Study On Triphenylene And Azobenzene Based Disc-Rod Mesogen Hybrid Systems And Their Side-Chain Liquid Crystalline Polymers

Liquid crystals(LCs)with a combination of mobility and self-assembly order,possess various fascinating functions such as photoelectronic response,self-healing and stimuli-responsive properties.After many decades of development,the field of liquid crystals has expanded from early scientific curiosity to extensive high-tech applications.On the other hand,photoresponsive materials have long been a hot study topic for the multidisciplinary field of physics,material science and also for chemical research.Especially for azobenzene-containing liquid crystal systems,their self-assembling capability and ordered characteristics are of great importance for achieving photoresponsive functions due to the fact that LC order can amplify photoinduced orientation of photochromic moieties and stabilize the photo-aligned structures.Rod-like(calamitic)and disc-like(discotic)aromatic cores are two types of typical mesogens to form liquid crystals.However,for quite a long period,the development of LC systems containing only calamitic or discotic mesogens was mostly in parallel.Calamitic LCs usually form lamellar structure with two-dimensional(2D)electronic transport and discotic LCs mostly generate columnar organization through 1D stacking of discotic cores that may serve as 1D semiconductors.Recently,many efforts have been paid for constructing disc-rod hybrid systems to bridge the gap between these differently shaped mesogens based LCs.However,in most of the reported disc-rod examples the columnar mesophase were inhibited due to a deficient fraction of discotic mesogen within the hybrid molecule,and systematic investigation into the relationship between discotic and calamitic mesogens is still rare.Study on disc-rod hybrid systems can help one to better understand the interplay of discotic and calamitic mesogens and elucidate the relationship between lamellar and columnar mesophases and their transformation,and also probably construct novel organic semiconductors with potentially simultaneous 1D and 2D electronic transport.Here in this dissertation,we present disc-rod shape-amphiphiles of a series of newly synthesized disc-rod dimers TP6-AZOn(n=6,8,12,16),TP10-AZOn(n=6,12)and the first liquid crystalline polymer poly(TP6-AZO10)with disc-rod hybrid side groups based on triphenylene(TP)and azobenzene(AZO)in an intentionally designed unbalanced geometry in cross section of one-to-one TP-AZO hybrid mode.This unbalanced design with volume advantageous TP discotic moieties brings about the general tendency of columnar packing,and by modification of the alkoxy length of either peripheral substituents of TP or AZO tails,it is easy to tailor the volume ratio of discotic and calamitic moieties and relevant space allocation and packing order.With combination of differential scanning calorimetry(DSC),polarized optical microscopy(POM)and variable temperature small/wide angle X-ray scattering(SAXS/WAXS),a systematic and in-depth study of the competition and compromise between variant shape aromatic moieties in disc-rod shape-amphiphiles and the relevant thermotropic mesophase evolution is presented.For the dimers of TP6-AZOn and TP10-AZOn,lamello-columnar mesophases are characteristic for these disc-rod hybrids bridging the lamellar and columnar phases constructed by calamitic or discotic mesogens only.For TP6-AZOn,with the extension of AZO aliphatic tails,the mesophases evolve from a sequence of lamello-columnar mesophases Colr/L and slim Colrs/L for TP6-AZO6 and TP6-AZO8 with shorter hexyloxy or octoxy tails,to a single Colr/L mesophase for TP6-AZO12 and TP6-AZO16 with longer dodecyloxy or hexadecyloxy tails.For TP 10-AZOn,TP10-AZO6 demonstrates a fascinating phase transformation from simple lamellar phase Lam to thermodynamically stable neat columnar phase Colob-d with disordered oblique columnar organization through an intermediate lamello-columnar mesophase Colr/L.With further extension of AZO tail to dodecyloxy,TP10-AZO12 exhibits only a stable lamello-columnar mesophase Colr/L.For side-chain disc-rod polymer poly(TP6-AZO10),the methacrylate monomer macr(TP6-AZO10)demonstrates lamello-columnar mesophase Colr/L,while the corresponding polymer poly(TP6-AZO10)displays a simple lamellar phase due to the constraint of polymer environment with the polymethacrylate backbone.Besides peripheral substituents of TP and AZO tails,the spacer connecting the disc and rod moieties of the disc-rod shape-amphiphiles constitutes another important structure factor strongly affecting the mesophase behavior and organization structures.A comparative study has been conducted based on a series of disc-rod shape-amphiphiles linked through decyl(dec)or tetraglycol(tg)spacer of different flexibility and polarity,including disc-rod dimer TP6-tg--AZO12,TP6-dec-AZO12(the same as TP6-AZO12 as noted in the previous chapter)and disc-rod-disc hybrid trimer TP6-tg-AZO-tg-TP6,TP6-dec-AZO-dec-TP6.For the dimers,both TP6-tg-AZO12 and TP6-dec-AZO12 display lamello-columnar mesophase Colr/L,while TP6-tg-AZO12 with tetraglycol spacer shows slightly weakened packing order within TP and AZO sublayer due to incompatibility between polar tetraglycol spacer group and apolar aliphatic chains.For disc-rod-disc trimers TP6-tg-AZO-tg-TP6 and TP6-dec-AZO-dec-TP6,the thermodynamically stable mesophases can only be achieved after long time annealing at room temperature,wherein TP6-tg-AZO-tg-TP6 with the flexible tetraglycol ether spacer shows a tetragonal columnar phase Coltet resembling simple TP6 dimers linked through spacer without AZO unit.Its counterpart TP6-dec-AZO-dec-TP6 with molecular structure similar to TP6-AZO6 serving as an extreme case with densely packed AZO dyads,demonstrates a slim lamello-columnar mesophase ColrS/L.The comparative study demonstrates that the spacer of variant flexibility and polarity is another important adjusting factor for the mesophase structures of disc-rod hybrid systems.Azobenzene chromophores are the most widely used photochromic mesogenic units with reversible photoisomerization,photo-induced alignment and even macroscopic movement.However,the realization of trans/cis photo-induced isomerization requires sufficient free volume and the photoalignment of azobenzene requires even more free volume space.As a consequence,the photo-responsiveness of simple low molar mass azobenzene derivatives in solid state is usually remarkably suppressed.Azobenzene-containing polymers were intensively investigated and quite promising as photoresponsive solid films due to the free volume introduced by the polymer backbone.However there exists a major drawback for azobenzene-containing polymers due to the usually ill-defined composition of synthetic polymer systems.Employing a rigid or semirigid framework of well-defined molecular structure to ensure sufficient free volume seems to be a sound solution,so here in this dissertation,we present the first pillararene-based liquid crystalline macrocyclic azobenzene photoresponsive materials Pillar[5]-S(n)-Azo with different spacer length of methylene number n=4 or 10.Pillar[5]-S(n)-Azo exhibit wide temperature range smectic mesophases and good film formation property,and the tubular shaped pillar[5]arene framework.provided sufficient free volume to realize reversible photoisomerization and light-induced alignment.By simply modifying the property of incident light such as wavelength and incident angle,thin films of Pillar[5]-S10-Azo demonstrate excellent light-triggered modulation of surface free energy,wettability and even orientation of upper layer triphenylene based hexagonal columnar discotic liquid crystal.Such pillararene-based macrocyclic azobenzenes represent a novel class of well-defined photoresponsive materials for smart materials and modulation of surface property.