Porphyrin-Azobenzene-Bodipy Triads: Syntheses, Structures, and Photophysical Properties

Article abstract of DOI:10.1021/acs.orglett.7b00988

Cyclic and acyclic azobenzene bridged porphyrin-dipyrrin derivatives were successfully prepared via Suzuki-Miyaura coupling reaction of α,α′-diborylated dipyrromethane with bromoazophenyl porphyrin or reaction of borylated porphyrin with dibromoazophenyl

Full text of DOI:10.1021/acs.orglett.7b00988

Letter
pubs.acs.org/OrgLett
Porphyrin−Azobenzene−Bodipy Triads: Syntheses, Structures, and
Photophysical Properties
Bangshao Yin,† Taeyeon Kim,‡ Mingbo Zhou,† Weiming Huang,† Dongho Kim,*,‡ and Jianxin Song*,†
†Key Laboratory of Chemical Biology and Traditional Chinese Medicine, Ministry of Education of China, Key Laboratory of the
Assembly and Application of Organic Functional Molecules, Hunan Normal University, Changsha 410081, China
‡Spectroscopy Laboratory for Functional pi-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 03722, Korea
S
* Supporting Information
ABSTRACT: Cyclic and acyclic azobenzene bridged porphyrin−dipyrrin derivatives were successfully prepared via Suzuki−
Miyaura coupling reaction of α,α′-diborylated dipyrromethane with bromoazophenyl porphyrin or reaction of borylated
porphyrin with dibromoazophenyl dipyrrin, and the corresponding porphyrin−Bodipy derivatives were obtained by subsequent
boron complexation. The cyclic porphyrin−dipyrrin compound 3Ni was confirmed by X-ray diffraction. The low fluorescence
quantum yields of azobenzene bridged porphyrin−Bodipy can be ascribed to the presence of the intramolecular charge transfer
state.
switches,42,43 and image storage.44 Recently, we reported the
he design and synthesis of photosensitizers capable of
Tharvesting a large fraction of solar light has attracted a great synthesis of azobenzene bridged porphyrin nanorings and found
deal of interest.1,2 Porphyrin derivatives have been widely
one Azo-linker of trimer exhibits cis/trans isomerization with
external stimuli.45 To the best of our knowledge, azobenzene
employed as energy capturing antenna or electron donor units
bridged porphyrin−Bodipy has rarely been reported. As an
extension of our research, we describe the synthesis, structures,
and photophysical properties of cyclic and acyclic azobenzene
bridged porphyrin−Bodipy derivatives in this letter.
in artificial model compounds due to their structural resemblance
to natural chlorophylls.3,4 Porphyrin derivatives have an intense
absorption in 400−450 nm region and relative weak absorptions
in 500−700 nm region. However, porphyrin derivatives exhibit
insufficient absorption property in blue-green region, so they are
not ideally suited for broadband solar harvesting. In order to
improve the broadband light harvesting properties of porphyrin
derivatives, one strategy involves coupling green absorbing
chromophores to it and creating donor−acceptor array
complexes.5−12 Boron−dipyrrins (Bodipys) absorb strongly in
blue-greenregionandhaveanumberoffavoritepropertiessuchas
large molar absorption coefficients, high fluorescence yields,
relatively long excited state lifetimes, and excellent photo-
stability.9,13−18 Bodipys are widely used in the fields of
luminescent devices,19 biological labeling,20 chemical sen-
sors,21−25 fluorescent probes,26 artificial photosynthetic systems,
and molecular wires.27,28 In order to enhance the light-harvesting
efficiency throughout the solar spectrum, many Bodipy−
porphyrin conjugate dyads have been reported in the literature
taking advantage of Bodipy’s ability to act as an antenna
molecule.29−36 Azobenzene and its derivatives are very important
organic pigments, owning to their reversible cis/trans photo-
isomerization.37−41 Azobenzene and its derivatives are consid-
ered to be useful materials in the field of photoresponsive
conjugated molecules such as liquid crystal, light-driven
We found the Suzuki−Miyaura cross coupling of α,α′-
diborylated dipyrromethane 1, which was prepared by iridium-
catalyzed selective borylation of mesityldipyrromethane, with
bromoazophenyl nickel porphyrin 2Ni45 in the presence of a
[Pd2(dba)3]/PPh3 catalyst and cesium bases gave the cyclic
azobenzene bridged porphyrin−dipyrrin compound 3Ni in 14%
yield, whereas no cyclic 3Ni was separated when cross coupling of
1
5 with borylporphyrin monomer. The H NMR and high-
resolutionESI-TOF spectraof3Niconfirmitsexpected structure.
Freebase precursor 3H was also obtained in10% yield bythis way.
Compound 3H was converted into 3Zn upon treatment with
Zn(OAc)2 in quantitative yield. We also examined the Suzuki−
Miyaura coupling of 1 with dibromoazobenzene 4, which
provided dibromoazophenyl dipyrrin 5; further Suzuki−Miyaura
coupling of 5 with meso-borylated nickel porphyrin 6Ni or free
base porphyrin 6H gave the acyclic azobenzene bridged
porphyrin−dipyrrin compounds 7Ni and 7H in 30% and 28%
yields, respectively. Compound 7H was converted into 7Zn upon
Received: April 3, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00988
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Letter
Scheme 1. Synthesis of 3M, 7M, 8M, and 9M
treatment with Zn(OAc)2 in quantitative yield. The subsequent
boron complexation of 3M and 7M (M = Ni, 2H) with BF3·OEt2
afforded corresponding porphyrin−Bodipy derivatives 8M and
9M (M = Ni, 2H) all in decent yields. Compounds 8H and 9H
were all converted into 8Zn and 9Zn upon treatment with
Zn(OAc)2 in quantitative yields (Scheme 1). All these newly
synthesized azobenzene bridged porphyrin−Bodipy derivatives
8M and 9M and the precursors 3M and 7M were fully
are tilted by 42.7° and 54.7° relative to the adjacent pyrrole rings
of porphyrin, while the tilted angles of the azobenzene moieties
relative tothe adjacent pyrrole rings of dipyrrinare 2.6°and 27.7°,
implying that π−π conjugation exists between the benzene rings
and the dipyrrin moiety. The dihedral angles between the two
benzene rings in the same azobenzene are 23.2° and 36.2°,
respectively, while the dihedral angle between the two benzene
rings in different linkages adjacent to the porphyrin is 88.7°, and
that adjacent to the dipyrrin is 38.4°. The optimized structures of
3Zn, 7Zn, 8Zn, and 9Zn were obtained at CAM-B3LYP/6-
31G(d) level (Figure S36) displaying dihedral angles between
units. Unlike similar dihedral angles between porphyrin and
azobenzene units in 3Zn to 8Zn and 7Zn to 9Zn, dihedral angles
between azobenzene and Bodipy in 8Zn and 9Zn are almost 20°
larger than those between azobenzene and dipyrrin units in 3Zn
and 7Zn. This difference seems to originate from steric hindrance
between hydrogen atom in benzene and fluoride atom in Bodipy
unit, which in turn can relatively obstruct through-bond
interaction between Bodipy and azobenzene unit. The most
important difference is that the dihedral angle between porphyrin
and azobenzene unit is relatively large in 7Zn and 9Zn compared
with those in 3Zn and 8Zn.
1
characterized by high-resolution mass spectrometry, H NMR,
UV/vis absorption, and fluorescence spectroscopy (see the
Supporting Information).
Single crystal of 3Ni was obtained by the slow diffusion of i-
PrOH vapor into its PhCl solution. The molecular structure of
3Ni was unambiguously confirmed by X-ray crystallography
study (Figure 1). The porphyrin ring is nearly parallel to the plane
defined by the dipyrrin. The porphyrin core is slightly ruffled, as is
often seen for nickel porphyrins. The maximum displacement of
theβ-carbons from theporphyrin mean plane, whichiscomposed
of carbon and nitrogen atoms, is 0.39 Å. The two azobenzene
moietiesadopt trans conformations, and theazobenzenemoieties
The UV/vis absorption spectra of 3Zn, 7Zn, 8Zn, and 9Zn
were measured in toluene (Figure 2). The acyclic compounds
7Zn and 9Zn show Soret bands at 424 and 421 nm, respectively,
and when compared to porphyrin monomer,46 slightly extended
Q bands ranging from 518 to 646 nm appear. In contrast to 7Zn
and 9Zn, the cyclic compounds 3Zn and 8Zn exhibit broader and
red-shifted Soret bands than 7Zn and 9Zn centered at 430 nm.
The broadened Soret bands of the cyclic compounds imply there
is larger electronic interaction between porphyrins and dipyrrins
or Bodipys than that in the acyclic compounds. The higher
extinction coefficients of Soret bands of 7Zn and 9Zn are
attributable to two factors: (1) the number of porphyrin moieties;
7Zn and 9Zn have onemore porphyrin moiety than 3Zn and 8Zn
definitely leading to the higher extinction coefficient; and (2)
conformational heterogeneity; 3Zn and 8Zn seem to exist in a
Figure 1. X-ray structure of 3Ni: (a) Top view and (b) side view. The
thermal ellipsoids are 50% probability level. tert-Butyl and methyl groups
are omitted for clarity.
B
DOI: 10.1021/acs.orglett.7b00988
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Letter
moieties seems to play an important role in electronic structure,
decay with double exponential functions, lifetimes of 103 (96%)
and 609 ps (4%) for 3Zn and 20 (65%) and 48 ps (35%) for 8Zn,
respectively. Since their lifetimes and coefficients well correspond
to the fluorescence lifetimes obtained from TCSPC, and they did
not display any spectral shift or change in shape of excited state
absorption (ESA), we may conclude that they exist in a range of
conformation and also their lifetimes of singlet excited state are
lying in some range of time scale. Therefore, decay profiles were
fitted with double exponential functions, and ICT state of 3Zn
and 8Zn is directly populated by excitation. The excited state
dynamics of 7Zn and 9Zn, however, lies in a more complex
manner than those of 3Zn and 8Zn. A contrasting feature in TA
spectra of 7Zn and 9Zn was observed that the excited state
dynamics cannot be explained only by the fluorescence process
from the lowest electronic excited state unlike the cases of 3Zn
and 8Zn. TA spectra of 7Zn and 9Zn evolves to relaxed S1 state
with additional component. (ICT process in 7Zn and 9Zn is not
full-charge transfer or electron transfer as discussed in absorption
spectra.)
Figure 2. (a) UV/vis absorption and (b) fluorescence (λex = 550 nm)
spectra of 3Zn (black), 8Zn (green), 7Zn (red), and 9Zn (blue) in
toluene.
range of conformation on account of broader Soret band features
in line with stronger electronic communication between
porphyrin and azobenzene unit. Remarkably, complex 8Zn and
9Zn do not show any characteristic absorption of Bodipy and its
derivatives, which have high extinction coefficient in the visible
region.9,47 This rare phenomenon indicates that two azobenzene
and dipyrrin (or Bodipy) units are not independent, rather, these
twounitsbehaveasanunit;hence, wewillmentiononlytwounits,
hereinafter, porphyrin and azobenzene units. Furthermore, from
the sharp Soret bands of 7Zn and 9Zn considering that 7Zn and
9Zn are in a condition where these complexes are not locked in as
3Zn and 8Zn of cyclic structure, we may assume that porphyrin
moiety in 7Zn and 9Zn possesses a highly independent character
despite their plausible conformational heterogeneities. On the
contrary, porphyrin and azobenzene units in 3Zn and 8Zn are
relatively more strongly coupled to each other, which could be
understood by their much smaller dihedral angles between
porphyrin and azobenzene units than those of 7Zn and 9Zn.
Thus, their absorption spectra can be more influenced by
conformational factor. No distinct feature was observed from the
fluorescence spectra (Figure 2) except for the quantum yield
(QY) that all the complexes exhibit very low QY, less than 1%,
whichcan beascribed tothe intramolecular charge transfer (ICT)
process upon photoexcitation as Zn porphyrin monomer or
Bodipy showed much stronger fluorescence.9,46,47
Therefore, we analyzed the excited state dynamics of 7Zn and
9Zn by extracting decay-associated spectra (DAS) using global
fitting method(Figure S40e,f).48 In both compounds, three decay
components were found: lifetimes of 1.2, 425, and 1525 ps for
7Zn and 5, 330, and 933 ps for 9Zn, respectively. These
components seem to originate from ICT, charge recombination
(CR), and relaxation of porphyrin triplet state, respectively, since
a fast decrease of stimulated emission (SE) was observed around
600 to 650 nm, which is indicative of ICT process.47,49
Furthermore, electron density difference map (EDDM) for the
lowest electronic transition also implies that ICT process may
occur after population of Franck−Condon (FC) state (Figure
S41). Interestingly, 7ZnshowedverystrongESAsignalcompared
to 9Zn even overwhelming the ground state bleaching (GSB)
signal,whichcanbeascribedtomanypossibletransitionsfromthe
lowest excited state to the higher states in line with time-
dependent density functional theory (TDDFT) calculations
showing a number of states around 350 nm (Figure S42).
Insummary,wehaveachievedthesynthesisofcyclicandacyclic
azobenzene bridged porphyrin−Bodipy derivatives via Suzuki−
Miyaura coupling and boron complexation. The structures of
these compounds were characterized by high-resolution mass
1
spectrometry, H NMR, UV/vis absorption, and fluorescence
spectroscopy. The structure of 3Ni was confirmed by X-ray
diffraction analyses, and optimized structures of 3Zn, 7Zn, 8Zn,
and 9Zn were analyzed focusing on the dihedral angle. Time-
correlated single-photon counting (TCSPC), femtosecond
transient absorption (fs-TA) measurements, and electron density
difference map (EDDM) indicate that, while CT state is directly
populated in 3Zn and 8Zn, CT state is populated via additional
ICT process fromFCstate in7Znand9Znupon photoexcitation.
Investigations of other elegant Bodipy−porphyrin conjugated
systems are underway in our laboratories.
To understand their radiative properties, we carried out time-
correlated single-photon counting (TCSPC) measurements in
toluene as shown in Figure S39. Compounds 3Zn and 8Zn
decayed with double exponential functions, which are 100 (95%)
and 620 ps (5%) for 3Zn and 22 (76%) and 48 ps (24%) for 8Zn,
respectively. However, 7Zn and 9Zn decayed with single
exponential functions, which are 490 ps for 7Zn and 340 ps for
9Zn. Double exponentialdecay in 3Znand 8Znmaybeduetothe
conformational heterogeneities in line with the above discussion
in the steady-state absorption spectra. Despite of their low
quantum yields, complexes with Bodipy (8Zn and 9Zn) showed
clear tendencies to have decreased lifetimes and increased
radiative and nonradiative rates compared with complexes with
dipyrrin (3Zn and 7Zn). Their optical properties are listed in
Table S1.
ASSOCIATED CONTENT
■
S
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00988.
In order to get further information on excited state dynamics of
the complexes, femtosecond transient absorption (fs-TA)
measurements were carried out (Figure S40). All experiments
were conducted with excitation wavelength of 555 nm in toluene.
Compounds 3Zn and 8Zn, where π−π interaction between
Detailedsyntheticproceduresandspectroscopiccharacter-
ization, 1H (13C 11B) NMR spectra, computational results
of 3Zn, 7Zn, 8Zn, and 9Zn (PDF)
Crystallographic files for compound 3Ni (CIF)
C
DOI: 10.1021/acs.orglett.7b00988
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Organic Letters
Letter
(18) Frath, D.; Massue, J.; Ulrich, G.; Ziessel, R. Angew. Chem., Int. Ed.
2014, 53, 2290−2310.
AUTHOR INFORMATION
■
Corresponding Authors
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*E-mail: dongho@yonsei.ac.kr.
*E-mail: jxsong@hunnu.edu.cn.
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Am. Chem. Soc. 2000, 122, 968−969.
ORCID
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Dongho Kim: 0000-0001-8668-2644
Jianxin Song: 0000-0002-1756-5898
́
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Notes
The authors declare no competing financial interest.
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N.; De Borggraeve, W. M.; Boens, N.
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ACKNOWLEDGMENTS
■
The work at Hunan Normal University was supported by the
National Natural Science Foundation of China (Grant No.
21272065;21602058)andtheScientificResearchFoundationfor
the Returned Overseas Chinese Scholars, State Education
Ministry, Scientific Research Fund of Hunan Provincial
Education Department (Grant No. 16A125). The work at Yonsei
was supported by the Global Research Laboratory (GRL)
Program (2013K1A1A2A02050183) of the Ministry of Educa-
tion, Science and Technology (MEST) of Korea. The quantum
calculations were performed using the supercomputing resources
of the Korea Institute of Science and Technology Information
(KISTI).
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