Indian Journal of Chemistry Vol. 42A, December 2003, pp. 3041-3043 Synthesis of a new cesium selective calix[4]arene based chromoionophore V Arora, H M Chawla*, T Francis, M Nanda & S P Singh Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India Received J J November 2003 A novel azo-pyridyl calix[4]arene, (5,1 I, 17,23-tetrakis(4'-azopyridyl)-25,26,27,28-tetrahydroxycalix[4]arene), has been synthesized in th e 1,3-alternate conformation by coupling debutylated calix[4]arene with a diazotized solution of 4-aminopyridine. The synthesized chromoionophore gives a large bathochromic shift (119 nm) with an increase in the inten- sity of absorption on addition of a methanolic solution of CS 2 C0 3 . The selectivity coefficients for Cr 3 + and Ni 2 + has been determined to be Kc/+, c'+ = 6.6 and K Ni 2 +, c'+ = 66, while other alkali and alkaline earth metal ions do not interfere. The synthesized calixarene derivative has the potential to be used for development of ionic filters . The design and synthesis of stimuli responsive chro- mogenic and fluorogenic ionophores is the current focus of research in supramolecular chemistry'. Calixarenes are recently discovered phenolic metacy- clophanes which possess a cavity in their molecular architecture and hydrophilic and hydrophobic lower and upper rims respectively2- 5. Several reports on calixarenes have appeared which suggest their useful- ness as molecular platforms for obtaining highly effi- cient new optical sensor materials 6 - '2 . For example, a tribenzo modified 1,3-calix[ 4 ]-bis-crown-6 has been reported to be highly selective for cesium ion 13 . Se- lective removal of cesium from model nuclear waste solutions have been achieved by using unsymmetrical calix[4]arene-bis-crown-6 after attachment to a poly- siloxane backbone '4 . A few research groups are in- volved in the synthesis and evaluation of calixarene based crown ethers with azo benzene unit in their molecular framework '5 . '6 to achieve photo-responsive switching in crown ethers probably because these calixarene derivatives are expected to combine the advantages of preorganization of ligating functionali- ti es and chromophoric properties of the azo benzene. Since such photoswitchable calix[4]crown ethers in cis-azo appendage at the lower rim for selective binding of Rb+ and Cs+ are difficult to obtain in rea- sonable yields 17 , there is a need for easily synthesiz- able and characterisable calixarene-based photo re- sponsive ligands for cesium and/or other alkali metal Ions. To date very little published work is available on calixarenes with appended pyridyl units at their lower rim IS . 19 • No chromogenic molecular receptor based on calix[4 ]arene with pyridine moiety attached to the upper rim has so far been reported. In the present work we describe our preliminary studies on the syn- thesis of azo-pyridyl calix[4]arene (1) and its extraor- dinary selectivity for recognition of cesium ions. The concepts can be translated into real life applications involving design of molecular filters for cesium from other alkali and alkali earth metal ions. Materials and Methods 4-Aminopyridine (Spectrochem) was used as plied and all other chemicals and metal salts used were of AR grade. UV -vis absorption spectra of the ligands and their complexes were recorded in methanol on Perkin- Elmer Lambda 20 UV -vis spectrophotometer in the range 800-200nm. NMR spectra were recorded on a 300 MHz Bruker DPX 300 instrument; IR spectra were recorded on a Nicolet Protege 460 spectrometer in KEr disks while CHN analysis was obtained using a Perkin-Elmer 240C elemental analyzer. Mass spec- trum was recorded on a Jeol SX-I02 spectrometer. Synthesis of 5, 11 ,17,23-tetrakis(4'-azopyridyl)-25,26, 27, 28-tetrahydroxy calix! 4 Jarene( l) An ice cold solution of 4-aminopyridine (0.615g, 6.55 mmol), sodium nitrite (excess) and conc. HCl (5 ml) in water (10 ml) was slowly added (over a pe- riod of about 10 minutes) into an ice cold (0-5°C) THF : pyridine (5: 1) solution of tetrahydroxy- calix[4]arene (0.530g, 1.16 mmol) (Scheme 1) with constant stirring to give a dark violet suspension. Af- ter stirring the suspension for 15 minutes at the same