Photophysical transient absorption spectra

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

One laser photophysical study (6) has already been reported by Fischer et al. in which a variety of natural water samples and solutions of humic substance standards were excited at 355 nm (near the absorption maximum for excitation of humic substance fluorescence). Extensions of their work appear in this symposium. They resolved two components of transient absorption common to all the samples studied a component with maximum absorbance at 475 nm with a lifetime of several microseconds and a signal at 700 nm which was attributed to a solvated electron on the basis of is its sensitivity to N2O and on its absorption spectrum. 

The use of triplet state acceptors has generally two roles. (1) Where the triplet state is to be removed and the transferred energy degraded as heat. Any passive absorber with lower triplet state energy than the donor will act in this way. Such a triplet state acceptor is also a photochemical stabiliser. (2) Where the triplet energy is to be trapped in a triplet state to be used for measurement or some specific function, e.g. energy transfer to an acceptor of known transient triplet absorption spectrum or emission. Here the acceptor triplet state photophysics and photochemistry must be known. Identification of energy transfer to such another known triplet state is often used as confirmatory evidence that a triplet state species is involved in the reaction under study. 

---