Intramolecular charge transfer interactions and molecular order of rod like mesogens

Abstract

A mesogenic 4-((4-(alkoxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)benzoate series with terminal chains varying from C₂ to C₁₂ carbons (even number carbons only) are synthesised and their mesophase transitions are examined by hot-stage optical polarising microscopy as well as differential scanning calorimetry. Accordingly, enantiotropic nematic mesophase for all the homologs and an additional smectic A phase for the C₁₂ homolog is observed. Powder X-ray diffraction studies confirm the interdigitated bilayer organization in the smectic A phase for the C₁₂ homolog. It is remarkable that the mesogens under investigation only differ in the linking unit i.e. ester versus imine in contrast to recently reported mesogens, yet show a large difference in certain properties. Accordingly, the crystal structure of the C₄ homolog reveals a triclinic lattice with P1 space group in which the molecules are packed in a slipped co-facial configuration. Additionally, a detailed investigation of the C₁₂ mesogen by UV-visible and fluorescence spectroscopy as well as computational methods unveils interesting features. The fluorescence spectrum of the C₁₂ mesogen is observed at 366 nm with a shoulder at 433 nm and a large solvent polarity induced red-shift is noticed in contrast to a structurally similar homolog examined recently. Further, the C₁₂ mesogen in solvents such as ethyl acetate, dichloromethane, chloroform, tetrahydrofuran, acetonitrile and dimethyl sulfoxide exhibited dual emission. Therefore, density functional theory and time dependent density functional theory calculations are utilized to obtain insight. Besides variation in the dihedral angle between rings B and C for the C₁₂ mesogen, it is found that the highest occupied molecular orbital (HOMO) is localized on the N,N-dimethylaminobenzene moiety while the lowest unoccupied molecular orbital (LUMO) is mostly concentrated on the phenyl benzoate unit. Time dependent-density functional theory (TD-DFT) calculations disclose the orbitals involved in the dominant excited state electronic transitions and their corresponding energies together with oscillator strength. The high resolution 1D and 2D separated local field (SLF) solid state ¹³C NMR investigation of the C₁₂ mesogen lead to the orientational order parameters of the phenyl rings of the core in the SmA_d phase. The temperature versus alignment induced chemical shifts reveals an increase in chemical shifts with a decrease in temperature in the smectic A_d phase in concurrence with order parameter values. Thus, understanding the photophysical properties of mesogens with dimethylamino moieties would facilitate better design of molecules for application in organic light emitting diodes for polarized emission.