−1.81 (1H, s, Chl-NH); MS (FAB) found: m/z = 873.
Calcd for C52H53N6O7: MH+, 873.
2.6.2. Spiropyran-appended (31R) 3-devinyl-3-(1-hydroxyethyl)-pyropheophorbide-a (3R)
Transesterification gave black solids of 3R in 41% yield (FCC 16% Et2O/CH2Cl2). Mp
128–132°C; vis (CH2Cl2) λmax 661 (relative intensity, 0.38), 605 (0.17), 537 (0.12), 505 (0.12),
410 (0.90), 347 nm (1.00); 1H NMR (CDCl3) δ(R/S) 9.69/9.68, 9.52/9.50, 8.52/8.51 (each 1H, s,
Chl-5-, 10-, 20-H), 7.90/7.91 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.81/7.68 (1H, d, J = 3 Hz, Bp-5-H),
7.06/6.91 (1H, dt, J = 1, 8 Hz, In-6-H), 6.98/6.98 (1H, dd, J = 1, 7 Hz, In-4-H), 6.80/6.70 (1H,
dt, J = 1, 7 Hz, In-5-H), 6.61/6.60 (1H, d, J = 9 Hz, Bp-8-H), 6.55/6.29 (1H, d, J = 10 Hz,
Bp-4-H), 6.45 (1H, m, Chl-3-CH), 6.45/6.40 (1H, d, J = 8 Hz, In-7-H), 5.58/5.37 (1H, d, J = 10
Hz, Bp-3-H), 5.20/5.07, 5.15/5.06 (each 1H, d, J = 20 Hz, Chl-132-H2), 4.39 (1H, dq, J = 1, 7
Hz, Chl-18-H), 4.27 (1H, m, Chl-17-H), 4.10–3.84 (2H, m, COOCH2), 3.69/3.68 (2H, q, J = 8
Hz, Chl-8-CH2), 3.67/3.65, 3.42/3.43, 3.26/3.26 (each 3H, s, Chl-2-, 7-, 12-CH3), 3.20–2.97 (2H,
m, NCH2), 2.63–1.96 (4H, m, Chl-17-CH2CH2), 2.15/2.14 (3H, d, J = 7 Hz, Chl-31-CH3),
1.78/1.76 (3H, d, J = 7 Hz, Chl-18-CH3), 1.70/1.68 (3H, t, J = 8 Hz, Chl-81-CH3), 1.15/1.12,
1.00./0.94 (each 3H, s, In-3-CH3), 0.39 (1H, br, Chl-NH), −1.79/−1.80 (1H, s, Chl-NH); MS
(FAB) found: m/z = 887. Calcd for C53H55N6O7: MH+, 887.
2.6.3. Spiropyran-appended (31S) 3-devinyl-3-(1-hydroxyethyl)-pyropheophorbide-a (3S)
Transesterification gave black solids of 3S in 45% yield (FCC 16% Et2O/CH2Cl2). Mp

125–128°C; vis (CH2Cl2) λmax 661 (relative intensity, 0.38), 605 (0.17), 537 (0.12), 505 (0.12),
410 (0.90), 347 nm (1.00); 1H NMR (CDCl3) δ(R/S) 9.69/9.66, 9.46/9.44, 8.49/8.49 (each 1H, s,
Chl-5-, 10-, 20-H), 7.91/7.89 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.81/7.67 (1H, d, J = 3 Hz, Bp-5-H),
7.07/6.92 (1H, dt, J = 1, 8 Hz, In-6-H), 6.98/6.98 (1H, dd, J = 1, 7 Hz, In-4-H), 6.81/6.70 (1H,
dt, J = 1, 7 Hz, In-5-H), 6.61/6.59 (1H, d, J = 9 Hz, Bp-8-H), 6.56/6.29 (1H, d, J = 10 Hz,
Bp-4-H), 6.46/6.38 (1H, d, J = 8 Hz, In-7-H), 6.36/6.36 (1H, q, J = 7 Hz, Chl-3-CH), 5.55/5.38
(1H, d, J = 10 Hz, Bp-3-H), 5.17/5.04, 5.12/5.05 (each 1H, d, J = 20 Hz, Chl-132-H2), 4.38 (1H,
dq, J = 1, 7 Hz, Chl-18-H), 4.23 (1H, m, Chl-17-H), 4.11–3.90 (2H, m, COOCH2), 3.68/3.66
(2H, q, J = 8 Hz, Chl-8-CH2), 3.63/3.61, 3.39/3.38, 3.24/3.23 (each 3H, s, Chl-2-, 7-, 12-CH3),
3.11–2.97 (2H, m, NCH2), 2.65–1.96 (4H, m, Chl-17-CH2CH2), 2.13/2.12 (3H, d, J = 7 Hz,
Chl-31-CH3), 1.75/1.73 (3H, d, J = 7 Hz, Chl-18-CH3), 1.68/1.67 (3H, t, J = 8 Hz, Chl-81-CH3),
1.15/1.12, 1.00./0.94 (each 3H, s, In-3-CH3), 0.32 (1H, br, Chl-NH), −1.83/−1.84 (1H, s,
Chl-NH); MS (FAB) found: m/z = 887. Calcd for C53H55N6O7: MH+, 887.
2.6.4. Spiropyran-appended 3-devinyl-3-methoxymethyl-pyropheophorbide-a (4)
Esterification gave black solids of 4 in 71% yield (FCC 12% Et2O/CH2Cl2). Mp
132–136°C; vis (CH2Cl2) max 661 (relative intensity, 0.44), 603 (0.07), 535 (0.09), 502 (0.09),
410 nm (1.00); 1H NMR (CDCl3) δ(R/S) 9.55/9.52, 9.47/9.46, 8.56/8.54 (each 1H, s, Chl-5-, 10-,
20-H), 7.92/7.92 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.83/7.73 (1H, d, J = 3 Hz, Bp-5-H), 7.06/6.91
(1H, dt, J = 1, 8 Hz, In-6-H), 6.98/6.97 (1H, dd, J = 1, 8 Hz, In-4-H), 6.80/6.69 (1H, dt, J = 1, 7
Hz, In-5-H), 6.63/6.61 (1H, d, J = 9 Hz, Bp-8-H), 6.58/6.35 (1H, d, J = 10 Hz, Bp-4-H),
6.46/6.44 (1H, d, J = 8 Hz, In-7-H), 5.72/5.71 (2H, s, Chl-3-CH2), 5.62/5.38 (1H, d, J = 10 Hz,
Bp-3-H), 5.22/5.17, 5.09/5.07 (each 1H, d, J = 20 Hz, Chl-132-H2), 4.40 (1H, dq, J = 1, 7 Hz
Chl-18-H), 4.28 (1H, m, Chl-17-H), 4.15–3.90 (2H, m, COOCH2), 3.71/3.71 (2H, q, J = 8 Hz,
Chl-8-CH2), 3.70/3.70, 3.69/3.67, 3.42/3.41, 3.27/3.27 (each 3H, s, Chl-2-, 7-, 12-CH3,

3-COCH3), 3.24–2.95 (2H, m, NCH2), 2.58–1.95 (4H, m, Chl-17-CH2CH2), 1.79/1.77 (3H, d, J
= 7 Hz, Chl-18-CH3), 1.70/1.69 (3H, t, J = 8 Hz, Chl-81-CH3), 1.16/1.13, 1.00./0.93 (each 3H, s,
In-3-CH3), 0.41 (1H, br, Chl-NH), −1.72 (1H, s, Chl-NH); MS (FAB) found: m/z = 887. Calcd
for C53H55N6O7: MH+, 887.
2.6.5. Spiropyran-appended zinc 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (Zn2)
Zinc metalation gave dark green solids of Zn2 in 93% yield (HPLC: retention time
was 6.6 min.). Mp 235–238°C; vis (CH2Cl2) λmax 650 (relative intensity, 0.72), 604 (0.10), 564
1
(0.07), 520 (0.05), 424 nm (1.00); H NMR (CDCl3) δ(R/S) 9.53/9.50, 9.32/9.29, 9.37/9.35
(each 1H, s, Chl-5-, 10-, 20-H), 7.92/7.90 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.79/7.65 (1H, d, J = 3
Hz, Bp-5-H), 7.03/6.92 (1H, dt, J = 1, 8 Hz, In-6-H), 6.98/6.97 (1H, dd, J = 1, 8 Hz, In-4-H),
6.78/6.70 (1H, dt, J = 1, 7 Hz, In-5-H), 6.61/6.59 (1H, d, J = 9 Hz, Bp-8-H), 6.41/6.41 (1H, d, J
= 8 Hz, In-7-H), 6.40/6.06 (1H, d, J = 10 Hz, Bp-4-H), 5.76/5.75 (2H, s, 3-CH2), 5.76/5.50 (1H,
d, J = 10 Hz, Bp-3-H), 5.16/5.13, 5.11/5.07 (each 1H, d, J = 20 Hz, Chl-132-H2), 4.36 (1H, m,
Chl-18-H), 4.23 (1H, m, Chl-17-H), 4.05–3.82 (2H, m, COOCH2), 3.74/3.76 (2H, q, J = 8 Hz,
Chl-8-CH2), 3.65/3.62, 3.35/3.34, 3.31/3.27 (each 3H, s, Chl-2-, 7-, 12-CH3), 3.21–3.08 (2H, m,
NCH2), 2.58–1.99 (4H, m, Chl-17-CH2CH2), 1.80 (3H, d, J = 7 Hz, Chl-18-CH3), 1.71/1.69 (3H,
t, J = 8 Hz, Chl-81-CH3), 1.13/1.08, 0.95/0.85 (each 3H, s, In-3-CH3); MS (FAB) found: m/z =
935. Calcd for C52H51N6O7Zn: MH+, 935.
2.6.6. Spiropyran-appended zinc (31R) 3-devinyl-3-(1-hydroxyethyl)-pyropheophorbide-a
(Zn3R)
Zinc metalation gave dark green solids of Zn3R in 92% yield (HPLC: retention time
was 6.4 min.). Mp 193–196°C; vis (CH2Cl2) λmax 648 (relative intensity, 0.75), 601 (0.17), 554
1
(0.11), 512 (0.10), 421 nm (1.00); H NMR (10%CD3OD/CDCl3) δ(R/S) 9.54/9.53, 9.45/9.44,

8.31/8.30 (each 1H, s, Chl-5-, 10-, 20-H), 7.92/7.91 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.83/7.72
(1H, d, J = 3 Hz, Bp-5-H), 7.04/6.94 (1H, dt, J = 1, 8 Hz, In-6-H), 6.98/6.98 (1H, dd, J = 1, 7
Hz, In-4-H), 6.80/6.73 (1H, dt, J = 1, 7 Hz, In-5-H), 6.61/6.61 (1H, d, J = 9 Hz, Bp-8-H),
6.57/6.34 (1H, d, J = 10 Hz, Bp-4-H), 6.43/6.39 (1H, d, J = 8 Hz, In-7-H), 6.19 (1H, m,
Chl-3-CH), 5.60/5.43 (1H, d, J = 10 Hz, Bp-3-H), 5.12/5.04, 5.10/5.03 (each 1H, d, J = 20 Hz,
Chl-132-H2), 4.35/4.35 (1H, dq, J = 1, 7 Hz, Chl-18-H), 4.22 (1H, m, Chl-17-H), 4.04–3.84 (2H,
m, COOCH2), 3.75/3.75 (2H, q, J = 8 Hz, Chl-8-CH2), 3.66/3.64, 3.64/3.63, 3.41/3.41 (each 3H,
s, Chl-2-, 7-, 12-CH3), 3.23–2.99 (2H, m, NCH2), 2.57–1.89 (4H, m, Chl-17-CH2CH2),
2.07/2.06 (3H, d, J = 7 Hz, Chl-31-CH3), 1.79/1.77 (3H, d, J = 7 Hz, Chl-18-CH3), 1.72/1.70
(3H, t, J = 8 Hz, Chl-81-CH3), 1.16/1.14, 1.00./0.97 (each 3H, s, In-3-CH3); MS (FAB) found:
m/z = 949. Calcd for C53H53N6O7Zn: MH+, 949.
2.6.7. Spiropyran-appended zinc (31S) 3-devinyl-3-(1-hydroxyethyl)-pyropheophorbide-a
(Zn3S)
Zinc metalation gave dark green solids of Zn3S in 94% yield (HPLC: retention time
was 6.6 min.). Mp 192–195°C; vis (CH2Cl2) λmax 648 (relative intensity, 0.75), 601 (0.17), 554
1
(0.11), 512 (0.10), 421 nm (1.00); H NMR (10%CD3OD/CDCl3) δ(R/S) 9.52/9.52, 9.51/9.51,
8.30/8.29 (each 1H, s, Chl-5-, 10-, 20-H), 7.93/7.92 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.85/7.75
(1H, d, J = 3 Hz, Bp-5-H), 7.07/6.97 (1H, dt, J = 1, 8 Hz, In-6-H), 7.00/7.00 (1H, dd, J = 1, 7
Hz, In-4-H), 6.81/6.76 (1H, dt, J = 1, 7 Hz, In-5-H), 6.64/6.64 (1H, d, J = 9 Hz, Bp-8-H),
6.61/6.40 (1H, d, J = 10 Hz, Bp-4-H), 6.47/6.42 (1H, d, J = 8 Hz, In-7-H), 6.30/6.29 (1H, q, J =
7 Hz, Chl-3-CH), 5.64/5.48 (1H, d, J = 10 Hz, Bp-3-H), 5.13/5.04, 5.11/5.03 (each 1H, d, J = 20
Hz, Chl-132-H2), 4.35 (1H, m , Chl-18-H), 4.20 (1H, m, Chl-17-H), 4.09–3.89 (2H, m,
COOCH2), 3.74/3.74 (2H, q, J = 8 Hz, Chl-8-CH2), 3.65/3.65, 3.64/3.63, 3.38/3.38 (each 3H, s,
Chl-2-, 7-, 12-CH3), 3.21–3.04 (2H, m, NCH2), 2.58–1.94 (4H, m, Chl-17-CH2CH2), 2.12/2.11

(3H, d, J = 7 Hz, Chl-31-CH3), 1.76/1.74 (3H, d, J = 7 Hz, Chl-18-CH3), 1.71/1.69 (3H, t, J = 8
Hz, Chl-81-CH3), 1.17/1.16, 1.02./0.98 (each 3H, s, In-3-CH3); MS (FAB) found: m/z = 949.
Calcd for C53H53N6O7Zn: MH+, 949.
2.6.8. Spiropyran-appended zinc 3-devinyl-3-methoxymethyl-pyropheophorbide-a (Zn4)
Zinc metalation gave dark green solids of Zn4 in 93% yield (HPLC: retention time was
8.5 min.). Mp 200–202°C; vis (CH2Cl2) λmax 648 (relative intensity, 0.73), 600 (0.12), 553
1
(0.07), 512 (0.05), 422 nm (1.00); H NMR (CDCl3) δ(R/S) 9.55/9.54, 9.28/9.27, 8.36/8.35
(each 1H, s, Chl-5-, 10-, 20-H), 7.93/7.92 (1H, dd, J = 3, 9 Hz, Bp-7-H), 7.85/7.74 (1H, d, J = 3
Hz, Bp-5-H), 7.06/6.96 (1H, dt, J = 1, 8 Hz, In-6-H), 6.98/6.98 (1H, dd, J = 1, 8 Hz, In-4-H),
6.80/6.74 (1H, dt, J = 1, 7 Hz, In-5-H), 6.63/6.62 (1H, d, J = 9 Hz, Bp-8-H), 6.57/6.33 (1H, d, J
= 10 Hz, Bp-4-H), 6.46/6.43 (1H, d, J = 7 Hz, In-7-H), 5.61/5.60 (2H, s, Chl-3-CH2), 5.61/5.41
(1H, d, J = 10 Hz, Bp-3-H), 5.15/5.12, 5.10/5.06 (each 1H, d, J = 20 Hz, Chl-132-H2), 4.36 (1H,
m, Chl-18-H), 4.23 (1H, m, Chl-17-H), 4.03–3.87 (2H, m, COOCH2), 3.75/3.74 (2H, q, J = 8
Hz, Chl-8-CH2), 3.75/3.74, 3.65/3.64, 3.40/3.39, 3.32/3.31 (each 3H, s, Chl-2-, 7-, 12-CH3,
3-COCH3), 3.22–2.95 (m, 2H, NCH2), 2.40–1.90 (4H, m, Chl-17-CH2CH2), 1.78/1.78 (3H, d, J
= 7 Hz, Chl-18-CH3), 1.68/1.67 (3H, t, J = 8 Hz, Chl-81-CH3), 1.16/1.13, 1.00./0.94 (each 3H, s,
In-3-CH3); MS (FAB) found: m/z = 949. Calcd for C53H53N6O7Zn: MH+, 949.
3. Results and discussion
3.1. Synthesis of spiropyran-appended zinc chlorins
Spiropyran-appended zinc chlorins were prepared from methyl pyropheophorbide-a which
was obtained by modification of natural chlorophyll-a (Scheme 1). The 3-vinyl group was

oxidized to a formyl group, and the 3-formyl group was transferred to a 3-methoxymethyl group
by reduction and acid-catalyzed condensation with methanol. The 17-propionic methyl esters of
the 3-formyl and 3-methoxymethyl chlorins were hydrolyzed to carboxylic acids, and esterified
with alcoholic spiropyran 1 to afford the corresponding spiropyran-appended chlorins.
Reduction of the 3-formylchlorin was performed, and the 3-hydroxymethyl- (2) and
3-methoxymethyl (4) chlorins were zinc metalized to give zinc chlorin–spiropyran dyads Zn2
and Zn4, respectively. The obtained dyads were mixtures of stereoisomers with different
configuration at the chiral 2’-position of spiropyran moiety. The dyads have additional chiral
carbons at the 17- and 18-positions (17S and 18S) of the chlorin macrocycle, and their 1H-NMR
spectra indicated that the ratios of the diastereomers were 1/1 regardless of the substituent at the
3-position. In the following experiments, we used these dyads as their diastereomeric mixtures.
The other spiropyran-appended zinc chlorins 3 with 3-(1-hydroxyethyl) group were prepared by
non-stereoselective hydration of the 3-vinyl group. The obtained 31-hydroxy product was a 1:1
mixture of 31R and 31S epimers. The diastereomeric mixture was zinc metalated, and the 31R
and 31S diastereomers were separated by a single reversed phase HPLC run with a packed ODS
column. The epimerically pure zinc chlorins were demetalated by acid treatment, and the free
base chlorins transesterified with the spiropyran 1 followed by zinc metalation to afford the 31R
isomer (Zn3R) and the 31S isomer (Zn3S).
3.2. Spectroscopic properties of spiropyran-appended (zinc) chlorins
UV-Visible absorption spectra of the closed form of zinc chlorin–spiropyran dyad Zn2c in
methanol showed 653- and 427-nm maxima of Qy and Soret bands, respectively (dashed line of
Fig. 3A). These bands are identical to those of the corresponding zinc 3-hydroxymethyl-chlorin.
An additional weak absorption band at around 330 nm appeared in the spectrum due to the

contribution of closed form of spiropyran moiety. When the methanol solution of Zn2c was
irradiated by UV light (352 nm) for 3 min, a new absorption band appeared at around 500–600
nm with a little decrease in absorption band at around 330 nm (solid line of Fig. 3A). The
spectral change is ascribed to the ring-opening reaction of the spiropyran moiety to afford
merocyanine appended zinc chlorin, Zn2o. The formation of merocyanine chromophore
accompanied a slight decrease in Qy absorption band of the zinc chlorin, suggesting that a little
interaction between the two chromophoric units appeared in the dyad. The merocyanine moiety
of Zn2o was re-isomerized to the original spiropyran form Zn2c by irradiation of visible light
(540 nm) for 15 min, and the initial absorption spectrum of Zn2c was completely recovered. A
relatively slow thermal ring closing reaction of the merocyanine form Zn2o to the spiro form
was also observed in about 1 h in the dark at room temperature. The other (zinc)
chlorin–spiropyran dyads Zn3, Zn4 and 2–4 showed the similar reversible isomerization of the
spiropyran moiety, i.e. the photochromic isomerization of spiropyran was not disturbed by the
covalently linked (zinc) chlorin moiety.
Fluorescence emission spectra of the dyads Zn2c and Zn2o were measured upon
excitation at 430 nm where the zinc chlorin chromophore was selectively excited. The corrected
fluorescence spectrum of Zn2c measured in methanol showed a strong emission band at 658 nm
(dashed line of Fig. 3B). The shape and amplitude of the spectrum is essentially identical to
those of the fluorescence spectrum of methyl ester form of zinc 3-hydroxymethyl-chlorin
(R=CH3 in the right drawing of Fig. 1) solution with the same concentration. Therefore, the
covalently connected spiropyran moiety in Zn2c did not disturb the fluorescence emission of
the zinc chlorin moiety. The Zn2c solution was UV-irradiated to convert the Zn2o solution, and
fluorescence spectrum was recorded with 430-nm excitation (solid line of Fig. 3B). A
fluorescence emission band of the merocyanine chromophore might be expected at around 640

nm, and the amplitude of the merocyanine emission was too small to be found in the spectrum.
The fluorescence emission band of zinc chlorin moiety similarly appeared at 658 nm, but its
intensity decreased to 45% of the Zn2c emission. The strong fluorescence emission of zinc
chlorin moiety was recovered, when the opened merocyanine form was converted to the closed
spiropyran form by visible light irradiation. These results showed that the opened merocyanine
form quenched the singlet excited state of the zinc chlorin chromophore. The similar
photoswitchable fluorescence quenching of a tetrapyrrolic chromophore was reported in
spiropyran-appended porphyrins. The mechanism of quenching is considered to be singlet
energy transfer from the photo-excited porphyrin to merocyanine moiety [36]. In our zinc
chlorin–merocyanine dyad, quenching of zinc chlorin singlet excited state (em = 658 nm) by
opened merocyanine moiety (abs = 530 nm) is thermodynamically unfavorable. Therefore, the
quenching might be assigned to other photochemical pathways such as intramolecular electron
transfer.
3.3. Self-aggregation of spiropyran-appended zinc 31-hydroxychlorins
Self-aggregates of the chlorophyllous pigments were prepared by dilution of Zn2–4
solutions with a large volume of the poor solvent. The preparation method has been widely used
in making in vitro aggregate of BChls-c and their synthetic analogues. The zinc
chlorin–spiropyran dyads were well soluble in polar solvents such as methanol, tetrahydrofuran,
and DCM, and less soluble in non-polar organic solvents such as hexane and cyclohexane. A
DCM solution of Zn2c (closed form) was diluted with a large volume of hexane to prepare the
self-aggregate of the chlorophyll derivative. Visible absorption spectrum of the aggregated
Zn2c in 30% DCM / hexane showed a Qy absorption band at 724 nm with a weak absorption
band of the remaining monomeric zinc chlorin at 647 nm (dashed line of Fig. 4A). The ratio of

the absorption intensities was A724 / A650 = 3.19, and most of the zinc chlorin Zn2c molecules
self-aggregated to form oligomer in the solvent. The aggregate of Zn2c provided intense
inverse-S type CD signals at the red-shifted wavelength. These spectroscopic features are
characteristic of formation of chlorosome-type exiton-coupled J-aggregate. The freebase
chlorin–spiropyran dyad 2c did not show any red-shifted absorption band in the same solvent,
and formation of the chlorosome-type J-aggregate required in central zinc in the chlorin
chromophore, which contributes an intermolecular coordination bond with the oxygen at the
31-position of the neighboring molecule. The self-aggregative Zn2c was converted to the
merocyanine form Zn2o by UV-irradiation in DCM, and the solution was diluted with hexane.
The resulting solution gave Qy absorption bands at 650 and 722 nm concomitant with the
absorption band of the merocyanine chromophore at around 580 nm (solid line of Fig. 4A). The
ratio of the absorption band intensities of the monomeric and aggregated zinc chlorin apparently
decreased in Zn2o (A722 / A650 = 0.60). Similarly, the intensities of CD signals decreased in
Zn2o solution (solid line of Fig. 4D). The merocyanine-appended chlorophyllous pigment Zn2o
aggregated less than Zn2c possessing the hydrophobic spiropyran moiety. The polar
merocyanine moiety at the 17-position of Zn2o might prevent the formation of the
chlorosome-type aggregate in which the side chain at the 17-position oriented toward outside of
the assembly. The miscibility of the chlorophyllous tail in non-polar organic media was changed
by photoisomerization of spiropyran moiety, which affected the self-aggregation behavior of
chlorophyllous pigment.
A similar photoisomerization effect on self-aggregation of zinc chlorins was found in zinc
chlorins Zn3R and Zn3S with a chiral 3-(1-hydroxyethyl) group, which is found in naturally
occurring chlorosomal BChls (Figs. 4B, 4C, 4E and 4F). Previous in vitro aggregation studies
showed that the absolute configuration at the chiral 3-(1-hydroxyethyl) group affect the
structure of the chlorosomal self-aggregates [10]. The closed spiropyran-appended zinc

(31R)-3-(1-hydroxyethyl)-chlorin Zn3Rc showed a red-shifted band at 706 nm with a small
band of monomeric form at 647 nm. The ratio of the oligomeric and monomeric bands A706
/
A647 was 1.42, which was larger than that found in the 31S isomer Zn3Sc (A702 / A647 = 0.78).
The 31R isomers self-aggregate more easily than the corresponding 31S isomers. However, these
oligomer / monomer ratios in Zn3c were smaller than that of zinc 3-hydroxymethyl-chlorin
Zn2c. These observations are similar as self-aggregation of BChl-c and their analogues. The
31-methyl group in Zn3 slightly disturbed the hydrogen bond and coordination bond of the
neighboring 31-hydroxy group, and the configuration of the methyl group might affect the
organization of the chlorophyllous assemblies. The merocyanine form of the zinc
3-(1-hydroxyethyl)-chlorins Zn3o hardly self-aggregated without showing intense CD signals at
around 700 nm. The less aggregative Zn3 showed a larger photoisomerization effect in the
pigment aggregation.
Photoisomerization of the spiropyran moiety was examined in the aggregate solutions.
When UV light was irradiated to the aggregated Zn2c prepared in 30% DCM / hexane, a new
absorption band of the merocyanine form appeared at around 580 nm. However, the absorption
band of aggregated zinc chlorin chromophore at 722 nm was not affected by the irradiation.
Therefore, the photoisomerization of the spiropyran to merocyanine in the dyads did not change
the suprastructure of the aggregated zinc chlorin. The intermolecular interaction in aggregated
zinc chlorin molecules, possibly C=O···H-O···Zn bonding as well as - interaction, would be
enough strong to maintain the suprastructure of the Zn2o aggregate. In addition, visible light
irradiation to the zinc chlorin-merocyanine form Zn2o in 30% DCM / hexane did not show any
change in absorption spectrum except decreasing the 580 nm band of the merocyanine form.
Similar results were obtained in the spiropyran-appended zinc 3-(1-hydroxyethyl)chlorins Zn3
aggregates. The suprastructures of the already prepared zinc 31-hydroxy-chlorin aggregate were
not perturbed by photoisomerization of spiropyran tail.

3.4. Self-aggregation of spiropyran-appended zinc 31-methoxychlorin
Zinc chlorins possessing a 31-methoxy group can form aggregate with red-shifted
absorption band [11,23]. The oxygen at the 31-methoxy group would coordinate to the central
zinc of another molecule, which contributes to organizing the J-aggregate. Because the modified
chlorophyllous pigments miss the 31-hydroxy group for the intermolecular hydrogen bond, the
zinc 31-methoxy-chlorins would associate loosely compared with the corresponding
31-hydroxy-chlorins. The spiropyran-appended zinc 3-methoxymethyl-chlorin Zn4c was
dispersed in 5% DCM / hexane. The solution showed 419- and 645-nm bands characteristic of
monomeric zinc chlorin (Fig. 5A). The UV-irradiation to the monomeric Zn4c solution induced
red-shift in the absorption spectrum. New absorption bands appeared at 435 and 668 nm
concomitant with a broad absorption band at around 580 nm of the merocyanine form of Zn4o
(Fig. 5B). These red-shifted bands of zinc chlorin moiety might be ascribable to formation of a
small aggregate such as dimer. Moreover, the red-shifted absorption bands of the dimeric form
disappeared by visible-light irradiation (Fig. 5C). The dimerization of 31-methoxy chlorophyll
derivative Zn4 was reversibly controlled by photoirradiation, which was unambiguously caused
by photoisomerization of the spiropyran tail. The formation of polar merocyanine form
destabilized the monomeric zinc chlorin in the non-polar solvent, 5% DCM / hexane. The
miscibility change in the chlorophyllous tail could switch on and off the aggregation behavior of
weakly bound zinc 31-methoxy-chlorin.
4. Conclusion
Chlorophyll derivatives with covalently linked photochromic spiropyran showed

photoswichable properties. The spiropyran moiety was reversibly isomerized in the dyads
without any perturbation of the connected chlorophyllous pigment. Therefore, the ground and
excited state of the closed spiropyran and opened merocyanine forms were not affected by the
photosynthetic chromophores. The freely isomerizable spiropyran unit in the dyad provided
unique aggregation properties of the chlorophyllous dyads. The hydrophobic closed spiropyran
tail in the chlorophyllous dyads was favorable for the aggregation of chlorosomal chlorophylls
possessing a hydrophobic alkyl chain at the same position. Tunable self-aggregation activity of
the chlorosomal BChl models was achieved by using the photoswichable polarity change of the
appended spiropyran group. In particular, self-aggregation of weakly associated zinc
31-methoxychlorin molecules could be controlled by the photochromic group.
References
[1] B. Grimm, R. J. Porra, W. Rüdiger, H. Scheer, Eds., Chlorophylls and
Bacteriochlorophylls, Springer, Dortrecht, 2006.
[2] R. E. Blankenship, Molecular Mechanisms of Photosynthesis, Wiley-blackwell, Oxford,
2002.
[3] G. S. Orf, R. E. Blankenship, Chlorosome antenna complexes from green photosynthetic
bacteria, Photosynth. Res. 116 (2013) 315–331.
[4] G. T. Oostergetel, H. van Amerongen, E. J. Boekema, The chlorosome: a prototype for
efficient light harvesting in photosynthesis, Photosynth. Res. 104 (2010) 245–255.
[5] J. Harada, T. Mizoguchi, Y. Tsukatani, M. Noguchi, H. Tamiaki, A seventh bacterial

chlorophyll driving a large light-harvesting antenna, Sci. Rep. 2 (2012) 671–675.
[6] S. Ganapathy, G. T. Oostergetel, P. K. Wawrzyniak, M. Reus, A. Gomez Maqueo Chew,
F. Buda, E. J. Boekema, D. A. Bryant, A. R. Holzwarth, H. J. M. de Groot, Alternating
syn-anti bacteriochlorophylls form concentric helical nanotubes in chlorosomes, Proc.
Natl. Acad. Sci. U. S. A. 106 (2009) 8525–8530.
[7] J. Chiefari, K. Griebenow, N. Griebenow, T. S. Balaban, A. R. Holzwarth, K. Schaffner, J.
Phys. Chem. 99 (1995) 1357–1365.
[8] H. Tamiaki, M. Amakawa, Y. Shimono, R. Tanikaga, A. R. Holzwarth, K. Schaffner,
Synthetic zinc and magnesium chlorin aggregates as models for supramolecular antenna
complexes in chlorosomes of green photosynthetic bacteria, Photochem. Photobiol. 63
(1996) 92–99.
[9] T. Miyatake, H. Tamiaki, Self-aggregate of natural chlorophylls and their synthetic
analogues in aqueous media for making light-harvesting systems, Coord. Chem. Rev. 254
(2010) 2593–2602.
[10] T. Miyatake, H. Tamiaki, Self-aggregates of bacteriochlorophylls-c, d and e in a
light-harvesting antenna system of green photosynthetic bacteria: Effect of
stereochemistry at the chiral 3-(1-hydroxyethyl) group on the supramolecular
arrangement of chlorophyllous pigments, J. Photochem. Photobiol. C 6 (2005) 89–107.
[11] H. Tamiaki, M. Kuno, M. Ohata, Self-aggregation of synthetic zinc chlorophyll
derivatives possessing 31-hydroxy or methoxy group and 131-mono- or dicyanomethylene
moiety in nonpolar organic solvents as models of chlorosomal bacteriochlorophyll-d
aggregates, Photochem. Photobiol. 90 (2014) 1277–1286.
[12] T. Miyatake, H. Tamiaki, A. R. Holzwarth, K. Schaffner, Self-assembly of synthetic zinc
chlorins in aqueous microheterogeneous media to an artificial supramolecular
light-harvesting device, Helv. Chim. Acta 82 (1999) 797–810.

[13] V. I. Prokhorenko, A. R. Holzwarth, M. G. Müller, K. Schaffner, T. Miyatake, H.
Tamiaki, Energy transfer in supramolecular artificial antennae units of synthetic zinc
chlorins and co-aggregated energy traps. A time-resolved fluorescence study, J. Phys.
Chem. B 106 (2002) 5761–5768.
[14] H. Tamiaki, S. Miyata, Y. Kureishi, R. Tanikaga, Aggregation of synthetic zinc chlorins
with several esterified alkyl chains as models of bacteriochlorophyll-c homologs,
Tetrahedron 52 (1996) 12421–12432.
[15] H. Tamiaki, R. Shibata, T. Mizoguchi, The 17-propionate function of
(bacterio)chlorophylls: Biological implication of their long esterifying chains in
photosynthetic systems, Photochem. Photobiol. 83 (2007) 153–162.
[16] S. Shoji, T. Hashishin, H. Tamiaki, Construction of chlorosomal rod self-aggregates in
the solid state on any substrates from synthetic chlorophyll derivatives possessing an
oligomethylene chain at the 17-propionate residue, Chem. Eur. J. 18 (2012)
13331–13341.
[17] N. Takahashi, H. Tamiaki, Y. Saga, Synthesis and self-assembly of amphiphilic zinc
chlorophyll derivatives possessing a crown ether at the 17-propionate residue,
Tetrahedron 69 (2013) 3638–3645.
[18] R. Nishimori, T. Mizoguchi, H. Tamiaki, S. Kashimura, Y. Saga, Biosynthesis of
unnatural bacteriochlorophyll c derivatives esterified with ,-diols in the green sulfur
photosynthetic bacterium Chlorobaculum tepidum, Biochemistry 50 (2011) 7756–7764.
[19] A. R. Holzwarth, K. Schaffner, On the structure of bacteriochlorophyll molecular
aggregates in the chlorosomes of green bacteria. A molecular modelling study,
Photosynth. Res. 41 (1994) 225–233.
[20] V. I. Prokhorenko, D B. Steensgaard, A. R. Holzwarth, H. J. M. de Groot, Exciton
dynamics in the chlorosomal antennae of the green bacteria Chloroflexus aurantiacus and

Chlorobium tepidum, Biophys. J. 79 (2000) 2105–2020.
[21] S. Valleau, S. K. Saikin, D. Ansari-Ohhol-Beig, M. Rostami, H. Mossallaei, A.
Aspuru-Guzik, Electromagnetic study of the chlorosome antenna complex of Chlorobium
tepidum, Nano Lett. 8 (2014) 3884–3894.
[22] B.-J. van Rossum, D. B. Steensgaard, F. M. Mulder, G. J. Boender, K. Schaffner, A. R.
Holzwarth, H. J. M. de Groot, A refined model of the chlorosomal antennae of the green
bacterium Chlorobium tepidum from proton chemical shift constraints obtained with
high-field 2-D and 3-D MAS NMR dipolar correlation spectroscopy, Biochemistry, 40
(2001) 1587–1595.
[23] T. Miyatake, S. Tanigawa, S. Kato, H. Tamiaki, Aqueous self-aggregates of amphiphilic
zinc 31-hydroxy- and 31-methoxy-chlorins for supramolecular light-harvesting systems,
Tetrahedron Lett. 48 (2007) 2251–2254.
[24] H. M. D. Bandara, S. C. Burdette, Photoisomerization in different classes of azobenzene,
Chem. Soc. Rev. 41 (2012) 1809–1825.
[25] M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Photochromism of diarylethene
molecules and crystals: Memories, switches, and actuators. Chem. Rev. 114 (2014)
12174–12277.
[26] R. Klajn, Spiropyran-based dynamic materials, Chem. Soc. Rev. 43 (2014) 148–184.
[27] H. Dürr, H. Bouas-Laurent, Photochromism: Molecules and systems, Elsevier,
Amsterdam (2003).
[28] S. Yagai, A. Kitamura, Recent advances in photoresponsive supramolecular
self-assemblies, Chem. Soc. Rev. 37 (2008) 1520–1529.
[29] F. Ercole, T. P. Davis, R. A. Evans, Photo-responsive systems and biomaterials:
photochromic polymers, light-triggered self-assembly, surface modification, fluorescence
modulation and beyond, Polym. Chem. 1 (2010) 37–54.

[30] D. Wang, X. Wang, Amphiphilic azo polymers: molecular engineering, self-assembly and
photoresponsive properties, Prog. Polym. Sci. 38 (2013) 271–301.
[31] L. Ma, J. Li, D. Han, H. Geng, G. Chen, Q. Li, Synthesis of photoresponsive
spiropyran-based hybrid polymers and controllable light-triggered self-assembly study in
toluene, Macromol. Chem. Phys. 214 (2013) 716–725.
[32] D. S. Achilleos, T. A. Hatton, M. Vamvakaki, Light-regulated supramolecular
engineering of polymeric nanocapsules, J. Am. Chem. Soc. 134 (2012) 5726–5729.
[33] M. Tamasulo, S. L. Kaanumal, S. Sortino, F. M. Raymo, Synthesis and properties of
benzophenone–spiropyran and naphthalene–spiropyran conjugates, J. Org. Chem. 72
(2007) 595–605.
[34] X. Guo, D. Zhang, Y. Zhou, D. Zhu, Synthesis and spectral investigations of a new dyad
with spiropyran and fluorescein units: Toward information processing at the single
molecular level, J. Org. Chem. 68 (2003) 5681–5687.
[35] L. Kong, H.-L. Wong, A. Y.-T. Tam, W. H. Lam, L. Wu, V. W.-W. Yam, Synthesis,
characterization, and photophysical properties of Bodipy–spirooxiazine and –spyropyran
conjugates: Modulation of fluorescence resonance energy transfer behavior via
acidochromic and photochromic switching, Appl. Mater. Interfaces 6 (2014) 1550–1562.
[36] J. L. Bahr, G. Kodis, L. de la Garza, S. Lin, A. L. Moore, T. A. Moore, D. Gust,
Photoswitched singlet energry transfer in a porphyrin–spiropyran dyad, J. Am. Chem. Soc.
123 (2001) 7124–7133.
[37] D. E. Williams, J. A. Rietman, J. M. Maier, R. Tan, A. B. Greytak, M. D. Smith, J. A.
Krause, N. B. Shustva, Energy transfer on demand: Photoswitch-directed behavior of
metal–porphyrin frameworks, J. Am. Chem. Soc. 136 (2014) 11886–11889.
[38] Y. Shiraishi, E. Shirakawa, K. Tanaka, H. Sakamoto, S. Ichikawa, T. Hirai,
Spiropyran-modified gold nanoparticles: Reversible size control of aggregates by UV and

Visible light irradiations, Appl. Mater. Interfaces 6 (2014) 7554–7562.
[39] R. Rosario, D. Gust, M. Hayes, F. Jahnke, J. Springer, A. A. Garcia, Photon-modulated
wettability changes on spiropyran-coated surfaces, Langmuir 18 (2002) 8062–8069.
[40] M. Zhang, X. Hou, J. Wang, Y. Tian, X. Fan, J. Zhai, L. Jiang, Light and pH cooperative
nanofluidic diode using a spiropyran-functionalized single nanochannnel, Adv. Mater. 24
(2012) 2414–2428.
[41] S. Scarmagnani, Z. Walsh, C. Slater, N. Alhashimy, B. Paull, M. Macka, D. Diamond,
Polystyrene bead-based system for optical sensing using spiropyran photoswitches, J.
Mater. Chem. 18 (2008) 5063–5071.
[42] K. Sumaru, T. Takagi, T. Satoh, T. Kanamori, Photo-induced reversible proton
dissociation of spirobenzopyran in aqueous solutions, J. Photochem. Photobiol. A: Chem.
261 (2013) 46–52.
[43] J. Andersson, S. Li, P. Lincoln, J. Andréasson, Photoswitched DNA-binding of a
photochromic spiropyran, J. Am. Chem. Soc. 130 (2008) 11836–11837.
[44] H. Tamiaki, S. Machida, K. Mizutani, Modification of 3-substituents in
(bacterio)chlorophyll derivatives to prepare 3-ethylated, methylated, and unsubstituted
(nickel) pyropheophorbides and their optical properties, J. Org. Chem. 77 (2012)
4751–4758.
[45] H. Tamiaki, N. Ariki, H. Sugiyama, Y. Taira, Y. Kinoshita, T. Miyatake, Synthesis of
3,20-disubstituted chlorophyll-a derivatives and reactivity of the substituents, Tetrahedron
69 (2013) 8412–8421.

Figure Captions
Scheme 1. Synthesis of spiropyran-appended zinc chlorins: (i) OsO4, NaIO4, HOAc / THF,
H2O; (ii) tBuNH2·BH3 / CH2Cl2; (iii) MeOH / H2SO4 (50°C); (iv) conc. HCl; (v) 1, EDC·HCl,
DMAP / CH2Cl2; (vi) Zn(OAc)2·2H2O / MeOH, CH2Cl2; (vii) HBr / HOAc (60°C), H2O, MeOH
/ H2SO4; (vii) reversed-phase HPLC run with ODS packed column, 2% aq. HCl; (viii) 1,
(nBuClSn)2O / toluene, reflux.

Fig. 1. Structure of bacteriochlorophylls-c, d, e, f and zinc chlorin models.
Fig. 2. Photoisomerization of spiropyran 1.
Fig. 3. UV-Visible absorption spectra (A) and fluorescence emission (B: exc = 430 nm) of
spiropyran-appended zinc 3-hydroxymethyl-chlorin Zn2 before (dashed line) and after (solid
line) UV irradiation at 352 nm in methanol.
Fig. 4. UV-Visible absorption (A–C) and CD (D–F) spectra of spiropyran-appended zinc
chlorins Zn2 (A, D), Zn3R (B, E) and Zn3S (C, F) in DCM / hexane (Zn2: 3 / 7, Zn3R and
Zn3S: 1 / 4; v / v); dashed lines: closed spiropyran form; solid lines: open merocyanine form.
Fig. 5. UV-Visible absorption spectra of spiropyran-appended zinc chlorin Zn4 in DCM /
hexane (1 / 18; v / v). (A) initial state, (B) after UV irradiation, and (C) after VIS irradiation of
the aggregated Zn4.

Scheme 1

HO
31
R7
R3
R8
N
N
N
N
N
N
N
R20
Mg
Zn
N
R12
17
17
O
O
O
O
OR
OR
BChl-c: R7=CH3, R20=CH3
BChl-d: R7=CH3, R20=H
BChl-e: R7=CHO, R20=CH3
BChl-f: R7=CHO, R20=H
R8=CH2CH3, CH2CH2CH3, etc.,
R12=CH3, CH2CH3, etc.,
R=farnesyl, stearyl, etc.
Zinc Chlorin Models:
R3=CH2OH, CH(OH)CH3,
CH2OCH3,etc.,
R=CH3, (CH2)nH,
(CH2CH2O)nH, etc.
Fig. 1
Fig. 2

1.0
0.5
0.0
(A)
(B)
0.5
0.0
300 400 500 600 700 800
Wavelength / nm
550 600 650 700 750 800
Wavelength / nm
Fig. 3
(C)
(B)
(E)
(A)
(D)
0.5
0.0
0.5
0.5
0.0
0.0
10
20
0
(F)
20
0
-10
-20
0
-20
-40
-20
-40
-60
300 400 500 600 700 800 300 400 500 600 700 800 300 400 500 600 700 800
Wavelength / nm
Wavelength / nm
Wavelength / nm
Fig. 4