Transient photoinduced absorption

The poly(542) pristine film, representative of the more crystalline polymers, exhibited a broader transient photoinduced absorption band around 900-1000 nm. This transient optical density decayed rapidly on the... 

Fig. VC-3) [181]. Both the magnitude and the lifetime of transient PC increase by addition of a few percent C o to the pure polymer. Transient photoinduced absorption studies demonstrated rapid (sub-picosecond) photo-induced electron transfer from the polymer to C q thereby minimizing early time recombination and enhancing the quantum yield for mobile carrier generation [175,176]. If neutral excitons were the primary excitations, the observed charge transfer would be thermally activated this is not observed.

K. The observation that the density of band tail and mid-gap states is very sensitive to the microstructure of the film [116] may explain why the mentioned peak is not observed in every sample. Moreover, transient photoinduced absorption and photoconductance have not been measured in the same film, and it is not certain that the carrier lifetime or photoexcited carrier concentration does not exhibit a peak at T approx. 250 K in some samples. 

The decay of the transient photoinduced absorption signal in oxygenated films has been fitted using a stretched exponential temporal decay fit) excitation wavelength of 605 nm (2.0 eV). This type of non-exponential carrier relaxation is frequently encountered in systems exhibiting dispersive transport, such as hydrogenated amorphous silicon [158,159]. The temperature dependence of the parameters and t... 

The technique of transient or quasi steady slate photoinduced absorption (PIA allows the population of excited states and also probing the optical transitions... 

Figure 4. Transients of DMABI films, (a) Up-conversion PL intensity decays of two self-trapped exciton states of DMABI at excitation energy 3.14 eV.(b) Time-dependence of photoinduced absorption monitored at 1.93 eV for different energy densities. Excitation at energy 2.15 eV. Reprinted with permission from Ref. [16].

The transient triplet absorption for the common polyfluorene PF2/6 is shown in Fig. 9, the spectra was measured by quasi-cw photoinduced absorption. This lock-in technique allows the absorption of long-lived states to be measured with high sensitivity. It is possible to estimate the lifetime of the state using the quasi-cw experiment [46] however, more accurate methods in-... 

Note that charge carriers can be extracted even after several milliseconds at lower temperatures, which is consistent with the frequently observed millisecond lifetime often observed in transient absorption and photoinduced absorption studies at lower temperatures. 

The dynamics of the electron spin polarization of the quinone radical-anion generated from photoinduced electron transfer between duroqinone (DQ) and A/, A/ -dimethylaniline (DMA) as well as its p-halogen substituents were investigated by using transient optical absorption and FT EPR spectroscopyThe radical yield decreases as the atomic number of the halogen in the donor rises. The intrinsic enhancement factors of net absorptive polarization due to the... 

While DFWM and other NLO measurement techniques yield information on response and decay times as well, as some of the data cited above indicate, additional data are obtainable from such techniques as pump-and-probe ps or fs transient absorption spectroscopy. Some of the first such data were collected by the Shank group at the erstwhile Bell Labs [524]. Fig. 12-31 shows their data for a 2.0 eV pump for cis-and trans-P(Ac), whilst Fig. 12-32 shows photoinduced absorption, at 1.4 eV, of P(cis-Ac) on a ps time scale. In the latter case, the rise time is less than 150 fs, the... 

Another important result from the photophysical studies on conjugated polymer/fullerene composites is the complete absence of any photoinduced interaction between fullerenes and either polyacetylene (PA) [54] or the polydiacetylenes (PDAs) [62], respectively. The results of absorption and emission spectroscopy, subpicosecond and millisecond photoinduced absorption spetroscopy, and picosecond transient and near steady state photoconductivity show that in contrast to the high quantum efficiency photoinduced electron transfer definitively established in the PPV and the P3AT composites with Cgo, photoinduced electron transfer from PA and PDAs onto Ceo is inhibited [62]. 

An important question is whether the decay of the photocurrent as seen in Figure 9.27 is due to carrier recombination or changes in the mobility. Comparison of the transient photocurrent and photoinduced absorption decays reveals similar relaxation times in these two types of measurement [32,11,151]. Since the photo-induced absorption signal is sensitive only to the... 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Excitation of an aqueous solution of poly(A/St/Phen) with a 355-nm, 22-ps laser pulse in the presence of MV2+ generated a transient absorption band peaking at about 600 nm due to MV + [120]. As shown in Fig. 16, the buildup of the 600-nm band completes immediately after the pulse excitation, indicating that the photoinduced ET from the singlet-excited Phen residue ( Phen ) to MV2 + occurs on a time scale comparable to or shorter than the duration of the laser pulse (ca. 22 ps) [120], Figure 16 also shows that a fast decay of the absorbance at 600 nm owing to the back ET from MV + to the Phen cation radical (Phen+ )... 

Transient absorption spectra of the CeoN cluster (C6oN" ) -MePH system following laser excitation at 355 nm indicate that the photoinduced intermolecular electron-transfer from the triplet excited state of PH to the QqN cluster (CfioN) occurs as shown in Figure 15.9a. 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

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