Transient bleach

Transient bleaching of low spin species followed by thermal relaxation t s 46-192 ns at 255-205 K... 

Spiro-oxazine (NOSH) photo-induced ring closure reactions were first described by Bohne et al., who used two-laser two-color excitation in the UV and visible regions [71,72]. In this work, they found that photoexciting the merocyanine in cyclohexane leads to a bleached product which recovers quantitatively to the merocyanine form over 30 xsec. The transient bleach state had an absorbance... 

In summary, the merocyanine lifetime depends on its particular isomeric form and this, itself, is affected by the solvent and substituents. The longest merocyanine isomer singlet-state lifetimes reported are 4 nsec and the shorter ones are tens of picoseconds. After excitation of the merocyanine, a bleached state can form, which leads to the closed form in polar solvents, but the full ring closure is blocked in nonpolar media and the bleached state recovers to its original state. The bleached state is possibly an isomer cis about the central p-methine bond. Scheme 17 describes the transient bleaching of the merocyanine state. 

Transient bleaching and recovery rates of CdS excitonic absorption, determined by picosecond pump-probe spectroscopy, depended on [H20]/[A0T] ratio and micellar surface. Fluorescence spectra and lifetimes depended on [Cd2+]/[S2 ] ratios... 

The 3.5- and 8-ntn nanoparticles show well-resolved peaks at 362 and 473 nm, respectively, as well as other features at higher energies. The 4.5-nm particles show a well-resolved peak at 400 nm and a shoulder at 450 nm. It is tempting to assume that in each case, the lowest energy absorption corresponds to the lowest allowed transition (the A exciton) in bulk M0S2. Polarization spectroscopy can be used to determine if this is the case. The lowest allowed transitions in bulk material, the A and B excitons, are polarized perpendicular to the crystallographic c axis. If the lowest allowed transition correlates to the A exciton, then it would be expected to also be a planar (xy polarized) oscillator. However, tire results of polarization studies reveal that the actual situation is more complicated. A combination of time-resolved polarized emission and one-color time-resolved polarized absorption (transient bleach) studies facillitate assignment of the polarizations of the observed nanoparticle transitions. The 3.5-nm particles are emissive and the polarization of the several of the lowest transitions may be determined... 

A. Photon-Mode Superresolution by Transient Bleaching of Phthalocyanines... 

Excitation of phthalocyanines resulted in transient bleaching at the 620 to 600-nm region and transient absorption with a peak at about 530 nm as shown... 

Upon excitation of both the C6o-oFL dyads 5a-c and C6o-oFL-C60 triads 6a-c transient features develop that agree with these seen in the oFL references, 4a-c, namely transient bleaching below 450 nm and transient maxima around 700 nm (Fig. 8.12). Thus, such observations attest—in close agreement with the ground... 

Kamat, P. V. Ebbesen, T. W. Dimitrijevic, N. M. Nozik, A. J. Photoelectrochemistry in semiconductor particulate systems. Part 12. Primary photochemical events in CdS semiconductor colloids as probed by picosecond laser flash photolysis, transient bleaching, Chem. Phys. Lett. 1989, 157, 384. 

Intensity dependent bleaching of poly(methylmethacrylate) doped with acridine or diphenyl butadiene and an acridine-containing methacrylate copolymer, has been observed using excimer lasers (XeF, KrF). Threshold-like irreversible bleaching was seen with the copolymer more gradual transient bleaching with the mixtures. The implications for submicron microlithography are discussed. 

In addition, for reasons that we cannot explain, the curve of Figure 3 is sharper than the theory of reference 13 would predict, and is sharper than would be obtained from a linear photobleaching system of the same initial optical density (28). Thus there appear to be significant advantages in this type of nonlinear transient bleaching, although more work is needed to understand it. 

We observed (Fig. 3) that in the absence of surface quinones, the relaxation of QD absorption bleach band (A,ex=528 nm) corresponding to 1 lSelSh> state reflects the trapping excited charge carriers at the surface. The picosecond kinetic analysis shows that in the presence of tCl-l,2-BQ the short time component of the transient bleach formation at A.reg=530 nm is additionally shortened from 93 ps down to 27 ps. It reflects the appearance of the additional non-radiative relaxation channel for electrons from QD conduction band to the lowest unoccupied molecular orbitals of quinone (LUMO). These results are in an agreement with calculations presented in [4]. We believe that long component (r> 3 ns) may reflect the electron shuttling from LUMO of the quinone to the QD valence band. 

The pressure effects on spin relaxation dynamics for these iron(II) complexes have been examined using laser flash photolysis techniques. For Fe(pyim) the two spin states are in equilibrium with a K = 0.56 in 298 K acetone with a partial molar volume difference AV = +8.1 cm mol [34]. Photoexcitation (2ex = 532 nm) leads to transient bleaching of the low spin isomer s MLCT bands followed by first order relaxation to the original spectrum with a 45-ns lifetime. Transient bleaching and subsequent return of the MLCT absorption was attributed to formation of the HS isomer and subsequent spin relaxation. The pressure dependence of the relaxation lifetimes was used to determine the activation volumes of the spin relaxation rates for a variety of FeL in different solvents. It was found that AV j fell into a remarkably narrow range of values (-5.5 + 1 cm mol ) and it was concluded that the spin crossover for these species follows a common mechanism via a transition state located midway between the high and low spin states [33]. 

An extremely fast relaxation of the 625 nm charge-transfer absorption band of the blue copper protein, azumin, has been studied by subpicosecond light pulses.The blue protein solution is transiently bleached but the absorption reversibly reappears with a time constant of 1.6 0.2 ps) owing to reverse charge-transfer. A fast transient (<0.5ps) is attributed to excited-state vibrational relaxation/internal conversion. Hydrophobic properties of phtyochrome have been examined using 8-anilinonaphthalene-l-sulphonate as probe. ... 

The differential absorption AOD of pi sample between the pump (/l=1.08 pm) and the probe Z=13 pm) beams is presented in Fig. 2. It is defined as AOT>=-lg(7y7 o), where To and T are the transmission of the probe beam without and with pump beam present, respectively. All the samples demonstrate the transient bleaching with a two-component decay. Moreover, time constants of relaxation are QD size dependent. The experimental data are fitted within the framework of the two-exponential decay model ... 

A previous flash photolysis study (12) of aqueous Ru(NH3)5py reported transient bleaching of the MLCT band followed by relatively slow decay to substrate and aquation products at a rate, in part, inversely proportional to [H ]. Since the transient bleaching was too long-lived to represent the behavior of an excited state, this observation was interpreted... 

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