Four kinds of N-salicylideneaniline derivatives with different conjugation units in different configurations were considered to extract some useful guidelines for multi-component and/or multi-functional materials. The coupling strength of the electronic properties and the proton transfer was found to depend on the size of the conjugation system. Electronic properties (such as the conducting properties and some nonlinear optical properties) can be improved through the configuration modulation via the proton transfer. The most important conclusion is that, in case of the weak electron correlation, some properties possess the additivity rule, which can be used in the design of molecular electronic devices. The ground-state geometry and charge density of the whole molecule is essentially the simple combination of the subunits. Besides the quantitative additivity, other properties have the qualitative additivity - they have the same tendency with the same components. Both the quantitative and qualitative additivity can be taken as the inferences of the molecular topology, which promise the probability for the prediction of unknown properties on the basis of some knowing topological invariants.