Columnar discotic mesophases from novel non-symmetrically substituted (octylsulfanyl) porphyrazines

Article abstract of DOI:10.1080/15421400701834088

Synthesis and mesomorphism of novel non-symmmetric mono -aryl (octylsulfanyl)porphyrazines and their Ni(II) complexes were reported. A recently disclosed asymmetrization procedure led to mono -bromo substituted porphyrazines, which provided access to aryl

Full text of DOI:10.1080/15421400701834088

Mol. Cryst. Liq. Cryst., Vol. 481, pp. 56–72, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701834088
Columnar Discotic Mesophases from Novel
Non-symmetrically Substituted (Octylsulfanyl)
Porphyrazines
Sandra Belviso, Mario Amati, Margherita De Bonis,
and Francesco Lelj
`
Universita della Basilicata, Dipartimento di Chimica,
Potenza, Italy
Synthesis and mesomorphism of novel non-symmmetric mono b-aryl (octylsulfa-
nyl)porphyrazines and their Ni(II) complexes were reported. A recently disclosed
asymmetrization procedure led to mono b-bromo substituted porphyrazines, which
provided access to aryl substituted derivatives through Suzuki cross-coupling. UV-
Vis spectroscopy showed a bathochromic shift for bromine and aryl substituted
nickel complexes. Optical Microscopy and DSC showed that Br derivatives
3a–3b and H-heptakis Ni(II) complex 2b display two columnar mesophases (Colh
and Colr) stable even at RT and a wide DT (100ꢀC). Introduction of the aryl
substituent on the macrocycle caused the appearance of only the Colh mesophase
and lower DT.
Keywords: columnar discotic mesophase; non-symmetric porphyrazines; Suzuki
cross-coupling
INTRODUCTION
Among the tetrapyrrole macrocycles the porphyrazines display
peculiar physical and chemical properties both as ‘free-base’ and as
transition metal complexes. Many recent investigations showed their
rich coordination chemistry, excellent chemical, thermal and photo-
chemical stability as well as the technological applications of these
macrocycles [1,2].
Dedicated to late Professor Augusto Sirigu
`
This work has been supported by the Italian Ministero dell’Istruzione, dell’Universita
e della Ricerca PRIN 2005 200503527 grant.
`
Address correspondence to Prof. Francesco Lelj Garolla di Bard, Universita della
Basilicata, Dipartimento di Chimica, Via Nazario Sauro, 85, 85100 Potenza, Italy.
E-mail: francesco.lelj@unibas.it
56

Mesomorphic Non-symmetric S-octyl Porphyrazines
57
In particular, the alkyl(sulfanyl)porphyrazines, bearing thioether-
groups at the b-positions of the azaporphyrin ring, present attractive
electrochemical and optical properties as well as high self-organizing
capabilities, giving rise to different types of condensed phases such
as discotic liquid-crystals (LC) and Langmuir-Blodgett (LB) films.
[1h-m,2] The octakis(alkyl)sulfanylporphyrazine ring can be easily
modified by synthetic means, allowing the tuning of the structural
and optical properties of the azaporphyrin ring [1a-m,3]. Such struc-
tural flexibility makes these derivatives suitable to investigate in
detail the molecular self-organizing phenomena of tetrapyrroles
with the aim to find the relationship between the molecular struc-
ture and the occurrence of mesophases, with possible applications
in opto-electronic technology [1e,4]. In recent papers, we have also
studied the mesomorphic behaviour of transition metal complexes
of octakis alkyl(sulfanyl)porphyrazines [2,5], providing a detailed
pattern of the individual role of the size and stiffness of the periph-
eral tails and of the nature of the metal on the structure and stab-
ility of the LC state. Taking into account the great interest risen by
non-symmmetric tetrapyrroles and their transition metal complexes,
which display a great potential for optical, electrical, catalytic
properties and therapeutic application [6], we further studied the
self-aggregation properties of non-symmetric alkylsulfanylporphyra-
zines [7], easily available by a new synthetic route recently disclosed
in our laboratories [8]. These studies demonstrated that highly
non-symmetric ‘free-base’ 2,7,8,12,13,17,18-heptakis(octylsulfanyl)-
5,10,15,20-porphyrazine 2a displays discotic mesomorphism, unlike
its symmetric counterpart 1a [2,5]. The same behaviour is shown
by 2-bromoheptakis(octylsulfanyl)porphyrazine 3a derived by simple
bromination of 2a. Probably in these compounds, the permanent
dipole induced by the non-symmmetric shape of the macrocycle
plays a role in stabilising ordered LC-phases, a feature which might
have relevance to the potential application of these discotic mole-
cules in molecular electronics. We then decided to investigate if
the peripheral substitution of such non-symmetric porphyrazines
with moieties able to modify both their charge distribution and their
molecular shape could influence the optical, electrical, and self-
aggregation properties of these macrocycles. We then report herein
the synthesis of a series of novel non-symmetrically substituted
mono b-aryl (octylsulfanyl)porphyrazines and their Ni(II) complexes.
A study on the optical and mesomorphic properties of these com-
pounds was then undertaken with the aim to identify the influence
of the substituents.

58
S. Belviso et al.
EXPERIMENTAL
General
All chemicals and solvents (Aldrich Chemicals Ltd.) were of reagent
grade. Solvents were dried and distilled before use according to stan-
dard procedures. Solvents used in physical measurements were of
spectroscopic or HPLC grade. Silica gel used for chromatography
was Merck Kieselgel 60 (70–230 mesh).
1H and 13C NMR spectra were recorded on an 500 MHz INOVA
Varian spectrometer with SiMe4 as internal standard. MALDI-MS
spectra were obtained with a MALDI DIE-PRO Applied Biosystems
instrument. Samples for mass spectrometric analysis were prepared
by depositing 1 mL of porphyrazine solution (<10ꢁ4M in CH2Cl2)
on the probe and allowing this solution to evaporate. 1 mL of the
matrix, prepared by dissolving a-cyano-4-hydroxycinnammic acid in
CH3CNꢁCF3COOH (70:30, v=v), was then added. MALDI-MS spectra
of 4a and 5b were obtained with an Ettan MALDI-ToF=Pro
(Amersham Biosciences) instrument. The matrix was prepared by
dissolving a-cyano-4-hydroxycinnammic acid in CH3CNꢁCF3COOH
(100:1, v=v). Samples for mass spectrometric analysis were prepared
by dissolving 4 mL of porphyrazine solution (<10 ꢁ 4 M in CH2Cl2)
directly in 4 mL of the matrix. 0.4 mL of this mixture was deposited
on the probe tip and allowed to evaporate. Time-of-flight mass spec-
trum (TOF) was obtained by irradiating the sample with 10 ns pulses
of a Nd(YAG) laser operating at 337 nm. Solution electronic spectra in
1 cm path length quartz cells in the region 250–800 nm were per-
formed on a UV-Vis-NIR 05E Cary spectrophotometer. Optical obser-
vations of textures as a function of the temperature were performed
with a Axioplan-Zeiss polarizing microscope equipped with a Linkam
microfurnace. Images were captured using a Zeiss MC80 Microscope
Camera and samples were studied between two silica glass plates
(diameter 16.0 mm). Transition temperatures were measured by dif-
ferential scanning calorimetry with a Perkin-Elmer DSC7 instrument
operated at scanning rates of 20ꢀC minꢁ1. The sample pans and covers
were made of aluminium (diameter 5.0 mm). The uncertainty in each
measured temperature was ꢂ0.1ꢀC.
Syntheses
The symmetrically substituted porphyrazines 1a and 1b were
prepared according to the procedure previously described in ref [2,5].

Mesomorphic Non-symmetric S-octyl Porphyrazines
59
The non-symmetrically H-substituted (octylsulfanyl)porphyrazine
2a and the corresponding Br-substituted porphyrazine 3a were pre-
pared following the procedure previously reported in ref. [8] and [7],
respectively.
[2H-3,7,8,12,13,17,18-Heptakis(octylsulfanyl)-5,10,15,20]-
porphyrazinato nickel (II) (2b). The complex was synthesised using
the same procedure as for the preparation of the similar heptaki-
s(ethylsulfanyl)-nickel (II) complex [8]. The purification of the result-
ing dark solid by column chromatography on silica gel using
CH2Cl2ꢁn-hexane (3:7, v=v) as eluent (first band) afforded the desired
1
pure compound (yield ca. 30%). H NMR (500 MHz, CDCl3, 297 K),
d=ppm: 8.51 (s, 1 H, pyrrole), 4.11 (t, SCH2), 4.05 (m, SCH2), 3.97
(m, SCH2), 3.79 (t, SCH2), 3.51 (t, SCH2), 2.16 (m, CH2), 1.82 (m,
CH2), 1.55 (m, CH2), 1.23 (m, CH2), 0.79 (t, CH3). UV-Vis (CHCl3)
k
max=nm: 328, 338 sh (Soret); 473; 673 (Q band). MALDI-MS: m=z
1381.10 [M þ H]þ (calc. 1379.70).
[2-Bromo-3,7,8,12,13,17,18-Heptakis(octylsulfanyl)-5,10,15,20]-
porphyrazinato] nickel (II) (3b). The complex was prepared according
to ref. [2] and [5] using 3a as starting porphyrazine. The crude product
was passed through a silica gel column using CH2Cl2ꢁn-hexane (3:7,
v=v) as eluent (second band). Removal of the solvent gave a yield of
1
ca. 70% of the desired pure compound. H NMR (500 MHz, CDCl3,
297 K), d=ppm:4.19 (t, SCH2), 4.01 (m, SCH2), 3.97 (t, SCH2), 3.90 (t,
SCH2), 1.85 (m, CH2), 1.58 (m, CH2), 1.25 (m, CH2), 0.80 (t, CH3).
UV-Vis (CHCl3), kmax=nm: 328, 340 sh (Soret); 485; 661 (Q band).
MALDI-MS: m=z 1460.03 [M þ H]þ (calc. 1457.62).
Suzuki Cross-Coupling Reaction. General Procedure
Non-symmetric brominated porphyrazine 3a or 3b (0.07 mmol), pot-
assium carbonate (0.56 mmol), tetrakis(triphenylphosphine)-palladium
(0) (10%mol) and the corresponding boronic acid (0.28 mmol) were
refluxed in a dry DMF=toluene (2:3, v=v) mixture (10 mL) under N2.
The reaction was stopped after 18 h or after 4 h when the substrate
was 3a or 3b, respectively. After cooling to room temperature water
was added and the solution was extracted with CH2Cl2. The organic
fractions were collected, dried over sodium sulphate and filtered. After
removal of the solvent under reduced pressure, the crude product was
purified by column chromatography on silica gel using a CH2Cl2ꢁ
n-hexane mixture as eluent.
2-Phenyl-3,7,8,12,13,17,18-Heptakis(octylsulfanyl)-5,10,15,20-21H,
23H-porphyrazine (4a). According to the conditions of the General

60
S. Belviso et al.
Procedure the reaction of ‘free-base’ 3a with phenylboronic acid
resulted in the isolation of 4a after purification by chromatography on
silica gel with a CH2Cl2ꢁn-hexane (4:6) mixture as eluent (third band).
Yield ca. 20%. 1H NMR (500 MHz, CDCl3, 297 K), d=ppm: 8.38 (d, 2H),
7.76 (t, 2H), 7.64 (t, 1H), 4.12 (m, SCH2), 4.02 (m, SCH2), 3.90 (t, SCH2),
1.85 (m, CH2), 1.55 (m, CH2), 1.20 (m, CH2), 0.88 (m, CH3), 0.78 (m,
CH3), ꢁ1.12 (s, NH). UV-Vis (CHCl3), kmax=nm: 363, (Soret); 503; 639,
708 (Q bands). MALDI-MS: m=z 1400.12 [M þ H]þ (calc. 1399.82).
[2-Phenyl-3,7,8,12,13,17,18-Heptakis(octylsulfanyl)-5,10,15,20]-
porphyrazinato] nickel (II) (4b). The reaction of nickel(II) complex 3b
and phenylboronic acid gave the desired pure b-arylated porphyrazine
4b after chromatographic purification on silica gel with a CH2Cl2ꢁ
n-hexane (4:6) mixture as eluent (second band). Yield ca. 30%. 1H
NMR (500 MHz, CDCl3, 297 K), d=ppm: 8.32 (d, 2H), 7.73( t, 2H),
7.62 (t, 1H), 4.03 (m, SCH2), 3.96 (t, SCH2), 3.77 (t, SCH2), 1.85 (m,
CH2), 1.55 (m, CH2), 1.20 (m, CH2), 0.78 (m, CH3). UV-Vis (CHCl3),
k
max=nm: 332, 341 sh (Soret); 488; 665 (Q band). MALDI-MS: m=z
1457.21 [M þ H]þ (calc. 1455.74).
2-(4-biphenyl)-3,7,8,12,13,17,18-Heptakis(octylsulfanyl)-5,10,15,20–
21H,23H-porphyrazine (5a). Reaction of ‘free-base’ 3a with biphenyl-
boronic acid resulted in the isolation of 5a after purification by chro-
matography on silica gel with a CH2Cl2ꢁn-hexane (4:6) mixture as
eluent (third band). Yield ca. 20%. 1H NMR (500 MHz, CDCl3,
297 K), d=ppm: 8.53 (d, 2H), 8.02 (d, 2H), 7.84 (d, 2H), 7.56 (t, 1H),
7.45 (d, 2H), 4.12 (m, SCH2), 4.02 (t, SCH2), 3.98 (t, SCH2), 3.94 (t,
SCH2), 1.89 (m, CH2), 1.55 (m, CH2), 1.19 (m, CH2), 0.88 (m, CH3),
0.79 (m, CH3), ꢁ1.09 (s, NH). UV-Vis (CHCl3), kmax=nm: 346, 3.62
sh) (Soret); 507; 639, 710 (Q bands). MALDI-MS: m=z 1477.10
[M þ H]þ (calc. 1475.85).
[2-(4-biphenyl)-3,7,8,12,13,17,18-Heptakis(octylsulfanyl)-5,10,15,20]–
porphyrazinato] nickel (II) (5b). From the reaction of 3b with biphe-
nylboronic acid was isolated 5b after purification by chromatography
on silica gel with a CH2Cl2–n-hexane (4:6) mixture as eluent (second
band). Yield ca. 50%. 1H NMR (500 MHz, CDCl3, 297 K), d=ppm:
8.47 (d, 2H), 8.00 (d, 2H), 7.84 (d, 2H), 7.56 (t, 1H), 7.46 (d, 2H),
4.05 (m, SCH2), 3.98 (t, SCH2), 3.86 (t, SCH2), 3.81 (t, SCH2), 1.85
(m, CH2), 1.55 (m, CH2), 1.21 (m, CH2), 0.80 (m, CH3). UV-Vis (CHCl3),
k
max=nm: 327, 338 sh (Soret); 487, 664 (Q band). MALDI-MS: m=z
1532.70 [M þ H]þ (calc. 1531.77).
[2-(4-N,N-dimethylammino-Phenyl)-3,7,8,12,13,17,18-Heptakis-
(octylsulfanyl)-5,10,15,20]–porphyrazinato] nickel (II) (6). Following
the general procedure the reaction of nickel(II) complex 3b and

Mesomorphic Non-symmetric S-octyl Porphyrazines
61
4-N,N-dimethylammino-phenylboronic acid led to the isolation of 6
after chromatographic purification on silica gel with a CH2Cl2–
n-hexane (4:6) mixture as eluent (second band). Yield ca. 30%. 1H NMR
(500 MHz, CDCl3, 297 K), d=ppm: 8.44 (d, 2H), 7.08 (d, 2H), 4.02 (m,
SCH2), 3.83 (t, SCH2), 3.79(t, SCH2), 3.18 (s, CH3), 1.84 (m, CH2),
1.56 (m, CH2), 1.20 (m, CH2), 0.78 (t, CH3). UV-Vis (CHCl3), kmax=nm:
nm: 328, 340 sh (Soret); 513; 669, 694 sh (Q bands), 750. MALDI-MS:
m=z 1499.51 [M þ H]þ (calc. 1498.78).
RESULTS AND DISCUSSION
Synthesis
We recently set up [8] a new preparative strategy (Scheme 1) to highly
non-symmetric (alkylsulfanyl)porphyrazines by hydrogen replace-
ment of one alkylsulfanyl tail of the fully substituted porphyrazines 1,
resulting in the non-symmetrical b-H-substituted porphyrazines 2.
Such approach displays the great advantage of a very clean reac-
tion, easy purification procedures and yield larger then 40% of the
desired non-symmetrically substituted product. The subsequent one-
step replacement of the pyrrole b-hydrogen of 2 by a bromine atom
leads to 3, suitable intermediates for introducing different substitu-
ents on the macrocycle [7]. Herein we show that the easily accessible
SCHEME 1 Synthesis if non-symmetrically substituted (octylsulfanyl)
porphyrazines.

62
S. Belviso et al.
mono-bromo porphyrazines 3 can be then efficiently arylated by
Pd-catalysed Suzuki coupling reactions [9], providing access to a wide
range of non-symmetric b-aryl substituted derivatives 4–6.
According to our protocol, symmetric ‘free-base’ and Ni(II) meta-
lated porphyrazines 1a and 1b were treated with the one-electron
donor CrCl2, leading to the non-symmetric 2a,b [8]. The latter were
then brominated with NBS, affording 3a,b [7]. The brominated com-
pounds were then submitted to Suzuki cross-coupling, by reaction
with the corresponding boronic acid in the presence of excess of
K2CO3 and catalytical amounts of Pd(PPh3)4 (10%mol). Spectral data
(1H NMR, MALDI-MS) of arylated products were consistent with
assigned formulations.
To the best of our knowledge these are the first examples of Suzuki
cross coupling reactions on porphyrazine macrocycles. Following this
approach we introduced aryl moieties that we expected to exert specific
shape and electronic effects on the macrocycles influencing their aggre-
gation properties. The phenyl (4) and biphenyl (5) groups were chosen
for their mainly steric effects, while the 4-N,N-dimethylamminophenyl
moiety in 6 was expected also to act as an electron donor, giving rise to
a push-pull system, suitable for molecular electronics. Preliminary
DFT computations shows, [10,11] in fact, that in all the studied aryl
substituted porphyrazines the LUMO, LUMO þ 1, and the HOMO
are porphyrazine-localised orbitals, whereas only in the case 4-
NMe2Ph substituted complex 6, the HOMO is localized on the phenyl
ring and characterized by less negative energy value (Fig. 1).
Furthermore according to TDDFT computations the first excited
state of 6 can be assigned to the HOMO-LUMO transition, with a
strong charge transfer from the phenyl ring to the porphyrazine ring.
The existence of this excited state is confirmed by the higher absor-
bance at longer wavelengths in the UV-Vis spectra (vide infra).
Interestingly, the Suzuki coupling on the ‘free base’ brominated
heptakis-octylthio porphyrazine 3a was very sluggish, while the same
reaction provided higher yields and faster reaction when performed on
the Ni(II) complex 3b. This behaviour resembles what observed in the
case of metal-catalyzed cross coupling on porphyrin macrocycles [12].
Spectroscopy
The substituent effect on the optical properties of the studied non-
symmetric porphyrazines were investigated by UV-Vis absorption spec-
troscopy of their Ni(II) complexes. The optical spectra (CHCl3) of the
non-symmetric Ni(II) porphyrazine 2b shows an intense band
at ꢃ 328 nm (Soret band), an almost equally intense band at ꢃ 673 nm
(Q band) and an absorption at about 473 nm (‘‘extra band’’) (Fig. 2) [13].

Mesomorphic Non-symmetric S-octyl Porphyrazines
63
FIGURE 1 Computed HOMO and LUMO Kohn-Sham orbitals of 6.
The main spectral change in respect to the symmetric parent 1b is
the observed blue shift (10–15 nm) of the extra band, as already
observed in non-symmetric S-ethyl substituted porphyrazines [8].
The macrocycle substitution with bromine, phenyl and biphenyl
moieties induces, on the contrary, a red shift (12–15 nm) in the
FIGURE 2 UV-Vis spectra in CHCl3; scan rate 600 nm=min.

64
S. Belviso et al.
400–550 nm range in respect to 2b. As inferred from Figure 2, the shift
is further increased up to 40 nm in 4-NMe2-phenyl substituted com-
plex 6, which also display a widening of the Q band (ꢃ669 nm). Such
bathochromic shifts reflect an effective electronic delocalization
between the porphyrazine core and the peripheral aryl groups, as
observed in the case of tetrakis arylethynyl substituted porphyrins
[14], and make these compounds very attractive for application in
non-linear optics [15]. The peculiar spectral behaviour of the 4-N,N-
dimethylaminophenyl substituted compound 6 is also confirmed by
its evident solvatochromism, not displayed by the other studied
complexes.
As inferred from the optical spectra of 6 (Fig. 3) the Q and the extra
bands exhibit strong ipsochromism when passing from CHCl3 to
toluene, shifting to the blue by 24 nm and 7 nm, respectively. In tolu-
ene are also removed other typical spectral features of this compound
like the marked asymmetry of the extra band and the weak absorption
at longer wavelengths (780 nm).
Mesomorphism
To gain insight on the effect of peripheral substitution on the self-
organizing capability of the non-symmetric porphyrazines in LC
phases, we studied the mesomorphic behavior of the synthesized
FIGURE 3 UV-Vis spectra of compound 6 in CHCl3 and toluene; scan rate
600 nm=min.

Mesomorphic Non-symmetric S-octyl Porphyrazines
65
‘free-base’ and nickel compounds using DSC and polarizing microscopy.
In Table 1 the thermal phase transitions data collected for all the
studied compounds are reported and compared with literature data
for compounds 1a-b [2], 2a, and 3a [7].
The optical microscopic observations were performed placing the
sample between two glass slides. Values of the mesophase thermal
stability range (DT) were usually measured cooling the isotropic melt.
In fact, on cooling the mesophase textures and phase transitions were
best resolved. In some compounds the presence of two different
mesophases and the temperatures of their transitions could be
detected only by optical analysis. In many cases the samples obtained
from evaporation of solutions exhibited at room temperature viscous
and birefringent trace when rubbed on the glass plate and observed
TABLE 1 Optical and Thermal Properties of Compounds 1–6
Phase transitions
T(ꢀC)a
Thermal stability
range DT(ꢀC)
Compound
1a
1b
2a
K 81.5 Ib
–
51.2
48.2
97.0
K 67.6 Colh 118.8 Ib
K 51.7 Colh 99.7 Ic
I 92 Colh-5 K
2b
3a
3b
M 51.7 Colh 92.5 I
I 71.8 Colh 8.0 Colr–30.0 K
Colh 40.8
Colh 63.8; Colr 38
DTtot 101.8
76.4
Colh 77.2; Colr 24.5
DTtot 101.7
Colh 72.6
K 37.3 Colh 113.7 Ic
I 105.2 Colh 28.0 Colr 3.5 K
M 47.4 Colh 120.0 I
I 92.0 Colh 35.1 Colr 7.0 K
Colh 56.9; Colr 28.1
DTtot 85.0
4a
4b
5a
5b
6
K 46.2 I
I 38.5 K
M 43.4 I
I 37.0 Col 1.0 K
M 77.2 I
I 57.0 Colh 43.0 K
M 62.5 I
I 39.5 Colh 27.0 K
K 77.1 I
–
–
–
36.0
–
14.0
12.5
–
I 27.7 K
–
aFirst line report transition temperatures measured on heating, second line report
temperatures on cooling. K ¼ solid, M ¼ unidentified mesophase, Colr ¼ columnar
rectangular mesophase, Colh ¼ columnar hexagonal mesophase, I ¼ isotropic liquid.
bData taken from ref. [2].
cData taken from ref. [7].

66
S. Belviso et al.
under crossed polarizers. This behavior was typical of a LC phase
[16,17] therefore, even if the texture of the mesophase was not detect-
able, we defined these phases as ‘‘unidentified mesophase’’ (M) in
Table 1. The calorimetric analysis (Table 1) points out that all the
investigated compounds display enantiotropic mesomorphism, being
the reverse phase transformations always well resolved, although
some supercooling of the transitions did regularly occur.
In the ‘free base’ porphyrazines the substitution of one of the per-
ipheral S-alkyl chain with a H atom in 2a, induces the appearance
of an ordered mesophase, not present in the parent symmetric azama-
crocycle 1a. This compound displays focal-conic or fan-shaped texture,
typical of columnar hexagonal mesophases (Colh) [2,5,16–18]. The
introduction of Ni inside the macrocycle ring enhances, both in sym-
metric and non-symmetric porphyrazines, the mesomorphic behav-
iour. In fact, earlier studies [2] showed that even though symmetric
‘free base’ counterpart 1a misses the LC phase, its Ni complex 1b dis-
plays a Colh mesophase. The non-symmetric Ni complex 2b displays
very interesting liquid crystalline properties. During the first heating
of the viscous phase of 2b, obtained by solution evaporation, the typi-
cal focal-conic texture of the Colh mesophase appeared at 51.7ꢀC and
cleared at 95.2ꢀC. Subsequent fast (20ꢀC=min) cooling from the
isotropic liquid led at 71.8ꢀC (after a supercooling of ca. 23ꢀC) to
the appearance, under crossed-polarizer, of focal-conic texture and of
hexagonal dendritic structures, more clearly visible without crossed
polarizers, both related to a Colh mesophase [18,19,20]. This LC phase
evolved, at 8.0ꢀC, to a fingerprint texture which is usually displayed by
columnar rectangular mesophases (Colr) [18,20,21]. Finally, further
cooling led to crystallization of the sample at ꢁ30ꢀC. The measured
liquid crystalline DT for 2b (101.8ꢀC) was then wider both of the one of
its non-symmetric ‘free base’ 2a and of the symmetric Ni complex 1b.
A similar behavior was displayed by the bromine substituted non-
symmetric porphyrazine 3a and by its Ni complex 3b (Fig. 4).
In both compounds, upon cooling from the isotropic liquid a tran-
sition from a Colh to a Colr LC phase was observed. The stability
ranges of their mesophases were comparable with the ones displayed
by the H substituted porphyrazines 2a and 2b but, surprisingly, the
Br substituted Ni complex 3b showed lower DT than the ‘free base’
counterpart 3a. It is noteworthy that all the non-symmetric porphyr-
azines 2a–3a and their Ni(II) complexes 2b–3b display a wide range
of mesophase stability and that their columnar arrangement is
retained until well below room temperature. Such tendency of the
molecules to maintain the same organization at low temperatures is
important for the preparation of single-domain LC films required for

Mesomorphic Non-symmetric S-octyl Porphyrazines
67
FIGURE 4 Liquid-crystalline textures shown on cooling by 3b at 77ꢀC (top)
and at 18ꢀC (bottom). Crossed polarizers microscope.
photovoltaic applications [17,19]. Against all cursory expectation, in
these compounds the less ordered mesophase Colr was observed at
lower temperature than the most ordered Colh phase, a behaviour
recently observed in columnar LC of non-symmmetrically substituted
porphyrins [16].
As inferred from Table 1 the b-aryl substitution of the porphyra-
zines in 4–6 destabilizes the mesomorphic properties of these azama-
crocycles. Also in these cases, however, the introduction of the Ni
within the macrocycle provides higher stabilization of the mesophases.
The phenyl-substituted ‘free base’ porphyrazine 4a does not
show any mesomorphic behaviour, while its Ni(II) complex 4b exhibits
a liquid-crystalline texture (between 1ꢀC and 37ꢀC) as shown in
Figure 5 (top).
The presence of a biphenyl substituent induces the formation of
a stable Colh mesophase even in the ‘free base’ porphyrazine 5a
(Fig. 5, bottom), although displaying a quite small thermal stability
range (14ꢀC). A small DT is also observed in the corresponding Ni(II)
complex 5b, which shows a columnar hexagonal LC phase between

68
S. Belviso et al.
FIGURE 5 (top) Liquid-crystalline texture shown on cooling by 4b at 20ꢀC;
(bottom) Liquid-crystalline textures shown on cooling by 5a at 49ꢀC. Crossed
polarizers microscope.
27.0ꢀC and 39.5ꢀC. As already observed for compounds 2a,b–3a,b the
aryl substituted Ni complex 4b displays mesomorphic behaviour at
room temperature, even if with a rather low Ti (37ꢀC). It is then poss-
ible that the aryl moieties sterically interfere with the intermolecular
interactions responsible of the mesomorphic aggregation. Unfortu-
nately, also the 4-N,N-dimethylamminophenyl substituted derivative
6, which, on the basis of calculations and spectral analysis (vide
supra), was the most promising one for applications in NLO, did not
show any mesophase. This behaviour could be due to the competition
between S and N interactions with Ni in the porphyrazine core. Work
is in progress for clarifying the origin of this behaviour in order to
design new derivatives displaying at the same time charge-tranfer
properties and more stable ordered mesophases.
In alkyl(thio)porphyrazines the formation of LC phases is determ-
ined by the concomitant presence of different intermolecular forces,
as p–p interactions between the aromatic rings (for the ‘free-base’
azaporphyrins), Van der Waals interactions of the peripheral chains,
static dipole-dipole interactions (for the non-symmetric porphyra-
zines), and metal-sulfur interactions between adjacent rings (for

Mesomorphic Non-symmetric S-octyl Porphyrazines
69
metalloporphyrazines) [1h-m,2,3,5,7,22]. Furthermore, from a DG
point of view the different availability of side chains torsional states
might have large influence on the DS contribution on the phase
transition temperatures [23].
Molecular Dynamics simulations [23] have shown that on cooling a
starting organization of the porphyrazine core is necessary. This
means that core-core interactions are the driving force for the achieve-
ments of columnar organization. In this respect the most interesting
parameter is represented by the isotropization temperature (Ti).
Considering the temperature values reported in Table 1 it is possible
to suggest that the two most relevant interactions are the static
dipole-dipole and the metal-heteroatom ones [3c–d,5,7] complemented
by strong stereochemical perturbation due either to the very large
moieties like the biphenyl or to the very small H- substituent. In fact,
the Ti value decreases as follow 3b > 1b > 3a > 2a > 2b >> 5a > 5b.
The largest value of Ti for 3b is probably due both to the larger dipole
than 1b and to the fact that the Br atom achieves the same interation
toward Ni as S atom does. On the other hand, 3a has a larger dipole
than 2a and thus more significant dipole-dipole interactions occur.
In the case of 2b the ‘‘hole’’ in the peripheral substitution of the macro-
cycle probably determines a large perturbation in the ordering of the
side chains during the liquid to mesophase transition and increases
the entropy due to a large number of S-Ni interactions. The more stiff
and bulky biphenyl substituent engenders very large perturbation in
the stacking and thus decreases the conformational freedom of the
side chains.
CONCLUSIONS
We described herein the synthesis of novel non-symmmetric b-aryl
(octylsulfanyl) porphyrazines by applying, for the first time on these
macrocycles, the palladium-catalyzed Suzuki cross-coupling reactions.
The electronic effects of the aryl substitution were investigated by
UV-Vis absorption spectroscopy of their Ni(II) complexes, observing
in all these compounds a spectral red shift in the 400–550 nm range,
more pronounced in the case of 4-NMe2-phenyl substituted complex 6,
which also displays a Q band widening. Such bathochromic shifts
reflect an actual electronic interplay between the porphyrazine core
and the peripheral aryl groups making these compounds very attract-
ive for application in non-linear optics. These non-symmetric porphyr-
azines as well as their H- and Br-substituted parent compounds,
showed also very interesting mesomorphic properties. Compounds
3a and 3b showed the most remarkable liquid-crystal behaviour,

70
S. Belviso et al.
displaying two columnar mesophase, hexagonal and rectangular, the
latter being stable until well below room temperature. In these
compounds a very wide thermal stability range was observed (ca.
100ꢀC), making them very attractive for technological applications.
On the other hand, the aryl substituted non-symmetric derivatives,
although displaying promising charge-transfer properties, showed
hexagonal columnar mesophases with lower DT and melting tempera-
ture values. Although the aryl substituted porphyrazines 4–6 are then
not suitable for LC application yet, this study shows that it is possible
to obtain porphyrazine macrocycle displaying both mesomorphic and
charge-transfer properties. The synthetic approach followed herein
then discloses the way to a wide range of molecular architectures,
allowing the tuning of the electronic and mesomorphic properties of
these macrocycles by insertion of suitable moieties capable of extend-
ing the electronic conjugation and stabilizing the intermolecular inter-
actions. Further work is in progress in our laboratory in order to
match useful technological properties for this class of molecules.
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