Effect of core twisting on self-assembly and optical properties of perylene bisimide dyes in solution and columnar liquid crystalline phases

DOI: 10.1002/chem.200600891

Effect of Core Twisting on Self-Assembly and Optical Properties of Perylene

Bisimide Dyes in Solution and Columnar Liquid Crystalline Phases

Zhijian Chen,^[a]Ute Baumeister,^[b]Carsten Tschierske,^[c]and Frank Würthner*^[a]

Abstract: A series of highly soluble

and fluorescent core-twisted perylene

bisimide dyes (PBIs) 3a–f with differ-

ent substituents at the bay area

(1,6,7,12positions of the perylene core)

were synthesized and fully character-

Vis measurements and VPO analysis

revealed that these core-twisted p-con-

jugated systems show distinct self-di-

merization equilibria in apolar solvent

methylcyclohexane (MCH) with dime-

rization constants between 1.310⁴and

30m^À1. The photoluminescence spectra

of the dimers of PBIs 3a–f exhibit

bathochromic shifts of quite different

magnitude which could be attributed to

different longitudinal or rotational off-

sets between the dyes as well as differ-

ences in the respective p–p stacking

distance. In condensed state, quite a

few of these PBIs form luminescent

rectangular or hexagonal columnar

liquid crystalline phases with low iso-

tropization temperatures. The effects of

the distortion of the p systems on their

p–p stacking and the optical properties

of the resultant stacks in solution and

in LC phases have been explored in

detail. In one case (3a) a particularly

interesting phase change from crystal-

line into liquid crystalline could be ob-

served upon annealing that was accom-

panied by a transformation from non-

fluorescent H-type into strongly fluo-

rescent J-type packing of the dyes.

1

ized by H NMR, UV/Vis spectroscopy,

MS spectrometry, and elemental analy-

sis. The p–p aggregation properties of

these new functional dyes were investi-

gated in detail both in solution and in

condensed phase by UV/Vis and fluo-

rescence spectroscopy, vapor pressure

osmometry (VPO), differential scan-

ning calorimetry (DSC), polarizing op-

tical microscopy (POM), and X-ray dif-

fraction. Concentration-dependent UV/

Keywords:

dyes/pigments

·

liquid crystals · perylene bisimide ·

pi–pi stacking · self-assembly

Introduction

p systems, for example, triphenylenes,^[2]porphyrins,^[3]phtha-

locyanines,^[4]hexabenzocoronenes,^[5]and perylene bisimides

(PBIs)^[6]have been intensively investigated aiming to their

application as optical recording media,^[7]organic photocon-

ductors,^[8]semiconductors,^[9]and solar cells.^[10]In recent

years, emphasis has been given on the organization of tail-

ored functional p systems by self-assembly leading to one-

dimensional stacks of p–p aggregates in solution,^[11]in orga-

nogels,^[12]and in columnar liquid crystalline (LC) phases.^[13]

The major interest in p–p aggregated assemblies stems from

their potential applications as conductive nanowires^[14]in or-

ganic electronics due to the effective p-orbital interactions

among the molecules which facilitate the hopping of charge

carriers.^[15]At a first glance, it might be expected that charge

carrier mobility should benefit from a large size planar aro-

matic core of the functional dyes.^[16]However, in our recent

studies, we have demonstrated that a high charge carrier

mobility is feasible for nonplanar PBI dyes with chlorine

substituents at bay area, thus, the concept of “core-twisted p

systems” has been established.^[17]The pronounced distortion

of the p system of these dyes, which is caused by the steric

encumbering effect of the substituents at bay area (1,6,7,12

Self-assembled p-conjugated systems have attracted tremen-

dous attention in the past years owing to their potential ap-

plication in organic electronics.^[1]Numerous functional

[a] Z. Chen, Prof. F. Würthner

Universität Würzburg, Institut für Organische Chemie

and Rçntgen Research Center for Complex Material Systems

Am Hubland, 97074 Würzburg (Germany)

Fax : (+49)931-888-4756

E-mail: wuerthner@chemie.uni-wuerzburg.de

[b] Dr. U. Baumeister

Martin-Luther-Universität Halle-Wittenberg

Institut für Physikalische Chemie, Mühlpforte 1

06108 Halle (Germany)

[c] Prof. C. Tschierske

Martin-Luther-Universität Halle-Wittenberg

Institut für Organische Chemie, Kurt-Mothes Strasse 2

06120 Halle (Germany)

Supporting information for this article is available on the WWW

under http://www.chemeurj.org/ or from the author: Additional UV/

Vis, fluorescence, VPO, DSC, X-ray diffraction data, and other meas-

urements of the PBI dyes.

450

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Chem. Eur. J. 2007, 13, 450 – 465

FULL PAPER

positions in the perylene core), leads to a unique two-di-

mensional organization of the molecules in the solid state

with close contacts that is not available for the planar un-

substituted PBI dyes. This contact pattern facilitates the

charge transport and leads to an isotropic charge carrier mo-

bility up to 0.14 cm²V^À1s^À1as revealed by pulse radiolysis-

time resolved microwave conductivity^[17]measurements.

More recently, Nuckolls and co-workers have reported an-

other example, that is, a contorted hexabenzocoronene

system, which exhibits a field-effect transistor (FET) mobili-

ty of 0.02cm ²V^À1s^À1.^[18]These studies have shown that core-

twisted p-conjugated discotic systems are of great interest

for the exploration of new organic electronic materials. In

this regard core-twisted PBIs are particularly promising as

PBIs are among the best n-type organic semiconductors to

date.^[19]In comparison with planar unsubstituted PBI dyes,

contorted derivatives exhibit in general much higher solubil-

ity and significantly lower isotropization points (up to 1908C

lower than the unsubstituted PBIs (see below) that are fa-

vorable properties for the fabrication of electronic devi-

ces.^[20]

Herein, we report the synthesis of a series of new, core-

twisted PBI dyes containing tridodecylphenyl substituents at

the imide positions and various substituents at bay area. The

advantage of the tridodecylphenyl substituents is that the

fluorescence quantum yield can be largely improved in com-

parison with the analogue dyes bearing the alkyloxyphenyl

substituents^[21]without losing valuable self-assembly proper-

ties such as the liquid crystallinity of the compounds. The

self-assembly and optical properties of the present PBI dyes

in solution and liquid crystal phase have been investigated

by UV/Vis and fluorescence spectroscopy, vapor pressure

osmometry (VPO), polarizing optical microscopy (POM),

differential scanning calorimetry (DSC) and X-ray diffrac-

tion. Furthermore, the effect of core twisting on the self-as-

sembly and optical properties of p–p stacks of these func-

tional dyes is discussed in detail.

Results

Synthesis: The N,N’-di(tridodecylphenyl)-substituted pery-

lene bisimides (PBIs) 3a–d were synthesized by condensa-

tion of the respective perylene tetracarboxylic bisanhydrides

(PBAs) 1a–d bearing different types of bay substituents

with 3,4,5-tridodecylaniline (2)^[22]according to Scheme 1.

We have previously reported that direct bromination of un-

substituted PBA affords a regioisomeric mixture of 1,6- and

1,7-dibromo PBA in a ratio of about 1:4.^[23]Since these iso-

mers possess similar reactivity, their subsequent reaction

leads to regioisomeric product mixtures that are often diffi-

cult to purify by silica gel column chromatography. We

could overcome this problem by repeated recrystallization

of the regioisomeric mixture of compound 4 (see below) to

obtain the isomer-free 1,7-dibrominated product as precur-

sor for further regioisomerically pure 1,7-disubstituted

PBIs.^[23]Thus, the saponification of 4 gave isomerically pure

1

1,7-dibrominated PBA 1d (verified by 600 MHz H NMR),

which condensed with aniline 2 to afford 1,7-dibrominated

PBI 3d (Scheme 1). The subsequent nucleophilic substitu-

tion of the bromine atoms in 3d by appropriate phenol de-

rivatives afforded the corresponding isomer-free dyes 3e

and f. Similarly, the nucleophilic reaction of 1,7-dibrominat-

ed PBI 4 with pyrrolidine gave the corresponding 1,7-dipyr-

rolidinyl PBI derivative, which was subsequently hydrolyzed

to afford the pyrrolidinyl-substituted PBA 1c. The imidiza-

tion of the latter bisanhydride with 2 afforded 3c. The PBIs

3a–f are fully characterized by ¹H NMR, UV/Vis, fluores-

cence spectroscopy, MS spectrometry, and elemental analysis.

Optical properties of bay-substituted PBI monomers: The

optical properties of 3a–f were investigated by UV/Vis and

fluorescence spectroscopy. The absorption spectra of these

dyes in CH₂Cl₂are depicted in Figure 1 and the optical data

are summarized in Table 1. These dyes show broad S₀–S₁ab-

sorption bands with absorption maxima in the range of 524–

Scheme 1. Synthesis of PBIs 3a–f: a) ZnACHTRE(UNG OAc)₂, quinoline, 1808C, 3 h, yields: 52% (3a), 53% (3b), 65% (3c), and 44% (3d); b) p-tert-butylphenol (for

3e) or pentafluorophenol (for 3 f), K₂CO₃, N-methyl-2-pyrrolidone (NMP), 1208C, 2h, yields: 40% ( 3e), and 61% (3 f).

Chem. Eur. J. 2007, 13, 450 – 465

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451

F. Würthner et al.

705 nm. Depending on their l_abs, the solutions of these dyes

exhibit various colors from orange (3a) to deep green (3c).

The absorption maxima of the PBI derivatives, which con-

tain electron-withdrawing substituents such as Cl (3a), Br

(3d), or pentafluorophenoxy groups (3 f) at the bay posi-

tions are only slightly shifted with respect to that of the un-

substituted reference PBI 6 (see preceding paper, l_abs

=

527 nm in CH₂Cl₂).^[22]But, for PBI derivatives with electron-

donating groups a large bathochromic shift of l_abswas ob-

served relative to the unsubstituted PBI 6 (56 nm for 3b,

181 nm for 3c, 22 nm for 3e), reflecting pronounced elec-

tronic interactions between the perylene bisimide core and

the electron-donating groups in bay area. The absorption

spectra of these dyes displayed vibronic structures in the S₀-

S₁absorption bands,^[24]indicating that the electronic transi-

tion is coupled with the vibration of the perylene skeleton.

In comparison with the spectrum of unsubstituted PBI 6, the

lineshape of the spectra for the bay-substituted dyes 3a–f

are broader and display less vibronic structures due to the

loss of planarity of the perylene core and the lower molecu-

lar symmetry caused by the bay substituents. (Several X-ray

crystal structures of bay-substituted PBIs revealed a signifi-

cant distortion of the perylene skeleton). For all these dyes,

a second absorption band was observed at around 400 nm

that can be attributed to the transition from the ground

state to a higher excited state.^[24]Previously, it was pointed

out that the S₀–S₂transition is symmetry forbidden for un-

substituted PBIs.^[24b]However, due to the twisted nature and

lower symmetry of the present bay-substituted PBIs such

transition is not strictly forbidden, thus the apparent absorp-

tion coefficients of these bands were increased in compari-

son with that of unsubstituted PBIs.

All of these dyes 3a–f are photoluminescent as their fluo-

rescence spectra in CH₂Cl₂revealed (Figure 2). The emis-

sion bands of these compounds are approximately the

mirror image of their S₀–S₁absorption bands with Stokes

shift between 17 and 40 nm (corresponding to 590 to

760 cm^À1in wavenumbers). The emission maxima (l_em) of

the bay-substituted PBIs 3a–f depend on the electronic

properties of the bay substituents. For 3a and d with elec-

tron-withdrawing halogen atoms l_emat 524 and 544 nm, re-

spectively, were observed, while the dye 3c with two elec-

tron-donating pyrrolidine groups showed l_emat 745 nm. As

expected, from our previous work on 6^[22a]the fluorescence

quantum yields (F_fl) of dyes 3b (0.95) and 3e (0.87) are

greatly improved in comparison with those of analogous

dyes bearing trialkoxyphenyl groups, instead of trialkylphen-

yl groups, at the imide positions (F_fl=0.23).^[21]While for

these phenoxy-substituted compounds 3b, e and f high F_fl

values (0.79–0.95) were observed, the halogen-containing

derivatives 3a and d exhibit relatively low fluorescence

quantum yield (Table 1) that may be attributed to fluores-

cence quenching by photoinduced electron transfer process-

es. In accordance with this assumption, the quantum yield of

the dyes 3a and 3d is significantly improved from 0.03 and

0.26, respectively, in polar solvent CH₂Cl₂to 0.37 and 0.67,

respectively, in apolar methylcyclohexane (MCH).

Figure 1. UV/Vis absorption spectra of dyes 3a–f in CH₂Cl₂.

Table 1. Optical properties of PBIs 3a–f in CH₂Cl₂.

Compound

l_abs[nm]

e _max[m^À1cm^À1

]

l_em[nm]

F_fl

3a

3b

3c

3d

3e

3 f

524

580

705

527

546

530

48100

50100

48200

57800

56000

71300

543

611

745

544

575

547

0.03

0.95

0.25

0.26

0.87

0.79

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Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

aggregation. Interestingly, dyes 3a and 3b showed a batho-

chromic shift of the absorption maximum (5 nm for 3a and

10 nm for 3b), while a hypsochromic shift of the absorption

maximum was observed for the remaining dyes (41 nm for

3c, 34 nm for 3d, 27 nm for 3e and 32nm for 3 f). Such

shifts of absorption maxima imply the formation of J-type

or H-type excitonic coupling regimes in these aggregates

(see Discussion).

However, only from the UV/Vis spectral data it cannot be

assessed whether dimer or higher aggregates prevail.^[25]

Therefore, vapor pressure osmometry (VPO) was employed

to determine the size of the aggregated species of the pres-

ent PBIs. For the reference PBI 5 (structure see above) con-

taining very bulky substituents, the obtained colligative con-

centrations were identical with the stoichiometric concentra-

tion of the MCH stock solution, indicating that this com-

pound does not aggregate in the concentration range ap-

plied. VPO measurements for dye 3d were performed at 5

different concentrations in the range of 0.0016 to 0.0132

molal (about 0.0013 to 0.0104m) and a reduction of the colli-

gative concentrations to about half of that for 5 was ob-

served (Figure S3 in the Supporting Information). Based on

the measured colligative concentrations, the aggregation

numbers N of this dye were calculated as 1.3 to 1.7 at differ-

ent concentrations. For the remaining dyes 3a–c and e,f, N

values between 1.1 and 1.8 were obtained. These results are

in compliance with a dimerization process in the given

range of concentration for the present bay-substituted PBIs

in MCH. The smaller N values observed for 3a and b (1.1

and 1.4, respectively) can be attributed to the relatively

small dimerization constants (see Table 2) of these dyes and

the coexistence of dimers and a large amount of monomeric

species at the measured concentration. For comparison, in

the case of planar PBI dye 6 the formation of oligomeric

stacks N > 5 was observed,^[22]underlining the significant

difference in p–p aggregation properties of the planar and

distorted PBI dyes.

Figure 2. Normalized steady-state fluorescence spectra of compounds 3a–

f in CH₂Cl₂.

Formation of p–p-stacked dimers of PBIs and their photolu-

minescence properties in solutions: The aggregation behav-

ior of PBIs 3a–f was investigated by concentration-depen-

dent UV/Vis spectroscopy in MCH. Upon increasing the

concentration of the dyes pronounced spectral changes, in-

cluding hypochromism (decrease of the apparent absorption

coefficients) and shifts of the absorption peaks were ob-

served (Figure 3 and Figures S1–2in the Supporting Infor-

mation), which clearly indicate the formation of aggregates

in this apolar solvent. For all these dyes, a broadening of the

spectra and a loss of the fine structures were observed upon

By nonlinear regression analysis of the UV/Vis spectral

data with the dimer aggregation model, which has been suc-

cessfully used for other dyes,^[26]the dimerization constants

(K_D) and the Gibbs free energy changes (DG^o) at 298 K

were determined. The results are summarized in Table 2and

a plot of the molar fraction of dimerized dyes a_dimerat differ-

ent concentrations is depicted in Figure 4. The curves calcu-

lated from the dimer model fit very well with the experi-

mental data points. For compound 3e and f with two phen-

oxy and pentafluorophenoxy bay substituents, respectively,

Table 2. Dimerization constants and corresponding Gibbs free energy

changes at 298 K for the dyes 3a–f in MCH.

Compound

K_D[Lmol^À1

]

ÀDG₂^o₉₈[kJmol^À1

]

3a

3b

3c

3d

3e

3 f

30

8.4

9.110²

1.310⁴

4.610³

7.910³

7.310³

16.9

23.5

20.9

22.2

22.0

Figure 3. Concentration-dependent UV/Vis spectra in MCH for a) 3a

(5.210^À6to 7.710^À2m); b) 3b (2.210^À6to 1.210^À2m); c) 3d (5.1

10^À6to 1.410^À2m); and d) 3e (5.110^À7to 2.610^À3m). The arrows indi-

cate the spectral changes upon increasing concentration.

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453

F. Würthner et al.

mirror image of the corresponding UV/Vis absorption spec-

tra (Figure 5 and Figure S5 in the Supporting Information).

These spectra show comparable lineshape as those of the

monomer emission spectra and small bathochromic shifts

Figure 4. Plot of the molar fraction of the dimerized molecules a_dimer

versus the total concentration c_Tof the dyes 3a–e obtained from the con-

centration-dependent UV/Vis spectral data in MCH at 258C (for clarity,

the curve and data points of 3 f are not shown since they completely

overlap with those of 3e). Inset: a_dimerof compound 3d as a function of

c_Tobtained from concentration-dependent UV/Vis spectroscopy (trian-

gles) and VPO (circles) at 448C.

nearly identical K_Dvalues were observed. It is noteworthy

that for 3b and e the aggregation constants are about two

orders of magnitude smaller than those previously observed

for PBIs containing the same substituents at bay positions,

but more electron-rich tridodecyloxyphenyl substituents at

imide positions.^[21]Apparently, in the case of tridodecyloxy-

phenyl-substituted PBIs either better stacking or additional

donor-acceptor interactions are provided by the more elec-

tron-rich alkoxy groups and the electron-deficient perylene

units, which lead to a stronger binding between the chromo-

phores.^[27]Thus, for such compounds much larger aggrega-

tion constants were observed.

Figure 5. Normalized fluorescence spectra in MCH for a) 3a (at 1.010^À6

and 1.410^À2m); b) 3b (at 1.010^À6and 1.010^À2m); c) 3e (at 2.010^À6

and 1.010^À3m); and d) 3 f (at 1.010^À6and 1.010^À2m). In all cases, a

dashed line is used for the lower concentration and a solid line for the

higher concentration.

For a better comparison of the results obtained from UV/

Vis spectroscopy and VPO, the K_Dof 3d was additionally

determined at 448C (the temperature for VPO experiments)

by UV/Vis spectroscopy as a representative example and a

K_Dvalue of 1200m^À1was obtained (Figure 4, inset and

Figure S4 in the Supporting Information). On the other

hand, K_Dcan be estimated at a single concentration with

for the 0–0 bands (9 nm for 3a and 31 nm for 3b). Unlike

the persistent spectral profiles of the tetrasubstituted dyes,

significant changes of the fluorescence spectra were ob-

served for disubstituted 3c–f upon dimerization. For 3c, an

obviously raised band was observed at around 750 nm to-

gether with the monomer emission band. The emission max-

imum of 3d was bathochromically shifted from 532to

569 nm with the loss of fine structure and significant broad-

ening of the spectrum. More significantly, the emission spec-

tra of 3e,f displayed new, broad and unstructured bands

with bathochromic shifts of about 100 nm for both dyes

(2700 cm^À1for 3e and 3000 cm^À1for 3 f). These spectral fea-

tures are similar to those observed previously for aggregates

of dye 6^[22]and can be related to a pronounced structural

and energetical relaxation process of the excited dimer ag-

gregate.^[28]

the equation K_D= (1ÀF)/c

(2FÀ1)²for dimerization equi-

_TA

librium, where F is the osmotic coefficient and equals to the

reciprocal of the average aggregation number N.^[26a]Based

on the N values obtained from the VPO measurements for

3d, K_Dvalues in a range of 670–1600m^À1were estimated at

five different concentrations (Table S1 in the Supporting In-

formation). These values are in good agreement with that

obtained from UV/Vis spectroscopic data. The molar frac-

tion of dimerized molecules a_dimercan be calculated from

the VPO results as a_dimer= 2À2/N and the data points fit

well with the curve obtained from UV/Vis spectroscopic

measurements (Figure 4, inset).

For the two strongly photoluminescent dyes 3b and e

(Table 1), the change of the quantum yields upon dimeriza-

tion was evaluated. The fluorescence spectra of the non-ag-

gregated dye 5 (F_flꢀ1) and 3b were measured at the same

concentration (1.010^À2m, a_dimer=0.8 for 3b) and optical

setup. The integrated fluorescence intensity (area under the

The photoluminescence properties of dyes 3a–f upon di-

merization were studied by steady-state fluorescence spec-

troscopy. The fluorescence spectra of the tetrasubstituted

dyes 3a and b at high concentrations in MCH resemble the

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Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

spectra) of 3b amounts to 92% of that of 5 (after calibration

by using different absorption coefficients at excitation wave-

lengths for 5 and 3b). This result confirms that for 3b the

fluorescence quenching upon aggregation is negligible,

which closely resembles the observation made for an ana-

logue of PBI 3b that contains trialkoxyphenyl groups at

imide positions.^[21]In contrast to that of 3b, the fluorescence

of 3e is drastically quenched upon aggregation. Comparison

of the spectra of 3e and 5 measured under the same condi-

tions (1.010^À3m, a_dimer=0.8 for 3e), the fluorescence quan-

tum yield of 3e is reduced from 0.87 (Table 1) for the mono-

mer to only ꢀ0.19 upon aggregation. Based on these data

and taking into account about 80% of 3e is dimerized in

1.010^À3m solution, a fluorescence quantum yield of 0.02

can be calculated for the dimeric species. The shape-persis-

tent and nondiminished fluorescence of 3b on aggregation,

on one hand, and the drastically quenched fluorescence of

3e with large Stokes shift and unstructured new band, on

the other hand, imply different stacking and relaxation

modes of these dyes in their aggregates (see Discussion).

Formation and optical properties of columnar liquid crystal-

line phases: The combination of large-sized aromatic cores

and flexible long alkyl chains has a strong effect on the self-

organization properties of bulk materials and may lead to

the formation of columnar liquid crystalline (LC) phases

due to the anisometric shape of the molecules and micro-

segregation of alkyl chains and aromatic cores.^[29]The ther-

motropic behavior of the PBI dyes 3a–f was studied by dif-

ferential scanning calorimetry (DSC), polarizing optical mi-

croscopy (POM), and X-ray diffraction (XRD). These stud-

ies revealed that the halogen- and 4-tert-butylphenoxy-sub-

stituted dyes 3a, b, d, and e form liquid crystalline meso-

phases upon cooling from the melt, while only formation of

an isotropic glass was observed for compounds 3c and f with

pyrrolidine and pentafluorophenoxy substituents, respective-

ly. The 4-tert-butylphenoxy-substituted dyes 3b and e were

isolated as viscous birefringent substances in contrast to the

halogen-substituted compounds 3a and d which are crystal-

line at room temperature. Only one reversible transition

from the birefringent phase to the isotropic liquid phase was

observed for the phenoxy-substituted dyes 3b and e (see

Figure 6b,d), indicating that only one LC phase is formed

and that there is no crystallization or only a very slow crys-

tallization of these mesophases. The DSC heating and cool-

ing curves of the dibromo-substituted compound 3d are dif-

ferent (Figure 6c). For this compound, in addition to a small

peak corresponding to the transition from the LC to the iso-

tropic liquid state at 1608C (DH=2.7 kJmol^À1), several

overlapping endothermic transitions were observed in the

first heating cycle around 808C between the pristine crystal-

line phase and the LC phase (ꢀDH=127 kJmol^À1). Broad

transitions with similar enthalpy changes at the same tem-

perature range were previously reported for other liquid

crystalline PBIs.^[17b]No subsequent recrystallization was ob-

served for the LC phases of the present PBIs at the em-

ployed cooling rate and exclusively the Iso-to-LC transition

Figure 6. DSC curves (108C min^À1) obtained for a) 3a, b) 3b, c) 3d and

d) 3e for the first heating-cooling cycle (solid lines) and the second cycle

(dashed lines).

is seen in the second and following heating curves. The most

complex DSC thermogram was observed for the tetrachloro

compound 3a (Figure 6a) which showed upon first heating

two crystal–crystal transitions, prior to the melting transition

at 1108C. No corresponding transitions occurred in the fol-

lowing cooling and heating scans. Upon cooling a quite

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455

F. Würthner et al.

sharp transition occurs at 808C, whereas the second and all

following heating scans are characterized by a rather broad

transition at 1058C. This indicates a supercooling, which is

often observed for columnar mesophases of relatively large

disc-like mesogens and a rather high degree of intracolum-

nar order.

The thermodynamic properties of the LC phases of com-

pounds 3a, 3b, 3d, and 3e are summarized in Table 3. The

entropy and enthalpy changes associated with the meso-

phase to isotropic transitions of the disubstituted com-

pounds 3d and 3e are much lower compared with those of

the tetrasubstituted compounds 3a and 3b bearing similar

substituents. For instance, the DH value of the dibromo-sub-

stituted compound 3d is about one third of that for the tet-

rachloro-substituted compound 3a and the ratio of DH for

the diphenoxy- (3e) and tetraphenoxy-substituted (3b) com-

pounds is about 1:4. A similar trend was observed for the

DS values, which hints at a higher degree of order in the

mesophases of the tetrasubstituted compounds 3a and b

compared with the disubstituted dyes 3d and e.

Table 3. Clearing temperature T, transition enthalpies DH, and entropies

DS for the LC-isotropic transitions.

Figure 7. Optical textures of the liquid crystalline phases of a, b) 3a at

708C; c, d) 3b at 2508C; e) 3d at 1108C and f) 3e at 1058C as seen be-

tween crossed polarizers (the directions of polarizer and analyzer is

shown by the arrows in f); a, c, e, f) show the textures obtained after

slow cooling from the isotropic liquid, b, d) show the textures obtained

after shearing the samples a) and c), respectively. Image size: 0.86 mm

0.65 mm for a), b) and e)–f), 0.53 mm0.40 mm for c).

Compound Mesophase T_LC-Iso[8C] DH [kJmol^À1

]

DS [Jmol^À1K^À1

]

3a

3b

3d

3e

Col_ro

Col_ho

Col_hd

110

285

160

130

8.4

23.5

2.7

21.9

42.1

6.2

5.3

13.2

For these two groups of materials, there are also signifi-

More detailed information were obtained from X-ray dif-

fraction measurements, which were performed for bulk sam-

ples of compounds 3a, b, d, and e in the temperature re-

gions of their mesophases.

cant differences in the textures of the mesophases seen

under a polarizing microscope between crossed polarizers.

The two disubstituted compounds 3d and e display spheru-

litic textures that are very typical for columnar LC phases

(Figure 7e,f). Extinction crosses parallel to polarizer and an-

alyzer were observed which implies that the columns in the

domains are parallel to the substrate and bend into circles

around the brush center.^[30]The position of the extinction

brushes indicates that there is no uniform tilt of the aromat-

ic cores within the columns.

In contrast, for the tetrasubstituted compounds 3a and b

the formation of lancet-like domains was observed (Figure

7a,c), which reflect a certain rigidity of the columns. How-

ever, for both compounds also some spherulites coexist with

the lancet-like domains (see for example Figure 7c), indicat-

ing that bending of the columns is still possible. Also in

these spherulites the extinction crosses are parallel to polar-

izer and analyzer, again indicating the absence of a uniform

tilt. In all cases shearing of the samples is possible and this

gives rise to complete removal of the well-developed tex-

tures, which are replaced by nonspecific highly birefringent

and nearly homogeneously aligned textures (columns pre-

dominately parallel to the substrate), as shown in Figure 7b

and d for compounds 3a and b, respectively. This clearly in-

dicates the fluid liquid crystalline character of the meso-

phases of all four compounds.

The Guinier powder pattern of the diphenoxy-substituted

compound 3e at 808C shows two reflections with a ratio of

1

the d values of 1:1/3 ⁼(see Table S2in the Supporting Infor-

2

mation) indicating a hexagonal two-dimensional lattice. In

the wide angle region, there is a diffuse scattering, revealing

the fluid liquid crystalline state of the mesophase. Hence,

the phase can be assigned as hexagonal columnar LC phase

with only short-range order of the molecules along the

stacks (Col_hdphase).

For the tetraphenoxy-substituted dye 3b, the three reflec-

tions in the small angle region have a ratio of the reciprocal

1

=

2

d spacings of 1:3 :2which have been indexed as the 10, 11,

and 20 reflections of a two-dimensional hexagonal lattice

(Guinier powder pattern at 1508C, see Table S2in the Sup-

porting Information). As for 3e, the wide angle region is

characterized by a diffuse scattering, but in this case an ad-

ditional rather sharp reflection can be observed, which

could be attributed to a periodic intracolumnar distance of

the molecules along the stacking direction. This stacking

period of d=0.45 nm is identical with that for the analogous

PBI dye with tridodecyloxylphenyl groups at the imide posi-

tions.^[21]Hence, the LC phase at this temperature can be as-

signed as Col_ho.

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Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

A similar phase assignment results from the Guinier

powder pattern and the 2D images of the scattering of an

aligned sample of the dibromo-substituted dye 3d (Col_ho

phase, see Figure 8a,b). The d value for the outer sharp re-

flection of 0.38 nm is considerably smaller than that seen for

the tetrasubstituted compound 3b. This is reasonable, be-

cause there are only two bromine atoms in the bay positions

which should allow a denser packing of the perylene cores

than the four more bulky tert-butylphenoxy groups in com-

pound 3b. The 2D diffraction patterns of partially aligned

samples are shown in Figure 8a,b. In this case the alignment

was achieved by slow cooling of a small droplet of the mate-

rial on a glass substrate. The X-ray beam was applied paral-

lel to the substrate surface. In both samples the columns are

aligned on the substrate with their long axes parallel to the

substrate, whereas the direction of the X-ray beam with re-

spect to the majority of the columns is different in Figure 8a

and b. In the diffraction pattern shown in Figure 8a, the X-

ray beam is nearly parallel to the long axes of most of the

columns and in this alignment the hexagonal lattice is seen.

In the diffraction pattern 8b, the columns are predominately

perpendicular to the X-ray beam (the columns are nearly

horizontal, but slightly inclined). In this diffraction pattern

the rather sharp reflections at d=0.38 nm are seen as lines

parallel to the meridian, located on the equator (see also

the c scan along the equator in Figure S6a in the Supporting

Information). This proofs that the scattering at d=0.38 nm

is due to a repeat distance within the columns (p–p stack-

ing) and that the molecules adopt a non-tilted organization

within the columns. The elongation of the reflections per-

pendicular to the equator shows, that there is no correlation

of this intracolumnar stacking between the columns, that is,

there are arbitrary displacements along the long axes of

neighboring columns or they can freely shift with respect to

each other along their long axes.^[31]

Figure 8. X-ray diffraction patterns a) of a surface-aligned sample of 3d

at 1408C (X-ray beam parallel to the columns) showing the hexagonal

lattice in the small angle region with indices of the observed reflections;

b) of a partially surface-aligned sample of 3d at 1408C with X-ray beam

perpendicular to most of the columns showing the intracolumnar stacking

(for c scans, see Figure S6a,b in the Supporting Information);^[31]c) for

compound 3a at 908C showing the Miller indices of the small-angle re-

flections and the maxima of the wide-angle scattering (a, b, and c).

The X-ray diffraction pattern of the tetrachloro com-

pound 3a is different from those of the other compounds

and more complex. Figure 8c shows the diffraction pattern

for a sample at 908C as obtained upon cooling from the iso-

tropic phase. The five Bragg reflections observed in the

small angle region can be indexed according to a two-di-

mensional centered rectangular lattice (c2mm) with satisfac-

maximum c), corresponding to a d value of about 0.34 nm.

To our best understanding, this unusual diffraction could be

attributed to Cl–Cl close contacts in the stacks.^[33]Taking the

repeat distance of the perylene cores along the columnar

axis (d=0.43 nm) as the c axes of a three-dimensional or-

thorhombic unit cell (Table 4), its volume can be calculated

to V_u=5.8 nm³. The volume of one molecule in the packing

[32]

tory accuracy (Table S2in the Supporting Information).

In the wide angle region, there are two diffuse scatterings

and one more sharp scattering. The comparatively sharp

peak at 2q=218 (peak b) is indicative of a periodicity in the

structure with a repeat distance of 0.43 nm. Since only

powder-like samples have been investigated, no direct prove

could be obtained from the X-ray measurements for the di-

rection of this periodicity within the structure. However, the

d value of 0.43 nm is characteristic for a close contact of the

p systems in perylene derivatives^[17b]and this can be taken

as a hint to a columnar phase with the perylene cores peri-

odically packed along the columnar axis. Interestingly,

beside the diffuse halo at 2q=188 which could be attributed

to “liquid-like” disordered alkyl chains, there is a second dif-

fuse halo at 2q ꢀ268 (as a rough estimate, see Figure 8c,

Table 4. LC phases, measured temperature and the corresponding lattice

parameters a and b, and the repeat distance along the columns c.

Compound

LC phase

T [8C]

Lattice parameters [nm]

a

b

c

3a

3b

3d

3e

Col_ro

Col_ho

Col_hd

90

4.59

3.20

3.020.38

3.04

2.92

0.43

0.45

150

140

80

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457

F. Würthner et al.

has been estimated according to Immirzi et al.^[34]and

amounts to 2.6 nm³. Accordingly, there should be about two

molecules in the unit cell, supporting the assumption of a

centered lattice. According to these XRD data, the meso-

phase of compound 3a is assigned as Col_ro.

the peak on the equator of the pattern in Figure 8b (Dq=

0.98 nm^À1, see Figure S6c, Supporting Information) to x=1/

Dq ꢀ1 nm, with a distance of the molecules within the col-

umns of 0.38 nm the correlation reaches over about three

molecules.^[36]

The unit cell parameters of the columnar LC phases of all

four mesogenic compounds are summarized in Table 4. For

the hexagonal columnar phases of compounds 3b, d, and e

the diameter of the columns (corresponds to a_hex) are 3.2,

3.0, and 3.0 nm, respectively. These diameters are signifi-

cantly shorter than the long molecular axes (about 5 nm) of

the dyes bearing alkyl chains in their fully extended all-trans

conformation. The difference could be attributed to folding

and interdigitation of the alkyl chains as well as rotational

offsets along the stacking axis of the dyes.^[22,35]This leads to

an in average circular cross section of the columns which

gives rise to the packing in a hexagonal lattice. Only the tet-

rachloro-substituted dye 3a shows a rectangular columnar

phase with a c2mm lattice, which can be regarded as a

slightly distorted hexagonal lattice (the average diameter of

the columns is also about 3.0 nm). In the case of disc-like

molecules, the formation of a rectangular columnar phase is

often associated with a tilted organization of the disc-like

cores in the columns, which gives rise to an elliptical shape

of the column cross section. For the PBI 3a, however, there

is no indication of a tilted organization of the perylene cores

(see discussion of the optical textures). Moreover, the pery-

lene core of compound 3a itself is not disc-like, but it has a

twisted conformation (see Discussion), so that an additional

tilt of these cores within the columns is not very likely. It is

more reasonable to assume that the distortion might arise

from the restricted rotational disorder of the perylene cores

due to the strong twist in the core (see Discussion). The

close Cl–Cl interactions along the columns are possible also

due to the segregation of these halogen regions from the re-

gions of the flexible alkyl chains. This means that the rota-

tional disorder of the twisted board-like PBIs around the

column axis is reduced.

There are also clear differences in the X-ray wide angle

scattering for the diphenoxy-substituted dye 3e, on one

hand, and compounds 3a, b, and d on the other. PBI 3e

shows only a broad diffuse scattering (Col_hd), whereas for

the other three compounds a much narrower peak is super-

imposed. This indicates that for compounds 3a, b, and d

there is a periodicity (with medium-range correlation)

caused by the interaction of the p systems. The appearance

of the comparatively sharp reflection at about 0.45 is the

criterion used for the assignment of these phases as Col_ho

and Col_ro. However, it should be pointed out that in general

there is no distinct transition between Col_hoand Col_hd

phases. Instead, there is a continuous increase of the correla-

tion length of the periodicity within the columns. In the

Col_ho/Col_rophases of the compounds reported herein, the

full width at half maximum (FWHM) is still rather large,

thus the correlation length should not exceed a few mole-

cules. A rough estimate for the correlation length in the

case of 3d may be calculated from the FWHM value Dq of

It is also noteworthy that the isotropization temperatures

of all compounds are about 60–1508C lower than those for

the previously reported structurally similar PBIs with dode-

cyloxy side chains.^[17b,21]This relates well to the lower bind-

ing constants observed for aggregate formation in solution

and indicates smaller cohesive forces between the p–p-

stacked molecules. Also the substituents at the bay area

have an effect on the clearing point. Thus, the isotropization

temperature of the tetrachloro-substituted dye 3a (1108C) is

about 1908C lower than that of the unsubstituted PBI 6

(3048C)^[22b]with the same imide substituents. Considering

2008C as an upper limit for the use of flexible plastic sub-

strates in device fabrication,^[37]the compounds 3a, d, and e

are very promising for such applications because they pos-

sess relatively low isotropization temperatures in the range

of 110–1608C.

Furthermore, UV/Vis and photoluminescence studies

were carried out for LC thin films of these dyes. As shown

in Figure 9, the UV/Vis spectrum of a solution-cast thin film

of dye 3a displayed an absorption maximum at 493 nm with

two less intense absorption bands at around 533 and

414 nm, respectively. However, no fluorescence could be ob-

served for this thin film. Upon annealing at 1058C, the

sample transformed into the LC phase according to the mor-

phological changes under the optical polarizing microscope

(cp. also DSC, Figure 6a) and pronounced spectral changes

occurred (the optical textures of the thin films correspond-

ing to the spectra in Figure 9a are shown in Figure S7 in the

Supporting Information). Thus, the absorption maximum for

the LC phase appeared now at 537 nm which is bathochrom-

ically shifted by 44 nm with respect to the pristine state.

Concomitantly, the initially non-fluorescent thin film

became fluorescent upon annealing. The fluorescence spec-

trum shows an emission maximum at 554 nm with well-re-

solved vibronic structure and has a nearly identical line-

shape with that observed for aggregates of 3a in MCH, but

with a bathochromic shift of 9 nm. The different optical

properties of the solution-cast and annealed thin films of 3a

reflect the different packing feature of 3a molecules in pris-

tine state and LC phase.

In the case of phenoxy-substituted dyes 3b and e, only

one phase below the isotropization point can be observed

by DSC measurements. Thus, the solution-cast thin films of

these compounds are in LC state. For 3b, the emission spec-

trum of the thin film is a mirror image of the S₀–S₁absorp-

tion band with a Stokes shift of 32nm. In comparison with

the dimer spectra in MCH, the spectra in thin film are

slightly bathochromically shifted. For dye 3e, the lineshape

of absorption spectrum of the thin film is highly comparable

with that of the dimer, apart from a bathochromic shift of

about 35 nm. In the fluorescence spectrum of the thin film

of 3e, a broad red-shifted band around 680 nm was ob-

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Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

distorted.^{[17,23,38b–d]}Depending on the number and steric

demand of the substituents, twist angles (q) of 2 4 to 387

were found in the solid state for bay-substituted PBIs. To

our knowledge, crystal structures for diphenoxy-, dipenta-

fluorophenoxy-, and dipyrrolidinyl-substituted PBIs are not

reported in literature. However, as twist angles assessed by

semiempirical AM1 calculations for the other four com-

pounds agree well with those determined by X-ray analysis

(see Table 5), these values can be estimated with high fideli-

ty.

Table 5. Experimental and calculated twist angles of bay-substituted

PBIs.

Structure

Bay substituents

q [8]^[a]

q [8]^[b]

without substituents

1,7-diphenoxy

1,7-dipentafluorophenoxy

1,7-dibromo

1,7-dipyrrolidinyl

1,6,7,12-tetrachloro

1,6,7,12-tetraphenoxy

0^[38a,b]

0

[c]

15

18

24

25

36

27

[c]

24^[23]

Figure 9. a) Normalized UV/Vis absorption spectrum of the thin film of

3a cast from MCH solution (dashed line) together with the UV/Vis ab-

sorption and fluorescence spectra of the LC thin film (after annealing,

solid lines). b) Normalized UV/Vis absorption and fluorescence spectra

of the thin film of 3b cast from MCH solution. c) Normalized UV/Vis ab-

sorption and fluorescence spectra of the thin film of 3e cast from MCH

solution.

[c]

37^[17a]

25^[38b–d]

[a] Twist angle (dihedral angle associated with the 4 carbon atoms C6-

C6’-C7’-C7) in the perylene core determined by single crystal X-ray anal-

ysis. [b] Calculated by semiempirical AM1 method (CAChe 5.0). [c] Data

not available.

served, which was about 35 nm bathochromically shifted

with respect to that observed in MCH solution.

For the dibromo-substituted dye 3d, despite the existence

of a crystal-LC transition, no spectral changes can be ob-

served in the thin film cast from MCH solution upon an-

nealing. The UV/Vis spectrum displays a l_maxat 483 nm with

a lower energy band at 540 nm (Figure S8 in the Supporting

Information). The thin film is weakly fluorescent with an

emission maximum at 624 nm. The observation of broad

emission bands of the thin film of 3d, e with very large

Stokes shifts (>140 nm) and the fact that no fine structure,

in contrast to that of dyes 3a, b, was observed clearly indi-

cate that the p–p stacking of the dyes 3d,e is distinct from

that of 3a,b in the LC state.

The different aggregation behavior of planar and core-

twisted PBI dyes, that is, formation of p–p-stacked dimers

versus extended p–p stacks, respectively, is a consequence

of different intermolecular interactions originating from dis-

tinct geometry. In flat PBI 6 monomer, the two p surfaces

are equal with respect to the approach of another molecule

from the top or bottom face to form dimers and the dimers

have the same feature as the monomers, thus, extended ag-

gregates can be formed. However, this situation is changed

when the molecular plane is twisted due to the substituents

at bay area. For example, the structure of dipyrrolidinyl-sub-

stituted PBI obtained from molecular modeling shows that

the two substituents point toward the same side of the mo-

lecular plane (Figure 10a), making the two p-surfaces of the

molecule distinct. One side is sterically much more encum-

bered than the other side due to the bay substituents. Thus,

in the self-assembly process the two sterically less hindered

faces of the molecules stack together to form an energy-

minimized dimeric unit through larger p–p overlap and to

minimize the energy of the system. Indeed, such dimeric

units are found in the crystal structure of compound 4.^[23]As

revealed by this structure, a rotational displacement around

the stacking axis may reduce the steric congestion of the

substituents and enforce the p–p contact (Figure 10b). Fur-

ther aggregation of such dimeric units should be disfavored

due to the steric hindrance of the bulky substituents at the

accessible p faces. Accordingly, the dimerization of bay-sub-

stituted PBI dyes in solution is favored with respect to fur-

ther aggregation.

Discussion

Here we will discuss elaborately some of our pertinent re-

sults presented in the preceding sections. The concentration-

dependent UV/Vis spectroscopic studies and VPO measure-

ments have clearly shown that the present bay-substituted

PBIs 3a–f favorably aggregate to give p–p-stacked dimers

in MCH at millimolar concentrations. By contrast, at bay

area unsubstituted PBI 6 formed extended oligomeric p–p

stacks under comparable conditions according to our recent

investigation.^[22]This distinct aggregation behavior of at bay

area substituted versus unsubstituted PBIs can be explained

in terms of their quite different molecular geometry. X-ray

crystallographic analysis showed that the p system of unsub-

stituted PBIs is perfectly planar,^[38a,b]while the perylene

cores of bay-substituted derivatives, which contain the same

bay substituents as the present dyes 3a–d, are significantly

Chem. Eur. J. 2007, 13, 450 – 465

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459

F. Würthner et al.

pends on the geometry of the perylene bisimide core and

the respective bay substituents have only a minor effect on

the p–p stacking energy (with the exception of 3c). This is a

reasonable result because for aggregates of twisted p sys-

tems we have to expect either an increased distance be-

tween the p planes (as observed in the LC phases of 3b,d)

or a pronounced displacement between the dyes leading to

a smaller p–p contact area (as observed in the LC phase of

3a).

Indeed, the core twisting of these PBIs has significant

effect on the molecular packing in the LC phase. Dye 3a

with the largest twist angle (378) formed a Col_rophase, while

for the less distorted dyes 3b (278) and 3d (248), Col_ho

phases were observed. The diphenoxy-substituted 3e (twist

angle 158) and the planar PBI 6 showed a Col_hdphase. Thus,

obviously, in the case of less twisted dyes, the intracolumnar

order decreases and the lattice structure changes from rec-

tangular to hexagonal. A pronounced distortion of the skel-

eton interferes with the translational and rotational fluctua-

tion of the molecules in the columns and thereby increases

the ordering within the columns, which is also reflected in

the optical texture of the LC phases of such dyes (Figure 7)

as for 3a and b lancet-like domains were observed, while for

3d and e, spherulitic textures were observed. For dye 3a,

the rotational displacement of the dye molecules, that is a

prerequisite for the formation of a hexagonal lattice is hin-

dered, due to the bulky chlorine atoms. In contrast, these

dyes stack on top of each other with longitudinal offsets

along the long molecular axis to form a rectangular phase

(Figure 12). With four bulky phenoxy substituents the core

of 3b is nearly disk-like. Taking into account a tilt of the

molecular cores with respect to the columnar axis as found

for the analogous compound with alkoxy side chains,^[21]rota-

tional disorder is not necessarily needed (even if possible to

some extent) to form columns with a circular cross-section.

Conversely, such a rotational displacement is necessary for

the PBI molecules 3d and e to compensate the considerably

differing dimensions of the long and short axis of the mole-

cules.^[39]This assumption is supported by the observation of

identical column diameters for the dyes 3d and e, although

the bay substituents in these compounds are quite different

(dibromo for 3d and diphenoxy for 3e). For the crystal

structure of dibromo-substituted PBI 4,^[23]a rotational dis-

placement of about 358 (Figure 10b) between the long axis

of the molecules was observed, implying that such displace-

ment could be also feasible in the LC state of disubstituted

PBIs.

Figure 10. a) Molecular structure of dipyrrolidinyl-substituted PBI (view

along the N–N axis) obtained from molecular modeling. b) Top view of

the dimeric unit of compound 4 in crystal structure with a rotational dis-

placement of about 358. c) Side view of the dimeric unit of compound 4

in crystal structure.^[23]

Our results have shown that the aggregation constants of

the bay-substituted PBI dyes are 2–4 orders of magnitude

smaller than those of the corresponding unsubstituted dyes

with the same imide substituents. This can be attributed to

the reduced p–p interaction energy in the case of bay-substi-

tuted PBIs due to the twisting of the perylene core. Indeed,

a good correlation between the twist angle and the Gibbs

free energy of p–p stacking, except for 3c, was observed

(Figure 11). Upon increasing the twist angle from 08 for the

unsubstituted PBI 6 to the 378 for the tetrachloro-substitut-

ed PBI 3a, the aggregation constant decreased strongly.

Thus, the degree of twisting has a pronounced effect on PBI

dye aggregation through p–p interaction. The rather good

correlation between DG8 and q, despite the different bay-

substituents, indicates that the magnitude of DG8 mainly de-

The twisted p core not only plays a decisive role for the

structure and thermodynamic stability of p–p-aggregated

PBIs, but it has also a strong effect on the excited state

properties of PBI aggregates. For all the present dyes, bath-

ochromic shifts of the aggregate emission bands with respect

to monomer emission were observed, indicating a lowering

energy of the emissive aggregate state compared to the mo-

nomer species. The reduction of the energy can be quanti-

fied by the spectroscopic energy difference Dn (in wavenum-

bers) between the monomer (0–0) emission maximum and

Figure 11. Plot of the Gibbs dimerization energy DG8 against the twisting

angle q of the bay-substituted PBIs 3a–f and 6 obtained from calculation

(Table 5).

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Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

the value of the Stokes shift between the absorption and

emission maximum (875 cm^À1). Accordingly, we can con-

clude that the energetic relaxation process in the excited

state is not significantly more pronounced in dimer aggre-

gates of the distorted dyes 3a, b than for their monomeric

species. On the other hand, an explanation is requested for

the very significant stabilization of the excited state in the

case of dimer aggregates of the more flat derivatives 3e

and f.

The above suggested p–p-stacking pattern of the core-

twisted dyes in their LC phase and the observed UV/Vis

spectroscopic features of p-stacked dyes in solution and thin

film can be rationalized by molecular exciton theory.^[41]

Based on this theory, excitonic coupling between closely

stacked chromophores causes changes in the optical spectra

as observed for various types of dyes, including PBIs. It has

been pointed out that either hypsochromic or bathochromic

shifts of the absorption maximum with respect to that of the

monomer can occur depending on the stacking geometry

and the selection rule for the respective optical transitions.

In the most simple case, when two dyes are stacked on top

of each other with parallel transition dipole moments only

one out of two excitonic transitions is allowed. In most

cases, this is the transition to the higher energy state (corre-

sponding to a hypsochromically shifted, that is, H-type

band) and only in rare cases of pronounced longitudinal dis-

placements between the two dyes it is the transition to the

lower energy state (corresponding to the bathochromically

shifted J-type band).^[42]On the other hand, when the transi-

tion dipoles are rotationally displaced both bathochromical-

ly and hypsochromically shifted bands can be observed.^[41]

This situation has been observed for the vast majority of ag-

gregates of unsubstituted PBIs and recently elucidated by

calculated spectra for a variety of possible geometries based

on molecular exciton theory.^[25]

In contrast to the parent flat PBIs, for 3a and b only

small bathochromic shifts and only minor changes in the

band shape were observed upon aggregation in the concen-

tration-dependent UV/Vis spectra in MCH. Both observa-

tions imply that the excitonic couplings are weak in the

dimers. This is a reasonable result because these twisted p

cores cannot approach into close proximity for steric rea-

sons. In addition, according to the packing model in Fig-

ure 12a, the longitudinal displacement is characterized by a

slipping angle close to the magic angle 54.78. Accordingly,

the electronic coupling remains small for these dye aggre-

gates.^[41]In contrast, significantly stronger excitonic coupling

effects are observed for the aggregates of the less twisted

dye 3c–f. The most evident features of their spectra are hyp-

sochromically shifted absorption maxima in concentration-

dependent UV/Vis spectra in MCH and in LC phases that

is, however, accompanied with the simultaneous occurrence

of another absorption band or at least a shoulder at longer

wavelengths (Figure 3 and Figure S1–2in the Supporting In-

formation). This pattern is indicative of cofacial-type dimers

of these dyes with rotational displacements among the long

molecular axis.^[25,41]

Figure 12. Schematic representation of the proposed packing patterns

and the orientation of the transition dipole moments in columnar LC

phases of the core-twisted PBI dyes: a) J-type p–p stacking of 3a, and b)

cofacial p–p stacking of 3d,e.

the maximum of the aggregate emission band (a quantity in-

troduced to estimate the stabilization energy for exci-

mers^[40]). As shown in Figure 13, this energy difference Dn is

highly dependent on the twist angle q. The planar dye 6 ex-

hibits the largest Dn value with a bathochromic shift of

4600 cm^À1.^[22]Upon twisting of the perylene skeleton the Dn

value decreases approximately linearly, despite different

electronic properties of the bay substituents. For the dyes 3e

and f bearing only two phenoxy substituents at the perylene

core, the bathochromic shift is still very significant while it is

of minor importance for the dyes with four bay substituents.

For instance, the value Dn=309 cm^À1for 3a is smaller than

Figure 13. Plot of the energy difference Dn (in wavenumbers) between

the 0–0 emission band of the monomeric dyes and the emission maxima

of the aggregated dyes in MCH versus the core twisting angle q.

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F. Würthner et al.

Whilst calculations based on the excitonic coupling theory

could successfully describe the spectral changes arising in

the UV/Vis absorption spectra for flat PBI upon dimer ag-

gregate formation,^[25]they cannot explain the far more pro-

nounced bathochromic shifts in the fluorescence spectra that

are observed for the flat PBI 6^[22a]or the slightly twisted

PBIs 3c–f. Because these fluorescence spectra exhibit many

features that are typically observed for excimers,^[28]that is, a

pronounced bathochromic shift and a structureless broad

band, we suggest that a major structural reorganization pro-

cess takes places in these dimer aggregates from the original

Franck-Condon excited state to the final energetically re-

laxed state from which emission occurs. Most likely, the p–p

distance is reduced in this relaxed state and, thus the elec-

tronic coupling between the two dyes is increased. Because

the radiative transition from such an excited state leads to

an energetically unfavorable state on the ground state po-

tential surface, the fluorescence spectra resemble those ob-

served for excimers.^[28]Therefore, even that the dimeric spe-

cies of PBIs 3c–f are stable in the ground state, bathochrom-

ically shifted broad and structureless emission bands are ob-

served for these dyes.

A very interesting phenomenon is the “switch on” of the

fluorescence of compound 3a during the transition from the

pristine phase into the LC phase upon annealing. This is in-

dicative for a process, in which the chromophore packing is

changed to a J-type geometry (Figure 12a) with transition

dipole moments parallel.^[42]In the pristine thin film, the dye

molecules may stack in a more H-type geometry where the

lowest energy transition is forbidden and accordingly no

fluorescence is observed.^[42]Upon phase transition, the dyes

are slipped into a more longitudinally displaced geometry

(Figure 12a), leading to the formation of luminescent J-type

aggregates. It is noteworthy that such spectral changes might

be of interest for various applications, for example, to record

the thermal history of material or to inscribe information into

a recording medium (e.g., CD-ROM) by the thermal effect of

laser light. Similarly, dye 3b also exhibits the features of a J-

type packing in the LC phase according to the UV/Vis ab-

sorption and fluorescence spectra of the LC thin film in

Figure 9. On the other hand, thin LC films of 3d,e show both

higher and lower energy bands where the higher energy

bands possess the larger oscillator strengths (Figure 9c and

Figure S8 in the Supporting Information). This is in good

agreement with our packing model that suggests rotational

displacements between the transition dipoles. In such colum-

nar p–p stacks, a pronounced relaxation process of the excit-

ed state may occur as suggested by the large Stokes shifts and

excimer-type emissions of these two dyes. Interestingly, stack-

ing by rotational displacement seems to be more favored by

the less twisted disubstituted PBIs than by the strongly twist-

ed tetrasubstituted dyes, which prefer a longitudinal offset

giving rise to J-type spectra. Although only few examples of

the screw-type stacking were observed in the crystal struc-

tures of PBIs,^[38b]rotational displacements play an important

role in the packing of these dyes in LC phases as observed

here for 3d,e and previously for unsubstituted PBIs.^[21,22]

Conclusion

A series of new, highly photoluminescent core-twisted PBI

dyes were synthesized that bear trialkylphenyl groups at the

imide positions and two or four substituents at bay area.

These compounds are characterized by distortions of the

perylene planes with dihedral angles in the range of 15–408

according to crystallographic data and molecular modeling

studies. In contrast to the extended oligomeric p stacks

formed for planar unsubstituted PBIs, aggregation of this

family of dyes was limited to p–p-stacked dimers in apolar

solvent due to the distorted aromatic core. Upon excitation

of these solutions, fluorescence was observed for the dimer

aggregates of all these compounds. Even in condensed state,

several PBIs exhibit luminescence as well as mesophase

properties, that is, they form thermotropic columnar liquid

crystal phases. The core twisting has a significant effect on

the p–p-stacking mode of the molecules in solution and in

the columnar stacks of the LC phases. In both cases, the

more twisted tetrasubstituted dyes prefer longitudinally slip-

ped stacks of the dyes with J-type emissive properties, whilst

for the less twisted disubstituted dyes, the feature of rota-

tional displacement between the molecules in cofacial aggre-

gates was confirmed. A remarkable linear relationship be-

tween the dihedral angle and the ground state dimerization

Gibbs free energy as well as the bathochromic shift of the

aggregate emission band has been found. The relationship

between the degree of distortion of the aromatic core and

its self-assembly properties demonstrated here for these

PBIs provides useful information for the supramolecular

control of electronic and optical properties of aggregates of

this type of functional dyes. Possessing high fluorescence

quantum yields, low isotropization temperatures, n-type

semiconducting and unique self-assembly properties, these

dyes are promising materials for organic electronics.

Experimental Section

Materials and methods: 1,7-Dipyrrolidinylperylene-3,4:9,10-tetracarbox-

ylic acid bisanhydride (1c) was synthesized by the procedures described

previously.^[23]1,7-Dibromoperylene-3,4:9,10-tetracarboxylic acid bisanhy-

dride (1d) was synthesized by saponification of isomer-free N,N’-dicyclo-

hexyl-1,7-dibromo PBI 4. NMR spectra were recorded at 300 K on

Bruker 400 MHz or 600 MHz spectrometers by using TMS (d=0.0) as in-

ternal standard. The solvents for spectroscopic studies were of spectro-

scopic grade and used as received. UV/Vis spectra were measured on

Lambda 40P spectrometer equipped with a Peltier System as the temper-

ature controller. The vapor pressure osmometry (VPO) measurements

were performed on a Knauer osmometer with a universal temperature

measurement unit. Benzil was used as standard and a calibration curve in

terms of R in Ohm versus molal osmotic concentration (moles per kg

MCH) was constructed up to 0.01 molal. DSC measurements were per-

formed using a TA Q1000 calorimeter with a heating/cooling rate of

108C min^À1. At least two heating–cooling cycles were performed for each

compound. Optical textures of the LC phases at crossed polarizers were

obtained with an Olympus BX-41 polarization microscope equipped with

a Linkam THMS 600 hot stage and a temperature controller unit.

X-ray diffraction of LC: Powder X-ray investigations were carried out

with a Guinier film camera (Huber Diffraktionstechnik, Germany). Sam-

462

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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

ples were kept in glass capillaries with a diameter of 1 mm (exception:

sample of 3b on a Be plate) in a temperature-controlled heating stage,

quartz-monochromatized Cu_Karadiation was used (30 to 60 min exposure

decylaniline (0.26 g, 0.43 mmol) and zinc acetate (0.040 g, 0.22 mmol).

According to the procedure described above for compound 3a, a green

viscous substance was obtained (210 mg, 65%). ¹H NMR (400 MHz,

CDCl₃, 300 K, TMS): d=8.55 (s, 2H, H_pery), 8.49 (d, 2H, J=8.1 Hz,

time, calibration with the powder pattern of PbACHTREU(GN NO₃)₂). 2D patterns were

H

_pery), 7.79 (d, 2H, J=8.1 Hz, H_pery), 6.95 (s, 4H, Ar-H), 3.78 (broad, 4H,

recorded for aligned samples on a glass plate on a temperature-control-

led heating stage (alignment at the sample/glass or sample/air interface)

with a 2D detector (HI-STAR, Siemens) using Cu_Karadiation mono-

chromatized by a Ni filter.

H_Pyrrolidinyl), 2.88 (broad, 4H, H_Pyrrolidinyl), 2.65 (m, 12H, Ar-CH₂), 2.05

(broad, 8H, H_Pyrrolidinyl), 1.8–1.2(m, 120H, CH ₂), 0.88 (m, 18H, CH₃); MS

(FAB, matrix: p-octyloxynitrobenzene): m/z: calcd for C₁₁₆H₁₇₆N₄O₄:

1689.4; found: 1689.6 [M]⁺; elemental analysis (%) calcd for

Fluorescence measurements: The steady state fluorescence spectra were

C

₁₁₆H₁₇₆N₄O₄: C 82.41, H 10.49, N, 3.31; found: C 83.14, H 10.68, N, 3.29;

recorded on

a PTI QM4/2003 spectrofluorometer. The fluorescence

quantum yields were determined by optical dilute method.^[43]Fluorescein

(F_fl=0.92in 1 N aqueous NaOH, for 3a, d), cresyl violet (F_fl=0.54 in

MeOH, for 3c) and N,N’-di(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxy-

perylene-3,4:9,10-tetracarboxylic acid bisimide (F_fl=0.96 in CHCl₃, for

3b, e, f) were used as references. The given quantum yields are averaged

from values measured at three different excitation wavelengths.

UV/Vis (CH₂Cl₂): l_max(e)=705 (48200), 654 (24600), 435 (18900), 314

(27900m^À1cm^À1); fluorescence (CH₂Cl₂): l_max=745 nm; fluorescence

quantum yield: F_fl=0.25.

N,N’-Di(3,4,5-tridodecylphenyl)-1,7-dibromoperylene-3,4:9,10-tetracar-

boxylic acid bisimide (3d): Prepared from 1,7-dibromoperylene-3,4:9,10-

tetracarboxylic acid bisanhydride (0.26 g, 0.47 mmol), 3,4,5-tridodecylani-

line (0.60 g, 1.0 mmol) and zinc acetate (0.10 g, 0.55 mmol). According to

the procedure described above for compound 3a, except that CHCl₃/n-

hexane 1:1 was used for chromatography, to give a orange-red powder

(350 mg, 44%). ¹H NMR (400 MHz, CDCl₃, 300 K, TMS): d=9.54 (d,

2H, J=8.1 Hz, H_pery), 8.96 (s, 2H, H_pery), 8.76 (s, 2H, J=8.2Hz, H _pery),

6.95 (m, 4H, Ar-H), 2.66 (m, 12H, Ar-CH₂), 1.8–1.2(m, 120H, CH ₂),

0.88 (m, 18H, CH₃); MS (FAB, matrix: p-octyloxynitrobenzene): m/z:

calcd for C₁₀₈H₁₆₀Br₂N₂O₄: 1707.1; found: 1707.3 [M]⁺; elemental analysis

(%) calcd for C₁₀₈H₁₆₀Br₂N₂O₄: C 75.85, H 9.43, N1.64, Br 9.34; found: C

75.90, H 9.38, N 1.58, Br 9.08; UV/Vis (CH₂Cl₂): l_max(e)=527 (57800),

491 (39400), 464 (16000), 388 (6800), 269 (36300 mol^À1dm³cm^À1); fluo-

rescence (CH₂Cl₂): l_max=544 nm; fluorescence quantum yield: F_fl=0.26.

1,7-Dibromoperylene-3,4:9,10-tetracarboxylic acid bisanhydride (1d):

N,N’-Dicyclohexyl-1,7-dibromoperylene-tetracarboxylic acid bisimide

(140 mg, 0.20 mmol), KOH (1.0 g, 18 mmol) was mixed with water

(0.5 mL) and tBuOH (20 mL). The reaction mixture was stirred at 908C

for 18 h and the color of the solution became bright green. After cooling

to room temperature, the reaction mixture was poured into acetic acid

(50 mL) and allowed to stay over night. Then the precipitate was collect-

ed by centrifugation and dried in vacuum. The resulting red solid was dis-

solved in H₂SO₄(15 mL) and this solution was slowly added into water

(100 mL). The precipitate was collected by filtration and washed with

large amount of water, and dried in vacuum at 1008C for 24 h to give a

deep red powder (96 mg, 90%). The purity of this material was enough

for further reaction. ¹H NMR (600 MHz, D₂SO₄, 300 K, external TMS):

d=9.65 (d, 2H, J=8.3 Hz, H_pery), 8.98 (s, 2H, H_pery), 8.76 (d, 2H, J=

8.4 Hz, H_pery); MS (EI, 70 eV): m/z (%): 549.8 (100) [M]⁺, 505.8 (36)

[MÀCO₂]⁺, 469.9 (6.4) [MÀBr]⁺.

N,N’-Di(3,4,5-tridodecylphenyl)-1,6,7,12-tetrachloroperylene-3,4:9,10-tet-

racarboxylic acid bisimide (3a): 1,6,7,12-Tetrachloroperylene- 3,4:9,10-

tetracarboxylic acid bisanhydride (0.13 g, 0.24 mmol), 3,4,5-tridodecylani-

line (0.30 g, 0.50 mmol) and zinc acetate (0.055 g, 0.30 mmol) were mixed

in quinoline (15 mL). The reaction mixture was stirred at 1808C for 2h.

After cooling to room temperature, the mixture was poured into MeOH

(30 mL). The precipitate was collected by filtration, washed with metha-

nol, and then dried in vacuum. The crude product was further purified by

silica gel column chromatography (CH₂Cl₂) and then slowly precipitated

from CH₂Cl₂/methanol 1:1. The product was isolated as an orange

powder by filtration (210 mg, 52%). ¹H NMR (400 MHz, CDCl₃, 300 K,

TMS): d=8.72(s, 4H, H _pery), 6.93 (s, 4H, Ar-H), 2.67 (m, 12H, Ar-CH₂),

1.8–1.2(m, 120H, CH ₂),0.88 (m, 18H, CH₃); MS (FAB, matrix: p-octyl-

oxynitrobenzene): m/z: calcd for C₁₀₈H₁₅₈Cl₄N₂O₄: 1687.1; found 1687.1

[M]⁺; elemental analysis (%) calcd for C₁₀₈H₁₅₈Cl₄N₂O₄: C 76.74, H 9.42,

N 1.66, Cl 8.39; found: C 76.76, H 9.38, N 1.60, Cl 8.52; UV/Vis

(CH₂Cl₂): l_max(e)=523 (48100), 490 (33800), 428 (11500m^À1cm^À1); fluo-

rescence (CH₂Cl₂): l_max=543 nm; fluorescence quantum yield: F_fl=0.03.

N,N’-Di(3,4,5-tridodecylphenyl)-1,7-di(4-tert-butylphenoxy)perylene-

3,4:9,10-tetracarboxylic acid bisimide (3e): Compound 3d (0.10 g,

0.058 mmol), tert-butylphenol (0.035 g, 0.23 mmol) and K₂CO₃(0.030 g,

0.22 mmol) were mixed in anhydrous N-methylpyrrolidinone (5 mL). The

reaction mixture was stirred at 1208C for 2h under argon atmosphere.

After cooling to room temperature, MeOH (20 mL) and 1n HCl (5 mL)

was added into the reaction mixture. The precipitate was collected by fil-

tration, washed with methanol, and then dried in vacuum. The crude

product was further purified by silica gel column chromatography

(CH₂Cl₂) and then precipitated from CH₂Cl₂/methanol 1:1 to give a dark

red solid (43 mg, 40%). ¹H NMR (400 MHz, CDCl₃, 300 K, TMS): d=

9.69 (d, 2H, J=8.5 Hz, H_pery), 8.65 (d, 2H, J=8.4 Hz, H_pery), 8.33 (s, 2H,

H_pery), 7.45 (m, 4H, Ar-H), 7.10 (m, 4H, Ar-H), 6.91 (s, 4H, Ar-H), 2.62

(m, 12H, Ar-CH₂), 1.8–1.2(m, 138H), 0.88 (m, 18H, CH ₃). MS (FAB,

matrix: p-octyloxynitrobenzene): m/z: calcd for C₁₂₈H₁₈₆N₂O₆: 1847.4;

found: 1847.6 [M]⁺; elemental analysis (%) calcd for C₁₂₈H₁₈₆N₂O₆: C

83.15, H 10.14, N 1.52, found C 82.50, H 10.01, N 1.55; UV/Vis (CH₂Cl₂):

l_max(e)=546 (56000), 511 (37800), 403 (10567), 268 (49800m^À1cm^À1);

fluorescence (CH₂Cl₂): l_max=575 nm; fluorescence quantum yield: F_fl=

0.87.

N,N’-Di(3,4,5-tridodecylphenyl)-1,7-di(pentafluorophenoxy)perylene-

3,4:9,10-tetracarboxylic acid bisimide (3 f): Prepared from compound 3d

(0.050 g, 0.029 mmol), pentafluorophenol (0.53 g, 0.23 mmol) and K₂CO₃

(0.36 g, 0.22 mmol). According to the procedure described above for

compound 3e, a deep red solid was obtained (34 mg, 61%). ¹H NMR

(400 MHz, CDCl₃, 300 K, TMS): d=9.52(d, 2H, J=8.4 Hz, H_pery), 8.75

(d, 2H, J=8.4 Hz, H_pery), 8.19 (s, 2H, H_pery), 6.92(s, 4H, Ar-H), 2.64 (m,

12H, Ar-CH₂), 1.8–1.2(m, 102H, CH ₂), 0.87 (m, 18H, CH₃); MS

(MALDI-TOF, matrix: DTCB): m/z: calcd for C₁₂₀H₁₆₀F₁₀N₂O₆: 1915.2;

found: 1915.2[ M]⁺; elemental analysis (%) calcd for C₁₂₀H₁₆₀F₁₀N₂O₆: C

75.20, H 8.41, N 1.46, found: C 75.31, H 8.63, N 1.47; UV/Vis (CH₂Cl₂):

l_max(e)=530 (71300), 494 (45300), 465 (17200), 381 (7800m^À1cm^À1);

fluorescence (CH₂Cl₂): l_max=547 nm; fluorescence quantum yield: F_fl=

0.79.

N,N’-Di(3,4,5-tridodecylphenyl)-1,6,7,12-tetra(4-tert-butylphenoxy)pery-

lene-3,4:9,10-tetracarboxylic acid bisimide (3b): Prepared from 1,6,7,12-

tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisanhy-

dride (0.15 g, 0.15 mmol), 3,4,5-tridodecylaniline (0.19 g, 0.31 mmol) and

zinc acetate (0.033 g, 0.18 mmol). According to the procedure described

above for compound 3a. A dark-red viscous substance was obtained

(170 mg, 53%). ¹H NMR (400 MHz, CDCl₃, 300 K, TMS): d=8.22 (s,

4H, H_pery), 7.23 (d, 8H, J=10.7 Hz, Ar-H), 6.86 (m, 12H, Ar-H), 2.58 (m,

12H, Ar-CH₂), 1.8–1.2(m, 156H), 0.88 (m, 18H, CH ₃); MS (FAB,

matrix: p-octyloxynitrobenzene): m/z: calcd for C₁₄₈H₂₁₀N₂O₈: 2143.6;

found 2143.7 [M]⁺; elemental analysis (%) calcd for C₁₄₈H₂₁₀N₂O₈: C

82.86, H 9.87, N 1.31; found: C 82.70, H 9.94, N 1.35; UV/Vis (CH₂Cl₂):

l_max(e)=580 (50100), 541 (31400), 453 (17400), 267 (51600m^À1cm^À1);

fluorescence (CH₂Cl₂): l_max=611 nm; fluorescence quantum yield: F_fl=

0.95.

N,N’-Di(3,4,5-tridodecylphenyl)-1,7-dipyrrolidinylperylene-3,4:9,10-tetra-

carboxylic acid bisimide (3c): Prepared from 1,7-dipyrrolidinylperylene-

3,4:9,10-tetracarboxylic acid bisanhydride (0.10 g, 0.19 mmol), 3,4,5-trido-

Chem. Eur. J. 2007, 13, 450 – 465

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

463

F. Würthner et al.

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Acknowledgements

We are grateful to the Deutsche Forschungsgemeinschaft for financial

support of this work within the research graduate school GK 1221 “Con-

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464

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Chem. Eur. J. 2007, 13, 450 – 465

Perylene Bisimide Dyes

FULL PAPER

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Received: June 22, 2006

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Article Doi

Effect of core twisting on self-assembly and optical properties of perylene bisimide dyes in solution and columnar liquid crystalline phases

DOI: 10.1002/chem.200600891

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