Relaxation dynamics of photoexcitations in polydiacetylenes and polythiophene

The relaxation dynamics of photoexcitations in cast films of the polydiacetylenes, poly[4, 6-decadiyne-1, 10-diol bis([(n-butoxycarbonyl)methyl]urethane)] (PDA-3BCMU) and poly[5, 7-dodecadiyne-1, 12-diol bis([n-butoxycarbonyl) methyl]urethane)] (PDA-4BCMU), and in electrochemically prepared films of poly(3-methylthiophene) (P3MT) were investigated by femtosecond absorption and picosecond luminescence spectroscopies. ¹B_u free excitons, generated by 100-fsec pump pulses, relaxed to the self-trapped (ST) state with time constants of 150 ± 50 fsec in PDA-3BCMU and 70 ± 50 fsec in P3MT. The difference was explained in terms of the relative sizes of the side chains. PDA-3BCMU has bulky side chains that are linked by hydrogen bonds, while P3MT has a repeating unit that contains a ring structure with a small methyl group. The lifetimes of the ST excitons at 10 K (290 K) were 2.0 ± 0.1 (1.5 ± 0.1) psec in PDA-3BCMU, 3.0 ± 0.3 (2.1 ± 0.2) psec in PDA- 4BCMU, and 800 ± 100 (800 ± 100) fsec in P3MT. The decay kinetics of the ST exciton were explained by potential crossing and tunneling between two potential curves of the ST exciton and the ground state. The weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. The relaxation dynamics of fluorescence from the PDA-4BCMU film cannot be represented by a single exponential decay; however, it can be described in terms of a random walk in the fractal dimension. By applying the fractaldimension model, the spectral dimension was determined to be between 0.50 and 0.85. The spectral changes that are due to several nonlinear-optical processes, i.e., hole burning, Raman gain, and the dynamic Stark effect, were also observed in femtosecond-resolved spectra. The third-order susceptibility was determined for these nonlinear processes, such as absorption saturation at the exciton transition, Raman gain, and the resonant Kerr effect from the observed spectral change. The third-order susceptibility that corresponds to absorption saturation in PDA-3BCMU and in P3MT was obtained as Im[χ⁽³⁾₁₁₁₁(-ω; ω, ω, -ω)] equal to -2.6 × 10^-9 and -3.5 × 10^-10 esu, respectively, for ħω = 1.97 eV The third-order susceptibility that corresponds to the Raman gain in PDA-3BCMU and in P3MT was determined to be Im[χ⁽³⁾₁₁₁₁(-ω₂; ω₂, ω₁, -ω₁)] equal to 5.8 × 10^-10 and -1.5 × 10^-10 esu respectively, for ħω₁ = 1.97 eV and ħω₂ = 1.79 eV. From the resonant Kerr experiment in the P3MT film, Δ χ⁽³⁾ ≡ χ⁽³⁾₁₁₁₁ (-ω₂; ω₂, ω₁, -ω₁) - χ⁽³⁾₁₁₂₂ (-ω₂; ω₂, ω₁, -ω₁) = 3.9 × 10^-11 esu was determined for ħω₁ = 1.97 eV and ħω₂ = 1.88 eV.