Rationally designed phthalocyanines having their main absorption band beyond 1000 nm

Article abstract of DOI:10.1021/ja208481q

Highly air-stable phthalocyanines (Pcs) having their main absorption band beyond 1000 nm have been synthesized using main-group elements as peripheral and central (core) substituents. The resultant [(PhS)₈PcP(OMe) ₂][PF₆]

Full text of DOI:10.1021/ja208481q

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pubs.acs.org/JACS
Rationally Designed Phthalocyanines Having Their Main Absorption
Band beyond 1000 nm
Nagao Kobayashi,* Taniyuki Furuyama, and Koh Satoh
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
S Supporting Information
b
Scheme 1. Molecular Design of NIR Phthalocyanines
ABSTRACT: Highly air-stable phthalocyanines (Pcs) hav-
ing their main absorption band beyond 1000 nm have been
synthesized using main-group elements as peripheral
and central (core) substituents. The resultant [(PhS)8-
PcP(OMe)2][PF6] and [(PhSe)8PcP(OMe)2][PF6] show
a single Q-band peak at 1018 and 1033 nm, respectively,
which was achieved by carefully taking into account the
spectroscopic properties of Pcs and the characteristics of the
frontier orbitals. The large red shift can be considered to
affect the position of the Q band.2 Careful inspection of the
originate from synergistic effects involving both the group-
molecular orbitals (MOs) of Pcs4 reveals that (2) results from the
15 and -16 elements.
fact that the MO coefficient of the α-carbon of the HOMO is
large, causing the introduction of electron-donating groups at
this position to destabilize the HOMO energy, thereby shifting
the Q band to longer wavelengths.5 Furthermore, the MO
onsiderable effort has been made to develop various types of
near-IR organic compounds because of their potential appli-
C
coefficient of the pyrrole nitrogens of the LUMO is quite large,
from which it is easily inferred that the introduction of
electron-withdrawing groups at the pyrrole nitrogen would dra-
matically stabilize the LUMO orbitals, thereby making it possible
to shift the Q band to longer wavelengths. If high-valency ele-
ments with a small ionic radius (i.e., strongly electron-deficient
groups) are used, the (LU)MOs of Pcs should be strongly stabi-
lized. On the basis of this rational concept, we chose main-group
elements as substituents because of their notable features such as
effective orbital interactions and diversity of coordination. In this
communication, we report the design of phosphorus(V) Pcs con-
taining electron-donating group-16 elements (S and Se), with
which we have succeeded in shifting the Q band beyond 1000 nm
through synergistic effects (Scheme 1).
We chose phosphorus(V) instead of transition metals for the
following reasons: (i) P has a large electronegativity (ca. 2.3) and
in particular can have a +5 oxidation state that can withdraw
electrons strongly, thus satisfying the requirements of this study.
(ii) The interaction between P and the Pc ligand is anticipated to
be larger than those between transition metals and the Pc ligand
because C and N, the elements that constitute the Pc ligand, are
closer to P than to transition metals in the periodic table. Hence,
the electron-withdrawing effect of P should be stronger than that
of a transition metal in the same oxidation state.
cations in various fields, such as organic solar cells, photodynamic
therapy for cancer, heat absorbers, and near-IR imaging.1 With
respect to their practical implementation, the compounds must
be easy to produce in a small number of steps and are required to
have high stability. Phthalocyanines (Pcs) are good candidates in
this regard, since they have been mass-produced (annually more
than 5000 tons) in relatively high yields for many years and are
known to be one of the most robust organic compounds. However,
most Pcs have their main absorption band (called the Q band) at
650ꢀ700 nm, and it is unusual to find Pcs having this band
beyond 800 nm.2,3 Although the Q-band position can be shifted
to the near-IR by benzoannulation to form naphthalocyanine
(Nc) and further anthracocyanine (Ac), this has the disadvantage
that the HOMO level destabilizes, making the resulting Ncs and
Acs prone to oxidation and hence sensitive to air. Considering
also that the reaction sequences leading to Ncs and Acs are longer
and their yields lower than those of Pcs,4 it is of considerable
interest to design new Pcs having main bands in the deep near-IR
region, particularly when we consider practical applications in
high-tech fields.
To achieve our goal in developing these new types of Pc,
we have to take into account several factors that influence the
position of absorption band of Pcs. (1) Since the Q band is
assigned to a transition between the HOMO and the LUMO or
LUMO+1,4,5 the HOMOꢀLUMO band gap must be smaller
than that of normal Pcs. (2) Electron-donating substituent groups
at the so-called α-positions generally shift the Q band to longer
wavelengths, but simultaneously they undesirably destabilize the
HOMO level.5 (3) Deformation of the Pc plane shifts the Q band
to longer wavelengths, but again this is due to the undesired
destabilization of the HOMO level.6 (4) Central elements also
The synthetic procedures for the Pcs are shown in Scheme 2.
Free-base Pcs were prepared in moderate yield by the so-called
lithium method7 at 120 °C. Phosphorus oxybromide was used for
the introduction of a phosphorus atom into the center of the Pc
ring. After the reaction was completed, the mixture was quenched
Received: September 8, 2011
Published: November 10, 2011
r
2011 American Chemical Society
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Scheme 2. Synthesis of Substituted Phthalocyaninesa
a Reagents and conditions: (i) PhSH (8 equiv), K2CO3 (4 equiv),
DMSO, rt, 14 h, 66%; (i)0 PhSeSePh (1.4 equiv), NaBH4 (3.2 equiv),
DMSO, rt, 3 h, 37%; (ii) Li (excess), 1-butanol, 120 °C, 2 h, 56ꢀ74%;
(iii) POBr3 (excess), pyridine, reflux, 1.5 h (X = S), rt, 30 min (X = Se);
(iv) CH2Cl2/MeOH, rt, 30 min, then KPF6 (4 equiv), CH2Cl2, rt, 12 h,
16ꢀ40%.
Figure 1. Thermal ellipsoid (50%) diagrams of the molecular structure
of 4a: (a) top view; (b) side view (peripheral substitutents omitted).
H atoms, the counteranion of 4a, and the solvent molecule have been
omitted for clarity.
Figure 2. (bottom) UVꢀvisꢀNIR and (top) MCD spectra of 3a (red),
4a (blue), and 4b (green) in CHCl3.
with methanol, which provided axial methoxide ligands for
the central phosphorus atom. The resulting phosphorus(V) Pcs
were cationic salts containing a mixture of several counteranions
(mainly [OH]ꢀ). With the aim of isolating a discrete counter-
anion for further characterization, counteranion exchange with
potassium hexafluorophosphate was carried out, producing the
desired stable Pcs as the hexafluorophosphate salts 4a (for S) and
4b (for Se) in yields of 40 and 16%, respectively. The 1H NMR
spectrum of 4a in CD2Cl2 shows only one kind of peripheral
phenylthio group (7.76 and 7.53 ppm), one signal for the Pc β-
hydrogens (7.29 ppm), and a doublet assignable to axial methoxy
groups at high field (0.6 ppm). The 31P NMR spectrum exhibits
only one peak at ꢀ179 ppm and follows the same trends of
[(porphyrin)P(V)(L)2].8 Hence, the conformation of 4a appears
to retain a highly symmetrical structure in solution, with the
phosphorus atom lying in the center of the macrocycle and
methoxy groups bound to the phosphorus (JPꢀH = 24 Hz). The
methoxy groups are affected by the strong shielding effect of the
diatropic ring current resulting from the macrocycle.
To our knowledge, no X-ray crystallographic structure of a
phosphorus(V) Pc has been previously reported. To determine
the detailed structure of the P(V) Pcs (Figure 1), single crystals
of 4a suitable for X-ray crystallography were grown by slow
diffusion of n-hexane into a chloroform solution of 4a.9 The ionic
radius of P(V) (0.38ꢀ0.52 Å, six-coordinate octahedral)10 is small
enough to allow the P(V) ion to sit in the center of the Pc 4N
mean plane (Δ4N < 0.005 Å). On the other hand, 4a is not planar
but rather is severely ruffled (Δr = 0.472 Å),11 as reported for
P(V) porphyrins,8 which helps to shift the Q band to longer
wavelengths relative the free base 3a, based on the above
characteristic (3). The peripheral phenyl rings are oriented
essentially perpendicular to the 4N mean plane, suggesting only
a weak electronic interaction between the substituent phenyl
groups and the Pc core. Since 4a has two axial ligands and
sterically hindered substituents, no PcꢀPc packing interaction
was observed [Figure S1 in the Supporting Information (SI)].
Figure 2 shows the electronic absorption and magnetic
circular dichroism (MCD) spectra of 3a and 4a and the absorp-
tion spectrum of 4b. Free base 3a shows a single Q-band peak
at 809 nm. The position of the Q band is similar to that of
H2(BuS)8Pc (805 nm in THF),5 so we can conclude that the
phenyl rings of 3a only marginally affect the electronic nature of
the Pc. In contrast, the Q band of P(V)-inserted 4a is at 1018 nm,
confirming a large red shift upon insertion of the P(V) ion due to
an effective orbital interaction typical of main-group elements.
The absorption spectrum in chloroform is typical of nonaggrega-
ted Pcs, which strictlyfollow theBeerꢀLambert law (FigureS2).12
In a similar manner, free base 3b and P(V)-inserted 4b show Q
bands at 812 and 1033 nm, respectively. Since the Q band of
substituent-free H2Pc is at ca. 678 nm (=center of the Q x00 and
Q y00 bands) in the same solvent,2 the above data indicate that
eight chalcogen atoms such as S and Se at the so-called α-positions
can shift the Q band to the red by ca. 130 nm (2420 cmꢀ1), while
the central P(V) ion can provide an additional red shift of
210ꢀ220 nm (2570ꢀ2630 cmꢀ1). The absorption coefficients
of 4a (and 4b) are smaller than those of the corresponding metal-
free species 3a, but the reason for this is not yet known (several
metallo-Pcs having Q bands at longer wavelength are also known
to have weaker Q bands).3d The Faraday A-term MCD spectra
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Figure 4. Cyclic voltammetry data for 3a (bottom, black lines) and 4a
(top, red lines). Cyclic voltammograms were acquired from 1.0 mM
solutions of analyte in 0.1 M nBu4NClO4/o-DCB. Ferrocene was used as
an internal standard and set to 0 V.
α-(HSe)8H2Pc 3b0 and its [P(OH)2]+ derivative 4b0 can also be
explained in a manner similar to that for 3a0 and 4a0 (Figure S7).
Natural bond orbital (NBO) analysis was carried out to eva-
luate further the marked effect of phosphorus insertion.13 Inter-
actions between the lone pairs of the group-16 elements and the
π* orbital of the Pc core were revealed (Table S1 in the SI). While
the values of the NBO donorꢀacceptor interaction energies for
free-base Pcs were relatively small and close in value, those of the
phosphorus Pcs were much larger than the free-base values and
depended significantly on the substituent elements at the Pc peri-
phery. The energies of the S- and Se-substituted phosphorus Pcs
were, for example, ca. 2.5 times larger than that of an O-substituted
phosphorus Pc. This effective orbital interaction affects the stabi-
lization of the LUMOs. Thus, S (or Se) and P cooperate in
narrowing the ΔHL.
Figure 3. Energy levels of frontier MOs and their contour plots
obtained from calculations. Blue and red plots indicate occupied and
unoccupied MOs, respectively. Calculations were performed at the
B3LYP/6-31G* level (for details, see the SI).
Cyclic voltammograms of 3a and 4a were measured in o-
dichlorobenzene (o-DCB) for comparison with the above calcula-
tions (Figure 4). tert-Butylated H2Pc shows redox couples at 0.94,
0.62, ꢀ0.82, and ꢀ1.19 V in CH2Cl2 (E1ox ꢀ E1red = 1.44 V).14
Compound 3a showed couples at 0.39, 0.16, ꢀ1.23, ꢀ1.48,
and ꢀ2.00 V (E1ox ꢀ E1red = 1.39 V), while 4a exhibited these at
0.37, ꢀ0.55, ꢀ0.98, and ꢀ1.83 V (E1ox ꢀ E1red = 0.92 V). Thus,
in the Q00 band region further indicate that the practical chromophore
symmetry of 4a is close to D4h, similarly to regular metallo-Pcs.
Our Pcs are remarkably air- and photostable. For example, the
UVꢀvisꢀNIR spectrum of 4a remained unchanged when 4a was
stored as a solid in air under ambient light for over 6 months. In
air-saturated CHCl3 under ambient light, the decrease in inten-
sity of the Q band of 4a was less than that of tert-butylated CuPc,
which is considered to be one of the most stable metallo-Pcs
(Figure S4).
To enhance the interpretation of the electronic structure of 4a
and 4b, MO calculations were performed for unsubstituted H2Pc
and its [P(OH)2]+ derivative and for α-(HS)8H2Pc 3a0 and its
[P(OH)2]+ derivative 4a0 (Figure 3). The optimized structure of
4a0 is also ruffled (Δr = 0.477 Å), in good agreement with the
crystal structure of 4a. As has already been reported,5 the HOMOꢀ
LUMO energy gap (ΔHL) becomes smaller on going from
unsubstituted H2Pc (ca. 2.2 eV) to α-(HS)8H2Pc 3a0 (ca. 1.8 eV).
The introduction of strongly electron-withdrawing [P(OH)2]+
into the Pc center stabilizes both the HOMO and LUMO signi-
ficantly, reducing ΔHL to ca. 2.0 and 1.5 eV, respectively, for the
unsubstituted and α-(HS)8-substituted species, making them
robust against oxidation and shifting the Q band to even longer
wavelengths. An important implication of these calculations is
that the extent of the red shift upon element insertion is different
from ligand to ligand. Thus, in the above case, the shift is larger
for the α-(HS)8-substituted species. The frontier orbitals of
in agreement with the above calculations, the value of E1ox
ꢀ
E1red decreases in going from normal H2Pc to α-octaphenylthio-
substituted 3a and further to [P(OMe)2]+-inserted 4a. In
particular, the introduction of the central [P(OMe)2]+ greatly
stabilizes the LUMO level (cf. the change of the first reduction
potential from ꢀ1.23 to ꢀ0.55 V), as anticipated in the intro-
ductory section from the large MO coefficient of the pyrrole
nitrogen in the LUMO.
In conclusion, we have succeeded in producing highly stable
Pcs having Q bands beyond 1000 nm as single chromophores by
considering several effects synergistically. The structure and elec-
tronic properties of the target phosphorus(V) Pcs were investiga-
ted by single-crystal X-ray diffraction(solid phase), NMRspectro-
scopy (solution phase), and DFT calculations (gas phase). In 4a
and 4b, destabilization of the HOMO by the introduction of
electron-donating elements at the α-positions, deformation of
the planarity of the Pc plane, and stabilization of both the HOMO
and LUMOs by a strongly electron-withdrawing, high-valency
central elementfunctionsynergistically. The Pccompounds repor-
ted here can be synthesized in only a few steps from commercially
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available compounds in moderate yields, while this magnitude of
red-shifted wavelength could not be achieved even for anthraco-
cyanines.2 This is important, since stable Pcs such as those in this
study can be used in society for various purposes. Furthermore,
the concept of taking the size of the MO coefficient into account
as described here is simple and should be applicable to other
organic dyes in adjusting the absorption wavelengths. Further
work is currently underway to expand this concept and synthe-
size other NIR dyes.
(7) Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever,
A. B. P., Eds.; VCH: Weinheim, Germany, 1989.
(8) Akiba, K.; Nadano, R.; Satoh, W.; Yamamoto, Y.; Nagase, S.; Ou,
Z.; Tan, X.; Kadish, K. M. Inorg. Chem. 2001, 40, 5553–5567.
(9) Crystallographic data for 4a: C83H55Cl3F3N8O2P1.50S8, MW =
1662.63; monoclinic, space group C2/c (No. 15); a = 17.407(4) Å, b =
37.264(8) Å, c = 29.795(6) Å, β = 101.927(3)°; V = 18909(7) Å3, Z = 8,
F
calcd = 1.168 g cmꢀ3; T = 100 K; R1 = 0.0879 [I > 2σ(I)], Rw = 0.2526
(all data), GOF = 0.845; CCDC-843317.
(10) (a) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767.
(b) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925. 1970,
B26, 1046–1048.
’ ASSOCIATED CONTENT
(11) Δr was calculated as the square root of the average of the
squares of the deviations of the atoms from the mean plane. The values
of Δr for ruffled phosphorous porphyrins are 0.38ꢀ0.54 Å. See ref 8.
(12) The position of the Q band did not depend on the counteran-
ion, so we can conclude that the counteranion does not affect the
electronic structure of phosphorous(V) Pc (see Figure S3).
(13) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899–926.
S
Supporting Information. Additional spectroscopic data,
b
full details of experimental and calculation procedures, and X-ray
crystallographic data of 4a (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(14) Lever, A. B. P. In Phthalocyanines: Properties and Applications;
Leznoff, C. C., Lever, A. B. P. , Eds.; VCH: Weinheim, Germany, 1993;
Vol. 3, pp 1ꢀ69.
Corresponding Author
nagaok@m.tohoku.ac.jp
’ ACKNOWLEDGMENT
This work was partly supported by a Grant-in-Aids for
Scientific Research on Innovative Areas (20108007, “pi-Space”)
and Scientific Research (B) (23350095) from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT).
T.F. thanks the Dean’s Grant for Exploratory Research Program
for Young Scientists from Tohoku University for financial support.
The authors thank Prof. Takeaki Iwamoto and Dr. Shintaro
Ishida (Tohoku University) for X-ray measurements and Dr. Soji
Shimizu (Tohoku University) for the collection of X-ray data for
4a. Some of the calculations were performed using supercomput-
ing resources at the Cyberscience Center of Tohoku University.
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