Syntheses of Chalcone-Type Chlorophyll Derivatives Possessing a Bacteriochlorin, Chlorin or Porphyrin π-System and Their Optical Properties

Article abstract of DOI:10.1111/php.13044

C3-(Trans-2-arylethenyl)carbonylated chlorophyll derivatives possessing a bacteriochlorin or chlorin π-system were synthesized by cross-aldol (Claisen–Schmidt) condensation of methyl pyrobacteriopheophorbide-a or 3-acetyl-3-devinyl-pyropheophorbide-a bearing the C3-acetyl group with p-(un)substituted benzaldehydes under basic conditions. The corresponding porphyrin-type chlorophyll derivatives were prepared by the oxidation (17,18-didehydrogenation) of the chlorin-type. Their Qy absorption and fluorescence emission maxima in dichloromethane correlated well with Hammett substituent constants of the p-substituents. Several electron-withdrawing p-substituents suppressed the emission due to photoinduced electron transfer quenching in a molecule. The substitution sensitivities for their maxima and fluorescence quantum yields decreased in the order of bacteriochlorin-, chlorin- and porphyrin-type derivatives.

Full text of DOI:10.1111/php.13044

Article type
: Research Article
Syntheses of Chalcone-Type Chlorophyll Derivatives Possessing a
Bacteriochlorin, Chlorin, or Porphyrin π-System and Their Optical
Properties
Kota Ohashi, Yusuke Kinoshita and Hitoshi Tamiaki*
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, Japan
_
*
_____________________________________
Corresponding author email: tamiaki@fc.ritsumei.ac.jp (Hitoshi Tamiaki)
ABSTRACT
C3-(Trans-arylethenyl)carbonylated chlorophyll derivatives possessing a bacteriochlorin or
chlorin π-system were synthesized by cross-aldol (Claisen-Schmidt) condensation of methyl
pyrobacteriopheophorbide-a or 3-acetyl-3-devinyl-pyropheophorbide-a bearing the C3-acetyl
group with p-(un)substituted benzaldehydes under basic conditions. The corresponding
porphyrin-type chlorophyll derivatives were prepared by the oxidation
(17,18-didehydrogenation) of the chlorin-type. Their Qy absorption and fluorescence emission
maxima in dichloromethane correlated well with Hammett substituent constants of the
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p-substituents. Several electron-withdrawing p-substituents suppressed the emission due to
photoinduced electron transfer quenching in a molecule. The substitution sensitivities for their
maxima and fluorescence quantum yields decreased in the order of bacteriochlorin-, chlorin-,
and porphyrin-type derivatives.
INTRODUCTION
Photosynthesis is one of the most efficient energy conversion systems in nature.
(Bacterio)chlorophylls [(B)Chls] play significant roles such as light absorption, energy
transfer, and charge separation in the initial stage of the photosynthetic processes (1,2).
Naturally occurring (B)Chls are cyclic tetrapyrrole pigments (3,4). They have various
substituents at their peripheral positions and different fundamental π-systems, which largely
affect their optical properties in the monomeric forms (5). In fact, Chls-a/b occurred in green
plants and prochlorophytes possess a chlorin π-system (trans-17,18-dihydroporphyrin), while
7
they have CH /CHO groups at the C7-position (R in Fig. 1 left). The C7-substituents impact
3
their singlet excited energy levels known as site energies, which causes an efficient excitation
energy transfer from Chl-b to Chl-a in the photosynthetic light-harvesting antennas (6).
Furthermore, Chl-c /c observed in chromophytes have a porphyrin π-skeleton (Fig. 1 middle).
1
2
They are utilized as accessory pigments like carotenoids in their photosynthetic
light-harvesting antennas, and transfer excitation energy to Chl-a (6). BChl-a found in purple
and green photosynthetic bacteria is a bacteriochlorin (trans-7,8-dihydrochlorin) with an
electron-withdrawing acetyl group at the C3-position (Fig. 1 right), which yields a largely
red-shifted Qy maximum at 771 nm in diethyl ether compared with Chl-a (661 nm). Therefore,
BChl-a is able to absorb light at a near-infrared region (7). As mentioned above, (an)oxygenic
phototrophs utilize three types of (B)Chl pigments depending on the illuminated light in their
cultivation environments.
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Investigating the correlation between chemical structures of natural (B)Chl pigments and
their optical properties is of considerable interest in the field of photobiological chemistry.
Several functional groups are available for peripheral substituents of natural (B)Chls (3,8):
3
-acetyl, 3-vinyl, 7-formyl, 8-vinyl, 13-carbonyl, 17-acrylate groups, and so on. A lot of
(B)Chl derivatives possessing a variety of substituents at their peripheral positions have been
synthesized by the chemical modification of natural (B)Chls to understand the substitution
effects on their optical properties based on several factors (5,9,10): the modification position,
π-conjugation expansion, and electronic/steric effects. However, a limited number of reports
are available for the substitution effects on (B)Chl derivatives possessing different
fundamental π-systems (bacteriochlorin, chlorin, and porphyrin) (5,11−14).
Here, we prepared C3-(trans-2-arylethenyl)carbonylated (B)Chl derivatives possessing a
bacteriochlorin 1a−f, chlorin 2a−f, or porphyrin π-system 3a−f via cross-aldol
(Claisen-Schmidt) condensation (Scheme 1). We compare optical properties between
chlorin-type Chl derivatives 2a−f and their structural isomers 4a−f, and the substitution effect
on three types of synthetic chalcone-(B)Chls 1/2/3a−f.
MATERIALS AND METHODS
Apparatus. Electronic absorption spectra were measured with a Hitachi U-3500
spectrophotometer. Fluorescence emission spectra and quantum yields were obtained with a
1
19
Hamamatsu Photonics C9920-03G spectrometer. H-NMR and F-NMR spectra were
measured with a JEOL AL-400 (400 and 376 MHz) spectrometer; chemical shifts (δs) are
expressed in parts per million relative to the residual CHCl (δ = 7.26 ppm) or CDHCl (δ =
3
H
2
H
5
.32 ppm) as an internal reference, and C F (δ = −164.90 ppm) as an external reference. High
6 6 F
resolution mass spectra (HRMS) were recorded on a Bruker micrOTOF II spectrometer: a
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time-of-flight analyzer, atmospheric pressure chemical ionization (APCI), and positive mode
in an acetonitrile solution.
Materials. Commercially available organic solvents and reagents were used without
further purification. Acetonitrile, dichloromethane, N,N-dimethylformamide (DMF),
tetrahydrofuran (THF), and toluene were purchased from Nacalai Tesque (spectroscopy
grade). Thin layer chromatography (TLC) and flash column chromatography (FCC) were
performed with silica gel (Kieselgel 60, 40−63 μm, 230−400 mesh).
Methyl pyrobacteriopheophorbide-a (1g) (12), methyl
3
3
-acetyl-3-devinyl-pyropheophorbide-a (2g) (9,13), methyl
-acetyl-3-devinyl-protopyropheophorbide-a (3g) (12), and methyl pyropheophorbide-a (4g)
(9,15) were prepared according to reported procedures.
Synthetic procedures of chalcone-bacteriochlorins/chlorins/porphyrins are described
below, and their spectral data are available in the Supporting Information file. All the reactions
were done under N in the dark and monitored by TLC and visible (Vis) absorption
2
spectroscopy.
Cross-aldol (Claisen-Schmidt) condensation for the synthesis of 1a−c and 2a−c.
3
-Acetyl-bacteriochlorin/chlorin 1/2g (10.2/10.9 mg, 18/19 μmol) and benz-, p-anis-, or
p-tolualdehyde (12/10 mmol) were dissolved in a 1:1 mixture of methanol and chloroform (16
ml), to which 28% sodium methoxide in methanol (1/0.5 ml, 5/3 mmol) was added. The
reaction mixture was refluxed for 1 h. After cooling down to room temperature, the reaction
mixture was diluted with CH Cl , washed with aq. 2% HCl, aq. sat. NaHCO , and distilled
2
2
3
water, dried over Na SO , and filtered. All the solvents were evaporated and the residue was
2
4
purified by FCC (6−9% Et O−CH Cl ) and recrystallization (CH Cl −hexane) to give the
2
2
2
2
2
corresponding 3-[trans-2-(phenyl, p-anisyl, or
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p-tolyl)ethenyl]carbonyl-bacteriochlorin/chlorin 1/2a, 1/2b, or 1/2c. Isolated yields were
6
6/30% for 1/2a (R = H), 53/59% for 1/2b (R = OCH ), and 47/33% for 1/2c (R = CH ).
3
3
Cross-aldol (Claisen-Schmidt) condensation for 1d−f and 2d−f.
3
-Acetyl-bacteriochlorin/chlorin 1/2g (10.9/10.7 mg, 19 μmol) and p-trifluoromethyl-,
p-cyano-, or p-nitrobenzaldehyde (3/6 mmol) were dissolved in a 1:1 mixture of methanol and
chloroform (16 ml), to which 28% sodium methoxide in methanol (1/0.5 ml, 5/3 mmol) was
added. The reaction mixture was stirred in an ice bath for 1 h. The same work-up as mentioned
above afforded the corresponding 3-[trans-2-(p-trifluoromethyl-, p-cyano-, or
p-nitro)phenylethenyl]carbonyl-bacteriochlorin/chlorin 1/2d, 1/2e, or 1/2f. Isolated yields
were 14/27% for 1/2d (R = CF ), 11/10% for 1/2e (R = CN), and 25/13% for 1/2f (R = NO ).
3
2
DDQ oxidation of chlorins 2a−f to porphyrins 3a−f. According to reported procedures
12), zinc metallation of chlorins 2a−f, 17,18-oxidation by 1.2 equivalent of
(
2
,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in dry acetone for a few minutes, and
demetallation by treatment with 10% (v/v) sulfuric acid in methanol gave the corresponding
porphyrins 3a−f.
RESULTS AND DISCUSSION
Synthesis of chalcone-type bacteriochlorins/chlorins/porphyrins 1/2/3a−f
Methyl pyrobacteriopheophorbide-a (1g) bearing an acetyl group at the C3-position and a
bacteriochlorin π-skeleton was semi-synthesized by modifying natural BChl-a extracted from
Rhodobacter sphaeroides as previously reported [steps (i) and (ii) in Scheme 1] (12).
3
-Acetyl-bacteriochlorin 1g was treated with benzaldehyde in a 1:1 mixture of methanol and
chloroform in the presence of 28% sodium methoxide in methanol under reflux for 1 h (16) to
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give 3-(trans-styryl)carbonyl-bacteriochlorin 1a as the product of cross-aldol
Claisen-Schmidt) condensation (17) in 66% isolated yield [step (iii)]. Under the above
(
refluxing conditions, ketone 1g reacted with p-anisaldehyde or p-tolualdehyde to give
3
3
-arylated 3-acryloyl-bacteriochlorin 1b (53%) or 1c (47%), respectively. In contrast, the
reaction of 1g with benzaldehyde substituted with an electron-withdrawing group at the
p-position, p-trifluoromethyl-, p-cyano-, or p-nitrobenzaldehyde, under refluxing conditions
gave no chalcone-bacteriochlorins 1d/e/f as products. Under milder conditions in an ice bath,
the desired products 1d/e/f were successfully obtained in 14/11/25% isolated yields,
respectively. Thus, the introduction of an electron-withdrawing group to the p-position of
benzaldehyde accelerated the reaction due to an increase of the electrophilicity of the formyl
group, and would lead to undesired over-reactions (not determined) at the refluxing
temperature. These observations are consistent with the previous results for Claisen-Schmidt
condensation of p-(un)substituted acetophenone with a 3-formyl-chlorin (16).
We then prepared 3-acetyl-chlorin 2g from natural Chl-a extracted from spirulina as
previously reported [steps (iv), (v), and (vi) in Scheme 1] (9). The reactions of 2g with
p-(un)substituted benzaldehydes afforded chalcone-chlorins 2a−f (10−59% isolated yields)
[
step (iii)] in a similar manner for the synthesis of 1a−f.
To obtain chalcone-porphyrins 3a−f bearing a porphyrin π-system, the following
pathways were examined. The chlorin ring of 2g was oxidized through zinc metallation, DDQ
oxidation, and acidic demetallation, to give 3-acetyl-porphyrin 3g [step (vii) in Scheme 1] (12).
Cross-aldol condensation of 3-acetyl-porphyrin 3g with benzaldehyde was performed in a
similar manner for the synthesis of 1/2a [step (iii)]. Unfortunately, the isolation of 3a from the
reaction mixture by silica gel chromatography was very difficult due to its broadening on the
gel. Chalcone-porphyrins 3a−f were successfully synthesized by the oxidation of
chalcone-chlorins 2a−f as mentioned above [step (vii)]. All the synthetic compounds 1/2/3a−f
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1
were fully characterized by their Vis, H-NMR, HRMS spectra (see Supporting Information).
3
2
3
The coupling constant J(3 H−3 H) of 16 Hz in the olefin moiety at the C3-position indicates
that the products were trans-isomers.
UV-Vis absorption spectra of chalcone-chlorins 2a−f and their structural isomers 4a−f in
dichloromethane
UV-Vis absorption spectra of 3-styrylcarbonyl-chlorin 2a and its p-substituents 2b−f were
measured in dichloromethane to investigate the substitution effects on their Qy absorption
maxima, and compared with those of their structural isomers 4a−f bearing the
3
-benzoylethenyl group with(out) p-substituents (Fig. 2) (16).
A diluted dichloromethane solution of 2a gave the intense Qy(0,0) band at 678 nm (green
line in Fig. 3). The Qy(0,0) maximum of 2a (3-COCH=CHPh-trans) in dichloromethane was
blue-shifted by 5 nm than that of 2g (3-COMe) as shown in Fig. 3. This hypsochromic shift
1
could be explained by considering the following two factors. (i) The 3 -vinyl group of 2a is
3
able to be π-conjugated with the 3 -phenyl ring and is more electronegative than a methyl
group (18), resulting in a red-shifted Qy(0,0) maximum. (ii) The larger 3-acyl group of 2a than
the 3-acetyl group of 2g induced its larger steric repulsion with the neighboring C2-CH /C5-H
3
1
and the rotation around the C3−C3 bond to give a less π-conjugated conformer between the
C3-substituent and the chlorin π-system, which resulted in a blue-shifted Qy(0,0) maximum.
The factor (ii) would be superior to the factor (i), which caused such a hypsochromic shift for
2
g → 2a. Molecular modeling calculations (19) also supported the above conformations
1
3
(
Supporting Information, Fig. S1). The dihedral angle between the 3 -CH=CH and 3 -phenyl
ring in 2a was estimated to be 2˚ and the styryl group was almost planar, while that between the
1
3
-CO and 3 -CH=CH was 24˚, giving a partial π-conjugation between the carbonyl and styryl
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1
1
groups. In addition, the dihedral angles of C2–C3–C3 –O and C4–C3–C3 –O were 14˚ in 2a
and larger than 1–2˚ in 2g. The 3-acryloyl group in 2a was rotated from the chlorin π-plane, but
the 3-acetyl group in 2g was almost coplanar to the chlorin π-skeleton. The blue shift of the
Qy(0,0) maximum is consistent with our previous works for 3-acyl-chlorins (20): 683
(
3-COMe, 2g) > 679 (3-COCH=CH ) > 675 nm (3-COCH=CHMe-trans).
2
UV-Vis absorption spectra of chalcone-chlorins 2a−f in dichloromethane were compared
with those of isomeric reverse chalcone-chlorins 4a−f. The Qy(0,0) maximum of 2a
3-COCH=CHPh-trans, green line in Fig. 3) was blue-shifted by 12 nm than that of the
(
corresponding isomer 4a (3-CH=CHCOPh-trans, pink line in Fig. 3). The blue shifts shown in
Table 1 could be caused by a steric factor. The 3-substituents of 2a−f interacted with their
neighboring C2-CH /C5-H more largely than those of 4a−f, so the former should give less
3
π-conjugated conformers with a chlorin π-system than the latter, resulting in the hypsochromic
shifts. The steric factor is more effective than the electronic factor that the more
electron-withdrawing C3-carbonyl group of 2a−f than the C3-vinyl group of 4a−f moves a
Qy(0,0) maximum to a longer wavelength: 668 (3-CH=CH , 4g) < 683 nm (3-COMe, 2g). The
2
observed blue shifts are consistent with our previous works (16,20): 675
(3-COCH=CHMe-trans) < 685 nm (3-CH=CHCOMe-trans). The estimated dihedral angles of
1
the 3-C=O with the chlorin moiety and the 3 -CH=CH in 2a were 14˚ and 24˚, respectively, and
2
larger than those of the 3-CH=CH with chlorin and 3 -C=O which were 7˚ and 12˚,
respectively. Based on the calculations, the π-conjugation of chalcone-chlorin 2a would extend
along the molecular y-axis less than that of reverse chalcone-chlorin 4a, supporting the
observed blue shifts.
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Table 1. Qy(0,0) absorption maxima λ [Qy(0,0)] (nm) of 2/4a−f in CH Cl .
abs
2
2
R
2 [3-COCH=CHPh(p-R)]
678 (2a)
4 [3-CH=CHCOPh(p-R)]
690 (4a)
H (a)
OCH (b)
676 (2b)
688 (4b)
3
CH (c)
677 (2c)
689 (4c)
3
CF (d)
681 (2d)
693 (4d)
3
CN (e)
682 (2e)
696 (4e)
NO (f)
683 (2f)
697 (4f)
2
2
We have previously reported that there was a good correlation (correlation coefficient r =
0
.959) between the Qy(0,0) maxima of 4a−f and Hammett substituent constants (σ) of the
p-substituents (R) (16). Similarly, the relationship between the Qy(0,0) maxima of 2a−f and
the σ-values (21) was investigated (Fig. 4). As expected, these two parameters showed a good
2
correlation (r = 0.995). The estimated slope (6.3) for 2a−f was smaller than the reported 8.1
3
for 4a−f. This result indicates that the electronic effect of the p-substituents on the C3 -phenyl
ring on Qy(0,0) maxima of 2a−f is less than those of 4a−f, which would be due to less
π-conjugation of 2a−f along the molecular y-axis than that of 4a−f (vide supra).
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Electronic absorption spectra of chalcone-bacteriochlorins/chlorins/porphyrins 1/2/3a−f
in dichloromethane
Diluted dichloromethane solutions of 1/2/3a showed totally different electronic absorption
spectra due to their different fundamental π-systems, to give Qy(0,0) maxima at 749/678/644
nm, respectively (Fig. 5).
3
The effects of the p-substituents in the C3 -phenyl ring on the Qy(0,0) maxima were
compared among 1/2/3a−f (Supporting Information, Table S1). Similarly as in 2a−f, good
2
correlations between the Qy(0,0) maxima and the σ-values in 1/3a−f were observed with r =
0
1
.993/0.941 (Figs. 6-7). The slopes for the estimated lines decreased in the following order:
0.9 (1a−f) > 6.3 (2a−f) > 3.5 (3a−f). This tendency should be considered as follows. The
π-electrons of a bacteriochlorin ring are more localized along the molecular y-axis than those
of a chlorin, and those of a porphyrin ring are delocalized over the π-skeleton, which may
suppress the sensitivity to the substitution effects in the above order.
Fluorescence emission data of chalcone-bacteriochlorins/chlorins/porphyrins 1/2/3a−f in
dichloromethane
Upon the excitation of 1/2/3a in dichloromethane at the intense Soret maxima (354/411/423
nm), their main emission peaks λ were observed at 768/693/652 nm, respectively (Table 2
em
−
1
and Fig. S2). The Stokes shifts ∆ of 1/2/3a were calculated to be 330/320/190 cm ,
3
respectively (Table S2). The effects of the p-substituents in the C3 -phenyl ring on the
λem-values were also examined among 1/2/3a. Except for the nitro-substituents 1/2f, there were
good correlations between λ and the σ-values (Figs. S3−S5): the reason of the deviation is
em
currently being investigated.
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Table 2. Fluorescence emission maxima λ (nm) of 1/2/3a−f in
em
CH Cl (excitation at the intense Soret maxima).
2
2
R
1
2
3
H (a)
768 (1a)
766 (1b)
768 (1c)
775 (1d)
775 (1e)
769 (1f)
693 (2a)
691 (2b)
692 (2c)
698 (2d)
700 (2e)
697 (2f)
652 (3a)
651 (3b)
652 (3c)
654 (3d)
655 (3e)
655 (3f)
OCH (b)
3
CH (c)
3
CF (d)
3
CN (e)
NO (f)
2
Fluorescence emission quantum yields Φ of 1/2/3a−f were measured in
em
dichloromethane by the absolute determination method. As shown in Table 3, the introduction
3
of strongly electron-withdrawing groups (CN and/or NO ) on the C3 -phenyl ring reduced
2
their Φ -values. Especially, the Φ of bacteriochlorin 1 was largely suppressed by the
em
em
substitution with cyano and nitro groups as in 1e and 1f (14% → 4% and 0%). The introduction
of a nitro group as in chlorin 2a → 2f similarly reduced the Φ -value (22% → 1%). A slight
em
decrease was caused by the substitution with a nitro group of porphyrin 3a to 3f: Φ = 3% →
em
2
%. The sensitivity for Φ is obeyed in the order of 1 > 2 > 3, which is consistent with the
em
above observation in the Qy(0,0) maxima.
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Table 3. Fluorescence emission quantum yields Φ (%) of 1/2/3a−f in CH Cl (excitation at
em
2
2
the intense Soret maxima).
R
1
2
3
F
H (a)
14 (1a)
14 (1b)
13 (1c)
11 (1d)
4 (1e)
0 (1f)
22 (2a)
24 (2b)
22 (2c)
19 (2d)
18 (2e)
1 (2f)
3 (3a)
3 (3b)
3 (3c)
3 (3d)
3 (3e)
2 (3f)
or
OCH (b)
3
the
CH (c)
3
furth
er
CF (d)
3
inve
stiga
tion
of
CN (e)
NO (f)
2
the decrease in the Φ -values of 1/2/3 by the substitution with strongly electron-withdrawing
em
groups, the Φ of 1/2/3a as well as those of their trifluoromethyl- 1/2/3d, cyano- 1/2/3e, and
em
nitro-substituents 1/2/3f were measured in various solvents (Table 4). Their Φ -values of
em
bacteriochlorins, chlorins, and porphyrins in toluene which is a less polar solvent than
dichloromethane were relatively close for their substituents, 13−16%, 23−27%, and 4−5%,
respectively. As the dielectric constant (ε) of the solvents increased, the distinct decreases in
the Φ -values of 1d/e/f, 2e/f, and 3f were observed: Φ (C H CH ) = 14/14/13%, 23/25%,
em
em
6
5
3
and 5% → Φ (CH CN) = 10/3/1%, 12/1%, and 2%, respectively (Table 4). These
em
3
observations are consistent with the reported data that the Φ -value of 4f bearing a nitro group
em
was dependent on the solvent polarity (16). Consequently, the decrease in the Φ -values of
em
3
1
/2/3a by the p-substitution with CF , CN, and/or NO groups on the C3 -phenyl ring would be
3
2
caused by intramolecular photoinduced electron transfer quenching.
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Table 4. Fluorescence emission quantum yields Φ (%) of 1/2/3a−f in various solvents
em
(excitation at the intense Soret maxima).
Compound [R]
C H CH
THF
14
14
13
1
CH Cl2
DMF
13
10
5
CH CN
6
5
3
2
3
1
1
1
1
2
2
2
2
3
3
3
3
a [H]
16
14
14
13
27
24
23
25
5
14
11
4
17
10
3
d [CF3]
e [CN]
f [NO2]
a [H]
0
1
1
21
22
22
8
22
19
18
1
20
21
16
1
23
20
12
1
d [CF3]
e [CN]
f [NO2]
a [H]
4
3
2
4
d [CF3]
e [CN]
f [NO2]
4
4
3
3
4
4
4
3
2
3
5
4
2
1
2
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CONCLUSIONS
Chalcone-(B)Chls 1a−f, 2a−f, and 3a−f possessing bacteriochlorin, chlorin, and porphyrin
π-systems, respectively, were successfully synthesized through a similar manner for the
synthesis of isomeric reverse chalcone-chlorins 4a−f. The redmost Qy absorption and main
fluorescence emission maxima of all of the synthetic chalcone analogs 1/2/3/4a−f were
3
dependent on the p-substituents of the C3 -phenyl ring. The sensitivities to their Hammett
σ-values decreased in the following order: 1 > 4 > 2 > 3. Furthermore, the substitution effects
on the quantum yields by the p-substituents are dependent on the π-conjugation degree of
cyclic tetrapyrroles and the solvent polarities. These results indicate that the use of different
fundamental π-systems and chemical modification of peripheral substituents of (B)Chls are
techniques to regulate their singlet excited energy levels.
Acknowledgements—We would like to thank Drs. Shin Ogasawara and Sunao Shoji of
Ritsumeikan University for experimental assistance. This work was partially supported by
JSPS KAKENHI Grant Number JP17H06436 in Scientific Research on Innovative Areas
4
“
Innovation for Light-Energy Conversion (I LEC)”.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the online version of this article:
Figure S1. Energy-minimized molecular models of 2a/2g/4a estimated by MM+/PM3
calculations.
Figure S2. Normalized fluorescence emission spectra (excitation at the intense Soret
maxima) of 1/2/3a in CH Cl .
2
2
Figures S3, S4, S5. Correlation of main emission maxima λ of 1/2/3a−f in CH Cl with
em
2
2
Hammett substituent constants σ.
Table S1. Qy(0,0) absorption maxima λ [Qy(0,0)] (nm) of 1/2/3a−f in CH Cl .
abs
2
2
−
1
Table S2. Stokes shifts ∆ (cm ) of 1/2/3a−f in CH Cl .
2
2
Spectral data of chalcone-bacteriochlorins/chlorins/porphyrins 1/2/3a−f
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FIGURE CAPTIONS
Figure 1. Chemical structures of naturally occurring (B)Chl pigments.
Figure 2. Chemical structure of reverse chalcone-type Chl derivatives 4a−f as the isomers of
2
a−f.
Figure 3. UV-Vis absorption spectra of 2a (green line), 2g (black line), and 4a (pink line) in
CH Cl . All the spectra were normalized at the intense Soret maxima.
2
2
Figure 4. Correlation of Qy(0,0) absorption maxima λ [Qy(0,0)] of 2a−f in CH Cl with
abs
2
2
Hammett substituent constants σ.
Figure 5. Electronic absorption spectra of 1a (red line), 2a (green line), and 3a (blue line) in
CH Cl . All the spectra were normalized at the intense Soret maxima.
2
2
Figure 6. Correlation of Qy(0,0) absorption maxima λ [Qy(0,0)] of 1a−f in CH Cl with
abs
2
2
Hammett substituent constants σ.
Figure 7. Correlation of Qy(0,0) absorption maxima λ [Qy(0,0)] of 3a−f in CH Cl with
abs
2
2
Hammett substituent constants σ.
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Scheme 1. Synthesis of chalcone-type (B)Chl derivatives bearing a bacteriochlorin 1a−f,
chlorin 2a−f, or porphyrin skeleton 3a−f through Claisen-Schmidt condensation of 1/2g with
p-(un)substituted benzaldehyde: (i) CH OH, acetone; (ii) (a) aq. 2% HCl/CH Cl , (b) collidine,
3
2
2
reflux, (c) H SO /CH OH; (iii) p-RC H CHO, NaOCH /CHCl −CH OH [R = H (a), OCH
3
2
4
3
6
4
3
3
3
(
b), CH (c), CF (d), CN (e), and NO (f)]; (iv) CH OH; (v) (a) H SO /CH OH, (b) collidine,
3 3 2 3 2 4 3
reflux; (vi) (a) HBr/AcOH, (b) H O, (c) CH N /Et O, (d) CH N(O)(CH CH ) O,
2
2
2
2
3
2
2 2
Pr NRuO /CH Cl ; (vii) (a) Zn(OAc) 2H O/CH Cl −CH OH; (b) DDQ/dry acetone, (c)
4
4
2
2
2
2
2
2
3
H SO /CH OH.
2
4
3
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This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.