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

Abstract

A bottom-up modelling strategy is adopted to discuss the linear and nonlinear optical spectra of a prototypical push-pull dye, 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 observed spectral properties in terms of ∼7 aligned molecules, reducing by an order of magnitude the size of the aggregate estimated in the hypothesis of linear amplification, as expected for non-interacting molecules. This has important implications for 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.

Understanding TADF: a joint experimental and theoretical study of DMAC-TRZ

“…In the spirit of essential state models (ESM) 36,37,49,50 we select the minimal set of electronic diabatic basis states to describe the ground and the low-energy excited states. As for singlet states, the two-state model, proposed and extensively validated for D-A dyes, 36,37,51 also applies to DMAC-TRZ. The electronic basis states are selected as the neutral DA state, N, and the zwitterionic D + A À state, Z.…”

Optical spectra of molecular aggregates and crystals: testing approximation schemes

“…It is therefore very important to discuss approximation schemes to cut the basis dimension and particularly so for vibrational states. Indeed the HL electronic basis is already very small, and we already discussed how the electronic basis in the complete model can be reduced by fixing a maximum number of excitons (M e = 3 seems to work pretty well in most cases of interest in this study, even if this approximation is untenable for clusters of polar and polarizable dyes 18,20 ).…”

“…Recently, 20 discussing J-aggregates of polar dyes, we realized that for largely delocalized excitons only the vibrational modes in the close proximity of the center of the Brillouin zone are effectively coupled to the electronic degrees of freedom so that, instead of accounting for N local harmonic oscillators, reasonable results are obtained accounting for the single oscillator with q = 0. This of course leads to an enormous reduction of the basis dimension.…”

“…Along these lines, analytical solutions have been obtained of the electronic problem that offer a reliable basis for understanding spectral properties of molecular aggregates and crystals. The approximations of the exciton model were discussed in the original papers 14,15 and have been recently addressed in relation with aggregates of polarizable molecules with polar [18][19][20] or quadrupolar character. [21][22][23][24] Molecular vibrations add another layer of complexity to the physics of molecular aggregates and crystals: the deformation of the molecular structure upon excitation is responsible for the Franck-Condon structure of absorption and fluorescence spectra of isolated molecules in solution, but the shape of absorption and fluorescence spectra of molecular aggregates often largely deviates from the Franck-Condon behavior, 11,25 as first recognized in the narrow and structureless absorption and emission spectra of aggregates of cyanine dyes.…”

“…Accordingly, the model does not apply to so-called charge-transfer dyes, i.e., p-conjugated molecules with electron-donor and acceptor groups. Model for aggregates of polar 18,20 and quadrupolar dyes, [22][23][24]27 have been proposed recently.…”