Metal(II)-induced synthesis of asymmetric fluorescence benzimidazoles complexes and their dye-sensitized solar cell performance as cosensitizers

Four asymmetric complexes with different conjugate structures based on benzimidazole derivatives with formulas of [ZnL¹Cl₂] (1) (L¹: 1-((quinoline-2-yl) methylene)-2-quinoline-2-yl benzimidazole), [CdL²Cl₂]_n (2) (L²: N¹,N²-bis((quinoline-2-yl) methylene)o-phenylenediamine), [HgL³Cl₂] (3) (L³: 2-(quinoline-2-yl)benzimidazole), and (HgCl₄)(L⁴)₂(C₆H₇)₂ (4) (L⁴: 1,2,3-((quinoline-2-yl) methylene) benzimidazoleion) are obtained. Interestingly, L¹−L⁴ are derived from the half-condensed asymmetrical Schiff base ligand [N¹-((quinoline-2-yl) methylene) N²-(H-(quinoline-2-yl) methylene-hydroxy)o-phenylenediamine] (L) by means of a metal(II)-induced method. To the best of our knowledge, it is the first example of one Schiff base ligand transforming into three different benzimidazole derivatives from the single quinoline ring (L³), double quinoline rings (L¹) to three quinoline rings (L⁴). Complexes 1, 3, and 4 are new mononuclear complexes, while complex 2 is a one-dimensional (1D) chlorine bridged Cd(II)-bis Schiff base (L²) coordination polymer. Complexes 1−4 are driven mainly by a C−H···Cl hydrogen bond and π−π stacking interactions packing to three-dimensional (3D) supramolecular metal−organic frameworks (SMOFs), respectively. Structural analysis shows that complexes 1−4 possesses a 7-connected, (6,6)-connected, 5-connected, (4,6,6)-connected SMOFs topology network with a Schlafl i symbol of {3⁴·4⁹·5⁸}, {4⁹·6⁵}₂, {4⁶·6⁴}, and {3³·4²·5}{3³·4²·5³·6⁷}{3³·4³·5³· 6⁶}, respectively. Under the excitation of 330 nm ultraviolet light, 1−4 show stronger blue luminescent emissions in solution comparing to the nearly nonluminous L. The order of the luminous intensity is 2 > 1 > 3 > 4. The strongest luminous intensity from complex 2 is due to its 1D chain conjugated system with the highest extent of electron-donating nature. The absent absorption of ruthenium complex N719 could be overcome by those complexes in the low wavelength region of the visible spectrum. These complexes could reduce the charge recombination of injected electrons and offset the competitive visible light absorption of I₃⁻. Complexes 1−4 could be utilized as cosensitizers in combination with N719 to examine the effect on enhancing the performance of dye-sensitized solar cells (DSSCs). After cosensitization, the incident-photon-to-current conversion efficiency is increased, and the dark current is reduced. Because of utilizing cosensitizers of 1/N719 and 4/N719, the DSSC devices show an overall conversion efficiency of 8.30% and 8.21% with a short circuit current density of 18.69 mA cm⁻2 and 18.61 mA cm⁻², and an open circuit voltage of 0.75 V and 0.74 V, respectively, under AM 1.5 G solar irradiation. The overall conversion efficiency is considerable, 26.72% and 25.34% higher than that by N719. Accordingly, the performance of N719 sensitized solar cells could be enhanced by the prepared fluorescence complexes serving as cosensitizers.