PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Self-assembled porphyrin microrods and observation of structure-inducediridescence†

Cicero Martelli,abc John Canning,*ab Tony Khoury,d Nina Skivesen,a Martin Kristensen,a George Huyang,bd

Paul Jensen,d Chiara Neto,d Tze Jing Sum,d Mads Bruun Hovgaard,a Brant C. Gibsone andMaxwell J. Crossley*d

Received 27th August 2009, Accepted 19th October 2009

First published as an Advance Article on the web 5th January 2010

DOI: 10.1039/b917695k

Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(CnH2n+1)3-NO2P]} and

microplates {based on 5,10,15,20-tetraheptylporphyrin [(C7H15)4-P]} are fabricated and characterised

using optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM).

The length of the alkyl chains and the deposition surface are found to influence the optical

properties and microrod self-assembly. When the deposition surface is silica (a-quartz), 5-nitro-

trialkylporphyrins, (C5H11)3-NO2P, (C7H15)3-NO2P and (C11H23)3-NO2P all form microrods

of 0.7–0.8 micron diameter; the average length of the microrods varies from 170 microns for

(C5H11)3-NO2P to about 11 microns for (C7H15)3-NO2P and (C11H23)3-NO2P, whereas

(C19H39)3-NO2P with much longer alkyl chains only gives powders. Controlling the precipitation is

crucial in preventing the disordered aggregation of assembled layers observed in the bulk. Very

interestingly, the microrods formed from (C7H15)3-NO2P show marked iridescent character. When

(C7H15)3-NO2P is deposited on silicon, however, longer curved microrods which do not show

iridescence are produced. Single crystal X-ray crystallography of (C7H15)3-NO2P reveals the packing of

the bulk material which explains the packing topology of the layers observed by AFM but not the

iridescence. The observed structural colour of the (C7H15)3-NO2P microrods is explained by staggering

of the layers to produce a corrugated surface with a period of 125 nm, as measured by AFM.

Introduction

A commonly used approach to self-assemble porphyrin struc-

tures uses both electrostatic and hydrophobic interactions of

porphyrins containing ionic substituents in aqueous solutions,1–3

as well as organic solvents.4 In these solutions, water soluble

porphyrins can be forced to aggregate by controlling the pH, the

ionic strength, and temperature.3–7 Without a supporting matrix,

H- or J-aggregates are formed, detected by the blue (H) or red (J)

shift of the exciton bands.6,8 Ionic self-assembly9 enables the

formation of porphyrin nanotubes in an aqueous solution.10–12

The expounded mechanism relied on electrostatic interactions

between two oppositely charged porphyrins [one with Sn(IV)],

which in addition to the van der Waals, hydrogen-bonding, axial

coordination and other weak intermolecular forces enhanced the

structural stability of the system. Tunnelling electron microscopy

(TEM) images showed these nanotubes to be hollow, microns in

length and between 50 and 70 nm in width. Fringe analysis,

aiNANO & Department of Physics and Astronomy, University of Aarhus,DK-8000 �Arhus C, DenmarkbInterdisciplinary Photonics Laboratories, School of Chemistry, TheUniversity of Sydney, NSW 2006, Australia. E-mail: j.canning@usyd.edu.aucDepartamento de Engenharia Mecanica, Pontifıcia Universidade Cat�olicado Rio de Janeiro, RJ 22453-900, BrasildSchool of Chemistry, The University of Sydney, NSW 2006, AustraliaeQuantum Physics Victoria, School of Physics, The University ofMelbourne, Vic 3010, Australia

† CCDC reference number 742688. For crystallographic data in CIF orother electronic format see DOI: 10.1039/b917695k

2310 | J. Mater. Chem., 2010, 20, 2310–2316

together with spectrophotometer measurements, suggests that

these nanotubes are stacks of offset J-aggregated porphyrins

likely in the form of cylindrical lamellar sheets and X-ray

diffraction studies reveal moderate crystallinity. Studies in acid

were unable to rule out the role of water molecules. Another

important observation of porphyrin self-assembly was the

formation in alcohols of rod-like micelles on the nanoscale of

a cobalt(II) porphyrin.13 In this case, the structure is thought to be

in a reverse micellular arrangement of face-to-face aggregate

having a hydrophobic corona around a polar core. In addition to

ionic self-assembly, porphyrin thin films have been fabricated in

the holes of photonic crystal fibres, showing interaction between

the tin(IV) porphyrin and the silica of the fibres.14 Examples of

self-assembly based on topology packing in two and three

dimensions of porphyrins and related systems such as phthalo-

cyanines and corroles have also been reported.15,16

We report self-assembly studies of porphyrin arrays on

a-quartz and silicon and show that in nonaqueous solutions the

incorporation of a polar NO2 group transforms the planar 2-D

self-assembly of 5,10,15,20-tetraheptylporphyrin [(C7H15)4-P]

into 1-D microrods as 5-nitro-10,15,20-triheptylporphyrin

[(C7H15)3-NO2P]. These structures are characterised using

optical microscopy, atomic force microscopy (AFM) and scan-

ning electron microscopy (SEM). The optical transparency

obtained with the microrods after drying suggests potential

photonic transport applications. The observed iridescence

reveals a lamellar period, suggesting novel optical functionality

exploiting these periodic structures is possible.

This journal is ª The Royal Society of Chemistry 2010

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Experimental

Sample and surface preparation

a-Quartz slides were purchased from Starna Pty. Ltd. and their

surfaces were rinsed and ultrasonic cleaned in water for 15 min.

To increase hydroxy termination, the slides were heated to 70 �C

in basic piranha solution (ammonia–hydrogen peroxide–water at

1 : 1 : 5) for 5 min. After heating, the slides were rinsed in hot

water and dried under continuous nitrogen flow. Distilled water

was used in all processes. The porphyrins were dissolved in

dichloromethane (CH2Cl2) (0.1 mg mL�1) and subsequently

mixed with N,N-dimethylformamide (DMF). Micro-droplets

(3 mL) of this mixture were deposited onto the cleaned quartz

slides with a secured calibrated micropipette.

Optical microscopy was carried out on an Olympus BX61

Motorised System Microscope. SEM was carried out using

a Nova 200 NanoLab (FEI Company) SEM/FIB. AFM imaging

was performed using a Veeco Multimode (Santa Barbara, CA) in

tapping mode.

Synthesis of 5-nitro-10,15,20-tripentylporphyrin [(C5H11)3-NO2P]

A solution of 5,10,15,20-tetrapentylporphyrin17 (300 mg, 0.508

mmol) was dissolved in CH2Cl2 (300 mL) and a solution of

nitrogen dioxide in light petroleum (1 M) was added portion-

wise with stirring until mono-nitration was complete. The

progress of the reaction was monitored by TLC analysis on

silica plates (light petroleum–CH2Cl2, 2 : 1) and upon comple-

tion, the reaction mixture was then evaporated to dryness and

the crude residue was further purified by column chromatog-

raphy over silica (light petroleum–CH2Cl, 2 : 1). The major dark

green band was recrystallised from CH2Cl2–methanol to afford

(C5H11)3-NO2P (59.8 mg, 20.8%) as a dark purple microcrys-

talline solid, mp 198–200 �C. Found: C, 74.9; H, 8.1; N, 11.4.

C35H43N5O2 requires C, 74.3; H, 7.7; N, 12.4%. (HR-ESI-FT/

ICR found: [M + H]+ 566.3493. C35H44N4NO2 requires

566.3490.) nmax (CHCl3)/cm�1 3317w (NH), 3022s, 2961s, 2930s,

2895s, 2872s, 2858s, 1558w, 1522m (NO2), 1506m, 1339m

(NO2), 1323m, 1283w, 1244w, 1163w, 1130w, 1103w; lmax

(CHCl3)/nm 308sh (log 3 4.12), 328sh (4.20), 370sh (4.47), 419

(5.26), 524 (4.02), 567 (3.91), 595 (3.78), 654 (3.75) nm;

d (400 MHz, CDCl3) �2.73 (2H, br s, inner NH), 0.96–1.02 (9H,

two overlapped t, ChH3), 1.50–1.56 (6H, m, CdH2), 1.71–1.77

(6H, m, CgH2), 2.39–2.44 (6H, m, CbH2), 4.68 (4H, t, J 8.1 Hz,

CaH2), 4.77 (2H, t, J 8.1 Hz, CaH2), 9.20 (2H, d, J 5.0 Hz,

b-pyrrolic H), 9.22 (2H, d, J 4.9 Hz, b-pyrrolic H), 9.32 (2H, d,

J 4.9 Hz, b-pyrrolic H), 9.34 (2H, d, J 5.0 Hz, b-pyrrolic H); m/z

(ESI) 566.5 ([M + H]+ requires 566.3).

Synthesis of 5-nitro-10,15,20-triheptylporphyrin [(C7H15)3-NO2P]

Treatment of (C7H15)4-P18 (0.300 g, 0.427 mmol) dissolved in

CH2Cl2 (300 mL) with nitrogen dioxide in light petroleum (1 M)

and work-up as above gave (C7H15)3-NO2P (29.6 mg, 10.7%) as

a dark purple microcrystalline solid (CH2Cl2–methanol), mp

108–110 �C. Found: C, 75.2; H, 9.3; N, 8.4. C41H55N5O2 requires

C, 75.6; H, 8.5; N, 10.8%. (HR-ESI-FT/ICR found: [M + H]+

650.4438. C41H56N5O2 requires 650.4429.) nmax (CHCl3)/cm�1

3317w (NH), 2955s, 2924s, 2854s, 1582w, 1520m (NO2), 1466m,

This journal is ª The Royal Society of Chemistry 2010

1366m (NO2), 1319m, 1288m, 1242w, 1126w; lmax (CHCl3)/nm

308sh (log 3 4.04), 328sh (4.13), 368sh (4.38), 419 (5.20), 524

(3.94), 567 (3.84), 594 (3.70), 655 (3.68); d (400 MHz, CDCl3)

�2.48 (2H, br s, inner NH), 0.89–0.94 (9H, two overlapped t,

ChH3), 1.34–1.40 (12H, m, C3H2 and CzH2), 1.52–1.57 (6H, m,

CdH2), 1.73–1.83 (6H, m, CgH2), 2.41–2.51 (6H, m, CbH2), 4.79

(4H, t, J 7.9 Hz, CaH2), 4.87 (2H, t, J 8.2 Hz, CaH2), 9.23 (2H, d,

J 5.0 Hz, b-pyrrolic H), 9.32 (2H, d, J 5.0 Hz, b-pyrrolic H), 9.41

(2H, d, J 3.2 Hz, b-pyrrolic H), 9.42 (2H, d, J 2.9 Hz, b-pyrrolic

H); m/z (ESI) 650.5 ([M + H]+ requires 650.4).

Synthesis of 5-nitro-10,15,20-triundecylporphyrin [(C11H23)3-NO2P]

Treatment of 5,10,15,20-tetraundecylporphyrin19,20 (300 mg,

0.323 mmol) dissolved in CH2Cl2 (300 mL) with nitrogen dioxide

in light petroleum (1 M) and work-up as above gave (C11H23)3-

NO2P (44.4 mg, 16.8%) as a dark purple microcrystalline solid

(CH2Cl2–methanol), mp 88–90 �C. (HR-ESI-FT/ICR found:

[M + H]+ 818.6314. C53H79N5O2 requires 818.6307.)

nmax (CHCl3)/cm�1 3319m (NH), 2957s, 2926s, 2854s, 1518m

(NO2), 1491w, 1468m, 1366w, 1340m (NO2), 1323m, 1246w,

1163m, 1107m; lmax (CHCl3)/nm 308sh (log 3 4.04), 328sh (4.11),

371sh (4.39), 419 (5.17), 524 (3.94), 566 (3.93), 595 (3.68), 655

(3.67); d (400 MHz, CDCl3) �2.42 (2H, br s, inner NH), 0.85–

0.89 (9H, two overlapped t, ChH3), 1.26–1.33 (36H, m, C3H2,

CzH2, ClH2, CqH2, CiH2 and CkH2), 1.47–1.53 (6H, m, CdH2),

1.74–1.88 (6H, m, CgH2), 2.42–2.50 (6H, m, CbH2), 4.83 (4H, t,

J 8.1 Hz, CaH2), 4.90 (2H, t, J 8.0 Hz, CaH2), 9.24 (2H, d, J 5.0

Hz, b-pyrrolic H), 9.37 (2H, d, J 4.9 Hz, b-pyrrolic H), 9.44

(2H, d, J 5.1 Hz, b-pyrrolic H), 9.46 (2H, d, J 4.9 Hz, b-pyrrolic

H); m/z (ESI) 818.7 ([M + H]+ requires 818.6).

Synthesis of 5-amino-10,15,20-triheptylporphyrin

[(C7H15)3-NH2P]

A solution of (C7H15)3-NO2P (10.0 mg, 0.0154 mmol) in a HCl–

ether mixture (4 M, 4 mL) was stirred with tin(II) chloride

dihydrate (36.0 mg, 0.160 mmol) in the dark for 2 h. The reaction

mixture was poured onto ice (10 g) and when the ice melted,

CH2Cl2 (100 mL) was added. The organic layer was washed with

water (50 mL), sodium carbonate solution (10%, 2 � 50 mL),

water (50 mL), dried over anhydrous sodium sulfate, filtered and

the filtrate evaporated to dryness to give (C7H15)3-NH2P (9.0 mg,

94%) as a purple solid, mp > 300 �C. (HR-ESI-FT/ICR found:

[M + H]+ 620.4678. C41H58N5 requires 620.4687.) nmax (CHCl3)/

cm�1 3302w, 3202w, 3124w, 2955s, 2924s, 2854s, 1666w, 1574w,

1520m, 1466m, 1373w, 1350m, 1265m, 1095w, 1018w; lmax

(CHCl3)/nm 310sh (log 3 3.92), 420 (4.46), 426sh (4.45), 523sh

(3.54), 523sh (3.54), 594sh (3.37), 625 (3.21), 676 (3.19); aggre-

gation was obtained in 1H NMR spectrum and manifested as

broad resonances; m/z (ESI) 620.7 ([M + H]+ requires 620.5).

Results and discussion

The porphyrin that provided the most regular self-assembled

microrods on silica was the (C7H15)3-NO2P (Scheme 1), which

was prepared using nitro-dealkylation of the (C7H15)4-P,18

carried out by nitrogen dioxide in light petroleum solution.

Nitration was assumed to follow a similar mechanism to that

reported in the literature for nitration of nickel(II)

J. Mater. Chem., 2010, 20, 2310–2316 | 2311

Scheme 1 Synthesis of (C7H15)3-NO2P: (i) NO2–light petroleum,

CH2Cl2.

Fig. 1 X-Ray crystal structure of (C7H15)3-NO2P shown as an ORTEP

plot. Thermal ellipsoids are drawn at the 50% probability level.

Fig. 2 Chemical structure of (C5H11)3-NO2P and (C11H23)3-NO2P.

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tetrapropylporphyrin.21 An X-ray crystal structure of the

(C7H15)3-NO2P‡ was successfully obtained (Fig. 1). The nitro

group is canted relative to the nearly planar porphyrin ring. The

(C5H11)3-NO2P and (C11H23)3-NO2P counterparts were also

synthesised following the same method (Fig. 2).

Micro-droplets (3 mL) of each of the nitroporphyrins in

CH2Cl2–DMF solution were deposited onto the cleaned quartz

slides with a secured calibrated micropipette. After deposition,

the nitroporphyrin crystallised rapidly from CH2Cl2–DMF

solution as the CH2Cl2, in which nitroporphyrin is highly

soluble, evaporates at room temperature (CH2Cl2 bp ¼ 40 �C,

‡ Crystal structure data for (C7H15)3-NO2P: molecular formulaC41H55N5O2, M 649.90, triclinic, space group P�1(#2), a 10.485(2),b 12.451(2), c 15.458(2) A, a 94.241(4), b 96.867(4), g 95.615(4)�, V1986.4(6) A3, Dc 1.087 g cm�3, Z 2, crystal size 0.49 by 0.05 by0.02 mm, colour red, habit needle, temperature 150(2) K, l(MoKa)0.71073 A, m(MoKa) 0.067 mm�1, T(SADABS)min,max 0.853, 0.999,2qmax 50.92, hkl range �12 12, �14 14, �18 18, N 28 303, Nind

7138(Rmerge 0.0440), Nobs 4196(I > 2s(I)), Nvar 459, residuals* R1(F)0.0764, wR2(F2) 0.2594, GoF(all) 1.043, Drmin,max �0.390, 0.595 e A�3.

2312 | J. Mater. Chem., 2010, 20, 2310–2316

DMF bp ¼ 153 �C). The nitroporphyrins have low solubility in

DMF. The experiments were repeated using CH2Cl2 and

different DMF : CH2Cl2 ratios. Within the scope of these

experiments, the longest, straightest, and most crystal-like

microrods were obtained with equal solvent ratios.

When a droplet of 1 : 1 solution of (C7H15)3-NO2P is deposited

on a-quartz, molecules self-assemble into microrods (Fig. 3a and b),

through the alignment in two dimensions of the heptyl chains,

and staggering of aromatic regions, influenced by dipole-align-

ment arising from the dipoles created by having the NO2 group—

this is consistent with ionic self-assembly. Visual iridescence of

the (C7H15)3-NO2P solid after recrystallisation is very different to

the bulk solid form, which has a dark brown-red chemical col-

ouration characteristic of its absorption bands. These microrods

(diameter f > 500 nm, lengths > 10 mm, see below) are an order

of magnitude larger than natural occurring photonic crystals that

were reported in the literature.22,23 It is clear that the preparation

conditions are critical to the success of the self-assembled layer

formation. In contrast, under the same conditions, (C7H15)4-P

crystallises into 2-D microplates [area > (10 � 50) mm2]—Fig. 3e

and 7b. Although transparency is high, the original colour,

characteristic of the bulk starting material, is observed. No

evidence of structural colour is seen in the microplates. Reduc-

tion of the nitro group of (C7H15)3-NO2P gave (C7H15)3-NH2P

which did not form microrods under the same deposition

conditions. The NH2 group has no significant polar contribution.

This highlights the additional and essential role of electronic

dipole orientation to both alter the spectroscopy processes and

improve optical transparency and to guide the self-assembly

process.

When CH2Cl2 alone was used, less organised self-assembly of

the (C7H15)3-NO2P was observed in the form of clusters of

aggregated multiple self-assembled crystals which show less

iridescence since the clustering is disordered (Fig. 3c and d). This

This journal is ª The Royal Society of Chemistry 2010

Fig. 3 (a) Optical micrograph of the (C7H15)3-NO2P microrods assembled on a-quartz; (b) observed iridescence of the (C7H15)3-NO2P microrods

deposited on a-quartz when white light is shone at an angle �45� to the surface; (c) optical micrograph of (C7H15)3-NO2P crystal cluster obtained by

crystallisation from CH2Cl2; (d) close up of shard from cluster shown in (c); (e) optical micrograph of the 2-D self-assembled microplates of (C7H15)4-P.

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gives rise to the dark colouration. X-Ray crystal structure

packing of (C7H15)3-NO2P from a single shard of crystal (Fig. 4)

shows p–p interaction of each three porphyrin molecules where

the nitro groups are in one direction, followed by another three

packed porphyrins where the nitro groups are observed in the

opposite direction. This is the basic underlying structure in which

the unit porphyrin cells pack and is similar to thin film structures

using longer chains characterised by scanning tunnelling

microscopy (STM) and reported in the literature on ordered

graphite and Au surfaces.20,24 Those reports used STM to probe

the structure of films deposited, raising the possibility that

Fig. 4 X-Ray crystal structure packing of the (C7H15)3-NO2P as viewed appr

for the disordered heptyl chain. Colour code: green is carbon, blue is nitroge

This journal is ª The Royal Society of Chemistry 2010

attachment to an ordered substrate was critical for good

assembly. Our results suggest that such self-assembly is essen-

tially spontaneous, dependent only on molecular topology, with

the environment primarily affecting the quality and extent of

assembly. No attachment to a substrate is necessary. These

results also indicate that far more ordered aggregates are possible

under ideal precipitation conditions.

In contrast to the resonant enhanced scattering used to char-

acterise aggregates,25 the highly iridescent nature of the reflected

structural colours arises from multiple reflections. Such angle-

dependent colour is characteristic of diffraction associated with

oximately along the b-axis (with c horizontal). Only one position is shown

n, red is oxygen, hydrogens are not shown.

J. Mater. Chem., 2010, 20, 2310–2316 | 2313

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well defined, periodic layers, analogous to some natural organic

supra-structure colours26 (for example, deep iridescence of

butterfly wings,22,27 bird feathers,23 beetle’s exocuticles27 and

plants28). It can therefore provide further evidence of the layered

structure anticipated from X-ray diffraction. The �1.5 to 2 nm

separation of porphyrin chains shown in the X-ray diffraction is

too small to explain the iridescence alone. It is thus likely that

sheets of these chained layers with a greater period are respon-

sible for the iridescent colour.

AFM images of the microrods reveal a layered (2 nm) surface

with a staggered period of 125 nm (Fig. 5a and b). The 2 nm

period of the layers is consistent with the 2 nm structure revealed

by X-ray scattering (Fig. 4). This suggests that the microrod

structure arises from a staggered configuration of layered sheets

extending along the microrods. The out-of-plane curvature is

introduced by the strain arising from dipole orientation and is

offset between nitro groups, where the dipoles are oriented

opposite to each other. The striking, iridescent structural colour

is consistent with scattering from a multilayer structure or

corrugated surface with such a period and is readily interpreted

from Bragg analysis. From the AFM results, these multiple

interference, Bragg-like, effects are due to the layers with

an approximate period, L z 125 nm. A green iridescence,

l z 530 nm, is observed at an angle of q z 45� to the incident

white light. If the incident light is assumed to be orthogonal to

the plane of the layers making up the microrods, the refractive

index, n, of the layers can be estimated from:

n ¼ (l sin q)/(m2L) (1)

For a first order grating where m ¼ 1, the refractive index is

calculated to be �1.5, typical for these organic systems.29 The

angular spectral separation, along with the likelihood of only

a few layers (since the diameter of the microrods is �1 mm),

indicates a gap between layers giving strong index contrast. If the

structure is a shell around a central region, it is likely that there

are contributions by multiple interference reflections from both

sides, and some resonance enhanced scattering should be

observable.

The role of the substrate was also explored by depositing

microrods on silicon (Si) and silica (SiO2, a-quartz), key opto-

electronic materials. The Si thermal oxide layer has a different

Fig. 5 (a) AFM image of the edge of a (C7H15)3-NO2P microrod

showing the layered structure; and (b) cross-section of this AFM image.

2314 | J. Mater. Chem., 2010, 20, 2310–2316

distribution of surface point defects than the quartz slide.30 Any

assembly based on dipole alignment may differ to that observed

on quartz. Common to both, the best self-assembly process was

observed when using 1 : 1 (DMF : CH2Cl2) co-precipitation.

Fig. 6a and b show the optical and AFM images of the

microrods generated on the Si substrate. No crystal-like struc-

tures were observed, with the little amount of colour accounted

by simple thin film scattering or some resonance enhanced

scattering. The AFM results (Fig. 6b) show a smooth surface

with no periodic features on the scale required, explaining the

absence of iridescence. This is further evidence that the surface

corrugation with period 125 nm is responsible for the observed

iridescence. SEM imaging of this sample further confirmed the

ultra smooth nature of the surface (Fig. 7a). The SEM image of

(C7H15)4-P microplates showed the surface to be similarly ultra

smooth (Fig. 7b) and again these are not iridescent. The differ-

ence between substrates indicates that substrate surface proper-

ties may play a role in determining the formation of the staggered

periodic structure.

Microrod formation with porphyrins with varying hydro-

carbon chain length [(C5H11)3-NO2P, (C7H15)3-NO2P and

(C11H23)3-NO2P] was evaluated more closely. The strongest

iridescence was always observed with the heptyl chains, indi-

cating that whilst ionic self-assembly drives the porphyrin

microrods formation, the regularity and uniformity of the

structures (at least in terms of the corrugated surface) require an

optimal hydrocarbon length for the conditions found. Microrods

which have precipitated in the centre of the deposited drop after

evaporation have been measured for each compound. (C5H11)3-

NO2P microrods have a similar width and iridescence compared

to the (C7H15)3-NO2P, but a much larger distribution length

(10–250 mm). In contrast, (C11H23)3-NO2P microrods have the

Fig. 6 (a) Optical micrograph of the (C7H15)3-NO2P microrods assem-

bled on a Si surface; (b) AFM image of the (C7H15)3-NO2P microrods.

Fig. 7 (a) SEM image of a single (C7H15)3-NO2P microrod; (b) SEM

image of sheets of (C7H15)4-P.

This journal is ª The Royal Society of Chemistry 2010

Fig. 8 (a) Optical micrograph of the (C5H11)3-NO2P microrods

assembled on a-quartz; (b) optical micrograph of the (C11H23)3-NO2P

microrods assembled on a-quartz.

Table 1 Distribution of the (C5H11)3-NO2P, (C7H15)3-NO2P and(C11H23)3-NO2P microrods lengths measured in the centre of a droplet.(C19H39)3-NO2P did not form microrods

SampleAve.length/mm

Std.dev./mm

Ave.width/mm

Std.dev./mm

Length/widthratio

(C5H11)3-NO2P 169 81.0 0.81 0.11 208.6(C7H15)3-NO2P 10.8 3.6 0.67 0.12 16.1(C11H23)3-NO2P 11.1 3.5 0.69 0.18 16.1

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same size range as the (C7H15)3-NO2P, but show very little

iridescence (Fig. 8). These may be better structures for optical

transport where diffractive effects need to be avoided. Beyond

this chain length no iridescence is observed. The compound

(C19H39)3-NO2P with much longer alkyl chains did not show

microrod formation. Table 1 summarises the average lengths of

each microrod family, all measured optically.

Conclusion

In conclusion, we have formed porphyrin-based microrods which

are an order of magnitude larger than those previously reported,

up scaling self-assembly into the micron domain. By tailoring

the polarity of the end group we have fabricated porphyrin

2-D microplates from (C7H15)4-P and microrods from

(C7H15)3-NO2P consistent with ionic self-assembly of the

microrods. In addition to comparison with previous work,

a variety of methods were used to characterise the microrods,

including optical microscopy, optical and X-ray diffraction

analysis, AFM, and SEM. These measurements permitted

quantification of some of the optical properties associated with

well organised stacked layers and the resulting corrugated

surface. The optical transparency of the microrods on a-quartz

suggests that such individual structures could form the basis of

photonic microrods and other device components (including

periodic structures). Additionally, the ability to either remove or

add iridescence through such structures adds a new level of

optical shaping and control of supramolecular structures for

both passive and active applications. These can eventually be

integrated onto silicon (or other material) photonic circuits or

within silica optical fibres. We note the potential role of such

microrods, with novel surface control, in enhancing photovoltaic

applications. 5,10,15,20-Tetraarylporphyrins and the octa-

substituted porphyrins related to haem have been the subject of

many thousands of reports and have found many uses. Previ-

ously, very little attention has been directed at the chemistry,

This journal is ª The Royal Society of Chemistry 2010

properties and applications of 5,10,15,20-tetraalkylporphyrins

and their derivatives. The self-assembly properties of this class of

porphyrins demonstrated in this study and their propensity to

form highly ordered monolayers on surfaces20,24 suggest that

they will also prove to be very useful in a variety of emerging

technologies.

Acknowledgements

C. Martelli acknowledges travel funding from the Australian

Research Council (ARC) Networks ARNAM and ACORN,

and funding from a Denmark–Japan NEDO project grant

(Kristensen). J. Canning acknowledges a Villum Kann Ras-

mussen Professorship during time at iNANO in Denmark.

Australian Research Council project grants (DP0770692,

DP0879465—Canning), DIISR-ISL project grant (CG130013—

Canning, Crossley) are acknowledged for providing various

funding on this project. N. Skivesen acknowledges a Carlsberg

Fellowship. G. Huyang acknowledges a studentship funding

from Canning and Crossley.

Notes and references

1 J. M. Ribo, J. Crusats, F. Sague, J. Claret and R. Rubires, Science,2001, 292, 2063.

2 M. de Napoli, S. Nardis, R. Paolesse, M. G. H. Vicente, R. Lauceriand R. Purrello, J. Am. Chem. Soc., 2004, 126, 5934.

3 L. M. Scolaro, A. Romeo, M. A. Castriciano and N. Micali, Chem.Commun., 2005, 3018.

4 G. de Luca, A. Romeo and L. M. Scolaro, J. Phys. Chem. B, 2005,109, 7149.

5 T. S. Balaban, A. D. Bhise, M. Fischer, M. Linke-Schaetzel,C. Roussel and N. Vanthuyne, Angew. Chem., Int. Ed., 2003, 42, 2140.

6 R. F. Pasternak, C. Bustamante, P. J. Collings, A. Gianetto andE. J. Gibbs, J. Am. Chem. Soc., 1993, 115, 5393.

7 W. I. White, in The Porphyrins Handbook, ed. D. Dolphin, AcademicPress, New York, 1978, vol. V, p. 303.

8 E. G. McRae and M. J. Kasha, J. Chem. Phys., 1958, 28, 721.9 C. F. J. Faul and M. Antonietti, Adv. Mater., 2003, 15, 673.

10 A. D. Schwab, D. E. Smith, C. S. Rich, E. R. Young, W. F. Smith andJ. C. de Paula, J. Phys. Chem. B, 2003, 107, 11339.

11 Z. Wang, C. J. Medforth and J. A. Shelnutt, J. Am. Chem. Soc., 2004,126, 15954.

12 K. Hosomizu, M. Oodoi, T. Umeyama, Y. Matano, K. Yoshida,S. Isoda, M. Isosomppi, N. V. Tkachenko, H. Lemmetyinen andH. Imahori, J. Phys. Chem. B, 2008, 112, 16517.

13 M. Yasuda, K. Oyaizu, A. Yamagachi and M. Kuwakado, J. Am.Chem. Soc., 2004, 126, 11128.

14 C. Martelli, J. Canning, J. R. Reimers, M. Sintic, D. Stocks,T. Khoury and M. J. Crossley, J. Am. Chem. Soc., 2009, 131, 2925.

15 J. A. A. W. Elemans, R. Van Hameren, R. J. M. Nolte andA. E. Rowan, Adv. Mater., 2006, 18, 1251.

16 R. Van Hameren, J. A. A. W. Elemans, D. Wyrostek, M. Tasior,D. T. Gryko, A. E. Rowan and R. J. M. Nolte, J. Mater. Chem.,2009, 19, 66.

17 J. S. Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney andA. M. Marguerettaz, J. Org. Chem., 1987, 52, 827.

18 M. J. Crossley, P. Thordarson, J. P. Bannerman and P. J. Maynard,J. Porphyrins Phthalocyanines, 1998, 2, 511.

19 M. A. Fox, J. V. Grant, D. Melamed, T. Torimoto, C. Y. Liu andA. J. Bard, Chem. Mater., 1998, 10, 1771.

20 B. Hulsken, R. van Hameren, J. W. Gerritsen, T. Khoury,P. Thordarson, M. J. Crossley, A. E. Rowan, R. J. M. Nolte,J. A. A. W. Elements and S. Speller, Nat. Nanotechnol., 2007, 2, 285.

21 O. Siri, L. Jaquinod and K. M. Smith, Tetrahedron Lett., 2000, 41,3583.

22 P. Vukusic and I. Hooper, Science, 2005, 310, 1151.23 J. Zi, X. Yu, Y. Li, X. Hu, C. Xu, X. Wang, X. Liu and R. Fu, Proc.

Natl. Acad. Sci. U. S. A., 2003, 100, 12576.

J. Mater. Chem., 2010, 20, 2310–2316 | 2315

Dow

nloa

ded

by U

nive

rsity

of

Sydn

ey o

n 13

Feb

ruar

y 20

13Pu

blis

hed

on 0

5 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

7695

K

View Article Online

24 B. Hulsken, R. van Hameren, P. Thordarson, J. W. Gerritsen,R. J. M. Nolte, A. E. Rowan, M. J. Crossley, A. A. W. Elemensand S. Speller, Jpn. J. Appl. Phys., 2006, 45, 1953.

25 R. F. Pasterneck and P. J. Collings, Science, 1995, 269, 935.26 S. M. Doucet and M. G. Meadows, J. R. Soc. Interface, 2009, 6, S115.27 K. Michielsen and D. G. Stavenga, J. R. Soc. Interface, 2008, 5, 85.28 P. Vukusic, Phys. World, 2004, 17, 35.

2316 | J. Mater. Chem., 2010, 20, 2310–2316

29 M. M€uller, R. Zentel, T. Maka, S. G. Romanov and C. M. SotomayorTorres, Adv. Mater., 2000, 12, 1499.

30 S. P. Karma, H. A. Kurtz, A. C. Pineda, W. M. Shedd andR. D. Pugh, in Defects in SiO2 and Related Dielectrics: Science andTechnology, ed. G. Pacchioni, L. Skuja and D. L. Grissom, NATOScience Series II, Kluwer Academic Publishers, Dordrecht, 2000,p. 599.

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