Push–Pull <i>N</i>,<i>N</i>-Diphenylhydrazones Bearing Bithiophene or Thienothiophene Spacers as Nonlinear Optical Second Harmonic Generators and as Photosensitizers for Nanocrystalline TiO₂ Dye-Sensitized Solar Cells

Abstract

A series of push-pull heterocyclic <i>N</i>,<i>N</i>-diphenylhydrazones were prepared to study the effect of structural modifications (different π-spacers and electron-withdrawing groups) on the optical (linear and nonlinear) and electronic properties of the molecules. The photovoltaic response of dye-sensitized solar cells assembled using nanocrystalline titania photosensitized with the synthesized dyes was also studied. These heterocyclic push-pull conjugated dyes involve <i>N</i>,<i>N</i>-diphenylhydrazones as electron donors linked to bithiophene or thieno[3,2-<i>b</i>]thiophene spacers and were functionalized with carboxylic acid, cyanoacetic acid, or dicyanovinyl acceptor groups. A combination of Suzuki-Miyaura cross-coupling, Vilsmeier formylation, and condensation reactions was used to synthesize the intermediates and final products. Density functional theory (DFT) and time dependent-DFT calculations were used to obtain information on conformation, electronic structure, and electron distribution, both for the free dyes and those adsorbed on TiO₂. The results of this multidisciplinary study indicate that dyes <b>5b</b> and <b>6b</b> have the strongest second-order nonlinear optical response with hyperpolarizability values in the range of β = 2330 × 10^-30 to 2750 × 10^-30 esu, whereas photovoltaic power conversion efficiencies reach values in the range of 0.7-3.0% for dyes <b>5a-b</b> and <b>7c</b> and were enhanced by coadsorbing deoxycholic acid (0.8-5.1%).