Influence of Thiophenes on Molecular Order, Mesophase, and Optical Properties of π-Conjugated Mesogens

Abstract

Increasing interest in π-conjugated aromatic cores built essentially with thiophene rings is recognized owing to their applications in optoelectronics. In this investigation, an attempt is made to understand the influence of terminal thiophene rings on the molecular order, mesophase, and optical properties of mesogens in which phenyl benzoate is part of the core. Accordingly, mono-, di-, and terthiophene units are linked to two phenyl ring core by Suzuki cross coupling reaction. The synthesized thiophene-based πconjugated mesogens exhibit enantiotropic nematic and smectic phases with excellent mesophase range. The tendency for smectic phases and the mesophase range enhanced with increased thiophene rings. The layer ordering in smectic A and smectic C phase is established by powder X-ray diffraction, while the orientational order of all the rings of core unit is accomplished by 13 C NMR spectroscopy. Thus the 13 C− 1 H dipolar couplings determined from 2D separated local field NMR experiments show a very high value for terminal C−H of thiophene ring (∼9−11 kHz) irrespective of number of thiophenes in the mesogenic core. The density functional theory and timedependent density functional theory calculations indicate the intramolecular charge-transfer transition between the phenylthiophene to phenyl benzoate unit. The solution absorption and fluorescence spectral studies reveal interesting features. The monothiophene-based mesogen is nonfluorescent, while those based on bithiophene and terthiophene show intense fluorescence. The well-resolved vibronic peaks observed in fluorescence spectra of mesogens are characteristic of oligothiophenes. Furthermore, the fluorescence excitation anisotropy measured by monitoring the vibronic features of the mesogens is found to be similar, signifying that the emission originates from the identical electronic energy level. Therefore, the investigation encompassing wide-ranging techniques manifests that the insertion of more thiophenes in the mesogenic core favors polymesomorphism and intense emission, enabling them for application in polarized emission.

13C NMR investigations of molecular order of rod‐like, bent‐core, and thiophene mesogens

“…In order to estimate the actual dipolar couplings from the values obtained experimentally, the scaling [55] M-8 Nematic, SmA d Guruprasad Reddy et al [56] M-9 Nematic, SmC, tilted hexatic Kesava Reddy et al [57] M-10 Nematic Lobo et al [51] M-11 B 1 Kesava Reddy et al [58] M-12 B 2 Kesava Reddyet al [59] Abbreviations: SmA, smectic A; SmC, smectic C; SmF, smectic F. [60] M-14 Nematic Santhosh Kumar Reddy et al [60] M-15 Nematic, SmC Santhosh Kumar Reddy et al [60] M-16 Nematic Rajasekhar Reddy et al [61] M-17 Nematic, SmC Rajasekhar Reddy et al [61] M-18 Nematic Rajasekhar Reddy. [62] M-19 Nematic, SmA Rajasekhar Reddy et al [62] M-20 Nematic, SmC Rajasekhar Reddy et al [62] M-21 Nematic, SmC Kesava Reddy et al [58] (Continues) factor of the experiment needs to be determined. For this purpose, the fully proton-coupled 1D 13 C spectrum of a 13 C-labeled CHCl 3 sample oriented in a nematic liquid crystal at room temperature was first obtained, and the 1 H─ 13 C dipolar coupling obtained from the spectrum was used as the reference.…”

“…[15] Banana mesogens constructed with benzene ring at the center and sidearm cores made up of two/three-ring were synthesized (M-11 and M-12) and examined by 13 C NMR spectroscopy. [58,59] The 2D SLF was used to obtain the 13 C─ 1 H dipolar couplings in smectic C phase at 180°C (Adapted with permission from Rajasekhar Reddy et al [62] Copyright 2016 American Chemical Society) of carbons of the center ring as well as in the side-wing phenyl rings. [58] Figure 8 shows 2D SLF spectrum for the M-11 in B 1 phase at 145°C where the center ring methine 13 C─ 1 H dipolar couplings were found to be larger (9.5-10.2 kHz) compared with those of the sidewing rings (1.6-2.1 kHz).…”

“…Significant effort has been made to investigate thiophenebased mesogens by our group using 13 C NMR measurements. [47,51,58,[60][61][62][63][64][82][83][84][85][86] It was well recognized that the replacement of phenyl ring with thiophene in mesogen results in decrease of melting point, increase of optical anisotropy, reduction in viscosity, promotion of negative dielectric anisotropy, and fast switching times. [87,88] Samulski et al showed that replacement of phenyl ring with thiophene would bring profound changes in molecular shape.…”

“…In order to understand the influence of thiophene rings on the mesophase properties as well as photophysical properties, mono (M-18), di (M- 19), and terthiophenes (M-20) were linked to two phenyl ring core unit. [62] The 13 C─ 1 H dipolar couplings determined from 2D SLF experiments were used for computing the order parameters of phenyl as well as thiophene rings (Figure 13). Interestingly, for mono thiophene mesogen M-18, the order parameter of thiophene ring was found to be lower than phenyl ring II, whereas for M-19 and M-20 mesogens, a reverse trend was noticed ( Table 7).…”

Ionic liquid crystals with aggregation-induced emission properties based on pyrrolo[3,2-b]pyrrole salt compounds

“…The d/L of DPP-2Py-9 remained constant (0.95) at 180 1C and 190 1C, indicating that the nature of the smectic mesophase of DPP-2Py-9 was smectic A (SmA) phase. 26 The schematic diagram of the SmA phase of DPP-2Py-9 molecules is shown in Fig. 6.…”