Structure-charge transport relationship of 5,15-dialkylated porphyrins

Article abstract of DOI:10.1039/c2cc31137b

5,15-Dialkyl-substituted porphyrins that are symmetrically capped with ethyl (C₂-Por), butyl (C₄-Por) and hexyl (C ₆-Por) were synthesized and characterized. Molecular structure versus physical property relationship has been established through the analysis of planar charge transport using thin film transistor (TFT) structure.

Full text of DOI:10.1039/c2cc31137b

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COMMUNICATION
Structure–charge transport relationship of 5,15-dialkylated porphyrinswz
Li Zhou,ya Zong-Xiang Xu,yb Ye Zhou,b Yan Feng,a Xiang-Ge Zhou,a Hai-Feng Xiang*a and
V. A. L. Roy*b
Received 15th February 2012, Accepted 3rd April 2012
DOI: 10.1039/c2cc31137b
5,15-Dialkyl-substituted porphyrins that are symmetrically capped
with ethyl (C2-Por), butyl (C4-Por) and hexyl (C6-Por) were
synthesized and characterized. Molecular structure versus physical
property relationship has been established through the analysis of
planar charge transport using thin film transistor (TFT) structure.
Organic electronics, having advantages of low manufacturing
cost, light-weight, and high device flexibility, are receiving a
surge of interest.1–5 Understanding and control of the relation
between the electronic transport properties and molecular
structures of organic semiconductors is crucial for further
improving device performance in organic electronics. Molecular
alignment determined by molecular structure is one of the
important aspects of controlling the film morphology and
crystallinity, which will lead to the enhancement of intermolecular
charge transport. As has been widely demonstrated, carrier trans-
port in organic molecular solids is governed by the extent of
intermolecular orbital coupling (transfer integrals), where larger
orbital coupling can afford higher mobility.6 One of the promising
approaches to enhance orbital coupling is to use molecules that
have extended p-conjugated structure. Another effective approach
is to use the self-organizing nature of organic molecules.7 This
has been demonstrated by thiophene or Cn-quinacridone-based
OTFTs, in which long alkyl groups can act as the driving force for
molecular ordering in the solid state owing to the van der Waals
intermolecular interaction between the alkyl groups, or the so-called
molecular fastener effect, that renders the semiconducting core
to pack tightly, thereby enhancing carrier mobility.8,9
Scheme 1 The reported approaches and the new approach (in this work)
on modifying chemical structures of porphyrins for the application in
OTFTs.
As p-conjugated planar molecules, porphyrin and its metal
derivates are widely known to have useful applications in areas
such as photovoltaic cells, thin film transistors (TFTs) and
organic light-emitting devices (OLEDs).10–15 In order to improve
the OTFTs’ performance, chemical structures of porphyrins
have been modified at b-positions (2, 3, 7, 8, 12, 13, 17, and
18-position) or meso-positions (5, 10, 15, and 20-position) of
porphyrin,16 as shown in Scheme 1. Porphyrin complexes with
various metal ions, such as Cu(II), Pt(II), Zn(II), and Ni(II), have
also been studied.11,16,17 However, no studies on charge transfer
properties of 5,15-disubstituted porphyrin molecules have been
found. With these molecular design strategies, a series of 5,15-
dialkylated porphyrins (Cn-Por, n = 2, 4, and 6) (Scheme 1) has
been designed, prepared, and characterized. The charge transfer
properties were studied using OTFTs structure. With the help of
alkyl groups, these well self-organizing 5,15-dibutyl porphyrins
a Institute of Homogeneous Catalysis, College of Chemistry,
Sichuan University, Chengdu, 610041, China.
E-mail: xianghaifeng@scu.edu.cn; Fax: +86 28-8541-2291;
Tel: +86 28-8541-2290
b Department of Physics and Materials Science,
City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong SAR. E-mail: val.roy@cityu.edu.hk;
Fax: +852-2788 7830; Tel: +852-3442 2729
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
z Electronic supplementary information (ESI) available: Details of
synthesis and characterization of the 5,15-dialkylated porphyrin com-
pounds and as-fabricated thin films, including 1HNMR, 13CNMR,
MALDI-TOF-MS, elemental analysis, thermo gravimetric analysis,
single crystal X-ray diffraction, atomic force microscopy, grazing
incidence X-ray diffraction, thin film transistor fabrication and charge
mobility measurement. CCDC 867177 and 867178. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc31137b
(C4-Por) show field-effect mobility close to 3 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1
,
which would be new promising molecular organic semi-
conductors. The main focus of this study is not on achieving
a high mobility device, nevertheless we want to demonstrate
the influence of molecular alignment through alkylation.
The 5,15-dialkylated porphyrins were simply synthesized in
two steps. The detailed synthesis and characterization of porphyrin
materials are described in the ESIz. With the presence of
alkyl groups, 5,15-dialkylated porphyrins have good solubility
y These authors contributed equally.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5139–5141 5139

in nonpolar organic solvents, such as CH2Cl2, toluene, and
CH3CN, and poor solubility in polar organic solvents, such as
MeOH, EtOH, and H2O. The single crystals of C2-Por and
C4-Por were obtained from slow diffusion of MeOH into a
CH2Cl2 solution of C2-Por and C4-Por, respectively (ESIz,
Table S1). Vacuum deposited thin films of 5,15-dialkylated
porphyrins were studied by grazing incidence X-ray diffraction
(GIXRD) and atomic force microscopy (AFM). The crystal
structures of C2-Por and C4-Por, determined by single crystal
X-ray diffraction (XRD), reveal that controlling the length of
the peripheral alkyl chain on the porphyrin ligand could improve
the thin film crystallinity and intermolecular charge transfer.z
The 5,15-dialkylated porphyrins were used to fabricate organic
transistors using a bottom-contact gate structure by high-vacuum
evaporation and measured as reported.9 The crystalline-phase
purity of all 5,15-dialkylated porphyrins were studied by powder
X-ray diffraction (PXRD). Except for C6-Por, the experimental
PXRD patterns of the other 5,15-dialkylated porphyrins
matched the corresponding patterns simulated using the single-
crystal X-ray data. The small deviation in the relative peak
intensities is attributed to the preferred orientation caused by
the crystallite shape anisotropy (Fig. S1 and S2 in ESIz).
Table 1 shows thermal stability and TFT performance of
Cn-Por compounds. The 5,15-dialkylated porphyrins exhibit
good thermal stability (Table 1) with decomposition temperatures
(Td) in the range of 360–391 1C, which were extracted from the
thermo gravimetric analysis (TGA) measurement (Fig. S3, ESIz).
All the 5,15-dialkylated porphyrins exhibited p-type field-
effect behavior (Table 1). C2-Por showed a hole mobility (m) of
2.5 ꢂ 0.3 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1. An increase in the n-alkyl chain
length (from ethyl to butyl chains) led to an increase in
mobility (m) up to 2.8 ꢂ 0.2 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 for C4-Por.
However, further increase in the n-alkyl length led to a
decrease in the mobility value, as revealed in the case of
Fig. 1 The current–voltage characteristics: (a) output and (b) transfer
characteristics of a bottom-contact transistor fabricated with C4-Por.
C6-Por (m = 7.6 ꢂ 0.2 ꢀ 10ꢁ4 cm2 Vꢁ1
s
ꢁ1). Fig. 1 shows
the output and transfer characteristics of a C4-Por-based
device exhibiting the best performance. Upon increasing the
gate voltage Vg, the drain current Ids increases, revealing a
p-type field-effect transistor (FET) behavior. The as-fabricated
transistor showed a highest mobility of 3 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1
,
a threshold voltage (Vt) of ꢁ10 V and a current on/off ratio
(Ion/Ioff) of 103. In addition, the as-fabricated devices could be
operated under open atmosphere at room temperature and
there was no change in the device performance. After a period
of more than 1 year, this device, kept in a glove box, showed a
mobility of 2.8 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 with less than 10% decrease
in the initial mobility value, revealing the device stability.
Fig. 2 GIXRD patterns of vacuum-deposited thin films of Cn-Por
compounds (the peak positions are shown in a 2y scale). Inset: molecular
length of C2-Por and C4-Por.
on SiO2/Si substrates were recorded (Fig. 2). In general,
the C2-Por-based thin film displayed broad (full width at
half maximum of 0.8–1.01 2y) and weak diffraction peaks
(o 100 counts sꢁ1), revealing the poor crystallinity and almost
amorphous film structure. The C4-Por-based thin film exhi-
bited a strong single diffraction peak, revealing that the film
was highly crystalline and textured with well-ordered mole-
cular alignment in one favorable direction. The GIXRD
pattern of the C4-Por-based thin film displayed a diffraction
peak at 5.61 (2y) corresponding to an interplanar d-space of
15.8 A, which is close to its molecular size (15.7 A derived
from the single crystal X-ray data), as well as for the case
of C2-Por (d-space of 12.8 A close to its molecular size
Both crystallinity and morphology play important roles in
OTFT performance. OTFT behavior is usually observed for
those materials that exhibit a regular crystalline packing arrange-
ment. To acquire structural information, GIXRD patterns of
thin films (50 nm thick) of 5,15-dialkylated porphyrins deposited
Table 1 Thermal stability and TFT performance of Cn-Por compounds
Mobility
Compound Td/1C (cm2 Vꢁ1 sꢁ1
Ion/
Ioff Threshold voltage/V
)
C2-Por
C4-Por
C6-Por
360
367
391
2.5 ꢂ 0.3 ꢀ 10ꢁ4 102 ꢁ18 ꢂ 3
2.8 ꢂ 0.2 ꢀ 10ꢁ2 103 ꢁ10 ꢂ 2
7.6 ꢂ 0.2 ꢀ 10ꢁ4 102 ꢁ20 ꢂ 2
c
5140 Chem. Commun., 2012, 48, 5139–5141
This journal is The Royal Society of Chemistry 2012

In summary, a series of 5,15-dialkylated porphyrins was
synthesized and the charge transport properties were studied
using OTFTs structure by the vacuum deposition method. The
charge transport properties are strongly affected by the surface
morphology, crystallinity, and molecular packing of the thin
film. Properly substituted alkyl chain of C4-Por led to the highest
crystallinity of the as-deposited film and best charge transport
behavior, which makes it an effective organic semiconductor for
potential practical application in organic electronics. The main
aim of this study is to reveal the structure–property relationship
of macromolecules and to serve as a guideline for materials
design and development for organic electronics.
Fig. 3 AFM images of vacuum-deposited thin films of (a) C2-Por;
(b) C4-Por and (c) C6-Por.
(12.9 A derived from the single crystal X-ray data)). Upon
further increasing the substituted alkyl chain to hexyl, the
as-deposited film of C6-Por exhibited a diffraction peak at
4.71 (2y) with a sharp decrease in the diffraction intensity.
These interplanar distances suggest that the molecular planes of
5,15-dialkylated porphyrins are roughly perpendicular to the
SiO2/Si substrate surface, thereby enabling a well-ordered face-to-
face molecular alignment along one direction, which is advanta-
geous for intermolecular charge transport (Fig. S4, ESIz).
The surface morphology of these thin films was studied by
tapping mode atomic force microscopy (AFM). The peripheral
alkyl substituents were found to affect the morphology of the films
of 5,15-dialkylated porphyrins and hence the charge mobility
of the as-fabricated devices. Fig. 3 shows the AFM images of
50 nm-thick thin films of 5,15-dialkylated porphyrins deposited on
the SiO2/Si substrate. Although the grain sizes of 5,15-dialkylated
porphyrins-based thin films were found to be similar, the reduced
ordering in the series of C4-Por > C6-Por > C2-Por is consistent
with the GIXRD measurement and charge carrier mobility
results. The C4-Por based thin film exhibited the best crystalline
grains (Fig. 3b) and strongest GIXRD diffraction peak (Fig. 2),
both of which led to the highest charge carrier mobility among
5,15-dialkylated porphyrins.
This work was supported by the National Natural Science
Foundation of China (no. 21172160), Ministry of Education
of China (no. 20100181120039) and City University of Hong
Kong (Project No. 7002672).
Notes and references
z Crystal data for C2-Por: M = 366.46, monoclinic, a = 13.1977(4) A,
b = 12.7229(4) A, c = 11.5658(4) A, a = 90.001, b = 103.008(2)1,
g = 90.001, V = 1892.21(11) A3, T = 296(2) K, space group P2(1)/c,
Z = 4, 24 410 reflections measured, 4303 independent reflections
(Rint = 0.0520). The final R1 value was 0.0642 (I > 2s(I)). The final
wR(F2) value was 0.1917 (I > 2s(I)). The final R1 value was 0.0871
(all data). The final wR(F2) value was 0.2123 (all data). Crystal data for
C4-Por: M = 422.56, monoclinic, a = 9.8980(6) A, b = 9.3449(6) A,
c = 12.9197(8) A, a = 90.001, b = 107.261(4)1, g = 90.001, V =
1141.20(12) A3, T = 296(2) K, space group P2(1)/c, Z = 2, 12 012
reflections measured, 2312 independent reflections (Rint = 0.0507).
The final R1 value was 0.0744 (I > 2s(I)). The final wR(F2) value was
0.2107 (I > 2s(I)). The final R1 value was 0.1290 (all data). The final
wR(F2) value was 0.2826 (all data).
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The degree of film crystallinity, morphological continuity,
concentration of the defects and grain boundaries within the
vacuum-deposited thin films could be affected by intermolecular
packing of the molecules.18 All 5,15-dialkylated porphyrins have
similar molecular orientation in the thin film state, where the
molecules are aligned nearly perpendicular to the substrate
(Fig. S4, ESIz). Changing the peripheral alkyl substituent from
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5.61 (2y) (Fig. 2), revealing that this film had the highest degree of
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porphyrin complexes incorporated with various metal ions for the
application in OTFTs is now under way.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5139–5141 5141