Supramolecular assembly of dipeptide functionalized benzo[ghi]perylene monoimide directs white light emission via donor-acceptor interactions

Article abstract of DOI:10.1039/c5ra11804b

An aromatic dipeptide Phe-Phe (FF) functionalized benzo[ghi]perylene monoimide (BPI) is synthesized and its optical behaviour and self-assembly nature in different organic solvents are studied. Diels-Alder reaction of perylene and N,N′-maleoyl-Phe-Phe-OMe

Full text of DOI:10.1039/c5ra11804b

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Supramolecular Assembly of Dipeptide Functionalized Benzo[ghi]perylene
Monoimide Directs White Light Emission via Donor-Acceptor Interactions
Manoj K. Manna,[a] Dnyaneshwar B. Rasale,[a] and Apurba K. Das*[a]
[a]Department of Chemistry, Indian Institute of Technology Indore, Indore 452017, India
*To whom correspondence should be made
Dr. Apurba K. Das
Email: apurba.das@iiti.ac.in
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ABSTRACT
An aromatic dipeptide PheꢀPhe (FF) functionalized benzo[ghi]perylene monoimide (BPI) is
synthesized and its optical behavior, selfꢀassembly nature in different organic solvents are
studied. DielsꢀAlder reaction of perylene and N,NʹꢀmaleoylꢀPheꢀPheꢀOMe produces a new
fluorophores (BPIꢀFFꢀOMe) which is highly soluble in a wide range of organic solvents without
any swallow alkyl chain. The dipeptide appended benzo[ghi]perylene monoimide (BPIꢀFFꢀOMe)
selfꢀassembles into nanospheres both in methanol and toluene solvents. The nanospherical
structures are driven by hydrogen bonding and πꢀπ stacking interactions among aromatic
dipeptide appended benzo[ghi]perylene monoimide. Selfꢀassembled nanoꢀarchitecture of BPIꢀ
FFꢀOMe is studied by a number of microscopic techniques. Tunable solvent dependent
photophysical behavior of BPIꢀFFꢀOMe is also studied. The synthesized BPIꢀFFꢀOMe exhibits
positive solvatochromic emission (λem (toluene) = 508 nm; λem (methanol) = 566 nm) as a
function of solvent polarity with higher lifetimes (τtoluene = 8.4 ns; τmethanol = 5.7 ns) and quantum
ꢂ
ꢃ
yields ( ꢀꢁ = 0.55 toluene ; ꢀꢁ = 0.39ꢂmethanolꢃ . The BPIꢀFFꢀOMe shows white light
emission upon energy transfer between donor pyrenebutyric acid (PyBA) to acceptor BPIꢀFFꢀ
OMe molecule. In order to study the role of peptide sequence PheꢀPhe attached with perylene
moiety of BPIꢀFFꢀOMe molecule, an amino acid functionalized BPIꢀLꢀOMe molecule is also
synthesized. BPIꢀLꢀOMe also shows white light emission upon energy transfer from donor
pyrenebutyric acid (PyBA) molecules to acceptor BPIꢀLꢀOMe molecules. Benzo[ghi]perylene
moiety plays an important role for energy transfer from donor pyrene moiety to acceptor
benzo[ghi]perylene moiety that exhibits white light emission.
2

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1. INTRODUCTION
Selfꢀassembled πꢀconjugated perylene based amphiphiles have attracted considerable interest in
recent years.1ꢀ3 Selfꢀassembled perylene based amphiphiles have been used in the fabrication of
supramolecular 1D, 2D, and 3D based micro and nanoꢀarchitectures for the applications in
biology and supramolecular electronics.4ꢀ6 A variety of perylene based molecules have been
designed for the development of perylene dye chemistry through extension of perylene core via
functionalizing the bay position of perylene.7ꢀ13 Extensive work has been done on perylene
diimides (PDI).14 The stable supramolecular architectures are achieved by designing the
molecular structure of selfꢀassembled materials as well as optimizing the response of suitable
stimuli.15ꢀ19 Various stimuli have been used to drive the selfꢀassembly of organic moieties via
hydrogen bonding, πꢀπ stacking and hydrophobic interactions. Zang et al. reported several
examples of PDI based selfꢀassembly.20ꢀ22 Selfꢀassembly of amino acid functionalized perylene
bisimides is also reported in literature.23ꢀ25 Recently, selfꢀassembly of peptides functionalized
perylene bisimides has been reported.26ꢀ27 In general, low molecular weight hydrophobic peptides
have been used for the evolution of selfꢀassembled architectures.28ꢀ32
Schenning et al. reported mixed fluorescent coꢀoligomer systems for the creation of whiteꢀlightꢀ
emitting gels.33 Ajayaghosh et al. reported supramolecular organogel composed of intertwined
twisted helical fibrillar assemblies made of OPPV decorated with cholesterol units.34 Meijer and
coꢀworker also described a supramolecular copolymer resulting from the selfꢀassembly in films
of blue, green, and red πꢀconjugated oligomers.35 Zhang and coꢀworkers described photoꢀ
facilitated aggregation and stable whiteꢀlight emission in solution.37 Zhang et al. also reported
bright whiteꢀlight emission from a novel donorꢀacceptor small organic molecule in the solid state
via intermolecular charge transfer.38 Number of strategies have been used to fabricate white light
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emitting materials. These include metal mediated white light emission,39 combination of three
primary colors (red, green and blue), polymers40 and selfꢀassembly of organic molecules.41
Development of low molecular weight white light emitting material is particularly interesting
due to their good solubility in distinct solvents. They can easily be applied on solid surface or
can be soaked on solid materials. There are various reports (Table S1) but none of the examples
show acceptor perylene monoimide functionalized small organic molecules based white light
emission upon donorꢀacceptor interactions. Thus, keeping all these aspects in mind, we have
explored an aromatic benzo[ghi]perylene monoimide (BPI) functionalized diphenylalanine
methyl ester (BPIꢀFFꢀOMe) and BPIꢀLꢀOMe for the evolution of white light emission. The BPIꢀ
FFꢀOMe is highly soluble in various organic solvents and shows solvochromic behavior in
different solvents.
Herein, our aim is to explore selfꢀassembly of benzo[ghi]perylene monoimide (BPI)
functionalized dipeptide for the evolution of white light emission via donorꢀacceptor interactions.
White light emitting materials have become an area of interest because of their optoꢀelectronic
applications. Till date, several research groups have emphasized the development of white light
emitting materials. Acceptors benzo[ghi]perylene monoimides (BPIs) and donor pyrenebutyric
acid lead to white light emission via donorꢀacceptor interactions. Benzo[ghi]perylene monoimide
functionalized dipeptide (BPIꢀFFꢀOMe) and amino acid (BPIꢀLꢀOMe) are a new class of
acceptor molecules which show white light emission with pyrene via energy transfer from donor
to acceptor molecular interactions (Scheme 1).
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2. RESULTS AND DISCUSSION
2.1 Synthesis of Dipeptide Functionalized Benzo[ghi]perylene Monoimide
In general, the widely followed pathway to synthesis benzo[ghi]perylenemonoimide is a Dielsꢀ
Alder reaction of perylene with maleic anhydride and subsequent condensation reaction42ꢀ45 with
primary aliphatic amines.46ꢀ49 Herein, we have considered an alternative method to synthesize
benzo[ghi]perylenemonoimide to obtain the product with high yield in respect of bulky hindered
substitute. The dienophile imide derivative has been synthesized by reaction of maleic anhydride
with a dipeptide diphenylalanine. Crucial steps involved in the synthesis the dienophile are (i)
the reaction of H2NꢀLꢀPheꢀLꢀPheꢀOMe with maleic anhydride for the formation of N,Nʹꢀmaleylꢀ
L
ꢀPheꢀ
L
ꢀPheꢀOMe and (ii) and subsequent cyclization of N,Nʹꢀmaleylꢀ
LꢀPheꢀLꢀPheꢀOMe to form
N,Nʹꢀmaleoylꢀ
L
ꢀPheꢀ ꢀPheꢀOMe using 1 equiv. of ZnCl2 and 1.5 equiv. of HMDS in benzene
L
solvent.50ꢀ51 Finally, a dipeptide PheꢀPheꢀOMe appended benzo[ghi]perylenemonoimide 1 was
obtained using DielsꢀAlder reaction of perylene with N,NʹꢀmaleoylꢀPheꢀPheꢀOMe as orange
color solid powder with 96% yield. Synthetic methodology of Nꢀdipeptideꢀfunctionalized
benzo[ghi]peryleneꢀ1,2ꢀdicarboxylic monoimide (BPIꢀFFꢀOMe 1) and other derivatives have
been shown in the supporting information (Scheme S1). Dipeptide attached with BPI core helps
to increase the solubility of compound BPIꢀFFꢀOMe in a range of organic solvents, which acts as
an alternative of mostly used ‘swallow’ alkyl chains directly attached to the main perylene or
PDI moieties.
2.2 Synthesis of Leucine Methyl Ester Functionalized Benzo[ghi]perylene Monoimides
Similarly, BPIꢀLꢀOMe 2 was synthesized by (i) the reaction of H2NꢀLꢀLeuꢀOMe with maleic
anhydride for the formation of N,Nʹꢀmaleylꢀ
LꢀLeuꢀOMe 6b and (ii) and subsequent cyclization of
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N,Nʹꢀmaleylꢀ
LꢀLeuꢀOMe to form N,NʹꢀmaleoylꢀLꢀLeuꢀOMe 7b using 1 equiv. of ZnCl2 and 1.5
equiv. of HMDS in benzene solvent.50ꢀ51 Finally,
a
LeuꢀOMe appended
benzo[ghi]perylenemonoimide 2 was obtained using DielsꢀAlder reaction of perylene with N,Nʹꢀ
maleoylꢀ ꢀLeuꢀOMe 7b as a red solid powder with 76% yield.
L
2.3 Photophysics of Dipeptide Functionalized Benzo[ghi]perylene Monoimide
UVꢀvis absorption study was performed to determine the optical and electronic properties of
synthesized BPIꢀFFꢀOMe. UVꢀvis absorption spectra of BPIꢀFFꢀOMe were recorded in various
solvents of different polarity (Figure 1A and Figure S1). UVꢀvis absorption spectra show
characteristics absorption bands between 325ꢀ500 nm in an aprotic solvent toluene. The strongest
absorption peak at 341 nm with molar absorptivity 51000 Mꢀ1cmꢀ1 is attributed to π→π*
transition of benzo[ghi]perylene monoimide system. The lowest energy transition at 483 nm with
a shoulder at 455 nm corresponds to S0→S1 transition. The absorbance at 455 nm is attributed to
the transition at highest excited vibrational level. The minimal blue shift was observed in the
UVꢀvis absorbance spectra of BPIꢀFFꢀOMe when changing the solvent from aprotic nonꢀpolar
toluene to protic polar solvent methanol (Figure 1A). In general, polar solvents plays important
role in stabilization of molecules in ground state and excited state which lead to either
hypsochromic or bathochromic shift in the absorption maxima.52 The λmax of BPIꢀFFꢀOMe in ten
different solvents are listed in Table 1.
Later, we tried to evaluate the fluorescence properties of the fluorophore BPIꢀFFꢀOMe. The
emission of BPIꢀFFꢀOMe strongly depends on solvent polarity. Herein, we observed that effect
of solvent polarity on the emission maxima was more pronounced than the absorption maxima
(Figure 2A and S2). The emission maxima of BPIꢀFFꢀOMe in various aprotic to protic solvents
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are listed in Table 1. Excitation wavelength at 337 nm was used to collect the emission spectra of
compound BPIꢀFFꢀOMe. The emission of BPIꢀFFꢀOMe shifts from green emission to yellow
emission in the visible region as a function of increasing solvent polarity (Figure 2A). BPIꢀFFꢀ
OMe exhibits emission maxima at 508 nm in aprotic toluene solvent whereas the emission was
shifted to 566 nm in polar protic methanol solvent. The strong red shift of about 58 nm was
observed with decrease in both the fluorescence intensity as well as the quantum yield (Table 1)
upon increase in solvent polarity. The peak broadening was also observed with increase in
solvent polarity. The shift in fluorescence maximum was attributed due to the solvent
polarizability factors (ꢁf) of different solvents. The excited state dipole moment (µe) of a
molecule is different in toluene and methanol which leads to the variation of change in dipole
moment ꢁµ (ꢁµ = µeꢀ µg). This dipole moment (ꢁµ) directs the excited state energy levels of a
molecule in a solvent.46 The excited state energy level is lower in methanol than toluene.
Therefore, energy gap difference is lower in methanol than toluene and corresponding
fluorescence emission is shifted to longer wavelength (red shift). Similarly, BPIꢀLꢀOMe is also
synthesized to determine the role of dipeptide in emissions. Spectroscopic analysis of BPIꢀLꢀ
OMe exhibits similar emission to BPIꢀFFꢀOMe (Figure S3). Thus, benzo[ghi]perylene moiety
plays an important role in the emission as well as chromic effects for two synthesized
compounds.
The excitation spectrum for the 564 nm emission band of BPIꢀFFꢀOMe in methanol solvent
shows the peaks at 315 nm, 327 nm, 369 nm, 384 nm, 453 nm and 476 nm which is similar to the
absorption spectrum with a 30 nm red shift (Figure S4). The fluorescence quantum yields (ꢀꢄꢅꢃ
of BPIꢀFFꢀOMe in ten different solvents are listed in Table 1. The fluorescence quantum yield of
BPIꢀFFꢀOMe was found to be in between 0.39 in methanol and 0.55 in toluene. The nature of
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solvents plays an important role in emission of BPIꢀFFꢀOMe. Similar observation was reflected
from the optical images of BPIꢀFFꢀOMe in different solvents under UV light at a wavelength of
365 nm. The compound BPIꢀFFꢀOMe showed green fluorescence in aprotic nonꢀpolar solvent
toluene while yellow fluorescence was observed in polar protic methanol solvent (Scheme 1).
A mirror image relationship between the electronic transitions of S0
→S1 (absorption) and S1→S0
(emission) of BPIꢀFFꢀOMe in aprotic nonpolar toluene solvent is clearly observed at the
wavelength of 410 nm ꢀ 590 nm (Figure 2B).46ꢀ47 Solvent dependent Stokes shifts for BPIꢀFFꢀ
OMe was interpreted in terms of the LippertꢀMataga equation. Stoke shifts calculated from lower
energy transition of BPIꢀFFꢀOMe molecule (Figure 3A). Figure 3B represents the Lippertꢀ
Mataga plot of S1→S0 Stokes’ shift ꢁν versus ꢁꢆ for molecule 1 in the solvents listed in Table 1.
The plot depicts a good linear relationship of ꢁν and ꢁꢆ.
Time Correlated Single Photon Counting (TCSPC) experiment was performed to determine the
average fluorescence lifetime of BPIꢀFFꢀOMe. To measure the average fluorescence decay traces
of BPIꢀFFꢀOMe, the samples were excited at 376 nm and the emissions were monitored at 508
nm for toluene and 566 nm for methanol. The excited state lifetime (τ) of BPIꢀFFꢀOMe
decreases with increasing in solvent polarity from 8.4 ns in toluene to 5.7 ns in methanol (Figure
4).
2.4 Theoretical Calculation
DFT calculation was performed to determine the energy minimized geometry of BPIꢀFFꢀOMe in
ground state. Theoretical calculation of highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of synthesized compound BPIꢀFFꢀOMe were calculated
using the B3LYP/6ꢀ31G* basis set of Gaussian 09 program. The highest occupied molecular
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orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of BPIꢀFFꢀOMe 1
molecule are ꢀ5.57 eV and ꢀ2.50 eV respectively. HOMO of the molecule is mostly found on
conjugated BPI core and the LUMO mostly exists at imide nitrogen of BPI core (Figure 5).
HOMO/LUMO calculation determines the band gap energy of molecules.53 The calculated band
gap energy of BPIꢀFFꢀOMe is 3.06 eV. The optical band gap 2.6 eV was calculated from the
experiment performed on UVꢀVisible spectroscopy in methanol. It is found that Nꢀcapped
dipeptide appended BPI molecule has no effect on the HOMO and LUMO distribution of BPI
core.
2.5 Self-assembly Study of BPI-FF-OMe
1
Variable temperature dependent H NMR study was performed in DMSOꢀd6 to understand the
various nonꢀcovalent interactions responsible for the selfꢀassembly nature of aromatic dipeptide
PheꢀPheꢀOMe functionalized benzo[ghi]perylenemonoimide (BPIꢀFFꢀOMe). The change in
chemical shift (δ) of aromatic BPI ring protons and peptide NH proton with different
temperatures suggests the presence of intermolecular Hꢀbonding and πꢀπ stacking interactions of
BPIꢀFFꢀOMe molecules. A significant change in chemical shift (δ) was observed for NH proton
of dipeptide backbone. The chemical shift of NH proton was shifted to upꢀfield (8.86 ppm to
8.60 ppm) with increase in temperature from 303 K to 343 K (Figure 6). The upꢀfield shifting is
due to the weaker in Hꢀbonding between BPI molecules and corresponding disꢀassembly on
warming.54ꢀ57 Moreover, the downꢀfield δ shift of BPI protons with increasing temperature is the
1
evidence in favor of πꢀπ stacking interactions. H NMR signals of all benzo[ghi]perylene ring
protons shifted to downꢀfield because the aromatic ring protons of benzo[ghi]perylene moieties
experience deꢀshielding effect from the ring current at high temperature where the interacting π
clouds of aromatic protons are loosely associated to each other than the lower temperature.58ꢀ59
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Therefore, Hꢀbonding and πꢀπ stacking interactions play the crucial role in the stabilization of
selfꢀassembly structure of BPIꢀFFꢀOMe molecule where the amide functionality of peptide
backbone provides hydrogenꢀbonding interaction and benzo[ghi]perylene monoinide protons
contributes in πꢀπ stacking interactions. The amphiphile BPIꢀFFꢀOMe form selfꢀassembled
nanospherical structures in different solvents. To further investigate the formation, stability and
equilibrium state of nanospheres, the particle size distribution was analyzed by dynamic light
scattering (DLS) experiments in methanol and toluene at 25 °C (Figure 7). The hydrodynamic
diameters of nanospheres 857 nm and 613 nm were observed in DLS analysis (Table S2).
Polydispersity index was calculated as 0.47 in toluene and 0.30 in methanol solutions. These
results suggest that nanospheres are polydisperse both in toluene and methanol solutions. The
estimated zetaꢀpotential of formed nanospheres of BPIꢀFFꢀOMe were ꢀ25.89 mV and ꢀ17.97 mV
in toluene and methanol respectively. These zeta potential determine the surface charge of
nanospheres.
2.6 Morphological Characterization: Supramolecular Nanospheres
Scanning electron microscope (SEM), transmission electron microscope (TEM) was used to
explore the morphology of BPIꢀFFꢀOMe in an aprotic solvent toluene and protic solvent
methanol. The solution of BPIꢀFFꢀOMe in toluene and methanol showed well organized uniform
nanosphere architectures (Figures 8A and 8B). The possible mechanism for nanosphere
formation could lead via micellar formation in the initial stage followed by higher order selfꢀ
assembly into the nanosphere architectures (Scheme 2). An aromatic dipeptide PheꢀPheꢀOMe
functionalized benzo[ghi]peryleneꢀmonoimide (BPIꢀFFꢀOMe) selfꢀorganized into nanospheres
via intermolecular hydrogen bonding and πꢀπ stacking interactions in toluene and methanol.
SEM results are consistent with the results obtained from TEM analysis (Figure 8C and 8D).
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Atomic force microscopic images were recorded upon drop casting of a solution of BPIꢀFFꢀOMe
on glass substrate. The AFM images showed the similar characteristics features of nanosphere
architectures which were observed for the SEM and TEM analysis. The average heights of
nanosphere architectures are 176 nm in methanol and 203 nm in toluene (Figures 9A and 9B).
Scheme 2 depicts the possible selfꢀassembly process of BPIꢀFFꢀOMe into nanospherical
architectures.
2.7 Donor-acceptor Interactions Lead to White Light Emission
The aromatic πꢀπ stacking interaction of BPIꢀFFꢀOMe was utilized in white light emission
through the coꢀassembly with 1ꢀpyrenebutyric acid (PyBA). PyBA was selected because it
contains πꢀconjugated aromatic moiety. Both the aromatic donor and acceptor molecules can
participate in aromatic πꢀπ stacking interaction and lead to energy transfer. Solution of donor
PyBA to acceptor BPIꢀFFꢀOMe 1 in methanol show white light emission upon exposure to a UV
lamp at 365 nm (Scheme 1).
UVꢀVis spectroscopic studies were performed to examine spectral characteristic features of
different molar ratios of donor and acceptor molecules (Figure 10A). Emission spectra were also
collected to establish the mechanism of energy transfer from donor PyBA to acceptor BPIꢀFFꢀ
OMe 1 molecules. Titrating PyBA with BPIꢀFFꢀOMe 1, emission spectra were recorded at an
excitation wavelength of 341 nm. The gradual addition of BPIꢀFFꢀOMe 1 to PyBA, a steady
decrease in fluorescence intensity at 375 nm, 395 nm, 415 nm and a concomitant increase of a
new peak at 568 nm (Figure 10B) was observed. The energy transfer from donor PyBA to
acceptor BPIꢀFFꢀOMe is well noticeable upon quenching emission peak of donor and
subsequently enhancement of emission of acceptor molecules in methanol (Figure 10C). In
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general, white light is constituted by many wavelengths. The emission spectra of equimolar
mixture of donor PyBA and acceptor BPIꢀFFꢀOMe show various emission peaks in the range of
350 nm to 650 nm (Figure S5).
The individual emission of BPIꢀFFꢀOMe and PyBA in a protic methanol solvent was analyzed.
BPIꢀFFꢀOMe emits yellow fluorescence upon exposure to UV lamp at 365 nm. However, a
solution of PyBA shows blue emission under UV lamp at 365 nm (Figure 10D). The
combination of donorꢀacceptor pair at different molar ratio shows remarkable changes in the
emission. A white light emission was observed when a pair of donor (PyBA)ꢀacceptor (BPIꢀFFꢀ
OMe) (10:1) was exposed to UV lamp at 365 nm in methanol (Figure 9B and D). The white light
emission was observed due to the energy transfer between donor and acceptor. The white light
emitting solution of PyBA:BPIꢀFFꢀOMe coated over the silica plate shows excellent white light
emission (Figure S6). Thus, the solution PyBA:BPIꢀFFꢀOMe can be used for the coating purpose
in various materials. Similarly, BPIꢀLꢀOMe also exhibits white light emission upon mixing with
different molar ratio of PyBA (Figure S7 and S8) which is confirmed by fluorescence
spectroscopy (Figure S8).
3. CONCLUSION
In this work, we have synthesized an aromatic benzo[ghi]perylene monoimide capped
derivatives BPIꢀFFꢀOMe and BPIꢀLꢀOMe. The optical behavior of BPIꢀFFꢀOMe was studied in
a wide range of organic solvents. BPIꢀFFꢀOMe preferably forms nanosphererical architecture
both in protic polar and aprotic solvents like methanol and toluene. Intermolecular hydrogen
bonding and aromatic πꢀπ stacking interactions play important role in the formation of
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nanospherical architecture of dipeptide functionalized benzo[ghi]perylene monoimide (BPIꢀFFꢀ
1
OMe). Temperature dependent HꢀNMR studies support the intermolecular interactions in
solution phase selfꢀassembly of BPIꢀFFꢀOMe. The microscopic techniques including SEM, TEM
and AFM elucidate the nanostructural morphology of BPIꢀFFꢀOMe. The theoretical studies also
support the experimental results. The acceptor benzo[ghi]perylene monoimide BPIꢀLꢀOMe
molecule also shows white light emission through efficient energy transfer between donor
acceptor pairs where pyrenebutyric acid was used as an efficient donor molecule. These results
could be an efficient approach for the construction of new organic light emitting devices.
4. EXPERIMENTAL SECTION
4.1 Materials
All reagents and solvents were purchased from commercial sources and used without
purification. Solvents were distilled from appropriate drying agents prior to use.60 MilliꢀQ water
was used whenever was required.
4.2 General characterization
All NMR characterizations were carried out on a Bruker AV 400 MHz spectrometer at 300 K.
Compound concentrations were in the range 5ꢀ10 mmol Lꢀ1 in CDCl3 and (CD3)2SO. Mass
spectra were recorded on a Bruker micrOTOFꢀQ II by positive mode electrospray ionisation.
Dynamic light scattering (DLS) and zeta potential measurements were made with a NanoPlusꢀ3
zeta/nano particle analyzer (Micromeritics Instrument). The errors for h and ξ values were ±2
d
nm and ±10%, respectively. Field Emission Scanning Electron Micrograps (FEꢀSEM) were
recorded in a Carl Zeiss supra 55 operated at an accelerating voltage 200 kV. For SEM study,
BPIꢀFFꢀOMe (6 mmol Lꢀ1) solutions were dried on a glass slide from respective solvents toluene
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and methanol and coated with gold. TEM measurements were performed on a PHILIPS electron
microscope (model: CM 200), operated at 200 kV. Samples were prepared by placing a drop of
the solution on carbon coated copper grids and dried under lamp. AFM measurements were
performed on a scanning probe microscope AISTꢀNT instrument (model no. smartSPMꢀ1000).
The samples were prepared by drop casting on glass substrate and dried under vacuum.
Compound 1 was dissolved in respective solvents. The samples were allowed to stand for 48
hours at room temperature. Then the solutions drop casted on to the glass substrate and allow to
dry for scanning electron microscopic study.
UVꢀVis absorption spectra of samples were recorded using a Varian Cary100 Bio UVꢀVis
spectrophotometer.
Fluorescence spectra of samples were recorded on a Horiba Scientific Fluoromaxꢀ4
spectrophotometer with a 1 cm path length quartz cell at room temperature. The slit width for the
excitation and emission was set at 2 nm and 1 nm data pitch. Excitations of samples were
performed at 338 nm and the data range was in between 348 to 666 nm.
The fluorescence quantum yields in various solvents were calculated using steadyꢀstate
comparative method using fluorescein as a standard (ꢀꢇꢈꢉ = 0.79).61ꢀ62
ꢑ
ꢊ
ꢐ
ꢅ ꢒ
ꢋꢌ
ꢍꢎꢏ
ꢋꢌ
ꢀꢄꢅ = ꢀꢇꢈꢉ
×
×
×
(1)
ꢑ
ꢊ
ꢍꢎꢏ
ꢐ
ꢅ ꢒ
ꢋꢌ
ꢍꢎꢏ
ꢀꢄꢅ is the quantum yield of the unknown sample and ꢀꢇꢈꢉ is the emission quantum yield of the
standard compound. Astd and Aun represent the absorbance of the standard and the sample at the
excitation wavelength, while Sstd and Sun are the integrated emission band areas of the standard
and the sample respectively. nDstd and nDun are the solvent refractive index of the standard and
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the sample respectively. Here, “un” stands for unknown sample and “std” stands for standard
sample. The values are quite similar to the previously studied structurally similar fluorophores.63
Time resolved studies were performed using a time correlated single photon counting (TCSPC)
system from Horiba Yovin (Model: Fluorocubeꢀ01ꢀNL). Samples were excited at 376 nm using a
picosecond diode laser (Model: Pico Briteꢀ375L).
The amplitudeꢀweighted lifetime is estimated by
ꢅ
〈 〉
ꢓ = ꢔ ꢕꢖꢓꢖ
ꢂ4ꢃ
ꢖꢗꢘ
where ꢓꢖ are the fluorescence lifetime of various fluorescent species, ꢕꢖ are the
normalized preꢀexponential factors. The goodness of the fit is judged by the reduced chiꢀ square
(ꢙꢚ) value.
2.1 Synthesis of BPI-FF-OMe
H2NꢀLꢀPheꢀ
LꢀPheꢀOMe 5 was synthesized according to the literature through
intermediates
3 and 4.64 1.63 g (5 mmol) of H2Nꢀ
L
ꢀPheꢀLꢀPheꢀOMe 6was taken in 8 mL ethyl acetate. Maleic
anhydride (0.735 g, 7.5 mmol) was dissolved into the ethyl acetate and added to the reaction
mixture at room temperature. After stirred over 24 h, the mixture was fully washed with H2O,
saturated brine and dried over Na2SO4. The above solution was concentrated by rotary
evaporator to obtain the product Nꢀmaleylꢀ
LꢀPheꢀLꢀPheꢀOMe 6a as white powder. 1.50 g (3.5
mmol) of Nꢀmaleylꢀ ꢀPheꢀ ꢀPheꢀOMe 6a was dissolved in benzene and put into a doubleꢀneck
L
L
flask. The solution was heated to 50 °C and 0.48 g (3.5 mmol) ZnCl2 was added. Then, the
solution of 0.85 g (5.25 mmol)1,1,1,3,3,3ꢀhexamethyldisilazane (HMDS) in benzene was added
drop wise into the reaction mixture at 80 °C, and kept 12 h with stirring. Then, the mixture was
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poured into 600 mL 0.5 (N) HCl and stirred until clear solution was obtained. The organic layer
was separated, the aqueous phase was extracted with 300 mL EtOAc. The combined organic
layer was washed with 3×100 mL NaHCO3 and brine solution and dried over Na2SO4. The
organic layer was evaporated under vacuum to obtain the product Nꢀmaleoylꢀ
LꢀPheꢀLꢀPheꢀOMe
7a. Perylene (0.504 g, 2 mmol), Nꢀmaleoylꢀ ꢀPheꢀ ꢀPheꢀOMe 7a (1.2192 g, 3 mmol) and
L
L
chloranil (1.0326 g, 4.2 mmol) were thoroughly mixed and heated at 240 °C with stirring for 3 h.
The mixture solidified after cooling at room temperature. Chloroform was added to dissolve the
solidified mass. The mixture was purified using a silica gel column with chloroform/ether (1:1)
as eluent. The compound benzo[ghi]peryleneꢀ1,2ꢀdicarboxylic(
collected as orange solid.
L
ꢀPheꢀ
LꢀPheꢀOMe)imide 1 was
1
Nꢀmaleylꢀ
L
ꢀPhe(1)ꢀ
L
ꢀPhe(2)ꢀOMe 6a: Yield 3.0 g (7.07 mmol, 94.33%) H NMR (400 MHz,
DMSOꢀd6): δ = 9.65 (d, 1H, J = 8.82 Hz), 9.09 (d, 1H, J = 7.76 Hz), 7.60ꢀ7.69 (m, 10H,
aromatic protons), 6.76 (d, 1H, J = 12.52 Hz), 6.66 (d, 1H, J = 12.56 Hz), 5.05 (m, 1H, CαH of
Phe(2)), 4.91 (m, 1H, CαH of Phe(1)), 3.99 (s, 3H, ꢀOCH3), 3.44 (m, 2H, CβHs of Phe), 3.35 (m,
1H, CβH of Phe), 3.16 (m, 1H, CβH of Phe). MS (ESI) m/z for C23H24N2O6Na (M+Na)+ calcd.:
447.1532, found: 447.1577.
1
Nꢀmaleoylꢀ
L
ꢀPhe(1)ꢀ
L
ꢀPhe(2)ꢀOMe 7a: Yield = 1.3912 g (3.42 mmol, 96.86%). H NMR (400
MHz, CDCl3): δ = 6.98ꢀ7.21 (m, 10H, aromatic protons), 6.48 (s, 2H), 6.35 (d, 1H, J = 7.24 Hz,
NH), 4.76ꢀ4.82 (m, 2H, CαHs of Phe(1) and Phe(2)), 3.65 (s, 3H, ꢀOMe), 3.28ꢀ3.30 (m, 2H,
CβHs), 3.07ꢀ3.12 (m, 1H, CβH), 2.97ꢀ3.02 (m, 1H, CβH). MS (ESI) m/z for C23H22N2O5Na
(M+Na)+ calcd.: 429.1426, found: 429.1470.
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Benzo[ghi]peryleneꢀ1,2ꢀdicarboxylic(LꢀPheꢀLꢀPheꢀOMe)imide 1: Yield = 1.26 g (1.93 mmol,
96.92%).1H NMR (400 MHz, CDCl3) δ = 8.60 (d, 2H, J = 8Hz ), 8.39 (d, 2H, J = 8Hz), 7.65ꢀ
7.73 (m, 6H), 7.23 (d, 2H, J = 8 Hz), 7.04 (d, 2H, J = 4 Hz), 7.12 (t, 2H), 7.01 (t, 2H), 6.93 (t,
2H), 6.85 (d, 1H, NH ), 5.28 (m, 1H, Cα H of Phe(1)), 4.97 (m, 1H, Cα H of Phe(2)), 3.68 (s, 3H,
ꢀOCH3), 3.23 (d, Cβ H of Phe(1)), 3.18 (d, Cβ H of Phe(1)), 3.07 (d, Cβ H of Phe(2)), 3.03 (d, Cβ
H of Phe(2)). MS (ESI) m/z for C43H30N2O5Na (M+Na)+ calcd.: 677.2052, found: 677.2096.
1
Maleylꢀ
L
ꢀLeuꢀOMe 6b: Yield 0.78 g (3.46 mmol, 98.99%) H NMR (400 MHz, DMSO d6): δ =
13.76 (s, 1H), 9.06 (d, 1H, ꢀNHꢀ), 6.35 (d, 1H), 6.28 (d, 1H) 4.36 (m, 1H, Cα H of Leu), 3.63 (s,
3H), 1.57 (m, 2H, Cβ H of Leu), 1.19 (m, 1H, CγH of Leu), 0.88 (d, 3H, CδH of Leu, J = 6.24
Hz), 0.85 (d, 3H, Cδ H of Leu, J = 6.28 Hz).
1
Maleoylꢀ
L
ꢀLeuꢀOMe 7b: Yield 1.62 g (6.65 mmol, 90.07%) H NMR (400 MHz, CDCl3): δ =
6.74 (s, 2 H), 4.75 (m, 1H, Cα H of Leu), 3.72 (s, 3H), 2.23 (m, 1H, CβH of Leu), 1.87 (m, 1H, Cβ
H of Leu), 1.14 (m, 1H, Cγ H of Leu), 0.92 (d, 3H, Cδ H of Leu, J = 3.24 Hz), 0.90 (d, 3H, Cδ H
of Leu, J = 3 Hz).
Benzo[ghi]peryleneꢀ1,2ꢀdicarboxylic(LꢀLeuꢀOMe)imide 2: Yield = 0.7259g (1.99 mmol,
76.74%).1H NMR (400 MHz, CDCl3): δ = 9.14 (d, 2 H, J = 9.04Hz), 8.98 (d, 2H, J =7.76 Hz),
8.19 (d, 2H, J = 7.8 Hz), 8.15 (d, 2H, J = 9Hz), 8.08 (t, 2H), 5.18 (m, 1H, Cα H of Leu), 3.84 (s,
3H), 2.52 (m, 1H, Cβ H of Leu), 2.13 (m, 1H, Cβ H of Leu), 1.72 (m, 1H, Cγ H of Leu), 1.08 (d,
3H, Cδ H of Leu, J = 6.52 Hz ), 1.02 (d, 3H, Cδ H of Leu, J = 6.8 Hz).
ACKNOWLEDGMENTS
This work was partially supported by Council of Scientific and Industrial Research (CSIR)
Project No. 02(0056)/12/EMRꢀII. We thank SAIF, IITB for the assistance of EM facility.
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Supporting information: Optical images, spectroscopic data, NMR and Mass data.
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Table 1. Spectroscopic and photophysical properties of BPIꢀFFꢀOMe in different solvents of
increasing polarity.
c
S.
No.
Solvent
λabs
(nm)
εmax
λem
τ(ns)
∆ꢛ
̅
∆ꢆ
ꢀꢁ
(Mꢀ1
(nm)a
(cmꢀ1)b
cmꢀ1)
1
2
3
4
5
6
7
8
9
Toluene
oꢀxylene
Benzene
Tetrahydrofuran
Chloroform
Dimethylformamide 338/390/479 50875 535
Dimethyl sulfoxide 340/391/480 72875 540
Acetonitrile
Ethanol
341/391/484 51000 508
341/391/483 53750 508
341/390/483 52500 508
337/389/475 80875 517
341/390/487 53125 528
976
0.013 8.4
0.029 8.4
0.001 7.5
0.209 7.9
0.149 6.0
0.274 7.4
0.263 7.2
0.305 6.7
0.288 6.2
0.309 5.7
0.55
0.51
0.56
0.43
0.42
0.51
0.43
0.39
0.38
0.39
1018
1018
1710
1594
2185
2314
2460
3011
3340
337/387/479 63000 543
337/387/477 47500 557
337/386/476 42500 566
10
Methanol
b
aEmission data obtained at λmax for each solvent. Stokes shift was calculated based on the
c
absorption at lower energy. Quantum yields measured using fluorescein standard in 0.1 (N)
NaOH.
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Scheme 1. Schematic demonstration represents white light emission upon energy transfer
between donor and acceptor (PyBA : BPIꢀFFꢀOMe). BPIꢀFFꢀOMe molecules shows efficient
solvatochromic effects in protic polar solvent methanol and aprotic solvent toluene. BPIꢀFFꢀ
OMe selfꢀassembled to form spherical nanosphere in solution phase.
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Scheme 2. Schematic presentation shows a possible mechanism of BPIꢀFFꢀOMe for nanosphere
formation.
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Toluene
Methanol
Fluoroscence
B Absorbance
benzene
THF
A
1.0
0.5
0.0
1.0
0.5
0.0
CHCl
3
DMF
EtOH
350
400
450
500
550
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Figure 1. (A) Normalized UVꢀVis absorption spectra of BPIꢀFFꢀOMe (8 × 10ꢀ6 mol Lꢀ1) in
various solvents. (B) Normalized UVꢀVis absorption and fluorescence spectra in a non polar
solvent toluene (blue) and polar solvent methanol (red). Optical images of BPIꢀFFꢀOMe show
(green emission in toluene and yellow emission in methanol under 365 nm UV light.
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Toluene
Absorption in toluene
Emission in toluene
A
B
oꢀXylene
benzene
THF
1.0
0.5
0.0
0.9
0.6
0.3
0.0
CHCl3
DMF
DMSO
ACN
EtOH
MeOH
500
550
600
650
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 2. (A) Normalized fluorescence spectra of BPIꢀFFꢀOMe in various non polar to polar
solvents listed in Table 1. (B) Fluorescence emission spectra of BPIꢀFFꢀOMe in toluene
normalized with respect to absorption peak at 483 nm.
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0.08
Toluene
oꢀXylene
Benzene
THF
A
B
10
3000
2000
1000
9
CHCl3
DMF
DMSO
ACN
EtOH
MeOH
7
8
0.04
0.00
6
4
5
2
3
1
400
450
500
550
0.0
0.1
0.2
0.3
Wavelength (nm)
ꢀ
f
Figure 3. (A) UVꢀVis absorption spectra of BPIꢀFFꢀOMe in different solvents from listed Table
1 (B) LippartꢀMataga plot: Stokes shift ∆ꢛ of BPIꢀFFꢀOMe versus Lippart solvent polarizability
̅
parameter ꢁf. The number refers to the solvents in the Table 1. The straight line represents the
best linear fit to the 10 data points.
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10000
1000
100
BPIꢀFFꢀOMe (Toluene)
BPIꢀFFꢀOMe (Methanol)
τ = 8.4 ns
τ = 5.7 ns
IRF
10
20
30
40
Time (ns)
Figure 4. Time correlated single photon counting decay traces of BPIꢀFFꢀOMe (9 × 10ꢀ5 mol Lꢀ1)
in toluene λem = 508 nm (red) and methanol λem = 566 nm (blue).
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RSC Advances
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DOI: 10.1039/C5RA11804B
BPIꢀFFꢀOMe
HOMO
LUMO
Calculated Optical
Band gap
(eV)
band
gap(eV)a
3.06 eV
2.6 eV
ꢀ5.57 eV
ꢀ2.50 eV
Figure 5. HOMO and LUMO frontier orbital of BPIꢀFFꢀOMe at the B3LYP/6ꢀ31G (d) level.
[a]Optical band gap of BPIꢀFFꢀOMe in methanol was calculated from UVꢀVis spectroscopy.
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