A large body of experimental data point towards a charge transfer instability
of parent insulating cuprates to be their unique property. We argue that the
true charge transfer gap in these compounds is as small as 0.4-0.5\,eV rather
than 1.5-2.0\,eV as usually derived from the optical gap measurements. In fact
we deal with a competition of the conventional (3d9) ground state and a
charge transfer (CT) state with formation of electron-hole dimers which evolves
under doping to an unconventional bosonic system. Our conjecture does provide
an unified standpoint on the main experimental findings for parent cuprates
including linear and nonlinear optical, Raman, photoemission, photoabsorption,
and transport properties anyhow related with the CT excitations. In addition we
suggest a scenario for the evolution of the CuO2​ planes in the CT unstable
cuprates under a nonisovalent doping.Comment: 13 pages, 5 figures, submitted to PR