  The optical conductivity sigma(omega) is calculated at finite temperature T
for CuO_2 chain clusters within a pd-Hubbard model. Data at T = 300 K for
Li_2CuO_2 are reanalyzed within this approach. The relative weights of
Zhang-Rice singlet and triplet charge excitations near 2.5 and 4 eV,
respectively, depend strongly on T, and a rather dramatic dependence of
sigma(omega) on the ratio of the first to second neighbor exchange integrals is
predicted. On the basis of these results, information about exchange
interactionsfor frustrated edge-shared cuprates can be obtained from
T-dependent optical spectra. Our results are also relevant for magnetically
weakly coupled wide-gap insulators in general.
