GUEST COMMENTARY pubs.acs.org/JPCL
Luminescent One-Dimensional Nanoobjects. Where Do We Go from Here?
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wo Perspectives in this issue of J. Phys. Chem. Lett. present interesting synopses of absorption by and emission from two different types of one-dimensional luminescent nanoobjects. Whereas the inorganic nanowires described by Giblin and Kuno1 as well as those from other recent articles in this journal2,3 are separated from each other, the organic and organometallic fibrous objects treated by Babu, Kartha, and Ajayaghosh4 are parts of three-dimensional gel networks.5 Their high aspect ratios imbue both types of nanoobjects with absorption and luminescence properties that are not found in bulk materials of the same composition. The differences are a result of both the sizes (and surface area to volume ratios) and shapes of the nanoobjects. This Commentary will address, in a very subjective manner, a simple question, “Where do we go from here in the field of luminescent one-dimensional nanoobjects?” Instead of proposing answers, it will attempt to identify challenges at different length scales for applying luminescent one-dimensional nanoobjects as light- and energy-harvesting materials. It has been known for many years that the mode of packing of atoms and molecules in crystals can have a large influence on the efficiency of their absorption of electromagnetic radiation and on the properties of the resultant excited states. An example is the very high quantum yield (>0.64) and monomer-like spectrum of the fluorescence of anthracene in single crystals.6 Both can be attributed in part to the herringbone-like orientation of neighboring molecules, which does not allow π-π stacking. More recently, the Bordeaux group has performed very detailed and insightful investigations into the relationship between the orientation of linearly fused polycyclic aromatic molecules and their luminescent properties in fibers.7,8 In order to transfer excitons (or charge) through conjugated systems over long distances within an ordered array, π-stacking has been shown by theory and experiment to be highly advantageous, if not a necessity. One of the earliest demonstrations of the advantages of π-stacking was by Saeva et al.,9 who examined charge and electron transfer from arrays of molecules in columnar liquid crystals. Later, Markovitsi and co-workers10 extended the field by their elegant experiments and calculations of one-dimensional energy and electron-hopping rates within columnar arrays. In each of these examples, it was not possible a priori to know how molecules would pack within the one-dimensional objects, the dimensions and dispersions of the cross sections, and the aspect ratios! Those uncertainties still attend the field; experimentation and observation are the methods employed; no operational theory exists to cover a wide variety of atoms or molecules aggregating into one-dimensional objects. Such a theory is sorely needed. Additionally, only under rigorously controlled conditions can nanowires and organofibers be grown so that their ensembles are oriented along one Euclidean axis. Without employing treated surfaces, magnetic or electric fields, or flow for directed growth or postgrowth orientation, the long axes of nanowires and organofibers lie along all possible directions
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and are frequently connected as bundles or networks. Even in those cases where external perturbations are applied to orient one-dimensional objects along one Cartesian coordinate, specific control over where each will reside within the plane defined by the two orthogonal coordinates is still lacking. Although control is possible at larger length scales using nanolithography techniques11 and over small areas using AFM and related methods, a general set of methods to orient and distribute nanowires and organofibers over large areas is not. Methods that are applicable at the time of or after growth of the one-dimensional species are needed so that the shape anisotropy of these objects can be exploited fully. Finally, in the absence of electrical fields, charge migration within nanowires is scalar. Excitation energy along the long axis of an organofiber is as well. In fact, both are best described by one-dimensional random walk models. If it were possible to make these migrations vectoral, the number of applications of both types of nanoobjects would increase enormously. A very important future challenge will be to find methods to make the migration of charge or exciton energy unidirectional over many molecules or units in the absence of externally applied perturbations. To accomplish these goals—to construct a parallel, patterned array of one-dimensional nanoobjects capable of transmitting excitons or charges vectorally—will require very new approaches to the design of one-dimensional materials and the application of new concepts.
Richard G. Weiss Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, United States ACKNOWLEDGMENT The National Science Foundation is thanked for its support of the author's research in this field at Georgetown.
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Giblin, J.; Kuno, M. Nanostructure Absorption: A Comparative Study of Nanowire and Colloidal Quantum Dot Absorption Cross Sections. J. Phys. Chem. Lett. 2010, 1, 3340–3348. Gu, Z.; Liu, F.; Li, X.-F.; Howe, J.; Xu, J.; Zhao, Y.-L.; Pan, Z.-W. Red, Green, and Blue Luminescence from ZnGa2O4 Nanowire Arrays. J. Phys. Chem. Lett. 2010, 1, 354–357. Nair, P. V.; Thomas, K. G. Hydrazine-Induced Room-Temperature Transformation of CdTe Nanoparticles to Nanowires. J. Phys. Chem. Lett. 2010, 1, 2094–2098. Babu, S. S.; Kartha, K. K.; Ajayaghosh, A. Excited-State Processes in Linear π-System-Based Organogels. J. Phys. Chem. Lett. 2010, 1, 3413–3424. Terech, P.; Weiss, R. G. Low-Molecular Mass Gelators of Organic Liquids and the Properties of their Gels. Chem. Rev. 1997, 97, 3133–3159.
Received Date: October 21, 2010 Accepted Date: November 5, 2010 Published on Web Date: December 02, 2010
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DOI: 10.1021/jz1014373 |J. Phys. Chem. Lett. 2010, 1, 3425–3426
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Katoh, R.; Suzuki, K.; Furube, A.; Kotani, M.; Tokumaru, K. Fluorescence Quantum Yield of Aromatic Hydrocarbon Crystals. J. Phys. Chem. C 2009, 113, 2961–2965. (7) Placin, F.; Desvergne, J.-P.; Belin, C.; Buffeteau, T.; Desbat, B.; Ducasse, L.; Lassegues, J. C. Molecular Arrangement in the Gel Fibers of 2,3-Didecyloxyanthracene (DDOA): A Spectroscopic and Theoretical Approach. Langmuir 2003, 19, 4563– 4572. (8) Olive, A. G. L.; Raffy, G.; Allouchi, H.; Leger, J. M.; Del Guerzo, A.; Desvergne, J.-P. Striking Correlation between the Unusual Trigonal Crystal Packing and the Ability to Self-Assemble into Nanofibers of 2,3-Di-n-alkyloxyanthracenes. Langmuir 2009, 25, 8606–8614. (9) Saeva, F. D.; Reynolds, G. A.; Kaszczuk, L. Liquid-Crystalline Cation-Radical Charge-Transfer Systems. J. Am. Chem. Soc. 1982, 104, 3524–3525. (10) Marguet, S.; Markovitsi, D.; Millie, P.; Sigal, H.; Kumar, S. Influence of Disorder on Electronic Excited States: An Experimental and Numerical Study of Alkylthiotriphenylene Columnar Phases. J. Phys. Chem. B 1998, 102, 4697–4710. (11) Lim, J. K.; Lee, B. Y.; Pedano, M. L.; Senesi, A. J.; Jang, J. W.; Shim, W.; Hong, S.; Mirkin, C. A. Alignment Strategies for the Assembly of Nanowires with Submicron Diameters. Small 2010, 6, 1736–1740.
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