Novel Organic CT Salts Employing 1,3-Bis(Dicyanomethylidene)Indan Anion As a Donor

Abstract

[Introduction] Organic charge transfer crystals (CTCs) can be interesting candidates as light absorbers of solar cells to eliminate large voltage loss inherent in dye-sensitized and bulk hetero junction types, in which carrier generation relies on the energy cascades. We have synthesized novel CTCs by forming salts between 1,3-bis(dicyanomethylidene)indan anion (TCNIH-) as a donor and viologen cations as acceptors, namely, N,N’-alkyl substituted 4,4’-bipyridiniums (Fig. 1, methyl = MV2+, ethyl = EV2+, heptyl = HV2+ and octyl = OV2+). [Experimental] Mixed salts of TCNIH- and viologens were obtained by slowly evaporating solvent at room temperature from their 2 : 1 mixed solution in ethanol. While their crystal structures were examined on powder samples and single crystals, their optical properties were studied by measuring UV-Vis and PL spectra between 77 and 298 K on a Horiba Fluorolog-3 equipped with a liq. N2 cryostat. [Results and Discussion] Colorless TCNIH2 turns into deep blue in ethanol due to deprotonation from its methylene carbon to become TCNIH- anion (Fig. 1). While its salt with Na+ was purplish black, having similar absorption features as the solution of TCNIH- with its onset around 800 nm, co-crystals with MV2+ and EV2+ were black with metallic shine to exhibit extended CT absorption in NIR up to ca. 1,000 nm (Fig. 2). On the other hand, those with HV2+ and OV2+ are quite similar to the Na+ salt with only small extension. All of the mixed crystals exhibited XRD patterns different from those of original TCNIH2 and viologen halides, indicating formation of their salts. Single crystal XRD structural analysis performed on large grains revealed monoclinic structure for MV2+ salt, while triclinic for the rest, in which TCNIH- and viologen cations are alternately stacked in 2 : 1 ratio. On increasing the length of the alkyl substituents, expansion and distortion of lattice occurs as seen from their lattice constants summarized in Table 1. The close packing of TCNIH- and MV2+/EV2+ by coulombic interaction obviously contributed to the enhancement of CT, whereas bulky HV2+ and OV2+ prohibited their intimate interaction. PL spectra were measured for the salts (Fig. 3). The Na+ salt indicates a spectrum very similar to that of the TCNIH- solution with not much enhancement and peak shift between 77 and 298 K, indicating emission from the exciton localized in a single TCNIH-. The salts with MV2+ and EV2+ exhibit broad emissions in the NIR range, which are greatly enhanced and resolved into two peaks at 77 K. Exciton-phonon coupling to delocalize exciton within CTCs explains these observations. PL spectra of HV2+ and OV2+ salts are unique, with their energy clearly smaller than that of the Na+ salt, despite of their very similar energy of excitation. Also, clear enhancement and resolution into two peaks at 77 K in case of OV2+ salt suggest relaxation of the TCNIH- localized exciton down to the CT states within these crystals. Acknowledgment The present work was supported by Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers, “Advanced Next Generation Energy Leadership (R2601, FY2014-2016)” of Japan Society for the Promotion of Science (JSPS). Figure 1