The synthesis, linear and second-order nonlinear optical (NLO) properties of a series of ionic indoline and N,N-dimethylaniline based chromophores are described. In order to optimise the second-order NLO response of these compounds four basic variations to the molecules and/or their environments have been implemented and their effect studied. These consist of changes to the counter-anion (from iodide to p-toluenesulfonate to naphthalene-2-sulfonate), variation of the solvent polarity [dimethyl sulfoxide (DMSO) to methanol (MeOH) to chloroform (CHCl3)], annelation of the acceptor unit (pyridinium to quinolinium) and a change in the donor used (N,N-dimethylaniline to indoline). It is found that the strongest enhancements to the quadratic NLO response occur when the acceptor moiety is changed from pyridinium to quinolinium, while compounds with an indoline donor group are slightly superior to those containing dimethylaniline as donor. There is no real discernable change in the first hyperpolarizability on altering either the counter-anion or solvent used for the measurements. X-ray crystallographic data were obtained for two of the compounds, and they are found to pack in centrosymmetric fashion, and have a small degree of twisting between the donor and acceptor. From this data it is seen that there is a general agreement between the degree of bond-length alternation (BLA) and the NLO response of the chromophores, and that for charged chromophores such as these a BLA value of approximately 0.1 angstrom is optimal. (C) 2010 Elsevier B.V. All rights reserved.