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46663-AC10
Study of Aggregation of Organic Dyes on Metal Oxide Nanoparticle Surfaces

Elena Galoppini, Rutgers, the State University of New Jersey (Newark)

Metal oxide nanoparticles with bound dyes find applications in electrochromic displays, dye-sensitized solar cells (DSSCs), and other devices. Organic dyes are promising alternatives to the inorganic dyes, typically Ru(II)-bpy complexes. Aggregation, however, influences most applications of organic dye/semiconductor systems. Here we describe our progress in developing two strategies to prevent and study aggregation effects (Fig 1): (a) the use of tripodal linkers having a large footprint (Dye-tripod) to control the spacing between molecules and (b) the encapsulation of dyes in host molecules that bind to the surface (Dye@host).
(a) Dye-Tripod/TiO₂ Electron injection in the semiconductor is the primary process in a number of devices such as the dye-sensitized solar cell and also in the photographic process.  Some of the earliest fundamental studies in interfacial electron transfer have been performed utilizing model compounds prepared from polycyclic aromatic hydrocarbons.  The interest in this type of organic chromophores derives from the fact that aromatic hydrocarbons possess well-characterized spin states, and well-defined vibronic structures and transition dipole orientations.  Excimers have often characteristic fluorescence emission spectra. The key results for this part of the proposed project are summarized below.
a) We have synthesized and studied the binding and photophysical properties of large footprint tripodal pyrene derivatives bound to TiO₂ and ZrO₂ semiconductor nanoparticles films through meta or para oriented anchoring groups (Fig 2).
b) Excimer emission was observed on the surface of insulating ZrO₂  (Ebg= 5eV) nanoparticle films  suggesting that excimer may form in the necking regions (i.e. where the nanoparticles come in contact) and between dyes trapped in pores. We are testing whether rapid intermolecular energy transfer to a pyrene excimer on the surface of the semiconductor may also occur.
c) Preliminary charge recombination dynamics suggest a dependence on the linker and on the binding group position on the aromatic ring (meta vs para). 
            d) Since monomer emission was observed on insulating flat sapphire slides, it appears that the morphology of the substrate is of crucial importance. To determine the morphology effect was one of the original goals of the proposal. We will study such dyes bound to flat titanium anatase or rutile and zirconium oxide and determine the influence of the footprint on the binding process and photophysical processes.
            e) As originally proposed, the synthesis of the compounds in Figure 2 was supported by simple computational work where we the footprint was attached to anatase 101 crystal surface.  Preliminary calculations indeed suggest that anchoring groups in meta position have a better match for binding sites.  This working hypothesis will be tested experimentally.
            f)  The proposed incapsulation of pyrene in hosts (see below) with TiO2-bound cyclodextrins was tested. Preliminary data suggest that this route is not particularly promising and that the use of a designed linker with anchoring groups is crucial for strong  binding.
            (b) Dye@Host/TiO₂ Part of the second strategy involved the encapsulation of viologens in a host and attachment of the resulting 1:1 viologen@host complex to the surface of semiconductors for the development of electrochromic devices. Viologens are a class of organic compounds which are transparent in the bleached state and intensly blue after two electron reduction process. They have been successfully bound to TiO₂ for the development of electrochromic windows by a number of groups.  Unfortunately, the process is not truly reversible, because the radical cation gradually rearrange themselves, degrade or  dimerize. Cucurbituril CB[7] exclusively forms a 1:1 host-guest complex with viologens and we wanted to test whether a) could be bound or physisorbed to TiO₂ films and b) whether encapsulated and bound viologens  exhibit improved properties over the directly bound compounds. The inclusion of viologen derivatives (commercially available and new compounds) in cucurbituril[7], their binding to metal oxide surfaces and the testing of electrochromic windows from them, one of the original goals of the proposal, has been achieved.
a) We have used the commercially available methyl viologen  (MV²⁺) as well as a newly synthesized compound NV²⁺ shown above. The inclusion of MV²⁺in CB [7] was  performed in water. Viologen derivative (NV²⁺) and CB [7] were dissolved in deionized water and the formation of the complexes in water (0.05 mM MV²⁺@CB and NV²⁺@CB) was monitored by ¹H NMR.  Their electrochemical properties were studied by cyclic voltammetry. (Fig3)
b) The binding of CB complexed or directly bound  viologens to TiO₂ and ZrO₂ thin films was studied by IR and electrochemistry. Indeed, we observed that the chemisorption of CB’s and the MV@CB[7] complex at nanocrystalline TiO₂ is possible. (Fig 4)
c) Photochromic cells exhibiting reversible color changes and reversible redox chemistry are currently being tested.

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