Hetero-association of aromatic molecules in aqueous solution

Abstract

Knowledge of the physical chemistry of small molecules complexation (the hetero-association) in aqueous solution is increasingly important in view of the rapidly emerging branch of supramolecular chemistry dealing with the formation of heterogeneous polymeric structures having specific functional roles. In this paper, the 50-year history of scientific studies of hetero-association of heterocyclic aromatic molecules in aqueous solution has been reviewed. Some important correlations of structural and thermodynamic parameters of complexation have been reported based on large data-set of hetero-association parameters accumulated to date. The fundamental problem of 'energetic composition' of π-stacking is extensively discussed. The review has shown that there are some gaps in our understanding of heteroassociation, which provides a challenge for further studies in this area.

Interrelation of Entropic Contributors to $$\pi $$ π -Stacking in Solution

“…In contrast to H exp , the negative magnitudes of S exp do not have an apparent interpretation. The common view, currently dominating in literature, states that classical hydrophobic interactions give the largest contribution to π -stacking in solution, or the hydrophobic and van der Waals/electrostatic factor commensurate at best [2,16]. It suggests that S exp should take large positive values or small values close to zero (negative or positive).…”

“…(i) the π -stacking complexation follows standard linear enthalpy-entropy compensation, suggesting no apparent deviation of the complexation process from common thermodynamical behavior of small molecules in aqueous solutions, (ii) the H exp quantity is always negative independent of the type of molecule X or Y involved in the aggregation process, (iii) the S exp quantity also takes negative values (with few exceptions) independent of the type of molecule X or Y , (iv) the thermodynamic patterns for the hetero-association X +Y and self-association of its components, i.e., X or Y , are generally similar suggesting that the overall thermodynamic properties of self-and hetero-association may be considered complementary to each other (the specific deviations of hetero-association from self-association are discussed in detail in [16]). …”

“…Analysis of the thermodynamic data reported in [16] allows drawing the following set of conclusions:…”

“…Recently, the most complete set of thermodynamical data on aggregation of small aromatic molecules (experimental enthalpy and entropy changes, H exp and S exp ) were reported [16]. It contains more than 50 self-(X + X ) and heterocomplexation (X + Y ) systems measured under comparable experimental conditions (i.e., aqueous solution, pH ∼ 7, I ∼ 0.1).…”

“…In the present work, the thermodynamic dataset [16] is analyzed in terms of the contribution of various entropic factors to π -stacking in aqueous solution.…”