Novel liquid crystalline fluorene and fluorenone MIDA boronates: influence of alkyl vs alkoxy side chains on the mesomorphic properties

Figures & data

Scheme 1. Overview of preliminary work on liquid crystalline fluorenes, fluorenones and MIDA boronates.

Scheme 2. Synthesis of the fluorene and fluorenone MIDA boronates Flu(n)MIDA, FluO(n)MIDA with alkyl side chains.

Scheme 3. Synthesis of the alkoxy-substituted fluorene or fluorenone MIDA boronates Flu(oO-n)MIDA, FluO(O-n)MIDA.

Figure 1. (Colour Online) POM images of 17(O-10) at (a) 82°C and (b) 45°C and (c) 17(O-12) at 85°C and (d)75°C (200 × magnification, taken upon cooling from the isotropic phase (5 K · min⁻¹).

Figure 2. DSC traces of (a) 17(O-10) and (b)17(O-12); 2^nd heating/cooling (5 K · min⁻¹).

Table 1. Phase transition temperatures (in °C) and phase transition enthalpies (in kJ/mol) of 17(O-n). ^a,b.

Compound	Cycle	Cr	Tm(ΔH)	SmE	T(ΔH)	SmA	Tc(ΔH)	I
17(O-10)	2. Heating	●^c	71 (0.7)	●	96 (11.0)			●
	2. Cooling	●	27 (−21.2)	●	94 (−10.9)			●
17(O-12)	2. Heating	●	74 (23.4)	●	81 (21.2)	●	90 (5.4)	●
	2. Cooling	●	46 (−30.1)	●	78 (−1.4)	●	89 (−5.0)	●
17(O-14)	2. Heating	●	83 (41.0)			●	87 (8.1)	●
	2. Cooling	●	61 (−39.9)	●	69 (−2.1)	●	85 (−5.1)	●
17(O-16)	2. Heating	●	87 (42.5)					●
	2. Cooling	●	73 (−36.9)	●	81 (−3.8)			●

^aThe following phases were observed: crystalline (Cr), smectic E (SmE), smectic A (SmA), isotropic (I).; ^bheating/cooling rate 10 K/min.; ^cCr-Cr transition at 67°C (26.6 kJ/mol).

Figure 3. (Colour Online) (a) SAXS profile and (b) WAXS profile of 17(O-12) at 57°C (black) and 85°C (blue).

Figure 4. (Colour Online) Proposed packing model of the SmE phase of 17(O-12) in (a) top view, (b) side view and the proposed packing model of the SmA bilayer (c).

Table 2. X-ray diffraction data of bromoalkoxyfluorene 17(O-12). The measurements were performed during cooling from the isotropic liquid phase. The halo was determined from WAXS.

Compound	Mesophase	Lattice constant/Å	Layer spacing/Å exp. (calc.)		Miller Indices
17(O-12)	SmE at 76°C		27.24	–	(001)
		a = 8.52	4.81	4.81	(110)
		b = 5.83	4.26	4.26	(200)
		c = 27.24	3.50	3.44	(210)
	SmA at 85°C	–	28.34	–	(001)
		–	4.60	–	halo

Figure 5. (Colour Online) DSC traces of (a) Flu(12)MIDA, (b) Flu(O-12)MIDA, (c) FluO(12)MIDA (phase transitions (T_g) determined via POM due to the absence in DSC traces) and (d) Flu(O-12)MIDA; 1^st heating/cooling of (a-c) 10 K · min⁻¹ and 1^st, 2^nd, 3^rd heating/cooling of d), 10 K · min⁻¹).

Figure 6. (Colour Online) Results from DSC measurements (rounded values, 1^st heating cycle; rate 10 K·min^{− 1}) of Flu(n)MIDA, Flu(o-n)MIDA, FluO(n)MIDA and FluO(O-n)MIDA. Gray: crystalline phase; blue: smectic a phase.

Table 3. Phase transition temperatures T (in °C) and phase transition enthalpies (in kJ/mol) of Flu(n)MIDA, FluO(n)MIDA, Flu(o-n)MIDA and FluO(O-n)MIDA.^a,b

Compound	Cycle	Cr	Tm (ΔH)	SmA	Tc (ΔH)	I
Flu(10)MIDA	1. H	●	166 (14.6)	●	252 (2.7)	●
	1. C	G^c	128 (−)	●	250 (−2.4)	●
Flu(12)MIDA	1. H	●	186 (0.5)	●	257 (0.9)	●^d
Flu(14)MIDA	1. H	●	162 (8.6)	●	251 (2.0)	●^d
Flu(16)MIDA	1. H	●	174 (16.9)	●	258 (1.5)	●
	1. C	G^c	115 (-)	●	255 (−1.4)	●
Flu(O-10)MIDA	1. H	●	199 (18.8)	●	226 (1.8)	●^d
Flu(O-12)MIDA	1. H	●	211 (26.4)	●	253 (2.4)	●
	1. C	●	153 (−21.1)	●	252 (−2.4)	●
Flu(O-14)MIDA	1. H	●	205 (20.5)	●	248 (1.5)	●^c
Flu(O-16)MIDA	1. H	●	203 (20.2)	●	225 (-)	●^c,d
FluO(10)MIDA	1. H	●	154 (1.6)	●	211 (1.1)	●
	1. C	G^c	124 (-)	●	204 (−0.7)	●
FluO(12)MIDA	1. H	G^c	171 (-)	●	233 (1.6)	●
	1. C	G^c	140 (-)	●	231 (−1.6)	●
FluO(14)MIDA	1. H	●	173 (1.6)	●	247 (1.7)	●
	1. C	G^c	137 (-)	●	245 (−1.6)	●
FluO(16)MIDA	1. H	●	163 (4.0)	●	240 (1.0)	●^d
FluO(O-10)MIDA	1. H	●	194 (3.6)	●	203 (-)	●^c
	1. C	●	138 (− 4.3)	●	202 (−2.2)
FluO(O-12)MIDA	1. H	●	201 (16.4)	●	238 (1.7)	●
	1. C	●	178 (−14.8)	●	237 (−1.7)	●
FluO(O-14)MIDA	1. H	●	198 (13.5)	●	247 (1.7)	●^d
FluO(O-16)MIDA	1. H	●	192 (10.8)	●	250 (0.9)	●^c

^aThe following phases were observed: Crystalline (Cr); glass-like (G); smectic A (SmA); isotropic liquid(I). ^b Values from DSC 1^st or 2^nd cooling cycle with cooling/heating rates of 10 K · min⁻¹; ^cPhase transitions determined via POM due to the absence in DSC traces (cooling/heating rates of 2 K · min⁻¹). ^dDecomposition during first heating cycle.

Figure 7. Out-of plane and in-plane conformations of alkyl and alkoxy chains on six-membered aromatic rings. The figure was adapted from ref [85].

Figure 8. (Colour Online) POM images of (a) Flu(12)MIDA at 220°C; (b) Flu(O-12)MIDA at 228°C; (c) FluO(12)MIDA at 225°C; (d) FluO(O-12)MIDA at 229°C (200 x magnification, taken upon cooling from the isotropic phase (5 K · min⁻¹).

Table 4. X-ray diffraction data for fluorene Flu(12)MIDA and fluorenone FluO(14)MIDA. The measurements were performed during cooling from the isotropic liquid phase. The halo was determined from WAXS.

Compound	Mesophase	Layer spacing/Å exp. (calc.)		Miller Indices
Flu(12)MIDA	SmA at 212°C	43.3 (–)	–	(001)
		5.16 (–)		halo
FluO(14)MIDA	SmA at 197°C	43.69 (–)	–	(001)
		5.23 (–)	–	halo

Figure 9. (Colour Online) (a) SAXS profile and (b) WAXS profile of Flu(12)MIDA and (c) SAXS profile and (d) temperature-dependant layer spacing of FluO(14)MIDA with the corresponding 2D-pattern. (WAXS profile see ESI fig. S9).

Figure 10. (Colour Online) Proposed packing model of the SmA bilayer of FluO(14)MIDA.

Arakawa Y, Ishida Y, Shiba T, et al. Effects of alkylthio groups on phase transitions of organic molecules and liquid crystals: a comparative study with alkyl and alkoxy groups. Cryst Eng Comm. 2022;24(10):1877–1890. doi: 10.1039/D1CE01470F

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