Spectroscopy photoinduced absorption

Cappel UB, Gibson EA, Hagfeldt A, Boschloo G (2009) Dye regeneration by spiro-MeOTAD in solid state dye-sensitized solar cells studied by photoinduced absorption spectroscopy and spectroelectrochemistry. J Phys Chem C 113 6275-6281... 

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 ... 

In good electron acceptor solvents, such as carbon tetrachloride and chloroform, the photodegradation of carotenoids is significantly increased as compared to other solvents (Christophersen et al. 1991, Mortensen and Skibsted 1999), because of a direct photoinduced electron-transfer reaction from the excited singlet state of the carotenoids to the solvent, as determined by transient absorption spectroscopy (Jeevarajan et al. 1996, Mortensen and Skibsted 1996,1997a,b, El-Agamey et al. 2005), Equation 12.2 ... 

The photophysics of fluorophores undergoing photoinduced charge transfer and/or internal rotation(s) is often complex. Time-resolved fluorescence experiments, transient absorption spectroscopy measurements, quantum chemical calculations, and comparison with model molecules are helpful in understanding their complex photophysical behavior. 

Photoinduced electron injection is by no means a new development. This process has already been applied in areas such as silver halide photography. In this discussion, only sensitized TiC>2 surfaces will be considered. Many experiments have shown that the charge injection into the semiconductor surface is very fast. In order to study these processes, fast spectroscopic techniques are preferred. Whether or not charge injection takes place can be studied conveniently on the nanosecond time-scale by using transient absorption spectroscopy. However, to address the injection process directly, experiments are carried out on the femtosecond time-scale, while recombination and charge separation require the nanosecond to microsecond range. 

The fundamentals of electron transfer from an adsorbed molecule to a solid substrate has been considered by Willig and co-workers [9]. In this study, the photoinduced electron injection from the excited state of [Ru(dcbpy)2(NCS)2] into nanocrystalline TiC>2 was investigated both in high vacuum and in methanol containing 0.3 M LiCl. The central issue addressed in this investigation was to determine the time-scale on which the photoinduced electron injection from the molecular component to the solid substrate occurs. Transient absorption spectroscopy was used to follow this charge separation. The measurement was based on... 

In the first section, steady-state spectroscopy is used to determine the stoichiometry and association constants of molecular ensembles, emphasize the changes due to light irradiation and provide information on the existence of photoinduced processes. Investigation of the dynamics of photoinduced processes, i.e. the determination of the rate constants for these processes, is best done with time-resolved techniques aiming at determining the temporal evolution of absorbance or fluorescence intensity (or anisotropy). The principles of these techniques (pulse fluorometry, phase-modulation fluorometry, transient absorption spectroscopy) will be described, and in each case pertinent examples of applications in the flelds of supramolecular photophysics and photochemistry will be presented. 

The short component reveals a quenching due to photoinduced electron transfer from the singlet ZnNc to Qolm, as proven by transient absorption spectroscopy. From the above values of decay times, the rate constant and quantum yield for electron transfer can be calculated according to the following relations ... 

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

Fig. 7.16. Characterization of the photoinduced charge-separated state of a porphyrin-fullerene diads with flexible polyether linkers by transient absorption spectroscopy. The differential absorption spectrum is given for a 50 ns delay after excitation at 532 nm with a 10 ns laser. (Reprinted with permission from ref. [41]).

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

First, 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. 

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