Molecular Self-Assembly of Mixed High-Beta Zwitterionic and Neutral Ground-State NLO Chromophores

We investigate a strategy for producing organic NLO materials with high chromophore densities by assembling Langmuir monolayers and Langmuir–Blodgett films containing mixtures of two hyperpolarizable chromophore amphiphiles. When deposited at an air–water interface, the amphiphilic molecules are oriented with their molecular hyperpolarizabilities aligned, and their dipole moments antialigned. We find that mixed chromophore Langmuir monolayers are more stable than pure ones, suggesting that electrostatic interactions aid self-assembly and ordering. Upon transfer of the monolayers to glass and silicon substrates, aggregates with well-defined topological features appear. We characterize aggregate formation using fluorescence microscopy, atomic force microscopy, and nonlinear optical ellipsometry, and we propose a mechanism for aggregate formation. Understanding molecular interactions between strong chromophores should enable fabrication strategies that prevent aggregation and optimize chromophore alignment. Overall, our results suggest that electrostatic forces can be successfully applied to the assembly of the chromophores within bulk nonlinear optical materials containing densely packed, highly ordered, mixed chromophore systems.