Theoretical Study of Interaction Between N-phenolate Betaine Dyes and Some Surfactants

Heterocyclic betaines are known for having electronic spectra thatreveals existence of the strong band associated with the low-lyingelectronic state of the charge-transfer (CT) character[1]. There aremany important theoretical works describing the nonlinear opticalproperties of of betaine dyes [2-3].Interaction of various solutes with the micellar phase andencapsulation of molecules inside micelle are studied as models tosee the nature of interaction between the anionic phosphate groups ofDNA and cationic heterocyclic dyes [4].Very important for the understanding of the functioning of suchsurfactant-dye blends are the positions of the HOMO and LUMOenergy levels of the dye with respect to the surfactant. If both theHOMO and LUMO energies of the dye are between those of thesurfactant, both electrons and holes can be trapped on the dyemolecules. If both LUMO and HOMO energies of the dye are lowerthan the corresponding energies of the surfactant trapping mayhappen successively: first electrons are trapped by the dye moleculesand next the holes by their Coulomb interaction with the electrons. Inthe reverse situation, the roles of electrons and holes would beinterchanged. Most previous research were concentrated on 4-(1-pyridinium-1-yl)phenolate (1) molecule [5], the smallest derivativeof the well-known Reichardt’s betaine dye. There are also a fewstudies of its ortho isomer [6]. Surprisingly, there have been noliterature reports about either isomer with oxygen in meta position ,or any of its derivatives.We present here a computational study of orto-, para- and metapyridiniumN-phenolate betaine in presence of hexadecyl tri methylammonium bromide (C-TAB) and P-(1,1,3,3, tetramethyl/buthyl)polyoryethelene (TX-100) surfactants in two parallel andperpendicular situations with respect to C-TAB and TX-100molecules. The aim of the present study is to investigateintermolecular interaction between orto-, para- and meta- pyridiniumN-phenolate betaine dyes in two paraller and perpendicular situationwith respect to C-TAB molecules and finding out the best situationfor these kinds of interactions by Density-functional theory. Inprinciple, DFT cannot be used to calculate LUMO energies, due tothe wellknown band-gap problem. In particular, energies ofunoccupied states are generally underestimated in DFT [7]. However,we will concentrate here not on absolute values but on trends inHOMO and LUMO energies and assume that these are welldescribed in DFT. We will especially pay attention to the trends inthe calculated values for the HOMO and LUMO energies of the dyeswith variation of the orto- para- and meta- structure. We will finallydiscuss the relative positions of the HOMO and LUMO levels of thedyes and those of the surfactants and the consequences for chargeand energy transport.