Pyrene based D-π-A architectures: synthesis, density functional theory, photophysics and electron transfer dynamics.

Pyrene derivatives show immense potential as sensitizers for dye-sensitized solar cells (DSCs). Therefore, this work focuses on the impact of π-spacers on the photophysical, electrochemical and photovoltaic properties of pyrene based D-π-A dyes, since the insertion of π-spacers is one of the doable strategies to improve the light harvesting properties of the dye. In this respect, three new pyrene based D-π-A dyes have been synthesized and characterized by 1H, 13C NMR, and elemental analyses and EI-MS spectrometry. The selected π-spacers are benzene, thiophene and furan. Compared with a benzene spacer, the introduction of a heterocyclic ring spacer reduces the band gap of the dye and brings about the broadening of the absorption spectra to the longer wavelength region through intramolecular charge-transfer (ICT). Combined experimental and theoretical studies were performed to investigate the ICT process involved in the pyrene derivatives. The profound solvatochromism with increased nonradiative rate constants (knr) has been construed in terms of ICT from the pyrene core to rhodanine-3-acetic acid via conjugated π-spacers. Electrochemical data also reveal that the HOMO and LUMO energy levels are fine-tuned by incorporating different π-spacers between pyrene and rhodanine-3-acetic acid. On the basis of the optimized DSC test conditions, the best performance was found for PBRA, in which a benzene group is the conjugated π-spacer. The divergence in the photovoltaic behaviors of these dyes was further explicated by femtosecond fluorescence and electrochemical impedance spectroscopy.

Unravelling the effect of anchoring groups on the ground and excited state properties of pyrene using computational and spectroscopic methods.

Anchoring groups play an important role in dye sensitized solar cells (DSCs). In order to acquire a suitable anchoring group for DSCs, a deeper understanding of the effect of anchoring groups on the ground and excited state properties of the dye is significant. In this context, various anchoring group connected pyrene derivatives are successfully synthesized and well characterized by using (1)H, (13)C-NMR, FT-IR and EI-MS spectrometry. The anchoring groups employed are carboxylic acid, malonic acid, acrylic acid, malononitrile, cyanoacrylic acid, rhodanine and rhodanine-3-acetic acid. The optimized geometries, HOMO-LUMO energy gap, light harvesting efficiency (LHE) and electronic absorption spectra of these dyes are studied by using density functional theory (DFT) calculations. The results show that pyrene connected with anchoring groups with weak electron pulling strength (PC, PAC and PMC) has a larger HOMO-LUMO energy gap, whereas that connected with anchoring groups with strong electron pulling strength (PCC, PMN, PR and PRA) has a reduced HOMO-LUMO energy gap. These molecules with a reduced energy gap are primarily preferred for DSC applications. Moreover, P, PC, PAC and PMC molecules undergo π→π* transition, whereas PCC, PMN, PR and PRA molecules show significant charge transfer along with π→π* transition. UV-visible absorption spectral studies on these dyes reveal that connecting various anchoring groups with different electron pulling abilities enables the pyrene chromophore to absorb in the longer wavelength region. Notably, an efficient bathochromic shift is observed for PCC, PMN, PR and PRA molecules in both electronic absorption and fluorescence spectral measurements, which suggests that the excitation is delocalized throughout the entire π-system of the molecules. Both theoretical and spectral studies reveal that dyes with an ICT character (PCC, PMN, PR and PRA) are suitable for dye sensitized solar cell applications.

A diminutive modification in arylamine electron donors: synthesis, photophysics and solvatochromic analysis--towards the understanding of dye sensitized solar cell performances.

An electron rich donor moiety plays an important role in dye sensitized solar cells (DSCs). In order to attain a suitable donor moiety for DSCs, a deeper understanding of the role of a donor moiety in the dye excited state is significant. In this context, different arylamine dye-based electron donor moieties (TRA, CRA and PyRA) were successfully synthesized and well characterized using (1)H-NMR, (13)C-NMR and EI-MS spectrometry. Their photophysical properties and solvatochromic behavior were studied using UV-visible absorption, steady state and time resolved fluorescence spectroscopic techniques. The absorption of arylamine dyes is due to intramolecular charge transfer (ICT) between the donor and rhodanine-3-acetic acid via a π-bridge, which is further confirmed by DFT calculations. Lippert-Mataga analysis on the solvatochromic data implies that these molecules are more polar in the excited state, which is additional support for ICT. Furthermore, nanocrystalline TiO2-based dye sensitized solar cells (DSCs) were fabricated using these dyes to investigate the influence of donor moieties on their photovoltaic performance. The overall power conversion efficiencies of 2.57%, 1.68% and 1.25% were obtained for the TRA, PyRA and CRA dyes, respectively. The enhanced power conversion efficiency of TRA is due to a longer lifetime of injected electrons as demonstrated by the electrochemical impedance spectroscopy (EIS) measurements.