Superlinear amplification of the first hyperpolarizability of linear aggregates of DANS molecules

A bottom-up modeling strategy is adopted to discuss linear and nonlinear optical spectra of 4-dimethylamino-4′-nitrostilbene (DANS) in different environments, from solutions to linear aggregates, fully accounting for the molecular polarity and polarizability. In particular, we demonstrate a large amplification of the first hyperpolarizability of linear aggregates with a superlinear dependence on the aggregate size. Results are discussed with reference to recent experiments for DANS molecules aligned inside single-wall carbon nanotubes, leading to a complete and internally consistent description of the oberved spectral properties in terms of ~7 aligned molecules, reducing by an order of magnitude the size of the aggregate estimated on the hypothesis of a linear amplification, as expected for non-interacting molecules. This has important implications for the material design: it is possible to obtain a large amplification of the first hyperpolarizability by aligning just a few DANS molecules (or more generally a few polar dyes showing normal solvatochromism) without the need to grow large ordered systems.