Exciton coupling is of fundamental importance and determines functional properties of organic dyes in (opto-)electronic and photovoltaic devices. Here we show that strong exciton coupling is not limited to the situation of equal chromophores as often assumed. Quadruple dye stacks were obtained from two bis(merocyanine) dyes with same or different chromophores, respectively, which dimerize in less-polar solvents resulting in the respective homo- and heteroaggregates. The structures of the quadruple dye stacks were assigned by NMR techniques and unambiguously confirmed by single-crystal X-ray analysis. The heteroaggregate stack formed from the bis(merocyanine) bearing two different chromophores exhibits remarkably different ultraviolet/vis absorption bands compared with those of the homoaggregate of the bis(merocyanine) comprising two identical chromophores. Quantum chemical analysis based on an extension of Kasha's exciton theory appropriately describes the absorption properties of both types of stacks revealing strong exciton coupling also between different chromophores within the heteroaggregate.

We have synthesized a series of new, polymethine chain extended merocyanine dyes 1−4 bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues 5−8, which contain two methine groups less, by UV−vis and electrooptical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of π-extended merocyanines are markedly redshifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC 61 BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the π-extended merocyanines exhibit lower short-circuit current densities (J SC ) as well as decreased open-circuit voltages (V OC ) and power conversion efficiencies (PCE) compared with those of their respective lower homologues. The unexpected decreased J SC values could be explained in terms of looser packing features of π-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (1/5 and 4/8) of these dyes. By optimization of device setup PCE of 2.3% has been achieved with the π-extended donor material 4.

We have synthesized a large series of bis(merocyanine) dyes with varying spacer unit and investigated in detail their self-organization behavior by concentration- as well as solvent-dependent UV/Vis spectroscopy. Our in-depth studies have shown that the self-organization of the present bis(merocyanine) dyes is subtly influenced by the nature of the spacer unit. The utilization of rigid spacers results in the formation of self-associated bimolecular complexes with high binding strength, while flexible spacers drive the respective bichromophoric dyes to intramolecular folding. Our thorough investigations on the impact of alkyl spacer chain length on the folding tendency of the present series of bis(merocyanine) dyes revealed a biphasic behavior, that is, a steep increase of the folding tendency for the dyes containing C4 to C7 chains and then a gentle decrease for dyes with longer alkyl spacer chains as evidenced by free energy (ΔG) values for the folding of these dyes. Furthermore, analyses of aggregates' optical properties based on exciton theory as well as quantum chemical calculations suggest a bimolecular aggregate structure for the dye possessing a rigid spacer and a rotationally twisted pleated structure for the bis(merocyanine) dyes having spacer units with less than seven carbon atoms, while the application of longer alkyl chain linkers (≥C7) provides enough flexibility to orient the chromophores in electrostatically most favored antiparallel fashion.

A novel bis(merocyanine) dye has been synthesized and its folding and aggregation behavior has been elucidated in dependence on the solvent polarity. Extended cofacially p-stacked H-aggregates could be prepared in nonpolar solvents based on the preorganized folded dimer structure of the dye exhibiting a more than 120 nm hypsochromic shift of the absorption band. Furthermore, incorporation of such H-aggregate domains in bulk heterojunction (BHJ) solar cells by careful adjustment of processing conditions afforded a nearly twofold increase in the photocurrent generation. Our detailed investigations on the external quantum efficiencies of these blends reveal that the increase of the shortcircuit current density J SC stems from the H-aggregated dye manifold in the photoactive layer of the solar cell devices. Results and discussion SynthesisThe target bis(merocyanine) dye 4 was synthesized according to the route depicted in Scheme 1. In the rst step, the literature

A series of new symmetrical donor-acceptor-donor (D−A−D) dyes based on s -indacene-1,3,5,7(2 H ,6 H )-tetraone as an acceptor unit containing varying electron donating moieties and analogous donor-acceptor (D−A) chromophores with indane-1,3-dione as an acceptor are synthesized. By employing these two sets of dyes, the infl uence of a scaffold change from unsymmetric push-pull (D−A) to symmetrical (D−A−D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D−A−D dyes compared to those of the reference D−A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC 61 BM or PC 71 BM as acceptors afforded signifi cantly improved performance for devices based on D−A−D blends (up to a factor of 4 compared to the respective D-A reference) with power conversion effi ciencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D−A−D chromophores exhibit unexpectedly high fl uorescence quantum yields Φ em of up to unity, in striking contrast to their weakly fl uorescent D-A counterparts.