Superbenzene-Porphyrin Conjugates

Article abstract of DOI:10.1002/anie.201407053

A free-base porphyrin carrying two hexabenzocoronene (HBC) substituents in a trans arrangement and its zinc complex have been prepared. The compounds were characterized extensively and found to form tricationic dimers in the gas phase. X-ray crystallography confirms for the zinc complex a profound π-stacking of the HBC moieties. In contrast, the free-base porphyrin incarcerates n-heptane which essentially prevents π-stacking. Upon excitation of the HBC substituents, efficient energy transfer to the central porphyrin is observed. Super size me! The first porphyrin conjugate with two superbenzene units shows properties induced by the superbenzene and porphyrin moieties, but also behavior that differs distinctly from that of the individual buildung blocks. The molecule is characterized by standard spectroscopic methods and modern mass spectrometry, and the first X-ray structure of such a conjugate is presented.

Full text of DOI:10.1002/anie.201407053

Angewandte
Chemie
DOI: 10.1002/anie.201407053
Carbon-Rich Porphyrins
Superbenzene–Porphyrin Conjugates**
Dominik Lungerich, Jakob F. Hitzenberger, Mario Marcia, Frank Hampel, Thomas Drewello,*
and Norbert Jux*
Abstract: A free-base porphyrin carrying two hexabenzocor-
onene (HBC) substituents in a trans arrangement and its zinc
complex have been prepared. The compounds were charac-
terized extensively and found to form tricationic dimers in the
gas phase. X-ray crystallography confirms for the zinc complex
a profound p-stacking of the HBC moieties. In contrast, the
free-base porphyrin incarcerates n-heptane which essentially
prevents p-stacking. Upon excitation of the HBC substituents,
efficient energy transfer to the central porphyrin is observed.
solubility of fivefold tert-butylated HBC aldehyde 1[6] in
common organic solvents, such as dichloromethane, makes
this molecule a versatile building block for the construction of
larger carbon-rich architectures and is key to a successful
porphyrin synthesis. Compound 1 was successfully condensed
with functionalized dipyrromethane 2[7]—chosen because it
provides further handles for functionalization—to obtain the
HBC–porphyrin conjugate 3 in high yield (Scheme 1). The
reaction was carried out under mild Lindsey conditions,[8] with
I
n recent years, efforts have been made to extend the
substituents on porphyrins from common phenyl and aryl
groups to larger polycyclic aromatic hydrocarbons (PAHs)
including naphthyl, pyrenyl, and even coronenyl groups.[1]
Typically substitution is associated with a bathochromic shift
of the absorption of these compounds compared to the
absorption of the parent tetraphenylporphyrin (TPP). The
properties can be further influenced upon fusion of porphyrin
and substituents through the formation of an additional bond
between the substituent and the b-pyrrolic position of the
porphyrin core.[2] However, in these systems, the porphyrin
moiety is still the dominating part determining the properties
of the molecular superstructure. Only little is known about
carbon-rich porphyrins with structural features that are
dominated by large PAHs. While in TPP the phenyl moieties
mainly impart solubility, large PAHs are predestined to have
a profound impact on the molecular properties paving the
way towards novel materials for nanoelectronics. Due to its
size and electronic properties, hexa-peri-hexabenzocoronene
(HBC)—“superbenzene”[3]—comes to mind as the almost
ideal candidate to investigate this matter.[4] Nonetheless, only
little is known about superbenzene–porphyrin conjugates,
since their synthesis faces considerable difficulties.[5]
Herein we present a straightforward synthetic strategy
towards those carbon-rich conjugates taking advantage of
soluble superbenzene derivatives. The impressively high
Scheme 1. Syntheses of free-base conjugate 3 and its ZnII derivative 4.
a) cat. BF3·OEt2, cat. EtOH, CHCl3, RT, then DDQ (M=2H) (10%);
b) Zn(OAc)2·2H2O, THF, reflux (M=Zn) (100%).
[*] D. Lungerich, M. Marcia, Dr. F. Hampel, Prof. Dr. N. Jux
Department Chemie und Pharmazie & Interdisciplinary Center for
Molecular Materials (ICMM)
application of catalytic BF3·OEt2 as a Lewis acid, followed by
oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ). Conjugate 3 was obtained as a dark red solid after
chromatography. Subsequent metalation of 3 by reaction with
Zn(OAc)2·2H2O in THF under reflux gave the ZnII derivative
4 in quantitative yield. The first structural evidence for the
desired conjugate 3 came from the 1H NMR spectrum:
a pronounced downfield shift towards d = 10.18 ppm for the
four HBC protons adjacent to the porphyrin core, a broad
multiplet at around d = 9.37 ppm for the remaining twenty
HBC protons, doublets at d = 9.15 and 8.83 ppm for the b-
pyrrylic protons, and the inner-NH protons of the porphyrin
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg
Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: norbert.jux@fau.de
J. F. Hitzenberger, Prof. Dr. T. Drewello
Department Chemie und Pharmazie
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
E-mail: thomas.drewello@fau.de
[**] We thank the DFG (SFB 953, Synthetic Carbon Allotropes) for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407053.
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Communications
moiety at d = À2.11 ppm. The sharp signals indicate that there
is no aggregation in solution.
The X-ray diffraction analysis (Figure 1) of a single crystal
of 3, grown at room temperature from a mixture of CH2Cl2, n-
heptane, and methanol, represents the first crystallographic
characterization of a superbenzene–porphyrin derivative and
establishes its structure beyond any doubt.[9] The molecule
ꢀ
crystallizes in a triclinic lattice with the space group P1 and
the unit cell contains six n-heptane molecules. The two HBC
Figure 2. MALDI-TOF mass spectrum of 3 in the positive-ion mode;
matrix: trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malono-
nitrile (DCTB); radical cations are depicted as (3)n+·. a) Reflectron
mode detection, b) linear mode detection, c) as (b) with magnification
of the larger clusters.
(Figure 2). Reflectron mode detection reveals that the
clusters de-aggregate considerably as they pass through the
+
mass spectrometer; only dimer (3)2 C in small amounts could
be detected. This behavior is in line with the noncovalent
+
nature of the bonding within the (3)n C cluster ions. In
experiments in which the decay behavior of size-selected
cluster ions is studied (post source decay, PSD), the release of
several moieties of 3 from the selected precursor ion is
observed, which supports the assumption that the molecules
of 3 retain their molecular integrity within the clusters. 3 itself
showed the expected stability and formed abundantly intact
molecular ions even under direct laser desorption/ionization
(i.e. without matrix, LDI).
Figure 1. Crystal structure of free-base HBC–porphyrin conjugate 3
(hydrogen atoms are omitted for clarity). a) ORTEP model, b) crystal
packing.
moieties are rotated by about 62.28 with respect to the
porphyrin plane; the two ortho-disubstituted phenyl substitu-
ents are tilted by about 87.78 and stand almost perpendicular
to the porphyrin plane. Both HBCs and the porphyrin core
experience distortion in the crystal packing and lose their
planarity. Although the HBC units are packed in parallel, the
average distance between two overlapping HBC moieties is
7.36 ꢀ and therefore p–p interactions in the solid state can be
ruled out. In the rather large cavities between the HBC
planes, n-heptane is entrapped rather unusually due to
hydrogen–p interactions at a distance of 2.9–3.0 ꢀ and acts
essentially as an insulator. This is in striking contrast to what
we found in a preliminary crystal structure of the zinc
complex 4.[10]
The propensity of 3 to form aggregates under the present
conditions is quite remarkable compared to the clustering
behavior of pentacene (5) and hexa-tert-butyl-hexa-peri-
hexabenzocoronene (6). Clusters of compound 5 were
observed previously in a dedicated high-pressure MALDI
experiment, which promoted ion/molecule reactions and
proved the existence of pentacene aggregates of the type
(5)n C with up to n = 5.[11] However, under the present
+
conditions, neither 5 nor 6 show signs of higher aggregates,
while 3 shows efficient cluster formation, underpinning the
propensity of 3 to form aggregates under the applied
conditions.
The ESI-MS shows the quasi-molecular ion peak of the
protonated conjugate 3, 3H+, the isotope pattern of which
confirms its identity and reveals the co-formation of doubly
protonated dimer, 32H22+. Besides protonation, oxidation of 3
occurs as an additional, competing ion-formation process.[12]
The oxidation of 3 leads to the formation of the dication 32+,
Interesting observations regarding the formation of
aggregates of 3 have been made by mass spectrometry.
MALDI-, LDI-, and ESI-MS were employed to characterize
3. In MALDI, the observed molecular ion is abundant and
+
accompanied by several cluster ions of the type (3)n C, as
3+
detected in linear (n = 2–6) and reflectron (n = 2) modes
the trication 33+, and the trication of the dimer of 3, (3)2 C.
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repulsion by separating the charges from each other as much
as possible.
We assume that the dimer is formed in the electrospray
process by the aggregation of two neutral molecules or by the
clustering of one neutral and one already charged species.
This process may be followed by further oxidation leading to
the observed trication. Formation of the dimer from two
already ionized monomers would require overcoming the
Coulombic repulsion. The high oxidation state of the dimer
may be favored under the applied spray conditions, since
monomeric 3 also appears in the 2 + and 3 + charge states,
while the radical cation is entirely absent. The fact that
aggregates of 3 are formed both in MALDI and ESI
experiments is indicative of the effective noncovalent inter-
action within the cluster. In light of the structural features of
3, p–p radical cation interactions seem a likely source of the
attractive forces in the gas phase, whereas X-ray and NMR
analysis suggest a different explanation in the solid state and
in solution. Clearly, the solvent plays a decisive structural role
which is reflected in the results obtained by the methods of
characterization.
Electronic interactions between the two HBC moieties
and the porphyrin core have been thoroughly investigated in
solution utilizing absorption and emission spectroscopy. The
spectra are shown in Figure 4 and the spectroscopic details
can be found in Table 1. The spectral features of porphyrin 3
are quite remarkable and the characteristic peaks of both
HBC and porphyrin building blocks are visible. The same is
true for the metalated derivative 4. The absorption features of
the conjugates are also affected by the electronic intra-
molecular communication between the HBC periphery and
the porphyrin core. A comparison of their spectra with those
of reference compound 6 and reference porphyrin 7[7] allows
for a better understanding of the optical properties of the
conjugates.
2+
Small amounts of the dication (3)2 are also detected
(Figure 3). The formation of a triply ionized dimer is most
remarkable and particularly rare. It requires that the strength
of supramolecular bonds between the two HBC–porphyrin
conjugates 3 exceeds the Coulombic repulsion of the charges.
The charges are distributed as in 32+ 3 , since the dimer
+
À
dissociates in collision-induced dissociations into the mono-
meric di- and radical cations. A possible charge distribution
may involve the large HBC moieties, reducing Coulombic
First of all, both the b- and the p-band of the HBC residue
are slightly hypsochromically shifted (ca. 2 nm) for 3 and 4,
with respect to reference 6. Bearing in mind
that two HBC units are present in the
conjugate, the extinction coefficient (e) at
the lmax value of the HBC region is
decreased for both 3 and 4 with respect to
the reference. For 3 this decrease in emax
amounts to approximately 34%, which is
already quite substantial, and for 4 it is even
larger (ca. 52%). Furthermore, both 3
and—to a lesser extent—4 exhibit a splitting
of the porphyrin Soret band (B-band): the
two peaks lie at 426 and 438 nm for 3 while
they are at 434 and 444 nm for 4. Addition-
ally, the B-band of 3 is bathochromically
shifted (on average 12 nm) and consider-
ably reduced in intensity (by ca. 50%) with
respect to reference porphyrin 7. A similar
red-shift, of about 5 nm, is observed for the
Q-bands as well. Finally, the presence or
absence of the central metal atom has
a profound effect on the position/intensity
of both the B- and Q-bands of the porphy-
Figure 3. ESI-TOF mass spectrum of 3 in the positive-ion mode; the inserts show measured
and simulated isotope patterns.
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with respect to reference 6. Upon excitation at lmax of the b-
band of HBC in the conjugate, however, strong fluorescence
from the porphyrinꢁs Q-bands can be detected. Upon
excitation at lmax,Porph in the conjugate, no emission from the
HBC moiety is present and only fluorescence from the
porphyrinꢁs Q-bands is observed. A possible explanation for
the quenching of the HBC emission may be the occurrence of
an electron/energy transfer from HBC towards the porphyrin
core, which has been observed previously for structurally
related porphyrin conjugates.[13] Interestingly, excitation at
lmax,HBC in the conjugate results in a significantly higher
intensity for the emission profile of 3 and 4 than excitation at
lmax,Porph. Additionally, excitation at lmax,Porph leads to a higher
emission for the metalated derivative 4. Further investiga-
tions, such as the determination of redox properties,[15] are
necessary in order to shed more light on this phenomenon
though.
In conclusion, a novel and versatile HBC-aldehyde
building block (1) has been designed and used to construct
the new porphyrin derivatives 3 and 4. A single-crystal X-ray
diffraction of HBC–porphyrin conjugate 3 gave insight into
the structural packing of such carbon-rich porphyrin conju-
gates. MALDI measurements revealed the pronounced
Figure 4. Absorption spectra of 3, 4, reference HBC 6, and reference
porphyrin 7; c=2.5·10À7 m in THF.
Table 1: Optical characterization in THF (see the Supporting Informa-
tion).
+
tendency of 3 to form clusters as big as (3)6 C. Furthermore,
ESI experiments showed that 3 undergoes a remarkable
lHBC [nm]
lPorph [nm]
lFluo,HBC
[nm]
lFluo,Porph [nm]
(FSoret)
3+
dimer formation to the species (3)2 C, whose attractive forces
[a]
(logemax
)
(logemax
)
(Fb)[a]
(presumably p–p or p–CH interactions) are strong enough to
overcome Coulombic repulsion. Absorption and emission
spectroscopy revealed a strong electron/energy transfer
between the two chromophores, which results in unique
absorption and emission features. The combination of these
two unique subunits opens the door to further detailed optical
characterization by, for example, ultrafast spectroscopy and
spectro-electrochemistry. Moreover, such derivatives could
pave the way for the construction and investigation of tailor-
made carbon-rich supramolecular architectures, with feasible
applications in the field of molecular electronics. Other HBC
substitution patterns as well as other HBC–porphyrin con-
jugates are currently under investigation.
[mÀ1 cmÀ1
]
[mÀ1 cmÀ1
]
6
7
3
344, 360
(5.33), 389
–
–
474 (0.586), 483,
492, 517
–
–
419 (5.69), 515, 549,
592, 650
426, 438 (5.39), 519, 469, 483
652 (0.033),
718
653 (0.059),
720
342, 358
(5.45), 388 555, 594, 650
(0.073),[b] 493,
516
4
342, 357
(5.31), 386 600
434, 444 (5.52), 561, 469, 484 (0.111), 606 (0.076),
495 658
[a] With reference to fluorescein in NaOH 0.1m (F=0.925).[14]
[b] Underestimated due to lamp interference.
Received: July 9, 2014
Published online: September 22, 2014
rin unit. Comparing the spectra of 3 and 4, it becomes evident
that lmax,Porph is red-shifted upon metal insertion, from 438 nm
to 444 nm. This is likely due to THF binding to the Zn ion.
Additionally, the extinction coefficient of the higher peak of
the B-band for the metalated derivative 4 (331000mÀ1 cmÀ1) is
drastically increased (ca. 26%) with respect to the corre-
sponding free-base porphyrin 3 (245000mÀ1 cmÀ1). Upon
metal insertion, position and intensity of the Q-bands are
also affected with respect to free-base porphyrin. In our case,
the absorption spectrum of 3 contains four Q-bands situated
at 519, 555, 594, and 650 nm with decreasing intensity, while in
the case of 4 only two bands are visible, located at 561 and
600 nm.
Keywords: arenes · clusters · organic synthesis · porphyrinoids ·
.
structure elucidation
[1] V. V. Diev, C. W. Schlenker, K. Hanson, Q. Zhong, J. D.
Zimmerman, S. R. Forrest, M. E. Thompson, J. Org. Chem.
2012, 77, 143 – 159.
[2] J. P. Lewtak, D. T. Gryko, Chem. Commun. 2012, 48, 10069 –
10086.
[3] a) S. Ito, P. T. Herwig, T. Bçhme, J. P. Rabe, W. Rettig, K.
Mꢂllen, J. Am. Chem. Soc. 2000, 122, 7698 – 7706; b) J. Wu, M. D.
Watson, N. Tchebotareva, Z. Wang, K. Mꢂllen, J. Org. Chem.
2004, 69, 8194 – 8204.
[4] J. Wu, W. Pisula, K. Mꢂllen, Chem. Rev. 2007, 107, 718 – 747.
[5] J. M. Englert, J. Malig, V. A. Zamolo, A. Hirsch, N. Jux, Chem.
Commun. 2013, 49, 4827 – 4829.
To highlight the profound electronic communication
between the HBC and the porphyrin units, color contour
plots of excitation versus emission for both conjugates 3 and 4
are presented in the Supporting Information (see Figures S3,
S7, and S11). The superbenzene–porphyrin conjugate intro-
duces a drastic quenching of the HBC emission in both 3 and 4
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Chemie
[6] A. Hirsch, A. J. Kratzer, D. Lungerich, J. M. Englert, F. W.
Heinemann, N. Jux, Faraday Discuss. 2014, DOI: 10.1039/
C4FD00069B.
[10] X-ray structure analysis of a non-single crystal of 4, grown from
CH2Cl2, indicates a p–p interaction distance of the HBC planes
of 3.5–3.9 ꢀ; see Figures S12 and S13 in the Supporting
Information.
[7] N. Jux, Org. Lett. 2000, 2, 2129 – 2132.
[8] J. S. Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney, A. M.
Marguerettaz, J. Org. Chem. 1987, 52, 827 – 836.
[9] Refined formula C214H260N4O4, Mr = 2952.26, crystal dimensions
[11] S. V. Shcherbyna, D. K. Bohme, J. Am. Soc. Mass Spectrom.
2006, 17, 222 – 229.
[12] a) T. Drewello, H. Frauendorf, R. Herzschuh, A. A. Goryunkov,
S. H. Strauss, O. V. Boltalina, Chem. Phys. Lett. 2005, 405, 93 –
96; b) M. P. Barrow, X. Feng, J. I. Wallace, O. V. Boltalina, R.
Taylor, P. J. Derrick, T. Drewello, Chem. Phys. Lett. 2000, 330,
267 – 274.
3
ꢀ
0.2878 ꢃ 0.0858 ꢃ 0.0451 mm , triclinic, space group P1, unit-cell
dimensions a = 16.7770(7), b = 16.7834(7), c = 20.4679(8) ꢀ, a =
101.505(4), b = 95.831(3), g = 119.864(4)8, V= 4758.7(3) ꢀ3, Z =
1, 1calcd = 1.030 mgmmÀ3
, linear absorption coefficient m =
0.448 mmÀ1, radiation/wavelength CuKa/0.15418 nm, tempera-
ture of measurement 173.0(2)8C, 2qmax = 6.26 – 117.868, no. of
measured reflections 28484, no. of independent reflections
13628, Rint = 0.0494, final R indexes [I ^ 2s(I)]: R1 = 0.0806,
wR2 = 0.2516, final R indexes [all data]: R1 = 0.0957, wR2 =
[13] C. Aurisicchio, R. Marega, V. Corvaglia, J. Mohanraj, R.
Delamare, D. A. Vlad, C. Kusko, C. A. Dutu, A. Minoia, G.
Deshayes, O. Coulembier, S. Melinte, P. Dubois, R. Lazzaroni, N.
Armaroli, D. Bonifazi, Adv. Funct. Mater. 2012, 22, 3209 – 3222.
[14] D. Magde, R. Wong, P. G. Seybold, Photochem. Photobiol. 2002,
75, 327 – 334.
0.2728, residual electron density 0.98/À0.31 eꢀÀ3
. CCDC
945391 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data request/cif.
[15] For preliminary results, see Figures S19 and S20 in the Support-
ing Information.
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