Transient absorption measurements

So, to summarize, the hole transfer rate between Py and Ptz was determined using the transient absorption measurements of Py + and Ptz +, and we have shown that the hole transfer rate from Py + to Ptz depends on the distance and sequence between Ptz and Py. In other words, the hole transfer rate in DNA is modulated by designing the sequence of DNA and conjugated molecules. It is suggested that DNA may be utilized as a molecular wire by introducing several organic molecules at the appropriate site in DNA. 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

Aramaki and Atkinson were also active in work on the spiro-oxazines [65]. They noted that for NOSH in many polar and nonpolar solvents the picosecond time-resolved resonance Raman spectra simply built up over 50 psec with no shape evolution. The same finding was concluded from transient absorption measurements over the same time scale. The spectra/absorbances were then constant for 1.5 nsec. These authors suggest that only two isomers can be expected to contribute to the merocyanine spectra because those trans about the y-methene bridge bond attached to the naphthalene ring are sterically crowded due to short interproton distances. There was no evidence for the X transient in their study however, the 50-psec convoluted pulse profile may be expected to mask this sortlifetime species even if it were present. 

Excitation wavelength dependence of the Raman lines presents another complication that is not a problem for transient absorption measurements. For example, in comparing spectra taken at two different excitation wavelengths, one must consider not only which and in what proportion each molecular species is pumped, but also keep in mind what differences in relative intensity of the Raman lines result from excitation at the two exciting wavelengths. The complication can usually be overcome, especially when the spectra of the two species are well resolved. 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

Nanosecond transient absorption measurements provided a further indication of efficient mediator transport within the nanopores of the gel, showing basically no difference in dye regeneration using the liquid iodide/iodine precursor and the quasi-solid-state polymeric electrolyte. 

We performed transient absorption measurements on BP(OH>2 with a spectrometer based on two noncollinearly phase matched optical parametric amplifiers (NOPAs) pumped by an homebuilt regenerative Ti Sapphire laser system or a CPA 2001 (Clark-MXR) [1,7]. The tunable UV pump pulses are generated by frequency doubling the output of one of the NOPAs. The other NOPA provides the visible probe pulses. The cross correlation between pump and probe pulses has a typical width (FWHM) of 40 fs. The sample is a cyclohexane solution of BP(OH)2 pumped through a flow cell with a 120 pm thick channel. 

The present results indicate that DMABN-F4 emits a strongly red shifted fluorescence with a mostly forbidden character. In contrast to DMABN, the red-shifted fluorescence is also observed in non-polar solvents. Transient absorption measurements give evidence that the CT state is formed in less than 1 ps, in both acetonitrile and hexane. In acetonitrile, this is significantly faster (> 6 times) than for DMABN. 

Fig. 1. Top Reference spectra for femtosecond transient absorption measurements S-S abs. in solution (thin solid lines), oxidized dye (dye+) abs. in solution (thick solid line), fluorescence for solution (dotted line), steady-state absorption ofNKX-2311/ZnO (dotted-dashed line), and absorption of electrons in the conduction band (dashed line). Bottom Transient absorption spectra of NKX-23ll/ZnO in the spectral range between 600 and 1350 nm at the 2 (thick solid line), 10 (dotted line), 100 ps (thin solid line) delay times after excitation at 540 nm by the femtosecond pulse with the intensity of about 10 pJ.

Since the initial reports of the C-P-Q triads, a number of other molecules of the D-D -A or D -D-A types have been described. Triad 12, prepared by Wasielewski and coworkers, is a relative of the C-P-Q series in which the secondary donor is an aniline derivative (D), rather than a carotenoid [63]. The bicyclic bridges were introduced in order to add rigidity to the system. The fluorescence lifetime of the porphyrin moiety of 12 was found to be <30ps. This result is consistent with rapid electron transfer to the quinone to yield D-P+-QT. This result was confirmed by transient absorption measurements. The absorption results also revealed that this intermediate charge separated state decays with a rate constant of 1.4 x 1010 s-1 to a final charge separated state D+-P-Qr. Thus, the decay pathways are similar to those shown in Fig. 3 for the C-P-Q triads. This final state has a lifetime of 2.45 ps in butyronitrile (which is similar to that found for 4 in acetonitrile) [44], and is formed with a quantum yield of about 0.71. Thus, the efficiency of the transfer analogous to step 4 in Fig. 3 for this molecule is also about 0.71. 

Conclusive evidence for the nature of the energy-transfer processes came from transient absorption measurements with 8 in the presence of variable concentrations of 7. We have followed the time evolution of the characteristic singlet excited state features of 8 upon 420 nm excitation to identify the spectral features of the resulting photoproducts and to determine the absolute rate constants for the underlying deactivation processes. As reference, the transient absorption features... 

Thus, from fluorescence lifetime and transient absorption measurements we gathered the electron-transfer rate constants, i.e. both for charge-separation and for charge-recombination. Next, we plotted these rate constants as a function of donor-acceptor distance. From the resulting linear dependence (Fig. 9.26) it is possible to determine the attenuation factors p for the presented donor-acceptor... 

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