Singly and Doubly Quinoxaline-Fused BIII Subporphyrins

Article abstract of DOI:10.1002/chem.201904151

B-Phenyl B^III subporphyrin-α-diones prepared in a three-step reaction sequence from the parent subporphyrin were condensed with 1,2-diaminobenzenes to give the corresponding quinoxaline-fused subporphyrins in variable yields. Quinoxaline-fused B-phenyl-5,10,15-triphenyl B^III subporphyrin was transformed to the corresponding subporphyrin-α-dione in the same three-step reaction sequence, which was then condensed with 1,2-diaminobenzene to give doubly quinoxaline-fused subporphyrin. These quinoxaline-fused subporphyrins exhibit redshifted absorption and fluorescence spectra compared with the parent one. A singly quinoxaline-fused subporphyrin bearing three meso-bis(4-dimethylaminophenyl)aminophenyl substituents shows blueshifted fluorescence in less polar solvent, which has been ascribed to emission associated with charge recombination of intramolecular charge transfer (CT) state.

Full text of DOI:10.1002/chem.201904151

DOI: 10.1002/chem.201904151
Communication
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Pi Interactions |Hot Paper|
Singly and Doubly Quinoxaline-Fused BIII Subporphyrins
Koki Kise and Atsuhiro Osuka*[a]
Abstract: B-Phenyl BIII subporphyrin-a-diones prepared in
a three-step reaction sequence from the parent subpor-
phyrin were condensed with 1,2-diaminobenzenes to give
the corresponding quinoxaline-fused subporphyrins in var-
iable yields. Quinoxaline-fused B-phenyl-5,10,15-triphenyl
BIII subporphyrin was transformed to the corresponding
subporphyrin-a-dione in the same three-step reaction se-
quence, which was then condensed with 1,2-diaminoben-
zene to give doubly quinoxaline-fused subporphyrin.
These quinoxaline-fused subporphyrins exhibit redshifted
Scheme 1. b-p-Extended subporphyrins and quinoxaline-fused porphyrins.
absorption and fluorescence spectra compared with the
parent one. A singly quinoxaline-fused subporphyrin bear-
ing three meso-bis(4-dimethylaminophenyl)aminophenyl
substituents shows blueshifted fluorescence in less polar
solvent, which has been ascribed to emission associated
with charge recombination of intramolecular charge trans-
fer (CT) state.
sively developed by Crossley and co-workers.[6] These fused
porphyrins were synthesized by the condensation of porphy-
rin-a-diones with 1,2-diaminobenzenes and the easiness of
these condensation reactions allowed various types of qui-
noxaline-fused porphyrins. As related examples, p-extended
phthalocyanines were actively prepared by Sastre-Santos et al.
by imine-formation reaction of diaminophthalocyanines with
cyclohexane-1,2,3,4,5,6-hexaone.[7] Our recent synthesis of sub-
porphyrin-a-diones encouraged us to examine the exploration
of quinoxaline-fused subporphyrins.[8]
Subporphyrinatoboron(III) (hereafter called as subporphyr-
in),[1–5] is the genuine ring-contracted porphyrin with 14p aro-
matic circuit consisting of three pyrroles and three bridging
methine carbons. Since the first synthesis in 2006,[1a] the
chemistry of subporphyrins has been extensively studied,
showing their unique properties and reactivities. Different from
meso-aryl-substituted porphyrins, most of meso-aryl substitu-
ents in subporphyrins can rotate rather freely, hence causing
large substituent effects on the optical and electronic properti-
es.[2,3e] Taking advantage of this feature, many interesting ex-
amples have been explored. b-Substituted subporphyrins have
been also developed,[4] such as b-halogenated,[4a] b-nitrated,[4b]
b-aminated,[4b] and b-sulfanylated subporphyrins.[4c,e] But there
are only limited examples of b-p-extended subporphyrins and
only tribenzosubporphines 1[1a] and meso-triphenyl-tribenzo-
subporphyrins 2 were reported.[2a] Herein, we report the syn-
thesis and properties of singly and doubly quinoxaline-fused
subporphyrins as rare examples of b-p-extended subporphyr-
ins (Scheme 1).
The synthetic route to quinoxaline-fused subporphyrins is
shown in Scheme 2. Subporphyrin-a-dione 5 was prepared by
the reported procedure[8] in a three-step sequence from sub-
porphyrin 4 and was reacted with 1,2-diaminobenzene 6 in a
mixture of THF and MeOH (1:1) at 408C for four hours to give
quinoxaline-fused subporphyrin 7 in 91% yield. In the conden-
sation reaction with 8, the starting subporphyrin 5 was con-
sumed at room temperature within 15 minutes, giving 9 in
moderate yield of 42%. The observed accelerated reaction for
9 may be attributed to high nucleophilicity of 8. On the other
hand, the reaction of 5 with 10 required longer reaction time
Quinoxaline-fused porphyrins 3 were one of the representa-
tive b-p-extended porphyrins, which were originally and exten-
[a] K. Kise, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
Scheme 2. Synthesis of monoquinoxalinosubporphyrins 7, 9, and 11. Reac-
tion conditions: [a] 5 (1.0 equiv), 6 (2.1 equiv), 408C, 4 h, 91% yield. [b] 5
(1.0 equiv), 8 (1.2 equiv), room temperature, 15 min, 42% yield. [c] 5
(1.0 equiv), 10 (1.2 equiv), 608C, 33 h, 15% yield.
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201904151.
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(33 h) and higher temperature (608C) to complete the conver-
sion. This reaction afforded 11 in a poor yield of 15%.
Next, we attempted to synthesize doubly quinoxaline-fused
subporphyrin 15 (Scheme 3). Subporphyrin-a-dione 14 was
prepared in the three-step sequence from 7 in a total yield of
Scheme 3. Synthesis of bisquinoxalinosubporphyrin 15.
29%. Namely, 7 was brominated with bis(2,4,6-trimethylpyri-
dine)bromonium hexafluorophosphate in refluxing THF to give
a crude mixture containing b-monobrominated subporphyrin,
b-dibrominated subporphyrin, and the starting material, which
were difficult to separate. Then, this mixture was roughly puri-
fied through a silica-gel column and was reacted with benzal-
dehyde oxime in the presence of sodium hydride in DMSO at
608C for 30 min to give b-hydroxy subporphyrin 13 as a crude
mixture. Then, this mixture dissolved in DMF was treated with
[bis(trifluoroacetoxy)iodo]benzene (PIFA) at 08C, to furnish sub-
porphyrin-a-dione 14. Finally, 14 was condensed with 1,2-di-
aminobenzene 6 in a mixed solvent of THF and MeOH (1:1) at
408C, giving doubly quinoxaline-fused subporphyrin 15 in
Scheme 4. Synthesis of meso-functionalized monoquinoxalinosubporphyr-
in 21.
1
78% yield. The H NMR spectrum of 15 in CDCl3 shows a Cs-
symmetric pattern, indicating the equivalence of both quinoxa-
line parts (Scheme 3).
Further, we attempted the synthesis of a quinoxaline-fused
subporphyrin bearing electron-donating meso-aryl substitu-
ents, 21, to examine whether the characteristic p-aminophenyl
substituent effect is still active for the quinoxaline-fused sub-
strate (Scheme 4).[2c] Similarly to the synthesis of 5 and 14, sub-
porphyrin-a-dione 19 was prepared in the three-step sequence
from
B-phenyl-5,10,15-tris(4-bromophenyl)subporphyrin 16
Figure 1. X-Ray crystal structures of (a) 7 and (b) 15. Solvent molecules are
omitted for clarity. The thermal ellipsoids are scaled at 50% probability level.
(see section 2.7–2.10 in Supporting Information for details).
Then, 19 was reacted with 1,2-diaminobenzene 6 in a mixed
solvent of THF and MeOH (1:1) at 408C for 4 h, affording 20 in
good yield of 81%. Finally, 20 was reacted with bis(4-dimethy-
laminophenyl)amine in the presence of tris(dibenzylideneace-
tone)dipalladium(0) (Pd2dba3), 2-dicyclohexylphosphino-2’,4’,6’-
triisopropylbiphenyl (XPhos), and sodium tert-butoxide in tolu-
ene at 1008C for 28 h. Subsequent separation by silica-gel
column chromatography and recrystallization from CH2Cl2 and
MeOH gave 21 in 54% yield.
the Supporting Information). The bowl depths are 1.38 ꢁ for 7,
1.33 ꢁ for 9, and 1.24 ꢁ for 15.[9] Characteristically, the b-b
bond lengths at the fused side are significantly longer (1.438–
1.444 ꢁ) compared with those of non-fused sides (1.367–
1.379 ꢁ). This implies the increased single bond character of
the b-b bonds at the fused side. On the other hand, the C(b)À
N bond lengths in 7 and 15 (1.315–1.321 ꢁ, Figure S4-2 in the
Supporting Information) are close to that of CÀN double bond
(1.29 ꢁ) rather than CÀN single bond (1.47 ꢁ), implying the sig-
The structures of 7, 9, and 15 were unambiguously deter-
mined by X-ray diffraction analysis (Figure 1 and section 4 in
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nificant double bond character in C(b)ÀN bonds. Previously,
these trends of bond lengths were theoretically predicted for
oligo(quinoxaline-fused)porphyrins by Lꢂ et al.[6a]
more complicated Soret-like and Q-like bands (Figure 2a). Fluo-
rescence was observed at 613 nm for 7 and 609 nm for 15
with quantum yields of 0.044 and 0.060, respectively, which
are smaller than that of 4 (FF =0.16). Introduction of electron-
donating (-NMe2) or electron-withdrawing (-NO2) substituents
at the quinoxaline part resulted in the redshifts in Soret-like
bands (Figure 2b). Especially, compound 11 displays a broad
band reaching around 700 nm and is non-fluorescent in
CH2Cl2. Although 7 and 20 show almost the similar UV/Vis ab-
sorption spectra, 21 exhibits a quite different panchromatic ab-
sorption spectrum up to around 750 nm. Interestingly, the fluo-
rescence behavior of 21 is dependent sensitively on the sol-
vent polarity, which is different from 4 and 7 (Figure S3–6-3-7
and Table S3-2 in the Supporting Information). Although practi-
cally non-fluorescent in CH2Cl2 (FF <0.001), 21 showed a weak
emission over 750 nm in toluene (FF =0.055) and exhibited a
slightly blueshifted emission at 737 nm with increased fluores-
cence quantum yield of 0.253 in a binary solvent (toluene/cy-
clohexane = 1:9) as summarized in Table 1. The results may be
The absorption and fluorescence spectra of the quinoxaline-
fused subporphyrins in CH2Cl2 are shown in Figure 2. Unlike
the reference B-phenyl-5,10,15-triphenylsubporphyrin 4, singly
quinoxaline-fused subporphyrin 7 exhibits a split Soret-like
band at 345 and 380 nm and Q-like bands at 500 and 539 nm,
whereas doubly quinoxaline-fused subporphyrin 15 exhibits
Table 1. Summary of optical properties of quinoxalinosubporphyrins 4, 7,
9, 15, 20, and 21 in CH2Cl2.
[a]
Compd. FF
tF [ns][a] kr [sÀ1
]
knr [sÀ1
]
Stokes shift [cmÀ1
]
4
7
9
15
20
21
21[b]
21[d]
0.16
2.0
0.044 1.8
0.081 1.3
0.060 2.3
0.008 0.31
8.1ꢃ107 4.2ꢃ108
2.4ꢃ107 5.2ꢃ108
6.3ꢃ107 7.2ꢃ108
2.6ꢃ107 4.0ꢃ108
2.5ꢃ107 3.2ꢃ109
1240
2240
1210
1150
2060
–
<0.001
0.055 1.7
0.253 4.2
–
–
–
3.3ꢃ107 5.7ꢃ108
6.1ꢃ107 1.8ꢃ108
>2510[c]
2510
[a] Excited at Soret-like bands (see Figures S3-1 and S3–5 in the Support-
ing Information for detail). [b] In toluene. [c] Stokes shift was not exactly
determined since the emission peak top was out of detection range.
[d] In toluene/cyclohexane (1:9).
understood by considering the following mechanism. Franck–
Condon excited state of 21 relaxes to an intramolecular
charge-transfer state even in non-polar solvent systems, such
as toluene and toluene/cyclohexane (1:9). This CT state emits
fluorescence associated with charge recombination to regener-
ate the ground state in non-polar solvents, but does not emit
fluorescence in polar solvents probably due to very rapid
charge recombination. The energy level of the CT state is
slightly higher in toluene/cyclohexane (1:9) than in toluene in
agreement with the observed blue shift in the mixed solvent.
Density functional theory (DFT) calculations have been per-
formed for quinoxaline-fused subporphyrins. Molecular orbital
(MO) diagrams of 4, 7, 15, and 21 are shown in Figure 3 (Fig-
ures S6-1–S6-3 in the Supporting Information). Compared with
the reference subporphyrin 4, singly quinoxaline-fused subpor-
phyrin 7 possesses largely stabilized LUMO (À2.66 eV), which
results in a diminished HOMO–LUMO gap of 2.79 eV. This is
consistent with the redshifted absorption and fluorescence of
7. Installation of fused quinoxaline unit continuously caused
decrease in the energy level of a2-like orbital and increase in
Figure 2. UV/Vis absorption (solid) and fluorescence (dashed) spectra of (a) 4
(gray), 7 (black), and 15 (red), (b) 7 (black), 9 (orange), and 11 (green), and
(c) 7 (black), 20 (pink), and 21 (blue) in CH2Cl2.
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Figure 3. Molecular orbital diagrams of 4, 7, 15, and 21 calculated at the level of B3LYP/6–311G(d).
the energy level of a1-like orbital. As a result, HOMOs in 4 and
7 are a2-like orbitals, but that in 15 is a1-like orbital and
HOMO–LUMO gap of 15 (2.76 eV) is almost similar to that of 7
(2.79 eV). These calculated results are consistent with the ab-
sorption and electrochemical data (see below). On the other
hand, 21 showed largely destabilized HOMO (À4.23 eV), appa-
rently due to the strong effect of the electron-donating meso-
substituents. This explains the further redshift in absorption
and fluorescence spectra. Especially, the calculated S0–S1 transi-
tion (l=744 nm) of 21 is mainly assigned to HOMO–LUMO
transition, according to time-dependent (TD) DFT calculations
(Figure S6-4 and Table S6-1–S6-6 in the Supporting Informa-
tion). The calculation indicates that HOMO and LUMO in 21
are, respectively, localized in the meso-aryl groups and quinox-
alinosubporphyrin moiety, implying intramolecular charge-
transfer character.
energy gaps with correlation coefficient (R2) of 0.940 (Fig-
ure S5-2 in the Supporting Information).
In summary, rare b-p-extended subporphyrin, namely, singly
and doubly quinoxaline-fused subporphyrins, were synthesized
by the double condensation of subporphyrin-a-diones with
1,2-diaminobenzenes. Introduction of electron-donating amino
groups in meso-aryl substituents gave push-pull type subpor-
phyrin 21 with redshifted absorption and solvent-sensitive
fluorescence from intramolecular CT state. The subporphyr-
in 21 can be regarded as the first example of subporphyrin
functionalized simultaneously at meso- and b-positions, reveal-
ing that electronic properties can also be tuned effectively by
modifying meso-aryl substituents in b-p-extended subporphyr-
in.
Acknowledgements
Finally, electrochemical measurements were conducted. The
first oxidation and reduction potentials, electrochemical
HOMO–LUMO gaps, and theoretically calculated HOMO–LUMO
gaps are summarized in Table 2 (Table S5–1 in the Supporting
Information for detail). Electrochemical HOMO–LUMO gaps
(Eox.1ÀEred.1) were correlated well with theoretically calculated
This work was supported by JSPS KAKENHI of Scientific Re-
search (A; 18H03910) and Challenging Exploratory Research
(18K19074). K.K. acknowledges JSPS fellowship for young scien-
tists.
Conflict of interest
Table 2. First oxidation and reduction potentials, electrochemical HOMO–
LUMO gaps, and theoretically calculated HOMO–LUMO gaps of 7, 9, 11,
15, 20, and 21.
The authors declare no conflict of interest.
Keywords: fused
quinoxalines · subporphyrins · pi extensions
structures
·
push-pull
systems
·
Compd.
Eox.1 [V]
Ered.1 [V]
Eox.1ÀEred.1 [eV]
ELUMOÀEHOMO [eV][c]
7
9
11
15
20
21
0.72[a]
0.46[b]
0.82[a]
À1.76[b]
À1.92[b]
À1.07[a]
À1.66[b]
À1.53[b]
À1.77[a]
2.48
2.38
1.89
2.43
2.33
1.54
2.79
2.86
2.47
2.76
2.79
2.02
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[9] Bowl depth is defined by the distance between the boron atom and the
mean plane of the peripheral six b-carbons of subporphyrin macrocycle.
Manuscript received: September 9, 2019
Revised manuscript received: September 30, 2019
Version of record online: && &&, 0000
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COMMUNICATION
Push-pull systems: Singly and doubly
quinoxaline-fused subporphyrins were
synthesized by condensation of subpor-
phyrin b,b-diketone with 1,2-diamino-
benzenes. These fused subporphyrins
exhibit redshifted absorption and fluo-
rescence spectra. A singly quinoxaline-
fused subporphyrin bearing three meso-
bis(4-dimethylaminophenyl)aminophen-
yl substituents exhibits charge-recombi-
nation fluorescence of intramolecular CT
state (see scheme).
&
Pi Interactions
K. Kise, A. Osuka*
&& – &&
Singly and Doubly Quinoxaline-Fused
BIII Subporphyrins
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