The solid-state emission spectra of clusters such as 2a display
two distinct bands: a very intense lower-energy (LE) band and
an extremely weak higher-energy (HE) band (Figure 5). The
marked temperature dependence of the apparent color of the
luminescence of these clusters does not arise from a shift in the
emission maximum, λmax
em, but from a significant change in the
relative intensities of the two bands. At room temperature, the
HE band is barely detectable and the LE band is largely
dominant, whereas below 80 K, the HE band overcomes the
other.7 For the sake of strictness, it must be mentioned that a
shift in λmax
em can actually be observed for the LE band (580
nm at 25 °C, 619 nm at −196 °C, Figure 5), but it is too small
to justify any dramatic change in color