Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial Hydroporphyrin Dyads with Strong Through-Bond and Through-Space Electronic Interactions

Article abstract of DOI:10.1021/acs.joc.9b00731

Photoisomerization of 3,4-di(methoxycarbonyl)-enediyne linker in hydroporphyrin (chlorin or bacteriochlorin) dyads leads to thermally stable cis isomers, where macrocycles adopt a slipped cofacial mutual geometry with an edge-to-edge distance of ～3.6 ? (d

Full text of DOI:10.1021/acs.joc.9b00731

Article
pubs.acs.org/joc
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial
Hydroporphyrin Dyads with Strong Through-Bond and Through-
Space Electronic Interactions
Adam Meares, Zhanqian Yu, Ganga Viswanathan Bhagavathy, Andrius Satraitis, and Marcin Ptaszek*
Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland
21250, United States
S
* Supporting Information
ABSTRACT: Photoisomerization of 3,4-di(methoxycarbonyl)-enediyne linker in hydroporphyrin (chlorin or bacteriochlorin)
dyads leads to thermally stable cis isomers, where macrocycles adopt a slipped cofacial mutual geometry with an edge-to-edge
distance of ∼3.6 Å (determined by density functional theory (DFT) calculations). Absorption spectra exhibit a significant
splitting of the long-wavelength Qy band, which indicates a strong electronic coupling with a strength of V = ∼477 cm−1 that
increases to 725 cm−1 upon metalation of hydroporphyrins. Each dyad features a broad, structureless emission band, with large
Stokes shift, which is indicative of excimer formation. DFT calculations for dyads show both strong through-bond electronic
coupling and through-space electronic interactions, due to the overlap of π-orbitals. Overall, geometry, electronic structure,
strength of electronic interactions, and optical properties of reported dyads closely resemble those observed for photosynthetic
special pairs. Dyads reported here represent a novel type of photoactive arrays with various modes of electronic interactions
between chromophores. Combining through-bond and through-space coupling appears to be a viable strategy to engineer novel
optical and photochemical properties in organic conjugated materials.
Rhodopseudomonas viridis, contains a pair of bacteriochlor-
ophyll b,11 which absorbs at 960 nm.7 The strength of
excitonic coupling varies from ∼200 cm−1 in B85012 to 550
cm−1 in P867,9 up to ∼900 cm−1 in P960.7 Thus, the extent of
excitonic interaction tunes the absorption properties of
photosynthetic pigments and, in turn, the energy of the
excited states, which collectively allows for a more efficient
harvesting of solar photons and assures the flow of the
excitation energy to the reaction centers.2 Moreover, excitonic
coupling is proposed to play a crucial rule in the ultrafast and
essentially quantitative energy transfer between light-harvesting
antenna and reaction center,2,9 as well as in ultrafast charge
separation, for example, through vibronic coherence.13−16
Excitonic coupling is currently viewed as the consequence of
two phenomena: (a) electrostatic interaction between
transition dipole moments of chromophores, as described in
Kasha’s model (aka long-range coupling)1,17 and (b) short-
range excitonic coupling, which resulted from the through-
space overlap of π-orbitals of interacting chromophores.17 For
chromophores separated by a close distance, like bacterio-
INTRODUCTION
■
Excitonically coupled chromophores1 are key components of
light-harvesting antennae and photosynthetic reaction centers;
as such, they are crucial for initial processes in photosynthesis,
that is, light harvesting and charge separation.2,3 Bacterial
photosynthetic light-harvesting antenna are constructed from
bacteriochlorophyll a clusters with increasing interpigment
excitonic coupling, which leads to a shift of the long-
wavelength Qy absorption band from 800 nm in the nearly
noninteracting pigments found in B800 to 850 and 875 nm, in
B850 and B875, respectively.2,3 This shift is, at least in a large
part, due to the excitonic interactions between bacteriochlor-
ophylls.2,3 Chlorosomes, i.e., light-harvesting antennae found
in green sulfur bacteria, are composed of self-assembled
bacteriochlorin c molecules, where strong interpigment
interactions lead to ∼100 nm bathochromic shift in the Qy
absorption band.4 Excitonic coupling is also evident in plant2,5
and bacterial6−9 photosynthetic reaction centers between
closely positioned chlorophylls and bacteriochlorophylls,
respectively. Thus, the reaction center from purple photo-
synthetic bacteria Rhodobacter sphaeroides contains a special
pair (P867) of two excitonically coupled bacteriochlorophyll a,
absorbing at 867 nm,3,10 whereas P960, a special pair from
Received: March 14, 2019
Published: May 22, 2019
© XXXX American Chemical Society
A
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Chart 1. Structures of Dyads and Monomers Discussed in This Paper and Numbering System for Chlorins (Ms = 2,4,6-
trimethylphenyl)
chlorophylls in light-harvesting antenna and special pairs,
short-range excitonic coupling seems to play a dominant role.18
To mimic coupled systems observed in the natural
photosynthetic apparatus, a large number of excitonically
coupled or strongly conjugated porphyrin arrays have been
studied, including directly linked,19,20 cofacial,21−23 slipped
cofacial,24 and ethynyl- or butadienyl-linked arrays.25 However,
nature utilizes hydroporphyrins, which are partially saturated
porphyrin congeners (chlorins and bacteriochlorins) as key
photo- and redox-active components in the photosynthetic
apparatus.2 Hydroporphyrins possess a much more intensive S0
→ S1 transition (Qy band) relative to porphyrin.26−28
Moreover, hydroporphyrins feature a lower magnitude of
E1/2 and E1/2 values (i.e., they are easier to oxidize and
harder to reduce) compared to corresponding porphyrins.29
Therefore, it is expected that the structure−excitonic coupling
relationship and properties of excitonically coupled hydro-
porphyrins will be different from that of porphyrins. Synthetic
hydroporphyrin arrays with excitonic coupling have been
studied to a much lesser degree than porphyrinic architectures.
There have been extensive studies performed on chlorosome
models, in which semisynthetic chlorins or bacteriochlorins
self-assemble into oligomers,25,30 where individual hydro-
porphyrins strongly interact with one another. There are also
several examples of synthetic, covalently linked cofacial31−52 or
directly linked53,54 arrays. For some of such arrays, evidence for
excitonic coupling is observed; however, hydroporphyrin
arrays have been investigated much less extensively than
corresponding porphyrinic systems.
include strongly conjugated arrays, where hydroporphyrins are
connected by ethynyl, butadiynyl, or vinyl linkers,55−57 meso−
meso directly linked hydroporphyrin dyads,58−60 and weakly
conjugated arrays, where hydroporphyrins are connected by
phenylene linkers.58,60 In strongly conjugated dyads, we
observed a significant bathochromic shift and splitting of the
Qy band, which is a result of through-bond π-conjugation. In
directly linked arrays, splitting of the Qy band is most likely
due to short-range excitonic coupling only, since the
perpendicular mutual arrangement of macrocycles effectively
prevents direct π-electronic coupling.59
Continuing our studies on the dependence of photophysical
properties of strongly conjugated hydroporphyrin dyads on the
linking moiety, we intended to prepare a series of arrays where
chlorins and bacteriochlorins are connected by a linker
comprising alternating carbon−carbon triple and double
bonds, i.e., hexa-3-en-1,5-diyne (enediyne) unit. Conjugated
chromophores containing this unit possess unusual optical and
electrochemical properties, as was demonstrated by Die-
derich61−66 and others.61,67 The Diederich group also reported
a conjugated porphyrin dimer, connected by an enediyne
unit.68 Moreover, conjugated enediynes undergo trans−cis
photoisomerization, a trait that has been utilized for the
development of photochromic and redox-active mole-
cules.69−73 Overall, we have anticipated that the combination
of enediynes with hydroporphyrins will lead to molecules with
interesting and useful optical and photochemical properties.
Here, we report the synthesis and properties of dyads
composed of two identical hydroporphyrins: chlorin (cis-2C,
cis-2ZnC, and trans-2C) or bacteriochlorin (cis-2BC, Chart
1), connected through their respective 13-positions by 3,4-
0/+
−/0
We recently reported arrays composed of hydroporphyrins
with varying degree of electronic interaction. These dyads
B
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dimethoxycarbonyl-hexa-3-en-1,5-diynyl motif. We selected
this particular diester-substituted enediyne linker primarily
due to the anticipated straightforward synthesis through the
Sonogashira reaction, which utilizes dimethyl dibromofuma-
rate.69−71 It is also known that diester-substituted enediynes
undergo trans−cis photoisomerization.69−71 As a reference, we
also prepared dyad phen-2C, where two chlorins are
connected by o-diethynylphenylene linker. As benchmarks,
we included known monomers: C-PE57 and BC-2PE55,56
(Chart 1). We found that enediyne-linked hydroporphyrin
dyads exhibit trans−cis photoisomerization, leading to
thermally stable cis isomers, with a slipped cofacial arrange-
ment of macrocycles, in which both strong through-bond and
through-space interactions occur.
of starting material, and no 3 could be isolated; apparently, 3 is
unstable at room temperature, similar to analogous enediynes
reported previously.76 Therefore, we pursued the syntheses of
dyads through the Sonogashira reaction of 1 with ethynyl-
substituted chlorin Ch-E57 and bacteriochlorin BC-E,55 which
syntheses were reported previously.
The Sonogashira reaction between Ch-E and 1 in the
presence of Pd2(dba)3/P(o-tol)3 in toluene/triethylamine
(5:1) at 60 °C for 2 h, performed without any protection
from ambient light, afforded two dyads, which were identified
as cis-2C (28% yield) and trans-2C (minor product, <5%
yield, Scheme 1). The low combined yield of both products is
attributed to the formation of a large amount of butadiynyl-
linked b-2C as a side product (∼30%, yield refers to a
semipurified sample). The analogous Sonogashira reaction,
when performed in the dark (reaction mixture was rigorously
protected from light at each stage: reaction, workup,
purification, and analysis of products), leads to isolation of
trans-2C (21% yield) and cis-2C (trace). As observed for
reaction performed in the presence of light, a large amount of
butadiyne-linked dyad b-2C was also isolated. cis-2C and
trans-2C can be separated by column chromatography in the
dark; however, when exposed to ambient light, either dyad
equilibrates within a few hours to the mixture of cis−trans
isomers in approximately 10:1 ratio of cis-2C to trans-2C. cis-
2ZnC was obtained in the reaction of cis-2C with Zn(OAc)2
in CHCl3/MeOH in 67% yield. The Sonogashira reaction of
bacteriochlorin BC-E with 1, conducted in the presence of
ambient light, leads to the formation of a mixture of cis-2BC as
the major product (35% yield, Scheme 2) and other product
(presumably trans-2BC) in >10:1 ratio; however, attempts to
separate both products using chromatography were unsuccess-
ful. A large amount of homodimer b-2BC55 was also isolated in
this case (20−30%).
RESULTS AND DISCUSSION
■
Our synthetic approach is outlined in Scheme 1. Dimethyl 2,3-
dibromofumarate 1 was prepared by bromination of dimethyl
acetylene dicarboxylate with pyridinium perbromide, according
to the previously reported procedure.74,75 Initially, we intended
to perform synthesis via enediyne 3 (Scheme S1); however,
attempted deprotection of 2 leads to complete decomposition
Scheme 1. Synthesis of trans-2C and cis-2C
Dyad phen-2C was synthesized in the Sonogashira reaction
of Ch-E with 1,2-dibromobenzene in 38% yield (Scheme 3).
Analogously to reactions described above, some amount of
dyad b-2C was also isolated (the yield of b-2C was not
determined).
The structures of the final dyads were confirmed by the
detailed analysis of their NMR spectra. The 1H NMR spectrum
of cis-2C exhibits significant shifts of the resonances of
methylene protons at the 17-position of chlorin macrocycle
(H-17 to 2.11 ppm, compared to δ 4.67 in the Ch-E) and
geminal methyl groups at the 18-positions (Me-18 to −0.48
ppm, compared to 2.01 ppm in Ch-E). Similarly, the spectrum
of cis-2BC exhibits a significant shift of the H-17 (to 2.81 ppm,
compared to 4.10 ppm in BC-E) and Me-18 (to 0.55 ppm,
compared to 1.93 ppm in BC-E) in bacteriochlorins.
Resonances of 7-methylene protons and geminal 8-methyl
substituents appear at 4.41 ppm and 1.88 ppm, respectively,
1
which is comparable to that observed for monomer BC-E. H
NMR spectrum of trans-2C shows resonances of H-17 and
Me-18 at 4.73 ppm and 2.10 ppm, respectively, which is
comparable to that observed for Ch-E. The detailed assign-
ment of chemical shifts in chlorin dyads and monomers is
presented in Table S1.
The observed significant changes in chemical shift of certain
protons between monomers and dyads are consistent with cis
stereochemistry of the enediyne linker, which forces macro-
cycles to adopt a slipped cofacial geometry so that some
protons of one macrocycle are placed in the shielding cone of
the aromatic system of the other macrocycle (Figure 1). To
C
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Scheme 2. Synthesis of cis-2BC
Scheme 3. Synthesis of phen-2C
Figure 1. DFT energy-minimized structures of cis-2C and cis-2BC.
Hydrogen atoms are omitted for clarity. The green spheres represent
centroids for the macrocycles.
further confirm this hypothesis, we analyzed dyad phen-2C,
where two chlorins are connected by o-diethynylphenylene
linker. In phen-2C, macrocycles should adopt a similar
geometrical arrangement as in cis-2C; thus, the similarity of
1H NMR spectra of both cis-2C and phen-2C will be further
1
indirect confirmation of the structure of the former. H NMR
spectrum of phen-2C shows similar features as for cis-2C, e.g.,
resonances of H-17 and Me-18 appear at 2.25 and −0.37 ppm,
respectively. Detailed analysis of NMR spectra and density
functional theory (DFT) calculations lead to the conclusion
that the most likely conformation of cis-2C, cis-2BC, and
phen-2C is that with a slipped antiparallel arrangement of
macrocycles (Figure 1). Such a structure is supported by a
D
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significant upfield shift of H-17 and Me-18, and a smaller
upfield shift for H-20, H-2, and H-3 resonances for dyads
versus monomer. Furthermore, there are no notable shifts of
methyl groups of mesityl substituents at the 10-position of
chlorin nor for H-7 and H-8 for dyads versus monomer (see
Table S1, Supporting Information). DFT calculations support
this structure, showing that such an antiparallel slipped cofacial
geometry is associated with the lowest energy for cis
configuration of the linker. The center-to-center distance
between chlorin macrocycles in such an arrangement is 7.17 Å,
while the edge-to-edge distance (measured as the distance
between carbon atoms C-15) is 3.64 Å. Similarly, for cis-2BC,
the center-to-center distance is 7.20 Å and the edge-to-edge
distance is 3.58 Å. In cis-2ZnC, these distances are shorter, i.e.,
6.59 and 3.55 Å, respectively. For phen-2C, the corresponding
distances are 7.22 and 3.65 Å, respectively (all distances above
are obtained from DFT calculations for energy-minimized
structures).
Figure 3. Changes in the absorption spectrum of cis-2C upon
irradiation at the maximum of the B band absorption in toluene
(absorption spectra were taken in 1 min intervals).
The 1H NMR spectra also suggest that the structures
presented in Figure 1 undergo fast conformational inter-
conversion on the NMR time scale since no differentiation
between the “inner” and “outer” Me-18 and H-17 positions of
macrocycles was observed. Such an interconversion would
involve simultaneous rotation of both macrocycles along
respective ethyne linker back and forth and “inverting” the
stacking arrangement of macrocycles in dyads. Note that the
proposed structures are axially chiral and such an inter-
conversion transforms one enantiomer into the other.
1
light of cis-2C in NMR tube results in H NMR spectrum
consistent with that observed for a cis/trans mixture of 10:1
ratio. Both cis-2C and trans-2C are stable in the dark since no
changes in their absorption spectra were observed (see
Supporting Information).
The photochemical trans−cis isomerization of compounds
possessing an enediyne moiety is well known;69−73 however,
the nearly quantitative and rapid conversion of trans-2C to cis-
2C is rather unexpected, since DFT calculations show that
trans-2C is more stable than cis-2C by 13.8 kJ/mol. We
hypothesize that the π−π interactions77 between large
aromatic systems of the two hydroporphyrins might stabilize
cis isomer and facilitate trans−cis isomerization. It is generally
accepted that π−π aromatic interactions are not well
reproduced by a simple DFT method, and dispersion-corrected
DFT are required for an accurate account for these
interactions.78
Photoisomerization of Enediyne-Linked Chlorin
Dyads. The results discussed above indicate that the
enediyne-linked hydroporphyrin arrays undergo fast trans−cis
photoisomerization. Both cis-2C and trans-2C isomers are
stable in the solid state in the dark at room temperature (no
changes observed for months). In solution, pure cis-2C and
pure trans-2C, when exposed to ambient light, undergo
equilibration within a few hours to the mixture containing cis-
2C and trans-2C in 10:1 ratio. In a more controlled
experiment, we irradiated solutions of cis-2C and trans-2C
in toluene at the maxima of their B bands. This resulted in the
equilibration of each solution to cis/trans ratio of 10:1 within
23 min (Figures 2 and 3). The isomerization time scale was
much the same upon irradiation of trans-2C at the maximum
Absorption and Emission Properties. Absorption
spectra of cis-2C and trans-2C and equilibrated mixture of
cis-2BC, as well as spectra of phen-2C, are presented in Figure
4. The Qy-type band in trans-2C (λQ = 701 nm) is broadened
y
and significantly shifted bathochromically, compared to that in
C-PE (λQ = 656 nm).57 The maximum of Qy band in trans-
y
2C is located between ethynyl-linked dyad (λQ = 689 nm) and
1
y
of the Qy band (700 nm). H NMR spectroscopy further
vinyl-linked dyad (λQ = 707 nm) reported previously.56,57
confirmed a trans−cis isomerization, as exposure to ambient
y
Absorption spectrum of cis-2C contains a split Qy-type band,
with the maximum of the more intensive band centered at 659
nm and a second maximum centered at 704 nm (the second
maximum coincidentally overlaps with the maximum of Qy
band in trans-2C; therefore, we cannot rule out that a trace
amount of trans-2C is present even in a sample purified under
rigorous exclusion of light). Similar splitting is observed for cis-
2BC, where corresponding maxima are centered at 752 and
811 nm. Observed splitting of the Qy bands in dyads is
consistent with the excitonic interaction between closely
positioned macrocycles in the “oblique” orientation of
transition dipole moments,1 which is consistent with a slipped
cofacial geometry of dyads, as discussed above. Such excitonic
interaction results in the formation of two new states, P− with
lower energy and P+ with higher energy, compared to the
excited state of monomers. In a slipped cofacial geometry, both
transitions (S0 → P− and S0 → P+) are allowed. The electronic
coupling between macrocycles (V, calculated as one-half of the
splitting energy of Q-like band) is V = 474 cm−1 for cis-2C and
Figure 2. Changes in the absorption spectrum of trans-2C upon
irradiation at the maximum of the B band absorption in toluene
(absorption spectra were taken in 1 min intervals).
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Figure 5. Emission spectra of dyads in toluene at room temperature:
phen-2C (black), cis-2ZnC (blue), cis-2C (green), cis-2BC (red).
Emission spectrum of trans-2C is identical with that of cis-2C and it
is not shown.
low Φf observed for cis-2C, cis-2ZnC, and cis-2BC are
characteristic for excimers.79−84 Excimer is formally formed by
the mixing of localized excited states and charge-resonance
transfer states,83,85 and its formation, which is often also
accompanied by large geometrical relaxation, leads to the
reduction of the excited-state energy.86,87 It is characteristic
that both Stokes shift and FWHM of the emission band
parallel the degree of electronic coupling calculated from
splitting of the Qy-type band. It is interesting to note that a
large Stokes shift and broad emission band were also reported
for P867, for which the primary electron acceptor (bacterio-
chlorophyll a monomer) was reduced.88 The weak shoulder on
the short wavelength side of the emission bands of dyads is
attributed either to the emission from the localized, monomer-
like excited state or to the trace amount of strongly fluorescent
monomeric impurities. Further research is being performed to
clarify this issue.
Emission spectrum of trans-2C is identical with that for cis-
2C. Excitation spectrum for trans-2C overlaps very well with
the absorption spectrum of cis-2C. These suggest that the
excitation of trans-2C leads first to isomerization of trans-2C*
to cis-2C* followed by emission from the excited state
corresponding to the latter isomer. This assumption is
additionally supported by the near-identical Φf for both
trans-2C and cis-2C in toluene (Table 1).
Contrary to dyads discussed above, phen-2C exhibits
narrow, monomer-like emission, with a small Stokes shift,
though with split maxima. Such splitting has been reported
previously for cofacial chlorin dyads, and it was attributed to
the presence of two conformers in the excited state.50 This is
likely the case here. Relative to monomer C-PE, Φf for phen-
2C is much reduced (1.6-fold) but not as extensively as in the
case of cis-2C (4.1-fold, Table 1).
DFT Calculations. The absorption and emission features of
cis-2C, cis-2ZnC, and cis-2BC indicate strong interactions
between chromophores, which leads to optical properties
substantially different from those observed for both linear
strongly conjugated and simple cofacial hydroporphyrin dyads.
The overall interactions between hydroporphyrins in these
dyads, in principle, are the product of (1) dipole−dipole
interactions between chromophores (i.e., long-range excitonic
coupling),1 (2) through-space interaction between overlapped
orbitals of closely positioned macrocycles (i.e., short-range
excitonic coupling or exchange interactions),17 and (3)
Figure 4. Normalized absorption spectra of dyads in toluene at room
temperature: phen-2C (black), cis-2ZnC (blue), cis-2C (green),
trans-2C (purple), cis-2BC (red). The lower panel presents the
expanded region of Qy bands.
V = 483.7 cm−1 for cis-2BC. The excitonic coupling for cis-
2ZnC V = 724 cm−1 is much larger than that observed for the
free base, despite nearly identical geometry of both dyads. The
absorption spectrum of phen-2C does not show a distinct
splitting but a shoulder at the long-wavelength side of the Qy
band. The fitting of Gaussian curves gave two peaks with
maxima at 653 and 671 nm, which indicates a V = 205 cm−1.
The coupling strength between hydroporphyrins in cis-2C
and cis-2BC is thus comparable to that observed in the purple
bacteria special pair P867 (V = 550 cm−1). For cis-2ZnC, the
coupling is slightly lower than that observed for P960 (V = 900
cm−1).
Emission Properties. Emission properties of dyads in
toluene are given in Figure 5 and Table 1. Dyads cis-2C, cis-
2ZnC, and cis-2BC feature structureless, broad (full width at
half-maximum (FWHM) > 900 cm−1) emission, with a
relatively large Stokes shift (i.e., 449, 832, and 311 cm−1 for
cis-2C, cis-2ZnC, and cis-2BC, respectively). These emission
features differ significantly from those observed for monomers
or previously reported ethynyl-, butadienyl-, or vinyl-linked,
strongly conjugated hydroporphyrin arrays, which all exhibit
relatively narrow (FWHM < 400 cm−1) emission with Stokes
shift of < 200 cm−1.55,57 In addition, the fluorescence quantum
yield (Φf) is substantially reduced in comparison to respective
monomers (4-fold for cis-2C compared to that of C-PE and 6-
fold for cis-2BC compared to that of BC-2PE), which also
differs from that observed for previously reported strongly
conjugated hydroporphyrin dyads, which typically exhibit
enhanced Φf, compared to the corresponding monomers.55,57
Broad emission spectra with a relatively large Stokes shift and
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a
Table 1. Absorption and Emission Properties of Dyads and Monomers Discussed in This Paper
b
c
compound
λB
398
401
404
408
364
402, 417
379
λQ
λQ
λem (FWHM )
Stokes shift (cm−1
)
Φf
x
y
trans-2C
cis-2C
551
507
508
701
660, 704
653
628, 690
752, 811
656
727 (1008 cm−1
727 (1008 cm−1
657, 678
)
)
510
449
0.092
0.082 (<0.01)
0.21 (0.20)
not determined
0.032 (<0.01)
0.33 (0.34)
0.19
phen-2C
cis-2ZnC
cis-2BC
732 (1475 cm−1
)
832
311
69
535
506
523
832 (957 cm−1
659 (285 cm−1
753 (323 cm−1
)
)
)
d
C-PE
e
BC-2PE
752
18
a
b
All data were acquired in air-equilibrated toluene at room temperature unless noted otherwise. Full width at half-maximum.
Tetraphenylporphyrin (Φf = 0.07089) was used as a standard for Φf determination. Values in parentheses are Φf in air-equilibrated
c
d
e
dimethylformamide. Data taken from ref 57. Data taken from ref 58, determined in Ar-purged toluene.
a
Table 2. DFT-Calculated MOs Energies for Dyads and Model Compounds
entry
compound
trans-2C
HOMO − 1 (eV) (ΔE)
HOMO (eV) (ΔE)
LUMO (eV) (ΔE)
LUMO + 1 (eV) (ΔE)
1
2
3
4
5
6
7
8
−5.10, −5.08 (0.02)
−5.12, −5.12 (0.00)
−5.08, −5.09 (0.01)
−5.30, −5.19 (0.11)
−5.14, −5.07 (0.07)
−5.13, −5.05 (0.08)
−5.14, −5.07 (0.07)
−5.40, −5.27 (0.13)
−5.24, −5.12 (0.12)
−5.21, −5.11 (0.10)
−5.08, −5.00 (0.08)
−5.00, −4.76 (0.24)
−4.95, −4.94 (0.01)
−4.77, −4.99 (0.22)
−5.13, −4.89 (0.24)
−4.96, −4.87 (0.09)
−4.96, −4.78 (0.18)
−4.96, −4.88 (0.08)
−5.07, −4.84 (0.23)
−4.85, −4.80 (0.05)
−4.80, −4.58 (0.22)
−2.79, −2.28 (0.51)
−2.36, −2.36 (0.00)
−2.76, −2.27 (0.49)
−2.78, −2.38 (0.40)
−2.38, −2.32 (0.06)
−2.44, −2.27 (0.17)
−2.38, −2.31 (0.07)
−2.71, −2.35 (0.36)
−2.30, −2.27 (0.03)
−2.85, −2.53 (0.32)
−2.53, −2.45 (0.08)
−1.95, −1.75 (0.20)
−1.77, −1.77 (0.00)
−1.96, −1.75 (0.19)
−1.98, −1.92 (0.06)
−1.78, −1.74 (0.04)
−1.76, −1.73 (0.03)
−1.78, −1.74 (0.04)
−1.90, −1.80 (0.10)
−1.64, −1.63 (0.01)
−1.97, −1.30 (0.67)
−1.20, −1.15 (0.05)
b
trans-2C disconnected
c
trans-2C twisted
cis-2C
cis-2C disconnected
phen-2C
phen-2C disconnected
cis-2ZnC
cis-2ZnC disconnected
cis-2BC
cis-2BC disconnected
b
b
b
9
10
11
b
−4.61, −4.53 (0.08)
a
b
All calculations were performed employing the DFT B3LYP 6-31G* method. MOs were calculated for prior energy-minimized structures of
corresponding dyads, for which the fumarate (or 1,2-phenylene) part of the linker was removed and replaced by hydrogens, which effectively
corresponds to two 13-acetylene-substituted hydroporphyrins, with mutual distance and orientation identical as in corresponding dyads. MOs
c
calculated for the structure where the dihedral angle between the macrocycle plane and the plane formed by enediyne linker is 26°.
through-bond electronic coupling, provided by a conjugated
linker.25 The rigorous delineation of effects of each of these
three interactions on the observed absorption and emission
properties of enediyne dyads, and the evaluation of their
possible synergistic effects are far beyond the scope of the
current paper and will be discussed in the subsequent
publication. Here, we use DFT calculations for the preliminary
evaluation of through-bond electronic conjugation and
through-space orbitals’ overlap contribution to the overall
coupling. We focus here on the effect of the electronic
interactions between hydroporphyrins on HOMO − 1, highest
occupied molecular orbital (HOMO), lowest unoccupied
molecular orbital (LUMO), and LUMO + 1, since configura-
tional interactions between these MOs give rise to the
spectroscopic properties in tetrapyrrolic macrocycles90 (Table
2). First, we calculated the MO energies for each dyad
reported here, for their energy-minimized geometries. For each
dyad, MOs, which are characteristic for monomers, are split
into a pair of new orbitals with the same symmetry and
delocalized on both macrocycles. Such splitting is a signature
of a strong electronic interaction between macrocycles and is
observed for noncovalent bacteriochlorins, investigated as
models for photosynthetic reaction centers,91,92 and for each
strongly conjugated and directly linked hydroporphyrin dyad
reported previously.56,59,93 The splitting energies (ΔE) for
trans-2C, cis-2C, cis-2ZnC, and cis-2BC, are comparable to
ethynyl- and butadienyl-linked dyads,56,59 suggesting compa-
rable strength of electronic interaction. Next, we calculated the
electronic structure for nonconnected acetylene-substituted
hydroporphyrin pairs, with interpigment distances and mutual
orientations identical as in corresponding dyads (i.e., beginning
from the energy-minimized structure of each enediyne dyad,
the fumarate is removed, and for phen-2C, the o-phenylene is
removed (Table 2, entries 2, 5, 7, 9, and 11)). These
calculations enable the evaluation of the degree of ground-state
through-space electronic interaction in cis isomers of dyads. As
is shown in Table 2, the nonconnected analogs of cis dyads also
exhibit splitting of relevant orbitals, albeit with ΔE significantly
smaller (>2-fold decrease), than in covalent dyads. As
expected, the nonconnected analog of trans-2C (entry 2)
shows virtually no splitting and instead two sets of degenerate
orbitals, localized on a single chlorin, each.
The through-bond π-conjugation between macrocycles in cis
isomers is possibly due to the relatively small angle in which
the macrocycle is twisted versus the double bond in enediyne
linker (around 25°). Such a twist causes only a small
perturbation in electronic π-conjugation, when compared to
that in the fully planar geometry, as is shown by calculations
performed on trans-2C, where chlorin macrocycles and the
plane defined by enediyne linker are twisted by 25° (entry 3,
the corresponding angles in the energy-minimized structure are
7°).
These results show that in cis dyads substantial through-
space and through-bond electronic interactions contribute to
the dipole−dipole, long-range excitonic coupling between
hydroporphyrins, and their combined effect gives rise to the
observed absorption properties. Through-space interactions
between chlorins seem to be nearly the same for cis-2C and
phen-2C (compare entries 5 and 7, Table 2), which is
expected given very similar distances and mutual orientations
G
DOI: 10.1021/acs.joc.9b00731
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The Journal of Organic Chemistry
Article
DFT calculations were performed, and the results were visualized
using Spartan 10 for Windows (Wavefunction Inc., Irvine, CA).
B3LYP 6-31G* method in vacuum was used for each calculation. Full-
structure optimization was performed on each dyad, and several
different conformations were examined to find the global energy
minimum. For “disconnected” dyads, the computations were
performed on a previously fully optimized corresponding dyad
structure, after removing the fumarate fragment in the linker and
replacing by hydrogen atoms on the remaining acetylene fragments,
without any further structure optimization.
of macrocycles in both dyads. Therefore, a substantially
stronger electronic interaction between chromophores in cis-
2C (as inferred from the larger splitting of both Qy-type band
in absorption spectrum and splitting of relevant MOs) is due
to stronger through-bond interactions provided by enediyne
linker, compared to those by 1,2-diethynylphenylene linker.
The DFT calculations, performed on model dyad connected by
1,4-diacetylenephenylene-linker, indeed showed lower values
of ΔE than for trans-2C (see Table S2, Supporting
Information). These results are in accord with the previously
reported data for enediyne derivatives. For example, cis-1,6-
diphenylhexa-3-en-1,5-diyne features absorption maximum at
slightly longer wavelength compared to 1,2-bis-
(phenylethynyl)benzene (352 nm62 vs 343 nm,94 respectively).
Known compounds: Ch-E57 and BC-E55 were prepared following
reported procedures. Synthesis of dimethyl dibromofumarate 1 was
reported numerous times,74,75 and here, 1 was synthesized following a
slightly modified procedure.95
Dimethyl 2,3-Dibromofumarate (1). Following a reported
procedure,95 a solution of dimethyl acetylene dicarboxylate (2.86 g,
20.0 mmol) in anhydrous CH2Cl2 (20 mL) was treated by pyridinium
tribromide (2.47 g, 28.1 mmol). The resulting mixture was stirred
under argon at room temperature for 6 days. The reaction mixture
was treated by an excess of 10% aqueous Na2S2O3, and the resulting
mixture was extracted by diethyl ether. The organic phase was washed
(water and brine), dried (MgSO4), and concentrated. The resulting
oil was purified by column chromatography (silica, CH2Cl2/hexanes
(1:1) → neat CH2Cl2) to afford a pure isomer as a white solid: 2.31 g,
38%. Characterization data (1H and 13C NMR) are consistent with
those reported previously for dimethyl 2,3-dibromofumarate.75
Sonogashira Reaction between Ch-E and 1 Performed without
Protection from Light. Following the general procedure, a mixture of
Ch-E (26.0 mg, 0.054 mmol), 1 (8.2 mg, 27.0 μmol), Pd2(dba)3 (7.4
mg, 8.1 μmol), and P(o-tol)3 (4.1 mg, 0.014 mmol) in anhydrous
toluene/Et3N (5:1, 12.0 mL) was stirred at 60 °C overnight. After 16
h, the reaction mixture was concentrated. Column chromatography
[silica, hexanes/CH2Cl2 (3:7)] afforded b-2C (first fraction, red-
brown, yield not determined), followed by cis-2C as a dark green
solid (8.2 mg, 28%). A small amount of trans-2C was also isolated.
CONCLUSIONS
■
Trans−cis photoisomerization of the enediyne linker leads to
thermally stable slipped cofacial hydroporphyrin dyads, where
the distance and mutual orientation of hydroporphyrins
resemble those observed for the special pairs of reaction
centers in photosynthesis. The close positioning of hydro-
porphyrins and the presence of a conjugated enediyne linker
lead to strong through-bond and through-space electronic
interactions between macrocycles, which enhanced the di-
pole−dipole, long-range excitonic coupling. A strong inter-
chromophoric interaction is manifested by a large splitting of
the Qy-type band and splitting of the MOs. Evidence for the
formation of an excimer, i.e., broad emission band, with an
unusually large Stokes shift, has been found. The emission
features resemble those observed for the special pair. In-depth
discussion of the spectroscopic properties and correlation
between electronic and spectroscopic properties will be
presented in the next paper. We anticipate that since both
the geometry and spectral properties of resulting dyads
resemble those observed in special pairs of photosynthetic
reaction centers, they will be useful components of energy and
electron-transfer arrays, for modeling photosynthetic reaction
centers and development of artificial solar energy conversion
systems. Moreover, the connection of chromophores by a
conjugated enediyne linker and subsequent trans−cis photo-
isomerization appears to be a viable strategy for the
development of photonic materials with multiple modes of
electronic interactions and thus novel optical properties.
1
Data for cis-2C: H NMR (400 MHz, CDCl3): δ −2.14 (bs, 2H),
−1.43 (bs, 2H), −0.48 (s, 12H), 1.91 (s, 12H), 2.11 (s, 4H), 2.60 (s,
6H), 4.13 (s, 6H), 7.25 (s, 4H), 7.67 (s, 2H), 8.43 (d, J = 4.7 Hz,
2H), 8.45 (d, J = 4.4 Hz, 2H), 8.80 (d, J = 4.4 Hz, 2H), 8.93 (s, 2H),
8.95 (d, J = 4.7 Hz, 2H), 9.23 (s, 2H), 9.54 (s, 2H); 13C{1H} NMR
(100 MHz, CDCl3): δ 21.4, 21.6, 28.8, 29.6, 29.9, 44.8, 49.3, 53.7,
93.0, 93.9, 96.1, 100.2, 106.3, 114.9, 121.6, 124.3, 127.0, 128.0, 128.8,
129.1, 132.1, 132.8, 133.0, 135.7, 137.5, 138.0, 139.1, 139.7, 141.6,
152.6, 152.8, 163.3, 164.9, 176.2; HRMS (electrospray ionization
time-of-flight, ESI-TOF) m/z [M + H]+ calcd for C72H65N8O4,
1105.5123; found 1105.5175.
1
Data for b-2C: H NMR (CDCl3): δ −1.79 (bs, 2H), −1.41 (bs,
2H), 1.90 (s, 12H), 2.09 (s, 12H), 2.64 (s, 6H), 4.71 (s, 4H), 7.29 (s,
4H), 8.48 (d, J = 4.1 Hz, 2H), 8.86−8.84 (m, 6H), 8.92 (d, J = 4.6
Hz, 2H), 9.19 (d, J = 4.6 Hz, 2H), 9.37 (s, 2H), 9.71 (s, 2H);
13C{1H} NMR (CDCl3): δ 21.4, 29.8, 31.2, 46.8, 51.8, 94.7, 95.2,
EXPERIMENTAL SECTION
■
General. 1H NMR (400 or 500 MHz) and 13C NMR (100 or 125
MHz) spectra were collected at room temperature in CDCl3 unless
noted otherwise. Chemical shifts (δ) were calibrated using solvent
peaks (1H signals: residual proton signals: 7.26 ppm for CHCl3, 13C
signals: 77.16 for CDCl3). Commercially available anhydrous solvents
were used for palladium coupling reactions. All other solvents and
commercially available reagents were used as received. Fourier
transform ion cycloctron resonance (FT-ICR) analyzer was used for
high resolution mass spectrometry (HRMS) analyses.
106.8, 108.9, 115.1, 121.5, 124.5, 127.9, 129.1, 129.8, 132.1, 132.8,
133.0, 135.7, 137.4, 138.0, 139.2, 140.8, 142.1, 152.7, 152.8, 163.5,
176.8; HRMS (ESI-TOF): m/z [M + H]+ calcd for C66H58N8,
963.4857; Found 963.4890.
Sonogashira Reaction between Ch-E and Dimethyl 2,3-
Dibromofumarate (1) Performed with Protection from Light. The
reaction mixture was protected from light using an aluminum foil at
each stage of the reaction and purification. All operations were
performed in the dark, using a red light-emitting diode headlamp as
an only source of light. Following the general procedure, a mixture of
Ch-E (37.9 mg, 0.079 mmol), 1 (11.9 mg, 0.039 mmol), Pd2(dba)3
(10.1 mg, 0.011 mmol), and P(o-tol)3 (10.8 mg, 0.035 mmol) in
anhydrous toluene/Et3N (5:1, 18.0 mL) was stirred at 60 °C. After 21
h, the reaction mixture was concentrated. Column chromatography
[silica, hexanes/dichloromethane (2:1)] yielded trans-2C (9.3 mg,
21%) as a brown solid. cis-2C was also observed on thin-layer
chromatography (TLC) as a minor product but was not isolated
cleanly in this procedure. 1H NMR (400 MHz, CDCl3): δ −1.69 (bs,
2H), −1.26 (bs, 2H), 1.89 (s, 12H), 2.10 (s, 12H), 2.65 (s, 6H), 4.37
General Procedure for Palladium Cross-Coupling Reactions.
All reagents and solvents with the exception of palladium catalyst were
placed in a Schlenk flask, and the contents were degassed by two
cycles of freeze−pump−thaw, at which time the catalyst is added, and
a third cycle of freeze−pump−thaw is performed. The reaction
mixture was stirred under N2 and heated in an oil bath at an indicated
temperature.
Characterization. All NMR spectra were acquired on 400 MHz
NMR or 500 MHz NMR. All HRMS data were acquired on 12T FT-
ICR MS.
H
DOI: 10.1021/acs.joc.9b00731
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The Journal of Organic Chemistry
Article
(s, 6H), 4.74 (s, 4H), 7.29 (s, 4H), 8.46 (d, J = 4.3 Hz, 2H), 8.81 (s,
2H), 8.83 (s, 2H), 8.83 (d, J = 4.3 Hz, 2H), 8.92 (d, J = 4.6 Hz, 2H),
9.17 (d, J = 4.6 Hz, 2H), 9.57 (s, 2H), 9.68 (s, 2H); HRMS (ESI-
TOF) m/z [M + H]+ calcd for C72H65N8O4,1105.5123; found
1105.5136.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mptaszek@umbc.edu.
ORCID
phen-2C. Following the general procedure, a mixture of Ch-E
(30.0 mg, 0.062 mmol), 1,2-dibromobenzene (7.3 mg, 0.031 mmol),
Pd2(dba)3 (8.6 mg, 9.3 μmol), and P(o-tol)3 (8.5 mg, 0.028 mmol) in
anhydrous toluene/Et3N (5:1, 6.0 mL) was stirred at 60 °C. After 16
h, the reaction mixture was concentrated. Column chromatography
[silica, hexanes/dichloromethane (2:1)] yielded a green solid (12.3
mg, 38%). Note that a red fluorescent impurity nearly coeluted with
the desired product, and early fractions of the product were discarded.
1H NMR (400 MHz, CDCl3): δ −2.28 (bs, 2H), −1.68 (bs, 2H),
−0.36 (s, 12H), 1.88 (s, 12H), 2.25 (s, 4H), 2.58 (s, 6H), 7.22 (s,
4H), 7.56 (dd, J1 = 3.3 Hz, J2 = 5.8 Hz, 2H), 7.78 (s, 2H), 7.99 (dd, J1
= 3.3 Hz, J2 = 5.8 Hz, 2H), 8.46 (d, J = 4.3 Hz, 2H), 8.48 (d, J = 4.6
Hz, 2H), 8.83 (d, J = 4.3 Hz, 2H), 8.94 (s, 2H), 8.99 (d, J = 4.6 Hz,
2H), 9.41 (s, 2H), 9.61 (s, 2H);13C{1H} NMR (100 MHz, CDCl3): δ
21.7, 21.8, 29.1, 44.9, 49.8, 77.6, 89.7, 94.1, 96.0, 96.2, 106.7, 117.2,
120.8, 123.9, 126.4, 128.1, 128.5, 128.8, 128.9 131.8, 132.8, 133.1,
133.2, 135.2, 137.9, 138.0, 139.3, 139.9, 141.2, 152.2, 152.8, 163.5,
175.7; HRMS (ESI-TOF) m/z [M + H]+ calcd for C72H63N8,
1039.5170; found 1039.5146.
Marcin Ptaszek: 0000-0001-6468-6900
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF (under award CHE-
1301109) and, in part, by UMBC (start-up and SRAIS award).
The authors thank Dr. Joshua Akhigbe for the synthesis of
compound 1 and Drs. Dewey Holten (Washington University
in St. Louis) and David F. Bocian (UC Riverside) for a
valuable discussion.
REFERENCES
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1
dark green solid (6.0 mg, 67%). H NMR (400 MHz, CDCl3): δ
−0.08 (s, 12H), 1.91 (s, 12H), 2.44 (s, 4H), 2.56 (s, 6H), 4.10 (s,
6H), 7.19 (s, 2H), 7.60 (s, 2H), 8.32 (d, J = 4.3 Hz, 2H), 8.39 (d, J =
4.3 Hz, 2H), 8.75 (d, J = 4.3 Hz, 2H), 8.85 (d, J = 4.4 Hz, 2H), 8.88
(s, 2H), 9.06 (s, 2H), 9.39 (s, 2H); HRMS (ESI-TOF) m/z [M + H]+
calcd for C72H61N8O4Zn2, 1229.3393; found 1229.3453.
cis-2BC. A mixture of BC-E (10.5 mg, 20 μmol), 1 (3.0 mg, 10
μmol), P(o-tol)3 (3.7 mg, 12 μmol), and Pd2(dba)3 (2.8 mg, 3.1
μmol) in toluene (10 mL) and Et3N (2 mL) was stirred at 60°C for 5
h. The resulting mixture was concentrated, and the residue was
purified with column chromatography [silica, hexanes/ethyl acetate
(2:1)] to afford a red-purple solid (4.1 mg, 35%). Approx. 20% of b-
2BC55 was also isolated. H NMR (CDCl3, 400 MHz) δ −1.96 (s,
1
2H), −1.50 (s, 2H), 0.54 (s, 12H), 1.87 (s, 12H), 2.80 (s, 4H), 4.18
(s, 6H), 4.40 (s, 4H), 4.47 (s, 6H), 7.40−7.51 (m, 6H), 7.82 (d, J =
7.3 Hz, 4H), 7.88 (s, 2H), 8.47 (s, 2H), 8.56 (d, J = 2.1 Hz, 2H), 8.87
(d, J = 1.9 Hz, 2H), 8.96 (s, 2H); 13C{1H} NMR (CDCl3, 100 Hz) δ
29.7, 31.0, 44.8, 45.3, 48.1, 49.9, 53.7, 64.8, 87.0, 93.4, 94.4, 96.5,
97.3, 98.3, 100.6, 113.9, 114.1, 124.3, 125.9, 126.9, 127.9, 128.5,
128.7, 131.9, 132.1, 134.9, 135.56, 135.60, 138.2, 157.0, 161.0, 165.1,
169.6, 171.3; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C76H69N8O6, 1189.5335; found 1189.5274.
ASSOCIATED CONTENT
■
S
* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.9b00731.
Additional synthetic scheme, assignment of signals in 1H
NMR spectra, results of additional DFT calculations,
results of the studies on thermal isomerization of cis-2C
and trans-2C, atom coordinates and absolute energies
1
for dyads, and copies of H and 13C NMR spectra for
new compounds (PDF)
I
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