Transient spectra

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Fig. 20 Deconvolution of the transient spectrum obtained upon the application of a 25-ps laser pulse to a solution of [hexamethylbenzene, NO+] charge-transfer complex showing the Wheland intermediate (430 nm) and the hexamethylbenzene cation radical (495 nm). Courtesy of S.M. Hubig and J.K. Kochi, unpublished results.

Irradiation of a benzene solution of DABA at room temperature with a nitrogen laser (Horn and Schuster, 1982) gives the transient absorption spectrum shown in Fig. 3. This spectrum was recorded 50 ns after irradiation of the diazo-compound and decays over a period of ca 250 ps by a path exhibiting complex kinetic behavior. This transient spectrum is essentially identical with the low temperature optical spectrum described above, and thus is similarly assigned to 3BA. 

The laser flash photolysis (Xex=351 nm) of a TBP/BP-MDI solution in benzene (Figure 7) yields a transient spectra with distinct maximum at 370 nm which can most likely be attributed to a substituted diphenylmethyl radical. (Similar results are obtained in other solvents such as DMF). No detectable transient species were generated above 350 nm by the laser flash photolysis (Xex=351 nm) of the 60/40 mixture of TBP and benzene alone. Results for the TBP/MDI-PU (7.0 X 10 2 g/dL) system in Figure 8 show, as in the case of the model BP-MDI (Figure 7), that the transient spectrum of MDI-PU obtained indirectly through tert-butoxy radicals has a maximum at 370 nm. This provides additional support for assignment of the transient species responsible for the 370 nm absorbance to a diphenylmethyl radical. 

The naphthopyran ring-opening reactions have not been as well studied as they have for the spiropyrans and spiro-oxazines. Aubard et al. [75] recently reported that in acetonitrile and hexane, irradiation of CHRl resulted in a broad transient spectrum after 0.8 psec, having three maxima at 360-370 nm, 500 nm, and 650 nm. At 1.8 psec, a well-defined band forms at 425 nm. From 10 to 100 psec, there is little further evolution except for a continued growth in the peak at 425 nm of about 15%. There is also a decrease in the overall bandwidth. The mechanism in Scheme 9 has been proposed, where B2 and B, are isomers of the mero-form. Three isobestic points were identified in the transient spectra at different times, suggesting four transient states. Forming between 0.8 and 1.6 psec, the Bi state was assigned as the cis isomer. This had a spectrum similar to that obtained for Tamai s X transient of the spiro-oxazine NOSH, which was obtained at subpicosecond time scales [26]. 

Laser flash photolysis (266 nm) of phenyl azide in pentane at 233 K produces a transient absorption spectrum with two sharp bands with maxima at 335 and 352 nm (Fig. 11.4). Spectrum 1 was measured, point by point, 2 ns after the laser pulse. In later work, the spectrum of 33s was reinvestigated and an additional very weak, long wavelength absorption band at 540 nm was observed (Spectrum 2). The transient spectrum of Figure 11.4 was assigned to singlet phenylnitrene in its lowest open-shell electronic configuration ( 2). 

Figure 11.4. Transient spectrum of singlet phenylnitrene produced upon LFP of phenyl azide. Spectrum 1 was recorded 2 ns after the laser pulse (266 nm, 35 ps) at 233 K. Long-wavelength band (2) was recorded with an optical multichanal analyzer at 150 K (with a 100-ns window immediately after the laser pulse, 249 nm, 12 ns). The computed positions and oscillator strengths (/, right-hand axes) of the absorption bands are depicted as solid vertical lines. For very small oscillator strength, the value multiplied by 10 is presented (/ X 10). [Reproduced with permission from N. R Gritsan, Z. Zhu, C. M. Hadad, and M. S. Platz, J. Am. Chem. Soc. 1999, 121, 1202. Copyright 1999 American Chemical Society.]...

The assignment of the transient absorption spectrum of Fig. 2 to PN is supported by the similarity of its spectrum to that of the longer-lived per-fluorinated singlet arylnitrenes. " The decay of this transient absorption is accompanied by the formation of cyclic ketenimine K. Furthermore, the electronic absorption spectrum of PN in the A2 state calculated at the CASPT2 level is in good agreement with the transient spectrum (Fig. 2). 

This hypothesis was supported by analysis of the transient spectrum obtained upon LFP of 2-fluorophenyl azide, which reveals the presence of triplet nitrene 20a despite the small ratio of kisc/koss- This is clearly evident in Fig. 16 (Insert Spectrum 1), which presents the spectrum of the products formed from the decay of singlet nitrene 16a at room temperature. This spectrum is the sum of the spectrum of triplet nitrene 20a (narrow band at 303 nm and weak absorption below 450 nm) and ketenimine 18a (broad band at 350 nm). This complicated spectrum can be eompared with the simpler spectrum of ketenimine 18b (Spectrum 2) and the spectrum of triplet nitrene 20a observed as a persistent species in a low-temperature matrix (Spectrum 3). It is clear that the yield of triplet nitrene 20a is significant at room temperature. However, if one postulates that azirine 17a does not inter-eonvert with singlet nitrene 16a (Scheme 6, -r). then the yield of... 

From the transient spectrum recorded a few picoseconds after the excitation a quantum yield for the photoionization of 45 15 % is determined. The given error takes into account the un-... 

Fig. 2 Transient spectrum taken in the ns-laser photolysis (15 mJ)of a 02 purged solution of 10"3mol dm"3 4-methyl-2,6-di-tert.-butylphenol in n-butyl chloride taken ( ) 10 ns and (>) 8 s after the pulse. The inset shows the photonity in the transient maxima where the 450 nm curve was threefold amplified (absorption given for I0= 300 mV).

For the reference compound 4, two different parts in the transient spectrum were found (data not shown) a negative signal (480...600 nm) resulting from a combined ground state bleaching and excited state absorption, and second positive part (>600 nm) due to excited state absorption. Both decay on a ns time scale. Close inspection of the spectra reveils another process with a time constant of 10 ps, which in accordance with previous results [1] is attributed to vibrational/solvent relaxation. 

Irradiation of cyclohexane-02 solutions gave rise74 to the transient spectrum shown in Fig. 10. Identification of this spectrum with the cyclohexyl peroxy radical was based on the following observations ... 

Similar results have been obtained with 4T (quaterthiophene) in dichloromethane (absorption maximum at 390 nm). The observed transient spectrum appears structured with two maxima at 560 nm and 610 nm. The lifetime of the corresponding triplet 34T is on the order of 35 (is. The excitation of 6T in dichloromethane (absorption maximum at 432 nm) leads to a transient triplet with an absorption maximum at 680 nm and a lifetime of about 24 (is. 

At all studied nT (n = 3-6), the first spectra which arise during absorption of the exciting pulse (FWHM 25 ps) are the induced fluorescence F and the absorption Ax. With a long delay, the second transient spectrum A2 appears in the spectral region between F and A. Both these decays of each member of the nT series are identically. The comparison of both fluorescence and absorption decay times demonstrates that the A band of each nT stems from the corresponding fluorescence state Si. 

The inclusion of 5 wt% PCBM [l-(3-methoxycarbonyl)propyl-l-phe-nyl-[6,6]C6i] in the spin-coating solutions resulted in efficient polymer emission quenching for all the polythiophenes studied. The transient absorption spectra of the amorphous poly(541)/PCBM blend film. At 10 ps exhibited an absorption peak around 700 nm, similar to that observed for the poly(541) pristine film. The shape of the transient spectrum varied with time, with the absorption peak shifting from 700 nm at 10 is to 900 nm for time delays >100 (is, demonstrating the formation of two distinct transient species in the blend film. The monoexponential lifetime was t = 8 (is under Ar atmosphere and significantly shortened under 02 atmosphere. Monoexponential phase is therefore assigned to the decay of poly(541) triplet excitons. 

Flash photolysis of the analogous dinitrogen complex trans-IrCl (N2)(PPh3)2 (17) demonstrates that flash photolysis leads in both cases to immediate appearance of a transient spectrum the same, within experimental uncertainty, as that attributed above to... 

Fig. 16.4. Typical transient spectrum showing the wavepacket dynamics in the strongly bound B state of I2. The period of 300 fs reflects the oscillatory motion between the two turning points. Adapted from Bowman, Dantus, and Zewail (1990).

Although the transient spectrum of a radical-ion pair was recorded in a picosecond-nanosecond time domain in the flash photolysis of Co(III) alkylcobalamins,123 there has been no direct spectroscopic observation of the LMCT excited states. The observed photochemical behavior may be simply described by the following sequence of events (1) absorption of radiation produces a Franck-Condon excited state, which (2) rapidly loses its excess vibrational energy (k > 10ns ) to form the thermalized excited state, followed by (3) product formation and internal conversion to the ground state. The existence of LMCT excited states with a finite lifetime in the... 

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