Neutral photoexcitations

Finally, we would like to point that one of the easiest ways to influence the electronic properties of conjugated oligomers and polymers is to vary the chain 

In summary, all the transitions expected for the neutral states of a model system for conjugated polymers, the m-LPPP, were observed and described and all of these transitions also show clearly resolved vibronic replicas due to coupling to vibronic modes of the backbone. 

In order to study the charged photoexcitations in conjugated materials in detail their contribution to charge transport can be measured. One possible experiment is to measure thermally stimulated currents (TSC). Next, we will compare the results of the TSC-experiments, which are sensitive to mobile thermally released charges trapped after photoexcitation, to the temperature dependence of the PIA signal (see Fig. 9-17) which is also due to charged states as discussed previously. 

we would like to address the question how sample quality influences the observed results. Synthesis and sample treatment influence the electronic properties of conjugated materials in a defined way [23]. We have already shown [31] that the shape and intensity of photoinduced absorption spectra in different representatives of the LPPPs may vary (see Fig. 9-16), indicating at least different trap densities but also different electronic properties of these traps, depending on the synthesis and subsequent treatment of the polymers. However, the electronic properties for this class of polymers can be imderstood in terms of effective conjugation length [23-25] charge transfer by photoexcitation or redox reactions [31] and also photo-oxidation upon intense visible irradiation under the influence of oxygen [23]. Therefore, by optical spectroscopy (absorption, photoluminescence, or photoinduced absorption) we can assess the quality of a sample. 

For the characterization of the trap levels we applied the thermally stimulated current technique following the initial rise method [26] which was successfully applied to other conjugated polymers [27]. The device consisted of a sapphire substrate with an interdigital gold electrode structure on top of which a 200 nm thick polymer film was drop-cast. The gold electrodes had an overall length of 

---

MEH-PPV and P3MBET, were used. As a measure of the efficiency of the photo-induced charge transfer, the degree of luminescence quenching and the ratio of the charged photoexcitation bands to the neutral photoexcitation bands were taken. These two numbers are plotted in Figure 15-15 versus the electrochemical reduction potential. A maximum in the photoinduced electron transfer was determined for Cbo. 

In this chapter we have reviewed the main cw techniques used to study photoexcitations in undoped conducting polymers. In particular, we have given an in-depth description, both experimental and theoretical, of the PM and ADMR techniques. We have used these techniques to study photoexcitations in two systems of conducting polymers—(r Aj, -(CH), with its unique twofold degeneracy and polythiophene, a representative of the NDGS polymers. We have identified long-lived charged and neutral photoexcitations in both systems and verified their spin states. We have also studied the relations between the various photoexcitations and the PL emission in these two polymer systems. 

X. Wei, B. C. Hess, Z. V. Vardeny, and F. Wudl, Studies of photoexcited states in polyacetylene and poly(paraphenylenevinylene) by absorption detected magnetic resonance the case of neutral photoexcitations, Phys. Rev. Lett. 68 666 (1992). 

The PIA band at 1.35 eV in the pure MEH-PPV shows a strong response at the (forbidden) half field resonance of spin = 1, indicating triplet character for this PIA band and dominance of the neutral photoexcitations in MEH-PPV (Figure 8.13). There is a small, residual spin = 1/2 response even within the pure... 

The results of photoinduced absorption in polymer solutions and oligothiophene films and solutions (see Sections 3 and 4) show a profound effect of the surrounding medium on the character of the photo-excited species. Based on these results, we inferred that for pristine conjugated polymers embedded in a nonpolar medium, neutral photoexcitations are favored, whereas in polar media, charged and neutral photoexcitations co-exist. In conjugated polymer/C6o mixtures, embedded in a non-polar medium, energy transfer and triplet sensitization are favored, whereas in polar media, photoinduced electron transfer phenomena clearly dominate. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

It lias also been suggested that photoexcited benzoyl peroxide is somewhat more susceptible to induced decomposition processes involving electron transfer than the ground state molecule. Rosenthal et c//.15 reported on redox reactions with certain salts (including benzoate ion) and neutral molecules (e.g. alcohols). 

Figure 4. Wavepacket dynamics of photoexcitation, shown as snapshots of the density (wavepacket amplitude squared) at various times. The model is a 2D model based on a single, uncoupled, state of the butatriene redical cation. The initial structure represents the neutral ground-state vibronic wave function vertically excited onto the A state of the radical cation.

Fig. 2. Evolution of twist angle around the P-bond (grey) and I-bond (black) after photoexcitation of the neutral form of GFP chromophore in the gas phase (left panel) and solvated by 150 water molecules (right panel). Solid lines are population-weighted averages over die trajectory basis functions. Dashed lines represent the twist angles for the individual trajectory basis functions. The sense of rotation for the two torsions is defined such that HT motion corresponds to both angles moving towards more negative values.

When pyridoxamine with a dipolar ionic ring structure (Fig. 14-9) and an absorption peak at 30,700 cm-1 (326 ran) is irradiated, fluorescence emission is observed at 25,000 cm 1 (400 ran). When basic pyridoxamine with an anionic ring structure and an absorption peak at 32,500 cm 1 (308 nm) is irradiated, fluorescence is observed at 27,000 cm-1 (370 nm), again shifted 5500 cm 1 from the absorption peak. However, when the same molecule is irradiated in acidic solution, where the absorption peak is at 34,000 cm 1 (294 nm), the luminescent emission at 25,000 cm 1 is the same as from the neutral dipolar ionic form and abnormally far shifted (9000 cm ) from the 34,000 cm-1 absorption peak.185186 The phenomenon, which is observed for most phenols, results from rapid dissociation of a proton from the phenolic group in the photoexcited state. A phenolic group is more acidic in the excited state than in the ground state, and the excited pyridoxamine cation in acid solution is rapidly converted to a dipolar ion. 

---