Photoinduced IR absorption

It is knownis that small metallic particle effects arising from random depolarization factors in highly anisotropic metallic grains in ceramic samples can lead to artifacts in the infrared reflection (and absorption) spectrum. Since the samples used in the photoinduced absorption experiments are insulating micron sized crystallites embedded in an insulating medium, such small particle effects will not affect either the infrared spectrum or the photoinduced IR absorption spectrum where the measured AT/T is of order i0-3. 

Figure 8.27. Photoinduced IR absorption spectra of P30T and P3OT/C60 (5%) at 80 K obtained by pumping with an argon ion laser at 2.41 eV, 50 mW/cm. (Reproduced by permission of The American Physical Society from ref 78.)...

Fig. 7. UV/visible/near-ir absorption spectra of emeraldine base (top) in solutions of NMP near-steady-state photoinduced-absorption spectra of a thin film of EB on quartz substrate at 15 K for E pump=2.54 eV, pump intensity of 250 mW/cm2, and chopper frequency of 21 Hz (middle) and long time PA spectra of EB in KBr at 80 K for Epump=2.54 eV (bottom) (from Ref. 24).

From electrochemical studies it is known that an irreversible oxidation of C6o occurs at +1.76 V vs SCE in benzonitrile [165]. One of the first methods to generate the radical cation by radiation was performed by "/-irradiation of C6o in glass matrix at 77 K [166], A new absorption in the near IR at 980 nm was assigned to the C 6o (Fig. 19) [19]. Despite the sufficiently high reduction potential of +2.0 V of dicyanoanthracene, first attempts to generate the radical cation by photoinduced electron transfer were unsuccessful [Eq. (5)] [19]. One reason may be a fast back-electron transfer that competes with ionic dissociation in benzonitrile [19]. 

As shown in Fig. 25, the rise of absorption spectra at the visible region due to radical formation and at the near-IR region due to the CR band was observed in less than 1 ps upon a fs laser excitation at 400 nm. These results indicated that the dimer radical cations were formed immediately after the photoinduced electron transfer reaction. The CR band at 900 nm in NS+TPB- decayed single exponentially (t = 3.3 ps) [68], The transient absorption at 580 nm showed... 

To determine the photoinduced structure, the orange crystals that were formed after irradiation of benzene solutions of 6-phenoxy-5,12-naphthacenequinone were used.46 It turned out that the elemental composition of the photoinduced form coincided with the ana-quinone composition. In addition, the interaction of 6-phenoxy-5,12-naphthacenequinone with ammonia and aniline in benzene resulted in the formation of compounds that were identified as derivatives of ana-naphthace-nequinone by the counter synthesis. The ana structure of the photoinduced form was supported by the similarity of the absorption spectrum of the ana form to the spectra of unsubstituted and 5-bromo derivatives of 6,12-naphthacenequinone46 as well as by the analysis of the IR spectra of initial and photoinduced forms of 6-phenoxy-5,12-naphthacenequinone. 4... 

The structural similarity of derivatives of phenoxynaphthacenequinone and phenoxyphthaloylpyrene and the photoinduced changes of absorption (Table 7.12) and IR spectra suggest an identical mechanism for photochromic transformations. 

It appears that no absorption measurements of (S,Cu) in the near IR has been reported, but absorption measurements of the isoelectronic donor centres associated with Sa and Sb have been performed at lower energies under continuous photoexcitation with a Nd-YAG laser operated at 1.06 or 1.32pm (1.17 or 0.939eV) by Beckett et al. [18]. At LHeT, the creation in the triplet state is predominant and the ground state for the EM spectra is therefore, the triplet states Sa° and Sb°. The photoinduced spectrum so obtained is displayed in Fig. 6.39. 

Figure 10. Influence of counterion valence (X , n = i, 2) on the time dependence offemtosecoridy photoinduced, electron-transfer trajectories in aqueous ionic solutions (X , nCl, X = Na , Mg ). The upper part represents the absorption signal rise time at 1.77 eV following the femtosecond UV excitation of aqueous Cl (2X4 eV). The difference in the signal rise times of Na and Mg is due to the balance between two electronic transitions e iR e hyd fCl e X ...

Schaniel and coworkers studied the properties of the photoinduced ON and r/ -NO isomers in Na2[Fe(CN)5N0]-2H20 embedded in meso-pores of sihca xerogels by X-ray diffraction, steady-state low-temperature absorption, nanosecond transient absorption spectroscopy, and IR spectroscopy (34). They determined that the electronic structures and activation energies of these / -ON and tf -NO isomers were not dependent on the particle size (statistically distributed molecules or nanoparticles) and as such the isomers were essentially quasi-free inside the pores of the gel (34). 

Two Pt azido complexes, czs -Pt (PPh3)2(N3)2 and Pt (dppp)(N3)2 (dppp = 1,3-bis(diphenyl-phosphino)propane) have been studied with a combination of flash photolysis and ps-TRIR. " UV excitation was found to cause photoinduced electron transfer and loss of an azidyl radical ( N3) to form Pt (PPh3)2(N3) and Pt (dppp)(N3), identified by their IR azido absorptions at ca. 2050 cm For the PPh3 complex, photoisomerization to tra w-Pt (PPh3)2(N3)2 was a competitive pathway. Both Pt complexes were found to decay via intramolecular electron transfer to the coordinatively unsaturated products, Pt°(PPh3)2 and Pt°(dppp). 

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