Picosecond conformational relaxation of singlet excited polyfluorene in solution

Poly[9,9-di(ethylhexyl)fluorene] was studied by steady-state and time-resolved fluorescence techniques in solution in cyclohexane, methylcyclohexane, tetrahydrofuran, and decalin over the temperature range from 343 to 77 K. A decrease in temperature leads to a decrease in the inhomogeneous broadening of the emission band. Fluorescence decays were biexponential, consistent with a two-state model involving two different polymer conformers. Global analysis of the time profiles of luminescence collected at different emission wavelengths shows a long decay-time of 371.5+/-1.5 ps, which is temperature and solvent independent. The second shorter time (29+/-3 ps at 313 K and 100+/-3 ps at 233 K in methylcyclohexane) appears as a decay-time at the onset of the emission spectrum and as a risetime at longer wavelengths. Whilst the slow process was independent of temperature, the fast process showed Arrhenius type behavior, with an activation energy value of 0.10 eV found in both methylcyclohexane and decalin solutions. However, the risetime in the more viscous decalin was longer than that in methylcyclohexane. The observed behavior is interpreted in terms of fast conformational relaxation of the initially excited polymer, leading to a more planar conjugation segment. (C) 2003 American Institute of Physics.