Light-Emitting Chiral Nematic Dimers with Anomalous Odd-Even Effect

DOI: 10.1002/cphc.201900716

Articles

1

2

3

4

5

6

7

8

9

Light-Emitting Chiral Nematic Dimers with Anomalous

Odd-Even Effect

B. N. Veerabhadraswamy,^[a]Sachin A. Bhat,^[a]Uma S. Hiremath,^[a]and

Channabasaveshwar V. Yelamaggad*^[a]

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

In this report, based on the results derived from the extensive

study into the thermal and photophysical properties, an

anomalous mesomorphic behavior of photoluminescent, chiral

nematic (N*) liquid crystalline dimers, belonging to two differ-

ent series has been revealed. They comprise cholesterol and

fluorescent three-ring Schiff base or salicylaldimine core inter-

linked via an ω-oxyalkanoyloxy spacer of varying length and

parity. The effect of molecular structure on the liquid crystal

(LC) behavior and photophysical properties of both the series

has been probed by varying the length of the terminal n-alkoxy

tails for a fixed (odd or even) parity of the spacer. The detailed

investigations using complementary techniques not only

evidenced the existence of the N* phase in all the dimers

synthesized but also the occurrence of an intriguing odd-even

effect; blue phases (BPs) exist in all the dimers comprising even-

membered spacer, which surprisingly remains totally absent in

their odd-membered counterparts. While the results reported

hitherto are exactly opposite to the aforesaid findings, this

atypical behavior has been interpreted in terms of the over-all

shape of the dimers rendered by the orientation of terminal

tails. Photophysical studies carried out clearly revealed the

intrinsic light emitting feature of the dimers not only in their

dilute solutions but also in their three condensed states viz.,

solid, N* phase, and isotropic liquid state; the emission

intensities of the N* phase varies with the change in temper-

ature, as expected. CD spectra of the N* phase recorded as a

function of temperature show bisignate CD band characteristi-

cally, signifying large chiral correlations in the molecular self-

assembly, while the origin of bands from positive to negative

region suggests a right-handed twist of the N* helix.

1. Introduction

a flexible spacer of varying length and parity.^{[2a,3a,d,4,7–22]}Precisely,

they have shown unprecedented rich phase sequences as well

as frustrated and technologically useful LC phases over wide

thermal width. Moreover, they provide an opportunity to imbue

the desired functional properties; for example, they can be

made photo-responsive systems by incorporating photoactive

mesogens derived from azobenzene^[14]and stilbene cores.^[9d,19a]

The foregoing briefly reviewed research results clearly

demonstrate the novelty of cholesterol-based dimers especially

in terms of their molecular structural design wherein any

functional characteristics can be introduced with a great ease.

In light of such highly inspiring findings, the current study

aimed to prepare, and evaluate the mesomorphism and photo-

luminescence of LC dimers comprising cholesterol and stilbene

fluorophore interlinked through a flexible spacer of varying

length and parity; the present work especially intended to

stabilize the photoluminescent chiral nematic (N*) phase over a

wide temperature width suitable for technological endeavors.

This particular project with an aforesaid molecular architec-

turing was chosen for the following genuine reasons. Mesogens

capable of exhibiting LC phase(s) accompanied by fluorescent

behavior have been attracting a lot of attention of both

researchers and technologists.^[23]Such an attention stems due

to the reason that non-polymerisable nematic and smectic

mesophases can be used in devices such as organic light-

emitting diodes (OLEDs), organic field-effect transistors (OFETs),

photovoltaics etc. Most importantly, non-polymerisable glass

forming N* LCs can be employed for circularly polarized

photoluminescence (PL) and electroluminescence (EL) for

practical applications in optical information display devices,

Among a wide variety of non-conventional liquid crystals (LCs)

reported hitherto,^[1]oligomeric LCs (OLCs) have drawn special

attention due to their true multi-functional characteristics

stemming purely from the combined properties of different

mesogenic segments they comprise.^[2]In fact, they not only

possess the features of both monomeric and polymeric LCs but

also exhibit a rich phase transitional behavior.^[1d,e,2]Of all these

OLCs, LC dimers,^[2a,3,4]the first members of OLCs, in which two

mesogens are interlinked covalently by a flexible spacer, have

been investigated rather exhaustively not only because of their

ability to serve as model systems for main-chain/side-group LC

polymers but also they display rich mesomorphism differing

from that of the corresponding conventional mesogens.^[2a,3]In

fact, the molecular chirality^[5]has significantly influenced the

phase transitional properties of both symmetric^[2a,6]and non-

symmetric dimers^[2a,4,6a]which respectively comprise two chemi-

cally identical and non-identical mesogens. This is especially

noteworthy in the case of optically active (chiral) nonsymmetric

dimers that are made by covalently linking cholesterol with a

variety of conventional calamitic (non-cholesterol) core through

[a] Dr. B. N. Veerabhadraswamy, S. A. Bhat, Dr. U. S. Hiremath,

Dr. C. V. Yelamaggad

Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao

Road, Jalahalli, Bengaluru 560 013, INDIA

E-mail: yelamaggad@cens.res.in

yelamaggad@gmail.com

Supporting information for this article is available on the WWW under

https://doi.org/10.1002/cphc.201900716

ChemPhysChem 2019, 20, 1–17

1

��

These are not the final page numbers!

Articles

processing and storage.^[23,24]Such materials are scarcely

reported,^[23]and thus, there is need for the design and develop-

ment of such chiral materials. In this regard, it is worth noting

from a wide range of experimental results from our own

laboratory as well as by literature reports that the vast majority

of cholesterol-based dimers display chiral nematic (N*) phase

(also known as the cholesteric phase), meaning that cholesterol

is a pro-cholesteric core, and possess the ability to widen the

thermal range of the N* phase and also freeze this structure

into a glassy state.^[14,20]On the other hand, stilbenes, apart from

their intrinsic photophysical behavior, exhibit a wide range of

functional properties and thus they can be used for different

applications; for example, they have been used as laser dyes,

optical brighteners, and media in light emitting diodes, field

effect transistors, photovoltaic cells etc.^[24–30]Thus, we intended

to incorporate a stilbene (fluorophore) mesogenic core in the

molecular design of dimers derived from cholesterol. In

previous investigations it is shown that the usage of 2,3,4-

trialkoxybenzene core not only aids in stabilizing the LC phase

at ambient temperature (or close to this temperature) but also

widens the thermal span.^[31,32]Thus, we set our goal to introduce

a stilbene core substituted with 2,3,4-trialkoxy tails. Accordingly,

we have prepared and characterized two new series of optically

active, photoluminescent dimers that are made by covalently

joining cholesterol with either Schiff base mesogen or salicy-

laldimine core. The molecular structures of these new dimers

derived from cholesterol are shown in Scheme 1. For conven-

ience and to ease out the description, the mnemonics SBD-m,n

and SAD-m,n have been used to describe these two series of

mesogens. Here, SBD and SAD denote Schiff base dimer and

salicylaldimine dimer respectively while n and m refer to the

number of carbon atoms in the terminal alkyl chain and the

number of methylene units (not including the carbonyl carbon)

in the ω-oxyalkanoyloxy spacer respectively.

base (pyridine) in THF to get cholesteryl ω-bromoalkanoates

(5a–d), which were reacted with either 4-hydroxybenzaldehyde

or 2,4-dihydroxybenzaldehyde under the Williamson-ether syn-

thesis protocol to obtain cholesterol ω-(4-formylphenoxy)alka-

noates (6a–d) and cholesteryl ω-(3-hydroxy-4-formylphenoxy)

alkanoates (7a–d). The final products belonging to SBD-m,n

and SAD-m,n series were prepared by condensing aldehydes

6a–d and 7a–d with two-ring primary amines 4a–d in the

presence of a catalytic amount of AcOH in ethanol. The

molecular structure of all the dimers and their intermediates

were characterized by spectroscopic analyses and microanalyt-

ical data (see Experimental section & SI for details).

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

2.2. Evaluation of Liquid Crystalline Properties

The thermotropic mesomorphism of the synthesized dimers

belonging to SBD-m,n and SAD-m,n series was evaluated using

a combination of polarizing optical microscope (POM) and

differential scanning calorimeter (DSC). X-ray diffraction (XRD)

study has been carried out for some representative samples to

ensure the mesophase structures. The sample sandwiched

between clean glass slides was used for the POM study. The

phase sequences ascertained based on POM study, and

transition temperatures and enthalpies of transitions noted

from the DSC thermograms of the first heatingÀ cooling cycles

À 1

°

scanned at a rate of 5 Cmin , are presented in Tables 1 and 2.

Figure S1 depicts the LC properties (phase sequences) estab-

lished for these two series of compounds while cooling them

from their optically isotropic liquid state.

Thermal characterization data presented in Table 1 & 2 clearly

illustrate that all the thirty two non-symmetric dimers belonging

to Schiff base (SBD-m,n) and salicylaldimine (SAD-m,n) series

exhibit enantiotropic mesomorphism. In both series, the LC

behavior of even-membered dimers differs from that of their

neighboring odd-spaced counterparts. Interestingly, when these

results are compared with those of the hitherto reported ones,

these two series of dimers were found to display a reverse

transitional behavior. In the following sections, these aspects are

presented in detail. It is immediately perceptible from Table 1 and

Figure S1a that the three sub-series of Schiff base dimers SBD-3,n,

SBD-5,n and SBD-7,n respectively possessing even-parity spacer

such as 4-oxybutanoyloxy (m=3; m+1=4), 6-oxyhexanoyloxy

(m=5; m+1=6) and 8-oxyoctanoyloxy (m=7; m+1=8) display

an identical LC behavior with an exception of two dimers.

Specifically, these dimers display blue phase (BP) and chiral

nematic (N*) phase except for dimers SBD-3,12 and SBD-7,12

which show smectic A (SmA) phase additionally; in fact, the former

dimer shows a short-temperature lived twist grain boundary (TGB)

phase as well. Likewise, all the salicylaldimine dimers with an

even-parity spacer belonging to SAD-3,n, SAD-5,n and SAD-7,n

sub-series show BP and N* phase with an exception of an dimer

SAD-3,12 which stabilizes the SmA additionally. The existence of

these thermodynamically stable mesophases, and their individual

nature as well as phase transitions with the temperatures were

determined using polarizing microscopy which primarily involved

the temperature dependent texture investigation. When a thin

2. Results and Discussion

2.1. Synthesis and Molecular Structural Characterization

The proposed cholesterol-based dimers were synthesized using

the reaction steps depicted in Scheme 1. The Michaelis-Arbuzov

reaction of triethyl phosphite with 4-nitrobenzyl bromide gave

diethyl 4-nitrobenzylphosphonate (1) in 70% yield. 2,3,4-

Trihydroxybenzaldehyde was O-alkylated with lauryl bromide/

myristyl bromide/cetyl bromide/stearyl bromide by following

the Williamson ether synthesis protocol to obtain 2,3,4-tris(n-

alkoxy)benzaldehydes (2a–d). The Wittig-Horner reaction of

these aromatic aldehydes 2a–d with the diethyl ester 1 in the

presence of a strong base, potassium tert-butoxide (t-BuOK), in

THF afforded 1,2,3-tris(n-alkoxy)-4-(4-nitrostyr-yl)benzenes (3a-

d), which upon subjecting to reduction using indium powder in

the presence of hydrochloric acid in a mixture of THF-water

yielded 4-(2,3,4-tris(n-alkoxy)styryl)anilines (4a–d) in good

yields. Cholesterol was treated with either 4-bromobutyroyl

chloride or 5-bromopentanoyl chloride or 6-bromohexanoyl

chloride or 8-bromooctanoyl chloride in the presence of a mild

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

2

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

°

Scheme 1. Reagents and conditions: (i) Triethylphosphite, 130 C, N₂atm., 6 h. (ii) n-Bromoalkanes, anhyd. K₂CO₃, KI, DMF, 80 C, 12 h. (iii) tBuOK, 1, THF, reflux,

°

30 min. (iv) Indium powder, HCl, THF-water (9 : 1), rt, 2 h. (v) n-Bromoalkynoyl chlorides, pyridine, THF, 0–5 C – rt. (vi) 4-Hydroxbenzaldehyde or 2,4-

dihydroxybenzaldehyde, anhyd. K₂CO₃, Butanone, reflux, 12 h. (viii) 4a–d, Ethanol, AcOH, reflux, 6 h.

film of the sample contained between two untreated glass

substrates and cooled from the isotropic phase, the dimer SBD-

transforms into a characteristic Grandjean texture (Figure 1b)

wherein the helix axis is positioned normal to the glass substrate;

as expected, the background blue color seen for light incidence,

due to optical Bragg reflection from the helical structure of the

phase, does not vary or disappear with a rotation of microscope

stage onto which the sample was mounted. On cooling the

sample further, the focal-conic fan texture of the N* phase sharply

3,12 shows the N* phase with a typical focal-conic fan texture

(Figure 1a),^[5,33]at 120.3 C (ΔH=0.8 kJmol ), where the incident

À 1

°

light scatters and thus, no Bragg reflection persists due to a

random orientation of the constituent helices. Indeed, upon

mechanically perturbing by shearing, the fan texture instantly

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

3

��

These are not the final page numbers!

Articles

a

°

Table 1. Phase sequences, phase transition temperatures ( C) and

enthalpies (ΔH, kJmol^{À 1}) of transitions obtained for dimers belonging SBD-

m,n (Schiff base) series. Cr, Cr₁& Cr₂: Crystal; SmA: Smectic A phase; TGB:

Twist grain boundary phase; N*: Chiral nematic phase; BP: Blue phase; I:

Isotropic liquid state.

Table 2. Phase sequences, phase transition temperatures ( C) and

enthalpies (ΔH, kJmol^{À 1}) of transitions obtained for dimers belonging to

SAD-m,n (salicylaldimine) series of dimers.

1

2

3

Salicylaldimine

Phase Sequence

Heating

4

Dimer (SAD)

Schiff base

dimers (SBD)

Phase Sequence

Heating

Cooling

5

6

7

8

SAD-3,12

SAD-3,14

SAD-3,16

SAD-3,18

SAD-4,12

SAD-4,14

SAD-4,16

SAD-4,18

SAD-5,12

SAD-5,14

SAD-5,16

SAD-5,18

SAD-7,12

SAD-7,14

SAD-7,16

SAD-7,18

Cr 49.34 [39] Cr₁65.81 [5.5] N* 159 [0.8] I

I 158.5 BP 157.9 [0.9] N* 66.4^bSmA 26.01 [11.2] Cr

Cr 73.4 [34.7] N* 164.1 [1.9] I

I 162.1 BP 161.4 [1.3] N* 71.2 [28.3] Cr

Cr 59.3 [47.8] N* 138.1 [3.4] I

I 136.2 BP 135.3 [1] N* 54.3 [37.3] Cr

Cr 63.1 [33.3] N* 156.1 [3.4] I

I 154.4 BP 153.5 [2.1] N* 64.3 [21.9] Cr

Cr₁70.3 [19.1] Cr₂72.4 [34.6] N* 164.3 [3.9] I

I 162.1 [2.8] N* 68.1 [31.4] Cr

Cr 66 [27.8] N* 160.5 [4] I

I 158.3 [3.7] N* 64.2 [25.4] Cr

Cr 65.1 [56.7] N* 150.3 [5.7] I

I 148.3 [3.9] N* 63.4 [41.2] Cr

Cr 67.2 [51.4] N* 147.6 [5.8] I

I 143.8 [3.9] N* 63.1 [38.7] Cr

Cr 32.8 [52.5] N* 129.2 [3.4] I

I 74.2 BP 73.7 [3.2] N* 62.1 [42.4] Cr

Cr 62.3 [54.7] N* 144.9 [3.2] I

I 142.8 BP 142.1 [2.4] N* 59 [47.9] Cr

Cr 60.1 [28.7] N* 141.3 [1.3] I

I 140 BP 139.1 [0.9] N* 58 [21.8] Cr

Cr 63.0 [28.3] N* 138.3 [1.9] I

I 136.3 BP 135.4 [1.1] N* 53.4 [21.3] Cr

SBD-3,12

SBD-3,14

SBD-3,16

SBD-3,18

SBD-4,12

SBD-4,14

SBD-4,16

SBD-4,18

SBD-5,12

SBD-5,14

SBD-5,16

SBD-5,18

SBD-7,12

SBD-7,14

SBD-7,16

SBD-7,18

Cr₁62 [10.9] Cr₂109.3 [25] N* 124.6 [0.9] I

I 120.3 [0.8] N* 85 TGB-SmA 67^bCr

Cr 93.4 [34.8] N* 130.4 [0.8] I

I 129.8 BP 129.3 [0.7] N* 64 [26.9] Cr

Cr 82.6 [53.5] N* 139.8 [0.8] I

I 138 BP 137.3 [0.7] N* 68.9 [47] Cr

Cr 81.5 [37.8] N* 126.2 [1] I

I 125.1 BP 124.3 [0.9] N* 63.8 [26.2] Cr

Cr 67.3 [38.4] N* 124.1 [1.1] I

I 121.4 [0.8] N* 63.5 [26.3] Cr

Cr₁66.8 [21.5] Cr₂92.9 [40.1] N* 107.7 [0.9] I

I 106.3 [0.7] N* 49.1 [25.5] Cr

Cr 105.5 [51.4] N* 137.7 [1.2] I

I 136.1 [1] N* 64.4 [41.9] Cr

Cr 60.8 [21.9] N* 111.8 [1.3] I

I 109.7 [1.2] N* 54.1 [19.3] Cr

Cr₁54.9 [14.1] Cr₂85.8 [26.3] N* 105.8 [1.1] I

I 103.5 BP 101.6 [0.7] N* 41.6 [6.4] Cr

Cr₁70.7 [57.8] Cr₂75.4 [13.8] N* 137.7 [1] I

I 135.3 BP 134.2 [0.9] N* 46 [48.4] Cr

Cr 79.8 [57.5] N* 132 [1.2] I

I 127.4 BP 126.8 [0.9] N* 57.3 [52.6] Cr

Cr 55.7 [34.3] N* 106.1 [0.6] I

I 105.1 BP 104.5 [0.3] N*^c

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Cr 72.6 [37.2] N* 136.6 [1.5] I

I 136.7 BP 135.9 [1.3] N* 45.1 [24.6] Cr

Cr₁64.9 [17.1] Cr₂75.5 [37.6] N* 126.9 [1.2] I

I 122.6 BP 121.9 [1.1] N* 108.6 [46.8] Cr

Cr₁48.5 [12.7] Cr₂59 [12.9] N* 122.1 [0.9] I

I 121.1 BP 120 [2.9] N* 44.2 [19.7] Cr

Cr 50.3 [37.1] N* 93.8 [3.1] I

Cr 50.4 [33.8] N* 90.4 [1.1] I

I 89.4 BP 88.3 [0.9] N* 47.3 SmA^c

Cr 56.3 [19.3] N* 86.3 [1.2] I

I 84.9 BP 84.2 [1] N*^c

Cr 51.3 [12.3] N* 85.4 [1.3] I

I 83.5 BP 82.6 [0.9] N*^c

I 90.4 BP 89.7 [0.9] N* 54.3 [0.7] Cr

Cr 52.4 [29.8] N* 80.4 [0.8] I

^aTransition temperatures recorded from the POM observation and/or peak

79.2 BP 78.3 [0.6] N*^c

values of the DSC traces pertaining to the first heatingÀ cooling cycles

^aTransition temperatures obtained from the polarizing optical microscopic

study and/or peak values of the DSC traces pertaining to the first

obtained at a rate of 5 Cmin

.

À 1

°

À 1

b

°

heatingÀ cooling cycles obtained at a rate of 5 Cmin

.

This transition

c

was seen only under POM.

The mesophase supercools till room

temperature as evidenced by POM study.

°

changes (at 85 C) to a pattern consisting of both pseudo-isotropic

and filament textures (Figure 1c) of the homeotropically aligned

SmA and TGB phases respectively; in some regions of the slide,

the co-occurrence of the N*, TGB and SmA phases could be seen

(Figure 1d). While the TGB phase exists for short thermal range,

the SmA phase occurs till the sample crystallizes. The occurrence

of the TGB phase was especially ensured based on observation of

a filamentary texture^[33]when the homeotropically aligned SmA

phase, before crystallization, was heated tending towards the N*

phase. The SmA phase exhibited a focal-conic texture exclusively

or a pseudo-isotropic texture solely when examined using surface-

coated slides, treated for planar orientation or homeotropic

alignment of mesogens respectively. Thus, the first member SBD-

3,12 of the SBD-3,n sub-series shows N*, TGB and SmA phases.

The next members of this series viz., SBD-3,14, SBD-3,16

and SBD-3,18, unlike the previous dimer, display BP besides the

N* phase, which we shall describe later. An identical behavior

was confirmed, through a meticulous textural observation, not

only for the other even-membered Schiff base dimeric sub-

Figure 1. Microphotographs of the textures observed for the LC phases of

dimer SBD-3,12. (a) A focal-conic texture of the N* phase; (b) a Grandjean

planar texture of the N* phase formed after shearing the texture-(a); (c) a

texture consisting of pseudo-isotropic & filament domains of homeotropically

aligned SmA and TGB phases respectively; (d) a pattern showing textures of

N*, SmA and TGB phases.

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

4

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

°

Figure 2. The DSC traces of the first heating-cooling cycles recorded at a rate of 5 C/min. for the representative dimers SBD-7,12 (a) and SAD-3,12 (b) of SBD-

m,n and SAD-m,n series respectively.

series viz., SBD-5,n and SBD-7,n but also for the even-

membered salicylaldimine dimers belonging to three sub-series

viz., SAD-3,n, SAD-5,n and SAD-7,n. However, the transition

from isotropic liquid phase to BP could not be traced in the

DSC profiles recorded for these samples. As representative

cases, the DSC traces obtained for first heating-cooling scans of

dimers SBD-7,12 and SAD-3,12 are presented in Figure 2a and

2b respectively.

(3D) periodic structure respectively having simple-cubic and

body-centered cubic symmetries. Owing to their 3D cubic

structure (orientational ordering) with lattice periods of several

hundred nanometers (~500 nm), they show selective Bragg

reflections in visible light regime. While the platelets are the

domains of crystalline BP ordering, the intense color stems from

the scattering in the direction of the viewer of those wave-

lengths satisfying the Bragg condition. As shown in Figure 3c,

the Grandjean plane textural pattern of the N* phase was

obtained when the BPI/II phase is subjected to mechanical

stress, as expected. When the sample is cooled slowly further

from the natural platelet texture of BPI/BPII, the N* phase

begins to appear over the platelet pattern (Figure 3d) which

eventually fills field of view to yield an archetypal focal-conic

fan texture; as expected, Grandjean texture was observed when

the top glass substrate was pressed gently. The N* phase

remains unaffected until the samples crystallize. Interestingly, in

dimers SBD-7,14, SBD-7,16 and SBD-7,18 the N* phase super-

cools till room temperature, which remains as such for a long

time. In essence, the even-nonsymmetric chiral dimers of both

the series generally, and unusually show BP and N* phases.

On the other hand, the odd-members of both the Schiff

base series (viz., SBD-4,12, SBD-4,14, SBD-4,16 & SBD-4,18) and

salicylaldimine series (SAD-4,12, SAD-4,14, SAD-4,16 and SAD-

4,18) behaved identically but certainly different from the

corresponding even-members of both the series. They display

thermodynamically stable N* phase solely omitting the BP that

was found present in almost all even-membered dimers as

described before. This odd-even behavior is contrary to what

has been usually reported by many researchers in the field.

Precisely, it is by and large observed that optically active (chiral),

non-symmetric dimers with an odd-numbered spacer, unlike

the even dimers, display BPs^{[2a,3,6a,34,35]}meaning that the form

chirality show dependence on the parity of the central

spacer.^[2a,6a]This behavior has been typically attributed to

several important aspects such as bent-conformation of odd-

membered dimers, high chirality^[36]– the short pitch N* phase,

lower orientational order and smaller elastic constant. Specifi-

cally, the bent-conformation of the dimeric mesogens shortens

the pitch of the cholesteric phase that originates from the

smaller twist elastic constant.^[2a,6a]In fact, it can also be

supposed that the need for the bent-core mesogens to move

As described in the succeeding text, the presence of the

BPI/II was authenticated from its defect texture. On slow cooling

°

(0.1 to 0.3 C/min.) the isotropic liquid of any one of these

aforesaid dimers confined between untreated glass substrates,

and viewed under POM, a striking mosaic texture composed of

platelets emanating from black background of the isotropic

phase was observed (Figure 3a). Upon progressive slow cooling,

platelets fill the entire field of view rapidly (Figure 3b) which

has been generally ascribed to the presence of BPI/BPII. In fact,

BPI and BPII, which comprise double-twist cylinders and

disclinations like that of the BPI, possess fluid three-dimensional

Figure 3. Photomicrographs of the textures of mesophases seen under POM

for dimer SBD-3,16: (a) A platelet/mosaic texture of BPI/II appearing just

below the isotropic liquid phase; (b) the fully-grown platelet texture of BPI/II;

(c) the Grandjean plane pattern of the N* phase formed after shearing the

platelet texture of BPI/II, (d) a texture showing the coexistence of BPI/II and

N* phases.

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

5

��

These are not the final page numbers!

Articles

apart from each other just below the isotropic phase (at the

elevated temperature) cannot be achieved by expansion, thus

they may twist to higher angles. This leads to an increased

twist, resulting in a shorter pitch N* phase above which the BPs

exist. However, it is interesting to note that comparable helical

twisting powers have been obtained for both odd- and even-

membered dimers in the host nematic phase of conventional

LCs.^[6a]Generally speaking, the archetypal odd-even behavior of

LC dimers is ascribed to, apart from the obvious spacer-parity

dictated molecular shape (the eventual shape) of dimeric

mesogens in their all-trans conformation, two other important

aspects such as population of conformers (bent or linear) and

inter- & intra-molecular energies. The relative population of

conformers depends on both inter-molecular & intra-molecular

energies.^[37]Some of these aspects may be applicable to the

present odd-even dimers provided the average molecular shape

posed by the terminal chains is considered rather than the

effect of parity of the spacer. On the whole, here the existence

of BPs in even-members than the odd dimers implies that there

are more bent-conformations for the former dimers where the

two mesogenic groups are inclined to each other than there are

for an odd-numbered central spacer.

The aforesaid atypical stabilization of BPs by even dimers,

unlike the odd dimers, has also been observed in one of our

earlier investigations;^[13a]this behavior can be interpreted in

terms of the average molecular-shape of the dimers that are

formed by covalently linking five-ring (long) banana-shaped

mesogen and cholesterol entities via an ω-oxyalkanoyloxy

spacer. Here, unlike the usual scenario, the dimers with even-

numbered spacer exhibiting BP must be gaining the bent-

conformation in which the mesogenic cores incline with respect

to one another. Such conformers may be stemming from the

shape-persistence/dominance of bent-core segment in the

dimeric molecular architecture. In yet another interesting study,

we have observed that the cholesterol-based dimers comprising

a short bent-core unit in the form of two-ring chalocone

fluorophore exhibit BPs followed by the obvious short pitch

chiral nematic phase, irrespective of length and parity of the ω-

oxyalkanoyloxy spacer they possess; thereby, it was demon-

strated that a small bent-core attached to cholesterol entity

enhances the chirality of the mesophases.^[38]Thus, the foregoing

discussion clearly suggests that the bent-cores (either-short or

long) covalently attached to cholesterol favor the stabilization

of BPs. In essence, the overall bent-conformation of dimers

enforced by bent-cores appears to be the root cause for the

existence of BPs in all even- and odd-membered dimers.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 4. The most stable fully-stretched (energy minimized) space-filling

models of an even-membered dimer SBD-3,12 (a) and an odd-membered

dimer SBD-4,12 (b). Seemingly, the even/odd dimers unusually gain bent-

shape/rod-shape which is seemingly dictated by the long n-alkoxy chains

attached to the terminal benzene ring of three-ring Schiff base unit.

or salicylaldimine cores where the effect of parity of the spacer

is perhaps overlooked. Needless to say, the odd-numbered

dimers, as shown in Figure 4b, attain an extended rod-like

conformation; it appears, therefore, that the comparison of LC

behavior of the present series of dimers with that of the

aforementioned dimers^[13a,15b]is rather difficult as they differ

significantly in their molecular structures, especially in terms of

the number of terminal chains and their positions on the

terminal benzene ring. Indeed, the difference in molecular

shape between the even- and odd-membered dimers could

have been figured out by X-ray data using d/L ratio, where d is

the measured layer spacing and L is the estimated all-trans

molecular length of these dimers. Such a comparative study is

precluded as the SmA phase is seen only in the even-

membered dimers viz., SBD-3,12, SBD-7,12 and SAD-3,12; that

is, none of the odd-membered dimers show SmA phase (see

Tables 1 & 2). However, logically speaking, the bent-core

molecule should have higher molecular width (W) and thus,

higher W/L ratio. These were figured out using the models

shown in Figure 4; the W (W/L) were found to be 24 Å (0.44)

and 20.5 Å (0.36) for dimers SBD-3,12 and SBD-4,12 respec-

tively. This means that the dimer SBD-3,12 with an even-parity

spacer attains bent-conformation.

In the following we compare and discuss the phase transi-

tional behavior of Schiff base (SBD-m,n) and salicylaldimine

(SAD-m,n) series of dimers. In both series, the N* phase exists

°

The cause for the occurrence of BPs in aforementioned

cholesterol-based dimers cannot be extrapolated to the

currently studied analogous dimers as they do not possess

bent-core/banana-shaped mesogen. However, this can be

explained by considering the probable influence of the terminal

tails on the overall shape of even-/odd-parity materials. As

shown in Figure 4a for a representative case, the even-

membered dimers in their most extended all-trans state

perhaps attain a bent-conformation rendered by the orienta-

tion/disposition of the two n-alkoxy chains present at 3,4-

position of the terminal benzene ring of three-ring Schiff base

enantiotropically over the thermal range of 35 to 100 C. Some

interesting observations were made during the POM textural

observations of the N* phase of these two series of compounds.

It was especially noted that the homogeneously oriented N*

phase of all the salicylaldimines showed temperature-depend-

ent color variation; that is, the N* phase changing reflected

color (visible light), which will have a characteristic wavelength

determined by the pitch of the structure, as a function of

temperature was observed. However, the uniform color regis-

tered at a given temperature does not alter/fade away when

the sample held between crossed polarizers is rotated. The

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

6

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 5. POM texture images (a & c) and the selective reflection (transmission) spectra (b & d) recorded as a function of temperature for the planarly aligned

N* phase of dimers SAD-3,12 (a & b) & SBD-3,16 (c & d). Notice that the temperature-dependent color (pitch) variation in the N* phase can be, unlike for

Schiff base SBD-3,16, effectively seen for the salicylaldimine SAD-3,12

planar textures of the N* phase obtained for a representative

dimer SAD-3,12 at a fixed position but at different temperature

are shown in Figure 5a where the color change can be seen

convincingly. The aforementioned phenomenon was observed

quantitatively by recording selective reflection (transmission)

spectra as a function of temperature in the N* phase belonging

to the dimer SAD-3,12. Initially a thin film of the sample was

prepared by holding a small amount of the material (say 1–

3 mg) between the two clean quartz plates and heated to

isotropization temperature using a programmable hot stage; in

this isotropic state, the cell was pressed hard with shearing to

ensure the uniform distribution of the sample. After a while, the

sample was slowly cooled and the N* phase appearing just

below the isotropic liquid state was sheared repeatedly to

attain the planar orientation. While cooling the sample

Figure 6. Temperature-dependence of light wavelength λ_minof the N* phase

formed by the dimer SAD-3,12 indicating the critical dependence of pitch

(p) on the temperature.

°

progressively the transmittance spectra were recorded at 5 C

intervals in the thermal range of the N* phase using a UV-VIS

spectrophotometer (wavelength range 400–800 nm). The trans-

mission spectra of different temperatures are shown in Fig-

ure 5b. In fact, these spectra comprise a Bragg reflection band

centered at λ_min, which is related to the pitch p as λ_min=pn_avg

where n_avg= (n_o+n_e)/2 is the average of the ordinary (n_o) and

extraordinary (n_e) refractive indices of the system. It can be seen

clearly in Figure 5b that with the decrease in temperature the

selective reflection light by the N* phase shifts from shorter- to

longer-wavelengths covering the light in the wavelength range

of 525–735 nm where the reflective bandwidth gradually

becomes broader. In other words, continuous lowering of the

N* phase temperature, “red” shift of the wavelength (λ_min) of

selective reflection occurs with a gradual increase of the

spectral band half-width. Figure 6 evidently depicts the temper-

ature dependence of λ_minin the entire range of the N* phase of

the dimer SAD-3,12 indicating a strong variation in the pitch as

the temperature is varied. Surprisingly, however the textures

color seen under the microscope for the planarly aligned N*

phase of Schiff base dimer SBD-3,16, like other analogous

dimers, does not vary with the change in temperature (Fig-

ure 5c); that is, the pitch remains practically independent of the

temperature. This microscopic observation was supported by

transmission (selective reflection) spectra of the sample re-

corded as a function of temperature (Figure 5d). In essence, the

N* phase of salicylaldimine dimers shows the characteristic

temperature-pitch relationship which is not the case with that

of the Schiff base dimers. While it is rather difficult to interpret

this observation, the presence/absence of hydroxyl group

appears to play an important role in determining the influence

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

7

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 7. 1D intensity vs 2θ profiles of the N* (a & c) and SmA (b & d) phases of dimers SAD-3,12 (a & b) and SBD-7,12 (c & d).

of temperature on the pitch/wavelength (λ_min) of selective

reflection of the N* phase.

presented. As can be seen in Figure 7a (Table S1), all the XRD

diffractograms of the N* phase obtained as a function of

temperature for the dimer SAD-3,12, which appear one and the

same, comprise a diffuse reflection solely in the wide angle

As expected, the o-hydroxy Schiff bases (salicylaldimine

dimers) show higher clearing temperatures when compared to

the corresponding Schiff base homologues; this can be ascribed

to the fact that salicylaldimines comprise a quasi six-membered

rigid ring formed due to intramolecular hydrogen (H)-bonding

between the H-atom of the hydroxy group and the nitrogen

(N)-atom of the imine linkage. Seemingly, in both series of

compounds the odd-effect on the phase transitional behavior is

unremarkable. In general, the increase in the length of the

spacer or terminal chains leads to the decrease in the clearing

temperatures. None of the dimers of the series stabilize the N*

phase near room temperature, which is unexpected.

The N* and SmA phases shown by salicylaldimine SAD-3,12

and Schiff base SBD-7,12 dimers, as representative cases, were

probed with the aid of X-ray diffraction (XRD) technique using

CuK_∝(λ=0.15418 nm) radiation. The unoriented specimens

contained in Lindemann capillaries were used for the measure-

ments. The samples heated to their isotropic liquid state were

cooled slowly in to the N* phase where XRD profiles were

recorded as a function of temperature. Following this, the XRD

traces of the SmA phase as a function of temperature were

recorded. Figure 7 presents XRD traces essentially showing the

1D intensity profiles as a function of the scattering angle 2θ,

obtained for the mesophases at the chosen temperatures. In

Table S1, the XRD data, especially the spacing corresponding to

the wide-angle peak and spacings (d) of the low-angle

reflections, along with the molecular length (L) in the highly

stretched form with an all-trans conformation of the terminal n-

alkoxy chains and the flexible spacer, and d/L ratio, have been

°

region (2θ �20 ) which not only confirms the liquid-like order

of the fluid chiral structure but also represents the distance

between dimeric molecules along their width; here, the absence

of layer reflection peak(s) especially supports assignment of the

N* phase. On the other hand, as shown in Figure 7c all the

alike-looking XRD profiles of the N* phase recorded at different

temperatures for the mesogen SBD-7,12 not only show a

diffuse peak in the wide angle but also a relatively sharp but

°

weak (low intensity) peak in the small angle region (0<2θ <5 )

whose respective measured spacings are 4.61 Å and ~25 Å

(averaged) (see Table S1). While the wide angle reflection is

indicative of liquid like in-plane order of the system, the peak in

the low angle region suggests the presence of short range/

cybotactic order. As listed in Table S1, the spacing values of the

low angle weak peak, ranging between 23.59-25.89 Å, are a

little lesser than that of half of the length of fully stretched (all-

trans) conformation of dimeric molecule SBD-7,12 (L=55 Å)

suggesting a local intercalated organization of mesogens; as

discussed below, this proposition has been supported by the

XRD results of the SmA phase.

The results derived from the XRD measurements clearly

suggested that the structure of the SmA phase depends on the

length of the flexible spacer. As discussed below, a monolayer

organization is observed for the dimer SAD-3,12 having a

shorter (oxybutanoyloxy) spacer, whereas for the dimer SBD-

7,12 with a longer (oxyoctanoyloxy) spacer an intercalated SmA

(SmA_c) phase is formed. Indeed, these results are fully consistent

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

8

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 8. Sketches illustrating the self-organization of dimeric molecules SAD-3,12 and SBD-7,12 respectively in the monolayer SmA phase (a) and intercalated

SmA (b) phase.

with the previous reports where the change over from a

monolayer to an intercalated organization on increasing spacer

length has been observed for a variety of cholesteryl-based

dimers.^{[2a,3d,4,10d,j]}The intensity data plotted against the diffrac-

tion angle 2θ for the SmA phase of dimers SAD-3,12, and SBD-

7,12 at different temperatures have been respectively pre-

sented in Figures 7b and 7d. As can be seen, all the identically

looking X-ray diffractograms comprise a sharp reflection in the

small angle region and a diffuse peak in the wide angle area

clearly evidence the layer structure of the LC phase under

investigation. The wide angle diffuse band, needless to say, is

the typical of a liquid like organization of the mesogens within

the layers. While the layer spacings (d) noted in the SmA phase

the vicinity of smectic (Sm)À N transition.^[5c]The occurrence of such

cybotactic nematic phases, which are characterized by a short

range translational order, is associated with small enthalpy change

of SmÀ N phase transition.^[5c,3d]Mostly, the structure of smectic

(Sm) phase occurring below the nematic phase gets reflected in

the form of smectic cybotactic clusters in the latter phase. Thus, in

the present study, the occurrence of intercalated orthogonal

smectic (SmA_c) clusters in the N* phase is expected as the dimer

SBD-7,12 exhibits SmA_cÀ N phase transition. In fact, a wide variety

of non-symmetric dimers show SmA_cphase which comprise two

microphase separated regimes. While the central spacer and the

terminal chains form a (flexible) regime, two chemically dissimilar

mesogens mix-up to yield another (rigid) domain. Most impor-

tantly, a specific inter-molecular interaction among the two

structurally dissimilar mesogens, which is anisotropic in nature,

has been regarded as the main driving force for the formation of

SmA_cphase by non-symmetric dimers.^[39]Such a specific associa-

tion has been regarded as an electrostatic quadrupolar interaction

between groups having quadrupole moments of opposite

signs.^[2a,3e,39]Besides, the mixing of the dissimilar mesogenic cores

necessary to maximise the extent of interaction is entropically

more favourable than permitting the dimers to microphase

separate into regimes comprising like mesogenic units, spacers

and terminal chains. So, the inter-molecular interaction between

the two dissimilar mesogens, which favours the formation of SmA_c

phase, will partly remain in the N* phase. Thus, the presence of

SmA_c-like short range order in the N* has been observed in the X-

ray profile of dimer SBD-7,12.

°

at 60 C and 50 C temperatures respectively are 46.66 Å and

44.0 Å, the calculated length L of the dimer SAD-3,12 in the

most extended form is about 49 Å. Apparently, the d-values are

quite closer to the all-trans (fully stretched) molecular length of

dimer SAD-3,12. That is, the ratio d/L is about 0.9 suggesting

that the mesophase under observation is a monolayer SmA

phase (Figure 8a), which may have stemmed from the van der

Waals interactions among the cholesterol unit and the alkyl

chains.^[10j]On the other hand, the d-values of the SmA phase

°

obtained at 45 C, 35 C and room temperature (25 C) for the

dimer SBD-7,12 are 25.35 Å, 25.3 Å and 25.2 Å respectively (see

Table S1). Seemingly, the d-values are lesser than the calculated

length L of the dimer in its most extended conformation. That

is, the d-values are close to 0.5 times the calculated molecular

length L=55 (Table S1) implying that the dimer SBD-7,12

stabilizes an intercalated orthogonal SmA (SmA_c) phase in

which the different molecular structural segments of mesogens

overlap (Figure 8b). Precisely, as shown in Figure 8b, unlike

mesogenic (cholesterol & Schiff base) entities are mixed yielding

one (rigid) domain whereas the central spacer and the terminal

chains form another (flexible) regime.

2.3. Photophysical and Chiroptical Studies

Given the fact that the stilbene fluorophore has been incorpo-

rated on purpose in the molecular architecture of the present

two series of dimers, they are expected to display intrinsic

photophysical properties. Thus, they were probed systematically

to demonstrate their absorption and photoluminescence ability.

It is widely known that the cybotactic achiral neamtic (N)/

cholesteric (N*) phases, which comprise a small number of smectic

clusters, exist as consequence of pre-transitional phenomena in

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

9

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 9. Absorbance (a & c) and photoluminescence (b & d) spectra recorded for the dilute DCM solutions of dimers belonging to SBD-3,n and SAD-3,n, sub-

series of dimers (for other spectra see SI). Insets in panels (b) and (d) are the photographs of DCM solutions of dimers SBD-3,12 & SAD-3,12, under UV

illumination (365 nm)

In particular, the absorption and emission spectra of these

luminous samples were recorded not only for their dilute

dichloromethane (DCM) solutions but also in their condensed

(solid) and mesomorphic states. While the spectra of the DCM

solution and crystalline (drop-casted thin-film) states of all the

substances prepared were recorded at room temperature, the

photophysical profiles of the N* phase were obtained as a

function of temperatures for several randomly chosen dimers

from each series, as representative cases. The electronic

absorption and photoluminescence spectra of the solutions (3–

5×10^{À 4}molL^{À 1}) are respectively shown in left-hand side (LHS)

and right-hand side (RHS) panels of Figure 9. In this figure, the

profiles obtained for even-membered dimers of both the series,

viz., SBD-3,n (Figures 9a & 9b) & SAD-3,n (Figures 9c & 9d) and

the spectra recorded for all other dimers belonging to both the

series are presented in SI (Figures S2 & S3). However, for the

sake of comparison and ease of interpretation, the relevant

photophysical data of all the compounds derived from their

respective spectrum are collected in Table S2.

photoluminescence spectra of solutions, after excitation with

light of 320 nm, were recorded. The spectra of the all

compounds were found to be alike as expected, which

comprised the emission maxima (λ_em) centered around 398–

411 nm, see Figure 9b and Figures S2b, S2d & S2f. As shown in

the inset of Figures 9b & 9d, the yellowish-green emission was

observed by the naked eyes on exposing these solutions to a

light of 365 nm stemming from a handheld UV-Vis lamp.

The optical absorption and emission spectra of all the

salicylaldimine dimers (SAD-m,n series) in DCM solution are

respectively shown in Figure 9c (Figures S3a, S3c & S3e) and

Figures 9d (Figures S3b, S3d & S3f) and the corresponding

optical data obtained are listed in Table S2. The absorption

spectra of these dimers, appearing identical, show two sharp

absorption bands in the ranges of 361–369 nm and 292–

313 nm which can be respectively assigned to the n-π* and π-

π* transitions. For the higher wavelength bands the molar

extinction coefficient values were found to be in the range of

1.31–4.2×10⁴Lmol^{À 1}cm^{À 1}. It may be pointed out here that the

absorption bands of these salicylaldimines, when compared to

those of Schiff bases (SBD-m,n series dimers), show spectral

shift towards higher wavelengths (red-shift) that can be

ascribed to extended π-conjugation due to the presence of

hydroxyl group of the mesogens. It is apparent from Figure 9d

and Figures S3b, S3d & S3f that the corresponding emission

spectra obtained for the compounds, after exciting them with

320 nm light display, analogous to Schiff base dimers, the

emission maxima centered around 403–418 nm. As listed in

Table S2, these salicylaldimines exhibit rather small Stokes shifts

(38–50 nm) implying the occurrence of self-quenching process

in these fluorophores; that is, the resonance energy transfer

As can be seen in Figure 9a and Figures S2a, S2c & S2e, the

pattern of UV-Vis spectra of all the sixteen dimers belonging to

SBD-m,n series (Schiff bases) appear alike displaying two

primary absorption bands in the UV region (250 to 380 nm)

owing to high absorption coefficients meaning that these

mesogens absorb photons readily. In particular, these bands

occur in the ranges of 276–287 nm and 310–362 nm; while the

latter longer wavelength band corresponds to n-π* transition,

the former shorter absorption peak can be ascribed to the π-π*

transition. The molar extinction coefficients (ɛ) for higher

wavelength band were determined to be in the range of 0.9×

10⁴to 4.88×10⁴Lmol^{À 1}cm^{À 1}(see Table S2). The corresponding

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

10

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 10. UV-Vis (a & c) and emission (b & d) spectra obtained for the thin-films (solid samples) of dimers of SBD-3,n and SAD-3,n sub-series of dimers (for

other profiles see SI). Insets in panels (b) & (d) are the photographs taken under UV illumination (365 nm) of the powder samples & thin-films of dimers SBD-

3,12 & SAD-3,12.

among the mesogens (homotransfer) appears to occur. In other

words, small Stokes shift imply that the change in dipole

moment/electronic property in ground and excited state is

negligible.^[40]It may be mentioned here that the overall shape

of emission or absorption spectra of all the members of SBD/

SAD series should match, as the change in the lengths of spacer

or terminal tails do not contribute to these factors. Such spectra

basically originate from the functional groups of dimers such as

imine linkage, double bonds, lateral (OH) substitutions etc., they

possess. In the present study, however, the dissimilarities are

seen in the shape and/or intensity of emission or absorption

spectra. Such variations are known to evolve from the differ-

ences in the concentration of solutes in solvents.^[41]

at room temperature for these thin films are shown in

Figure 10.

Figures 10a & 10b and 10c & 10d respectively show the

spectra obtained for a set of even-dimers belonging to SBD-3,n

and SAD-3,n series. The profiles obtained for odd-membered

dimers viz., SBD-4,n & SAD-4,n have been shown in SI along with

other dimers. Such absorption and emission spectra obtained for

other dimers are presented in Figure S4 and S5. The correspond-

ing photophysical data are shown in Table S2. The insets of

Figures 10b and 10d respectively show the photographs of the

thin-films of dimers SBD-3,12 and SAD-3,12 illuminated with UV

light at 365 nm originating from a handheld UV lamp. It is

immediately apparent from the Figure 10a (Figures S4a, S4c and

S4e) and Figure 10c (Figures S5a, S5c and S5e) that the UV-Vis

spectral patterns of thin-films of Schiff bases and salicylaldimines

are nearly comparable to spectra of corresponding dilute DCM

solutions (see Figure 9 & Figures S2 and S3). The UV-Vis spectra of

Schiff bases (Figure 10a & Figures S4a, S4c and S4e) comprise two

absorption peaks in the ranges of 304–312 nm and 364–378 nm.

Likewise, the absorption spectra of thin-films of salicylaldimines

(Figure 10c & Figures S5a, S5c and S5e) show two bands in the

range of 279–293 nm and 310–368 nm. In both the cases, the

shorter absorption peak pertains to the π-π* transition whereas

the longer wavelength band arise due to n-π* transition.

Seemingly, the absorption bands of the thin-films of Schiff bases

exhibit a bathochromic (red) shift when compared to those of

dilute solutions, and surprisingly such a feature is not apparent for

salicylaldimines. The photoluminescence spectra of the thin-films

recorded, after irradiating with 320nm excitation wavelength, for

the Schiff bases and salicylalsdimines are respectively shown in

Generally, fluorescent mesogens tend to aggregate in

condensed phases viz., solids, LC phases and liquids, leading to

a phenomenon called aggregation-caused quenching (ACQ)^[42]

where intermolecular energy transfer exists.^[43]So, circumvent-

ing ACQ difficulty by some strategy is absolutely necessary to

realize novel solid or liquid crystalline fluorescent materials.

However, needless to say, one of the simple ways of achieving

this is to synthesize new light-emissive materials, where the

fluorophore is intentionally incorporated in the molecular

architecture, as in the present study, and examine for their

fluorescence behavior. Thus, absorption and emissive properties

of all the solid mesogens pertaining to both the series as well

as the N* phase of some representative compounds were

investigated. The thin-films of the mesogens (solids) required

for the study were prepared on quartz substrate by drop-

casting method. The absorption and emission spectra recorded

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

11

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 11. Temperature dependent photoluminescent spectra of the N* phase of LC dimers SBD-3,16 (a) and SAD-3,12 (b).

Figure 10b (Figures S4b, S4d & S4f) and Figures 10d (Figures S5b,

S5d & S5f). Seemingly, all the spectra characteristically comprise a

broad emission band; Schiff base dimers show a broad emission

band at 433–455 nm where as the spectra of salicylaldimine

dimers consist of a emission maxima centered around 513–

521 nm. It can be noticed from Table S2 and Figures 10d

(Figures S5b, S5d & S5f) that the emission peak position of the

thin-films of salicylaldimine dimers, as compared to that of the

corresponding dilute solutions, shifts remarkably towards a longer

wavelength (red shift); however, such a shift appears to be

moderate in the case of Schiff bases. The Stokes Shift, which is the

difference between wavelengths of the emission and excitation

maxima, was found to be in the range of 64–65 nm for Schiff

bases and 146–201 nm for salicylaldimine dimers (Table S2). These

findings are indeed in good agreement with the general

observation that the emission spectrum shifts to longer wave-

lengths than the excitation (absorption) spectrum. The observed

higher Stokes shift (over 140 nm) for salicylaldimine dimer is

remarkable given the fact that it is rather easier to distinguish

clearly and separate emission light from that of excitation light.

That is, such large values rule out the overlap of emission and

absorption spectra enabling the detection of fluorescence without

any interference; this also eliminates fluorescence quenching.

The N* phase, being highly sensitive and responsive/

sensitive to a number of external stimuli viz., the temperature,

light, electric field, and mechanical stress,^[5,23a,44]formed by a

Schiff base dimer SBD-3,16, and a salicylaldimine dimer SAD-

3,12, as representative cases, was probed for its photophysical

property. The thin-films of the samples prepared during the

selective reflection measurements were employed. The spectra

were recorded, while cooling the samples from their isotropic

liquid state, as a function of temperature using a programmable

3,12, upon excitation with 320 nm and 360 nm respectively,

show photoluminescence; as we shall discus later, the meso-

phase exhibits thermo-sensitivity with regard to emission peak

intensities. It can be seen that the spectral pattern of both

liquid and mesomorphic states appear alike with exception of

band intensities (see Figure 9 and Table S2). While the N* phase

spectra of salicylaldimine dimer SAD-3,12 comprise only a

fluorescence maximum at around 480 nm (Figure 11b), two

broad emission bands around 470 nm and 500 nm, along with

a small peak at 610 nm, were observed in the emission N*

phase spectra of Schiff base dimer SBD-3,16 (Figure 11a).

Thus, it is interesting to note that spectral pattern of the N*

phase of these dimers differ perceivingly. In both cases, while the

overall pattern of the emissive spectra recorded as a function of

temperature remains nearly the same, their intensity increase

progressively with the decrease in temperature. Such a phenom-

enon occurs perhaps due to the thermal energy activating the

molecular motion leading to the non-radiative transition or

breaking of larger aggregates.^[45]The fluorescence of the N* phase

of these materials is remarkable given the fact that in LC

(condensed) phase(s) there is a high likelihood of emission getting

quenched due to thermally activated molecular motions.^[46]In

order to examine how the intensity of emitted radiation varies

when the sample SBD-3,16 transforms from Cr state to N* phase,

and N* phase to isotropic liquid state, epifluorescence microscopic

images were obtained as a function of temperature. Figure S6a

depicts the epifluorescence microscopic pictures recorded in the

°

solid state at 25 C, N* phase at three different temperatures viz,

°

65 C, 85 C and 115 C and isotropic state at 140 C; a plot

depicting the mean intensity (green channel) of epifluorescence

against measured temperatures has been presented in Figure S6b.

These experimental results clearly support the fact that the

photoluminescence intensity of these LC dimeric fluorophores

decreases progressively when the phase transition occurs from

solid to LC state to isotropic liquid state.

°

hot-stage having an accuracy of about �0.6 C variations. The

UVÀ Vis absorption spectra recorded and their analysis/interpre-

tations have already been discussed earlier (see Figures 5b &

5d). The emission spectra obtained as a function of temperature

in the entire thermal width of N* phase exhibited by dimers

SBD-3,16, and SAD-3,12 have been respectively shown in

Figures 11a & 11b; the corresponding photophysical data have

been summarized in Table S3. It is immediately apparent from

the emission spectra (Figures 11a & 11b) that the isotropic

liquid state and the N* phase of dimers SBD-3,16 and SAD-

It is well known that theoretically calculated energy HOMO-

LUMO band gap and closely related factors hint about the

electronic properties of materials. For instance, it helps in

knowing the wavelengths at which the compound can absorb

light. On the other hand, by measuring the electronic

absorption wavelength of a given compound one can deter-

mine the HOMO-LUMO gap. Thus, density functional theory

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

12

��

These are not the final page numbers!

Articles

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Figure 12. CD spectra recorded as a function of temperature in the N* phase exhibited by the LC dimers SBD-7,14 (a) and SAD-7,14 (b).

(DFT) calculations were carried out using ORCA 4.0 software^[47]

on two representative dimers SBD-3,16 and SAD-4,12 not only

to reveal the possible structure of HOMO and LUMO orbitals

but also to determine the theoretical band gap. The details of

this theoretical work have been given in SI. However, an

important point worth mentioning is that in both the dimers

SBD-3,16 and SAD-4,12, the electron distribution in the LUMO

is largely contained around the imine linkage, and the HOMO

orbital is localized mainly over the alkoxy-substituted benzene

(electron rich) ring (Figure S7; see SI for details)

The chiroptical behavior of the N* phase stabilized by

dimers SBD-7,14 and SAD-7,14, as representative cases, was

investigated with the aid of CD spectroscopy. This study was

undertaken not only to formally evidence the helicoidal macro-

scopic structure but also to know the screw sense of the helical

N* phase. The thin-films of the aforesaid compounds were

fabricated using quartz glass substrates as discussed earlier. It

must be mentioned here that prefabricated (commercial) cells

with the known thickness could not be employed for the

quantification of measurements as the CD bands obtained in

such experiments were found to go beyond the measurable

range of the instrument. To overcome this CD band saturation

problem, the aforementioned cells were used and naturally the

CD measurements reported herein are purely qualitative. To

begin with, the CD spectra were recorded in the isotropic liquid

state of the samples where the CD response remained nearly

absent meaning that the chromophores are not affected by the

molecular chirality, but they are influenced by the macroscopic

chirality of the N* fluid phase where the CD activity was

registered as expected, which is described as follows. The

samples from their isotropic state were slowly cooled into the

N* phase and the CD spectra were recorded as a function of

ity of bisignate CD bands in the N* phase of both the dimers

was verified by obtaining the CD spectra at six different

orientations achieved by in-plane rotation of the cells by 60 ;

°

the spectra recorded in these orientations were found to be

virtually matching not only with each other but also with one

recorded in the original (original/unrotated) position meaning

that the CD activity is not due to linear dichroism (LD) artefact.

Furthermore, as expected, the LD activity could not be traced

when the samples were subjected to direct LD measurements

by using inbuilt program of the CD instrument. The bisignate

CD band of the N* phase formed by the dimer SBD-7,14

comprise a positive less intense peak centered around 440 nm

and an intense negative peak centered around ~360 nm with a

cross over at ~400 nm. Similarly, the bisignate CD band,

comprising a positive (less intense) peak centered at ~448 as

well as a negative strong peak at ~412 nm, was observed in

each spectrum of the N* phase formed by the salicylaldimine

dimer SAD-7,14. In general, the bisignate structure points to

the through-space coupling of two to several chromophores in

the chiral macroscopic structure. In particular, such CD profiles

suggest the helical disposition of at least two chromophores in

the structure, which is consistent with the helical organization

of dimers in the N* phase. It is readily apparent from both the

profiles shown in Figures 12a & 12b that the intensity of the CD

bands decreases with increase in temperature which can be

attributed to breaking of larger stacks and thermally activated

radiationless process. In essence, the origin of a bisignate band

from positive to negative side suggests a right-handed twist of

the cholesteric helix, whereas the bisignate CD bands signify

large chiral correlations in the molecular packing.

temperature. Figures 12a and 12b respectively depict the CD 3. Conclusion

spectra obtained for the N* phase of dimers SBD-7,14 and SAD-

7,14. The chiroptical data derived from these CD spectra have

been shown in Table S4.

With the prime objective of realizing optically active, photo-

luminescent chiral nematic (N*) LCs, thirty two new non-symmetric

dimers belonging to two series have been prepared and their

molecular structure, liquid crystallinity, photophysical properties

and chiroptical behavior were characterized. They are made by

covalently joining cholesterol with a fluorescent (three-ring)

calamitic mesogen, which is either a Schiff base or a salicylaldi-

As can be seen, the N* phase of both the dimers shows a

significant bisignate CD band (i.e., CD with split Cotton effect)

characteristics in every spectrum obtained at a chosen measur-

ing temperature. These bands are indeed directly related to

macroscopic chirality/helicity of the fluid phase. The authentic-

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

13

��

These are not the final page numbers!

Articles

Yvon. The thin-films of the solid samples required for absorption

and emission measurements were prepared as follows. About

0.1 mg of the solid samples were dissolved in 1 ml of DCM (~

0.1 mg/ml) and then about 100 μl of the same was dropped on the

quartz substrate and solvent was allowed to evaporate naturally. ¹H

and ¹³C nuclear magnetic resonance (NMR) spectra were recorded

mine core, via an ω-oxyalkanoyloxy spacer. The influence on the

thermal (and other) properties of both central spacer and terminal

chain length are studied systematically. An odd-parity (5-oxy-

pentanoyl) spacer along with three even-parity (4-oxybutanoyl, 6-

oxyhexanoyl & 8-oxyoctanoyl) spacers are used; the three-ring

mesogenic core is substituted with n-dodecyloxy, n-tetradecyloxy,

n-hexadecyloxy and n-octadecyloxy tails. The results derived from

several complementary techniques not only evidence the occur-

rence of BPs and/or the N* phase but also reveal an anomalous

spacer-parity (odd-even) effect. That is, all the dimeric compounds

with an even-numbered spacer exhibit blue phases (BPs), besides

N* phase, and those with an odd-membered spacer display the N*

phase solely expelling BPs. This reverse trend as compared to the

hitherto reported results has been interpreted in terms of

geometrical (rod or bent) shapes of dimeric mesogens dictated by

the orientations of the three long terminal alkoxy chains rather

than the parity of the relatively short spacers used. UV-Vis and

photoluminescence spectra of the dilute solutions, solids, LC (N*)

phase and isotropic liquid state have been recorded. Our study

reveals that the photoluminescence behaviors vary during the

thermal phase transitions. The temperature dependence of the

photoluminescence intensity of the N* phase observed for these

materials is especially noteworthy considering the fact that

thermo-responsive luminescent property can be used for practical

applications. DFT calculations disclosed the possible structure of

HOMO and LUMO orbitals, and also determined the theoretical

band gap. The CD spectra measured as a function of temperature

in the N* phase suggested a right-handed twist of the helicoidal

structure.

1

2

3

4

5

6

7

8

9

1

using a Bruker AMX-400 spectrometer operating at 400 MHz for H

and 100 MHz for ¹³C. Chemical shifts (δ) are reported in parts per

million (ppm) relative to TMS using the residual CHCl₃peak in

CDCl₃solution as internal standard (δ H 7.26 and δ C 77.0 relative

to TMS). Multiplicities of the NMR peaks are presented as s (singlet),

d (doublet), t (triplet) and m (multiplet). Coupling constants (J) are

given in Hertz (Hz). Microanalysis was performed on a Perkin Elmer

Elemental Analyzer Series II 2400 analyzer. Mass spectra were

determined on JEOL JMS-600H spectrometer in FAB⁺mode where

3-nitrobenzylalcohol was used as a liquid matrix. CD spectra were

obtained with the help of Jasco J-810 spectropolorimeter. The

epifluorescence images of the condensed phases viz., solid, LC

phase and liquid state were acquired using a Leitz Metalux 3

microscope (25× objective, 0.4 NA), having epifluorescence setting,

equipped with a CCD camera (DXC 390P, Sony). The color intensity

of the epifluorescence images were determined using ImageJ

software. DFT studies were carried out using ORCA 4.0 software on

the ultra high frequency (UHF) energy optimised molecules with

single point B3LYP calculation using the RIJCOSX approximation. J-

auxiliary basis set for Coulomb integrals and numerical COSX

integration (def2/J) for the Exchange integrals (Default COSX grid

chosen automatically).

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Liquid crystalline materials were evaluated for their thermal proper-

ties using an Olympus BX50 (Model BX50F4) polarizing optical

microscope (POM), attached to

a Mettler FP82HT hot stage

(programmed by an FP90 central processor) as well as to a digital

camera. Differential scanning calorimeter (DSC) thermograms were

°

recorded at

a

scanning rate of 5 C/min using Perkin-Elmer

Diamond DSC equipment. Prior to use, DSC was calibrated using

pure indium as a standard. X-Ray diffraction (XRD) experiments

were performed with CuK_∝(λ=0.15418 nm) radiation using a

PANalytical X’Pert PRO MP machine consisting of a focusing

elliptical mirror and a fast resolution detector (PIXCEL). The powder

samples placed in Lindemann capillaries (0.5 mm diameter) and

carefully flame sealed at both the ends were used for XRD studies.

Experimental

General Information (Materials and Methods)

The requisite chemical such as triethyl phosphate, 4-nitrobenzyl

bromide, 2,3,4-trihydroxybenzaldehyde, lauryl bromide, myristyl

bromide, cetyl bromide, stearyl bromide, cholesterol, 4-bromobuty-

ric acid, 5-bromovaleric acid and 6-bromohexanoic acid were

procured from Sigma-Aldrich Company and were utilized without

any purification. While the common commercial organic solvents

employed for various general purposes were distilled prior to use,

the organic solvents used in organic synthesis were dried following

standard drying protocols. Analytical thin-layer chromatography

(TLC) was performed using TLC plates consisting of a thin layer of

silica gel (Merck, Kieselgel60, F254) on an aluminium foil backing.

This technique was especially used to monitor the progress/

completion of reactions. In fact, TLC plates were also used to assess

the purity of the intermediates. The spots were visualized using

revealing sources such as UV light at 254 nm or KMnO4 stains.

Wherever necessary, the column chromatography was carried out

using glass columns packed, by wet method, with either Merck

silica gel (60-120/100–200 mesh) or neutral alumina as the solid

support. Electronic absorption (UV-Vis) spectra were recorded using

either a Perkin Elmer’s, Lambda 20 UV-Vis spectrometer or a Perkin–

Elmer’s Lambda 750, 2015 NIR spectrophotometer. Perkin-Elmer

Spectrum 1000 FT-IR spectrometer was used to record IR spectra of

the samples; the spectral positions are given in wave number

(cm^{À 1}) unit. Photoluminescence spectra of the solid samples and

their solutions spectra (in CH₂Cl₂) as well as mesomorphic state

were measured using a spectrofluorometer Flurolog-3, Horiba Jobin

Synthetic Procedures and Characterization

Considering the fact that many of the intermediates (1, 2a–d, 3a–

d, 4a–d, 5a–d, 6a–d and 7a–d) prepared in this study are known

materials, their synthetic protocols and characterization data have

been given in the Supporting Information (SI). However, a general

but an extremely simple synthetic procedure employed for the

synthesis of Schiff base (SBD-m,n; series-I) dimers and salicylaldi-

mine (SAD-m,n, series-II) dimers has been presented below. Here,

as representative cases, the characterization data are given for a

Schiff base dimer (SBD-3,12) as well as for a salicylaldimine dimer

(SAD-3,12), since the only difference in the characterization data,

especially the NMR spectra relates to the number of carbons in the

alkoxy chains as well as alkylene spacer and presence or absence of

oxygen atom in the form of hydroxyl (OH) group. The full-

characterization data acquired for other dimers of both series are

presented in the SI.

General Procedure for the Synthesis of Schiff Bases (SBD-m,n)

and Salicylaldimines (SAD-m,n)

A mixture of cholesteryl ω-(4-formylphenoxy)alkanoate (6a–d) or

cholesteryl-ω-(3-hydroxy-4-formyl-phenoxy)alkanoate

(7a–d)

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

14

��

These are not the final page numbers!

Articles

[1] a) D. Demus, Liq. Cryst. 1989, 5, 75–110; b) C. Tschierske, Annu. Rep.

Prog. Chem. Sect. C 2001, 97, 191–267; c) C. Tschierske, Angew. Chem.

Int. Ed. 2013, 52, 8828–8878; Angew. Chem. 2013, 125, 8992–9047;

d) J. W. Goodby, I. M. Saez, S. J. Cowling, V. Gortz, M. Draper, A. W. Hall,

S. Sia, G. Cosquer, S.-E. Lee, E. P. Raynes, Angew. Chem. Int. Ed. 2008, 47,

2754–2787; e) J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H.

Gleeson, P. Raynes, (Editors), Handbook of Liquid Crystals; Volume 5:

Non-Conventional Liquid Crystals, 2^ndEdition, Weinheim, Wiley-VCH,

2014.

[2] a) C. T. Imrie, P. A. Henderson, Chem. Soc. Rev. 2007, 36, 2096–2124 and

references cited therein b) A. S. Achalkumar, U. S. Hiremath, D. S. S. Rao,

C. V. Yelamaggad, Liq. Cryst. 2011, 38, 1563–1589; c) R. J. Mandle, M. P.

Stevens, J. W. Goodby, Liq. Cryst. 2017, 44, 2046–2059.

[3] a) C. T. Imrie, in Structure and Bonding – Liquid crystals, II, Ed: D. M. P.

Mingos, Springer-Verlag., 1999, p. 149–192; b) C. T. Imrie, P. A. Hender-

son, Curr. Opin. Colloid Interface Sci. 2002, 7, 298–311; c) C. T. Imrie, G. R.

Luckhurst, in Handbook of liquid crystals. Vol-2B, Eds: D. Demus, J. W.

Goodby, G. W. Gray, H.-W. Spiess, V. Vill, Wiley-VCH, Germany, 1998,

part – III, p. 801–833; d) S. K. Pal, S. Kumar, Liquid Crystal Dimers,

Cambridge University Press, UK, 2017. e) A. E. Blatch, I. D. Fletcher, G. R.

Luckhurst, Liq. Cryst., 1995, 18, 801–809.

(0.1 mmol, 1 equiv.), 4-(2,3,4-tris(n-alkyloxy)styryl)-aniline (4a–d)

(0.15 mmol, 1.2 equiv.), absolute ethanol (10 mL) and a catalytic

amount of acetic acid was refluxed under inert (N₂) atmosphere for

2 h. The dull-yellow product separated upon cooling the reaction

mixture was collected by filtration, washed with warm ethanol

repeatedly and air-dried. The crude product was recrystallized with

absolute ethanol (AR grade) until the transition temperature,

especially the isotropization temperature, remained unaltered.

Yields: Schiff bases À 63 to 72% and salicylaldimines À 68 to 78%).

1

2

3

4

5

6

7

8

9

SBD-3,12: Pale yellow solid; yield: 0.129 g (66%); IR (KBr Pellet): ν_maxin

cm^{À 1}2916, 2848, 1734, 1572, 1465, 1437, 1297, 1171 and 1088; UV-VIS:

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

1

λ

_max=365 nm, ɛ=3.41×10²Lmol^{À 1}cm^{À 1}; H NMR (400 MHz, CDCl₃): δ

8.42 (s, 1H, HC=N), 7.85 (d, 2H, J=8 Hz, Ar), 7.52 (d, 2H, J=8 Hz, Ar),

7.40 (d, 1H, J=16 Hz, vinylic), 7.29 (d, 2H, J=8 Hz, Ar), 7.21 (d, 2H, J=

8 Hz, Ar), 7.03 (d, 1H, J=16 Hz, vinylic), 6.98 (d, 1H, J=8 Hz, Ar), 6.68

(d, 1H, J=4 Hz, Ar), 5.38 (brs, 1H, olefinic), 4.63–4.61 (m, 1H, OCO=CH),

4.05-3.98 (m, 8H, 4×OCH₂), 1.81–0.67 (m, 116 H, 8×CH₃, 43×CH₂, 6×

CH); ¹³C NMR (100 MHz, CDCl₃): 173.21, 158.91, 148.32, 145.36, 139.65,

138.88, 136.0, 134.08, 130.52, 128.32, 127.08, 125.71, 123.87, 122.73,

121.93, 121.34, 120.35, 114.73, 105.51, 103.02, 74.18, 68.85, 67.01,

56.73, 50.07, 39.55, 35.82, 31.95, 30.41, 29.76, 29.71, 29.66, 29.63, 29.61,

28.04, 26.21, 26.16, 23.86, 22.83, 22.71, 22.58, 19.34, 18.74, 14.12, 11.88;

Anal. Calcd. for C₈₈H₁₃₉NO₆. C, 80.86; H, 10.72; N, 1.07. Found: C, 80.38;

H, 10.32; N, 1.28.

[4] C. V. Yelamaggad, G. Shanker, U. S. Hiremath, S. K. Prasad, J. Mater.

Chem. 2008, 18, 2927–2949.

[5] a) H.-S. Kitzerow, C. Bahr (Eds), Chirality in Liquid Crystals, Springer-

Verlag, New York Inc., 2001; b) M. M. Green, R. J. M. Nolte, E. W. Meijer

(Eds.), Materials Chiralty: Volume 24 of Topics in Stereochemistry, John

Wiley & Sons Inc., New Jersey, 2003; c) I. Dierking, Symmetry 2014, 6,

444–472; d) J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H.

Gleeson, P. Raynes, (Editors). Handbook of Liquid Crystals; Volume 3:

Nematic and Chiral Nematic Liquid Crystals, 2nd Edition, Weinheim,

Wiley-VCH, 2014.

[6] a) A. E. Blatch, I. D. Fletcher, G. R. Luckhurst, J. Mater. Chem. 1997, 7, 9–

17; b) I. Nishiyama, J. Yamamoto, J. W. Goodby, H. A. Yokoyama, J.

Mater. Chem. 2001, 11, 2690–2693.

[7] a) F. Hardouin, M. F. Achard, J.-I. Jin, J.-W. Shin, Y.-K. Yun, J. Phys. II,

1994, 4, 627–643; b) S. W. Cha, J.-I. Jin, M. F. Achard, F. Hardouin, Liq.

Cryst. 2002, 29, 755–763.

SAD 3,12: Pale yellow solid; yield: 0.140 g (71%); IR (KBr Pellet): ν_max

in cm^{À 1}3433, 2920, 2840, 1735, 1631, 1560, 1498, 1302, 1172, 1090

and 963; UV-VIS: λ max=352 nm, ɛ=1.5×10³Lmol^{À 1}cm^{À 1}; ¹H NMR

(400 MHz, CDCl₃): δ 13.73 (brs, 1H, -OH), 8.57 (s, 1H, HC=N), 7.54 (d,

2H, J=8 Hz, Ar), 7.40 (d, 1H, J=16 Hz, vinylic), 7.32–7.25 (m, 2H, Ar),

7.21–7.18 (m, 2H, Ar), 7.02 (d, 1H, J=16 Hz, vinylic), 6.61 (d, 1H, J=

8 Hz, Ar), 6.43–6.39 (m, 2H, Ar), 5.38 (brs, 1H, olefinic), 4.63–4.56 (m,

1H, À C=C-CH), 4.11–4.06 (m, 8H, 4×OCH₂), 1.80-0.68 (m, 116 H, 8×

CH₃, 43×CH₂, 6×CH); ¹³C NMR (100 MHz, CDCl₃): 173.08, 158.40,

148.10, 145.48, 143.10, 139.49, 137.96, 137.59, 134.15, 132.18,

130.21, 128.05, 127.03, 125.29, 123.47, 122.53, 121.59, 121.23,

120.21, 113.37, 105.29, 103.01, 74.30, 68.19, 66.41, 56.18, 50.01,

39.69, 35.68, 31.61, 30.38, 29.57, 29.51, 29.43, 29.39, 29.09, 26.97,

26.41, 26.30, 24.18, 22.97, 22.66, 22.38, 19.03, 18.11, 13.89, 11.41;

Anal. Calcd. for C₈₈H₁₃₉NO₇. C, 79.89; H, 10.59; N, 1.06. Found: C,

79.48; H, 10.28; N, 1.15.

[8] A. Zep, S. Aya, K. Aihara, K. Ema, D. Pociecha, K. Madrak, P. Bernatowicz,

H. Takezoe, E. Gorecka, J. Mater. Chem. C 2013, 1, 46–49.

[9] a) F. Hardouin, M. F. Achard, J.-I. Jin, Y. K. Yun, J. Phys. II, 1995, 5, 927–

935; b) F. Hardouin, M. F. Achard, J.-I. Jin, Y. K. Yun, S.-J. Chung, Eur.

Phys. J. B1 1998 47–56; c) C. V. Yelamaggad, A. Srikrishna, D. S. S. Rao,

S. K. Prasad, Liq. Cryst. 1999, 26, 1547–1554; d) J.-W. Lee, Y. Park, J.-I. Jin,

M. F. Achard, F. Hardouin, J. Mater. Chem. 2003, 13, 1367–1372; e) Y.

Tian, X. Xu, Y. Zhao, X. Tang, T. Li, Liq. Cryst. 1997, 22, 87–96; f) K. C.

Majumdar, P. K. Shyam, D. S. S. Rao, S. K. Prasad, J. Mater. Chem. 2011,

21, 556–561.

[10] a) A. T. M. Marcelis, A. Koudijs, E. J. R. Sudholter, Recl. Trav. Chim. Pays-

Bas 1994, 113, 524–526; b) V. Surendranath, Mol. Cryst. Liq. Cryst. 1999,

332, 135–140; c) A. T. M. Marcelis, A. Koudijs, E. J. R. Sudholter, Liq. Cryst.

2000, 27, 1515–1523; d) A. T. M. Marcelis, A. Koudijs, E. A. Klop, E. J. R.

Sudholter, Liq. Cryst. 2001, 28, 881–887; e) C. V. Yelamaggad, S. A.

Nagamani, U. S. Hiremath, G. G. Nair, Liq. Cryst. 2001, 28, 1009–1015;

f) C. V. Yelamaggad, M. Mathews, Liq. Cryst. 2001, 28, 1009–1015; g) C. V.

Yelamaggad, M. Mathews, Liq. Cryst. 2003, 30, 125–133; h) A. T. M.

Marcelis, A. Koudijs, E. J. R. Sudholter, Mol. Cryst. Liq. Cryst. 2004, 411,

193–200; i) C. T. Imrie, P. A. Henderson, G.-Y. Yeap, Liq. Cryst. 2009, 36,

755–777; j) T. Donaldson, H. Staesche, Z. B. Lu, P. A. Henderson, M. F.

Achard, C. T. Imrie, Liq. Cryst. 2010, 37, 1097–1110.

[11] G. Shanker, C. V. Yelamaggad, New J. Chem. 2012, 36, 918–926.

[12] a) C. V. Yelamaggad, U. S. Hiremath, D. S. S. Rao, Liq. Cryst. 2001, 28,

351–355; b) C. V. Yelamaggad, U. S. Hiremath, S. A. Nagamani, D. S. S.

Rao, S. K. Pasad, Liq. Cryst. 2003, 30, 681–690; c) K. C. Majumdar, P. K.

Shyam, Mol. Cryst. Liq. Cryst. 2010, 528, 3–9; d) U. S. Hiremath, G. M.

Sonar, D. S. S. Rao, C. V. Yelamaggad, J. Mater. Chem. 2011, 21, 4064–

4067; e) K. C. Majumdar, P. K. Shyam, D. S. S. Rao, S. K. Prasad, Liq. Cryst.

2012, 39, 1117–1123; f) D. D. Sarkar, R. Deb, N. Chakraborty, G.

Mohiuddin, R. Kanti Nath, N. V. S. Rao, Liq. Cryst. 2013, 40, 468–481;

g) M. Kumar, T. Padmini, K. Ponnuvel, J. Saudi Chem. Soc. 2017, 21,

S322–S328.

Acknowledgments

The corresponding author (CVY) gratefully acknowledges the

Science and Engineering Research Board (SERB), Department of

Science and Technology, Government of India, for providing funds

to this study through a research project No. EMR/2017/000153. We

profoundly thank Dr. D. S. Shankar Rao for providing XRD data

generously.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: chiral nematic phase · dimers · liquid crystals · odd-

even effect · photoluminescence

[13] a) C. V. Yelamaggad, I. Shashikala, G. Liao, D. S. S. Rao, S. K. Prasad, Q. Li,

A. Jakli, Chem. Mater. 2006, 18, 6100–6102; b) G. Liao, I. Shashikala, C. V.

Yelamaggad, D. S. S. Rao, S. K. Prasad, A. Jakli, Phys. Rev. E. 2006, 73,

051701–1–051701-5.

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

15

��

These are not the final page numbers!

Articles

[14] a) N. Tamaoki, Y. Aoki, M. Moriyama, M. Kidowaki, Chem. Mater. 2003,

15, 719–726; b) V. Ajay Mallia, N. Tamaoki, J. Mater. Chem. 2003, 13,

219–224; c) V. Ajay Mallia, N. Tamaoki, Chem. Mater. 2003, 15, 3237–

3239; d) V. Ajay Mallia, N. Tamaoki, Chem. Commun. 2004, 2538–2539;

e) K.-N. Kim, E.-D. Do, Y.-W. Kwon, J.-I. Jin, Liq. Cryst. 2005, 32, 229–237;

f) W.-K. Lee, K.-N. Kim, M. F. Achard, J.-I. Jin, J. Mater. Chem. 2006, 16,

2289–2297; g) C. Wu, Mater. Lett. 2007, 61, 1380–1383; h) Y. Kim, N.

Tamaoki, ACS Appl. Mater. Interfaces 2016, 8, 4918–4926.

[15] a) A. S. Pandey, R. Dhar, A. S. Achalkumar, C. V. Yelamaggad, Liq. Cryst.

2011, 38, 775–784; b) V. S. Pandey, R. Dhar, A. Kr. Singh, A. S.

Achalkumar, C. V. Yelamaggad, Phase Transitions 2010, 83, 1049–1058.

[16] C. V. Yelamaggad, M. Mathews, U. S. Hiremath, D. S. S. Rao, S. K. Prasad,

Tetrahedron Lett. 2005, 46, 2623–2626.

pitch has been used to indicate the strength of chirality of the system.

The chiral nematic (N*) phase, having a helical structure, is generally

labeled as strongly or weakly chiral depending upon whether the pitch

of the phase is short or long. Almost as a rule, BPs are observed when

the pitch of cholesteric (N*) phase is short.

1

2

3

4

5

6

7

8

9

[37] D. A. Dunmur, G. R. Luckhurst, M. R. de la Fuente, S. Diez, M. A. P.

Jubindo, J. Chem. Phys. 2001, 115, 8681–8691.

[38] a) C. V. Yelamaggad, A. S. Achalkumar, N. L. Bonde, A. K. Prajapati, Chem.

Mater. 2006, 18, 1076–1078; b) C. V. Yelamaggad, N. L. Bonde, A. S.

Achalkumar, D. S. S. Rao, S. Krishna Prasad, A. K. Prajapati, Chem. Mater.

2007, 19, 2463–2472.

[39] G.-Y. Yeap, T.-C. Hng, S.-Y. Yeap, E. Gorecka, M. M. Ito, K. Ueno, M.

Okamoto, W. A. K. Mahmood and Corrie T. Imrie, Liq. Cryst. 2009,

36,1431–1441.

[40] a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer 3rd

ed., 2006; b) X. Peng, F. Song, E. Lu, Y. Wang, W. Zhou, J. Fan, Y. Gao, J.

Am. Chem. Soc. 2005, 127, 4170–4171; c) W. Lin, L. Yuan, Z. Cao, Y. Feng,

J. Song, Angew. Chem. Int. Ed. 2010, 49, 375–379; Angew. Chem. 2010,

122, 385–389.

[41] a) X. K. Ren, B. Sun, C. C. Tsai, Y. F. Tu, S. W. Leng, K. X. Li, Z. Kang,

M. V. H. Ryan, X. P. Li, M. F. Zhu, C. Wesdemiotis, W. B. Zhang, S. Z. D.

Cheng, J. Phys. Chem. B 2010, 114, 4802–4810; b) Y. Shao, G.-Z. Yin, X.

Ren, X. Zhang, J. Wang, K. Guo, X. Li, C. Wesdemiotis, W.-B. Zhang, S.

Yang, M. Zhu, B. Sun, RSC Adv. 2017, 7, 6530–6537; c) Y. Shao, X. Zhang,

K. Liang, J. Wang, Y. Lin, S. Yang, W.-B. Zhang, M. Zhu, B. Sun, RSC Adv.,

2017, 7, 16155–16162

[42] a) M. Shimizu, T. Hiyama, Chem. Asian J. 2010, 5, 1516–1531; b) A. C.

Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz, A. B. Holmes, Chem. Rev.

2009, 109, 897–1091; c) J. Z. Liu, J. W. Y. Lam, B. Z. Tang, Chem. Rev.

2009, 109, 5799–5867.

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

[17] K. C. Majumdar, S. Mondal, R. K. Sinha, New J. Chem. 2010, 34, 1255–

1260.

[18] B. Pradhan, N. Chakraborty, R. K. Gupta, G. Shanker, A. S. Achalkumar,

New J. Chem. 2017, 41, 879–888.

[19] a) S. W. Cha, J.-I. Jin, M. Laguerre, M. F. Achard, F. Hardouin, Liq. Cryst.

1999, 26,1325-1337; b) C. V. Yelamaggad, S. A. Nagamani, D. S. S. Rao,

S. K. Prasad, U. S. Hiremath, Mol. Cryst. Liq. Cryst. 2001, 363, 1–17;

c) D. S. S. Rao, S. K. Prasad, V. N. Raja, C. V. Yelamaggad, S. A. Nagamani,

Phys. Rev. Lett. 2001, 87, 085504–1–085504-4; d) C. V. Yelamaggad, M.

Mathews, T. Fujita, N. Iyi, Liq. Cryst. 2003, 30, 1079–1087; e) C. V.

Yelamaggad, I. Shashikala, U. S. Hiremath, D. S. S. Rao, S. K. Prasad, Liq.

Cryst. 2007, 34, 153–167; f) S. K. Gupta, D. P. Singh, R. Manohara, U. S.

Hireamth, C. V. Yelmaggad, Liq. Cryst. 2012, 39, 1125–1129.

[20] S. Abraham, V. Ajay Mallia, K. V. Ratheesh, N. Tamaoki, S. Das, J. Am.

Chem. Soc. 2006, 128, 7692–7698.

[21] M. Gupta, S. K. Pal, Liq. Cryst. 2015, 42, 1250–1256.

[22] U. S. Hiremath, G. G. Nair, and D. S. S. Rao, Liq. Cryst. 2016, 43, 711–728.

[23] a) M. P. Aldred, R. Hudson, S. P. Kitney, P. Vlachos, A. Liedtke, K. L. Woon,

M. O’Neill, S. M. Kelly, Liq. Cryst. 2008, 35, 413–427 and references cited

therein; b) M. O’Neill, S. M. Kelly, Adv. Mater. 2003, 15, 1135–1146; c) S.

Mery, D. Haristoy, J.-F. Nicoud, D. Guillon, S. Diele, H. Monobe, Y.

Shimizu, J. Mater. Chem. 2002, 12, 37–41; d) M. P. Aldred, A. J. Eastwood,

S. M. Kelly, P. Vlachos, A. E. A. Contoret, S. R. Farrar, B. Mansoor, M.

O’Neill, W. C. Tsoi, Chem. Mater. 2004, 16, 4928–4936.

[24] K. L. Woon, M. O’Neill, G. J. Richards, M. P. Aldred, S. M. Kelly, A. M. Fox,

Adv. Mater. 2003, 15, 1555–1558.

[25] H. Meier, Angew. Chem. Int. Ed. 1992, 31, 1399–1420; Angew. Chem.

1992, 104, 1425–1446.

[43] D. D. Prabhu, N. S. S. Kumar, A. P. Sivadas, S. Varghese, S. Das, J. Phys.

Chem. B 2012, 116, 13071–13080.

[44] a) M. Mitov, Adv. Mater. 2012, 24, 6260–6276; b) H. K. Bisoyi, T. J.

Bunning, Q. Li, Adv. Mater. 2018, 30, 1706512(1–35); c) H. K. Bisoyi, Q. Li,

Angew. Chem. Int. Ed. 2016, 55, 2994–3010; Angew. Chem. 2016, 128,

3046–3063; d) Liquid Crystals Beyond Displays: Chemistry, Physics, and

Applications (Ed.: Q. Li), John Wiley & Sons, Hoboken, NJ, 2012; e) H.

Wang, H. K. Bisoyi, L. Wang, A. M. Urbas, T. J. Bunning, Q. Li, Angew.

Chem. Int. Ed. 2018, 57, 1627–1631; Angew. Chem. 2018, 130, 1643–

1647.

[45] a) X. Bu, D. Zhu, T. Liu, Y. Li, S. Cai, H. Wang, Z. Zeng, Dyes Pigm. 2017,

145, 324–330; b) S. Yamada, K. Miyano, Tsutomu Konno, Tomohiro

Agou, Toshio Kubotab, Takuya Hosokai, Org. Biomol. Chem. 2017, 15,

5949–5958; c) Y. Sagara, T. Muramatsu, N. Tamaoki, Crystals 2019, 9,

92(1–10); d) T. Liua, D. Zhua, M. Zhanga, M. Cuia, Q. Lia, Y. Fana, H.

Wanga, Z. Zenga, Dyes and Pigments 2018, 159, 115–120; e) S. Yamada,

M. Morita, T. Agou, T. Kubota, T. Ichikawa, T. Konno, Org. Biomol. Chem.

2018, 16, 5609–5617.

[46] a) C. V. Yelamaggad, A. S. Achalkumar, D. S. S. Rao, S. K. Prasad, J. Am.

Chem. Soc. 2004, 126, 6506–6507; b) C. V. Yelamaggad, A. S. Achalku-

mar, D. S. S. Rao, S. K. Prasad, J. Mater. Chem. 2007, 17, 4521–4529;

c) C. V. Yelamaggad, A. S. Achalkumar, D. S. S. Rao, S. K. Prasad, J. Org.

Chem. 2007, 72, 8308–8318; d) C. V. Yelamaggad, A. S. Achalkumar,

D. S. S. Rao, S. K. Prasad, J. Org. Chem. 2009, 74, 3168–3171; e) A. S.

Achalkumar, U. S. Hiremath, D. S. S. Rao, S. K. Prasad, C. V. Yelamaggad,

J. Org. Chem. 2013, 78, 527–544; f) S. K. Pathak, S. Nath, R. K. Gupta,

D. S. S. Rao, S. K. Prasad, A. S. Achalkumar, J. Mater. Chem. C 2015, 3,

8166–8182; g) A. S. Achalkumar, B. N. Veerabhadraswamy, U. S. Hire-

math, D. S. S. Rao, S. K. Prasad, C. V. Yelamaggad, Dyes Pigm. 2016, 132,

291–305; h) S. K. Pathak, R. K. Gupta, S. Nath, D. S. S. Rao, S. K. Prasad,

A. S. Achalkumar, J. Mater. Chem. C 2015, 3, 2940–2952.

[26] M. Lehmann, C. Kohn, H. Meir, S. Renker, A. Oehlhof, J. Mater. Chem.

2006, 16, 441–451.

[27] J. H. Burroughes, D. D. C. Bradeley, A. R. Brown, R. N. Marks, K. Mackay,

R. H. Friend, P. L. Burn, A. B. Holmes, Nature 1990, 347, 539–541.

[28] G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, P. L. Burn, J. H.

Burroughes, R. H. Friend, N. C. Greenham, R. W. Gymer, D. A. Halliday,

R. W. Jackson, A. Kraft, J. H. F. Martens, K. Pichler, I. D. W. Samuel, Synth.

Met. 1993, 55–57, 4031–4040.

[29] G. Yu, J. Wang, J. McElvain, A. J. Heeger, Adv. Mater. 1998, 10, 1431–

1434.

[30] T. Piok, C. Brands, P. J. Neyman, A. Earlacher, C. Soman, M. A. Murray, R.

Schroeder, W. Graupner, J. R. Heflin, D. Marciu, A. Drake, M. B. Miller, H.

Wang, H. Gibson, H. C. Dorn, G. Leising, M. Guzy, R. M. Davis, Synth. Met.

2001, 116, 343–347.

[31] G. Shanker, M. Prehm, C. V. Yelamaggad, C. Tschierske, J. Mater. Chem.

2011, 21, 5307–5311.

[32] G. Shanker, A. Bindushree, K. Chaithra, P. Pratap, R. K. Gupta, A. S.

Achalkumar, C. V. Yelamaggad, J. Mol. Liq. 2019, 275, 849–858.

[33] I. Dierking, Textures of Liquid Crystals, Wiley-VCH Verlag GmBH & KGaA,

Weinheim 2003.

[34] a) V. Padmini, P. N. Babu, G. G. Nair, D. S. S. Rao, C. V. Yelamaggad,

Chem. Asian J. 2016, 11, 2897–2910 and references cited therein; b) R.

Nayak, S. A. Bhat, G. Shanker, C. V. Yelamaggad, New J. Chem. 2019, 43,

2148–2162 and references cited therein.

[47] Frank Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73–78.

[35] a) J. Rokunohe, A. Yoshizawa, J. Mater. Chem. 2005, 15, 275–279; b) A.

Yoshizawa, RSC Adv., 2013, 3, 25475–25497; c) T. Hirose, A. Yoshizawa, J.

Mater. Chem. C, 2016, 4, 8565–8574

[36] Given the fact that chirality (handedness) is a symmetry property it can

be either present or absent. However, in practice, the magnitude of the

Manuscript received: July 22, 2019

Revised manuscript received: September 12, 2019

Accepted manuscript online: September 13, 2019

Version of record online: ■■■, ■■■■

ChemPhysChem 2019, 20, 1–17

www.chemphyschem.org

16

��

These are not the final page numbers!

Articles

ARTICLES

1

2

3

4

5

6

Dr. B. N. Veerabhadraswamy, S. A.

Bhat, Dr. U. S. Hiremath, Dr. C. V. Ye-

lamaggad*

7

8

1 – 17

Light-Emitting Chiral Nematic

Dimers with Anomalous Odd-Even

Effect

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Photoluminescent chiral nematic

dimers exhibit an atypical odd-even

effect. While the even members

unusually show blue phases (BPs)

besides the chiral nematic (N*) phase,

their odd-parity counterparts display

the N* phase only.

Article Doi

Light-Emitting Chiral Nematic Dimers with Anomalous Odd-Even Effect

DOI: 10.1002/cphc.201900716

Source and publish data:

Authors:

Article abstract of DOI:10.1002/cphc.201900716

Full text of DOI:10.1002/cphc.201900716

Products guided by the article

R&D Labs maybe for 1190604-91-1

Relevant to this article

Hot Product