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COMMENTS Comment on “Ultrafast Dynamics of Polarons in Conductive Polyaniline: Comparison of Primary and Secondary Doped Forms” Alexey A. Melnikov* and Sergey V. Chekalin Institute of Spectroscopy RAS, 142190, Troitsk, Moscow Region, Russia ReceiVed: March 27, 2009; ReVised Manuscript ReceiVed: July 30, 2009 In a recent paper,1 Kim, Park, and Scherer reported the results of femtosecond pump-probe investigations of primary and secondary doped polyaniline (PD and SD PANI). They measured changes of transmission at selected wavelengths from 650 to 1025 nm after illumination of the sample by a 30 fs laser pulse with a central wavelength of 800 nm. The main features of photoinduced spectra were interpreted as a result of excitation of valence electrons into the polaron band, their subsequent cooling and, finally, relaxation via an intermediate state. Recently, we have performed analogous experiments on PD PANI using femtosecond pump pulses at two wavelengthss400 and 800 nm. Inside the 650-1025 nm interval photoinduced spectra were similar to those measured in ref 1. However, due to the broader spectral range in our measurements (from 400 to 1050 nm), it became possible to observe new features. A detailed description of our results will be published elsewhere. In this comment, we discuss several arguable statements in ref 1 and present some new experimental facts that are inconsistent with the model of PD PANI charge carrier dynamics developed by the authors of ref 1. (i) The main features of the photoinduced response of the sample according to ref 1 are PA1, PA2, and PA3. PA1 is photoinduced absorption near time zero in the 900-1025 nm range due to “hot” charge carriers. As time delay varies from -100 to +100 fs, the detected signal changes sign several times. In this case, pump and probe pulses overlap in the time domain, and the resulting photoinduced response is usually referred to as a coherent artifact. Its temporal shape strongly depends on the parameters (both spectral and temporal) of pump and probe pulses, optical properties of the sample, and photodetector spectral characteristics. It is very important to take into account that the change of ∆OD value near time zero (when pump and probe pulse overlap) does not necessarily reflect the change of population of energy levels. Indeed, a typical alternating temporal shape recorded in ref 1 can be observed even if a central wavelength of the pump pulse falls inside the transparency range of the sample. Then, all transitions to higher lying energy states are virtual. Therefore, a proper treatment of the signal near time zero must include an analysis of the way the electrical field of the pump pulse affects energy levels of PANI molecules. A similar approach is used in the theory of the alternating current (ac) Stark effect and third-order nonlinear interactions. (ii) As for PA2 and PA3, the authors of ref 1 ascribe them to the “cooling” of free charge carriers excited by the pump pulse * Corresponding author. E-mail:
[email protected].
Figure 1. Differential spectra of PD PANI obtained with 800 and 400 nm pump pulses at three time delays.
from valence to polaron band. While PA1 reflects the possibility of transitions of “hot” population to higher lying states inside the polaron band induced by the probe pulse, PA2 and PA3 stand for the same process for “cold” charge carriers. This model implies that there exists a well-defined range of states inside the polaron band which are final for transitions involving light quanta of the pump pulse and states in the valence band. In this case, photoinduced spectra must strongly depend on the central wavelength of the pump pulse. Because radiation at 400 nm promotes electrons to higher lying states than pulses at 800 nm, one can expect spectral positions of PA1 and of ground state bleaching to change significantly in this case. Neither was detected in our experiments. Figure 1 shows photoinduced spectra of PD PANI obtained using 40 fs pulses at 800 and 400 nm with excitation fluence of about 1011 W/cm2 (of the same order as in ref 1 according to our estimates). Only minor difference can be detected; the temporal behavior is also identical on all wavelengths used. We would like also to emphasize the presence of a transient absorption feature near 460 nm. It had not been observed in ref 1 due to smaller spectral range and cannot be interpreted by the model proposed there. (iii) As stated in ref 1, PA3 manifests itself as a “dip” in decay traces from the interval 650-700 nm during about 1 ps. In our experiments, we observed the same feature in transients at wavelengths of 600-700 nm and also at 900-1000 nm. An alternative approach that treats this feature as a rise (i.e., delayed bleaching/absorption) allows avoiding the use of an additional component for the interpretation of dynamics. To make the presence of the rise clearer, decay traces at 730 (at the inset) and 960 nm are shown on Figure 2 together with fitted delayed transients. The delay is blurred due to 200 fs temporal resolution of our setup but, nevertheless, is distinguishable, especially if we recall that the duration of the pump pulse is 40 fs. (iv) Besides PA3, one more feature was associated in ref 1 with cooling of charge carrierssthe shift of NIR photoinduced absorption to shorter wavelengths in subpicosecond time. It is
10.1021/jp902776f CCC: $40.75 2009 American Chemical Society Published on Web 09/15/2009
Comments
Figure 2. Decay traces at wavelengths 730 (inset) and 960 nm shown with delayed bleaching/absorption transients (red color) used for fitting. The trace at 840 nm illustrates the temporal overlap of bleaching (in pure form at 815 nm) and delayed absorption.
clearly seen at Figure 1 as a shift of the point at which difference signal changes sign from 880 to 820 nm. Processing the data by a global analysis procedure, we found in contrast to ref 1 that such behavior results from the presence of about 100 nmwide (fwhm) bleaching centered near 815 nm. The decay of this bleaching is not monoexponential with a maximal characteristic time value of 0.7 ps. As this component overlaps (Figure 2) with longer-living signal of the opposite sign (absorption), a shift is observed in differential spectra. So, we found no clear evidence of direct free charge carrier (polaron) generation by a femtosecond pump pulse. On the contrary, it seems reasonable to treat initial photoexcited species as neutral and their subsequent decay as a source of charges. Such a model can account for the delay (or dip) in decay traces
J. Phys. Chem. B, Vol. 113, No. 40, 2009 13455 and several specific spectral features and avoid the use of a band model, whose applicability for PANI is questionable. In brief, we found that the differential spectrum obtained from electroabsorption measurements in the 600-1050 nm range2 is similar to spectra recorded in our pump-probe experiments after 1 ps. Both can be well-approximated by a combination of the first and the second derivatives of the linear spectrum. The derivativelike character of the 460 nm transient absorption feature is even more pronounced. In the case of electroabsorption, the electrical field applied to the sample causes shift and/or splitting of energy levels and induces spectral changes. We propose that analogous transmission changes in pump-probe experiments are due to a local field that is created by charge carriers on polyaniline chains. They are the result of decay (or relaxation) of some neutral excited species with a ground state absorption centered near 815 nm (charge transfer excitons or exciplexes3,4) and thus appear with a delay. Subsequent dynamics (7 ps relaxation) reflects the recombination of charge carriers. References and Notes (1) Kim, J.; Park, S.; Scherer, N. F. J. Phys. Chem. B 2008, 112, 15576. (2) Premvardhana, L.; Peteanua, L. A.; Wang, P.-C.; MacDiarmid, A. G. Synth. Met. 2001, 116, 157. (3) Electronic processes in organic crystals and polymers; Pope, M., Swenberg, C. E., Eds.; Oxford University Press: New York, 1999; p 740. (4) Primary doping of PANI causes a shift of the energy levels (at least of π-π* transition) at 0.4 eV. (Shimano, J. Y.; MacDiarmid, A. G. Synth. Met. 2001, 123, 251). This may lead to the shift of the exciton band position from 633 to 815 nm after primary doping. This is right in the center of the region where bleaching was observed.
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