The field of nonlinear optics (NLO) has been recognized as a key domain for the development of high-bit-rate communications and optical information processing. [1] In the quest for even more efficient materials for quadratic NLO, chemists and physicists have joined forces to design ever-increasingly hyperpolarizable molecular and supramolecular units.[2±5]Early efforts aimed at synthesizing one-dimensional push±pull chromophores combining large hyperpolarizability (b) and dipole-moment values, and various strategies were imagined to enforce acentric arrangements of the chromophores: electrical poling of grafted polymers, [6] stepwise construction of multilayers, [7] use of thin films and Langmuir±Blodgett films, [8] polymerization, [9] self-assembly, [10] chirality, [11] inclusion in low-dimensional compounds, [12] and crystal engineering based on metal±organic-coordination networks.[13]More recently, the concept of octopolar nonlinearity has also stimulated intense research, [14] and very efficient non-polar molecular ªNLO-phoresº, such as metal complexes with D 3 symmetry, have been synthesized over the past decade.[15]However, although such non-polar materials should not experience intermolecular dipole±dipole interactions, only a few self-assembled NLO-active supramolecular or macromolecular architectures have been obtained so far, [16] whereas optical poling has been shown to be an attractive and efficient way to orient octopoles by multipolar coherent interference of oneand two-photon absorption processes.[17]Along the more traditional dipolar route, promising improvements of dipolar NLO materials were recently achieved by Dalton and co-workers, who designed strategies to minimize dipole±dipole interaction by either isolating the chromophores in a dendritic envelope or by introducing bulky substituents. [18,19] Alternative approaches consist of pre-organizing hyperpolarizable species in multichromophoric units by means of b-cyclodextrins [20] or dendrimers.[21±23] In a given preorganized unit, random mutual orientations of the chromophores are strongly reduced, and b values as large as 3000 10 ±30 esu (1 esu = 3.712 10 ±21 C m 3 V ±2 ) have been reached. However, the number of chromophores present in such units remains relatively small. The only macromolecular unit achieved so far is a glutamate-based polypeptide (molecular weight MW = 550 000 g mol ±1 ) with a helix configuration and a dipolar moment of 6000 D (1 D = 3.335 10 ±30 C m), but the b value remains quite low because of poor electronic delocalization.[24]We have designed an entirely different approach for synthesizing nanoparticles exhibiting giant b values, evidenced by the spontaneous alignment of several hundreds of dipolar chromophores in each particle. The strategy to engineer these supramolecular assemblies of NLO-phores involves a combination of several areas of chemistry: J-aggregates of organic dyes, intercalation in layered materials, coordination chemistry, and control of nanoparticle growth.Back in 1994, some of us reported that h...
