Electric-field-induced singlet and triplet exciton quenching in films of the conjugated polymer polyspirobifluorene

An experimental investigation into the low-temperature electric-field-induced quenching of the transient triplet absorption and the prompt and delayed fluorescence of the conjugated polymer polyspirobifluorene is presented. A maximal instantaneous triplet exciton quenching of about 25% is observed for an electric field of $2.5\times {10}^6\mathrm{V}∕\mathrm{cm}$. The fluorescence intensity under such conditions is quenched by 97%, which, to our knowledge, is the highest value ever reported for a conjugated polymer. A comparison of the absolute singlet and triplet exciton quenching yields a singlet exciton binding energy of approximately $0.38\mathrm{eV}$. The delayed fluorescence in the above polymer is known to be caused by bimolecular triplet annihilation, which has been further substantiated using electric-field quenching experiments. Further, earlier experiments, which seem to verify the geminate pair origin for the delayed fluorescence of the laddertype polymer poly(para-phenylene) (MeLPPP), are discussed and re-evaluated using the above polyfluorene derivative.

Figure 1

These graphs show the transient triplet absorption (a), phosphorescence (b), prompt fluorescence (c), and absorption spectra (d) of the polyspirobifluorene under investigation, see inset for chemical structure. The dashed lines indicate the excitation and the transient absorption probe energies, respectively.

Figure 2

Compendium of time-resolved transient triplet absorption graphs where delayed (with respect to the end of the excitation pulse) electric fields were applied as indicated. Photoexcitation was provided from $-600\mu \mathrm{s}$ until time zero.

Figure 3

Comparison of the singlet (probed by fluorescence) and triplet exciton (change in the transient absorption at the end of a $100\mu \mathrm{s}$ delayed-applied $10\mu \mathrm{s}$ pulse) quenching on electric-field strength. Error bars are indicated in both graphs and the quenching is defined in the text.

Figure 4

Double logarithmical presentation of the spectrally integrated delayed fluorescence versus time with superimposed electric-field pulses as indicated. The light integration time for the luminescence was $1\mu \mathrm{s}$ and the excitation was provided by a $600\mu \mathrm{s}$ optical pulse prior to time zero. The straight solid line is a guideline for an algebraic decay with slope $-1$. The inset magnifies the time around the switch on of the electric field in a linear fashion and was measured using $100\mathrm{ns}$ detection width.

Figure 5

Dependency of the quenching of the delayed fluorescence on field strength, during and after application of a delayed electric pulse under conditions as in Fig. 4. For the purpose of comparison, the prompt fluorescence quenching from Fig. 3 is included as well. Also shown (solid circles) is the delayed fluorescence intensity from $20.2\text{to}20.3\mu \mathrm{s}$ after excitation as a function of electric field.