DOI: 10.1002/adfm.200600931 Tris(4-cyanophenyl)amine: Simple Synthesis via Self-assembly; Strong Fluorescence in Solution, Nano/microcrystals, and Solid** By Abhijit Patra, S. Philip Anthony , and T. P. Radhakrishnan* 1. Introduction Strongly fluorescent molecular materials are prime candi- dates for applications such as light-emitting diodes, molecular switches, and sensors. [1] Even though several classes of mole- cules show bright fluorescence in dilute solutions, only a small fraction of them retain the fluorescence or show enhanced light emission in the aggregated and solid states; [2–11] some of them show a very high quantum yield for solid state fluorescence. [3] The enhanced fluorescence observed in these materials has been attributed to a variety of mechanisms including efficient energy transfer from monomers to aggregates, [5] molecular conformational changes on assembly, [6] hindrance to aggrega- tion, and hence energy transfer, [7] intermolecular interactions restricting geometry relaxation, [8,9] and efficient fluorescence of novel aggregate structures. [10] Development of new mole- cules that show strong fluorescence in the aggregated state is not only of interest from the applications perspective, but also facilitates the exploration of fundamental mechanisms in- volved in the photophysical changes induced by molecular as- sociation. In addition to the bulk solid state, if the nano- or mi- crocrystals in the colloidal state of the molecule also show strong light emission, [10–12] the material’s optical properties can potentially be exploited in applications that require fluids or doped polymers. Photophysical properties of symmetric triarylamines have at- tracted considerable attention. [13] An important member of this family is tris(4-cyanophenyl)amine (TCPA). The methodolo- gies employed to synthesize TCPA have either involved multi- ple reaction steps [14] or specialized reagents. [15] Linear and non- linear optical properties of TCPA in the solution state have been reported. [16,17] We have synthesized TCPA from easily available 4-fluorocyanobenzene (FCB) through a simple one- step process. A stable colloid of TCPA was prepared through the reprecipitation protocol leading to highly monodisperse nano/microcrystals with well-defined morphology. Strong blue fluorescence was observed in the solution as well as in colloidal and solid states (Fig. 1); the colloid shows dual emission with a very small Stokes shift. The range of photophysical phenomena observed are explained through a crystal structure investiga- tion of TCPA coupled with detailed computations on the su- pramolecular clusters. Adv. Funct. Mater. 2007, 17, 2077–2084 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2077 – [*] Prof. T. P. Radhakrishnan, A. Patra, Dr. S. P. Anthony [+] School of Chemistry, University of Hyderabad Hyderabad 500046 (India) E-mail: tprsc@uohyd.ernet.in [+] Present address: National Creative Research Center for Block Copoly- mer Self-Assembly, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784 (Korea) [**] Financial support from the DST and CSIR, New Delhi; and infrastruc- ture support from the National Single Crystal X-ray Diffractometer Facility at the School of Chemistry, Centre for Modeling, Simulation and Design, and the UPE Programme of the UGC, New Delhi are ac- knowledged. We thank Dr. Suresh Das and Mr. Shibu Abraham for help with the solid state fluorescence and lifetime measurements, Dr. N. Hebalkar and Dr. B. Sreedhar for providing the TEM images, and Prof. M. Durga Prasad, Prof. D. Basavaiah, and Mr. T. Mondal for fruitful discussions. A.P. thanks the UGC, New Delhi for a Senior Research Fellowship. Supporting Information is available online from Wiley InterScience or from the author. The threefold symmetric molecule, tris(4-cyanophenyl)amine (TCPA) is synthesized from 4-fluorocyanobenzene by treatment with potassium carbonate in dimethylsulfoxide; the occurrence of the reaction without any amine reagent suggests the involve- ment of a novel self-assembly process. A reprecipitation strategy provides stable colloids containing highly monodisperse nano/ microcrystals with well-defined cubic morphology and sizes tunable from 250 to 500 nm. The solution, colloid, and solid states of TCPA exhibit strong blue fluorescence; the colloid shows dual emission with an unusually small Stokes shift. Computational investigations are carried out on the molecule and supramolecular assemblies derived from the crystal structure. Coupled with detailed spectroscopic studies, they show that the emission in the colloidal and solid states can be attributed to energy levels re- sulting from the intermolecular interactions within different aggregation motifs in the condensed phase and energy cascades be- tween them. The computations also reveal the presence of cooperative interactions in the molecular crystal contributing to its high thermal stability. The strong light emission exhibited by TCPA, concurrently in the solution, nano/microcrystal, and solid states establishes it as a novel molecular material of potential practical utility; it has led to the exploration of the underlying mechanism that describes the phenomena observed in the different physical states. FULL PAPER