Evaluation of the parameters of 1:1 charge transfer complexes from spectrophotometric data by non-linear numerical method

The non-linear numerical method for evaluation of equilibrium constants and molar extinction coefficients of molecular complexes from a spectrophotometric experiment is described, which in contrast to linear models has no limitations with respect to concentrations of the components. The proposed procedure is applied to donor-acceptor interaction in solution between N-ethyl carbazole (EtCz) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) or n-hexyl 2,5,7-trinitro-9-dicyanomethylenefluorene-4-carboxylate (HexDTFC) to evaluate the method and to obtain the parameters of charge transfer complexes (CTCs) formation. Association constants (K) and molar extinction coefficients (epsilon) of CTCs derived from non-linear approach (EtCz-TCNQ: K = 2.49+0.19 M(-1); epsilon = 2950 +/- 160 M(-1) cm(-1). EtCz-HexDTFC: K = 12.1 +/- 0.3 M(-1); epsilon = 1335 +/- 24 M(-1) cm(-1)) are close to that from linear models but show lower standard errors in parameter estimations.