Charge-transfer absorption band enhancement

Figure 12 Vibrational enhancement selectivity available from resonance Raman spectroscopy. The UV-visible spectrum of a P. aeruginosa azurinis shown together with two different Raman spectra (frozen solution at 77 K) that derive from laser excitation within the S(Cys) — Cu(II) charge-transfer absorption band at 625run (647.1 nm) and away from the absorption (488.Onm). Excitation within resonance leads to dramatically increased Raman scattering from the Cu active site, whereas off-resonance excitation produces a spectrum dominated by bands of nonchromophoric ice...

Based on the absorption spectrum of the above material there is a strong charge-transfer (CT) band at 626.6 nm. This is an indication of an enhanced charge separation and greater nonlinearity. 

While the band position in the spectra of acylmetallocenes is close to that of the parent, above 325 m its intensity is enhanced with the introduction of the —COR. (This is in contrast to the alkylmetallocenes, whose spectra are almost identical to the parent metallocene.) Below 300 m(x, however, the intense, broad end-absorption in the spectrum of the parent metallocene is replaced by two intense peaks (for acylferrocenes these appear near 230 and 270 m j.). These are believed to be associated in part with electronic transitions of the substituent with the ring, superimposed upon metal to ring charge-transfer bands.7... 

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