Photophysical properties of di-Schiff bases: Evaluating the synergistic effect of non-covalent interactions and alkyl spacers in enhanced emissions of solids

Article abstract of DOI:10.1039/c6ra08582b

Photophysical properties of di-Schiff base compounds in the solution and solid states are explored. The di-Schiff bases with alkyl spacers (ethyl, butyl and hexyl) showed enhanced light emitting properties in the solid state, while quenching was observed for di-Schiff bases with a hydrazine spacer. The photoluminescence spectra and the crystal structure were compared with bis-pyridyl-ethyl-di-imine and bis-pyridyl-butyl-di-imine. The crystal structure analysis of the compounds rationalizes the observed results. The concentration dependent ¹H-NMR and NOESY spectra showed the aggregation behaviour of the compounds.

Full text of DOI:10.1039/c6ra08582b

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ARTICLE TYPE
Photo Physical Properties of DiꢀSchiff Bases: Evaluating the Synergistic
Effect of Non Covalent Interactions and Alkyl Spacer in Enhanced
Emissions of Solids
Moyna Das, Fayaz Baig and Madhushree Sarkar*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Photo physical properties of diꢀSchiff base compounds in the solution and solid states are explored. The
diꢀSchiff bases with alkyl spacer (ethyl, butyl and hexyl) showed enhanced light emitting properties in
solid state, while quenching was observed for diꢀSchiff bases with hydrazine spacer. The
10 photoluminescence spectra and the crystal structure were compared with bisꢀpyridylꢀethylꢀdiꢀimine and
bisꢀpyridylꢀbutylꢀdiꢀimine. The crystal structure analysis of the compounds rationalizes the observed
results. The concentration dependent 1HꢀNMR and NOESY spectra showed the aggregation behaviour of
the
compounds.
coworkers have reported the non conjugated Salen compounds
with unique AIE, where the small πꢀconjugated system resulted in
aggregationꢀinduced emission (AIE) with large Stokes shifts and
50 high fluorescence quantum yields was observed.10e
Introduction
15 The potential application1 of light emitting materials includes
organic lightꢀemitting diodes (OLEDs), sensors and biological
imaging, which requires them to have enhanced luminescence
efficiency in the aggregated state. Tang and coworkers in 2001
first explored the aggregation based emission (AIE) and reported a
20 series of tetraphenylethene derivatives.2 Park and coꢀworkers3,
Tian and co workers4 have reported several material systems with
AIE properties. The AIE behavior is rationalized by considering
the restriction of Intramolecular Rotation, formation of Jꢀ
aggregates and intramolecular planarization.5
25 The solid state arrangement of the molecules, in general, decides
the properties of a compound, so the crystal structure analysis will
provide information on arrangement of the molecules. The
knowledge on supramolecular packing of the molecules and its
relation with the photophysical property will pave the way to
30 develop the strategies to design new materials targeting the light
emitting properties. The synthesis of materials by utilizing non
covalent interaction is an efficient route and is a handy method to
tailor made light emitting solids. Draper and co workers have used
supramolecular synthesis and tuned the solid state luminescence
35 of 2ꢀcyanoꢀ3(4ꢀ(diphenylamino)phenyl)acrylic acid by reacting
them with substituted pyridines and amines.6 The review on nonꢀ
covalent methods to synthesize solid state emitting molecular
materials and their applications is highlighted by Varughese.7
The Schiff Base based compounds are explored widely because of
40 their easy synthetic procedures, photophysical properties and
biological activity.8 Kawasaki et al. have reported the solid state
PL spectra for a series of Salen compounds (Schiff Bases) with
varying alkyl chain length. They have observed a relation of photo
physical property with the alkyl chain length.9 Xiang et al. have
45 reported AIE in salicylaldehyde azine where despite having planar
geometry, no quenching was observed.10 Recently, Xiang, Liu and
In the present work, we have studied the photophysical properties
of di Schiff base molecules (Scheme 1) in the solid state and in
solution. We have rationalized the observations on the following
grounds: (i) Effect of alkyl chain length; (ii) Presence of methyl
55 group on the methinine carbon; (iii) Comparison of the light
emitting properties with the pyridyl based Schiff base compounds.
Recently our group has reported the photo physical properties
along with the structural description of pyridyl based Schiff bases,
where we have concluded that the flexible spacer (alkyl bridge) in
60 these molecules helped in aggregating the molecules which results
in enhanced emission.11
R
R
65
(CH2)n
N
Ar
Ar
N
70
75
80
a
Ar = phenyl and R = H:
L1
n = 0:
= 2: L2
L3
Ar = phenyl and R = CH3: b
Ar = 2ꢀpyridyl and R = H: c
Ar = 2ꢀpyridyl and R = CH3:
= 4:
= 6: L4
d
Scheme 1: Di Schiff Base Compounds
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There are reports on the crystal structure analysis of L1a
,
L1b and
of benzaldehyde (2.0ml, 20 mmol.) in ethanol(40ml).The mixture
L2a along with their property studies.12 The absorption spectra of 60 was refluxed at 80ºC for 6 hrs. The solvent was removed under
Azines and Dianils, which included L1a
,
L1b
,
L2a and L2b
,
vacuum and the orange semiꢀsolid product was crystallized with
hexane. Yield: 38%., Melting point: 60ºC. IR (cmꢀ1 KBr pellet):
were reported by Ferguson and Goodwin in 1949.12a Sinha
reported the crystal structure of L1a and reported the effect of
conjugation on the bond lengths.12b Glaser et al. have reported the
5
3418(m),
3271(w),
3055(w),
3032(w),
2908(w),
2843(m),1674(w),
1643(vs), 1574(w),1450(m), 1373(m),
1
solid state structures of series of acetophenone azines including 65 1281(w), 157(w), 1018(s), 972(w), 918(w) (Figure S7). HꢀNMR
L1b12c while the structure of L2b is reported by the research
group of Tiekink12d
The electrochemical behavior of the
(400 MHz DMSO) δppm:8.34 (s,1H), 7.71 (dd,J=7.2,2.5 Hz, 2H),
7.48ꢀ7.36 (m,3H), 3.54ꢀ3.31 (m, 2H) (Figure S8). 13CꢀNMR(101
MHz DMSO) δppm:162.42,136.56,131.07,129.09,128.27,61.17
(Figure S9).
.
10 acetophenone azines were studied by Workentin et al., where the
effect of substituents on aryl group on the electrochemical
potential was studied.12e Research group of Ding reported the high 70 N1,N2ꢀbis(1ꢀphenylethylidene)ethaneꢀ1,2ꢀdiamine(L2b): 0.5ml
pressure spectral analysis of L1a and observed pressure induced
phase transformation.12f Xiang et al. have studied the emission
15 properties of L1a and reported that no AIE properties was
observed.10a
(10 mmol.) ethylene diamine was added dropwise to a solution of
acetophenone (2.3 ml, 20 mmol.) in ethanol(40 ml).The mixture
was refluxed at 80ºC for 6h. The solvent was removed under
vacuum and brown semiꢀsolid product was recrystallized with
75 hexane. Yield: 36%., Melting point: 107.5ºC. IR (cmꢀ1 KBr
pellet): 3418(m), 3248(w), 3063(w), 3016(w), 2885(m), 2824(m),
1628(vs), 1574(w), 1489(w), 1443(m), 1373(m), 1265(vs),
1180(w), 1080(m), 1041(m), 910(m),756(vs) (Figure S10). 1Hꢀ
Experimental Section
20 General: Infra Red Spectra were taken in FTIR ABB Bomen
MBꢀ3000. UVꢀVisible Spectra and Fluorescence Spectra were
NMR (400 MHz, DMSO) δ ppm: 7.92 (dd, J = 6.6, 3.1 Hz, 2H),
taken in Shimadzu spectrophotometer model UVꢀ2450 and 80 7.51 – 7.44 (m, 3H), 3.48 – 3.30 (m, 2H), 2.28 (s, 3H) (Figure
Fluorimaxꢀ4 0426C0809, respectively. Proton and Carbonꢀ13
Nuclear Magnetic Resonance (1H NMR and 13C NMR) spectra
25 were recorded on a 400 MHz spectrophotometer (Bruker). Powder
XꢀRay Diffraction (XRD) were recorded with a Rigaku miniflex
S11). 13CꢀNMR (101 MHz, DMSO) δ ppm: 165.09, 140.97,
129.84, 128.45, 126.89, 52.74, 15.17 (Figure S12).
N1,N2ꢀdibenzylidenebutaneꢀ1,4ꢀdiamine(L3a):
0.5
ml
(10mmoles) diaminobutane was added dropwise to a solution of
ll, λ =1.54, Cu Kα. Fluorescence quantum yields are determined 85 benzaldehyde (2.0 ml, 20 mmol.) in ethanol (40ml).The mixture
by using quanta phi instrument (Jobin Horiba, Fluoremaxꢀ4).
The compounds L1 L4 were synthesized by the usual method of
30 preparation of Schiff bases, which involved the condensation
reaction of primary diamines with an aldehyde/ketone precursor in
alcoholic solution under reflux conditions.
1,2ꢀdibenzylidenehydrazine (L1a): Hydrazine hydrate (0.5 ml,
10 mmol.) was added drop wise to an ethanolic solution of
35 benzaldehyde (2.0 ml, 20 mmol.). The mixture was kept for 6 hrs.
under refluxing condition. The solvent was removed under
was refluxed at 80ºC 6h. The solvent was removed under vacuum
and the brown color semiꢀsolid product was recrystallized with
hexane. Yield: 49%., Melting point: 50ºC. IR (cmꢀ1 KBr pellet):
3394(w), 3271(w), 3055(w), 3024(w),2932(m), 2847(m),2646(w),
90 1643(vs), 1605(w), 1574(w), 1443(s), 1381(w), 1288(w),
ꢀ
1
965(w),756(vs) (Figure S13). HꢀNMR (400 MHz, DMSO)δ
ppm: 8.34 (s, 1H), 7.73 (dd,
(m,3H), 3.60 (d, J = 1.1 Hz 2H), 1.67 (Dt,
(Figure S14).13Cꢀ NMR (101 MHz, DMSO) δ ppm: 161.05,
J
= 6.6, 3.1 Hz, 2H), 7.49ꢀ7.39
J
= 5.6, 3.0 Hz, 2H)
vacuum and the yellow color solid product was recrystallized 95 136.70, 130.96, 128.97, 128.18, 60.31, 28.58 (Figure S15).
with methanol. Yield: 36%., Melting point: 86.5ºC. IR (cmꢀ1 KBr
pellet): 3418(w), 3171(w), 3047(w), 3001(w), 2947(m), 1666(m),
40 1628(vs), 1574(w), 489(w), 1443(m), 1304(w), 1203(w), 957(m),
856(w) (Figure S1). 1HꢀNMR (400 MHz, DMSO) δ ppm:
N1,N2ꢀbis(1ꢀphenylethylidene)butaneꢀ1,4ꢀdiamine(L3b): 0.5 ml
(10 mmol.) diaminobutane was added dropwise to a solution of
acetophenone (2.3 ml, 20 mmol.) in ethanol (40 ml). The mixture
was refluxed at 80ºC for 6h. The solvent was removed under
8.73(s,1H), 7.90 (dd,
J =7.4,2.2 Hz,),7.51 (dq, J= 6.6, 3.2 Hz, 3H) 100 vacuum and the green color semiꢀsolid product was recrystallized
(Figure S2).13Cꢀ NMR (101 MHz DMSO) δ ppm: 161.99, 134.24,
131.84, 129.38, 128.84 (Figure S3).
with hexane. Yield: 50%., Melting point:62ºC IR (cmꢀ1 KBr
pellet): 3418(m), 3240(w), 3086(w),3047(w), 2932(m), 2878(m),
1628(vs), 1574(w), 1489(w), 1443(w), 1350(m), 1281(m),
1180(w), 1080(w), 918(w),764(vs) (Figure S16). 1HꢀNMR (400
45 1,2ꢀbis(1ꢀphenylethylidene)hydrazine (L1b):
mmoles) hydrazine hydrate was added dropwise to a solution of
acetophenone (2.3 ml, 20 mmoles) in ethanol (40 ml). The 105 MHz, DMSO)
0.5ml (10
δ
ppm:7.82 (dd,J=6.8,2.9 Hz,2H),7.43ꢀ7.35
mixture was refluxed at 80ºC for 6hrs. The solvent was removed
under vacuum and the yellow color solid product was
(m,3H), 3.52ꢀ3.45(m, 2H), 2.20 (s,3H), 1.84ꢀ1.75 (m, 2H) (Figure
S17). 13Cꢀ NMR (101 MHz, DMSO) δ ppm: 163.85, 141.02,
129.73, 128.09, 127.33, 51.52, 28.98, 15.06 (Figure S18).
50 recrystallized
with
methanol.
Yield:
46%.,
Melting
point:123ºC.IR(cmꢀ1 KBr pellet): 3418(m), 3055(m), 2962(m),
N1,N2ꢀdibenzylidenehexaneꢀ1,6ꢀdiamine
(L4a):
0.5ml
2916(w), 1643(vs), 1566(vs), 1489(w), 1443(vs), 1358(vs), 110 (10mmol.) hexamethylene diamine was added dropwise to a
1281(s), 1072(m), 1018(m), 756(vs) (Figure S4). 1HꢀNMR(400
MHz, DMSO) δppm:7.92 (dd, J=6.6,3.1 Hz,2H), 7.50ꢀ7.44
55 (m,3H), 2.28 (s, 3H) (Figure S5).13CꢀNMR(101 MHz DMSO)
δppm:157.77,138.77,130.54,128.79,126.80,15.19 (Figure S6).
solution of benzaldehyde (2.0 ml,20 mmol.) in ethanol (40
ml).The mixture was refluxed at 80ºC for 6h.The solvent was
removed under vacuum and the brown semiꢀsolid product was
recrystallized with hexane. Yield: 48%., Melting point:170ºC. IR
0.5 115 (cmꢀ1 KBr pellet): 3394(w), 3148(m), 3001(w), 2939(w), 2854(w),
N1,N2ꢀdibenzylideneethaneꢀ1,2ꢀdiamine
ml(10mmol.) ethylene diamine was added dropwise to a solution
(L2a):
2716(w), 2106(w),1643(s),1589(w),
1551(vs), 1379(vs),
2
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1234(w), 1173(w), 1065(w), 949(w),825(s) (Figure S19). 1Hꢀ
NMR(400 MHz, DMSO) δ ppm:7.89 (s, 1H), 7.44ꢀ7.23 (m, 3H),
2.70 (dd, J = 59.6 Hz, 2H), 1.63ꢀ1.45 (m,2H), 1.40ꢀ1.18 (m, 2H)
(Figure S20). 13CꢀNMR (101MHz, DMSO) δ ppm:170.46, 139.62,
129.80, 129.42, 127.83, 27.84, 25.92 (Figure S21).
60
65
70
75
5
N1,N2ꢀbis(1ꢀphenylethylidene)hexaneꢀ1,6ꢀdiamine(L4b): 0.5 ml
(10 mmol.) hexamethylene diamine was added dropwise to a
solution of acetophenone (2.3 ml, 20 mmol.) in ethanol (40
ml).The mixture was refluxed at 80ºC for 6hrs. The solvent was
10 removed under vacuum and the brown semiꢀsolid product was
recrystallized with hexane. Yield:47%, Melting point: 47.5ºC IR
(cmꢀ1KBr pellet): 3811(w), 3433(m), 3256(w), 3055(w), 2924(m),
2854(w), 1628(vs), 1582(w), 1443(w), 1412(m), 1381(m),
1281(w), 794(m), 694(s),571(w) (Figure S22). 1HꢀNMR (400
15 MHz, DMSO) δ ppm:7.84 – 7.76 (m, 2H), 7.45 – 7.29 (m, 3H),
3.39 (d, J = 8.9 Hz, 2H), 2.17 (S, 3H), 1.64 (dt, J = 33.6, 6.9 Hz,
2H), 1.54 – 1.27 (m, 2H) (Figure S23). 13CꢀNMR (101 MHz,
DMSO) δppm: 164.09, 141.13, 129.71, 128.26, 126.75, 51.40,
31.07, 27.55, 15.40 (Figure S24)
20
Results and Discussion
Figure 1: Illustration of Crystal Structure of L1a12b: (a) Asymmetric Unit
of L1a; (b) Arrangement of adjacent molecules; (c) Packing of the
In order to analyze the relation of the photophysical properties of
the compounds with the geometry and supramolecular
25 arrangement of the molecules, the crystal structure description of
80 molecules (Figures were generated from the data obtained from CCDC
No. 1118104)
some compounds were studied. The crystal structure of L1a12b
,
L1b12c L2b12d and L2c12g were reported by other groups while our
groups had reported the crystal structure description of L1d, L2d
85
90
95
and L3c11. The following features were observed while analyzing
30 the structural aspects of the compounds.
Crystal Structure Analysis of L1a12b
orthorhombic bcn space group and only half of the molecule was
: The compound L1a had
P
present in the asymmetric unit (Figure 1a). The molecules in L1a
35 were packed in such a way that the NꢀN bonds of the adjacent
molecules were arranged in parallel fashion with a distance of
3.810Å between the two nitrogen of the neighboring molecules
(Figure 1b). The aromatic moieties of the two adjacent molecules
have inclined/tilted arrangements with centroidꢀtoꢀcentroid
40 distance of 4.873Å (Figure 1c). This results in a 1D arrangement
of the molecules, which were further assembled in 3D via
aromatic interactions between the inclined aromatic rings
(centroidꢀtoꢀcentroid distance of 5.011 Å) (Figure 1c).
100
45 Crystal Structure Analysis of L1b12c
crystallized in monoclinic 21/n space group with one molecule
: The compound L1b was
P
present in the asymmetric unit. The molecule of L1b had a non
planar geometry, where the CꢀNꢀNꢀC torsion angle was 138.72º
and the two phenyl rings of a molecule were inclined at an angle
50 of 64.62º (Figure 2a). Although inclined faceꢀtoꢀface stacking of
the molecules were observed in this case due to the non planar
geometry of the molecules, but the significant number of π•••π
interactions resulted in packing of the molecules (Figure 2c). The
phenyl ring of a molecule of L1b was in the vicinity of
55 neighboring phenyl rings such that the centroidꢀtoꢀcentroid
distance between the aromatic rings was in the range of 4.6ꢀ4.9Å
(Figure 2b).
105
110
Figure 2: Illustration of Crystal Structure of L1b12c: (a) Asymmetric Unit
of L1b: Notice the non planar geometry of the molecule; (b) Centroidꢀtoꢀ
Centroid distance between the adjacent aromatic rings; (c) Packing of the
molecules (Figures were generated from the data obtained from CCDC
115 No. 1207284)
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Crystal Structure Analysis of L1d11: The crystal structure
analysis of L1d showed that there are three molecules in the
asymmetric unit, which differed in the orientation of the pyridyl
and imine moieties. The two molecules were arranged parallel
such that they form dimer (Figure 3). The distance between the
C=N of the two molecules were 3.906 and 3.943 Å, while the
centroid to centroid distance between the two pyridyl rings were
3.888Å and 4.009Å. The third type of molecule interacted with
the other two molecules via CꢀH•••N and aromatic π•••π
60
65
70
75
80
5
10 interactions and helped in the formation of dimers.
15
20
25
30
85 Figure 4: Illustration of Crystal Structure of L2b12d: (a) Asymmetric Unit;
(b) Arrangement of the molecules: Notice that the aromatic centroidꢀtoꢀ
centroid distance is more than 5Å; (c) Herringbone arrangement of the
molecules of L2b (Figures were generated from the data obtained from
CCDC No. 608471)
90
35
Figure 3: Illustration of Crystal Structure of L1d11: (a) Packing of the
molecules (Hydrogen atoms are removed for clarity); (b) Three types of
molecules of L1d are present in the asymmetric unit, which are shown in
different colors. (Figures were generated from the data obtained from
40 CCDC No. 963345)
95
Crystal Structure Analysis of L2b12d
crystallized in 21/n space group with half of the molecule in the
: The compound L2b was 100
P
asymmetric unit (Figure 4a). The ethyl group in the molecule
45 adopted all anti conformation. The molecules were stacked in
parallel manner to form a oneꢀdimensional network (Figure 4b).
The 3D arranged of the molecules can be viewed as herringbone 105
arrangement of the one dimensional networks (Figure 4c).
50 Crystal Structure Analysis of L2d11: The crystal structure of
L2d was reported by our group11 and it showed that there are two
molecules in the asymmetric unit (Figure 5a). The ethyl spacer of 110
L2d adopted anti conformation thereby resulting in linear
geometry of the molecule. The neighboring molecules were
55 arranged in a offset manner to form a one dimensional network
(Figure 5b). The overall supramolecular network can be viewed as
Figure 5: Illustration of Crystal Structure of L2d11: (a) Asymmetric Unit;
(b) Offset arrangement of the molecules to form 1D network; (c)
Herringbone arrangement of the molecules of L2d (Hydrogen atoms were
herringbone arrangement of these one dimensional network 115 removed for clarity) (Figures were generated from the data obtained from
(Figure 5c). CCDC No. 962823)
4
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Crystal Structure Analysis of L2c12g and L3c11: The
experimental spectra of other compounds are shown in Figure S25
ꢀS29.
asymmetric unit in L2c has half of the molecule and the ethyl
spacer adopts gauche conformation. The CꢀH•••N hydrogen bond
interactions between the pyridyl N and CꢀH of methinine resulted
in the formation of "nonꢀcovalently bonded chromophore" unit
(Figure 6a and 6b). The crystal structure analysis of L3c showed
that it has half of the molecule in the asymmetric unit (Figure 6c).
Further it was observed that the butyl spacer of L3c adopted
60
65
70
75
80
85
5
gaucheꢀantiꢀgauche conformation and not expected all anti
10 conformation. The pyridylꢀN•••HCꢀ(Methinine) interactions
resulted in formation of a non covalently bonded "macrocyclic"
moiety (Figure 6d). The butyl chain conformation resulted in
formation of corrugated layers and offset packing of the layers
results in the overall 3D arrangement of the L3c (Figure 6e).
15
20
25
30
Figure 7: (a) Calculated PXRD of L1a; Generated from Crystal Data
CCDC No. 1118104; (b) Experimental PXRD of L1a; (c) Calculated
90 PXRD of L1b; Generated from Crystal Data CCDC No. 1207284; (d)
Experimental PXRD of L1b; (e) Calculated PXRD of L2b; Generated
from Crystal Data CCDC No. 608471; (f) Experimental PXRD of L2b
35
UVꢀVisible Absorption Spectra: The solid state UVꢀVisible
95 spectra of L1a showed absorption maxima at 196nm (π→π*) and
300nm (n→π*) while L1b shows λmax at 206nm (π→π*) and
266nm (n→π*) (Figure 8a and 8b). The absorption maxima for
L1d11 were at 253 nm (π→π*) and 286 nm (n→π*); which clearly
indicated the effect of changing phenyl group with pyridyl group.
40
Figure 6: Illustration of Crystal Structure of L2c12g and L3c11: (a)
45 Asymmetric unit in L2c; (b) Packing of the molecules of L2c via CꢀH•••N
interactions to form non covalent macrocyclic moiety; (c) Asymmetric
Unit in L3c; (d) Non covalent "macrocyclic" moiety in L3c; (e) Offset
packing of the corrugated layers in L3c (Hydrogen atoms are removed for
clarity) (Figures were generated from the data obtained from CCDC No.
50 721559 (L2c) and 930055(L3c)).
100 The solid state UVꢀVisible absorption maxima for L2a
,
L2b
, L3a,
L3b
,
L4a and L4b were in the range 192ꢀ194 nm (π→π*) and
223ꢀ237 nm (n→π*) (Table 1, Figure 8cꢀ8h), whereas for the L2
and L3 compounds with pyridyl group, the λmax was observed in
the range of 232ꢀ235nm (π→π*) and 269ꢀ271 nm (n→π*)11.
105 The UVꢀVisible absorption spectra in methanolic solution for the
Powder XRD Spectra of the compounds: Powder XRD spectra
compounds L1a, L1b, L2a, L2b, L3a, L3b, L4a and L4b showed
were recorded for L1a
,
L1b
,
L2a
,
L2b
,
L3a
,
L3b
,
L4a and L4b
.
red shift (7ꢀ12nm) in the π→π* peak position compared to their
solid state spectra (Figure S30ꢀS37); which indicates the
possibility of parallel stacking of aromatic rings in solid state to
110 form faceꢀtoꢀface stacking of the aromatic moieites/Hꢀ
aggregates.13
The experimental powder XRD spectra of the compounds L1a
,
L1b, and L2b were compared with the simulated powder XRD
55 spectra from the single crystal XRD data. Figure 7 shows the
phase purity of the compounds L1a, L1b, and L2b. The
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Photoluminiscence Spectra of L2a and L2b: The PL spectra of
L2a and L2b in solid state as well as in methanolic solutions
showed the enhanced emission properties with increase in
concentration. The observation from the concentration dependent
35 PL spectra of L2a showed the appearance of a new peak on
increasing concentration.
5
10
15
40
45
50
55
20
60 Figure 9: PL spectra of L2a in different concentrations
At 10ꢀ4 M concentration, by fixing the excitation wavelength at
300 nm, structured bands in PL spectra was observed with λmax. at
330 and 364 nm, whereas at 10ꢀ3 M concentration, an extra
25
shoulder peak appeared at 426 nm and at 5×
10ꢀ1 M, band at
Figure 8: UVꢀVisible Absorption Spectra of compounds in Solid State (a)
L1a; (b) L1b; (c) L2a; (d) L2b; (e) L3a; (f) L3b; (g) L4a; (h) L4b
65 around 359 nm disappeared completely and the only peak at 426
nm was observed. But when the excitation wavelength was fixed
at 450 nm, concentration lower than 10ꢀ3M didn't show any
emission and on increasing the concentration to 10ꢀ1 M, an intense
emission peak appeared at 518 nm (Figure 9). The PL spectra of
70 L2b showed a structured emission spectra (λmax. at 330 and 365
nm) at a concentration of 10ꢀ4 M, when the excitation wavelength
was kept at 300 nm while a shoulder peak at 424 nm appeared at
Table 1: UVꢀVisible absorption maxima for compounds L1a, L1b, L2a,
30 L2b, L3a, L3b, L4a and L4b
Compd Solid State (λmax in Compds. in MeOH (conc. 10ꢀ4 M)
nm)
(λmax in nm)
L1a
L1b
L2a
L2b
L3a
L3b
L4a
L4b
196, 300
206, 266
192, 223
197, 238
194, 229
197, 237
194, 239
194, 236
206, 299
205, 265
209, 247
207, 241
204, 223
205, 237
203, 223
206, 237
10ꢀ3 M. On changing the excitation wavelength to 450 nm, 10ꢀ1
M
concentration showed an intense peak at 500 nm (Figure 10). The
75 PL spectra of L2c and L2d was reported previously by our group.
In L2d, the PL spectra showed an increase in emission intensity
along with a red shift of the peaks and the solid state PL showed a
λmax at 453nm when excitation wavelength is at 400nm. The PL
spectra of L2c showed the appearance of new peak on increasing
80 concentration. It was suggested that the presence of pyridyl group
resulted in CꢀH•••N interaction and hence a non covalently
bonded species appeared at high concentration.11
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Photoluminiscence Spectra of L3a, L3b, L4a and L4b: The PL
spectra of L3a L3b L4a and L4b in methanolic solution also
,
,
showed enhanced emission on increasing the concentration. The
60 compound L3a showed a strong emission spectra at λmax. 514 nm
(excitation wavelength at 440 nm) with a 10ꢀ1 M concentration,
while L3b showed the emission maxima at 508 nm (excitation
wavelength at 450 nm) with a 10ꢀ1 M concentration (Figure 11, 12
and S38). The compounds L4a and L4b at concentration of 10ꢀ1
5
10
15
20
65
M also showed emission maxima at 519 nm (excitation
wavelength at 450 nm) and 420 nm (excitation wavelength at 493
nm), respectively (Figure S39). The PL spectra of L3c showed
appearance of new peak at high concentration while L3d showed
intensity increase along with red shift as reported previously by
70 us.
75
80
85
90
95
25
Figure 10: PL spectra of L2b in different concentrations
30
35
40
45
50
100
Figure 12: PL spectra of L3b in different concentrations
Solid State PL Spectra: The PL spectra of compounds L1a
, L1b,
105 L2a
,
L2b L3a L3b L4a and L4b were recorded in their powder
,
,
,
form and crystalline form to get a comparative analysis of the
emissions in different states. Compounds L1a and L1b didn't
show any PL spectra in powder as well as in crystalline state. The
solid state PL of L2a showed an intense peak at 531 nm when the
110 excitation wavelength was fixed at 350 nm, while L2b showed an
intense peak at 549 nm. The solid state PL of L2a
, L2b, L3a,
L3b L4a and L4b are shown in Figure S40ꢀS44. Table 2 and
,
table S1 shows the comparison of λmax. For the compounds in
solid state and in different concentrations in MeOH.
55 Figure 11: PL spectra of L3a in different concentrations
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Concentration and Excitation Dependence of PL: The steady
state PL measurements have shown that the emission of the
compounds depends on concentration as well as excitation
wavelength. On increasing concentration, at an excitation
wavelength of around 300 nm, a red shift of emission maxima is
observed along with the disappearance of peak in the region of
300nm (Table 2). Figure 13 shows the absorption and emission
L2a
Emission-10-1M
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Emission-10-3M
5
Absorption-10-5M
443
204
359
spectra of compound L2a. The absorption and emission
422
wavelengths have very less overlap in the 300 nm range. Table 3
248
35
10 shows the molar extinction coefficients of UVꢀVis spectra of L2a
in wavelengths near 300nm. Although the molar extinction
coefficient values are very less in the 300nm region, but
reabsorption at higher concentration can be expected and to some
extent may be reason for disappearance of peak at high
15 concentration. Figure S45 and table S2 shows the overlap of
absorption and emission spectra (at an excitation ~300 nm) and
molar extinction coefficient of UVꢀVis spectra in wavelength
200
250
300
350
400
450
500
550
region 300nm, respectively of the compounds L2b
, L3a, L3b,
Wavelength in nm
L4a and L4b
.
40
20
Table 2: λmax. in the PL Spectra of the compounds
Figure 13: Absorption and emission spectra (excitation 300nm) of L2a
λmax. in Methanolic Solution of different λmax. in Solid
concentration
L2a
L2b
L3a
L3b
330, 364 (Excitation 439 (Excitation
300nm; 10-4M) 300nm; 10-1M)
531 (Excitation
350nm)
Table 3: Molar Extinction Coefficient of UVꢀVis spectra of L2a in
518 (Excitation 450 518 (Excitation 450
wavelengths near 300nm
nm; 10-4M)
nm; 10-1M)
330, 366 (Excitation 464
(Excitation 549 (Excitation
Compounds
Molar
Extinction Wavelength
in nm
300nm;10-4M
300nm; 10-1M)
350 nm)
coefficient (ϵ)
(Mꢀ1cmꢀ1)
195.2
489 (Excitation 450 500 (Excitation 450
nm;10-4M) nm; 10-1M)
365, 386 (Excitation 463 (Excitation 320
320nm; 10-3M) nm;10-1M)
479 (Excitation 420 509 (Excitation 420
nm;10-4M) nm; 10-1M)
376, 460 (Excitation 382 (Excitation 320 510(Excitation
L2a
10ꢀ3M
330
524(Excitation
10ꢀ4M
829
330
300
289
280
330
300
289
280
248
205
440 nm)
1095
3533
4793
3190
4510
7970
10610
61690
91310
10ꢀ5M
340nm; 10-2M)
478 (Excitation 504 (Excitation 450
420nm; 10-4M) nm; 10-2M)
nm;10-1M)
450 nm)
L4a
L4b
300, 370 (Excitation 514 (Excitation 280
280nm; 10-4M) nm;10-1M)
517 (Excitation 450 517 (Excitation 450
nm; 10-4M) nm; 10-2M)
350, 420 (Excitation 465 (Excitation 320 534(Excitation
320nm; 10-3M) nm; 10-1M)
460 nm)
484 (Excitation 420 490 (Excitation 420
nm; 10-4M) nm; 10-2M)
515 (Excitation
440 nm)
45
TimeꢀResolved Fluorescence Technique for Lifetime Analysis:
With the help of the timeꢀcorrelated singleꢀphoton counting
(TCSPC) technique, we have measured the excitedꢀstate decay
Further, the dependence of emission wavelength on the excitation
wavelength is observed from table and table S1. The
dependence is more pronounced at lower concentration. In L2a, at
25 10ꢀ4 M concentration, emission maxima are observed at 330nm,
364 nm (when the excitation wavelength is 300nm) while
emission maxima is observed at 518 nm (when the excitation
wavelength is 450 nm). This dependence clearly shows the
formation of aggregate species, which are responsible for different
30 emission peaks at different excitation.
50 curve of the compounds (Figure S46ꢀS51). The compounds
showed a triꢀexponential decay, with three lifetimes in the range
of 1ꢀ2 ns, 4ꢀ7ns and 0.1 ꢀ0.2 ns at concentration more than 10ꢀ4M.
The shortest lifetime may be attributed to the deactivation
(quenching) of excited aromatic groups while the longer lifetime
55 is associated with the unquenched decay of the compounds. Table
4 and Table S3 summariz the lifetime measurements of the
compounds.
2
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L4b
10ꢀ1
λ Emission 540nm
1.42
(α1 0.11)
5.52
(α2 0.37)
0.19
(α3 0.52)
2.29
(χ2 1.18)
10ꢀ2
1.56
(α1 0.09)
6.02
(α2 0.36)
0.17
(α2 0.36)
2.39
λ
Emission 530nm
(χ2 1.29)
5
Methanolic solution of L2b of concentrations 10ꢀ1 M, and 10ꢀ4
M
15 showed a tri exponential decay when excited at 375 nm and
emission was monitored at 457 nm, while a bi exponential decay
was observed at a concentration of 10ꢀ6 M. The average lifetime of
10ꢀ1M was 1.209 ns, 10ꢀ4 was 1.17 and 10ꢀ6 M was 0.90 ns (Figure
14). The concentration dependence of the TCSPC profile of the
20 L2b suggested the aggregation of the molecules at higher
concentration. The increase in the average lifetime with
concentration suggests that with aggregation in concentrated
solution, the unquenched decay processes increases, which may
be considered to be aggregation induced emission.
10
Figure 14: TCSPC decay profiles of L2b in different concentration in
MeOH with excitation at 375 nm
25 Fluorescence Quantum Yield: Fluorescence quantum yield was
determined for the compounds L2a, L2b, L3a, L3b, L4a and L4b
Table 4: Fluorescence lifetime data for L2a, L2b, L3a, L3b, L4a and 4b
in spectroscopic grade methanol using optically matching
solutions of quinine sulfate (Φf = 0.546 in 0.5 H2SO4 at an
excitation wavelength of 345 nm) as standard. In solid state,
30 sample was mixed with BaSO4 to measure the absolute quantum
yield.
λ
Excitation = 375nm
τ1 (ns)
τ2 (ns)
τ3 (ns)
τavg (ns)
L2a
10ꢀ1
1.55
(α1 0.30)
5.66
(α2 0.10)
0.23
(α3 0.60)
1.153
λ Emission 520 nm
(χ2 1.24)
10ꢀ4
Table 5: Absolute Quantum Yield in solution and in solid
λ Emission 449 nm
1.23
(α1 0.35)
0.26
(α2 0.54)
4.69
(α3 0.11)
1.076
(χ2 1.05)
10ꢀ1
M
10ꢀ4
M
10ꢀ1
M
Compd.
10ꢀ4 M
Solid Compd.
Solid
L2b
10ꢀ1
1.48
(α1 0.38)
4.17
(α2 0.13)
0.23
(α3 0.49)
1.209
L2a (Φf)
1.06
4.04
5.81
350
530
L2b (Φf)
0.89
1.52 2.28
λ Emission 457nm
(χ2 1.15)
Excitation
(nm)
Excitation
(nm)
10ꢀ4
λ Emission 457nm
1.51
(α1 0.37)
5.92
(α2 0.08)
0.21
(α3 0.55)
1.172
(χ2 1.11)
300
450
300
450
350
10ꢀ6
λ Emission 457nm
Emission
(nm)
Emission
(nm)
0.79
(α1 0.97)
4.56
(α2 0.03)
ꢀꢀ
0.90
365
515
365
515
545
(χ2 1.40)
L3a (Φf)
Excitation
(nm)
1.24
5.48
13.57 L3b (Φf)
1.09
2.68 9.98
L3a
Excitation
10ꢀ1
1.39
(α1 0.21)
5.43
(α2 0.11)
0.17
(α3 0.0.68)
1.00
λ Emission 500nm
(χ2 1.23)
320
440
440
524
(nm)
320
450
420
492
10ꢀ2
λ Emission 463nm
0.99
(α1 0.19)
4.53
(α2 0.09)
0.12
(α3 0.72)
0.682
Emission
(nm)
Emission
(nm)
(χ2 1.11)
380
505
390
505
L4a (Φf)
Excitation
(nm)
1.98
7.58
18.68 L4b (Φf)
1.32
4.48 14.46
L3b
10ꢀ1
2.50
(α1 0.19)
7.43
(α2 0.24)
0.24
(α3 0.57)
2.416
Excitation
λ Emission 477nm
(χ2 1.14)
340
370
450
515
450
515
(nm)
340
370
420
490
460
530
10ꢀ5
λ Emission 473nm
6.24
(α1 0.93)
4.83
(α2 0.07)
ꢀ
0.903
(χ2 1.34)
Emission
(nm)
Emission
(nm)
L4a
10ꢀ1
1.93
(α1 0.17)
6.90
(α2 0.11)
0.15
(α3 0.72)
1.201
λ Emission 464nm
(χ2 1.20)
It was observed that the quantum yield of the compounds in dilute
solution (10ꢀ4 M in MeOH) was in the range of 0.89ꢀ1.98, while
35 on increasing the concentration to 10ꢀ1 M, Φf were observed in
range of 1.52ꢀ7.58. The absolute quantum yield in solid state
showed a further increase and values were in the range 2.28ꢀ
10ꢀ4
λ Emission 460nm
0.86
(α1 0.43)
5.29
(α2 0.11)
0.19
(α3 0.46)
1.021
(χ2 1.21)
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molecules may involve parallel faceꢀtoꢀface stacking of the
molecules (or HꢀAggregates) (Scheme 3).
18.68. An interesting trend was observed in the quantum yield for
these compounds (Table 5). The compounds L2a L3a and L4a
showed a higher quantum yield than that of L2b L3b and L4b
,
,
,
60
65
70
75
respectively. Further, on increasing the alkyl spacer length, the
quantum yield was observed to increase.
5
N
H3C
H3C
H2C
CH3
Correlating the Enhanced Emission of the compounds with
N
N
their Crystal Structure: The absence of fluorescence in L1a
L1b L1c and L1d can be explained by observing their crystal
structures. In the solid state of L1a, the molecule has planar
,
CH3
,
N
N
CH3
N
10 geometry. The aromatic rings of the neighboring molecules are
tilted and inclined faceꢀtoꢀface stacking is observed. The UVꢀ
Visible absorption maxima of crystalline/solid L1a showed a blue
shift compared to that in MeOH (10ꢀ4 M). Both the crystal
N
CH3
CH3
H3C
H3C
N
N
structure as well as the absorption spectra indicated that in L1a
,
N
N
15 the formation of Hꢀaggregates in the solid state and in
concentrated solution, which may be attributed to the absence of
fluorescence.
CH3
H3C
N
L2b
Alkyl Spacer
N
N
H3
C
H3C
H3C
N
N
20
25
30
35
N
N
N
N
N
N
N
CH3
N
CH3
N
N
N
H3C
N
CH3
N
N
N
HꢀType Aggregates due to faceꢀtoꢀface stacking
N
N
N
H3
C
80 Scheme 3: Arrangement of the molecules of L2b; L2a, L3a, L3b, L4a
and L4b are also expected to have similar arrangements in the solid state.
Notice the faceꢀtoꢀface stacking of the molecules
L1a
L1d
L1b
In L2d, the attainment of the planar geometry and offset
85 arrangement of molecules resulted in increased emission in PL
spectra on increasing concentration and in the solid state and
similar behavior is expected for L3d in solid state (Scheme 4).
HꢀType Aggregates due to faceꢀtoꢀface stacking
Scheme 2: Arrangement of the molecules of L1a, L1b and L1d in the
solid state; Notice the faceꢀtoꢀface stacking of the molecules. Although it
is inclined in case of L1a and L1b, no fluorescence is observed in solid
state
In L1b, the non planar geometry of the molecule along with faceꢀ
toꢀface stacking of the molecules has resulted in quenching of
N
90
H3C
N
fluorescence, while in L1d
explained by the dimer formation in the solid state (Scheme 2).
40 Although the crystal structure of L1c is not reported, we can
speculate that non radiative decay due to the headꢀtoꢀhead
arrangement of the molecules.
, quenching of fluorescence is
N
N
H3C
N
CH3
CH3
N
N
N
N
H3C
N
N
N
CH3
CH3
H3C
N
N
N
N
N
CH3
N
N
H3C
N
In L2b, the molecules attain planarity in the solid state and faceꢀ
toꢀface stacking of the molecules is observed. This is similar to
45 the formation of Hꢀaggregates, which is defined as the parallel
alignment of the molecules and blue shifted absorption bands. The
blue shifted UVꢀVisible bands (about 8 nm) and the crystal
structure of L2b shows that they form Hꢀaggregates. The
appearance of the new peak in the PL spectra at higher
50 concentration and red shift of that peak indicates that a new
emissive species has formed in the high concentration. The crystal
structure analysis shows that although Hꢀaggregates are formed in
L2b, these aggregates are reason for its high emission properties.
N
CH3
N
N
L2d
H3C
N
95
JꢀType Aggregate
The crystal structures of L2c, L3a, L3b, L4a and L4b are not
55 reported, but the similar trend in UVꢀVisible absorption spectra
and PL spectra indicates that the solid state arrangement of the
Scheme 4: Arrangement of the molecules of L2d and L3d in the solid
100 state; Notice the offset stacking of molecules
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protons. From the crystal structure analysis of L1a, it was
55 observed that the molecules have tilted faceꢀtoꢀface arrangement
and the aromatic moieties are clustered together.
In L2c and L3c, it was reported previously by our group, that the
formation of a new "chromophore" species at concentration and in
solid state resulted in increased emission spectra. The crystal
structure analysis of L3c showed that the non covalent
interactions led to the arrangement of the molecules in "non
covalently bonded macrocycles" (Scheme 5).
5
60
65
70
N
10
N
H
N
H
N
N
H
15
N
H
N
N
Non Covalently Bonded Macrocyclic Ring
Scheme 5: Arrangement of the molecules of L2c and L3c in the solid
75
20 state; Notice the formation of new "Chromophore" on aggregation and in
the solid state
Figure 16: 1H NMR spectra of L3b in different concentrations in CDCl3
1
The concentration dependent H NMR spectra of L2b and L3b
NMR Assay to Detect the Aggregation in Concentrated
Solution: The effect of changing the concentration on the NMR
spectra of aggregating compounds include changes in the
80 was recorded in CDCl3 (Figure 16, Figure S52). In both the cases,
it was observed that the protons in the aromatic region resonated
in the deshielded region when the concentration was increased
from 10ꢀ2 M to 1M, whereas the methyl protons resonated in the
shielded region on increasing the concentration. The crystal
85 structure analysis of L2b showed the parallel stacking of the
aromatic rings, which may be associated with the deshielding
effects of aromatic protons on increasing concentrations, while on
closely observing the structure, it was seen that the herringbone
arrangement of the parallel stacked layer resulted in positioning of
90 the methyl groups in such a way that they fall just above the
aromatic rings (Scheme 6), which explains the shielding effect of
the methyl protons on increasing concentration.
25 chemical shifts, shape and intensity of the peaks on changing the
1
concentration.14 The HꢀNMR of compounds L1a
,
L1b
,
L2b and
L3b were recorded in different concentrations in CDCl3. The
indication of the possible molecular arrangement on
aggregation/in concentrated solution may be obtained from final
30 arrangement of the molecules in the solid state.
35
95
Methyl Group on the shielded region
N
N
40
H3C
H3
C
N
N
Parallel Stacked Layers
100
N
N
CH3
CH3
H3
C
H3C
H
N
45
N
Figure 15: 1H NMR spectra of L1a in different concentrations in CDCl3
105
The concentration dependent 1HꢀNMR of L1a showed that the
50 aromatic protons and the methinine protons resonated in
deshielded region when the concentration was increased from
Scheme 6: Arrangement of the molecules in L2b and L3b resulted in
deshielded aromatic proton and shielded methyl protons
0.01M to 1M (Figure 15). The aggregation of molecules at higher 110
concentration has resulted in the change in chemical of the
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1
The concentration dependent HꢀNMR of L2d in CDCl3 showed
some of these compounds have given us the idea about interplay
of non covalent interactions and crystal packing on the photo
that the increase in concentration from 10ꢀ2 M to 1M resulted the
aromatic proton to resonate in shielded region (Figure 17, Figure 60 physical properties of the compounds. Apart from the compounds
S53). The shielded aromatic proton in concentration solutions of
L2d may be due to the offset arrangement of the molecules. So
faceꢀtoꢀface stacking of aromatic rings are absent in L2d, which in
with hydrazine spacer i.e. L1, all the other compounds have small
πꢀconjugated systems linked with alkyl spacers. In L1, where
extended conjugation is present throughout the molecule didn't
show any fluorescence, while other compounds with alkyl spacer
65 show good emission properties. The emission properties of these
compounds are related to the arrangement of the molecules in
aggregated states. In L1, the Hꢀaggregate formation/ dimer
formation resulted in quenching of fluorescence. In L2b, the Hꢀ
aggregate formation was evident from crystal structure and UVꢀ
70 Visible absorption spectra, but AIE properties were observed. The
alkyl spacer in L2b is preventing the nonꢀradiative decay
processes usually observed for Hꢀaggregated. The compounds
5
10
15
20
case of L1a, L2b and L3b has resulted in deshielding effects.
L2a, L3a, L3b, L4a and L4b were also expected to show similar
packing as that of L2b and the enhanced emission properties of
75 these compounds are attributed to the presence of the alkyl
spacers.
Acknowledgement
Figure 17: 1H NMR spectra of L2d in different concentrations in CDCl3
We gratefully acknowledge the financial support from DST
(SR/FT/CSꢀ24/2011) and Instrumentation facility from the DST
80 FIST & UGCꢀSAP to the Department of Chemistry, BITS Pilani,
Pilani Campus. We also acknowledge the DST FIST supported
Powder XRD facility of the Department of Physics, BITS Pilani,
Pilani Campus.
25 The NOESY of L2b at 2M concentration in CDCl3 was taken to
observe the interaction between different types of protons at
higher concentration due to aggregation. The spectrum showed
interaction between the aromatic protons 'a' and aromatic protons
'b', 'c' & between aromatic protons ('a', 'b', 'c') and the methyl
30 proton ('d') (Figure 18). This suggested that at high concentration,
the methyl protons interact with the aromatic protons and a
structure similar to scheme 6 may be possible. Further the
interaction between the aromatic protons also suggest a faceꢀtoꢀ
face stacking of the molecules at higher concentration.
Notes and references
85 Department of Chemistry, Birla Institute of Technology and Science ,
Pilani, Pilani Campus, Rajasthan, India. Fax: +91-1596-244183; Tel:
+91-1596-515679; E-mail: msarkar@pilani.bits-pilani.ac.in
† Electronic Supplementary Information (ESI) available: [IR, 1H NMR,
13C NMR, UV absorption spectra, Photo Luminiscence Spectra and other
90 supplementary material.]. See DOI: 10.1039/b000000x/
35
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
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Figure 18: NOESY of L2b in CDCl3 at a concentration of 2M
55 Conclusions
Systematic study of unique AIE properties of di Schiff base
compounds were carried out. The crystal structure analysis of
12
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DOI: 10.1039/C6RA08582B
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Photo Physical Properties of Di-Schiff Bases: Evaluating the Synergistic Effect of Non
Covalent Interactions and Alkyl Spacer in Enhanced Emissions of Solids
Moyna Das, Fayaz Baig and Madhushree Sarkar*
The di-Schiff bases with alkyl spacer (ethyl, butyl and hexyl) showed enhanced light emitting
properties in solid state, while quenching was observed for di-Schiff bases with hydrazine
spacer. The crystal structure analysis of the compounds showed that the packing of the molecules
via non covalent interactions along with the flexible spacer played the role in dictating the
emission properties.