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J. Phys. Chem. B 2009, 113, 16110–16117
Thermochromism of a Poly(phenylene vinylene): Untangling the Roles of Polymer Aggregate and Chain Conformation Chun-Chih Wang,† Yuan Gao,† Andrew P. Shreve,‡ Chang Zhong,‡ Leeyih Wang,§ Kumara Mudalige,| Hsing-Lin Wang,*,† and Mircea Cotlet*,| Physical Chemistry and Applied Spectroscopy, Chemistry DiVision, Mail Stop J567, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Center for Integrated Nanotechnologies, Materials Physics and Applications DiVision, Mail Stop K771, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Center for Condensed Matter Sciences and Institute of Polymer Science and Technology, 106 National Taiwan UniVersity, Taipei, Taiwan, and BrookhaVen National Laboratory, Mail Stop 735, Upton, New York 11973 ReceiVed: July 14, 2009; ReVised Manuscript ReceiVed: October 28, 2009
We report reversible thermochromism of a conjugated polymer, poly{2,5-bis[3-(N,N-diethylamino)-1oxapropyl]-1,4-phenylenevinylene} (DAO-PPV), in diluted solutions of toluene and 1,2-dichlorobenzene. By means of temperature- and solvent-dependent steady-state spectroscopy, picosecond time-resolved photoluminescence spectroscopy, and dynamic light scattering, we provide new insights into the role of polymer aggregates in defining the thermochromic behavior of PPVs. We find DAO-PPV to exhibit a low temperature state with vibronically structured red visible absorption and emission spectra. Structurally, this low temperature state is a densely packed and disordered polymer aggregate, which contains a fraction of well-ordered, packed polymer chains. These ordered regions serve as low energy trap sites for the more disordered regions in the aggregate, thus regulating the final emission of the aggregate and imposing a vibronically resolved emission spectrum, which is usually associated with emission from one or a few chromophores. The high temperature state of DAO-PPV is a loose aggregate, with structureless absorption and emission spectra in the green visible range. Structurally, the loose aggregate is a well-solvated aggregate retaining the physical dimension of the dense aggregate but for which interchain interactions are diminished with the increase of temperature. As a result, the spectroscopic behavior of the loose aggregate is very similar if not identical to that of the single polymer chain. Increased solubility untangles polymer aggregates into single, dispersed, polymer chains, as we demonstrate here for DAO-PPV in 1,2-dichlorobenzene and at high temperature. 1. Introduction Conjugated polymers with reversible thermochromic properties have potential applications toward smart windows, colorimetric sensors,1 or light emitting diodes.2 Thermochromism in conjugated polymers such as substituted polythiophenes was first reported by Yoshino et al.,3 and it was attributed to thermal vibrations increasing side-chain interactions and resulting in twisting of the thiophene rings that would decrease the overall electronic conjugation. To date, substituted polythiophenes remain one of the most extensively studied conjugated polymers with respect to their thermochromic properties.4-7 Thermochromic behavior has been reported for other conjugated polymers such as polyaniline,8-11 polydiacetylene,12 and poly(phenylene vinylene) (PPV).13-15 Similar to polythiophene, the reversible thermochromism of PPVs has been attributed to thermal vibrations imposing twisting of benzene rings resulting in a less coplanar polymer conformation with a decreased conjugation length. Leger et al. reported reversible temperature-dependent color changes from red (298 K) to yellow (373 K) for MEH-PPV gel films and related the * Corresponding author. H.-L.W.: phone, (505) 667-9944; fax, (505) 6670041; e-mail,
[email protected]. M.C.: phone, (631) 344-7778; fax, (631) 344-7765; e-mail,
[email protected]. † Chemistry Division, Los Alamos National Laboratory. ‡ Materials Physics and Applications Division, Los Alamos National Laboratory. § National Taiwan University. | Brookhaven National Laboratory.
thermochromic behavior to conformational changes in the polymer chain.16 These authors related the thermochromism to changes in the energetics of the HOMO and LUMO by ruling out aggregation among different chains because MEH-PPV was embedded in a gel matrix. Alternatively, the thermochromism in PPVs may be the result of an enhanced solubility that minimizes interchain interaction by untangling polymer aggregates into single polymer chains.17,18 Because of the relatively small number of reports on the thermochromism of PPVs and of the difficulties in clearly distinguishing between conformation and aggregate effects, the rationale behind the direct connection between thermochromism and the polymer aggregate in solution has been less convincing. The role of polymer aggregates in defining the roomtemperature optical properties of conjugated polymers has been intensively investigated in the past.19-21 It is well agreed that aggregate formation is the result of interchain interactions, leading to corresponding interchain excitations. These excitations are often red-shifted in absorption and emission properties as compared to the intrachain exciton and have low photoluminescence quantum efficiency when compared to the single polymer chain species.22,23 This low quantum efficiency of the aggregates is supposedly due to the presence of quenching sites in the aggregate leading to self-quenching. Such quenching sites become more prominent in thin polymer films than in solution, as cast polymeric films often have lower quantum efficiency.24 Diluting the polymer solution from which a film is cast may increase the quantum efficiency by diminishing interchain
10.1021/jp906645d 2009 American Chemical Society Published on Web 11/23/2009
Thermochromism of a Poly(phenylene vinylene)
J. Phys. Chem. B, Vol. 113, No. 50, 2009 16111
Figure 1. (a) Chemical structure of poly{2,5-bis[3-(N,N-diethylamino)1-oxapropyl]-1,4-phenylenevinylene}, DAO-PPV. (b) Temperatureinduced photoluminescence color changes in a toluene solution of DAOPPV under UV (360 nm) illumination.
interactions. Because most of the polymeric thin film devices are prepared by casting from a sample of polymer solution, the optical properties of the thin polymeric film will be dictated, in part, by those of the precursor polymeric solution. Therefore, it is of fundamental importance to understand the role of polymer aggregates and polymer chain conformation in defining the optical properties of PPVs and other conjugated polymers. The temperature-dependent optical properties of conjugated polymers are becoming increasingly important, especially due to their commercial applications. Understanding how ways of processing can lead to changes in polymer conformation and aggregates will allow optimizing/tailoring the optical properties of polymeric devices. Here, we report reversible thermochromism in toluene and 1,2-dichlorobenzene (DCB) for poly{2,5-bis[3-(N,N-diethylamino)-1-oxapropyl]-1,4-phenylenevinylene}, DAO-PPV (Figure 1a). DAO-PPV is a conjugated polymer with pH-dependent optical properties.22 DAO-PPV has tertiary amine side chains that are subject to protonation upon addition of acid molecules. The protonation leads to color changes from red (pH >9.0) to green (pH
