[5-(p-alkoxy)phenyl-10,15, 20-tri-phenyl] porphyrin and their rare earth complex liquid crystalline

Article abstract of DOI:10.1002/poc.1132

Three series of porphyrin liquid crystalline compounds, [5-(p-alkoxy)phenyl-10, 15, 20-tri-phenyl] porphyrin and their rare earth complexes (Tb (III), Dy (III), Er (III), Yb (HI)), with a hexagonal columnar discotic columnar(Col_h) phase have be

Full text of DOI:10.1002/poc.1132

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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 229–235
Published online 2 April 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1132
[5-(p-alkoxy)phenyl-10, 15, 20-tri-phenyl] porphyrin and
their rare earth complex liquid crystalline
1
2
1
1
1
*
Miao Yu, Wen-yang Zhang, Yong Fan, Wen-ping Jian and Guo-fa Liu
1College of Chemistry, Jilin University, Changchun 130023, P.R. China
2College of Materials and Engineering, Jilin University, Changchun 130023, P.R. China
Received 13 July 2006; revised 21 September 2006; accepted 21 October 2006
ABSTRACT: Three series of porphyrin liquid crystalline compounds, [5-(p-alkoxy)phenyl-10, 15, 20-tri-phenyl]
porphyrin and their rare earth complexes (Tb (III), Dy (III), Er (III), Yb (III)), with a hexagonal columnar discotic
columnar(Colh) phase have been synthesized. These compounds were characterized by elemental analysis, molar
conductances, UV-visible spectra, infrared spectra, luminescence spectra, and cyclic voltammetry. These compounds
exhibit more than one mesophases, which transition points of temperature change from ꢀ33.6 to 16.0 8C, and
transition points of temperature for isotropic liquid also increase from 4.9 to 38.2 8C, with increasing chain length.
Their surface photovoltage (SPV) response have also been investigated by the means of surface photovoltage
spectroscopy (SPS) and field-induced surface photovoltage spectroscopy (EFISPS). It was found that their SPV bands
are analogous with the UV-visible absorption spectra and derived from the same transition. Copyright # 2007 John
Wiley & Sons, Ltd.
KEYWORDS: porphyrins; lanthanide; liquid crystals; cyclic voltammetry; SPS and EFISPS
INTRODUCTION
their rare earth porphyrin complexes have focused on a
certain aspect property such as fluorescence, besides their
liquid crystallinity. In our lab, extensive efforts have been
to devoted this kind of porphyrins and their rare earth
complexes, some good results have been reported.15–17 In
this paper, we synthesized three series of porphyrin liquid
crystals whose structures are shown in Fig. 1, investigated
their surface photovoltage spectra (SPS), electrochemical
behavior, luminescence spectra, and liquid crystalline
properties.
Porphyrins, as a model hemoglobin, myoglobin, and
cytochrome P450, have been widely studied, because
of their interesting excited state, catalytic behavior,
and ubiquitous electron-transfer processes.1–4 While
their transition metal complexes have been extensively
investigated, fewer studies of rare earth porphyrin
complexes have been reported comparatively. The work
in this area is limited to specific NMR shift reagents,
heavy atom probes for electron microscopy and X-ray
structure determination, and agents for photodynamic
therapy et al.5–8 As we know, liquid crystalline system is
the combination of the ordered structure of a crystal phase
and the molecular mobility of an isotropic (liquid) phase,
which allow liquid crystal mesophases to self-correct
structural defects.9–11 Thus, they can exhibit electric or
magnetic responses and have potential application in the
field of electronic devices, that is, information sto-
rage.12–13 Porphyrins have been revealed to be a class of
fascinating liquid crystalline materials due to their
synthetic versatility, thermal stability, large p-electron
system, and photochemical properties.14 However, the
studies on porphyrin liquid crystal compounds, as well as
RESULTS AND DISCUSSION
Luminescence spectra
Tables 1 and 2 give the excitation and emission spectral
data of the ligands and complexes. The quantum yields
(Ff) were calculated by the following equation:
Ffs ꢁ n2 ꢁ As ꢁ If
Ff ¼
n2s ꢁ A ꢁ Ifs
In the above equation, ns, As, and Ifs represent the
refractive index, absorbance, and integrated intensity of
standard sample at excited wavelength, respectively.
Meso-tetraphenylporphyrin zinc, ZnTPP was used as
standard sample, Ffs ¼ 0.033.18
*Correspondence to: M. Yu, College of chemistry, Jilin University,
Changchun 130023, P.R. China.
E-mail: yumiao@jlu.edu.cn
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M. YU ET AL.
Figure 1. Porphyrin liquid crystalline compounds: [5-(p-alkoxy)phenyl-10, 15, 20 -tri-phenyl]porphyrin(a), n ¼ 11, 13, 15
correspond to 12L, 14L, 16L, respectively; [5-(p-alkoxy)phenyl-10, 15, 20-tri-phenyl ] porphyrin lanthanide complex(b), n ¼ 11,
13, 15; Ln ¼ Yb, Er, Dy, Tb, correspond to 12ErOH, 12DyOH, 12TbOH, 14YbOH, 14ErOH, 14DyOH, 14TbOH, 16YbOH, 16ErOH,
16DyOH, 16TbOH, respectively
Two fluorescence bands S2 (B, Soret band) and S1 (Q
band) are observed in porphyrin complexes, which are
attributed to transition from the second excited singlet
state S2 to ground state S0, S2 ! S0 and S1 ! S0 of Q band
emission, respectively. The Soret fluorescence is about
two orders of magnitude weaker than the S1 ! S0 of Q
band emission. Its quantum yield is very low and
sometimes, it becomes unobservable. In our experiment,
this fluorescence emission can not be observed at room
temperature at excited wavelength, 420 nm. Soret bands
in three ligands and eight complexes are split into two
bands, as compared with their UV-visible absorption
spectra. Other spectral bands undergo only a small
change. Q (0–0) fluorescence bands of the complexes are
in the region 600–606 nm, while Q (0–1) fluorescence
bands of the complexes in the region 651–655 nm and
Q (0–2) bands 712–720 nm, see Fig. 2. They are mirror
symmetric to the absorption spectra. Quantum yields (Ff)
of Q band for the complexes are in the range
0.0007–0.2067 and ligands 0.09508–0.2098. Obviously,
the quantum yields of complexes are much less than those
of ligands.
two radiationless processes S1
S0 and S1
Tn.
According to the known result, the fluorescence quantum
yields of the porphyrin complexes are low.18 Therefore, in
our experiment, because the spin forbidden
process S1
Tn plays a predominant role in radiation
absent deactivation of S1 in porphyrin complexes, the
fluorescence quantum yields of complexes are much less
than 0.21.18
Surface photovoltaic spectroscopy (SPS) and
electric field-induced surface photovoltaic
spectroscopy (EFISPS)
The surface photovoltage spectroscopy technique, as
a very sensitive characterization method to detect the
change of charge distribution on functional semiconduc-
tor surfaces, is related to electron transition processes,
caused by light absorption. So it can directly reflect the
properties of photogenerated charge separation and
As we know, the S1 ! S0 quantum yield depends on
Table 2. Emission spectral data of ligands and complexes
(lex ¼ 420 nm)
the relative rates of the radiative process S1 ! S0 and
Peak values (nm)
Quantum
Table 1. Excitation spectral data of ligands and complexes
Compounds
Peak values(l per nm)
Compounds
Q(0–0)
Q(0–1)
Q(0–2)
yields(Ff)
12L
14L
16L
408
405
417
410
418
419
419
416
411
418
415
419
419
415
415
431
431
428
430
437
519
518
515
520
517
515
519
519
516
516
511
514
518
516
517
548
549
552
554
558
557
554
556
549
556
554
552
553
551
552
594
593
592
589
592
592
590
591
589
585
589
593
593
594
594
12L
14L
16L
604
605
603
604
604
605
603
600
606
603
603
606
606
603
603
653
653
654
655
651
653
652
654
654
652
653
652
652
653
653
717
716
712
716
718
715
720
717
716
716
716
719
713
712
718
0.1255
0.09508
0.2098
0.0008
0.0205
0.0308
0.0787
0.0172
0.0157
0.0261
0.0312
0.0740
0.2067
0.1409
0.0478
12YbOH
12ErOH
12DyOH
12TbOH
14YbOH
14ErOH
14DyOH
14TbOH
16YbOH
16ErOH
16DyOH
16TbOH
12YbOH
12ErOH
12DyOH
12TbOH
14YbOH
14ErOH
14DyOH
14TbOH
16YbOH
16ErOH
16DyOH
16TbOH
426
430
437
430
431
427
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 229–235
DOI: 10.1002/poc

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PORPHYRIN LIQUID CRYSTALLINE COMPOUNDS
231
molecular orbital) to LUMO (the lowest unoccupied
molecular orbital), while the Q and Soret bands arised
from the coupling of the two transitions between the
HOMO (the highest occupied molecular orbital) and
LUMO.20
By comparison of the spectral peaks of the 12L
compound in Figs. 3(a) and (b), some change of
‘simultaneous response’ in the positive or negative
electric field intensity was observed. The surface
photovoltaic (SPV) response exhibits positive in positive
electric field and negative in negative field. This kind of
simultaneous response to the electric field can all be
assigned to p–pꢂ transitions. However, the variation rates
of Soret and Q bands, with an increase in the positive or
negative electric field intensity are noticeably different.
Since the common upper energy level transition in the p
system is pꢂ(eg), the difference mainly stems from the
lower energy levels. Briefly, the lower energy levels of
these transitions are at different depths of the valence
band, resulting in different photogenerated hole diffusion
lengths. The response of the Q band is induced to a
positive electric field. When employed in a negative
electric field, the photogenerated electrons in the
conduction band diffuse towards the bulks, thus, increase
the probability of recombination with photogenerated
holes, whereas diffusion lengths are shortened, leading to
decreasing of the SPV response, even reversal. Compar-
ing the SPS and EFISPS of 12L and 12ErOH, it is found
that the SPV response of 12ErOH is different from that of
the free ligand 12L, which shows the decrease of the
peaks. Symmetric increase of the complexes is supposed
to cause this phenomenon. In the meantime, the
photovoltaic action spectra follow the absorption spectra
response well, indicating that they are derived from the
similar electron transition process.
Figure 2. Emission spectra of 12L (. . .. . .) and 12TbOH (–) at
room temperature in CHCl3
charge transfer. Its sensitivity is up to 108 q cmꢀ2, or about
one elementary charge per 107 surface atoms, increasing
by several orders of magnitude, compared with the X-ray
photoelectron spectroscopy.19
The SPS and EFISPS of the ligand 12L and its complex
12 ErOH are presented in Fig. 3. Their photovoltaic spec-
tral bands are given in Table 3. The photovoltaic spectra of
the ligand and complexes are similar to their UV-visible
spectra, even the shape and the numbers of Soret and Q bands
do not change. Therefore, it is deduced that the same
electron transition processes occurred in both spectra.
Porphyrin molecule is a conjugated p bonding system,
in which p-orbitals are analogous to energy band of
organic semiconductor, and pꢂ-orbital to the conduction
band. Photogenerated charge carriers in p system are
nonlocalized, their motion is free in the valence band,
the same as those photogenerated electrons in the
conduction band. In this kind of system, the band to
band transition is characterized as a p–pꢂ transition. The
band at 300–400 nm is named as P band, corresponding to
the higher energy level transition, b2u(p) ! eg(pꢂ), while
Q and Soret bands are due to a2u(p) ! eg (pꢂ) and a1u
(p) ! eg (pꢂ) transitions, respectively.20 The P band is
transition from NHOMO (the next highest occupied
Liquid crystals
Small-angle X-ray diffractions of 16L and 16DyOH have
been measured at room temperature. The d-spacing of
Table 3. Spectral bands of SPS and EFISPS of the compounds
Maximum peaks(lmax per nm)
Compounds
12L
External field voltage(V)
P band
Soret(B) band
Q band
0
326
328
330
332
328
324
326
364
363
362
362
360
430
429
428
429
425
430
428
424
425
424
425
423
516
518
516
519
520
517
518
550
552
551
550
547
551
552
552
548
551
553
550
595
591
591
591
592
588
592
588
580
589
583
583
649
649
652
651
650
652
652
0.0313
0.0625
0.125
ꢀ0.0313
ꢀ0.0625
ꢀ0.125
0
12ErOH
0.125
0.25
518
518
518
ꢀ0.125
ꢀ0.25
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

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232
M. YU ET AL.
Figure 3. SPS and EFISPS of 12L and 12ErOH (a): SPS of 12L (b): EFISPS of 12L (c): SPS of 12ErOH (d): EFISPS of 12ErOH
˚
˚
˚
16L are d100 ¼ 45.5 A, d110 ¼ 38.4 A, and d200 ¼ 23.0 A,
chain length (from 4.9 8C for 12ErOH to 38.2 8C for
16TbOH). This advantage may have potential application
for this kind of porphyrin crystalline liquids in the future.
respectively, while those of 16DyOH are 45.3, 27.1 and
˚
22.6 A. The ratios of the d-spacing, 1:1/H3:1/2, are
characteristic of a hexagonal columnar phase (Colh).
Their wide-angle X-ray diffraction both gave a broad
peak, each centered at 2u ¼ 20 8, which is derived
from the molten alkyl chains. The X-ray diffraction
results of other compounds are similar to those of 16L and
16DyOH.
EXPERIMENTAL SECTION
General methods
The transition temperatures and enthalpies of the
porphyrin ligands and their complexes are given in
Table 4. The liquid crystalline phase of all compounds
were characterized by differential scanning calorimetry
(DSC) and the optical texture was observed by viewing
the birefringence of the sample between crossed
polarizers in a polarizing microscope. The DSC data of
complexes are summarized in Fig. 4. Figure 5 shows the
birefringence texture for 16TbOH and 16L. From Table 4
and Fig. 4, more than one mesophases were found in
these compounds. The complexes exhibited a systematic
variety in their liquid crystalline, that is, the transition
points of temperature tend to room temperature with
increasing chain length (from ꢀ33.6 8C for 12ErOH
to 16.0 8C for 16YbOH), and the transition points of
temperature for isotropic liquid increase with increasing
Infrared spectra were recorded on a Nicolet 5PC-FT-IR
spectrometer, using KBr pellets in the region 400–
4000 cmꢀ1. UV-visible spectra were recorded on a
Shimadzu UV-240 spectrophotometer in the range
350–700 nm using chloroform as solvent. 1H NMR
spectra were recorded in deuterated chloroform using a
Varian-Unity-400 NMR spectrometer and employing
tetramethylsilane (TMS) as internal reference. Molar
conductances of 10ꢀ3mol dmꢀ3 chloroform solution, at
25 8C were measured on a DDX-111A conductometer.
Elementary analysis was measured by a Perkin-Elmer
240C auto elementary analyzer. Optical microscopical
properties were observed by a XinTian XP1 (CCD:
TOTA-500 II) polarized light microscope, equipped
with a variable temperature stage (Linkam TMS 94).
Transition temperatures and heats of fusion were
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 229–235
DOI: 10.1002/poc