ISSN 0959-9428

www.rsc.org/materials Volume 20 | Number 1 | 1 January 2010 | Pages 1–196

PAPERPaolo Samori et al.Phase separation and affinity between a fluorinated perylene diimine dye and an alkyl-substituted hexa-peri-hexabenzocoronene

PAPERChristoph Weder et al.Bio-inspired mechanically-adaptive nanocomposites derived from cotton cellulose whiskers

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

Phase separation and affinity between a fluorinated perylene diimide dyeand an alkyl-substituted hexa-peri-hexabenzocoronene†

Giovanna De Luca,ab Andrea Liscio,a Manuela Melucci,a Tobias Schnitzler,c Wojciech Pisula,c

Christopher G. Clark, Jr,c Luigi Mons�u Scolaro,b Vincenzo Palermo,*a Klaus M€ullen*c and Paolo Samorı*ad

Received 29th July 2009, Accepted 27th August 2009

First published as an Advance Article on the web 24th September 2009

DOI: 10.1039/b915484a

Fluorination of alkyl groups is a known strategy for hindering miscibility, thus promoting phase

separation, when blends are prepared with a hydrocarbon compound. A new perylene

bis(dicarboximide) derivative functionalized with branched N-perfluoroalkyl moieties (BPF-PDI) has

been synthesized as electron acceptor to be potentially used in conjunction with the electron donor

hexakis(dodecyl)hexabenzocoronene (HBC-C12) in bulk heterojunction solar cells. Aiming at

controlling self-assembly between the two components at the supramolecular level, stoichiometric

blends in CHCl3 have been prepared either by spin- or drop-casting onto silicon surfaces, and further

subjected to solvent vapour annealing (SVA) treatment in a CHCl3-saturated atmosphere.

Spectroscopic investigation in solution shows the formation of supramolecular BPF-PDI–HBC-C12

dimers, with an association constant Kass ¼ (2.1 � 0.3) � 104 M�1, pointing to a strong and unexpected

affinity between the two species within the mixture. Characterization through optical and atomic force

microscopies of the deposited samples revealed that the self-assembly behaviour of the blends on SiOx is

remarkably different from mono-component films, thus confirming the absence of a macroscopic

phase-separation between the two components featuring isolated domains of the neat acceptor or

donor compound. In addition, X-ray studies provided evidence for the existence of a local-scale phase

separation. These findings are of importance for organic photovoltaics, since they offer a new strategy

to control the phase separation at different scales in electron acceptor–donor blends.

Introduction

Achieving full control over the phase separation in bi-component

films of electron-acceptor and electron-donor materials is crucial

for the fabrication of organic photovoltaic devices.1–4 The ideal

phase-separation for such applications should be on the local

scale, and in particular on the nanometer scale, i.e. comparable

to the mean exciton diffusion length. Several approaches have so

far been employed to develop phase-segregated bi- or multi-

component architectures with a nanoscale precision, including

block copolymers,5,6 monomolecular dyads,7–14 as well as blends

of dissimilar systems with15–18 or without fluoro-substitution on

the side-chains.19

aIstituto per la Sintesi Organica e la Fotoreattivit�a – Consiglio Nazionaledelle Ricerche, via Gobetti 101, 40129 Bologna, Italy. E-mail: palermo@isof.cnr.itbDipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica –Universit�a di Messina, Salita Sperone 31, Vill. S. Agata, 98166 Messina,ItalycMax Planck Institute for Polymer Research, Ackermannweg 10, 55128Mainz, Germany. E-mail: muellen@mpip-mainz.mpg.dedNanochemistry Laboratory, ISIS – CNRS 7006, Universit�e de Strasbourg,8 all�ee Gaspard Monge, 67000 Strasbourg, France. E-mail: samori@isis-ulp.org

† Electronic supplementary information (ESI) available: Explanation ofdense network formation during BPF-PDI drop-casting; AFM and OMimages of PDI7 and HBC-C12 samples cast on SiO2; equatorialintegrations of the 2D WAXS patterns; synthesis and characterizationof BPF-PDI. See DOI: 10.1039/b915484a

This journal is ª The Royal Society of Chemistry 2010

Nanographenes,20,21 and in particular hexa-peri-hex-

abenzocoronenes,22–26 are building blocks exhibiting high

charge carrier mobility, which is key for the fabrication of solar

cells with high quantum efficiencies.27 On the other hand,

perylenebis(dicarboximide)s (PDIs)28–34 are well-known n-type

semiconductors of interest as active components for the fabri-

cation of field-effect transistors (FETs)35–41 and solar cells.42–47

Fluorinated PDIs have been used in the last few years for

application in FETs, as a strategy to hinder the planarity of the

conjugated core as well as to change the stability of the device in

the air.48–52 Above all, the fluorination of PDI makes them

stronger electron acceptors.

Previous work on blending hexakis(dodecyl)hexabenzocor-

onene (HBC-C12)53,54 as a good electron donor, with PDI con-

taining branched aliphatic side chains55,56 as a good electron

acceptor, showed that while interaction between the aromatic

cores tends to form ADADAD periodic columnar stacks, as

observed by X-ray diffraction measurements,57 the side chains

can either hinder or favour this stacking. Making use of different

side chains and deposition conditions, it was possible to vary the

extent of the phase separation from the molecular57 to the

micron27 scale.

Herein we describe the characterization of a novel PDI

derivative with branched fluorinated chains (BPF-PDI, Fig. 1a)

and its self-assembly behaviour, either neat or as a blend with

hexakis(dodecyl)hexabenzocoronene (HBC-C12, Fig. 1b). A

thorough optical characterization in solution has been carried

out by absorption spectroscopy to gain a deep understanding on

J. Mater. Chem., 2010, 20, 71–82 | 71

Fig. 1 Chemical formulae of (a) BPF-PDI and (b) HBC-C12.

Scheme 1 Reagents and conditions: (i) NaH, Rf(CH2)3I, DMF, 80 �C, 16 h,

95%; (ii) KOH, EtOH, 90 �C, 16 h, 70%; (iii) 180 �C, 30 min, 87%; (iv)

BH3$THF, THF, reflux, 16 h, 95%; (v) phthalimide, diethylazodicarbox-

ylate, THF, rt, 72 h, 82%; (vi) N2H4$H2O, EtOH–THF (5:1), reflux, 16 h,

90%; (vii) AcOH, N-methyl-2-pyrrolidone, 160 �C, 16 h, 63%.

the intimate interaction between BPF-PDI and HBC-C12, both in

terms of stoichiometry of the potential supramolecular complex

and of the association equilibrium constant. The formation of

a complex, either a donor/acceptor dimer or a higher aggregate,

is hence hypothesized in solution owing to supramolecular

interactions between the two aromatic species, which are elec-

tronically complementary.58 An atomic force microscopy59

investigation of the blend deposited on surfaces has shown that

this interaction leads to different morphologies as compared to

the parent neat compounds. This provided evidence for the

absence of a macroscopic phase separation between the two

components. X-ray studies on thin films or bulk samples pointed

out the occurrence of a phase separation on the local scale.

Differently from other studies, where the fluorine end-groups

were attached directly to the bay area of PDI to lower the

HOMO and LUMO energy levels,52 our molecule is intentionally

fluorinated only on the side chains, to very affect only slightly the

molecular energy levels. In particular, we used long, double-

armed side chains, which give some solubility to the molecule

even if partially fluorinated. In this way, the different behaviour

observed can be ascribed to the self-assembly properties of the

molecule, and not to the electronic properties of its aromatic

core.

Experimental

Synthesis of PDI with branched perfluorinated sidechains

The monoacid 1 was prepared starting from substitution of

a malonic ester with two fluorinated alkyl chains, followed by

saponification of the esters and decarboxylation of the resulting

diacid 2 (Scheme 1).60,61

The alcohol 3 was obtained in high yield by reduction of the

monoacid 1 in anhydrous THF with the BH3$THF complex as

reducing agent. This alcohol was reacted with phthalimide under

Mitsunobu conditions to form the phthalimide derivative 4.

After treatment of compound 4 with hydrazine hydrate in

72 | J. Mater. Chem., 2010, 20, 71–82

a boiling ethanol–THF (5:1), the fluorinated branched amine 5

was received in 90% yield.

The doubly perfluoroalkyl-swallow-tailed perylene diimide 6

was achieved by condensing amine 5 with 3,4:9,10-perylenete-

tracarboxylic dianhydride in NMP with acetic acid to give the

crude product 6, which was purified by washing with THF and

subsequent recrystallisation from chloroform.

Additional details on synthetic procedures and spectroscopic

characterization can be found in the ESI†.

Self-assembly and characterization

All chemicals and solvents were purchased from Sigma-Aldrich

and used as received. All solvents were HPLC grade, whereas

hydrogen peroxide (30% w/w) and ammonium hydroxide ($25%

w/w) solutions were semiconductor grade.

N,N0-Bis(1-heptyloctyl)-3,4:9,10-perylene-bis(dicarboximide)55

and hexakis(dodecyl)hexabenzocoronene53 (PDI7 and HBC-C12,

respectively) were synthesized according to literature methods.

After cleaning of the Si/SiOxsamples (�5 � 5 mm2) by a stan-

dard RCA procedure,62 20–40 mL drops of the solutions in

CHCl3 (500 mM total dye concentrations) were either drop-cast

or spun (2000 rpm, 60 s) on the substrates in the air. During drop-

casting, solvent evaporation was slowed down by placing the

sample under a watch glass, to favour self-assembly. Due to the

low solubility of the fluorinated species in CHCl3, its solutions

were heated prior to use until no aggregates were detectable by

eye (40–50 �C). Solvent vapour annealing (SVA) post-treatments

were carried out as described elsewhere.63–66 Optical microscopy

(OM) images were recorded with a Nikon Eclipse 80i microscope

equipped with CFI Plan Fluor Series objectives and a DS-2Mv

digital camera.

This journal is ª The Royal Society of Chemistry 2010

Tapping-mode atomic force microscopy (AFM)67–69 was

employed, recording both the height signal (output of the feed-

back signal) and the phase signal (phase lag of the tip oscillation

with respect to the piezo oscillation). While the first type of image

provides a topographical map of the surface, the latter is

extremely sensitive to structural heterogeneities on the sample

surface, being therefore ideal to identify different components in

a hybrid film.70,71

Scanning Probe Image Processor (SPIP; image metrology

ApS, Lyngby, Denmark) was used for AFM image processing.

Scientist for Windows (version 2.1, Micromath Scientific Soft-

ware, Saint Louis, Missouri USA) was employed for least-mean-

square fitting procedures.

Solutions for UV/vis extinction spectra were prepared by

means of Hamilton syringes, and measured on a Jasco model

V-560 double-beam spectrophotometer using quartz cells of

different path lengths (0.1–40 mm) depending on their concen-

tration. Normalized extinction spectra were obtained by dividing

measured absorbance values for the corresponding concentra-

tions and optical path lengths.

Fluorescence (emission and excitation) spectra were recorded

on a Jasco model FP-750 spectrofluorimeter equipped with

a Hamamatsu R928 photomultiplier, and were not corrected for

absorption of the samples. In the case of highly absorbing

samples a front-face geometry was adopted. A UV filter (Hoya

glass type UV-34, cut-off: 340 nm) was used in order to cut off

the UV component of the instrument’s light sources, avoiding the

formation of HCl by photodecomposition of CHCl3.72

The thin films for structural investigation were prepared as

described above. The fibres were prepared by filament extrusion

using a home-built mini-extruder. If necessary, the material was

heated up to a phase at which it became plastically deformable,

and was extruded as 0.7 mm thin fiber by a constant-rate motion

of the piston along the cylinder.

The 2D-WAXS experiments were performed by means of

a rotating anode (Rigaku 18 kW) X-ray beam with pinhole

collimation and a two-dimensional Siemens detector. A double

graphite monochromator for CuKa radiation (l ¼ 0.154 nm)

Fig. 2 (a) Dots: extinction at 527 nm vs. concentration for BPF-PDI (6 � 10

for optical path lengths. Solid line: Beer’s law. Fitting parameters: 3F-PDI ¼ (6.

to solutions presenting extensive aggregation phenomena in solution, and they

the solutions relative to (a), experimental data having been divided for optica

This journal is ª The Royal Society of Chemistry 2010

was used. A q–q Siemens D500 Kristalloflex with a graphite-

monochromatized CuKa X-ray beam was used for the investi-

gation of the structure in the thin film. The diffractogram was

presented as a function of the scattering vector s with s¼ (2sinq)/

l where 2q is the scattering angle.

Results and discussion

Self-assembly in solution

The substitution of PDI with fluorinated swallow-tailed N-alkyl

groups (Fig. 1a) causes low solubility of BPF-PDI in CHCl3; the

associated aggregation phenomena were studied by Beer’s law

experiments in a concentration range between 6 � 10�7 M and

2 � 10�4 M. As shown in Fig. 2a, BPF-PDI displays a behaviour

compatible with the presence of monomeric species for

concentrations up to 70 mM. A linear fit of the data obeying

Beer’s law yields an extinction coefficient 3527 ¼ (6.4 � 0.1) �104 M�1 cm�1, which is in good agreement with the values

usually observed for this class of chromophores.73 A sudden

drop in the absorbance at lmax and the simultaneous appearance

of extended fibrillar aggregates precipitating from solution are

observed for concentrations higher than 70 mM (red dots in

Fig. 2a),74 accompanied by a slight broadening of the absorption

bands (Fig. 2b). At first, the sudden absorbance reduction

observed in the Beer experiment going from 70 to 122 mM

(Fig. 2a) may suggest the existence of a critical aggregation

concentration. However, one day after the samples were

prepared (by dilution from a warm stock solution) and their

spectra recorded (3 hours after sample preparation), aggregates

can also be detected in the 70 mM BPF-PDI solution, pointing to

slower aggregation kinetics on decreasing BPF-PDI concentra-

tion. Future effort in our laboratories will be addressed towards

the exploration of the kinetics of the aggregation processes.

Differently, HBC-C12 displays a more conventional aggrega-

tion behaviour in CHCl3 than the perylene derivative solutions,

with a smoother deviation from Beer’s law on increasing its

concentration (Fig. 3a). The value of the extinction coefficient of

�7 M to 2 � 10�4 M). Experimental extinction values have been corrected

4 � 0.1) � 104 M�1 cm�1 (l ¼ 527 nm); R2 ¼ 0.9995. Red dots correspond

have been excluded from the fitting. (b) Normalized extinction spectra of

l path length and BPF-PDI concentration.

J. Mater. Chem., 2010, 20, 71–82 | 73

Fig. 3 (a) Dots: extinction at 391 nm vs. concentration for HBC-C12. The red line, reported to highlight the deviation of this system from Beer’s law, has

been obtained considering only the first seven points for the linear fit. [HBC-C12]¼ 6 � 10�7 M to 5 � 10�4 M. Experimental extinction values have been

corrected for optical path lengths. Fitting parameters: 3HBC-C12¼ (3.9� 0.1)� 104 M�1 cm�1 (l¼ 391 nm); R2¼ 0.999. (b) Normalized extinction spectra

of the solutions relative to (a), experimental data having been divided for optical path length and HBC-C12 concentration.

this molecule in CHCl3 has been determined experimentally at

391 nm by considering only the spectra of the most diluted

solutions (from 6 � 10�7 M to 8 � 10�5 M), and the linear fit

yielded a value of (3.9 � 0.1) � 104 M�1 cm�1. From this, the

extinction coefficient in CHCl3 at lmax (361 nm) can be calculated

as 1.15 � 105 M�1 cm�1, which is almost one order of magnitude

larger than the value reported by Kastler et al. for HBC-C12

solutions in 1:3 CHCl3–CH3OH (�2 � 104 M�1 cm�1).75

As far as the acceptor–donor mixtures are concerned, an

unexpected affinity between BPF-PDI and HBC-C12 species can

be noticed at first glance. In fact, unlike the behaviour exhibited

in CHCl3 by the two isolated molecules, their 1:1 mixtures do not

exhibit any evident sign of aggregation, even when the concen-

tration of each component reaches 1 mM. At the same time,

evident colour changes occur as soon as the two species are

mixed: immediately upon mixing CHCl3 solutions of BPF-PDI

(whose bright orange colour is mainly due to fluorescence) and

HBC-C12 (which is faintly yellow), an intense magenta solution is

obtained (Fig. 4). This evidence points to the existence of a strong

interaction between the two molecules in solution, which is not

Fig. 4 Left to right: BPF-PDI (50 mM), HBC-C12 (50 mM), and their 1:1

blend (50 mM each), in CHCl3 solutions.

74 | J. Mater. Chem., 2010, 20, 71–82

observed upon mixing HBC-C12 with PDIs bearing conventional

alkyl chains. The propensity of BPF-PDI and HBC-C12 to asso-

ciate was examined by a Job continuous variation experiment

(Fig. 5).76 This was accomplished by varying BPF-PDI:HBC-C12

molar ratios while keeping the solution total molarity constant

(50 mM). By plotting the function:77

Yi ¼ Absi,l � 3l(BPF-PDI)[BPF-PDI]i � 3l(HBC-C12)[HBC-C12]i (1)

against the molar fraction ci of one component (ci¼ [BPF-PDI]i/

([BPF-PDI]i + [HBC-C12]i)), and by fitting these data with

a quadratic equation, the vertex of the corresponding parabola

(Fig. 5, red line) is found at cBPF-PDI ¼ 0.5. This is the value

Fig. 5 A Job continuous variation experiment: molar fraction of BPF-

PDI is varied while keeping the total concentration of HBC-C12 and

BPF-PDI constant at 5.0 � 10�5 M. Dots correspond to absorbance

differences at 552 nm (Y, as defined in eqn (1)) vs. BPF-PDI molar

fractions. The vertex of the parabola is found at 0.5, the value expected

for a 1:1 ratio of donor and acceptor molecules in their supramolecular

adduct (hetero-dimer). Model equation and fitting parameters: y¼ a + bx

+ cx2; a ¼ 0 � 0.007; b ¼ 0.65 � 0.03; c ¼ �0.65 � 0.3; R2 ¼ 0.97.

This journal is ª The Royal Society of Chemistry 2010

Fig. 6 (a) UV/vis spectral change upon dilution of a HBC-C12–BPF-PDI blend (1:1 ratio), highlighting the dissociation equilibria exhibited by the

hetero-dimer. Experimental data have been divided for optical path length and D/A total concentration, arrows marking the decrease in hetero-dimer

concentration from 8.0 � 10�4 M to 10�6 M. (b) Normalized UV/vis spectra of most concentrated (red) and most diluted (blue) solutions, principally

containing the hetero-dimer or isolated species, respectively. The sum of normalized UV/vis spectra of the two isolated species (green line) is reported for

comparison.

Fig. 7 Dots: normalized extinction at 527 nm vs. hetero-dimer total

concentration; solid line: theoretical curve corresponding to the hetero-

dimer association equilibrium (see eqn (2)). Fitted parameters: Kass¼ (2.1

� 0.3)� 104 M�1; 3¼ (1.2� 0.2)� 104 M�1 cm�1 (l¼ 527 nm); R2¼ 0.998.

expected in case of the presence of a 1:1 stoichiometry in

a complex, and it substantiates the existence of donor–acceptor

dimers (or larger aggregates with a 1:1 ratio) of HBC-C12 and

BPF-PDI in their mixtures in CHCl3.

Moreover, systematic dilution experiments have been carried

on concentrated solutions containing a 1:1 ratio between BPF-

PDI and HBC-C12, enabling investigation of the equilibrium

between the two isolated species and their hetero-dimer,

according to the following:78

HBC-C12 + BPF-PDI # HBC-C12$BPF-PDI (2)

This allowed the determination of the related association

constant Kass. Fig. 6a shows the normalized UV/Vis spectral

change observed upon dilution of a CHCl3 solution containing

the two species at a concentration of 800 mM each. The

extinction spectrum of this starting solution (Fig. 6b, red line)

displays broad features both in the zone typical of HBC-C12

(275–425 nm) and in that related to BPF-PDI (425–600 nm). In

this latter spectral range, the band centered around 543 nm

disappears upon dilution from 800 mM to 1 mM (Fig. 6a).

Moreover, sharp features typical of monomeric PDI derivatives

arise from the starting less structured spectrum, with a slight

blue-shift of their maxima and a marked increase of absorbance

(Fig. 6b, blue line). In addition, in the spectral window where

HBC-C12 presents its typical absorption profile, the broad

features of the concentrated mixture undergo a small blue-shift,

become markedly sharper, and increase in intensity. At the end,

the profile of the most diluted samples can be satisfactorily

described by the sum of the spectra of the isolated species (green

line in Fig. 6b), in agreement with the dissociation of the dimer

at high dilution.

By analyzing the absorption variation at 527 nm (Fig. 7, dots)

with the hetero-dimerization model described in eqn (2), the

theoretical curve shown as a red line in Fig. 7 is obtained through

a least-mean-square fitting process. This analysis yields a value for

the dimer extinction coefficient at 527 nm of (1.2� 0.2)� 104 M�1

cm�1, while the equilibrium constant of dimer formation Kass is

calculated to be (2.1 � 0.3) � 104 M�1.

This journal is ª The Royal Society of Chemistry 2010

The fluorescence emission exhibited by diluted solutions

(10 mM) of HBC-C12 in CHCl3 (Fig. 8a; black lines) is in good

agreement with the emission properties already reported in the

literature for this molecule,75 as is the emission of 10 mM solu-

tions of BPF-PDI (Fig. 8a; red lines) with respect to previous

data concerning other PDI derivatives.73 In neither case does the

fluorescence profile depend on the excitation wavelength,

pointing to the presence of only one emitting species in each

solution. When the two molecules are mixed in a 1:1 ratio at

a low concentration (10 mM total dye), and the sample is illu-

minated with light that both dyes can absorb (l < 420 nm), the

resulting emission features (Fig. 8a, blue lines) can be described

as the sum of contributions from two non-interacting fluo-

rophores: the bands centred around 470 and 491 nm are analo-

gous to those of neat HBC-C12, whereas the peaks at 536, 576

and 623 are reminiscent of the emission of the perylene moiety.

This conclusion is supported by the excitation spectra (Fig. 8b),

J. Mater. Chem., 2010, 20, 71–82 | 75

Fig. 8 Normalized fluorescence (a) emission and (b) excitation spectra

of CHCl3 solutions of the systems under study, showing that the emission

profiles of both concentrated and diluted blends are due to contributions

from the isolated chromophores in equilibrium with the quenched BPF-

PDI–HBC-C12 dimer. Experimental conditions: (black) 10 mM HBC-C12;

(red) 10 mM BPF-PDI; (green) mix of 400 mM BPF-PDI and 400 mM

HBC-C12, front-face geometry; (blue) mix of 5 mM BPF-PDI and 5 mM

HBC-C12. (a) Excitation wavelengths: 352 nm, 550 nm (main and inset,

respectively). (b) Emission wavelengths: 491 nm, 576 nm (solid and

dashed lines, respectively).

in particular by that corresponding to the emission at 576 nm

(Fig. 8b, blue dashed line). Its profile coincides with both fluo-

rescence excitation and UV/vis absorption spectra of BPF-PDI

(Fig. 8b, red dashed line, and Fig. 2b, respectively), and shows no

contribution from HBC-C12 absorption, also indicating the

absence of energy transfer between the two molecules under these

conditions. In the concentrated equimolar mixture (800 mM total

dye), where the equilibrium shifts from the two isolated dyes

towards the supramolecular hetero-dimer, the emission profile is

still very similar to that of the diluted mixture (Fig. 8a, green

lines).79 This evidence remains even valid when exciting the

concentrated mixture at 550 nm (Fig. 8a, inset), where the

extinction coefficient of the dimeric species is larger than that of

the neat chromophores. Moreover, the excitation spectra recor-

ded for this solution (Fig. 8b, green lines) indicate that the

various emission features are due to contributions from the non-

interacting HBC-C12 and BPF-PDI monomers, the luminescence

76 | J. Mater. Chem., 2010, 20, 71–82

from the perylene moiety showing no contribution from the

excitation of HBC-C12 molecules. All this evidence suggests that

the HBC-C12–BPF-PDI complex is non-emitting and that the

fluorescence exhibited by the concentrated mixture is due to the

residual HBC-C12 and BPF-PDI monomers in equilibrium with

the dimers, although a very weak emission of the complex cannot

be entirely excluded as it might be hidden underneath the strong

emission of the perylene monomers.

If this high affinity between BPF-PDI and HBC-C12 is

considered together with the increase of the total dye concen-

tration as CHCl3 evaporates after casting the mixture on a silicon

surface, the formation of a blend with no macroscopic phase

separation can be foreseen. In the present case, these interactions

are strong enough to cause the formation of a stable hetero-

dimer between BPF-PDI and HBC-C12 already in solution,

probably strengthened by additional contributions from the

presence of the long, fluorinated side chains of the perylene

derivative. This binary complex may behave as a new single

building block, allowing easier processability for the blended

material.

Deposition at surfaces of mono-component films of branched

BPF-PDI or PDI7

Self-assembly of neat films for BPF-PDI on surfaces has been

studied by depositing CHCl3 solutions of this molecule on Si/SiOx,

either by spin-coating or drop-casting. We have performed a direct

comparison of the behaviour of this system with previously inves-

tigated PDI derivatives, i.e. PDI2 (having 1-ethyl-propyl side

chains) and PDI7 (having 1-heptyl-octyl side chains).63,64 For this

purpose, rather concentrated CHCl3 solutions (500 mM) have been

employed and, due to the low solubility of BPF-PDI in CHCl3,

deposition experiments have been carried out after heating the

starting solutions until no aggregates were detectable by eye.

An AFM characterization of the BPF-PDI sample prepared

by spin-coating is displayed in Fig. 9a; it shows a multilayered

structure, each layer having a height of (2.1 � 0.2) nm, similar to

previous observation on PDIs exposing non-fluorinated side-

chains (see Fig. S1†).64 A closer look revealed that these films

have a grainy substructure, leading to a reduced surface flatness

if compared to layers formed by PDI7 (Fig. S1), as proven by

measuring the root-mean-square roughness (RRMS) on images

with a scan length of 1 mm. Indeed, while BPF-PDI films had

RRMS¼ 0.41� 0.04 nm, PDI7 films had RRMS¼ 0.18� 0.02 nm.

This higher roughness for the BPF-PDI films can be attributed to

two reasons. On the one hand, the longer fluorinated side-chains

have a larger degree of freedom than the shorter alkyl groups of

PDI7, possibly leading to a less ordered structure. On the other

hand, the same fluorinated chains might cause the formation of

globular or micellar aggregates representing a first level of

organization, that might further assemble into the observed

multilayer.80,81 These factors might also be the origin of the

difference observed between BPF-PDI and PDI7 on comparing

the heights of their multilayered structure.

An improvement in the degree of order exhibited by these

BPF-PDI deposits has been attempted through solvent vapour

annealing (SVA) treatments by placing the spin-coated

samples in a CHCl3-saturated atmosphere for three days at

room temperature. In fact, spin-coating generally leads to the

This journal is ª The Royal Society of Chemistry 2010

Fig. 9 (a) AFM topography of a BPF-PDI sample spun on SiOx from a warm CHCl3 solution, showing the formation of a well-defined multilayered

structure. The height histogram is shown in (b), while (c) highlights the uniform thickness of (2.1� 0.2) nm of each layer. (a) z-range: 9.2 nm. (d) R2¼ 0.997.

formation of kinetically trapped aggregates due to fast solvent

evaporation, and the annealing of the samples in a solvent

vapour atmosphere may enhance the mobility of the deposited

molecules on the surface, directing the self-aggregation towards

thermodynamically favoured structures.82 As a result of the SVA

treatment, the molecules constituting the starting multilayered

structures migrate on the SiOx surface to yield well-ordered

objects having a high aspect ratio and very sharp edges

(Fig. 10a,b), with lengths of 1–4 mm and widths of 100–250 nm,

sometimes organized in bundles (indicated by a white arrow in

Fig. 10a). By analyzing the height histogram (Fig. 10c,d) it

becomes evident that the heights of these fibrillar objects are

quite regular: a thickness value of (6.4 � 0.5) nm is common to

several self-assembled fibers, while taller ones show a uniform

Fig. 10 (a) AFM topography of a BPF-PDI sample spun on SiOx from a war

(SVA) in saturated CHCl3 atmosphere at room temperature for 3 days. Upo

100–250 nm. (b) Height profile taken across the long dimension of the fiber ind

image is shown in (c), while (d) highlights the uniform increase in the fibers’ th

This journal is ª The Royal Society of Chemistry 2010

increase in thickness with steps of (2.1 � 0.2) nm. This value

matches that measured for the multi-layered structure prior to

SVA, although in that case a smaller number of layers are

formed, pointing to an overall increase in the degree of order

upon annealing. Furthermore, the 3:1 ratio observed between the

thickness of the first layer and that of the upper layers suggests

that the first layer might be three molecules thick, while the upper

layers are one molecule thick.

BPF-PDI samples deposited on SiOx by drop-casting appear

very different from the spun ones; also a result of the larger

amount of material that is deposited on the surface using this

method. When 40 mL of a warm BPF-PDI solution in CHCl3(500 mM) are cast on the silicon surface, the formation of a very

dense, entangled network of self-assembled fibers is observed

m CHCl3 solution ([BPF-PDI]¼ 500 mM), after solvent vapour annealing

n SVA, BPF-PDI forms fibers having lengths below 4 mm and widths of

icated by the white arrow in (a). The height histogram of the topographic

ickness by steps of (2.1 � 0.1) nm. (a) z-range ¼ 23.3 nm. (e) R2 ¼ 0.999.

J. Mater. Chem., 2010, 20, 71–82 | 77

(Fig. 11 and ESI†). The profound entanglement of the BPF-PDI

self-assembled network is revealed by AFM imaging of the

sample surface (Fig. 11), highlighting the presence of overlapping

bundles of fibers completely covering the SiOx surface. In the

case of drop-cast samples, SVA treatment does not cause any

significant change in sample morphology.

Deposition at surfaces of bi-component films

As mentioned above, blends of HBC-C12 and BPF-PDI were

prepared to obtain a nanoscale phase separation between

electron donor and acceptor components in a simple, single-

step deposition process. When the warm equimolar mixture

between BPF-PDI and HBC-C12 (500 mM total dye concen-

tration) is deposited onto SiOx by spin-coating, the surface gets

covered by a uniform double-layer network comprised of

unstructured assemblies (Fig. 12), the topmost layer having

a height �1.6 nm. The bottom layer exhibits 94% coverage,

with small voids having an approximate area of 120 nm2, while

the upper layer has a less dense network, with hollow sites of

about 600 nm2 and a 30% surface coverage. This morphology is

quite different from those exhibited either in the neat BFP-PDI

or HBC-C12 spin-cast from CHCl3 on SiOx (see Fig. 9 and

Fig. S2†).64

In order to establish if the observed lack of macroscopic phase

separation is due to kinetic trapping, as a result of the spin-

coating procedure employed, drop-casting was also used to

deposit the blend onto SiOx, making it possible to exploit the

slower solvent evaporation, and aiming at operating in a more

thermodynamically controlled regime that might lead to

a detectable phase separation. When the blend solution (500 mM

total dye concentration) is drop-cast onto SiOx and evaporation

Fig. 11 AFM (a) topography and (b) x-gradient images (respectively) of

a BPF-PDI sample drop-cast on SiOx from a warm CHCl3 solution

([BPF-PDI] ¼ 500 mM), consisting of a membrane-like, densely packed

network of fibres. (a) z-range ¼ 496 nm.

Fig. 12 (a,b) AFM topography and phase images (respectively) of BPF-PDI

concentration), consisting in a uniform, porous network made of unstructu

histogram in (c). (a) z-range ¼ 5.9 nm.

78 | J. Mater. Chem., 2010, 20, 71–82

is slowly carried out, optical and AFM microscopies reveal that

most of the material self-organizes into objects with high aspect

ratio, with lengths between 10–80 mm, widths around 2–5 mm and

maximum height of ca. 100 nm (Fig. 13a,c,d). They have well

defined growth directions without bending along their length,

sharp edges and very regular surface organization. The silicon

surface surrounding these objects is covered by a bilayer

comprised of unstructured assemblies, the topmost level being

�3 nm thick (Fig. 13b). The morphology of these samples is

markedly different from that observed for drop-cast samples of

the neat mono-component films (Fig. 11 and Fig. S3†, respec-

tively), and gives no indication of phase separation at the

hundreds of nanometers scale.

The spin-coating and drop-cast samples of the BFP-PDI–

HBC-C12 blend were also subjected to SVA in CHCl3 atmo-

sphere for three days at room temperature, to evaluate the

possibility of promoting phase separation within the films. On

the one hand, after SVA on the spun samples, AFM (Fig. 14)

shows a surface mostly covered by flat, high aspect ratio objects

of (150� 70) nm in length, (35� 15) nm width, and around 4 nm

in height (Fig. 14b), whereas in proximity of a defect, such

a scratch or solvent droplet, the morphology evolves toward

larger (�20 mm long) fibrous structures (Fig. 14c). On the other

hand, the drop-cast samples (Fig. 14d) revealed minimal changes,

with bundles of micron-sized fibers. In addition, residuals of the

bilayer structure are found on the silicon surface as nano-sized

needles surrounded by less structured material (not shown).

Ultimately, although a change in the morphology of the

deposited blend has occurred in both cases, the interaction

between donor and acceptor molecules appears to be strong

enough to lead to only one kind of morphology on the silicon

surface, with no evidence of phase separation at a microscopic

scale, differently from other acceptor/donor blends.27,83–85 For

the sake of comparison, Fig. 15a shows the blend of a conven-

tional alkyl-substituted PDI co-deposited with HBC-C12: upon

SVA, the two materials tend to phase-separate, forming

different structures, unlike the HBC-C12–BPF-PDI blends

(Fig. 15b).

To better understand the organization in the equimolar BPF-

PDI–HBC-C12 mixture, in particular to gain information about

their tendency to phase-separate on a local scale, X-ray diffrac-

tion analysis of the thin films was performed (Fig. 16). The dif-

fractogram of a freshly drop-cast film of the mixture (black line)

–HBC-C12 blend spun on SiOx from a CHCl3 solution (500 mM total dye

red assemblies, the upper layer being �1.6 nm thick as per the height

This journal is ª The Royal Society of Chemistry 2010

Fig. 13 (a) OM and (b–d) AFM images of BPF-PDI–HBC-C12 blend

drop-cast on SiOx from a CHCl3 solution (500 mM total dye concentra-

tion), consisting of needle-like microcrystals surrounded by a thin bilayer

of unstructured nano-aggregates, the topmost layer being �3 nm thick.

(b) Zoom-in on the bilayer; topography and phase (respectively), z-range

¼ 7.8 nm. (c,d) Zoom-in on a microcrystal; topography and x-gradient

(respectively), z-range ¼ 47.0 nm.

Fig. 14 (a) OM and (b,c) AFM topographic images (the boxes are

indicated) of BPF-PDI–HBC-C12 blend spun on SiOx from a CHCl3solution (500 mM total dye concentration), after SVA in saturated CHCl3atmosphere at room temperature for 3 days. (b) Upon SVA, the initial

porous network changes into flat, high aspect ratio objects having (150�70) nm length, (35 � 15) nm width, and �4 nm height. (c) Fibrous

structures of 20 mm approximate length, which form in the presence of

a defect (here a solvent droplet) on the surface. (d) OM of the aforemen-

tioned blend drop-cast on SiOx, after SVA in saturated CHCl3 atmosphere

at room temperature for 3 days. z-ranges: (b) 7.8 nm; (c) 970 nm.

Fig. 15 (a) Fluorescence microscopy of a blend of HBC-C12 and non-

fluorinated PDI2 after SVA in THF. Phase separation of the two mole-

cules leads to the growth of two structures, long red fibers formed by PDI

and shorter yellow crystals (due to HBC-C12). See refs. 63 and 64 for

more details. (b) Fluorescence microscopy of a blend of HBC-C12 and

BPF-PDI after SVA in CHCl3. CHCl3 was used instead of THF due to

better solubility of BPF-PDI in this solvent. No phase separation is

observed. The reddish background is due to the high sensitivity needed to

collect the much lower fluorescence from the blend.

This journal is ª The Royal Society of Chemistry 2010

mainly reveals reflections related to the donor material (green

line), due to the pronounced order of neat HBC-C12, and no

scattering intensities from the perylene derivative (blue line), the

layer of pure BPF-PDI showing only poor intensity peaks. Given

that for a homogenous mixture new scattering intensities would

be expected,86 our results strongly suggest the occurrence of

a phase separation between the donor and acceptor components.

To gain further information on the molecular packing in the

mixture, fiber wide-angle X-ray scattering experiments have been

performed on bulk samples prepared in the same way from the

solution as for the thin films. The typical 2D pattern in Fig. 17a

indicates crystalline order of pure HBC-C12 with tilted molecules

in columnar structures,87 whereas crystalline BPF-PDI is

arranged in columnar stacks, but in a staggered fashion

(Fig. 17b).88 The sample prepared by casting the blend on SiOx

from solution shows only a few scattering intensities in the

pattern, and the peaks in the diffractograms display a low

intensity (Fig. 17c), indicating that the two molecules are influ-

encing each other’s ordering but not completely eliminating

Fig. 16 X-ray diffraction of thin films drop-cast from CHCl3: (blue)

BPF-PDI; (green) HBC-C12; (black) equimolar BPF-PDI–HBC-C12

blend. The diffractograms were recorded 30 �C.

J. Mater. Chem., 2010, 20, 71–82 | 79

Fig. 17 2D WAXS patterns of a) HBC-C12, b) BPF-PDI, and

BPF-PDI–HBC-C12 c) before and d) after annealing at 135 �C. All

patterns were recorded at 30 �C. Dashed circles indicate characteristic

reflections of the pure compounds related to intracolumnar packing of

the molecules.

phase separation. This conclusion is further verified by the

comparison of the equatorial reflections, which are related to

intercolumnar arrangement, between the starting materials and

the drop-cast blend (Fig. S4†).

An improvement of the molecular packing in the individual

domains was obtained by annealing the samples at 135 �C, as

shown by (i) the increase in intensity of the reflections of the

annealed thin films while their positions remained unchanged

(Fig. 16, red line), and (ii) the presence of reflections only due to

the pure compounds after annealing the fibres of the mixture

(Fig. 17d). This confirms a distinct phase separation between

BPF-PDI and HBC-C12, with a considerable improvement in the

local order upon thermal treatment.

The occurrence of a phase separation at the local scale might

be due to the dissimilar solubilities of the two starting species

which, during the evaporation of the solvent, precipitate at

different critical concentrations, leading to the phase-separated

blend. Nonetheless, the surface morphologies of both donor and

acceptor in the blends drop-cast on SiOx are indeed markedly

different from the neat films, confirming the key role played by

the intermolecular interactions between the two dissimilar

molecular systems when deposited at the surface. It is worth

noting that HBC-C12 has shown a similar behaviour when mixed

with other alkyl-substituted PDI derivatives;89 moreover, Wang

et al.86 recently reported phase separation for truxene derivatives

depending on the molar ratio between donor and acceptor, while

the morphology remained identical for all samples. In the present

case, a deeper understanding of the actual organization of the

BPF-PDI–HBC-C12 blend has been obtained by combining

the complementary information concerning morphology and

structure given by AFM and X-ray studies.

80 | J. Mater. Chem., 2010, 20, 71–82

The origin of the strong interaction between BPF-PDI and

HBC-C12 detected in solution still remains to be fully under-

stood. The supramolecular interactions involving fluorinated

organic compounds has been studied in various papers, where

the formation of C–H/F hydrogen bonds (in analogy with

O–H/F bonds) has been hypothesised.90–93 In our case,

although different kinds of interactions between the various

parts of the two molecules have to be considered (like stacking of

the aromatic cores,94 Calkyl–H/F,95 Carom–H/F,96 C–H/O]C,97), some assumptions can be made on the relative strength

of the different contributions. In fact, the unusual interaction

between BPF-PDI and HBC-C12, and the lack of phase separa-

tion both in solution and solid, cannot be due to simple stacking

of the aromatic moieties, which is present even in non-fluorinated

PAH blends.27 Hence, the strong binding between the donor and

acceptor molecules under study should be due to the presence of

the long, fluorinated chains on the perylene derivative, and their

interaction with the HBC-C12 molecule. In particular, given that

the side-chains have a greater ability to arrange and interact in

view of their conformational flexibility and peripheral position,

the strong interaction observed between the two species should

mainly involve the (CF2)n moieties of the PDI and the (CH2)n

chains of the HBC. From XRD studies present in the literature

on fluorinated compounds, a typical C–H/F bond length is 2.7–

2.9 A, which added to the C–F and C–H bond lengths (�1.4 A

and �1.1 A, respectively)98 gives a possible interaction distance

between the two flexible chains that is quite large (5.2–5.4 A).

This interaction distance is much larger than a typical p-stacking

between aromatic cores (3.34 A).94 These proposed F/H inter-

actions might coexist with stacking of the aromatic fragments,

and even with direct C–F/p interactions, which have been

previously observed in organic crystals.96 Even if it is much

weaker than a ‘‘classical’’ hydrogen bond,99 the C–H/F–C

interaction is strong enough to change significantly the crystal

packing of various molecules and increase their melting

point.95,97 Here, the large number of C–F and C–H bonds (68 and

150 respectively) present in the side chains of BPF-PDI and

HBC-C12, together with the ease of these chains to interact due to

their flexibility, might favour the formation of a high number of

C–H/F–C contacts. These could operate in a cooperative way,

and in synergy with the stacking of the aromatic cores, to give the

very strong intermolecular interaction observed between these

two molecules in the blends. It must be noted that simple linear

alkanes and perfluoroalkanes, when mixed together, tend to

phase-separate, mainly due to the weak intermolecular forces

acting on the perfluoroalkanes. Such a phase separation is

hindered in our case since the alkyl and perfluoroalkyl moieties

are forced to interact with each other, being covalently attached

to poly-aromatic cores which tend to form stacks.

Conclusions

A new perylene molecule BPF-PDI, functionalized with

branched, highly fluorinated alkyl side chains, has been studied

both in solution and on the surface. This molecule exhibits

a strong tendency to self-assemble in organic solvents; when

deposited on surfaces it forms either multi-layers or nano-needles,

according to processing conditions. In particular, this electron-

accepting molecule shows a strong tendency to interact with

This journal is ª The Royal Society of Chemistry 2010

electron-donating alkyl-substituted hexa-peri-hexabenzocor-

onene molecules. Spectroscopic studies in solution show that

BPF-PDI and HBC-C12 form supramolecular complexes with

a 1:1 stoichiometry and featuring a relatively high association

constant of Kass ¼ (2.1 � 0.3) � 104 M�1. When deposited on

a surface, these bi-component systems self-assemble in aniso-

tropic structures, with no evidence of a macroscopic phase sepa-

ration, even after solvent vapour annealing treatments. On the

other hand, X-ray studies provided unambiguous evidence for

a local-scale phase separation between the two components. Such

a phase separation may be of future interest for applications in the

fabrication of amphiphilic materials and solar cells, where

macroscopic phase separation of the different molecules

composing the active material is often an issue, reducing interface

area and exciton conversion efficiency.

Acknowledgements

This work was supported by the EC FP7 ONE-P large-scale

project no. 212311, the French National Agency (ANR), the

German Research Foundation (Special program Organic

Photovoltaics) and the EC through the NanoSciEra-SENSORS

project, the ESF-SONS2-SUPRAMATES project, the Regione

Emilia-Romagna PRIITT PROMINER Net-Lab, and the

International Center for Frontier Research in Chemistry (FRC,

Strasbourg).

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