A series of indoline-pi-acceptor chromophores has been synthesized and the members first hyper-polarizabilities measured to investigate the effect of conjugation length, the use of various substituents and configurational locking on the nonlinear optical response. While increasing the length of the conjugated interconnect enhances the optical nonlinearity, ring locking was found to have little effect, although this is thought to be due to the electronic properties of the substituent groups, rather than structural factors. Nonetheless, all of the compounds were found to have high molecular hyper-polarizabilities with values of up to 1485 x 10(-30) esu when measured in chloroform at 1300 nm, findings which confirm that these chromophores are excellent candidates for further study. X-ray crystallographic studies were performed on three of the compounds and bond length alternation values obtained; these were found to correlate well with the observed molecular responses. (C) 2010 Elsevier Ltd. All rights reserved.