Synthesis of 5,10,15-triazaporphyrins - Effect of benzo-annulation on the electronic structures

Article abstract of DOI:10.1039/c2cc30625e

Novel triazaporphyrins were synthesized using 1,9-dibromodipyrromethene as a key starting material. These triazaporphyrins exhibit comparatively intense Soret and Q bands in the UV/vis region due to their hybrid properties between porphyrins and phthalocy

Full text of DOI:10.1039/c2cc30625e

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Synthesis of 5,10,15-triazaporphyrins—effect of benzo-annulation on the
electronic structureswz
Soji Shimizu,* Yuki Ito, Kazuaki Oniwa, Shoma Hirokawa, Yoshiaki Miura,
Osamu Matsushita and Nagao Kobayashi*
Received 28th January 2012, Accepted 28th February 2012
DOI: 10.1039/c2cc30625e
Novel triazaporphyrins were synthesized using 1,9-dibromodipyrro-
methene as a key starting material. These triazaporphyrins exhibit
comparatively intense Soret and Q bands in the UV/vis region due to
their hybrid properties between porphyrins and phthalocyanines.
in which meso-carbon atoms are partially replaced by nitrogen
atoms.5 The spectral features such as positions and intensities
of the Soret band and Q bands of azaporphyrins vary relative to
the number of meso-nitrogen atoms.6 In addition to their hybrid
optical properties between porphyrins and phthalocyanines, the
presence of meso-carbon atoms in azaporphyrins gives us greater
scope for further functionalization at the meso-positions,
which is not possible for normal phthalocyanines. Despite their
potential application in various fields in place of porphyrins and
phthalocyanines, azaporphyrins have been rarely investigated
mainly due to the lack of synthetic availability. Conventionally
tetrabenzo-5,10,15-triazaporphyrins have been synthesized
from o-phthalonitrile in the presence of alkyl or aryl Grignard
reagents or alkyl lithium or lithium alkoxide,5–8 and recently a
series of tetrabenzo-azaporphyrins including 5-monoaza, 5,10-
diaza, and 5,10,15-triazaporphyrins have been synthesized from
reactions of 3,6-dialkylphthalonitriles with Grignard reagents.6b
Herein we will add another facile synthetic method of novel
dibenzo-5,10,15-triazaporphyrin using 1,3-diiminoisoindoline
and 1,9-dibromodipyrromethene. From similar reactions using
2,5-pyrrolinediimine and 1,3-diiminobenz[f]isoindoline in place of
1,3-diiminoisoindoline, benzene-ring-contracted and -expanded
5,10,15-triazaporphyrins were successfully obtained. In addition
to their hybrid properties between porphyrins and phthalocyanines,
effects of benzo-annulation on the electronic structures were also
revealed based on electronic absorption and magnetic circular
dichroism (MCD) measurements as well as theoretical calculations.
From a mixed-condensation reaction of 5-pentafluorophenyl-
1,9-dibromodipyrromethene 19 and 5,6-di-p-tert-butylphenyl-
1,3-diiminoisoindoline 2a in p-xylene and dimethylaminoethanol
(DMAE), dibenzo-5,10,15-triazaporphyrin 3a was obtained in
15% yield along with octakis-p-tert-butylphenylphthalocyanine
(Scheme 1). The yield is moderate considering those of tetra-
benzo-5,10,15-triazaporphyrins ranging from 13% to 27%.5–8
In place of 2a, 6,7-di-p-tert-butylphenyl-1,3-diiminobenz[f]isoindo-
line 2b and 3,4-di-p-tert-butylphenyl-2,5-pyrrolinediimine 2c were
utilized to provide dinaphtho-5,10,15-triazaporphyrin 3b and
5,10,15-triazaporphyrin 3c in 1.6% and 0.11% yields, respectively.
The significantly low isolated yields of these compounds were due
to low reactivity of 2c and difficulty in separating 3b from
byproducts, which were mainly brominated species of 3b.
Porphyrins and phthalocyanines have been some of the best
commercially available dyes due to their facile syntheses and
the high molar absorptivity of the UV/vis light, and a number
of intensive studies have been aimed at application fields such
as solar cell, photodynamic therapy, and organic semi-conducting
devices.1–3 One of the most obvious differences in properties
between porphyrins and phthalocyanines is their spectral features
despite their similar 18p-electron conjugation systems. Both
porphyrins and phthalocyanines exhibit two main absorptions
in the UV/vis region, referred to as Soret and Q bands, but their
relative intensities are totally different; porphyrins generally
exhibit an intense Soret band at around 400 nm and weak
and forbidden Q bands in the region of 500–600 nm, whereas
phthalocyanines exhibit intense Q bands in the region of
650–700 nm and a rather broad and ill-defined Soret band at
around 340 nm. As theoretically illustrated by Gouterman with
his so-called four-orbital theory,4 non-degeneracy of the first and
second HOMOs of phthalocyanines arising from the presence of
electron-deficient nitrogen atoms at the meso-positions in
place of carbon atoms and annulation of benzene rings at
the b,b0-positions lessens configuration interaction between
the Soret and Q band transitions, and consequently red-shift
and intensification of the Q bands of phthalocyanines as
compared to porphyrins are observed. The main group
elements at the meso-positions and benzo-annulation, thus, play
a crucial role in determining the spectral features of porphyrins
and phthalocyanines. A class of intermediate molecules between
porphyrins and phthalocyanines is known as azaporphyrins,
Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai 980-8578, Japan.
E-mail: ssoji@m.tohoku.ac.jp, nagaok@m.tohoku.ac.jp;
Fax: +81 22 795 7728; Tel: +81 22 795 7728
w This article is part of the ChemComm ‘Porphyrins and Phthalo-
cyanines’ web themed issue.
z Electronic supplementary information (ESI) available: Syntheses,
spectroscopic data, and theoretical calculations. CCDC 862131 and
864016. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc30625e
1
The H NMR spectra of these triazaporphyrins in CDCl3
for 3a and 3c and in toluene-d8 for 3b exhibited two doublet
Chem. Commun., 2012, 48, 3851–3853 3851
c
This journal is The Royal Society of Chemistry 2012

Fig. 2 Absorption (bottom) and MCD (top) spectra of 3a (blue), 3b
(green), and 3c (red) in CHCl3.
than that of the LUMOs (DLUMO). 3a exhibits intense
fluorescence at 638 nm with a Stokes shift of 124 cmꢀ1
(see ESIz). The fluorescence quantum yield of 0.4113 falls
between those of meso-tetraphenylporphyrin (0.11 in benzene)14
and tetra-tert-butylphthalocyanine (0.77 in benzene).15 These
absorption and fluorescence spectra can be rationalized in terms
of intermediate properties of porphyrins and phthalocyanines.
3b and 3c also exhibit split Q band absorptions at 655 and
635 nm for 3b and at 637 and 564 nm for 3c, and corresponding
to these split Q bands, Faraday B terms are observed at 658 and
633 nm for 3b and at 637 and 564 nm for 3c in the MCD spectra
(Fig. 2). It is generally known that benzo-annulation on going
from porphyrazine to phthalocyanine and naphthalocyanine
causes a red-shift of the lower energy Q band, an increase in the
molar absorptivity, and a decrease in the splitting energy
between the Qx and Qy bands of the free base compounds.10
Although the energy differences of the Q bands follow this
order (2032 cmꢀ1 for 3c, 1009 cmꢀ1 for 3a, and 481 cmꢀ1 for
3b), it is contradictory to the trend that the lower energy Q band
of 3c appears at slightly longer wavelength region than that of
3a. The fluorescence measurements also revealed different
behavior of 3c. The fluorescence spectrum of 3b exhibits an
intense emission at 662 nm with a Stokes shift of 161 cmꢀ1 and
a fluorescence quantum yield of 0.31,13 whereas 3c does not
exhibit any fluorescence with respect to the S1–S0 emission,
probably due to a fast non-radiative decay process in the excited
state of 3c. This discrepancy in the optical properties of 3c
from those of 3a and 3b implies nonnegligible effects of the
substituent at meso-positions and b-positions on the electronic
structure of 3c.
Scheme 1 Syntheses of a series of 5,10,15-triazaporphyrins. Conditions:
(a) p-xylene, DMAE, 145 1C, 1 h.
signals of the b-pyrrolic protons (8.43 and 8.22 ppm for 3a,
8.28 and 7.80 ppm for 3b, and 9.16 and 8.74 ppm for 3c) and
four doublet signals due to the o- and m-protons of the tert-
butylphenyl groups (see ESIz). In addition to these peaks,
non-peripheral proton signals of the benzo and naphtho
moieties were observed at 9.14 and 8.98 ppm for 3a and at
9.19, 9.15, 8.22, and 8.21 ppm for 3b, respectively. Compared to
the chemical shifts of these compounds, the peaks of the b-pyrrolic
protons were gradually up-field shifted in the order of 3c, 3a, and
3b, which is indicative of reduction of the paratropic ring current
effect in the same order, although the effect of solvents is
not negligible. Similar reduction of ring current effects upon
annulation of benzene rings was observed from tetraazaporphyrin
to phthalocyanine and naphthalocyanine.10
The structure of 3a was unambiguously elucidated as its
ruthenium complex11 by X-ray diffraction analysis on crystals
obtained from slow diffusion of hexane into a chloroform
solution of the ruthenium complex in the presence of pyridine.
The ruthenium ion is coordinated in an octahedral fashion
with 3a, and carbonyl and pyridyl axial ligands (Fig. 1).y
The absorption spectrum of 3a exhibits split Q bands at 633
and 595 nm and similarly an intense single Soret band at
376 nm (Fig. 2). Corresponding to the split Q bands, Faraday
B terms are observed at 633 and 596 nm in the MCD spectrum
of 3a, which is indicative of non-degenerate excited states due to the
molecular symmetry lower than C3.3,12 The negative-to-positive
sequence of the MCD signals in ascending energy infers a greater
energy difference between the first and second HOMOs (DHOMO)
DFT and TDDFT calculations at the B3LYP/6-31G(d)
level were performed on model compounds of 3a–c, in which
the tert-butylphenyl substituents were replaced by phenyl
substituents for simplicity. A molecular orbital (MO) diagram
of 3a–c related to transitions in the Q band region is depicted
in Fig. 3 (see Table S1 in ESIz). Density distribution patterns
reveal an essentially similar HOMO and LUMOs (LUMO and
LUMO + 1) to those of phthalocyanines; a1u-like HOMO and
eg-like LUMOs for 3a–c.1–3 The second-to-fourth HOMOs are
delocalized on the phenyl moieties at b-positions. The energies
and order of these orbitals vary upon annulation of benzene
rings. The energy differences between the HOMO and the
LUMO are 2.57 eV for 3c, 2.48 eV for 3a, and 2.33 eV for 3b.
The TDDFT calculations on 3a and 3b predict two main bands
in the Q band region at 556 (oscillator strength, f = 0.27) and
539 nm (0.32) for 3a and at 583 (0.37) and 579 (0.47) nm for 3b.
Fig. 1 X-ray crystal structure of the ruthenium complex of 3a, top
view (left) and side view (right). The thermal ellipsoids were scaled to
the 50% probability level. Hydrogen atoms and p-tert-butylphenyl
substituents were omitted for clarity in the side view.
c
3852 Chem. Commun., 2012, 48, 3851–3853
This journal is The Royal Society of Chemistry 2012

here has potential utility in creating azaporphyrin analogues
with suitable optical properties for specific applications.
This work was supported by a Grant-in-Aid for Scientific
Research (C) (No. 23550040) from JSPS and a Grant-in-Aid
for Scientific Research on Innovative Areas (No. 20108007,
‘‘pi-Space’’) from MEXT. The authors thank Prof. T. Iwamoto
and Dr S. Ishida in Tohoku Univ. for X-ray measurement.
Notes and references
y Crystallographic data for 1: C15H5F5N2Br2, Mw = 468.03, mono-
clinic, space group C2/c (no. 15), a = 13.432(6), b = 15.534(7), c =
7.285(3) A, b = 93.759(6)1, V = 1516.6(12) A3, Z = 4, rcalcd
=
2.050 g cmꢀ3, T = ꢀ173(2) 1C, 5116 measured reflections, 1332 unique
reflections (Rint = 0.0508), R = 0.0455 (I 4 2s(I)), Rw = 0.1309
(all data), GOF = 1.056, CCDC 862131. Crystallographic data for
the ruthenium complex of 3a: C77H65F5N8ORuꢁCHCl3, Mw
=
1433.81, monoclinic, space group P21/c (no. 14), a = 16.596(4), b =
18.842(4), c = 25.025(6) A, b = 101.216(3)1, V = 7676(3) A3, Z = 4,
rcalcd = 1.241 g cmꢀ3, T = ꢀ173(2) 1C, 71 588 measured reflections,
13 527 unique reflections (Rint = 0.1299), R = 0.0847 (I 4 2s(I)),
Rw = 0.2338 (all data), GOF = 0.958, CCDC 864016.
Fig. 3 Partial MO diagram of 3a (middle), 3b (right), and 3c (left).
1 The porphyrin handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2000, vol. 1–10; The
porphyrin handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2003, vol. 11–20.
2 Handbook of Porphyrin Science, ed. K. M. Kadish, K. M. Smith
and R. Guilard, World Scientific Publishing, Singapore, 2010,
vol. 1–15.
3 Phthalocyanines: properties and applications, ed. C. C. Leznoff and
A. B. P. Lever, VCH, New York, NY, 1989, vol. 1–4.
4 (a) M. Gouterman, J. Chem. Phys., 1959, 30, 1139;
(b) M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138.
5 (a) J. H. Helberger and A. V. Rebay, Justus Liebigs Ann. Chem., 1937,
531, 279; (b) C. E. Dent, J. Chem. Soc., 1938, 1; (c) P. A. Barrett,
R. P. Linstead and G. A. P. Tuey, J. Chem. Soc., 1939, 1809;
(d) P. A. Barrett, R. P. Linstead, F. G. Rundall and G. A. P. Tuey,
J. Chem. Soc., 1940, 1079.
6 (a) A. N. Cammidge, M. J. Cook, D. L. Hughes, F. Nekelson and
M. Rahman, Chem. Commun., 2005, 930; (b) A. N. Cammidge,
I. Chambrier, M. J. Cook, D. L. Hughes, M. Rahman and L. Sosa-
Vargas, Chem.–Eur. J., 2011, 17, 3136.
These bands are mainly composed of transitions from the
HOMO to the LUMOs. On the other hand, the TDDFT
calculation on 3c reveals four bands with smaller oscillator
strengths of 0.0056–0.080 at 563, 553, 536, and 526 nm in
addition to intense bands at 484 (0.21) and 457 (0.24) nm. The
main components of these bands are transitions from the
HOMO ꢀ 1, HOMO ꢀ 2, and HOMO ꢀ 3 to the LUMOs
in addition to those from the HOMO to the LUMOs. Since the
electron densities are delocalized on the phenyl moieties in the
HOMO ꢀ 1 and HOMO ꢀ 2 of 3c, transitions from these
orbitals to the LUMOs, which are delocalized on the macro-
cycle, can be regarded as a kind of intramolecular charge
transfer transitions. This charge transfer contribution in the Q
band absorption would plausibly cause the observed significant
red-shift of 3c. Similar contribution can be seen in the band
components of 3a and 3b as well, but the Q band components
comprising transitions from the HOMO to the LUMOs become
more predominant in the case of 3a and 3b due to the larger
difference in energies between the HOMO and the second-to-
fourth HOMOs. Considering the fact that TDDFT calculations
predict Q bands at much shorter wavelength regions (528 and
497 nm) for a model compound of 3c in which peripheral
phenyl substituents are not included, the intramolecular
charge transfer contribution cannot be negligible in the case
of 3c (see Table S2 in ESIz).
7 (a) C. C. Leznoff and N. B. Mckeown, J. Org. Chem., 1990,
55, 2186; (b) Y. H. Tse, A. Goel, M. G. Hu, A. B. P. Lever,
C. C. Leznoff and J. E. Van Lier, Can. J. Chem., 1993, 71, 742.
8 (a) N. E. Galanin, L. A. Yakubov, E. V. Kudrik and
G. P. Shaposhnikov, Russ. J. Gen. Chem., 2008, 78, 1436;
(b) Y. B. Ivanova, Y. I. Churakhina, A. S. Semeikin and
N. Z. Mamardashvili, Russ. J. Gen. Chem., 2009, 79, 833.
9 1 was synthesized from a bromination reaction of 5-pentafluoro-
phenyldipyrromethane with NBS in THF at ꢀ78 1C, followed by
oxidation with DDQ. For details see ESIz.
10 N. Kobayashi, S. Nakajima, H. Ogata and T. Fukuda, Chem.–Eur. J.,
2004, 10, 6294.
11 Ruthenium complex of 3a was synthesized by following the
In summary, a series of 5,10,15-triazaporphyrins were
synthesized from mixed-condensation reactions of 1,9-dibromo-
dipyrromethene and 1,3-diiminoisoindoline derivatives.
Compared to other synthetic reactions of 5,10,15-triaza-
porphyrin reported to date, this reaction opens new access
to asymmetric 5,10,15-triazaporphyrin analogues, in which
substituents at meso-positions and peripheral ring units
can be varied using different dibromodipyrromethenes16 and
1,3-diiminoisoindolines. Since the electronic structures of
5,10,15-triazaporphyrins are significantly perturbed by substi-
tuents as can be seen for 3c, the synthetic reaction described
literature procedure; M. S. Rodrıguez-Morgade, M. Planells,
´
T. Torres, P. Ballester and E. Palomares, J. Mater. Chem., 2008,
18, 176.
12 J. Mack, M. J. Stillman and N. Kobayashi, Coord. Chem. Rev.,
2007, 251, 429.
13 Quantum yields were determined relative to cresyl violet (FF = 0.54 in
methanol): D. Magde, J. H. Brannon, T. L. Cremers and J. Olmsted,
J. Phys. Chem., 1979, 83, 696.
14 P. G. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969, 31, 1.
15 D. S. Lawrence and D. G. Whitten, Photochem. Photobiol., 1996,
64, 923.
16 5-p-Tolyl-1,9-dibromodipyrromethene was similarly reacted with
1,3-diiminoisoindoline to give a triazaporphyrin compound.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3851–3853 3853