Synthesis and controlled self-assembly of covalently linked hexa- peri -hexabenzocoronene/perylene diimide dyads as models to study fundamental energy and electron transfer processes

DOI: 10.1021/ja211504a

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Journal of the American Chemical Society p. 5876 - 5886 (2012)

Update date:2022-08-04

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Authors:

Doessel, Lukas F.

Kamm, Valentin

Howard, Ian A.

Laquai, Frederic

Pisula, Wojciech

Feng, Xinliang

Li, Chen

Takase, Masayoshi

Kudernac, Tibor

De Feyter, Steven

Muellen, Klaus

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Article abstract of DOI:10.1021/ja211504a

We report the synthesis and photophysical characterization of a series of hexa-peri-hexabenzocoronene (HBC)/perylenetetracarboxy diimide (PDI) dyads that are covalently linked with a rigid bridge. Both the ratio of the two components and the conjugation o

Full text of DOI:10.1021/ja211504a

Article

pubs.acs.org/JACS

Synthesis and Controlled Self-Assembly of Covalently Linked Hexa-

peri-hexabenzocoronene/Perylene Diimide Dyads as Models To

Study Fundamental Energy and Electron Transfer Processes

Lukas F. Dossel,^†Valentin Kamm, Ian A. Howard, Frederic Laquai, Wojciech Pisula,^†Xinliang Feng,

Chen Li, Masayoshi Takase,^§Tibor Kudernac,^‡Steven De Feyter,^‡and Klaus Mullen*

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany

* Supporting Information

ABSTRACT: We report the synthesis and photophysical characterization of a

series of hexa-peri-hexabenzocoronene (HBC)/perylenetetracarboxy diimide

(PDI) dyads that are covalently linked with a rigid bridge. Both the ratio of the

two components and the conjugation of the bridging element are

systematically modified to study the influence on self-assembly and energy

and electron transfer between electron donor HBC and acceptor PDI. STM

and 2D-WAXS experiments reveal that both in solution and in bulk solid state the dyads assemble into well-ordered two-

dimensional supramolecular structures with controllable mutual orientations and distances between donor and acceptor at a

nanoscopic scale. Depending on the symmetry of the dyads, either columns with nanosegregated stacks of HBC and PDI or

interdigitating networks with alternating HBC and PDI moieties are observed. UV−vis, photoluminescence, transient

photoluminescence, and transient absorption spectroscopy confirm that after photoexcitation of the donor HBC a photoinduced

electron transfer between HBC and PDI can only compete with the dominant Forster resonance energy transfer, if facilitated by

an intimate stacking of HBC and PDI with sufficient orbital overlap. However, while the alternating stacks allow efficient electron

transfer, only the nanosegregated stacks provide charge transport channels in bulk state that are a prerequisite for application as

active components in thin film electronic devices. These results have important implications for the further design of functional

donor−acceptor dyads, being promising materials for organic bulk heterojunction solar cells and field-effect transistors.

attributed to the horizontal phase separation within the active

layer, providing a large interfacial area and close packing

between D and A domains. However, at present these blends

lack any means of control of the supramolecular ordering and

phase separation at the nanometer scale, leading to defects that

promote charge trapping, charge recombination, and competing

INTRODUCTION

■

In the emerging field of organic opto-electronics, control of the

supramolecular organization within the active layer has been

found to be a key factor affecting the properties and the

performance of thin film devices.¹⁻⁸Especially when blends of

self-organizing donor and acceptor moieties are utilized, their

mutual orientation and the degree of supramolecular ordering

are of importance in controlling the fundamental energy and

electron transfer processes and the existence of continuous

percolation pathways for charge carriers.⁹⁻¹³For this reason,

small discotic molecules such as hexa-peri-hexabenzocoronene

(HBC) and perylenetetracarboxy diimide (PDI) were found to

be promising candidates, as they can be functionalized to self-

assemble into columnar superstructures in bulk state with a

pronounced phase separation on the nanometer scale, creating

efficient transport channels for charge carriers while maintain-

ing very thorough donor−acceptor mixing.^14,15Electron-

donating (D) HBC exhibits the highest charge carrier mobility

of all discotic liquid crystals,^16,17while the crystalline acceptor

(A) PDI possesses a high electron mobility^18,19and excellent

chemical, thermal, photochemical, and photophysical stabil-

ity.^20,21Based on a fast intermolecular photoinduced electron

transfer at the donor−acceptor interface, blends of HBC and

PDI have been successfully used to build photovoltaic devices

with remarkably high external quantum efficiencies by simple

solution processing.^15,22,23The excellent performance is mainly

processes such as Forster type energy transfer within the active

layer.

In contrast, controlled phase separation at this desired scale

can be introduced by chemical design of organic semi-

conducting molecules. Covalent linkage in D−A dyads can

offer full control over the self-organization and orientation of

the molecular units to one another.^2,24Furthermore, by using a

rigid bridge, the distance between D and A can be set within

the nanoscopic dimension to optimize charge carrier mobilities

and achieve fast exciton dissociation to suppress energy transfer

processes. Such dyads incorporating HBC and perylene

derivatives have recently been synthesized, and their self-

assembly and electronic properties have been investigated.^2,25,26

Highly ordered and nanosegregated π-stacks were found to

exhibit an ambipolar behavior in a field-effect transistor device

with high charge carrier mobilities.²To further broaden the

scope of potential applications for such dyads, a detailed

Received: December 8, 2011

Published: March 6, 2012

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Scheme 1. Chemical Formulas of the HBC-PDI Dyads 1−5

Scheme 2. Synthesis of the Asymmetric PDI Building Blocks 10a, 10b, and 15

Reagents and conditions: (i) N-methyl-2-pyrrolidone (NMP), HCl, 130 °C, 34% (a), 26% (b); (ii) TIPS-acetylene, Pd(PPh₃)₄, CuI,

tetrahydrofuran, Et₃N, 70 °C, 83% (a), 73% (b); (iii) TBAF, tetrahydrofuran, rt, 95% (a), 97% (b); (iv, v) synthesized according to literature;^27,28

(vi) p-bromoaniline, propanoic acid, 54%; (vii) TIPS-acetylene, Pd(PPh₃)₄, CuI, piperidine, 70 °C, 67%; (vii) TBAF, tetrahydrofuran, rt, 99%.

understanding of the interactions between donor and acceptor

moieties, the resulting supramolecular organization, and its

effect on the fundamental photophysical processes is required.

In this study, we investigated whether HBC-PDI dyads can be

designed to self-assemble into well-defined supramolecular

structures with tailored energy and charge transfer processes

after photoexcitation.

Herein, we present a series of novel HBC-PDI dyads 1−5

(Scheme 1) with rigid covalent linkers that were fully

characterized regarding their fundamental interactions between

donor and acceptor moieties. Driven by noncovalent

intermolecular interactions, the dyads assembled into well-

ordered two-dimensional structures with controllable mutual

orientations and distances between D and A at a nanoscopic

scale, both in solution and in the solid state. As the pronounced

π-stacking tendencies of both HBC and PDI are known to

reduce solubility, limit purification, and hamper analytical

methods, sterically demanding alkyl chains were introduced in

the periphery of the dyads 1−5. After self-assembly, the donor

moiety was selectively excited to study intramolecular and

intermolecular decay channels of the generated excitons. To

promote a fast intramolecular charge transfer, a fully conjugated

p-phenyleneethynylene linker was used in dyad 5, whereas for

1−4 an additional nonconjugated ethylene unit was introduced

to inhibit direct charge recombination. The ethynylene bridging

had the advantage of allowing a Sonogashira coupling that

promised high yields, fewer side products, and reduced steric

hindrance in comparison to a single bond.²The rigidity of the

linker was required to prevent intramolecular formation of a

charge-transfer (CT) complex between D and A. Both in bulk

and in solution intermolecular D−A interactions were found to

be the dominant forces for phase separation and self-

organization, leading to alternating stacks of HBC and PDI,

unless inhibited by a nonstoichiometric ratio that resulted in

strictly segregated columns. Depending on the type of

supramolecular ordering, either an efficient intermolecular

electron transfer was observed for alternating stacks or a fast

intramolecular energy transfer for segregated stacking, with the

latter preserving potential percolation pathways for both holes

and electrons (ambipolar transport). Scanning tunneling

microscopy (STM) of physisorbed aggregates and the

solvent−HOPG interface was utilized to study self-assembly

in solution and interactions with the graphite surface, while

bulk samples were analyzed by two-dimensional wide-angle X-

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Scheme 3. Synthesis of the HBC-PDI Dyads 1−5 via Sonogashira−Hagihara Cross-Coupling Reaction

Reagents and conditions: (i) Pd(PPh₃)₄, CuI, piperidine/tetrahydrofuran, 80 °C, 51% (1), 31% (2), 10% (3), 86% (4), 5% (5).

ray scattering (2D-WAXS) in extruded filaments. Photophysical

processes after photoexcitation were investigated in solution

and in thin films by UV−vis, steady state, and transient

photoluminescence (PL) spectroscopy.

tetrahydrofuran, toluene or dichloromethane, due to the

rigidity of the rest of the molecule. This hampered purification

and spectroscopic characterization. Subsequent Sonogashira

coupling reaction (Pd(PPh₃)₄, CuI, piperidine) with TIPS-

acetylene was used to introduce the ethynyl group in PDI 14

with 67% yield, resulting in a dramatic increase in solubility.

Finally the TIPS protecting group was cleaved off with TBAF at

room temperature to obtain PDI 15 with a free ethynyl

function. Despite the reduced solubility, column chromatog-

raphy quantitatively yielded PDI 15 as a red solid (99%). All

PDI compounds (10a/b and 15) could be characterized by

field desorption (FD) mass spectrometry, NMR spectroscopy,

and elemental analysis.

In a final reaction step, the PDI compounds were covalently

connected to the corresponding HBC core via Sonogashira

cross-coupling (Scheme 3). To avoid homocoupling of the

alkyne compounds, the reaction mixtures were thoroughly

degassed by several “freeze−pump−thaw” cycles, before the

catalysts were added. For the star-shaped HBC-6PDI

compounds 1 and 2 a 6-fold functionalized hexakis(4-iodo)-

hexa-peri-hexabenzocoronene 16 was used, which had been

synthesized following a known literature procedure.³²As the

HBC compound 16 exhibited very poor solubility, the reaction

had to be conducted in a solvent mixture of piperidine and

tetrahydrofuran at 70−80 °C overnight. At the same time, 12

equiv of PDIs 10a/10b had to be used in order to reduce the

amount of incompletely substituted side products. Even though

the reaction conditions were carefully optimized, matrix-

assisted laser desorption/ionization time-of-flight (MALDI-

TOF) MS detected 5-fold substituted side products that could

not be separated using recrystallization or column chromatog-

raphy. At this point, recycling preparative gel permeation

chromatography (GPC) was employed to isolate the desired

HBC-6PDI dyads 1 and 2 with maximum purity. After workup,

both target compounds could be obtained as red solids with

yields of 51% and 31%, respectively. Originating from strong

intermolecular π−π interactions, both samples showed very

limited solubility in common organic solvents, which hampered

analysis by NMR spectroscopy. In conformity with previous

reports in the literature, satisfactory elemental analysis results

could not be obtained, possibly due to incomplete combustion

and soot formation of the extended aromatic cores.³³However,

MALDI-TOF MS (Figure 1) and high resolution STM

experiments were able to verify the structure and the purity

of HBC-6PDI dyads 1 and 2.

RESULTS AND DISCUSSION

■

Synthesis. Dyads 1−5 were synthesized according to the

synthetic procedure depicted in Schemes 1 and 2. A palladium-

catalyzed Sonogashira cross-coupling reaction of an ethynyl-

PDI (10a, 10b, or 15) with a halogenated HBC moiety was

utilized in the final reaction step. This was in contrast to

previously reported dyads of HBC and PDI, where an ethynyl-

HBC and a halogenated PDI were coupled. As potential Glaser-

type cross-linking reactions of 2-fold or 6-fold ethynyl-

functionalized HBCs would dramatically reduce the yields,

this was avoided by introducing the ethynyl function to the

PDI. Synthesis of the asymmetric PDI compounds 10a and 10b

could be achieved by a statistic imidization of commercial

perylenetetracarboxy dianhydride 6 with 2 equiv of 4-

bromophenethylamine 7 and 10-nonadecylamine (8a) and 2-

decyl-tetradecylamine (8b), respectively, which had to be

synthesized following known literature procedures.²⁹⁻³¹Due to

the limited solubility of the starting material, the reaction had to

be conducted at 130 °C, to finally yield 26−34% of PDIs 8a/b

after purification via column chromatography on silica gel as

red waxy solids. The iodo function now allowed the

introduction of a triisopropylsilyl-protected (TIPS) ethynyl

function via Sonogashira coupling reaction (Pd(PPh₃)₄, CuI,

piperidine) to give 9a/b in high yields (73−83%). Subsequent

desilylation with tetra-n-butylammonium fluoride (TBAF) gave

the final PDIs 10a and 10b with a nonconjugated ethylene

linker and an unprotected ethynyl function in almost

quantitative yields (95−97%) as red solids. To prevent

oxygen-promoted Glaser homocoupling during storage, the

compounds were kept refrigerated under argon atmosphere.

Attempts to also obtain the asymmetric PDI 13 (Scheme 2)

with a fully conjugated linker via statistical imidization did not

yield any product. Due to a limited reactivity of the aromatic p-

bromoaniline in comparison to aliphatic primary amines, only a

2-fold reaction of the 2-decyl-tetradecylamine was observed. So

a more complex literature-known synthetic route had to be

used, employing the selective alkaline hydrolysis of the

symmetric PDI 11 to obtain the perylene monoimide-

monoanhydride 12.^27,28Under harsh reaction conditions (160

°C, 6 d) and use of 5 equiv of p-bromoaniline the asymmetric

PDI 13 could be obtained in 54% yield after column

chromatographic purification. Despite the presence of a

sterically demanding alkyl chain in the periphery, 13 showed

only limited solubility in common organic solvents like

Following the same synthetic procedure toward HBC-2PDI

3, 3 equiv of PDI 10b was reacted with the 2-fold halogen-

functionalized 1,10-dibromo-4,7,13,16-tetra(3,7-dimethyl-

octyl)-hexa-peri-hexabenzocoronene 17 (Scheme 3), which

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interactions, H NMR experiments of dyads HBC-1PDI 4 and

HBC=1PDI 5 in dilute solutions and at elevated temperatures

still revealed a pronounced upfield shift of the aromatic proton

resonances. PDI proton signals were even shifted by more than

1.5 ppm in comparison to the isolated PDI building blocks 10a

and 10b (see Supporting Information). This effect was caused

by additional intermolecular donor−acceptor interactions

within aggregates.^38,39For the star-shaped HBC-6PDIs 1 and

2 that carry multiple chromophores, a more pronounced

aggregate formation was observed, leading to strong broadening

of the signals in NMR experiments and no distinct effect of

concentration or temperature, suggesting the formation of

stable aggregates. Here, STM experiments at the liquid−solid

interface were utilized to image dyads 1 and 2 after adsorption

on Au(111) substrates.

Figure 1. MALDI-TOF MS of dyads 1−5 after purification via

recycling GPC, proving full removal of incompletely substituted side

products. For details see Experimental Section.

Self-Assembly at the Solid−Liquid Interface (STM).

STM imaging of the HBC-6PDIs 1 and 2 after physisorption of

submonolayers onto Au(111) surfaces from dilute solutions in

1-phenyloctane and 1,2,4-trichlorobenzene allowed imaging of

the molecules with molecular resolution. Independent of the

solvent used, both dyads showed a strong tendency to form

dimer-like structures where both HBC and PDI were adsorbed

on the gold surface in a staggered conformation (face-to-face).

In the STM height images (Figure 2a−c), the circular structure

had been prepared according to literature.³⁴To increase

solubility, vacant positions on the HBC were decorated with

four branched 3,7-dimethyloctyl alkyl chains. Due to a lowered

reactivity of the bromo as compared to the iodo function, a

complete 2-fold substitution could not be achieved either by

applying more drastic reaction conditions or by using a larger

excess of PDI 10b. Finally, the raw product mixture was

purified by column chromatography and subjected to another

Sonogashira coupling reaction with 2 equiv of PDI 10b and

fresh catalysts to increase the amount of HBC-2PDI 3 in the

reaction mixture. After purification utilizing the recycling GPC

technique, dyad HBC-2PDI 3 could be isolated in 10% yield as

a dark red solid that showed limited solubility, solely allowing

for analysis with MALDI-TOF MS (Figure 1).

To synthesize HBC-1PDI 4, 1-bromo-4,7,10,13,16-penta-

(3,7-dimethyl-octyl)-hexa-peri-hexabenzocoronene 18³⁴was

coupled with an excess of PDI 10b, following the same

synthetic protocol as the other dyads (Scheme 3). Dyad HBC-

1PDI 4 exhibited excellent solubility in common organic

solvents and allowed for easy purification with recycling GPC,

yielding 86% of a dark red solid. MALDI-TOF MS, elemental

analysis, H NMR spectroscopy, and two-dimensional H−H

NOESY NMR experiments confirmed the structure and purity

of the molecule. For the directly conjugated HBC=1PDI 5, 1.5

equiv of PDI 15 was reacted with HBC 18. Due to the

pronounced rigidity, the resulting dyad 5 showed strong self-

aggregation tendency and low solubility, even at low

concentrations. Finally repetitive usage of the recycling GPC

system allowed isolation of dyad HBC=1PDI 5, yielding 10% of

a dark red solid. In addition to MALDI-TOF MS (Figure 1),

the product could also be characterized by ¹H NMR

spectroscopy at elevated temperatures.

In Figure 1, the relevant sections of the superimposed

MALDI-TOF MS of dyads 1−5 are presented, showing the

compounds after recycling GPC purification and proving full

removal of incompletely substituted side products. The HBC-

6PDIs 1 and 2 with molecular weights of more than 5000 g/

mol required a solvent-free sample preparation^35,36and

increased laser power to create a sufficient signal-to-noise

ratio. Under these conditions fragmentation occurred, which

correlated with the excitation energy and could no longer be

observed at low laser powers.

Figure 2. STM height images of HBC-6PDIs 1 and 2 in

submonolayers at the liquid−solid interface using 1-phenyloctane

and 1,2,4-trichlorobenzene (TCB) as the solvents. Some dimer-type

aggregates are highlighted with red boxes, a single dyad with a green

box. (a) 2, phenyloctane/Au(111), 39.0 × 39.0 nm², I_set=100 pA, V_bias

= 560 mV; (b) 2, TCB/Au(111), 69.7 × 69.7 nm², I_set= 80 pA, V_bias

423 mV; (c) 1, TCB/Au(111), 107.2 × 107.2 nm², I_set= 109 pA, V_bias

= 352 mV; (d) schematic model of the aggregates (dimers) on the

surface with staggered configuration.

of the aggregates can clearly be observed with the HBC in the

center and 12 surrounding PDI moieties (red boxes), as

depicted in the schematic model in Figure 2d. Occasionally,

individual structures with the outer ring composed of only six

bright spots could be observed, originating from individual

molecules (green box). The more frequent occurrence of

dimers suggested that even at low concentrations the dyads

self-assembled into well-defined aggregates in solution.

Depending on the polarity of bias used for the experiments,

differences of contrasts between HBC and PDI were observed,

indicating that these units behave as isolated species. This was

in good agreement with the desired properties, as a

nonconjugated ethylene bridge was intentionally introduced

Self-Assembly in Solution (NMR). A characteristic feature

of HBC and PDI is their tendency to aggregate in a face-to-face

fashion in the solid state as well as in solutions. Aggregation of

HBC has been observed even at concentrations as low as 10⁻⁹

mol/L.³⁷Because of the pronounced intermolecular π−π

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to prevent ground-state charge-transfer and fast charge

recombination after a photoinduced electron transfer. In a

next step, the self-organization in solid state was investigated.

Bulk Organization (2D-WAXS, 2D-SAXS). Intermolecular

interactions are promoted in the solid state, so the supra-

molecular organization of dyads 1−5 was investigated in bulk

by X-ray scattering to gain information about the orientation of

the HBC and PDI subunits within the columnar super-

structures. As described in the Introduction, the molecular

arrangement of the donor−acceptor units within the stacks

plays a major role for the charge carrier transport and exciton

dissociation. The two-dimensional patterns recorded for 1 in

the wide and small-angle range (Figure 3a,b) pointed toward

uncommon behavior was related to the high molecular aspect

ratio of 3 and 4. Since no additional reflections appeared in the

small-angle scattering region, the small intercolumnar distance

determined from the WAXS pattern of only 2.02 nm for 3 and

2.10 nm for 4 implied mixed columns consisting of alternating

HBC and PDI subunits. Since 3 and 4 carry the same alkyl side

chains and bear the same spacer between the subunits, the

intracolumnar distance is identical for both compounds and is

independent of the molecular ratio between HBC and PDI. It

can be assumed that in the case of 3 the subunits stack in the

columns in a more random way due to the higher

concentration of PDI, while for the more balanced ratio a

strictly alternating arrangement of donor and acceptor units

could be found. Recently, the application of solvent vapor

diffusion allowed a fine control over the self-assembly of 4 on a

surface in thin films, leading to a broad range of different well-

defined microstructures with similar alternating arrangements

of HBC and PDI.⁴⁰

Optical Characterization. With HBC-6PDI dyads 1 and 2

packing into segregated columnar structures (both in solid state

and in solution) and HBC-2PDI 3 as well as HBC-2PDI 4

packing into alternating stacks, the influence of the supra-

molecular ordering on the inter- and intramolecular energy and

charge transfer processes was studied. After selective photo-

excitation of the donor HBC a desired photoinduced electron

transfer between HBC and PDI would compete with HBC

emission, Forster-type resonance energy transfer, and other

decay chanels.

UV−vis in Solution. Due to the strong tendency of HBC

and PDI to aggregate in the solid state as well as in solution, the

absorption and emission spectra often exhibit special features,

especially at concentrations typically used for transient

fluorescence spectroscopy experiments. Figure 4 shows the

Figure 3. (a) 2D-WAXS and (b) 2D-SAXS (inset) of 1 as powder with

the corresponding integration (Miller’s indexes are used to assign the

reflections), (c) 2D-WAXS of 4 as extruded fiber,⁴⁰(d) Schematic

schematic representation of the columnar stacks of star-shaped dyads 1

(left) and 2 and interdigitating stacks of the linear dyad 4 (right).

the formation of columnar stacks that arranged in a hexagonal

lattice. The hexagonal intercolumnar distance fitted well with

the molecular size of 5.86 nm and indicated that the stacks

consisted of individual molecules, as schematically illustrated in

Figure 3d. We assume that this type of packing was forced by

the local phase separation between the flexible alkyl side chains

and the rigid PDI-HBC-PDI cores and the nonstoichiometric

ratio between HBC and PDI that did not allow any

superstructures with alternating D−A stacking. An identical

observation has been made for compound 2. For both systems,

1 and 2, no reflections related to intracolumnar order appeared,

being characteristic for a so-called disordered columnar

structure. This disorder was attributed to the bulky, branched,

and long alkyl side chains, which were needed in this case to

ensure processability from solution but at the same time

decreased the molecular interactions.

Figure 4. Normalized absorption (dashed lines) and emission spectra

(solid lines) of dyads 1−5 in toluene, each at a concentration of 10⁻⁵

mol/L. For photoluminescence measurements HBC moieties were

selectively excited at 375 nm with a 100 fs laser pulse.

Compounds 3 and 4 (Figure 3c) showed a different type of

molecular organization. After extrusion into fibers they self-

assembled into well-organized discotic stacks with a typical π-

stacking distance of 0.34 nm, unlike the above-discussed

molecules 1 and 2. Interestingly, the equatorial positions of the

corresponding reflections in the pattern (Figure 3c) suggested

that in comparison to other discotics the columnar structures

were oriented perpendicular to the alignment direction. This

absorption spectra of the two star-shaped HBC-6PDI dyads

molecules 1 and 2 as well as of the linear dyads 3−5 in toluene

at a concentration of 10⁻⁵mol/L, thus at a concentration at

which aggregation was likely to occur. Clearly, the absorption

spectra were composed of a superposition of the spectrum of

the HBC core (325−450 nm) and the PDI chromophores

(450−600 nm). Furthermore, the shape of the absorption

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spectra indicated ground-state aggregation of the PDI

chromophores in all dyads 1−5, as the absorption differed

significantly from that typically seen for single PDI

chromophores in diluted solutions.⁴¹In fact, the reduced

intensity of the 0−0 vibronic with respect to the 0−1 vibronic

could be interpreted as a clear signature of H-aggregate

formation of PDI and was in line with previous reports.³⁸

However, the extent to which the PDI chromophores

aggregated varied between the individual dyads. It is worth

mentioning that no additional absorption below the typical

optical band gap absorption of HBC and PDI could be

observed, which would otherwise have pointed to the presence

of ground state CT states.

PL in Solution. After selective photoexcitation of the HBC

the emission spectra of the investigated molecules (also shown

in Figure 4) dyads 1−3 and 5 clearly exhibited the typical

vibronic progressions of the fluorescence of PDI chromophores

(520−620 nm), while no typical structureless emission

spectrum of HBC could be observed between 500 and 600

nm. This exclusive PDI fluorescence detected in the PL

measurements indicated a fast and efficient energy transfer

from the photoexcited HBC core to the PDI chromophores.

This was expected, since the emission spectrum of HBC

overlapped well with the absorption of PDI and allowed

Figure 5. Flourescence decay transients of the dyads 1−5 in toluene at

a concentration of 10⁻⁵mol/L measured by the Streak Camera

technique.

the PDI chromophores, resulting in solely PDI fluorescence.

This was in good agreement with the results of the steady state

emission experiments (Figure 4).

For the linear dyads 3−5 the lifetime of the emission at 700

nm was determined to be ∼10 ns, which was longer than the

fluorescence lifetime of separated PDI chromophores but

shorter than values typically observed for the PDI excimer

emission.⁴⁴To further support the conclusion of a charge-

transfer state, we compared transient absorption of both the

star-shaped HBC-6PDI 1 and the linear dyad HBC-1PDI 4 in

solution (see Supporting Information). The linear dyad showed

the characteristic photoinduced absorption peaks of the PDI

anion, while the star-shaped molecule did not exhibit any

anion-induced peaks, confirming the presence of a charge-

transfer state for the linear dyad only. Taking into account our

2D-WAXS measurements, we propose that this emission is

related to intermolecular CT-states whose population became

possible by the alternating packing motif of HBC and PDI

chromophores in these materials.⁴⁵Since the ratio of HBC and

PDI in HBC-1PDI 4 was 1:1, a complete conversion of the

excitons into intramolecular CT states was possible, and indeed

no residual emission from individual PDI chromophores could

be observed, while for the linear dyad HBC-2PDI 3, where the

ratio of HBC to PDI was 1:2, concurrent emission from single

PDI chromophores and intramolecular HBC-PDI CT states

was found. The increased conjugation in the HBC=1PDI dyad

5 in comparison to HBC-1PDI 4 had no significant impact on

the photophysical properties of the dyad, except that the ratio

of PDI fluorescence and CT state emission differed from those

observed for the nonconjugated ethylene bridge.

UV−vis/PL in Drop-Casted Films. We further compared

the absorption and photoluminescence measurements of the

star-shaped HBC-6PDIs 1 and 2 and the linear dyads 3−5 in

diluted solution to drop-casted films, to further elucidate if the

CT-state formation is an intramolecular process, i.e., charge

transfer through the ethylene bridge, or an intermolecular

charge-transfer state, originating from the stacking pattern (see

Figure 6) and causing interaction between HBC and PDI

chromophores on two successive molecules in the stack. This

also provides insight into the solid-state photophysics that are

more relevant to photovoltaic devices. The absorption bands of

the molecules in the solid state were blue-shifted compared to

efficient Forster-type energy transfer. However, compared to

the emission of individual PDI molecules in solution, the

emission spectra of HBC-6PDI 1 and 2 were significantly

broadened and the lower energy vibronic replica of the first

electronic transition were more intense than expected and

typically observed for individual PDI chromophores. This

observation was in line with the interpretation of the absorption

spectra, indicating H-aggregate formation. In the present case

of the star-shaped HBC-6PDI dyads the PDI emission spectra

showed a superposition of the single chromophore emission

and a broad and featureless excimer emission, similar to spectra

observed previously in related materials.⁴²For the linear dyads

3−5 an additional emission band between 650−800 nm could

be observed in the PL spectra, which was of different origin.

This band had previously been assigned to a charge-transfer

state emission involving electron transfer between HBC and

PDI.²²

To confirm energy transfer processes after selective photo-

excitation of the HBC core in dyads 1−3 and 5 and electron

transfer processes in dyads 3−5, time-resolved photolumines-

cence (Figure 5) and absorption measurements (see Support-

ing Information) were performed. Due to very characteristic

lifetimes of the different excited states, this method allowed a

clear differentiation between electron and energy transfer

processes.

Transient PL/Absorption Spectroscopy in Solution.

Photoluminescence decay transients were measured at 560 nm,

where isolated individual chromophores (both PDI and HBC)

emit and at 700 nm in the red spectral region, where the

charge-transfer state recombination emission of the linear dyads

3−5 was observed. The kinetics at 560 nm could clearly be

assigned to the emission of PDI chromophores with a lifetime

of 4.5 ns (described as single-exponential decay), which was

well within the range of fluorescence lifetimes typically

observed for separated PDI chromophores. The absence of

any spectral or decay component from the HBC core for dyads

1−5, which should typically be in the range of 40 ns,⁴³

suggested that excitons were rapidly transferred from the HBC

core by Forster-type fluorescence resonance energy transfer to

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transfer in self-organizing D−A dyads, stressing the impact of

supramolecular ordering on these processes.^46,47

Summary and Conclusions. To investigate the inter-

molecular interactions between covalently linked electron

donor−acceptor dyads and their influence on supramolecular

organization and fundamental photophysical processes, a series

of five HBC-PDI model compounds have been synthesized by

Sonogashira coupling reaction. By introducing the ethynyl

function to the PDI, multialkynes could be avoided that would

otherwise have undergone Glaser-type cross-linking side

reactions. This way, even 6-fold substitution toward star-

shaped HBC-6PDIs 1 and 2 could be achieved in high yields.

Employing rigid linkers, it was possible to control the ratio and

the respective distance between donor and acceptor moieties.

Dyads 1−5 showed a pronounced self-assembly into well-

defined superstructures in solution and in the solid state. These

aggregates have been extensively studied by STM, 2D-WAXS,

and photophysical means to investigate intra- and intermo-

lecular interactions and energy and charge transfer processes. It

could be shown that the star-shaped HBC-6PDIs 1 and 2

assembled into columnar structures with nanosegregated stacks

of HBC and PDI and a defined intramolecular donor−acceptor

distance of 2.1 nm. In contrast to this, the linear dyads HBC-

2PDI 3, HBC-1PDI 4, and HBC=1PDI 5 formed interdigitat-

ing networks with alternating HBC and PDI moieties along the

columns, resulting in a defined intermolecular D−A distance of

only 0.35 nm and direct π−π interactions.

Figure 6. Normalized absorption (dashed lines) and emission spectra

(solid lines) of dyads 1−5 in drop-casted films, λ_exc= 375 nm.

in solution, indicating increased H-aggregation of the PDI

chromophores. The solid-state emission bands of the HBC-

6PDIs 1 and 2 were also red-shifted compared to solution and

found to be broad and featureless. Compared to the CT-

emission bands of dyads 3−5 observed in solution between

650−800 nm, the emission peaks of HBC-6PDIs 1 and 2 now

emerged between 600−700 nm. This emission is typical for the

excimer state of aggregated PDI molecules.⁴⁴We observed a

further red-shift of the emission of the HBC-6PDI 1 with 10-

nonadecyl alkyl chains with respect to HBC-6PDI 2 bearing the

2-decyl-tetradecyl chains. This could be easily understood, since

the longer alkyl chains of HBC-6PDI 2 should affect the

aggregation of the PDI chromophores. Furthermore, the

emission of the linear HBC-PDI dyads 3−5 was exclusively

dominated by the CT state emission, whereas any isolated PDI

chromophore or excimer emission was absent. We noted that

similar to the spectra in solution, no emission from the HBC

core could be observed here, supporting the notion of fast

energy or electron transfer to the PDI.

In sum, we conclude from the time-resolved PL experiments

that all excitons generated in the star-shaped HBC-6PDIs 1 and

2 rapidly created an excimer state, which was likely to be a

consequence of the interaction between two PDI chromo-

phores. In the linear dyads 3−5 the photogenerated excitons

underwent a charge transfer process, which was too fast to be

resolved in our experiments. However, the nature of the CT-

state must have been intermolecular, caused by the alternating

donor−acceptor stacking structure, and not by desirable

intramolecular electron transfer. The close packing of the

molecules led to an overlap of the HBC and PDI π-systems and

thus increased the amount of CT-emission. In fact, we could

only observe the CT-emission in those dyads 3−5 that showed

an alternating packing of HBC and PDI, as demonstrated by

the morphology experiments. Hence, intermolecular electron

transfer was only possible if an alternate packing of HBC and

PDI was obtained leading to a reduction of the distance

between the chromophores to 0.35 nm, whereas intramolecular

charge transfer was impeded by the rigid linker of the dyads,

since charges had to overcome a distance of 2.1 nm, possibly

with poorer wave function overlap considering the direction-

ality of the π-bonding system. However, intramolecular energy

transfer processes from HBC to PDI were observed. These

results are consistent with results from kinetic studies on charge

For dyads 1 and 2, an efficient Forster-type resonance energy

transfer (presumably intramolecular) from the HBC to the PDI

was observed, promoted by a high degree of spectral overlap

between the HBC emission and the PDI absorption spectra. A

photoinduced electron transfer between HBC and PDI, the

basic working principle of an organic bulk heterojunction solar

cell, did not occur within these aggregates, regardless of

whether donor or acceptor were excited. In solution as well as

in thin films only emission from PDI chromophores and

excimer states could be detected. This indicated that the

efficiency of charge transfer was hampered by the distance of

more than 1 nm between donor and acceptor and the limited

donor−acceptor orbital overlap in the plane of the molecule.

However, the linear dyads 3−5, which possessed a direct

donor−acceptor overlap between two successive molecules in

an aggregate stack, showed CT-emission after HBC excitation

in solution. The effect became even more pronounced in the

solid state due to enhanced intermolecular interaction

compared to in solution. This suggests that intermolecular

CT state formation was possible in these systems, but the

nanomorphology that enabled this charge transfer was

unfortunately unable to provide continuous transport channels

for charge carriers as they would be required in electronic

devices. The conjugation of the linker in dyads HBC-1PDI 4

and HBC=1PDI 5 had no influence on the photophysical

properties, supporting the conclusion that the intermolecular

D−A distance was the key factor accelerating the electron

transfer. However, the star-shaped HBC-6PDIs 1 and 2

provided potential transport channels for both holes and

electrons along the columns. As no electronic interactions

between donor and acceptor were found, these independent

channels would represent a type of “coaxial cable” that could be

used in electronic applications like field-effect transistors. This

is currently being investigated in our group.

It could be shown that a careful molecular design is of utmost

importance to control the supramolecular organization of

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yield 404 mg of PDI 10a as a red solid (93%, 0.525 mmol). EA found:

C, 80.6; H, 7.1; N, 3.3. (calcd for C₅₃H₅₆N₂O₄: C, 81.1; H, 7.2; N,

3.6); ¹H NMR (250 MHz, d₂-DCM) δ 8.63 (m, 8H, CH), 7.38 (d, J =

8,13 Hz, 2H, CH), 7.25 (d, J = 8.16 Hz, 2H, CH), 5.08 (s, 1H, CH),

4.32 (m, 2H, CH₂), 3.03 (s, 1H, acetylene), 2.98 (m, 2H, CH₂), 2.16−

1.15 (m, 33H), 0.75 (m, 6H, CH₃); ¹³C NMR (125 MHz, d₂-DCM) δ

163.33, 140.66, 134.72, 134.23, 132.80, 131.27, 129.78, 129.65, 129.42,

126.44, 126.34, 123.38, 123.28, 120.75, 84.09, 77.39, 55.28, 41.93,

34.55, 32.97, 32.50, 30.21, 29.91, 27.66, 23.26, 18.08, 14.44, 12.95; MS

(FD, 8 kV) m/z (%) 785.6 (100) [M+] (calcd for C₅₃H₅₆N₂O₄784.4).

Mp (°C): 238.

discotic liquid crystalline materials and by this an effective

control over the fundamental photophysical processes like

electron and charge transfer and the occurrence of continuous

percolation pathways for charges. To promote intramolecular

electron transfer, the length of the bridging moiety between

HBC and PDI could be reduced or the energy transfer could be

impeded by utilizing a different acceptor with less spectral

overlap with the HBC emission. With this knowledge, one can

now readily develop dyads with targeted properties for a wide

variety of potential applications.

N-(2-Decyl-tetradecyl)-N′-(4-iodphenylethyl)-perylene-

3,4,9,10-bis(dicarboximid) (8b). In a single-neck round-bottom

flask equipped with a reflux condenser were suspended 2.38 g of

perylenetetracarboxy dianhydride 6 (6.07 mmol), 4.29 g of 1-amino-2-

decyl-tetradecane (12.14 mmol), and 3.00 g of p-(iodophenyl)-

ethylamine²⁹7 (12.14 mmol) in 100 mL of N-methyl-2-pyrrolidone

(NMP), and the mixture was stirred at 130 °C for 3 d under argon

atmosphere. Then the solution was neutralized with 2 M HCl, and the

product was extracted with dichloromethane (three times) before the

organic phase was washed with water and dried over anhydrous

magnesium sulfate. After evaporation of the solvent in vacuo the crude

product was purified using column chromatography (silica gel, eluent

dichloromethane) to yield 1.51 g of PDI 8b as a red solid (26%, 1.57

mmol). EA found: C, 70.3; H, 7.1; N, 3.0. (calcd for C₅₆H₅₅IN₂O₄: C,

EXPERIMENTAL SECTION

■

N-(10-Nonadecyl)-N′-(4-iodophenylethyl)-perylene-3,4,9,10-

bis(dicarboximide) (8a). In a single-neck round-bottom flask

equipped with a reflux condenser were suspended 860 mg of

perylenetetracarboxy dianhydride 6 (2.19 mmol), 1.30 g of 10-

aminononadecane (4.58 mmol), and 1.00 g of p-(iodophenyl)-

ethylamine²⁹7 (4.58 mmol) in 40 mL of N-methyl-2-pyrrolidone

(NMP), and the mixture was stirred at 130 °C for 2.5 h under argon

atmosphere. Then the solution was neutralized with 2 M HCl, and the

product was extracted with dichloromethane (three times) before the

organic phase was washed with water and dried over anhydrous

magnesium sulfate. After evaporation of the solvent in vacuo the crude

product was purified using column chromatography (silica gel, eluent

dichloromethane) to yield 661 mg of PDI 8a as a red solid (34%, 0.74

mmol). EA found: C, 69.2; H, 6.3; N, 3.3. (calcd for C₅₁H₅₅IN₂O₄: C,

69.1; H, 6.3; N, 3.2%); ¹H NMR (250 MHz, d₂-DCM) δ 8.65 (s, 8H,

CH), 7.65 (m, 2H, CH), 7.13 (m, 2H, CH), 5.18 (m, 1H, CH), 4.37

(m, 2H, CH₂), 3.00 (m, 2H, CH₂), 2.20−0.76 (br m, 39H, CH₂and

CH₃); ¹³C NMR (125 MHz, d₂-TCE, 373 K) δ 163.98, 163.52, 140.24,

138.67, 135.02, 134.77, 132.09, 131.43, 131.38, 129.85, 126.72, 126.67,

124.30, 124.14, 124.06, 92.26, 45.32, 42.31, 37.84, 34.66, 33.02, 31.20,

30.80, 30.77, 30.42, 27.61, 25.98, 25.81, 25.65, 25.49, 25.33, 25.18,

23.66, 14.51; MS (FD, 8 kV) m/z (%) 886.3 (100) [M⁺] (calcd for

C₅₁H₅₅IN₂O₄886.3). Mp (°C): thermal decomposition >200 °C.

N-(10-Nonadecyl)-N′-(4-(triisopropylsilylethinyl)-

phenylethyl)-perylene-3,4,9,10-bis(dicarboximide) (9a). In a

dried Schlenk tube were dissolved 705 mg of N-(10-nonadecyl)-N′-

(4-iodophenylethyl)-perylene-3,4,9,10-bis(dicarboximide) (8a, 0.795

mmol) and 365 mg of TIPS-acetylene (1.99 mmol) in a solvent

mixture of 10 mL of tetrahydrofuran and 2.5 mL of triethylamine, and

the solution was degassed with three freeze−vacuum−thaw cycles.

Then 22.5 mg of tetrakis(triphenylphosphino)-palladium(0) (32

μmol) and 9.1 mg of copper(I) iodide (48 μmol) were added to the

reaction mixture, which was then stirred at 70 °C for 14 h under argon

atmosphere. Finally, the reaction solution was poured into methanol,

and the precipitate was filtered off and repeatedly washed with

methanol. After drying in vacuo the crude product was purified using

column chromatography (silica gel, eluent dichloromethane with 1.5%

triethylamine) to yield 621 mg of PDI 9a as a red solid (83%, 0.66

mmol). EA found: C, 78.8; H, 8.1; N, 2.8. (calcd for C₆₂H₇₆N₂O₄Si: C,

70.3; H, 6.9; N, 2.9); H NMR (500 MHz, d₈-THF) δ 8.16 (s, 8H,

CH), 7.68 (d, J = 7.9 Hz, 2H, CH), 7.19 (d, J = 7.9 Hz, 2H, CH), 4.27

(m, 2H, CH₂), 4.04 (d, J = 6.8 Hz, 2H, α-CH₂), 3.03 (m, 2H, CH₂),

1.55−1.20 (m, 41H, CH₂), 0.86 (t, J = 6.5 Hz, 6H, CH₃); ¹³C NMR

spectra could not be recorded due to the strong aggregation tendency

of the material; MS (FD, 8 kV) m/z (%) 955.3 (100) [M⁺] (calcd for

C₅₆H₅₅IN₂O₄956.4). Mp (°C): 235−237.

N-(2-Decyl-tetradecyl)-N′-(4-(triisopropylsilylethinyl)-

phenylethyl)-perylene-3,4,9,10-bis(dicarboximide) (9b). In a

dried Schlenk tube were dissolved 761 of mg N-(2-decyl-tetradecyl)-

N′-(4-iodophenylethyl)-perylene-3,4,9,10-bis(dicarboximide) (8b,

0.795 mmol) and 365 mg of TIPS-acetylene (1.99 mmol) in a solvent

mixture of 5.5 mL of tetrahydrofuran and 1.4 mL of triethylamine, and