There has been a great interest in recent years in organic nonlinear optical materials for their applications in photonics and optoelectronics. In particular, materials displaying twophoton absorption and efficient two-photon fluorescence are attractive for applications such as optical limiting, [1,2] optical data storage, [3] two-photon fluorescence microscopy, [4] and two-photon pumped lasing.[5] Two-photon pumped lasing is of increasing interest for making smaller, simpler and cheaper visible laser systems, as the gain medium can be pumped at red or near-infrared wavelengths for which laser diodes are inexpensive and more powerful than lasers needed for single-photon pumping. In addition, as the gain medium can be optically pumped at a wavelength longer than that of the laser emission, there is potential to increase the lifetime and photostability of the material.[6] Two-photon pumped lasing in organic small molecules has been demonstrated in the past in dye solutions [7,8] as well as in dye-doped organogels [9] and dye-doped polymer films.[10]In this context, organic semiconductors emerge as attractive candidates, owing to their high gain coefficients, low concentration quenching and optical losses, and ease of processing. [11][12][13] In addition, large molecules with delocalized electrons can display substantial optical nonlinearities and these properties can be further enhanced and tuned by means of synthetic chemistry. [14][15][16] Among these organic materials, conjugated dendrimers are a special category of organic semiconductors where different properties can be assigned to different parts of the macromolecule, allowing for greater flexibility in the design of materials with specific properties.[17]These molecules commonly consist of a central core containing a conjugated chromophore that determines the optical and electronic properties, a number of dendrons branching out of the core that control the intermolecular interactions and a set of surface groups that control the solubility of the molecule. [18][19][20] Changing any of these properties only requires change of part of the structure, which increases the flexibility for tailoring the materials to particular applications. In addition, they are highly processable from solution, meaning that thin films can be deposited onto various substrates by simple techniques such as spin-coating, allowing for an easy and cost-effective integration into devices. Dendrimers have previously shown great potential for use as gain media in lasers [21,22] and have also been studied in the context of nonlinear absorption, for which the effects of the excited state delocalization and the generation of the dendrimer on the nonlinear optical properties were investigated. [23,24] In this Communication, we report on the study of two-photon absorption and photoluminescence properties of a series of first-generation blue-emitting bisfluorene-cored dendrimers. We show that by adding suitable dendrons we may substantially increase the two-photon absorption while not strongly af...
