Molecular-Level Control of the Photoluminescence from PPV Nanostructured Films Valtencir Zucolotto, A ˆ ngelo D. Faceto, Felipe R. Santos, Cleber R. Mendonc ¸ a, Francisco E. G. Guimara ˜ es, and Osvaldo N. Oliveira Jr.* IFSC, USP, CP 369, 13560-970, Sa ˜ o Carlos, SP, Brazil ReceiVed: December 10, 2004; In Final Form: January 28, 2005 The fabrication of varied molecular architectures in layer-by-layer (LbL) films is exploited to control the photoluminescence (PL) of poly(p-phenylene vinylene) (PPV) in an unprecedented way. This was achieved by controlling the Fo ¨rster energy transfer between PPV layers (donors) and layers of a commercial azodye, Brilliant Yellow (BY) (acceptors). Energy transfer was controlled by inserting spacer layers of inert polymers between PPV and BY layers and by photoaligning the BY molecules via trans-cis-trans isomerization. The PPV/BY LbL films displayed polarized PL whose intensity could be varied almost continuously by changing the time of photoalignment, which was carried out by impinging a linearly polarized laser light simultaneously to the PL experiments. For PPV/BY films with no spacer layers, PL was completely quenched, but its intensity increased with the number of spacing layers. Further increase in PL was obtained by photoaligning the BY molecules perpendicularly to the PPV molecules. This minimizes energy transfer, since Fo ¨rster processes are directional, dipole-dependent resonant transfers. Energy transfer is also controlled by imparting a preferential orientation of the PPV chains on PPV/BY LbL films deposited onto flexible Teflon substrates that may be stretched. 1. Introduction The layer-by-layer (LbL) technique has been widely exploited for fabricating heterostructures where interactions between the components are controlled at the molecular level. 1-7 For instance, the intensity of photoluminescence in LbL films of poly(p-phenylene vinylene) (PPV) has been controlled by electron transfer between PPV and an electron acceptor layer, viz., C 60 or poly(thiophene acetic acid) (PTAA). 3 Recovery of the fluorescence signal was obtained by interposing inert spacer layers between PPV and the electron acceptor layers, in a process that could be modeled with electron transfer theories. 4 In LbL films of PPV and poly(p-phenylene) (PPP), fluorescence quenching was caused by energy transfer rather than electron transfer, 8 which could also be recovered using spacer layers. Fo ¨rster energy transfer processes were used to explain the experimental data. Control over energy transfer in resonant systems is also useful to tune the color of emission in dye- doped PPV films. 9 In this work, the molecular control of luminescence was achieved in nanostructured films where PPV layers were alternated with layers of an azodye, Brilliant Yellow (BY), 10 which acts at the receiving end of energy transferred from photoexcited PPV molecules via the Fo ¨rster transfer mecha- nism. 11 The intensity of photoluminescence from the PPV LbL films is controlled by interposing inert bilayers of polyelectro- lytes between PPV and BY layers. The novelty lies in the possibility of photoaligning BY molecules which makes it possible to further control photoluminescence as the energy transfer is hampered when the BY orientation 12 is perpendicular to the electric field of the polarized emission from PPV. The PPV LbL films were deposited onto quartz and polymeric Teflon substrates. After film deposition, the Teflon + PPV films were subjected to mechanical stretching, which imparted a preferential orientation of PPV chains, leading to an increase in the polarization of the emitted light. A host of applications are envisaged with such fine control of luminescence properties, including optical switches, displays, and antenna devices. 2. Experimental Section Poly(allylamine hydrochloride) PAH, poly(vinyl sulfonic acid) (PVS), and poly(xylylidenetetrahydrothiophenium chlo- ride) (PTHT) were purchased from Aldrich Co. PTHT was purified before use, and the others were used as received. Brilliant Yellow (BY) was purchased from TCI. The PPV layers were deposited using PTHT, a polycationic PPV precursor, assembled with long chain ions of a sodium salt of dodecyl- benzenesulfonate (DBS). 9 The chemical structures of the materi- als employed are shown in Figure 1. PTHT/DBS bilayers were deposited from aqueous solution at pH 5.0 and a deposition time of 1 min. The acceptor layers comprised PAH/BY bilayers adsorbed from pH 8.0 solutions with 3 min of deposition. These experimental conditions also applied to the PAH/PVS spacer inert layers. The deposition times employed here were chosen on the basis of previous studies 16 and were appropriate for the assembly of good optical-quality films. The final architecture of the films corresponds to four blocks of PTHT/DBS and four blocks of PAH/BY, where each block contained five bilayers of PTHT/DBS or PAH/BY deposited onto quartz or Teflon substrates. The PAH/PVS spacing layers were interposed between the blocks. After assembly, the films were treated at 110 °C for 20 min under vacuum to allow the thermal conversion of PTHT into PPV. Photoluminescence (PL) mea- surements were carried out with a He-Cd linearly polarized laser operating at 441 nm as excitation light. The emitted light was collected by a monochromator. The analyses were per- formed at room temperature under vacuum to avoid photodeg- radation. * Corresponding author. E-mail: chu@if.sc.usp.br. 7063 J. Phys. Chem. B 2005, 109, 7063-7066 10.1021/jp0443613 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/22/2005