Tuning the electronic nature of mono-bay alkynyl-phenyl-substituted perylene bisimides: Synthesis, structure, and photophysical properties

Article abstract of DOI:10.1002/chem.201400099

Perylene bisimide (PBI) derivatives with various alkynyl-phenyl substituents at a single bay position have been synthesised by Sonogashira coupling. NMR spectroscopic studies reveal the unsymmetric nature of the dyads. All of the dyads undergo two reversible reductions, which demonstrates their structural and electrochemical rigidity. The synthesised dyads show a remarkable redshift in their absorption maxima and sharp vibronic progression. Electron-rich substituents facilitate efficient charge transfer from the substituent HOMO to the electron-deficient PBI core. The most interesting spectral signatures were exhibited by a PBI with a strongly electron-donating ethynyl(dimethylaminophenyl) substituent. The steady-state features of this PBI showed a broad absorption that covered almost the whole visible region with no emission. A twisted intramolecular charge-transfer (TICT) process, related to the rotational motion of ethynyl(dimethylaminophenyl) PBI, was also demonstrated. Computational investigations shed light on the coplanarity of the various substituents with respect to the PBI core; the PBI core itself remains flat without any noticeable deformation even after mono-functionalisation. This illustrates that mono-functionalisation exerts meagre steric hindrance on the bay positions relative to disubstituted analogues. Despite several previous reports on the structural characterisation of 1,7-disubstituted PBI derivatives, we present the first structural characterisation of a mono-bay ethynyl-phenyl substituted PBI. The solid-state structure of the phenyl derivative has a flat PBI core without any noticeable steric constraints from the substituents, as predicted. In contrast, single-crystal X-ray analysis for the mono-bromo bay-substituted PBI shows that the bromine substituent is not in the plane of the PBI core. Single minded: Perylene bisimide derivatives that contain various alkynyl-phenyl substituents at a single-bay position have been synthesized and characterized (see scheme). Detailed studies on the electrochemical and the photophysical properties were explored.

Full text of DOI:10.1002/chem.201400099

DOI: 10.1002/chem.201400099
Full Paper
&
Conjugated Materials
Tuning the Electronic Nature of Mono-Bay Alkynyl–Phenyl-
Substituted Perylene Bisimides: Synthesis, Structure, and
Photophysical Properties
Ruchika Mishra,[a] Jong Min Lim,[b] Minjung Son,[b] Piyush Panini,[a] Dongho Kim,*[b] and
Jeyaraman Sankar*[a]
Abstract: Perylene bisimide (PBI) derivatives with various
alkynyl–phenyl substituents at a single bay position have
been synthesised by Sonogashira coupling. NMR spectro-
scopic studies reveal the unsymmetric nature of the dyads.
All of the dyads undergo two reversible reductions, which
demonstrates their structural and electrochemical rigidity.
The synthesised dyads show a remarkable redshift in their
absorption maxima and sharp vibronic progression. Electron-
rich substituents facilitate efficient charge transfer from the
substituent HOMO to the electron-deficient PBI core. The
most interesting spectral signatures were exhibited by a PBI
with a strongly electron-donating ethynyl(dimethylamino-
phenyl) substituent. The steady-state features of this PBI
showed a broad absorption that covered almost the whole
visible region with no emission. A twisted intramolecular
charge-transfer (TICT) process, related to the rotational
motion of ethynyl(dimethylaminophenyl) PBI, was also dem-
onstrated. Computational investigations shed light on the
coplanarity of the various substituents with respect to the
PBI core; the PBI core itself remains flat without any notice-
able deformation even after mono-functionalisation. This il-
lustrates that mono-functionalisation exerts meagre steric
hindrance on the bay positions relative to disubstituted ana-
logues. Despite several previous reports on the structural
characterisation of 1,7-disubstituted PBI derivatives, we pres-
ent the first structural characterisation of a mono-bay ethyn-
yl-phenyl substituted PBI. The solid-state structure of the
phenyl derivative has a flat PBI core without any noticeable
steric constraints from the substituents, as predicted. In con-
trast, single-crystal X-ray analysis for the mono-bromo bay-
substituted PBI shows that the bromine substituent is not in
the plane of the PBI core.
Introduction
various chemical modifications allow us to achieve characteris-
tic photophysical properties in these p-conjugated organic
molecules.[6] For instance, these systems display a broad ab-
sorption window, a prerequisite for efficient light harvesting
and excellent excited-state properties, for example, CT process-
es. They allow wide chemical functionalities to further fine-
tune their optoelectronic properties to complement existing
molecules. Specifically, CT moieties that mimic natural photo-
synthetic processes are in huge demand due to their utility in
alternative energy-harvesting systems.[7] Various factors, which
include precise control of geometry, enhancement of conjuga-
tion length and control of electronic distribution, remain para-
mount for perturbation of the excited-state dynamics of such
systems for CT studies.[8] In this regard, the search for robust
materials with a rigid scaffold based on CT characteristics has
been greatly extended in the past few decades.[9] Among the
various p-conjugated organic chromophores, perylene bis-
imides (PBI), and their derivatives, with their intense absorption
and fluorescence in the visible spectral region emerged as po-
tential electron-transport systems (i.e. n-type semiconduc-
tors).[10] These molecular systems have near-unity fluorescence
quantum yields, excellent CT characteristics, high photochemi-
cal and thermal stability, strong electron-accepting capacity
and high charge carrier mobilities.[11–16] From correlation of the
Organic photovoltaics (OPV) comprised of p-conjugated organ-
ic molecules[1] have received significant scientific interest due
to their potential applications in many fields, such as light-
emitting diodes,[2] solar cells[3] and nonlinear optical materials.[4]
Recently, great progress has been made in the development of
new p-conjugated organic materials based on push–pull sys-
tems. Such materials exhibit excellent charge-transfer (CT)
characteristics for modern optoelectronic applications.[5] The
ease of synthesis, high processability and tunability offered by
[a] R. Mishra, P. Panini, Prof. Dr. J. Sankar
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Indore Bypass Road, Bhouri, Bhopal - 462066 (India)
Fax: (+91)755-6692 392
E-mail: sankar@iiserb.ac.in
[b] Dr. J. M. Lim, M. Son, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems
Department of Chemistry
Yonsei University, Seoul-120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400099.
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theoretical and experimental data derived for PBI systems, it
can be envisaged that substitution at the imide nitrogen does
not alter the optical properties to a noticeable extent. Howev-
er, covalent modification in the bay area influences the optical
properties efficiently. Thus, various bay substituents, including
electron-donating or -accepting moieties, were examined to
control the electronic structure of PBI systems with intramolec-
ular charge-transfer (ICT) characteristics.[17] The most common
synthetic methods applied for bay substitution involve either
Suzuki coupling or methoxy-aryl group substitution; both
methods are well documented for 1,6- and 1,7-dibromo PBIs.[18]
After the Suzuki coupling strategy directly introduces substitu-
ents to the bay area, the resultant molecular framework often
has non-planar distorted structures. This is attributed to steric
hindrance between the bay substituents and the PBI core. As
a result, the photophysical properties of these systems are
compromised relative to PBIs that are not derivatised in the
bay area. There are a handful of reports on the functionalisa-
tion of the bay areas through Sonogashira and Stille coupling
strategies to achieve acetylenyl-linked 1,6- and 1,7- bay-substi-
tuted planar conjugated systems.[19] In these cases, the effect
of substitution is prominent in the photophysical properties of
PBI systems (fluorescence, excited-state lifetimes, CT character-
istics) as a result of the acquired planarity. Nevertheless, limited
information is available for similar functionalisation and sys-
tematic studies of 1-substituted PBIs relative to the 1,6- and
1,7-substituted PBIs.[20] Thus, studies on mono-bay substitution
of PBI derivatives for photophysical and molecular electronic
applications are still in their infancy. Though it would have
been extremely useful to design unsymmetrical dyads, which
can show interesting photophysical properties, mono-bay
functionalisation of PBIs will provide a synthetic handle to
tune the CT characteristics by a simple single-point derivatisa-
tion. Therefore, in the present work, PBI systems are linked to
various phenylene-based substituents through an acetylene
bridge rather than an alkenyl or single bond (Scheme 1). It is
expected that an ethynyl bridge will impart structural robust-
ness and also restrict any untoward stereochemical implica-
tions by maintenance of the coplanarity of the PBI core with
phenylene substituent. We have also investigated the photo-
physical properties of these derivatives, which are good refer-
ences for evaluation of CT processes without any noticeable
PBI-core distortions. The planarity of the PBI core is further
substantiated by structural characterisation. In addition, theo-
retical simulation helps to predict the unusual behaviour of
the newly synthesised derivatives to an appreciable extent.
Scheme 1. Synthetic scheme for the preparation of mono-bay-substituted
PBIs. DIA=diisopropylamine.
is limited. Interestingly, the introduction of alkyl/aryl amine
substituents to the anhydride converts the PDA to correspond-
ing PBI that is soluble in common organic solvents.[21] As men-
tioned earlier, substitution at the bay region is an efficient ap-
proach to fine-tune the electronic structure and optoelectronic
properties of PBIs; the frontier molecular orbital energy levels
are modified because there are significant HOMO and LUMO
coefficients in the bay area.[22] One of the easiest methods to
functionalise the bay area is selective bromination with neat
bromine in sulfuric acid, pioneered by Bçhm et al. in 1997.[23]
This method yields a mixture of 1,6,7,8-tetrabromo-PDA and di-
bromo-PDA, along with a small amount of monobromo-PDA.
Wꢁrthner et al. reported the existence of regioisomers of di-
bromo-PBI in 2004.[24] It was realised that the earlier examples
of 1,7-disubstituted derivatives were actually mixtures of 1,6-
and 1,7-regioisomers. On the other hand, bromination reac-
tions of PBIs are not quantitative and mostly result in dibromo
isomers as the major product, along with monobromo- and
unreacted PBI. Poor yields of the monobromo derivative ob-
tained from the above procedure hindered its further explora-
tion in photophysical studies. There was no simple way to se-
lectively brominate only one position of the bay area of PBIs
until Rajasingh and co-workers[25] developed an interesting and
convenient method for the controlled bromination of 1. Ac-
cording to this procedure, 1 was simply stirred with neat bro-
mine in dichloromethane at ambient conditions to yield the
mono-brominated PBI in good yields, along with a small
amount of dibromo-PBI. This method offers an efficient
method to singly functionalise PBIs for further studies.
Results and Discussion
Synthesis
As depicted in Scheme 1, various phenyl groups were selected
to explore how the electronic features of the phenyl moieties
influence the electronic nature of the PBI derivatives. The pri-
mary starting material for synthesis of the PBI derivatives is
perylenetetracarboxydianhydride (PDA), which is insoluble in
common solvents, thus its use in photophysical investigations
In this work, an attempt has been made to study the proper-
ties of the mono-substituted PBI derivatives by functionalisa-
tion of 1-bromo-PBI with various aryl substituents. The synthet-
ic protocol used to obtain the derivatives is depicted in
Scheme 1. The parent PBI 1 was synthesised by a literature
procedure.[26] PBI 2 was then formed by mono bromination
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and further reaction with aryl alkynes with electron-donating
and electron-withdrawing substituents. 1-(Trimethylsilylacety-
lenyl)-PBI as a starting material for the coupling led predomi-
nantly to homo-coupled PBI. Therefore, efforts have been
made to link mono-bromo PBI 2 with aryl alkynes to get excel-
lent yields. Similar to 1,6- and 1,7-disubstituted PBIs, the
mono-bay-substituted PBIs synthesised by the aforementioned
protocol are moderately soluble in common organic solvents.
Wherever there are two substituents on the phenyl ring, the
substituents have been located meta to each other. Single sub-
stituents on the phenyl rings have been placed only at the
para positions. In the case of 9, chromatographic separation
proved to be tedious and led to lower yields than other exam-
ples. Details of the 1H and 13C NMR spectroscopic data are
given in the Supporting Information. Synthetic procedures and
characterisation details for all of the compounds are provided
in the Experimental section.
vations that the expansion of conjugation by incorporation of
various phenyl-based substituents leads to a bathochromic
shift and retention of the distinct vibronic spectra, except for
PBI 9. This is in good agreement with the 1,7-disubstituted PBI
derivatives reported by Marder et al.[19b] Electron-accepting
substituents on the arylene unit make the PBI more electron
deficient and lead to a small bathochromic shift. Further in-
sight can be obtained by comparison of the absorption
maxima of PBIs 4, 5 and 6, with meta-disubstituted bay sub-
stituents to that of the unsubstituted PBI 3. The derivatives 3
and 4 both have lmax ꢀ547 nm, whereas 5 and 6 have lmax
ꢀ539 and 542 nm, respectively, with a blueshift of approxi-
mately 5 nm compared to 3 due to the strong negative induc-
tive effect of fluorine-based substituents on the ring. Similarly,
for the PBI derivatives with a para-substituted phenyl group,
one can see that 7 and 8 both exhibit lmax =543 nm with
a slight blueshift relative to unsubstituted PBI 3. In this case,
the trifluoromethyl-substituted PBI 7 shows higher molar ab-
sorptivity than cyano-substituted PBI 8.
Compound 9 exhibits significantly different, but interesting,
characteristics relative to 1–8, and shows broad absorption
that covers almost the whole visible range. PBI 9 shows an in-
tense absorption in the l=486–510 nm region in dichloro-
methane, as well as in chloroform, which moderately retains
the vibronic features. A broad, structureless band with a maxi-
mum of lꢀ610 nm is also observed, which corresponds to the
CT transition from the HOMO of the N,N-dimethylamino-aryl-
ene to the LUMO of the PBI core. The blueshifted band at l=
510 nm can be assigned to the transition that originates from
the PBI HOMO to the PBI LUMO.
UV/Vis absorption studies
UV/Vis absorption spectra for all compounds were obtained
from dilute solutions in dichloromethane, chloroform or THF.
The spectral signatures of the compounds look similar in most
of the solvents. All of the synthesised molecules display the
absorption spectra of typical PBI-based dyes (Figure 1). The un-
Fluorescence emission and lifetime studies
To gain further insights into the role of the substituents in de-
termining the optical properties of mono-bay-substituted PBI
systems 1–9, fluorescence spectroscopic studies were per-
formed in chloroform and THF. All molecules, except 9, were
strongly fluorescent in solution in chloroform with an orange-
shaded emission and near-unity quantum yields (Figure S1 in
the Supporting Information). This reinstates the fact that
mono-bay substitution of PBIs does not negatively affect their
fluorescent behaviour to a noticeable extent (Figure 2).
In general, the S1!S0 emission of the PBIs is redshifted and
broadened in the region l=550–650 nm with an increase in
the extent of conjugation. For most cases, the emission spectra
displayed a mirror image of the absorption spectra. For PBIs 3
and 4, a broad emission peak with a low-energy shoulder was
observed. Stokes shift values of n˜ =570–800 cmꢁ1 were ob-
served for the present set of molecules. However, the fluores-
cence of 9 was totally quenched, which further confirms the
efficient CT from the dimethylamino moiety to the PBI core.
The fluorescence quantum yields were obtained by using
parent PBI 1 as a reference.
Figure 1. Normalised UV/Vis absorption spectra of PBIs 1 (c), 3 (d) and
9 (g) measured in chloroform (molar concentrationꢀ10ꢁ5 m). O.D.=opti-
cal density.
substituted PBI 1 has a low-energy absorption maximum
(lmax =525 nm) with well-defined vibronic structures. This peak
can be attributed to the S0!S1 electronic transition, which can
also be described as a HOMO (p)!LUMO (p*) transition for
this class of molecules. Introduction of various arylene sub-
stituents in the bay area leads to significant bathochromic
shifts up to 20 nm with high molar extinction coefficients (e)
and band broadening. It can be inferred from the above obser-
For further understanding of the interesting emission prop-
erties displayed by the synthesised molecules, fluorescence
lifetime measurements were performed by using the time-cor-
related single-photon counting (TCSPC) technique. All of the
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lected PBI systems. Because of their relatively poor solubility
the PBI dyes were dissolved in toluene, dichloromethane or
THF. Negligible blueshifts (less than 6 nm) in both the absorp-
tion and emission spectra were observed as the solvent polari-
ty increased (Figure S2 and Table S1 in the Supporting Informa-
tion). The fluorescence signals of PBI 3 exhibited almost identi-
cal decay profiles in toluene and THF with time constants of
t=5.4 and 5.5 ns, respectively. In contrast, the PBIs with elec-
tron-donating substituents revealed solvent-dependent excit-
ed-state dynamics. The fluorescence lifetime of PBI 4 became
shorter in a polar environment; t=6.0 and 2.3 ns in toluene
and THF, respectively. The fluorescence quantum yield of PBI 4
was reduced to FF =0.25 in THF.
In the case of the non-fluorescent PBI 9, we have investigat-
ed the ICT characteristics of the system, as well as its perturba-
tion by the rotational motion of the dimethylamino group. Be-
cause the CT transition from the dimethylamino group to the
PBI unit might be blocked under acidic conditions,[27] we have
performed a spectrophotometric titration of PBI 9 in toluene
with trifluoroacetic acid (TFA). Upon protonation, the broad ab-
sorption band (l=550–750 nm) of PBI 9 became diminished
and a new absorption structure similar to that of typical PBI
dyes with an intense transition at l=547 nm appeared (Fig-
ure 3a; Figure S3 in the Supporting Information). Moreover,
Figure 2. Normalised fluorescence emission spectra of PBIs 1 (c), 3 (d),
4 (c) and 5 (d) measured in chloroform (molar concentrationꢀ10ꢁ5 m).
F.I.=fluorescence intensity.
Table 1. Spectroscopic and electrochemical data of compounds 1–9 in
chloroform and dichloromethane, respectively.
absorption
max
emission
max
Compound
l
emax
[mꢁ1 cmꢁ1
l
FF
t
Ered
[nm]
]
[nm]
[ns] [V]
1
3
4
5
6
7
8
9
525
78300
59200
57500
59800
63100
64100
53100
538, 575
1 (ref.) 4.5
ꢁ0.7
ꢁ0.82
ꢁ0.65
ꢁ0.78
ꢁ0.65
ꢁ0.77
ꢁ0.61
ꢁ0.75
ꢁ0.62
ꢁ0.75
ꢁ0.64
ꢁ0.78
ꢁ0.62
ꢁ0.76
ꢁ0.72
ꢁ0.84
547
547
539
542
543
543
572
575
0.95
0.97
5.7
6.0
5.1
5.3
5.4
5.4
–
556, 593[sh] 0.97
560, 596[sh] 0.93
562, 599[sh] 0.84
562, 599[sh] 0.96
510, 611 16300,
7200
–
–
[sh]=Shoulder peaks.
molecules were excited at l=470 nm and probed at their re-
spective steady-state emission maxima. Fluorescence decay
profiles for compounds 1–8 can be well fitted to a mono-expo-
nential decay (see Table 1 for details). The decay time constant
(t) observed for the reference compound 1 was 4.5 ns, which
is characteristic of an unsubstituted PBI. Fluorescence lifetimes
(t) were observed to be in the range of 5–6 ns depending
upon the substitution on the phenyl ring; PBI 4 displays the
slowest fluorescent decay of t=6.0 ns relative to unsubstitut-
ed PBI 3.
Solvent dependence of the PBI systems
Figure 3. UV/Vis absorption and fluorescence emission (a) spectra of PBI
9; a) comparison between neutral (d) and protonated (c) forms and
b) in toluene (d) and versus toluene/paraffin oil (c).
To identify the CT characteristics, we have examined the sol-
vent-dependent absorption and emission behaviours of the se-
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the fluorescence emission in the spectral region l=500–
750 nm was recovered in the acidic environment. To confirm
the role of the rotating dimethylamino group without electron
affinity change, the absorption and emission features of PBI 9
were examined in the viscous mixture of toluene and excess
paraffin oil (Figure 3b). Although no striking change was ob-
served in the absorption spectra, the fluorescence emission of
PBI 9 in the spectral region l=600–800 nm only appeared in
paraffin oil, which supports that the rotational motion might
be interrupted in a viscous environment. These results indicate
that the rotational motion of the dimethylamino group pre-
dominantly influences the electronic structures, especially the
excited states of PBI 9.
Femtosecond transient-absorption studies
We have comparatively investigated the excited-state dynam-
ics of the PBI dyes by using the femtosecond transient absorp-
tion (fs-TA) technique (Figure 4). To support our findings, the
temporal evolution of the TA spectra was globally analysed.
The ground-state bleaching (GSB) and excited-state absorption
(ESA) signals for PBIs 3 and 4 in toluene and THF showed long
decay profiles in our experimental time window (3 ns), which
are in agreement with their fluorescence behaviours (Figure S4
in the Supporting Information).[28] The transient species with
a shorter lifetime exhibited a more blueshifted stimulated
emission (SE) signal than those with a longer lifetime.
However, for a solution of PBI 9 in toluene, which showed
broad redshifted ground-state absorption bands, the temporal
profiles of the transient species revealed ultra-fast rise and
decay dynamics. After the rise profile, accompanied by a spec-
tral blueshift, appeared at the spectral region l=550–650 nm,
the transient species, which exhibits characteristic spectral fea-
tures of PBI 9 in its ground state, showed a decay profile with-
out noticeable spectral shifts. The TA spectrum signals were
analysed globally and we have distinguished two principal
transient species with t=10.2 and 893 ps, respectively (Fig-
ure S5 in the Supporting Information). The transient species
with the shorter time constant (t=10.2 ps) exhibited broad
negative signals in the spectral region l=600–750 nm, where-
as that with the longer time constant corresponds well to the
steady-state absorption feature of PBI 9. It should also be
noted that the spectral features of the transient species corre-
lates with the fluorescence emission of PBI 9 in paraffin oil
(Figure 3b). In other words, the fluorescence signal observed
under viscous conditions, in which the rotational motion is
blocked, disappeared with a lifetime of t=10.2 ps. Based on
these results, we can assume that the non-radiative relaxation
process from the ICT state of PBI 9 might be accelerated by
the rotational motion of the dimethylamino group. Interesting-
ly, in contrast to PBIs 3 and 4, the transient species of PBI 9
was shown to be sensitive to the solvent polarity. The rise and
decay time constants of PBI 9 in THF were estimated to be t=
1.3 and 9.7 ps, respectively. This feature indicates that the ex-
cited states of PBI 9, which show prominent CT absorption
bands, are more sensitive to the surrounding solvent environ-
ments than the ground states. Considering that the CT transi-
Figure 4. Femtosecond transient-absorption spectra and decay profiles
(insets) of PBI 9. lexcitation =510 nm.
tion might stimulate the intersystem-crossing process (STICT
!
T1),[29] we assume that the TICT state and the related excited-
state dynamics of PBI 9 are sensitive to the solvent environ-
ment.[30]
Computational studies
To gather further insights into the delocalisation of the HOMO
and LUMO of the PBIs and the electronic features upon intro-
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duction of various substituents,
DFT calculations were performed
at the B3LYP/6-311G** level by
using the Gaussian 09 suite of
programs.[32] The alkyl groups on
the nitrogen of the imide groups
were substituted with hydrogen
atoms for computational simplic-
ity. An interaction between the
HOMOs of the individual alkyn-
yl–phenyl substituents and the
PBI is expected to raise the
HOMO energy level of the PBI
core and to destabilise the
HOMO of the substituent.[19g]
Figure 5. DFT-B3LYP/6-311G** optimised frontier orbitals of selected PBI derivatives.
This explains the bathochromic shift observed in the p–p*
transitions for the PBI derivatives relative to the parent PBI. As
expected, the LUMO electron density is localised on the PBI
framework (Figure 5). Except for 9, the HOMO electron density
is spread all over the molecule for the PBI derivatives. In com-
pound 9, the HOMO electron density is mainly confined to the
dimethylaminophenyl substituent due to its electron-rich
nature. The LUMO electron density of 9 lies predominantly on
the PBI core, similar to the other derivatives. This accounts for
the CT transition observed in the steady-state absorption spec-
trum of 9.
In all optimised structures, the PBI core remains largely
planar. This feature suggests that mono-bay substitution has
very little influence on the geometry of the main framework of
the PBI systems, unlike that of 1,6- and 1,7- substituted PBI de-
rivatives. This may be attributed to the minimum steric con-
straint imposed by the alkynyl group on the bay area. The gas-
eous-state structure optimisation reveals an interesting obser-
vation about the dihedral angles of the sterically crowded bay
positions in these derivatives. The optimised geometry of 3
has a C1-C12a-C12b-C12 torsional angle of 15.68. Other deriva-
tives have torsional angles that range from 14.5–16.38. Further-
more, all of the substituted phenyl rings are slightly tilted with
respect to the mean PBI plane, which may be due to the mini-
mal flexibility offered by the connecting alkynyl group.
Electrochemical studies
Cyclic voltammetry (CV) studies have been carried out on
these derivatives to understand the redox nature of the PBI
systems. Compounds 1–9 were dissolved in dichloromethane
that contained [nBu4N][PF6] (0.1m) for the electrochemical
studies. The electrochemical data with respect to the saturated
calomel electrode (SCE) are summarised in Table 1. The reduc-
tion potentials obtained for the PBI derivatives were compared
to those for the parent PBI 1. The PBI derivatives showed two
reversible cathodic waves in the range Ered =ꢁ0.8 to ꢁ0.6 V, in
line with PBI 1. These observations are similar to those report-
ed for 1,7-bis(phenylalkynyl) PBI by Marder et al.[19b] Regardless
of the nature of the substituents on the phenyl ring, the alkyn-
yl–phenyl derivatives 3–8 are more readily reduced than the
parent PBI. Furthermore, the reversibility of these waves also
Figure 6. Cyclic voltammograms of selected PBI derivatives in CH2Cl2 with
[nBu4N][PF6] (0.1m) as the supporting electrolyte (scan rate=100 mVsꢁ1).
emphasises the structural robustness of these molecules
(Figure 6). The dimethylaminophenyl-substituted PBI 9 exhibit-
ed higher reduction potentials than other substituted PBIs.
This stems from the fact that 9 is harder to reduce and indi-
cates the electron-rich nature of the PBI p backbone. Thus, it
could be surmised that the derivatives other than 9 are elec-
tron-deficient relative to the parent molecule. The reduction
potentials further reiterate that the LUMOs of the PBI deriva-
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tives are altered only a little by substitution on the phenyl
ring.
is far away from the PBI core. Therefore, it can be envisaged
that the alkynyl group in 3 resembles that of the CN moiety, as
reported by Wasielewski et al.,[11c] and imposes a minimal steric
constraint. In the absence of any noticeable steric hindrance
from the bay substituent, the observed planarity of the PBI
core may be explained by consideration of the packing forces
that act on the molecule in the solid state.
Single-crystal X-ray diffraction studies
An unambiguous structural proof is imminent to visualise the
planarity of the entire p system to ascertain the observed
properties. As documented in the literature, a solid-state struc-
tural proof for a bay-substituted PBI remains a challenge to
obtain. A recent report on the structure of a 1,7-disubstituted
PBI by Wꢁrthner et al.[31] describes a flat PBI core. This planarity
of the PBI core might be induced by the proximity of the bulki-
er substituents to the PBI scaffold, which exerts steric strain
from both above and below the molecular plane. The planarity
achieved by this approach was shown to be responsible for at-
taining sharp vibronic progression in the absorption spectra.
After repeated trials, X-ray quality single crystals of 3 were
obtained by slow vapour diffusion of hexane into a solution of
3 in chloroform. Except for the two 2,4-dimethylpentyl groups
on the imide functionality, which protrude above and below
the plane, the rest of the molecule possesses a planar geome-
try. Notably, the present example is the first structural charac-
terisation of alkynyl-linked mono-bay substituted PBI deriva-
tive. To our surprise, the C1-C12a-C12b-C12 dihedral angle is
only 1.88. This is interesting in the context that bay-substituted
PBI derivatives often deviate from planarity because of steric
congestion at the bay positions. Figure 7 depicts the top and
side views of the structure obtained from the X-ray diffraction
measurements. The planar structure obtained in the solid state
is different from that obtained from the computational optimi-
sation. In the present case, the alkynyl–phenyl substituent at
the bay position is neither bulky, nor induces any steric effect
on the crowded bay area. Furthermore, the phenyl substituent
The ethynyl-phenyl group appears to be positionally disor-
dered at two crystallographically equivalent positions (C1 and
C12) in the bay area with an equal occupancy. Overall, the
main PBI frame remains planar, which indicates that electronic
perturbation at the phenyl rings eventually influences the opti-
cal properties of the PBI frame. Thus, the planar scaffold of 3 is
a noteworthy example of a favourable structure–property cor-
relation. Careful selection of linkers and substituents could be
an efficient approach to achieve structural planarity, as well as
to fine-tune the optoelectronic properties.
Motivated by the flat structure of 3, we were prompted to
probe the structure of the mono-brominated precursor 2. The
solid-state structure has previously been reported only for 1,7-
PBI-Br2,[24,25] in which both bromine atoms protrude away from
the PBI plane; this is reflected even in its derivatives. Surpris-
ingly, even though the precursor PBI-Br (2) has been known for
nearly a decade, there has not been any structural characterisa-
tion reported for this molecule. A thorough understanding of
the structure of 2 would help to predict the possible geome-
tries of its derivatives, which prompted us to investigate the
structure of our precursor 2. Initial attempts to obtain X-ray
quality single crystals were discouraged by aggregation. Finally,
a suitable single crystal was obtained from vapour diffusion of
hexane into a solution of 2 in chloroform at 48C (Figure 8).
In contradiction to our expectation derived from the planar
structure of 3, the CꢁBr bond is not coplanar to the PBI core in
the solid-state structure. Rather, it has a torsional angle of
ꢀ228 above the plane of the PBI core. This is in sharp contrast
to the structure of 3, in which the PBI core and phenyl group
Figure 7. Single-crystal X-ray diffraction structure of 3 (top and side views).
Figure 8. Single-crystal X-ray diffraction structure of 2 (top and side views).
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are coplanar. However, this did not affect the PBI core, which
remains largely planar.
tion studies. The X-ray structure of 2 shows that the bromine
atom protrudes out of the plane of the PBI core, whereas 3 ex-
hibits a coplanar structure. This hints at the possibility of ex-
ploitation of the mono-substitution of PBIs to obtain flatter an-
alogues to preserve their superior photophysical properties.
The easy synthesis and good solubility of the present systems
could be useful in various future applications (charge-trans-
porting devices, non-linear optics, etc.), in which the tuning of
photophysical properties is required by means of simple func-
tionalisation.
In the case of 2, the compound crystallises in a centrosym-
metric monoclinic space group P21/c with Z=2 (Z’=0.5). The
true molecule (mono-bromo substitution at the ring) does not
have any symmetry. Therefore, the requirement of Z’=0.5 sug-
gests the presence of crystallographic disorder around the cen-
tral ring of the molecule, which generates the second half of
the molecule around the centre of inversion. This also leads to
positional disorder associated with the bromine atom at four
positions which were refined with an equal occupancy ratio
(0.25:0.25:0.25:0.25) at four positions (C1, C6, C7, C12). The dis-
order associated with the terminal isopropyl group was mod-
elled with the PART command in SHELXL97 at two occupan-
cies. Therefore, it can be understood that the bromine atom
protrudes outside the plane of the bay area, probably due to
steric hindrance. The functionalisation of the bromo derivative
with acetylenyl linkers avoids this steric effect and therefore
enables the molecule to become coplanar. Thus, the present
study demonstrates the versatility of the PBI framework, which
has the structural flexibility to anchor any desired functional
group without compromising the core geometry.
Experimental Section
X-ray crystallography
CCDC-959994 (3) and 959995 (2) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Synthesis
Compound 1: The parent PBI was synthesised according to the lit-
erature procedure.[26] A solution of 2,4-dimethylpent-3-ylamine
(0.688 g, 2.35 equiv), PDA (1 g, 1 equiv) in imidazole (10.2 g,
60 equiv) was stirred at 1408C for 4 h, then the mixture was dilut-
ed with ethanol. This was followed by the addition of 2m HCl
(100 mL). The reaction mixture was allowed to settle down. The
precipitates formed were collected by vacuum filtration, oven
dried and purified by silica gel chromatography (chloroform) to
give 1 (1.17 g, 80%). 1H NMR (500 MHz, CDCl3): d=8.69 (m, 8H),
4.78 (t, J=14 Hz, 2H), 2.74 (m, 4H), 1.14 (d, J=7 Hz, 12H),
0.96 ppm (d, J=7 Hz, 12H); 13C NMR (126 MHz, CDCl3): d=165.22,
164.10, 139.32, 134.60, 134.54, 131.33, 129.61, 126.48, 123.19,
123.11, 123.07, 114.09, 65.19, 29.72, 29.15, 21.85, 20.63 ppm; HRMS:
m/z calcd: 587.2904 [M+H]; found: 587.2903.
Conclusion
We have demonstrated the synthesis, photophysical proper-
ties, computational investigations and first structural character-
isation of alkynyl–phenyl mono-bay-functionalised PBI deriva-
tives. Incorporation of various phenylene substituents with
acetylene linkers into a single bay position of PBI facilitates ef-
ficient tuning of the electronic nature of the derivatives. Exten-
sion of conjugation here leads to a rigid PBI core rather than
a distorted scaffold. We also described the first single-crystal X-
ray structures of a mono-bromo PBI and mono-bay alkynyl–
phenyl-substituted PBI. Our investigations not only highlight
the PBI geometric framework and its effect on the optoelec-
tronic properties, but also focus on the apparent importance
of the physical structure of the phenylene substituents. Mono-
functionalisation preserves the planarity of the PBI core due to
minimal steric hindrance, thus preserves the photophysical
properties of the dyads. In addition, we have investigated the
CT characteristics of selected derivatives. Contrary to the un-
substituted PBI 3, CT transitions between the electron-donat-
ing bay substituent and PBI core are revealed in 4 and 9. In
the case of 9, the electron-rich nature of the dimethylamino
group further enhances the intramolecular CT efficiency from
the alkynyl–phenyl substituent to the PBI core. Time-resolved
spectroscopy revealed that the excited-state dynamics of the
PBI dyes are sensitive to the polarity of their surrounding
medium, in contrast to their ground-state behaviour. Particular-
ly, for PBI 9, which has a rotating dimethylamino group on the
substituent, a TICT process that accelerates the relaxation dy-
namics was observed. The electronic nature of the substituents
on the aryl motif may influence the photophysical and CT char-
acter of the parent PBI due to a possible push–pull mecha-
nism. For the first time, we have experimentally supported the
prediction of a planar PBI core by single-crystal X-ray diffrac-
Compound 2: Excess bromine (6.94 mL, 80 equiv) was added to
a solution of 1 (1 g, 1 equiv) in CH2Cl2 and the reaction mixture
was stirred at 23–258C for 28 h. The progress of the reaction was
monitored by TLC. The excess bromine was removed by gentle
purging of air inside the solution. The compound was purified by
repetitive flash chromatography with hexane/chloroform as elu-
ents. The first band eluted was characterised as dibromo-perylene-
bisimide. The second band was characterised as 2 (0.65 g, 57%).
1H NMR (500 MHz, CDCl3): d=9.83 (m, 1H), 8.96 (d, J=17 Hz, 1H),
8.75 (m, 5H), 4.77 (m, 2H), 2.76 (sep, J=6 Hz, 4H), 1.14 (m, 12H),
0.96 ppm (m, 12H); 13C NMR (126 MHz, CDCl3): d=165.44, 165.06,
164.98, 163.84, 162.69, 139.62, 138.98, 133.67, 133.62, 131.65,
131.14, 130.95, 130.42, 128.98, 128.15, 127.11, 123.83, 123.80,
123.31, 123.02, 120.99, 65.46, 65.40, 65.26, 65.24, 29.16, 29.15,
29.10, 29.06, 21.83, 21.75, 20.60, 20.54, 20.53 ppm; HRMS: m/z
calcd: 665.2009 [M]+; found: 665.2004.
Compound 3: A dry Schlenk tube was charged with [PdCl2(PPh3)2]
(1.3 mg, 4 mol%), CuI (0.6 mg, 2 mol%), dry THF (3 mL) and N,N-
diisopropylamine (2 mL). The resultant solution was degassed vig-
ourously under argon. Compound 2 (30 mg, 1 equiv) and phenyl-
acetylene (6 mL, 1.2 equiv) were added and the reaction mixture
was heated at reflux (70–808C). The progress of the reaction was
monitored by TLC at regular intervals. After the reaction was com-
plete, the solvent was evaporated under vacuum. The reaction
mixture was mounted on a column with silica gel (230–400 mesh)
and was eluted with hexane/chloroform (20:80). Further purifica-
Chem. Eur. J. 2014, 20, 5776 – 5786
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tion was done by recrystallisation in hexane/CH2Cl2 to give 3 as
a red solid (29 mg, 97%). H NMR (500 MHz, CDCl3): d=10.35 (two
127.13, 126.80, 125.97, 125.90, 125.87, 125.84, 119.37, 97.04, 92.94,
65.38, 65.35, 65.25, 29.15, 29.14, 29.11, 29.07, 21.82, 21.76, 20.60,
20.56, 20.54 ppm; 19F NMR (400 MHz, CDCl3): d=ꢁ62.93 ppm;
HRMS: m/z calcd: 755.3091 [M+H]; found: 755.3082.
1
merged doublets, J=8.24 Hz, 1H), 8.93 (d, J=13.12 Hz, 1H), 8.74
(m, 5H), 7.73 (m, 2H), 7.52 (m, 3H), 4.79 (m, 2H), 2.75 (m, 4H), 1.15
(d, J=6.67 Hz, 12H), 0.96 ppm (d, J=6.77 Hz, 12H); 13C NMR
(126 MHz, CDCl3): d=165.35, 165.18, 164.72, 164.24, 164.10, 164.07,
163.60, 138.89, 138.26, 134.57, 134.32, 134.23, 131.86, 131.71,
131.63, 130.99, 130.91, 129.76, 129.19, 128.92, 128.54, 127.36,
127.07, 126.84, 123.96, 123.82, 123.62, 123.58, 123.20, 123.04,
122.92, 122.75, 122.28, 122.17, 120.34, 99.50, 91.11, 91.08, 65.21,
30.96, 29.16, 29.12, 29.09, 21.85, 21.83, 21.78, 20.62, 20.58,
20.55 ppm; HRMS: m/z calcd: 686.3139 [M]+; found: 686.3144.
Compound 8: Compound 8 was synthesised from 4-ethynyl-ben-
zonitrile (6.8 mg, 1.2 equiv) by the procedure employed for 3.
Yield: 30.4 mg, 95%. 1H NMR (500 MHz, CDCl3): d=10.16 (two
merged doublets, J=8.26 Hz, 1H), 8.91 (d, J=13.2 Hz, 1H), 8.76 (m,
5H), 7.80 (m, 4H), 4.79 (m, 2H), 2.75 (m, 4H), 1.15 (two merged
doublets, J=6.68 Hz, 12H), 0.96 ppm (d, J=6.76 Hz, 12H); 13C NMR
(126 MHz, CDCl3): d=165.21, 165.07, 165.01, 164.09, 163.97, 163.89,
163.43, 138.65, 138.04, 135.12, 131.5, 127.14, 131.50, 127.14,
123.72, 122.35, 118.21, 112.92, 96.42, 96.37, 94.68, 94.66, 65.41,
65.37, 65.29, 29.14, 29.13, 29.09, 29.06, 21.81, 21.76, 20.59, 20.55,
20.53 ppm; HRMS: m/z calcd: 712.3170 [M+H]; found: 712.3182.
Compound 4: Compound 4 was synthesised from 1-ethynyl-3,5-di-
methoxy-benzene (8.7 mg, 1.2 equiv) by the same procedure em-
ployed for 3. Yield: 40 mg, 92%. 1H NMR (500 MHz, CDCl3): d=
10.32 (two merged doublets, J=8.23 Hz, 1H), 8.92 (d, J=13.67 Hz,
1H), 8.74 (m, 5H), 6.85 (brd, 2H), 6.62 (brs, 1H), 4.79 (m, 2H), 3.9
(s, 6H), 2.74 (m, 4H), 1.14 (d, J=6.65 Hz, 12H), 0.96 ppm (d, J=
6.75 Hz, 12H); 13C NMR (126 MHz, CDCl3): d=165.34, 165.16,
164.72, 164.23, 163.61, 160.93, 138.42, 134.36, 134.26, 131.74,
131.62, 131.03, 130.91, 129.19, 127.36, 127.05, 126.82, 123.47,
123.08, 120.16, 109.59, 109.56, 102.99, 99.70, 90.50, 65.32, 65.30,
65.21, 55.62, 29.16, 29.12, 29.08, 21.83, 21.77, 20.62, 20.57,
20.54 ppm; HRMS: m/z calcd: 747.3429 [M+H]; found: 747.3437.
Compound 9: Compound 9 was synthesised from 4-ethynyl-N,N-
dimethylaniline (7.8 mg, 1.2 equiv) by the procedure employed for
3. Purification of the reaction mixture was a tedious task. The reac-
tion mixture was purified by repetitive column chromatography
with hexane/ethyl acetate as eluents. Stacking of the compound
on silica gel hindered the yield (21 mg, 64%). 1H NMR (500 MHz,
CDCl3): d=10.43 (two merged doublets, J=8.28 Hz, 1H), 8.89 (d,
J=13.48 Hz, 1H), 8.74 (m, 5H), 7.60 (t, J=14.60 Hz, 2H), 6.80 (m,
2H), 4.80 (m, 2H), 3.12 (s, 6H), 2.75 (m, 4H), 1.15 (t, J=11.50 Hz,
12H), 0.97 ppm (m, 12H); 13C NMR (126 MHz, CDCl3): d=207.01,
165.46, 165.32, 164.89, 164.36, 164.23, 163.77, 151.05, 138.90,
138.28, 135.05, 134.26, 133.35, 132.70, 132.61, 131.81, 131.52,
131.08, 131.03, 130.80, 130.31, 129.26, 128.07, 127.44, 126.89,
126.52, 123.52, 123.48, 122.89, 122.68, 122.56, 121.99, 121.68,
112.00, 108.53, 99.98, 90.80, 65.22, 65.12, 40.16, 30.96, 29.17, 29.13,
29.10, 21.87, 21.84, 21.79, 20.65, 20.63, 20.59, 20.56 ppm; HRMS:
m/z calcd: 730.3639 [M+H]; found: 730.3647.
Compound 5: Compound 5 was synthesised from 1-ethynyl-3,5-
bis(trifluoromethyl)benzene (12.8 mg, 1.2 equiv) by the same pro-
cedure employed for 3. Yield: 34.7 mg, 94%. 1H NMR (500 MHz,
CDCl3): d=10.14 (two merged triplets, J=2.38 Hz, 1H), 8.92 (d,
J=13.84 Hz, 1H), 8.76 (m, 5H), 8.12 (d, J=5.60 Hz, 2H), 7.98 (s, 1H),
4.79 (t, J=16.63 Hz, 2H), 2.75 (m, 4H), 1.15 (d, J=6.69 Hz, 12H),
0.96 ppm (d, J=6.78 Hz, 12H); 13C NMR (126 MHz, CDCl3): d=
165.20 165.00, 164.96, 164.49, 164.09, 163.89, 163.84, 163.35,
138.28, 137.91, 134.95, 134.59, 134.27, 134.07, 133.79, 133.74,
133.57, 133.48, 132.75, 132.48, 132.27, 131.78, 131.56, 131.06,
130.89, 129.19, 128.99, 128.97, 127.31, 127.05, 126.74, 124.55,
123.88, 123.79, 123.39, 123.36, 122.97, 122.91, 121.71, 118.44, 95.00,
93.66, 65.42, 65.39, 65.30, 29.16, 29.14, 29.10, 29.06, 21.82, 21.75,
20.60, 20.55, 20.53 ppm; 19F NMR (400 MHz, CDCl3): d=
ꢁ63.06 ppm; HRMS: m/z calcd: 823.2965 [M+H]; found: 823.2966.
Compound 6: Compound 6 was synthesised from 1-ethynyl-3,5-di-
fluoro-benzene (7.45 mg, 1.2 equiv) by the same procedure em-
ployed for 3. Yield: 31.2 mg, 96%. 1H NMR (500 MHz, CDCl3): d=
10.16 (two merged doublets, J=8.23 Hz, 1H), 8.89 (d, J=13.0 Hz,
1H), 8.75 (m, 5H), 7.32 (t, J=12.66 Hz, 2H), 6.99 (t, J=17.46 Hz,
1H), 4.79 (m, 2H), 2.75 (m, 4H), 1.15 (two merged doublets, J=
6.65 Hz, 12H), 0.97 ppm (d, J=6.77 Hz, 12H); 13C NMR (126 MHz,
CDCl3): d=165.25, 165.05,165.04, 164.57, 164.14, 164.04, 163.94,
163.43, 162.05, 161.94, 138.74, 138.13, 134.87, 134.64, 134.22,
133.91, 132.67, 131.72, 131.60, 131.36, 131.00, 130.88, 129.17,
128.81, 127.30, 127.07, 126.77, 124.76, 124.27, 123.68, 123.29,
123.30, 122.87, 119.04, 114.84, 114.66, 105.91, 96.14, 92.36, 65.27,
29.15, 21.82, 21.76, 20.61, 20.56, 20.54 ppm; 19F NMR (400 MHz,
CDCl3): d=ꢁ108.2 ppm; HRMS: m/z calcd: 723.3029 [M+H]; found:
723.3032.
Acknowledgements
The work at Bhopal was supported by IISER Bhopal and De-
partment of Science and Technology, New Delhi (DST/TSG/PT/
2009/115 & DST/PHY/2011/017). RM thanks UGC, New Delhi for
a Senior Research Fellowship. The work at Yonsei was support-
ed by the Global Frontier R&D Program of the Center for Multi-
scale Energy System (2012-8-2081), Mid-career Researcher Pro-
gram (2010-0029668) of National Research Foundation (NRF)
grant funded by MEST of Korea and AFSOR/AOARD grant
(FA2386–09–4092). The authors sincerely thank the referees for
the insightful comments to improve the manuscript.
Keywords: charge transfer · conjugation · fluorescence ·
perylene bisimides · structure elucidation
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