Dipole-Dipole Interaction Driven Self-Assembly of Merocyanine Dyes: From Dimers to Nanoscale Objects and Supramolecular Materials.

Abstract

π-Conjugation between heterocyclic donor (D) and acceptor (A) groups via a polymethine chain leads to dyes with dipole moments greater than 10 D. These dipole moments direct the self-assembly of the dyes into antiparallel dimer aggregates, even in dilute solution, with binding strengths that are far beyond those observed for other π-scaffolds whose self-assembly is driven primarily by dispersion forces. The combination of directionality and exceptional binding strength of dipolar interactions between D-π-A dyes indeed resembles the situation of the hydrogen bond. Thus, similar to the latter, dipolar interactions between merocyanine dyes, a unique class of D-π-A chromophores, can be utilized to construct sophisticated supramolecular architectures of predictable geometry, particularly in low polarity environments. For bis(merocyanine) dyes it has been demonstrated that the self-assembly pathway is encoded in the tether between the two constituent merocyanine chromophores. If the tether enables the antiparallel stacking of the two appended dyes, folding takes place, which may be followed by further self-assembly into extended H-aggregate π-stacks at higher concentrations in solvents of low polarity. For tethers that do not support folding, the formation of bimolecular complexes of four merocyanine units, cyclic oligomers, and supramolecular polymers has been observed. For the former case, that is, formation of a bimolecular stack of four merocyanine units from tweezer-type molecules, association constants >10(9) M(-1) were measured in chloroform. On the other hand, because only one π-face is utilized in the formation of supramolecular polymers from bis(merocyanine) dyes, higher hierarchical structures typically originate in which the other π-face is surrounded by an antiparallel π-stacked neighbor molecule. Among the observed self-assembled structures, nanorods in particular have attracted considerable attention because their self-assembly into well-defined H-aggregates falls under kinetic control and is slowed tremendously with decreasing solvent polarity. Co-assembly of achiral and chiral merocyanine building blocks or two enantiomers of a chiral merocyanine in different ratios provided insight into "majority rules" and "sergeant-and-soldiers" effects as well as the autocatalytic fiber growth process. With regard to materials applications, it is important to note that the high propensity for dipolar aggregation was disadvantageous for many envisioned applications of these dyes in the area of nonlinear optics. However, this aggregation behavior proved to be advantageous for the recently demonstrated applications of D-π-A dyes, in particular, merocyanines as p-type organic semiconductors in organic electronics and photovoltaics. Thus, organic transistors with hole mobilities >0.5 cm(2)/(V s) and organic solar cells with power conversion efficiencies >6% could be achieved with merocyanine-based organic semiconductor molecules.