Control of absorption properties of tetraazaporphyrin group 15 complexes by modification of their axial ligands

Article abstract of DOI:10.1039/c4cc07408d

Tetraazaporphyrin (TAP) complexes with group 15 elements (phosphorus(v) or arsenic(v)) containing two axial OH ligands showed reversible spectroscopic changes with acid or base doping. Spectroscopic and theoretical analysis revealed that the modification of axial ligands can tune the interaction between peripheral substituents and the TAP macrocycle.

Full text of DOI:10.1039/c4cc07408d

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Control of absorption properties of
tetraazaporphyrin group 15 complexes by
Cite this:DOI: 10.1039/c4cc07408d
modification of their axial ligands†
Received 19th September 2014,
Accepted 10th October 2014
Taniyuki Furuyama, Mitsuo Asai and Nagao Kobayashi*
DOI: 10.1039/c4cc07408d
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Tetraazaporphyrin (TAP) complexes with group 15 elements having hydroxyl groups as axial ligands. In the case of reported
(phosphorus(V) or arsenic(V)) containing two axial OH ligands showed porphyrin phosphorus(V) complexes with hydroxyl groups, the
reversible spectroscopic changes with acid or base doping. Spectro- proton of the hydroxyl group could be easily removed using a
scopic and theoretical analysis revealed that the modification of axial base, resulting in switching of the crystallographic structure of
ligands can tune the interaction between peripheral substituents and the porphyrin macrocycle.6 We anticipated that the modification
the TAP macrocycle.
of the axial ligands of TAP by external stimuli would tune the
optical properties of TAPs.
Controlling the spectral arrangement of optical properties in a
The synthetic procedure of TAPs is shown in Scheme 1.
reversible manner by external stimuli (‘‘stimuli-responsive’’) is Bulky 3,5-di-tert-butylphenyl groups were used as peripheral aryl
one of the most interesting and essential topics in chemistry, as groups to improve the stability of the macrocycle against reduction
well as in material and biological sciences.1 In this regard, by a base, i.e. bulky groups prevent the approach of a base to the
azaporphyrinoids, such as tetraazaporphyrins (TAP) and phthalo- macrocycle.3b Mg complex 6 was finally characterized by X-ray
cyanines (Pc), are good candidates as core structures of diffraction analysis of crystals as a species7 having one H2O
functional molecules, since they have intense absorption bands in molecule coordinated at the axial position (6ꢀH2O, Fig. S1, ESI†).
the UV and visible regions (termed the Soret and Q bands), which Bulky peripheral groups surround the macrocycle, while the
can be fine-tuned using appropriate peripheral modifications.2
Recently, we proposed a novel strategy for tuning the optical
properties of azaporphyrinoids, where the combination of a
simple macrocyclic ligand and main-group elements was used,
accompanied by simple synthetic procedures.3 In particular,
TAP phosphorus(V) complexes (PTAPs) having eight aryl
(Ar) groups at peripheral positions showed unique absorption
properties.4 An intense charge-transfer (CT) band5 appeared
between the Soret and Q bands in the absorption spectrum of
PTAP, so that it can absorb light across the entire UV-vis region.
Moreover, substitution at the peripheral groups of PTAP can
tune both the position and intensity of the CT band in a
rational manner. Hence, an appropriate combination between
the core PTAP and substitution groups can establish a finely-
tuned light harvesting over the complete UV-visible region.
Herein, we report the synthesis and optical properties of TAP
complexes with group 15 elements (phosphorus(V) or arsenic(V))
Department of Chemistry, Graduate School of Science, Tohoku University,
Sendai 980-8578, Japan. E-mail: nagaok@m.tohoku.ac.jp; Tel: +81 22 795 7719
† Electronic supplementary information (ESI) available: Additional spectroscopic
data, full details of experimental and calculation procedures for all studied
compounds. CCDC 1024678. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc07408d
Scheme 1 Synthesis of tetraazaporphyrin group 15 complexes. Reagents
and conditions: (i) (nBuO)2Mg (1 eq.), nBuOH, reflux, 15 h, 83%; (ii) CF3COOH,
rt, 3 h, 68%; (iii) POBr3 (25 eq.), pyridine, reflux, 20 h, then ROH, reflux, 3 h;
(iv) NaClO4, CH3CN/CH2Cl2, rt, 12 h, 18% (for 1), 19% (for 2); (v) AsCl3 (460 eq.),
pyridine, reflux, 5 h; (vi) HPyBr3 (7 eq.), rt, 30 min, and (vii) ROH, rt, 15 min, then
NaClO4, rt, 15 h, 6% (for 3), 46% (for 4). Ar = 3,5-di-tert-butylphenyl.
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central magnesium ion deviates by 0.53 Å from the 4N mean entire UV-visible region. AsTAP 3 also showed a similar rever-
plane as a result of the axial coordination. The macrocycle has a sible spectrum change (Fig. S5, ESI†). However, a larger excess
saddled structure (Dr8 = 0.22), although the reported crystal (B200 eq.) of triethylamine was required to complete the
structure4 of PTAP exhibited a ruffled structure. The free-base spectral transformation. A similar spectroscopic change was
TAP 7 was reacted with excess phosphorus oxybromide in observed upon addition of DBU, but in this case, the spectral
pyridine, as previously reported.4 After the reaction, the axial change occurred more efficiently, such that only 1 eq. of DBU
ligands may be Br at this stage. The reaction was quenched with was required to change the spectrum. This difference in the
water or methanol, such that hydroxy or methoxy groups could amount of base depends on their basicity, i.e. pKas in aceto-
be introduced at the axial position of the phosphorus. Finally, nitrile of DBU and Et3N are 24.16 and 18.63, respectively,11
the counter anion was replaced by excess NaClO4, producing so that DBU is a stronger base. For species containing two OMe
the desired complexes 1 (HO–P–OH) and 2 (MeO–P–OMe) as the axial ligands (2 and 4), this kind of spectral change was not
perchlorate salts. Arsenic complexes containing different axial observed upon addition of excess amounts of an acid or a base,
ligands (3 and 4) were also synthesized from 7 using AsCl3 and so that we can conclude that the axial hydroxyl groups of the
HPyBr3,3b and obtained as the perchlorate salts. All TAPs were group 15 elements are crucial for the reversible switching of the
characterized by 1H and 31P (1 and 2) NMR and MALDI-TOF-MS absorption properties to occur. Magnetic circular dichroism
spectroscopy. These compounds exhibit good stability in air- (MCD) spectra of both the acidic (1ꢀacidic) and basic (1ꢀbasic)
saturated solution under ambient light.
forms of 1 show typical features of phosphorus(V) and metal-
The absorption spectra of 1–4 in CH2Cl2 are very similar in ated TAPs, respectively (Fig. S6, ESI†). Here, typical Faraday
shape (Fig. S2, ESI†). All compounds show three intense bands A terms were detected for both the Soret and Q band regions, in
at ca. 635–650, 510–535, and 335–340 nm, corresponding to the agreement with the approximate D4h symmetry of both 1ꢀacidic
Q, CT, and Soret bands, respectively. The CT band is characteri- and 1ꢀbasic in solution. Since the MCD intensity is associated
stic of PTAPs, as we have already reported, and can be assigned with changes in orbital angular momenta between ground and
to CT transitions from the peripheral aryl groups to the TAP excited states, the intense MCD signals for the Q band regions
macrocycle.4,9 The CT bands of AsTAPs appear in a longer indicate that both species have 18p aromatic structures. Thus,
wavelength region than those of PTAPs while the intensities oxidation or reduction of the TAP macrocycle does not occur by
are also larger than those of PTAPs. However, the differences in adding an acid or a base.
absorption properties between PTAPs and AsTAPs appear to be
To enhance the interpretation of the spectral changes,
1
small. Interestingly, the color of the PTAP with hydroxyl groups a H NMR titration experiment of 1 in CDCl3 was carried out
1 in CH2Cl2 solution gradually changed from purple to green (Fig. 2a). Under an excess amount of an acid or a base, a single
upon addition of triethylamine (Fig. S3, ESI†). The spectro- set of peaks assignable to the peripheral aryl groups appeared
scopic changes across the entire UV-vis region are shown in in the normal aromatic region. During the addition of triethyl-
Fig. 1a. The Q band of 1 at 636 nm shifted to shorter wavelength amine or trifluoroacetic acid, the peaks gradually shifted. Under
(at 617 nm), while the intense CT band at 515 nm diminished and around neutral conditions, the peaks broadened. From the
shifted to 479 nm. After adding excess (B2 eq.) triethylamine, the absorption and MCD spectra, the reason for the broad peaks
spectral envelope resembled those of metalated TAP (Fig. S4, ESI†) can be attributed to an equilibrium between two similar species
and electron withdrawing group-substituted PTAP.4 The original of approximate D4h symmetry. The sharp peaks under both acidic
spectrum of 1 was restored upon addition of trifluoroacetic acid and basic conditions indicate that intermediates of 1 are dia-
(Fig. 1b) and the clear isosbestic points during this transforma- magnetic, hence a one electron oxidation and/or reduction of the
tion indicate an equilibrium between two species in solution. TAP ligand through the addition of an acid or a base can be
Reversible switching of the optical properties of Pcs by addition ruled out. Although the difference in peaks under the two states
of an acid or a base has been reported previously, but spectral can be distinguished, the small difference in a chemical shift
changes occurred only in the Q band region.10 The switching of (o0.1 ppm) reveals that the electronic structure of the peripheral
1 observed here is the first example of a change across the aryls and the TAP macrocycle was only marginally changed by
the addition of an acid or a base. To confirm the effect at the
central phosphorus atom, 31P NMR spectra of 1 under acidic and
basic conditions were also measured (Fig. 2b). Only one kind of
peak at ꢁ193 or ꢁ173 ppm was observed under acidic or basic
conditions, respectively. Both kinds of peaks were consistent
with those of hexacoordinated phosphorus representations,12
lying within the central cavity of the TAP macrocycle. The large
(20 ppm) downfield shift under basic conditions, which was
also observed in porphyrin phosphorus complexes with axial
hydroxyl groups,6 supports the conclusion that the nature of the
phosphorus atom changed as a result of the addition of an acid
or a base. Based on the results of the spectroscopic changes,
proposed structures of 1 under acidic and basic conditions are
Fig. 1 Spectral changes of 1 solution by (a) adding Et3N (0–2 eq.) and then
(b) adding TFA (B4.5 eq.) in CH2Cl2.
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Fig. 3 Partial molecular energy diagrams and orbitals of the basic and acidic
states of 10 (top) and their calculated absorption spectra (bottom). Blue and
red plots indicate occupied and unoccupied MOs, respectively. For 10ꢀbasic,
an axial-oxygen-centered orbital, HOMO ꢁ 1 (255), was omitted for clarity
since transitions from this MO have no intensity. Calculations were performed
at the LC-BLYP/6-31G*//B3LYP/6-31G* level (for details, see the ESI†).
Fig. 2 (a) 1H NMR titration of 1 solution in CDCl3 with Et3N (base) or
CF3COOD (acid). (b) 31P NMR spectra of the basic and acidic states of 1.
(c) Proposed structures of the basic and acidic states of 1.
shown in Fig. 2c. The proton at the hydroxyl group can be easily adopts a planar structure (Dr = 0.09). The bond lengths of the
removed by a base, so that the cationic form of PTAP is central phosphorus and axial oxygen of 10ꢀbasic (1.711 and
neutralized. Thus, the bonding characteristic of the central 1.519 Å) are either shorter or longer than those of 10ꢀacidic
phosphorus atom in the basic state of 1 may be hexacoordinated (1.676 and 1.675 Å), in good agreement with the crystal struc-
bearing a PQO double bond, as previously found by X-ray diffrac- tures of reported porphyrin phosphorus complexes having a
tion in some porphyrin phosphorus complexes.6 The difference in P–OH single bond and a PQO double bond.6 The calculated
the bonding characteristics of the phosphorus atom correlates the MO energies of 10ꢀbasic are higher than those of 10ꢀacidic, since
quite different absorption properties between acidic and basic the electronic structure of 10ꢀbasic is neutral. For both states,
conditions. The difference in the amount of base required to the HOMO, LUMO, and LUMO + 1 are dominated by the TAP
change the absorption spectra of AsTAP 3 can also be explained. orbitals, which corresponded to the a1u-, egy- and egx-like orbitals
13
The different acidity (pKa = 2.12 for H3PO4 and 2.22 for H3AsO4
)
in Gouterman’s model.15 Therefore, these calculated transitions
of the proton at P–OH or As–OH appears to be the origin of the at 672 and 670 nm (for 10ꢀacidic) and 633 and 626 nm (for
different reactivity between 1 and 3 to a base.
10ꢀbasic) can be assigned to the experimental Q bands. The
Molecular orbital (MO) calculations were performed for calculated HOMO–LUMO energy gap of 10ꢀbasic is slightly
proposed structures of PTAPs with acidic and basic states. larger than that of 10ꢀacidic, so that the position of the Q band
Model structures (10ꢀacidic and 10ꢀbasic) whose peripheral sub- of 1 shifts slightly to shorter wavelength when a base is added.
stituents were replaced by phenyl groups were used for simpli- The calculated bands (401, 397, and 392 nm) of 10ꢀacidic are
city. Partial MO energy diagrams of the model structures are composed of transitions from the HOMO ꢁ 1, HOMO ꢁ 2,
shown in Fig. 3, with the results of TDDFT calculations at the HOMO ꢁ 3 and HOMO ꢁ 4 to the degenerate LUMOs. The
LC-BLYP4,14/6-31G*//B3LYP/6-31G* level of theory summarized HOMO ꢁ 1 to HOMO ꢁ 4 are localized on the peripheral phenyl
in Table S2 (ESI†). The optimized structure of 10ꢀacidic is a rings, so that these bands can be assigned to CT transitions, as
distorted ruffled structure (Dr = 0.17), while that of 10ꢀbasic previously calculated for PTAP(OMe)2.4 Similar CT transitions
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were also calculated at slightly higher energy (352 and 345 nm) Challenging Exploratory Research (No. 25620019) and Young
for 10ꢀbasic. However, these bands were estimated to be weaker Scientist (B) (No. 24750031) from the Ministry of Education,
than those of 10ꢀacidic, in agreement with the experimental Culture, Sports, Science, and Technology (MEXT). The authors
observations. In the MCD spectrum of 1ꢀbasic, a weak Faraday thank Prof. Takeaki Iwamoto and Dr Shintaro Ishida (Tohoku
A term9 was observed (495 and 443 nm) between the Soret and University) for X-ray measurements. Some of the calculations were
Q bands, indicating that the corresponding absorption band at performed using supercomputing resources at the Cyberscience
479 nm can be assigned to transitions to degenerate orbitals Center of Tohoku University.
(Fig. S6, ESI†). Hence, the calculated CT transitions of 10ꢀbasic
clearly explain the spectroscopic alternation between 10ꢀacidic
Notes and references
1 (a) N. L. Bill, J. M. Lim, C. M. Davis, S. Bahring, J. O. Jeppesen, D. Kim
and 10ꢀbasic. The calculated MOs also suggest that the electronic
¨
configuration of the central group 15 atom can switch not only
the structures of the TAP macrocyclic core, but also the effect of
the peripheral aryl moieties.
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In summary, ‘‘stimuli-responsive’’ TAP group 15 complexes
have been developed, in which the optical properties across the
entire UV-visible region can be altered by the addition of an
acid or a base. Titration experiments suggest that the modifica-
tion of the axial ligands is crucial for controlling the optical
properties of TAPs. 31P NMR spectra of acidic and basic condi-
tions indicate that the bond configuration of phosphorus(V) can
be changed after the proton at the hydroxyl group of axial ligands
has been removed by a base, without structurally modifying the
p-conjugated system of the TAPs. Finally, the bond configuration
under basic conditions was assigned to a hexacoordinated
phosphorus(V) atom having a PQO double bond. The results
of MO calculation also support the model, revealing that the
interaction between the peripheral aryl moieties and the TAP
macrocyclic core (i.e. the CT band) can be switched by altering
the electronic configuration of the central group 15 element.
Further work is currently underway to synthesize TAP complexes
with group 15 elements having various axial and peripheral
ligands, with the aim of developing novel chemo- or biosensing
probes through the ensemble of the TAP macrocycle, peripheral
substituents and axial ligands.
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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),
Chem. Commun.
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