Intrachain Energy Migration to Weak Charge-Transfer State in Polyfluorene End-Capped with Naphthalimide Derivative Emanuelle R. Simas, † Marcelo H. Gehlen,* ,† Melissa F. S. Pinto, ‡ Jonathas Siqueira, ‡ and Lino Misoguti ‡ Instituto de Quı ´mica de Sa ˜o Carlos and Instituto de Fı ´sica de Sa ˜o Carlos, UniVersidade de Sa ˜o Paulo, 13560-590, Sa ˜o Carlos - SP, Brasil ReceiVed: August 27, 2010; ReVised Manuscript ReceiVed: October 21, 2010 Polyfluorene end-capped with N-(2-benzothiazole)-1,8-naphthalimide (PF-BNI) is a highly fluorescent material with fluorescence emission modulated by solvent polarity. Its low energy excited state is assigned as a mixed configuration state between the singlet S 1 of the fluorene backbone (F) with the charge transfer (CT) of the end group BNI. The triexponential fluorescence decays of PF-BNI were associated with fast energy migration to form an intrachain charge-transfer (ICCT) state, polyfluorene backbone decay, and ICCT deactivation. Time-resolved fluorescence anisotropy exhibited biexponential relaxation with a fast component of 12-16 ps in addition to a slow one in the range 0.8-1.4 ns depending on the solvent, showing that depolarization occurs from two different processes: energy migration to form the ICCT state and slow rotational diffusion motion of end segments at a longer time. Results from femtosecond transient absorption measurements agreed with anisotropy decay and showed a decay component of about 16 ps at 605 nm in PF-BNI ascribed to the conversion of S 1 to the ICCT excited state. From the ratio of asymptotic and initial amplitudes of the transient absorption measurement, the efficiency of intrachain ICCT formation is estimated in 0.5, which means that, on average, half of the excited state formed in a BNI-(F) n -BNI chain with n ) 32 is converted to its low energy intrachain charge-transfer (ICCT) state. Introduction Conjugated polymers (CP) have been extensively studied due to their interesting optical and electronic properties besides applications in organic electronics. The excitons formed on CP chains by light absorption are able to migrate along the π-conjugated system before deactivation that occurs by radiative and nonradiative steps to the ground state or by dissociation into charge carriers. Therefore, CP application in organic electronics depends on the relationship between the chemical structure and the properties of excited states. For instance, photovoltaic cells require materials with an efficient charge separation, and thus high exciton mobility is needed to enhance the probability of exciton migration to a charge-separation site during its lifetime. 1 On the other hand, materials for light emitting organic diodes must be highly fluorescent, and therefore, exciton mobility should be moderate during its lifetime to avoid migration (and the transfer of the excitation energy) to a quenching site or low energy dark state. 2 The fluorescence color emission of CP can be tuned by copolymerization of functional building blocks with fluoro- phores. In particular, polyfluorene (PF) has been extensively modified to obtain emitter copolymers over the whole visible spectral region. 3 The tuning of fluorene copolymer emission is provided by exciton migration (or energy transfer) from the fluorene moieties (“blue emitter”) to the lower energy acceptor units. The energy migration (and transfer) process is largely influenced by molecular interactions and polymer chain packing. In isolated chains, the resonant energy transfer provides a way of exciton migration along the polymer chain to an acceptor site. In a solid state thin film the close contact between the chains creates additional paths for exciton migration and, in general, transfer becomes faster and more efficient than in solution. 4 Among several comonomers applied in copolymerization with fluorene, N-substituted-1,8-naphthalimides have been chosen due to their good electron-acceptor properties. 5 An advantage of using N-substituted-1,8-naphthalimides (NI) is that the fluores- cence of these compounds can be modulated by the medium polarity. 6,7 Energy transfer from fluorene to NI has been observed when NI is incorporated into PF chains as end-capping unit 8-10 or randomly in the copolymer chain. 11 The color emission in this type of copolymer is tuned by varying the chemical structure or the ratio of NI units in the copolymer. It has been shown that emission of the NI is much more pronounced in the solid state than in dilute CP solution. Although these results are very interesting, the optical properties and photophysics of these copolymers have not been fully exploited. In this contribution, poly(9,9′-di-n-octylfluorene) end-capped with N-2-benzothiazole-1,8-naphthalimide (PF-BNI, vide infra Schemes 1 and 2) was synthesized to investigate the photo- physical and exciton migration properties of this CP system. To get information on the behavior of isolated chains, steady- state and time-resolved fluorescence allied with transient absorp- tion measurements were made in dilute solution of different solvents. The exciton migration was evaluated by means of time- resolved fluorescence anisotropy and transient absorption. For comparison with the PF-BNI results, polyfluorene homopolymer (PF) was synthesized as standard. N-2-Abenzothiazole-1,8- naphthalimide (BNI) was synthesized as model compound and its steady-state properties was also investigated. * Corresponding author. E-mail: marcelog@iqsc.usp.br. † Instituto de Quı ´mica. ‡ Instituto de Fı ´sica. J. Phys. Chem. A 2010, 114, 12384–12390 12384 10.1021/jp108168f  2010 American Chemical Society Published on Web 11/09/2010