4-Alkoxybenzoic acid

The same study has shown that the organization completely changes for the corresponding benzoic acid 23 (Fig. 10) where the COOH groups form cyclic dimers with a linear shape.10 In this case there is no interdigitation of the Rp-chains and a (tilted) double layer structure is formed (see Fig. 11c), which is in line with the dll ratio of 1.6-1.7 observed for these benzoic acids. Related types of LC structures have been discussed for other partly fluorinated 4-alkoxybenzoic acids [126], and succinimidyl benzoates 24 [121]. 

Recently fluorinated chains were also attached to amine based dendrimers, like PPI and poly(amido amine) (PAMAM), by means of ionic self assembly with semiperfluorinated alkanoic acids (Fig. 71, 216, 217). All dendrimer complexes 216 with perfluorinated alkanoic acids show SmA phases except the fifth generation dendrimer which shows a Colrec phase [378]. Also semiperfluorinated 4-alkoxybenzoic acids 217 [379] and mixed systems combining fluorinated and nonfluorinated alkanoic or benzoic acids (216, 217) have been reported [379-381]. In most cases layer structures (SmA) and for higher generation dendrimers also modulated SmA phases (Colrec phases) were observed. The layer thickness of the SmA phases of the mixed complexes 216 depends on the ratio of fluorinated and non-fluorinated acids, where smaller d values were observed for dendrimers with low Rp-content for Rp-rich dendrimers the more rigid Rp-chains provide... 

It is of interest that supramolecular polyamides incorporating 4-alkoxy-benzoic acids with polar and nonpolar groups at the 3 position exhibit meso-phases until ca. 200 °C, while no stable complexation behavior is observed when the simple 4-alkoxybenzoic acid is used for complexation [62, 105, 106]. 

A dynamic equilibrium for closed dimers and open hydrogen bonded structures has been proposed for the liquid crystalline phase of an 4-alkoxybenzoic acid. See E.F. Carr and L.S. Chou, J. Appl. Phys., 44 3365 (1973). 

In fact, hydrogen bonding in liquid crystals is a very old concept and it has been known for some time that, for example (Figure 22), the mesogenic behavior of 4-alkoxybenzoic acids (21) depends totally on the fact that they exist as hydrogen-bonded dimers. Other simple, elegant examples are the undeca-2,4-dienoic acids (22). ... 

Conceptually similar is work from Janietz and cowork-ers" who prepared a 2,4,6-triarylamino-l,3,5-triazine core (25) which was itself mesomorphic showing Colh phases. When hydrogen bonded to alkoxybenzoic acids, these triazines gave rise to complexes (26) showing Colh mesophases between about 35 and 80 °C for 4-alkoxybenzoic acids (Figure 25) while Coir phases (non-centered, P2m) were seen between 60 and 98 °C for complexes with 3,4-dialkoxybenzoic acids (not shown). 

Ambrozic, G., Zigon, M. (2005a), Hydrogen-bonded liquid-crystalline polyurethane complexes with 4-dodecyloxybenzoic acid, Acta Chimica Slovenica, 52,207-14. Ambrozic, G., Zigon, M. (2005b), Hydrogen-bonded polyurethane complexes based on 4-alkoxybenzoic acids as the low molar mass components. Polymer International, 54, 606-13. 

The polarity and polarizability of the central groups of mesogenic compounds are more clearly correlated with thermal stability. Thus, 4-alkoxy-4 -biphenyl carboxylic acids, form more stable mesophases than corresponding 4-alkoxybenzoic acids. Similarly, esters of cholesterol form generally more thermally stable mesophases than esters of cholestanol. These differ only by the presence of a double bond, as shown in Fig. 5. Again, certain conjugated unsaturated aliphatic carboxylic acids show liquid crystalline phases, while normal aliphatic... 

To provide sufficient rigidity to the core at least two ring systems are usually required. Rare exceptions are the 2,4-alkyldienoic carboxylic acids in which dimers are formed by hydrogen bonding of the carboxylic acid groups (which becomes a pseudo-ring system) indeed, most carboxylic acid systems behave in the same manner, e.g., 4-alkoxybenzoic acids (6). 

More recently El-Waki et al. reported the synthesis and characterization of a series of 4-alkyoxybenzoyloxypropyl cellulose (ABPC-n) samples, obtained via the esterification of hydroxypropyl cellulose (HPC) with 4-alkoxybenzoic acid bearing different numbers of carbOTi atoms (n= 1, 2, 3, 4, 5, 7, 8, 12). In this series the authors noted that only for the higher alkoxy chain lengths (7, 8 or 12) a thermotropic liquid crystal behavior was observed however all the esters of HPC within the series presented lyotropic behavior (El-Wakil et al. 2010). 

Edgar KJ, Buchanan CM, Debenham JS, Rundquist PA, Seiler BD, Shelton MC, Tindall D (2001) Advances in cellulose ester performance and application. Prog Polym Sci 26(9) 1605-1688 El-Wakil NA, Eahmy Y, Abou-Zeid RE, Dufi-esne A, El-Sherbiny S (2010) Liquid crystalline behavior of hydroxypropylcellulose esteiified with 4-alkoxybenzoic acid. Bioresources 5(3) 1834-1845... 

---