Cooperative Two‐Photon Absorption Enhancement by Through‐Space Interactions in Multichromophoric Compounds

Abstract

In the last decade, a major effort has been devoted to the achievement of efficient organic materials for nonlinear optics (NLO). Optimization has been sought at both the molecular and the supramolecular level. Of major importance is the awareness that the NLO properties of organic-based materials can be different from the sum of the properties of the isolated molecules. [1][2][3][4] In many cases, intermolecular interactions are found to be detrimental to the achievement of good performances for either second-order (electro-optical organic materials) [3] or third-order materials (e.g., twophoton absorption, TPA).[5] It has been shown, however, that significant cooperative TPA enhancement can be obtained in conjugated branched [6][7][8][9] or conjugated dendritic structures [10][11][12] as a result of through-bond coherent coupling between chromophoric subunits. Enhancement can also be achieved in the solid state through strong environment (charges) effects. [13,14] In contrast, the role of through-space interactions as a means to enhance TPA responses has seldom been considered.Recently, we reported a strongly nonadditive behavior in covalent antiparallel dimers of a dipolar chromophore. [15] Dipole-dipole interactions were found to be responsible for a reduction of the TPA response per chromophoric unit, while modeling suggested that changing the relative orientation/ distance of the chromophores would allow cooperative TPA enhancement to be achieved. [15] This prompted us to design and investigate multichromophoric molecular structures in which through-space interactions-instead of through-bond interactions [16,17] -would be exploited to increase the TPA response. In contrast to supramolecular chemistry, our aim was to use covalent bonding to confine chromophores and control their number and relative distance/positioning by grafting on suitable platforms.Following this strategy, herein we report the amplification of the TPA response of a series of well-defined and soluble (as opposed to aggregates or nanocrystals) multistilbazole architectures (Scheme 1), and investigate how the different topologies and number of dipolar chromophores influence the Scheme 1. A series of multichromophoric compounds built from the confinement of push-pull stilbazole chromophores: reference monomer (M); dimers obtained by grafting two chromophoric subunits to a central benzenic platform in the para (D p ), meta (D m ), and ortho (D o ) positions; trimer (T); dendrimers of the first (G1) and second generation (G2) bearing 12 and 24 chromophoric subunits, respectively.

Dipolar versus Octupolar Triphenylamine‐Based Fluorescent Organic Nanoparticles as Brilliant One‐ and Two‐Photon Emitters for (Bio)imaging

“…Such modifications of the TPA spectra can be related to through‐space interactions between chromophores in the densely packed nanoaggregates. It has been shown earlier in simpler systems (i.e., dimers, trimers, and dendrimers) that interchromophoric interactions between close polar and polarizable chromophores could lead to TPA enhancement or reduction—as well as a shift of the TPA band—depending on the relative orientations and distances between the chromophores 44, 45. The situation is much more complex in this case due to the number of molecules and range of relative orientations/distances between interacting chromophores within the nanoaggregates.…”

Molecular-Based Fluorescent Nanoparticles Built from Dedicated Dipolar Thienothiophene Dyes as Ultra-Bright Green to NIR Nanoemitters

“…Indeed, dipolar interactions are known to strongly influence the 2PA response of polar and polarizable chromophores [31]. Due to molecular confinement, such effects are expected to be quite strong within FONs made from dipolar dyes, depending both on the relative orientation and distance of the dyes and on their polarity/polarizability [32,33].…”

“…The colloidal stability is not the only goal of bottom-up engineering of FONs for bioimaging purposes. In fact, in many cases, where the colloidal stability is found to be suitable (i.e., no change of size and morphology over time), the luminescence properties of FONs are found to decrease markedly over time [32], in relation with slow surface rearrangements processes. These characteristics are related to the structure of chromophoric subunits and their interactions within FONs and thus are expected to be strongly influenced by the structure of the molecular subunit.…”

Ready Access to Molecular Rotors Based on Boron Dipyrromethene Dyes-Coumarin Dyads Featuring Broadband Absorption

“…The design of molecules featuring two or more chromophoric units is of great interest [1][2][3][4]. One can envisage new applications of such systems by taking advantage of the interactions that may develop between the chromophoric units, i.e., energy transfer [5,6], or electron transfer pathways [7], enabling the design of light harvesters with broadband absorption and high pseudo-Stokes shifts [8][9][10][11], or photosensitizers for photovoltaic devices [12,13] mimicking the natural photosynthesis [14,15].…”