Halogen bond induced phosphorescence of capped γ-amino acid in the solid state

Article abstract of DOI:10.1039/c3cc44231d

The Boc and N,N′-dicyclohexylurea capped γ-amino acid upon monobromination showed phosphorescence in the solid state. The compound exhibited different photoluminescence intensity and lifetimes in crystals obtained from ethyl acetate and methanol. X-ray cr

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Halogen bond induced phosphorescence of capped γ-amino acid in solid
state†
Suman Kumar Maity, Santu Bera, Arpita Paikar, Apurba Pramanik and Debasish Haldar*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
phosphorescence. In chloroform, the foldamer 2 shows very
weak fluorescence. But the compound exhibits stronger
photoluminescence intensity and longer lifetime in crystals
obtained from methanol than ethyl acetate solution.
The foldamer 1 containing mꢀamino benzoic acid and
capped with Boc and N,N′ꢀdicyclohexylurea has been
synthesized by conventional solutionꢀphase methodology. The
foldamer 2 obtained by bromination of foldamer 1 with one
equivalent Nꢀbromosuccinimide in DCM for 48 hours. The
The Boc and N,N'-dicyclohexylurea capped γ-amino acid on
monobromination showed phosphorescence in solid state. The
compound exhibited different photoluminescence intensity
and lifetime in crystals obtained from ethyl acetate and
10 methanol. The X-ray crystallography revealed that the
intermolecular C=O....Br halogen bond direct the heavy atom
effect to produce the phosphoresence.
55
Organic compounds are rarely phosphorescent1 in nature
because the emission from the longꢀlived triplet excited state
15 is nonꢀradiative due to some inhibitory processes such as
thermal perturbations, intramolecular motions, soluteꢀsolute
and soluteꢀsolvent collisions, and intermolecular interactions
with oxygen and humidity. Moreover, the electrons in the
organic compounds are tightly bonded than that of the metal
20 containing organic materials, which makes them less efficient
to emit from triplet excited state.2 Hence, phosphorescence is
regarded mainly as a property of inorganic and organometallic
compounds.3 Phosphorescent materials are commonly used in
organic electronics,4 as well as chemical5 and biological
25 sensing.6 Purely organic compounds exhibit phosphorescence
under impractical conditions such as in low temperature (77
K), in rigid “glasses” of frozen solvents or inert atmospheres.7
Tang and coꢀworkers have reported the crystallization induced
phosphorescence of benzophenone with heavy halogen
30 atoms.8 Kim et. al. have designed crystal from organic
compounds with efficient phosphorescence.9 Kabe et. al.
have enhanced the phosphorescence in dibenzophosphole
chalcogenide mixed crystal.10 Lee and coꢀworkers have
reported the room temperature phosphorescence of metalꢀfree
35 organic materials in amorphous polymer matrices.11
60 compounds were purified, characterized, and studied. The
assumption was that the incorporation of heavy halogen atom,
bromine would provide the halogen bonding10 which brings
aromatic carbonyl and halogen atom in close proximity in
crystal state. This halogen bonding joins the bromine to the
65 carbonyl oxygen of neighbouring molecules in a nonꢀcovalent
interaction that delocalizes the oxygen electrons partially onto
the bromine, thereby amplifies triplet generation and
activating triplet emission, phosphorescence.
To study the effect of bromination on selfꢀassembly
70 behaviour, the solid state FTIR experiments of foldamers 1
and 2 have been performed (ESI Fig. 1). Though the position
and intensity of most of the peaks corresponding to NH and
C=O stretching frequencies remain same for both foldamers,
one of the two NHs is non hydrogen bonded in the bromo
75 substituted foldamer 2 as the stretching frequency has shifted
to higher frequency at 3319 cmꢀ1. Moreover one peak
corresponding to C=O stretching frequency at 1628 cmꢀ1 has
disappeared in the bromo substituted foldamer 2.
The selfꢀassembly and morphology of the foldamer 2 was
80 further studied by atomic force microscopy (AFM) and fieldꢀ
emission scanning electron microscopy (FESEM). A solution
of the corresponding compound in ethyl acetate or methanol
was drop casted on clean microscopic cover slips and allowed
to dry under vacuum at 30 ºC for 2 days. The AFM study
85 reveals that the compound formed nanofibrillar morphology
(diameter 100ꢀ150 nm) from ethyl acetate solution and
spheres (diameter 300ꢀ350 nm) from methanol solution (ESI
Fig. 2). FESEM images of the foldamer 2 from ethyl acetate
(Fig. 1a) shows the fibrillar morphology. From methanol
90 solution the foldamer 2 exhibits spherical morphology with
diameter ranging from 300ꢀ350 nm (Fig. 1b). These results
demonstrate the unique property of the reported compoud to
selfꢀassemble in completely different pattern from polar
aprotic (ethyl acetate) and polar protic (methanol) solvents.
Previously, we have reported that the foldamer 1 containing
mꢀamino benzoic acid and capped with Boc and N,N'ꢀ
dicyclohexylurea formed an antiparallel hydrogen bonded
dimer in the solid state.12 In this work, we present the bromo
40 substituted capped γꢀamino acid foldamer 2 that promotes the
halogen bond induced selfꢀassembly and efficient
Department of Chemical Sciences, Indian Institute of Science Education
and Research Kolkata, Mohanpur, West Bengal 741252, India, Fax:
45 +913325873020; Tel: +913325873119; E-mail: deba_h76@yahoo.com;
deba_h76@iiserkol.ac.in
† Electronic Supplementary Information (ESI) available: Synthesis and
characterization of foldamers, 1H NMR, 13C NMR, solid state FTIR
spectra, Fugures ESI S1ꢀ7, Figure S1ꢀS6. other electronic format see
50 DOI: 10.1039/b000000x/
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65
Xꢀray crystallography‡ shed some light on this fact at
atomic level. Foldamer 1 containing two molecules in the
asymmetric unit and there is no intramolecular hydrogen
bond. In higher order, foldamer 1 formed an antiparallel
hydrogen bonded dimer in the solid state (Fig. 2a). Foldamer
5
10
15
20
70 2 crystallizes with one molecule in the asymmetric unit from
both ethyl acetate and methanol solution (ESI Fig. 5). There
exist a six member intramolecular hydrogen bond. Two
molecules of the foldamer 2 form antiparallel duplex by
reciprocal intermolecular hydrogen bonding interactions
75 between maba NH and urea C=O (Fig. 2b). In higher order
assembly each duplex is linked to four other duplexes via
halogen bonding between maba C=O and bromine atom
(C7=O3....Br1, 3.15Å, 4.31Å, 159° from methanol, N3–
H3....O4, 3.29Å, 4.43Å, 159° from ethyl acetate) and thereby
80 forms a two dimensional layer like structure along the
crystallographic b direction (ESI Fig. 6). Hence, due to the
closer proximity between the carbonyl oxygen and bromine
(3.15 Å) in the crystals from the methanol solution, they
should show stronger phosphorescence compared to that
85 obtained from ethyl acetate (3.29 Å) (Fig. 2c and 2d).
Hydrogen bonding parameters are listed in Table 1.
Fig. 1 The FESEM images of foldamer 2 from (a) ethyl acetate and (b)
methanol solution showing fibrillar and spherical morphology
respectively. (c) The photoluminescence spectra of the foldamer 2 in
25 chloroform solution (blue), crystals from ethyl acetate (black) and crystals
from methanol (red).(d) Photographs of chloroform solution of foldamer
2 under laboratory light and under UV light. (e) and (f) showing the
origin of fluorescence and phosphorescence for the compound.
90
Moreover, the bromine containing foldamer
2 exibits
30 diverse photophysical properties in solution state and in solid
state. The compound has absorbance maxima at 288 nm (ESI
Fig. 3). In chloroform solution the compound shows weak
fluorescence having maxima at 380 nm when excited at 288
nm. But, the crystals obtained from ethyl acetate solution
35 exibit photoluminescence comprising with fluorescence
similar to the solution state, as well as vibronically structured
phosphorescence bands at 467, 482 and 492 nm (Fig. 1c).
However, the crystals obtained from methanol solution show
95
the
fluorescence
with
decreased
intensity
and
100
40 phosphorescence with increased intensity (Fig. 1c). These
observations can be encountered by examining the distance
between the aromatic carbonyl and the bromine atom in three
different cases, stated above. In solution state there is large
separation between the triplet state producing aromatic
45 carbonyl and the heavy atom bromine. Heavy atoms like
bromine facilitates the intersystem crossing by spinꢀorbit
coupling, thereby enhances the triplet state population and
emission, the phosphorescence. Thus in solution state, it is
expected that there will be no phosphorescence (Fig. 1d). But
50 in crystalline state due to ‘directed’ heavy atom effect, there is
significantly close proximity between the aromatic carbonyl
and bromine which leads to the partial delocalization of
electrons of carbonyl oxygen to bromine (Fig. 1e and 1f).
Moreover, in crystalline state the nonradiative relaxation from
55 the triplet state due to the thermal vibration is reduced as the
atoms are relatively fixed in crystal lattice positions. These
facts synergistically enhancec the triplet sate population and
radiation from there in crystalline state. The differences in the
photoluminescence properties of the crystals obtained from
60 ethyl acetate and methanol can be attributed by the effect of
crystallization conditions and crystal packing. Emission
spectra of foldamer 2 in glassy solutions (77 K) exhibit no
additional peak in the 460ꢀ500 nm. The foldamer 1 shows no
significant luminescence (ESI Fig. 4) in solid state.
105
Fig. 2 The antiparallel duplex by (a) foldamer 1 and (b) foldamer 2.
The C=O….Br bond in foldamer 2 crystal from (c) ethyl acetate and
(d) methanol solution. Cyclohexyl groups here appear as purple
110 spheres and tꢀbutyl groups as pink spheres.
Table 1. Hydrogen bonding parameters of foldamers 1 and 2a.
Foldamer
1
DꢀH….A
N3ꢀH3….O4c
N6ꢀH6….O8k
H….A(Å)
2.12
2.05
2.23
2.02
D…A(Å)
2.97(7)
2.89(7)
3.05(6)
2.64(6)
3.02(3)
2.65(4)
DꢀH…A (°)
172
167
163
128
162
2, EtOAC N3ꢀH3….O4e
N2ꢀH5….O3
2, MeOH N1ꢀH1….O4e’
N3ꢀH3….O3
2.19
2.06
125
aSymmetry equivalent c = 2ꢀx, ꢀy, 1ꢀz ; k = ꢀx, 1ꢀy, ꢀz ; e = 1ꢀx, ꢀy, ꢀz ; e’
= 1ꢀx, 2ꢀy, 2ꢀz.
115 The photoluminescence lifetime of foldamer 2 in chloroform
solution and in crystals obtained from ethyl acetate and methanol
2
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were measured. For all three cases, the excitation wavelength was
290 nm and the emission was acquired at 467 nm. The crystals
were gently crushed in a mortar pestle and put into the groove of
the solid sample probe for lifetime measurements. The data
obtained were well fitted with the single exponential decay
model. The results show that in chloroform solution the lifetime
of the compound is in nanosecond regime (4 ns) (ESI Fig. 7)
responsible for fluorescence. Whereas the lifetime of the
compound in crystals obtained from ethyl acetate solution is in
India. S. K. Maity and A. Pramanik thank C.S.I.R, India and
60 S. Bera and A. Paiker thank UGC, India for fellowship.
Notes and references
5
‡ Crystallographic data: Foldamer 2 from EtOAc: C25H36BrN3O4, Mw
=
522.47, monoclinic, space group P21/c, a = 13.649(13), b = 16.921(16), c
= 12.008(12) Å, α = 90°, β = 108.526(15)°, γ = 90°V = 2629(4) Å3, Z = 4,
65 dc = 1.320 Mgmꢀ3, T = 100 K, R1 = 0.0526 and wR2 = 0.1191 for 4442
data with I >
2σ(I). From MeOH: C25H36BrN3O4, Mw = 522.48,
monoclinic, space group P21/c, a = 13.479(19), b = 16.843(2), c =
11.834(17) Å, α = 90°, β = 107.838(2)°, γ = 90° V = 2557.5(6) Å3, Z =4,
dc = 1.357 Mgmꢀ3, T = 100 K, R1 = 0.0364 and wR2 = 0.0914 for 4450
10 microsecond regime (11 ꢀs) which corresponds to
phosphorescence (Fig. 3a). Fig. 3a inset shows the green
phosphorescence as evident from the photographs under 254 nm
UV light. The crystal obtained from methanol solution exhibits
increased phosphorescence lifetime (13 ꢀs) compared to the
15 crystals from ethyl acetate (Fig. 3b). Fig. 3b inset shows the
strong green phosphorescence under 254 nm UV.
70 data with I > 2σ(I). Intensity data were collected with MoKα radiation for
foldamer 2 at 100 K using Bruker APEXꢀ2 CCD diffractometer. Data
were processed using the Bruker SAINT package and the structure
solution and refinement procedures were performed using SHELX97.14
For foldamer 2 nonꢀhydrogen atoms were refined with anisotropic
75 thermal parameters. The data for foldamer 2 from EtOAc and MeOH
have been deposited at the CCDC with reference number CCDC 942338ꢀ
942337 respectively.
1.
2.
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Fig. 3 The time resolved decay curves of the crystals obtained from (a)
ethyl acetate and (b) methanol solution. Insets, the photographs of the
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50 In summary, we have shown the phosphorescence from purely
organic compounds in solid state at normal condition. The
increase in phosphorescence lifetime and intensity
significantly depends on the extent of halogen bonding
interaction where distance between the carbonyl oxygen and
55 the bromine atom plays an important role to the triplet state
population and emission. The results provide atomic level
insights over the halogen bonding and phosphorescence.
110
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The work was supported by DST (SR/FT/CSꢀ041/2009),
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