Fig. 3. Time-dependent results from EEI2D spectroscopy. (a) Measured and (b) calculated evolution of integrated fifth-order signals as a function of population time. The colors indicate the varying length of the polymers. The rectangles determine the regions within which the signal exceeds 90% of its maximum (see text for details). The data were normalized on the decaying tail. (c) Exemplary illustration of excitons in P5 and P18 polymers. The excitons are delocalized over a significantly larger fraction of the polymer in P5 than in P18 and therefore interact sooner after photoexcitation.