Picosecond lasers reactive intermediates

Picosecond-resolved thermochemical information can be extracted from the evolution of a transient grating produced by the crossing of two laser pulses and interrogated with a third short pulse of light. Several groups have applied this method to thermodynamic questions about the decay of excited states and the evolution of excited states into reactive intermediates. 

The reactive intermediates leading to the charge-transfer deligation of the bis(arene)iron(II) acceptors in Eqs. (59) and (60) are examined by time-resolved picosecond spectroscopy immediately following the application of an 18-psecond laser pulse. The time-resolved spectrum from the (HMB)2Fe2+ complex with ferrocene consists of the superposition of the... 

The reactive intermediates leading to the (charge-transfer) photodecomposition of the 6w(arene)iron(II) acceptor are revealed by picosecond time-resolved spectroscopy. For example, photoexcitation of the CT absorption band of the ferro-cene-(HMB)2Fe complex (HMB = hexamethylbenzene) with the second harmonic output (at 532 nm) of a mode-locked Nd YAG laser (25-ps pulse width) generates a transient spectrum with an absorption maximum at 580 nm (see Figure 11 A). Careful deconvolution of this absorption spectrum reveals the superposition of the absorption bands of ferrocenium (Imax = 620 nm, e = 360 cm [162]) and (HMB)2Fe+ (2 ,ax = 580 nm, = 604 M" cm" [163]). 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

If an intermediate is not sufficiently stable to be isolated, it might nevertheless be formed in sufficient concentration to be detected spectroscopically. Techniques used for this purpose include UV—vis spectroscopy in stopped-flow kinetics experiments for relatively stable intermediates or IR spectroscopy in matrix isolation spectroscopy for more reactive species. For photochemical reactions, we can detect transient spectra of intermediates in the millisecond to microsecond ( conventional" flash spectroscopy) or nanosecond to picosecond or femtosecond (laser flash spectroscopy) time scale. In all cases we must be certain that the spectra observed are indeed indicative of the presence of the proposed intermediate and only the proposed intermediate. Theoretical calculations have been useful in determining the spectroscopic properties of a proposed intermediate, whether it is likely to be sufficiently stable for detection, and the t)q e of experiment most likely to detect it. In addition, kinetic studies may suggest optimum conditions for spectroscopic detection of an intermediate. ... 

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