J. Phys. Chem. C 2007, 111, 18759-18764
18759
Spiroconjugation-Enhanced Intramolecular Charge-Transfer State Formation in a Polyspirobifluorene Homopolymer S. M. King,* S. I. Hintschich, D. Dai, C. Rothe, and A. P. Monkman Durham Photonic Materials Institute, Department of Physics, Durham UniVersity, South Road, Durham, DH1 3LN, U.K. ReceiVed: July 14, 2007; In Final Form: September 3, 2007
In this work we demonstrate the complex excited-state behavior of polyspirobifluorene in the solid state, which, due to the interaction of the spiroconjugated side group, readily forms a charge-transfer (CT) excited state of lower energy than the singlet exciton. The polymer is compared to standard polyfluorene materials using pump-probe and field-assisted pump-probe spectroscopy. It has been found that the small energy barrier between the singlet exciton and the CT state allows for population of the emissive singlet state at room temperature giving rise to a long tail in the singlet lifetime compared to an apparent single-exponential lifetime at low temperature. It is proposed that in devices the CT state is populated on charge recombination before the singlet states are excited by electron transfer from the CT state.
Introduction As the technology of polymer LEDs becomes more advanced and closer to becoming a commercial reality there is a drive for better materials with regard to emission efficiency, charge transport, and stability.1-5 One of the advantages of using conjugated polymers for light emission is the ability to make multifunctional emissive polymeric materials; that is, by modification of the side chains or copolymerization, specific properties can be adjusted. For example, incorporation of efficient emitters with moieties to enhance the stability or charge transport of the polymer can improve device efficiency. However, modification of the backbone often leads to a change in the excited-state properties invariably causing a change of the emission efficiency or wavelength; in some cases a gain is achieved in one area with a loss in another. For example, a moiety which offers a gain in stability or charge transport can lead to a reduction in efficiency of photoluminescence.6,7 The complex excited-state behavior of such highly functional polymers warrants investigation; by understanding the excitedstate behavior and how it relates to the chemical structure in these systems the polymer can be further tailored to enhance performance in devices. The first step in any investigation of photophysical properties is usually done in dilute solutions; understanding the behavior of isolated molecules is fundamentally important. There is of course a caveat to this; it is often found that the nature and dynamics of excited states in conjugated polymers alter dramatically in the solid state compared with dilute solution.8,9 This is to be expected considering that interactions between polymer chains are possible when in close proximity to one another. These effects can generally be categorized into two groups: the interactions of the unexcited polymer and the excited-state interactions. The ideal conjugated polymer is totally amorphous, and the relationships between neighboring chains are entirely random. However, many polymers are able to adopt a microcrystalline ordering in the solid state, often due to steric or electrostatic interactions of the polymer and its side chains.10,11 * Corresponding author.
Thus, one conformation or another becomes favored for the polymer backbone. In addition to the ground-state interactions, the close proximity of the polymer chains has a great effect on the excited states; the excitons become much more mobile as they are now able to hop between neighboring chains easily. This dramatically increases the way defect or trap states affect the excitation dynamics and emission quantum yield of the polymer as excitons are able to very quickly reach quenching or dopant sites.12-14 One example where an improvement to the chemical stability and color purity of emission has been made through chemical modification of the side groups of a polyfluorene backbone is the polymer studied in this paper, a polyspirobifluorene derivative (PSBF). PSBF is based on the familiar polyfluorene backbone with an additional fluorene unit attached at the 9 position in a spiro conformation (Figure 1). In common with other spiro compounds this prevents the formation of greenemitting defects by improving the resistance of the 9 position to photo-oxidation as well as allowing the polymer to form good amorphous films.3,15,16 As an aid to solubility and electron transport, electron-donating alkoxy side chains are added to the side group fluorene. The chemical modifications have proved successful at improving both the polymer stability and the device performance. PSBF has been demonstrated to be a good electron-transporting polymer; thus, device performance can be further improved when copolymerized with a hole-transporting moiety.17,18 However, the advanced nature of the polymer structure has led to complex behavior in the excited state, both in solution and the solid state. Isolated polymer chains in solution have been investigated in our previous paper by Hintschich et al.19 It was found that excited-state relaxation plays a significant role in the rate of the decay of the singlet exciton; the presence of such solvent viscosity based effects suggests that in the solid state the conformation of the molecule and relative orientation of the side group compared to the main chain could be important in determining the excited-state behavior. The conventional thinking on conjugated polymers is that the side chains play no part in the electronic properties of the material; for polyfluorene-type materials this means that the
10.1021/jp0755138 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
18760 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Figure 1. Normalized absorption and fluorescence of PSBF film (solid), dilute toluene solution (dot), compared to films of PF2/6 (dash). The inset shows the structure of PSBF.
substitutions at the 9 position are generally only important for control of the interaction between the polymer chain and its surroundings, for example, solubility, aggregate formation, and the morphology of the polymer in the bulk; it is generally assumed that the excited state does not extend into the side chains. This approach must be modified for the case of spirofunctionalized molecules as work on small molecule spiro systems has shown that when two separate π-systems are connected by a spiro linkage, there is an interaction between the two conjugated segments.20 A red-shift in the absorption edge has been noted as evidence for the interaction, and theoretical modeling confirms the interaction of the two π-systems. Of greater relevance to the polymer PSBF is the fact that incorporation of heteroatoms into the spiro side group of the polymer backbone can be used to donate electron density to the π-system through the spiro carbon.21 Density functional theory calculations in our previous work on the polymer have shown that the excited state does indeed extend into the side group of the polymer. Additionally, previous studies of spirobridged systems have shown that the spiro linkage can enhance the effect of any donor-acceptor system and aid the formation of charge-transfer (CT) states.22,23 Experimental Section The focus of the work is on the polyspirobifluorene derivative (PSBF) shown in the inset of Figure 1; to help explain the excited-state behavior it is compared with the prototypical polyfluorene PF2/6. Pump-probe experiments were carried out using a conventional femtosecond pump-probe system. This comprises an amplified titanium sapphire laser (Coherent Mira 900F and RegA 9000) and optical parametric amplifier generating
