Synthesis of a tetrabenzotriazacorrole μ-oxo dimer and investigation of its stacking effect

Article abstract of DOI:10.1039/c4cc01115e

A μ-oxo dimer of phosphorus(v) tetrabenzotriazacorrole (PTBC) has been synthesized and characterized for the first time. Spectroscopic and theoretical analysis support the opinion that two PTBC chromophore groups interact with each other, while the low-symmetric PTBC structure is also efficient for generating singlet oxygen.

Full text of DOI:10.1039/c4cc01115e

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Synthesis of a tetrabenzotriazacorrole l-oxo
dimer and investigation of its stacking effect†
Cite this: Chem. Commun., 2014,
50, 4312
Taniyuki Furuyama, Yusuke Sugiya and Nagao Kobayashi*
Received 12th February 2014,
Accepted 25th February 2014
DOI: 10.1039/c4cc01115e
www.rsc.org/chemcomm
A l-oxo dimer of phosphorus(V) tetrabenzotriazacorrole (PTBC) has phosphorus tribromide in pyridine, to give the TBC phosphorus
been synthesized and characterized for the first time. Spectroscopic complex 2. The reaction was quenched with water, and the axial
and theoretical analysis support the opinion that two PTBC chromo- ligands of 2 were OH at this stage. The solution of 2 was heated in
phore groups interact with each other, while the low-symmetric PTBC the presence of triphenylsilyl chloride, and both the triphenylsiloxy-
structure is also efficient for generating singlet oxygen.
capped TBC monomer 3 and the desired dimer 4 were obtained. The
bulkiness of the triphenylsilyl group appeared efficient for stabili-
Phthalocyanines (Pcs) and related tetrapyrrolic macrocycles are zation of the dimer. The structure of 3 was finally elucidated by X-ray
attractive target molecules in a variety of fields. These molecules diffraction analysis of crystals (Scheme 1b and Fig. S1, ESI†).‡ The
have intense absorption bands in the UV-visible region, and their TBC macrocycle of 3 has a highly planar structure as previously
unique optical properties can be used as a key characteristic for reported for phosphorus(V) corrolazine4c and corrole6 complexes.
constructing functional molecules. Since the optical properties of The phosphorus ion is just the right size to fit into the center of the
Pcs depend significantly on their structure, a number of Pc TBC ligand, while phosphorus(V) Pc has a ruffled structure7 due to
derivatives have been proposed and synthesized for over a century.1 mismatching between phosphorus and a central core of Pc.
Among several methods in this regard, lowering of the molecular
1H and 31P NMR spectra of 3 and 4 in CDCl3 are shown in Fig. S2
symmetry of Pcs is a good strategy.2 However, separation of low- (ESI†). Four sets of singlet assignable to the a-position of 3 are
symmetry Pcs is generally not easy, and the yield is often low. present at low magnetic field (9.11–9.02 and 8.34 ppm), reflecting the
Properties resulting from aggregation and/or oligomerization of Pcs lower-symmetric TBC macrocycle. The 31P NMR spectrum of 3
are also important in various fields, such as materials science and exhibits only one peak, at ꢀ216 ppm, similar to porphyrin8 and
photochemistry.3 We focus on tetrabenzotriazacorrole (TBC) as
a variant of low-symmetry Pcs. TBC is known as a 18p aromatic
Pc–corrole analogue, in which one meso nitrogen atom of Pc is
missing. The direct pyrrole–pyrrole bond results in reduced
symmetry (C2v) compared to the D4h of regular metallo Pcs. TBCs
can be synthesized from the corresponding symmetric Pcs in
moderateyieldsbyusing asimple procedure, andareeasytohandle.4
In order to learn more about TBCs, we have chosen phosphorus(V)
complexes which havetwo axialligands.4a,c Herein, we report the first
synthesis and properties of a TBC m-oxo dimer as a cofacial model of
low-symmetry Pc oligomers. m-oxo oligomers have been synthesized
in Pc chemistry, and unique stacking effects observed.5
The synthetic route of the TBC m-oxo dimer (4) is shown in
Scheme 1a. Free-base b-(PhO)8Pc 1 was reacted with excess
Department of Chemistry, Graduate School of Science, Tohoku University,
Scheme 1 (a) Synthesis of the tetrabenzotriazacorrole m-oxo dimer. Reagents
and conditions: (i) PBr3 (20 eq.), pyridine, reflux, 1 h, 55%; (ii) Ph3SiCl, toluene,
reflux, 3 h in the dark, 60% (3) and 3% (4). (b) The X-ray crystal structure of 3.
The thermal ellipsoids were scaled to the 50% probability level. H atoms and
axial ligands have been omitted for clarity.
Sendai 980-8578, Japan. E-mail: nagaok@m.tohoku.ac.jp; Tel: +81 22 795 7719
† Electronic supplementary information (ESI) available: Additional spectroscopic
data and full details of experimental and calculation procedures for all studied
compounds. CCDC 985980. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc01115e
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Fig. 2 Cyclic voltammetry data for 3 (black), and 4 (red). Cyclic voltammo-
grams were acquired from 1.0 mM solutions of an analyte in 0.1 M nBu4NClO4/
o-DCB. Ferrocene was used as an internal standard and set to 0 V.
peak potentials, Epc, were used as indexes of the first reduction
potential. The close Epc values clearly show that the LUMO energy
levels of 3 and 4 are close. These voltammograms indicate that the
gap between the HOMO and the LUMO decreases after the m-oxo
dimer formation of the TBC macrocycle.
In order to enhance the interpretation of the electronic
structure of the TBC m-oxo dimer and understand the origin of the
blue-shifted absorption spectrum of 4 with a narrower energy gap
compared to that of monomer 3, model structures (30 and 40), whose
peripheral substituents and axial triphenylsilyl groups are replaced
Fig. 1 Magnetic circular dichroism (top), and electronic absorption (middle)
spectra of 3 (black) and 4 (red) in CHCl3, and theoretical absorption spectra
(bottom) of 30 (black) and 40 (red). Calculations were carried out at the
B3LYP/6-31G(d) level of theory.
phthalocyanine7 phosphorus(V) complexes. Both H and 31P NMR by hydrogen, were considered and used in the calculations. The
spectra of 4 exhibit only one set of peaks for the TBC unit, and each torsion angle (y) of the optimized structure of 40 is 41.41 (Fig. S5,
TBC moiety could not be distinguished even at low temperature ESI†), which is larger than that of the crystallographic structure of
(Fig. S3, ESI†). The peaks of 4 were observed at higher field than the m-oxo SiPc dimer (y = 36.61).5e The optimized structures where y
those of 3. A similar upfield shift was also observed in the case of the was restricted were also obtained, but their energy difference was
m-oxo dimer of SiPc,5c indicating that each TBC moiety is affected by smaller than 4 kcal molꢀ1 (Fig. S6, ESI†), thus allowing for the two
1
the diatropic ring current generated in the other TBC moiety.
TBC macrocycles to undergo a rapid rotation, supporting the variable
UV-vis and MCD spectra of compounds 3 and 4 in CHCl3 are temperature 1H NMR results.
shown in Fig. 1. The absorption spectrum of capped monomer 3
The calculated absorption spectra of 30 and 40 are shown at the
exhibits two intense bands at 662 nm and 451 nm for the Q and bottom of Fig. 1, while partial MO energy diagrams are shown in
Soret bands, respectively. Both the absorption coefficients and Fig. S7 (ESI†), with the calculated transition energies, oscillator
positions of the Q and Soret bands are close to those of the spectrum strengths ( f ), and configurations summarized in Table S1 (ESI†).
of 2 (Fig. S4, ESI†). In the MCD spectrum, pseudo Faraday A terms of As previously reported for the electronic structure of silicon(IV)
almost comparative intensity appeared corresponding to the Q and corrolazine,9 the absorption spectrum of monomer 30 can be
Soret absorption bands. On the other hand, the absorption spectrum explained by Gouterman’s ‘‘four-orbital’’ theory.10 Both the calcu-
of dimer 4 is completely different from that of 3. Here, the absorp- lated Q and Soret bands split into two due to the low-symmetry of
tion bands are blue-shifted to 635 and 425 nm with concomitant the TBC chromophore. These bands are composed of transitions
broadening, although the apparent intensity is close to that of the from the HOMO ꢀ 1 and HOMO to the LUMO and LUMO + 1,
monomer. In the near-IR region (680–800 nm), a weak, broad band, similar to typical aromatic azaporphyrinoids. The frontier MOs of
monotonously decreasing toward a longer wavelength, is seen, m-oxo dimeric 40 are delocalized over the entire complex. Here,
suggesting the presence of a low-energy forbidden transition for the HOMO–LUMO gap of 40 (1.93 eV) is narrower than that of 30
the dimer. In the MCD spectrum, an intense pseudo Faraday A term (2.28 eV). On the other hand, the calculated HOMO–LUMO transi-
appeared, corresponding to the Q absorption peak, indicating that tion at 809 nm is very weak ( f = 0.01). The main Q bands calculated
the MCD spectrum of cofacial dimers does not sensitively reflect the at 583 and 565 nm are relatively strong, and the sum of the two f
molecular symmetry of the constituent monomer.
values (0.46) are close to that (0.41) of the calculated Q band for
The electrochemical properties of 3 and 4 were examined by monomeric 30 (595 and 563 nm), as is indeed observed experimen-
cyclic voltammetry (Fig. 2). 3 showed one reversible oxidation couple tally for 3 and 4. Based on the components of transitions for 40, the
at 0.19 V, while 4 showed two reversible oxidation couples at 0.44 lowest-energy transition is forbidden, and the HOMO ꢀ 3, HOMO ꢀ 2,
and ꢀ0.02 V vs. Fc+/Fc. The first oxidation couple (Eox1) shifted and HOMO ꢀ 1 dominate in the intense Q absorption band for 40.
cathodically by ca. 0.2 V compared to that of the monomer 3. Since Therefore, the transition component of the intense band in theQ band
the exact reduction potentials could not be estimated, their cathodic region of 4 is completely different from that of 3, as a result of the
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the design of sensitizers. Further work is currently underway to
tune the rotation properties by external stimuli.
This work was partly supported by a Grant-in-Aids for
Scientific Research on Innovative Areas (25109502, ‘‘Stimuli-
responsive Chemical Species’’), Scientific Research (B) (No.
23350095), Challenging Exploratory Research (No. 25620019)
and Young Scientist (B) (No. 24750031) from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT).
The authors thank Dr Eunsang Kwon (Tohoku University) for
X-ray measurements. Some of the calculations were performed
using supercomputing resources at the Cyberscience Center of
Tohoku University.
Fig. 3 Comparison of the rate of decay of DPBF sensitized by 1 (red), 3
(blue), 4 (green) and Std-ZnPc (purple) in CHCl3 as shown by the decrease
in the absorbance at 415 nm. The absorption coefficient was normalized
by the rates of the light absorption coefficient over 600 nm.
Notes and references
‡ Crystallographic data for 3: C116H78N7O10PSi2, Mw = 1817.00, ortho-
rhombic, space group Pbca (no. 61), a = 12.5583(2) Å, b = 31.0602(6) Å,
c = 47.1556(8) Å, V = 18393.7(6) Å3, Z = 8, rcalcd = 1.312 g cmꢀ3, T = ꢀ183 1C,
orbital interaction between the two TBC chromophores. These 202 162 measured reflections, 16 834 unique reflections (Rint = 0.0934),
R = 0.0974 (I 4 2s(I)), Rw = 0.3134 (all data), goodness-of-fit on F2
=
spectroscopic features can also be interpreted conceptually by
considering exciton coupling interactions between the co-facially
arranged monomer units (H-type aggregate).11 In the dimer, the
orientation of two excitons derived from the monomer units is not
perfectly parallel. However, as a first approximation, the band on
the lower energy side of the Q band can be expressed as the
difference of two electric dipole moments, and that on the higher
energy side as the sum of the two moments; thus the band at
higher energy becomes much stronger.
1.035, largest diff. peak/hole 1.007 and ꢀ0.317 e Åꢀ3, CCDC 985980.
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TBCs12 by determining their singlet oxygen quantum yields
(FD) by a steady-state method using 1,3-diphenylisobenzofuran
(DPBF) as the chemical quencher. A b-(4-t-BuOPh)8Pc zinc
complex (Std-ZnPc) was used as a standard (FD = 0.73 in
CHCl3).13 The photobleaching decays are shown in Fig. 3.
TBC monomer 3 showed a high quantum yield (FD = 0.75),
while free-base Pc had a low FD value. Compound 3 consists
only of main-group elements above the third period, and
therefore, its high FD value is considered not to be caused by a
heavy-metal effect, but due to the low-symmetry chromophore14
of TBC. On the other hand, dimer 4 (FD = 0.25) was much less
efficient than 3. In general, H-type aggregation in water reduces
the lifetime of the excited state due to enhanced radiationless
excited-state dissipation.15
In summary, a m-oxo phosphorus(V) TBC dimer has been
synthesized for the first time. In accordance with the narrower
HOMO–LUMO gap derived from cyclic voltammograms, the
dimer showed a weak Q absorption band with a forbidden
character at a longer wavelength than the Q band of the
monomer. The strong band of the dimer was observed to be
blue-shifted than that of the constituting monomer. These
spectral features could be explained by theoretical calculations,
and conceptually by an exciton coupling model. The low-symmetry
structure of TBC showed enhanced efficiency in generating singlet
oxygen, so that the molecular symmetry appears to be important in
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4314 | Chem. Commun., 2014, 50, 4312--4314
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