Charge separation (CS) in molecular systems usually takes place in weakly coupled donor–acceptor dyads where an electron charge moves from the donor to the acceptor in the local excited state of a chromophore. Herein, we present a two-step charge-separation process in a newly synthesized diketopyrrolopyrrole-pyrrolopyrrole (DPP-PP) dyad (AD), which starts from the initial photoexcited bright exciton and goes through a partial charge-transfer (CT) state before finally reaching the charge-separated (CS) state. The evolving CT character in the excited state is demonstrated through the complementary use of transient absorption, broad-band fluorescence upconversion, and transient impulsive stimulated Raman spectroscopy. The bright exciton state of the dyad relaxes to a partial CT state with 1 and 20 ps during solvent and structural fluctuations in toluene, respectively, and with 700 fs for the solvent fluctuations occurring in tetrahydrofuran. This is evident from the characteristic excited-state absorption spectra and the reduced fluorescence intensity observed on the adiabatic potential energy surface. AD in THF additionally evolves to the diabatic potential energy surface of the CS state, whose absorption spectrum converges to that of a DPP anion for which fluorescence is completely quenched. The trend of shifting for certain vibrational frequencies also supports the proposed CT dynamics and mechanism; furthermore, it gives quantitative insight into the CT characters of the bright state (0.1 e) and intermediate partial CT state (0.5 e), as determined by the linear relationship that exists between the vibrational frequency of the marker modes and the CT character. We have found that as the structure of the bridge between donor and acceptor enables an intermediate level of electronic communication, the charge-separation can rapidly occur through a distinct partial charge-transfer state. It seems that the exceptionally strong electronic communication at positions 2 and 5 of the pyrrolo­[3,2-b]­pyrrole core is a crucial element for this charge-transfer mechanism, which could be applied to organic photovoltaics or light-emitting diodes requiring efficient charge separation.

A highly efficient procedure for the synthesis of bis-coumarins fused at the pyranone ring has been developed. The electron-rich phenols reacted with esters of coumarin-3-carboxylic acids, leading to substituted chromeno[3,4-c]chromene-6,7-diones. The reaction is catalyzed by both Lewis acids and 4-dimethylaminopyridine. The most probable mechanistic pathway involves Lewis acid catalyzed or DMAP catalyzed transesterification, followed by intramolecular conjugate addition of α,β-unsaturated esters to phenols and subsequent oxidation of the initially formed intermediate. The reaction is compatible with various functionalities such as NO2, Br, and OMe. Not only benzene derivatives but also dihydroxynaphthalenes are reactive in this reaction, and the structure of the product can be controlled by adjusting the reaction conditions. Furthermore, a double addition is possible, leading to a horseshoe-shaped system comprised of seven conjugated rings. Compounds with four structurally unique skeletons have been obtained and have been shown to strongly absorb in the violet, blue, and/or green regions of the visible spectrum. Most of them display strong greenish yellow fluorescence, which can be modulated by both structural changes and the character of the solvents. Again, introduction of an electron-donating group in the chromeno[3,4-c]chromene-6,7-diones caused a significant red shift in both the absorption and emission maxima, and the effect became especially noteworthy in the case of amino substituents.