The role of ion pairs in the second-order NLO response of 4-X-1-methylpiridinium salts

A series of 4-X-1-methylpyridinium cationic nonlinear optical (NLO) chromophores (X=(E)-CH=CHC6H5; (E)-CH=CHC6H4-4’- C(CH3)3; (E)-CH=CHC6H4-4’-N(CH3)2; (E)-CH=CHC6H4-4’-N(C4H9)2 ; (E,E)-(CH=CH)2C6H4-4’-N(CH3)2) with various organic (CF3SO3 , p- CH3C6H4SO3 ), inorganic (I , ClO4 , SCN , [Hg2I6]2 ) and organometallic (cis-[Ir(CO)2I2] ) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their secondorder NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second-order NLO response, investigated by the electric-field-induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10 3m, and their dipole moments were determined. Generally, below 5 10 4m, ion pairs start to dissociate into ions with parallel increase of the second-order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10 3m, some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second-order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self-aggregate.