Intermolecular Interactions and Energetics in the Crystalline π-π Stacks and Associated Model Dimer Systems of Asymmetric Halogenated Diketopyrrolopyrroles

Article abstract of DOI:10.1021/acs.cgd.5b01656

Four novel structurally analogous asymmetric, halogenated N-benzyl substituted diketopyrrolopyrroles (DPP) have been synthesized, and their crystal structures obtained. All four crystal structures exhibit π-π stacks with very small displacements along the

Full text of DOI:10.1021/acs.cgd.5b01656

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Article
Intermolecular interactions and energetics in the
crystalline #-# stacks and associated model dimer
systems of asymmetric halogenated diketopyrrolopyrroles
Jesus Calvo-Castro, Monika Warzecha, Iain D. H. Oswald, Alan R.
Kennedy, Graeme Morris, Andrew J McLean, and Callum J McHugh
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01656 • Publication Date (Web): 13 Jan 2016
Downloaded from http://pubs.acs.org on January 19, 2016
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Intermolecular interactions and energetics in the crystalline πꢀπ stacks and associated
model dimer systems of asymmetric halogenated diketopyrrolopyrroles
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Jesus Calvo-Castro,* a Monika Warzecha, b Iain D.H. Oswald, b Alan R. Kennedy, c Graeme Morris, a
Andrew J. McLean a and Callum J. McHugh*a
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aSchool of Science and Sport, University of the West of Scotland, Paisley, PA1 2BE, UK.
bStrathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, Glasgow, G1
1XL, UK.
ABSTRACT: Four novel structurally analogous asymmetric, halogenated Nꢀbenzyl substituted
diketopyrrolopyrroles (DPP) have been synthesised and their crystal structures obtained. All four
structures exhibit πꢀπ stacks with very small displacements along their short molecular axes, which is a
characteristic of Nꢀbenzyl substitution. Intermolecular interaction energies were computed for extracted
crystal πꢀπ dimer pairs using density functional theory to investigate the most energetically favoured
position of the halogen atoms in FBDPP and ClBDPP structures. Effective stabilisation energies arising
from both benzyl and halogen substitution in these derivatives and in BrBDPP and IBDPP πꢀπ dimer
pairs were also determined to probe the impact of these groups on dimer stability. Effects of the
intermonomer displacements along the long molecular axis, which have been shown by us previously to
significantly influence wavefunction overlap and effective electronic coupling, were investigated in detail
using aligned and antiꢀaligned model systems of ClDPP and BrDPP. The predictions of these model
systems are remarkably consistent with the observed displacements in their crystal derived πꢀπ dimer pair
equivalents, offering insight into the effective role of intermolecular contacts in stabilising crystal
structures involving this DPP molecular motif.
Dr. C. J. McHugh*
University of the West of Scotland
School of Science and Sport
Paisley, UK
PA1 2BE
Tel: +44 (141) 848 3210
Fax: +44 (141) 848 3204
Email: callum.mchugh@uws.ac.uk
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Intermolecular interactions and energetics in the
crystalline πꢀπ stacks and associated model dimer
systems of asymmetric halogenated
diketopyrrolopyrroles
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Jesus Calvo-Castro,* a Monika Warzecha, b Iain D.H. Oswald, b Alan R. Kennedy, c Graeme
Morris, a Andrew J. McLean a and Callum J. McHugh*a
aSchool of Science and Sport, University of the West of Scotland, Paisley, PA1 2BE, UK.
bStrathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, Glasgow,
G1 1XL, UK.
cDepartment of Pure & Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, UK.
*Corresponding authors: jesus.calvo@uws.ac.uk, callum.mchugh@uws.ac.uk
ABSTRACT: Four novel structurally analogous asymmetric, halogenated Nꢀbenzyl substituted
diketopyrrolopyrroles (DPP) have been synthesised and their crystal structures obtained. All four
crystal structures exhibit πꢀπ stacks with very small displacements along their short molecular
axis, which based upon our previous studies involving symmetrical DPPs is a characteristic of
Nꢀbenzyl substitution. Intermolecular interaction energies were computed for extracted crystal
πꢀπ dimer pairs by means of M06ꢀ2X density functional at 6ꢀ311G(d) level to investigate the
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most energetically favoured position of the halogen atoms in FBDPP and ClBDPP structures. In
addition, effective stabilisation energies arising from both benzyl and halogen substitution in
these derivatives and in BrBDPP and IBDPP πꢀπ dimer pairs were determined in order to probe
the impact of these groups on the resulting dimer stability. Effects of the intermonomer
displacements along the long molecular axis, which have been shown by us previously to
significantly influence wavefunction overlap and effective electronic coupling, were investigated
in detail using aligned and antiꢀaligned model systems of ClDPP and BrDPP. The predictions of
these model systems are remarkably consistent with the observed displacements in their crystal
derived πꢀπ dimer pair equivalents, offering insight into the effective role of intermolecular
contacts in crystal structures involving this molecular motif, particularly with a view towards
crystal engineering in these systems. As a result, we believe that this study should be of
significant interest to the growing DPP based materials community and in general to those
investigating the detailed manner by which substituents can be employed in the supramolecular
design of crystalline molecular architectures.
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INTRODUCTION
Diketopyrrolopyrroles (DPPs) are widely employed in industry as high performance pigments on
account of their desirable properties such as brightness, low solubility and light and weather
fastness.1ꢀ5 More recently there has been an increasingly large surge of interest in DPP based
materials employed as charge transfer mediators in functional devices, either in the form of small
molecules or polymers.6ꢀ17 Currently, we are engaged in the inꢀdepth investigation and rational
design of crystalline small molecule DPPs, specifically to develop an understanding of their
behaviour and application in optical and optoelectronic environments.18ꢀ20 We are particularly
interested in the exploitation of structureꢀproperty relationships involving these materials,19,20
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targeted towards rational design of functional charge transfer mediating materials. Previous
studies of Nꢀsubstituted DPP single crystal systems19ꢀ22 have shown that they tend to crystallise
forming slipped πꢀπ stacking interactions and not the characteristic herringbone crystal structures
observed in nonꢀsubstituted acenes, thiophenes23ꢀ26 or DPP pigments27ꢀ29. In such systems it is
crucial to optimise intermonomer displacements along the short and long molecular axes via
judicious molecular design in order to achieve optimum intermolecular interactions as well as
wavefunction overlap, which is critical for effective charge transport.19,26,30ꢀ32 Given the high
sensitivity of hole (th) and electron (te) transfer integrals to small intermolecular displacements, it
is also desirable to maximise thermal integrity with respect to intermonomer slip by optimising
the strength of intermolecular interactions in πꢀconjugated systems.19,26,31,33 Understanding the
nature of nonꢀcovalent intermolecular interactions in πꢀconjugated systems and how these
interactions can be modified via systematic and rational substitution is therefore of fundamental
importance in advancing the design of supramolecular architectures in crystal engineering as
well as in drug design and biological probes. Consequently, studies such as the one presented
herein are important in providing a deeper insight into the manner by which substituents can be
employed in supramolecular design.
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The nature of the intermolecular interactions stabilising πꢀconjugated systems is of considerable
recent interest.34ꢀ60 The πꢀresonance based model proposed by Hunter and Sanders34,35,38,39 and
the π/polar model of Cozzi and Siegel36,37 have been shown to inaccurately represent
intermolecular interactions between πꢀconjugated architectures since they neglect the crucial role
played by dispersion effects in this type of interaction. Recent studies by Wheeler and
Houk48,49,55,60 and Sherrill and coꢀworkers (using symmetryꢀadapted perturbation theory)41ꢀ44,52ꢀ54
have shown that πꢀπ dimers in benzene derived pairs are primarily stabilised via local bond
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dipole/bond dipole and induced bond dipole interactions rather than HOMO/LUMO based
donorꢀacceptor global charge transfer type interaction suggested previously. Despite this
progress, there is a clear lack of experimental studies34,55,61ꢀ64 which test these theoretical
predictions, particularly in relation to their influence on those interactions which are derived
from crystal structures and associated model systems. Importantly, dispersion interactions denote
the main point of discrepancy between experimental and theoretical results, given that most
experimental work has been conducted in solution where electrostatic effects are dominant.38,55,65
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Herein, we aim to enhance the understanding of the impact of structural variation on the πꢀπ
stacking interactions in crystalline DPP systems by reporting the synthesis, determination and
characterisation of four asymmetric monoꢀhalogenated Nꢀsubstituted DPP single crystal
structures (Scheme 1), that systematically vary in only one atom out of sixty, located at the para
position of one of the two phenyl rings linked to the central DPP core. Previously, we have
highlighted the effect of varying both halogen atoms on the intermolecular interaction energies
and charge transfer integrals in a series of symmetric diꢀhalogenated DPP single crystal
structures. In the following report, we examine, in essence, “real” cropped equivalents from our
initial study, thus facilitating a deeper understanding of the effective influence of the additional
halogen atom on the important πꢀπ stacking interactions and their associated energetics. The four
reported structures were given names with a form of XYDPP arising from their topology where
X and Y denote the substitution on the para position of one out of the two phenyl rings attached
the central DPP core and the lactam nitrogen atoms respectively. These four structures were
based on fluoro (FBDPP), chloro (ClBDPP), bromo (BrBDPP) and iodo (IBDPP) substituted
DPPs (with B = Nꢀbenzyl substitution). The particular interest in halogen substitution can be
readily understood from the enhanced optoelectronic behaviour observed in materials containing
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these groups66,67 as well as the potential for monoꢀhalogenated DPP based materials to be
employed as an interesting platform for the further synthesis of structurally related systems.20
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O
O
EtO2C
Ph
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EtO2C
K2CO3
Cl
NH4OAc
CO2Et
CO2Et
CO2Et
Ph
Ph
O
N
H
Ph
N
O
N
i. PhCNX, Na, t-amyl alcohol
FBDPP (X = F)
ClBDPP (X = Cl)
BrBDPP (X = Br)
IBDPP (X = I)
ii. BnBr, K2CO3, DMF, 120 °C
O
X
Scheme 1. Synthesis of asymmetric monoꢀhalogenated DPPs.
EXPERIMENTAL SECTION
Reagents and instrumentation. Unless otherwise specified, all starting materials and reagents
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were purchased from SigmaꢀAldrich and used as received without further purification. H NMR
and 13C NMR spectra were determined using a JEOL ECS400 400 MHz spectrometer (in
CDCl3). Elemental analyses were carried out using the service provided at Jagiellonian
University in Krakow, Poland. FTIR analyses were carried out on the neat samples by attenuated
total reflectance using a PerkinElmer Spectrum One FTIR Spectrometer with Universal ATR
Sampling Accessory.
Synthesis. 2,3ꢀDihydroꢀ2ꢀoxo 5ꢀphenylꢀ1Hꢀpyrroleꢀ4ꢀcarboxylic acid ethyl ester (Pyrroline
ester).68 A suspension of ethyl benzoylacetate (20.2 g, 18.3 ml, 105 mmol), K2CO3 (15.2 g, 110
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mmol), NaI (2.0 g), and ethyl chloroacetate (13.2 g, 11.6 ml, 108 mmol) in acetone/DME 120
ml:80 ml was heated under reflux for 24 hours. After cooling to room temperature the salts were
filtered and washed with acetone. The combined filtrates were evaporated to dryness under
reduced pressure affording an oily, brown intermediate product with sufficient purity for the next
stage of the preparation. The crude product was dissolved in a mixture of acetic acid (200 ml)
and ammonium acetate (78.7 g, 1.02 mol) and the reaction mixture was then stirred at reflux for
3 hours. After cooling to room temperature, the reaction mixture was added to iceꢀwater (800
ml). The precipitate was filtered and washed with water. The residue was recrystallized from
ethanol/water 4:1. After drying under reduced pressure the title product (17.87 g, 73 % starting
from benzoylacetate) was obtained as a whiteꢀgrey powder. 1H NMR (400 MHz, CDCl3): 1.15ꢀ
1.19 (t, 3H, CH3), 3.49 (s, 2H, CH2), 4.08ꢀ4.14 (q, 2H, CH2), 7.42–7.45 (m, 3H, ArH), 7.59–7.62
(m, 2H, ArH), 8.49 (br s, 1H, NH). 13C NMR: 14.5 (CH3), 39.1 (CH2), 60.4 (CH2), 105.0 (C=C),
128.6 (C=C), 129.1 (C=C), 130.0 (C=C), 130.9 (C=C), 151.6 (C=C), 163.6 (C=O), 177.5 (C=O).
Melting point: 173ꢀ175 °C.
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3ꢀ(4ꢀfluorophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]pyrroleꢀ1,4(2H,5H)ꢀdione (FDPP). Under nitrogen
atmosphere and vigorous stirring 4ꢀfluorobenzonitrile (0.32 g, 2,7 mmol) and the pyrroline ester
(0.45 g, 1.96 mmol) were added to a solution of sodium tꢀamyloxide (obtained from 0.14 g, 6.1
mmol sodium metal dissolved in 20 ml dry tꢀamyl alcohol). The colour changed to dark red and
a red solid precipitated. The mixture was stirred under reflux for 2hours. After cooling, 20 ml of
iceꢀcold methanol with 2 ml of hydrochloric acid were added. The red precipitate was filtered,
washed with methanol and water and then dried under vacuum to give FDPP as a bright red
powder (0.22 g, 37 % yield), which was used without further purification or characterisation.
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3ꢀ(4ꢀchlorophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]pyrroleꢀ1,4(2H,5H)ꢀdione (ClDPP). As per the
method described for FDPP using 4ꢀchlorobenzonitrile (0.74 g, 5.4 mmol) and the pyrroline
ester (0.9 g, 3.92 mmol) to give ClDPP as a bright red powder (0.85 g, 68 % yield).
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3ꢀ(4ꢀbromophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]pyrroleꢀ1,4(2H,5H)ꢀdione (BrDPP). As per the
method described for FDPP using 4ꢀbromobenzonitrile (0.5 g, 2,7 mmol) and the
pyrrolinecarboxylate ester (0.45 g, 1.96 mmol) to give BrDPP as a bright red powder (0.46 g,
65 % yield).
3ꢀ(4ꢀiodophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]pyrroleꢀ1,4(2H,5H)ꢀdione (IDPP). As per the method
described for FDPP using 4ꢀiodobenzonitrile (1.24 g, 5.4 mmol) and the pyrrolinecarboxylate
ester (0.9 g, 3.92 mmol) to give IDPP as a bright red powder (1.22 g, 76 % yield).
2,5ꢀdibenzylꢀ3ꢀ(4ꢀfluorophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]ꢀ1,4ꢀ(2H,5H)ꢀdione
(FBDPP).
A
suspension of FDPP (0.20 g, 0.65 mmol) and K2CO3 (1.05g, 0.65 mmol) in dry DMF (20 ml)
was heated at 120 °C under nitrogen atmosphere. At this temperature and under vigorous stirring,
a benzyl bromide (1.92 ml, 16 mmol) solution in DMF (10 ml) was added over 20 minutes.
Stirring and heating at 120 °C were continued for 1.5 hours. After cooling to room temperature,
salt was filtrated and washed with DMF. The remaining filtrate was collected and ice cold
methanol/water was added to give an orange precipitate that was washed with water and then
recrystallized from chloroform/hexane to afford FBDPP as an orange powder (0.14 g, 31 %).
1H NMR (CDCl3): 4.96 (2H, s, CH2), 4.98 (2H, s, CH2), 7.09ꢀ7.19 (5H, m, ArH), 7.24ꢀ7.32 (7H,
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m, ArH), 7.41ꢀ7.49 (3H, m, ArH) 7.73ꢀ7.78 (4H, m, ArH). C NMR (CDCl3): 45.70 (CH2),
45.70 (CH2), 116.18 (C=C), 116.39 (C=C), 124.14 (C=C), 124.17 (C=C), 126.66 (C=C), 126.77
(C=C), 127.51 (C=C), 127.58 (C=C), 127.87 (C=C), 128.89 (C=C), 128.97 (C=C), 129.13
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(C=C), 131.48 (C=C), 131.56 (C=C), 137.40 (C=C), 137.47 (C=C), 147.94 (C=C), 149.20
(C=C), 162.78 (C=O), 162.86 (C=O). IR (ATR)/cmꢀ1: 3031 (ArH), 2943 (CH2), 1654 (C=O),
1603 (C=C), 1495 (C=C), 821 (ArH), 736 (ArH), 690 (ArH). Anal. Calcd for C32H23FN2O2: C,
79.00; H, 4.76; N, 5.76. Found: C, 78.98; H, 4.84; N, 5.55. Melting point: 267ꢀ269 °C.
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2,5ꢀdibenzylꢀ3ꢀ(4ꢀchlorophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]ꢀ1,4ꢀ(2H,5H)ꢀdione (ClBDPP). As per
the method described for FBDPP using ClDPP (0.40 g, 1.23 mmol), K2CO3 (2.28 g, 1.23 mmol)
and benzyl bromide (2.4 ml, 20 mmol) to give ClBDPP as an orange powder (0.51 g, 82 %).
1H NMR (CDCl3): 4.95 (2H, s, CH2), 4.97 (2H, s, CH2), 7.17ꢀ7.18 (4H, m, ArH), 7.23ꢀ7.32 (6H,
13
m, ArH), 7.35ꢀ7.47 (5H, m, ArH) 7.68ꢀ7.70 (2H, d, ArH) 7.72ꢀ7.75 (2H, d, ArH). C NMR
(CDCl3): 45.71 (CH2), 45.71 (CH2), 126.33 (C=C), 126.67 (C=C), 126.77 (C=C), 127.53
(C=C), 127.61 (C=C), 127.82 (C=C), 128.90 (C=C), 128.98 (C=C), 129.00 (C=C), 129.15
(C=C), 129.32 (C=C), 130.44 (C=C), 131.65 (C=C), 137.36 (C=C), 137.41 (C=C), 137.62
(C=C), 147.64 (C=C), 149.57 (C=C), 162.73 (C=O), 162.82 (C=O). IR (ATR)/cmꢀ1: 3030
(ArH), 2928 (CH2), 1655 (C=O), 1601 (C=C), 1494 (C=C), 841 (ArH), 732 (ArH), 690 (ArH).
Anal. Calcd for C32H23ClN2O2: C, 76.41; H, 4.61; N, 5.57. Found: C, 75.72; H, 4.88; N, 5.24.
Melting point: 238ꢀ240 °C.
2,5ꢀdibenzylꢀ3ꢀ(4ꢀbromophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]ꢀ1,4ꢀ(2H,5H)ꢀdione (BrBDPP). As per
the method described for FBDPP using BrDPP (0.30 g, 0.81 mmol), K2CO3 (1.5 g, 0.81 mmol)
and benzyl bromide (2.4 ml, 20 mmol) to give BrBDPP as an orange powder (0.51 g, 82 %).
1H NMR (CDCl3): 4.95 (2H, s, CH2), 4.97 (2H, s, CH2), 7.17ꢀ7.18 (4H, d, ArH), 7.23ꢀ7.32 (6H,
m, ArH), 7.41ꢀ7.49 (3H, m, ArH) 7.54ꢀ7.56 (2H, m, ArH), 7.60ꢀ7.62 (2H, m, ArH), 7.73ꢀ7.75
(2H, d, ArH). 13C NMR (CDCl3): 45.71 (CH2), 45.71 (CH2), 126.12 (C=C), 126.68 (C=C),
126.78 (C=C), 127.53 (C=C), 127.62 (C=C), 127.83 (C=C), 128.90 (C=C), 128.99 (C=C),
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129.14 (C=C), 130.54 (C=C), 131.66 (C=C), 132.29 (C=C), 137.36 (C=C), 137.41 (C=C),
147.68 (C=C), 149.59 (C=C), 162. 71 (C=O), 162.82 (C=O). IR (ATR)/cmꢀ1: 3056 (ArH), 2943
(CH2) 1673 (C=O), 1604 (C=C), 1493 (C=C), 828 (ArH), 730 (ArH), 689 (ArH). Anal. Calcd for
C32H23BrN2O2: C, 70.21; H, 4.23; N, 5.12. Found: C, 70.25; H, 4.34; N, 4.99. Melting point:
235ꢀ236 °C.
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2,5ꢀdibenzylꢀ3ꢀ(4ꢀiodophenyl)ꢀ6ꢀphenylpyrrolo[3,4ꢀc]ꢀ1,4ꢀ(2H,5H)ꢀdione (IBDPP). As per the
method described for FBDPP using IDPP (1.0 g, 2.4 mmol), K2CO3 (4.5 g, 2.4 mmol) and
benzyl bromide (2.4 ml, 20 mmol) to give IBDPP as an orange powder (0.68 g, 47 %). 1H NMR
(CDCl3): 4.94 (2H, s, CH2), 4.97 (2H, s, CH2), 7.16ꢀ7.18 (4H, m, ArH), 7.22ꢀ7.32 (6H, m, ArH),
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7.41ꢀ7.49 (5H, m, ArH) 7.72ꢀ7.78 (4H, m, ArH). C NMR (CDCl3): 45.71 (CH2), 45.71 (CH2),
126.66 (C=C), 126.78 (C=C), 127.29 (C=C), 127.53 (C=C), 127.61 (C=C), 127.84 (C=C),
128.89 (C=C), 128.98 (C=C), 129.00 (C=C), 129.13 (C=C), 130.45 (C=C), 131.65 (C=C),
137.37 (C=C), 137.41 (C=C), 138.23 (C=C), 147.83 (C=C), 149.59 (C=C), 162.72 (C=O),
162.80 (C=O). IR (ATR)/cmꢀ1: 3031 (ArH), 2943 (CH2), 1682 (C=O), 1592 (C=C), 1487 (C=C),
824 (ArH), 736, 691 (ArH). Anal. Calcd for C32H23IN2O2: C, 64.66; H, 3.90; N, 4.71. Found: C,
65.19; H, 3.96; N, 4.60. Melting point: 201ꢀ202 °C.
Preparation of Crystals for Single Crystal XꢀRay Diffraction analysis. FBDPP: Slow
evaporation of a cooled solution of FBDPP in hexane/chloroform (1:1). ClBDPP: Slow
evaporation of a cooled solution of ClBDPP in hexane/chloroform (1:1). BrBDPP: Slow
evaporation of a cooled solution of BrBDPP in hexane/chloroform (1:1). IBDPP: Slow
evaporation of a cooled solution of IBDPP in hexane/chloroform (1:1).
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Crystal structure determination. For BrBDPP data were measured at Station I19 of the
DIAMOND synchrotron light source.69 All other data were measured using laboratory based
instruments and monochromated Mo radiation. All structures were refined to convergence, on F2
and against all unique reflections with SHELXꢀ97.70 Highly disordered and partially present
solvent was present in channels parallel to the b axis in both BrBDPP and IBDPP. As this could
not be identified or modelled, the SQUEEZE routine of PLATON was implemented to remove
the effects of approximately 58 electron equivalents from 221 Å3 of unit cell space for BrBDPP
and 56 electron equivalents from 247 Å3 of unit cell space for IBDPP.71 Table 1 summarises the
different selected crystallographic data and refinement parameters for the crystal structures
herein reported.
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Table 1. Selected crystallographic data and refinement parameters for XBDPP compounds
Compound
Formula
Formula
Weight
Xꢀray
FBDPP
ClBDPP
BrBDPP
IBDPP
a
a
C32H23FN2O2
C32H23ClN2O2
C32H23BrN2O2
C32H23IN2O2
486.52
502.97
547.43a
594.42a
micosource
Monoclinic
microsource
Monoclinic
synchrotron
Monoclinic
rotating anode
Monoclinic
Source
Crystal
system
Space
P 21/c
P 21/c
P 21/c
P 21/c
Group
λ Å
0.71073
0.71073
0.6889
0.71075
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12.7952(11)
15.4086(14)
5.9945(5)
97.086(5)
1172.83(18)
123(2)
13.0195(6)
15.3154(7)
6.1283(3)
22.15(4)
5.300(8)
23.88(4)
22.758(11)
5.3375(19)
24.226(11)
114.517(5)
2677(2)
100(2)
a Å
b Å
c Å
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β°
97.371(2)
1211.88(10)
123(2)
115.082(17)
2539(7)
100(2)
4
Volume Å3
Temp. K
Z
2
2
4
Refls.
18254
52.84
2407
21168
52.18
2398
20642
50.0
15140
52.0
Collected
2θ max °
Refls.
4831
5253
Unique
Refls. Obs.
Rint
1806
1946
3054
4069
0.0360
0.0282
0.1109
0.0641
Goodness
of Fit
1.078
1.127
1.100
1.181
R[I>2s(I)],
F
0.0404
0.1030
0.0604
0.1314
0.1057
0.3123
0.0810
0.1610
Rw, F2
Max/min
electron
density
eÅꢀ3
0.149/ꢀ0.204
0.269/ꢀ0.428
0.868/ꢀ0.920
0.511/ꢀ0.711
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Not including traces of disordered solvent which were removed from the model using the
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SQUEEZE routine implemented in PLATON – see experimental text.
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Computational details. All molecular modelling studies were carried out using the Truhlar
M06ꢀ2X density functional72 and 6ꢀ311G(d) level as implemented in Spartan10 software.73 This
density functional has been shown to give good account of the dimer interaction energies of πꢀπ
interacting systems.32,55 Dimer interaction energies, ꢁECP, were all corrected for Basis Set
Superposition Error (BSSE) using the counterpoise correction method of Boys and Bernardi.74
The computation of the dimer interactions of an H2DPP model system were performed following
the method previously described by us.19 Herein, the triple zeta 6ꢀ311G(d) basis set was also
employed for the model system and a comparison of these results with those previously reported
employing a 6ꢀ31G(d) basis set was conducted. The halogen substitution effects on these systems
were evaluated by means of two analogous model systems (denoted aligned and antiꢀaligned in
line with previous nomenclature56) of a monoꢀsubstituted BrDPP system (which represents the
nonꢀbenzylated analogue of BrBDPP) using the density functional M06ꢀ2X at 6ꢀ311G(d) level.
Note the intermonomer separation (represented as ꢁz) for the BrDPP model system was set to
ꢁz = 3.6 Å as optimised for the H2DPP analogue by scanning the intermolecular distance at
0.3 Å increments while holding the monomers in a fully eclipsed relative orientation,19 given that
as extracted from halogen substituted crystal structures the presence of halogen atoms does not
lead to any significant/systematic changes to the intermonomer distance along the z axis.
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RESULTS AND DISCUSSION
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Structural description. Structures of the diꢀhalogenated species X2BDPP (X = H, Cl, Br, I and
B = Nꢀbenzyl) have been described by us previously.19 They were found to form a structural
series where, despite some slight, systematic variations, each compound displayed similar
conformation and packing. An exception was one of the two observed polymorphs of Cl2BDPP
which differed from the others in terms of molecular conformation, packing and even colour.
The four monoꢀsubstituted structures described here, XBDPP (X = F, Cl, Br, I), fall into two
structural groups with the fluoroꢀ and chloroꢀderivatives being mutually isomorphous and
isostructural and the bromoꢀ and iodoꢀderivatives forming a second mutually isomorphous and
isostructural pair. Interestingly, the F and Cl structures are also isomorphous with the “typical”
polymorph of Cl2BDPP, i.e. the polymorph that fits well into the X2BDPP structural series. The
X2BDPP structures all contain crystallographically centrosymmetric molecules (Z’ = 0.5) and
despite the nonꢀsymmetrical monoꢀsubstitution of FBDPP and ClBDPP these structures also
feature molecules that are situated at crystallographic centres of symmetry. The F and Cl atoms
are thus disordered over two sites with 50 % occupancy. For ClBDPP this structural
approximation leads to somewhat elongated thermal displacement parameters as the reported C,
N and O atom sites are in fact averages of two slightly different geometries. FBDPP with a
closer size match between X and H does not show the same distortions. The molecular
conformations of FBDPP and ClBDPP are also in good agreement with the main conformation
found for the X2BDPP species. In all structures, the plane of the benzyl group ring approaches a
perpendicular relationship with the DPP ring plane whilst the phenyl and haloꢀbenzene rings are
much less twisted with respect to the DPP ring plane (80.31(8) and 78.40(10); 23.18(16) and
24.25(17) ° for F and Cl species respectively). Presumably because of the increased geometrical
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difference between H and X, the bromoꢀ and iodoꢀ structures do not feature crystallographically
imposed molecular centrosymmetry, thus all four substituents are free to adopt their own
conformations. Despite this, the conformation adopted is closely related to that seen for the
fluoroꢀ, chloroꢀ and diꢀsubstituted analogues. The only difference is a slight increase in the
degree of twist seen between the DPP plane and the planes of the conjoined aromatic rings
(35.2(5) and 29.2(4) ° for the phenyl and haloꢀbenzene rings of the bromoꢀderivative and 34.9(3)
and 30.9(3) ° for the equivalent angles in the iodoꢀderivative; compare to a range of 20.6 to
24.3 ° for others).
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Analysis of the intermolecular contacts of the four XBDPP species shows that all make three
groups of contacts with intermolecular separations less than the sum of Van der Waals radii.
These are πꢀπ interactions, haloꢀπ interactions and CꢀH…O/N hydrogen bonds. The latter are
largely long range (H…O/N 2.35 to 2.58 Å) and relatively constant over the four structures and
so will not be discussed further. The πꢀπ stacking interactions are of particular interest with
respect to opticalꢀelectronic properties and are found in each structure.
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Figure 1. Short πꢀπ contacts between a pair of FBDPP molecules assuming aligned orientation.
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The disordered F atom sites and all H atoms have been omitted for clarity.
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For the fluoroꢀ and chloroꢀ derivatives, close interactions are found between the DPP ring
system and the phenyl and/or haloꢀbenzene rings, as illustrated in Figure 1. Note that the disorder
in the X atom position makes it impossible to determine the relative orientation of the molecules
in each pair. Each molecule makes interactions with two neighbouring molecules to give πꢀπ
stacks that propagate parallel to the c direction. The bromoꢀ and iodoꢀderivatives make similar
interactions and stacks, but it is apparent that short contacts involve only the DPP and phenyl
rings (i.e. the halogenated rings are not involved) and as these are well ordered structures it can
be seen that each close pair has a head to head orientation. In each pair, a second interaction
between two benzyl group aromatic rings also occurs, illustrated in Figure 2.
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Figure 2. Short πꢀπ contacts between a pair of BrBDPP molecules showing the observed
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aligned orientation. All H atoms have been omitted for clarity.
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These interactions combine to give πꢀπ stacks that propagate parallel to the crystallographic b
direction. As with the fluoroꢀ and chloroꢀ species this is the short unit cell dimension. For the
four XBDPP structures, the closest DPP to π C…C contacts are 3.290(2), 3.274(4), 3.357(11)
and 3.453(8) Å for X = F, Cl, Br and I respectively. Thus, as well as making fewer such close
contacts per molecule, the Br and I derivatives also make longer contacts. Indeed for IBDPP the
shortest contact is longer than the sum of Van der Waals radii and the benzyl to benzyl πꢀπ
contact is shorter (3.380(10) Å) than the DPP to π contact. All four XBDPP structures make
short haloꢀπ contacts (shortest X…C 3.047(3), 3.086(4), 3.343(11) and 3.453(8) Å for X = F, Cl,
Br and I respectively). For the Br and I structures these are X to benzyl interactions that link
pairs of stacks whilst for the F and Cl structures these interactions also link stacks but now they
do so by forming contacts between the nonꢀbenzyl benzene ring derivatives. The BrBDPP and
IBDPP structures feature partially present solvent layers at approximately a = 0.5 and thus
bilayers of XBDPP form parallel to the bc plane.
Intermolecular interaction energies, ꢁECP. Given the crucial role played by πꢀπ stacking
motifs in determining the charge transfer properties of organic materials,31,32,75 we devote the
remainder of the paper to an inꢀdepth investigation of substituent effects on the πꢀπ dimer pairs
of these systems. In order to widen our understanding of halogen substitution effects in DPPs as
well as to further investigate headꢀtoꢀhead (aligned) versus headꢀtoꢀtail (antiꢀaligned) relative
intermonomer orientations with respect to the position of the halogen atoms, the intermolecular
interactions, ꢁECP for the πꢀπ dimer pairs of the monoꢀhalogenated XBDPP systems were
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computed. The FBDPP and ClBDPP structures exhibit πꢀπ dimer pairs which are stacked along
the c crystallographic axis, analogous to the πꢀπ dimer pair of their isomorphous “typical”
polymorph of Cl2BDPP (vide supra).19 In turn, πꢀπ dimer pairs of BrBDPP and IBDPP stack
along the b crystallographic axis as opposed to their diꢀhalogenated counterparts where the πꢀπ
dimer pairs were observed to stack along the a and c crystallographic axes for Br2BDPP and
I2BDPP respectively.
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Figure 3. XBDPP crystal structures determined with short and long molecular axes in blue and
red, respectively. Benzene axis denoted in green for comparison (A). Long (right) and short (left)
molecular axis perspective views of the HBDPP,19 FBDPP, ClBDPP, BrBDPP and IBDPP (B).
aassuming aligned orientation.
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Table 2. Intermonomer displacements exhibited by the πꢀπ dimer pairs of FBDPP, ClBDPP,
BrBDPP and IBDPP.
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FBDPP
4.68
ClBDPP
4.88
BrBDPP
3.57
IBDPP
3.55
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ꢁx / Å
ꢁy / Å
ꢁz / Å
0.20
0.30
0.23
0.05
3.54
3.48
3.42
3.66
The πꢀπ dimer pairs of FBDPP, ClBDPP, BrBDPP and IBDPP were observed to exhibit similar
intermonomer displacements (see Table 2 for details). The relative alignment of monomers in
each of the πꢀπ dimer pairs appears to vary systematically according to the increased
polarisability of the substituent on the para position of the phenyl rings attached to the central
core, with IBDPP displaying the smallest degree of long molecular axis slip (see Figure 3).
Significant differences were observed in the displacements of the πꢀπ dimer pairs of BrBDPP
and IBDPP systems along their long molecular axes compared to the diꢀhalogenated equivalents
(ꢁx = 8.44 and 9.40 Å for Br2BDPP and I2BDPP respectively), whereas ꢁy shift measured for
ClBDPP was more in line with the equivalent diꢀhalogenated system, Cl2BDPP (ꢁx = 5.13 Å).
Displacements along the short molecular axis were observed to be in line with those exhibited by
other Nꢀbenzyl substituted DPP systems.19,20 Accordingly, these systems reinforce the
importance of Nꢀsubstitution in crystalline DPPs over and above a recognised solubilising effect.
Similarly to the diꢀhalogenated symmetrical analogues, in asymmetric halogenated Nꢀsubstituted
DPPs the presence of benzyl groups on the lactam nitrogen definitively preclude any significant
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displacement along the short molecular axis, which is in sharp contrast to the non Nꢀsubstituted
pigmentary equivalents such as H2DPP, Cl2DPP and diphenylDPP for which 1.8 Å < ꢁy < 5.6
Å.27ꢀ29
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Figure 4. Illustration of ꢁy shifts in the πꢀπ dimer pairs (left) and herringbone vs slipped cofacial
packing motifs (right) of ClDPP (top) and ClBDPP (bottom) respectively.
An additional effect of Nꢀbenzylation is in the disruption of the herringbone structure, which is
characteristic of DPP pigments, to a slipped stack as illustrated in Figure 4 for ClDPP and
ClBDPP. Nꢀsubstituted DPP crystal structures have been previously reported21 employing alkyl
chain solubilising groups instead of benzyl substituents. The measured displacement (ꢁy =
1.80 Å) of the monomers in the πꢀπ stack of the latter were observed to be more in line with the
pigment analogues and to exceed that of the Nꢀbenzyl counterparts. Thus, our previous studies,
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and those structures reported herein highlight the important role played by benzyl groups in
facilitating the design of crystalline DPP systems which maximise spatial and hence
wavefunction overlap, widely recognised as being critical in the optimisation of effective charge
transport behaviour. 19,26,31,33
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We initially investigated the position of the halogen atoms in the disordered structures of
FBDPP and ClBDPP (vide supra) as well as the role of benzyl stabilisation of these πꢀπ dimer
pairs (see Figure 5 for details). Intermolecular interaction energies were computed for the
different dimer pairs possible (where dimer pairs A, B and C denote the relative position of the
halogen atom in the bottom and top monomer as inꢀin, inꢀout and outꢀout respectively, as
illustrated in Figure 5) from the disordered model and from these the role of benzyl, halogen and
simultaneous benzyl/halogen substitution was rationalised through a series of systematically
cropped dimer pairs. In short, the benzyl (B) and halogen (X) substituents were removed and
replaced with H atoms, first individually, and then simultaneously, resulting in XDPP, BDPP,
and DPP structures respectively, where XBDPP denotes the uncropped dimer pair. Table 3
summarises the computed ꢁECP for these systems.
Table 3. Counterpoise corrected intermolecular interactions energies, ꢁECP for a series of
structurally modified and nonꢀstructurally modified πꢀπ dimer pairs of FBDPP/ClBDPP.
ꢁECP / KJ molꢀ1
Dimer pair
XBDPP
XDPP
BDPP
DPP
ꢀ62.35/ꢀ72.54 ꢀ39.09/ꢀ49.23 ꢀ63.06/ꢀ63.75 ꢀ39.90/ꢀ40.55
A
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ꢀ63.58/ꢀ67.05 ꢀ40.10/ꢀ44.86 ꢀ63.06/ꢀ63.75 ꢀ39.90/ꢀ40.55
ꢀ63.80/ꢀ61.44 ꢀ39.92/ꢀ39.84 ꢀ63.06/ꢀ63.75 ꢀ39.90/ꢀ40.55
B
C
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The πꢀπ dimer pairs of FBDPP exhibit negligible (< 1 kJ molꢀ1) fluorine induced stabilisation as
extracted from the computed intermolecular interaction energies arising for the examined dimer
pairs. This finding is consistent with the absence of any intermolecular short contact interactions
involving fluorine atoms and the low polarisability of CꢀF bonds.76,77 Consequently, none of
these dimer pairs is significantly more energetically favoured in relation to the position of the
halogen atoms. Benzyl induced stabilisation can be identified via comparison of the BDPP and
DPP cropped dimer pairs. In line with other DPP systems characterised by similar intermonomer
displacements,19 benzyl induced stabilisation in this case arises from the additive contribution of
slipped cofacial interactions57,59 between the benzylic phenyl rings, a soꢀcalled Tꢀshape
interaction42,52 between the benzylic phenyl rings and the phenyl rings attached to the central
core and an attractive Hꢀbonding interaction between the electronegative carbonyl oxygen and
the electropositive methylene hydrogens, which are separated by 3.082 Å. Analogous benzyl
induced stabilisation was computed for the various dimer pairs of ClBDPP, which is consistent
with similar intermonomer displacements along the short and long molecular axes compared to
the fluorinated equivalent.
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Dimer pair A
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ꢁECP XBDPP / kJ molꢀ1
Figure 5. Computed ꢁECP for XDPP vs XBDPP dimer pairs of ClBDPP. Inset illustrates
different πꢀπ dimer orientations investigated.
Unlike its fluorinated equivalent, computed intermolecular interactions for the various πꢀπ dimer
pairs of ClBDPP were observed to be influenced by the relative dimer orientation. In this regard,
dimer pair A (inꢀin) was computed to be the most stable as a result of halogen induced
stabilisation (ꢁECP = ꢀ72.54, ꢀ67.05 and ꢀ61.44 kJ molꢀ1 for dimer pairs A, B and C of ClBDPP
respectively), arising solely from dipole/induce dipole attractive intermolecular interactions
between the CꢀCl bond and the CꢀC linking the phenyl rings to the central DPP core. In this case
no significant halogenꢀbenzyl induced stabilisation was observed. Figure 5 illustrates the linear
relationship (r2 = 0.999) exhibited by computed ꢁECP for XBDPP and XDPP for the various
orientated dimers of ClBDPP. It is worthwhile to note that the role of halogen substitution is
additive in these systems, as observed from plots of computed ꢁECP for XBDPP and XDPP
versus number of halogen atoms in the dimer pair (r2 = 1.000 and 0.998 respectively), which is
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consistent with current theories describing the role of halogen atoms involved in nonꢀcovalent
interactions.52,53,56,58,60
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Intermolecular interaction energies were also computed for the crystallographically observed πꢀπ
dimer pairs of BrBDPP and IBDPP and the role of benzyl/halogen in stabilising these dimer
pairs was investigated analogously to the fluorinated and chlorinated derivatives (vide supra).
Table 4 summarises the computed intermolecular interactions for BrBDPP and IBDPP.
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Table 4. Counterpoise corrected intermolecular interactions energies, ꢁECP for a series of
structurally modified and nonꢀstructurally modified πꢀπ dimer pairs of BrBDPP and IBDPP.
ꢁECP / KJ molꢀ1
Compound
XBDPP
ꢀ79.16
ꢀ79.36
XDPP
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ꢀ45.40
BDPP
ꢀ76.45
ꢀ76.81
DPP
ꢀ41.90
ꢀ41.25
BrBDPP
IBDPP
Intermolecular interactions, ꢁECP of ꢀ79.16 and ꢀ79.36 kJ molꢀ1 were computed for the πꢀπ dimer
pairs of BrBDPP and IBDPP respectively. These almost identical values of ꢁECP are consistent
with the very subtle structural effects on progression from Br to I substituted Nꢀbenzyl DPPs
such as the torsion of the ring bearing the halogen substituent with respect to the phenyl core
(29.90 and 29.44° for BrBDPP and IBDPP respectively) and the intermonomeric displacement
along the long molecular axis (vide supra). The intermolecular interactions exceed in all cases
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those computed for the πꢀπ dimer pair of Rubrene (ꢀ35.60 kJ molꢀ1, M06ꢀ2X/6ꢀ311G(d)). Thus,
we propose, in line with our previous report19, a greater thermal integrity of monoꢀhalogenated
DPP single crystals compared with Rubrene; a very desirable property given the high sensitivity
of the charge transfer integrals for both hole and electrons (th and te respectively) to very small
intermolecular displacements.19,26,31,33 In both πꢀπ dimer pairs, the contribution of the halogen
atoms to the total intermolecular interactions is identified via comparison of XBDPP with BDPP
and XDPP with DPP. In each case, the action of cropping the halogen atoms from both
monomers results in very small, ca 2ꢀ4 kJ molꢀ1 destabilisation of the dimer pair interaction
energy.
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Figure 6. πꢀπ dimer pairs of IBDPP (top) and I2BDPP19 (bottom) illustrating the intermonomer
contacts for their significantly different slips along the long molecular axis.
Of note is the small destabilisation computed on progression from XBDPP to BDPP (2.71 and
2.55 kJ molꢀ1 for BrBDPP and IBDPP respectively) which contrasts to the computed values for
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analogous dimer pairs of the diꢀhalogenated equivalents (16.1 and 23.49 kJ molꢀ1 for Br2BDPP
and I2BDPP respectively).19 This can be readily explained through analysis of the different
displacements exhibited by mono and diꢀhalogenated πꢀπ dimer pairs along their long molecular
axis (vide supra) as illustrated in Figure 6. In short, the computed destabilisation observed on
cropping the halogen atoms in the diꢀhalogenated systems arises from close intermolecular
electrostatic interactions between the electropositive phenylic hydrogens and the electronegative
halogen atoms, which in BrBDPP and IBDPP are precluded given the shorter dimer
displacements along the long molecular axis. In turn, the role of the benzyl substituents in
stabilising these πꢀπ dimer pairs can be understood from the comparison of XBDPP with XDPP
and BDPP with DPP. In light of the very small halogen contribution to ꢁECP, the effect of
cropping the benzyl groups should be similar by comparing both pairs. In fact, variations in
intermolecular interactions of 34.65/34.55 and 33.96/35.56 kJ molꢀ1 were computed for XBDPP
vs. XDPP and BDPP vs. DPP for BrBDPP and IBDPP respectively. The large benzyl
stabilisation of these πꢀπ dimer pairs was identified to arise from three different contributions: a
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slipped phenylꢀphenyl type between the two phenyl rings within the benzyl groups,57,59
a
Tꢀshaped interactions between the benzylic phenyl rings and the phenyl rings attached to the core
of the DPP42,54 and a third electrostatic contribution arising from the close contact between the
electronegative carbonyl oxygen of one monomer and the electropositive methylene/phenylic
hydrogen atoms of the other monomer. The importance of Nꢀbenzyl substitution, over and above
a solubilising effect and steric influence on precluding significant shifts along the short
molecular axis, is therefore clearly exhibited by an enhancement of the stabilisation of these πꢀπ
dimer pairs. This finding is in contrast with previous reports of DPP systems which employ
alkyl chain as Nꢀsubstituents.21
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Given our interest in developing charge transfer mediating materials based on Nꢀsubstituted
DPPs, we calculated the hole (th) and electron (te) transfer integrals for the πꢀπ dimer pairs of
BrBDPP and IBDPP by means of an H2DPP model system (vide infra and SI2) since both of
these dimer pairs exhibit negligible intermonomer displacement along the short molecular axis.
We propose on the basis of the calculated integrals that both πꢀπ dimer pairs of BrBDPP and
IBDPP should be characterised by larger electron than hole transfer (th = 2.84 and te = 14.13 kJ
molꢀ1 at ꢁx = 3.6 Å, SI2) in light of their displacement along the long molecular axis.
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In conclusion, we find that removal of the benzyl groups on progression from BDPP to DPP
results in decreasing the ꢁECP (ꢀ79.16 and ꢀ79.36 kJ molꢀ1 for BrBDPP and IBDPP respectively)
by 43.65 and 44.81 % for BrBDPP and IBDPP respectively. In turn, removal of the
bromine/iodine atoms on progressing from XDPP to DPP results in a destabilisation of the total
ꢁECP by 3.30 % for BrBDPP and 5.23 % for IBDPP. Systematic halogen substitution in DPPs
therefore appears to play an important role in defining the intermonomer displacements along the
long molecular axis, which facilitate determination of intermolecular interactions and associated
charge transfer integrals in these systems.19 Accordingly, we dedicate the remainder of the paper
to an inꢀdepth investigation of halogen substitution in DPP structures of this type through
analysis of a series of novel model dimer systems.
Dimer model system. We have previously reported a DPP model system formed by two nonꢀ
substituted H2DPP monomers where the top monomer is shifted with respect to the bottom one
from a fully eclipsed geometry through the long molecular axis across a distance of 15.3 Å in
0.3 Å increments while retaining ꢁy = 0.00 Å and ꢁz = 3.60 Å. Herein, we revisit this model
system employing an enhanced triple zeta basis set and the analogies in the computed data are
briefly discussed. It is of interest that πꢀπ dimer pairs of FBDPP and ClBDPP are characterised
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by shifts (ꢁx = 4.68 and 4.88 Å respectively) closely related to the local minima at ca 5.1 Å and
that those of BrBDPP and IBDPP exhibit displacements along their long molecular axes (ꢁx =
3.57 and 3.55 Å respectively) coinciding with the global minimum predicted by the potential
energy surface (PES) of the model system. It is also of particular interest that both monomers in
the dimer pairs of BrBDPP and IBDPP are aligned with respect to the relative orientation of the
halogen atoms. In this regard, we report two additional model systems and compute their
interaction energies as a function of long molecular axis dimer displacement, where the relative
orientation of the halogen atoms on each monomer is aligned and antiꢀaligned with respect to
one other.
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ꢁx / Å
Figure 7. Counterpoise corrected H2DPP model dimer interaction energy as a function of
intermonomer slip (ꢁx) at M06ꢀ2X/6ꢀ31(d) (yellow filled circles) and M06ꢀ2X/6ꢀ311G(d) (blue
filled circles). Inset illustrates linear regression between computed ꢁECP for 6ꢀ31G(d) and 6ꢀ
311G(d) with red filled circles representing the dimer pairs with ꢁx ≤ 1.2 Å.
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Figure 7 illustrates the computed PES for the H2DPP model system determined using double and
triple zeta basis sets. Significantly greater intermolecular interactions energies (ꢁECP = ꢀ38.52
and ꢀ54.46 kJ molꢀ1 for 6ꢀ31G(d) and 6ꢀ311G(d) respectively at ꢁx = 3.6 Å) were computed
throughout the entire studied range of displacements along the long molecular axis by means of
the triple zeta basis set. The inset in Figure 7 illustrates the relationship between the computed
ꢁECP by means of the two employed basis sets. Both potential energy surfaces denote the same
number of inflections located at identical shifts along the long molecular axis. However, the inset
in Figure 7 illustrates that the linear relationship (r2 = 0.992) exhibited between computed ꢁECP
by means of 6ꢀ31G(d) and 6ꢀ311G(d) is diminished for dimer pairs characterised by ꢁx ≤ 1.2 Å
(denoted by red filled circles in the inset in Figure 7). In order to further investigate this finding,
we reproduced our previous cropped dimer model19 at the 6ꢀ311G(d) level of theory (SI1); where
we previously reported repulsive interaction energies for fully eclipsed/quasiꢀeclipsed (ꢁx ≤ 0.6
Å) phenylꢀphenyl (ꢁECP = 3.07 kJ molꢀ1 at ꢁx = 0.00 Å) interactions using a 6ꢀ31G(d) basis set.
Contrary to our previous findings and to common chemical intuition, attractive intermolecular
interaction energies were computed for these cropped dimer pairs (ꢁECP = ꢀ3.05 kJ molꢀ1 at ꢁx =
0.00 Å) by means of the wider 6ꢀ311G(d) basis set. In fact, this is consistent with current studies
on charge penetration effects at interplanar distances lower than 4 Å reported by Sherrill and coꢀ
workers,53 given that at such intermonomeric distances the electron clouds do not repel each
other as much as point charges do, since intermolecular electronꢀnuclear attractive term becomes
larger than the sum of the electronꢀelectron and nucleusꢀnucleus repulsive contributions.
Accordingly, in the following all of the reported model dimer systems were computed by means
of the M06ꢀ2X density functional at 6ꢀ311G(d) level. Along these lines, we observed the location
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