Article pubs.acs.org/JPCC
Photoinduced Energy and Electron Transfer in Micellar Multilayer Films Maciej Kopeć,† Wiktor Niemiec,†,§ Andre Laschewsky,‡,# Maria Nowakowska,*,† and Szczepan Zapotoczny*,† †
Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland Fraunhofer-Institut, Angewandte Polymerforschung (FhG-IAP), Geiselbergstrasse 69, D-14476 Potsdam-Golm, Germany # University of Potsdam, Institute of Chemistry, Karl-Liebknecht-Str. 25 D-14476 Potsdam-Golm, Germany ‡
S Supporting Information *
ABSTRACT: Micellar multilayer films were prepared from an amphiphilic comb-like polycation (“polysoap”) and the polyanion poly(styrene sulfonate) (PSS) using alternate polyelectrolyte layer-by-layer (LbL) selfassembly. Linear growth of the film thickness was evidenced by UV−vis spectroscopy and spectroscopic ellipsometry. Imaging by atomic force microscopy (AFM) indicated that the micellar conformation adopted by the polycation in solutions was preserved in the films. Thus, hydrophobic photoactive molecules, which were solubilized by the hydrophobic nanodomains of the micellar polymer prior to deposition, could be transferred into the films. Photoinduced energy transfer was observed in the nanostructured multilayers between naphthalene (donor) and perylene (acceptor) molecules embedded inside the polymer micelles. The efficiency of the energy transfer process can be controlled to some extent by introducing spacer layers between the layers containing the donor or acceptor, revealing partial stratification of the micellar LbL films. Also, photoinduced electron transfer was evidenced between perylene (donor) and butyl viologen (acceptor) molecules embedded inside the multilayers by steady-state fluorescence spectroscopy. The obtained photoactive nanostructures are promising candidates for solar-to-chemical energy conversion systems. deposition of ultrathin films based on the alternate adsorption of oppositely charged polyelectrolytes has been successfully used for the fabrication of functional nanostructures.11−15 Förster resonance energy transfer (FRET)16−24 and PET25−29 as well as multistep cascade processes30−32 have been reported to occur in polyelectrolyte multilayers (PEMs) deposited either on planar surfaces, or as multilayered shells on micro/ nanocapsules. However, the chromophores are usually covalently attached to the main chain18,22,25−30 or co-deposited with standard polyelectrolytes.19,21,31−33 The efficiency of FRET or PET in such systems is reduced due to aggregation of the photoactive molecules34 as well as interpenetration of the subsequent layers that generally occurs in LbL films.11,35,36 Thus, chromophore aggregation has to be avoided in order to improve the efficiency of photochemical reactions within multilayers. Recently, we showed that amphiphilic micellar polyelectrolytes can be used to incorporate isolated photoactive molecules into PEMs.37−41 The solubilized molecules in these
1. INTRODUCTION Photoinduced energy and electron transfer (PET) are the crucial steps for converting light to chemical energy in natural photosynthesis. Light is harvested by so-called antenna complexes and funnelled via energy transfer to the reaction center where charge separation occurs, followed by a multistep unidirectional electron transfer.1 One of the approaches undertaken to mimic photosynthesis by artificial systems was to use synthetic photoactive polymers, also referred to as “photozymes”. “Photozymes”, as introduced by Guillet and coworkers, are amphiphilic polyelectrolyte copolymers that adopt a micellar conformation in an aqueous solution and that are able to capture light via antenna chromophores which form the hydrophobic micellar core. The excitation energy is transferred to such hydrophobic nanodomains able to host molecules, which can undergo photosensitized reactions.2−6 The efficiency of the PET process in organic systems is limited by a relatively short distance range of the electron transfer, which is spatially limited to the donor−acceptor interface. Therefore, nanostructured self-assemblies are promising candidates for future photoconversion devices, as they provide an excellent level of control over the distance between the photoactive centers.1,7−10 The layer-by-layer (LbL) © 2014 American Chemical Society
Received: November 2, 2013 Revised: January 7, 2014 Published: January 8, 2014 2215
dx.doi.org/10.1021/jp410808z | J. Phys. Chem. C 2014, 118, 2215−2221
The Journal of Physical Chemistry C
Article
Scheme 1. Chemical Formulas of the Polyelectrolytes and the Dyes Studied
(AFM) (Picoforce, Bruker) working in tapping mode in air. Standard silicon cantilevers (Bruker) with nominal spring constant 40 N/m and the tip radius
