ARTICLE pubs.acs.org/JPCC
Triplet Transport to and Trapping by Acceptor End Groups on Conjugated Polyfluorene Chains Paiboon Sreearunothai,†,‡ Alexis Estrada,§ Sadayuki Asaoka,|| Marta Kowalczyk,§ Seogjoo Jang,§ Andrew R. Cook,† Jack M. Preses,† and John R. Miller*,† Chemistry Department, Brookhaven National Laboratory and ‡Sirindhorn International Institute of Technology, Thammasat University, Pathum Thani 12121, Thailand § Department of Chemistry and Biochemistry, Queens College, City University of New York, 65-30 Kissena Boulevard, Flushing, New York 11367, United States Department of Biomolecular Engineering, Kyoto Institute of Technology, Kyoto, Japan
)
†
bS Supporting Information ABSTRACT: Triplet excited states created in polyfluorene (pF) molecules having average lengths up to 170 repeat units were transported to and captured by trap groups at the ends in less ∼40 ns. Almost all of the triplets attached to the chains reached the trap groups, ruling out the presence of substantial numbers of defects that prevent transport. The transport yields a diffusion coefficient D of at least 3 104 cm2 s1, which is 30 times typical molecular diffusion and close to a value for triplet transport reported by Keller (J. Am. Chem. Soc. 2011, 133, 1128911298). The triplet states were created in solution by pulse radiolysis; time resolution was limited by the rate of attachment of triplets to the pF chains. Naphthylimide (NI) or anthraquinone (AQ) groups attached to the ends of the chains acted as traps for the triplets, although AQ would not have been expected to serve as a trap on the basis of triplet energies of the separate molecules. The depths of the NI and AQ triplet traps were determined by intermolecular triplet transfer equilibria and temperature dependence. The trap depths are shallow, just a few times thermal energy for both, so a small fraction of the triplets reside in the pF chains in equilibrium with the end-trapped triplets. Trapping by AQ appears to arise from charge transfer interactions between the pF chains and the electron-accepting AQ groups. Absorption bands of the end-trapped triplet states are similar in peak wavelength (760 nm) and shape to the 760 nm bands of triplets in the pF chains but have reduced intensities. When an electron donor, N,N,N0 , N0 -tetramethyl-p-phenylenediamine (TMPD), is added to the solution, it reacts with the end-trapped triplets to remove the 760 nm bands and to make the trapping irreversible. New bands created upon reaction with TMPD may be due to charge transfer states.
’ INTRODUCTION The transport of excitons requires electronic interactions that mix the wave function of the exciton at its current location into an adjacent region. For singlet excitons the interactions are the sum of matrix elements from the Coulomb (F€orster in the dipole limit) mechanism and the exchange (Dexter) mechanism within the approximation of configuration interaction singles. For triplet excitons the contribution of the Coulomb mechanism is negligible in general, leaving only the Dexter contribution. Dexter exchange matrix elements depend on the overlaps of electronic wave functions. Their exponential decrease with distance is rapid and is given approximately by the product of electron and hole transfer matrix elements.1,2 This gives triplet transfer rates an exceptionally steep dependence on distance36 and makes triplet transport among like sites extremely sensitive to structure. Indeed triplet transport in crystals,711 polymers,1214 and films12,1518 shows an astonishingly wide variation, and a change from naphthalene to naphthalene choleric acid crystals diminishes triplet diffusion by a factor of 105. While crystalline order can be important for exciton transport,11,19,20 triplet diffusion is sometimes faster in disordered medium than in crystals.14 The recent r 2011 American Chemical Society
measurements of Najafov et al.11 indicate triplet diffusion lengths of ∼5 μm in rubrene crystals. Whether in crystals or in more disordered solids, electronic mixing between separate molecules may constitute weak links for triplet transfer and transport. By contrast conjugated molecular chains provide strong, continuous π-interactions that would seem to furnish the intimate contact needed for fast triplet transport. On the other hand triplet motion along chains is often seen to be exquisitely sensitive to defects thought to reduce πinteractions along conjugated chains.2123 Triplet excitons have been found to be localized, often to one or two repeat units,2430 in contrast to more delocalized states for electrons, holes, or singlet excitons. Triplets are also different as King has shown that the transition moments for phosphorescence in polyfluorenes are perpendicular to the chain in contrast to singlet transitions.30 In addition to their sensitivity to structure, triplets have long lifetimes, so it is of great interest to investigate one-dimensional Received: June 21, 2011 Revised: August 30, 2011 Published: August 31, 2011 19569
dx.doi.org/10.1021/jp205828q | J. Phys. Chem. C 2011, 115, 19569–19577
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Conjugated polymer with traps (red) at each end, which captures triplets of naphthalene created in solution using pulse radiolysis. The structures of the polymers (pF) with naphthylimide (NI) and anthraquinone (AQ) end traps are depicted.
Table 1. Triplet Energies, E(T1) E(S0) = ET, of Molecules Used in This Study namea
ET (eV)
benzophenone
2.9746
naphthalene
2.6246
2.29 2.7046
p-xylene TMPD
3.4946 2.8,49 2.950
2.9246
acetophenone
3.4, 3.546
2.19
31
pFO
2.3
48
NI AQ fluorene a
2.2946
47
pF2/6
ET (eV)
fluoranthene
2.337,45
pF2/6
name
0
pF2/6 is poly-9,9 -(2-ethylhexyl)fluorene, for which two groups report slightly different triplet energies; pFO is poly(octyl)fluorene. AQ and NI are anthraquinone and naphthylimide; TMPD is N,N,N0 ,N0 -tetramethyl-p-phenylenediamine.
triplet transport along conjugated molecular chains, but only a few measurements have been made.3134 For single chains in solution, triplet motion can be studied by triplettriplet annihilation, which indicated that triplets move slowly by hopping.31,32 Monkman et al. found that generation of up to 30 triplets per poly(p-phenylenevinylene) (PPV) chain still resulted in little triplettriplet annihilation,32 pointing to slow transport. In films of conjugated polymers, it is often difficult to delineate the role of intrachain transport. A promising method is to determine the capture of triplets by traps attached to the ends of chains or incorporated at locations along the chain.33,34 Schanze photoexcited platinum phenylethynylene that produce triplet states by rapid intersystem crossing. The triplets were rapidly (>108 s1) and efficiently quenched by 5% copolymerized 2,5-thienylene (Th) groups. Rapid intersystem crossing promoted by the Pt metal was prevented by singlet quenching. Funston utilized a poly(phenylene ethynylene) having terthiophene traps at each end. Triplets were created in the solvent and transferred to the polymer. Triplet transport to the end traps was found to occur in
