Intrachain photoluminescence properties of conjugated polymers as revealed by long oligothiophenes and polythiophenes diluted in an inactive solid matrix

The intrinsic intrachain photoluminescence (PL) dynamics of conjugated polymers in the solid state is investigated. We focus on the PL properties of long $\beta$-substituted oligothiophenes (8-mer, 12-mer, and 16-mer) and regio-regular (RR) and regio-random poly(3-octylthiophenes) (P3OTs) diluted in the inactive solid matrix polypropylene (PP). The oligothiophenes have well-resolved 0-0 PL and absorption peaks at $4\mathrm{K}$ and show a good linear relationship with the reciprocal of the ring number. Franck-Condon analyses on the PL spectra reveal that the strength of the electron-phonon coupling represented by the Huang-Rhys factor becomes weak with increasing chain length in the oligomers. In contrast, the presence of stronger electron-phonon coupling is confirmed in RR P3OT despite it being of a much longer chain length than oligomers. This is probably due to its stereoregular chain conformation. The materials used in this work diluted in PP exhibit single-exponential PL decays with lifetimes that decrease with increases in the chain length. The presence of intrachain exciton migration is shown by the time-resolved PL measurements, except for the case of RR P3OT. The energy shifts attributed to intrachain migration are much smaller than those ascribed to interchain migration. We conclude that a change in the torsion angle around the thiophene ring bonds occurs within $50\mathrm{ps}$ following the absorption transition in thiophene derivatives and that the torsional change gives rise to a constant Stokes shift $(60–70\mathrm{meV})$, regardless of the chain length. We propose that the Stokes shift can be utilized for estimating the magnitude of exciton migration in thiophene derivatives, even in the case of undiluted films.

Figure 1

The chemical structure of oligothiophenes used in this study: $n=2$ for the 8-mer $\left(8\mathrm{T}\right)$, $n=3$ for the 12-mer $\left(12\mathrm{T}\right)$, and $n=4$ for the 16-mer $\left(16\mathrm{T}\right)$, where $n$ is the number of repetitions.

Figure 2

The absorption (lower right), photoluminescence excitation (PLE) (upper right), and steady-state PL (left) spectra excited at $2.54\mathrm{eV}$ at $4\mathrm{K}$ for regio-regular (RR) and regio-random (RRa) poly(3-octylthiophene)s (P3OTs) diluted in polypropylene (PP). The horizontal solid and dotted lines represent the level of the baselines for the PLE and PL spectra, respectively. The arrows indicate the energy positions at which the PLE spectra were measured. The PLE spectrum for RR was measured at the second vibronic peak $\left(1.82\mathrm{eV}\right)$. The PLE spectra for RRa were measured at the second vibronic peak $\left(1.88\mathrm{eV}\right)$ and at $2.25\mathrm{eV}$.

Figure 3

The absorption (lower right), photoluminescence excitation (PLE) (upper right), and steady-state PL (left) spectra excited at $2.54\mathrm{eV}$ at $4\mathrm{K}$ for oligothiophenes ($8\mathrm{T}$, $12\mathrm{T}$, and $16\mathrm{T}$) diluted in PP. The horizontal solid and dotted lines represent the level of the baselines for the PLE and PL spectra, respectively. The arrows indicate the energy positions at which the PLE spectra were measured. The PLE spectra were measured at the second vibronic peak; $2.09\mathrm{eV}$ for $8\mathrm{T}$, $1.97\mathrm{eV}$ for $12\mathrm{T}$, and $1.93\mathrm{eV}$ for $16\mathrm{T}$.

Figure 4

A plot containing the 0-0 peaks of the absorption and PL spectra (left) and Huang-Rhys factors $S$ (right) at $4\mathrm{K}$ for the $8\mathrm{T}$, $12\mathrm{T}$, and $16\mathrm{T}$ versus the reciprocal of ring number $m$. The straight lines represent the linear fits of the 0-0 peak data set.

Figure 5

The PL time profiles of (i) $16\mathrm{T}$ at $4\mathrm{K}$, (ii) $16\mathrm{T}$ at $298\mathrm{K}$, (iii) RR-P3OT at $4\mathrm{K}$, and (iv) RRa P3OT at $4\mathrm{K}$, diluted in PP.

Figure 6

The time-resolved PL spectra measured at $4\mathrm{K}$ for RR P3OT, RRa P3OT, and $16\mathrm{T}$ diluted in PP. The spectra were obtained by integrating signals over given time width. From the top, the integrated times are $0\pm 50\mathrm{ps}$, $100\pm 20\mathrm{ps}$, $250\pm 40\mathrm{ps}$, $600\pm 50\mathrm{ps}$, and $1500\pm 100\mathrm{ps}$. The average time in each spectrum is indicated in the figures. The spectra were scaled as to give nearly the same PL intensity. The dotted lines in the figures indicate the PL peak position in the steady-state PL spectrum.

Figure 7

Plots of the average spectral shift of the spectra shown in Fig. 6 based on the determination of the spectral first moment. The spectral shift is defined with a deviation from the moment in the spectrum at $1500\pm 100\mathrm{ps}$ that has a peak near to that of each steady-state PL spectrum.

Figure 8

Examples of the results of the best-fit Franck-Condon analysis (dotted lines) of the PL spectra (solid lines) measured at $4\mathrm{K}$ for $16\mathrm{T}$ and RR P3OT. For clarity, the dotted lines are offset from the spectral fit.

Figure 9

A schematic description of the energy diagram illustrating the absorption and PL transitions. The notion of the configuration coordinate with a harmonic-oscillator type vibrational mode is assumed in the layout of this diagram. The horizontal axis shows a displacement of the equilibrium separation of the primary $\mathrm{C}\mathrm{C}$ stretching vibrational mode. The axis from the front to the back of the figure represents the degree of ring-torsion motion. The potential energy curves are drawn with the broken and the straight lines, respectively, for the states before and after the change in the ring-torsion angle. Intrachain migration is assumed to be present. $k_{\mathrm{tor}}$ and $k_{\mathrm{mig}}$ represent the processes of the molecular distortion (torsional change) and intrachain exciton migration, respectively. $\Delta Q_{\mathrm{ab}}$ and $\Delta Q_{\mathrm{PL}}$ represent the displacements of the configuration on the absorption and PL transitions, respectively. $\Delta E_{\mathrm{t},\mathrm{ex}}$ and $\Delta E_{\mathrm{t},\mathrm{G}}$ represent the energy changes induced by the molecular distortion in the excited and ground states, respectively.