Guest-Host Interactions in Symmetrical Carboxy Heptamethine Cyanine Dyes-Titanium Dioxide Systems: Synthesis, Theoretical Calculations, Aggregation Properties, and Application in Dye-Sensitized Solar Cells

Rodrigo Da Costa Duarte,Fabiano Severo Rodembusch,Matheus Costa De Oliveira,Marcos José Leite Santos,Josene Maria Toldo,Paulo Fernando Bruno Gonçalves,Takuya Fujimura

doi:10.1155/2021/8852979

Abstract

In this work, the role of deoxycholic acid (DCA) as a coadsorbent was investigated in the sensitization of mesoporous TiO2 layers (host) with symmetrical carboxy heptamethine cyanine dyes (guest). Different approaches have been tested, aimed at reducing the H-aggregation and minimizing the competition between cyanine molecules and DCA for active sites of the host, thus improving solar cell efficiency. Heptamethine cyanines containing carboxylic anchoring groups were obtained with good yields. The cyanines present UV-Vis absorption in methanol and dimethylformamide solutions ascribed to fully allowed electronic transitions ( 1 π π ∗ ), as well as fluorescence emission in the NIR region, with any evidence of aggregations in both ground and excited states. TD-DFT calculations were also performed in order to study the geometry and charge distribution of these compounds in their ground and excited electronic states. Solid-state photophysics indicates that the cyanines showed excellent adsorption on TiO2, which can be justified by the presence of the -COOH moieties in the structure. Photophysical measurements have revealed the best concentrations of dye and DCA, which resulted in efficient inhibition of cyanine H-aggregates on the TiO2 surface in addition to allow large dye loading. HOMO and LUMO energy levels of the dyes were identified by cyclic voltammetry, showing oxidation and reduction potentials within acceptable limits for application as a photosensitizer in dye-sensitized solar cells (DSSCs) based on a TiO2 mesoporous photoanode. Assembled DSSCs have shown a large improvement of the electrical parameters and efficiency when a balance between dye aggregation and the competition to the host active sites was reached.

Highlights

Heptamethine cyanines are organic cationic or neutral compounds, structurally constituted by two heterocyclic nitrogenous portions interconnected by a polymetallic chain [1]
The photophysical characterization in methanol and DMF was performed in both ground and excited states using UV-Vis absorption and fluorescence emission spectroscopies and TD-Density Functional Theory (DFT)/PCM calculations
The photophysical characterization in the solid state indicated that these compounds presented excellent adsorption on TiO2, which can be justified by the presence of the carboxylic groups

Summary

Introduction

Heptamethine cyanines are organic cationic or neutral compounds, structurally constituted by two heterocyclic nitrogenous portions interconnected by a polymetallic chain [1] This structural configuration gives these compounds bands of absorption and fluorescence emission that vary from 700 to 1000 nm and can be modulated according to the number of vinyl groups present in the unsaturated chain [1]. Due to their photophysical properties in the near-infrared, this class of dyes has been explored in many technological applications, such as in hybrid solar cells [2] and nonlinear absorbing materials [3, 4], and in bioanalysis, such as enzymatic assays, immunoassays, cancer detection, diagnosis modalities, identification of metallic ions, and pH studies, and as a drug carrier [5,6,7,8,9,10]. The experimental and theoretical photophysical characterizations of these compounds are presented and discussed, and sensitized solar cells were assembled with an optimized dye : coadsorbent ratio

Experimental

Synthesis

Results and Discussion

Theoretical Calculations

Conclusions